Horizontal tectonic deformation geometries in a late Archaean sedimentary sequence, Belingwe greenstone belt, Zimbabwe

Abstract

In the Belingwe greenstone belt of Zimbabwe, structural evidence from the circa 2.65 Ga old sedimentary Cheshire Formation which overlies and is imbricated with a mafic volcanic unit is consistent with thrusting of the greenstone sequence. The Cheshire Formation consists of a karstified carbonate ramp sequence overlain by siliciclastic turbidite deposits that formed in a southeast deepening basin. The earliest deformational structures formed during a syndepositional to postdepositional, thin-skinned thrusting event (D 1) that affected poorly consolidated sediments and is recorded in bedding-parallel ductile shear zones, boudins, folds, and block-in-matrix structures. D 1 shear zones separate the volcanic sequence and the overlying sediments and occur between the carbonate and siliciclastic units. Syntectonic sulphide mineralization and silicification of mainly fine-grained sediments along the main thrust faults gave rise to the formation of rocks similar in appearance to banded iron formations. Stratigraphic units were locally duplicated along the D 1 thrust faults, including a tectonic slice of mafic volcanics that was emplaced onto carbonates along a chaotic unit similar to a tectonic melange. The elongation of inclusions in the melange zone and lineations in ironstones together with kinematic indicators suggest that stratigraphic duplication resulted from northwestward directed tectonic transport. Deposition of the Cheshire Formation took place in an asymmetric, foreland-type basin contemporaneously with thrusting. Soon after deposition the formation was incorporated into the thrust stack. Subsequent deformation events include tight upright folding, gentle cross-folding, and dextral strike-slip faulting of the greenstone succession. Evidence for early thrusting suggests that horizontal tectonic processes played an important role in the evolution of the Belingwe greenstone belt.