Influence of Gravity on the Geometry of Martian Normal Faults

Introduction: Extensional fault scarps on Mars bear a striking resemblance to fault scarps on Earth, including geometric features such as horsts, grabens, relay ramps and breached relay ramps, en echelon arrangement of faults and grabens, and scaling characteristics [1], [2], [3]. Other surface features associated with extensional faulting on Mars are lines (" chains ") of circular topo-graphic depressions, referred to as pit craters. These pit chains are easily observed in remotely sensed data from Mars. Although rare, pit chains also occur on Earth associated with extensional deformation. Based on the similarity in map view, one might surmise that the subsurface geometry of faults on Mars would similarly mimic structural style on Earth. However, a fundamental difference exists between Earth and Mars, which may have a profound effect on fault architecture—the difference in the gravitational acceleration on the two planets. We consider likely fault dips and fault profiles on Mars by accounting for Mars' gravity, likely rock types, fluid pressure conditions, and analogs from Earth. The results demonstrate that fault profiles on Mars are likely to be similar in shape to analogs on Earth, but that depth-dependent fault dip transitions are likely to occur deeper than on Earth and occur more gradually with depth. These results provide the basis for more robust structural interpretations of remote sensing data from Mars, and technically more sound models of faulting and related processes on Mars. Dilational faulting, found in the upper ~2 km on Earth, may extend to depths of 5 km or more on Mars. This dilational faulting may influence subsur-face fluid flow, as well as dissolution and mineralization along faults. Similarly, the seismogenic zone on Mars is likely to be considerably deeper than on Earth. Failure modes, failure angles, and fault dip: Initial fault orientations are controlled by mechanical properties of the faulted rocks, and magnitudes and orientations of the effective principal stresses at the time of failure [4]. Failure angle is defined as the angle between the failure plane and the maximum principal compressive stress (F 1) at the time of failure. At low differential stress (F 1 ! F 3), failure is commonly in the tensile mode, producing fractures with failure angles of 0° (fault dip = 90°) that experience wall-normal extensional displacement. Failure at high differential stress is typically in the shear mode, producing fractures that are oblique to the maximum