Two‐dimensional velocity models of the Nazca Plate subduction zone between 19.5°S and 25°S from wide‐angle seismic measurements during the CINCA95 project

Abstract

The interpretation of seismic refraction/wide‐angle reflection data from the 1995 Crustal Investigations off‐ and on‐shore Nazca/Central Andes (CINCA95) 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 the coast between 19.5°S and 25°S, with three of the cross sections extending a farther 100–200 km inland. These sections define the major 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 subducting at an increasing angle of 9–25° down to 30–50 km depth near the coast, these cross sections show a portion of the Moho dipping eastward from 43–50 km near the coast to 55–64 km up to about 240 km inland. Owing 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 upper continental South American plate. However, a comparison with other seismological data suggests that the Moho identified here east of the coast defines the base of the continental crust of the upper South American plate immediately behind the downgoing Nazca plate. Thus the hypothesis of Delouis et al. [1996] that the zone of seismic coupling between the plates correlates with the region where continental crust is in contact with the plate boundary is supported. The cross sections also show between 20°S and 22.5°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 subducting Nazca plate. The discussion focuses on either that the boundary is a fossil extensional structure or preferably that the wedge below the boundary represents gabbroic material from layer 3 of the oceanic crust tectonically underplated onto the lower part of the upper plate.