Upper crustal Vp/Vs ratios along the northern East Pacific Rise derived from downward-continued streamer data

Abstract

SUMMARY Multi-channel seismic (MCS) imaging has been extensively used to investigate fast-spreading East Pacific Rise (EPR) crustal compressional wave velocity (Vp) structure and tectono-magmatic behaviors. However, its upper oceanic crust’s shear wave velocity (Vs) profile has remained a rarity. We first confirm that additional offset ranges can be derived for traveltime picking from downward-continued MCS data in fast-spreading tectonic settings for both early-arrival P and S waves. We then inverse independent 2-D Vp and Vs structures along a ∼80-km-long along-axis stretch of the northern EPR. The resulting Vp/Vs ratio exhibit that the upper crust comprises pillow lavas, transition zone and sheeted dykes. The average thickness of pillow lavas is ∼125 m, with Vp increasing from ∼2.8 to 3.2 km s−1 and Vs from ∼1.2 to 1.5 km s−1. The lava unit with a transition zone has high Vp/Vs ratios (∼2.1 ± 0.2), indicating that fracturing and alteration are variable but pervasive. The average thickness of the transition zone is ∼400 m, with Vp increasing from ∼3.2 to 5.3 km s−1 and Vs from ∼1.5 to 2.8 km s−1. The pillow lavas and the transition zone constitute the layer 2A with an average thickness of ∼525 m. The boundary of layer 2A/2B can be defined using a Vp/Vs ratio contour of 1.9. The layer 2B exhibits lower Vp gradients (∼1.51 s−1), Vs gradients (∼1.30 s−1) and Vp/Vs ratios (∼1.8–1.9) compared to the layer 2A (∼4.65 s−1, ∼2.98 s−1 and ∼2.1 ± 0.2). Porosity variation and crack morphology are critical in controlling the seismic velocities of layer 2A. The strong lateral heterogeneity of the Vp/Vs ratios indicates hydrothermal signatures in the upper crust. The high Vp/Vs anomalies indicate fluid pathways into and out of the oceanic upper crust. This study demonstrates that the Vp/Vs ratio can be obtained from seismic tomography of downward-continued streamer data and used as a reference to investigate the crustal structure and hydrothermal activities along fast-spreading ridges.