Deconvolved waveforms for two earthquakes (M-w: 6.0 and 5.8) show clear postcritical SsPmp arrivals for broadband stations deployed across the coastal plain of Georgia, allowing mapping of crustal thickness in spite of strong reverberations generated by low-velocity sediments. Precritical SsPmp arrivals are also identified. For a basement in which velocity increases linearly with depth, a bootstrapped grid search suggests an average basement velocity of 6.5 +/- 0.1 km/s and basement thickness of 29.8 +/- 2.0 km. Corresponding normal-incidence Moho two-way times (including sediments) are 10.6 +/- 0.6 s, consistent with times for events interpreted as Moho reflections on coincident active-source reflection profiles. Modeling of an underplated mafic layer (V-p = 7.2-7.4 km/s) using travel time constraints from SsPmp data and vertical-incidence Moho reflection times yields a total basement thickness of 30-35 km and average basement velocity of 6.35-6.65 km/s for an underplate thickness of 0-15 km.

In contrast to crustal deformation observed in the actively forming Himalayas, where shallowly dipping crustal detachments extend over hundreds of kilometers, prior work on the Paleozoic southern Appalachian orogeny inferred that the final continental collision occurred on a steeply dipping crustal suture, permitting collision models that are dominated by strike-slip motion. Here, we use scattered seismic phases to instead reveal the Appalachian (Alle-ghanian) crustal suture as a low-angle (

In contrast to crustal deformation observed in the actively forming Himalayas, where shallowly dipping crustal detachments extend over hundreds of kilometers, prior work on the Paleozoic southern Appalachian orogeny inferred that the final continental collision occurred on a steeply dipping crustal suture, permitting collision models that are dominated by strike-slip motion. Here, we use scattered seismic phases to instead reveal the Appalachian (Alle-ghanian) crustal suture as a low-angle (

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.

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.

The Central Andean Plateau (CAP), as defined by elevations in excess of 3 km, extends over 1800 km along the active South American Cordilleran margin making it the second largest active orogenic plateau on Earth. The uplift history of this high Plateau, with an average elevation around 4 km above sea level, remains uncertain as paleoelevation studies along the CAP suggest a complex, nonuniform uplift history. As part of the Central Andean Uplift and the Geodynamics of High Topography (CAUGHT) project, we image the S wave velocity structure of the crust and upper mantle using surface waves measured from ambient noise and teleseismic earthquakes to investigate the upper mantle component of plateau uplift. We observe three main features in our S wave velocity model including (1) a positive velocity perturbation associated with the subducting Nazca slab; (2) a negative velocity perturbation below the sub-Andean crust that we interpret as anisotropic Brazilian cratonic lithosphere; and (3) a high-velocity feature in the mantle above the slab that extends along the length of the Altiplano from the base of the Moho to a depth of similar to 120 km. A strong spatial correlation exists between the lateral extent of this high-velocity feature and the relatively lower elevations of the Altiplano basin suggesting a potential relationship. Determining if this high-velocity feature represents a small lithospheric root or foundering of orogenic lithosphere requires more integration of observations, but either interpretation implies a strong geodynamic connection with the uppermost mantle and the current topography of the northern CAP.

The Central Andean Plateau (CAP), as defined by elevations in excess of 3 km, extends over 1800 km along the active South American Cordilleran margin making it the second largest active orogenic plateau on Earth. The uplift history of this high Plateau, with an average elevation around 4 km above sea level, remains uncertain as paleoelevation studies along the CAP suggest a complex, nonuniform uplift history. As part of the Central Andean Uplift and the Geodynamics of High Topography (CAUGHT) project, we image the S wave velocity structure of the crust and upper mantle using surface waves measured from ambient noise and teleseismic earthquakes to investigate the upper mantle component of plateau uplift. We observe three main features in our S wave velocity model including (1) a positive velocity perturbation associated with the subducting Nazca slab; (2) a negative velocity perturbation below the sub-Andean crust that we interpret as anisotropic Brazilian cratonic lithosphere; and (3) a high-velocity feature in the mantle above the slab that extends along the length of the Altiplano from the base of the Moho to a depth of similar to 120 km. A strong spatial correlation exists between the lateral extent of this high-velocity feature and the relatively lower elevations of the Altiplano basin suggesting a potential relationship. Determining if this high-velocity feature represents a small lithospheric root or foundering of orogenic lithosphere requires more integration of observations, but either interpretation implies a strong geodynamic connection with the uppermost mantle and the current topography of the northern CAP.

