The flexural-slip mechanism

Abstract

Detailed field studies of turbiditic sequences from South Georgia (South Atlantic), North Devon (England) and Cardigan Bay (Wales) show that flexural slip occurs on discrete movement horizons between rock packets in which the beds have welded contacts. Stair-stepping displacements of sedimentary dykes and of early quartz veins show that the movement horizons generally have a decimetre to metre spacing and are marked by bedding-parallel quartz veins. These veins are from 1 mm to several cm thick and can be used to identify movement horizons in the absence of displaced markers; they consist of several sheets of quartz fibres which each carry a slickenfibre lineation and which together preserve a record of the displacements on an individual surface. Complex slickenfibre patterns, and departures from ‘ideal’ behaviour, in which slip occurs orthogonal to the fold hinge, probably result from changes in slip vector on the limbs of growing non-cylindrical folds. Movement horizons show many of the features associated with large-scale thrusting, such as ramps, duplexes and imbricate structures, and the shear sense given by fibre steps on these surfaces and by duplexes within stratigraphically-restricted packages changes across fold hinges. Chevron folds are thought to develop mainly by flexural flow in the early stages, with flexural slip becoming dominant later as the beds become lithified; new slip surfaces are generated as the dip of the fold limbs increases.