Geological mapping of the 1985 Chinese—British Tibetan (Xizang—Qinghai) Plateau Geotraverse route

Abstract

The 1:500,000 coloured geological map of the traverse route combines observations from the Geotraverse, previous mapping, and interpretation of orbital images. The position of all localities visited by Geotraverse participants and basic geological data collected by them along the traverse route are shown on a set of maps originally drawn at 1:100,000 scale, reproduced on microfiche for this publication. More detailed mapping, beyond a single line of section, was achieved in five separate areas. The relationships between major rock units in these areas, and their significance, are outlined in this paper. Near Gyanco, (Lhasa Terrane) an ophiolite nappe, apparently connected with outcrops of ophiolites in the Banggong Suture about 100 km to the north, was under thrust by a discontinuous slice of Carboniferous—Permian clastic rocks and limestone, contrary to a previous report of the opposite sequence. At Amdo, a compressional left-lateral strike-slip fault zone has modified relationships along the Banggong Suture. Near Wuli, (northern Qiangtang Terrane) limited truncation of Triassic strata at the angular unconformity below Eocene redbeds demonstrates that most of the folding here is of Tertiary age. The map of the nearby Erdaogou region displays strong fold and thrust-shortening of the Eocene redbeds, evidence of significant crustal shortening after the India- Asia collision began. In the Xidatan-Kunlun Pass area, blocks of contrasting Permo—Triassic rocks are separated by east-trending faults. Some of these faults are ductile and of late Triassic — early Jurassic age, others are brittle and part of the Neogene—Quaternary Kunlun leftlateral strike-slip fault system. Some more significant remaining problems that geological mapping might help to solve are discussed briefly, including evidence for a possible additional ophiolitic suture within the Qiangtang Terrane.

Late Oligocene-Early Miocene thrusting in southern East Kunlun Mountains, northern Tibetan plateau

“…Since 1980s, geologists from both China and overseas have jointly surveyed and studied the Tibetan plateau, including East Kunlun Mountains, and developed different tectonic models (Tapponnier et al, 2001;Yin and Harrison, 2000). Kidd et al (1988) compiled a small-scaled geologic-tectonic map across East Kunlun Mountains along the Qinghai-Tibet highway through route reconnaissance and remote sensing images. Kidd and Molnar (1990), Seismological Bureau of Qinghai Province and Institute of Crust Stress (1999), and van der Woerd et al (2002Woerd et al ( , 2000Woerd et al ( , 1998 (Wu et al, 2007).…”

Tertiary structural evolution of the Gangdese Thrust System, southeastern Tibet

“…Whether the presently high elevation of the Tibetan plateau was caused by the Indo-Eurasia collision alone, or was contributed to by crustal thickening prior to the collision, is a controversial [Kidd et al, 1988;England and Houseman, 1986;England and Searle, 1986] issue that is central to determining the crustal mass balance during collision [e.g., Le Pichon et al, 1992;England and Houseman, 1986;Richter et al, 1992]. Our preliminary mapping in the Chuoqin area, south-central Tibet (Figure 1), suggests that crust in this region was thickened by at least 40-50% over a belt 40-70 km wide by thrusting and folding.…”

“…This fault was first mapped by Chinese geologists [Wang et al, 1983] and later included in regional compilations of southern Tibetan geology [Burg, 1983;Liu et al, 1988;Kidd et al, 1988]. The RZT is clearly younger than the GT, because its hanging wall strata locally thrust over the trace of the GT, putting both the hanging wall and footwall rocks of the GT below its footwall.…”

Finding of high-pressure mafic granulites in the Amdo basement, central Tibet

“…It is predominately composed of strongly foliated orthogneisses that contain small but abundant mafic amphibolite lenses [25][26][27][28]. The gneisses were intruded by widespread, generally undeformed granitoids of variable composition (granite, monzonite, granodiorite, quartz-syenite, and diorite) during Mesozoic time [25][26][27][28][29][30]. The presence of sillimanite, K-feldspar, and garnet in the paragneisses of the Amdo basement, as well as the migmatites, suggest upper amphibolite-facies metamorphic conditions [27].…”