PLATE BOUNDARY GEOMETRY TO MOHO DEPTHS WITHIN THE 9-DEGREES-03'N AND 12-DEGREES-54'N OVERLAPPING SPREADING CENTERS OF THE EAST PACIFIC RISE

Multichannel seismic reflection data from the East Pacific Rise provid e a three-dimensional view of overlapping spreading centers (OSCs) at 9 degrees 03'N and 12 degrees 54'N. Previous studies of the 9 degrees 03'N OSC, based on position of the axial magma chamber top, are extend ed to the base of the crust using the observation that the Moho reflec tion is disrupted in the vicinity of the ridge axis. The smaller 12 de grees 54'N OSC is investigated for comparison. Like the shallower feat ures, the Moho disruption is offset within the OSCs. At the larger 9 d egrees 03'N OSC, misalignment between the morphologic ridge axis and t he axial magma body extends to the base of the crust, where the axial gap in the reflection Moho does not consistently underlay the shallowe r structures. Spatial relationships within the OSCs between axial topo graphy, axial magma chamber and axial gap in reflection Moho, suggest that ridge tips propagate first by brittle fracturing of the upper cru st, followed by intracrustal magma residence, followed finally by esta blishment or capture of magma pathways from the mantle. Interpreting a ll three features as demarcating the axial zone of weakness, along whi ch new magma enters the crust, the axial plane within the 9 degrees 03 'N OSC appears to dip and to rotate with depth. The sense of overlap i n the shallow crust is dextral while the sense of offset in the deep c rust is sinistral. Spatial relationships within the 9 degrees 03'N OSC suggest that the northern, propagating ridge tip has already captured a portion of the deep crustal magma conduit connected with the southe rn ridge.