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Tectonophysics 407 (2005) 65 – 80 www.elsevier.com/locate/tecto The Liaonan metamorphic core complex, Southeastern Liaoning Province, North China: A likely contributor to Cretaceous rotation of Eastern Liaoning, Korea and contiguous areas Liu Junlai a,*, Gregory A. Davis b,c, Lin Zhiyong d, Wu Fuyuan e a State Key Laboratory of Geological Processes and Mineral Resources and Key Laboratory of Lithospere Tectonics and Lithoprobing Technology of Ministry of Education, China University of Geosciences, 29, Xueyuan Road, 100083, Beijing, China b Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740, USA c College of Earth Sciences and Mineral Resources, China University of Geosciences, Beijing 100083, China d Research and Development Center, China Geological Survey, Beijing 100037, Beijing, China e Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing P.O.Box 9825, 100029, Beijing, China Received 18 July 2004; received in revised form 1 July 2005; accepted 9 July 2005 Available online 16 August 2005 Abstract The Mesozoic Liaonan metamorphic core complex (mcc) of the southeastern Liaoning province, North China, is an asymmetric Cordilleran-style complex with a west-rooting master detachment fault, the Jinzhou fault. A thick sequence of lower plate, fault-related mylonitic and gneissic rocks derived from Archean and Early Cretaceous crystalline protoliths has been transported ESE-ward from mid-crustal depths. U–Pb ages of lower plate syntectonic plutons (ca. 130–120 Ma), 40 Ar–39Ar cooling ages in the mylonitic and gneissic sequence (ca. 120–110 Ma), and a Cretaceous supradetachment basin attest to the Early Cretaceous age of this extensional complex. The recent discovery of the coeval and similarly west-rooting Waziyu mcc in western Liaoning [Darby, B.J., Davis, G.A., Zhang, X., Wu, F., Wilde, S., Yang, J., 2004. The newly discovered Waziyu metamorphic core complex, Yiwulushan, western Liaoning Province, North China. Earth Science Frontiers 11, 145– 155] indicates that the Gulf of Liaoning, which lies between the two complexes, was the center of a region of major crustal extension. Clockwise crustal rotation of a large region including eastern Liaoning province and the Korean Peninsula with respect to a non-rotated North China block has been conclusively documented by paleomagnetic studies over the past decade. The timing of this rotation and the reasons for it are controversial. Lin et al. [Lin, W., Chen, Y., Faure, M., Wang, Q., 2003. Tectonic implication of new Late Cretaceous paleomagnetic constraints from Eastern Liaoning Peninsula, NE China. Journal of Geophysical Research 108 (B-6) (EPM 5-1 to 5-17)] proposed that a clockwise rotation of 22.58 F 10.28 was largely post-Early Cretaceous in age, and was the consequence of extension within a crustal domain that tapers southwards towards the Bohai Sea (of which the Gulf of Liaoning is the northernmost part). Paleomagnetic studies of Early Cretaceous * Corresponding author. Tel.: +86 010 8232 2156. E-mail address: [email protected] (J. Liu). 0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2005.07.001 66 J. Liu et al. / Tectonophysics 407 (2005) 65–80 strata (ca 134–120 Ma) in the Yixian–Fuxin supradetachment basin of the Waziyu mcc indicate the non-rotation of North China and the basin [Zhu, R.X., Shao, J.A., Pan, Y.X., Shi, R.P., Shi, G.H., Li, D.M., 2002. Paleomagnetic data from Early Cretaceous volcanic rocks of West Liaoning: Evidence for intracontinental rotation. Chinese Science Bulletin 47, 1832–1837]. Such upper-plate non-rotation supports our conclusion that the lower plates of the Waziyu and Liaonan metamorphic core complexes were displaced ESE-ward in an absolute sense away from the stable North China block, thus contributing to the rotation of Korea and contiguous areas. Rotation is inferred to have affected only the upper crust above mid-crustal levels into which we believe the Jinzhou and Waziyu detachment fault zones flattened. If this is the case, the regional Tan Lu fault that lies between the two core complexes was truncated at mid-crustal depth, since in areas to the south it forms the boundary between the North and South China lithospheric blocks. It is noteworthy that the two extensional complexes lie not far north of the Bohai Bay, the area proposed by Lin et al. [Lin, W., Chen, Y., Faure, M., Wang, Q., 2003. Tectonic implication of new Late Cretaceous paleomagnetic constraints from Eastern Liaoning Peninsula, NE China. Journal of Geophysical Research 108 (B-6) (EPM 5-1 to 5-17)] as the site of the pole of rotation for Korea’s clockwise displacement. Lin et al. [Lin, W., Chen, Y., Faure, M., Wang, Q., 2003. Tectonic implication of new Late Cretaceous paleomagnetic constraints from Eastern Liaoning Peninsula, NE China. Journal of Geophysical Research 108 (B-6), (EPM 5-1 to 5-17)] were unaware of the Liaonan and Waziyu mcc’s and argued that most of the regional block rotation was post-Early Cretaceous and, in part, early Cenozoic. However, the ca. 130–120 Ma ages of the two Liaoning mcc’s and a Songliao basin mcc (Xujiaweizi), the latter discovered only by recent drilling through its younger stratigraphic cover, support our and some Korean coworkers’ conclusions that most of the clockwise rotation was Early Cretaceous. D 2005 Elsevier B.V. All rights reserved. Keywords: Liaonan metamorphic core complex; Early Cretaceous; Crustal rotation; Eastern Asia 1. Introduction The late Mesozoic and early Cenozoic was a period of major lithospheric thinning and crustal extension in the North China (Sino-Korean) bcratonQ following a long and complicated history of late Paleozoic and earlier Mesozoic contractional deformation (Menzies and Xu, 1998; Griffin et al., 1998; Xu, 2001; Davis et al., 2001). This period was characterized by the widespread development of Early Cretaceous half graben sedimentary basins (e.g., Zhang, 1997; Hu et al., 1998; Ma, 2001; Ren et al., 2002; Meng, 2003), by