Exhumation of the Western Cordillera of the Peruvian Andes (5-12°S)
Abstract
The topography of mountain belts is the first-order morphological expression of the dynamical forces that drive plate tectonics. Despite recent advances in techniques that quantify tectonic and geomorphologic processes, characterizing the near and far-field forces that form mountainous topography at a continental scale remains challenging. The South American Andes are an example of a long-lived orogen that is morphologically and tectonically segmented. For example, the Northern Peruvian Andes are a region characterized by flat-slab subduction, no active volcanism and a relatively narrow section of high topography, in contrast to the central Andes, which contain the broad Altiplano-Puna plateau, active volcanism and normal-angle subduction. Current models of Andean orogenesis based on paleoelevation estimates, thermochronology data, and structural analysis suggest that most of the high topography evolved sometime in the last 20Ma, possibly characterized by a period of punctuated uplift from 10-6Ma. However, as many of these data are from the central Andes, they may not be directly applicable to the entire Andean chain. Therefore, it is critical to investigate areas outside of the central Andean region using similar techniques to test the applicability of these models elsewhere in the Andes. For this study, we use (U-Th)/He low-temperature thermochronology to characterize the shallow crustal cooling history from three sites in the Western Cordillera of the Peruvian Andes, across seven degrees of latitude (5°S to 12°S). Near-vertical sample transects were taken in three river valleys that expose the Jurassic through Eocene granitoids of the 1,600km long Coastal Batholith of Peru. By using both apatite and zircon (U-Th)/He thermochronometers we are able to quantify crustal exhumation rates as well as detect temporal and spatial accelerations and decelerations of exhumation. This new dataset, combined with existing crystallization ages of the plutonic sequence of the Coastal Batholith and previous apatite and zircon fission-track work provides a more complete understanding of the cooling history and uplift of the range. Zircon (U-Th)/He results show exhumation and/or cooling in the Oligocene through early Miocene (~35-18Ma), shortly following the last stages of plutonic emplacement. Preliminary apatite (U-Th)/He results indicate cooling through the shallow (~2km crustal depth) apatite closure isotherm in the late Miocene. Previous studies based on stratigraphical, structural and sedimentological relationships associate the Eocene 'Incaic' tectonic phase with major shortening in the Western Cordillera, and call for a period of tectonic quiescence during the Oligocene 'Aymara' phase. Some of our zircon (U-Th)/He ages are within this tectonically quiet 'Aymara' phase. At present, with this preliminary dataset, it is challenging to determine what this phase of cooling represents; river incision, adjustment of the thermal field, or surface uplift. In summary, we show i) a cooling phase of the Coastal Batholith in the Oligocene to late Miocene, shortly after the end of the plutonic emplacement period, ii) corroboration with previous AFT and ZFT results, and iii) emerging evidence for an additional phase of more shallow crustal cooling, through the apatite closure isotherm, in the late Miocene.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T21E2614M
- Keywords:
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- 1140 GEOCHRONOLOGY / Thermochronology;
- 8104 TECTONOPHYSICS / Continental margins: convergent;
- 8175 TECTONOPHYSICS / Tectonics and landscape evolution;
- 9360 GEOGRAPHIC LOCATION / South America