Modification of Thickened Orogenic Crust by a Subducting Ridge: Disruption of the Andean Lower Crust of Southern Peru by the Subducting Aseismic Nazca Ridge

Abstract:

The subduction of oceanic plateaus or aseismic ridges represent significant perturbations to the long term development of subduction systems and associated orogenies, the consequences of which are variable and determined by the physical characteristics of both the overriding and subducting plates. Flat subduction of the ~18 km oceanic crust of the aseismic Nazca Ridge under the 50 km to 65 km continental crust of the Peruvian Andes provides an opportunity to investigate these consequences. Through analysis of 2233 teleseismic P-wave receiver functions from 55 broadband seismometers deployed in southern Peru for the PULSE, CAUGHT and PeruSE seismic experiments we have identified the South American continental Moho and subducted Nazca oceanic Moho to a higher degree of detail than previously possible in the region.

We find that the continental Moho beneath the Western and Eastern Cordilleras of the Peruvian Andes is at a depth >60 km to the north and south of the subducted Nazca Ridge but at <50 km depth directly above the subducted ridge, forming an arch-like structure approximately perpendicular to the structural strike of the Andean surface geology and to the Peruvian Trench. Under this arch, the subducted oceanic Moho of the Nazca Ridge occurs at a depth of ~70-80 km and increases in depth north and south of the ridge with at least one apparent tear or offset to the north. Along the southern edge of the subducted ridge, the separation between the South American continental Moho and subducted Nazca oceanic Moho is less than 10 km, requiring the complete removal of any South American continental lithosphere and potentially lateral displacement of the lower-most Andean crust. The positioning of this trench perpendicular structure connects with an uplifted segment of the Peruvian forearc above the subducting ridge and the Fitzcarrald Arch, a ~500 m uplift in the Amazonian foreland basin that lies along strike of the subducting ridge. In contrast to the forearc and foreland features, the arched Moho below the high cordillera is not clearly associated with a change in surface elevation. This suggests the thick Andean crust above the flat slab responds to ridge subduction by thinning. In addition, these observations suggest the ridge remains strongly and continuously coupled to the upper plate for a distance >500 km from the trench.