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Shallow crustal structure of the Endeavour Ridge segment, Juan de Fuca Ridge, from a detailed seismic refraction survey Cudrak, Constance Frances


The Endeavour Ridge is a segment of the Juan de Fuca Ridge, an active spreading centre which lies off western North America between the Pacific and Juan de Fuca plates. This segment is a bathymetric high and a site of hydrothermal activity—both characteristics suggest an underlying heat source such as an axial magma chamber which is associated with crustal generation. To investigate the creation and evolution of oceanic crust, a detailed refraction survey was carried out over the Endeavour Ridge in the fall of 1985. As a component of this survey, a diamond-shaped array consisting of eight OBS along a 20-km line across the ridge and two OBS placed along it at distances of 10 km on either side of the cross-ridge line was deployed to define the shallow crustal structure near and beneath the ridge, especially the possible existence of an axial magma chamber. Airgun shots at 0.2 km intervals along ~300 km of profiles provide conventional reversed and unreversed refraction lines as well as multiple full azimuthal coverage of the region. Travel-time and amplitude data from fifteen in-line airgun profiles recorded on the inner array were forward modelled using an algorithm based on asymptotic ray theory with a starting model obtained from a concurrent study. Two-dimensional models were constructed and then combined to obtain the three-dimensional structure of the region. These models consist of four layers, with the average model correlating well to the classic model of oceanic crust. Layer 2A averages 0.40 km in thickness and has velocities of 2.6 km/s and 2.8 km/s at the top and bottom of the layer, respectively. To achieve such a low velocity, Layer 2A must consist of highly fractured vesicular basalts. A sharp velocity increase to 4.8 km/s marks the transition to Layer 2B. This velocity discontinuity is also visible as a reflector on a. multichannel reflection line obtained through the centre of the study region and is caused by an abrupt decrease in porosity. Layer 2B averages 0.67 km in thickness, has a velocity of 5.4 km/s at its base and consists of less fractured pillow basalts and sheet flows. The Layer 2B-Layer 2C interface is a velocity increase to 5.8 km/s and is the pillow basalt-sheeted dike contact. A small velocity increase from 6.3 to 6.5 km/s delineates the base of the 0.95 km-thick Layer 2C which is the boundary between the sheeted dikes and cumulate gabbros in Layer 3. Layer 3 has the lowest velocity gradient (0.30 s⁻¹) and a velocity of 7.3 km/s at 4.65 km below the seafloor, the maximum depth constrained by the modelling. Lateral heterogeneities on the scale of 2-3 km are superimposed on this basic velocity structure. These heterogeneities are effects of porosity changes, differential pressure changes, and alteration caused by hydrothermal circulation. Layer 2A thins and increases in velocity away from the ridge; ridge-parallel cracks create a velocity anisotropy of ~10-25%, the faster direction parallel to the ridge. Velocities within Layers 2B and 2C also increase by 0.1 km/s away from the axis of the ridge. Layer 3 velocities decrease by 0.1 km/s for arrivals travelling under the ridge. Increased Layer 2 velocities at the ridge crest reveal high lateral velocity constrasts in very young crust, but within 0.03 Ma the oceanic crust at the ridge has matured to the off-ridge structure. No firm evidence exists for a large magma chamber under Endeavour Ridge. Although the bathymetric high and high-temperature hydrothermal discharges are evidence for a magma chamber, the lack of recent sheet flows at the ridge crest and the presence of a rift along the crest indicate the magma chamber is waning and must be of a size (<1 km in width) not resolvable by seismic refraction data.

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