ABSTRACT

Significant subduction‐zone complexity in southern Peru has been attributed to the subducting Nazca ridge. In this study, we constrained the seismic structure of the Nazca plate to the region where the ridge is subducting and its plate geometry transitions from flat to normal. We applied harmonic decomposition of teleseismic receiver functions (RFs) to retrieve isotropic and anisotropic structures of the oceanic crust and uppermost mantle of the plate, and provide model parameters that best represent the observed structures and clarify the geodynamic processes associated with plate subduction. Harmonic decomposition of the RFs revealed enhanced positive‐polarity amplitudes for the dipping oceanic Moho, indicating the presence of a strong anisotropic (10%–20%) layer near/at the oceanic Moho. This condition requires either anisotropic oceanic crust (AOC) or anisotropic mantle lid (AML) with a predominant trench‐normal fast axis of symmetry for the dipping segment of the slab. The seismic anisotropy produced by the AOC model may indicate strong crystal‐preferred orientation of minerals within the oceanic crust or/and hydrous minerals associated with outer‐rise faulting that developed prior to subduction. The presence of AML may point to frozen olivine fabric in the uppermost mantle of the slab, in alignment with the direction of paleoplate spreading. In the flat‐slab region, the fast‐axis orientation changes abruptly from trench‐normal in the dipping segment to a random direction in the flat segment of the slab. Such disrupted patterns in the fast axes indicate slab fabric deformation due to a high degree of tectonic coupling between the two plates in response to subduction of the Nazca ridge and/or a change in slab dip during the slab‐flattening process. Toward the southeast in the normal dip‐slab region, the presence of frozen olivine fabric in AML is less evident, suggesting that fossil fabric may be overprinted by earlier tectonic processes.

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