Towards Understanding the History and Mechanisms of Martian Faulting

Introduction: An understanding of the sources of stress, and the expected style, orientation, and magnitudes of stress and associated strain is important for understanding the evolution of faulting on Mars and its relationship to loading, thus leading to an improved understanding of Martian geologic history and surface morphology. The Tharsis province, characterized by radial graben sets and circumferential wrinkle ridges [1, 2, 3], has been intensely studied since the Viking missions. More recently, the analysis was extended using MGS topography and gravity data [4], and [5] furthermore argued that the faulting is explained by membrane flexure alone. [4] calculated the deflection of the lithosphere due to the Tharsis load alone while satisfying the long wavelength signal of present day topography and gravity. The resulting extensional component of the stresses and strains is consistent with normal faulting on preexisting faults radial to Tharsis and away from the load, e.g. Memnonia, Thaumasia, southern Claritas, and Tempe Fossae. However, the faulting extending from northern Claritas Fossae north to Tantalus and Alba Fossae is not well explained by the membrane model, which predicts zero extension there, where the density of normal faults is high [1]. Therefore, these faults may have formed under different conditions (topography and gravity) than we see today [4]. In particular, while the bulk of the crust formed 4.5 Ga and later additions were volumetrically minor [6], gravity and elastic thicknesses are unlikely to have remained unchanged for the last 4.5 Gy. It is conceivable that mantle circulation rather than flexure played a significant role in the early support of Tharsis, producing a different gravity field during that time [7]. We consider a different source of stress – stress associated with internal buoyancy forces, i.e., gravitational potential energy (GPE), constrained by MGS topography and crustal thickness models of [8]. To test the validity of these stress models, we compare results with the style and orientation of the surface faults. Lithospheric Stress Models: We solve the forcebalance equations for the vertically integrated deviatoric stress field associated with topography and crustal thickness variations as the unique solution that balances the body force distribution, in this case GPE differences, while providing a global minimum of the second invariant of stress [9]. The solution is computed over a global grid of 0.25 × 0.25 resolution, assuming ρcrust = 2900 kg.m, ρmantle = 3500 kg.m, g = 3.7 m.s, various lithospheric depths and rheologies. Results: The Martian deviatoric stress field associated with horizontal GPE gradients for an incompressible elastic rheology shows, to first order, deviatoric extension over topographically high areas transitioning to deviatoric compression at topographically low areas (Fig. 1) [see also 10]. A notable exception to this pattern is areas with low topography but thin crust, which exhibit propensity for deviatoric extension, e.g., Isidis Planitia and to a smaller extent Utopia, Argyre, and Acidalia Planitae.