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Destruction geodynamics of the North China Craton and its Paleoproterozoic plate tectonics ZHU RiXiang† & ZHENG TianYu Paleomagnetism and Geochronology Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

Much attention has been paid in the last two decades to the physical and chemical processes as well as temporal-spatial variations of the lithospheric mantle beneath the North China Craton. In order to provide insights into the geodynamics of this variation, it is necessary to thoroughly study the state and structure of the lithospheric crust and mantle of the North China Craton and its adjacent regions as an integrated unit. Based on the velocity structure of the crust and upper mantle constrained from seismological studies, this paper presents various available geophysical results regarding the lithosphere thickness, the nature of crust-mantle boundary, the upper mantle structure and deformation characteristics as well as their tectonic features and evolution systematics. Combined with the obtained data from petrology and geochemistry, a mantle flow model is proposed for the tectonic evolution of the North China Craton during the Mesozoic-Cenozoic. We suggest that subduction of the Pacific plate made the mantle underneath the eastern Asian continent unstable and able to flow faster. Such a regional mantle flow system would cause an elevation of melt/fluid content in the upper mantle of the North China Craton and the lithospheric softening, which, subsequently resulted in destruction of the North China Craton in different ways of delamination and thermal erosion in Yanshan, Taihang Mountains and the Tan-Lu Fault zone. Multiple lines of evidence recorded in the crust of the North China Craton, such as the amalgamation of the Archean eastern and western blocks, the subduction of Paleo-oceanic crust and Paleo-continental residue, indicate that the Earth in the Paleoproterozoic had already evolved into the plate tectonic system similar to the present plate tectonics. destruction of North China Craton, seismic imaging, mantle flow, Paleoproterozoic, plate tectonics

Stable cratons are the most important tectonic elements of the lithosphere. Thus, their evolution is of substantially importance for understanding the whole Earth’s dynamic system. Only the cratons carry understanding the information about the Earth’s dynamic processes in the Precambrian. Moreover, craton destruction is crucial for distinguishing between regional perturbations and precursors to continental dispersal. Despite enormous effort, there are remaining controversies over the formation mechanism of cratonic roots. For instance, roots could be linked to vertical stacking of subducted plates, or to igneous processes associated with deep mantle-derived magmas[1]. The oldest material documented on the Earth was formed at 4.4 Ga ago,

which provides the upper limit of the cratonic age[2]. The thick subcontinental lithospheric mantle (SCLM) root (~200 km) underlying craton is geochemically depleted and refractory, compositionally dehydrated, physically buoyant, and mechanically strong (high viscosity and yield strength), which makes cratons stable after formation. However, several lines of evidence suggest that the cratonic lithosphere cannot be permanently stable. As examples the East African rift system is affected by manReceived April 15, 2009; accepted June 2, 2009; published online July 3, 2009 doi: 10.1007/s11434-009-0451-5 † Corresponding author (email: [email protected]) Supported by the Key Program of National Natural Science Foundation of China (Grant No. 90814000)

Citation: Zhu R X, Zheng T Y. Destruction geodynamics of the North China Craton and its Paleoproterozoic plate tectonics. Chinese Sci Bull, 2009, 54: 3354―3366, doi: 10.1007/s11434-009-0451-5

1 Geochemical studies of the lithospheric destruction in NCC 1.1 Evidence of the lithospheric thinning In the fifties of the last century, the concept of “platform reactivation” in the NCC was put forward by Chinese geologists[6,7]. Presently, it is the consensus that the SCLM underlying the NCC was thinned and modified during the Mesozoic-Cenozoic[8,9] Kimberlitic magma originates from the mantle at a depth of ~200 km, where diamond can crystallize, as this region is characterized by high temperature, high pressure and low oxygen fugacity. Therefore, the diamondiferous kimberlites and entrained mantle xenoliths/ minerals can provide invaluable information on the lithospheric structure through which the magma passes. Occurrence of refractory peridotitic xenoliths in the diamond-bearing kimberlites at Mengyin of Shandong Province, and Fuxian of Liaoning Province, implies the existence of a thick (~200 km) and refractory mantle lithosphere in the middle Ordovician. The high Fo number and the ancient Re-Os ages for these samples document an Archean age for the SCLM in the area during the early Paleozoic[8−10]. In contrast, studies of the Cenozoic basalts and enclosed peridotitic xenoliths in eastern China suggested a different property of the lithospheric mantle then. Garnet harzburgite is a dominant kind of mantle xeno-

