The Structure of the Southern Central Andes (Chos Malal Fold and Thrust Belt)

Abstract

The Chos Malal fold and thrust belt, formed during the Andean orogeny, is characterized by the involvement of both the Paleozoic basement and Mesozoic strata of the Neuquén Basin into the deformation. Two detailed structural cross sections, built based on previous field mapping, new subsurface interpretations, and seismic and borehole data, allow characterizing the structural style of this orogenic belt. A close interaction between large thick-skinned structures (first order) and complex thin-skinned structures (second, third, and fourth order), related to the presence of multiple detachments in the sedimentary cover, is recognized. The largest thrusts form basement-involved duplex structures, with a lower detachment located at a depth of about 12–14 km and an upper detachment in the Jurassic evaporites of the Auquilco Formation. Displacement transmitted by these basement sheets in the inner zone of the Chos Malal fold and thrust belt produces a wide region of thin-skinned deformation, which contains second-order fault-bend folds that transfer deformation to the overlying Agrio Formation shales (Early Cretaceous) giving rise to third-order folds and thrusts involving this unit. In the outer zone, the basement-involved thrusts have less displacement and form monoclines and a complex thin-skinned deformation restricted to the deformation front, possibly caused by buttressing effect exerted by the overlying Miocene volcanic sequences. This impediment in forward deformation leads to an important out-of-sequence faulting, whose displacement is compensated by a passive-roof backthrust along the Cretaceous evaporites of the Bajada del Agrio Group forming a triangle zone. Second-order anticlines under this triangle zone, where the seismic data are of low quality, constitute important hydrocarbon oil fields such as El Porton and Filo Morado. Understanding the close relationship between the structures of different order cropping out in the inner zone of the Chos Malal fold and thrust belt is important to interpret the subsurface structures forming hydrocarbon oil fields in the outer zone as well as to identify other complex structures that may lead to new exploration opportunities. Restitution of the structural cross sections allowed calculating a tectonic shortening for this region in the order of 22–25 km (16–18%), higher than estimated by previous authors who generally simplified the thin-skinned deformation and considered the tectonic inversion of normal faults as the main mechanism of deformation in this orogen.