Peter Giese

Subduction and collision processes in the Central Andes constrained by converted seismic phases

The Central Andes are the Earths highest mountain belt formed by ocean–continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovičić discontinuity—generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro–eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10–20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

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.

The Variscan Belt in Central Europe: Main structures, geodynamic implications, open questions

Abstract The Variscan Belt of Europe originated from the confrontation of northern Europe and Gondwana, with intervening pre-Variscan blocks (Cadomian and older: Armorica, Tepla-Barrandean, Moravo-Silesian). Though a strike-slip component cannot be excluded, geological evidence suggests a subduction/collision process. Newly discovered features include basaltic rocks with MORB affinities, high-pressure metamorphic rocks, trench deposits, and large-scale allochthonism, which also affects the pre-Variscan basement, and has often led to an inversion of the metamorphic isogrades. Large-scale thrusting is evident at all crustal levels, and is also observable by geophysical methods. The structure of the Variscan crust is best described as horizontal-tectonic layering. Late Ordovician through Carboniferous convergence closed one Rhenohercynian and one Saxo-thuringian basin to the north, and at least one Mediterranean basin to the south of the pre-Variscan blocks. Subduction occurred from the north as well as from the south down under the dorsal core region. The exact timing and the driving mechanism of this bilateral activity remains a problem. Likewise uncertain are the exact limination of the pre-Variscan blocks, and the existence and width of oceanic areas. In the Saxothuringian Zone and in the south-facing part of the Moldanubian Zone, there is good evidence of early Palaeozoic rifting and of late Ordovician through Carboniferous plate convergence. Palaeomagnetic and palaeoclimatic constraints, however, suggest only a limited extent of oceanic basins in these areas, while they appear to allow a wider ocean at the Rhenohercynian-Saxothuringian boundary, which is relatively unobtrusive from the geological point of view. Research during the past decade has produced a wealth of information on the Variscan Fold belt in Europe. The resulting geodynamic models, however, are extremely diverging. Evidently, it is not the time yet for definitive statements. The present paper intends to review the state of knowledge, to deduce such basic items of a geodynamic model as appear reasonably well documented, and to point out the most urgent open questions. Stress will be laid on Central Europe, which offers a complete cross section through the northern flank of the Variscan Belt, and where we have carried out most of our studies during the last years.

Lithospheric split in the descending plate: Observations from the Northern Apennines

Abstract A NE-plunging extension of the Corsican continental crust beneath the Northern Apennines or Adriatic plate seems to contradict SW-dipping subduction during the Apenninic orogeny. However, such an antithetic crustal structure can be generated if the upper lithosphere i.e. the crust, of the descending (Adriatic) plate is progressively split from the continuously sinking lower lithosphere. Other examples in the Alps reveal that synorogenic detachment of crustal portions of the descending (European) plate seems to be an important additional feature of continental collision.

Partial Melting in the Central Andean Crust: a Review of Geophysical, Petrophysical, and Petrologic Evidence

The thickened crust of the Central Andes is characterized by several first-order geophysical anomalies that seem to reflect the presence of partial melts. Magnetotelluric and geomagnetic deep-sounding studies in Northern Chile have revealed a high conductivity zone (HCZ) beneath the Altiplano Plateau and the Western Cordillera, which is extreme both in terms of its size and integrated conductivity of > 20000 Siemens. Furthermore, this region is characterized by an extremely high seismic attenuation and reduced seismic velocity. The interrelation between the different petrophysical observations, in combination with petrological and heat-flow density studies, strongly indicates a huge area of partially molten rocks that is possibly topped with a thin, saline fluid film. The average melt fraction is deduced to be ∼20 vol.%, which agrees with typical values deduced from eroded migmatites. Based on the distribution and geochemical composition of Pliocene to Quaternary silicic ignimbrites in this area, this zone is thought to be dominated by crustally-derived rhyodacite melts with minor andesitic contribution. An interconnected melt distribution — typical for migmatites - would satisfy both the magnetotelluric and seismic observations. The high melt fraction in this mid-crustal zone should lead to strong weakening, which may be a main cause for the development of the flat topography of the Altiplano Plateau.

