Imaging the transition from flat to normal subduction: variations in the structure of the Nazca slab and upper mantle under southern Peru and northwestern Bolivia

Abstract

Two arrays of broad-band seismic stations were deployed in the north central Andes between 8 • and 21 • S, the CAUGHT array over the normally subducting slab in northwestern Bolivia and southern Peru, and the PULSE array over the southern part of the Peruvian flat slab where the Nazca Ridge is subducting under South America. We apply finite frequency teleseismic P-and S-wave tomography to data from these arrays to investigate the subducting Nazca plate and the surrounding mantle in this region where the subduction angle changes from flat north of 14 • S to normally dipping in the south. We present new constraints on the location and geometry of the Nazca slab under southern Peru and northwestern Bolivia from 95 to 660 km depth. Our tomographic images show that the Peruvian flat slab extends further inland than previously proposed along the projection of the Nazca Ridge. Once the slab re-steepens inboard of the flat slab region, the Nazca slab dips very steeply (∼70 • ) from about 150 km depth to 410 km depth. Below this the slab thickens and deforms in the mantle transition zone. We tentatively propose a ridge-parallel slab tear along the north edge of the Nazca Ridge between 130 and 350 km depth based on the offset between the slab anomaly north of the ridge and the location of the re-steepened Nazca slab inboard of the flat slab region, although additional work is needed to confirm the existence of this feature. The subslab mantle directly below the inboard projection of the Nazca Ridge is characterized by a prominent low-velocity anomaly. South of the Peruvian flat slab, fast anomalies are imaged in an area confined to the Eastern Cordillera and bounded to the east by well-resolved low-velocity anomalies. These low-velocity anomalies at depths greater than 100 km suggest that thick mantle lithosphere associated with underthrusting of cratonic crust from the east is not present. In northwestern Bolivia a vertically elongated fast anomaly under the Subandean Zone is interpreted as a block of delaminating lithosphere.

The role of ridges in the formation and longevity of flat slabs

“…Resolution at the location representing the far inboard extent of flat slab is weak and suffers from smearing. However, our conclusion on the far inboard extent of flat slab is supported by constraints from other studies 34,35 . Vertical resolution.…”

Lithospheric structure beneath the northern Central Andean Plateau from the joint inversion of ambient noise and earthquake‐generated surface waves

“…Additional evidence for the absence of undisrupted cratonic lithosphere is observed in a recent finite frequency teleseismic tomography study where Scire et al . [] observed slow‐velocity perturbations in both P and S wave tomography under (~90 km) the sub‐Andes and Chaco foreland basins. Our results cannot uniquely distinguish between a completely absent or a heavily altered (metasomatized) Brazilian cratonic mantle lithosphere beneath the sub‐Andes and Chaco foreland basin.…”

“…Lithospheric delamination has been long postulated as a contributing mechanism to the growth of the central Andes [ Kay and Kay , ; Kay et al ., ; Beck and Zandt , ; McQuarrie et al ., ; Asch et al ., ; Garzione et al ., ; Ghosh et al ., ; DeCelles et al ., ], but only recently has the data coverage and methodology become available to investigate the structure of the uppermost mantle in sufficient detail [e.g., Calixto et al ., ; Beck et al ., ; Scire et al ., ]. Although seismic models of the uppermost mantle alone cannot determine the timing and evolution of delamination events, they are particularly useful in discriminating against different models and stages of lithospheric delamination.…”

“…In a recent finite frequency tomography study that included P and S waves, Scire et al . [] imaged a similar positive S wave velocity perturbation roughly coincident with our sub‐Moho Altiplano anomaly in the uppermost mantle (<120 km) with a localized positive anomaly beneath the sub‐Andes extending to a depth of ~300 km and suggested that this anomaly may be delaminated lithosphere. Scire et al .…”

Mantle flow through a tear in the Nazca slab inferred from shear wave splitting

“…The South American subduction zone is one of the geographically longest continuous regions of active subduction and is characterized by lateral variations in slab dip angle [e.g., Kay and Coira, 2009;Scire et al, 2016;Portner et al, 2017]. In particular, two prominent regions of flat slab subduction are observed beneath Peru and beneath central Chile and Argentina.…”