AN OVERLAPPING PROPAGATING SPREADING CENTER AT 87-DEGREES-30'W ON THEGALAPAGOS SPREADING CENTER

In September of 1987 we completed a SeaMARC II (SMII) survey of the pr opagating spreading center located at 87 degrees 30'W on the Galapagos Spreading Center (GSC). The spreading rate in the area is intermediat e with a full rate of approximately 70 mm/yr and the spreading axis is marked by an axial high. Analysis of bathymetry, sidescan sonar and s urface magnetic data indicates 90-100 mm/yr eastward propagation of th e 26 km offset. The morphology of the feature is that of an overlappin g spreading center pair (OSC). It fits a kinematic model of overlappin g ridges with cyclic failure of the dying rift. There are marked diffe rences in the morphology and kinematics of this propagator compared to the propagator at 95 degrees 30'W where the spreading rate and offset are comparable. The 87 degrees 30'W propagator is marked by an axial high and greater than 40 lan of overlap in contrast to the axial rift valley and small amount of overlap associated with the 95 degrees 30'W propagator. Near-field stresses associated with variable axial topogr aphy contribute to decrease crack propagation forces at the 95 degrees 30'W propagator and increase those at the 87 degrees 30'W propagator. Differences in crack propagation forces and morphology may be due to a fundamental difference in the amount of available magma. This differ ence is evident in dominance of volcanism near 87 degrees 30'W as oppo sed to the dominance of tectonism near 95 degrees 30'W.