[1] Regional P and S wave travel time data were used to obtain three-dimensional seismic tomography models for V p , V s , and V p /V s above the subducting slab in central Chile and Argentina. In this region, there is an abrupt change from a normal subduction geometry south of 33°S to a flat subduction geometry to the north. We find low V p , low V s , and high V p /V s ratios in the southern half of our study area directly beneath the modern active volcanic arc, which we interpret as localized pockets of melt. In the northern half of our study area, above where the subducting Nazca plate flattens at 100 km depth, we find low V p , high V s , and low V p /V s ratios. These unusual results point to a lack of melt or hydrated mineralogies such as serpentine, both of which are characterized by high V p /V s values. The only mantle rocks that have low V p /V s and high V s are Mg-rich compositions, such as dehydrated serpentinite or orthopyroxenite. We suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite, formed by a transient fluxing of silica-rich fluids. Such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at $10 Ma. This would imply that the hydration of mantle material above a flat slab can be a transient phenomenon, which leaves little residual-free water behind but significantly alters the mantle chemistry.

The lithospheric structure of the Southeastern United States is a product of earlier episodes of continental collision and breakup. The region is located in the interior of the North American Plate, away from active plate margins. However, there is ongoing tectonism in the region with multiple zones of seismicity, uplifting arches, and Cenozoic intraplate volcanism. The mechanisms controlling this activity and the state of stress remain enigmatic. Two important factors are plate strength and preexisting, inherited structures. Here we present new tomographic images of the upper mantle beneath the Southeastern United States, revealing large‐scale structural variations in the upper mantle. Examples include the relatively thick lithospheric mantle of stable North America that abruptly thins beneath the Paleozoic Appalachian orogeny, and the slow upper mantle of the Proterozoic Reelfoot rift. Our results also indicate fast seismic velocity patterns that can be interpreted as ongoing lithospheric foundering. This provides a viable explanation for seismicity, uplifting, and young intraplate volcanism. We postulate that not only tectonic inheritance but also continuing lithospheric foundering may control the ongoing activity of the region long after it became a passive margin. Based on distinct variations in the geometry and thickness of the lithospheric mantle and foundered lithosphere, we propose that piecemeal delamination has occurred beneath the region throughout the Cenozoic, removing a significant amount of reworked/deformed mantle lithosphere. Ongoing lithospheric foundering beneath the eastern margin of stable North America explains significant variations in thickness of lithospheric mantle across the former Grenville deformation front.

Current end-member models for the geodynamic evolution of orogenic plateaus predict (a) slow and steady rise during crustal shortening and ablative subduction (i.e., continuous removal) of the lower lithosphere or (b) rapid surface uplift following shortening, which is associated with punctuated removal of dense lower lithosphere and/or lower crustal flow. This review integrates results from recent studies of the modern lithospheric structure, geologic evolution, and surface uplift history of the Central Andean Plateau to evaluate the geodynamic processes involved in forming it. Comparison of the timing, magnitude, and distribution of shortening and surface uplift, in combination with other geologic evidence, highlights the pulsed nature of plateau growth. We discuss specific regions and time periods that show evidence for end-member geodynamic processes, including middle–late Miocene surface uplift of the southern Eastern Cordillera and Altiplano associated with shortening and ablative subduction, latest Oligocene–early Miocene and late Miocene–early Pliocene punctuated removal of dense lower lithosphere in the Eastern Cordillera and Altiplano, and late Miocene–early Pliocene crustal flow in the central and northern Altiplano.

The Central Andes of southern Peru, Bolivia, Argentina and Chile (between 12 • S and 42 • S) comprise the largest orogenic plateau in the world associated with abundant arc volcanism, the Central Andean Plateau, as well as multiple segments of flat-slab subduction making this part of the Earth a unique place to study various aspects of active plate tectonics. The goal of this continental-scale ambient noise tomography study is to incorporate broad-band seismic data from 20 seismic networks deployed incrementally in the Central Andes from 1994 May to 2012 August, to image the vertically polarized shear wave velocity (V sv) structure of the South American Cordillera. Using dispersion measurements calculated from the cross-correlation of 330 broad-band seismic stations, we construct Rayleigh wave phase velocity maps in the period range of 8-40 s and invert these for the shear wave velocity (V sv) structure of the Andean crust. We provide a dispersion misfit map as well as uncertainty envelopes for our V sv model and observe striking first-order correlations with our shallow results (∼5 km) and the morphotectonic provinces as well as subtler geological features indicating our results are robust. Our results reveal for the first time the full extent of the mid-crustal Andean lowvelocity zone that we tentatively interpret as the signature of a very large volume Neogene batholith. This study demonstrates the efficacy of integrating seismic data from numerous regional broad-band seismic networks to approximate the high-resolution coverage previously only available though larger networks such as the EarthScope USArray Transportable Array in the United States.

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.