Natalia Sánchez

Tectonic Evolution of the Chos Malal Fold‐and‐Thrust Belt (Neuquén Basin, Argentina) From (U‐Th)/He and Fission Track Thermochronometry

In the southern central Andes at 37–38°S latitude, the Chos Malal fold-and-thrust belt (FTB), which results from the Late Cretaceous closure of the Neuquén Basin, has generated increasing interest because of its potential for hydrocarbon exploration. Using detailed field mapping, seismic reflection, and well data analysis, we have produced balanced cross sections, which combined with apatite and zircon (U-Th)/He, and fission track thermochronology from samples distributed along the FTB, bring new constraints on the chronology of the structural development of the Chos Malal FTB. Fully reset samples obtained from the Early Jurassic rocks at the bottom of the sedimentary sequence exposed in the Cordillera del Viento, a major basement-involved hinterland structure, permit to quantify its cooling rate from 5.4 ± 4.1 and 3.8 ± 3.2 °C/Ma between 70 and 55 Ma down to between 2.0 ± 1.3 and 1.3 ± 0.9 °C/Ma after 55 Ma until the present. Detrital apatite fission track ages from Late Jurassic and Early Cretaceous sandstones reveal that tectonically driven exhumation through basement-involved thrusting has occurred at ~15–7 Ma in both the inner and outer sectors of the FTB. Finally, the cooling and exhumation of the Las Yeseras-Pampa Tril basement-involved anticlines at the mountain front at ~9–7 Ma, slightly younger than previously assumed, suggests a normal sequence of faulting propagation. Our proposed thermostructural model of the Chos Malal FTB contributes to a better understanding of the tectonic evolution of this segment of the Andes.

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

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 Neuquen 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.