Observatorio San Calixto

The Central Andes is a key global location to study the enigmatic relation between volcanism and plutonism because it has been the site of large ignimbrite-forming eruptions during the past several million years and currently hosts the world’s largest zone of silicic partial melt in the form of the Altiplano-Puna Magma (or Mush) Body (APMB) and the Southern Puna Magma Body (SPMB). In this themed issue, results from the recently completed PLUTONS project are synthesized. This project focused an interdisciplinary study on two regions of large-scale surface uplift that have been found to represent ongoing movement of magmatic fluids in the middle to upper crust. The locations are Uturuncu in Bolivia near the center of the APMB and Lazufre on the Chile-Argentina border, on the edge of the SPMB. These studies use a suite of geological, geochemical, geophysical (seismology, gravity, surface deformation, and electromagnetic methods), petrological, and geomorphological techniques with numerical modeling to infer the subsurface distribution, quantity, and movements of magmatic fluids, as well as the past history of eruptions. Both Uturuncu and Lazufre show separate geophysical anomalies in the upper, middle, and lower crust (e.g., low seismic velocity, low resistivity, etc.) indicating multiple distinct reservoirs of magma and/or hydrothermal fluids with different physical properties. The characteristics of the geophysical anomalies differ somewhat depending on the technique used—reflecting the different sensitivity of each method to subsurface melt (or fluid) of different compositions, connectivity, and volatile content and highlight the need for integrated, multidisciplinary studies. While the PLUTONS project has led to significant progress, many unresolved issues remain and new questions have been raised.

Seismic anisotropy has been documented in many portions of the lowermost mantle, with particularly strong anisotropy thought to be present along the edges of large low shear velocity provinces (LLSVPs). The region surrounding the Pacific LLSVP, however, has not yet been studied extensively in terms of its anisotropic structure. In this study, we use seismic data from southern Peru, northern Bolivia and Easter Island to probe lowermost mantle anisotropy beneath the eastern Pacific Ocean, mostly relying on data from the Peru Lithosphere and Slab Experiment and Central Andean Uplift and Geodynamics of High Topography experiments. Differential shear wave splitting measurements from phases that have similar ray paths in the upper mantle but different ray paths in the lowermost mantle, such as SKS and SKKS, are used to constrain anisotropy in D″. We measured splitting for 215 same station-event SKS–SKKS pairs that sample the eastern Pacific LLSVP at the base of the mantle. We used measurements of splitting intensity(SI), a measure of the amount of energy on the transverse component, to objectively and quantitatively analyse any discrepancies between SKS and SKKS phases. While the overall splitting signal is dominated by the upper-mantle anisotropy, a minority of SKS–SKKS pairs (∼10 per cent) exhibit strongly discrepant splitting between the phases (i.e. the waveforms require a difference in SI of at least 0.4), indicating a likely contribution from lowermost mantle anisotropy. In order to enhance lower mantle signals, we also stacked waveforms within individual subregions and applied a waveform differencing technique to isolate the signal from the lowermost mantle. Our stacking procedure yields evidence for substantial splitting due to lowermost mantle anisotropy only for a specific region that likely straddles the edge of Pacific LLSVP. Our observations are consistent with the localization of deformation and anisotropy near the eastern boundary of the Pacific LLSVP, similar to previous observations for the African LLSVP.

The geometry of the Nazca plate is established on the basis of two-dimensional seismicity sections using NOAA hypocenters from 1963 to 1973. Seismicity as mapped in the region is not a true indicator of the subducted slab due to systematic errors in hypocenter location because there are no seismic stations in the Pacific Ocean. Very narrow seismicity sections, however, are shown to be effective in describing the slab's geometry. Despite the uncertainties, it is clear that the Nazca plate underthrusts the South American continent in individual, discontinuous tongue-like pieces at different dip angles. It is especially true that between latitudes 19° and 21° S, a significantly large change occurs in the direction of underthrusting, from ENE to almost E-W. Accordingly, we should expect to find a transition region somewhere between latitudes 19° and 20° S. Such a region is not detectable from the seismicity alone, however. A simple theoretical model of the collision between the continental and oceanic plates shows two systems of principal stresses: one parallel to the coastline and the other perpendicular to it. The latter system shows a conical concentration of stress lines opening toward the continent, clearly defining a change in underthrusting direction, a conical transition zone near Arica, and the small tongue-like components of the plate coincident with the seismicity data. Thus the slab seems to act as stress guide down to a depth of 250 km. East-west gravity profiles show a minimum in the Bouguer anomaly curve of the order of 300 mgal and agree very closely with the surface topography and the inflexion region of the downgoing slab, but it seems likely that the plate effect is too small to account for the observed Bouguer minimum. The tectonic pattern of Bolivia, studied by means of ERTS satellite images and correlated with the theoretical distribution of stresses, shows that faults, lineations, and other geomorphological features agree well with the two systems of lines of stress derived from the collision of the oceanic plate against the continental lithosphere. Seismicity, gravity, and tectonic data combined with the spatial distribution of mines in Bolivia show very intriguing relationships between the geometry of the slab, gravity minimum, tectonism, and ore deposits. Many questions, however, need to be answered before definite conclusions can be drawn. The ages of most host rocks seem to be too old to have any direct relationship with the present geophysical patterns, but the age of ore mineralization itself is not known. The age of the ore is the critical parameter to be established to see whether or not the mineral deposits are young enough to be attributed to the geometry of the downgoing slab.

Abstract The ratio of the vertical and horizontal components of the P phase provides a crustal transfer function which is independent of the frequency content of the source and is a function only of the angle of incidence of the rays and the crustal parameters of the site where the observation is taken. This ratio of the vertical and horizontal spectra corresponds to the tangent of the apparent angle of emergence and as such this apparent angle is a function of the frequency. Spectra of this type have been obtained in the central United States and in La Paz, Bolivia, for large magnitude teleseisms. The observed curves have been compared with theoretical universal curves corresponding to one- and two-layer models. Average results for several observations give a crustal thickness in the central United States of 42 kilometers and a mean P velocity of 6.6 kms/sec. For the Bolivian Andes at La Paz, the crustal thickness obtained is 64 kms and the mean P velocity 6.7 kms/sec. These results are in good agreement with similar determinations obtained by independent methods.