Seismically induced slump on an extremely gentle slope (<1°) of the Pleistocene Tecopa paleolake (California)

A superbly exposed similar to 5 km(2) slump is developed in middle Pleistocene lacustrine deposits of the Tecopa Basin (California, United States). A subhorizontal detachment (<1 degrees) with a single sheet similar to 8 m thick is displaced eastward nearly 100 m. The different parts of the slump are continuously exposed for more than 1 km, with extensional structures in the head zone and contractional features in the intermediate and toe zones. Meter-scale normal faults and thrusts coexist with meter- to decameter-scale open folds, all of them detached, on a clay bed rich in sepiolite. These well-exposed deformation structures were produced by liquidization of the clay bed, due to the thixotropic behavior of the sepiolite. Processes such as oversteepening or sedimentary overloading are discarded in light of sedimentological and paleoenvironmental analyses. Thixotropic behavior of sepiolites might have been caused by seismic shaking related to regional faults that were capable of producing large earthquakes during the middle Pleistocene (between 579 and 200 ka).