Flat-slab subduction occurs when the descending plate becomes horizontal at some depth before resuming its descent into the mantle. It is often proposed as a mechanism for the uplifting of deep crustal rocks ('thick-skinned' deformation) far from plate boundaries, and for causing unusual patterns of volcanism, as far back as the Proterozoic eon. For example, the formation of the expansive Rocky Mountains and the subsequent voluminous volcanism across much of the western USA has been attributed to a broad region of flat-slab subduction beneath North America that occurred during the Laramide orogeny (80-55 million years ago). Here we study the largest modern flat slab, located in Peru, to better understand the processes controlling the formation and extent of flat slabs. We present new data that indicate that the subducting Nazca Ridge is necessary for the development and continued support of the horizontal plate at a depth of about 90 kilometres. By combining constraints from Rayleigh wave phase velocities with improved earthquake locations, we find that the flat slab is shallowest along the ridge, while to the northwest of the ridge, the slab is sagging, tearing, and re-initiating normal subduction. On the basis of our observations, we propose a conceptual model for the temporal evolution of the Peruvian flat slab in which the flat slab forms because of the combined effects of trench retreat along the Peruvian plate boundary, suction, and ridge subduction. We find that while the ridge is necessary but not sufficient for the formation of the flat slab, its removal is sufficient for the flat slab to fail. This provides new constraints on our understanding of the processes controlling the beginning and end of the Laramide orogeny and other putative episodes of flat-slab subduction.

Flat or near-horizontal subduction of oceanic lithosphere has been an important tectonic process both currently and in the geologic past. Subduction of the aseismic Nazca Ridge beneath South America has been associated with the onset of flat subduction and the termination of arc volcanism in Peru, making it an ideal place to study flat-slab subduction. Recently acquired seismic recordings for 144 broadband seismic stations in Peru permit us to image the Mohorovičić discontinuity (Moho) of the subducted oceanic Nazca plate, Nazca Ridge, and the overlying continental Moho of the South American crust in detail through the calculation of receiver functions. We find that the subducted over-thickened ridge crust is likely significantly eclogitized ~350 km from the trench, requiring that the inboard continuation of the flat slab be supported by mechanisms other than low-density crustal material. This continuation coincides with a low-velocity anomaly identified in prior tomography studies of the region immediately below the flat slab, and this anomaly may provide some support for the flat slab. The subduction of the Nazca Ridge has displaced most, if not the entire South American lithospheric mantle beneath the high Andes as well as up to 10 km of the lowermost continental crust. The lack of deep upper-plate seismicity suggests that the Andean crust has remained warm during flat subduction and is deforming ductilely around the subducted ridge. This deformation shows significant coupling between the subducting Nazca oceanic plate and overriding South American continental plate up to ~500 km from the trench. These results provide important modern constraints for interpreting the geological consequences of past and present flat-slab subduction locations globally.

The effects of complex slab geometries on the surrounding mantle flow field are still poorly understood. Here we combine shear wave velocity structure with Rayleigh wave phase anisotropy to examine these effects in southern Peru, where the slab changes its geometry from steep to flat. To the south, where the slab subducts steeply, we find trench‐parallel anisotropy beneath the active volcanic arc that we attribute to the mantle wedge and/or upper portions of the subducting plate. Farther north, beneath the easternmost corner of the flat slab, we observe a pronounced low‐velocity anomaly. This anomaly is caused either by the presence of volatiles and/or flux melting that could result from southward directed, volatile‐rich subslab mantle flow or by increased temperature and/or decompression melting due to small‐scale vertical flow. We also find evidence for mantle flow through the tear north of the subducting Nazca Ridge. Finally, we observe anisotropy patterns associated with the fast velocity anomalies that reveal along strike variations in the slab's internal deformation. The change in slab geometry from steep to flat contorts the subducting plate south of the Nazca Ridge causing an alteration of the slab petrofabric. In contrast, the torn slab to the north still preserves the primary (fossilized) petrofabric first established shortly after plate formation.

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