POLYPHASE STRUCTURAL EVOLUTION RELATED TO A MAJOR DISCONTINUITY IN THE ANDEAN FORELAND: THE VALLE FÉRTIL FAULT SYSTEM

The Valle Fértil Fault System (VFFS) is a major NW discontinuity in the Andean foreland that controlled the continuous basement uplift of the Sierra de Valle Fértil, La Huerta and other ranges along 600 km in the Sierras Pampeanas structural province in the modern Pampean flat slab segment of the Andes. We studied in detail the Las Chacras Fault System (LCFS), a segment of the VFFS, where the polyphase structural evolution can be evaluated.

The trace of the VFFS and its subsidiary LCFS is coincident with a major NW crustal discontinuity representing the Early Paleozoic suture between the Cuyania terrane and the protomargin of Gondwana. This collisional event generated in the vicinity of LCFS a NW striking penetrative fabric dipping ~55° to the NE.

The second stage occurred during a Late Triassic - Early Jurassic extensional regime, when a number of rift basins formed on the hanging-wall of the Paleozoic sutures. Several depocenters controlled by the NW orientation of the basement fabric are located along the VFFS trend. In the study area Triassic sediments of the Marayes basin represent the rift infill related to the LCFS, where a NW trending, E dipping master fault was developed. Meter scale normal faults cutting Triassic sediments were measured showing the same NW orientation than the basement fabric.

The last evolution stage started at ~5 Ma with the uplift of the basement block of Sierra de la Huerta - Valle Fértil. Detailed structural analysis at LCFS and nearby basement fabric indicates a westward vergence displacement along NW trending fault zones. The most recent displacement of the LCFS is recorded in the offset of Quaternary deposits along the length of the fault zone. Kinematic indicators on fault planes show a reverse, west verging movement for the LCFS. Focal mechanisms located 15 km depth beneath the trace of LCFS indicate ongoing seismic activity related to NW orientated structures, coinciding with the orientation of faults observed at surface.

As a conclusion, the geometry and location of focal mechanisms, neotectonic features, Andean-age structures and normal fault systems that controlled the inception of the Triassic rifts along the VFFS were governed by the preexisting NW basement fabric that was presumably generated in Early Paleozoic times in response to the collision between Cuyania and Pampia Terranes.