Middle Triassic Turbidite Fan,Source Area and Plate Tectonics in Southern Guizhou and western Guangxi

Caused by tectonism, the Middle Triassic gravity flow deposits covering an area of 7000 square kilometres are widely distributed over southern Guizhou and western Guangxi. According to their origins. mudstone and conglomerate resulted from debris flow and turbidite from turbidity current may be distinguished. The turbidite fan model. the main sedimentary model, is characterized by multiple stages and progradation of the turbidite fan which trends to be elongated during the early stage of formation and radial during the later stage. varying with the features of the deposits. The results from sedimentary facies, palaeocurrent direction, petrology, geochemistry and mathematical geology show that the sedimentary system of gravity flow is developed in the marginal back-arc basins, and evolved due to tectonism from turbidite fan(Bannaian)through slope aprons(later Bannaian) to the mixed system of turbidite fan and slope aprons(Lanmuian). The North Vietnam oldland situated in the southwestern part of the turbidite basin is believed to be the main source area for the gravity flow system.