TECTONICS BLOG Rev. 2020-07-11;10-29-2020

Gregory Charles Herman, PhD
Flemington, New Jersey, USA

Old Pacific mayhem; three suspected bolide-impact events of Cretaceous age with global consequences.

Current problems with plate-tectonic theory * Methods * MPM1 * Manahiki * MPM2 * Discussion * References

Introduction

Google Earth (GE) is used to map three, suspected, bolide-impact (asteroid or comet) events that disrupted Cretaceous Pacific oceanic crust and raised both the Mid-Pacific Mountains (MPM) and the Manahiki Plateau (figs. 1). The largest and oldest is named MPM1 with an estimated age of 125.0 Ma, followed by Manahiki at about 101.5 Ma, and the youngest MPMP2 at about 94.5 Ma, the latter lying physically close and structurally overlapping MPM1. The older and younger events involve multiple craters lying close together in large strewn fields (fig. 2 to 4). These extraterrestrial, catastrophic agents structurally disrupted regions of the Pacific Oceanic basin and correlate to low-level mass-extinction events stemming from global volcanic, atmospheric, and sedimentation episodes that lasted eons (figs. 5 and 6). What began as an exercise to map the visually dominant MPM1 (est. 125 Ma) has resulted in the addition of the other two sites upon closer scrutiny of the regional physiographic, geological, geophysical, and temporal data.  Each of the three strewn fields lie amid thickened oceanic crust and have associated fractures that systematically flare out from the craters and verge toward the foreland along the interpreted trend of the bolide-descent trajectories. They also extend to great lengths in lateral sectors of each blast zone. The fracturing, faulting, and folding mapped for each impact event are the structural  components of "astroblemes" or "star wounds" that were coined by Dietz (1961), a plate-tectonic patriarch that helped discover sea-floor spreading. Each set of features occur in close proximity and provide comparable examples of how oceanic-impact events structurally imprint Earth. This chapter summarizes the mapping techniques and geological aspects of these three, large impact events in the old Pacific Basin. The interpretations borrow heavily from prior work that identified impact-tectonic far-field (ITFF) strains on the North American Plate stemming from the Chicxulub  (~66 Ma) and Chesapeake (~35 Ma) impact events where resulting, systematic crustal fracturing, faulting, and deep-seated, multi-ring lithospheric welting contribute to the tectonic architecture of the North American continent.

There is only one oceanic impact site included in the 159 known and confirmed bolide impact craters in Earth's impact-crater database despite two thirds of our planetary surface being covered by oceans or marginal seas. Scores of craters must lie undiscovered beneath thick blankets of sediment across the globe. The confirmation of oceanic impact structures however is expensive, risky, difficult, and hampered by having to drill the sea bed lying kilometers below the water surface to additional kilometers depth through post-impact pelagic and hemi-pelagic sediment to retrieve rock core containing the geological evidence of shock strains. Such evidence in oceanic, basic crust can include near-crater breccia, melts, mineral veining, crystal transformations and dislocations, and shatter cones.  But by closely examining the geometric expression of the fracture systems that gave rise to the oceanic mountains with respect to 1) seafloor age, 2) their gravitational and magnetic expressions, and by 3) applying historical and structural-geological principles, the geological mysteries of the oceanic realm begin to unfold.  As characterized here, many of these extensive fracture systems are secondary structures imparted by large (> 1 km diameter) asteroid or comet (bolides) impacts that occur periodically through time to impart far-reaching, tectonic strains in Earth's lithosphere when it is suddenly shocked and rumpled from an extraterrestrial strike.    

