Unraveling middle-crust conductive layers in Paleozoic Orogens through 3D modeling of magnetotelluric data: The Ossa-Morena Zone case study (SW Iberian Variscides)

摘要

In recent years a number of magnetotelluric (MT) surveys have been carried out in the southern branch of the Iberian Variscan Massif, namely in the Ossa Morena Zone (OMZ) and its neighboring areas, the South Portuguese Zone (SPZ) and the Central Iberian Zone (CIZ), in order to determine the geoelectrical structure of this transpressive orogen and gain insight into its internal architecture. New MT data collected across the OMZ complete a dense database that covers the whole belt and its boundaries. This paper presents an integrative three-dimensional model of the available MT data from the apparent resistivities (four components), impedance phases and geomagnetic transfer functions from 61 stations. Given the location of the study area, near the Atlantic Ocean, coast effect on the data was analyzed as well as the stability of the mesh due to the strong resistivity contrasts. A mesh of 80 × 77 × 76 cells was finally used, with an average cell size of 4 × 4 km in the horizontal direction, and increasing thickness in the vertical direction from 25 m in the shallower layers to 5 km in the deepest ones. The most striking feature is, in addition to the conductive anomalies associated with the sutures between the SPZ-OMZ and the OMZ-CIZ, a conductive layer which spans the whole Ossa Morena middle to lower crust. The top of this conductive layer is interpreted as a major décollement between the granulitic basement and the overlying meta-sedimentary pile.