Sedimentary chaotic melanges and their tectonic significance:Upper Cretaceous Zongzhuo Formation,Gyangze,Xizang

The sedimentary chaotic melanges in the Late Cretaceous Zongzhuo Formation, Gyangze region,Xizang are aligned to be oriented nearly EW in parallel with the Yarlung Zangbo suture,and consist of three parts,i.e."matrix",autochthonous and allochthonous blocks.The "matrix" comprises dominantly fine-grained clastic rocks.The autochthonous blocks were formed due to the deformation and displacement of the unconsolidated sedi-ments caused by gravitational and other geological processes.The allochthonous blocks are characterized by mixed compositions,diverse morphologies and varying sizes,and yield the Late Triassic-Early Cretaceous belemnites and bivalves.Generally,these blocks occur separately,and geomorphologically display prominent reliefs.Their metamorphosed degrees don't seem to be in tune with the "matrix".The subduction of the Himalayan slab toward the Gangdise-Nyainqentanglha slab from the Early Cretaceous onwards allowed a nearly EW-trending elongated trough to be created on the passive continental margins. When the overriding slab was subjected to intense fragmentation,the blocks of varying sizes then slumped down into the trough to form the sedimentary chaotic melanges.