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Subproject G4: Fluid- and melt-related properties of the Andean lithosphere in the image of electromagnetic methods

Research Field: Geophysics, Magnetotellurics

Staff

Project-Leader(s)

Dr. Heinrich Brasse

Prof. Dr. Volker Haak

Members

Dr. Yuguo Li

Working area of the subproject G4

g4_working_area

Fig. 1: Working area of project G4

In the past funding periods of the subproject several large electrical conductivity anomalies were detected in the middle and lower crust of the Central Andes. A) At 20?-21?S a midcrustal anomaly is associated with the West Fissure in the North Chilean forearc and interpreted as a pathway for deep crustal saline fluids possibly originating from the downgoing slab of the Nazca plate. B) Also the Atacama Fault near the coast is correlated with enhanced conductivities from depths of several km to midcrustal levels. C) At the same latitude the Bolivian Altiplano is characterized by one of the largest high conductivity zones (HCZs) ever found by electromagnetic measurements. D) Further south at 22?-24?S the active volcanic arc is associated with a deep crustal HCZ; it vanishes towards the north, which correlates with a decrease of seismic absorption and an increasing age of volcanism in the so-called Pica gap.

All these partly spectacular anomalies map zones of rheological weakness of the Andean lithosphere. While partial melts may be excluded as an interpretation of the forearc HCZs, this assumption cannot be ruled out in the arc and plateau regions due to the (known or modeled) temperature regime, which provides the necessary conditions for melting of wet granitic rocks. A largely acidic composition of the middle and deep crust of the Bolivian Altiplano was deduced from seismological observations, and low seismic velocities also led to the assumption of a partially molten midcrustal layer. There exist several young ignimbrite calderas on the Altiplano-Puna Plateau, where geochemical considerations may imply large, still partially molten volumes remaining in the crust. These areas (e.g., La Pacana in Chile) may thus be seen as a terrain to test the hypothesis.

Until now it is unclear if the Altiplano anomaly at 20?-21?S is characteristic for the whole high plateau. Sparse data of the Research Group "Mobility of active continental margins" (acquired in the eighties and of much poorer quality than is standard today) indicate that a HCZ of these dimensions does not exist below the Puna. The existent data base is yet completely unsatisfactory to carry out any 2-D or even 3-D modeling and to draw any general conclusions. We thus propose a single campaign in late 2002 to achieve a state-of-the-art data set of equal quality and spatial density as on the Altiplano and in the North Chilean Forearc; this campaign is also a follow-up experiment of the seismological PUNA97 experiment. The station array shall also include a 3-D layout covering La Pacana Caldera.

The knowledge of these zones of lithospheric weakness has immediate consequences for the understanding of the different deformation styles on both Altiplano and Puna. In the south the magmatic arc - coupled with a high conductivity zone - acts as a rheological buffer, and upper crustal deformation is halted and does not extend into the Puna, while in the north - where the arc is not connected with a HCZ - the deformation front extends towards the Eastern Cordillera. This interpretation requires close co-operation with subprojects dealing with deformation analysis, heat flow, geochemistry, seismic low velocity vs. high absorption zones, and laboratory-determination of petrophysical parameters.

The further investigation of the other prominent anomalies - the large shear zones parallel to the plate boundaries as the Atacama Fault and the Liqui?e-Ofqui Lineament - will comprise only modeling studies in this proposal in close co-operation with other subprojects.

As most of the high conductivity zones discovered up to now display a very complicated spatial pattern, both a 3-D layout of field stations and subsequent 3-D modeling are mandatory. As a promising methodical approach, the study of inter-station transfer functions of magnetic fields yielded a much smoother image of the subsoil conductivity distribution than the inclusion of telluric fields; this will comprise one of the main focusses of the planned research.

Objectives, methods, work plan, and schedule

Objectives and methods

The main objective of this subproject is the investigation of high conductivity zones in the Central and Southern Andes and their identification as high mobility zones, in particular as planes of fault movements and/or regions of partial melt in the deep crust. This can only be achieved in close co-operation with the structural-geological, petrophysical and geochemical working groups and in combination with the other geophysical sounding methods due to the non-uniqueness inherent to the interpretation of well-conducting features.

