STRATIGRAPHIC RESPONSE TO NEOGENE FLAT-SLAB SUBDUCTION, ARGENTINA

The flat-slab region of central Chile and Argentina (27-33°) is the type example of modern shallow subduction, and exhibits a contractional orogenic system defined by basement-involved foreland uplifts (Sierras Pampeanas), active seismicity, and cessation of arc magmatism. The influence of flat-slab subduction on upper plate deformation, magmatism and basin evolution is a fundamental component of convergent margins, yet the feedbacks among these processes remain enigmatic. Here we apply stratigraphic, geochronological, and isotopic constraints from the retro-arc foreland basin of Argentina to reveal changes in Neogene magmatism, depositional systems, sediment provenance, and accumulation rates as the region was structurally partitioned before and during slab flattening. Detrital zircon U-Pb age signatures and Hf isotopes from western (Calingasta hinterland basin), central (Talacasto and Bermejo wedge-top and foreland basins), and eastern (Pampean broken foreland basin) segments of the foreland system display variations in syndepositional magmatism, unroofing of multiple tectonic provinces, and reorganization of river drainage networks. Initial shortening in the Principal Cordillera at 25-20 Ma is represented in the Talacasto basin by accumulation of distal eolian facies with Oligo-Eocene detrital zircons from the Andean arc. The Calingasta basin records volcanism and hinterland basement involved shortening of the Frontal Cordillera at ~18-10 Ma, as marked by an upward coarsening fluvial to alluvial-fan succession. Thin-skinned shortening of the Precordillera at 12-5 Ma exhumed Calingasta and Talacasto basin fill and induced eastward advance of flexural subsidence in the Bermejo foreland basin. Within the foreland, basement-involved Sierra Pampeanas uplifts and eastern Precordillera backthrust systems partitioned the retro-arc basin system, contributing locally derived sediments. A progressive eastward advance of exhumation in the Principal Cordillera (25-20 Ma), Frontal Cordillera (18-10 Ma), Precordillera (12-5 Ma), and Sierras Pampeanas (5-0 Ma) was perturbed at 12-10 Ma by the inception of flat-slab subduction and associated changes in depocenter location, shifts in provenance, increased crustal contribution to magmatism, and eastward inboard sweep of volcanism.