Abstract. 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.

The chemical composition of impact melt rocks, breccias and ejecta layers is dominated mainly by the proportions and composition of target rocks. However, small quantities of vaporized and molten meteorite material mixed with them significantly alter the concentrations and ratios of certain elements and isotopes. The identification of this meteoritic signature is used to propose an impact formation for structures of uncertain origin, as well as a possible criterion for infemng the impactor type. The most common criteria applied to these studies are the detection of a positive siderophile element anomaly, the Re-Os isotopic

The only economic deposit of garnets in Spain is located in the Almería province (SE Spain), forming part of the Neogene Volcanic Complex of El Hoyazo. Due to the high quantity and diversity of the almandine garnets, this area is unique. Garnet occurs as isolated euhedral crystals in the volcanic rock, as the principal component of metapelitic xenoliths in dacite, and as a terrigenous deposit formed by the erosion of the volcanic dome. In this work, we provide new mineralogical and geochemical data, which should be taken into account during the processing and recovery of the garnet placer.