HIGH-RESOLUTION PLATE RECONSTRUCTION OF THE SOUTHERN MID-ATLANTIC RIDGE

We use recently acquired magnetic and SeaBeam bathymetric data to exam ine the spreading rates and plate boundary geometry of the Mid-Atlanti c Ridge 30 degrees-36 degrees S. Using a statistically rigorous estima tion of rotation poles we develop a precise spreading history of the A frican-South American plate boundary. The total opening rate for 1-4.2 3 Myr (Plio-Pleistocene) is nearly constant at 32.3 +/- 1 km Myr(-1). The spreading rate apparently is faster in the Late Miocene (7.3-5.3 M yr), though this may reflect inaccuracies in the geomagnetic time scal e. The rotation poles enable a plate boundary reconstruction with an a ccuracy of 2-3 km. The reconstructions also show that the plate bounda ry geometry underwent several changes since the late Miocene including the growth of one ridge segment from 40 to 105 km in length, and the reorientation of another ridge segment which has spread obliquely from 7 to 1 Myr. Pole calculations using both right- and left-stepping fra cture zones show an offset of 1-2 km between the deepest, most linear part of a fracture zone trough and the former plate boundary location. The high-resolution plate kinematics suggests that the plate boundary , as a whole, evolves 2-dimensionally as prescribed by rigid plates. O n a local scale, asymmetric accretion, asymmetric extension, small lat eral ridge jumps (<3 km), and intra-segment propagation result in mino r plate boundary adjustments and deformation to the rigid plates.