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. / Graduation date: 1981
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29200 |
| Date | 05 December 1980 |
| Creators | Cook, Jeffrey A. |
| Contributors | Bibee, L. Dale |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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