P and S wave delays beneath intraplate volcanic ridges and gravity lineations near the East Pacific Rise

Abstract

The Gravity Lineations Intraplate Melting Petrology and Seismic Expedition (GLIMPSE) explored the origin of two intraplate, non–hot spot ridge systems associated with gravity lineations in the south central Pacific. Using the 11 month ocean bottom seismometer (OBS) deployment of the GLIMPSE experiment, we determined the average P and S station delays for the region. Across the array, station delays varied by 1.22 s for P waves and 2.13 s for S waves. The presence of ultraslow shear velocity (<400 m/s) sediment layers of variable thickness throughout the region introduced S delays, determined from P‐to‐S‐converted phases generated at the crust‐sediment boundary, of 0.08 to 0.43 s. Consequently, we corrected the S delays for these sediments to examine seismic anomalies due to crustal and mantle structure. We used the sediment delay times and the relative amplitudes of the converted phases to estimate the average regional shear velocity and thickness of the sediment layer beneath the stations. The range of sediment thickness in the region is 10–70 m, and the average shear velocity in the sediments is 140 m/s. Both the average P and sediment‐corrected S delays show a positive correlation with seafloor age, with faster arrivals on older seafloor where the seismically fast lithosphere has thickened away from the spreading axis. The delay times also correlate with the residual mantle Bouguer gravity anomaly (rMBA), with slow arrivals coinciding with rMBA lows and the associated ridge systems. The delays are much too large to be caused by variations in crustal thickness and must represent anomalies within the mantle. These observations qualitatively support the either the channelized asthenospheric return flow model or the small‐scale convection hypothesis for the origin of the gravity lineations. These models predict both positively correlated density and velocity anomalies caused by thermal anomalies and the pressure‐release melting that migrates to the surface to form the volcanic ridges above the upwelling limbs of convection cells or channels.