Not only do impacts accrete new material to Earth, but they also instantaneously impart extensive brittle and plastic structural damage to the crust and lithosphere together with producing severe atmospheric disturbances, craters, and pulverization (communition) of the projectiles. At this time we are only beginning to recognize and catalogue far-field strains and study the manner in which ground energy is partitioned into the strain responses to account for these massive energy fluxes. There is a lack of experimentation and mathematical modeling to account for these far-reaching strains like those seen surrounding the two- well known, large, Cenozoic-aged impacts on marginal continental lithosphere of the North American Plate (see posts 1 and 2). Humans cannot reproduce the physical conditions of large, hypervelocity impacts because of the process scale, but laboratory tests do help constrain crater morphology (see Gault references). On the other hand, the seismic efficiency, or the amount of the bolide kinetic energy that gets transferred into ground energy upon impact is poorly constrained and will vary depending upon impact velocity, incident obliquity, and both projectile and target compositions. The velocity variable in particular is a primary factor, because when a strike opposes the host bodies' direction of planetary rotation and/or circulation, the resulting impact velocity, and physical expression of resulting strain fields would be additive, as is the case for the interpreted events mapped below.  Most laboratory experiments derive impact variables based on firing projectiles into static bodies, and the physics of colliding bodies is very complex and often limited by having one body at rest. Such conditions inadequately account for the tectonic energy arising when spinning bodies moving in opposite directions collide. The online Earth Impact Effects Program states that the minimum impact velocity on Earth is 11 km/s, with typical impact velocities of 17 - 51 km/s for asteroids and comets respectively. Maximum impact velocities of 72 km/s are attained when Earth collides with bodies moving in opposite directions.  Jutsi and others (2015) summarize the state of the art of modeling asteroid collisions and tectonic processes and conclude that there have been major advances in the past decade, but numerical simulations need to become of 'higher fidelity' in order to just match the results to conditions seen from physical experimentation.  Mathematical validation awaits as it's necessary to accurately identify the physical limits and impact parameters to achieve realistic models. 

These interpretations primarily used GE as a geological mapping tool. Recognition of the impact features rely principally on the physiographic expression of the seafloor, compiled from decades of work conducted by the U.S. National Oceanic and Atmospheric Administration (NOAA).  The three locations coincide with large, ocean-floor rises that are also large igneous provinces (LIPS) where oceanic crust is thickened by post-impact magmatism in addition to material accretion from bombardment. The number of craters and the composition of the bolides are unknown because deep-sea (DSDP) and ocean-drilling (ODP) programs have not sampled the suspected crater locations, although some are very close (figs, 2 to 5).  The suspected crater locations and dimensions are estimates based on the following aspects illustrated in figures 1 to 4.

1) GE sea-floor physiography
2) ocean floor ages with the 100 Ma magnetic isochron highlighted
3) regional magnetic and gravity potential-field maps
4) fracture geometry that systematically radiates outward from each site
5) crustal-floor fold geometry derived from the magnetics
6) far-field, radial fractures and lithosphere welts or 'rings'.

GE placemarks (points), polylines, and polygons were used to highlight and label features. Crater-rims and radial traces of hypothetical crustal arches lying circumferential to craters at 660, 1600, and 2900 km. were generated using the free GE application Range Rings to generate the polyline circles. For strewn fields involving multiple craters, the overlapping arch traces were merged and pared using QGIS so that only those traces enveloping all others of the same value were saved. That is, only the most external traces were saved to eliminate clutter. Vertical crustal deflections associated with these far-field, radial arches and intervening troughs are estimated to be about 0.5 to 1.5 km amplitude and are discussed further in the concluding discussion below.

The surface traces of systematic fractures and faults were digitized using a hand-held computer mouse with the caveats that they lie within 2900 km radius of the crater(s) and their extent is limited to oceanic crust that is older, or of the same age as the interpreted event.  A GE theme of the ocean-floor ages (Muller and others, 1997) is included in a 2012 GE computer file named Dynamic Earth.KMZ that was compiled by Laurel Goodell of Princeton University (2012) and used here to help constrain the timing of each event. The aforementioned KMZ file also includes the differentiated tectonic-plate boundaries included in the illustrations (Bird, 2012).

The fold interpretation in the old Pacific basin is derived from the aforementioned magnetics theme. Ocean-floor isochrons of approximate age 126.5 and 143 Ma age are highlighted in figure 2 (top right) to show that the trace of a major, central antiform lies parallel to the interpreted trend of the bolide-flight path. As portrayed, the anticline trace reaches over 2700 km distance. This impact event apparently wrinkled the ocean crust. I'm uncertain if all of the folding mapped in figure 1 and 2 stem from this one event, or if the triangular shape of the oldest ocean crust to the west was inherited from prior tectonic events. This will also be discussed further below.

To capture and compare the geometry of the fracture systems interpreted for each event, a global white mask was used as a backdrop to capture their surface-trace expression. All fractures were colored black, set to a line width of 2.0, then each fracture set was centered in the display before being captured, copied, and embedded into MS PowerPoint software for further illustration (fig. 8).

Other GE themes include a KMZ file of the results of the U.S.-funded deep-sea-drilling-project (DSDP) and subsequent ocean-drilling program (ODP) that provide detailed geological records of both ocean-crust basement and the sedimentary cover (fig. 8). The plate-drift vectors are from another KMZ compilation based on global-positioning-systems (GPS) measurements from ground-fixed receiving stations. 

