CENTRIFUGE MODELLING OF FOLDING IN HIGH-GRADE ROCKS DURING RIFTING

Abstract

Centrifuge modelling is used to simulate the progressive development of structures in a simplified crustal profile during rifting. This paper focuses on folding of ductile layers representing the middle to lower crust. Displacement along a pre-existing cut in the layer representing mantle lithosphere creates a broad shear zone in overlying ductile layers and an asymmetric rift in upper layers. The footwall of the ‘mantle lithosphere’ layer and overlying shear zone are rotated to sub-horizontal during rise of the basal ductile layer (representing asthenosphere) as an isostatic response to thinning in the extended model. Synthetic and antithetic faults develop in models with thicker ductile ‘crustal’ layers. Although upper, semi-brittle layers, representing upper crust constitute a simple fault-bounded or sag ‘basin’, the underlying ductile layers are complexly folded. Open, upright folds developed over the crest of the footwall ‘mantle lithosphere’ layer are tightened and increased in amplitude as they progressively rotate to a recumbent attitude. Refolding occurs during rise of the basal ductile material representing asthenosphere and from boudinage of overlying competent layers. This study suggests that regional-scale folds in some high-grade terrains previously interpreted as evolving in a convergent margin tectonic setting may instead be produced as a result of flow of the ductile crust during rifting.