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Magnetotelluric soundings have been made at 41 locations across the main tectonic structures in the Chaves region in northeast Portugal, near the frontier with Spain. The zone studied is dominated by a graben bounded by Hercynian granite and Silurian metamorphic schist. It is an elemental unit of a broad hydrothermal province, in which the low-enthalpy system of Chaves is the most important in the Portuguese mainland. Calculated impedance tensors at 27 sites were converted into invariant (determinant) apparent resistivity and phase curves and used to construct a regional 3-D model of the electrical resistivity in the crust. A conductive zone (resistivity of 100 ohm-m) was detected at a depth between 7 and 11 km. Based on geological information and on results from a previous 3-D dc model, a more detailed 3-D model of the graben area was constructed, by forward modeling. The detailed model of the graben was imbedded in the regional electrical model obtained from the invariant curves. The main shallow structure—a conductive body (resistivity of 12 ohm-m) at a depth ranging from 250 to 1000 m—appears to be related to the Hercynian and neotectonic activities. Our results indicate that a valuable picture of the regional electrical model can be obtained from 3-D interpretation of the determinant of the impedance tensors.

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