THE BANGONG SUTURE IN SOUTHEAST TIBET: A EXTENDED CONTINENTAL MARGIN OPHIOLITE

Mafic/ultramafic rocks and associated mélange units define the Bangong suture between the Lhasa and Qiangtang tectonic units, a suture that extends from western Tibet into SE China. In SE Tibet it contsist of at least three belts of mélange only two of which contain possible ophiolitc rocks. The northern most belt of mélange contains the largest mafic/ultramafic complex in SE Tibet where it contains pillow basalt unconformably overlain by deepwater Middle Jurassic limestone. The mafic/ultamafic rocks and enclosing mélange are unconformably overlain by redbeds of Upper Cretaceous age. To the south is a second belt of mélange with enclosed blocks of mafic/ultramafic rocks and serpentinite that trends NW into a belt of mélange but with only sedimentary blocks that finally ends farther NW into a broad area of Jurassic flysch. South of this belt are at least two areas of flysch with olistostromal blocks of only sedimentary rocks. None of these belts of rocks can be traced into one another and are truncated along their eastern sides by a major fault zone that juxtaposes them on the north against marine Mesozoic and Paleozoic strata of the Qiangtang tectonic unit that lacks evidence for a continental margin that might be expected adjacent to the Bangong oceanic (?) suture. This assemblage of rocks is interpreted as a rifted margin analogous to the Iberain-New Foundland margins adjacent to the present Atlantic Ocean, margins consisting of extended continental crust with grabens and ridges that lie inboard of the oceanic realm, a scenario fits the relations of a broad Bangong suture. The extended continental is crust underlain by mafic/ultramafic rocks in one or more grabens overlain by Jurassic and lower Cretaceous flysch before closing in mid-Cretaceous time. All these belts of grabens and ridges are truncated on their north and east side by a major strike-slip fault zone that juxtaposes the Qiangtang rocks against the extended continental margin rocks and the suture becomes cryptic farther to the southeast.