Gerhard Schwarz

Partial melting below the magmatic arc in the central Andes deduced from geoelectromagnetic field experiments and laboratory data

Abstract Magnetotelluric and geomagnetic deep soudings in northern Chile revealed a pronounced high conductivity zone (HCZ). Below the Western Cordillera, which constitutes the present magmatic arc with active volcanism of the South American continental margin, conductivities in the range of 1 S/m are observed. The anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km, are interpreted in terms of partial melting. Other geophysical observations, such as a zone of low seismic velocities (LVZ) at similar depths, high heat flow values (> 100 mW/m2) and a pronounced negative anomaly in the residual gravity field, are also considered. Impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities. At sub-solidus temperatures, electrical behavior is described by defect electrons with an activation energy of 1.34 eV and a conductivity of 2.5 mS/m at 900°C. Model calculations using a modified-brick-layer model (MBL) were compared with experimental observations. A good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 S/m at 1250°C assuming an activation energy of 1 eV. The same MBL model is used to calculate melt proportions beneath the Western Cordillera. Between 14 and 27 vol.% of interconnected melt are necessary to explain the observed HCZ. The stability of the melt rich crust is explained by a dynamic melting-crystallisation behavior during crustal anatexis and by magma filled dikes.

Electrical conductivity of the earth's crust and upper mantle

This review summarizes recent results of electrical resistivity studies of the earths crust and upper mantle. Where available, the data are discussed in the context of further regional geophysical information. Electrical resistivity is very sensitive to a wide range of petrological and physical parameters, e.g., to carbon, fluids, volatiles and enhanced temperatures, making electrical resistivity methods a powerful tool in crust and upper mantle investigations. Yet, the general increase in resistivity data of the crust and mantle has not ended the battle of explanations for ‘anomalous’ crustal conductivities.

Resistivity cross section through the southern central Andes as inferred from magnetotelluric and geomagnetic deep soundings

To investigate the upper lithospheric structure and subduction-induced processes across an active continental margin, magnetotelluric (MT) and geomagnetic deep sounding (GDS) experiments were done in the southern central Andes (21°–25°S) of northern Chile, southern Bolivia and northwestern Argentina. Using two-dimensional modeling, we have constructed an E-W resistivity cross section at latitude 21°45′S that involves all the Andean units from the Pacific coast to the lowland plains. The crust of the Coastal Cordillera has high resistivities of more than 5000 ohm m with some conductive structures embedded. Increased electrical conductivities between depths of 8 and 40 km may be related to the rise of fluids released from the subducted Nazca plate. The electrical resistivity of the ‘normal Andean’ crust seems to be low, having values of only 200 ohm m. A wide zone of extremely high electrical conductivity with an E-W extent of more than 330 km was detected. This high- conductivity zone (HCZ) is found at a depth of about 25 km under the Western Cordillera and has a total conductance of more than 23,000 S. Here, it may be caused by partially melted crust, an interpretation which would correlate with those from gravity and seismic data. The HCZ can be followed in the lower crust eastward below the Altiplano where it has a total conductance of about 15,000 S. In the western part of the Eastern Cordillera, total conductance first increases and then drops abruptly at about longitude 65°10′W. The latter HCZs can be explained in terms of thrust tectonics, where detachment zones may involve fluids as well as increased mineralizations. The lowlands of the sub-Andean and the Chaco are characterized by a cover of anisotropic low resistivity. The resistivity of the crust and uppermost mantle increases from west to east to more than 1000 ohm m, while the deeper mantle gets unexpectedly much more conductive.

ELECTROMAGNETIC STUDY OF THE ACTIVE CONTINENTAL MARGIN IN NORTHERN CHILE

Magnetotelluric and geomagnetic deep sounding measurements were carried out in the magmatic arc and forearc regions of northern Chile between 19.5° and 22°S to study the electrical conductivity structures of this active continental margin. The instruments used covered a very broad period range from 10−4 s to approx. 2 × 104 s and thus enabled a resolution of deep as well as shallow structures. n nIn this paper we focus on the interpretation of data from an east-west profile crossing Chile from the Pacific coast to the Western Cordillera at 20.5°S. A decomposition of the impedance tensors using the Groom-Bailey decomposition scheme shows that a two-dimensional interpretation is possible. The resulting regional strike direction is N9°W. Two-dimensional models were calculated in this coordinate frame and include the significant bathymetry of the trench as well as the topography of the Andes. The final model shows a generally high resistivity in the forearc and a very good conductor below the Precordillera. Unlike earlier models from areas further south, a good conductor is not observed below the magmatic arc itself. This correlates with the so-called Pica gap in the volcanic chain and a higher age of volcanic activity compared with adjacent areas.

