Basin development and provenance of the Lower to Middle Jurassic Ban Don Group in Indochina: Implications for the Jurassic Palaeo-Pacific subduction and drainage patterns

Reconstructions of the Mesozoic Palaeo-Pacific are contentious due to significant overprinting during Cenozoic tectonic reorganization in the South China Sea. Here we document rapid (c. 35 myr) basin development, infilling, and pervasive folding of the ~4 km thick Ban Don Group, a Lower to Middle Jurassic basin succession in south-central Vietnam. We address the tectonic significance of the Ban Don Group using sedimentary petrography, detrital zircon geochronology, and structural constraints. Petrography of sedimentary rocks of the Ban Don Group shows mixed recycled orogen character and volcanic arc provenance. Detrital zircon U-Pb geochronology supports sourcing from the Indochina cover units and basement blocks, such as the Kontum Massif, and importantly identifies contemporaneous volcanic arc sources. Structural constraints require a phase of pervasive shortening shortly after deposition of the Ban Don Group. Together, these datasets, along with regional tectonic constraints suggest that the Ban Don Group was deposited along a convergent margin, interpreted here as back-arc basin during Early-–Middle Jurassic subduction of the Palaeo-Pacific Plate under Indochina. Extension in the Indochina Block was likely a result of a change in the Palaeo-Pacific subduction angle. The NW basin axis of the Ban Don Group is compatible with Lower to Middle Jurassic NW-directed back-arc extension and associated NW-dipping subduction. The symmetric shape and orientation of the Ban Don Group salient strongly supports NW Palaeo-Pacific Plate motion in the Late Jurassic. Therefore, NW-dipping subduction of the Palaeo-Pacific Plate during the Jurassic was the driving force for Ban Don Basin subsidence and subsequent shortening.