Citation: | Jinlong Ni, Junlai Liu, Xiaoling Tang, Haibo Yang, Zengming Xia, Quanjun Guo. The Wulian Metamorphic Core Complex: A Newly Discovered Metamorphic Core Complex along the Sulu Orogenic Belt, Eastern China. Journal of Earth Science, 2013, 24(3): 297-313. doi: 10.1007/s12583-013-0330-5 |
The North China Craton has undergone largescale lithospheric thinning since the Late Mesozoic (Xu and Qin, 2009; Kusky et al., 2007; Liu et al., 2005; Deng et al., 2004; Wu et al., 2003; Xu, 2001; Menzies et al., 1993). Petrological, geochemical, and geophysical studies led to the construction of different tectonic models that elucidate the fundamental processes involved in the tectonic evolution and thinning of the cratonic lithosphere (Xu and Qin, 2009; Mao et al., 2007; Deng et al., 2004; Wu et al., 2003). The most frequently cited ones are the delamination (Wu et al., 2002), lithosphere derooting (Deng et al., 2004), mantle plume (Zheng et al., 2010; Zhi et al., 2001), coupled chemical thermo-mechanical (Xu and Qin, 2009), and mantle-cell convection (Ren et al., 2002) models.
As a significant tectonic style of continental lithospheric extension and crust thinning, metamorphic core complexes (MCCs) provide exceptionally important information on lithospheric evolution (Lister and Davis, 1989). The Late Mesozoic lithospheric evolution of North China, for example, is characterized by the formation of MCCs in the crustal level along with regional lithospheric extension and thinning. These geological structures include the Liaonan (Liu et al., 2006, 2005), Yiwulüshan (Waziyu) (Darby et al., 2004), and Xiaoqinling MCCs (Zhang et al., 2000). Similar to regional magmatism and mineralization, the exhumation of the MCCs resulted from the Late Mesozoic lithospheric evolution of North China.
As the southeastern margin of the North China Craton, the Jiaodong Peninsula also experienced lithospheric extension and thinning, thereby forming a series of extensional structures, such as the Queshan and Linglong (Charles et al., 2011) MCCs in the northern Shandong Peninsula. Although considerable research has been done on the formation and evolution of high-pressure (HP) and ultrahigh-pressure (UHP) rocks on the southern part of the peninsula, little information on Late Mesozoic extension in the area has been provided. In this study, we report a newly MCC located at the northern part of the Sulu orogenic belt near Wulian City. The discovery of this MCC and detailed analysis of its evolution in relation to the development Sulu orogenic belt can provide direct evidence for the regional lithosphere thinning process in the Late Mesozoic.
The Jiaodong Peninsula principally comprises three petrological tectonic units: the Jiaobei metamorphic block, Sulu UHP metamorphic block, and Mesozoic Jiaolai Basin, which is superimposed on the two blocks (Yang et al., 2002; Faure et al., 2001; Hacker et al., 1998). The Jiaobei metamorphic block is possibly a segment of the Qinling microplate, composed of banded ferromagnesian-felsic gneiss of amphibolite facies, and locally granulite facies (Hacker et al., 2006). The block is superimposed by the medium metamorphic to nonmetamorphosed Archean, Proterozoic, and Early Paleozoic Jiaodong, Fenzishan (Jingshan), and Penglai (Wulian) Groups (Wu et al., 2004). The lowermost part of the Sulu UHP metamorphic block consists of UHP metamorphic rocks. The upper section of the UHP metamorphic rocks consists mainly of the metamorphic rocks of the Jingshan Group (Fenzishan Group), with Neoproterozoic granites (Faure et al., 2001).
During the Mesozoic, the north and south blocks of the Jiaodong Peninsula evolved through multiple stages of magma intrusion and volcanic eruption, producing huge amounts of granitic and mafic rocks. The volcanic activity was centered in Jiaolai Basin (Yang et al., 2005; Zhang Y Q et al., 2005).
Tan-Lu fault is a very striking structure in the Jiaodong Peninsula. Some parts of the main fault zone form the western boundary of the Mesozoic Jiaolai Basin. A series of branch faults including the WulianQingdao-Yantai fault usually considered as the northern boundary of the Sulu orogenic belt (Zhou et al., 2008).
