Reworking of an ancient lithospheric anisotropy during the Permo-Triassic extension in southwestern Gondwana

L. GIAMBIAGI Y A. MARTINEZ

Mendoza

Simposio; Gondwana Symposium; 2005

The prolonged history of convergence of oceanic crust against the Pacific edge of Gondwana resulted in several episodes of deformation during Paleozoic to Cenozoic times. Overprinting relationships between different structures in a sector of the Andes comprised between 32º and 34º South latitude, preserve evidence for at least four deformational events occurred since the Early Paleozoic (Ramos, 1988; von Gosen, 1995; Cortés et al., 1997; Folguera et al., 2001): (1) an Early Paleozoic collisional event; (2) a Late Paleozoic compressional orogeny; (3) the Permo-Triassic extension; and (4) the Miocene to present Andean orogeny. The Late Paleozoic orogenic belt has a NNW trend and its location coincides with the inferred Early Paleozoic suture between the Cuyania and Chilenia exotic terranes. The Permo-Triassic evolution of southwestern Gondwana was characterized by the development of a great amount of volcanism under extensional conditions (Kay et al., 1989; Sato et al., 1990). This extensional regime continued during the Triassic time and led to the formation of a series of rift systems, with overall NNW trend, formed along the western margin of Gondwana. One of these basins, the Cuyo basin, corresponds to a NNW-trending narrow basin parallel to the inferred suture zone and the Late Paleozoic orogenic belt.   Detailed geological mapping and kinematic analysis of faults developed coetaneously with this volcanism, in the Uspallata-Potrerillos area, indicates that Permo-Triassic volcanic rocks are affected by normal and oblique-slip normal faults with WNW to NW trends and sinistral displacement. The great thickness and facies variations of these volcanic rocks near these structures, and the relationship between structures and the intrusion of Permo-Triassic plutonic rocks allow us to identify them as synmagmatic Permo-Triassic structures. We measured fault orientation and sense of displacement and used the orientation of the shortening and extension axes to obtain an approximate orientation of the principal strain rate axes. This fault slip analysis allows us to infer a N23ºE stretching direction, suggesting that the NW- and WNW-trending structures are the result of an oblique sinistral extensional regime. The clear parallelism between the trace of the inferred Early Paleozoic suture zone, the Late Paleozoic San Rafael orogenic belt, and the Permo-Triassic rifting, suggests that Early and Late Paleozoic tectonic inheritance permitted the reactivation of a NNW-trending long-live lithospheric weakness zone. The reactivation of this pre-existing weakness zone during Late Permian to Early Triassic time has resulted in the generation of a new complex fault system, which concentrated the oblique-slip normal displacement related to a NNE-SSW stretching.