Graduate Thesis Or Dissertation
 

Sonobuoy refraction study of the crust in the Gorda Basin

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  • The Gorda Basin is a young oceanic plate which comes in direct contact with the convergent margin of western North America. Two long sonobuoy refraction profiles crossing the basin provide nearly continuous data for computing the velocity structure of the crust and adjacent continental slope. Time-term analysis utilizing multiple receivers and overlapping profiles revealed a thick transition layer which averages 2.3 km but displays considerable lateral variation. The seismic compressional velocity of this layer is 5.3 km/sec. Th average thickness of Layer 3 is 3.4 km with a velocity of 6.9 km/sec. The base of the crust is marked by the seismic Moho, the velocity below which is 8.1 km/sec. Refraction and reflection studies of sediment cover indicate a thickening of turbidite deposits to the southeast from less than 100 meters to over 2.5 km along the continental margin. Ophiolite studies indicate that the top of Layer 3 marks the upper extent of amphibolite facies metamorphism of basaltic sheeted dikes. Lateral depth variations of this seismic boundary in the Gorda Basin may suggest the occurrence of isograd relief along the spreading center. The Moho marks the boundary between mafic and ultramafic rocks near the ridge but may represent the maximum depth of serpentinization in the crust after it moves away from the spreading axis. Thin crust (4-5 km) and deep bathymetry in the central portion of the basin have resulted from crustal formation processes occurring at ridge crest offsets and are coincident with recent seismicity in the area. The Gorda ridge offsets and asymmetrical fan spreading of magnetic anomalies are features observed in response to a regional change in spreading directions and encroachment of the Pacific and North American plates. The Gorda plate as a whole does not respond rigidly to the resulting north-south compression. Complex structures of the continental slope, revealed by seismic reflection, limited the reduction of refraction data using plane layer methods. A simplified seismic section was computed consisting of three probable sediment layers with velocities of 1.8, 2.5 and 4.0 km/sec overlying oceanic crust. The crust is observed to dip about two degrees towards the continent at the base of the slope. A model of subduction unique to the northern California margin is one whereby young crust is subducted slowly and quickly reheated so that no brittle portion remains at normal Benioff depths. Rapid sedimentation rates balance the subduction of the crust at the margin, preventing the formation of a deep trench.
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