The data set from the Southern Andes has only preliminarily been evaluated so far. Comprehensive 3-D modeling is necessary to explain the rather complicated pattern of the detected anomalies (e.g., in the Coastal Cordillera and the volcanic arc). As in the north, the calculation of inter-station magnetic transfer functions should yield a much smoother image of the conductivity distribution compared with the magnetotelluric results. After the planned on-offshore seismological campaign, a rather complete geophysical data set (gravity, EM, seismology) will also exist in South Chile by the end of 2001, which has to be interpreted integratively together with the other working groups. In the Central Andes remain several principal problems in connection with the large shear zones in the forearc, the worldwide unparalleled extremely high conductivity zone below the plateau and in particular the question of partial melts vs. saline brines as a cause for the observed conductivities. The following problems shall be addressed:

Large shear zones and fluid transport

There exists a good conductor in the area of the Liqui?e-Ofqui-Lineament (LOF) in Southern Chile, which was crossed by the MT profile. The depth extent of this anomaly and its connection with the magmatic arc proper are not yet clear. Due to missing surface manifestations at this latitude, the exact location of the fault trace still remains somewhat unclear; the detailed fault structure, its relevance to recent volcanism and the interaction with the fracture zones of the subducted plate have to be worked out in close co-operation with subproject F1.
The Falla Oeste in North Chile is connected with a HCZ at greater depths; closer to the surface the anomaly is much smaller and probably related to the influx of meteoric water. The deep structure of the fault has to be clarified together with SP F1. An important question concerns the connection of the deep HCZ with the Altiplano conductor. In contradiction to seismic results, which show the Quebrada Blanca Bright Spot to extend from the Pre- towards the Western Cordillera, MT results so far do not favor an electrical connection; there are, however, minor indications in the TM mode (tangential magnetic, referring to the polarization with the magnetic field parallel to the conductivity contrast) which do not completely exclude this possibility.
The situation is different near the coast, where the Atacama Fault System has to be modeled as a good conductor also at near-surface levels, although only long-period data have been measured so far. A more detailed image of this fault shall be obtained during a joint off-onshore campaign within the framework of the "Geotechnology Program."

Fluids in the slab

The induction arrows near the coast do not point away from the ocean at periods >1000s, blurring the strong coast effect and impeding the detection of the downgoing slab as a conductor. This is the case for the northern as well as the southern Andes. The task is to separate the EW-running inland anomaly from the one stemming from the ocean and to deduce possible high conductivities below the Coastal Cordillera from the remaining residual transfer functions. Can the upper part of the slab be resolved as a good conductor, which is more likely in South Chile (latitude 39?S) than in the north due to a younger oceanic plate age and a significant sedimentary filling of the trench?
Does the EW-running anomaly near the coast in South Chile image a fracture zone in the subducted Nazca plate and can it be traced towards the volcanic arc?

The electrical image of the Altiplano-Puna Plateau

Is the HCZ below the Altiplano at 20?-21?S a characteristic feature of the entire high plateau? To address this question two parallel profiles (one with a larger site separations) across the Chilean/Argentinian Puna are proposed which shall be measured at the end of 2002 (see Fig. 1). The sparse and insufficient (in terms of period band coverage and distances between sites) data available from the Research Group were collected in the eighties along the El Toro Lineament (partly in the Quebrada El Toro itself) and accordingly display strong distortions; this makes modeling and interpretation with state-of-the-art methods impossible. If the exceptional conductor is missing below the Puna, we will probably be able to resolve the asthenosphere at longest periods. Does an asthenospheric high exist below the Puna, as was deduced by Lezaeta et al. (2000) from the analysis of former data collected further east? These data will enable a contribution to the problem of delamination in the Puna, addressed by subproject G3.
Since a seismological network already exists on the Puna (PUNA97) and new measurements are planned on the Altiplano at 21?S, this gives a unique chance to interpret EM, seismological and gravity data jointly in a significant part of the plateau and to work out common and different features and relate them to the distinct tectonic styles and deformation patterns together with subprojects F2, F3, F4, G1, G2, G3 and G7. A fundamental question has to be addressed: Do the subsurface structures explain the surficial movements as determined by recent GPS measurements in the sense that the southern (i.e., south of 21?S) active volcanic arc acts as a rheological buffer and inhibits the transference of stress from the continental margin into the Puna, whereas the "cooled-out" northern arc (north of 21?S) provides no barrier and the upper crust slips undisturbed over the weak zone mapped by MT and receiver function analysis up to the Eastern Cordillera?

