CHAOTIC GEOLOGICAL BODIES IN CHINA
中国的混杂地质体

Chaotic geological bodies, an important indication for finding paleo-subduction zone and paleo-suture zone within continents, are the products of plates moving from each other. They may be divided into three types, i. e. ophiolitic melange (or serpentinite melange), tectonic melange (or argillo-arenaceous melange), and olistostrome. This paper describes in detail the chaotic geological bodies along the Yarlung Zangbo River, the Jinsha River, the West Qinling Mountains and Western Junggar which serve as examples to delineate the characteristics of the three types. They share a character in common of consisting of three parts, i. e. exotic blocks, autoclastic blocks and matrix. The fossil chaos are usually present in them.In China chaotic geological bodies are distributed chiefly in four greattectonic belts, i. e. Ancient Asian, Central China, Tethyan and the Circum-Pacific Ocean Tectonic Belts which are located among several rigid massives(median massives). Moreover, chaotic geological bodies concentrically occuralong paleo-subduction zones, suture zones and transform faults.The distribution of the three types reveals that they were all formed in subduction trench. The differences in the distances away from the central line of the trench and in the geological processes brought about various formative fashions.Ophiolitic melange belongs to a pure tectonic origin. It was formed in the central line of the trench. During or after the oceanic crust in the form of a young foldbelt fringed on a continental margin, an ophiolite nappe occurred along the subduction zone or collision zone between plates, and then was destroyed by overthrusting, thrusting and imbrication, resulting in the formation of ophiolitic melanges. Therefore, the ophiolitic melanges are a polytectonic product, in which the ophiolite is exotic, or high allochthon. But terrigenous sediments or shelf neritic deposits, which are younger than the tectonically overlying ophiolite, are autochthonous.Olistostrome was mainly formed by deposition further away from the central line of the trench. The important factors for its formation are the offscraping of the overriding plates to the sediments on the subduction oceanic crust, the self-collapsing of fore margin of the overriding plate, and the transportation of the products of collapsing by turbidity current. The components of blocks depend upon the location of a trench subduction zone. When the trench is located on the oceanic crust, ophiolitic olistostrome will be formed. Its embed ment in turbidite also proved its depositional origin.During subduction-collision of plates, tectonic melange was formed from olistostrome reworked by strong tectonism rather near the central line of the trench. In addition it may be also formed in such a way that normal sequences were broken up under strong tectonic compression. Its overlying and underlying beds are both marked by tectonic contact, i. e. thrusting (or fault) contact. The matrix of tectonic melange