GEOTECTONIC ATTRIBUTES AND TECTONIC EVOLUTION OF THE LUNPOLA AREA, XIZANG （TIBET）

Passing through the northern part of the Lunpola area, Xizang （Tibet）, the Bangong Lake-Nujiang deep fracture zone divides the area into two different first-order tectonic units, with the principal part lying south of the zone. North of the fracture zone, only part of the Meso-Cenozoic strata of the alpine tectonic layer is observed to be exposed sporadically. The strata are not thick and the types of formations are simple, and the tectonic disturbances and magmatic activity were relatively weak too. South of the fracture zone, the Caledonian and Variscan tectonic layers are both mainly represented by carbonate and clastic formations of shallowmarine origin, with a total thickness greater than 9345.15m. In the Caledonian tectonic layer almost no indications of tectonic disturbances and magmatic activity have been observed; while in the Variscan tectonic layer only a small number of gentle-dipping, short-axis folds and faults are present and magmatic activity was also very weak. The rocks in both tectonic layers have hardly undergone regional metamorphism. The Indosinian tectonic layer is exposed relatively sparsely, represented mainly by a flysch-like formation intercalated with grawacke and slliceous rock formations, with a total thickness of 1515.32m. It is characterized by basic-ultrabasic magmatic intrusions and linear fold and fracture structures. The Yanshanian tectonic sublayer is widespread, including flysch-like, siliceous, marine and continental volcanic, carbonate and clastic formations, with a total thickness greater than 10930.26m. In this sublayer, linear fold and fracture structures and magmatic activity are very well developed. The Tertiary System of the Himalayan tectonic sublayer is mainly distributed in the Lunpola and Baingoin basins, consisting mainly of dark-coloured pelite and clastic formations, in which folds, fractures and volcanic rocks are relatively well developed. The Quaternary System of the Himalayan sublayer is mostly composed of loose sands and gravels of fluvio-lacustrine facies and tillite facies. In this paper, on the basis of a summation of the characteristics of the development of various tectonic layers in the area, combined with the data of the regional settings and ideas of various schools, the authors suggest that south of the fracture zone the Paleozoic forms the sedimentary cover of a paraplatform and that the Mesozoic and Cenozoic are infillings of depressions and fault troughs respectively; besides, a corresponding division of geotectonic units throughout the area is also suggested. In the last part of the paper the history of tectonic evolution, both vertical and lateral, of the structures in the area is discussed.