LateTriassic initial subduction of the Bangong- Nujiang Ocean beneath Qiangtang revealed: stratigraphic and geochronological evidence from Gaize,Tibet
M. Zeng, X. Zhang, H. Cao, F.R. Ettensohn, W. Cheng and X. Lang
Journal name: Basin Research
Issue: Vol 28, No 1, February 2016 pp. 147 - 157
Info: Article, PDF ( 6.29Mb )
Sedimentological and geochronological studies along a north–south traverse across the Bangong-Nujiang suture zone (BNSZ) in Gaize, Tibet provide evidence for a Late Triassic–Jurassic accretionary wedge accreted to the south margin of Qiangtang. This wedge, preserved as the Mugagangri Group (MG), records evidence for the northward subduction of the Bangong-Nujiang Ocean (BNO) beneath Qiangtang. The MG strata comprise two coarser intervals (lower olistostromes and upper conglomerates) intercalated within sandy turbidites, which are consistent with timing and forearc stratigraphy during subduction initiation predicted by geodynamic modelling. Following the model, the northward subduction of the BNO beneath Qiangtang and subsequent arc-magmatism are inferred to have begun, respectively, at ca. 220 Ma and ca. 210 Ma, with respect to depositional ages constrained by youngest detrital-zircon ages. The initiation of arc-magmatism is also supported by provenance transition reflected by sandstone detrital modes and age patterns of detrital zircons. Previously, evidence for an incipient arc was lacking, but the timing of Late Triassic BNO subduction and related arc-magmatism is coincident with an important Late Triassic magmatic event in central Qiangtang that probably represents the ‘missing’ arc. Other Qiangtang events, such as exhumation of the Qiangtang metamorphic belt as a source area, and development of the Late Triassic Nadigangri deposits and bimodal volcanism, are more easily explained in the tectonic context of early northward subduction of the BNO beneath Qiangtang, beginning at about 220 Ma.