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Department of Geology, Box 1846, Brown University, Providence R1 02912
Amoco Production Research Co., Box 50879, New Orleans L A 70150
Amoco Production Company, Research Center, P.O. Box 591, Tulsa OK 74102
Petroleos del Peru, Apartado 3126, Lima, Peru

The crustal structure and tectonic framework of the central Peru Margin, between lat 7° and 10°S were interpreted using mainly a 102 km long, multichannel seismic section and existing geologic and geophysical data in the region. Thrust faulting occurs in upper layer 2 basalts as the Nazca plate descends beneath the continental slope. This produces basaltic ridges (slabs) within the trench axis and at least 26 km landward beneath the overriding continental plate. Broken low-frequency reflectors within this diffracting subduction complex suggest that ophiolitic slivers of basalt are being incorporated into portions of it, forming a sediment-basalt melange.

Three prominent forearc basins, Salaverry, Trujillo, and Yaquina Basins, occupy the central margin from east to west, respectively. Drill holes penetrated Tertiary sediments on the outer shelf and the nearby eastern flank of the Trujillo Basin and bottomed in a metamorphic arc massif. The massif is correlated with seismic refraction velocities greater than 5.7 km/sec and a density of 2.72 to 2.80 g/cm3 which underlie the continental shelf. Our interpretation of the seaward limit of the massif is uncertain and depends upon the geophysical cirteria used. Each of three forearc models position the arc massif at about 26, 61, or 115 km landward of the trench, with the subduction complex occupying the region between the trench and the massif. The massif-subduction complex interface should be located through drilling to test the proposed models.

The intramassif basins, Salaverry and Trujillo, have subsided during Tertiary time to allow the accumulation of 2 to 4 km of marine sediment. The Trujullo Basin apparently has not experienced much vertical movement since the late Miocene, based upon microfossil paleodepth indicators found in dolomicrites and glauconitic micrites dredged from the basin. However, abundant brecciated dolomicrites in the same dredges and disturbed strata in reflection records from both the Yaquina and Trujillo Basins suggest deformation of the basins during the Pliocene-Pleistocene by a compressional regime.

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