Present-day strain field on the South American slab underneath the Sandwich Plate (Southern Atlantic Ocean): A kinematic model

Abstract

This work analyses the present-day principal strain orientation on the downgoing slab of the South America Plate (SAM) beneath the Sandwich Plate (SAND). The strain regime was deduced from the study of 331 earthquake focal mechanism solutions examined by fault population analysis methods. In the slab, the maximum horizontal shortening direction (ey) rotates in trend in a clockwise direction from NE in the north, to SE in the south. Based on this rotation, three different areas were defined according to the prevailing focal mechanism type: (1) the North Zone, with ey oriented N058°E and reverse and strike-slip focal mechanisms; (2) the Central Zone, with only reverse focal mechanisms and ey striking N080°E; and (3) the South Zone, with ey oriented N1068E and reverse and strike-slip focal mechanisms. The strain field in the North Zone of the SAND involves decoupling of the slab at approximately 70 km depth. In contrast, the South Zone edge slab exhibits no decoupling and it exhibits different geometry (hook-like shaped) from the North Zone. Finally, we define the dextral strike-slip component acting at the South Sandwich Fracture Zone (SSFZ), according to focal mechanism solutions and the regional tectonic configuration.