Active Margins, Part 5—South American Trench, Profiles P-1304, P-1307, and P-1017
P. Lehner, H. Doust, G. Bakker, P. Allenbach, J. Gueneau, 1983. "Active Margins, Part 5—South American Trench, Profiles P-1304, P-1307, and P-1017", 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 South America trench, the oceanic crust of the Pacific underthrusts the Andean chains. Rough estimates indicate that some 600 km (373 mi) of oceanic lithosphere have been subducted since early Tertiary alone.The Andes, which rise abruptly from the coastal plains to elevations of 4 to 6 km (2.5 to 3.7 mi) have been the site of intense magmatic and tectonic activity since Paleozoic time. Most conspicuous are massive Mesozoic granitic intrusions. Rows of active volcanoes line up along axial rifts parallel to the chain. A foldbelt with folded and overthrusted sediments, ranging in age from Proterozoic to early Tertiary, follows the eastern slope of the chain, facing the craton.
The oceanic basement of the Nazca plate, which descends into the subduction zone, is of early Tertiary age in the Peru trench and of late Tertiary age in the Ecuador trench. Off Peru the oceanic basement drops from a depth of some 3,500 m (11,483 ft) in the open Pacific to 5,500 m (18,045 ft) in the trench, while in Ecuador it drops from 1,500 m (4,921 ft) on the Carnegie ridge to 3,500 m (11,483 ft) in the trench.
The accretionary wedge of folded and imbricated sediments, which forms the inner wall of the trench, is thin and rudimentary in Peru. On the other hand, in Ecuador it is unusually thick and reaches from the bottom of the trench to the continental shelf.The development of these fore-arc regions indicates two different subduction regimes. The Tertiary fill of the fore-arc basins in Ecuador overlies oceanic-type igneous basement of Cretaceous age. The basement of the fore-arc basins in Peru is continental with Paleozoic metamorphics and Mesozoic granites exposed along the coast. These differences may reflect processes of tectonic accretion on the one hand and tectonic erosion on the other.
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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.