KINEMATIC RECONSTRUCTION OF A TRIASSIC SYNDEPOSITIONAL DETACHMENT FAULT, ISCHIGUALASTO PROVINCIAL PARK, NORTHWESTERN ARGENTINA

The Ischigualasto Basin is a continental back-arc rift basin in which contain > 4 km of non-marine strata. In the east-central part of the basin, the Middle Triassic Los Rastros Formation (~230-233 Ma) contains a small area (~2 km²) where rocks exhibit both extensional and contractional deformation features. To the east, both high angle and listric normal faults are the dominant structures. To the west, deformation consists of reverse and thrust faults in addition to upright and overturned folds. Faults in both areas sole into a sub-horizontal detachment surface below which rocks are undeformed. The deformed rocks are overlain by the La Peña Member of the Upper Triassic Ischigualasto Formation. The detachment, roughly 10 km from the main bounding fault, formed at shallow depths (< 30 m from surface) in relatively shallow water environments. Paired extensional-contractional detachment systems that have been described in literature are most commonly associated with depositional loading and/or gravity gliding on low angle basin-ward dipping horizons in deltaic, continental shelf, and slope-rise strata along continental margins, and are 1-2 orders of magnitude larger than those found in the Los Rastros Fm. Although smaller, it can be assumed that the driving forces acting on this detachment were similar to those that act on larger detachment systems. The principal process involved in the development of faults is considered to be differential compaction resulting from depositional loading above a weak basin-dipping horizon at depth, usually evaporites or over-pressured shales). Given that there are no evaporites in the Los Rastros Fm., depositional loading or slip along an over-pressured basin-dipping horizon are the most likely causes of the detachment. What is more uncertain is the origin of the depositional load. The load that generated the gravitational potential to drive the detachment may have been created by Los Rastros Fm. deltaic sedimentation, or by fluvial-channel deposition in the overlying La Peña Mbr. In addition, low-angle regional fault-related tilting of Ischigualasto Basin strata may have also contributed to the development of the detachment.