Abstract:

The theory and method for 1∶50000 geological mapping in accretionary complex belt is a focus problem for present regional geological surveys. Based on 1∶50000 geological mapping, the characteristics of lithologies, collage style and structural framework for accretionary complex can be easily found, which then contributes to the knowledge of tectonic evolution in suture zone. After 1∶50000 geological mapping and 1∶10000 structurelithology mapping in Qomo Ri Indosinian accretionary complex in Central Qiangtang, it is discovered that this complex is comprised of intensivecleavaged matrix and tectonic blocks that weld each other by brittle, brittleductile and ductile shear faults forming at the oceanic subduction stage. The matrix are composed of fine plastic rocks from submarine fans, continental slope turbidite and deep ocean deposit, and also other strongly foliated rocks. Tectonic blocks mainly contain OIBtype ophiolite relics, mafic dike relics, seamount carbonate rock relics, exotic block relics etc. This complex is characterized by a stable regional foliation S2 of CCCtype, forming under a compressional background following oceanic subduction. It is also superimposed by postorogenic shallow level deformations showed by nonpenetrative foliation S3. The geological maps are expressed by means of a method named “threecomponent” and “fourfactor”. The above practice presents methods including outcrop observation, mapping unit classification, and graphic expression of map for 1∶50000 geological mapping in accretionary complex, which also provides implications for the present geological mappings in orogenic belt.

Key words: 1∶50000 geological mapping, accretionary complex, Qomo Ri, structure, orogenic belt