Structure and tectonic description of the Nazca plate subdiction zone near Peru

Abstract

A detailed interpretation of the erustal and upper mantle structure of the eastern Nazca Plate, Peru-Chile Trench, and Peruvian continental shelf, from 8° S to 12° S latitude, has been made from land seismic procedurcs in order to provide the data density necessary for this spedfic experiment, where a detailed cross-section was desired across the complcx subduction

zone structure using the results of five, two-ship reversed explosion refraction lines, over 20 ASPER airgun-sonobuoy seismic stations, and a 360 km refraction profile. The refraction profile technique, which resulted in 15 overlapping split refraction profiJes, was adop ted. On the Nazca Plate the unusually high velocity crust is underlain by a shallow (around 10 km deep) Moho, which is in turn underlain in place s by an apparent sub-Moho layer with a vclocity of about 8.7 km/sec. Along 8° 30'S across-section of the upper lithosphere shows deep faulting in the cníst: Along 12° S a more pronounced fault,with dip increasing with depth from an average of around 4°, causes the crust to underthrust itself about 250 km from the present axis of the trench. A bathymetric depression on the western (underthrusting) side of the fault, as well as the location of shallow and intermediate earthquakes in the same region, attest to the location and activity of the fault and suggest that it may extend deep into the mantle, Futher deep compressional faulting is depicted at the trench axis although shallow tensional features occur in the upper crust as it bends into the trench. Although not well defined, the velocity configuration under the continental slope is not inconsistent with imbricate thrust fault structure. The continental shelf is highly faulted at depth, with features which corre late wilh the local land geology, and has a crustal thickness of about 30 kms.