The Dabie UHP unit, Central China: a Cretaceous extensional allochthon superposed on a Triassic orogen Xianda Wang 1,2 , Franz Neubauer 1 *, Johann Genser 1 and Weiran Yang 2 1 Department of Geology and Palaeontology, University of Salzburg, Hellbrunner Straße 34, A-5020 Salzburg, Austria, 2 Faculty of Earth Sciences, China University of Geosciences, Wuhan city, Hubei province, 430074, China Introduction Over the past decade, many studies on the ultrahigh-pressure (UHP) meta- morphic rocks in the eastern Dabie Shan area, China, have been carried out. Most of these efforts focused on miner- alogy, petrology and geochronology of the UHP eclogites (e.g. Wang and Liou, 1991; Wang et al., 1992; Xu et al., 1992; Chen et al., 1993; Okay, 1993; Hacker and Wang, 1995; Ames et al., 1996; Rowley et al., 1997; Carswell et al., 1997). The occurrence of UHP mineral assemblages in country rocks of eclo- gites such as gneisses, marbles, and me- tapelites and the identical isotopic ages of UHP metamorphism in eclogites and their country rocks, in conjunction with geochemical data, demonstrate that these have undergone regional UHP metamorphism as a coherent slab (Wang and Liou, 1991; Wang et al., 1992; Ames et al., 1996; Rowley et al., 1997). Based on geochronological data ranging from 240 to 217 Ma (e.g. Ames et al., 1996; Chavagnac and Jahn, 1996; Rowley et al., 1997; Hacker et al., 1998; Li et al., 1989), the UHP metamorphism occurred during continent–continent collision between the Yangtze and North China blocks in the Early Triassic epoch. Subsequent exhumation to shal- low tectonic levels occurred between Late Triassic and Jurassic time (about 220–180 Ma) (e.g. Hacker et al., 1995). Structural data constraining burial and exhumation processes of the UHP unit are rare, however (Hacker et al., 1995; Xue et al., 1996; for reviews, see Hacker et al., 1996 and Liou et al., 1996). Pre-existing tectonic models for the exhumation of the UHP unit and its relationship to other units exposed in the Dabie metamorphic complex are controversial. Okay et al. (1993) attrib- uted exhumation of UHP rocks mainly to southward thrusting and erosion, and late out-of-sequence thrusts that transported the eclogite belt above the northern zone. Maruyama et al. (1994) proposed a tectonic model of a south- ward wedge-extrusion to be responsible for the exhumation of the UHP rocks during the Triassic. This was followed by doming of the North Dabie unit, which accommodated their S-dipping structure during the Cretaceous. In a similar tectonic model, Ernst and Liou (1995) considered isostatic uplift and buoyancy responsible for vertical ex- trusion. Hacker et al. (1995) suggested that exhumation of the UHP unit is due mostly to orogen-perpendicular, sub- horizontal shortening and subvertical extension originating from extrusion of an orogenic crustal wedge between 200 and 180 Ma. All of these tectonic mod- els for exhumation rest on N–S cross- sections (subparallel to the consistent strike of the regional foliation within the UHP/HP units) and stress the im- portance of post-subduction, Triassic convergence between the Yangtze and North China blocks, but do not con- sider a possible polyphase nature of the boundary between the Dabie UHP/HP units and the underlying Dabie orthog- neiss unit (exposed in the Luotian and Yuexi domes) in an orogen-parallel cross-section. Recent U–Pb zircon ages of 133–134 Myr indicate Cretaceous orthogneiss formation and granite in- trusions in the Luotian and Yuexi do- mes (Xue et al., 1997; Hacker et al., 1998), and, consequently, Cretaceous amphibolite and granulite facies meta- morphism there. Sufficiently large-scale geological maps of the eastern Dabie area are unavailable (few 1: 50 000 scale geolo- gical maps exist). Therefore, the regio- nal distribution of the UHP unit and its regional structural relationships with other units are not well known. Furthermore, severe high-temperature recrystallization and Cretaceous plu- tons have obliterated most deforma- tion structures along the basal bound- ary of the UHP unit. Moreover, in a 4 150 km-long N–S cross-section a blueschist unit, a high-pressure (HP) unit and an UHP unit successively oc- cur from south to north, respectively, from hanging wall to footwall. This displays a regular, overall SE-dipping, homoclinal structure, without reversal of metamorphic grades, which appears to be different from exposure of eclogi- tic belts in Alpine orogens (e.g. England and Holland, 1979; Chopin et al., 1991) and which is difficult to explain using existing tectonic models for exhumation of UHP rocks (e.g. see Platt, 1993). 260 *C 1998 Blackwell Science Ltd ABSTRACT Petrological, geochronological and structural data show that the eastern Dabie metamorphic complex resulted from two orogenic stages. Precursor rocks of the ultrahigh-pressure (UHP) and high- pressure (HP) units in the present hanging wall tectonic position were buried, penetratively deformed and subsequently exhumed by distributed, ESE-directed shearing during Triassic time. In contrast, rocks of the Dabie orthogneiss domes, now in a footwall tectonic position, were penetratively deformed during temperature-dominated, Early Cretaceous tectonic events, that are likely related to magmatic underplating. The Dabie orthogneiss domes and the UHP/HP units were juxtaposed during Early Cretaceous exhumation of Dabie orthogneiss domes by the formation of an ESE-directed low-angle ductile normal shear zone. Consequently, the UHP/HP units now represent an extensional allochthon in the hanging wall of the younger normal shear zone. The Cretaceous extensional structures are limited by boundary strike-slip faults. Consistent with the South China regional tectonic framework, ESE-directed lateral extrusion is considered to be the driving mechanism for extension and the present-day structure of the Dabie metamorphic complex. Terra Nova, 10, 260–267, 1998 Ahed Bhed Ched Dhed Ref marker Fig marker Table marker Ref end Ref start *Correspondence: Fax: +43/ 662 8044621; E-mail: franz.neubauer@sbg.ac.at Paper 200 Disc