Volker Haak

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.

Why is the electrical resistivity around the KTB hole so low

The low-resistivity anomaly close to the KTB borehole coincides with both a self-potential and a static magnetic anomaly. If this coincidence is not accidental, it may yield information about the conditions required for the existence of low-resistivity anomalies in the deeper crust. A possible answer to the question in the title of this paper is that the oxygen fugacity in the crust around the KTB hole must be low enough to stabilize graphite on a grain boundary scale. This could partially explain the extremely low resistivities of the graphitized cataclastic zones, the formation of magnetic pyrrhotite from ∼ 250 to 4000 m deep and the large self-potential anomaly. The latter requires a large vertical gradient of the oxygen fugacity and a continuous graphitic conductor through this gradient zone.

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. In 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.

Modelling European magnetic repeat station and survey data by SCHA in search of time-varying anomalies

Abstract The changing geomagnetic field interacting with conductivity anomalies in the Earth’s lithosphere should lead to time-varying magnetic anomalies, which would appear as anomalies in the observed geomagnetic secular variation. We studied 50 years of spatially dense magnetic repeat station data from Europe in search for such possible secular variation anomalies. The available data are highly non-uniform in space and time, which makes modelling difficult. Further, the accuracy of much of the data is only of the same order as the expected signal. Modelling the data by spherical cap harmonic analysis (SCHA) showed satisfactory results for low spatial indices of the harmonics. Large-scale regional secular variation can thus, be represented quite well. However, including harmonics with higher spatial index to investigate possible shorter-wavelength features or, equivalently, modelling residuals of the repeat station secular variation data after subtracting a normal secular variation field, led to numerically unstable results. The main reason for this is thought to be the relatively large data errors. A definite answer as to whether secular variation anomalies exist or not cannot yet be given. Simultaneous modelling of the data in space and time might give better results. Of greater importance, though, is a more careful elimination of external field variations from repeat station data to reduce the data errors.

Did the solar eclipse of August 11, 1999, show a geomagnetic effect?

The solar eclipse of August 11, 1999, presented good conditions for the study of associated geophysical effects. Ionospheric measurements clearly show a decrease of electron density due to the reduced solar irradiation during the eclipse. However, contrary to claims elsewhere, the decreased conductivity did not cause an obvious effect in the geomagnetic recordings at the Earths surface. Recordings of several European geomagnetic observatories and of a temporary variometer network, set up specially to observe an eclipse effect in detail, have been studied directly and in terms of equivalent currents in the ionosphere. We present the results of these studies and discuss possible current configurations that might explain the lack of an eclipse effect in geomagnetic recordings.

Introduction to Special Section: The KTB Deep Drill Hole

When we pose the question “What do we want to know about the Earth?”, the answer is “Everything!” We are demanding, we must be demanding. Our life, and the very survival of our species, depends on our understanding of the Earth. As long as we do not know when earthquakes will happen, when volcanoes will erupt, and how water circulates or energy is stored or minerals are concentrated, our life is a matter of chance. Fortunately, we are curious creatures, and our curiousity leads us to observe, measure, and interpret our surroundings.

Electrical Resistivity Studies in the Vicinity of the KTB Drill Site, Oberpfalz

Measurements of the electrical resistivity distribution are becoming more and more popular in studies of the Earth’s crust. Nevertheless, causes and interpretations still often remain a matter of speculation. The deep drilling project therefore offers a unique possibility to answer problems as to the existence in particular of low resistivity zones in the crust. It was the aim of the measurements described here to “forecast” the existence of such low resistivity zones in the vicinity of the drilling site. Since the final location of the drilling site was decided after most of the measurements had been concluded, it turned out that no electrical depth sounding was done at or even within a few kilometers of the drilling site. Nevertheless these results are, hopefully, representative for the drilling site.

