Seismic Profiles Across Active Margins—Introduction
P. Lehner, H. Doust, G. Bakker, P. Allenbach, J. Gueneau, 1983. "Seismic Profiles Across Active Margins—Introduction", 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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This selection of seismic profiles illustrates the structure of subduction zones along active margins. The profiles were recorded during the early 1970s as part of a marine deepwater roving program, at a time when Coastal State jurisdiction was still restricted to the continental shelf under Geneva convention rules. The goal of the program was a study of the geology of marginal basins on a global scale. The acquisition and processing parameters of these profiles, recorded some 10 years ago, were geared for rapid and economic retrieval of reconnaissance information.
Active margins are the sites of plate subduction and plate collision. Synonyms for active margins are Pacific type margins, converging margins and destructive margins. Characteristic features are oceanic trenches, island arcs, and folded mountain chains. The term "active" refers to earthquake activity. Active margins are connected with "sinks" of mantle convection. The oceanic crust is subducted; it re-enters the mantle and is eventually consumed. As a byproduct of this process, continental crust is formed. Subduction zones are generally asymmetric as one plate plunges below the other at an angle varying between 15 and 60°. On a global scale, two tectonic regimes can be distinguished, namely the island arc type and the Andean type.
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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.