TECTONIC AND SEDIMENTARY EVOLUTION OF TRIASSIC NANPANJIANG BASIN, SOUTH CHINA

As the lion’s share siliciclastic accumulation exposed in South China, Triassic turbidites within the Nanpanjiang basin are generally termed as “flysch” to support Indosinian orogensis between the South China and Indochina blocks. Detrital zircon geochronology from Early Triassic shows predominant ca. 250–300 Ma population, in contrast, Middle Triassic turbidites yield several comparable age groupings dominated by ca. 250–300, 350–400, 400–550, 900–1250 and minor 1550–1950 Ma. Hf isotopic compositions show both juvenile crustal growth and crustal reworking for all the age groupings, but Hf isotopic signatures for Phanerozoic populations are narrow and toward more radiogenic compositions. Paleocurrent restoration in conjunction with detrital zircon signatures suggest that the Early Triassic provenance was the Permo-Triassic plutono-volcanic belt in Northeastern Vietnam, whereas Middle Triassic turbidites were sourced by the Cathaysia block. Backstripping of two established 2-D cross sections, roughly perpendicular to the strike of intrabasinal faults, document the subsidence rate and magnitude attenuates northeastward and the “horst and graben” tectonic subsidence pattern of the basin across the intrabasinal faults. Identified deep-water architecture elements suggest that the Early Triassic witness the “fill-and-spill” stratigraphic development of deep-water gravity-flow deposits in the southern Nanpanjiang basin. During the Anisian period of Middle Triassic, the turbidity current transformed by river flooding fill the southern basin while the starved condition of the northern basin proceeded. Subsequent differential subsidence induced substantial turbidite influx arrived throughout the basin, with only the northernmost isolated platform (Great bank of Guizhou) developed during Ladinian. Our preferred interpretation is evolution from Early Triassic extensional back-arc basin, due to the northward subduction of the oceanic lithosphere underneath the Northeast Vietnam, to a transtensional basin controlled by ‘horse-tail’ tectonics. This study implies that Triassic tectonics of the South China block had a strong bearing on the interplay of clockwise rotation of the South China block and subduction of the paleo-Pacific.