LATE MIOCENE EXHUMATION OF THE MAGALLANES BASIN AND SUB-ANDEAN FOLD BELT, SOUTHERN CHILE: NEW CONSTRAINTS FROM APATITE U-TH/HE THERMOCHRONOLOGY

The thermal history of the Magallanes retro-arc foreland basin and the adjacent fold-thrust belt of southern Chile (51°S) offers significant insight into basin evolution, crustal deformation, and the exhumational processes responsible for forming the modern southern Andes. The Magallanes basin consists of Upper Jurassic through Lower Tertiary sedimentary deposits that record the transition from an early extensional phase of basin evolution to the subsequent contractile phase during the Andean orogeny. Crustal shortening began in the late Cretaceous and as it continued into the Neogene (?), the foreland basin deposits were progressively deformed and incorporated into the fold belt. In light of new data regarding the details of orogenesis at this latitude and elsewhere in the Andes, the complex interplay between tectonic and climatic processes in the fold belt, particularly in its late Cenozoic history, merit further inquiry.

Recent advances in both the methodology and application of low temperature thermochronology offer the capability to asses the complete exhumational history of the sub-Andean fold belt. Thermochronologic samples were collected from Upper Cretaceous to Lower Tertiary (?) Magallanes basin deposits in order to evaluate the detailed timing and magnitude of thrust-related unroofing and erosional exhumation in this sector of the Andes. Apatite (U-Th)/He data from 16 samples across the Rio Nutria and related thrust faults and folds in the Parque National Torres del Paine indicate unroofing along the eastern edge of the southern Patagonian fold belt ca. 11-4 Ma. These data record regional exhumation of the fold belt from > 5 km depth to within 1-3 km of the Earth's surface. Late Miocene exhumation may be attributed to late-stage thrust faulting, erosional denudation, and/or Miocene magmatic reheating. Many samples cluster ca. 6-5 Ma and are concurrent with the documented age of ice sheet formation in southern Patagonia. Additional thermal constraints provided by fission track and zircon (U-Th)/He dating, combined with improved structural control, are necessary to estimate exhumation rates and resolve evidence of post-Miocene thrust-faulting from the effects of large-scale erosional exhumation.