Late Triassic initial subduction of the Bangong‐Nujiang Ocean beneath Qiangtang revealed: stratigraphic and geochronological evidence from Gaize, Tibet
2016
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.
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