scattered and isolated development of metamorphic core complexes, including the Liaonan complex discussed here (e.g., Zheng and Zhang, 1994; Davis et al., 1996, 2002; Webb et al., 1999; Darby et al., 2004), and by alkalic A-type magmatism and associated gold mineralization (e.g., Zhang and Xu, 1998; Webb et al., 1999; Davis et al., 2002; Darby et al., 2004). The existence of extensional structures in the Liaonan area was first discussed by Xu et al. (1991) who proposed processes of contraction and extension related to Indosinian orogenesis at about 250 Ma. Xu et al. (1994) later suggested that the Liaonan mylonitic rocks could be attributed to N–S shortening during Indosinian (Triassic) to Early Yanshanian (Jurassic) deformation. The concept of an extensional Liaonan metamorphic core complex (mcc) was first applied by the Liaoning Bureau of Geology and Mineral Resources (LBGMR, 1994), and later amplified by Yang et al. (1996) and Yin and Nie (1996). Yang et al. (1996) suggested that the extensional complex was the consequence of magmatic upwelling and emplacement related to Triassic subduction of the Pacific plate beneath Eurasia. Yin and Nie (1996), relying on 40Ar/39Ar age determinations for lower plate mylonites and quartzofeldspathic dikes in the mylonites, proposed a Cretaceous age for mcc extension. This paper describes the geologic characteristics and age of the Liaonan metamorphic core complex and presents new data and interpretations on its tectonic significance in eastern Asia. The Liaonan complex is located in the southern part of the Liaodong Peninsula of Liaoning province (Fig. 1). It consists of three major elements—the Jinzhou master detachment fault, a lower plate of Archean metamorphic rocks, Early Cretaceous syntectonic (synextensional) intrusions and fault-related mylonitic and gneissic rocks, and an upper plate J. Liu et al. / Tectonophysics 407 (2005) 65–80 67 Fig. 1. Geological map of Liaonan mcc. supradetachment basin of Cretaceous age and its basement rocks (weakly deformed Neoproterozoic and Lower Paleozoic sedimentary rocks). 2. Structural associations and major components of the core complex 2.1. Jinzhou detachment fault and lower plate rock assemblages The controlling structural element of the Liaonan mcc is the Jinzhou fault, which has an arcuate map trace resulting from antiformal folding (Fig. 1). Its western segment strikes NNE and dips WNW, whereas a southerly segment, east of Jinzhou, strikes ENE and dips SSE (Fig. 1). Whether the folded detachment fault is due to synextensional folding at high angles to the extension direction, as observed in other mcc’s (cf. Davis and Lister, 1988; Lister and Davis, 1989; Davis et al., 2002), or formed after extension has not yet been resolved. Lower plate Archean gneiss was mainly derived from granitic plutons (Zhang et al., 1994; Gang et al., 1999). Supracrustal rocks are present as xenoliths in the Archean orthogneisses. All Archean rocks, both supracrustal and plutonic, are of lower amphibolite grade, and were retrograded under greenschist facies conditions (Yang, 1985; Zhang et al., 1994; Gang et al., 1999; Wang and Wang, 2001). The Archean orthogneisses are generally tonalitic, trondhjemitic and granodioritic in composition, with biotite and hornblende contents that vary from 18%– 31%. Pb–Pb dating of single grain zircons from the gneisses gave ages of 2467 F 18 Ma and 2773 F 50 Ma (LBGMR, 1994); more recently, LA-ICPMS dat- 68 J. Liu et al. / Tectonophysics 407 (2005) 65–80 ing of zircon grains from granodioritic gneisses in the complex yielded U–Pb ages of 2501 F17 Ma and 2436 F 17 Ma (Lu et al., 2004). Precambrian crystalline rocks underlying the detachment fault have been transformed by shearing and partial to complete recrystallization into mylonitic gneisses and related tectonites under conditions of the amphibolite to upper greenschist facies. Tectonites thus formed exhibit a clear increase in fabric development upwards towards the master detachment fault. The mylonitic rocks grade downwards into faultrelated schist and gneiss in which all minerals exhibit crystal–plastic recrystallization. These hotter, deeper fault-related rocks transit downward into the preextensional Precambrian basement assemblage. Mylonitic foliation generally parallels the Jinzhou fault, although locally the fault cuts early-formed (but fault related) mylonitic fabrics at small angles, reflecting a protracted history of uplift (cf. Lister and Davis, 1989; Davis et al., 2002). Near Jinzhou, the strike of mylonitic foliation changes gradually from NNE to ENE across a narrow (ca 1000 m wide) zone within the antiformal hinge area between the western and southern branches of the detachment fault (Figs. 1 and 2a,b). In contrast to the variation in foliation orientation, stretching and mineral lineations have consistent orientations in different parts in the footwall of the mcc (Fig. 2c). Both mineral and mylonitic lineations plunge ca 3108 in the western part of the detachment fault zone, and ca. 1308 in the southern zone (Figs. 1 and 2c). As discussed and interpreted in the following section, there is a gradual transition from magmatic mineral lineation in some lower plate plutons (e.g. Yinmawanshan pluton) to parallel mylo- nitic lineations in the detachment zone above them (Fig. 2c). Strongly foliated mylonitic rocks in uppermost levels of the lower plate are typically overprinted by brittle structures and microstructures. The cataclasized rocks just below the Jinzhou fault exhibit varying degrees of shattering, brecciation, grain size reduction by shearing with included lenses (phacoids) of relict mylonitic rocks, and local occurrence of pseudotachylite and gouge. Sparse pseudotachylite is present locally along the main fault and up to several tens of meters below in breccias and chloritic breccias. Due to involvement of fluid phases during deformation, most mafic minerals in the cataclasized tectonites become chloritized, leading to the occurrence of distinctive chloritic breccias (Lister and Davis, 1989; Lin et al., 2002). Pseudotachylite in the brecciated zone occurs as black, irregular networks of microcrystalline (now) veins with geometric characteristics of injection. The total thickness of ductile and brittle faultrelated rock units in the deformation zone beneath the Jinzhou fault ranges from about 700 m near Jinzhou to about 250 m near Pulandian; the zone dies out in areas north of Fig. 1. 