OROGEN-WIDE RESPONSE OF THE PERUVIAN ANDES TO ASEISMIC RIDGE INDUCED FLAT SUBDUCTION

The Peruvian flat slab is one of the largest modern occurrences of flat-slab subduction and its southern portion shows a clear spatial link with the ongoing subduction of the overthickened oceanic crust of the Nazca Ridge. Recent seismic observations show clear evidence that the subducting Nazca Ridge is in direct contact with the overriding South American plate >500 km from the Peru-Chile Trench and locally in direct contact with the thickened continental crust of the Peruvian Andes. When combined with constraints on timing provided by prior geologic observations of central Peru, these seismic observations reveal clear relationships in space and time between the progressive eastward motion of the shallowest part of the flat slab and the development of features in the overriding South American plate. Here we provide an overview of these developments, linking deep crustal, magmatic, and surface uplift processes to the subduction of the Nazca Ridge.

We show that beneath the high Andes the subducted Nazca Ridge’s oceanic Moho lies at ~70-80 km depth, placing the top of the subducted overthickened oceanic crust at ~50-60 km depth. The inboard extent of this shallowly subducted part of the ridge requires that shallow, flat subduction in this area began by ~4 Ma, coincident with the end of arc volcanism in the area and the start of uplift in the foreland’s Fitzcarrald Arch. We infer that complete closure of the asthenospheric mantle wedge, marked by termination of volcanism, occurred at this time and allowed for basal shear stress to be induced in the overriding continental lithosphere. This shear stress has resulted in the eastward displacement and removal of the Andean lithospheric mantle and up to ~10 km of the lowermost continental crust of the high Andes directly above the subducting Nazca Ridge. Inboard of the ridge, the continental crust bordering the fold-thrust belt appears to be ~5-10 km thicker than north and south of the subducting ridge, suggesting that a portion of the disrupted continental crust has been displaced beneath the fold-thrust belt. This displacement may also account for the broad (>500 km half-wavelength) foreland uplift of the Fitzcarrald Arch, which requires ~5 km of crustal thickening to explain its maximum of ~0.8 km of rock uplift relative to surrounding basins with little concurrent faulting.