We present new tomographic models of the Nazca slab under South America from 6 degrees S to 32 degrees S, and from 95 km to lower mantle (895 km) depths. By combining data from 14 separate networks in the central Andes, we use finite-frequency teleseismic P-wave tomography to image the Nazca slab from the upper mantle into the mantle transition zone (MTZ) and the uppermost lower mantle on a regional scale. Our tomography shows that there is significant along-strike variation in the morphology of the Nazca slab in the MTZ and the lower mantle. Thickening of the slab in the MTZ is observed north of the Bolivian orocline, possibly related to buckling or folding of the slab in response to the penetration of a near-vertical slab into the higher-viscosity lower mantle, which decreases the sinking velocity of the slab. South of the orocline, the slab continues into the lower mantle with only minor deformation in the MTZ. In the lower mantle, a similar difference in morphology is observed. North of 16 degrees S, the slab anomaly in the lower mantle is more coherent and penetrates more steeply into the lower mantle. To the south, the slab dip appears to be decreasing just below the 660 km discontinuity. This change in slab morphology in the MTZ and lower mantle appears to correspond to the change in the dip of the slab as it enters the MTZ, from steeply dipping in the north to more moderately dipping in the south.

We present new tomographic models of the Nazca slab under South America from 6 degrees S to 32 degrees S, and from 95 km to lower mantle (895 km) depths. By combining data from 14 separate networks in the central Andes, we use finite-frequency teleseismic P-wave tomography to image the Nazca slab from the upper mantle into the mantle transition zone (MTZ) and the uppermost lower mantle on a regional scale. Our tomography shows that there is significant along-strike variation in the morphology of the Nazca slab in the MTZ and the lower mantle. Thickening of the slab in the MTZ is observed north of the Bolivian orocline, possibly related to buckling or folding of the slab in response to the penetration of a near-vertical slab into the higher-viscosity lower mantle, which decreases the sinking velocity of the slab. South of the orocline, the slab continues into the lower mantle with only minor deformation in the MTZ. In the lower mantle, a similar difference in morphology is observed. North of 16 degrees S, the slab anomaly in the lower mantle is more coherent and penetrates more steeply into the lower mantle. To the south, the slab dip appears to be decreasing just below the 660 km discontinuity. This change in slab morphology in the MTZ and lower mantle appears to correspond to the change in the dip of the slab as it enters the MTZ, from steeply dipping in the north to more moderately dipping in the south.

Subduction of the Nazca and Caribbean Plates beneath northwestern Colombia is seen in two distinct Wadati Benioff Zones, one associated with a flat slab to the north and one associated with normal subduction south of 5.5 degrees N. The normal subduction region is characterized by an active arc, whereas the flat slab region has no known Holocene volcanism. We analyze volcanic patterns over the past 14 Ma to show that in the mid-Miocene a continuous arc extended up to 7 degrees N, indicating normal subduction of the Nazca Plate all along Colombia's Pacific margin. However, by similar to 6 Ma, we find a complete cessation of this arc north of 3 degrees N, indicating the presence of a far more laterally extensive flat slab than at present. Volcanism did not resume between 3 degrees N and 6 degrees N until after 4 Ma, consistent with lateral tearing and resteepening of the southern portion of the Colombian flat slab at that time.