EFFECT OF GRAVITY ON NORMAL FAULT PROFILES ON MARS VERSUS EARTH

The shape of normal faults on Earth is recognized as a key factor in determining the location and shape of basins and mountain ranges, sedimentation patterns in active fault systems, patterns of earthquake seismicity, and pathways for flow and accumulation of fluids. Extensional fault scarps on Mars and Earth have many similarities, including the morphology and segmentation of fault scarps, and their map-view arrangement, while lines of topographic depressions (pit chains) that are parallel or collinear with extensional faults are apparently unique to Mars. In this study, we use rock mechanical data for basalt and andesite along with gravity and depth vs. density profiles for Earth and Mars to calculate theoretical fault profiles. Active normal faults on Earth commonly have very steep dips (70°–90°) in the uppermost brittle crust (0-2km), steep dips (50°–70°) at intermediate depths (2–5 km), and moderate dips (35°–50°) in the lower part of the brittle crust (>5 km). Low angle dips of 0°–35° occur near the brittle-ductile transition and in extremely weak sedimentary layers. Consequently, normal faults tend to have concave upward profiles. In some cases, steep fault segments in the uppermost crust are dilational, experiencing volume increase. Controls on initial fault dip include mechanical properties of the deforming rocks (e.g., friction angle, tensile strength) and the stress gradient with depth. In the normal faulting regime, maximum principal effective stress is primarily a function of rock density, depth, and gravity. Because gravitational acceleration on Earth (9.81 m/s²) is higher than Mars (3.72 m/s²) by a factor of 2.64, stress within Earth is greater than Mars for any given depth. If faults form in similar situations on the two planets, and if rock types are similar, fault-dip transitions described above would occur at greater depths (approx. 2.64 times) on Mars than on Earth. Consequently, steep fault segments in the crust are likely to extend deeper (0–5 km) on Mars than their counterparts on Earth. Dilation of these steep segments associated with fault slip at depth could result in large volume increase in the uppermost crust. Highly dilational faults would help explain the occurrence of pit chains and would strongly influence fluid movement and mineralization in the Martian crust.