[1] A long-period magnetotelluric study was carried out in the central Andes between latitudes 19.5°S and 21°S along two almost parallel profiles of 220 and 380 km length, respectively. The investigation area extends from the Pacific coast to the southern Altiplano Plateau in the back arc of the South American subduction zone. The main geoelectrical structure resolved is a broad and probably deep-reaching highly conductive zone in the middle and deeper crust beneath the high plateau. Although the data show deviations from two-dimensionality, a two-dimensional approach is justified for large parts of the profiles. Sensitivity studies were carried out in order to constrain the depth extent. Another electrically conductive structure was resolved in the middle crust of the Chilean forearc, thought to be connected with the Precordillera fault system. The Andean Continental Research Program (ANCORP) seismic reflection profile, carried out along the same line at 21°S, revealed highly reflective zones below the Altiplano, in good correlation with the upper boundary of the Altiplano conductor. This highly conductive domain also coincides with low seismic velocities and a zone of an elevated v p /v s ratio and, although not well resolved, with low Q p seismic quality factors. Taking into account the enhanced heat flow and a derived temperature model, the most probable explanation lies in the assumption of granitic partial melts. The good conductor below the volcanic arc which was found in regions farther south at 22°S gradually vanishes toward the north; this is consistent with the results of seismic tomography concerning Q p values and a gap of recent volcanism.

Long-period magnetotelluric data have been used to image the deep electrical structure of the Cascadia subduction zone in British Columbia, Canada. Zones of elevated electrical conductivity were found in both the forearc and backarc regions and are interpreted as a consequence of the fluid release from subducting slab. A shallow zone of high conductivity beneath Vancouver Island is likely due to fluids that are trapped above the subducting plate. East of this structure is a conductive (ϳ0.03 S/m) forearc mantle wedge that also exhibits low seismic velocities and may be serpentinized. A free fluid phase is required to account for this enhanced conductivity. Elevated conductivities are observed in the upper mantle throughout the backarc (ϳ0.01 S/m) and strongly support the hypothesis of a shallow, convecting asthenosphere. This enhanced upper mantle conductivity can be explained by either hydrogen ion diffusion in olivine minerals, or by a few percent partial melting (Ͻ4%). Figure 1. Location of magnetotelluric (MT) sites in northern Cascadia subduction zone. Large symbols denote University of Alberta-POLARIS deployment in 2003 (circles represent LIMS instruments, inverted triangles, NIMS instruments). Small symbols show locations of previous LITHOPROBE MT stations from Kurtz et al. (1990) (squares) and Ledo and Jones (2001) (triangles). Solid triangles mark volcanoes.

Please cite this article as: Brasse, H., Kapinos, G., Li, Y., Miitschard, L., Soyer, W., Eydam, D., Structural electrical anisotropy in the crust at the South-Central Chilean continental margin as inferred from geomagnetic transfer functions, Physics of the Earth and Planetary Interiors (2008), doi:10.1016/j.pepi.2008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Orogenic gold deposits provide a significant source of the world’s gold and form along faults over a wide range of crustal depths spanning sub-greenschist to granulite grade faces, but the source depths of the gold remains poorly understood. In this paper we compiled thirty years of long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) data across western Victoria and south-eastern South Australia that have sensitivity to the electrical resistivity of the crust and mantle, which in turn depend on past thermal and fluid processes. This region contains one of the world’s foremost and largest Phanerozoic (440 Ma) orogenic gold provinces that has produced 2% of historic worldwide gold production. Three-dimensional inversion of the long-period MT and GDS data shows a remarkable correlation between orogenic gold deposits with > 1 t production and a < 20 Ω m low-resistivity region at crustal depths > 20 km. This low-resistivity region is consistent with seismically-imaged tectonically thickened marine sediments in the Lachlan Orogen that contain organic carbon (C), sulphides such as pyrite (FeS2) and colloidal gold (Au). Additional heat sources at 440 Ma due to slab break-off after subduction have been suggested to rapidly increase the temperature of the marine sediments at mid to lower crustal depth, releasing HS− ligands for Au, and CO2. We argue that the low electrical resistivity signature of the lower crust we see today is from a combination of flake graphite produced in situ from the amphibolite grade metamorphism of organic-carbon in the marine sediments, and precipitated graphite through retrograde hydration reactions of CO2 released during the rapid heating of the sediments. Thus, these geophysical data image a fossil source and pathway zone for one of the world’s richest orogenic gold provinces.