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|Amphibious magnetotellurics at the South-Central Chilean continental margin|
|Haupttitel||Amphibious magnetotellurics at the South-Central Chilean continental margin|
|Titelvariante||Amphibische Magnetotellurik am zentralen Teil des südchilenischen Kontinentalrandes|
Geburtsort: Ratibor, Polen
|Gutachter||Prof. Dr. Shapiro|
|weitere Gutachter||Prof. Dr. Kaufmann|
|Freie Schlagwörter||Andes; Marine magnetotellurics; Kramers-Kronig; Anisotropy|
|Zusammenfassung||In the austral summer of 2004-2005, the South-Central Chilean continental margin became the subject of an amphibious magnetotelluric experiment to image the conductivity distribution of the subduction zone, performed within the multidisciplinary TIPTEQ project (from The Incoming Plate to mega-Thrust EarthQuake processes). The enhanced conductivity is diagnostic of the presence of fluids, i.e water or partial melts. Magnetotellurics enables to assess qualitatively and quantitatively fluid volumes in the Earth's interior, and thus applies as an appropriate approach for investigating subductions zones, in which fluids play a key role, as a trigger and a controlling factor of rupture evolution, and earthquake nucleation, as well as partial melting reactions and volcanic activity.
The magnetotelluric data were collected at several on- and offshore sites predominantly aligned along a transect, running over 400km across the main geological and morphotectonic units. It extended from the backarc up to the Pacific coast on the land side, and beyond the trench on the sea side, and complemented previous measurements from 2000 in this area in South Chile.
A complete image of the conductivity distribution associated with dehydration and melting processes can only be provided if the data are acquired on both the sea and land side of the trench. This is because the fluids in the continental plate are electrically coupled with the ocean through the subduction zone via connecting pathways which can only be resolved by measurements in the sea.
Whilst the terrestrial magnetotelluric approach is well established for imaging electrical conductivity structures deep in the Earth's interior, electromagnetic investigation in marine environments does not permit a direct application of the terrestrial approach to offshore measurements with respect to acquisition, analysis, and interpretation of offshore data, due to the special conditions at the ocean bottom.
The highly conductive sea water acts like a low pass filter, and causes, at short periods, strong attenuation of the electric, and even more, of the magnetic field. Towards long periods the decay of the electric and magnetic fields is significantly different. The electric field penetrates the ocean layer from surface to the seafloor nearly unchanged, while the magnetic field experiences a strong decay, and reaches the ocean bottom with just a fraction of its surface value. However, the field decay depends strongly on the resistivity contrast between the ocean and the seafloor, and both fields approximate the decay that would be observed in a homogeneous half space if the resistivity of the basement decreases.
The induction process on the seabed is significantly affected by the seafloor relief. Even a gently changing bathymetry generates preferentially in the overlaying highly conductive ocean layer an enhanced concentration of electric currents flowing along the slope, and above instead of below the sensing points. At a continental margin, where the seafloor shallows towards land, the accumulated currents in TE mode induce an anomalous, and on the seafloor, oppositely directed magnetic field, that becomes predominant over the attenuated normal field. The effect of the secondary field on the impedances is manifested in TE mode by cusps in the apparent resistivities, accompanied by phases exceeding the "normal" quadrant. Unlike TM mode, the offshore apparent resistivities and phases are disturbed by strongly changing bathymetry, and the resulting concentration of electric currents, and cannot be linked together via Kramers-Kronig dispersion relations as can be the causal (analytical) responses at the surface.
The dimensionality analysis of the onshore impedances revealed predominantly a regional skew value below 0.3, which suggests a two-dimensional conductivity distribution, and justifies a 2-D approach and interpretation of the data. The estimated conductivity strike direction matches the roughly N-S striking land-sea boundary and is nearly perpendicular to the MT profile.
The isotropic 2-D modeling along the new, northern (TIPTEQ) profile, and renewed modeling along the central and southern profiles reveals a consistent conductivity image, with several zones of enhanced conductivity above the subducted Nazca Plate in the region of the volcanic arc, as well as in the forearc and backarc.
Although the resolved structures appear geologically plausible, and the model responses fit the magnetotelluric impedances quite well, inspection of the geomagnetic transfer function indicates that the 2-D approach does not exploit the whole information content inherent in the data. Neither 2-D nor 3-D isotropic approaches could explain the behavior of the real induction vectors, which point, at long periods (>1000 s), at all sites throughout the study area systematically NE, irrespective of the nearly N-S direction of the conductivity strike as determined by the impedances. It is thus obvious that the isotropic images do not express the "complete" conductivity distribution at the South-Central Chilean subduction zone. On the other hand, such a pattern of uniformly deflected induction arrows coupled with the fact that conductors can also arise in a model when if attempting to model anisotropic data by an isotropic approach, may be diagnostic of anisotropic properties of subsurface.
Indeed, geologically realistic models, which satisfactorily reproduce this behavior, are provided by the anisotropic approach, and are compatible with the conductivity distribution derived from the isotropic approach. They consist mainly of two crucial structures: the Pacific Ocean, and a structural anisotropy in the lower continental crust. This structural anisotropy might be associated with a deeply fractured crust, caused by subduction-related stress. The NE-SW anisotropic strike direction coincides with the distribution of the minor volcanic centers in the volcanic arc, and with the alignment of the feeder dikes underneath. While in the volcanic arc, partial melts, ascending through a pattern of fractures, might account for the preferred conductivity along the anisotropic strike, as well as for zones of enhanced conductivity revealed by the isotropic approach, in the forearc, water, released from the subducted plate and migrating upward through the fractured upper plate, might better explain the modeled isotropic and anisotropic features. Alternatively, solidified magma intrusions in pre-existing structures, enriched with metallic phases, can also explain, at least partly, the modeled enhanced conductivities.
Further zones of high conductivity are suggested by the behavior of induction vectors derived from a supplementary 3-D study around the active Villarrica volcano. The model derived from 3-D forward consists of the Pacific Ocean, a conductive anomaly directly beneath Villarrica, a conductive structure below the volcanoes Quetrupillan and Lanin, and a regional feature running NE-SW in accordance with conductive lineaments, which provide the strike direction in the anisotropic models. The modeling fits the geomagnetic transfer functions quite well at periods >10 s, but might be too simple to satisfactorily explain the apparent resistivities and phases. A more complex conductivity distribution can be provided by a denser network of sites measuring at short periods (in the AMT range) and by a 3-D inversion approach.
PDF-Datei von FUDISS_thesis_000000024773
|Seitenzahl||II, 155 S.|
|Tag der Disputation||12.07.2011|
|Erstellt am||22.08.2011 - 11:14:47|
|Letzte Änderung||30.08.2011 - 12:27:52|