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_Journal of Structural Geology 32 (2010) 423–444_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _Structural geometry of an exhumed UHP terrane in the eastern Sulu Orogen, China: Implications for continental collisional processes_ _Lu Wang a,b, Timothy M. Kusky a,b,*, Sanzhong Li a_ _a State Key Laboratory of Geological Processes and Mineral Resources and Three Gorges Geohazards Research Center, Ministry of Education, China University of Geosciences Wuhan, 388 Lumo Road, Wuhan 430074, China_ _b Department of Marine Geology, Ocean University of China, 238 Songling Road, Qingdao 266100, China_ _article info_ _Article history: Received 31 March 2009; Received in revised form 25 January 2010; Accepted 30 January 2010; Available online 17 February 2010_ _Keywords: Yangkou UHP metamorphism Structural mapping Sulu orogen Superimposed folding_ _abstract_ _High-precision 1:200–1:1000 mapping of Yangkou Bay, eastern Sulu orogen, defines the structural geometry and history of one of the world's most significant UHP (ultrahigh-pressure) belts. At least four stages of folds are recognized in UHP eclogites and associated quartzo-feldspathic gneiss. UHP eclogite facies rootless F1 and isoclinal F2 folds are preserved locally in coesite-eclogite. Mylonitic to ultramylonitic quartzo-feldspathic and coesite-eclogite shear zones separate small-scale 5–10-m-thick nappes of ultrama?c-ma?c UHP rocks from banded quartzo-feldspathic gneiss. These shear zones are folded, and progressively overprinted by amphibolite-greenschist facies shear zones. The prograde to retrograde D1–D5 deformation sequence is explained by deep subduction of offscraped thrust slices of lower continental or oceanic crust from the down-going plate, caught between the colliding North and South China cratons in the Mesozoic. After these slices were structurally isolated along the plate interface, they were rolled in the subduction channel during exhumation and structural juxtaposition with quartzo-feldspathic gneisses, forming several generations of folds, sequentially lower-grade foliations and lineations, and intruded by in situ and exotically derived melts. Shear zones formed during different deformation generations are wider with lower grades, suggesting that deep-crustal upper mantle deformation operates more efficiently, perhaps with more active crystallographic slip systems, than deformation at mid-upper crustal levels._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Understanding deep crustal deformational and metamorphic processes associated with continental collision has advanced greatly in recent years with numerous studies of exhumed HP and UHP metamorphic rocks from classical localities around the world, including the Dora Maria area of the western Alps, the Western Gneiss region of the Caledonian belt in Norway, and in the 1500 km long Qinling-Dabie Sulu orogen of eastern China (e.g. Baldwin et al., 2005; Brown and Piccoli, 2006; Brown and Rushmer, 2006; Brueckner and van Roermund, 2004; Chemenda et al., 1997; Chopin, 1984, 2003; Hacker et al., 2006, 2009; Liu et al., 2006; Maruyama et al., 1994; Molina et al., 2002; Okay and Sengor, 1992; Okay et al., 1993; Smith, 1984; Sobolev and Shatsky, 1990; Stefan et al., 2004; Wiesinger et al., 2006; Zheng et al., 2003). Analysis of metamorphic PeT conditions and processes has advanced at a faster pace than studies of the deformation mechanisms at UHP conditions, and of the detailed structure of the belts that contain the UHP rocks (e.g., Wallis et al., 1997, 1999, 2005; Xue et al., 1996). In this contribution we present a detailed structural analysis of an exceptionally well-exposed UHP terrain that contains some of the highest-pressure UHP rocks known on Earth, Yangkou Bay on the Shandong Peninsula of China's Sulu belt._ _Previous studies (Wallis et al., 1997, 1999; Lin et al., 2009; Faure et al., 2003; Ratschbacher and Hacker, 2000; Ratschbacher et al., 2006; Suo et al., 2000) on the multi-stage deformation in the Dabie-Sulu belt show that the main preserved deformation structures formed under amphibolite facies conditions. Ideas for why blocks containing UHP eclogite facies deformation fabrics are rare have been diverse, including: (1) few of the structural lenses may have been brought to depths great enough to experience UHP conditions; (2) the long exhumation path was associated with retrogression; (3) strong overprinting by younger deformation and metamorphism; and (4) localized development of the initial structures was restricted to narrow shear zones. Zhao et al. (2003) reported several localities of UHP shear zones in Sulu belt including Yangkou, the focus of this study._ _The reason Yangkou Bay is so important for geologists is because (1) it preserves one of most rare and significant occurrences of garnet peridotite and coesite-bearing UHP (ultrahigh-pressure) eclogite containing interstitial coesite, (2) it shows evidence for UHP conditions affecting granitic rocks, and (3) it preserves preeclogite facies deformation fabrics (Hirajima, 1996; Hirajima et al., 1993; Jahn, 1998; Liou and Zhang, 1996; Yoshida et al., 2004; Wallis et al., 2005). Petrologic study reveals rare intergranular coesite (Liou and Zhang, 1996; Ye et al., 1996; Wallis et al., 1997) and high concentrations of clinopyroxene, rutile and apatite exsolution lamellae in garnet within eclogite, which may indicate very deep continental subduction exceeding 200 km in a dry environment, followed by fast exhumation (Ye et al., 2000a,b). However, the interpretation of depths exceeding 200 km has recently been challenged by Hwang et al. (2007). Yangkou is also exceptional in that it is rare to find preserved locations in the Dabie-Sulu belt where different metamorphic stages and deformation events spanning the whole subduction and exhumation process can be documented, including details about the internal