Structure and Kinematics of a Syndepositional Paired Detachment System, Triassic Ischigualasto Basin, NW Argentina

Brian S. Currie¹ and Tony Albrecht^2
1Department of Geology, Miami University, Oxford, OH
²North America Gas Reserves and Renewal, BP America, Inc., Houston, TX

In the Ischigualasto basin of NW Argentina, lacustrine/fluvial deposits exhibit both extensional and contractional deformation features. Extensional structures consist of north-striking listric normal faults that are concentrated in an ~100 meter-wide zone at the eastern edge of the deformed area. 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 a west-vergent thrust system that contains two primary imbricates and carries upright to overturned folds with fault bend, fault propagation, and detachment geometries. Faults in both eastern and western areas have similar magnitudes of displacement (~80 m) and sole into a sub-horizontal detachment surface, below which deposits are undeformed. All deformed strata are overlain by undeformed fluvial deposits. Observed 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) in relatively shallow water (<15 m) environments. Although 2-3 orders of magnitude smaller, the overall geometry of the Ischigualasto detachment system is comparable to passive margin detachment systems. Based on these similarities, the deformation features, internal geometry, and reconstructed kinematics of the Ischigualasto detachment system may serve as a small-scale analogue for passive margin paired extensional-contractional systems.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas