TECTONICS BLOG
Rev. 2024-04-04 www.impacttectonics.org
TECTONICS BLOG
Rev. 2024-04-04 www.impacttectonics.org
Gregory Charles Herman, PhD, Flemington, New Jersey, USA
Introduction * Review * Geographic maps of eighteen lunar astroblemes * Seismological aspects * The South Pole - Aitken basin * Maria Imbrium and Serenetatis * Mare Nectaris * Mare Crisium * Mare Orientale * Mars and Earth models * Discussion * References * GE Pro Moon KMZ file
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Click on an image to enlarge it Two large bolide strain fields mapped on the North American tectonic plate using GE Pro Figure 1. ITFF crustal-strain fields mapped around two large impact craters on the North American tectonic plate. Circumferential blast patterns drawn around each crater include strain sectors dominated by compression and reverse faulting (C - down range), tension and normal faulting (T - up range), or mixed-mode (M) faulting within lateral sectors. Radial arching of the lithosphere is traced around each crater at 660, 1600, and 2900-km radii. The compressed foreland has thickened upper mantle and crust downrange of the crater where grounded impact energy is focused and refracted back to the surface at great distances. Crustal seismogenic zones mapped inside 90oN to 90oS latitudes and 30oE to 150oW longitudes mapped by Herman (2006). Mapped basins and uplifts mapped from Trehu and others (1989), Pindell and Kennon (2009), and Ewing and Lopez (1991). LU – Llano uplift. CP – Colorado Plateau. ETOPO1 surface base theme from Amante and Eakins (2008).  Figure
2
Large, km-scale meteorite impacts on Earth produce
enormous amounts of grounded shock energy resulting in far-field strains
in the upper mantle and crust reaching thousands of kilometers distance
from craters. Experimental
results from impact tests using small glass balls and seismological ray
paths for compressional body waves (P-waves) help constrain the geometry
of planetary-scale impact-tectonic, far-field (ITFF) strains (adapted
from Herman, 2022). The Earth model is part of a SU Pro CAD model.Target and projectile impact parameters 
Figure
3.
Model impact
parameters for an oblique impact including a missile, target, impact point (O) and
impact crater. AO is horizontal to ground surface at the impact point,
CO is vertical, BO is the missile trajectory (impact angle
Θ =
70o). Four equidimensional blast sectors include two up- and
down range ones and two lateral ones split along the plane of
trajectory. Real blast sectors are disproportional and skewed by
impact obliquity and inherited compositonal and structural heterogeneity of target materials.
Headings are horizontal in the plane of trajectory measured down range
relative to geographic north. Fracture strikes, patterns, and strain areas from glass-ball impact tests conducted at high (>60°), intermediate, and low (<30°) angles
Lunar chronology and Apollo anorthosite sample 600025 
Figure 5. Top - Lunar chronology including large impact basins with temporal placement and symbols scaled to outer-ring diameters (table 1). Twelve of the largest basins mapped in figure 6 are represented, most of which are thought to occur during a Nectarian and Early Imbrium period of heavy bombardment (~3.8 to 3.9 Ga). Basin ages and bombardment-rate curve adapted from Tartèse and others (2019) and Stöffler and Ryder (2001). Note the clustering of ages of the oldest impact melts and the oldest gneiss on Earth. 1 Age of the SPA from Morbidelli and others, 2012). The open circle labeled “V” represents the Vredefort impact crater (2.2 Ga), the oldest confirmed impact crater on Earth (Allen and others, 2019). Bottom - Apollo 16 surface sample 600025 photomicrograph and facts.Table 1. Locations, sizes, and interpreted headings of eighteen lunar astroblemes (fig. 5). Cells highlighted gray fall within +10% deviation of calculated ring dimensions using a Ö2 scaling ratio for adjacent rings. This ratio suits 64% (9/14) of outer rings 4 to 6, 33% (12/36) of inner rings 2 and 3, and 42% overall (21/50).  The structure of eighteen lunar astroblemes in geographic space using eleven geospatial themes 6A. NASA Digital Terrain Model 
6B. Structures and Geosamples
6E.
Bouguer Gravity (Watters, 2022)
6F.
JAXA SELENE Magnetic-Field Intensity
6G.
NASA LP Spectroscopy - Silicon
6H. NASA LP
Spectroscopy - Aluminum
6I. NASA LP Spectroscopy
- Iron
6J.
NASA LP Spectroscopy - Potassium
6K. NASA LP Spectroscopy
- Uranium
6M Circular histograms of
large-bolide headings Figure 6. Top - Eighteen multi-ring astroblemes mapped in geographic space using 11 geospatial themes. Maps include a colorized NASA DTM (5A), GRAIL free-air (5B) and Bouguer (5C) gravity, large faults and geosamples (D, a Bouguer gravity gradient map (5E) by Watters (2002), SELENE magnetic-field intensity (5F), and LP spectroscopy themes of elemental abundances in shallow ground for silicon (5G), aluminum (5H), iron (5I), potassium (5J), uranium (5K) and thorium (5L). Each multi-ring structure is labeled and includes concentric rings of variable radii corresponding to radial and concentric crustal features and ITFF strain limits. Ring dimensions and bolide headings summarized in table 1. Rings radii labeled for the Aiken and Mare Imbrium basins. Outer ring radii labeled for all other basins. NASA and JAXA data sources explained in the text. The orange line labeled SPA limits and large-basin limits are interpreted boundaries used for calculating basin and astrobleme areas. Bottom - Interpreted bolide headings are statistically tallied and compared to large-bolide headings mapped on Mars. Lunar cross section including seismological traces of refracted and reflected shock energy  Figure 7. A Moon profile and aligned seismic-velocity profile adapted from Weber and others (2011) used to model interior phase boundaries. Mantle and crustal regions having energetic shock-wave rarefaction reflections are highlighted yellow. The outer rings of some impact basins correlate well with shock reflections of 60° arising from material-phase boundaries with inverted acoustic impedances like the upper- and lower-mantle boundary (fig. 8; Telford and others, 1976). Seismic-wave energy coefficients vs. wave-incidence angles for reflected and transmitted compression waves
 Figure
8.
Graph
adapted from Telford and others (1976, fig. 4.18) of compression-wave energy
coefficients versus incidence angles for partitioning compression-wave
energy at layer boundaries of contrasting densities. Most shock energy
is transmitted across layers having increased density contrasts at
incidence angles less than 60°. But most of the seismic
energy is reflected off all boundaries at high incidence angles above 80°.
The most
reflected energy is focused at a 60°
incidence angle for boundaries having decreasing densities and compression-wave
velocities (inverted acoustic-impedance contrast).
Lunar seismic-compression-wave reflection geometry in the
upper mantle
 Figure 9. Compression-wave refraction and reflection paths for the Moon’s crust and upper mantle at 30°, 45°, and 60° incidence angles. P-wave refractions follow those of Wieczorek (2009). Note how head waves reflected off phase boundaries with negative impedance contrasts give rise to rarefaction waves that return to the surface with diminished energy but first-arrival tension that induces normal, brittle failure and localized upper-mantle melting along crustal faults. The South Pole - Aiken Basin (SPA)    Figure
10.
The SPA basin portrayed in GE Pro using ten
geospatial themes. Top pair includes NASA LOLA DEM (left) and Bouguer
Gravity by Watters (right; 2022). Middle pair is SELENE magnetics (left)
and GRAIL free-air gravity (right). The bottom panel
includes a stock GE
Pro surface image and NASA LP spectroscopy for silicon, aluminum, iron (wt.
%) and potassium and uranium (ppm). The blue stippled area along
longitude W180° is a 5° data gap that is explained
in the text. CA - Mare Crisium antipode, IA - Mare Imbrium antipode, SA
- Mare Serenetatis antipode. AC1 - Aitken basin center of Hurwitz and Kring (2014). AC2 - Aitken basin 890-km radius center ring center.Maria Imbrium and Serenitatis    Figure
11. Maria Imbrium and Serenitatis
portrayed in GE Pro using ten
geospatial themes. Top pair includes NASA LOLA DEM (left) and Bouguer
Gravity by Watters (right; 2022). Middle pair is SELENE magnetics (left)
and GRAIL free-air gravity (right). The bottom panel includes a stock GE
Pro surface image and NASA LP spectroscopy for silicon, aluminum, iron (wt.
%) and potassium and uranium (ppm). Geosamples locations noted.Mare Nectaris  Figure
12.
A GE Pro portrait of the Mare Nectaris astrobleme
overlapping an earlier one using the NASA blue
steel DTM (left),
JAX SELENE magnetic-field intensity (middle), and GRAIL free-air gravity intensity
(right). This astrobleme displays axial splitting with a type,
double-shear fault response. It likely was of moderately high obliquity
and northwest heading. It marks the base of the Nectarian lunar period
before the late-heavy bombardment period. Ring radii marked in kilometers.Mare Crisium  Figure
13.
A GE Pro portrait of the Mare
Crisium astrobleme using the NASA blue
steel DTM (left), JAX SELENE magnetic-field intensity (middle), and GRAIL free-air gravity intensity
(right). This
astrobleme also displays axial splitting and of moderately high
obliquity and eastward heading. Earlier faults from Mare Nectaris are
crosscut by Mare Crisium as seen in the circular histogram.Mare Orientale    Figure
14.
Mare Orientale in GE Pro (top) and cross section (bottom). Top maps include
the NASA Blue Steel DTM (left) and the GRAIL free-air gravity themes
(right). above maps using GE Pro stock imagery (left) and the Watters (2012) Bouguer gravity theme (right). Bottom diagrams include an E-W cross
section of the Mare Orientale basin above diagrams of Moore (1976)
illustrating maps and profiles of craters formed by oblique missile
strikes.Lunar impact tectogenesis  Figure 15. Profile diagrams of lunar, upper mantle and crustal ITFF strains, and a large igneous province stemming from impact tectogenesis that includes ring dikes intruded along concentric faults. A structural solution to having KREEP terrane registered spectroscopically on the lunar surface is to have deep-seated upper mantle and crustal faults bringing uppermost-mantle layers to the surface downrange of a large, oblique impact from rapid, reverse faulting and uplift stemming from focused, refracted shock energy. Cone, cup, and saucer reflection ITFF geometry  Figure 16. A SketchUp Pro 2022 CAD Moon model illustrating two overlapping sets of ITFF strain models comprising the South Pole -Aitken Basin astrobleme. This event involved a low- to moderate angle, very large, fragmented bolide with multiple projectiles having formed a vast strewn field including the Aiken basin where down-range upper-mantle wedging occurs from refracted, radial faulting that's overprinted by reflected rarefaction structures (fig. 17). SUP CAD model of the SPA and Maria Imbrium, Serenitatis, and Nectaris overlapping astroblemes occupying opposite hemispheres.  Figure 17. A SUPCAD model of the Moon includes structural details of the SPA and three other large astroblemes occupying the opposite hemisphere that include Maria Imbrium, Serenitatis, and Nectaris (fig. 18)The SPA is modeled with two impact centers, the more northerly having formed by spalling of the parent bolide forming the South Pole crater. Together they constitute an enormous impact strewn field. Low-intensity gravity anomalies peppering the Aitken basin downrange of the South Pole are likely secondary craters formed during this event. Three overlapping lunar astroblemes comprising Oceanus Procellarum  Figure 18. Maria Imbrium, Serenitatis, and Nectaris are three of the largest lunar astroblemes that overlap in close proximity on the Moon's nearside with ITFF strains that cut across and interfere with one another. A SU Pro model in semi-transparent mode is used to demonstrate the spatial alignment of mapped upper mantle and crustal faults caused by refracted and reflected shock-waves arising from the acoustic boundary between the upper and lower mantle. This surface has an inverted impendance contrast for compression waves that gave rise to rarefaction (pressure-release) waves that incited near-surface material failure under tension and upper-mantle magmatism from decompression melting. Igneous plutons intruded along the concentric, tensional faults correlate with localized high-intensity gravity flares. The shock-wave reflection geometry resembles cones and tea cups with saucers. The left two diagrams are overhead views of the overlapping astroblemes whereas the right diagram is a side view illustrating a cross-section perspective of the structural interference using the reflected 30o and 60o compression-wave models (fig. 16).
