Asymmetric deep crustal structure across the Juan de Fuca Ridge

A multichannel line was shot across the Endeavour segment of the Juan de Fuca Ridge. Careful processing of 25 km of the line centered about the rise axis shows that young crustal structure is asymmetric. An intermittent reflector from the top 1 km of crust follows topography on both the Pacific and the Juan de Fuca plates. It could be either the pillow basalt-diabase dike contact or a metamorphic front within the pillow basalts. A reflection exists 2-3 km below the rift-valley floor; diffractions from its edges indicate that the rift valley's velocity structure is markedly different from the structure of the flanks. The deep crustal structure is highly asymmetric. Moho exists within 4 km of the rift valley on the Pacific plate, in contrast to the Juan de Fuca plate, which shows a reflector at intermediate depths dipping away from the rise to reach Moho depth 12 km from the rift valley. This reflector's origin is enigmatic; we speculate that the thermal history and structure of the two plates differ because of local differences in mantle structure caused by either ridge migration or a melting anomaly.