Partial melts vs. fluids

What causes the observed high conductivities below the Altiplano and the volcanic arc at 22?-24?S? The role of fluids and/or partial melts is still controversially discussed - not only in the Andes - and due to insufficient knowledge of the temperature regime difficult to assess. However, there exist regions on both Altiplano and Puna where the hypothesis of large volumes of acidic partial melts, respectively post-magmatic fluids, as the main cause for the HCZs can be tested directly: the young large ignimbrite calderas, which erupted less than 2 Ma ago (Trumbull et al. 2000) and which may still contain molten material in the order of several 10.000 km3 at depths of several km. One of them, La Pacana Caldera, is situated in the western part of the planned profiles across the Puna and thus proves to be an ideal target for a detailed study which shall be carried out as a 3-D array of long- and short-period MT/AMT stations (see Fig. 1) during the Puna campaign at the end of 2001.
These results have to be compared with the laboratory measurements carried out by SP G5. Since the connectivity of rocks containing free hydrous fluids is quite different from that of partially molten rocks, G5 will concentrate on how the fluids connect on a microscale, which depends critically on the wetting behavior of the liquid. This behavior is expected to vary considerably with density and viscosity of the liquid phase. It is therefore expected that fluids and melts will have distinctly different effects on the electrical conductivity of a rock. Furthermore, it is necessary to address the scaling problem. This should be done by studying the segregation of melts and water from microscopic to mesoscopic and larger scales, also by analyzing and integrating modern field studies as in Tibet, East Africa and elsewhere. A further approach is the transfer of temperature models (subproject G1) into electrical conductivities.

Several new methodical approaches will be followed. The analysis of magnetic field gradients, which proved to be very successful in the ANCORP study, will again play a major role. Since the problem of regional strike estimation is still unsatifactorily solved in certain parts of the study area, new decomposition schemes which allow the calculation of regional strikes (if existent) for a set of frequencies and stations simultaneously, shall be applied. Although several approaches (3-D and 2-D-anisotropy modeling) have led to a far better understanding of the observed transfer functions than in the past, it has not yet been possible to interpret all components, especially regarding the vertical magnetic field. Thus full 3-D modeling including anisotropy seems to be the only alternative in some problematic areas (e.g, Falla Oeste and South Chile). Sensitivity studies and the incorporation of a-priori information from other methods are mandatory to constrain the geometry, the conductance and thus place bounds on the physical parameters. The proposed code for 3-D inversion (Mackie, pers. comm.) will be available by 2002 and will be applied to our data.

G4_1

Fig. 2: aus der Graphik ergänzen

Working plan and schedule

In this last proposed funding period the main emphasis shall be placed on comprehensive interpretation of the existing data sets, the integration of all relevant physical parameters in order to achieve a concluding image of the deduced conductivity structures and their relevance for the deformation processes. Therefore no additional field campaign is intended in the Southern Andes; the data sets collected there at the end of 2000 still have to be analyzed more thoroughly than has been possible to this point. Further activities are only planned within the "Geotechnology Programme."

A concluding field campaign is planned in the central Andean Puna which will focus on a network around La Pacana Caldera for an in situ test of the partial melt hypothesis and two profiles across the Puna to yield an electromagnetic image of the southern high plateau, which is entirely different with respect to tectonic styles from the Altiplano.

The time schedule is planned as follows:

Jan. - Jun. 2002:

2-D inversion of South Chile data, anisotropic modeling, focus on Liqui?e-Ofqui Fault and oceanic fracture zones together with subproject F1. Preparation of instruments and adaptation of software for field campaign in North Chile/Northwest Argentina. Travel (4 weeks) to Chile and Argentina for pre-site investigation and project coordination. Calibration of induction coils and test measurements near Berlin. Presentation of results from South Chile at DGG Annual Congress. Visit of A. Chave and preparation of the on/offshore experiment.

