Crustal evolution over the last 2 m.y. at the Mid-Atlantic Ridge OH-1 segment, 35 degrees N

We present the crustal and mantle velocity structure along the strike of th e eastern rift mountains at 35 degreesN on the Mid-Atlantic Ridge. These re sults were obtained by an inversion of similar to 1800 Pg/Pn and similar to 450 PmP travel times and by gravity modeling. As commonly observed at slow spreading mid-ocean ridges, thicker crust (9 km) occurs at the segment mid point, while thinner crust (7 km) is found toward the segment ends. This al ong strike variation occurs primarily in the lower crust, which is 7 km thi ck at the segment center and 4-6 km thick at the segment ends. In contrast, the thickness of the upper crust is relatively constant along strike. At t he segment ends, relatively low velocities extend for 10-15 km along strike and from the seafloor to 4 km depth. These low velocities may indicate an attenuated melt supply and/or fracturing and alteration within the shallow to mid-crust. Directly beneath a cluster of three seamounts at the segment center is a region of relatively high velocity (+0.5 km/s) in the mid-crust . This feature may correspond to a frozen magma chamber that fed the overly ing volcanoes. A synthesis of these results with those from two companion e xperiments along the rift valley and the conjugate flank provide a detailed record of crustal accretion and evolution at this segment. Specifically, t he crustal velocity structures of each flank are nearly identical, and they exhibit a thinner and 16% faster upper crust than is observed on axis. The lower crust is remarkably similar in all three settings, except for a low- velocity body on axis, which is interpreted as a partially molten zone. The maximum crustal thickness is also similar in all three profiles, but north of the segment center, zero-age crust is nearly 4 km thinner than beneath the eastern flank and 2 km thinner than beneath the western flank. These di fferences may indicate that segment-centered mantle upwelling varies on a t imescale of similar to2 m.y.