Pre-andean deformation in the southern central Andes (32¡Æ-33¡ÆS)

GIAMBIAGI, L. B.; MESCUA, J. F.; FOLGUERA, A.; MARTÍNEZ, A.

Niza, Francia

Simposio; VII International Symposium on Andean Geodynamics; 2008

IRD

Detailed outcrop-scale field analysis was conducted at 56 sites throughout the Southern Precordillera and eastern sector of the Cordillera Frontal. At each outcrop, we measured fault orientation, slip direction, average displacement or fault width and sense of displacement of the structures. We also measured and studied fold attitudes, the angular relationship between bedding and tectonic foliations and asymmetric folds in shear zones to obtain the vergence of Paleozoic structures. Fault-slip data were acquired by measurements on minor faults and were considered in terms of incremental strain (Cladouhos and Allmendinger, 1993). We used the kinematic hypothesis proposed by Marrett and Allmendinger (1990) and Twiss and Unruh (1998) to determine constraints on the orientation and magnitudes of the principal strain rates from a large set of fault-slip data. Principal strain axes have been computed using the moment tensor summation method (both unweighted and weighted by measured displacement) as implemented in FaultKin 2.1.1 stereonet program of Almendinger et al. (2001). Early Paleozoic rocks are affected by folds and faults which kinematic analysis indicates an E-W compressional direction and vergence toward the west. Late Paleozoic faults indicate a NW-SE to NNW-SSE compressional direction. We rotated both Early and Late Paleozoic structures 80¡Æ counterclockwise in order to reconstruct their orientation previous to Late Permian vertical rotation, and obtained a N-S compressional direction with southward vergence during the Early Paleozoic and a SW-NE compressional direction with double vergence for the Late Paleozoic. The kinematic axes of the Permo-Triassic faults indicate that this deformational phase was characterized by NNE-SSW oriented extension (N23¨¬E stretching direction). Cenozoic structures appear to be due to two interfering processes: a regional E-W shortening direction and sinistral strike-slip movements along preexisting NW trending crustal weakness zones. These Cenozoic strike-slip faults are affecting only the western sector of the Precordillera and the eastern sector of the Cordillera Frontal. Crosscutting relationships of Cenozoic structures in the western part of the Precordillera indicate that the E-W shortening event occurred first and thrusts and reverse faults were afterward cut by strike-slip faults. The shortening event is interpreted to be related to a compressional tectonic regime which evolved into a strike-slip regime at the time the compressional process migrated progressively further to the east, towards eastern Precordillera. This change from a compressional stress regime to a strike slip one in western Precordillera could have been due to a change in the vertical stress axes from ¥ò3 to ¥ò2.