José G. Viramonte

Young mafic back arc volcanic rocks as indicators of continental lithospheric delamination beneath the Argentine Puna Plateau, central Andes

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 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 0.7055) and Pb and nonradiogenic Nd ( eNd < −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.

Variation in the Crustal Structure of the Southern Central Andes Deduced from Seismic Refraction Investigations

A net of mainly reversed seismic refraction profiles has been measured in the years 1987 and 1989 in northern Chile, northern Argentina and southern Bolivia to investigate the crustal structure beneath the Andes from the coastal range to the Andean foreland. Regular blasts of the Chuquicamata copper mine, sea shots in the Pacific Ocean and land shots in Chile, Argentina and Bolivia were used as energy sources. One- and two-dimensional model calculations were applied to the data. Strong west-east as well as north-south variations in the crustal structure are observed which allow one to distinguish mainly three different crustal blocks.

UPPER CENOZOIC MAGMATIC EVOLUTION OF THE ARGENTINE PUNA—A MODEL FOR CHANGING SUBDUCTION GEOMETRY

The spatial and temporal distribution and chemistry of Late Oligocene to Recent Central Andean Puna volcanic rocks can be broadly explained by temporal changes in the dip of the subducting Nazca pl...

Large ignimbrite eruptions and volcano-tectonic depressions in the Central Andes: a thermomechanical perspective

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 km3 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.

The Cretaceous paleorift in northwestern Argentina: A petrologic approach

Abstract The development of the Cretaceous-early Eocene basin of northwestern Argentina can be divided into three main magmatic phases on the basis of preliminary petrologic data. The oldest phase (130-100 Ma) is divided into an early stage of anorogenic plutonism, with subalkaline, alkaline, and minor peralkaline granitic intrusives, and a volcanic stage in which alkaline rocks characterized by trachytes, basanites and foidites prevail. The second phase (80-75 Ma) is characterized by an alkaline suite with basanites, hawaiites and tephriphonolites. The last phase (65-60 Ma) consists of lamproitic sills and basic lava flows. The two first phases correspond, respectively, to prerifting and initial rifting stages. According to the magmatic, tectonic, and sedimentary features observed, it is suggested that this basin is a foreland paleorift, of low volcanicity type, that developed along the western side of South America from the Early Cretaceous to the Eocene—at which time the rift basin was closed by the Incaic diastrophic phase.

Cretaceous rift related magmatism in central-western South America

Abstract The Cretaceous–Paleocene Andean basin system of central-western South America, comprises northwestern Argentina and southwestern Bolivia. It is situated between 62°–68°W and 18°–27°S, but extends westward to northern Chile and northward to Bolivia and Peru. These basins have been interpreted as an aborted foreland rift. In a general sense, it may be possible to relate this rift to the opening of the South Atlantic Ocean, however it was directly associated, in a backarc position, with the subduction of the Nazca Plate below the South American Plate. Three main magmatic episodes were recognized: the pre-rift stage (130–120 Ma) which is characterized by an early phase of anorogenic plutonism, with subalkaline and alkaline granitic intrusives; the syn-rift volcanic episode which started with a mainly alkaline volcanic activity (110–100) in which alkaline rocks prevail; a second more voluminous volcanic episode (80–75 Ma) which is characterized by an alkaline suite represented by basanites and tephriphonolites; and the last volcanic episode (65–60 Ma) which consists of lamproitic sills and basic K rich lava flows. Petrography, chemistry and chronology of the Cretaceous plutonic bodies indicate anorogenic pre-rift related A-type granite complexes closely related to the further evolution of the Cretaceous rift basin. The petrology and geochemistry of the Cretaceous volcanic rocks show strong alkaline affinities and suggest a similar rift-related origin. The geochemical and isotopic characteristics of the alkaline basalts suggest that they originated through low degrees of partial melting of a depleted mantle subcontinental lithosphere which was previously enriched by processes such as the introduction of veins rich in amphibole, high Ti phlogopite, and apatite. Cretaceous plutonic and volcanic rocks from central-southwestern South America are related to an intracontinental rift environment and although their ages are correlative with those of the Parana volcanic province, their petrology, geochemistry and isotopic compositions reveal different source regions and petrogenetic processes.

