PATTERNS OF TURBIDITE SANDSTONE DEPOSITION IN THE NANPANJIANG BASIN: EVIDENCE TO CONSTRAIN THE TIMING OF PLATE TECTONIC COLLISIONS DURING THE ASSEMBLY OF CONTINENTAL BLOCKS IN SOUTH CHINA AND SOUTHEAST ASIA

The Nanpanjiang basin is surrounded by the Yangtze platform and contains several isolated carbonate platforms. It is bordered by the Precambrian Khamdian, Jiangnan, and Yunkai massifs on the west, northeast, and southeast respectively. It is bounded by potential suture zones, the Songma and the Ailaoshan, to the south and southwest.

Turbidites include the Lower 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 progradation of margins proximal to potential source areas. This study tests the hypotheses that clastic flux originated from Precambrian massifs or from an active convergence zone (volcanic arc) along the Songma and or Ailaoshan sutures, and that timing of clastic flux affected platform evolution.

Analysis of 1440 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.

QFL and QmFLT plots from point counts place the sandstones in the recycled orogen and dissected arc fields indicating a relatively mature source consistent with Precambrian massifs, although the Lower Triassic Shipao Fm has significant volcaniclastic component consistent with arc derivation.

Detrial zircon populations from 17 samples across the basin indicate: several samples have a few concordant zircon ages older than 3000 Ma⁽¹⁾, there are widespread peaks across the basin at 2500 Ma⁽²⁾, 1800 Ma⁽³⁾, 900 Ma ⁽⁴⁾ , and 440 Ma ⁽⁵⁾. Among the younger zircons a larger population dating to around 250 Ma⁽⁶⁾ occurs in the southwestern part of the basin and becomes less abundant to the northeast, where somewhat older populations of 260-290 Ma become more abundant. The detrital zircon data support the hypothesis of provenance from the Khamdian⁽²⁾, Jiangnan⁽⁴⁾, and Yunkai^(1,2,4,5) massifs as well as a volcaniclastic source from Permian-Triassic convergence⁽⁶⁾ in the southwest part of the basin.