Oblique-Impact Complex on Mars inlcuding Syria and Sinai Planum
Syria Planum and Sinai Planum lie at the center of a complex crustal structure interpreted to be the result of a multiple, hypervelocity (>6km/sec) impact event on Mars. Syria Planum is shown to be a composite, asymmetric crater formed by impact of at least two bolides whereas Sinai Planum is interpreted to be a result of a chain of tightly-clustered impacts (fig. 1), perhaps partly the result of spalled projectiles (Schultz and Gault, 1990). The impact complex is portrayed below as having tectonic elements including a a circumferential crustal welt, compressional crustal wedging, and an extensional regime having embedded volcanic activity including Olympus Mons and Tharsus Montes (fig. 1). The craters lie in the center of the crustal welt having about 135o surface span. Other tectonic features include near-crater fracturing and symmetric topographic ridges fanning outward from the wedge apex toward a prominent foreland scarp. Solis Planum and Thumasia Planum are probably plateau uplands lying foreland of the impact craters and behind foreland fault scarps forming the toe of the crustal wedge.
Figure 1. Part of the Topographic Map of Mars (2002) centered on the Syria, Sinai, and Solis Planums. Click on the central part of the image for a more detailed view.
NASA mission imagery in figure 2 shows details of the proposed craters. Figures 2a (uninterpreted), 2b (interpreted), and 2c show a series of faint outer rims within Syria Planum that probably result from multiple, overlapping impacts. Figure 2d covers the Tharsis Montes region and shows multiple, aligned impacts of Sinai Planum. The alignment of the craters of Sinai Planum are used to infer the horizontal component of the bolide trajectories as modeled in figures 3 and 4.
Figure 2. NASA photographs showing details of the crater region.
Figures 3 and 4 include primitive and rendered views of the impact complex mapped in kilometers using AutoCad software. Map sectors are defined relative to the impact center that include compressional (C), extensional (E), and marginal (M) regions. The compressional region of crustal wedging lies foreland of the craters in the direction of the inferred flight trajectories. The model shows a set of three overlapping wedges having structural components including lateral shear fracture systems, topographic ridges, and crustal scarps associated with crustal thickening and uplift. Valley Marineris is portrayed as lying within the compressional region along one lateral margin of the crustal wedging, and thereby coinciding with a zone of intense crustal fracturing, although this well known crustal feature may also have originated in part by other projectile impacts, perhaps oriented at very low angles to the planet surface...
The extensional region lies hinterland of the impact center where the crust and mantle were extended by steeply-dipping extension fractures penetrating deep into the mantle. These fractures probably initiared decompression melting of deep mantle material that subsequently ascended along these fracture forming volcanic pipes and feeder dikes. The inferred fractures are now buried beneath volcanic flows emanating from Tharsis Montes and Olympus Mons. However, the alignment of Tharsis Montes indicates that the strike of extension fracturing lies about normal to the bolide flight trajectory (fig. 3).
Figure 3. AutoCADR14 model of the impact complex. Click on any image for more details.
Marginal regions have the lowest topographic elevations in the circumferential crustal welt and correlate to areas that probably contain near-impact ejecta and crustal fractures having a different trend from those in the compressional region. The diameter of the circumferential welt roughly corresponds to the diameter of the Martian core (1700 km) as detailed in profile (fig. 4). The crustal welt is also cut by arcuate lineations assumed to parallel fracture dispersed in front of the impacts, as ripple marks cast in front of a stone splash. The bolide angle of descent is modeled below as being about 30o from horizontal, although this is highly speculative.
Figure 4. Profile diagram of Mars showing the geometric relations associated with the proposed impact structure.
Two global perspectives of the impact complex are shown in figure 5 using NASA's Mars Global Surveyor, orbital laser altimetry (fig. 5a) and gravity (5b) data. Digital traces of geological structures from the Viking Orbiter-based geologic maps of Mars are superimposed on the geophysical coverage that include undifferentiated grabens, calderas, wrinkle ridges, channels, crustal scarps, depressions, and crater rims (Skinner and others, 2006).
Figure 5. Mars Global Surveyor altimetry (left) and gravity (right) data rendered using ESRI ArcGlobe. Click on any image for more details.
The composite set of tectonic features are proposed to result from impact shock, crust and mantle fracturing and upheaval, and perhaps relaxation. Here, the impact tectonic features are frozen in time due to the lack of active orogenic processes that would otherwise mask such effects.
Figure 6. NASA Viking Orbiter image showing global view of Sinai Planum and Valley Marineris and other components of the proposed impact complex. Click on the image for a more-detailed view.
This impact event created a huge energy flux and solid-body disturbance on Mars, with body waves generated by impact perhaps reflecting and refracting from major phase boundaries in the planetary interior and leaving telltales scars of brittle deformation at the surface. The nature of the mantle deformation necessary to produce such a crustal signature is uncertain but may originate from primary reflections of compression waves gnerated by impact and driven into the mantle where they reflect off the core-mantle boundary back to the surface as giant welts in the Earth's crust. Similar geometric relationships are seen on Earth for known (Herman, 2006) and suspected (Cuvette Central of Rajmon, 2007) impacts . The relative timing and development of the various ductile and brittle strains and associated volcanic activity are also unknown. However, most of the brittle crustal disturbance must have been relatively instantaneous with both ductile flow and igneous activity ensuing. It's possible that tectonic activity in the form of mantle plumes, perhaps even mantle dynamics with associated crustal stains persist today, thereby reflecting long-lasting regional strain effects from this catastrophic event.
Herman, G. C, 2006, Neotectonic setting of the North American Plate in relation to the Chicxulub impact: Geological Society America Abstracts with Programs, Vol. 38, No. 7, p. 415
Rajmon, David , 2007, Suspected Earth Impact Sites database, April 13, 2007: Shell, Houston, TX, USA. http://eps.utk.edu/ifsg_files/SEIS/SEIS_database9.xls (Excel 708KB).
Schultz, P. H. and Gault, D. E., 1990, Prolonged global catastophes from oblique impactsm in Sharpton V.L., and Ward, P.D., eds., Global catastrophes in Earth hisotory; An interdisciplinary conference on impacts, volcanism, and mass mortality: Geological Society of America Special Paper 247, p. 239-261.
Skinner, J. A., Jr, T. M. Hare, and K. L. Tanaka 2006, Lunar and Planetary Science Conference XXXVII, abstract #2331
www.impacttectonics.org Rev. 10/30/2007