Jul. - Dec. 2002:

Transport of equipment to Chile (Antofagasta, Calama) and Argentina (Salta). Presentation of results from South Chile at the International Workshop on Electromagnetic Induction (Santa Fe, USA). Field campaign in the Chilean/Argentinian Puna with base at San Pedro de Atacama (Chile). Field work will consist of two parts: 1) 3-D array in the area of La Pacana Caldera, comprising short period stations (T = 0.001 - 100 s) with AMT instruments at altogether 32 sites, recording for one day at each site and use of reference stations. 2) 40 long-period LMT sites (T = 10 - 20.000 s) along two parallel profiles crossing La Pacana (see fig. 1), site separation 10 km. To avoid crossing border s with instruments, the Argentinian part of the profiles will be organized from Salta. The University of G?ttingen (Prof. Bahr) has agreed to lend their 10 LMT instruments; they will be sent to Salta independently. The Buenos Aires MT group will also participate in the campaign on the Argentinian part of the working area and will install their own equipment. The advance in computing facilities will allow processing and 2-D modeling already in the field. Preliminary results from this campaign and their integration into the existing central Andean data set will be presented at the AGU fall meeting in San Francisco.

Jan. - Jun. 2003:

Transport of equipment back to Berlin/Potsdam/G?ttingen/Buenos Aires. Data archiving. Maintenance of instruments. Remote-reference processing of all transfer functions from the Puna experiment, calculation of anomalous magnetic transfer functions, decomposition, 2-D inversion, for both LMT-Puna and AMT-La Pacana data. Incorporation of temperature models into MT forward modeling, systematical evaluation of laboratory-field scaling problems together with SP G5. Presentation of actual results at EGS Annual Congress. Preparation and submission of papers dealing with South Chile and the Puna. Tentative 3-D inversion

Jul. - Dec. 2003:

2-D anisotropy modeling for magnetotelluric and magnetic transfer functions of long-period data sets. First 3-D attempts on Puna data and full 3-D modeling of AMT data. AGU fall meeting, San Francisco. Project coordination and discussion of results in Chile and Argentina.

Jan. - Jun. 2004:

Final Preparation and submission of papers on Puna and La Pacana. Integrative publication with subprojects G3 and G5. 3-D modeling including anisotropy. Comprehensive interpretation of electrical anomalies with regard to other geophysical and petrological parameters.

Jul. - Dec. 2004:

Full 3-D inversion of data sets in the Southern and Central Andes. Finalizing of joint interpretation efforts. Presentation of integrative results at the Workshop on Electromagnetic Induction (India) and the AGU fall meeting, San Francisco.

Collaboration with external research groups

Names of important partners in the host countries

Argentina:

Prof. Dr. B. Coira, Universidad de Jujuy
Prof. Dr. A. Osella, Universidad de Buenos Aires
Prof. Dr. C. Pomposiello, CONICYT, Buenos Aires
Prof. Dr. J. Viramonte, Universidad Nacional de Salta

Bolivia:

Prof. E. Ricaldi, UMSA, La Paz
Dr. S. Tawackoli, SERGEOMIN, La Paz

Chile:

Dr. Klaus Bataille, Universidad de Concepci?n
Dr. L. Baeza, CODELCO Chile, Calama
Dr. G. Behn, CODELCO Chile, Santiago
Prof. Dr. G. Chong, Universidad Cat?lica del Norte, Antofagasta
Dr. A.J. Tomlinson, SERNAGEOMIN, Santiago
Dr. H. Wilke, Universidad Cat?lica del Norte, Antofagasta

Publications

Literature

Reviewed publications

Asch, G., Brasse, H., Giese, P., G?tze, H.J., Haberland, C., Wigger, P., Yuan, X. (1999): Fluid generated features in the convergence system of the Central Andes, imaged by geophysical observations.. - 24. EGS-Jahrestagung, Den Haag, extended Abstract, session SE6: 35; Den Haag.

BRASSE H. (1999): Magnetotelluric studies of the central Andes within the framework of an integrated geophysical project. - 24th EGS Annual Meeting, : ; Den Haag. - []

BRASSE H. (2000): Magnetotellurik in den Anden im Kontext anderer geophysikalischer Beobachtungen. - 18. Kolloquium elektromagnetische Tiefenforschung, : ; Altenberg. - []

BRASSE H., LEZAETA P., RATH V., SCHWALENBERG K., SOYER W. (1999): The Complex Pattern of Electrical Conductivity Anomalies in the North Chilean Andes. - AGU Fall Meeting, : ; . - []

Brasse, H. & Soyer, W. (2001): A magnetotelluric study in the Southern Chilean Andes. - Geophys. Res. Let..