structure, overprinting relationships between different generations of structures and metamorphic fabrics, and relationships to bounding units. All of these relationships are exposed clearly in nearly continuous coastal exposures within a range of 1 km2 at Yangkou, making this a classical location to test relationships between deformational and metamorphic processes associated with subduction and exhumation of continental rocks from 200 km to the surface. Despite Yangkou having proven to be a testing ground of metamorphic theories, few studies of the complex structural relationships in this region have been undertaken and the geologic history is not yet completely understood (Wallis et al., 1997; Fang and Zhao, 2004; Zhao et al., 2003, 2005)._ _Results of new detailed and targeted high-precision 1:200–1:1000 field mapping on the nearly continuous coastal exposures are presented here as a basis for understanding the structural sequence of the lenses that contain the UHP assemblages at Yangkou, and their relationship to surrounding HP and lower-grade rocks. Comprehensive field, structural and petrofabric studies of the UHP and other rocks in the Yangkou exposures reveal a sequence of four folding events at different metamorphic grades, associated with parts of the prograde and the retrograde path of the UHP rocks formed as they completed their circuit from the surface to >200 km depth, and back to the surface, separated by partial melting and igneous intrusion events. Preliminary data indicates that UHP metamorphism was associated with formation of a series of 5–10s of meters wide fold nappes bounded by cm-decameter thick shear zones, and that deformation at progressively lower grades of metamorphism was associated with formation of progressively wider shear zones as the metamorphism entered the realm of regional amphibolite and greenschist facies conditions._ _2. Regional geology_ _The Sulu belt of China's Shandong Peninsula represents the eastern extension of the Qinling-Dabie orogenic belt (Meng and Zhang, 2000; Oh and Kusky, 2007; Hacker et al., 2009; Li et al., 2009), formed by subduction of oceanic lithosphere leading to the collision of the Yangtze and North China cratons and deep subduction of intervening continental material (inset of Fig. 1), possibly including an eastern extension of the Qinling microcontinent from the Dabie orogen (Hacker et al., 2009). There have been many studies in the Sulu belt on the UHP metamorphism, petrology, geochronology, geochemistry, geophysics, and structural analysis. However, structural analysis has lagged behind the metamorphic studies and has been largely restricted to investigation of the deep-level continental subduction processes at the thin section to lattice scales (e.g., Jin et al., 2001; Zhang et al., 1996, 2006; Wang et al., 2010). The timing of the collision, nature of pre-collision subduction systems, intervening crustal blocks, geometries of subduction, and causes of the deep continental subduction and later exhumation have all been controversial topics (e.g., Cheng et al., 2000; Hacker et al., 1998, 2006, 2009; Ratschbacher et al., 2000, 2006; Katsube et al., 2008; Li et al., 2007, 2009; Liu et al._ Ключевые слова: sheath fold, bay, quartz grain, developed, geophysical, surface, outcrop, liou, hacker, gondwana, preserved, eclogite body, tonalite intruded, webb, dynamically recrystallized, fold hinge, early, fold generation, metamorphic, amphibolite, ultrahigh pressure, nicolas, lithos, ratschbacher, tectonic history, structural sequence, feldspathic, ultramylonite, down-going plate, center ministry, plate, wa, metamorphic process, layer, uhp, ma, uhp condition, uhp assemblage, eclogite facies, journal structural, uhpm terrains, dabie-sulu belt, earth, eastern, ha, uhp rock, structure, fabric, foliation, overriding plate, younger deformation, high, elsevier, brought, qinling microcontinent, strong foliation, geological processes, zircon, eclogite transition, exhumation, formed, fold, uhp metamorphism, mineral resources, basaltic eclogite, american, uhp unit, amphibolite facies, prograde path, process, greenschist, liu, ultrahigh, foliated eclogite, science, complex, garnet, retrogressed eclogite, fang, dabie, chinese, intergranular coesite, kusky, plastic deformation, deformation, nw ax, western alps, deep subduction, grain, uhp eclogites, figs, including, relationship, grained, coesite, shandong peninsula, crustal, depth, yangkou eclogite, geological society, london, dark green, igneous rock, assemblage, early fold, journal, shear zone, english, location, late warp, quartzo-feldspathic gneiss, mac-ultramac boudins, belt, north, garnet peridotite, study, petrology, green, wang journal, tectonic evolution, initial subduction, continental, zheng, light grey, wallis, yang, continental subduction, gneiss, metamorphism, eclogite, omphacite, ultrahigh-pressure, key laboratory, coesite eclogite, detailed, chopin, zone, structural geology, wide range, sulu belt, facies, area, ultramac rock, mineral, coarse grained, wang, sander, folded, tectonic, partial melting, early stage, sulu, tectonics, brown, subduction, structural, zhou, structural relationship, compositional layering, geological, isoclinal fold, series, eclogite unit, eastern extension, rock, deformed, quartzo feldspathic, quartz porphyry, isoclinal, structural geometry, geology, rutile, inclusion, pressure, event, typically, hinge, metamorphic stage, stage, gorges geohazards, zhang, retrogressed, structural window, unit, quartz, yangkou bay, yangkou, sengor, zhao, condition, doi, age, abstract, acta, peridotite, subduction channel, nature, gabbroic eclogite, detailed geochronology, exhumation tectonics, orogen, und, cong, quartzo-feldspathic, eclogite mylonite, collision, dabie-sulu orogen, exhumation process, lens, phengite, small dike, fold overprinted, hirajima, special, china, eastern china, map, history, orientation, song, quartzo, shear