337-5: TRACKING ANDEAN ARC MAGMATISM AND CRUSTAL SHORTENING DURING A SHIFT TO FLAT-SLAB SUBDUCTION USING DETRITAL ZIRCON GEOCHRONOLOGY FROM THE RETROARC FORELAND BASIN SYSTEM OF WESTERN ARGENTINA (30.5°S)

MACKENZIE, N.; CAPALDI, T.; ALVARADO, P.; HORTON, B.

Seattle

Conferencia; Geological Society of America Anual Meeting; 2017

GSA

The effect of flat-slab subduction on magmatism and contractional orogenesis is a fundamental component of convergent-margin tectonics, yet the feedbacks among subduction angle, volcanism, and surface processes remain enigmatic. New geochronological and sedimentological constraints on Cenozoic magmatism and upper crustal exhumation in the southern central Andes of western Argentina (30.5°S) reveal Neogene changes in volcanism, foreland/hinterland depositional environments, sediment provenance, and accumulation rates as the retroarc region was structurally partitioned by various Andean ranges before and during slab flattening. Detrital zircon U-Pb age signatures from western (Calingasta hinterland basin), central (Talacasto wedge-top basin), and eastern (Bermejo foreland basin) retroarc segments of western Argentina record syndepositional volcanism and unroofing of multiple tectonic provinces. 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 Andean arc detrital zircons. The Calingasta basin records volcanism and basement involved shortening of the Frontal Cordillera at ~18-10 Ma, as marked by an upward coarsening fluvial to alluvial-fan succession with a prolonged Permian-Triassic zircon age component. Thin-skinned shortening of the Precordillera at 12-5 Ma exhumed Calingasta and Talacasto basin fill and induced advance of flexural subsidence eastward to the Bermejo foreland basin. Progressive unroofing from the Principal Cordillera (25-20 Ma) to Frontal Cordillera (18-10 Ma) to Precordillera (12-5 Ma) was perturbed at 12-10 Ma with drastic changes in depocenter location, provenance trends, and eastward inboard sweep in volcanism, which coincided with the inception of flat-slab subduction at these latitudes. Patterns of Andean erosion and sedimentation record the initiation of shallow subduction and increased plate coupling, with eastward migration of magmatism, shortening, and foreland basin subsidence.