Active Margins, Part 3—Java Trench, Profiles P-7 and N-508
P. Lehner, H. Doust, G. Bakker, P. Allenbach, J. Gueneau, 1983. "Active Margins, Part 3—Java Trench, Profiles P-7 and N-508", 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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Along the southern rim of the Sunda arc, between India and Australia, the oceanic basement of the Indian Ocean plunges below the continental masses of southeast Asia. The associated Benioff zone of earthquake epicenters dips away from the Java trench to a depth of about 500 km (310 mi) below the Java Sea. To illustrate the structure of this subduction zone, two profiles across the Java trench were selected. These profiles are "classical" in the sense that they represented, in 1972, the first seismic confirmation of the subduction model postulated by plate theory.
The oceanic basement of the Indian Ocean, which descends into the Java trench, is of Late Jurassic to Early Cretaceous age. JOIDES corehole 271 in the Wharton basin, some 150 km (93 mi) to the south of the trench, encountered about 500 m (1,640 ft) of deep-sea clay with radiolarian and nannoplankton oozes, ranging in age from Early Cretaceous to Quaternary, and resting on oceanic basement.
Reflection seismic across the Java trench shows the downward plunge of the oceanic basement below a highly imbricated accretionary wedge of sediments, which forms its landward slope. The uppermost portion of the oceanic basement, which probably consists of bedded pillow basalts, is structurally deformed and partly imbricated. The accretionary prism forms a ridge parallel to the trench. The internal structure of the ridge is difficult to resolve from seismic, but sporadic reflections indicate its imbricated nature. The thrusts appear to become steeper away from the trench as would be expected for mechanical reasons. There is evidence on the migrated seismic profile that individual imbrications bend over toward the trench in their uppermost portion in adjustment to the slope topography. This process probably triggers submarine slides and turbidity flows.The sediment fill of the fore-arc basins of Java ranges in age from late Oligocene/early Miocene to Recent. Onshore western Java, folded paralic sediments of Eocene age with limestones and coals are unconformably overlain by Oligocene volcanics. Offshore wells in the fore-arc basin encountered Oligocene volcaniclastics below the base Miocene unconformity. The presence of reefs on the unconformity surface indicates that the fore-arc basin subsided to its present depth after an Oligocene orogenic pulse.
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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.