Crustal structure near Explorer Ridge from a marine deep seismic sounding survey

Abstract

In 1974, two reversed, 75‐km, deep seismic sounding profiles were recorded in the Explorer Ridge region of the northeast Pacific, one parallel and the other perpendicular to the ridge. Multichannel seismograms recorded at distances beyond 4 km were stacked, filtered, and statics‐ and amplitude‐corrected before compilation as record sections. The data have been interpreted using both travel time analysis and amplitude studies with the aid of synthetic seismograms. For the reversed profiles (74‐2 and 74‐2R) across the ridge, significantly different velocity‐depth curves were derived. The uppermost 2.5 km (subbottom) is similar with a rapid increase in velocity from about 2.5 to 5.6 km/s. However, the depth to and the form of the increase from about 5.6 to 6.8 km/s are different. Profile 74‐2 shows a small steplike increase followed by a substantial velocity gradient, while 74‐2R has a large steplike increase followed by a lesser gradient. The deeper crust as shown by 74‐2 consists of a monotonie velocity increase, but with changing velocity gradient, to the upper mantle. On 74‐2R, a deep crustal low‐velocity zone was required. It is suggested that this may be associated with a magma chamber beneath Explorer Ridge. For the two profiles (74‐1 and IR) parallel to the ridge, large velocity gradients in the uppermost crust also are derived. Our preferred interpretation to explain travel time delays and offsets involves crustal faulting with vertical offsets of approximately 5 km. An alternative interpretation in terms of a low‐velocity zone also is given. The total subbottom thickness of the oceanic crust varies between 8 and 10 km, except in the faulted regions, where it is thin. In the upper mantle, reversed velocities of 7.9 and 7.3 km/s are determined in directions perpendicular and parallel to the ridge; anisotropy is proposed to explain these different velocities. The interpreted velocity‐depth models are discussed in terms of petrologic models of the oceanic crust and recent determinations of the seismic velocity and lithologie structure of a New‐foundland ophiolite complex. In the uppermost crust, the large velocity gradients probably result from the closing of large cracks and pore spaces within the basalt. In the lower crust, our results are consistent with proposals involving gradual transitions between material of differing composition.