Description: The spatial distribution of some major and trace element and isotopic characteristics of backarc Plio-Quaternary basaltic to high-Mg andesitic (51% to 58% SiO2) lavas in the southern Puna (24°S to 27°S) of the Central Andean Volcanic Zone (CVZ) reflect varying continental lithospheric thickness and the thermal state of the underlying mantle wedge and subducting plate. These lavas erupted from small cones and fissures associated with faults related to a change in the regional stress system in the southern Puna at ≈ 2 to 3 Ma. Three geochemical groups are recognized: (1) a relatively high volume intraplate group (high K; La/Ta ratio 〈25) that occurs over a thin continental lithosphere above a gap in the modern seismic zone and represents the highest percentage of mantle partial melt, (2) an intermediate volume, high-K calc-alkaline group ( La/Ta ratio 〉25) that occurs over intermediate thickness lithosphere on the margins of the seismic gap and behind the main CVZ and represents an intermediate percentage of mantle partial melt, and (3) a small-volume shoshonitic group (very high K) that occurs over relatively thick continental lithosphere in the northeast Puna and Altiplano and represents a very small percentage of mantle partial melt. Mantle-generated characteristics of these lavas are partially overprinted by mixing with melts of the overlying thickened crust as shown by the presence of quartz and feldspar xenocrysts, negative Eu anomalies (Eu/Eu 〈 0.90; most 〈 0.80), and radiogenic Sr (〉 0.7055) and Pb and nonradiogenic Nd ( εNd 〈 −0.4) isotopic ratios. Mixing calculations show that the lavas generally contain more than 20% to 25% crustal melt. The eruption of the intraplate group mafic lavas, the change in regional stress orientation, and the high elevation of the southern Puna are suggested to be the result of the late Pliocene mechanical delamination of a block (or blocks) of continental lithosphere (mantle and possibly lowermost crust). The loss of this lithosphere resulted in an influx of asthenosphere that caused heating of the subducting slab and yielded intraplate basic magmas that produced extensive melting at the base of the thickened crust. Heating of the subducting slab led to formation of the seismic gap and trenchward depletion of the slab component. Backarc calc-alkaline group lavas erupted on the margins of this delaminated block, whereas shoshonitic group lavas erupted over a zone of relatively thick nondelaminated lithosphere to the north.

Description: The late Miocene and younger mafic back-arc lavas in the southern Puna of the central Andean plateau have been attributed to the aftermath of crustal and mantle lithospheric delamination or foundering. In this paper, we analyze in more detail the nature of the back-arc mafic suite magmas, including the conditions of magma generation in the mantle and of magma evolution during ascent and ponding in the crust, using extensive compositional data for phenocryst minerals and olivine-hosted melt inclusions in combination with published and new whole-rock chemical and isotopic data. We estimate that the primary melts last equilibrated with an enriched mantle source at temperatures near 1375°C and pressures near 2 GPa, which is near the base of the seismically determined ~60 km thick crust. A mantle source geochemically enriched by continental material introduced through delamination and subducted erosion processes is required to explain the coincidence of the high 87 Sr/ 86 Sr ratios (〉0·705) and high Sr concentrations (〉700 ppm) of the most primitive lavas (e.g. 9–10 wt % MgO, olivine Fo 88 ). The crystallization conditions inferred from mineral–melt equilibria indicate that olivine ( T = 1320–1220°C) was followed by clinopyroxene ( T = 1230–1140°C). Clinopyroxene–melt equilibration pressures of 0·7 to near 1 GPa in the most mafic samples indicate that the magmas crystallized at mid-crustal depths of 20–35 km, within a region of inferred partial melt accumulation based on the presence of low seismic velocity zones. Olivine-hosted melt inclusions indicate relatively dry melts (maximum 0·5 wt % H 2 O) with unusual high-Al basaltic compositions, which are attributed to the high-pressure suppression of plagioclase crystallization. A first stage of crustal contamination before mid-crustal accumulation and crystallization of the mafic magmas is suggested by high O-isotope ratios in olivine phenocrysts and negative Eu anomalies in clinopyroxene from the plagioclase-free mafic lavas. Mixing models based on trace elements and radiogenic isotopes suggest assimilation of silicic melt in the lower crust, similar to contemporaneous glassy dacites with steep REE patterns and negative Eu anomalies. A second stage of crustal assimilation at shallower depths is indicated by the mismatch of incompatible elements in clinopyroxene relative to bulk-rock compositions, by strong positive correlations of radiogenic isotopes with wt % SiO 2 , and by petrographic observation of partly resorbed and reacted quartz xenocrysts. Mixing calculations require the erupted magmas to have assimilated in total some 15–25% crust.

