Subduction and Denudation Dynamics Along the Andean Margin of Chile, Assessed by Low- Temperature Thermochronology.
The aim of this study is to reconstruct the evolution of the western Andean forearc between latitudes 20¡ã and 40¡ãS, during the early/middle Cenozoic. Little information is available for this time period, yet it is essential to reconstruct the thermal history of this period to have a full understanding of the early stages of the Andean orogeny. The dating techniques utilized are apatite (U-Th)/He, apatite fission-track (AFT) and biotite K-Ar. These techniques allow quantification of rock cooling and exhumation patterns in the forearc related to arc magmatism, changes in plate tectonics and/or erosion (burial). (U-Th)/He thermochronology is the most widely used here, since its closure temperature, between 40¡ãand 70¡ãC, is much lower than any used previously in regional Andean studies. Samples dated come from Mesozoic magmatic rocks outcropping in the Coastal Cordillera and Precordillera. In the Coastal Cordillera of northern Chile, biotite K-Ar ages (152¡À1 Ma), AFT ages (118-145 Ma) and (U-Th)/He (63-76 Ma) show a regional post-emplacement cooling and rapid exhumation pattern during the inland migration of the arc from late Jurassic to early Cretaceous times. A ¡Ý1-km vertical (U-Th)/He age vs. elevation trend, in the coastal cliff, reveals for the first time a regional younger phase of significant cooling at 45¡À5Ma Ma. This cooling phase is coeval with important changes in plate convergence rates, slab dip and plate coupling between the Nazca (Farallon) and South America plates. Based on our results, in this study we investigate how these changes might have effected the overriding plate during the onset of the first phase of significant uplift (Incaic phase) in Eocene times. In Central and Southern Chile, (U-Th)/He results for the Coastal Cordillera show larger differences in cooling ages. In Central Chile, ages range from 10 to 123 Ma. This geographical age distribution suggests a very different rock cooling and exhumation patterns along an area influenced by the present day flat slab and semi-arid climate (25¡ã-35¡ãS). Inland, results yield cooling ages in the Precordillera between 14 and 37 Ma. These ages reveal river incision rock cooling of between 50 and 70 m/myr during uplift. In southern Chile, with a humid temperate climate, we found differences in ages across the Coastal Cordillera. One west-east subvertical profile, at 37¡ãS, yields synorogenic ages (30-47 Ma) close to the coastline, while much older ages (88-107 Ma) a few tenths of kilometers inland. These results suggest much larger amounts of exhumation (2-3 km) in the coastal regions since the onset of uplift.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 3045 Seafloor morphology;
- and geophysics;
- 7240 Subduction zones (1207;
- 8170 Subduction zone processes (1031;
- 8177 Tectonics and climatic interactions