OPHIOLITIC SUBMARINE FANS AND THEIR PIATETECTONIC SETTINGS IN THE XIGAZE GROUP ALONG THE YARLUNG ZANGBO SUTURE ZONE IN XIZANG

The ophiolitic sandy and gravelly submarine fans are developed as a kind of distinctive sedimentary unit in the fine-grained flysch of the Xigaze Group along the Yarlung Zangbo suturc zone in Xizang. The characteristics of these submarine fans are as follows. (1) They are relatively small in scale. (2) The channel deposits are especially well developed in both upper and middle fans, and their sequences are very clear. (3) The channel are frequently migrated and braided. (4) The sandy and gravelly high-density turbidity current deposits and the cohesive debris flow deposits are prominant in these submarine fans. (5) The sediments are coarse-grained and contain a large amount of cobbles and boulders, indicating that they are relatively proximal and rapidly accumulated. (6)Much of the sediments are derived mostly from ophiolites and only a small part from island-arc volcanic-magmatic rocks and older sedimentary strata, which may make these submarine fans rare in both modern and ancient equivalents in the world.
These distinctive submarine fans were developed on the margins of the residual flysch basins during the closing of the Yarlung Zangbo Ocean and the onset of the collision of the Indian plate with the North Xizang plate. The collision of these plates resulted in the overthrusting and uplifting of the ophiolitic complexes and the oceanic crust, and thus in the formation of an outer arc orogenic zone which serve as the main provenance of these fans bodies. Meanwhile, intense overthrusting and napping may also introduce part of island-arc rodk and older strata to the margins of the residual basins which are interpreted as an additional source of sediments.