Geophysical study of the East Pacific Rise 15°N–17°N: An unusually robust segment

Abstract

Bathymetric, side‐scan sonar, magnetic and gravity data from the East Pacific Rise (EPR) between 15° and 17°N are used to establish the spreading history and examine melt delivery to an unusually robust spreading segment. The axial ridge between the Orozco transform fault (15°30′N) and the 16°20′N overlapping spreading center (OSC) has an average elevation of 2300 m which is 300 m shallower than typical EPR depths, and its cross‐sectional area is double the 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 east by more than 20% since 0.78 Ma. The orientation of magnetic isochrons and lineaments in the side‐scan sonar indicates a ∼3° 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]. However, the mean fault length (4.8 km) is 1.5 km shorter than expected for the spreading rate and suggests that extensive off‐axis volcanism has draped the faults. Gravity analysis shows that the inflated segment has a ∼12‐mGal bull's eye shaped low in residual mantle Bouguer anomaly. We offer several possible end‐member models for the anomaly, including a prism of 10% partial melt in the mantle and lower crust or a crustal thickness anomaly of 2.25 km. Kinematic modeling that is based on structure and magnetic data suggests that two large magmatic pulses occurred at approximately 0.8 Ma and 0.3 Ma and have reshaped the plate boundary geometry and inflated the segment.