The central Andes in South America is an ideal location to investigate the interaction between a subducting slab and the surrounding mantle to the base of the mantle transition zone. We used finite-frequency teleseismic P-wave tomography to image velocity anomalies in the mantle from 100 to 700 km depth between 18°S and 28°S in the central Andes by combining data from 11 separate networks deployed in the region between 1994 and 2009. Deformation of the subducting Nazca slab is observed in the mantle transition zone, with regions of both thinning and thickening of the slab that we suggest are related to a temporary stagnation of the slab in the mantle transition zone. Our study also images a strong low-velocity anomaly beneath the Nazca slab in the mantle transition zone, which is consistent with either a local thermal anomaly or a region of hydrated material. The shallow mantle (<165 km) under the Eastern Cordillera is generally fast, consistent with proposed underthrusting of the Brazilian cratonic lithosphere or a string of localized lithospheric foundering. Several discontinuous low-velocity anomalies are observed beneath parts of the Altiplano and Puna Plateau, including two strong low-velocity anomalies in the upper mantle under the Los Frailes volcanic field and the southern Puna Plateau, consistent with proposed asthenospheric influx following lithospheric delamination.

Lithospheric foundering or delamination has been long recognized as an important process in the formation of the Andes, but the scale, timing, and surface uplift consequences remain controversial. We use recently completed ambient noise tomography and finite-frequency P-wave tomography results and other geologic and geophysical information to identify two ~200-km-diameter regions of piecemeal delamination in the Puna region between 21°S and 27°S. One location in the northern Puna Plateau is centered under the 11–1 Ma large-volume silicic Altiplano-Puna volcanic center, and the other in the southern Puna Plateau is centered approximately between the Arizaro Basin and 6–2 Ma Cerro Galan volcanic field. The foundering in the northern location has progressed to the point where the main thermal anomaly resides in the middle and upper crust, and the surface volcanic flare-up and mantle thermal anomalies are both in a waning stage. In the southern location, the main thermal anomaly is still in its waxing stage in the lower crust and upper mantle, and the foundering mantle material is imaged in the mantle wedge. The differing patterns of back-arc volcanism in the two foundering centers suggest different styles and timing of delamination, with the foundering process coming to completion earlier in the north than in the south. Based on plate-motion reconstructions, the NE-SW–aligned Juan Fernandez Ridge swept southward through this area starting about ca. 14 Ma in the north and ca. 10 Ma in the south. Although we do not think the passage of the Juan Fernandez Ridge initiated foundering, it played an important role in facilitating delamination by increasing interplate coupling, and weakening and perhaps hydrating the upper plate, and its passage allowed the delaminated material to sink into the expanding space of the mantle wedge. Another important factor in this evolution is the upper-plate lithospheric strength variations inherited from the different geologic basements underlying the northern and southern Puna regions. As the larger-scale delamination progressed, leaving behind thin lithosphere and a mantle wedge with a mixture of continental lithospheric fragments and hot asthenosphere, smaller secondary Rayleigh-Taylor instabilities occurred beneath the southern Puna Plateau, influencing basin development, and subsequent melting of this “drip” material was the source of the ensuing low-volume mafic volcanism.

Abstract We used teleseismic P-wave receiver functions recorded by the Eastern Turkey Seismic Experiment to determine the crustal structure across an active continent–continent collision zone. Moho depth and V p / V s variations in the region are mapped by incorporating crustal multiples and later two-dimsional (2-D) seismic profiles are produced using a common conversion point technique with our crustal V p / V s estimates. Moho depths do not correlate with surface topography and reveal a relatively thin crust consistent with the high plateau being supported by hot asthenosphere near the base of the crust. Under the Arabian plate, the crust is thinnest ( c . 35 km) and exhibits high V p / V s (≥1.8) associated with mafic compositions. In the east, the crust gradually becomes thicker towards the north and exceeds 45 km in the northeastern side whereas in the west, the crust thickens sharply near the Bitlis suture and displays pronounced Moho topography within the Anatolian plate that suggests the presence of multiple fragments. V p / V s variations show an anomalously high V p / V s corridor (≥1.85) along the North Anatolian Fault and near the youngest volcanic units ( c . 3 Ma) and support the presence of partial melt. This corridor is spatially limited from both north and south by low V p / V s regions implying a change in crustal composition. Near the Bitlis suture, a layered V p / V s model points to the source of low V p / V s in the lower crust that may be rich in quartz. Furthermore, the seismic profiles indicate a prominent low velocity zone in the lower crust across a large area beneath the plateau that may act as a decoupling zone between the crust and upper mantle.

Abstract The Neogene ignimbrite flare-up of the Altiplano Puna Volcanic Complex (APVC) of the Central Andes produced one of the best-preserved large silicic volcanic fields on Earth. At least 15 000 km 3 of magma erupted as regional-scale ignimbrites between 10 and 1 Ma, from large complex calderas that are typical volcano-tectonic depressions (VTD). Simple Valles-type calderas are absent. Integration of field, geochronological, petrological, geochemical and geophysical data from the APVC within the geodynamic context of the Central Andes suggests a scenario where elevated mantle power input, subsequent crustal melting and assimilation, and development of a crustal-scale intrusive complex lead to the development of APVC. These processes lead to thermal softening of the sub-APVC crust and eventual mechanical failure of the roofs above batholith-scale magma chambers to trigger the massive eruptions. The APVC ignimbrite flare-up and the resulting VTDs are thus the result of the time-integrated impact of intrusion on the mechanical strength of the crust, and should be considered tectonomagmatic phenomena, rather than purely volcanic features. This model requires a change in paradigm about how the largest explosive eruptions may operate.