The anatomy of shallow-crustal transpressional structures: insights from the Archaean Carajas fault zone, Amazon, Brazil

The Carajas fault zone (CFZ) is the most prominent structure in the E-W-trending Carajas strike-slip system (CaSSS), an ancient upper crustal fault network of probable Late Archaean age that cuts across the Precambrian Amazonian Craton in Brazil. The subvertical faults reactivated pre-existing, high-grade basement shear-zone fabrics (Itacaiunas shear zone) and display a complex, long-lived history of movement ( >1 Ga) dominated by oblique- and strike-slip displacements. A postulated early regional phase of dextral movement downfaulted units of low-grade to unmetamorphosed basement rocks into dilational bends and offsets in the CaSSS. Subsequent sinistral transpression ca. 2.6 Ga led to faulting and folding during partial inversion of the cover rocks in these dilational sites with much of the associated deformation focused in the immediate vicinity (< 2 km) of the major fault traces. The youngest sequence of rocks affected by sinistral transpression along the CFZ-the Aguas Claras Formation-were apparently deformed prior to complete lithification and developed a complex assemblage of disharmonic, curvilinear folds and associated transpressional fault arrays. The structures are generally comparable to published descriptions of fold and fault assemblages from shallow-crustal transpressional settings, notably along the San Andreas Fault. However, the meso-scale deformation patterns observed in the Aguas Claras road section point to a hitherto unrecognised level of structural complexity that may exist adjacent to comparable transpressional fault zones in many Mesozoic and Cenozoic settings. Our findings suggest that the architecture and underlying dynamic controls of structural development in shallow crustal transpression zones have remained similar for at least the last 2.6 Ga. (C) 2000 Elsevier Science Ltd. All rights reserved.