An overlapping propagating spreading center at 87°30′W on the Galapagos Spreading Center

Abstract

In September of 1987 we completed a SeaMARC II (SMII) survey of the propagating spreading center located at 87°30′W on the Galapagos Spreading Center (GSC). The spreading rate in the area is intermediate 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 surface magnetic data indicates 90–100 mm/yr eastward propagation of the 26 km offset. The morphology of the feature is that of an overlapping spreading center pair (OSC). It fits a kinematic model of overlapping ridges with cyclic failure of the dying rift. There are marked differences in the morphology and kinematics of this propagator compared to the propagator at 95°30′W where the spreading rate and offset are comparable. The 87°30′W propagator is marked by an axial high and greater than 40 km of overlap in contrast to the axial rift valley and small amount of overlap associated with the 95°30′W propagator. Near-field stresses associated with variable axial topography contribute to decrease crack propagation forces at the 95°30′W propagator and increase those at the 87°30′W propagator. Differences in crack propagation forces and morphology may be due to a fundamental difference in the amount of available magma. This difference is evident in dominance of volcanism near 87°30′W as opposed to the dominance of tectonism near 95°30′W.