Understanding the Geological Setting of Northeastern Brazil

Abstract

The geological setting of northeastern Brazil comprises the Borborema Province and Cretaceous to Cenozoic sedimentary basins. The Borborema Province corresponds to a region ~900 km long and ~600 km wide in northeastern Brazil. It is composed of Neoproterozoic fold belts with Archean and Proterozoic basement inliers, which were deformed during the Brasiliano orogeny at ~750–500 Ma. The tectonic stability of this Province was achieved in the Cambrian. The Province was mostly deformed by ductile shear zones, which bound major tectonic terrains. The coastal area of the region and part of its interior comprise mainly Cretaceous sedimentary basins, which mark areas of rifting during the breakup of Pangea and the separation of the African and South American plates. The major faults of the region were formed during this period. These rift basins mostly formed along the continental margin as far as 300 km from the present coastline, which correspond to areas of extended crust and crustal thinning. Fault reactivation in the Cretaceous occurred mostly where shear zone forme terrain boundaries. Paleoseismological data indicate that earthquakes struck the region in the last ~100 ka. Earthquake at least 5.5–6.0 Ms induced soft-sediment deformation characterized mainly by dikes, dome-like load structures, and mixed layers that affect gravel and sand strata from 400 to 10 ka in alluvial valleys. Paleoseismic data also exhibit evidence of surface faulting that either occur on both favorably oriented and unfavorably oriented faults in response to the stress field. Marine terrace deposits that correspond to two sea-level highstands and oxygen-isotope stages (7c, 220–206 ka and 5e, 117–110 ka) mark sea-level changes and tectonics along the coast.