STRUCTURAL CONFIGURATION OF THE MESOZOIC ISCHIGUALASTO BASIN, NORTHWESTERN ARGENTINA

The Ischigualasto basin is one of a series of northwest-trending Mesozoic continental back-arc extensional basins located in central and western Argentina. The basin contains >3 km of Triassic and Jurassic strata. The Ischigualasto basin has been previously interpreted as half-graben with the main basin-bounding Valle Fértil fault situated along the NW margin of the basin, and the synthetic intrabasinal Alto fault trending obliquely to the north. Although Cenozoic-Recent basement-involved uplift of the Andean foreland has deformed original Mesozoic extensional structures in the region, outcrop and subsurface data can be combined to produce a generalized cross section of the basin outlining its original rift-related structural configuration. Data used in the construction of the cross section are derived from published seismic-reflection studies from the Bermejo Basin and Campo del Talampaya, as well as outcrop data from the Ischigualasto Provincial Park and Talampaya National Park. The new reconstruction of the rift-related structural configuration of the basin indicate the basin had the geometry of a broad graben formed between the east dipping Valle Fértil fault zone and the west dipping Alto Fault. In addition, the displacement on individual faults within the basement are interpreted to gradually loose displacement towards the basin margins, resulting in the lack of a prominent basin-bounding fault with significant footwall topography. While this geometry is different from the previous half-graben structural interpretation, it helps explain several observed stratigraphic relationships within the basin. First, the new interpretation helps explain why the thickest Triassic strata in the basin is beneath Campo de Talampaya, and why those units thicken towards the interpreted trace of the Alto fault before thinning dramatically in outcrops near Los Chañares. In addition, the interpretation of decreasing displacement along normal faults towards the basin margins explains why no basin-margin facies (i.e. alluvial fan deposits) have been identified in Middle-Upper Triassic rocks outcropping near the trace of the Valle Fértil and Alto faults.