Abstract:

The Galashan extensional detachment structure is located in the east slope of northern Great Xing’an Ranges. There develop SE-dipping foliation, interlayer fold, augen structure, low-angle normal fault, sheath fold and rodding structures, as well as various ductile deformation micro-textures, such as mica fish, rotated porphyroblast and quartz ribbon in the extensional detachment belt. The temperature of the ductile deformation is about 300-400 ℃. The asymmetric interlayer folds and lineations indicate a large-scale top to SE extensional detachment structure. The ⁴⁰Ar/³⁹Ar dating of the muscovite from the granitic gneiss and marble within the deformation zone gives 144-147 Ma age, which represents the ductile deformation age of the extensional detachment. Combined with the regional metamorphic core complex data, we suggest that the formation of Galashan extensional detachment structure should be related to the post-orogenic collapse of the thickened crust due to Mongo-Okhotsk orogeny, which resulted in the regional extension in the Great Xing’an Ranges area.

Key words: Great Xing’an Ranges, Galashan, extensional detachment, ⁴⁰Ar/³⁹Ar dating