High‐ to ultrahigh‐pressure (UHP) ductile shear zones in the Sulu UHP metamorphic belt, China: implications for continental subduction and exhumation

Abstract

High‐ to ultrahigh‐pressure (HP‐UHP) metamorphic rocks that resulted from deep continental subduction and subsequent exhumation in the Sulu orogenic belt, China, have experienced multiphase deformation and metamorphic overprint during its long journey to the mantle and return to the surface. HP‐UHP shear zones are strain‐localized weak zones on which the UHP slab is transported over long distances. HP‐UHP shear zones are well exposed along a 200‐km belt in the Sulu UHP metamorphic belt. The shear zones lie structurally below the UHP rocks and above the non‐UHP rocks, suggesting the early exhumation of the UHP rocks by thrusting. The large area distribution, HP‐UHP nature, high strain and structural association of the shear zones with the UHP rocks suggest that the shear zones are probably a regional detachment developed during the early stage of exhumation of the UHP rocks. Kinematic indicators suggest top‐to‐the N–NW motion of the UHP slab during the exhumation, which, combined with isotope signature in Mesozoic igneous rocks, leads us to the interpretation that the subduction polarity is the North China plate down to the south rather than the Yangtze plate down to the north in the Sulu region.

Assemblages and Textural Evolution of UHP Eclogites from the Chinese Continental Scientific Drilling Main Hole

“…This is compatible with the southeast-plunging regional mineral elongation lineation in association with a top-to-the-north sense of shear, synkinematic with D2 deformation in the Dabie area (Xue et al, 1996). However, Faure et al (2001) considered that the top-to-the-northwest ductile deformation was primarily coeval with retrograde amphibolite facies metamorphism, whereas Zhao et al (2003) suggest that the main ductile shear deformation probably took place under UHP conditions.…”

“…This structural feature is preserved in eclogites and country gneisses; hence it indicates a top-to-the-northwest sense of ductile shear deformation (Faure et al, 2001;Zhao et al, 2003). This is compatible with the southeast-plunging regional mineral elongation lineation in association with a top-to-the-north sense of shear, synkinematic with D2 deformation in the Dabie area (Xue et al, 1996).…”

Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms

“…The preservation of primary textures also implies low-stress conditions within the subduction zone. Although HP shear zones have been described from many localities (Pognante et al, 1985;Austrheim, 1987;Boundy et al, 1992;Camacho et al, 1997), UHP shear zones are much rarer, although they have been reported from the Dabie Sulu region in China (Zhao et al, 2003). The lack of evidence for structures forming at UHP conditions may be due to metamorphic or deformational overprinting, continuous development and reactivation during exhumation, or an unspecific and/or unrecognizable microstructural record (Stöckhert, 2002;Zhao et al, 2003).…”

Seismotectonics in the Pamir: An oblique transpressional shear and south-directed deep-subduction model

“…Since the beginning of the 21st century, arguments have been raised on the origin and exhumation mechanisms of ultrahigh-pressure metamorphic rocks in the Dabie-Sulu terrane of eastern China (Zheng, 2008). There, ultrahigh-pressure eclogites of Mesozoic age are exposed at the Eerth's surface, and show evidences of being overprinted by NE-striking left-lateral shear zones before their uplift (Zhang et al, 2003(Zhang et al, , 2005Zhao et al, 2003). According to the results of petrological, geochemical and isotopic geochemical research, these ultrahigh-pressure metamorphic rocks in the Dabie-Sulu terrane are considered to have resulted from deep-subduction and recrystallization of Mesozoic continental crust (Zheng, 2008).…”