Features and Data Sources * Interpretation Process *  Discussion * GCH_Google_Mars_2.0_KMZ

Remotely sensed geospatial data are used to conduct a structural and tectonic analysis of the very large impact craters and sedimentary basins seen on Mars. The analysis includes the physical dimensions of large craters, their surrounding crustal and lithospheric strain fields including linear traces of their structural expression on the plant's surface stemming from large, hypervelocity bolide (asteroid and comet) impacts. An orientation analysis of fault and fracture traces seen on Mars surface for 13 different astroblemes demonstrates that brittle crustal failure stemming from hypervelocity impacts adheres to Coulomb shear-failure criteria for material failing under uniaxial compression. Mars is devoid of tectonic-plate plate drift as seen on Earth and therefore, most tectonic features including crustal fractures, faults, and folds stem from extraterrestrial bombardment by asteroids and comets (bolides) striking the planetary surface at oblique incidence angles and sometimes forming large igneous provinces in their wake, or in the case of near-vertical impacts, antipodal igneous provinces. The largest impact events produce deep-seated faults in the planetary lithosphere as revealed from prior magnetic mapping.

Introduction

I focus my impact-tectonic research on Mars again here for a second time in two decades to show how mountains can rise and large basins form in terrestrial crust and lithosphere from instantaneous, catastrophic upheavals caused by large-bolide impacts. The first structural analysis of the red-planet's impact scars (figs. 1 and 2) in 2007-09 was spurred on by the many years of my staring at the US Geological Survey's topographic map of Mars hanging on the wall across the hallway from my cubicle in the geophysics wing of the NJ Geological Survey. That map is dominated by such notable Martian features as Valley Marineris, Olympus Mons and Tharses Montes. Those structures comprise an enormous impact blemish stemming from an asteroid shower with a strewn field centered on Syrian and Sinai Planums (fig. 1). The crustal disruption is extensive and stands out as an enormous red bruise (elevated topography) on the planetary surface with a circumferential welt spanning the distance of any major continent on Earth by comparison (fig. 2).  The definition of this astrobleme was included in Google Mars version 1.2 that was developed using Google Earth (GE) as a base map before Google Mars was released as an integral part of the standard revisions of GE Pro . As recently shown for a set of Cretaceous-age suspected impacts in the western Pacific ocean basin, the impact-generated fracturing, faulting, and folding mapped for a large terrestrial event like this one are the structural components of "astroblemes" or "star wounds" as defined by Dietz (1961), a plate-tectonic patriarch that helped discover sea-floor spreading. In my mind this term just as easily can be thought of as asteroid-impact blemishes (astero-blem).  These planetary-disrupting impact events bruised and welted the planetary surface during episodes of mass accretion from extraterrestrial bombardment. The astroblemes are the telltale signs.

This study defines structural elements associated with 13 of the largest bolide-impact scars and sedimentary basins on Mars (figs. 3 and ). The large sedimentary basins are included as they stem from very large impacts events, with most involving multiple bolide strikes from fragmented or clustered projectiles having a focused concentration within a relatively small region, or strewn field. These basins are now filled with oceanic or aeolian detritus that masks the craters, just like those on Earth recently discovered lying beneath kilometers of sedimentary overburden from subsurface petroleum and water geotechnical investigations, and corroborated with geophysical potential-field and seismic-reflection studies.   These impacts generated tectonic episodes of both sedimentary basin and mountain genesis, and in many cases spurred the development of large-igneous provinces (LIPs). Some impact events punched depressions into the lithosphere while others formed expansive regions of material accretion and structural disruption from having received multiple incident bolides within a strewn field (fig. 4).