Crustal High Conductivity Zones in the Southern Central Andes

Magnetotelluric (MT) and geomagnetic deep sounding (GDS) experiments were performed in the southern Central Andes from 1982 to 1989. There is evidence for high conductivity zones (HCZ) at crustal depth beneath parts of the Central Andes. MT data give quantitative results: a zone of extremely high electrical conductivity (total conductance of 20000 to 30000 Siemens) at shallow depth beneath the volcanic belt of northern Chile strikes about NNW-SSE, running into northwestern Argentina. The eastern border of the HCZ is found in the Eastern Cordillera of southern Bolivia and probably northwestern Argentina, where total conductance of the upper crust reaches more than 10000 S. The area in between both Cordilleras, the Altipiano, shows besides a well-conducting cover an increase in conductivity at much greater depth, i.e., at about 40 km in its western part and at about 20 km in the east (total conductance > 10000 S). The HCZs beneath the Andes have quite different origins and do not strictly correlate with the known structural units. They can be classified not only in their E-W but also in their N-S extent. The HCZs correlate with some other geophysical and geological features and may reflect the intense tectonic and magmatic evolution of this mountain belt.

Probing electrical conductivity of the Trans-European Suture Zone

The Trans-European Suture Zone (TESZ) is the largest tectonic boundary in Europe, crossing northwest-southeast through central Europe from the North Sea to the Black Sea. More than 2000 kilometers long, it constitutes a complex transition between the thick and cold East European Craton (EEQ/Baltic Shield, created more than 650 million years ago (Ma) during the Precambrian, and the warmer, younger Paleozoic (650 to 250 Ma) central European mobile belts.

Electrical conductivity studies in the Travale geothermal field, Italy

Abstract Magnetotelluric and geomagnetic depth soundings have been carried out in the area of the Travale high enthalpy geothermal field (central Tuscany, Italy) in 1980 and 1981 to study the distribution of electrical conductivity in the geothermal anomaly and the crust beneath. Within this project the possible contributions of electromagnetic investigations to geothermal research were to be tested and a geothermal model of the Travale area was to be developed. The time-varying electric and magnetic fields have been recorded in a broad period range from 6–10,000 s, mainly on two profiles, the one parallel, the other perpendicular to the Travale graben. Strong lateral variations of apparent resistivities have been observed. Up to periods of 50–100 s the Travale graben is the dominating 2-D structure, but for longer periods of investigation the three-dimensionality of electrical conductivity structures has to be considered. The apparent resistivities inside the geothermal anomaly are extremely low, reaching not more than 50 ohm · m, even in the lower crust, but they increase to 100–300 ohm · m north of the geothermal field. Total conductance also indicates the geothermal field as a local conductivity anomaly, whereas further to the north the poorly conducting “barrier” has been confirmed. The cause of the high conductivity structures inside the geothermal area is to be seen in a highly fractured basement within this zone, allowing upward movement of hydrothermal fluids.

Three-Dimensional Electrical Resistivity Image of the Volcanic Arc in Northern Chile—An Appraisal of Early Magnetotelluric Data

Magnetotelluric investigations were carried out in the late 1980s across all morphological units of the South American subduction zone with the aim to observe lithosphere structures and subduction-induced processes in northern Chile, southwestern Bolivia, and northwestern Argentina at ~xa022°S. Earlier two-dimensional forward modeling yielded a complex picture of the lower crust and upper mantle, with strong variations between the individual morphological units as well as between forearc and backarc. The principal result was a highly conductive zone beneath the volcanic arc of the Western Cordillera starting at ~xa025xa0km depth. Goal of this work is to extend the existing 2-D results using three-dimensional modeling techniques at least for the volcanic arc and forearc region between 22°S and 23°S in Northern Chile. Dimensionality analysis indicates strong 3-D effects along the volcanic arc at the transition zone to the Altiplano, in the Preandean Depression and around the Precordillera Fault System at ~xa022°S. In general, the new 3-D models corroborate previous findings, but also enable a clearer image of lateral resistivity variations. The magmatic arc conductor emerges now as a trench-parallel, N–S elongated structure slightly shifted to the east of the volcanic front. The forearc appears highly resistive except of some conductive structures associated with younger sedimentary infill or young magmatic record beneath the Precordillera and Preandean Depression. The most prominent conductor in the whole Central Andes beneath the Altiplano and Puna is also modeled here; it is, however, outside the station array and thus poorly resolved in this study.