The Wulian MCC is located in the northwestern section of the Sulu orogenic belt and lies to the east of the main fault belt of the Tan-Lu fault (Fig. 1). It covers an area from Zhucheng (ZhCh), Qingdao to the north, and Huangdun (HD), Rizhao to the south.
The Wulian MCC comprises of three different tectonic units: an NE-extending wavy-shaped detachment fault zone; a lower plate of Proterozoic granite gneiss and Triassic ultrahigh-pressure rocks, which were intruded by a mass of Mesozoic plutons; and an upper plate of weakly deformed Cretaceous superposed basins and a spot of Proterozoic metamorphic rocks (Figs. 1 and 2). These constituents form a typical Cordillera-type MCC (Lister and Davis, 1989).
The footwall is composed primarily of the upper Jingshan Group (Ar3–Pt1), Fenzishan Group (Pt1), and Lower Triassic UHP metamorphic rocks. The Fenzishan Groupis also called the Wulian Group in this area. Upper rocks are distributed limitedly, with the UHP rocks making up the majority of the composition (Yang et al., 2005).
The Jingshan and Fenzishan Group are distributed in an NW- or NEE-oriented zone or in a lens-shaped area. The formations are intruded by Proterozoic plutonic bodies and enclosed within the rocks as relic segments. Most of the rocks experienced intense ductile deformation, forming mylonites or gneisses (Li et al., 2004).
The Lower Triassic UHP block primarily comprises two sets of rocks, namely, layered meta-sedimentary rocks, and deformed and metamorphic Neoproterozoic granites. The layered meta-sedimentary rocks are distributed chiefly near Zhucheng in the northern part of the study area. Garnet porphyroclasts, up to 1 mm in diameter, are very common. Large eclogite lenses are typically present in the metamorphic rocks. The deformed and metamorphic Proterozoic granites are distributed principally in Wulian and Huangdun, with the appearance of massive to thick-layered coarse-grained gneisses. Given the large-scale eclogite that contains coesite in these rocks, it is considered to have gone through UHP metamorphism during Triassic (Enami et al., 1993). The surrounding rocks are thought to have undergone such metamorphism together with the eclogites (Ye et al., 2000).
Syntectonic rock mass The study site is composed of numerous but small-scale syntectonic rocks that occur primarily in the form of dikes or apophysis. The major rock type making up these stocks is diorite (Fig. 3a).
Both the foliations and stretching lineations are developed with different grades at varied locations in the ductile detachment zone. The lineations and foliations in the rocks are similar in the wall rocks and pluton, and the refraction phenomenon took place at the contact area of the rock body and wall rocks. Micrographs (Figs. 3b and 3c) and quartz EBSD fabric testing (Fig. 3d) show that the intrusive rocks underwent syntectonic deformation (Schofield and D'Lemos, 1998).
Based on the lattice-preferred orientations of the quartz c-axis, three types of pole density exist in rock mass, namely, types II, III, and IV (Fig. 3d). Types II and IV pole density surround the y-axis and denote mid-temperature deformation caused by simple shear movement during magma crystal formation. The included angle of the Type III pole density asymmetrically distributed in the Z-axis is 58°. These features characterize mid-lower temperature deformation, which may be caused by detachment extension movement during later stage or after magma crystal formation.
Post-tectonic plutons Post-tectonic plutons outcrop in a large area in the footwall, showing massive structural characteristics. The field survey shows that these rocks did not undergo ductile shear as did the Fangzi, Wulianshan, and Maershan plutons (Fig. 2).
The Fangzi and Maershan plutons intruded into the footwall gneisses and mylonites, cutting the ductile shear zone, which was similarly cut by later NNE or NE brittle faults. The rock types that make up the Fangzishan rock mass are mainly monzodiorite, monzonite, and monzogranite. Maershan rock is composed primarily of inequigranular hornblende-bearing biotite monzogranite, with the hornblende content decreasing locally to form biotite monzogranite.
The Wulianshan pluton is situated far from the detachment fault zone. This intrusion outcrop inside the Sulu UHP metamorphic belt occurs in the form of stock. In the field, this rock intrudes into granite gneiss, with the granite gneiss inclusions visible in the rock.
The ages of different post-tectonic plutons are relatively close. Zircon U-Pb, hornblende, and biotite 40Ar-39Ar chronology testing show that the rocks were formed at 116±4 to 125±4 Ma (Huang et al., 2006; Yang et al., 2005). Thus, the rocks can be regarded as indicative of contemporaneous emplacement. Later emplaced rock that cut the ductile shear zone can aid in determining the chronological constraints of the denudation history of the Wulian MCC.
