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Geomechanical and tectonic investigations into the geologic history of Mars at local, regional, and global scales
AdvisorSchultz, RIchard A
Geological Sciences and Engineering
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The primary goal of this dissertation is to understand the geologic history of Mars through the use of geomechanical techniques to investigate tectonic deformation of the lithosphere at local, regional, and global scales. Techniques that have their origins in terrestrial geology are applied to Mars to analyze tectonic deformation of the lithosphere at these scales. At the local, or outcrop scale, I determine the physical properties of a sedimentary rock unit at Meridiani Planum by utilizing data collected in situ by the Mars Exploration Rover Opportunity. The geological engineering technique of rock mass rating (RMR) was used to characterize the strength and deformability of a jointed outcrop of the upper unit of the Burns Formation. Results of this study show that the upper unit of the Burns formation has similar physical properties to terrestrial sedimentary rock masses such as siltstone, mudstone, and shale and that at the time of deposition, the modulus of deformation, cohesive strength, and tensile strength for the Burns Formation were ~50% lower than for present-day dry conditions. The hypothesis that the Thaumasia Highlands, located in southern Tharsis, formed as an orogenic belt is tested using critical taper wedge mechanics (CTWM). Key physical parameters such as the coefficients of friction for the wedge material and décollement and the pore fluid pressure ratio were varied between reasonable values for these parameters suggested by terrestrial and Venusian values. The topographic slope of the Thaumasia Highlands was measured from a digital elevation model derived from MOLA topographic data and, together with the physical parameters listed above, were used in a series of equations that describe the physical properties and geometry of a hypothetical critical-taper wedge. The results of this study suggest that regional slopes in the Thaumasia region are too small for the topography to achieve a critical wedge taper for reasonable values of décollement dip angle, pore fluid pressure ratios, and rock properties, which suggests that the Thaumasia Highlands likely did not form as an orogenic wedge but instead through a mechanism such as lithospheric flexure. I test the hypothesis that a period of enhanced contractional deformation occurred on Mars during the Late Noachian - Early Hesperian (3.8 - 3.6 Ga). The magnitude of fault-related strain and corresponding planetary radius decrease was calculated for contractional structures that formed during this time period. Additionally, I calculate the fault-related strain and planetary radius decrease for all of Mars' observable geologic record (3.95 - present; Middle Noachian - Late Amazonian). Comparison of these values to those predicted by thermal evolution models for Mars shows that thermal evolution models significantly overpredict the amount of strain and associated planetary radius decrease for both time periods. Results of this study also support the hypothesis that a "pulse" of enhanced contractional deformation occurred during the Late Noachian - Early Hesperian.