2.2. Synkinematic Cretaceous intrusions Syntectonic granitic intrusions, interpreted as Atype by Guo et al. (2004) and Wu et al. (2005), are important components of the Liaonan lower plate (Fig. 1). They are typically in either direct contact with the Jinzhou detachment fault or lie in close proximity to it. These intrusions were emplaced at different stages of core complex development (U–Pb Fig. 2. Stereographic projections (lower hemisphere) of foliation in detachment fault zones, and mylonitic and mineral lineations (c) in the Liaonan mcc. a. Projection of foliations in NNE detachment fault zone; b. projection of foliations in ENE detachment fault zone; c. point diagram for all mcc lineation (in total 45 data): solid triangles—NNE branch detachment fault zone (20 data); solid squares—ENE branch detachment fault zone (17 data); circles—Yinmawanshan monzogranite (8 data). J. Liu et al. / Tectonophysics 407 (2005) 65–80 zircon ages of ca. 129 to 122 Ma, Guo et al., 2004; Wu et al., 2005), but all exhibit concordant relationships with their wall rocks. A typical example of the major syntectonic intrusions in the mcc is the multiple-stage Yinmawanshan granodiorite–granite complex (Fig. 1; Guo et al., 2004). Two zones are recognized in the complex, an early marginal zone of granodiorite and porphyritic monzogranite, and a younger inner zone of finer grained granodiorite. Biotite defines a weak magmatic foliation in the inner zone. Although most minerals in the inner zone have weak dimensional and crystallographic orientations, they do not show obvious intracrystalline fabrics. The magmatic foliation in the inner zone, and the inner zone–outer zone boundary generally have very high dip angles (ca 608 to 808), suggesting that a vertical emplacement process dominated intrusion of the pluton’s core. The outer or marginal zone is composed of two different members, an older medium to coarse-grained granodiorite in the west and a younger porphyritic monzogranite in the east. Coarse tabular plagioclase crystals (up to 5 cm) in the monzogranite define a primary magmatic foliation and lineation. Hornblende grains and grain aggregates in the coarse-grained granodiorite are also oriented, with increasing fabric strengths approaching the detachment fault zone. A gradual transition from magmatic fabric to mylonitic foliation is observed within 30 m of the detachment fault zone. The transition is shown from microscopic fabrics of the rocks, i.e. from magmatic fabrics in the unsheared part, through mylonitized magmatic fabrics in the transitional zone, to mylonitic fabrics in the detachment fault zone. Oriented crystals in the magmatic fabrics do not show evidences for crystal plastic deformation, whereas mylonitic foliation is composed of strongly elongated and plastically deformed grains and grain aggregates. Both fabrics, i.e. foliation in magmatic rocks and that in mylonitic rocks, have identical orientation patterns, dipping WNW at the western part of the Yinmawanshan pluton. The shear sense in these granitic mylonitic gneisses is top to the WNW and is consistent with that of the Jinzhou detachment’s mylonitic footwall. The Chaoyangsi and Zhaotun granites (Fig. 1) are small lower plate intrusions within the southern detachment fault zone that we interpret as syntectonic. The Chaoyangsi granite is represented by several 69 small intrusions distributed rather randomly within the ductile shear zone. The intrusions appear to have been spatially controlled by the detachment fault zone and some of their original magmatic fabrics have been transposed into mylonitic fabrics containing relict magmatic crystals. Their shear zone rocks have equally developed mylonitic foliation and stretching lineation, and are therefore, S-L type tectonites. The Zhaotun granite, in contrast, is a granitic sheet with a welldeveloped stretching lineation. Foliation is so weakly developed in comparison with the lineation that LNNS tectonites are its major rock type. It is significant that magmatic lineations in the intrusions have consistent WNW–ESE orientations that are parallel to the stretching lineations in the overlying detachment fault rocks (Fig. 2c). This parallelism is suggestive of a syntectonic process of magmatic emplacement (Paterson et al., 1989; Vernon, 2000), i.e. that magmatic flow in the lower plate may have been influenced by relative slip between the upper plate and the igneous melt below it. Accordingly, we believe, as did Guo et al. (2004), that the Yinmawanshan pluton was deformed in close proximity to the Jinzhou detachment fault despite its map appearance on Fig. 1 of being distant from it. The western margin of the pluton has parallel magmatic and mylonitic fabrics, the latter comparable to those of the detachment fault’s lower plate. It is, therefore, likely that the western margin of the pluton lay not far below west-dipping detachment fault prior to subsequent erosion. 