Lunar tidal locking
 Figure 19. The Moon's heavy side has the densest concentration of mass (mascons) and always faces Earth. It's locked into orbit with Earth so that it spins once on its polar axis for each lunar orbit with its 'near side' always facing us. This is called tidal locking and explains why we only directly see half of the lunar surface. It also gives a spatial perspective on the powerful yet subtle gravitational forces operating over vast distances in space.A SUP CAD Mars model  Figure 20. A SUP 2022 Mars model incorporating the seismological constraint on interior layering and the mantle regions where shock rarefaction reflections occur relative to a large impact crater. The major velocity inversion at about 1500 km depth at the boundary between the plastic mantle and liquid core is a major reflective surface. Reflections of 30° to 60° incidence angles are highlighted yellow and correspond spatially with the surface extents of Mars' largest astroblemes at about 2500 km radius. A satellite-derived gravity-intensity map of Mars centered on the Syria-Sinai-Solis Planums astrobleme
A SUP CAD Earth model
A satellite-derived gravity-intensity map showing the ringed structural nature of the suspected Congo basin astrobleme  Figure 23.
A
satellite-derived, gravity-intensity theme for Earth made for GE Pro
portraying the crustal and upper-mantle stains centered on the Congo
basin, Africa. The 5000-km radius ring around the basin center
corresponds to the 60o rarefaction reflection off the
core-mantle boundary that directly aligns with the Triassic Newark rift
basins hosting the central Atlantic magmatic province (CAMP; fig. 23)
and the southern oceanic-spreading ridges framing Africa. This is also
when Pangaea
began splitting apart with India, North America, and
Australia drifting rapidly away. Stylized map reconstruction of Pangaea near the start of the Mesozoic Era showing how CAMP and continental rifting could have been spurred by a suspected bolide impact forming the Congo basin
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I recently proposed punctuated tectonic equilibrium as an alternative tectonic theory to uniformitarianism after mapping impact-tectonic far-field (ITFF) crustal and upper-mantle strains in many astroblemes on Earth and Mars using global physiographic, gravity and magnetic geophysical themes, geographic information systems (GIS) and three-dimensional (3D) virtual globes (figs. 1 and 2; Herman, 2022). The ITTF strains are global tectonic components arising from impact tectogenesis; when catastrophic, large-bolide (asteroid or comet) impacts suddenly disrupt a terrestrial bodies' crust and upper mantle during episodes of projectile bombardment and mass accretion. However the manner in which shock stresses are absorbed and dispersed within the target body were underdeveloped then for shock strains stemming from reflected energy because of my initial focus on refracted shock strains after recognition of foreland ITFF compressive strains occurring down range from the Chesapeake (35.5 Ma) impact on the eastern seaboard of north America (fig. 1). This work therefore advances the geometry and structural effects of reflected shock energy as part of the finite strains on terrestrial bodies resulting from impact tectogenesis. The lunar surface and interior are first portrayed with 2D and 3D illustrations including seismological constraints that limit the geometric solutions for where reflected shock energy is focused to produce ITFF strains. The multi-ring astroblemes (impact structures) of the Moon are measured and placed into context with the interior layering to show how km-scale bolides impacting the lunar surface at hypervelocity speeds have produced numerous, large, multi-ring astroblemes having strain components stemming from both refracted and reflected shock energy. Two of the largest impact events, the South Pole - Aitken Basin and Mare Imbrium ones, have played significant roles in lunar geological evolution by generating regional melt bodies upon impact that included major phases of mineral-differentiation and fractionation resulting in the formation of the bright, silicon-rich, anorthositic highlands surrounding concentrations of dark, iron-rich mare (ma-aire); the high density, mafic admixture of impact-melted and reconstituted crust and upper mantle stemming from impact tectogenesis. The Moon lacks an atmosphere and plate tectonics, so its surface geology stems from extraterrestrial bombardment and gives the clearest picture of ITFF strains manifest by targeted terrestrial bodies.
The seismological constraints from the lunar study are then applied to profile illustrations of Mars and Earth that spatially constrain regions around large terrestrial impact craters subject to focused shock strains. This work is done using geographic information systems (GIS), the Sketch Up (SU) Pro computer-aided 3D drafting system (CAD) and Google Earth Pro (GE Pro). It is an exercise in analytical geometry rather than numerical modeling, and structural insights are gained from applying empirical results obtained from missile-impact tests (Moore, 1976), from both traditional (Telford and others, 1976) and atomic-bomb seismology (Dienes and Fisher, 1961), and from prior geological analyses of the lunar surface (Wilhem and others (1987) and Watters and others (2022) among many others).
The concept of tectonic inheritance was also raised in the aforementioned work because ITFF strain fields stemming from different impacts overlap to form interfering surface structures with deep roots in terrestrial bodies. An astrobleme imposes structural heterogeneity in a target body that subsequently perturbs the shock responses made by subsequent impacts with superimposed strain fields. Many ITFF structures also exhibit planar geological symmetry with respect to impacts striking at oblique angles and producing down-range crustal wedging and thickening opposed to up-range crustal rifting where large tensile faults give rise to the sudden, concurrent production of magma in deep reaches from dynamic decompression (figs. 1 to 3). Mantle melting occurs along ITFF faults in both radial and concentric alignment to craters that mediated magmatic ascent into the crust to form large igneous provinces spurred by impact tectogenesis.
I begin by mapping and profiling the structural layering and seismological nature of the Moon to gain a spatial perspective on how refracted and reflected impact-shock energy produced its large impact basins and ITFF multi-ring surface structures. Structural details for eighteen multi-ring astroblemes are mapped using eleven geospatial themes to demonstrate how impact tectogenesis produced the contrasting physiography of the bright lunar highlands with the dark mare flooring deep, centralized impact basins that together constitute the face of the Old Man of the Moon. The largest lunar impact basins are portrayed as vast astroblemes where upper mantle and crustal layers have been melted, plasticized, and brecciated around craters with crustal compaction and thickening down range of oblique projectile strikes and tensional rifting up range and circumferential to craters with the latter stemming from reflected shock waves. LIP production apparently occurs in ways conforming to these two, incremental, ITFF strains mechanisms with recognizable seismological behavior and structural interference.
Figure 3 summarizes key geometric impact parameters used to characterize interpreted bolide or missile trajectories including the surrounding blast quadrants, angle of impact, and principal axis of compressive stress. Oblique impacts display symmetry with respect to the plane of trajectory as opposed to vertical impacts that are axis symmetric with more equally distributed radial strains given a homogeneous target (Monteux and Arkani-Hamed, 2019). I also demonstrated both axis and plane-symmetric impact strain fields in little glass spheres with a bench-top impact experiment that used a steel projectile fired from an air gun into the glass balls at oblique impact angles ranging between 35° and 85° (fig. 4; Herman, 2022). In the most energetic impact test set at a very steep impact angle, a down-range structural tongue was produced from absorbed shock energy that has a structural form resembling the geometry of refracted elastodynamic compression (P) waves arising from near-surface seismic sources in Earth’s crust (fig. 2). This experiment helped constrain the geometry of down-range, foreland ITFF crustal wedging and thickening seen at the regional scale for Earth and Martian astroblemes, but it didn’t adequately explore the possibility of reflected impact-shock energy contributing to the ITFF concentric faulting and crustal welting around large impact craters. Outboard crustal arching, intra-plate seismic zones and crustal drift of the North American tectonic plate (NAP) point to ITFF concentric welting including active intra-cratonic seismogenic zones that define sub-plate boundaries in North American lithosphere relative to the Chicxulub crater (fig. 1). The ITTF welting includes epierogenic arches and intervening troughs or moats that together form low-amplitude lithospheric waveforms having amplitudes on the order of few kilometers. These radial, curved structures are large mountain ranges and sedimentary basins that likely formed by impact tectogenesis. A good example of this is where southern most Mexico and the Central American isthmus arose from the seas along the 2900-km arch developed around the Chesapeake impact (fig. 3). The focus here is on exploring how grounded shock energy arising from large-bolide impacts is manifest in the upper layers of the Moon, Earth, and Mars. I use computerized geological models that are constrained by geophysical principles to illustrate how target bodies are layered, and how those layers systematically dispel impact shock waves resulting in ITFF regional strains including radial and concentric crustal faulting, concentric lithospheric welting and large-igneous provinces (LIPS).
The bench-top impact experiments of Herman (2022) using 60 mm glass balls and a hardened steel projectile displayed systematic variability of the strain-field areas with respect to impact obliquity, and the typical development of conjugate, secondary, brittle structures bracketing the crater that varied in their extent and density with respect to gentle (<30°), intermediate (30° - 60°), and high (>60°) impact angles when measured from the spherical surface (fig. 4). The geometry and area of each strain field varied with impact angle such that the near-normal impact has a circular shape and axial symmetry whereas those formed by moderate to shallow impact angles developed planar symmetry across the plane of trajectory (figs. 3 and 4). The moderate-angled impacts produced fan shaped strain fields, and impacts at the shallowest angle having the largest faults flaring out in lateral sectors in a direction normal to the impact headings (fig. 4). These forms are referenced below to help interpret an astroblemes impact obliquity, but impact angles are not tallied in table 1 owing to the high levels of uncertainty in their interpretations. More impact testing into spherical surfaces is necessary in order to derive statistically valid reference models more certain ones. Large crustal fault striking in conjugate arrangement bracketing an impact strewn field were also noted in the structural analysis of thirteen Martian astroblemes using remote sensing (Herman, 2022). The most notable aspect of these impact experiments and mapping exercises was the production of a down-range, fractured wedge from refracted shock energy that was focused downward within the target along the line of impact momentum (fig. 2). The structural tongue descends into the foreland blast sector where the focused energy was refracted back to the surface almost one-quarter of the surface span away from the crater. The downrange wedging stems from uniaxial compression of the targeted media and opposes the up-range sector where tensile fracturing occurs after the crust is first compressed, then stretched behind the down-range sector like that seen on Mars for the Syria-Sinai-Solis Planums astrobleme and up-range volcanism.