Description: A passive seismic array which operated between December 2007 and November 2010 has been deployed in the southern Puna plateau between 25°S to 28°S latitude and 65W to 70W longitude in Argentina and Chile to address fundamental questions on the processes that form, modify and destroy continental lithosphere and control lithospheric dynamics along Andean-type continental margins. This experiment consisted of 75 stations has been conducted by Argentine, Chilean, German and US scientists and was designed to improve our understanding of the evolution of the Central Andean plateau in an area where there is a very little geophysical data available.We present here some preliminary results form P and S receiver functions that enables us to address fundamental questions about the role of crustal and mantle lithospheric delamination in the evolution of plateau regions and the evolution of the continental crust in general. Our aim is to determine the seismic structure and thickness of the continental crust and the lithospheric mantle beneath the southern Puna plateau.We also want to determine the thickness of the mantle transition zone beneath the southern Puna and constrain the geometry and rheology of the subducting slab. As a collaborative project, our network has been designed to use other standard broadband seismological methods besides receiver function involving surface wave tomography, tomographic travel time inversion, and shear wave splitting analysis.

Description: The high elevation of the southern Puna plateau, the widespread melting of its crust, the gap in intermediate depth seismicity and the recent eruptions of ignimbrite complexes can be explained by delamination of the lithospheric mantle beneath it. To test this hypothesis, an array consisting of 73 broad band and short period seismic stations was deployed in the region for a period of 2 years starting in 2007. We inverted the data using the two plane wave approach and obtained 1-D and 3-D Rayleigh wave phase velocities. Our dispersion curve shows that at short periods (〈70 s) the phase velocities are slightly higher than those of the Tibetan plateau and lower than those of the Anatolian plateau. At periods of 100–140 s we observe a low velocity zone that might be remnant hot asthenosphere below a flat slab (7–10 Ma). We estimate the average continental lithosphere thickness for the region to be between 100 and 130 km. Our three-dimensional Rayleigh wave phase velocities show a high velocity anomaly at low frequencies (0.007, 0.008, and 0.009 Hz) slightly to the north of Cerro Galan. This would be consistent with the hypothesis of delamination in which a piece of lithosphere has detached and caused upwelling of hot asthenosphere, which in turn caused widespread alkaline-collision related volcanism. This interpretation is also corroborated by our shear wave velocity model, where a high velocity anomaly beneath the northern edge of Cerro Galan at 130 km depth is interpreted as the delaminated block on top of the subducting Nazca slab.

Description: We performed a teleseismic P wave tomography study using seismic events at both teleseismic and regional distances, recorded by a temporary seismic array in the Argentine Puna Plateau and adjacent regions. The tomographic images show the presence of a number of positive and negative anomalies in a depth range of 20–300 km beneath the array. The most prominent of these anomalies corresponds to a low-velocity body, located in the crust, most clearly seen in the center of the array (27°S, 67°W) between the Cerro Peinado volcano, the Cerro Blanco caldera and the Farallon Negro in the east. This anomaly (southern Puna Magmatic Body) extends from the northern most part of the array and follows the line with the highest density of stations towards the south where it becomes smaller. It is flanked by high velocities on the west and the east respectively. On the west, the high velocities might be related to the subducted Nazca plate. On the northeast the high velocity block coincides with the position of the Hombre Muerto basin in the crust and could be indicating an area of lithospheric delamination where we detected a high velocity block at 100 km depth on the eastern border of the Puna plateau, north of Galan. This block might be related to a delamination event in an area with a thick crust of Paleozoic metamorphic rocks at the border between Puna and Eastern Cordillera. In the center of the array the Southern Puna magmatic body is also flanked by high velocities but the most prominent region is located on the east and is interpreted as part of the Sierras Pampeanas lithosphere with high velocities. The position of the Sierras Pampeanas geological province is key in this area as it appears to limit the extension of the plateau towards the south.

