Subduction of the Nazca plate beneath Peru: evidence from spatial distribution of earthquakes

Summary. This paper reports a detailed re-examination of the spatial distribution of shallow- and intermediate-depth earthquakes located beneath Peru between latitudes 3° and 18°S. The events are classified into two groups according to the quality of the locations. The results of this paper confirm and considerably strengthen the findings of Barazangi & Isacks, which have taken on significance beyond Peru because of their implications regarding the tectonics of western North America during the Laramide orogeny. The more accurately located events that occurred beneath central and northern Peru clearly define a shallow-dipping (about 10°) inclined seismic zone that extends inland for a distance of about 700 km from the Peru trench. Although the intermediate-depth events are not uniformly and continuously distributed along the strike of the arc, the available data are best explained by a single Benioff zone that defines the descending Nazca plate beneath central and northern Peru. In southern Peru the Benioff zone is well determined and has a dip of about 30°. The transition from the relatively flat Benioff zone in central Peru and the steeper Benioff zone in southern Peru is abrupt and is interpreted as a tear in the descending Nazca plate. Considerable seismic activity occurs within the upper 50 km of the overriding continental South America plate. This activity is well separated from the shallow-dipping Benioff zone in central and northern Peru, and probably occurs in the crustal part of the South American lithosphere. All events that occur outside the spatial pattern of events as described above are based on few and generally insufficient data. Azimuthal coverage at teleseismic distances is poor, and no reliable pP readings are available to control hypocentral depths. A search for ScS-to-P converted phases at the upper boundary of the descending Nacza plate beneath central Peru has failed to provide any clear and consistent data. This is in contrast to the well-documented observations from Japan. The flat geometry of the descending Nazca plate beneath central and northern Peru requires that the thickness of the continental South American lithosphere does not exceed about 130 km in this region. A remarkable correlation exists between the flat geometry of the descending Nazca plate and the absence of Quaternary volcanism on the overriding South America plate. This is in marked contrast to the presence of abundant volcanism above the steeper segment of the descending Nazca plate in southern Peru, and suggests that generation of arc-type volcanism requires a mantle wedge of asthenospheric material between the subducted and overriding plates. The flat geometry of the descending plate may also be the cause for the broad deformation of the Andean Cordilleras. Many factors may be responsible for the observed flat geometry, such as the fact that the Nazca plate is relatively young (< 50 Myr old), and hence is thinner and more buoyant than the older western Pacific plate. Also, it may be significant that the upper South American lithosphere is thick, in comparison to the upper western Pacific plates, and is possibly actively overriding the descending Nazca plate. Finally, the width of the flat Peru segment along the strike of the arc is probably controlled by the existence of the aseismic submarine structural features near the southern limit (the Nazca ridge) and the northern limit (the Carnegie ridge) of the flat segment. These features are probably zones of weakness along which the Nazca plate is being transversely torn.