Large-scale Structural Manifestations of Phyllosilicate Generation and Deposition in Early Martian History

Introduction: Recent detections of phyllosilicate (clay) minerals in ancient terrains [1-3] have revolutionized our understanding of early hydrothermal activity on Mars. The spectral signatures of phyllosili-cates are seen exclusively in Noachian-aged outcrops and not in Hesperian or Amazonian materials [1-3]. Phyllosilicates are inferred to have been widely em-placed during a period of time (termed the " Phyl-losian " in [2]), that overlaps with, but ends before the end of, the Noachian. Afterward, conditions (such as pH) changed such that phyllosilicate formation was inhibited; instead, materials such as sulfates were favored. The potential existence of extensive phyllosili-cate deposits low in the stratigraphic column has enormous implications for the structural evolution of materials emplaced higher in the column. Clay layers are often zones of low strength, enabling enhanced deformation and slip in terrestrial settings, such as fault zones, accretionary wedges, and basal decollements beneath volcanoes [4-8]. The low hydraulic diffusivity of clays also makes lateral transport of pore water difficult , thus facilitating the buildup of high pore pressures in a decollement [9-11]. Here we develop the idea that ancient phyllosilicate deposits, generated by vigorous hydrothermal activity early in Martian history [e.g., 12] and concentrated by sedimentary processes, may constitute decollement zones that facilitate movement and deformation of younger superposed structures, such as large volcanic edifices, wrinkle ridge provinces, and thrust belts. Flank Movements at Large Volcanic Edifices: The 23 km tall, 600 km wide Olympus Mons edifice exhibits several characteristics of volcanic spreading, including concave-upward lower flank topography, faulting (both extensional and compressional), and a large basal scarp, sectors of which constitute the head-scarps of giant landslides (the Olympus Mons aure-oles) [13-15]. These features have been attributed to motion along a basal decollement rooted in a thick basal clay layer [15]. The asymmetric distribution of topography and tectonics seen at the Olympus Mons edifice, scarp, and aureole described are best explained by a gradient in basal friction beneath the edifice, increasing toward the southwest (e.g, with proximity to the Tharsis rise) [15]. Such a gradient is consistent with a decrease in sediment thickness with increasing elevation, as would be expected if sediments are trans