Fold nappes as asymmetric mantled gneiss domes and ensialic orogeny

Abstract

Descriptions are presented of the development of asymmetric gravity-driven structures rising from wedge-shaped bodies of light fluids overlain by denser fluids. Two programmes of experiments are described: 1. (1) the stabilization of wedges of oil overlain by glucose syrup in the field of gravity 2. (2) the stabilization while spinning in a centrifuge of wedge-shaped bodies of a light stiff silicone putty beneath denser and less viscous silicone putties. The development in these experiments of structures analogous to either basement cored domes or fold nappes typical of many orogenic belts is found to depend mainly on the inclination of the interface between the light and the overlying heavier fluids. Simple dome-like structures rise from this interface if it is horizontal or very shallow, whereas nappe-like structures alone develop if the interface is steeper than about 25° to the horizontal. A range of inclinations exist from which complex combinations of “domes” and “nappes” interfere to produce what would appear to be episodic deformations on small scales. It is argued that these experiments demonstrate that basement cored fold nappes are merely asymmetric mantled gneiss domes and that an Atlantic-type continental margin is, with an increase in the geothermal gradient, analogous to the unstable configuration of fluids at the start of the experiments. The conceptual model of ensialic orogeny due to Ramberg (1967) based on experiments like these is described and its implications discussed.