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).
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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
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.