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liths in the Paleozoic kimberlite; whereas the majority of the Cenozoic basalt-hosted xenolith is spinel-faces lherzolite. The Paleozoic peridotite xenoliths are refractory, representing residues after high degrees of melt extraction from the ancient mantle source. In contrast, the Cenozoic peridotite xenoliths are chemically fertile, representing residues after low degrees of melt extraction from the juvenile mantle source. Sr-Nd-Hf isotopic analyses indicate an enriched SCLM during Paleozoic, but it was depleted during the Cenozoic. Thermobarometric studies of the mineral inclusions in diamonds of kimberlites indicate that the Paleozoic geotherm was relatively cool of ca. 40 mW/m2, similar to that in other typical cratons of the world. In contrast, the Cenozoic geotherm rose to ca. 80 mW/m2, similar to that in the tectonic activated regions. These differences suggest an essential change in the physical and chemical properties of the SCLM in eastern NCC, indicating a lithospheric modification and even destruction of the NCC since the early Paleozoic[8,10]. 1.2 Ways to destroy the NCC In view of the stratified structure of the Earth, SCLM is an assemblage of some stable materials at giving temperature and pressure conditions in the uppermost mantle. With the support of new techniques, to determine the formation age, the composition and properties of the mantle-derived materials is the most fundamental approach to characterize temporal variations of SCLM. By analyzing the formation conditions (such as temperature, pressure, and compositions) of SCLM and related magmatism and regional tectonism, we hope to look for the processes controlling the lithospheric thinning beneath the NCC. To address this question, comprehensive studies have been conducted to determine the time, vertical thinning degree, spatial range, and dynamic mechanism of this important geodynamic phenomenon. The geochemical study of Gao et al.[11] indicate that the ancient mafic lower crust was foundered into the convecting asthenospheric mantle and subsequently melted. The melt was then interacted with the overlying mantle peridotite in the Yanshan region (at Xinglonggou in western Liaoning Province). This is considered as important evidence of the delamination model for lithospheric thinning. Their new study of the Early Cretaceous alkaline picrites and high-magnesium basalts in Feixian of western Shandong and Sihetun of western Liaoning provides further constraints on recycling of

Zhu R X et al. Chinese Science Bulletin | October 2009 | vol. 54 | no. 19

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tle upwelling[3], the intraplate volcanoes (hot spots) on the African and South American plates are linked to cratonic edge-driven convection[4]. Nevertheless, the most striking examples of wholesale destruction of cratonic roots are the Wyoming Craton and the North China Craton (NCC)[5]. The NCC consists of a relatively intact western part and a destroyed eastern part, and thus can be considered as a typical example to study the continental geodynamics. Petrological and geochemical analyses have documented the lithospheric thinning of the NCC and proposed potential candidates for the mechanism of its physical and chemical modifications. Here we summarize the geophysical data on the dynamical mechanism of lithospheric destruction, especially by a regional mantle flow system beneath the NCC, and further assess the initiation and modality of plate tectonic activity in the early Paleoproterozoic from the paleotectonic records preserved in the crust of the western NCC.

continental lithosphere by density foundering[12]. Alternatively, lithospheric thinning of the NCC has been attributed to thermo-mechanical erosion[13−17] based on studies of Cenozoic basalts and their peridotite xenoliths in the Tan-Lu Fault zone (Shanwang in Shandong and Nüshan in Anhui), and other localities. Zhang[18] studied the zoned structure of olivine xenocrysts/xenoliths entrained in the basalts. They suggested that a peridotitemelt reaction is responsible for the replacement of SCLM from the Paleozoic refractory mantle to the late Mesozoic fertile and enriched mantle. In summary, the petrologic and geochemical studies of samples derived form the Earth’s interior provide important constraints on the possible physical and chemical processes modifying the SCLM, and several mechanisms have been put forward to interpret the destruction of the NCC, including delamination, thermomechanical/chemical erosion, peridotite-melt reaction, mechanical extension, and lithospheric weakening by hydration[8,19].