Sequential impact-generated shock events on a planetary surface gradually hardens its exterior from repeated bombardment producing overlapping, far-field strains exhibiting tectonic inheritance (Herman, 2022). In other words, pre-existing geological heterogeneity of a terrestrial body will influence the seismological expression of absorbed energy and hence structural expression of successive, overlapping strain fields. But the ITFF strains occurring as regional, large-scale, epierogenic welts are poorly understand and their geodynamic mechanisms ill defined. I therefore attempt to address these aspects below by first mapping and parameterizing eighteen multi-ting lunar impact basins, and then using a 3D Moon model to portray seismologically constrained, geometric solutions for the shock strains stemming from km-scale bolide impacts resulting in multi-ring terrestrial astroblemes. Impact tectogenesis must also include the geological processes and sets of secondary structures leading to the formation of the lunar highlands where thickened crustal regions having elevated gravity intensities likely reflecting the spurred emplacement of basic- to granitic plutons along secondary faults and and mineral veins lying beneath impact-generated regolith and distal ejecta blankets that radiate outward beyond the crater for hundreds to thousands of kilometers distance (fig. 6 and Willhelms and others, 1986). Widespread epeirogeny, or the pronounced vertical tectonic shifts that have been noted since the advent of geology as a science, are placed into geospatial perspective below with the natural, but extraterrestrial-born, impact-tectonic agents of bolide bombardment and accretion that are integral agents of solar system evolution.
With respect to the Moon, much of its geologic
history is gained by studying the distribution and structural expression of
large, multi-ring astroblemes across its surface and from geological analyses of
about one-third ton of surface samples and core collected on the near side by
the U.S.A, Russia, and Chinas' manned and unmanned missions (figs. 5 and 6, and
table 1). According to Wilhelms and others (1987) impacts began to leave a
visible record about 4.2 billion years ago (Ga), after the crust and mantle had
differentiated and the crust had solidified. At least 30 basins and 100 times
that many craters larger than 30 km in diameter were formed before a massive
impact created the Nectaris basin about 3.92 Ga. Impacts continued during the
ensuing Nectarian Period at a lesser rate, whereas volcanism left more traces
than during pre-Nectarian time. The latest basin-forming impacts created the
giant and still-conspicuous Imbrium and Orientale basins during the Early
Imbrian Epoch, between 3.85 Ga and 3.80 Ga. The rate of crater-forming impacts
continued to decline during the Imbrian Period. Beginning in the Late Imbrian
Epoch, mare-basalt flows remained exposed because they were no longer obscured
by many large impacts. The Eratosthenian Period (3.2-1.1 Ga) and the Copernican
Period (1.1 Ga to present) were times of lesser volcanism and a still lower,
probably constant impact rate. Copernican impacts created craters whose surfaces
have remained brighter and topographically crisper than those of the more
ancient lunar features. But so far, no direct sampling has occurred on the far
side.
As impact craters increase in size, they become
increasingly complex and change from having central peaks or groups of peaks to
having central depressions surrounded by two or more mountainous rings (Hartmann
and Kuiper, 1962). This transition occurs for structures exceeding about 180 to
300 kilometers in diameter with the latter regarded as impact basins. More than
thirty multi-ring astroblemes are identified on the Moon as these structures
shape the lunar surface (Willhem and others, 1987; Fasset and others, 2012;
Tartèse and others, 2019; among many others). Eighteen of the largest and most
apparent multi-ring impact structures are mapped in figure 6 with the
corresponding ring radii and interpreted heading parameters recorded in table 1.
Hartmann and Wood (1971) provide a thorough review and synthesis of geological thought surrounding multi-ring impact basins of the Moon during the first decade of lunar exploration. Ideas for ring genesis ranged from frozen crustal tsunamis to collapse faulting from propagating standing waves, but they end their summary with the statement “stresses will be set up in the Moon by the violent dissipation of energy during basin formation; all authors agree that these stresses are likely to produce a series of concentric ring fractures surrounding the basin, through the detailed theoretical models differ.” A recurring aspect of this early work is the recognition of both concentric and radial fault systems, the former of which has been characterized as having a √2 ratio for inter-ring spacing (Hartmann and Wood, 1972). This is predicated on the assumption that circular plates sagging into an underlying fluid medium will fracture at distances near a √2 radius values based on the theoretical and experimental work of Lacke and Onat (1962).
The Moon lacks plate tectonics but has a long history of impact tectogenesis (fig. 5). It was likely born from an impact of proto Earth by a planetesimal about 4.5 billion years ago and its evolution since then includes continuous meteorite bombardment that added mass to its body (Hartman and Davis, 1975). Vast surface regions have been pulverized and melted from the heat and energy absorbed from the largest impacts. Upon the initial hardening and density stratification of its shell from an early, lunar magma ocean (LMO), the Moon was subsequently pounded by large projectiles during the formative stages of the solar system as the planets and their moons were settling into place. A hypothetical, late-heavy bombardment (LHB) phase early in the lunar chronology depicted in figure 5 from about 3.7 to 3.9 Ga is proposed to have been spurred by the orbital adjustments of the four largest planets that destabilized near-Earth asteroids resulting in a period of path clearing and heavy bombardment that apparently focused on the near side for the Moon with probable, coeval periods of impact tectogenesis on Earth and Mars (Gomes and others, 2005). But as Harrison and others (2018) and Boehnke and Harrison (2016) have pointed out, the most widely used evidence to support the LHB hypothesis yields unreliable impact histories but does not preclude the existence of such events.
Lunar multi-ringed astroblemes were instrumental in the development of Melosh and McKinnon’s’ (1978) ring tectonic theory that explains how large, concentric fault scarps develop around large impact craters from the transient collapse of a crater rim inwards and downwards from gravitational adjustments toward the crater center shortly after impact. The structural process involves a rigid, but weakened crust and lithosphere sitting atop fluid or ductile substrate that allows mantle flow from beneath to accommodate rigid fault slip and block rotations in response to gravitational instabilities. This is the only theory available at this time to account for the formation of ringed basins, and many numerical simulations have been run to test this theory under a wide range of conditions using variable impactor size, crustal thickness, and near-surface thermal gradients that reproduce similar results with empirical observations (Potter, 2015). But by their own admission they have not been able to reproduce model results that consistently agree with observed ring-spacing geometry using these methods. Currently, the computational time and fidelity of the numerical simulations can only be tested for impacts occurring normal to the surface and in two-dimensions. Three dimensional simulations including variable impact obliquity have not been attained. But my fundamental concern with ring-tectonic theory is the assumption of having a fluid layer below the crust to allow rapid creep of the mantle material that accommodate the differential movement of overlying solid fault blocks. Perhaps melting of the upper mantle immediately upon impact temporally provide the fluid substrate needed to accommodate crustal extension, but according to the lunar seismic data, there is no fluid substrate beneath the Moon’s crust showing seismic shear-wave dampening. The first apparent, internal seismological boundary in the upper mantle having reduced S-wave velocities occur at about 230 km depth but accompany increased P-wave velocities. This upper-mantle acoustic boundary therefore likely stems from mineral-phase transitions involving crystalline anisotropy that retard S-wave transmissions but are denser with more rapid P-wave velocities. The first noticeable acoustic-layering contrast in the upper mantle occurs at a depth of about 490 km and separates the upper and lower parts of the mantle with an inverted impedance contrasts for compression waves (figs. 7 to 9).
Very large impacts also produce widespread ITFF
strains in the form of radial and concentric faulting and folding with the
consequential development of large igneous provinces (LIPs) fed by
deep-penetrating faults that mediate the ascent and dispersal of mantle-derived
basic lava covering large areas like surface areas like Oceanus Procellarum
(fig. 6). Material ejected from impact basins has also been distributed over
vast surface reaches and provide useful markers in analyzing a meteorite's
horizontal heading and the geologic history of the Moon (Wilhelm and others,
1987; among others). For example, if a crater or other structure is
superimposed, or formed on top of such ejecta, then the crater is younger than
the impact basin. On the other hand, if a feature is partially buried by the
ejecta blanket, the feature must be older than the impact basin. By analyzing
stratigraphic superposition and cross-cutting structures across the Moon, it is
possible to derive a relative overview of the lunar geological history that is
partly constrained with absolute radiometric dating of collected surface samples
of mostly loose material, regolith, and boulder fragments (figs. 5 and 6).
We know from the NASA Grail mission that the lunar
crust ranges in thickness from near zero from excavation around cratered areas
to over 60 km thick in the lunar highlands (Miljkovic, 2018). Planetary
geologists generally use the starting assumption that the excavation depth of a
cratering event is about 10% of the excavated diameter (Melosh, 1989; Melosh and
Ivanov, 1999). If we use a average crustal thickness of 30 km for the Moon, then
upper-mantle excavation and mixing occurs for astroblemes with inner rings
exceeding 300 km diameter. As detailed below, only the five largest lunar
astroblemes exceed this size. Very large, deep craters are floored by mare,
melted crust + upper-mantle material. Mare is ponded within the cratered,
central regions of large impact basins (fig 6). This also happens sometimes on
Earth (French, 2004). Mascons are circular, high-intensity gravity anomalies
where ponded mare is concentrated (Phillips and others, 1997). Sampled lunar
basalts have an average crustal density of ~ 3.3 gm/cm3, whereas the bulk Moon
crust is ~ 2.95 gm/cm3 owing to the abundant plagioclase feldspar in
anorthosite-rich crust of the lunar highlands that is comparatively light and
thick with respect to mare.
Global geophysical themes have proven very useful
in demonstrating ITFF strains occurring in radial alignment around large craters
as part of planetary astroblemes, or ‘star wounds’ (Dietz, 1962; Buthman 2022;
Herman 2022). Eighteen large impact basins displaying multi-ring architecture
are listed in table 1 from the largest to smallest and mapped in figure 6 using
eleven different geospatial themes. The interpretations use publicly available
geospatial data from the United States of America (USA) National Aeronautics and
Space and Administration (NASA) and Japan’s National Space Development Agency
(NSDA). Each impact basin has at least three rings with the second largest (Mare
Imbrium) having six, and the largest (South Pole - Aitken Basins) having two
sets of rings and ITFF strains that span a global hemisphere (fig. 6).
Astrobleme structural analysis relied upon the detailed topographic, gravity,
and seismic-velocity themes to characterize ITFF strains and obtain model
dimensions for the Moon, Earth, and Mars. The largest lunar astroblemes contain
the curvilinear mountains chains surrounding impact basins as core components of
defined ITFF strain envelopes that cover between 0.5% to over 43% of the globe
(fig. 6 and table 1).
The computer methods used to map the astroblemes and
conduct a spatial comparison between mapped lithosphere strains and expected
seismological responses includes Google Earth Pro (GE Pro), QGIS desktop
software (ver. 3.16.14), and the SketchUp (SU) Pro 2020 computer-aided drafting
system (CAD). GE Pro and QGIS provide compatible file formats for exchanging
data files and exporting the results into SU Pro for 3D modeling of the Moon,
Earth, and Mars. The SU Pro extension spirix_textured_sphere by J. Hamilton
(ver. 05.29.2016) was downloaded from the Spirix website and used to wrap
geographic maps around virtual 3D Earth and Moon globes. Circular histogram
analyses were conducted on sets of mapped radial faults within the mapped limits
of the largest astroblemes using the QGIS Line Direction Histogram plugin by H.
