Understanding lower-crustal deformational processes and the related features that can be imaged by seismic waves is an important goal in active tectonics. We demonstrate that teleseismic receiver functions calculated for broadband seismic stations in Southern California reveal a signature of pervasive seismic anisotropy in the lower crust. The large amplitudes and small move-out of the diagnostic converted phases, as well as the broad similarity of data patterns from widely separated stations, support an origin primarily from a basal crustal layer of hexagonal anisotropy with a dipping symmetry axis. We conducted neighborhood algorithm searches for depth and thickness of the anisotropic layer and the trend and plunge of the anisotropy symmetry (slow) axis for 38 stations. The searches produced a wide range of results, but a dominant SW-NE trend of the symmetry axis emerged. When the results are divided into crustal blocks and restored to their pre-36 Ma locations, the regional-scale SW-NE trend becomes more consistent, although a small subset of the results can be attributed to NW-SE shearing related to San Andreas transform motion. We interpret this dominant trend as a fossilized fabric within schists, created from top-to-the-SW sense of shear that existed along the length of coastal California during pretransform, early Tertiary subduction or from shear that occurred during subsequent extrusion. Comparison of receiver-function common conversion point stacks to seismic models from the active Los Angeles Regional Seismic Experiment shows a strong correlation in the location of anisotropic layers with "bright" refl ectors, further affi rming these results.

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.

In order to better understand the tectonic evolution of the North American continent, we utilize data from the EarthScope Transportable Array network to calculate a three‐dimensional shear velocity model for the continental United States. This model was produced through the inversion of Rayleigh wave phase velocities calculated using ambient noise tomography and wave gradiometry, which allows for sensitivity to a broad depth range. Shear velocities within this model highlight the influence of orogenic and postorogenic events on the evolution of the lithosphere. Most notable is the contrast in crustal and upper mantle structure between the relatively slow western and relatively fast eastern North America. These differences are unlikely to stem solely from thermal variations within the lithosphere and highlight both the complexities in lithospheric structure across the continental U.S. and the varying impacts that orogeny can have on the crust and upper mantle.

[1] The Pampean flat-slab region, located in central Argentina and Chile between 29 and 34 S, is considered a modern analog for Laramide flat-slab subduction within western North America. Regionally, flat-slab subduction is characterized by the Nazca slab descending to $100 km depth, flattening out for $300 km laterally before resuming a more "normal" angle of subduction. Flat-slab subduction correlates spatially with the track of the Juan Fernandez Ridge, and is associated with the inboard migration of deformation and the cessation of volcanism within the region. To better understand flat-slab subduction we combine ambient-noise tomography and earthquake-generated surface wave measurements to calculate a regional 3D shear velocity model for the region. Shear wave velocity variations largely relate to changes in lithology within the crust, with basins and bedrock exposures clearly defined as low-and high-velocity regions, respectively. We argue that subduction-related hydration plays a significant role in controlling shear wave velocities within the upper mantle. In the southern part of the study area, where normal-angle subduction is occurring, the slab is visible as a high-velocity body with a low-velocity mantle wedge above it, extending eastward from the active arc. Where flat-slab subduction is occurring, slab velocities increase to the east while velocities in the overlying lithosphere decrease, consistent with the slab dewatering and gradually hydrating the overlying mantle. The hydration of the slab may be contributing to the excess buoyancy of the subducting oceanic lithosphere, helping to drive flat-slab subduction.

In this work, we compile several seismic velocity models publicly available from the Incorporated Research Institute for Seismology (IRIS) Earth Model Collaboration (EMC) and compare subcrustal mantle velocities in the models to each other and to the timing of tectonism across the continent. This work allows us to assess the relationship between the time elapsed since the most recent thermotectonic event and uppermost mantle temperatures. We apply mineral- and physics-based models of velocity-temperature relationships to calculate upper-mantle temperatures in order to determine cooling rates for the lower-crust and uppermost mantle following thermotectonic activity. Results show that most of the cooling occurs in the ∼300–500 million years following orogeny. This work summarizes current estimates of upper-mantle shear velocities and provides insights on the thermal stabilization of continental lithosphere through time.

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