Description: We present here the results obtained using the data form our passive seismic array in the southern Puna plateau between 25°S to 28°S latitude in Argentina and Chile. In first instance we have been able to calculate P and S receiver functions in order to investigate the Moho thickness and other seismic discontinuities in the study area. The RF data shows that the northern Puna plateau has a thicker crust and that the Moho topography is more irregular along strike. The seismic structure and thickness of the continental crust and the lithospheric mantle beneath the southern Puna plateau reveals that the LAB is deeper to the north of the array suggesting lithospheric removal towards the south. Later we performed a joint inversion of teleseismic and regional tomographic data in order to study the distribution of velocity anomalies that could help us to better understand the evolution of the Andean elevated plateau and the role of lithosphere-asthenosphere interactions in this region. Low velocities are observed in correlation with young volcanic centers (e.g. Ojos del Salado, Cerro Blanco, Galan) and agree very well with the position of crustal lineaments in the region. This is suggesting a close relationship between magmatism and lithospheric structures at crustal scale coniciding with the presence of hot asthenospheric material at the base of the crust probably induced by lithospheric foundering.

Description: The Central Andean margin has been proposed as a site of crustal destruction by forearc subduction erosion and by delamination or founding of thick continental crust as well as an area of crustal addition and reworking by arc magmatic processes. These processes are supported both by the magmatic record and by new slab and crustal earthquake distributions and tomographic and receiver function images from the 2007-2009 southern PUNA passive seismic experiment. The principal evidence for forearc subduction erosion comes from an ~50 km eastward displacement of the arc front at ~27°S to 28°S at 8 to 3 Ma as high Sm/Yb magmas erupted on the hinge of the shallowing slab to the south. The seismic images in this region show a sharp transition to a low Qp anomaly at 50 to 100 km in the southernmost CVZ arc at 27°S that shallows to 〈 50 km at 26.5°S in a region of a shallow seismic swarm. The underlying high Qp anomaly above the slab can be interpreted as a subduction channel. Further north, intermediate Qp velocities occur above the slab. In the backarc, the chemistry and distribution of the giant 6 to 2 Ma Cerro Galan ignimbrite eruptions and the 〈 7 Ma mafic calc-alkaline and intraplate like mafic lavas have been attributed to delamination of thickened mafic crust as a shallow subduction zone steepened after 7 Ma. The seismic tomographic images support this interpretation with Qp images in the Cerro Galan region showing an ~ 100 km long low Qp velocity anomaly to a depth of ~100 km and an ~ 100 km long west dipping high Qp anomaly at a depth of 100 to 150 km just to the east being interpreted as the delaminated crust and lithosphere. A seismic swarm at 〈 15 km beneath Cerro Galan supports the low Qp anomaly and receiver functions confirm high crustal velocities at depths of 〉75 km. A low Qp crustal to mantle anomaly to the south in the most concentrated region of young intraplate-like mafic lavas supports decompression mantle melting above an expanded mantle wedge.

Description: Earthquake hypocenters recorded in the Andean Southern Puna seismic array (25–28°S, 70–65°W) provide new constraints on the shape of the subducting Nazca plate beneath the Puna plateau, the transition into the Chilean‐Pampean flat slab and the thermal state of the mantle and crust. Some 270 new mantle hypocenters suggest that the subducting slab under the Puna shoals into the flat‐slab segment more abruptly and farther to the north than previously indicated. The revised geometry is consistent with the Central Volcanic Zone Incapillo caldera being the southernmost center with Pleistocene activity until reaching the southern side of the flat‐slab region. Evidence for the revised slab geometry includes three well‐defined hypocenter clusters in the Pipanaco nest (27.5–29°S, 68–66°W), which are interpreted to reflect slab‐bending stresses. A few low‐magnitude earthquakes with strongly attenuated S waves in the long‐recognized Antofalla teleseismic gap (25.5–27.5°S) support a continuous slab under the Southern Puna. The paucity of gap earthquakes and the presence of mafic magmas are consistent with a hot mantle wedge reflecting recent lithospheric delamination. Evidence for a hot overlaying Puna crust comes from new crustal earthquake hypocenters concentrated at depths shallower than 5 km. Two notable short‐duration swarms were recorded under the resurgent dome of the ~2 Ma back‐arc Cerro Galán caldera and the near‐arc Cerro Torta dome. New crustal earthquake focal mechanisms from 17 events in the array along with two existing mechanisms have strike slip, oblique reverse, and oblique normal solutions fitting with regional E‐W compression and N‐S extension.