SHORTENING UNDER EXTENDED CIRCUMSTANCES: REGIONAL STRUCTURE OF THE CRETACEOUS-NEOGENE PATAGONIAN FOLD-THRUST BELT AND MAGALLANES BASIN, CHILE AND ARGENTINA (51°30' S)

We present an ~ 170 km wide regional balanced cross section of the Patagonian fold-thrust belt and Magallanes foreland basin that integrates new geologic mapping and industry subsurface data to assess the kinematic history of crustal shortening across this segment of the Andes (51°30’ S). Additional age constraints from zircon U-Pb geochronology characterize multiple stages of deformation, syntectonic sedimentation, and basin paleogeography. Most published estimates of crustal shortening in the Patagonian Andes are constructed from north of 50° S, however, south of 50° S, the Patagonian thrust belt developed on oceanic and attenuated crust of the predecessor Rocas Verdes backarc basin. Inherited lithospheric differences, including reduced flexural rigidity and dense thrust blocks with oceanic affinity, had a pronounced effect on subsequent thrust belt and foreland basin evolution. Our palinspastic reconstructions of the thrust belt (to the Sarmiento Ophiolite backstop) indicate ~ 35 km of retroarc shortening (20%). Based on cross cutting relationships and stratigraphic evolution of the synorogenic basin fill we partition this strain into at least 4 stages of deformation. Stage I (< 88-80 Ma): crustal shortening of the Jurassic Tobífera and Lower Cretaceous Zapata Formations was likely accommodated across a narrow belt of stacked imbricate thrust sheets and faults that soled-out in metamorphic Paleozoic basement and the Jurassic Sarmiento Ophiolite (> 20 km shortening). Stage II (ca. 70?-28 Ma): The Upper Cretaceous foreland basin fill was uplifted and deformed during thin-skinned shortening, producing broad folds above ramp-flat transitions of subsurface thrust faults. This stage of deformation resulted in ~ 17 km of shortening within the Cretaceous foreland basin. Stage III deformation (ca. 22-20 Ma) is characterized by substantial uplift due to basement-seated thrust faults, imaged by 2D seismic-reflection data, that sole out at ~ 15 km depth and involve Jurassic and Paleozoic basement rocks. Along the eastern frontal monocline, tilting of the Cretaceous-Miocene basin fill is partly attributed to uplift of the Cretaceous depocenter along high-angle thrust faults (Stage IV: < 18-17 Ma), in conjunction with regional uplift of Patagonia associated with middle Miocene subduction of the Chile Ridge.