Field structural observations and microstructure analyses demonstrate that the Huangdun-WulianZhucheng (HD-WL-ZC) fault is an important detachment fault zone in the northwest section of the Sulu orogenic belt. This detachment fault zone aids the understanding of the tectonic distribution, evolutionary history, and extension-thinning mechanism in this region. It is distributed primarily throughout Huangdun, Rizhao City, Wulian City, and Zhucheng City, cutting through the Paleoproterozoic metamorphic rock series and UHP metamorphic rock mass. It also controlled the development of Early Cretaceous sedimentary rocks.
The strike of the HD-WL-ZC detachment fault changes from NNE to NE from south to north, forming a spacious antiform in a plane graph, whose sides are cut by the Maershan pluton at the fold axial trace (Fig. 2). As indicated by the change in geometric shapes and occurrences along the strike, the detachment fault has a basic wavy-shaped structure, with its southern part more apparent (Fig. 2).
The data on and analyses of the planar and linear structure of the Chuangfang (CF) and Chetong (CT) antiforms show the hinge orienting 272° 22° ∠ and 278°∠ 34°, respectively. The field measuring slickenside occurrence is oriented 274° 20°, which is in ∠ accordance with the 270° 21° direction of regional ∠ stretching lineation (Fig. 2).
Crossed by later transpressional faults, the wavy shape of the northern detachment fault was slightly destroyed; the data on foliation occurrence show that the preferred orientations of the strike are NE and NNE (Fig. 2). These orientations still reflect the wavy feature of the previous detachment fault.
The HD-WL-ZC detachment fault zone has an outcrop spanning 1–2 km. The fault tectonites observed along the detachment fault have certain differences in lithological association.
The differences in tectonite associations are manifested primarily in the integrity of the tectonite series in the detachment fault zone. The tectonites developed perpendicular to the detachment fault strike in eastern Chetong, Wulian City, and occurred in sequence as follows: fault gouge (Fig. 4a) > microbreccia (Fig. 4b) > cracked mylonite (Fig. 4c) > mylonite (Figs. 4d and 4e) > mylonitic gneiss (Fig. 4f). These are the products of different evolutionary layers when the detachment fault footwall withdrew to the surface; they also indicate whether there is a detachment fault structure (Lister and Davis, 1989).
The integrity of tectonite combination varies depending on area; such differences are possibly related to the dip angle of mylonitic gneiss. The tectonite series are generally integrated in segments with a gentle dip, such as the Chetong-Shimengou Reservoir Section in Wulian County. In the segments with a steep dip, the tectonite series are generally not intact, indicating that they are void of fault gouge or microbreccia in the upper detachment fault zone. The differentiation of the tectonite series may be relevant to the uplift range during footwall uplift. The areas characterized by large-scale uplift generally have higher terrain elevation and a steeper dip angle, which causes weathering remolds and destroys the tectonites.
The progressive deformation of detachment faults is recorded by microstructural features. These features include low-temperature deformation at the shallow crust level and medium- to high-temperature deformation characteristics at the middle crust level (Table 1 and Fig. 5).
The fabric features of the deformation of the medium and shallow levels show that the detachment fault underwent deformation and metamorphism, transforming from low amphibolite facies and high greenschist facies to low greenschist facies (Lister and Davis, 1989). These changes reveal that the rocks of the main detachment fault footwall experienced progressive exhumation and gradual extraction to the surface from deep to shallow crust levels.
Polished thin sections were cut along the directions perpendicular to the foliations and parallel to the lineations. The samples were taken to the State Key Laboratory of Geological Processes and Mineral Resources of the China University of Geosciences (Beijing). Subsequently, the lattice-preferred orientation data were obtained using a Hitachi S-3400N-II scanning electronic microscope (connected to a Nordlys EBSD Model NL-II probe) operated in interactive mode at an accelerating voltage of 15 kV and operating distance of 23 mm. The LPO was counted using HKL CHANNEL5 software. Lower hemisphere projection was adopted, with foliations parallel to the XY plane and lineations parallel to the X axis.
Six samples were collected from the section measured at the Chetong-Shimengou Reservoir (Fig. 6).
