Fault Plane Solutions and Tectonics of the South Atlantic and Scotia Sea.

摘要

Focal mechanisms are presented for 46 earthquakes that occurred in the South Atlantic Ocean, in the Scotia Sea, and in southern Chile during the period 1963-73. The slip vectors of shallow earthquakes indicate that the South American plate is moving directly west with respect to the Antarctic plate at the ridge-fault-fault triple junction in the South Atlantic. The directions of motion of Africa with respect to the South American (SA) and Antarctic (ANT) plates at the triple junction are N70 deg E and N47 deg E, respectively. The SA-ANT relative motion between the triple junction and the South Sandwich trench is best described by a pole of rotation at 80 deg S, 166 deg W, with an angular rotation rate of 0.24 deg/m.y. Shallow earthquakes along the South Sandwich trench indicate that the oceanic portion of the South American plate is being thrust under the South Sandwich arc in an east-west direction. Most of the earthquakes at the northern end of the arc are due to hinge faulting or bending stresses within the underthrust oceanic plate. The focal mechanisms of intermediate depth events beneath the arc indicate down-dip extension in the northern end of the downgoing slab and down-dip compression in the southern end. This change in the stress pattern may be caused by reduced negative buoyancy forces in the younger, southern half of the subducted plate. The seismicity and focal mechanisms suggest that the SA-ANT relative motion in the Scotia Sea region is taken up on both the north and the south Scotia ridges. However, the plate boundaries in this area and in southern Chile are not well defined.