The GE theme-transparency functions were used extensively to prepare the illustrations that are captured screen displays of GE, once each figure was fashioned to emphasize the intended physical relationships. The theme-transparency function is what affords GE such great utility in integrating complex scientific themes covering large areas in great detail. Earlier structures are set to a transparency of about 50% so that they become muted with respect to the younger, emphasized structures in turn. For example, MPM1 and Manahiki astoblemes were made semi-transparent to emphasize the MPM2 astobleme in figure 4. The following sections provide more detailed aspects of each event prior to a concluding discussion.

MPM1 is the oldest, most expansive and energetic event interpreted to stem from multiple strikes by a fragmented parent asteroid or comet.  The suspected craters lie beneath blankets of Cenozoic pelagic oceanic sediment that are generally less than 1 kilometer thick as sampled at DSDP site 463 (fig. 7). The names, locations and diameters of each crater are listed in table 1. As mapped, there are seven with the smallest about 60 km in diameter, and the largest either 80 or 300 km, depending upon whether it's a multi-ring structure or just a very large crater.  The crater form of these features was previously noted by Wilde (2010). The fracture, fold and fault geometry reflect a probable, moderate angle of projectile descent (<45^o and >30^o) from azimuth 085^o toward 265^o (fig. 2). The age and magnetic expression of the crust indicates that the lithosphere was wrinkled by these impacts, resulting in a series of major antclines and synclines in oceanic basement older than about 125.0 Ma. The folds have gently-plunging axes trending sub parallel to the interpreted flight paths (figs 1 and 2).  The fracture system lies symmetrically disposed about the strewn field and verges westward towards the direction of incidence along the trend of the central anticline axis.  The foreland sector situated downrange of the strewn field shows melting and magnetic alteration of the oceanic crust within a region emanating from the craters and fanning out to the west toward the Marianas trench (fig. 1 and 2). The foreland region is where the majority of the absorbed ground energy was focused (fig. 8). 

Other impact-tectonic far-field (ITFF) strains include the lithosphere arches and troughs lying circumferential to each crater as illustrated having arch traces located at 660, 1600, and 2900 km radial distance from crater centers. These welts have amplitudes on the order of 1.0 to 1.5 km, varying occurrences and physiographic expressions. They are clearly seen in places, like along the 2900 ring to the south and west of the strewn field where the seafloor attains elevation of almost 2 km structural relief from the nearby abyss. But in other places, they are not apparent because of uncertain reasons, two of which may be tectonic inheritance and/or tectonic overprinting. These points are discussed further at the end of this post.

The timing of MPM1 points to correlation with a global atmospheric carbon disturbance referred to as the oceanic anoxic event 1 (OAE1) when carbon-enriched silt and clay was deposited in worldwide basins for almost one-million years (fig. 5), and major perturbations of seawater chemistry and climate occurred (Schlanger and Jenkyns, 1976; Jenkyns, 2010).  It also happened just after the onset of the Mesozoic thermal maximum (fig. 6) when worldwide hot-spots were flaring to produce large-igneous provinces (LIPS). This was a period of global biological and tectonic change that was probably spurred on by this event involving multiple impacts in a strewn field covering an estimated 140,000 sq. km. in the center of the Pacific Ocean (figs. 1 and 2). The associated seamounts of the Mid-Pacific Mountains now reach over 2 kilometers relief from the nearby oceanic abyss and although many aspects of these events are poorly constrained, the geological and geophysical evidence pointing to their occurrence as major impact strewn fields is abundant.    

The current direction of plate drift is sub-parallel to the inferred descent azimuth indicting a possible link between impact momentum and plate traction. It's also possible that the Hawaiian-Emperor seamount chain developed along deep-seated faults imparted by this collision that tapped asthenosphere melts then, and continue today to feed magma at spots with fault-mediated ascent. The Emperor seamount chain is situated to the North of the strewn field and occupies a lateral blast sector with large faults that are mirrored in the opposite southern sector with surface traces thousands of kilometers long. The Hawaiian seamount-island chain rests in crust younger than 125 Ma and therefore the surface trace of those fractures were not traced as part of the MPM1 set. However, that is not to say that MPM1 didn't impart deeply penetrating  fractures that persist at lower lithosphere and upper asthenosphere depths to facilitate new oceanic crust growth in their wake. This too will be considered further below.

