GEOPHYSICAL-STUDY OF THE EAST PACIFIC RISE 15-DEGREES-N-17-DEGREES-N - AN UNUSUALLY ROBUST SEGMENT

Bathymetric, side-scan sonar, magnetic and gravity data from the East Pacific Rise (EPR) between 15 degrees and 17 degrees N are used to est ablish the spreading history and examine melt delivery to an unusually robust spreading segment. The axial ridge between the Orozco transfor m fault (15 degrees 30'N) and the 16 degrees 20'N overlapping spreadin g center (OSC) has an average elevation of 2300 m which is 300 m shall ower than typical EPR depths, and its cross-sectional area is double t he average value for the northern EPR. The total opening rate is 86 km /Myr, but the inflated segment appears to have spread faster to the ea st by more than 20% since 0.78 Ma. The orientation of magnetic isochro ns and lineaments in the side-scan sonar indicates a similar to 3 degr ees counterclockwise rotation of the spreading direction since 1.8 Ma (C2) and reflects a change in the Pacific-Cocos plate motion. The side -scan lineaments also show that the percentage of inward facing faults (83%) and the spacing between faults (1.5 km) are consistent with the spreading rate dependence shown by Carbotte and Macdonald [1994]. How ever, the mean fault length (4.8 km) is 1.5 km shorter than expected f or the spreading rate and suggests that extensive off-axis volcanism h as draped the faults. Gravity analysis shows that the inflated segment has a similar to 12-mGal bull's eye shaped low in residual mantle Bou guer anomaly. We offer several possible end-member models for the anom aly, including a prism of 10% partial melt in the mantle and lower cru st or a crustal thickness anomaly of 2.25 km. Kinematic modeling that is based on structure and magnetic data suggests that two large magmat ic pulses occurred at approximately 0.8 Ma and 0.3 Ma and have reshape d the plate boundary geometry and inflated the segment.