Deformational history of the Wutai Complex: implications for the Tectonic Evolution of the Trans-North China Orogen

Abstract

The Trans-North China Orogen (TNCO) has been considered as a Paleoproterozoic collisional belt along which two discrete continental blocks, called the Eastern and Western Blocks, amalgamated to form the North China Craton (NCC). However, controversy has surrounded the timing and tectonic processes involved in the amal-gamation of the two blocks. Intervening between the high-grade Hengshan and Fuping Complexes of the central segment of the TNCO, the low-grade Wutai Complex preserves various structural elements of polyphase deformation, which are directed toward understanding tectonic processes of the orogen. Detailed structural investigations reveal that the Wutai Complex underwent three episodes deforma¬tion (D1 to D3). D1 resulted from the initial NW-SE-directed collision between the Eastern and Western Blocks and was responsible for the prograde metamorphism (M1). The major D2 deformation successively overprinted the D1 fabrics under an approximately same NW-SE-directed convergent stress field and formed large-scale ‘Z’ shaped folds. During D2, a crustal-scale dextral transpression occurred due to the oblique compression of the two blocks, which resulted in the development of dextral transpressional ductile shear zones of variable scales. The superposition of D1 and D2 shaped the regional tectonic framework of the Wutai Complex, which is characterized by a fan shaped structural pattern as a result of opposite thrusting on the bilateral sides. The folding and thrusting during D2 caused doubly thickening of the crust, leading to the regional peak metamorphism (M2) in the Wutai Complex. D3 is characterized by NWW-trending F3 open folds and kink bands that formed under relatively low-pressure conditions. D3 is interpreted as a result of later exhumation and exten¬sion, accompanied with the near-isothermal decompression (M3) and late cooling metamorphism (M4).Structural data, combined with other available geological data of this region support the recently proposed model that the TNCO developed from a continental magmatic arc that formed at the western margin of the Eastern Block by the eastward subduction of an oceanic lithosphere between the two blocks, which collided to form the coherent basement of the NCC at ~1.85 Ga. (SS11; 27/05/2008; 10:40)