A photogeological reconnaissance of Viking mosaics and images of the Tharsis dome has been carried out. Fifteen new areas of transcurrent faulting have been located which, together with other structures previously detected, support a model in which the Thaumasia Plateau, the southeastern part of the Tharsis dome, is proposed to be an independent lithospheric block that experienced buckling and thrust faulting in Late Noachian or Early Hesperian times as a result of an E-W directed compression. Evidence is presented that this stress field, 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, moreover, to tentatively reconstruct a Martian Hesperian lithosphere similar in elastic thickness to the mean present terrestrial oceanic lithosphere, thus supporting the possibility of a restricted lithospheric mobility in that period. 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 strong to buckle. The possible causes of those stresses, and especially their relationships to a putative period of plate tectonics, are discussed. ¿ 2001 American Geophysical Union