Teleseismic P‐wave delay times in a major oceanic fracture zone

Abstract

Teleseismic P‐waves were recorded by four ocean‐bottom seismographs located in the Vema fracture zone near its western intersection with the median valley of the mid‐Atlantic ridge. J‐B travel‐time residuals range from +1.31 to +1.81 sec. The largest residuals are associated with the topographic depression that forms the extension of the median valley into the fracture zone. Differences in residuals between the stations are the result of structural variations near the receivers. The differences are too large to be explained by changes in crustal thickness. Instead, low velocities in the uppermost mantle are required by the relative residuals. These low velocities may be related to the extension into the fracture zone of a narrow zone of partial melting from beneath the ridge axis.