Seismic Images of Accretive and Erosive Subduction Zones from the Chilean Margin

Modern seismic imaging methods were used to study the subduction processes of the South American convergent margin. The data came from reflection and from wide-angle/refraction experiments acquired within the framework of the Collaborative Research Center SFB267 ‘Deformation Processes in the Andes’. Two areas of differing character and subduction type were investigated: an erosive margin to the north (19–26° S) and an accretionary margin to the south (36–40° S). Results from different seismic models yield three main transects that give an overall impression about the internal structure below the Chilean margin. At the erosive margin, we find that the upper part of the subducting oceanic lithosphere is characterized by a horst-and-graben structure that coincides with the coupling zone between the plates. Strong coupling between oceanic crust and fore-arc in the case of a horst-continent collision is also indicated by plate-parallel faults beneath the lower continental slope, which we interpret as the upper parts of the subduction channel. In this context, the subduction channel represents the downgoing Nazca Plate as well as those portions of the continental crust which moved landward. Low seismic velocities below the coastline also represent parts of the subduction channel and of the hydrofractured base of the upper crust near the plate interface. Between 45 and 60 km depth, a double reflection zone marks the upper and lower boundary of the subducted oceanic crust. Off southern Chile, the ocean bottom is characterized by relatively smooth morphology. In contrast, in the south, the trench is filled with sediments and contains an axial channel (Figs. 7.16 to 7.18) extending in N-S direction along the trench axis within the investigation area. The periodicity of the reflected seismic signal within these sediments correlates with the main glacial cycle during the Quaternary. The recent accretionary wedge is built up from strongly heterogeneous unconsolidated sediments. Frontal accretion takes place within the southern working area except for the region around the Arauco Peninsula, which shows uplift due to basal accretion and antiformal stacking. Below the Coastal Cordillera, the heterogeneity of the modern accretionary wedge and the antiformal stack structure of the Permo-Triassic accretionary wedge complicate imaging at depths greater than about 30 km. Thus, we obtain an image of the top of the subduction channel as a thin reflector segment only to about 25 km depth.

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.

Architecture of the Central Andes—a compilation of geoscientific data along a transect at 21°S

Abstract Based on numerous geoscientific data a section through the Central Andean active continental margin at 21°S has been compiled which shows the structure of the South American upper plate and the downgoing Nazca Plate.

The Salar de Atacama Basin: a Subsiding Block within the Western Edge of the Altiplano-Puna Plateau

The internally drained Salar de Atacama (SdA) Basin, located in the proximal fore-arc between the present magmatic arc (Western Cordillera) to the east and the North Chilean Precordillera (Cordillera de Domeyko) to the west, represents a prominent morphological anomaly in the Central Andean Plateau. The basin is a post-Incaic feature that developed contemporaneously with the initial plateau uplift. Before 38 Ma, the magmatic arc was positioned in the present-day Precordillera; as a result, the Cretaceous to Eocene sequences that underlie younger SdA sediments were deposited in a proximal back-arc location, where a westward extending arm of the Salta Rift interfered with the magmatic arc.

Geothermal Structure of the Central Andean Crust — Implications for Heat Transport and Rheology

This study aims to determine the central Andean geothermal structure by downward continuation of the surfcace temperature field. The heat flow density values along a trans-Andean profile show a minimum of 40 mW/m2 in the region of the Coastal Cordillera and Chaco and a maximum of 80 mW/m2 in the Western Cordillera and the western Altipiano. The necessary structural data are provided by deep seismic sounding measurements. The petrophysical parameters used for this study are taken from the literature. Steady state and one-dimensional conditions and pure conductive heat transport are assumed. For the region with heat flow values between 40 and 50 mW/m2 the temperature at the base of the crust does not exceed 700 °C. But in the zones with high heat flow density values of 60–80 mW/m2 the temperature in the lower crust reaches values exceeding 1000 °C. Such high temperatures seem to be unrealistic. In order to lower the temperature gradient an additional advective heat transfer is postulated. For the advecting material (fluids and volatiles) a Darcy velocity of about 1 mm/year results. For comparison the amount of upstreaming fluids and volatiles released by the downgoing oceanic crust is determined. This rough estimation yields the amount of fluids pervading the hanging lithosphere and is in the order of 0.3 mm/year. Because most parameters used for these calculations are only estimates the result may vary by a factor of 0.2–5 Finally the rheological response has been determined as the maximum possible stress as a function of depth for the different heat flux locations and a strain rate of 710-15 s-1.