ALBERT

Description: 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 (Vsv) 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 (Vsv) structure of the Andean crust. We provide a dispersion misfit map as well as uncertainty envelopes for our Vsv 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 low-velocity 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.

Description: In eastern Turkey, the ongoing convergence of the Arabian and African plates with Eurasia has resulted in the westward extrusion of the Anatolian Plate. To better understand the current state and the tectonic history of this region, we image crust and uppermost mantle structure with ambient noise tomography. Our study area extends from longitudes of 32° to 44°E. We use continuous data from two temporary seismic deployments, our 2006–2008 North Anatolian Fault Passive Seismic Experiment and the 1999–2001 Eastern Turkey Seismic Experiment, as well as from additional seismographs in the region. We compute daily cross-correlations of noise records between all station pairs and stack them over the entire time period for which they are available, as well as in seasonal subsets, to obtain interstation empirical Green's functions. After selecting interstation cross-correlations with high signal-to-noise ratios and measuring interstation phase velocities, we compute phase velocity maps at periods ranging from 8 to 40 s. At all periods, the phase velocity maps are similar for winter and summer subsets of the data, indicating that seasonal variations in noise sources do not bias our results. Across the study area, we invert the phase velocity estimates for shear velocity as a function of depth. The shear velocity model, which extends to 50 km depth, highlights tectonic features apparent at the surface: the Eastern Anatolian Plateau is a prominent low-velocity anomaly whereas the Kırşehir Massif has relatively fast velocities. There is a large velocity jump across the Inner Tauride Suture/Central Anataolian Fault Zone throughout the crust whereas the North Anatolian Fault does not have a consistent signature. In addition, in the southeastern part of our study area, we image a high velocity region below 20 km depth which may be the northern tip of the underthrusting Arabian Plate.

Description: Located in the central Andes, the Altiplano-Puna Volcanic Complex (APVC) is the location of an 11–1 Ma silicic volcanic field, one of the largest and youngest on Earth. Yet its magmatic/plutonic underpinnings have been seismically investigated in only a few widely spaced locations. Previous studies have identified an extensive (∼60,000 km2) low-velocity zone (LVZ) below the APVC referred to as the Altiplano-Puna Magma Body (APMB); however, insufficient seismic constraints have precluded uniquely measuring its thickness, and the volume of the APMB remains mostly constrained by varying estimates of plutonic to volcanic (P:V) ratios. Here we present new 3-D seismic images of the APVC crust based on a joint inversion of Rayleigh-wave dispersion from ambient seismic noise and P-wave receiver functions from broadband seismic stations recently deployed in the area. We identify a large ∼200 km diameter and ∼11 km thick LVZ that we interpret as the plutonic complex that sourced the voluminous APVC volcanics and show that its volume is much larger than previous estimates, perhaps as much as an order of magnitude larger. The large volume (∼500,000 km3) and shallow depth (4–25 km below sea level) of the LVZ centered on the observed surface uplift below the composite volcano Uturuncu provide strong evidence linking our imaged low-velocity body (APMB) with the presence of an amalgamated plutonic complex. We suggest the APMB retains a significant percentage (up to 25%) of partial melt, most likely in a melt-crystal mush state, and is related to the source of the continued ground deformation attributed to magma ascent beneath the APVC. The seismic imaging of this plutonic complex and the well-preserved and documented volcanic deposits allow us to make one of the best-constrained calculations of a plutonic to volcanic ratio. Although this calculation is still dependent on a few critical assumptions, the large volume of the newly imaged APMB requires a much larger ratio (20–35) than often cited in the literature. This large ratio has significant implications for both petrologic and tectonic models of this portion of the Andean arc.