Kinematics of a Hybrid Thick-thin-skinned Fold and Thrust Belt Recorded in Neogene Syntectonic Wedge-top Basins, Southern Central Andes Between 35° and 36°S, Malargüe, Argentina

Abstract

In this chapter, we study the geometry and evolution of the Andean fold and thrust belt located between 35 and 36S. The main goal of this chapter is twofold; the first is to propose a new Neogene chronostratigraphic scheme based on outcrop studies and a new 39Ar/40Ar and K-Ar dating of volcanic intervals. The second is to integrate the structural kinematics deduced from the age and internal architecture of Neogene strata with two balanced cross sections to define the main pulses of fold-belt activity during the Miocene and Pliocene. Fold-belt kinematics is interpreted applying the Coulomb critical wedge model, and a succession of at least two cycles of activity is proposed. The hybrid thick-thin-skinned structural style of the fold belt is characterized by deep basement structures transferring shortening to the sedimentary cover, which is deformed into forethrust, back thrusts, and duplexes complicated by local salt tectonics. A net shortening of 30 km (19 mi) was calculated via restoration of two balanced cross sections. Clastic, pyroclastic, and volcanic synorogenic deposits form two megasequences called the Pincheira-Ventana of Miocene age (16–7 Ma) and the Malargue of Pliocene age (5–1 Ma) composed of four tectonosequences, S1, S2, S3, and S4, respectively. Active structures acted as barriers bounding local independent synorogenic depocenters as is shown by palinspastic and paleogeographic reconstructions. Two main pulses of deformation and a general foreland migration of deformation are evidenced by the spatial and temporal arrangement of synorogenic depocenters. Fold-belt kinematics is interpreted as a succession of two cycles of activity applying the Coulomb critical wedge model. Each cycle included a supercritical stage with out-of-sequence reactivation of basement structures and foreland migration of deformation followed by a subcritical stage with relative low rates of structural uplift and consequent high relative depositional rates in synorogenic basins. The results of this work agree with a progressive north to south deactivation of the Neogene deformation front between 33 and 37 being still active to the north but fossilized during the Pliocene and Miocene in the central and southern parts, respectively.