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Abstract

A photogeoIogical reconnaissance ofViking mosaics and images ofthe Tharsis dome has been carried out. Fifteen new areas of transcurrent faulting have been located which, together with other struétures previously detected, support a modeI in which the Thaumasia PIateau, the southeastem part of the Tharsis dome, is proposed to be an independent lithospheric block that experienced buckling and thrust faulting in Late N oachian or Early Hesperian times as a result of an E-W directed compression. Evidence is presented that this stress fieId, rather than the Tharsis uplift, was decisive in the inception of Valles Marineris, which we consider a transtensive, dextral accident. The buckling spacing permits us, rnoreover, to tentatively reconstruct a Martian Hesperian lithosphere similar in eIastic thickness to the mean present terrestrial oceanic lithosphere, thus sí.lpporting the possibility of a restricted lithospheric mobility in that periodo Tharsis lithosphere was again subjected to shear stresses in Amazonian times, a period in which important accidents,such as strike-slip faults, wrinkle ridges, and straight and sigmoidal graben, were formed under a thin-skin tectonic regime, while the lithosphere as a: mechanical unit had become too thick and sttong to buckle. The possibIe causes of those stresses, and especially their relationships to a putative period of pIate tectonics, are discussed.