西藏江孜地区上白垩统宗卓组沉积混杂堆积基本特征及构造意义

分布于藏南江孜一带的上白垩统宗卓组沉积混杂堆积岩呈近东西向与雅鲁藏布江缝合带平行相伴,由“基质”、原地岩块、外来岩块三部分组成。“基质”以正常沉积的细碎屑岩为主;原地岩块是沉积物未完全固结时,由于重力或其它地质作用使之变形变位后形成的;外来岩块成分复杂、形态各异、大小悬殊,不同岩块中产晚三叠世—早白垩世的箭石、双壳类等化石。岩块间很少直接接触,地貌上明显突起,其变形变质程度与“基质”极不协调,它是早白垩世以来,喜马拉雅板片向冈底斯－念青唐古拉板片俯冲,于被动陆缘形成近东西向的狭长海槽,而仰冲板片受到强烈的破碎,大小不等的岩块滑落于海槽中构成的沉积混杂堆积岩。

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.