Long-Term Signals in the Present-Day Deformation Field of the Central and Southern Andes and Constraints on the Viscosity of the Earth’s Upper Mantle

As part of the South American Geodynamic Activities project we observed the present day deformation field in the territories of Chile and Argentina using the Global Positioning System. The results clearly show that the earthquake cycle dominates the contemporary surface deformation of the central and southern Andes. Compared to geological timescales, the transient elastic deformation related to subduction earthquakes presents a short-term signal which can be explained by interseismic, coseismic, and postseismic phases of interplate thrust earthquakes. We constructed the Andean Elastic Dislocation Model (AEDM) in order to subtract the interseismic loading from the observed velocities. The estimated parameters of the AEDM, and the amount and depth of coupling between the subducting Nazca and overriding South American Plates, represent long-term features and show that the seismogenic interface between both plates is fully locked and that the depth of coupling increases from north to south.

Crustal Evolution at the Central Andean Continental Margin: a Geochemical Record of Crustal Growth, Recycling and Destruction

Active continental margins are considered as the principal site for growth of the continental crust. However, they are also sites of recycling and destruction of continental crust. The Andean continental margin has been periodically active at least since the early Paleozoic and allows the evaluation of the long-term relevance of these processes. The early Paleozoic orogeny at ca. 0.5 Ga recycled and homogenized the ∼2 Ga old early Proterozoic crust of the Brazilian Shield, which was previously orogenized at ca. 1 Ga, consistent with global models of prominent crustal growth at 2 Ga and a near constant mass of continental crust in the Phanerozoic. The metamorphic and magmatic evolution and the isotopic signatures of the early Paleozoic rocks do not indicate significant crustal growth, either by accretion of exotic terranes and island arcs or by juvenile additions from a mantle source. The dominant inferred mode of crustal evolution in the Paleozoic was recycling of older crust. Destruction of continental crust by subduction erosion is prominent in sections of the present active margin and is also likely to have occurred in the past orogens. Voluminous juvenile magmatism is only observed in the Jurassic — lower Cretaceous extensional magmatic arc. Compositions of mantle-derived magmas from the early Paleozoic to the Cainozoic, as well as late Cretaceous mantle xenoliths, indicate that depleted mantle was already present beneath the early Paleozoic orogen. The old subcontinental, enriched mantle related to the Brazilian shield and bordering Proterozoic mobile belts was modified by asthenospheric mantle in the younger subduction systems. In summary, this transect of the Andes is not a site of major continental growth, but a site where long-term processes of growth, recycling and destruction balance out.

Seismological Studies of the Central and Southern Andes

The central Andes have formed by the complex interaction of subduction-related and tectonic processes on a lithospheric scale. The deep structure of the entire mountain range and underlying subduction zone has been investigated by passive and active seismological experiments. Detailed tomographic features are interpreted to represent the ascent paths of fluid and melts in the subduction zone and provide new insights about the mechanisms of lithospheric deformation. Receiver functions from teleseismic events have been used to observe the upper-plate continental Moho and subducted oceanic Moho, as well as the interaction of subducted oceanic lithosphere and mantle discontinuities. A second working area was established in the southern Andes to compare two different types of Andean subduction and to identify the principal controlling parameters. Besides the first accurate definition of the Wadati-Benioff zone in south-central Chile, a three-dimensional, tomographic velocity model based on local earthquakes in the southern Andes is presented.

Large-scale silicic volcanism - The result of thermal maturation of the crust

Shanaka de Silva, Department of Space Studies, John D. Odegard School of Aerospace Sciences, University of North Dakota, Grand Forks, ND 58202-9008, USA George Zandt, Department of Geosciences, University of Arizona, Tucson, AZ, 85721-0077, USA. Robert Trumbull, GeoForschungZentrum Potsdam, Telegrafenberg, P.B. 4.2, 14473, Potsdam, Germany. Jose Viramonte, Instituto GEONORTE and CONICET, Universidad Nacional de Salta, Buenos Aires 177, Salta 4400, Argentina.