Structure of a Subduction Complex
K.T. Biddle, D.R. Seely, 1983. "Structure of a Subduction Complex", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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High-quality seismic reflection data are commonly difficult to acquire across convergent margins. As a result, the structure of these areas can be hard to resolve. However, modern data collecting and processing techniques have alleviated some of the problems associated with structurally complicated terranes. Here we present several versions of a multichannel reflection seismic profile in which the larger structures of a subduction complex are relatively well displayed (Figure 2 through 4). We also present a geologic cross section without vertical exaggeration constructed from the seismic profile (Figure 5).
The seismic profile presented here crosses part of the continental margin of northernmost California (Figure 1). During the late Cenozoic, this area was the site of slow, oblique convergence and subduction of the Gorda plate beneath North America (Silver, 1969, 1971a, 1971b, 1971c; Atwater, 1970). The oceanic crust currently beneath the lower continental slope is about 5 m.y. old (Silver, 1969). The sedimentary section being deformed at the leading edge of the subduction complex is therefore younger than late Miocene. Material incorporated in the subduction complex becomes progressively older to the east, but we have no detailed age control there. The sedimentary section in the Eel River basin, at the eastern end of the profile, is primarily Neogene although some Paleogene rocks may be present.
Two major sets of structures are recognized along this segment of the California margin: a north-trending set of folds and faults that occur on the continental slope (Silver, 1969, 1971a), and a northwest-trending set of strike-slip faults and associated structures found higher on the margin and onshore (Silver, 1971a; Herd, 1978).