The Manahiki event appears to be a solo impact, although small, circular depressions occurring close by may prove to be smaller impacts, and/or spalled projectile fragments (fig 3). The crater as portrayed is 160-km in diameter and the bolide is assumed to have had a moderate angle of descent (<60^o and >45^o) from azimuth 115^o toward 295^o (fig. 2).  The strain expression of the event is the most uniform of the three having the associated fractures fanning symmetrically outward in the wake of the crater and verging northwest toward and into the foreland sector. However this event shows less foreland fracturing in comparison to MPM1, which may reflect a relatively higher incident angle of impact. Faults occupying lateral sectors are long and continuous and link together to form a basin depression extending into the foreland sector, probably formed from an impact, plunger effect. The astrobleme is remote to the other two which may explain its relatively pristine strain expression, reflecting inheritance of ordinary oceanic crust at the impact center itself. Therefore, the resulting scar serves as a "type" strain response to an oblique, hypervelocity bolide impact in oceanic crust, and a basis for comparing the geometry to that at other sites (fig. 8).

The timing of the Manahiki event is uncertain. It appears younger than MPM1 because the 2900-km arch stemming from MPM1 seems to limit the foreland disruption of the Manahiki event. That is, MPM1 ITFF strains interfere with Manahiki's scarring by limiting their foreland propagation. The regions around a large impacts like MPM1 become strain hardened from brittle and plastic strains distributed in the lithosphere and asthenosphere that include fracturing, folding, faulting and melting. These secondary structures strengthen material that they form in and give rise to regions having decreased compressibility and higher shear strength with respect to unstrained regions having slightly different material strain responses. The geometry of such interactions have only begun to be recognized and characterized and also occur where MPM2 strains overlap and interfere with MPM1 strains.

The MPM2 event is portrayed here as a multiple-impact event having three craters with diameter of 40, 60, and 100 km (table 1). The type of projectiles are unknown, but the associated fracturing appears to result from a set of oblique strikes having moderate- to low incidence (<45^o and >30^o) from azimuth 58^o toward 235^o (fig. 2).  Being the youngest event of the three, the associated ITFF strain features overlap and interfere with by the ITFF strain features mapped for both MPM1 and Manahiki (figs. 1 and 4). This astrobleme has a comparatively wide fault- and fracture expression extending to far reaches in the lateral sectors that may reflect a selective strain response from indenting previously strained lithosphere that hampers the foreland transmission of energy from this later event, as if 'running into a wall' that was erected by MPM1 in the form of the large central fold-limb (figs. 1, 2 and 4).

MPM2 is correlated to Oceanic Anoxic Event 2 (OAE 2) in the late Cenomanian that involved LIP volcanism as seen by increased accumulation rates of marine organic carbon in globally distributed basins from massive CO₂ atmospheric releases (Turgeon and Creaser, 2008; Du Vivier et al., 2014; Holmden et al., 2016) that may have limited available bio nutrients for marine organisms (Jenkyns and others, 2017). Stratigraphic, geophysical, and structural geological evidence point to a disruptive marine event that occurred at the Cenomanian-Albian (C/A) time boundary and coincident with marine mass extinctions.

The representation of buried craters lacking seismic-reflection coverage is risky because they await confirmation through expensive sea-borne drilling operations that gamble on their existence. Hence, the lack of confirmed oceanic impact structures. Even when there is seismic and drilling data, an impact interpretation is not always forthcoming because drilling is fraught with uncertainty stemming from sampling gaps and having but one point of geological observation that may be insufficient to confirm a large cratering event.  These suspected crater locations are constrained using only morphological, structural, and geophysical criteria and if they are in fact realized, their form may vary greatly from the circular craters represented here. Experimentation shows that low-incidence, oblique strikes can leave scars that are shaped in tear-drop or elliptical forms, and impacts striking at low-incident angles can ricochet off Earth's surface and expel fragments that splash down elsewhere.  Much work remains in proving their existence before they are accepted as part of our tectonic heritage. And in that respect, associated strain hardening of the lithosphere, ITFF strains, and punctuated, geodynamic perturbations also become part of this heritage, as seen for the Chicxulub impact crater that sits at the center of a tectonic hub fixed on the Gulf of Mexico having the surrounding tectonic plates rotating about it in concert with a lack of central seismicity to within 660-km radius of the crater (Herman, 2006). Strain hardening is a common repair response to deformation of material having the ability to heal, and as familiar as when our bones fracture and heal with more strength than before from calcium deposition during repair. The network of healed fractures, mineralized fault surfaces, and compacted material imparted by impacts cause the lithosphere to strain resulting in increased rigidity and hardness in comparison to unstrained material, and therefore will preferentially resist subsequent strains relative to unstrained or less-strained regions. The great transform faults that now span the mid- to eastern Pacific Ocean floor were probably inherited from these suspected events, among others that have yet to be recognized. The systematic flaring outward of the deeply penetrating fractures in the wake of each event occur in tectonic hinterlands (fig. 8) and are apparently utilized as subsequent sutures between plate segments that accommodate growth and adjustment as the plates drift and twist about on the surface of a spheroid. Overlapping ITFF strain fields are mapped around two, large, confirmed impacts on the North American plate (NAP, Chapters 1 and 2), and now there are three suspected oceanic impacts having overlapping strain fields that allow a comparison among each other and to the continental ones. The latter show ITFF epierogenic uplifts with radial arching at ~1600 and 2900-km radii from craters as seen with the Adirondack Mountains and the Colorado Plateau, whereas some of the deepest troughs lie at the intersections of overlapping rings.  Earth's crust is highly anisotropic from the combined effects of both orogenic and impact-born strains that must overlap and interfere with one another to influence the manner in which seismic energy becomes transmitted and dissipated from younger events.

