STRUCTURE AND KINEMATICS OF A SYNDEPOSITIONAL PAIRED-DETACHMENT SYSTEM, TRIASSIC ISCHIGUALASTO BASIN, NW ARGENTINA

In the Ischigualasto basin of NW Argentina, the Triassic Los Rastros and Ischigualasto formations contain a small area (~1 km²) where lacustrine/fluvial deposits exhibit both extensional and contractional deformation features. Extensional structures in the study area consist primarily of north-striking normal faults that are concentrated in an ~100 meter-wide zone at the eastern edge of the deformed area. The dominant structures in this area are three listric normal faults that primarily deform Los Rastros Fm. strata. Contractional structures are concentrated along an ~250 m wide zone at the western side of the deformed area. The primary structure in this area is an east-dipping thrust system. The thrust system is west-vergent, has a ramp-flat geometry, contains two primary imbricates and carries north-trending, upright to overturned folds with fault bend, fault propagation, and detachment geometries. A small thrust duplex also exists at the base of the primary thrust ramp. Faults in both eastern and western areas have similar magnitudes of displacement (~80 m) and sole into a sub-horizontal detachment surface, below which rocks are undeformed. Additionally, hanging-wall strata in both normal and thrust faults display evidence for sediment liquifaction/mobilization during the time of deformation. All deformed strata are overlain by undeformed fluvial deposits.

Observed extensional, contractional, and liquefaction structures are consistent with deformation associated with syndepositional gravity-induced displacement above an overpressured detachment horizon. The contractional frontal part of the system was driven by extension produced by gravity spreading/depositional loading of lacustrine delta platform and fluvial deposits, with lateral translation occurring above an overpressured shale décollement. Deformation occurred at shallow depths (<30 m). Gravitationally-driven, extensional-contractional fault systems, although unreported from fluvial-lacustrine depositional systems such as the Ischigualasto basin, are common features along passive margins. Although 2-3 orders of magnitude smaller, the overall geometry of the Ischigualasto detachment system is comparable to passive margin detachments systems.