Geotectonic Pattern And Geological Evolution Of The Gandise-Lasha Landmass In Tibet

The Gandise— Lasha landmass in Tibet, westward joining Keshimier arc massif andeastward extending to Tengchong landmass in West Yunnan, is located between theYarlungzangbojiang suture and the Ban-gong-Co-Nujiang suture. In geology circle, there aremany different opinions on the division of sub-units of tectonics in the landmass and itsgeotectonic attributes. On the bases of the new discoveries of the Asou tectonic mélange zoneand the late Jurassic Epoch--early Cretaceous Epoch magma arc in Wenbu and in Nixiong–Xurucuo and the N-S garben during the surveying of regional geology of the 1:250000 BangduoDistrict sheet and Cuomei District sheet in the middle of the landmass, the tectonic units intowhich can be divided from north to south in the middle of the landmass are as follows: ⑴ the Mianqianri Cretaceous relic sea basin: It is a relic sea basin formed after theconsumption of the small Asou ocean basin (margin sea basin) and characterized for a set ofstable carbonate sediments containing marine fossils. ⑵ the Asou tectonic mélange zone: Thezone, stretching in NNW strike along the Asou line in the middle-north study area, has thegreatest width of about 14km. The tectonic mélange consists of such rocks with differentproperties and various ages as ophiolite rock mass disconnected, flysch, deep sea siliceous rock,muddy rock, volcanic rock, carbonate rock, shallow sea clastic rock and split Permianmass .These rocks are mixed up with each other. ⑶ the Taricuo— Wenbu Yanshanian magmaarc: The magma arc, a mature type of island arc which was developed on the base of a early splitmass of late Palaeozoic strata, abounds with late Jurassic Epoch--early Cretaceous Epochcalc-alkaline volcanic rock and contemporary granitic plutons. Its northern border is the Niubaoformation faulted basin and the Taricuo faulted basin within arc,but its southern plutons had anintrusive contact with late Paleozoic strata. Its formation and evolution were in a close correlationwith the Ban-gong-Co-Nujiang ocean and its back arc small ocean basin subducting northward,forming the magma arc with superposition of poly-terms subvolcanism and plutonic magmatism.(4) the Cuoqin— Bangduo late Mesozoic compound back-arc basin: Stretching in E-W strike inthe middle study area, the basin has a northern border of the Zhala rupture and theTaricuo— Dangxiong Yanshanian magma arc and a southern border, namely theLonggeer— Namucuo-Chongsha rupture in the back arc faulted upwarping zone. The basinsediments consist of Luoshize group , Jiega formation ,Jingzhushan formation and LinzizongGroup.(5)the Nixiong— Geergeng faulted upwarping zone: Stretching in NWW strike alongNixiong— Geergeng--Cuomei in the middle study area, the faulted upwarping zone has a mainpart composed of Carboniferous Period— Permian Period strata and the Xurucuo ultra-unit of lateJurassic Epoch and the Nixiong ultra-unit of early Cretaceous Epoch, etc. Iron and copperPolymetallic deposits have been lately found in their contact. The formation of the faultedupwarping zone is closely relative to poly- activities of both of regional boundary great rupturezones, one in the south and the other in the north. (6)the Gandise –Chazi Himalayan magma arczone: As the main part of the Gandise marginal volcano— magma arc, the Gandise –ChaziHimalayan magma arc zone is a continental margin volcano— magma arc developed fromYarlungzangbojiang ocean subducting north during late Cretaceous Epoch--Eocene Epoch. Itsmain part is composed of the calc-alkaline series volcanic rock of Linzizong Group ofPalaeocene Epoch--Eocene Epoch and the contemporary granite. ⑺ theDanqiongcuo— Xurucuo Quaternary— Neogene graben: Distributed alongDanqiongcuo— Xurucuo in the eastern study area, the graben has a width of 5— 22kw and aS— N length of 70km and consists of a group of S— N ladder normal faulted subsides and lakebasins among them. The filling in...