Electrical conductivity beneath the Andes

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research area
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measurement results
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field work

The principal investigation area of the FUB magnetotellurics group are the Central and Southern Andes of Chile, Bolivia and Argentina. The studies have been carried out as a subproject of the Special Research Programme SFB 267 "Deformation Processes in the Andes" (funded by German Science Foundation DFG) together with the MT group at the GeoForschungsZentrum Potsdam, and since 2004 within the TIPTEQ programme (funded by Federal Ministry of Education and Research BMBF).

The investigations of the MT subproject in the last years have concentrated on 6 topics: 1) The Altiplano conductivity anomaly including sensitivity studies to constrain upper bounds of electrical conductivity and depth extent; 2) Detailed study of large shear zones in the central Andean forearc (Precordillera Fault System and Atacama Fault) in close co-operation with subproject C5B; 3) Re- and extended interpretation of older data sets covering the Chilean forearc as well as the Argentinian backarc; 4) Two traverses in South Chile at ~39°S carried out in 2000; 5) A comparative transect across the Altiplano at ~18°S, measured at the end of 2002 and 2004, respectively; 6) An amphibious experiment at ~38°S in austral summer 2004/2005 (TIPTEQ).

The studies have been carried out in cooperation with E. Ricaldi, P. Miranda and F. Ticona (UMSA La Paz ), G. Chong and H. Wilke (UCN Antofagasta), Miguel Muñoz (U Santiago) and K. Bataille (U Concepción). The offshore campaign was carried out in cooperation with A. Chave and J. Bailey (WHOI); their instruments were deployed on RV Sonne cruise SO 181.

The Altiplano anomaly - one of the great high conductivity zones in the world - extends from the Altiplano-Puna Volcanic Complex up to the Eastern Cordillera with core resistivities as low as 1 Ohm-m. While its upper boundary undulates between 10 and 30 km, it was not possible to resolve the lower depth limit, although sensitivity studies indicate that probably the whole middle and deep crust must be conductive. The upper margin of the structure coincides well with seismic bright spots detected during the ANCORP programme. Despite limited resolution concerning its depth extent there is also a very good correlation with the seismic low velocity zone and a zone of enhanced Vp/Vs ratios. Taking the temperature models into account, the most probable interpretation of the high conductivities lies in the assumption of large amounts of partial melts (>10%) in a wet, predominantly felsic to intermediate deep crust. In the Chilean forearc two major fault zones - the Atacama Fault and the Falla Oeste - have been modeled extensively as good and deep reaching conductors, including the development of a scheme to account for strong surficial 'current channeling' effects distorting the regional responses. Data at several sites at both fault zones exhibit very unusual magnetotelluric phases well above 90°, which was explained as 'edge effects' in 3-D modeling studies. Potentially the deflection of induction arrows can also be attributed to anisotropic structures as was shown by 2-D-anisotropy modeling.
The Precordillera anomaly is imaged by anomalous magnetic variations even clearer than in the magnetotelluric transfer functions and shows the classical response of a sub-vertical dike. There is no clear evidence for a connection of this anomaly to the Altiplano conductor and thus a relation to the Quebrada Blanca seismic bright spot, although 3-D models do not exclude this possibility. The near-surface electrical structure of the Falla Oeste was investigated together with subproject C5B at 21°S (Quebrada Huatacondo) and 23.5°S (Falla Limon Verde as a branch of the Falla Oeste) and correlated with structural geological findings. Unexpectedly only the core zones were detected as moderately good conductors (probably due to meteoric water), while the damage zones display normal resistivities, which hints at obviously closed fractures. Obviously the good conductors associated with the fault cores are not connected with the deeper anomalies mentioned above; this is a clear indication for deep-crustal brines as a cause for the deep HCZ. The re-interpretation of former data sets showed that the deep Precordillera anomaly extends well to the south of 23°S and can thus be regarded as a regional phenomenon which is apparently characteristic for the entire Falle Oeste.
Including rigorous decomposition and 2-D inversion, the re-examination of an older data set from the Argentinian backarc revealed a rise of the asthenosphere below the Eastern Cordillera and a high conductivity zone below Tuzgle volcano in the Puna in correlation with seismological attenuation studies.
Data  collected in South Chile along two transects from the Pacific coast to the Argentinian border display a completely different character compared to the Central Andes. Although east of the Liquiñe-Ofqui Fault (LOF) a zone of enhanced conductivities exists at depths of 20 km below the volcanic arc, it is constrained to the middle crust and its conductivities aren't nearly as high as encountered e.g. beneath the Altiplano. Another anomaly is correlated with the trace of the Lanalhue Fault in the Longitudinal Valley. A phenomenon not yet understood are the induction arrows (see TIPTEQ), which point consistantly northeastwards over the entire study area for long periods. They are thought to hint at anisotropy of the crust.

Below is a collection of two-dimensional models (achieved with the inversion code of Rodi and Mackie, 2001). Further information may be found in several papers and PhD/diploma theses.