Large-scale gravity sliding in the Miocene Shadow Valley Supradetachment Basin, Eastern Mojave Desert, California

The basal contacts of Shadow Valley slide sheets are characteristically knife sharp, show few signs of lithologic mixing of upper- and lower-plate rocks, and locally exhibit slickensided and striated, planar fault-like bases. Pronounced folding of overridden Miocene lacustrine and fan deposits beneath the Eastern Star allochthon extends to depths up to 40 m at widely scattered localities. We conclude that this slow moving slide sheet encountered isolated topographic asperities (hills) and that stress transfer across the basal slide surface produced folding of footwall strata. Synkinematic gypsum veins in footwall playa sediments, with fibers up to 12 cm long, have trends and shear senses compatible with the direction and sense of displacement of the overriding crystalline allochthon. The undisturbed veins, which closely parallel the base of the slide sheet, attest to high fluid presence and pressure in the playa sediments-factors facilitating allochthon movement across them. The long length of the fibers, indicative of a protracted dilational process, is incompatible with a catastrophic rate of emplacement. We believe that the only explanation for slow displacement of this allochthon and other gravity driven slide sheets across the landscape is that they formed as slumps on high, steep bedrock slopes and that their elevated heads drove their toes across lower fan and playa deposits. Initial detachments from bedrock sources were facilitated by pre-existing structural and stratigraphic anisotropies. Detachment of the Eastern Star allochthon from the bedrock of Shadow Mountain likely occurred by inversion along the playa-ward dip of a preexisting Mesozoic thrust fault within Proterozoic rock units.