GRAVITY-DRIVEN DEFORMATION IN DESICCATED LAKE SEDIMENTS OF RINC"N DE PARANGUEO, CENTRAL MÉXICO

The relations between gravity-driven deformation and near-surface lithology have been investigated at the bottom of a former maar lake. Near the shore, the sedimentary succession is formed by relatively competent mudstone deposited atop a sequence of pyroclastic and/or talus deposits. Closer to the depocenter occur plastic and low compressible organic-rich silt deposits with >60 vol% of water. Exposed lake sediments display an impressive array of structures which include normal faults, extension-related folds (rollovers and fault-propagation folds), dilation fractures (concentric and perpendicular to the shore), and, in certain places, folds and elongated chains of domes produced by mud injection. The most conspicuous structure in the crater is an annular, segmented, normal fault, which forms a steep, up to 15 m high scarp near the former shore line. Deformation styles and intensities vary along the ring fault system. Based on the occurrence of multiple, apparently rigid blocks separated by widely open fractures, on the western portion of the ring fault scarp, local gliding appears to be occurring on a planar detachment which sharply bends near its breakaway zone, whereas on the eastern part of the scarp movement appears to develop above: A) curved detachments that slope entirely downhill or B) curved detachments sloping uphill near the base of the topographic scarp. Significant differences in the toe structures may be related to far-field response to the extensional deformational domains: 1) on the western part of the crater all the blocks dip toward the depocenter and at the break of the topographic slope gentle folds and mud domes occur; 2) on the eastern portion, near the breakaway zone of the curved detachment, the lake sediments dip gently towards the listric fault and further away (closer to the depocenter) blocks dip in the opposite direction producing a rollover. In case (1), hydraulic overpressure caused by gravity-driven mass movement triggered mud injection producing a dome chain parallel to the scarp. Near the toes of both types of mass movements (1 & 2) megabreccias formed by randomly oriented and steeply inclined mud blocks may be found. We interpret the deformation as nearly rigid dry blocks of mudstone subsiding and translating on weak planes in a deformable substrate formed by plastic mud. Papiit IN109410