Reactivation history of the Carajas and Cinzento strike-slip systems, Amazon, Brazil.

Abstract

The Carajás-Cinzento fault system is centred within the Itacaiunas Shear Zone, the northern tectonic margin of the Archaean Sul do Pará Granite Greenstone Terrain of the Amazonian Craton, Brazil. The regional tectonostratigraphy can be subdivided based on the geological relationship of units to the main phase of ductile movements along the Itacaiúnas Shear Zone. A Basement Assemblage includes an older group of orthogneisses, migmatites, and granitic to dioritic plutons (Xingu Complex) and a later volcano-sedimentary sequence of ironstones, quartzites. amphibolites and schists (Igarapé Salobo Group). Intense ductile shearing and high grade metamorphism along the Itacaiúnas Shear Zone has led to widespread tectonic interleaving of the gneisses and volcano-sedimentary rocks and has obliterated all traces of the original unconformity between these units. A Cover Assemblage is represented by very low-grade volcanic and sedimentary rocks that are inferred to rest unconformably on the Basement Assemblage rocks deformed within the Itacaiúnas Shear Zone. Older clastic, volcanic and ironstone sequences (Igarapé Pojuca & Grao Pará Groups; ca.2.7 Ga) are overlain by a sequence of shallow-water marine to fluvial clastic deposits (Águas Claras Formation). Both Cover and Basement assemblages are intruded by ca.1.8 Ga A-type granitic plutons and basic dykes. All units are unconformably overlain by a thin, localised sequence of polymictic conglomerates (? Gorotire Formation).During the formation of the upper amphibolite facies within the Itacaiúnas Shear Zone, sub-vertical mylonitic fabrics, generally E-W-trending, were variably developed in the Basement Assemblage rocks. They preserve widespread sinistrally transpressional kinematic indicators. Radiometric dating suggests that the metamorphism and deformation in the shear zones occurred toward the end of the Archaean (ca.2.8 Ga). At least three cycles of brittle-ductile strike-slip reactivation at low metamorphic grades appear to post-date the development of the Itacaiúnas Shear Zone, leading to the formation of the Carajás and Cinzento faults. There is no stratigraphic, structural or sedimentological evidence to suggest that these faults were active during the deposition of the Cover Assemblage sequences. However, as most outcrops of Cover Assemblage rocks are presently localised within bends, branches and offsets of these fault systems it appears that, following their deposition, they were faulted down into dilational jogs formed during an initial phase of brittle dextral movements. The effects of a later episode of brittle-ductile sinistral transpression are widely preserved in both Basement and Cover assemblages, with intense deformation localised in the region of the major fault strands. The 1.8 Ga granites and dykes appear to relate to a regional extensional or dextral transtensional episode recognised in the Middle Proterozoic throughout the Amazon region. There is some circumstantial evidence for further minor fault reactivation during the Phanerozoic, and the region appears to be tectonically active in the present day, as illustrated by the occurrence of recent small-scale earthquakes and hot springs centred along the major fault traces. The influence of the basement architecture and the intensity of later reactivations appears to wane after a time of ca.1.0 Ga following the existence of a weakening effect on a lithospheric-scale with a finite life span, possibly originating in the underlying lower crust and mantle. There is widespread evidence that the mylonitic fabrics of the Itacaiúnas Shear Zone have controlled the orientation of later structures, including the Carajás and Cinzento fault systems. Long-term fault zone weakening mechanisms are recognised in the region. Brittle fracturing processes have caused increases in fault zone permeabilties allowing extensive ingress of fluids, some of which have caused e.g. gold and copper mineralisation.