Geometria, cinemática da história tectônica das rochas da Serra Norte, Carajás-PA

Abstract

The studied area is located on northeast portion of the Carajás Strike-Slip System, it corresponds to plateaus of Serra Norte, where the CVRD acts in iron ore mining. On this region there are chemical sedimentary rocks (banded iron formation) and volcanic rocks of the Carajás and Parauapebas formations (Grão Pará Group) respectively, with age of 2.759 ± 2 Ma U/Pb in zircon (Machado et al., 1991). Previous structural geology studies done in the region indicated recumbent folds with fold axis dipping at low angles to NW. These folds are pointed to be the main structures controlling the arrangement of rocks in the area. The present work indicates mutually adjusted folds in different scales. Centimetric folds are parasitic being present in limbs of asymmetric folds with metric size. That corresponds to layering folded, varying from open to isoclinal with axial plane striking NWSE and dipping with low angles to SW-NE. The folds are concentrated in zones ranging from dozen to hundred meters wide striking NW-SE with different shortening measure. Kilometer scale folds are featured by layering drawing a J shape fold in map view. This fold is recumbent with direction parallel to the minor scale folds observed in the region. There are two types of contacts between iron ore and volcanic rocks: (1) normal concordant folded contacts and (2) fault contacts. The rocks studied in the area were regionally deformed by transpression with simple shear dextral component and pure shear component, dominated by shortening from NE to SW. This is based on vergence sense, fold asymmetries and general trend of the mapped structures compared to the E-W regional trending of the Carajás Strike-Slip System. The deformation is particularly concentrated in NW-SE zones trending that follow the trace of major discontinuities, converging from the Carajás Fault. The deformation responsible for the geometric and kinematic history of the rocks takes place during transpressional sinistral deformation affecting all Carajás Strike Slip System, between 2.6 and 2.0 Ga.