Tveite (ver. 3.1.1, 2020). The Contour QGIS plugin by C. Crook and L. Roubeyrie
(ver. 2.0.12, 2023) was used to generate maps of magnetic-field intensity and
near-surface crustal, elemental abundances derived from reduced spectroscopy
data obtained by NASA’s Lunar Prospector (LP) and website (Feldman and others,
accessed 2023). The GE Pro plugin Range Rings for Google Earth by T. Davis and
R. Turner (ver. 2023) was used to generate polyline rings of specified radius
around impact points lying near the center of large craters. Bolide headings and
large faults seen in the various geospatial themes were manually digitized in
both GE Pro and QGIS using geographic spatial coordinates. The SU Pro global
models uses kilometer distance units.
NASA’s Lunar Reconnaissance Orbiter (LRO) Wide Angle Camera and the Lunar Orbiter Laser Altimeter (LOLA) instrument have enabled the accurate portrayal of the shape of the entire moon at high resolution (www.lunar.gsfc.nasa.gov/lola/index.html). The LRO was launched and began operations in 1997 that continue today. Topographic elevations were surveyed to about one-meter accuracy for each ~118-meter pixel in the global digital terrain model (DTM; Mazarico and others, 2013).
The Gravity Recovery and Interior Laboratory (GRAIL) was another NASA mission run in 2011 and 2012 to obtain high-quality gravitational field mapping of the Moon to help determine its interior structure. The GRAIL mission produced high-resolution maps of the Moon’s gravitational field, including global, free-air and Bouguer gravitational-intensity anomalies (figs. 6C and 6D; www.svs.gsfc.nasa.gov/4014). Bouguer anomalies are derived from the free-air data by correcting for gravitational effects of variable topography. The GRAIL mission flew twin spacecraft (Ebb and Flow) in tandem around the Moon to map variations in the lunar gravitational field. At the end of the mission, the probes were purposely crashed on the Moon (www.solarsystem.nasa.gov/missions/grail/in-depth/).
A second Bouguer gravity map used in this study is from Watters (2022) that is the basis for mapping very-large fault systems that penetrate the crust into the upper mantle (fig. 6E). This map uses a color scheme that highlights gravity gradients rather than gravity intensities as for the GRAIL themes. The Watter's map seems to emphasize features that have rapid field-intensity changes over small distances like where large crustal fault zones occur that are brecciated, mylonitized, and can locally penetrate to upper-mantle reaches to mediate the ascent of magma (figs. 12, 15, and ). The best examples of this are seen is the eastern margin of the South Pole - Aiken (SPA) Basin astrobleme where braided fault segments stream dark blue through the eastern, lateral reaches to partly bracket this enormous impact basin (fig. 10). These earliest anomalies are cross cut by younger, high-intensity anomalies that are concentric to impact craters and likely highlight impact-generated igneous dikes that arose from the upper mantle or were generated within the crust by decompression melting.
Global magnetometer data for the Moon were obtained by
Japan’s SELENE probe that are available to the public from Japan’s Aerospace
Exploration Agency (JAXA) in support of scientific and educational purposes
(fig. 6F; www.darts.isas.jaxa.jp/planet/pdap/selene/). Japan’s NSDA SELENE probe
was launched in 2007 and orbited the Moon collecting geospatial data until 2009
when it was directed to impact the surface. The magnetometer data were
downloaded as tabular data with gridded, 1° data points with the total magnetic
field-intensity (F) values measured at 100 km elevation.
Spectroscopic sensing of the Moon’s surface by NASA’s
Lunar Prospector (LP) returned major and trace-element abundances in the upper
30 centimeters of ground surface (Feldman and others, accessed 2023). Geospatial
variability of elemental concentrations in large impact basins like the Aitken
basin and Oceanus Procellarum have been tied to depths of impact excavation and
the generation of melt bodies and flood basalts originating from the upper
mantle (Hurwitz and Kring, 2014; Uemoto and others, 2017; Zhu and others, 2019).
Spectrographic data are free from NASA as gridded ASCII text files formatted
with either 1o or 5° geographic cells with center points tagged with either
elemental bulk-weight percentage (wt. % ) values or parts-per-million (ppm)
concentration units for aluminum (Al), silicon (Si), Iron (Fe), potassium (P),
thorium (Th) and uranium (U). Geographic maps of the various elemental
abundances were generated for each point theme by contouring the set of values
using three to five ranges of values, or quantiles, to colorize the map and
thereby allow a geospatial assessment of the elemental abundances for the major
impact basins (figs. 6G to 6L). Each geographic map was also added into a GE Pro
project as an image overlay further assess spatial comparisons using a virtual
globe (figs. 10 and 11). However, it is noted that using data recorded at cell
centers leaves small data gaps of either 1° or 5° along the frame of the
geographic boundaries. These data gaps are particularly noticeable along
longitude -180° when viewed using GE Pro (figs. 6 and 10).
A discussion of the
elemental distributions with respect to the larger impact basins is developed
below when focusing further on the structural and geophysical aspects of ITFF strains for
the six-largest astroblemes (figs. 10 to 14). The locations, sizes and
structural aspects for each of the eighteen multi-ring astroblemes therefore
relied upon visual inspection of observed surface features gleaned from
geophysical anomalies as visualized using geographic maps that are also used as
image overlays in the GE Pro virtual globe. Ring spacing noted in table 1 is
calculated and compared with the aforementioned √2 factor, and the interpreted
headings for the set of bolides are statistically analyzed using a circular
histogram that is compared to interpreted bolide headings derived for Mars (fig.
6M; Herman, 2022). The six largest multi-ring impact basins are further analyzed
using the NASA and JAXA geospatial data in GE Pro and the mapped dimensions of
the basin rings added to a SUP Moon model that incorporates seismological
constraints on the manner in which the Moon's interior layering will reflect
shock energy (figs. 7). A spatial comparison is then conducted on where
reflected shock energy would be focused in concentric alignment around large
craters using a CAD Moon model. This is used to highlight outlying regions
around large craters where large crustal faults are mapped based on global
topographic and gravity maps. Such faults radiate outward from craters with
steep dips and in many places correlate spatially with high-intensity gravity
anomalies likely stemming from basic, high-density upper-mantle melts (~3.34 gm/cm3
pervading lower-density anorthositic crust (~2.55 - 2.85 gm/cm3; fig.
7).
The astrobleme rings were mapped in hierarchical order
and recorded in table 1 with the innermost ring designated 1 and outlying rings
noted as 2 through 6. The inner rings either coincide with the crater rim
approximated by the first set of topographic fault scarps forming the central
depressions, or the limits of gravity mascons for most of the astroblemes (fig.
6 and table 1). Only the SPA basin and the Birkhoff (table 1, no. 9) and Korolev
(table 1, no. 16) astroblemes lack central, high-intensity gravity anomalies
exceeding 300 mGal, although the latter two have relatively high-intensity
gravity anomalies on the order of 100-200 mGal (fig. 6C and 6D). The innermost
ring of most basins is mapped along the edge of the central, negative gravity
anomaly (fig. 6C). The outer basin rings (2 through 6) are drawn along visible
fault scarps or near the limits of concentric, gravitational-intensity
anomalies, although fault scarps are not perfectly concentric to craters and
form to varying degrees of density and elevations around craters owing to the
variance of many impact-related physical factors. For example, there is good
evidence that the Mare Orientale impact was at a moderate- to high angle (> 60°)
impact from a projectile heading westward owing to having a nearly continuous
set of outer rings and cordilleran uplift in a sector downrange of the crater
like that seen in missile-test studies (fig. 14). The outer rings of Mare
Orientale have a muted topographic expression up range where fault scarps occur
and the crust has been stretched resulting in having mare ponded in ringed
crustal depressions (fig. 14).
Outer rings correspond with prominent, concentric
fault scarps lying outboard of mascons where large tensional fault systems
encircle craters and form semi-continuous fault chains that accommodated the
rapid, incremental strains accumulated in the target from the absorbed ground
energy of impact and subsequent gravitational relaxation. But in other places
the mapped rings denote the circumferential limits of thickened crust where
radial and concentric faulting spurred the coeval generation of crustal plutons
and associated surface volcanism. In these cases, the outer concentric sectors
of astroblemes are puffed up where the thickest crust occurs from
impact-generated faulting, magmatism and veining. The outermost rings of each
astrobleme are mapped to encompass most of the ITFF strains stemming from each
impact event with the notable exception of the SPA which has two sets of
cratered regions (figs. 6, 10, and 17). The majority of the outer ring correlate
with Turtle and others (2005) 'outer limit of deformation' defining the limits
of far-field impact strains.
There are also many places on the Moon where sharp,
gravity anomalies flare out along linear trends from the the margins of impact
basins that lie at the center of large astroblemes. These sharp gravity
anomalies stem from deeply penetrating faults that have low-intensity signatures
where the crust has been broken with cataclasis and density reduction. On the
other hand, high-intensity signatures probably represent faults that have tapped
low-titanium (Ti), high-density magma from the upper mantle leading to the local
deposition of diabase dikes and mare basalts. For the eighteen ringed basins
mapped here, only the SPA Basin doesn't have rings represented in figure 6
because the main impact crater in this vast strewn field lies close to the South
Pole with rings paralleling latitude lines that otherwise clutters the maps. A
more detailed discussion and representation of impact-generated rings of the SPA
is reserved for a later section using GE Pro that provides a clearer
representation of the structure than for the geographic maps that spatially
inflate and distort polar regions on 2D maps.
Many criteria are used to help interpret the meteorite
headings. Primary criteria include down-range crustal excavation together with
low-intensity gravity anomalies including the cratered region forming a
horseshoe shape with the meteorite heading bisecting the open ends, thereby
resembling a trident (Î)
with the open end pointing down range. This generally seems to be the case for
impacts of low- to moderate obliquity. Those of higher angle display more
axial-type of concentric strain fields with closure of the U's open end with
continuous uplands and mountain chains. Another strong criteria is having large
crustal faults striking in parallel alignment in the central region and
sometimes bracketing the crater with sets lying in conjugate arrangement to the
axial plane and bisected by the interpreted heading and the horizontal component
of the principal, compressive stress axis arising from impact. This phenomenon
in rock mechanics is called axial splitting and is one of the dominant,
brittle-rupture responses of rocks subjected to uniaxial compression under low
confining pressures (Chakraborty and others, 2019). In that respect, it is also
common to see radial crustal faults exhibiting a double-shear response giving
rise to conjugate fault systems bisected acutely by the meteorite heading as
demonstrated for Martian astroblemes and from the aforementioned glass-ball
impact experiments (Herman, 2022). Radial faulting arising from axial splitting
and conjugate faulting are seen in most of the mapped astroblemes with the most
notable exception being the overlapping Maria Imbrium- and Serenitatis, which
may have caused vast regions of the crust to be melted and then flooded by
Oceanus Procellarum mare (figs. 6 and 11). Another prime criteria is where the
free-air and Bouguer gravity maps show mascon axial elongation along the heading
line in the free-air theme and horseshoe-shaped anomalies in the Bouguer themes
with the open end bisected by the heading. This trend can also parallel
far-field magnetic striping within the limits of the astrobleme. The striping
consists of alternating, thin bands of varying intensity anomalies situated
outside of a cratered region and is best exemplified by the area between Maria
Imbrium and Serenitatis where the striping stems from the latter and is reset
and gone from inside Mare Imbrium's 490 km concentric ring (figs. 6 and 11) is
also exemplified by the gravity expression of the Chicxulub crater on the
Yucatan Peninsula of Mexico that was caused by an oblique impact (Gulick and
others, 2013). Basin proportionality and structural symmetry was also used to
interpret the bolide headings for each astrobleme which is grounded in the
observational records of oblique missile impacts like that depicted in figure 14
(bottom), with a basin’s long axis paralleling the missile heading and thickened
ground lying down range from structural compounding. Also, astrobleme symmetry,
patterns of impact ejecta, and the alignment of multiple craters in a strewn
field either arising from projectile fragmentation or bolide spalling provide
additional clues that help decipher suspected impact trajectories (Wilhems and
others, 1987).
