STRUCTURAL AND PALEOGEOGRAPHIC ELEMENTS OF THE NANPANJIANG BASIN, GUIZHOU, GUANGXI, AND YUNNAN PROVINCES, SOUTH CHINA. A COMPILATION FROM SATELLITE IMAGES, REGIONAL GEOLOGIC MAPS AND GROUND OBSERVATIONS

The Nanpanjiang basin was an embayment in the southern margin of the Yangtze micro-continent. Marine sedimentation continued from the Early Paleozoic to the Late Triassic when synorogenic turbidites filled the basin. Triassic platforms include a great diversity of architectures and evolutionary histories that reflect differences in tectonics, subsidence, carbonate production, and siliciclastic influx. The contrast between the high-relief karsted carbonate terrain and stream-eroded siliciclastic turbidites strikingly delineates platforms in satellite images. The Yangtze platform (YP) margin that rims the basin extends 530 km in a sigmoid from Guizhou to Yunnan. Five isolated platforms (IP), up to 150 km across occur in southern Guizhou and Guangxi. The YP and IP evolved from ramps with oolite margins in the Early Triassic to Tubiphytes reef margins in the Middle Triassic. The western YP and the northernmost IP developed high-relief, retreating, erosional escarpments following the reefs while at the same time the northern YP prograded 3km over basinal siliciclastics. The western YP was drowned and buried by turbidites in the Late Triassic whereas shallow-water sedimentation continued until burial by siliciclastics in the north. The IP show a southward pattern of earlier termination and burial by siliciclastics and a southward increase in number and thickness of felsic tuff horizons. These dramatic differences in platform architecture resulted primarily from differential tectonic subsidence probably reflecting Late Triassic arc collision to the south of the basin. Narrow faulted N-S and NE-SW trending synclines separated by broad anticlines stretch across the northern YP. The western YP contains a series of N-S, NW-SE and NE-SW trending folds that form complex triangular fold domains. This folding occurred between the Late Jurassic and Late Cretaceous. A spectacular, northward convex, fold and fault belt formed during the Late Tertiary and extends across the southern part of the basin.