The microstructures of the samples from the Chetong-Shimenzi Reservoir show that the detachment fault tectonites transformed from mylonitic gneiss to mylonite and microbreccia. The deformation of feldspar is manifested in various forms, such as ductile elongation (Sample 10-JDWL12), dynamic recrystallization (samples 10-JDWL9 and 10JDWL2), and intracrystal fractures (Sample 10-JDWL4). Quartz pervasively developed dynamic recrystallization, producing a series of subgrains or polycrystalline quartz aggregates that make up the mylonitic gneiss. The deformation features revealed by the microstructures shows that the deformation of the detachment fault tectonites followed a medium-to-low temperature pattern (Fig. 6 and Table 1).
The measurement of the preferred orientation of quartz demonstrates that the tectonites in this section embody many pole density types (Fig. 6). Among these, the c axis pole density (Schmid and Casey, 1986) region of the quartz fabric is located on the Y axis or on both sides of the axis. These axis sides are chiefly medium-temperature prism slip or intermediate fabric formed by rhombohedrons, with the slip system {10-10} < a > or {1011} < a > (Okudaira et al., 1995).
In addition, V-type pole density (Fairbairn and Robson, 1942) developed near the Z axis, while Type-III pole density developed on both sides of the Z axis. The angle of type-III pole density is ca. 60°, embodying the fabric of a low-temperature basal slip system, whose slip system and formation temperature are {0001} < a > (Festa, 2009). The superposition of two kinds of pole densities is typically visible in the same sample. For instance, Sample 10-JDWL4 exhibits the superposition of the Y axis (type-I pole density) and Z axis (V-type pole density) pole densities; the former represents a medium-temperature deformation environment, while the latter is indicative of a low-temperature environment. Overall, the quartz fabric is manifested as a transition from prism slip < a > or rhombohedron slip < a > to basal slip < a > . The tectonites corresponding to this manifestation transitioned from high greenschist facies to low greenschist facies, illustrating that the footwall underwent gradual denudation (from deep to low of crust) until it rose to the surface.
The direction of motion about the detachment fault is determined according to macroscopic, microstructural, and fabric features. As determined through the field observation, the silica lenses in the fault gouge (near the major fault plane of the HD-WL-ZC detachment fault) and the en echelon extensional fractures (which grew with the microbreccia) indicate that the footwall was rising (Figs. 4a and 4b, respectively).
Fairly clear foliation and lineation structures, which exhibit a W-E extension motion as reflected by stereographic projection, are present in the mylonite of the HD-WL-ZC detachment fault (Fig. 2). In the detachment fault tectonite, asymmetric fabrics generally developed, revealing the direction of shear motion. These fabrics include σ porphyroclast in feldspar (Figs. 5d and 5e), δ porphyroclast and domino structure (Fig. 5b), quartz subgrain obliquely arraying in a polycrystal quartz aggregation (Fig. 5g), and asymmetric fold (Fig. 4e). In addition, the lattice-preferred orientation of quartz significantly guided the determination of the shear strain direction. The numerous quartz fabrics indicate that the tectonite of the detachment fault exhibited a top-to-the-west sense of shear (Fig. 6). The above-mentioned asymmetric fabrics all suggest that the footwall moved eastward. Thus, the kinematics of the detachment fault system points to a nearly E-W extension, which is consistent with regional extension.
The superposed basin, which is an important part of the Wulian MCC, is the Zhucheng sag. It is one of the secondary tectonic elements in the south of Jiaolai Basin. The basin basement consists of the Proterozoic Fenzishan Group, Jinning Period granite, separated to the east by a master detachment zone, from a lower unit composed of the middle crust metamorphic rocks and syn-kinematic pluton (see details in the above-mentioned part about the lower plate).
The basin is covered with the K1 Laiyang, Qingshan, and K2 Wangshi groups (Fig. 2). The Laiyang Group is a set of fluvial-lacustrine sediments, with broadly developed parallel bedding, graded bedding, rhythmic stratification, and trough cross-bedding. Its rocks consist primarily of gray sandstone followed by shale and rudite (Fig. 2), which partly comprise volcanic rocks. According to the chronology test on hornblende and zircon in the basalt invading this set of rocks and fossil data, the rock formation occurred at approximately 135–125 Ma (Zhang Y Q et al., 2003).