The interpreted bolide headings are therefore constrained by the basins shape, topographic asymmetry, structural, and gravitational expression of the associated fault and fracture systems that commonly display systematic distribution about the impact-trajectory plane (figs. 3 and 4). Oblique strikes occurring at various impact angles produce variable ITFF strain fields including up- and down-range topographic variations and gravity signatures that reflect differential structural processes operating within different blast sectors (Moore, 1976). Missile-test crater studies of oblique projectile strikes have documented broken and tilted material occupying the down range sector whereas the up-range sector has open fracturing and little tilting of the ground. The zone of open fractures differs from the tilted and broken sector in that the original ground surface is often exposed and the original surface is level or displaced downward. Sparse, nearly vertical open fractures in the zone are concentric to the crater edge and confined to the up-trajectory side. Beneath the surface, shattered target material or conjugate fractures form criss-cross patterns that were exposed on the up-trajectory crater wall. The conjugate fractures and tensile fractures perpendicular to them form diamond-shaped blocks with acute angles pointing upward and downward. This fault pattern is seen up range in the Mare Orientale impact basin on the gravity and digital terrain themes (fig. 14). The tendency to have elevated topography down range from structural compounding and crustal thickening corresponds directly with high-intensity gravity anomalies opposing low-intensity, concentric ones situated up range. These relationships are complicated where multiple astrobleme ITFF strains overlap as seen for the five covering much of the near-side of the Moon (fig. 6). Astroblemes therefore commonly display bilateral symmetry with respect to interpreted headings that are also seen on the elemental-abundance maps for the largest impact basins, the SPA and Maria Imbrium-Serenitatis astroblemes and further visualized and discussed below with respect to the six largest, lunar, multi-rings impact basins (figs. 10 to 14).
The inclination aspect of the bolide trajectories are not included in table 1 because their interpretation is the most difficult to deduce along with impact velocities. Bolide sizes are somewhat constrained by crater dimensions (Turtle and others, 2005) but interpreted headings for some of the smaller astroblemes are less certain than the larger ones owing to the relative lack of manifest, widespread asymmetric, ITFF strains that help constrain them. It has been shown that impact obliquity influences crater morphology (Davis and Collins, 2022) and my glass-ball experiments show the transition from axis- to plane-symmetric strain responses that vary based on high (>60°), moderate (30°-59°) and low <30°) impact angles. Vertical impacts have nearly symmetric, radial strain fields and little sector-based structural variance in the respective blast sectors when compared to oblique impacts. Theoretically, the size and strain expression of a bolide will vary as a sine function of its obliquity with the most grounded strain imparted by vertical impacts and the least from very shallow ones. Bolide obliquity for each event is speculated on further in a section below focused on the structural expression of the six largest, lunar astroblemes mapped using GE Pro.
The mapped limits of Oceanus Procellarum and the Aitken Basin as represented in figure 6 are somewhat arbitrary as they are mostly based on visual scrutiny of the NASA DTM (fig. 6A). The area of Oceanus Procellarum listed in table 1 uses the digitized polygon boundary that loosely follows the 0-meter elevation contour. The respected period ages assigned to the various astroblemes is largely based on the work of Tartèse and others (2019), Morbidelli and others (2012) and Wilhelms and others (1987), and it is important to note that these ages will be modified, perhaps significantly based on more robust sampling of the lunar surface. We are still in the formative stages of understanding the absolute timing of impact events on the Moon owing to the difficulties of directly obtaining outcropping geological samples, particularly on the Moon's far side.
The plausibility of impact-generated shock energy giving rise to radial and concentric crustal faults, mountains and basins is placed into 2D and 3D perspectives below using maps, cross sections, and a CAD model of the Moon’s internal seismological boundaries deduced from a study of Moonquake and meteorite impacts recorded with the Apollo seismic network (fig. 7). The Apollo Passive Seismic Experiment (PSE) resulted from multiple deployments of a surface-based seismological network between 1969 and 1972 from five Apollo Moon landings, and the resulting transmission of seismological data to Earth until September 1977 (Wiezorik, 2009). Weber and others (2011) analyzed the PSE data and reached a consensus on a reference velocity model having eight components including seven concentric layers surrounding a solid inner core. Their velocity model was used to build a 3D geometric model of the Moon with its interior layers and core assembled and rendered using SketchUp Pro 2020 (fig. 7). The layered model includes the crust, two upper mantle layers, three lower mantle layers, and both an outer and inner core. Two, noticeable P-wave velocity inversions occur in the upper and lower mantle where strongly reflected rarefaction waves are returned to the surface from interior material-phase boundaries having reduced acoustic impedance contrasts (430 km and 1224 km radial depths). The spatial limits of the representative impact basins as defined by the concentric fault systems agree very well with the primary shock reflections arising off these two boundaries (figs. 7 and 15 to 18).
Meteorite impacts generate primary shock-compression waves like underground atomic-bomb blasts, but unlike normal earthquakes that stem from the elastic rupture and rebound of subsurface material which generates both P- and S- body waves and surface waves when the radiated energy reaches the ground. Km-scale bolides mostly strike target surfaces at oblique angles and shock compress the target leaving fluidized and plasticized regions around craters where pseudotachylyte melts, mylonite and cataclastic fault zones radiate outward beyond the crater (figs. 2 to 4 and 15). At some point in time and space, widespread plasticity cause by highly pressurized compression waves yields to elastic seismic responses when dissipating shock waves fall below the elastic limit in the medium in which they travel. The transition point from a fluid- plastic state to the elastic state is called the Huguenot elastic limit (HEL) whose extent can vary for the big craters on Earth that are buried and concealed deep beneath sedimentary basins and oceans. Field work around old, large, continental craters have shown that such fluidized, ductile, and brittle shock structures can also include shatter cones mapped at distances of tens to hundreds of kilometers from the crater where they have been raised to the surface and unroofed as with the Precambrian Vredefort (Colliston and Reimold, 1992; Spray, 1998; Allen and others, 2022) and Sudbury (Thomson and Spray, 1996) astroblemes. There is also field evidence in the central Appalachian Mountain region of penetrative, ITFF crustal strains reaching over 700 km distance from the impact crater located at the mouth of Chesapeake Bay and within the compressed, down-range blast sector extending northward through the central Appalachians of Pennsylvania and New Jersey (Mathur and others, Herman, 2022). As such, there is mounting geological evidence of remote ITFF strains occurring on Earth at thousands of kilometers radial distances from large craters, and it becomes a matter of planetary rheology and the seismological behavior of shock waves to place these types of ITFF strain into spatial perspective for the impact basins of The Moon (fig. 7), Mars (fig. 20), and Earth (fig. 22). On Earth, field evidence of ITFF, regional crystal plasticity in quartz, feldspar, and calcite also stem from impact-generated shock waves that instantly raise radial mountain belts and the intervening annular troughs and basins, but traditional thinking places mountain building on Earth, dynamo-thermal metamorphism, and far-field penetrative, secondary structures solely into the realm of gradual tectonic orogenesis which isn’t exclusive given the manner in which km-scale hypervelocity impacts plasticize and strain the crust across vast regions. We see clear evidence from remote sensing of these ITFF strains reaching radial distances of thousands of kilometers around large lunar Impact basins like Mare Imbrium basin crater (figs. 6 and 11). These same processes operate on Earth and Mars, but gradual orogenesis from plate tectonics on Earth and atmospheric weathering on both helps masks their effects.
A shock wave is a strong pressure wave in any elastic medium produced by phenomena that create violent changes in pressure at the wave front. The shock physics of elastic media is well known (Grady, 2017). The shock wave front is an expanding, spherical region of sudden and violent compression and change in stress, density, and temperature. Propagating shock waves cause tectonic disruption that occur above the HEL. Shocked Earth materials can be vaporized, liquefied, plasticized, and undergo solid-solid phase transitions depending upon pressure-intensity variations relative to the shock front and how the energy gets dispersed and absorbed (Bevan, 1994). Shock waves are directed through media in a similar manner as elastic compression waves with respect to radiating energy along wave fronts outward in directions aligned normal to the wave front called ray paths (fig. 9). Wave incidence angles are measured along ray paths from a reference axis aligned normal to a boundary plane, such as an interior compositional or density changes like the phase boundary in the Moon between the upper and lower mantle (~490 km depth; fig. 7). Low incidence angles have steep ray paths relative to the surface and high-incidence angles are oriented at low angles to the boundary. Dienes and Fisher (1961) determined from studying atomic bomb blasts and numerical modeling that shock energy introduced into a solid medium is both transmitted and reflected at solid-solid interfaces having measurable contrasts in material density and wave-transmission speeds that determine a layers acoustic impedance. This is also generally the case for acoustic, or elastic seismic waves (figs. 8 and 9; Telford and others, 1976). But shock waves differ from elastodynamic waves as they travel faster than sound, and their speed increases as the amplitude is raised (Dienes and Fisher, 1961). But the intensity of a shock wave also decreases faster than does that of an elastic wave, because some of the energy of the shock wave is expended to heat and fracture the medium in which it travels.
When a compressional wave moves through a solid body and encounters an interface with a medium of different acoustic impedance at angles other than normal, the wave energy splits between transmitted energy that continues into the “target” medium along a different ray path, and a newly reflected wave that carries a fraction of the original energy back into the “parent” medium (fig. 9). Dienes and Fisher (1961) found If the acoustic impedance ratio between adjacent layers is as large as 10, or as small as 1/10, the pressure transmitted through parent layer is generally 64% of the original. This is about the greatest reduction that might be expected with common materials. Therefore, for typical cases where acoustic impedance contrasts increase with depth at layer boundaries, most materials transmit roughly 2/3 of the original compression energy through a boundary with only about 1/3 of it being reflected back into the parent medium. But when the impedance contrasts decrease with depth, a layer boundary returns nearly 100% of the wave energy back into the parent material as a rarefaction (tensional, or ‘pressure-release’) wave (figs. 8, 9, and 15). In other words, when the pressure transmitted through a boundary layer is less than that in the incident wave, the reflected wave relieves pressure as a rarefaction wave that returns back into the parent medium, and when it exceeds that in the incident wave, the reflected wave is a compression wave. Shock waves also get diffracted, transmitted into and partially absorbed along layer boundaries at normal incident angles. But in particular, layer boundaries having high, negative impedance contrasts return the bulk of wave front energy as reflected waves back into the parent material at incidence angles exceeding 30o with maximum reflectance occurring at about 60° (fig. 8; Telford and others, 1976). Dienes and Fisher (1961) also report that strong, rarefaction waves lead to tensile fracturing in brittle material and possible spallation at free surfaces like the ground. It is therefore likely that the ringed cordilleran and basins constituting these large basins rise in part from the dispersion and absorption of shock seismicity radiating outward from and focused below large craters as both refracted, transmitted waves and reflected, rarefaction waves. The SU Pro CAD models of The Moon, Mars, and Earth show that the sizes of impact basins and their ringed-basin architecture are dimensionally similar to where primary reflections arise off layered, internal interfaces with inverted impedance contrasts in targeted bodies at ray-path incident angles between 30° and 60° (figs. 15 to 18).
