THE MURCHISON GREENSTONE BELT (SOUTH AFRICA): A GENERAL TECTONIC FRAMEWORK

摘要

This paper discusses the deformation pattern of the Murchison Greenstone Belt, which is located in the northeastern Kaapvaal Craton, South Africa. The belt strikes northeast to southwest, is surrounded by gneisses and granitoids, and contains Meso-Archaean metavolcanics and metasediments. It is narrow, strongly deformed, and deeply rooted in the centre with a hulk keel geometry. Regional fabrics consist of a sub-vertical foliation at a low angle to the belt strike and bears steeply plunging lineations. From available data and our observations, we constructed a map of the foliation trajectories. The map reveals the following: the deformations are distributed, both the belt and the northern bounding gneisses are deformed together in a ductile way, and the plutons were emplaced during regional deformation. The overall structural pattern, together with the existence of contemporaneous intrusive and extrusive rocks outcropping at the surface, supports a vertical tectonic model, with burial of the upper crustal rocks within the underlying weak material during horizontal crustal shortening. The occurrence of leucogranite intrusions is consistent with the melting of buried sediments belonging to the belt at depth. Finally, ages available on syn-kinematic granitoids (2.97 to 2.77 Ga) suggest a long-lasting tectonic process.