Internal Deformation of a Rapidly Exhuming Orogenic Wedge in a Paleozoic Microplate-Continent Collision, NW Argentina
Abstract
A significant ductile shear zone within the lower- to mid-crustal rocks of the Sierra de Pie de Palo tracks the internal deformation of a west-directed, lower-crustal exhumation wedge to shallow crustal levels, and indicates both changing structural style and progressive localization of strain with increasing exhumation. Preliminary results from the top-to-the-west, 80-m thick Bajo Pequeño Shear Zone (BPSZ) suggest that it provides a record of the timing, duration, and intensity of deformation during the latest stages of the early Paleozoic collision of the Precordillera microplate with Gondwana. Located in the Central Complex, which has been previously defined as a syn-convergent exhumation wedge, the BPSZ shows a transition from penetrative, simple-shear-dominated mylonites at the edges of the shear zone (defined by asymmetric σ-type porphyroclasts and S-C fabric) to pure-shear-influenced ultramylonitic deformation in the center of the shear zone (defined by symmetric θ- and φ-type porphyroclasts, and fractures aligned normal to the foliation in garnet and plagioclase porphyroclasts). Ongoing thermobarometric work is aimed at determining the P-T conditions of metamorphism and deformation associated with the BPSZ. Chemical zoning patterns in garnet indicate differing metamorphic histories between the metasedimentary footwall and orthogneiss-dominated hanging wall, marking the BPSZ as a significant lower-crustal boundary. Preliminary THERMOCALC results from syn-tectonic metamorphic assemblages outside of the shear zone in the footwall (10.8±0.9 kbar, 613±25 °C) and hanging wall (11.4±0.9 kbar, 664±35 °C), separated by only 200 m of section, suggest the onset of deformation in the lower crust during or after peak metamorphic conditions (~469 Ma). Biotite-garnet thermometry, including equilibrium biotite-garnet pairs from within the shear zone, also indicates an overall increase in temperature over the BPSZ, with a minor decrease in metamorphic temperature within the shear zone itself. Ensuing quartz c-axis thermometry will further elucidate the temperatures of deformation in the BPSZ and will better define relationship between metamorphism and deformation. Finally, thermochronologic 40Ar/39Ar muscovite and amphibole data (in progress) will constrain the timing of cooling and exhumation. Previous work suggests the onset of extremely rapid exhumation at ~439 Ma (as defined by coincident amphibole and muscovite ages) on the shear zones bounding the orogenic wedge, and thus these new data will help clarify the cessation of deformation associated with the Precordillera microplate collision.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T51A2315G
- Keywords:
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- 1140 GEOCHRONOLOGY / Thermochronology;
- 3651 MINERALOGY AND PETROLOGY / Thermobarometry;
- 8012 STRUCTURAL GEOLOGY / High strain deformation zones;
- 8104 TECTONOPHYSICS / Continental margins: convergent