2.3. Deformational history of the lower plate As is characteristic of the lower plates of other metamorphic core complexes, Jinzhou fault-related footwall rock assemblages record a history of progressive uplift under increasingly colder and brittle conditions. Such Jinzhou assemblages exhibit transitions in time and space (upwards in the complex) from fault-related gneiss to mylonitic schist and gneiss, retrograded schist and gneiss, brecciated mylonite, microbreccia, pseudotachylite and gouge (Fig. 3). These lithologic transitions document progressive overprinting of the fault’s footwall from depths below the crust’s brittle–ductile transition to near surface conditions as it was drawn up and out from beneath its hanging wall. 70 J. Liu et al. / Tectonophysics 407 (2005) 65–80 Fig. 3. Structural, microstructural characteristics and shear indicators in mylonitic rocks from the detachment fault zones in Liaonan mcc. a. Shear bands in mylonites; b. sigmoidal fabric in quartzofeldspathic mylonite; c. S-C fabric in sheared monzogranite; d. S-C fabric in sheared amphibolite (Scale: base line, a—50 cm; b–d—2.5 mm). Biotite–plagioclase gneiss and hornblende–plagioclase gneiss derived from the Archean gneiss units are the major types of fault-related gneisses. Rocks are completely recrystallized with all mineral phases, including quartz, plagioclase, biotite and hornblende, showing characteristics of plastic flow. In contrast, the gradationally overlying mylonitic zone is characterized by crystal–plastic deformation of some minerals, primarily quartz, and by the brittle behavior of most other silicates (Fig. 3). Outcrop and microscopic scale structural elements of this zone include mylonitic foliation, shear bands and extensional crenulation cleavage (Fig. 3a), j fabrics (Fig. 3b), S-C fabrics (Fig. 3c,d), stretching lineations, sheath folds and other types of a-folds. They are useful shear-sense indicators in the mylonitic rocks and indicate deformation in the crust’s brittle–ductile transition (Kawamoto and Shimamoto, 1997; Fig. 3 a–d) accompanying a relative southeast to northwest sense of shear. This shear sense is compatible with the absolute southeastward displacement of the footwall of the Jinzhou fault (see below). In the microscopic domain, quartz, plagioclase and biotite grains show distinct deformation microstructures. Quartz grains are intensely deformed via crystal–plastic mechanisms (e.g. Nicholas and Poirier, 1976; Passchier and Trouw, 1996; Lin et al., 2002), resulting in deformation microstructures (elongated grains and grain aggregates, undulose extinction, deformation lamellae), recovery microstructures (subgrains) and recrystallization microstructures (grain size reduction by subgrain rotation and dynamic recrystallization, Fig. 3a,b). Microstructures indicating both crystal–plastic and brittle deformation are preserved in plagioclase grains, indicating their superposed deformation from deeper to higher crustal levels (c.f. Tullis and Yund, 1987; Fig. 3c). 2.4. Upper plate Neoproterozoic and Paleozoic rocks (Fig. 4a–b, c–d) The Liaonan mcc upper plate includes Archean gneiss, and Neoproterozoic and Paleozoic strata J. Liu et al. / Tectonophysics 407 (2005) 65–80 (Wang et al., 2000; Zhang et al., 1994) (Fig. 4a–b, c– d). Brittle-deformed Neoproterozoic quartz sandstone, siltstone, calcareous shale, and limestone are the most common upper plate rocks. Their possible lower plate counterparts, however, have been transformed into quartzite (or quartz mylonite), mica schists, marble (and calc-mylonite). Upper plate Paleozoic sedimentary rocks include interlayered limestone, sandstone and, subordinate shale. 2.5. Cretaceous supradetachment basin An important, but relatively little studied, component of the Liaonan mcc is an upper plate supradetachment basin (LBGMR, 1994; Friedmann and Burbank, 1995; Davis et al., 2002) that covers more than 200 km2 along the western trace of the Jinzhou detachment (Figs. 1 and 4e–f). This Cretaceous half-graben unconformably overlies Archean and Neoproterozoic rocks in the west. To the east it lies above the west-dipping detachment fault, where its strata dip eastward at moderate to steep angles (ca. 308 to 508) into the fault. Andesitic 71 volcanic rocks have been reported in the base of the basin (LBGMR, 1994), but they are found higher in the section as well. Fluviatile and lacustrine sedimentary rocks are the dominant deposits in the basin (LBGMR, 1994). Clasts of quartz sandstone and limestone from Neoproterozoic and Paleozoic units, and schist and gneiss from the lower plate basement are found in the sediments. Sedimentary rocks in the basin contain Cretaceous fossils, supportive of an Early Cretaceous age, e.g. Dictyozamites, Cycadolepis, Elatocladus and Otozamites (LBGMR, 1994). Radiometric dating of the volcanic units is needed to more clearly establish the basin as being synchronous with mcc extension. West-dipping normal faults that cut Neoproterozoic to Lower Paleozoic (Cambrian–Ordovician) strata in the hanging wall of the detachment fault north of Jinzhou may be related to core complex development. Large-scale basin groups in the Gulf of Liaoning to the west and the Bohai Sea to the southwest may have developed under similar conditions, although only small basins are observed on the Liaodong Peninsula (Qiu et al., 1994; Zhang, 1997). Fig. 4. Geological sections across the Liaonan mcc (see Fig. 1 for locations). Pz–Pt unit: Partly metamorphosed Neoproterozoic to Paleozoic sedimentary rocks; Lower plate: Mainly Archean gneisses and Mesozoic plutons. ab section length, 25 km; cd section length, 5 km (revised from LBGMR, 1989); ef section length, 13 km. 72 J. Liu et al. / Tectonophysics 407 (2005) 65–80 3. Debate on core complex geochronology and recent data The age of the Liaonan mcc has been controversial (cf. Yang, 1985; Xu et al., 1991; Yang et al., 1996; Chen et al., 1999). Yang (1985) first identified the southern, ENE-striking segment of the detachment fault zone and named it the Dongjiagou ductile shear zone (after the Archean Dongjiagou Formation in the Anshan Group). It was thought of as an Archean ductile shear zone. Xu et al. (1991) first recognized and discussed extension and contraction in the area and considered both to reflect Indosinian deformation based on a whole rock Rb–Sr age (226 Ma) of a mylonite and K–Ar dating (225 Ma) of muscovite grains in a felsic dike intruding the mylonite. The LBGMR (1994) was first to apply the metamorphic core complex concept to Liaonan extensional deformation. Yang et al. (1996) later concluded that the Liaonan mcc was