A closer inspection of the radial and concentric structures of six of the largest lunar astroblemes follows using GE Pro. The two largest ones are covered first including the SPA and Maria Imbrium and Serenitatis, followed by Maria Nectaris, Crisium, and Mare Orientale. The Maria Imbrium and Serenetatis astroblemes are mapped and discussed together because they structurally overlap and their geophysical expression is closely linked because of their enormity, successive ages, and close proximities.
The focused structural analysis begins with the most structurally complex region on the far side of the Moon covered by the SPA. It's considered to be the oldest, largest, and deepest impact basin on the Moon but it hasn't been directly sampled and its absolute age is unknown (Spudis and others, 1994; Potter and others, 2012; Garrick-Bethell and Milković, 2018 among others). Hurwitz and Kring (2014) provide a through treatment of the SPA geology and cover both pre- and post-SPA conditions. The reader is referred to this work for a more through discussion of the geological aspects of this massive feature, and how obtaining an absolute age is crucial for constraining the chronology of our solar system evolution. The SPA is very old and has an aureole of anorthositic crust that locally has some of the highest crater densities on the Moon (Wu and others, 2022). The age of the SPA is probably about 4.3 to 4.4 Ga (Morbidelli and others, 2012).The SPA has an unique structural and geophysical expression involving two sets of regional gravity anomalies that together constitute a vast strewn field formed by multiple projectiles impacting the southern hemisphere. Based on its structural expression as principally defined using the Watters (2002) gravity map (fig. 6E, 10 and 17), the strain field is slightly larger than than mapped by the SPA limits depicted in figure 6, with ITFF strains likely covering half of the lunar surface and ejecta deposits likely spread over most of the globe early in its history. The largest, 'parent' crater is located close to the south pole with the astrobleme fanning out to the north from there and reaching a surface span of over 4800 km directly downrange along a heading of ~007° (figs. 6 and 10, table 1). The center of primary crater is mapped at latitude, longitude -89.5°, 165.5°, an approximate location because of the amount of subsequent, dense cratering occurring there. A second SPA ring is centered down range to the north at longitude 54.42 and latitude 173.48 that was generated with a 820 km radius to roughly follow the 0-meter land elevation along the rim of the basin as seen on the NASA Blue-Steel DTM (fig. 6A and 10). This ring radius best fits the circular set of geophysical anomalies situated down range that fall in line with the main crater, and only departs from the mapped limits of the basin on its northern edge where it is elongated beyond the circular limits along the interpreted heading (fig. 10).
Hurwitz and Kring (2014) characterized the SPA basin as a 2400 km long by 2050 km wide impact structure centered at 53°, 191°E. That point (AC1; fig. 10) falls very close to the center of the Aitken basin as defined for the aforementioned, secondary ring with center point AC2 on figure 10. The SPA limits therefore extend beyond the basin rim and encompass over 4800 sq. km. of the surface when accounting for radial and concentric faulting within surrounding regions of the lunar highlands. As such, the two cratered regions and associated ITFF strains cover about 47% of the Moon’s surface (table 1, figs. 6 and 10). Large faults also flare out laterally in its wake from the south pole center to the east and west, and other sets of large radial crustal faults flare out from the basin into the highlands along its heading down range (figs. 6 and 10). The strike of the large faults covered by the astrobleme are in agreement with the experimental results of Herman (2022) that show dominant fault strikes normal to the bolide heading for gently inclined (<30°) impacts (fig. 4). Figure 17 (right side)depicts a 200-km diameter bolide inclined at 45° to the surface in order to give this event perspective and illustrates how a moderately inclined, large projectile will tend to spall and shear the lithosphere because of the globe's curved surface.
The central region of this astrobleme is thought to be a vast impact melt sheet, or LIP that was likely differentiated through time with magmatic fractionation of the mare (Uemoto and others, 2017). Potter and others (2012) have determined that after the basin formed, melt remaining within the transient crater pooled to form an impact melt sheet with a radius of ~200 km and a depth of 50 km. The floor of the Aitken basin is at higher elevations and gravity intensities on its north end where it has likely been structurally compounded down range as a wedged section of crust and upper mantle in a manner similar to that depicted for the Mare Orientale basin and in the schematic profile representations of a lunar astroblemes (figs. 12 and 15). About halfway downrange in the basin the crust transitions from excavated, mantle-type lithologies to contracted and thickened mixed upper mantle and crustal rocks in a manner depicted in figure 15. KREEP terrane is mapped spectroscopically in the foreland and western margin of the Aitken basin near where the antipodes of Mare Imbrium and Serenetatis occur (figs. 6G to 6L and 10). The KREEP terrane has relatively high surface concentration of potassium, iron, uranium and thorium with relative depletion of aluminum and silicon (fig. 6G to 6L). This is only the second place on the lunar surface where there is satellite evidence of KREEP on the lunar surface in addition to Oceanus Procellarum and the associated Maria-Imbrium-Serenetatis-Nectaris astroblemes (figs. 10 and 11). The SPA KREEP may simply arise from excavated, upper mantle ejecta that was deposited down range but it may also signal the subtle uplift of KREEP terrane to the surface after being shoved and compacted by an oblique projectile (fig. 15). If this KREEP expression is solely attributed to ejecta deposition the we should see a more uniform spread of these upper-mantle materials across the foreland down range of the crater. But it's possible that this KREEP occurrence is an expression of secondary, anatectic magmatism generated by crustal compounding and heating of the upper mantle with resulting plutons and mineral veins emplaced along concentric, ITFF tensional structures stemming from reflected shock energy as demonstrated above. Because this basin appears to be the oldest, deepest, and largest impact basin on the Moon, it has been overprinted by repeated bombardment and younger craters but nevertheless retains it's dominant, oblong structural form. The ovoid shape of the basin is a primary criterion used for constraining the bolide’s heading that doesn’t quite align with basin's long axis, but is skewed a little to the west because the basin's shape has been subsequently modified by the younger, overlapping Apollo and Schröedinger multi-ringed basins, and perhaps from impact-antipodal, faulting, igneous flaring and epierogenic welting from the antipodal Maria Imbrium, Serenitatis, and Crisium impact events (Figs. 6 and 7A). Antipodal cracking, heating, magmatism and welting from a series of younger, overlapping, basin-forming events occurring on the opposite side of the globe may also help account for the anomalous KREEP expression here because the SELENE magnetic theme also shows close spatial agreement between high-intensity magnetic anomalies and the aforementioned MISC antipodes (figs. 6F and 10).
This basin likely therefore formed as a strewn field from large, low-angle impacts that was modeled by Schultz and Crawford (2011) as a 170-km diameter bolide impacting at a gentle trajectory with impactor decapitation prevalent. But the impact parameters resulting in this complex, double-ringed basin are speculative with the decapitation scenario likely as the Bouguer gravity expression of the Aitken basin shows numerous, teardrop-shaped craters resulting from spalled fragments that splashed down along a south-to-north heading and leading to secondary basin development (fig. 17). Bolide fragmentation before or after atmospheric entry is also possible that would have resulted in a few main projectiles and slews of smaller ones impacting the south-polar region. The distribution of the anorthositic lunar highlands relative to the SPA appears to be systematic, as if this event could have generated a expansive, associated phase of crustal melting and mineral fractionation leading to the formation or modification of the main lunar highlands. Because it hasn't been directly sampled yet, its possible that this main event, so early in the lunar chronology, formed the primary division between the excavated and melted basin area and the surrounding upland regions around 4.3 to 4.4 Ga (fig. 5).
The gravity expression of the lunar highlands outside of the SPA limits is different from that inside it. Anorthosite inside the SPA limits have higher-intensities than those outside, indicating that the SPA event not only melted and excavated the central cratered region of the astrobleme but also strained a great expanse of the surrounding lunar crust where the relative increase in gravity intensities likely reflect the infusion of iron-rich fractionated melts and secondary mineral veins throughout the anorthositic aureole surrounding the Aitken basin (fig. 6C and 10). Mixing of the primitive crust and upper mantle occurred in the astrobleme core but the diffusion of iron-laden fluids along faults flaring out and surrounding the core structure helps explain the genesis and distribution of pure (PAN) versus siderophilic (FAN) anorthosite as determined from remote spectroscopy and direct sampling (Nagaoka and others, 2023). Higher-intensity gravity signatures of the highlands inside the mapped SPA limits in comparison to lower-intensity ones outside of the SPA limits may represent anorthosite crust that has been magmatically intruded and mineralized with ferrous veins that pervaded this swath of the highlands on the far side within the defined ITFF reaches of the SPA astrobleme. So the overall higher-intensity gravity expression of the anorthositic aureole surrounding the Aitken basin could either have originated by primary fractionation of melted and mixed upper mantle and crustal protoliths by the heat and friction of impact, or they may have originated during secondary impact tectogenesis from anatectic crustal melting and intrusive mixing of basic material from the upper mantle resulting in ferrous veining and magmatism that metamorphosed primary PAN into FAN.
Astroblemes Maria Imbrium and Serenitatis combine to form most of the second largest region of crustal disruption on the lunar surface stemming from impact tectogenesis. They are ranked second and fourth in size with respect to all of the multi-ring structures (table 1) and major impact events separated slightly in time that are parts of a vast, overlapping strewn field involving other large impact basins including Maria Nectaris and Crisium, together covering most of the Moon's near side, and likely causing the Moon to be tidally locked into Earth so that we perpetually see the 'near' side (fig. 19). As currently interpreted, the relative ages of these four, near-side, large astroblemes is from oldest to youngest: 1) Nectaris, 2) Serenetatis, 3) Crisium, and 4) Imbrium, although this is speculative until absolute ages are obtained. Maria Nectaris and Imbrium are used to mark primary divisions of time in the Moon's history with the former discussed below in the following section.
