X.—Frictional Fusion Along a Himalayan Thrust

Abstract

At a height of 4400 metres in the Nepal Himalaya, a vitreous and vesicular rock was discovered by J. S. Scott in 1949. It is located along a line of intense brecciation in a region of high-grade granitic gneisses.This paper is based on a laboratory examination of a specimen of this rock and a few other specimens from the same locality. As some of these specimens have no exact counterpart hitherto described, a new but self-explanatory nomenclature has been employed in their description. The new rock-types—breccia with a cryptomylonite matrix, cryptomylonite and hyalomylonite—seem to represent, in this order, progressive stages in the degree of vitrifaction.Biotite, alkali feldspars and quartz participated in the vitrifaction, which was due to high temperatures and heat developed by friction between these minerals in a triturated rock medium of low thermal conductivity.An important function is attributed to the biotite which, in the more vitreous parts of the rocks, tends to disappear entirely. It is suggested that selective melting of this biotite has yielded to the liquid ferrous iron, and fluxing by this component is believed to be a critical factor although water must also be considered. Chemical analysis revealed no significant quantity of water in the glass-rich rock, but the development of vesicles and bubbles implies that there was more water in solution prior to its ebullition as a vapour phase.There appears to have been a remarkable variation in temperatures within extremely short distances, and there is some evidence that the specific melting points of the alkali feldspars may, in a few exceptional instances, have been attained.