TECTONIC FEATURES OF THE MOUNT JOLMO LUNGMA REGION IN SOUTHERN TIBET, CHINA
CHANG CHENG-FA AND ZENG SHI-LANG
This paper presents a brief account of the regional geology of the Jolmo Lungma region which lies south of the Yalutsangpo (Tsangpo) Eiver and between longitudes 85°30′E and 89°30′E.In view of the regional stratigraphy and tectonic history of the region, two belts could be recognized. Rocks of the southern belt lie upon a crystalline and metamor-phic basement, being a thick sequence of essentially unmetarnorphosed sediments, mainly marine, ranging in age from early Ordovician 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 Mount Jolmo Lungma and other lofty peaks, amounts to about 55 km while along the Yalutsangpo Valley a maximum thickness over 70 km is deduced.Near and roughly parallel to the above mentioned valley, a long, narrow, intermediate belt of ultramafic rocks occurs. This is the eastern continuation of the so-called "Indus Suture" and probably marks the line of collision, as postulated by some authors, where the northward drifting Indian Plate was thrusted under the Eurasian Plate. Northward, the Tahglha, the Kunlun and the Altyn Tagh ranges might be also outcomes of successive stages of subduction. In each case, a smaller southern plate impinged against and plunged beneath the ancient Eurasian Plate.