Geometry and kinematics of Seno Arcabuz Shear Zone, Patagonian Andes, Chile.

Abstract

Metamorphic units of different age and structural levels occur at Isla Diego de Almagro (51°30'S), where they are juxtaposed by the Seno Arcabuz Shear Zone (SASZ). The SASZ strikes NNW and exposes deeper structural levels to the west of it. The SASZ rocks are ductilely deformed and document, at least, two deformation events evidenced by coexistent superposed fabrics. An early mylonitic foliation and lineation, and a late low strain foliation and lineation originated by folding of the early fabric. The mylonitic lineation is well preserved and plunges moderately to the north (ca. 40°); the rock strain and metamorphic grade increase progressively to the south. Ductile deformation is triclinic, as documented by oblique stretching lineations, typical of transpressional shear zones. Kinematic indicators, which are mainly sinistral-reverse, are consistent with bulk transpressional kinematics. According to field geology, shear zone kinematics and available geologic and chronologic data, two possible scenarios are proposed to account for the spatial relationship between the SASZ and the two different structural levels juxtaposed by it: (1) The SASZ is coeval with the differential exhumation of the western block, and then the north-plunging lineations formed during sinistral-reverse motion or (2) The SASZ is not coeval with differential exhumation of the western block, in such case the mylonitic lineation formed in a formerly sinistral strike-slip shear zone and the exhumation of the western block took place later, by block tilting to the north.