Implications of centrifuge simulations of channel flow for opening out or destruction of folds

Abstract

Centrifuge models of modelling clay and silicone microlaminates upon silicone putty layers simulate folding and subsequent opening out and unfolding of a layered sedimentary cover sequence (the superstructure) upon a ductile substrate, e.g. migmatitic gneiss in the infrastructure of large hot orogens or Archaean terrains. After initial layer-parallel shortening a process akin to channel flow in nature is induced in models by removal of material to simulate foreland erosion. Isostatic readjustments due to lateral difference in gravitational loading without further shortening result in extrusion and exhumation of lower ductile layers and vertical collapse, opening out, and unfolding of microlaminate layers of the cover sequence. Many active folds developed in the superstructure during initial layer-parallel shortening open out and unfold during subsequent ductile flow of the infrastructure, especially where they are rounded in profile and a basal décollement to the cover sequence is present. Angular to chevron folds tend to be translated or passively rotated and undergo less increase in interlimb angle. Isolated synforms or synform–antiform pairs separated by broad areas of gentle undulations or plateaux in nature may be indicative that unfolding during channel flow has taken place. Fold axial surfaces, especially for folds of finely laminated layers, may also be rotated so that folds become hinterland-verging. Experiments suggest that where channel flow has occurred in nature, if unfolding were not recognised through detailed field observations, the amount of regional shortening would be underestimated. Field criteria to recognise unfolding in nature are suggested. Our models predict that veins and saddle reefs developed during folding may be potentially preserved in an area of open or sub-horizontal strata following unfolding, which has implications in exploration for fold-related mineralisation in large hot orogens or Archaean terrains.