Deformational characteristics of the granitic gneiss unit from the 1113-1600 m core of the CCSD main hole

Abstract：The lithologic section of the upper 2000 m core from the main hole of CCSD reveals that the granitic gneiss unit of the 1113-1600 m interval of core is the downward extension of granitic gneisses of the Shihu slice of the North Sulu ultrahigh-pressure metagranitic gneiss imbricate tectonic slice at the surface. The upper and lower boundaries of this unit are ductile shear zones, and in the interior of this unit minor ductile shear deformation is developed. The shear sense markers (rolled porphyroclastic structure, etc.) are only seen locally. They, mainly located in interbeds of weakness, are dominated by the SE-NW thrust shear sense with subordinate NW-SE normal-slip shear sense. The latter is new evidence for the existence of extensional dome structure in the Sulu area. In the 1140-1280 m lithologic interval there occurs discontinuous, weak stretching lineation, which generally plunges SE at angles of 10-36. Some quartz in granitic gneisses occurs as polycrystalline quartz bands. The dominant mineral feldspar in the granitic gneisses essentially shows no phenomenon of dynamic recrystallization and has only the feature of weak stretching (X∶Y=2 or so); foliation of the granitic gneiss unit generally dips 170°E at 20°on the average, which is notably different from the attitude of foliation the other lithologic units. Foliation of the granitic gneiss unit probably represents E-W-directed structure before exhumation and deformation; whereas other lithologic units are greatly influenced by exhumation and deformation. The foliation mainly represents NE-NNE structure formed in the exhumation phase. EBSD analysis of the quartz fabric and comparison of the analytic results with the results obtained by the Fedorov stage indicate that the granitic gneiss unit in the 1113-1600 m core has undergone medium- to low-temperature deformation, with the high-temperature fabric survived locally. The shear sense is mainly the vergence of thrusting from SE to NW. The high-temperature fabric and medium-temperature fabric both indicate the SE-NW thrusting shear sense, reflecting the consistency of the senses of relative shear of the slice in the early stage and main stage of exhumation. The weak deformation in the interior of the granitic gneiss unit is due to the fact that rocks of the granitic gneiss unit were so competent during exhumation and deformation that they were deformed difficultly. Brittle deformation in the late stage is mainly marked by tenso-shear faulting.