The Opening of the Valles Marineris Canyons on Mars: Stress Focusing along the Buried Dichotomy Boundary beneath Tharsis

Introduction: Valles Marineris is the largest tectonic canyon in the solar system, with a length of ~2500 km, depths of up to 10 km, and widths in excess of 100 km. The location of this canyon on Tharsis, and its orientation radial to the center of the rise suggests Tharsis control of Valles Marineris tectonism. Tharsis loading dominated the tectonic history of Mars, accounting for the radial graben, circumferential wrinkle ridges, and strike-slip faults around the rise [1-4]. However, the reason for the growth of this canyon to its extreme size is unknown. Valles Marineris is an order of magnitude larger than all other Tharsis-radial graben. Valles Marineris also represents a unique style of tectonism, without parallel in the solar system. Extensional tectonic environments generally develop into either narrow rifts (such as the East African rift on Earth, or the Thaumasia graben on Mars) or wide basin and range provinces (such as the southwestern US), depending on the strain rate and heat flux [5]. In contrast, Valles Marineris is a simple fault-bounded graben accomodating in excess of 10 km of extension. A clue to the mechanism responsible for Valles Marineris tectonism comes from its location south of and roughly parallel to the sub-Tharsis dichotomy boundary [6], suggesting that the buried boundary may be controlling the tectonism on this part of the rise. Prior to Tharsis formation, the sub-Tharsis dichotomy boundary marked a narrow transition between thick highlands and thin lowlands crust, similar to that seen to the west of the rise [6]. The emplacement of the Tharsis lavas over a pre-existing step function in topography and crustal thickness would have resulted in an abrupt change in the thickness of the load and the flexural response of the lithosphere along the boundary, leading to narrowly focused bending stresses. Modeling: Studies of loading and tectonism on Mars commonly employ inversions of the gravity and topography to solve for the membrane and flexural stresses in the lithosphere [2, 7]. However, because of its immense size, stresses predicted in the vicinity of the canyon based on the present day gravity and toporaphy are greatly affected by the canyon itself. In order to understand the stresses responsible for the initiation of tectonism and the formation of the canyon, we must rather use forward modeling techniques. Forward modeling requires a representation of the state of both the pre-Tharsis crust and the Tharsis load. Andrews-Hanna et al. [6] inverted the gravity and topography of Mars for the isostatic crustal roots and revealed a distinct transition in the root thickness beneath Tharsis that was interpreted as the sub-Tharsis −40 −20 0 20