The Qingshan Group, a set of volcanic eruptive rocks, is composed of andesitic volcanic breccia, rudite breccia, brecciated lava, and rudite brecciabearing tuff, with the bottom consisting of andesite tuffaceous rudite and sandstone. Numerous geochronological tests show that the age of this set of rocks range from 120 to 105 Ma (Ling et al., 2009; Zhang L C et al., 2003; Zhang Y Q et al., 2003).
The Wangshi Group, which has an unconformable contact under the Lower Cretaceous, is composed of a set of fluvial-lacustrine sediments, including mauvebrick red sandy conglomerate with marl. On the basis of geochronological data, the age of the rocks was determined to be 85–65 Ma (Zhang Y Q et al., 2003).
MCC exhumaiton is generally accompanied by the intrusion of rocks bodies and the ductile deformation of detachment faults, as was observed in the Liaonan MCC (Liu et al., 2005). Thus, the exhumaiton age of this MCC can be determined by the syntectonic rock chronology or by looking into the period at which the ductile deformation of the detachment fault occurred.
The analysis of the microstructures of the detachment fault zone and EBSD testing on the preferred orientation of quartz show that the tectonites of the detachment fault zone were deformed and metamorphosed from high greenschist facies to low greenschist facies. The deformation and metamorphic conditions of these facies are lower than those of the amphibolite facies during the rapid exhumation of the Sulu orogenic belt in the Late Triassic. Thus, the development and exhumaiton of the Wulian MCC is unrelated to the rapid exhumation of the Sulu UHP belt during the Late Triassic.
The muscovite 40Ar/39Ar chronology of the mylonites of the ductile shear zone reveals varied ages: 146.7±0.9 (Xu et al., 2003), 145.3±0.6, 128.2±0.7 (Webb et al., 2006), and 136.3±2.3 Ma (GSFSP, 2002). The first two are relatively close and agree with the compression-shear movement of Tan-Lu fault at Jiaodong and Liaodong peninsulas (Li et al., 2004; Yang et al., 2004), which may not be able to as the initial detaching movement time. The age of 136 Ma (GSFSP, 2002) is basically consistent with the formation age of the Laiyang Group of Jiaolai Basin (~135–125 Ma) (Zhang Y Q et al., 2003) and maybe more suitable for the initial development and denudation of the Wulian MCC. The age of 128 Ma (Webb et al., 2006) may show two extension detachment events of the ductile shear zone in the course of the Wulian MCC denudation. The formation of the Laiyang Group of Zhucheng Basin indicated that the footwall rocks underwent gradual exhumaiton from deep to shallow crust with the extension of Wulian area.
The chronology data demonstrate that the emplacement age of post-tectonic rocks is from 115±1 to 122±2 Ma, with the peak age at 122 Ma (Gao et al., 2008; Huang et al., 2006; Yang et al., 2005). The post-tectonic rocks cut the HD-WL-ZC detachment fault zone and maintained the features of the massive structure, showing that the post-tectonic rocks were not sheared by ductile deformation after intrusion. This phenomenon may also indicate that the Wulian MCC was no longer exhumed after 122 Ma.
Dabie and Sulu orogenes, separated by Tan-Lu fault, are suture zones between Yangtze and North China cratons. Thus, these orogenes possessed similar plate subductions, rapid exhumations, and other tectonic deformations. Wulian MCC, which developed in Sulu orogen, was an important tectonic type of Sulu orogen that formed under strong extensional setting during the Early Cretaceous (K1). Hence, two possible phenomena should be discussed, i.e., whether Dabie orogen underwent lithosphere thinning during K1 and whether Wulian MCC resulted from the rapid exhumation of Sulu orogen during the Late Triassic (T3) Period.
According to Ratschbacher et al. (2000), a Cordilleran-type MCC occurred during K1 in the Dabie orogen. Suo et al. (2012) considered a tectonic dome similar to the Cordilleran-type MCC. The MCC or dome created in the Dabie orogene proved that this orogen underwent strong stretching, similar to that of the Sulu orogen. Li et al. (2002) also conformed that a tectonic dome in the Dabai orogen, and considered it was resulted from lithophere delamination and thinning event during K1 (130–110 Ma).
The other phenomenon is whether Wulian MCC resulted from the rapid exhumation of the Sulu ultrahigh pressure zone.
