Paleocurrent Patters and Detrital Zircon Ages of Triassic Turbidites in the Nanpanjiang Basin South China: Implications for Tectonic History and Influence on Carbonate Platform Evolution

The Nanpanjiang basin forms an embayment in the vast Yangtze platform and contains several isolated carbonate platforms. It is bordered by the Khamdian, Jiangnan, and Yunkai Precambrian massifs on the west, northeast, and southeast respectively. Further, it is bounded by potential suture zones, the Songma and the Ailaoshan to the south and southwest. Collision along these zones may have occurred in the Triassic, however the timing is controversial.

Turbidites include the Early Triassic volcaniclastic Shipao Fm. in the southwest, the Middle Triassic Baifeng, Xinyuan, Lanmu and Bianyang fms. and the Late Triassic Laishike Fm. Carbonate platforms show a pattern of greater subsidence, earlier drowning and clastic burial in the southern basin and greater margin progradation proximal to potential source areas. This project uses paleocurrents to test the hypotheses that clastic flux from the south resulted from active convergence along the Songma and or Ailaoshan sutures and that timing of clastic flux affected platform evolution.

Analysis of 880 paleocurrent measurements indicates two major directions of sediment fill. The northern part of the basin was primarily sourced by the Jiangnan massif in the northeast, and the central and southern parts of the basin were primarily sourced from the south, possibly from the Yunkai or the Songma suture zone.

Preliminary analysis of detrial zircon populations from 17 samples across the basin indicate: (1) several samples have a few concordant zircon ages older than 3200 Ma, (2) there are widespread peaks across the basin at 1800 Ma and 2500 Ma, the latter of which may indicate derivation from the Khamdian massif, (3) a widespread 900 Ma population likely matching with the Jiangnan uplift, (4) a larger population of younger zircons dating to around 250 Ma occurs in the southwestern part of the basin and decreases to the northeast, where somewhat older populations of 260-290 Ma become more abundant, and (5) an additional peak centered at around 440 Ma is widespread across the basin. The detrital zircon data support the hypothesis of Triassic convergence and volcanic source in the southwest part of the basin as well as provenance from the Khamdian and or Jiangnan massifs in the northern part of the basin. Several populations do not match with previously reported basement ages and need evaluation as to whether they originated from the northerly massifs or yet unidentified terranes to the south.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California