Deep structure of Parecis Basin, Brazil from 3D magnetotelluric imaging

Abstract

The resistivity structure beneath the Parecis basin in central Brazil has been studied using broadband (0.001–5000 Hz) magnetotelluric (MT) data for 455 stations recorded along five lines in the Juruena sub-basin and over the Precambrian basement at the northern and southern basin borders. We found that the reconstructed resistivity-versus-depth profiles from our 3D MT inversion models satisfactorily match the average values of the resistivity well logs in the top 2200 m at two exploration well locations chosen for model validation. The low resistivity Parecis Formation at shallow depth can be traced across the region while an areally restricted conductive sequence at about 2000 m depth can be imaged in the central part of Juruena sub-basin and is interpreted as synrift deposits or remnants of a buried Neoproterozoic basin. Sedimentary formations deeper than 2200 m could be sensed but not individually resolved in our resistivity models. The deeper resistivity structure consists of a resistive upper crust, severely thinned over the Neoproterozoic fold belts bounding the basin to the north and south where it is underlain by conductive zones at mid-lower crustal depths. We interpret the deep crustal steep conductors as pre-existing shear-zones possibly associated with the Upper Cretaceous to Tertiary alkaline magmatism in the region.