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O arcabouço estrutural da região de Chega-tudo e Cedral, noroeste do Maranhão, com base em sensores geofísicos
(Universidade Federal do Pará, 2002-12-05) RIBEIRO, José Wilson Andrade; ABREU, Francisco de Assis Matos; http://lattes.cnpq.br/9626349043103626; PINHEIRO, Roberto Vizeu Lima; http://lattes.cnpq.br/3251836412904734
The central-northern continental segment of the Brazilian coast, which straddles the boundary between the Pará and Maranhão States, is known in the geological literature as the Gurupi Region. The region is characterized by the presence of two main erosional Precambrian windows which expose small-sized Proterozoic shields in Phanerozoic sedimentary basins. Advances in the understanding of the Precambrian geological history of this area have been limited by the lack and quality of exposures due to the deep weathering profile developed. The use of airborne geophysics proved to be a helpful tool allowing a better definition of some lithological contacts and the shape of major tectonic features. The study area covers part part of the Gurupi Shear Belt, located in one of the Precambrian windows mentioned. This segment of the belt exhibits a variety of sedimentary- and igneous-derived supracrustal rocks in association with plutonic-derived metamorphic rocks. These rocks are structurally organized in three main NW-trending strips which were correlated to the Maracaçumé Complex, Gurupi Group and Tromai Suite. Sheared- and hydrothermally-altered basic and ultrabasic intrusive rocks are often found tectonically interlayered with the Gurupi Group rocks. All the above-mentioned rocks have been metamorphozed under low green-schist to low amphibolite facies conditions and affected by hydrothermal alteration. Diabase dikes crosscut the main Precambrian lithological units along several structural trends. Alluvial deposits along active drainages constitute the youngest lithologic units in the area. The kinematic and geometric analysis of the main tectonic structures observed allows the definition of the area as a shear-belt segment, characterized by steeply-dipping planar structures which are indicative of an oblique left-hand continental collision. The area is also characterized by the presence of a relatively narrow NW-trending highlystrained zone developed along carbonaceous metapelitic lithologic units. This corridor is well defined in the geophysical data and represents an important zone of crustal weakening responsable for most of the deformation partitioning which took place in the area. A secondary NNW-trending shear zone branches off from this main deformation zone and follows the contact of the metapelites with the metavolcaniclastic rocks. The splay caused the detachment of the coarse-grained volcanic rocks from the metapelites. The dominant NW-trending tectonic fabric observed along the exposed rocks of the belt is crosscut by three main sets of fractures well defined by the geophysical surveys, trending N-S, EW and NE-SW. The N-S and E-W fracture sets are associated with drag folds showing sinistral and dextral displacements, respectively. The E-W faults correspond to those previously described by several authors as strike-slip faults. On the scale used, no displacement along the NE-trending fractures could be observed in the geophysical maps, although they control the main river courses in the study area. The NEtrending fractures themselves are not outlined in the magnetic data. In function of this, the available data can not support the idea of some authors who described them as major fault zones related to the the main devolopment of the belt. The N-S fault zones disrupt the NW-trending dominant fabric of the belt exibit left-hand displacements. The N-S, NW-SE and NNW-SSE tectonic fabric present in the region show multiple disruptions, crosscutting one another probably due to several reactivation episodes. The crosscutting relations observed in the geophysical data suggest the N-S fault zones are later in relation to the main NW-trending structures of the belt, although their timings could not be defined with the available data.