of Triassic age, although the pluton they dated is not clearly part of the mcc. Recent reliable dating has established a new and younger geochronological framework for the Liaonan mcc. Yin and Nie (1996) obtained 40Ar/39Ar cooling ages within the NNE ductile shear zones of ca. 110 to 113 Ma (biotite). Yang J.H. (personal communication, 2004) has dated lower plate mylonitic rocks in both shear zone segments and has obtained reasonably consistent 40Ar–39Ar ages with the following ranges: hornblende, 124.2 to 113.3 Ma; biotite, 122.6 to 111.3 Ma; K-feldspar, 118.2 to 112.0 Ma; and muscovite, 111.9 Ma. These Jinzhou fault-related ages are consistent with 121–113 Ma 40Ar/39Ar ages (muscovite, biotite, K-feldspar) from an extensional ductile shear zone across the Jurassic Heigou pluton in the southern Liaodong Peninsula (Yang et al., 2004); the shear zone is on strike with the ENE-trending segment of the Jinzhou detachment fault farther to the southwest. In contrast, U–Pb dating of syntectonic plutons in the Liaonan mcc is, not surprisingly, generally older than its lower plate cooling ages. For example, single zircon grains from the footwall Zhaotun monzogranite yield an age of 128 F 5 Ma (Guo et al., 2004; Wu et al., 2005). Yinmawashan U–Pb zircon ages vary within the composite pluton, including 122 F 6 Ma for an inner zone fine grained monzogranite, 129 F 2 Ma for a porphyritic granite along the northern margin of the pluton, and three ages varying between 120 F 4 Ma and 125 F 2 Ma for gneissic granitic rocks and associated dikes in its southwestern margin (Guo et al., 2004; Wu et al., 2005; Fig. 1). There is no conflict between crystallization ages of the synkinematic plutons and cooling ages of footwall rocks in the detachment fault zone. Collectively, they document a protracted period of crustal extension within the Liaonan mcc from ca. 130 Ma to 110 Ma. The final stages of lower plate uplift and erosion must be still younger than about 110 Ma. 4. Tectonic significance and implications of the Liaonan metamorphic core complex 4.1. The tectonic setting of the Liaonan mcc The Liaonan mcc is, to date, one of the two easternmost documented examples (Liaonan and Xujiaweizi) of core complex formation within a large region of southern Siberia, Mongolia, and eastern China affected by Cretaceous NW–SE to N–S crustal extension. The origin of this extensional province remains unresolved and constitutes one of the major tectonic problems of eastern Asia. This problem stems in part from (1) the vastness of the area of crustal extension (more than two million km2), (2) the suddenness of the onset of extension across this huge region (generally at 130–125 Ma), and (3) the complexities of the region’s earlier tectonics. Following closure of a Paleoasian Ocean along the northern margin of the North China plate in the Permian, and the Triassic collision of the North and South China (Yangtze) plates to the south, the North China (or Sino-Korean block) became the northern part of an amalgamated continental plate (Li, 1994; Yin and Nie, 1996; Zorin, 1999). In the late Mesozoic this plate was further affected by Jurassic–Early Cretaceous closure of the Mongol-Okhotsk ocean that separated it from Siberia, by subduction of Pacific plates beneath it, and by Cenozoic collision with the Indian plate to the south (Watson et al., 1987; Zhang et al., 2001; Ren et al., 2002). The Liaonan complex is one of six Early Cretaceous North China mcc’s that developed across the northern margin of the Archean-floored North China J. Liu et al. / Tectonophysics 407 (2005) 65–80 block (or bcratonQ as it is misleadingly called, given its Phanerozoic history of complex orogenesis; cf. Davis et al., 2001; Li et al., 2004). All developed, albeit in a scattered, isolated pattern, across areas of major Mesozoic north–south contraction and presumed crustal thickening related to thrust faulting, folding, and Jurassic and earliest Cretaceous plutonism (Zheng and Zhang, 1994; Li, 1994; Davis et al., 1996, 2002; Darby et al., 2004). The Liaonan mcc lies within just such an area of complicated prior tectonic history (Zhang and Wang, 1995; Yin and Nie, 1996; Chen et al., 1999; Gang et al., 1999; Wu et al., 2002). The youngest major thrusting event in the Yanshan area of the North China block has been dated at about 127 Ma in the Yunmeng Shan north of Beijing (Davis et al., 1996, 2001). This age approximates the beginning of distributed and somewhat diachronous Early Cretaceous crustal extension in eastern Asia (Li, 2000; Han et al., 2001; Davis et al., 1996, 2002; Ren et al., 2002; Meng, 2003; Darby et al., 2004). Much Mesozoic magmatism in eastern China prior to the Early Cretaceous has adakitic signatures suggesting melt generation in or below a thick (N 45 km) continental crust (Zhang et al., 2001). A correlation between the spatial and temporal occurrence of contractional structures in the Yanshan belt of North China and adakitic magmatism was recently proposed by Davis (2003). In contrast, the onset of crustal extension in eastern Asia generally coincides with regional intrusion of alkalic and subalkalic Atype granites, including the synkinematic plutons of the Liaonan mcc (Wu et al., 2002, 2005; Guo et al., 2004; Li et al., 2004). Many different tectonic models have been proposed to elucidate the regional transition between crustal contraction and extension. Amongst the most often discussed are (1) gravitational collapse of contractionally thickened crust, perhaps triggered by plutonism (Davis et al., 2001, 2002; Zhang and Xu, 1998); (2) delamination of North China lithosphere during subduction of the Izanagi plate under the Eurasian plate (Wu et al., 2002); (3) rollback of an Izanagi (or other Pacific) subducting plate (Traynor and Sladen, 1995; Davis et al., 2001); and (4) existence of a mantle plume and its effects on the overlying Eurasian lithosphere (Deng et al., 2003). As stated at the beginning of this section, the problem remains unresolved. 