Two-dimensional velocity models of the Nazca plate subduction zone between19.5 degrees S and 25 degrees S from wide-angle seismic measurements during the CINCA95 project

The interpretation of seismic refraction/wide-angle reflection data from th e 1995 Crustal Investigations off- and on-shore Nazca/Central Andes (CINCA9 5) project has resulted in the derivation of nine E-W two-dimensional (2-D) velocity cross sections for the region between the Peru-Chile trench and t he coast between 19.5 degrees S and 25 degrees S, with three of the cross s ections extending a farther 100-200 km inland. These sections define the ma jor lithospheric structures of the upper South American plate and the lower Nazca plate down to uppermost mantle depths of 30-60 km beneath this part of the present-day forearc region. In addition to showing the Nazca plate s ubducting at an increasing angle of 9-25 degrees down to 30-50 km depth nea r the coast, these cross sections show a portion of the Moho dipping eastwa rd from 43-50 km near the coast to 55-64 km up to about 240 km inland. Owin g to a gap of almost 50 km in the data coverage of the Moho near the coast, it is uncertain from this data set alone whether the Moho defined east of the coast should be correlated with the lower oceanic Nazca plate or the up per continental South American plate. However, a comparison with other seis mological data suggests that the Moho identified here east of the coast def ines the base of the continental crust of the upper South American plate im mediately behind the downgoing Nazca plate. Thus the hypothesis of Delouis et al. [1996] that the zone of seismic coupling between the plates correlat es with the region where continental crust is in contact with the plate bou ndary is supported. The cross sections also show between 20 degrees S and 2 2.5 degrees S, a boundary extending from upper crustal levels down to 25-30 km depth subparallel to and 5-10 km above the top surface of the subductin g Nazca plate. The discussion focuses on either that the boundary is a foss il extensional structure or preferably that the wedge below the boundary re presents gabbroic material from layer 3 of the oceanic crust tectonically u nderplated onto the lower part of the upper plate.