Morphology and geometry of duplexes formed during flexural-slip folding

Abstract

Bedding-parallel slip on the limbs of chevron-style folds during flexural-slip folding results occasionally in the formation of small-scale duplexes. The duplexes described here from Upper Carboniferous turbidites in SW England generally range in thickness from 0.3 to 30 cm, and are up to 2.4 m long. They have smooth nearly flat roofs, and both the internal thrusts and the slickenfibres on them are commonly oblique to the mean transport direction as indicated by slickenfibres on the floor and roof thrusts. The morphology of the duplexes suggests that they developed between active floor and roof thrusts and they show a characteristic lack of hangingwall anticlines on the link thrusts. Their shear sense always corresponds to that required by flexural slip, and the total bedding-parallel displacement across each is commensurate with that predicted by the flexural-slip model. All thrust surfaces in the duplexes are marked by quartz fibre veins (a feature which distinguishes them from soft-sediment and other pre-folding duplexes) and carry slickenfibres whose mean orientation and complex variations in slip direction on different fibre sheets on the same slip surface are identical to those on flexural-slip movement horizons from the same fold limb. The duplexes form either as a result of resistance to thrust propagation by local facies or bed thickness changes, or develop as transfer structures between the tips of movement horizons propagating along adjacent slip surfaces. Late-stage duplexes develop from imbricated fibre veins and also form on slip surfaces oblique to the axial planes of major folds.