Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p41a0393o&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #P41A-0393
Physics
5475 Tectonics (8149), 5480 Volcanism (8450), 8005 Folds And Folding, 8010 Fractures And Faults, 8015 Local Crustal Structure
Scientific paper
Current geophysical models of the mechanical structure of Tharsis can be delineated by the proposed strength of the base of the volcanic pile, at its interface with the Noachian crust. The 'welded base' class of models suggests that the base of the volcanic pile is brittle and effectively welded to the Noachian crust [e.g. Solomon and Head, 1982; Phillips et al., 2001]. Alternatively, a 'detached cap' model proposes that much of the base of Tharsis was ductile and slippery [e.g. Banerdt and Golombek, 1990; Tanaka et al., 1991]. In this abstract, we compare predicted thrust fault vergence frequencies for each class of models against a global set of MOLA-based thrust fault vergence frequencies. Determination of thrust fault vergence from topography is now possible (e.g. Okubo et al., 2001, 2002 fall AGU). Within a thrust fault-related fold (i.e. a wrinkle ridge or lobate scarp), the thrust verges from the shallowest sloping fold limb toward the steepest sloping fold limb. Using this relation, we extract thrust fault vergence directions along 36 evenly spaced topographic profiles constructed in 10° intervals radial to Tharsis. These topographic profiles are based on the MOLA Mission Experiment Gridded Data Records at 128 pixel/degree ( ˜455 m/pixel at the equator) resolution. Our results show that the percentage of thrusts that verge away from Tharsis (47.5% - 48.8%) is sub-equal with the percentage of thrusts that verge toward Tharsis for thrusts within 9000 km, 6000 km, and 4500 km of Tharsis. Based on mechanical analyses of a brittlely deforming plate [Davis and Engelder, 1985; Montési and Zuber, 2003], we infer that welded base models would produce sub-equal thrust fault vergence frequencies on all parts of Tharsis and in the surrounding Noachian crust. By comparison, in the detached cap model, if the base of the Tharsis load were sufficiently hot and weak in shear, we would predict sub-equal vergence frequencies above the central, ductile part of the base. This area would then be ringed by an annulus of outward-verging thrusts near the edge of the Tharsis load, where basal ductility decreases toward the colder, brittle periphery, causing shearing. Alternatively, if the base of the detached cap was stronger but still ductile, then we would again predict the formation of thrust faults that consistently verge away from the center of Tharsis but in a much larger annulus. In both detached cap scenarios, we would also predict sub-equal thrust fault vergence frequencies within the surrounding Noachian crust. Neither of the 'bull's eye' patterns of thrust fault vergence frequencies predicted for the detached cap model is observed in our profile results. Instead, we find that our observations of sub-equal thrust fault vergence frequencies support the welded base class of Tharsis models. Therefore, at the time when the thrust-related folds were forming (e.g. late Noachian to early Hesperian), lateral spreading of Tharsis would have occurred independent of regional shearing along mid-lithospheric ductile horizons. If shearing did occur along a detached Tharsis base, this process would have had to take place before the formation of the thrust-related folds (and evidence of this event would be buried by subsequent Tharsis volcanics). Further, episodes of widespread Tharsis-radial graben formation, which are contemporaneous with or post date thrust fold formation, must have also occurred in the absence of mid-lithospheric shear zones due either to a detached Tharsis base, or to shallow volatile-rich layers (e.g. Okubo and Schultz, in review, GSA Bulletin).
Okubo Chris H.
Schultz Richard A.
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