STRUCTURAL EVIDENCE FOR >150-KM LEFT-SLIP MOTION ALONG THE VALLES MARINERS FAULT ZONE ON MARS: IMPLICATIONS FOR THE DYNAMIC DEVELOPMENT OF THE THARSIS RISE

Radial extensional is postulated to be the dominant expression of Tharsis development on Mars and the Valles Marineris (VM) trough system is often cited as the most prominent manifestation of this process. The origin of the VM trough system has also been attributed to sub-surface dissolution and strike-slip faulting. We tested these models by investigating the Ius-Melas-Coprates (IMC) fault using THEMIS, CTX and HiRISE data. The fault trace is mostly expressed by linear and nearly vertical scarps. We examined the fault kinematics by mapping structures on and near the main fault. On the plateau between Coprates and eastern Candor Chamsta, left-slip faults parallel and adjacent to the IMC fault offset N-trending ridges with a total offset of ~ 4.5 km. Near this site, a drainage system is offset left laterally for ~13 km. NW-trending folds also occur on the plateau, which we interpret as en echelon folds produced by left-slip shear. On the valley floor of the IMC trough, NW-trending en echelon folds occurred in Amazonian valley fills. The folds are ~ 4 km long and ~ 2 km wide, forming topographic highs above the flat valley floor. We also observed 4-km left-slip offset of a debris-flow apron in eastern Coprates Chasma. In Ius Chasma to the west, NE-trending en echelon normal faults link with east-trending left-slip faults, forming several extensional left-steps. Because the above observed offsets are derived from subsidiary structures or offsets of syn-faulting sediments, the slip estimates are only minimum values. We determined the total offset of the IMC fault zone by evaluating three mutually consistent offset markers: (1) a possible impact basin at the present-day Melas Chasma, (2) the Thaumasia-Coprates Rise thrust front, and (3) a poorly preserved crater basin in the easternmost Coprates Chasma. This method yields a left slip of 170 ± 20 km and < 45 km extension. We suggest the IMC fault zone to be a left-slip transtensional structure linking NE-trending normal faults at its western and eastern ends.