Magnetotelluric investigation of the lithosphere in the Alto Paranaíba Ígneous Province / Investigação magnetotelúrica da litosfera na província Ígnea do Alto Paranaíba

AUTOR(ES)
DATA DE PUBLICAÇÃO

2001

RESUMO

Magnetotelluric (MT)soundings are presented for the region of Alto Paranaíba Igneous Province (APIP), an important Cretaceous diamondiferous province in western Minas Gerais State. In addition to conventional wideband soundings, suitable for crustal scale investigations, long period MT systems were used to investigate the mantle lithosphere. This study is focused on the potential of the MT technique to provide tectonic insights in an area with petrological and geochemical information, but is poorly known in a geophysical sense. By using wideband and long period data together we have worked in a dynamical range from about 0,001 s until 14000 s, which allowed us to get important information from the crust and the lithospheric mantle. Data analyses have indicated the presence of distinct electrical anomalies, affected by 3D induction effects and galvanic distortions due to shallow structures, anisotropy at intermediate crustal depths, azimuthal variations of the anisotropy, variations of the electrical strike along distance and with depth, and galvanic distortions on long period data due crustal anisotropy. Therefore, preinversion algorithms were applied to minimize such problems and consequently increase the accuracy of the results. The inversion of the short period data (up to 53 s)over a 100 km profile from southwestern half of the studied area has identified a uniform conducting zone with conductance of 25 S between 3 and 5 km. The source of the conduction mechanism could be saline fluids, interconnected graphite films, or a combina-tion of both. Because of its depth this feature has been interpreted as a possible fractured zone associated with the sole of thrust and fold faults of the Brasilia Belt, which is overthrusted onto crystalline terranes of southwestern border of São Francisco Craton. At about 10-15 km depth a conducting layer with conductance of 200 S over the same southwestern portion presents an anisotropy that could be related to macro structures crustal swam of dikes or deep fractured zones (micro properties). The complex behavior of the anisotropy requires further studies. The inversion of long period data has included sites across the entire APIP, in a 180 km long NE profile. The results show a heterogeneous mantle, more conductive beneath the borders of both Paraná and Alto Sanfran-ciscana basins and resistive in the central part of the profile, over the Brasília Belt. Such variations on the electrical properties of the mantle are compatible with the tectonic evolution of the APIP, developed between two sub-basins (Uberaba and Abaeté)with uplift at the central part and subsidence at the sub-basins. The agreement with petrochemical data suggesting the mafic and ultramafic intrusions originated at distinct depths from heterogeneous mantle sources, supports the implications of the conductive variations to mechanical and rheological characteristics.

ASSUNTO(S)

lithosphere condutividade magnetic surveys estudos magnéticos cratons conductivity geofÍsica espacial geomagnetismo litosfera crátons geomagnetism

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