73 4.2. Liaonan mcc and the Tan Lu fault The recent discovery of the Waziyu mcc in the Yiwulü Shan of western Liaoning (Fig. 1; Darby et al., 2004) with its WNW-dipping master detachment fault compliments the earlier discovery of the Liaonan mcc in southeastern Liaoning with its similar geometry. Both master detachment faults were active in the Early Cretaceous, and their coeval development both west and east of the NNE-striking Tan Lu fault raises interesting questions regarding the temporal, kinematic, and mechanical relationships of that eastern Asian strike-slip fault to the extensional core complexes. Early Cretaceous sinistral slip has been documented on the southernmost part of the Tan Lu fault in Anhui province, including areas along the eastern margin of the Dabie Shan collisional complex between the North and South China blocks. Zhu et al. (2001) have reported 6 whole rock 40Ar/39Ar plateau ages from fault zone mylonite, ultramylonite, and phyllonite ranging in age from 132.5 F 0.5 to 120.5 F 0.75 Ma and, subsequently (Zhu et al., 2004), a muscovite 40Ar/39Ar age of 127.6 F 0.2 Ma from a Tan Lu mylonite; all dated samples are characterized by horizontal stretching lineations. The age range of their dated samples, e.g. 132.5 to 120.5 Ma, is closely similar to timing constraints on the Liaonan mcc presented in this paper (ca. 130 to 110 Ma), and on the Waziyu mcc to the northwest (ca. 130 to 116 Ma; Zhang et al., 2003b; Darby et al., 2004). Despite evidence for Early Cretaceous Tan Lu fault displacement in southern areas, however, the occurrence of such slip on more northerly portions of the Tan Lu fault, specifically from the Shandong peninsula to the north, is considerably more problematic. For one reason, sinistral slip on the Tan Lu fault in these northern areas is stress and strain incompatible with concurrent major WNW–ESE crustal extension on both sides of the fault in the Gulf of Liaoning and in close proximity to it. In addition, paleomagnetic studies of the Korean Peninsula, eastern Liaoning, and eastern Shandong (respectively, Doh et al., 2002; Lin et al., 2003; Koo et al., 2003) are in agreement that no latitudinal displacement between those areas and the North China block (NCB) to the west is discernible from Cretaceous data, although the sensitivity of such studies does not preclude limited strike-slip displace- 74 J. Liu et al. / Tectonophysics 407 (2005) 65–80 ment on the intervening Tan Lu fault. As an aside, we note that Early Cretaceous sinistral slip has been documented in more outboard (coastal) regions of the east Asian margin, including South Korea (e.g., Chough et al., 2000), northeast Japan (e.g., Sasaki, 2003), and Sikhote Alin, Russia (e.g., Golozubov and Khanchuk, 1996). Recent Shandong peninsula and neighboring North China basin studies appear to be in agreement that Early Cretaceous deformation in those areas was not related to Tan Lu sinistral faulting. Zhang et al. (2003c) have proposed that the earliest Cretaceous Jiaolai basin in northern Shandong is a pull-apart basin within a dextral Tan Lu system, and that the middle Early Cretaceous was boverwhelmingly riftdominated and characterized by widespread intermediate volcanism, normal faulting, and basin subsidenceQ (op. cit., p. 243). Most large gold deposits in Shandong, including some on the northern margin of the Jiaolai basin that were controlled by a low-angle normal fault, are related to Early Cretaceous magmatism and were mineralized between 130 and 115 Ma (Yao et al., 2002; Zhang et al., 2003a). This age range is coincident with formation of the Liaonan mcc. Jin et al. (2002) relied on extensive seismic data in the North China basin to report that a NW-striking Middle to Late Mesozoic fold-thrust belt was succeeded in the Late Mesozoic by normal faults striking in the same direction, a history similar to that of the Yanshan foldthrust belt exposed to the north where a contractional to extensional tectonic transition occurred at about 130 Ma (Davis et al., 2001; Davis, 2003). 4.3. Liaonan mcc and large-scale crustal rotation The paleomagnetic studies of Cretaceous strata cited above (Doh et al., 2002; Lin et al., 2003; Koo et al., 2003), while not supporting latitudinal displacements across the Tan Lu fault in northern China, are in good agreement that the Korean peninsula, eastern Liaoning, and contiguous areas generally east of the fault have been rotated with respect to a western, previously amalgamated North China–South China block. Estimates of the clockwise rotation of the ELK block (East Liaoning–Korea; Lin et al., 2003) with respect to the SCB–NCB range from ca 348 to 208 (Zhao et al., 1999; Doh et al., 2002; Lin et al., 2003; Koo et al., 2003). Collective paleomagnetic data from Cretaceous strata within the rotated domain indicate an average clockwise rotation of 22.58 F 10.28 (Lin et al., 2003, their Table 3, Fig. 7). Most early hypotheses to explain the clockwise rotation favored large sinistral displacements on a curviplanar Tan Lu fault system, concave to the ESE in northeastern China (cf. Doh et al., 2002). However, the limited or absent Early Cretaceous slip on this fault as discussed above effectively negates this hypothesis. More recently, Lin et al. (2003) have attributed the rotation of the ELK to large-scale Cretaceous and Cenozoic extension within a south-tapering wedge- or triangular-shaped area, in northeastern China. This area contains the large Songliao basin and a number of smaller extensional basins of variable age (Xialiaohe, Zeya, Sanjiang). Lin et al. (2003) proposed that the pole of rotation for this rotating triangular crustal block lay in the southern part of the Bohai Sea near the western edge of the Shandong peninsula; the peninsula itself appears to be part of the NCB–SCB block. 