Impact effects including atmospheric entry, impact crater formation, fireball expansion and thermal radiation, ejecta deposition, seismic shaking, and the propagation of the atmospheric blast wave can be calculated at various distances from any impact point using an on-line computer program that requires projectile, impact, and target input parameters.  But determining the physical nature of suspected, hidden features is purely speculative and there is no way of determining if a strewn field was generated by projectiles that were fragmented before or after atmospheric entry, let alone their impact velocities. Also, having multiple craters of uncertain dimensions in a strewn field makes it nearly impossible to estimate input parameters for a single, parent projectile if there was only one. Bolide clusters or fragments can impact a planetary or moon surface with a series of tightly-spaced projectiles arranged like 'a string of pearls'. Humans recently witnessed such an event on Jupiter with the Shumaker-Levy event in 1997, the first modern, direct observation of an extraterrestrial collision of Solar System objects. Calculating impact energies stemming from big events like those is one thing, but portioning that energy into the various strain responses realized by the host is another, especially because we have such a poor understanding of seismic efficiency, or the amount of impact energy that becomes grounded. Much more experimentation is needed in order to understand how seismic efficiency varies with impact conditions before we truly can understand the limits and strain effects stemming from these catastrophic disturbances.

The ocean-floor folding mapped in association with MPM1 is the first instance that I know of where large-scale crustal folding that is not radial to an impact crater is attributable to a large-bolide impact on Earth.  If the MPM1 location and the proposed secondary structures from this suspected event are proven, then structural folding on the order of alpine scales becomes part of an astrobleme. As originally defined, they are the 'root structures' of ancient meteorites that are exposed by uplift and erosion over time (Dietz, 1961).  Large-scale, radial welting of the crust is reported by Dietz in this defining work:

"Practically nothing of the original crater remains, but geological study has revealed a worn-down "dome" of granite 26 miles in diameter surrounded by an upturned and even partially overturned collar of Pre-Cambrian rock. A great ring syncline (the trough of a fold in the rocks) surrounds the collar, making the entire deformation 130 miles in diameter. Geologists have traditionally attributed this huge structure to a long sequence of tectonic events. ".

The term 'astrobleme' therefore includes large-scale circumferential folding and is extended here to include the complete array of ITFF fracture, fault, and fold strains mapped for each event.

A comparison of the Phanerozoic record of climate change versus extinction intensity shows that biological extinction events occur during both hot and warm climates (fig. 6). But any causative link between the climate change and extraterrestrial-born events is speculative, but certain to reflect in some measure such large, energetic, disruptive events as those outlined here and prior chapters of this blog. And so to conclude, I offer some thoughts regarding the heaping of speculation. This work compliments and supports earlier observations and thoughts concerning ITFF strains that were first discovered and reported surrounding the Chicxulub and Chesapeake impact craters on the continental NAP (Herman, 2006). Epierogenic and mass movements associated with such events are the resulting plastic and brittle strain responses when hard-shelled, plastic spheroids spinning and circulating through space in time get struck. Impacts events like these should be soon recognized with more regularity from using widespread modern technology to portray our environment with unprecedented detail and eloquence. So although it may seem like speculative events are used to constrain other speculative events, I assure you that by closely inspecting what is proposed and detailed here, you will find plausible mechanisms and explanations for processes and features that are soon to become part of our tectonic tapestry as it continues to be woven.

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