Active Margins, Part 1—Japan Trench, Profile P-849
P. Lehner, H. Doust, G. Bakker, P. Allenbach, J. Gueneau, 1983. "Active Margins, Part 1—Japan Trench, Profile P-849", 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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The chain of islands that forms the Japan arc represents a segment of the orogenic belt that marks the subduction of the Pacific Ocean plates beneath the Eurasian continental plate. The islands are separated from the mainland of Asia by the Japan Sea.Profile P 849 crosses the Japan trench to the east of northern Honshu between North latitudes 39° and 40°. To provide a more coherent picture of trench morphology and trench tectonics, tracings of three additional profiles are included.
The seaward wall of the trench appears on seismic as a faulted flexure in the oceanic basement, which in this region is of Early Cretaceous age. The sediment cover, less than 500-m (1,640-ft) thick, consists of pelagic mudstones and siliceous shales. The most conspicuous features on this flexure are normal faults in a stepwise horst and graben arrangement. The spacing of the faults is between 5 and 10 km (3 and 6 mi). Individual fault throws are of the order of several hundred meters, rarely reaching 1,000 m (3,281 ft). Along this outer wall of the trench, the oceanic basement plunges over a distance of about 50 km (31 mi) from an average depth of 6 km (3.7 mi) in the West Pacific basin to a depth of nearly 8 km (5 mi) in the trench itself. This sudden plunge has been interpreted as the result of tectonic loading as observed in foredeep basins.
The landward wall of the trench is formed by an imbricated and folded wedge of sediments. Below it, the top of the oceanic basement can be traced westward to a distance of around 50 km (31 mi). The shape of the toe and the sporadic development of reflectors suggest that listric faults and submarine slides are present. This observation is in agreement with the findings of the JOIDES coreholes, which encountered only land-derived turbiditic shales in this position. Tectonic instability of the slope appears to have produced a continuous mass transport by submarine slides and turbidity flows into the trench. The pelagic sediments of the Pacific were buried below this heterogenous material derived from the imbricated wedge and the overlying Neogene and were subsequently folded and imbricated with it. This process repeated over and over again is thought to form a "melange."
Japan Sea. There, undisturbed, flat and level Neogene strata overlie oceanic basement. Folding and overthrusting of the thick Neogene sediments along the island arc indicate a renewal of compression and uplift since Pliocene time.
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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.