There is ample stratigraphic and structural evidence of Mare Imbrium being younger than Mare Serenetatis (Wilhems and others, 1986 among others) but they're modeled and discussed together here as a conjoined astrobleme that largely coincides with the surface expression of Oceanus Procellarum (figs. 6 and 11). They're complimentary, large lunar structures that melted, plasticized, and brecciated vast regions of Oceanus Procellarum in close succession. Mare Imbrium is thought to have occurred during the heavy bombardment phase of planetary evolution at around 3.92 ± 12 Ga based on U-Pb ages of impact-melt breccia, phosphates, and zircon grains from more than 20 different samples, five Apollo landing sites, and one meteorite sample (Nemchin and others, 2021; Zhang and others). The age of Mare Serenetatis is thought to be slightly older at about 3.98 Ga (Cadogan and Turrner, 1975). The Imbrium impact was numerically modeled by Zhu and others (2019) as a Moon-shattering event that excavated deep into the mantle with fracturing and spalling of the core. They used model projectiles in the 400 km to 1,000 km diameter range and impact velocities ranging between 3 and 17 km/s.
The successive punches from these large impacts occurring closely in time and space has resulted in ITFF strains occurring over a vast region with a profound structural and geophysical expression on the lunar surface. The outboard gravity expression of the Mare Imbrium astrobleme is enormous, covering about one quarter of the globe (figs. 6 and and table 1). The outer limits of crustal disruption is defined by a set of semi-continuous, sharp, high-intensity gravity anomalies that link up to surround the crater at about 1800 km radial distance from its center point, thereby giving this astrobleme a mascon diameter of 600 km and its overall structural expression exceeding 3600-km diameter. These sharp perimeter anomalies are crustal fault zones developed along the margin of the impact basin with increased material densities likely reflecting magmatic intrusion along deeply penetrating fault systems that likely tap melts generated in the upper mantle as depicted in figures 9 and 15. The strike of the basin-bounding faults veer away from concentricity where they formed in areas having inherited crustal and upper-mantle heterogeneities stemming from preceding large impacts like Maria Serenitatis and Nectaris.
The interpreted headings for Maria Imbrium and
Serenetatis lie at high angles to one another with the former located
immediately down range of the latter. Much of the compressed and foreland ITTF
sector of Serenetatis has therefore been excavated, mixed up and strewn about
the Imbrium basin. The depth of excavation for Mare Serenetatis appears to be
mostly confined to the crust as it generally lacks an upper-mantle KREEP
expression. The GRAIL and Watters Bouguer gravity themes also show that the Mare
Serenetatis mascon appears elongated to the northwest because is superimposed
upon at least two earlier, large impacts, with the largest one having been half
eroded by the younger Serenetatis. The earlier meteorite impact had a more
northerly heading based on the grain seen in the Watters theme (fig. 11). This
previously uncharacterized structure is tentatively referred to as the Hunterdon
(HU) astrobleme on figure 11 that's now incorporated into the Mare Serenetatis
astrobleme. A smaller impact crater occurs immediately to the north of HU and
further stretches the overall geophysical expression of the Mare Serenetatis
astrobleme out to the north.
The Mare Imbrium structure includes many other sets of
gravity 'hot spots' besides those distributed along the perimeter faults that
are clustered down range of the mascon and along the structural intersection
with Mare Serenetatis. Two of the most prominent sets of linear gravity and
topographic anomalies align with Montes Appeninus and Caucasus in concentric
arrangement around Mare Imbrium's center point at radii between ~700- to 800-km
(fig. 11). These mountain ranges are bisected along the Serenetatis heading and
flare out laterally from there which demonstrates how shock energy from the
Imbrium event became focused there when running into older, inherited ITFF
structures. Other spotty, high-intensity gravity anomalies are concentrated
immediately down range along the heading along the heading down range between
radial distances of 300 to over 1800 km distance (figs. 6 and 11). This sets of
radial anomalies likely reflects axial splitting focused downrange of an oblique
strike along a heading of about 174o. It is interesting to note that
the SELENE magnetic theme shows that magnetic banding occurs along this same
trend with the central spine of the structure running ~N-S and lacking a
magnetic signature, whereas the lateral margins of the spine show weak magnetic
striping paralleling the central structure (fig. 6F and 11). This could
represent the mobilization and expelling of Fe-rich fluids and material from the
core from pressure and heat or perhaps, marginal melt bodies accumulating along
the margins of the shocked core region. Such melting could happen in either or
both the upper mantle and crust. But the composite set of gravity flares
occurring down range indicates the likelihood of anatectic magmatism in addition
to melt emplacement originating from the upper mantle from crustal compounding,
thickening, and heating within a foreland, down-range structural wedge formed by
refracted shock energy. It is interesting to note that the spectroscopic
signature of the down-range blast sector where the excavated ejecta is focused
takes on a triangular form with a base aligned with the up-range chain of large
faults and its top pointing directly down range. This form suggests that the
KREEP expression of Mare Imbrium is not solely a result of thick ejecta blankets
that happen to align with basin-bounding, magmatically active fault systems but
also movements of tectonic blocks. The form of the basin boundary line is also
asymmetrically extended down range in comparison to up range where the basin
limits are nearly straight with a rectangular shape up-range but is more tapered
down range.
Mare Nectaris is the third largest multi-ring lunar
astrobleme and marks the beginning of the Nectarian period of lunar geological
history and span of heavy bombardment as proposed (fig. 5). The astrobleme is
located in the southern hemisphere and resulted from a moderately inclined
hypervelocity impact heading northwest that creased the lunar surface and
thickened lateral sectors of the strewn field with large ejecta block and an
ejecta blanket followed by mantle-derived magmatism emplaced along deeply radial
faults as inferred from the GRAIL high-intensity, Bouguer-gravity anomalies that
cluster along systematic, northwest-striking surface faults. The crust and upper
mantle are split along it's axis with large faults that have focused,
high-intensity gravity anomalies and therefore are likely intruded by magma.
Mare Nectaris itself overlaps an earlier large crater having a more northerly
heading as indicated by the Watters gravity theme. The age of Mare Nectaris was
determined using cratering-flux estimates based on its ejecta blanket named the
Jansenn Formation; a key, lunar stratigraphic unit. As proposed, the period
marks the beginning of a heavy, early, lunar impact flux that declined
exponentially with a short half-life (Stuart-Alexander and Wilhelms, 1975)
although other impact-flux theories favor a steadier and more constant post-mare
flux based on crater counts constrained by radiometric dates (Hartmann, 1970,
1972; Soderblom and Boyce, 1972).
The magnetic-intensity field theme shows that Mare
Nectaris retains a strong magnetic signature that's roughly concentric to the
crater but segmented and distorted by the aforementioned superimposed
structures. The Mare Nectaris astrobleme is a good example of how
impact-tectonic thickening of the crust occurs concentric to a large crater. The
outer rings of this astrobleme cannot solely be explained as ejecta blankets as
they have sharp, concentric, mountainous ridges spaced at ~440 and 850 km radii
that correspond with pronounced gravity and magnetic anomalies indicating that
this early, massive structure has deep roots with abundant ITFF igneous
plutonism that was subsequently overprinted by the younger trio of large
astroblemes including Maria Serenetatis, Crisium, and Smythii (fig. 6F). Age
dating of material within the outer rings and within a lateral sector return
three groups of ages including ones from anorthosite associated with Mare
Crisium (~3.9 Ga), and older one lateral margins of Mare Nectaris appear to have
been melted during formation of the younger Mare Crisium astrobleme.
The mare Crisium basin is ranked fifth in size of the eighteen mapped astroblemes with an outer ring reaching 957 km radial distance and covering about 7% of the Moon’s surface area. It is a multi-ring structure formed by a moderately-inclined impact heading east. The astrobleme morphology belies the meteorite's heading along a 079o from the basin center where the innermost ring is missing and an extended mare tract occurs in its place. The Bouguer gravity themes show a set of deep, radial conjugate faults that bracket the mascon, with the most noticeable set flaring out westward and up range (fig. 11). The circular histogram plot of large crustal faults shows a poor correlation with definitive axial splitting because of the scattered histogram plot of large faults within it's limits owing to the overlapping strains from the Maria Nectaris and Smythii astroblemes (fig.6 and 11).
The Soviet Luna 16, 20, and 24 robotic Moon missions were completed within the Mare Crisium astrobleme by drilling into regolith at the various sites and obtaining a mixture of basalts, metaclastic, and anorthositic rocks within 2-m long drill cores that have yielded some very interesting interpretations (Cadogan and Turner, 1977; Meyer, 2009). Among them are an age for the mare basalt in Mare Fecunditatis (Luna 16) of ~3.4—3.5 Ga (fig. 13), a Luna 20 metaclastic fragment dated at ~4.05 and 3.85 Ga that supports widespread cataclysmic bombardment of the moon at that time, and the presence of at least two isotopically distinct, non-radiogenic argon components in the Luna 20 anorthositic sample leading to model plateau ages of ~4.40 Ga to ~4.30 Ga that support SPA as having giving rise to large swaths of the Lunar highlands. But perhaps the most interesting aspect of the LUNA 24 samples is the relatively young age date of 3.3 Ga returned for the mascon mare. The magnetic-intensity theme shows that Mare Fecunditatis is physically connected to Mare Crisium rather than Mare Nectaris (fig. 12) and so the obtained age for outlying melts (3.4 - 3.5 Ga) is just slightly older than the mare within the cratered realm ~3.3 Ga (Meyer, 2009). Because Mare Crisium is assumed to have occurred during the hypothetical late, heavy bombardment (LHB) stage of the lunar chronology, the results of Luna 24 are dismissed as being younger than the age of the astrobleme, and presumed to stem from a younger impact. But if we relax the preconceived notion that Mare Crisium occurred during the LHB stage, then the younger age of this structure is indicated by the close ages Mare Fecunditatis and the crater mare (~3.3 Ga). It's not a far stretch to think that that volcanism within the cratered realm may have occurred over a protected time interval that terminated after outlying effects. This possibility would make the Mare Crisium astrobleme of Late Imbrium age rather than Nectarian age (fig. 5 and table 1).
The Mare Orientale astrobleme ranks sixth in size for all multi-ring, lunar structures and is one of the most apparent impact basin on the Moon's far side (figs. 5 and 9). It covers about 4% of the lunar surface (table 1) and has a spectacular multi-ring architecture with a ‘bull’s eye’ first reported by Hartmann and Kuiper (1962). Its location amidst the Lunar Highlands makes its structural expression clear on topographic, gravity, and photogrammetric maps. It’s also situated midway between two very large impact basins amid the lunar highlands with the Aiken basin to the south and Maria Imbrium to the north. The pupil of Mare Orientale’s eye is a dark circular Mare about 278 km diameter corresponding to the mascon where thick layers of impact-generated melts and/or basalt ponded (figs, 6, 13, and 15). The crater is surrounded by at least three concentric mountain ranges bounded inward by normal-fault scarps have elevated footwall blocks nearly forming continuous rings around the crater. The outer two rings reach between 460 to 580 km radial distances and bracket splotchy, high-intensity gravity anomalies that encircle the crater (figs. 15A). This pronounced outer-crustal arch of Mare Orientale is named the Cordilleran Ring (ref) and is topographically elevated over 2 km higher in the down-range sector than for the opposing, up-range one. This likely happened as a result of down-range reverse-shear faulting that compounded and thickened crust like that seen in missile-test craters formed by oblique impacts (fig. 14). The inner low-intensity gravity anomaly is asymmetric with a wider expression down range in comparison to the width up range.