2 Structure of the crust and the upper mantle beneath the NCC Cenozoic-Mesozoic mantle-derived magmas and their mantle xenoliths provide direct information about lithospheric compositions and physical properties, and the evidence for lithospheric thinning of the NCC. However, the limited spatial distribution of the samples has hindered our further understanding of the destruction of the NCC. To determine the corresponding dynamic process and mechanisms, we need to understand the mantle flow behavior beneath the NCC and the adjacent regions[20]. Geophysical surveys provide such an opportunity to construct a fine-scale structure of the present crust and mantle. With the technical developments and the publicly available seismic data available from the Global Digital Synchronization Network, records from many permanent and temporal seismic arrays produce a high-quality, dense, and homogenous data set, which can be used to study the fine-scale structure of the Earth’s interior. According to the velocity structure of the crust and upper mantle constructed by seismological studies for the recent data from the temporal seismic arrays in the NCC, we are in a position to decipher the lithosphere thickness, the nature of crust-mantle boundary, the upper mantle structure and deformation characteristics as well as their tectonic features and evolutional systematics. On this 3356

basis we propose that the interaction between the lithospheric mantle and the fast moving and unstable upper mantle beneath the NCC may result in the wholesale destruction of cratonic roots. We further propose that the subduction of Pacific plate played a pivotal role in the reactivation of the NCC. 2.1 Spatial variation of the lithospheric thickness The lithospheric structural images have been constructed along different linear profiles in the eastern NCC based on dense seismic array data. The lithospheric thickness of 60―80 km beneath the profile from Gaomi to Jinan approved the significant lithospheric thinning[21]. Mapping of lithosphere-asthenosphere boundary depth beneath the eastern NCC, which was produced by integrating the results for individual profiles, shows a thinned lithosphere and a general SE-NW deepening of the lithosphere-asthenosphere boundary, from 60―70 km in the southeast areas to >100 km in the northwest (Figure 1)[22]. The bulk lithospheric thinning is indicated by the 1-D and 2-D lithospheric structure beneath the eastern NCC as shown in Figure 1. The spatial variation of the lithospheric thickness suggests a progressive destruction from east to west.

Figure 1 Map of the lithosphere-asthenosphere boundary depth [22] for the eastern North China Craton (refer to Chen et al. ). The scale of lithospheric thickness is shown at the bottom. Short black bars give the SKS splitting results. BBB, Bohai Bay Basin; LU, Luxi uplift; TM, Taihang Mountains; YinM, Yinshan Mountains; YM, Yanshan Mountains.

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Zheng et al.[23] used regional seismic images of the crust-mantle boundary zones in the NCC to investigate the contrast structures and thus place further constraints on the mechanism of the NCC destruction. Teleseismic body waves from earthquakes produce a series of reflections, refractions, and conversions as they traverse the boundaries between regions of different seismic velocity and/or impedance. P-S wave conversion at the Moho is defined as a transmitted SV wave produced by an incident P wave at the Moho. We ranked the receiver functions stacked from each station along seven observed seismic profiles respectively as shown in Figure 2. The dominant phases near 4―5 s are clear in all the stations. In the Yanshan profiles (B and C), the Ordos profile (A), and the part of Hebi-Qingyang profile (F) (at the eastern Qinshui basin) another strong phase can be con- tinuously traced near 14―16 s, which is the PpPs phase. In contrast, the same counterparts do not appear in the other profiles. By comparing the synthetic receiver functions calculated from the inverted velocity models with the data, Zheng et al. [23] identified that the P-to-S convert wave energy near 4―5 s and the following PpPs phases near 14―16 s are generated by the crust-mantle boundary. Here, we confirmed that the distinct characteristics shown in the latter phases of the receiver function profiles (Figure 2) are mainly generated by the distinct structures of the crust-mantle boundary. As an example, the velocity models and the waveforms of receiver functions for stations FEN and MIY at the Yanshan region, and ZHB and CHC at the Taihangshan region are shown in Figure 3. It is clear that the thick crust-mantle transition zone results in diffuse and weakened PpPs phases at stations ZHB and CHC in the Taihangshan region, while the sharp crust-mantle boundary yields strong PpPs phases at stations FEN and MIY in the Yanshan region. One might wonder what kind of governing factors lead to the spatial variation of the crust-mantle boundaries that accompany the cratonic reactivation in the NCC. A sharp crust-mantle boundary revealed by the seismic image of the Yanshan region could be attributed to the direct contact of intruding mantle materials with the evolved higher-level crust. The geodynamic process of this phenomenon, associated with the NCC reactivation, is more likely due to the rapid foundering of the lower