4.4. ELK block rotation and its relationship to Liaoning province mcc’s Our studies in the Liaonan mcc and the recent discovery of the Waziyu mcc in western Liaoning (Darby et al., 2004) support the hypothesis of Lin et al. (2003) that the rotation of the ELK block is the consequence of Cretaceous extension along the eastern margin of the NCB. Those authors were unaware of the two mcc’s and their significant contribution to crustal extension in the southern region of their hypothesized extensional wedge (Fig. 5). Studies of core complexes in the lower Colorado River region of the southwestern United States (e.g. Davis and Lister, 1988; Lister and Davis, 1989; John and Foster, 1993), and elsewhere, have confirmed that core complex extension is accompanied by the active footwall (i.e. lower plate) uplift of mid-crustal rocks, leaving hanging walls more or less in situ with respect to such reference surfaces as sea level or the geoid. Both Liaoning master detachment faults, Waziyu and Jinzhou, root WNW-ward and both had footwall rock assemblages that were translated ESE-ward in Early Cretaceous time—ca. 130 to 110 Ma for the Liaonan complex (see above), and ca. 130 to 116 Ma for the J. Liu et al. / Tectonophysics 407 (2005) 65–80 22 ° lin S u Xmcc B x x Sikh 90 km ote-A A ture N Q 75 C B Gulf of Liaoning X in g -M e n g S u tu re Wmcc NCB ELK ds Lmcc ult se TanL u Fa Shandong Peninsula an Isl e an p Ja lu Su Da bie SCB Fig. 5. Tectonic map of eastern Liaoning, Korea, and contiguous areas (modified from Lin et al., 2003). The bold dashed lines define approximately a wedge-shaped area of Mesozoic extensional basins, including the Songliao basin and the Early Cretaceous Xujiaweizi (Xmcc), Waziyu (Wmcc), and Liaonan (Lmcc) metamorphic core complexes. The bold open circle is the pole of rotation inferred by Lin et al. (2003) for clockwise rotation of the ELK block relative to a stable North China block (NCB). The upper left inset shows the locations of the Waziyu and Liaonan mcc’s relative to the Gulf of Liaoning and the location of cross-section AB–B’C (Fig. 6) through them. Waziyu mcc to the northeast (Zhang et al., 2003b; Darby et al., 2004). We estimate that the horizontal component of extension along each of the master detachment faults was in tens of kilometers. Estimates of such large displacements can be deduced from the large transport needed to bring mid-crustal rocks deformed in and below the brittle–ductile transition to the surface along faults of probable low dip (V 308–358 based on worldwide studies; cf. John and Foster, 1993; Pease and Argent, 1999). Accordingly, we propose that (1) footwall displacements to the ESE in both mcc’s produced absolute, not relative, crustal displacements away from the stable NCB–SCB block, and that (2) these displacements contributed significantly to rotation of the ELK block in the vicinity of its pole of rotation. This interpretation is supported by recent paleomagnetic studies on NCB Early Cretaceous volcanic rocks in western Liaoning that were deposited within the Yixian–Fuxin supradetachment basin above the active west-dipping Waziyu detachment fault and its lower plate (Zhu et al., 2002). Over 400 oriented cores were taken from 55 lava flows in the Yixian Formation at Sihetun and Zhuanchengzi and from the Tuhulu and Dalinghe Formations of the Yixian– Fuxin basin. Fifteen samples from the Yixian For- 76 J. Liu et al. / Tectonophysics 407 (2005) 65–80 mation provided new K–Ar ages ranging from 133.6 F 2.6 Ma to 120.4 F 2.3 Ma, an age range that agrees well with recent 40Ar–39Ar dating of the Sihetun section (132.9 F 5.1 to 123.9 F 1.5 Ma; Chen et al., 2003). A Tuhulu Formation basalt from the Fuxin basin yielded a K–Ar age of 119.0 F 0.4 (Zhu et al., 2002). Detailed paleomagnetic analysis of these Cretaceous (K1) units led Zhu et al. (above) to conclude that the Yixian–Fuxin basin area has undergone neither significant latitudinal displacements nor rotation with respect to the NCB and Eurasia, and was not, therefore, involved in the rotation of the ELK. The likelihood that both master detachment faults root westward into the middle crust—as is typical of most mcc’s given that detachment fault-related mylonitic and gneissic rocks typically have metamorphic grades no higher than middle amphibolite facies— raises the interesting possibility that crustal rotation of the ELK may have involved only the upper crust above an intracrustal zone of detachment related to the brittle–ductile transition (Fig. 5). In this scenario, the Tan Lu fault between the Waziyu and Lionan mcc’s was cut off at mid-crustal depth, since in areas south of Bohai Bay the fault separates different SCB and NCB lithospheric blocks (Yin and Nie, 1998). Zheng et al. (2003) reported that seismic imaging of the Tanlu fault across the Luxi uplift in Shandong province clearly indicates that it crosses the entire crust and offsets the Moho by 4 km. 4.5. Timing of ELK block rotation and Liaonan core complex extension Our conclusion that formation of the Liaonan and Waziyu mcc’s contributed significantly to the rotation of the ELK crustal block is at variance with conclusions of Lin et al. (2003) regarding the timing of rotation. Although agreeing that crustal extension in the Songliao Basin within the wedge-shaped terrane north of the Bohai Sea began in the Late Jurassic or Early Cretaceous, they favor a later Mesozoic–Cenozoic time for most of the block rotation. One reason for this conclusion is their interpretation that Early (K1) and Late (K2) Cretaceous paleomagnetic poles from a very limited ELK data set (6 pole determinations) are bstatistically consistentQ (Lin et al. (2003), pages 5–11, Table 3), and thus require that most block rotation occurred at a time younger than the sampled rock units. The two bK2Q data sets come from poorly dated Dayu Group units in the Benxi area of eastern Liaoning. Lin et al. (2003) assigned an age range of 118–83 Ma to the group based on the assumption that its non-reversed magnetic signatures indicate formation during the long Cretaceous normal polarity field. Even if their assumption is correct, given that the K1–K2 boundary has been variably set at 96 Ma and 98.9 Ma (Remane et al., 2000), rocks of the Dayu Group could be wholly, or in part, Early Cretaceous. South Korean paleomagnetic studies support largely Early Cretaceous rotation. Zhao et al. (1999) collected new Cretaceous paleomagnetic data consistent with the hypothesis that regions south of the Okchon zone experienced up to 368 clockwise rotation from the NCB between 121 and 114 Ma (34.38 for the early Aptian, 24.98 for the middle Aptian, and 0.98 for the late Aptian) in the middle part of Early Cretaceous. Lin et al. (2003) dispute these interpretations, although they seem broadly compatible with the more recent Korean data-based conclusions of Doh et al. (2002). Based on a recent study of the Late Cretaceous Gongu basin in southwestern Korea (K–Ar volcanic rock ages ca. 82 Ma to 73.5 Ma) and an analysis of other Korean paleomagnetic data, Doh et al. have concluded that the bKorean Peninsula underwent clockwise rotation of 21.28 F 5.38 for the middle Early Cretaceous, 12.68 F 5.48 for the late Early Cretaceous, and 7.18 F 9.88 for the Late Cretaceous with respect to EurasiaQ (p. 737). Returning to the age of extension in the Songliao basin, Ren et al. (2002) have concluded that syn-rift deposition in the Songliao Basin began in the latest Jurassic (Yinsheng, Shahexi, Huoshiling Fms.), continued through the Early Cretaceous (Denglouku Fm.), and ended in the Aptian (K1; Quanton Fm.); no radiometric ages in support of these stratigraphic calls were provided. However, Zhang et al. (2000) report that Early Cretaceous extension in the Xujiaweizi area of the west-central Songliao Basin was accompanied by a mcc-related detachment fault active between ca. 133 Ma and 120 Ma based on K–Ar ages. Mylonitic rocks in the footwall of the detachment fault (recovered only from cores) have yielded a 40 Ar–39Ar age of 126.7 F 1.5 Ma (op.cit.). Collectively, these ages indicate synchrony with activity J. Liu et al. / Tectonophysics 407 (2005) 65–80 along the Waziyu and Liaonan detachment faults farther south, and support the interpretations for ELK rotation proposed by Lin et al. (2003) and amplified here. 5. Conclusions The Liaonan mcc of southeastern Liaoning province is an asymmetric Cordilleran-style metamorphic core complex with a master detachment fault, the Jinzhou fault. That fault roots to the WNW under the western Liaodong Peninsula and, in all likelihood, the Gulf of Liaoning. A thick sequence of lower plate fault-related mylonitic and gneissic rocks derived from Archean and Early Cretaceous crystalline rock protoliths has been transported to the surface from mid-crustal depths. U–Pb ages of lower plate syntectonic plutons (ca. 130–120 Ma), 40Ar–39Ar cooling ages in the mylonitic and gneissic sequence (ca. 120– 110 Ma), and an Early (?) Cretaceous supradetachment basin attests to the Early Cretaceous age of the extensional complex. The recent discovery of a comparable and coeval mcc in western Liaoning, the Waziyu mcc of the Yiwulü Shan (Darby et al., 2004), indicates that the Gulf of Liaoning region was a locus of profound crustal extension. This conclusion is supportive of the recent hypothesis of Lin et al. (2003) regarding the cause of the paleomagnetically documented clock- 77 wise rotation (ca 22.58 F 10.28) of the Korean peninsula and adjacent areas with respect to a non-rotated North China–South China continental block. They proposed that the rotation is the consequence of crustal extension within a wedge-shaped block, dominated by the Songliao basin, which tapers southwards towards the Bohai Sea (of which the Gulf of Liaoning is the northernmost part). Lin et al. (2003) were unaware of the Liaonan and Waziyu mcc’s and argued that most of the rotation was post Early Cretaceous and, in part, early Cenozoic. However, the ca. 130–120 Ma ages of the two Liaoning mcc’s and a Songliao basin mcc (Xujiaweizi), discovered only by recent drilling through its younger stratigraphic cover, support Korean workers’ conclusion that most of the clockwise rotation was Early Cretaceous. The well-dated Yixian–Fuxin supradetachment basin lies above the WNW-dipping Waziyu detachment fault. Paleomagnetic data from volcanic rocks in this upper plate section (ca. 134– 120 Ma) indicate that it, and therefore its basement, has not been rotated significantly with respect to the North China–South China block (Zhu et al., 2002). This finding leads to our conclusion that the lower plates of the Waziyu and Liaonan metamorphic core complexes have been displaced ESE-ward in an absolute sense away from the stable Eurasian block, thus contributing to the significant rotation of Korea and contiguous areas. The displacement is likely to have affected only the upper crust, with the Waziyuu and Fig. 6. Synchronous Early Cretaceous extension in the Waziyu and Liaonan metamorphic core complexes, Liaoning province, northern China. Middle crustal rocks below the brittle–ductile transition are drawn out from beneath the fixed upper crust of the North China Block (NCB) and are displaced upwards to surface levels in the two core complexes. The gray shaded zone is the brittle–ductile transition only at the time of core complex initiation. As rocks in the brittle–ductile transition and underlying crust are uplifted in the footwalls of the two detachment faults, new brittle–ductile transitions are progressively formed under appropriate T and P (depth) conditions across the uplifting units. Middle crustal rocks uplifted in the footwall of the western Waziyu mcc remain at depth farther east where they underlie the upper plate of the Liaonan complex. Although the diagram shows extension across the two complexes as additive, they are not aligned in a NW–SE direction and it is possible that the two complexes are only en echelon examples of more limited extension. 78 J. Liu et al. / Tectonophysics 407 (2005) 65–80 Jinzhou faults flattening into a mid-crustal shear zone below the brittle–ductile transition (Fig. 6). 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