Andean Basin Evolution Associated with Hybrid Thick- and Thin-Skinned Deformation in the Malargüe Fold-Thrust Belt, Western Argentina

Abstract:

Andean deformation and basin evolution in the Malargüe fold-thrust belt of western Argentina (34–36°S) has been dominated by basement faults influenced by pre-existing Mesozoic rift structures of the hydrocarbon-rich Neuquen basin. However, the basement structures diverge from classic inversion structures, and the associated retroarc basin system shows a complex Mesozoic-Cenozoic history of mixed extension and contraction, along with an enigmatic early Cenozoic stratigraphic hiatus. New results from balanced structural cross sections (supported by industry seismic, well data, and surface maps), U-Pb geochronology, and foreland deposystem analyses provide improved resolution to examine the duration and kinematic evolution of Andean mixed-mode deformation. The basement structures form large anticlines with steep forelimbs and up to >5 km of structural relief. Once the propagating tips of the deeper basement faults reached cover strata, they fed slip to shallow thrust systems that were transported in piggyback fashion by newly formed basement structures, producing complex structural relationships. Detrital zircon U-Pb ages for the 5–7 km-thick basin fill succession reveal shifts in sedimentation pathways and accumulation rates consistent with (1) local basement sources during Early–Middle Jurassic back-arc extension, (2) variable cratonic and magmatic arc sources during Late Jurassic–Cretaceous postrift thermal subsidence, and (3) Andean arc and thrust-belt sources during irregular Late Cretaceous–Cenozoic shortening. Although pulses of flexural subsidence can be attributed to periods of fault reactivation (inversion) and geometrically linked thin-skinned thrusting, fully developed foreland basin conditions were only achieved in Late Cretaceous and Neogene time. Separating these two contractional episodes is an Eocene–lower Miocene (roughly 40–20 Ma) depositional hiatus within the Cenozoic succession, potentially signifying forebulge passage or neutral to extensional conditions during a transient retreating-slab configuration along the southwestern margin of South America.