Deformation of the Paleozoic metamorphic basement in Tanumé-Pichilemu (34°30’ S), Central Chile

Abstract:

We present a new geodynamic evolution for the late Paleozoic accretionary complex of the Chilean coast, located between Tanumé-Pichilemu villages. The accretionary complex has been interpreted as paired metamorphic belt, with continuous exposures along the Coastal Cordillera between 34° and 38°S, was formed during subduction of the paleo-Pacific plate against the southwestern margin of Gondwana. This complex consists of two tectonometamorphic units: (i) a basally accreted HP/LT, the Western Series, formed by metabasites, metacherts, greenschists, blue-schists and marbles, with peak P-T conditions of 7-11 kbar and 350-400°C, that are superimposed on (ii) the frontally accreted Eastern Series, mainly formed by metaturbidites and deformed in HT/LP metamorphic conditions, coeval with the emplacement of the Pennsylvanian Coast Range batholith.

The structures observed in the Tanumé-Pichilemu transect has been affected by three deformation episodes related with Gondwanan orogeny (D1, D2, D3), developed in late Carboniferous-Early Permian times. In the Eastern Series, D1 is associated to recumbent isoclinals folds with NW trend and associated (S1) cleavage. The D2 event is related to SW vergent folds which are part of an antiform-synform pair, developing an axial plane cleavage (S2) that dips 30° to the NE. This regional tectonic foliation S2 has intertectonic garnets porphyroblasts. In the Western Series, the first episode (D1) is represented by a foliation (S1) formed under HP-LT metamorphic conditions and preserved in syn-D2 garnet and albite porphyroblast. A NE directed reverse ductile fault system (D2), developed under pressure decreasing conditions, overlaps the HP-LT Western Series, formed about 15-25 km depth, above the shallow HT/LP Eastern Series. In this context the D2 of the Eastern Series are formed previously to D2 episode of the Western Series.

The third deformative event D3 is linked to west-verging kilometric-scale antiform, related to a fragile reverse fault that thrust the Eastern Series above the Western Series. This episode is associated with a S3 cleavage. The D3 reverse fault presents a minor sinestral movement, probably related to the Andean orogenic cycle.

This work has been supported by FONDECYT grant 1130227, and TORANDES (CGL2012-38396-C03) Spanish I+D+i Plan.