Brasse, H., Burkhardt, H., Fiedler-Volmer, R., Rath, V., Schuster, K., Troschke, B. (1999): Geophysikalische und geologische Untersuchungen im Es-Safya-Graben (Nordwest-Sudan): Ein Beitrag zur Erkundung einer kleinr?umigen Struktur. In Nordost-Afrika: Strukturen und Ressourcen. - Nordost-Afrika: Strukturen und Ressourcen: 194-249; Wiley-VCH-Verlag Weinheim.

Brasse, H., Echternacht, F., Fiedel, S., Haak, V., Heise, W., Lezaeta, P., Schwalenberg, K., Tauber, S. (1998): Untersuchung von Schw?chezonen der andinen Lithosph?re mit elektromagnetischen Verfahren. - Deformationsprozesse in den Anden, Berichtsband des SFB 267, 1996 - 1998, extended Abstract: 261-287; Berlin.

Brasse, H., Lezaeta, P., Rath, V., Schwalenberg, K. Soyer, W. & V. Haak (2002): The Bolivian altiplano conductivity anomaly. - J. Geophys. Res., 107, Nr. B5.

Brasse, H., Rath, V. (1997): Audiomagnetotelluric investigations of shallow sedimentary basins in Northern Sudan. - Geophys. J. Int., 128: 301-314.

Brasse, H., Soyer, W. (2001): A uniform conductivity image along the Chilean margin? Observations from Central and Southern Andes. - AGU Fall meeting, San Francisco, California, session GP12B-03, extended Abstract, session GP12B-03; San Francisco, California.

Brasse, H., Soyer, W. (2001): Magnetotellurik in S?dchile: Erste Ergebnisse. - 61. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Frankfurt am Main session EM01, extended Abstract, session EM01; Frankfurt a.M..

Giese, P., Brasse, H., Haak, V. (2000): In: Lexikon der Geowissenschaften, Band 1,2. - Lexikon Geowissenschaften, Band 1,2, Spektrum Akademischer Verlag GmbH, Band 1,2; Heidelberg, Berlin.

Giese, P., Brasse, H., Haak, V. (2000): In: Lexikon der Geowissenschaften, Band 3,4. - Lexikon Geowissenschaften, Band 3,4, Spektrum Akademischer Verlag GmbH, Band 3,4: ; Heidelberg, Berlin. - []

Giese, P., Brasse, H., Haak, V. (2002): In: Lexikon der Geowissenschaften, Band 5,6. - Lexikon Geowissenschaften, Band 5,6, Spektrum Akademischer Verlag GmbH, Band 5,6: ; Heidelberg, Berlin. - []

Janssen, C., Hoffmann-Rothe, A., Tauber, S. & Wilke, H. (2001): Internal structure of the Precordilleran fault system (Chile): Insights from structural and geophysical observations. - Journal of Structural Geology.

LEZAETA P. (2000): A case of strong local current channelling in the Southern Central Andes. - 18. Kolloquium elektromagnetische Tiefenforschung, : ; Altenberg. - []

LEZAETA P. & BRASSE H. (1999): Electrical Conductivity beneath the Tuzgle Volcano and its surrounding shoshonitic volcanic centers, NW Argentina. - 4th International Symposium on Andean Geodynarnics, : ; G?ttingen. - []

Lezaeta, P. (2001): The confidence limit of the phase sensitive skew parameter in magneto-tellurics. - Special Issue of Earth, Planets and Space.

Lezaeta, P. (2002): The confidence limit of the magnetotelluric phase sensitive skew. - Earth, Planets and Space, : ; . - []

Lezaeta, P., Brasse, H. (2000): A complex distortion matrix tensor decomposition, a new method to detect and remove local strong current channeling. - The 15th Workshop on Electromagnetic Induction in the Earth, Cabo Frio, Brazil, extended Abstract: 39; Cabo Frio, Brasil.

