Geoelectric crustal structures off the SW border of the São Francisco craton, central Brazil, as inferred from a magnetotelluric survey

Details Geoelectric crustal structures off the SW border of the São Francisco craton, central Brazil, as inferred from a magnetotelluric survey Geoelectric crustal structures off the SW border of the São Francisco craton, central Brazil, as inferred from a magnetotelluric survey

Aug 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005geoji.162..357b&link_type=abstract

Geophysical Journal International, Volume 162, Issue 2, pp. 357-370.

Astronomy and Astrophysics

Astronomy

1

Alto Paranaíba Igneous Province, Central Brazil, Crustal Structure, Electrical Anisotropy, Magnetotellurics, São Francisco Craton

Scientific paper

A magnetotelluric (MT) survey of the crust beneath sedimentary basins and thrust sheets along the southwestern edge of the São Francisco craton, central Brazil, reveals intricate electrical characteristics that are interpreted in the context of Proterozoic collision tectonics and horizontal transport of allochthonous rock units, emplacement of Cretaceous hypabyssals, lavas and diatremes of ultrapotassic-mafic composition, and occurrence of induced seismicity. The data exhibit strong distortions represented by 3-D induction effects and galvanic disturbances resulting from shallow structures, frequency and site dependence of electrical strike, and inhomogeneous anisotropic layers with smoothly varying phase split, conductance and azimuth of the highly conductive direction. Geoelectrical and orthogonal phase difference directions, 2-D inversion, and forward modelling characterize three distinct subhorizontal sections showing two anisotropic conductors within a highly resistive crust, laterally segmented into unique blocks. The model for the uppermost crust section has E-W geoelectric directions and a 15-30 S anisotropic zone at a depth of approximately 1-2 km along the entire profile. This conducting layer is interpreted to represent a brine-filled fracture layer possibly controlled by the present-day state of crustal stresses, as disclosed from reservoir-triggered quakes. The mid-crust section presents a deeper conducting zone located at depths below 10 km. It is defined by stronger MT responses having phase split directions oscillating from WNW-ESE to E-W beneath sites in the central-western area (Paraná basin and allochthonous cover units) and NNE-SSW in the northeastern region (autochthonous platform units and Sanfranciscana basin). Anisotropy is greater than an order of magnitude in the highly conductive direction, with conductance in the range of 250-400 S. Conjecturally, the source of this anomalous feature would come from interconnected grain-boundary phases and hypersaline fluids, exsolved and precipitated from upwelling Cretaceous magma. In the central-western area, favourable trapping of conductors was constrained along a nearly E-W direction, feasibly associated with relic structures inherited from Brasiliano/Pan African continental collisions. Along the northeast, however, the coincidence with superficial NNE-SSW structural directions suggests a localized direct causal relationship with the trend of extension related to magma emplacement. The lower crust has a highly resistive quasi-1-D section along the entire profile that prevails also at uppermost mantle depths. Thus, whereas the brittle crust would have reconciled subhorizontal strain with fluid percolation related to uplift and magma emplacement, a mechanically coupled and stronger lower crust/upper mantle would have controlled the deep magma generation during Cretaceous distention pulses.

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