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.