Characteristics and occurrence of bedding-parallel slip surfaces and laminated veins in chevron folds from the Bendigo-Castlemaine goldfields: implications for flexural-slip folding

The occurrence of bedding slip planes and associated slip-striated, laminated, quartz veins in the Bendigo-Castlemaine goldfields, southeastern Australia was investigated. Bedding slip planes are concentrated in thin siltstone layers. Data from surface stratigraphic sections on the limbs of five major anticlines gave a mean slip-plane spacing of 8.9 m (N = 96), and a mean laminated vein thickness of 19 mm. Most laminated veins are < 20 mm thick and discontinuous along strike and around fold hinges; thicker veins are correspondingly more continuous and are traceable around fold hinges. The mean thickness of laminated veins from individual fold limbs increases with increasing mean spacing of slip planes on each limb, suggesting a sympathetic relationship between vein thickness and net slip; however, there is no such correlation between individual vein thickness and spacing. These relations are explained using a model for the progressive generation of bedding slip planes via shear failure at competent-incompetent layer interfaces as a result of shear strain rate incompatibilities between competent and incompetent beds during fold limb steepening. The model predicts flexural-slip activity continuing well into the stage of homogeneous flattening of the folds, and gives rise to a population of slip planes dominated by those with low net slip.