The formation of Valles Marineris: 1. Tectonic architecture and the relative roles of extension and subsidence

The formation of the Valles Marineris troughs on Mars is widely held to involve some combination of horizontal extension and vertical subsidence or collapse, but the specific tectonic mechanism is poorly understood. This study uses boundary element models of Valles Marineris formation to evaluate a range of fault dip angles for two end-member models of trough formation: simple extensional tectonism, and extension together with vertical accommodation at the base of the lithosphere representing either viscous lower crustal flow or some other form of deep-seated collapse. The models are constrained by the lack of footwall uplift on the plateau surface outside the troughs, and the lack of significant tectonic failure of the plateau in response to the stress changes induced by Valles Marineris formation. The results demonstrate that trough formation by extensional tectonism alone generates large surface uplifts in the surrounding plateau and enormous stresses in the lithosphere, in conflict with the observations. The only mechanism that is compatible with both the observed topography and the limitations imposed by the finite strength of the lithosphere is trough formation through displacement along steeply dipping to subvertical border faults, together with vertical accommodation at the base of the subsiding fault block. Fault dips are constrained to be greater than or equal to 85, leading to a ratio of vertical subsidence to horizontal extension exceeding 5.7 and a maximum extension of less than 4.6 km. These results confirm earlier studies suggesting that trough formation involved a substantial component of vertical subsidence or collapse, with only modest amounts of total extension.