Mantle, crust, and sediment structure near the Hawaiian ridge using seismic shear waves

Abstract

The Hawaiian Ridge represents a complex intra-plate volcanic system formed by plume or hotspot-related mantle melts rising through the oceanic lithosphere, constructing volcanic edifices on the Pacific Plate. At Ka‘ena Ridge, a submarine volcanic edifice west of O‘ahu, the seafloor rises from ~5 km to less than 2 km below sea level, while surrounding flexural moats extend to depths of ~6 km due to lithospheric loading. We use active-source seismic data to construct 2-D S-wave (Vs) velocity-depth and Vp⁄Vs models along a wide-angle seismic profile crossing Ka‘ena Ridge. The velocity models reveal a typical Pacific oceanic crustal structure, with a thin sub-seafloor low-velocity layer (< 2.3 km/s; seismic Layer 1) characteristic of volcaniclastic and oceanic sediments, a thicker low-velocity layer (2.5–3.4 km/s; seismic Layer 2) thought to be the upper oceanic crust composed of dikes and lava flows, and a deeper high-velocity layer (3.5–4.0 km/s; seismic Layer 3), associated with intrusive gabbroic rocks. Relatively high Vp⁄Vs values (1.85–1.91) are recorded in some areas of the upper crust, indicative of increased porosity and fluid-filled cracks. Below the Moho, Vs values range from 4.5 to 4.8 km/s, while Vp⁄Vs decreases to 1.69–1.72 along most of the seismic line, likely reflecting an increase in dry olivine content in the uppermost mantle with no strong indication of an abundance of serpentinite or underplating occurring. Thus, we find no far-reaching effects on oceanic lithosphere other than the flexure from the seamount itself.