A Cordilleran-type MCC comprises five parts. The supradetachment basin was an essential tectonic unit among them. However, no extensional basin was present during T3, thereby denoting that the Wulian MCC was not caused by the rapid exhumation of the Sulu UHP zone.
The seismic section perpendicular to the Wulian MCC long axis (Xu et al., 2003), and the general attitude of the foliation planes show that the structure of the Wulian MCC is consistent with a dome-like structure.
Plate collision or rapid exhumation of UHP zone may result in ductile shear zone in the deep lithosphere. Nevertheless, the thermochronologic data, combined with field and microstructural observations, suggest that the HD-WL-ZC detachment shear zone was active as an Early Cretaceous top-to-the-W detachment fault.
Chronology data revealed multiperiod deformations in the detachment fault zone during K1. The chronology of the Laiyang Group in Zhucheng sag coincided with the era of deformation in the HD-WL-ZC detachment shear zone. The preferred orientation of stretching lineation and cold scratches were in accordance with the stretching direction during K1, demonstrating that the exhumation of Wulian MCC resulted from strong crustal stretch movements in the Sulu orogen during K1 and not from the rapid exhumation of the Sulu orogen during T3.
The denudation of the Wulian MCC was not an isolated geological event in Jiaodong Peninsula and adjacent North China Craton. Its denudation was accompanied by a series of MCC denudation events, the rapid uplifting and denudation event of Jiaobei Block, and a change in the properties of numerous other intrusive masses in the North China Craton and its margin. It also followed the tectonic system changing from compressional shearing to transtension of the Tan-Lu fault zone (Wang, 2006; Li et al., 2004; Yang et al., 2004; Zhang Y Q et al., 2003; Zhu et al., 2001).
The similarities in denudation periods with that of the Wulian MCC are given as follows: the Linglong MCC (150–124 Ma) located in the north of Jiaodong Peninsula (Charles et al., 2011), Yagan MCC (150–126 Ma) on the north margin of the North China Craton (Wang et al., 2002), Xiaoqinling MCC (135–123 Ma) (Zhang et al., 2000), Liaonan MCC (130–110 Ma) (Liu et al., 2005), and Yunmengshan MCC of the Yanshan orogen (151–127 Ma) (Shi et al., 2009; Passchier et al., 2005).
As discussed in the front of this article, the development and denudation of Wulian MCC has nothing do with the rapid exhumation Sulu orogen during Triassic Period. Obviously, it was one of the regional extensional events. What led to these events?
We noted that along with the exhumation of a series of MCCs, the type of intrusive granitic magmas had changed.
Intrusive granitic magmas in the North China Craton indicate that there might be a thickened crust during 165–127 Ma but thinned after 127 Ma (Xiong et al., 2011; Hu et al., 2010; Wang et al., 2000). In view of this result, the denudation event of the Wulian MCC may be directly related to the crustal thickening and partial melting of the crust in the Jiaodong Peninsula and North China Craton. This large-scale crust melting event may have brought about the delamination of thickened crust and reduction of the lithosphere (Gao et al., 2009).
Tectogenesis at the crustal level generally signifies a shallow response to extensive lithospheric processes (Shao and Han, 2000). The formation and denudation of the Wulian MCC may be an important form of the extension and thinning of the lithosphere in the North China Craton and its adjacent area.
The conclusions drawn in this study are summarized as follows.
(1) The HD-WL-ZC regions in the Sulu orogen may have featured an MCC structure, which is a typical Cordilleran MCC possessing three typical factors and five components (hanging wall and superposed basin, detachment fault zone, footwall, syn-extension dykes, and post-extension plutons). Given the late compressional-shear fracture and rock mass invasion, the detachment fault zone was crossed along its strike and dip, making it a dismembered structure.
(2) The exhumation of the Wulian MCC was possibly from ~135 to 122 Ma. The superposed basin of the MCC is covered with the K1 Laiyang Group (~135–125 Ma), Qingshan Group (120–105 Ma), and the K2 Wangshi Group (85–65 Ma).
(3) The attitudes, microstructure, and preferred orientation of the mylonites in the detachment fault belt indicated that the footwall had gradually transformed from middle to shallow crustal level under the nearly W-E extensional kinematics.
(4) The development and exhumation of the Wulian MCC were unrelated to the rapid exhumation of Sulu UHP metamorphism zone during T3 yet not isolated events, which may be an important form of the extension and thinning of the lithosphere in the North China Craton and its eastern margin.
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