A magnetotelluric profile across the German Deep Drilling Project (KTB) area: Two‐ and three‐dimensional modeling results

Previous interpretations of magnetotelluric data from the vicinity of the German Deep Drilling Project (KTB) revealed two major structures: a midcrustal layer of increased conductivity and large, regional extent and a highly anisotropic upper crust. Nevertheless, a satisfactory combination of both structures explaining all measurements has not yet been achieved, mostly due to incomplete and qualitatively poor data. Simplified superposition of both structures could not yield an explanation of the observations. On the basis of a carefully processed new data set we apply different modeling approaches to verify the existence of both structures. Models calculated with a two-dimensional modeling program, which allows for general anisotropy, as well as a full three-dimensional code show that an anisotropic upper crust is overlaying a regional east-west striking high-conductivity structure. Nevertheless, this continuous conductive midcrustal layer with a conductance decreasing from north to south must be replaced by a quasi-anisotropic one, at least in the region of the KTB. The final model may still be oversimplified, considering the complexity of the true but unknown geology in this particular area, but it demonstrates which major electrically effective structures could be resolved.

Interpretation of CHAMP Crustal Field Anomaly Maps Using Geographical Information System (GIS) Technique

Crustal field models from CHAMP magnetic measurements are increasingly stable and reliable. In particular, they now allow for quantitative geological studies of crustal structure and composition. Here, we use a forward modeling technique to infer deep crustal structure of continental regions overlain by younger sediments. For this, a Geographical Information System (GIS) based technique has been developed to model the various geological units of the continental crust. Starting from geologic and tectonic maps of the world and considering the known rock types of each region, an average magnetic susceptibility value is assigned to every geological unit. Next, a vertically integrated susceptibility (VIS) is computed for each unit, taking into account the seismic crustal thickness, as given by models 3SMAC and CRUST2.1. From this preliminary VIS model, an initial vertical field anomaly map is computed at a satellite altitude of 400 km and compared with the corresponding CHAMP vertical field anomaly map. We demonstrate that significant geological inferences can be made from the agreement and the discrepancies between the predicted and observed anomaly maps. In particular, the lateral extent of Precambrian provinces under Phanerozoic cover is revealed.

Electrical double-dipole experiment in the German Continental Deep Drilling Program (KTB)

Among the most important rationales to drill the German Continental Deep Drilling Program (KTB) borehole was the necessity to calibrate geophysical methods. Deep and hitherto inaccessible seismic reflectors, high-conductivity layers, and temperature belong to this group of deep crustal properties which can be predicted from surface measurements, but whose depth and nature are a matter of dispute. One problem is the unknown influence of inhomogeneous superficial layers on the determination and resolution of the model parameters. In the case of electrical resistivity a number of presite experiments had detected a high-conductivity layer of regional extent at a mean depth of ∼10 km. Distorting superficial layers were expected to cause severe ambiguity in the interpretation of the specific properties of this layer, even feigning its existence at all. The drilling yielded direct evidence of high-conductivity material within the range of 8 km depth. After completion of the KTB a large-scale dipole-dipole experiment was carried out using a vertical electric receiver dipole with one of the electrodes in the main drill hole at 9065 m depth and a second in the earlier drill hole at 4000 m depth. The idea was to find out whether the buried electrode was close to a high-conductivity layer of regional extent. The surprising result was that the two apparent resistivity curves measured with the transmitter spread perpendicular and parallel to the NNW striking very highly conductive fracture zones are almost overlapping, even though these fracture zones are the cause of a strong structural anisotropy of the apparent resistivity measured with magnetotellurics. Such a strong anisotropy should also show up in the buried electrode experiment except when a high-conductivity layer close but above the buried electrode at 9000 m depth is introduced in the model, As a result, the interpretation of this experiment suggests a NE dipping electrically conductive fault system soling out into a high-conductivity horizontal layer at 7–8 km depth. The conductivity is increased due to graphite and high-salinity fluids, in a depth near the fossil Cretaceous brittle-ductile transition zone for quartz-rich rocks.