Forward mechanical modeling of the Mare Orientale basin rings by Nahm and others (2013) showed that the largest, outer ‘Cordilleran’ ring is bounded by large-scale normal faults with displacements of 0.8 to 5.2 km, fault dip angles of 54° to 80°, and vertical depth of faulting around 30 km. These faults and the distribution of mare inside the basin suggest that the transient crater had a diameter between 500 and 550 km. They don’t think that the difference in crustal thickness between the western and eastern sides of the basin is a result of the basin-forming event, although the variable topographic expression of the astroblemes agrees with crustal compounding and thickening focused down range like that observed in missile-test craters (fig. 16). There is good geospatial evidence of tectonic inheritance also, with the older and larger Aiken and Maria Imbrium-Serenitatis astroblemes having influenced the locations and forms of subsequent structures where the IFFF strains overlap and interfere with one another (figs. 5 – 10).
The concentric fault systems and mountainous, curved fault scarps have isostatically rebounded, rifted footwalls that reach over 5 km (or ~17,000 feet) above the central mare that likely resulted from the strain effects stemming from reflected shock waves from the acoustic discontinuity separating the upper and lower mantle (um2/lm1; fig. 7). As illustrated in figures 8 and 9, reflected shock waves return to the surface with about 60% of their original energy, but as pressure-release (rarefaction) waves that induce regional, concentric tensional, ITFF failure around the crater. These concentric faults are clearly accompanied by magmatic intrusions in the form of lava flows, ring dikes (Andrews-Hanna and others,2018) and plutons that pervade the shallow mantle and crust along both radial and concentric faults that mediated the ascent of mantle-derived magma (fig. 11B). It thus appears that rapid, shock reflections from the mantle spurred the development of vast, deeply penetrating crustal rifts that span thousands of kilometers length. The splotchy, high-intensity gravity anomalies of the Cordilleran ring probably signal the occurrence of dense, basic, crustal plutons that may have locally fractionated into more acidic intrusive complexes and volcanoes over time. Another possibility is that crustal anatexis has occurred in compounded and thickened highlands crust occurring down range of craters in most astroblemes. Differentiated, evolved, acidic magmas have been obtained from direct Lunar sampling, and the periodic magmatic genesis within down-range overlapping strain fields can help explain the various ages obtained (ref.). The structural expression of these combined processes is to puff up the outer collar of an astrobleme like that clearly seen in Mare Orientale.
The circular histogram plot of faults occurring within the basin (fig. 12A) have fault-strike maximums that do not parallel the interpreted heading and that likely reflect the inheritance of pre-existing faults in the region imparted by the larger, bracketing Aiken and Maria Imbrium-Serenitatis impacts. It therefore is possible that complex structural movements have occurred on the older faults caused by tectonic overprinting, and the likelihood of having older faults involved in younger magmatic activity. The primary criteria used for interpreting the Mare Orientale heading is the topographic expression of the basin with the heading pointing to the highest, down-range cordilleran and the cross section paralleling that trend (fig. 12). Another criterion arose from the subtle topographic ridges within the central mare as seen in the digital elevation models that are oriented at high-angles to the interpreted heading and that likely stem from post-mare isostatic, structural adjustments causing subtle fault scarps to develop above blind faults within the limits of the crater (fig. 15).
The same methods used to model the seismic responses to large impacts on the Moon are also applied to Mars (fig. 21) and Earth (fig. 23) using global seismic-velocity profiles together with 3D CAD models to derive ITFF strain solutions for impact-shock reflections. Mars and Earth are similar insofar as the only major, interior, inverted acoustic-phase boundary occurs at the liquid outer cores beneath overlying plastic mantles. Figures 21 and 22 show the interior layering of Mars shaving strong shock reflections off the liquid outer core in relationships to the Syria-Sinai-Solis Planums astrobleme in Mars. The model shows a direct spatial correlation between major concentric, crustal structures and the dimensions of strongly reflected shock rarefactions. Figure 23 and 24 show similar relationships for Earth where strong reflections rising back to the surface off the outer core spatially correlate with a 5000-km radius rings around the large, suspected Congo impact basin on Earth, and how that event may be responsible for spearheading the breakup of Pangaea and emplacement of the 200 Ma central Atlantic magmatic province (CAMP) that I have had the pleasure to map in parts of New Jersey and New York.
There is a large body of literature concerning the
Moon’s genesis and geological evolution as it plays a pivotal role in our
understanding of our place in our Universe. Recent numerical thermal and
geodynamic models of the Moon and large impacts resulting in the multi-ring
basins have been done in three dimensions including 2D geometry and time (Wieczorek
and Phillips, 2000, Zhu and others, 2019). What is gleaned from this work, among
many others, is that the Moon’s crust varies in thickness from about 30 to 65 km
in thickness with respect to the Maria and Highlands terrains, and with respect
to the two largest astroblemes, the SPA and Maria Imbrium-Serenitatis
astroblemes respectively. These three bolide-impact events imprinted and
significantly altered the expression of a primitive, lunar magma ocean (LMO)
that is thought to have fractionated into the lowland Mare and anorthositic
Highlands. As seen illustrated above, large regions of the Moon have been
repeatedly melted, fractionated, mixed up and tossed about from cosmic
bombardment and planetary accretion. Visual inspection of the geospatial data
show that the SPA carved out a vast section of the southern, polar region and is
collared by stretches of highlands that reach the maximum lunar altitudes far
down range where focused, refracted shock energy structurally thickened and
elevated the upper mantle and crust. It also appear that the largest impact
craters are concentrated in the southern hemisphere in comparison to the north
which is also the case for Mars and Earth. But most of the Lunar KREEP terrane
is focused within the mid-latitude Mare Imbrium astrobleme, and to a lesser
extent the SPA. Only the largest four multi-ring astroblemes show impact
excavation to upper-mantle levels (figs. 6, 10 and 11).
This study has outlined aspects of the 18 largest,
lunar, multi-ring astroblemes and provided a secondary glance at five of the
largest ones as a preliminary investigation into their structural nature that
appears to hinge on how the global interior is structured. Other geophysical
aspects and structural details surrounding the 12 smaller astroblemes await
further scrutiny but will undoubtedly shed more light on the nature of ITFF
scaling for larger versus smaller astroblemes. But what we really need from the
Moon are absolute ages for specific geological structures based on outcropping
rock samples rather than grab samples of loose surface materials and regolith
that can have ejecta origins located hundreds to thousands kilometers distance
from collection points. In this respect the Luna core samples of basalt-laden
regolith are amazing achievements but subject to the widespread deposition of
impact ejecta across lunar hemispheres, and then reworked into regolith by
successive cratering and ejecta depositional events. The vastness of Oceanus
Procellarum, comprising over 10% of the Moon’s surface area (table 1 and fig. 6)
testifies to the extreme distances impact-spurred volcanic basalts can flow
across the surface to infill crustal depressions and mask the visual
identification of craters predating the emplacement of the volcanic flows and
melt sheets.
The magnetic-field data show a north-south striking grain to the east of the Maria Imbrium, Serenitatis, and Nectaris astroblemes that may result from mineral-fractionation processes and compositional banding in the partially melted deep mantle from iron segregating outward from the compressed spine of the Imbrium impact as previously mentioned. It's herein noted that the most intense magnetic-field anomalies occur in the northwest rims of the SPA near the antipodes for the massive Maria Imbrium, Serenitatis, and Crisium multi-ring basins. This spatial correlation emphasizes the relative importance of antipodal ITFF strains in the upper-mantle and crustal from cracking and decompression melting appear significant only for the largest astroblemes.
Wieczorek and Phillips (2000) have shown how the lunar Moho is the top of the KREEP upper-mantle layer that has been differentially uplifted by tens of kilometers beneath young multi-ring basins. This behavior has commonly been attributed to the vast quantity of material that is excavated during the impact event and the subsequent rebound of the crater floor. Wieczorek and Phillips (1999) used this observation to argue that most of the lunar basins formed in accordance with the premise of proportional scaling. Specifically, they found that the depth/diameter ratio of the excavation cavity for most of the young basins was equal to about 0.1 independent of crater size. This work offers a complimentary explanation for the the differential occurrence of KREEP terrane occurring principally downrange of large craters coring asymmetric astroblemes and with large-basin antipodal welting. My intent in conducting this research was to spatially examine the geometric links between radiated impact-shock energy and the resulting ITFF radial and concentric crustal faulting. In that regard I am satisfied with the derived geometric association with interior seismogenic layering and crustal strain fields stemming from impact tectogenesis. But the correlation of strong rarefaction shock waves pulling the crust apart from beneath minutes after a meteorite collision were an unexpected aspect stemming from this work that helps explain rift tectonics on Earth.
This work compliments recent advances in impact tectonic theory by Buthman (2022) and Herman (2022) that show how, where, and when sudden and catastrophic tectonic episodes of mountain and basin formation have occurred on Earth and Mars from periodic bombardment by km-scale bolides striking at hypervelocity speeds. Impact-generated tectonic events suddenly punctuate geological time on Earth that otherwise reflects the slow, steady, and comparatively uniform tectonic processes operating near the planetary surface most of the time such as the slow-spreading oceanic ridges, subducting tectonic plates, and stratigraphic accumulation arising from ordinary tectonic orogenesis. But large, catastrophic impact events also correlate temporally with rapid and brief biological mass extinctions on Earth and regional magmatic events that help us partition geological time into the various Eras, Periods, and Epochs (Herman, 2022). Plutonism and volcanism are constructive geological processes that can topographically elevate and sometimes thicken the crust where large igneous provinces (LIPS) form beneath oceanic-spreading ridges, as oceanic seamounts, or topographic highlands and mountains of terrestrial bodies. It is shown herein that impact tectogenesis of the Moon stems from absorbed ground energy imparted by hypervelocity, km-scale meteorites, and the manner in which shock energy is absorbed can be partitioned into refracted and reflected shock strain mechanisms that account for what we can sense using satellite imagery. These concepts also apply to other terrestrial bodies like the mapped ITFF structures on Earth. The pronounced vertical tectonic shifts that have been noted since the advent of geology as a science can also stem from the extraterrestrial, impact-tectonic agents of bolide bombardment and accretion that are integral agents of solar system evolution.
It has been nearly one year since I began analyzing lunar structures in the hopes of gaining a better understanding of how ITFF welting happens on Earth. From this I have realized how fascinating the Moon is, have developed new vocabulary, and an alternative explanation for how ITFF ring structures on the Moon principally arise from primary shock reflections returning to the surface off interior layer boundaries having inverted, subjacent acoustic impedances. These hypotheses are only made possible through the gathering and sharing of global geophysical data by multi-national agencies. So even thought humanity struggles to coexist in harmony, this work proves that we effectively cooperate as a scientific community globally to advance our understanding of our natural surroundings. I hope that these contributions to lunar geology prove useful.
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Introduction * Review * Geographic maps of eighteen lunar astroblemes * Seismological aspects * The South Pole - Aitken basin * Maria Imbrium and Serenetatis * Mare Nectaris * Mare Crisium * Mare Orientale * Mars and Earth models * Discussion * References * GE Pro Moon KMZ file
