Kinematics of early paleozoic ductile deformation in the basement of NW Argentina

Abstract

Preliminary results are presented based on the analysis of kinematics and finite strain associated with pre-Carboniferous tectonometamorphic events recorded in various basement blocks of NW Argentina (Sierras Pampeanas, southern Puna). It would appear that various granitoids and their metamorphic country rocks have been affected by the same deformational phase: a N-S trending foliation, sometimes gently dipping, associated with a pronounced stretching lineation generally showing steep pitchs. Kinematic criteria indicate that movements were mostly vertical or involved thrusting of the eastern block over the western block. In some rare cases, the movement picture is reversed, with overthrusting of the western block onto the eastern block. Andean tectonic activity can cause local complications to these interpretations by introducing tilting or rotation of the blocks. However, on the basis of present study, it is possible to establish that tectonic events affecting the pre-Carboniferous basement involved thrusting with minor wrenching. Two main structural domains can be recognized: in the south, the Sierras Pampeanas, characterized by westward-directed thrusting; in the north, near the edge of the Puna region, more variable kinematics are associated with thrusting towards the east and also towards the west. In this way, it is possible to confirm the existence of an E-W compressive regime during pre-Carboniferous times across the entire western border of the Pre-Andean basement. This compression contributed to crustal thickening and the bringing together of previously widely separated blocks. The systematic observation of geometric and kinematic criteria associated with ductile deformation should enable us to place better constraints on geodynamic interpretations of the Pre-Andean domain.