FLEXURAL SUBSIDENCE AND HEAT FLOW IN THE MIOCENE VINCHINA BASIN, SIERRAS PAMPEANAS, ARGENTINA: A CASE STUDY FROM ABOVE THE CENTRAL ANDEAN FLAT-SLAB SUBDUCTION ZONE

The Mio-Pliocene Vinchina Basin in the La Rioja Province of Argentina straddles the boundary between the thin-skinned thrust belt of the Precordillera to the west and the basement-cored uplifts of the Sierras Pampeanas to the east. This basin, characterized by nearly 10 kilometers of sedimentary strata deposited in less than 12 My, has been tied to both dynamic subsidence associated with flat slab subduction as well as flexure, however the timing and mechanisms of basin formation remain controversial. Basin modeling presented in this study shows that flexure of weak crust along the Ordovician suture zone between the Famatinian and Precordilleran terranes can account for the anomalously thick sedimentary strata in the Vinchina basin. New U-Pb geochronology presented in this study shows that sedimentation rates may have been as high as 5.53mm/yr at ~ 9 Ma. New apatite fission track (AFT) and apatite (U-Th/He) (AHe) thermochronology from the Vinchina basin suggest that most of the sedimentary rocks were thermally reset during basin burial corresponding to a geothermal gradient between 19 – 24 ̊C/km. This thermal history contrasts with previous studies advocating for an anomalously low geothermal gradient (~ 12 ̊ C/km) during the mid-late Miocene coincident with flat slab subduction. Both AFT and AHe data record a pulse of uplift induced exhumation by 4 Ma as the Precordilleran fold and thrust belt cannibalized basin strata. Our data show that the timing and rate of subsidence in the Vinchina basin correspond with those of other foreland basin deposits along the Central Andes. This may suggest that Miocene sedimentary basin formation in the Sierras Pampeanas region above the central Andean flat slab is related to flexural loading related to the advancement of the fold and thrust belt, a process active along several thousand kilometers of the Andean Cordilleran system at this time, and is not necessarily tied to processes related to flat slab subduction in this region.