Causes and consequences of flat-slab subduction in southern Peru

Abstract

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.

Fluid and Melt Pathways in the Central Chilean Subduction Zone Near the 2010 Maule Earthquake (35–36°S) as Inferred From Magnetotelluric Data

“…This could explain why the backarc conductor, C7, is so much larger than the forearc conductors and farther east than similar conductors in Cascadia (e.g., Wannamaker et al, ). The Nazca subslab asthenosphere (A1) is only present when the slab is imposed but, with a value between 10 and 100 Ωm, can be explained by a small amount of partial melt and/or hydrated mantle (Bishop et al, ; Worzewski et al, ).…”

The Alaska Wrangell Arc: ~30 Ma of subduction‐related magmatism along a still active arc‐transform junction

“…There is no characteristic style of volcanism associated with flat‐slab subduction. In central Mexico, there is an active continental arc ~800 km inboard of the trench that migrated ~100's km inboard during the Miocene (Ferrari, Orozco‐Esquivel, Manea, & Manea, ), whereas in Peru, Miocene flat‐slab subduction has led to the almost complete shutting off of arc magmatism (Bishop et al., ). Furthermore, flat‐slab subduction adds a potential increased role for slab‐edge melt as a contributor to the geochemistry of arc‐transform systems (Portnyagin & Manea, ; Portynagin et al., ), especially along transforms and slab windows (Yogodzinski et al., ).…”

Peruvian Subduction Surface Model for Seismic Hazard Assessments

“…The model captures the characteristic of flat subduction, that previous studies have proposed a broad flat slab of relatively uniform depth [21][22][23]. It should be mentioned that between the range of distances of 200 and 400 km from the trench there is not many events, however, from 400 to 600 km there is evidence of a high deep intraplate seismicity.…”