Each astrobleme form reflects the many variables of impact energetics, but mainly the size, composition, velocity and trajectory of the bolide, the latter of which is geometrically assessed with respect to it heading, or the horizontal trace of its flight path, and its angle of incidence. No effort is made to estimate the size of the projectiles based on cratering criteria, as that is not the focus here. Those calculations can be partially constrained by the crater diameter and target composition, but my aim is to see how secondary geological structures spatially and geometrically manifest themselves with respect to each large crater or sedimentary basin, particularly with respect to interpreted  impact trajectories and the distribution of planetary, LIPs. Most impact events are oblique rather than normal-incident strikes. The character of those glancing off the planetary surface at low-incident angles (1-30^o) transmit less ground energy and fan out over broad regions from splashing down, ricocheting and fanning out downrange of the area with the largest craters.

Google Mars is used as part of the standard GE Pro engine to map and parameterize the most apparent astroblemes as summarized below in figure 3. The analysis is based on topographic and geophysical planetary-scale geophysical themes added to GE that show regions of tectonic disruption stemming from these catastrophic, and geologically instantaneous, large-impact events.  A set of planetary-scale images were captured for each astrobleme using five different base-image overlays serving as the basis for mapping geological structures interpreted to stem from each impact event. The base imagery include NASA Viking 2 colored photography, United States Geological Survey (USGS) Topography, and global laser altimetry, gravity, and magnetic themes.

The composite set of strain features of each astrobleme are mapped and organized as GE theme elements in folders within a GE KMZ file (fig. 3). The study methodology is detailed below including explanation of the organizational structure of the KMZ file containing the structural elements of this study as geospatial line elements used to visually emphasize each event (fig. 3). These elements specifically include center points for each astrobleme with some coinciding with visible craters while others are approximate centers of large crustal basins with hypothetical, hidden craters masked by thick sedimentary blankets. The digitized polyline traces of linear, curved, and circular features defining each astrobleme are organized in file subfolders (fig. 5).

Figure 3. 13 Martian astroblemes superimposed using GQIS

Interpreted headings and incident angles are estimated for each interpreted event based on the systematic arrangement of the spawned, secondary structures together with the crater morphology where exposed (figs. 6).  For moderate- to low-incident angle strikes, compressed foreland regions located down range systematically fan out in front of craters where the crust and lithosphere were shoved, displaced forward, and structurally compounded and thickened.  A graphical summary of the trajectory statistics compares the headings of these large-impact events based on the structural analyses (fig. 6).

The names of the 13 astroblemes are derived from place names seen in Google Mars, or on the the USGS topographic map that lie closest to the crater or basin center point, while others derive their names simply by location as in 'the North Pole' event (fig. 3). Prominent craters are mostly named with some having been targeted as bases for human extraterrestrial exploration. In other places, large impact carters are assumed to lie beneath deep sediment-filled basins or are covered by oceanic sediment from long-ago evaporated seas covering much of its northern hemisphere, or beneath thick blankets of loess deposited during frequent wind storms.

The thirteen named folders contained in the KMZ file hold the interpreted structural features including crater center points and both circular and linear paths distributed around craters that highlight systematic, far-field strains in each astrobleme resulting from impact tectogenesis. Some traces of linear features seen on the surface imagery are not assigned to any particular event here, and are being compiled separately as this is a continuing and evolving effort.  Other structural features representing tectonic elements like like-volcanic edifices, are also interpreted to stem from some impact events, and in those cases are also shown to be arranged with systematic geometry with respect to large impact craters or strewn fields and is likewise discussed in more detail later.

Because the nature of the impact target factors into the impact-tectonic effects, it is important to discriminate between continental and oceanic realms in study the physical effects of impacts. Because there is no freely available oceanic theme currently available for Google Mars, I built one using NASA's Mars Orbital Lander Altimetry (MOLA) theme. The oceanic theme uses the MOLA digital-elevation model from NASA he Mars Ocean at 0 and -30 m elevation using the Mars Orbital Laser altimetry that was downloaded and processed using a geographic information systems. The global digital-elevation-model (DEM) is available from the NASA data Annex. It's a gray scale raster image derived from the Mars Global Surveyor (1996-2001) mission.  I used QGIS (ver. 3.102) to process this DEM into a gray-shaded base-map image having darkened, low elevations and lighted higher ones (fig. 3). Polygons encompassing topography less than the 0 and -3 km elevation values over the northern hemisphere were generated and colored as blue with their opacities set to 10% (90% transparent).  This processed first involved generating 1-km hypsography from the DEM, then selecting the 0 and -3 km contours and manually closing them into polygons by adding line segment along the image perimeter.

