Deep Resistivity Structure of Basalt-Covered Central Part of Parana Basin, Brazil, From Joint 3-D MT and GDS Data Imaging

The deep structure beneath the basalt-covered sediments of Parana basin is not well understood. Magnetotelluric (MT) broadband (1,000 to 0.001Hz) data recorded at 296 selected stations along three new survey lines were first inverted in three dimensions to reveal the local scale resistivity structure of the central part of the basin where there are three deep exploration wells with resistivity logs for model validation. The 3-D MT inversion model correctly identified the resistive (>100m) flood basalts and postvolcanic cover layer, the underlying electrically conductive (similar to 10m) sediments, and the highly resistive (>1,000m) crystalline basement at the well locations and provides a reliable average crustal resistivity structure consisting of similar to 20-24km thick highly resistive upper crust and similar to 20km thick conductive lower crust above a resistive mantle. Subsequent basin-scale joint inversion of highest quality MT data sets (261 stations) and geomagnetic depth sounding array data imaged two prominent subparallel belts of N-S and NNE-SSW trending steep conductors in the deep crust and upper mantle, one located directly beneath the present-day axis of thickest sedimentary deposits in the basin and another near the coastal location where dolerite dyke swarms intrude the outcropping Precambrian basement. Other roughly orthogonal bands (E-W to NW-SE) of linear crustal conductors radiate from these two main belts. The resulting networks of steep conductors at upper to midcrustal level seem to mark the fossil plumbing system of Parana magmatic province and are interpreted as preexisting shear zones exploited by the voluminous Early Cretaceous magmatism in the region.