SHALLOW CRUSTAL STRUCTURE BENEATH THE JUAN-DE-FUCA RIDGE FROM 2-D SEISMIC REFRACTION TOMOGRAPHY

The formation of oceanic lithosphere along ocean ridges, and the role that crustal magma chambers play in the accretionary process, continue to be fundamental issues in plate tectonics. To address these issues, a multireceiver airgun/ocean‐bottom seismograph refraction line was shot across the Endeavour segment of the Juan de Fuca Ridge near 48° N, 129° W. 2‐D traveltime tomography applied to the data results in a three‐layer model for the upper crust. Layer 1 is 250–650 m thick, with v1= 2.5 km s−1 and ∇Vzv1= 0.5 s−1. Layer 2 is ∼800 m thick, v2= 4.8 km s−1 and ∇Vzv2= 1.0 s−1. Layer 1 and layer 2 probably represent the sequence of extrusives and the transition to layer 3 (v3= 5.8 km s−1, ∇Vzv3= 0.5 s−1) is associated with the extrusives‐dike complex transition. An abrupt velocity transition between layer 1 and layer 2 may be a metamorphic front within the basalt pillows or it may be the depth to which surface fissures penetrate. A low‐velocity anomaly (velocities decreased by < 0.45 km s−1) exists beneath the ridge in layer 2 and upper layer 3. It is interpreted as a zone of increased fracture porosity and/or permeability associated with axial hydrothermal circulation, and correlates reasonably well with a sub‐axial reflector. No evidence is found for the existence of a crustal magma chamber in the depth range of 1.5–3.5 km sub‐bottom. However, the velocity anomalies observed in layer 3 suggest that crustal temperatures in this layer are elevated by 150–250°C beneath and to the east of the ridge relative to temperatures west of the ridge. Copyright © 1990, Wiley Blackwell. All rights reserved