The structural interpretation of impact-tectonic strain fields is complicated and with many uncertainties and comprised by the inevitable misidentification and prejudice. Nevertheless, the exercise was completed after progressing through three interpretation passes, with the second and third refining the heading of some impacts based on the initial pass and the subsequent statistical analysis of line traces for each astrobleme (fig. 5). The latter two passes also assessed how the far-field strain fields appear to overlap and interfere with one another. Tectonic inheritance is seen where strain fields merge and overlap because the crust and lithosphere becomes strain hardened from the hypervelocity shock of impact and the consequent formation of healed, brittle structures in the lithosphere to suspected depths of about 600 km (fig. 6). Impact- and drift-related tectonic strains for ensuing tectonic events will propagate through previously shocked lithosphere differently than for less-shocked regions. I suppose this is good time as any to remind the reader that their are no 'un-shocked' or pristine regions on terrestrial planets or Moons, other than newly formed oceanic crust on planets having recycling tectonic plates like Earth, but for Mars the entire planet is formed from bombardment and accretion, even during initial cooling and condensation from a planetary embryo. Additional consideration was given to examine the strain fields with respect to the possibility that some of the large impacts occurred in areas covered by shallow seas on Mars northern hemisphere earlier in its history before they evaporated.

Once the custom imagery was compiled as a basis for visual scrutiny of astrobleme structural signatures, the first step in the interpretation process involved generating circumferential rings around each visible crater, or sets of crater in a strewn field, or approximated centers of large sedimentary basins that are assumed to stem from a very-large bolide-impact event. Circles around each crater were generated using the GE custom application RangeRings available from EarthSurvey.us.  The tool works by setting the center point and entering a specific radius, using kilometers for this project. Outboard circles of 600, 1600, and 2900 kilometer radii were thus generated around most craters or basin centers that represent circumferential, arched welts lying thousands of kilometers outboard of the craters with amplitudes of 1-3 km.  The circles represent the approximated traces of circumferential crustal arches that mark the edge of large sedimentary basins or fractured areas of high topographic relief lying outboard of the craters. These radial components represent far-field welts of low amplitude resulting from epeirogenic movements eons after the impact event from lithospheric flexure and ensuing asthenosphere responses. The timing and nature of such brittle-ductile processes remain hypothetical and await future modeling, testing and refinement. But from empirical measurements, they are remarkably similar in size to those seen on Earth. The tectonic manifestation of these features is elaborated further below.

Once each crater was delineated as a multi-ring impact structure, the next step was to manual digitize apparent surface features and remotely sensed geophysical anomalies that occurred in close proximity to the crater(s) that demonstrate symmetric arrangement about the crater in a plausible manner, as for example, by having faulting geometry reflecting Coulomb shear-failure criteria with conjugate sets striking within 30^o of the bolide heading (fig. 6). In some cases, pronounced sets of emergent, crustal faults stem from craters and radiate outwards, whereas in other places, secondary structures coincide with surface scarps or rims of crustal depressions, or arches lying circumferential to the crater but thousands of kilometers away (figs. 4 and 6). Structural traces of astrobleme elements were thus digitized manually using the five geospatial, planetary themes providing the visual contrast of color intensities and hues reflecting physiographic relief seen on the planetary surface or the more deep-seated, lithospheric structural elements interpreted based on potential-field gravity and magnetic anomalies. The various structural elements are represented using GE paths (vector polylines) having vertices at inflection points between connected and continuous line segments.

Figure 6. Structural analysis of 13 large, Martian astroblemes on a gray (continental) and blue (oceanic) base.