Lezaeta, P., Brasse, H. (2000): A case of strong current channelling in the Southern Central Andes. - The 15th Workshop on Electromagnetic Induction in the Earth, Cabo Frio, Brazil, extended Abstract: 116; Cabo Frio, Brasil.

Lezaeta, P., Brasse, H. (2001): Electrical conductivity beneath the volcanoes of the NW Argentinian Puna. - Geophys. Res. Let., Vol. 28, No. 24: 4651-4654.

Lezaeta, P., Brasse, H. (2002): Electrical conductivity beneath the volcanoes of the NW Argentinian Puna. - Geophys. Res. Lett., : ; . - []

Lezaeta, P., Munoz, M. & Brasse, H. (2000): Magnetotelluric image of the crust and upper mantle in the backarc of the NW Argentinian Andes. - Geophys. J. Intern., 142: 841-854.

Schilling, F.R., Giese, P., Asch, G., Brasse, H., Haberland, Ch., Rietbrock, A. (2000): Fluid/Rock interactions in the Central Andean subduction zone imaged by geophysical observations. - 17. Geowissenschaftliches Lateinamerika-Kolloquium, Stuttgart, extended Abstract.

Schilling, F.R., Giese, P., Patzig, R., Brasse, H., Rietbrock, A., Wigger, P., Asch, G., Kind, R., Haberland, Ch. (2000): Fluid/Rock interactions in the Central Andean subduction zone imaged by geophysical observations. - 31st International Geological Colloquium, Rio de Janeiro, Brasil, extended Abstract 17-3 E3; Cabo Frio, Brasil.

SCHWALENBERG K. (2000): 2D-Modellierung von MT-Daten aus den Zentralen Anden: Am Rande der Aufl?sbarkeit.. - 18. Kolloquium elektromagnetische Tiefenforschung, : ; Altenberg. - []

SCHWALENBERG K., LEZAETA R, SOYER W. and BRASSE H. (1999): Electrical Conductivity Anomalies around the ANCORP-Profile: An Overview of new Results. - 4th International Symposium on Andean Geodynamics, : ; G?ttingen. - []

Schwalenberg, K., Brasse, H., Lezaeta, P., Rath, V., Soyer, W., Haak, V. (2000): The conductivity structure of the Southern Central Andes. - 17. Geowissenschaftliches Lateinamerika-Kolloquium, Stuttgart, extended Abstract; Stuttgart.

Schwalenberg, K., Brasse, H., Lezaeta, P., Rath, V., Soyer, W., Haak, V. (2000): Two-dimensional modeling of magnetotelluric data from Central Andes. - The 15th Workshop on Electromagnetic Induction in the Earth, extended Abstract: 94; Cabo Frio, Brasil.

Schwalenberg, K., Haak, V., and Rath, V. (2002): The application of sensitivity studies on a two-dimensional resistivity model from the Central Andes.. - Geophys. J. Int., : ; . - []

SOYER W. (2000): Geomagnetische Variationsanomalien in den s?dlichen zentralen Anden.. - 18. Kolloquium elektromagnetische Tiefenforschung, : ; Altenberg. - []

Soyer W. & Brasse H. (2000): Investigation of anomalous magnetic field variations in the central Andes of N Chile and SW Bolivia. - Geophys. Res. Lett., : ; . - []

Soyer, W. & Brasse, H. (2001): A magneto-variation array study in the central Andes of N Chile and SW Bolivia. - Geophys. Res. Let..

Soyer, W., & Brasse, H. (2001): Anomalous geomagnetic variations in the Andes. - Geophys. Res. Let.

Publikationen in Tagungsbänden

Brasse, H. (1997): Geoelektrik an der Fränkischen Linie - Vergleich verschiedener Methoden der Sondierung und Kartierung. - 57. Jahrestagund der DGG, extended abstract: 103; Potsdam.

Brasse, H., Friedel, S., Lezaeta, P., Schwalenberg, K. (1998): The Ancorp Project: a Seismic and Magnetotelluric Study of the Central Andean Subduction Zone. - The 14th Workshop on Electromagnetic Induction in the Earth, extended Abstract: 77; Sinaia, Romania.

Echternacht, F., Brasse, H. (1997): The resistivity structure of the forearc in Northern Chile at 20?S.. - EOS, Transactions, AGU Fall Meeting, 78: 46; San Francisco.