SOME TECTONIC FEATURES OF THE MT. JOLMO LUNGMA AREA, SOUTHERN TIBET, CHINA

Abstract

This paper presents a brief account of the regional geology of the Mt. Johmo Lungma area which lies south of the Yalu Tsangpo (Tsangpo) River and between the longitudes 85°30E and 89°30E. In view of the regional stratigraphy and tectonic history of the region, two belts may be recognized. The Southern Belt lies upon a crystalline and metamorphic basement, being a thick sequence of essentially unmetainorphosed sediments mainly marine, ranging iu age from Early Ordovieian to Early Tertiary with many horizons rich in fossils. Rocks are generally of shallow and stable water origin and seem to have nothing to do with geosynclines. So far as we know, the oldest fossiliferous beds in the Northern Belt belong to the Permo-Carboniferous. From the Permian on, the deposits differ from those of the Southern Belt and the difference becomes more distinguished during the Mesozoic. Here the rock types are highly varied, including flysch, graywackes, basic volcanics and radiolarian cherts. This thick series of geosynclinal origin is intensely folded and often thoroughly metamorphosed. From gravimetric and seismological data, it is estimated that the crustal thickness of the main Himalayas, including the Mt. Jolmo Lungma and other lofty peaks, amounts to about 55 km while along the Yalu Tsangpo Valley a maximum thickness over 70km is deduced. Near and roughly parallel to the above-mentioned valley, a long, narrow, intermittent belt of ultramafic rocks occurs. This is the eastern continuation of the so-called Indus Suture and presumably marks the line of collision where, as postulated by some authors, the northward drifting Indian Plate had been subducted under the Eurasian Plate. Northward, the Tanglha, the Kunlun and the Altyn Tagh ranges resulting from progressively older orogenies, might also be considered as the outcome of successive stages of subduction. In each case, a smaller southern plate impinged against and plunged beneath the ancient Eurasian Plate.