crust and the part of SCLM, upwelling of the asthenospheric mantle to replace the volume formerly occupied by the sunken lower crust and SCLM. On the other hand, a number of plausible causes could explain the thicker crust-mantle transition zone, for example the mixing of crustal rocks and mantle rocks by underplating, the shear deformation of mantle rocks, and the phase transformation of mafic rocks. The dynamic thickening of the crust-mantle transition zone revealed by the seismic image of the Taihangshan region, as well as the Tan-Lu Fault zone, would be better fit by a secular interaction between the crust and the underlying mantle, which would result in thermomechanical/chemical erosion associated with the lithospheric destruction. The seismic images document that the contrast in structural features of the crust-mantle boundary zones are inherent to the eastern NCC, which means that the change in SCLM can be attributed to distinct processes. We suggest that the lithospheric destruction can occur in different ways in different regions, including protracted thermal erosion and underplating, as well as rapid delamination. 2.3 Regional variation of the deformation in the upper mantle Lattice preferred the orientation of anisotropic minerals, developed in response to deformation, can produce seismic anisotropy. The orientation of the seismic fast axes relative to the strain directions depends on the deformation mechanism and history, and on rheological variables. The contemporaneous mantle deformation and the coherent deformation in the crust and SCLM throughout their history are often proposed to explain the origin of seismic anisotropy under continents. The anisotropy pattern of the upper mantle in the NCC is substantially variable[24−26]. The results of shear wave splitting measurements in the Luxi uplift and Bohai Bay basin show that the fast polarization directions trending NW-SE agree with the orientation of extensional structures in this region. This suggests that the lithosphere beneath the Eastern Block retains the history of Late Mesozoic to Early Cenozoic deformation structures. In the Taihangshan region, the fast polarization directions show good consistency trending NE to NNE, which is subparallel to the trend of the tectonic features that may retain the history of compressive deformation in the region. Shear-wave splitting results indicate complex seismic anisotropy beneath the NCC. Rapid


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2.2 Distinct structures of crust-mantle transition zone-diverse ways to destroy the NCC

Figure 2 Cross-sections of P-receiver functions along the seismic observed profiles from teleseismic data stacked for each station. The inset shows the seismic station distribution within the North China Craton; triangles represent seismic stations marked in the same color with corresponding receiver function profiles. A, Ordos profile; B, E-W profile in the Yanshan region (data in the western section from the Capital Seismic Network in China, and in the eastern section from the NSFC Program 40474022); C, Yanshan-Xingmeng profile; D, E-W profile in Taihangshan region; E, N-S profile in Taihangshan region (data from the Capital Seismic Network in China); F, Hebi-Qingyang profile (data from the NSFC Key Program 90814002); G, Tan-Lu-Luxi profile. If not specified, the data are from the North China Interior Seismic Experiment (NCISP).

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changes in the anisotropy pattern occur across the boundary between the Central and the Eastern NCC Blocks. The results in the Yanshan region reveal a complex pattern of mantle deformation. At the cratonic edge, null splitting results indicate undetectable anisotropy beneath the stations. This may be due to mantle upwelling or chaotic ascension of mantle flow. At the western edge of the NCC, weak anisotropy was observed. In contrast, obvious splitting was recorded in the central and eastern Ordos Block with the majority of fast directions trending NW-SE and the central segment having a larger average delay time of 1.8 s. The spatial change in fast directions from east to west may imply a spatially varying deformation and flow pattern of the upper mantle. The anisotropy pattern of the upper mantle can be used to identify a mantle upwell, which could be one of

Figure 4 Shear-wave splitting analysis results in the Tan-Lu Fault [27] zone region (refer to Zheng et al. ). Each bar indicates measured SKS splitting parameters at one station. The orientation represents the polarization direction of the fast shear wave, and the length is proportional to the delay time between the slow and fast shear waves. LXU, Luxi uplift; TLFZ, Tan-Lu Fault zone.

Seismological results also reveal that the present lithosphere beneath the Gaomi-Jinan profile consists of two parts with different structural features throughout[27]. The contrasting structure demonstrates that the Tan-Lu fault zone was operated as a vertical lithospheric shear zone, inducing stress concentration and facilitating the NCC reactivation in the Mesozoic. The crustal structural


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Figure 3 Waveform analysis for P-S conversion wave at Moho [23] (refer to Zheng et al. ). (a) Waveform comparison for the observed receiver functions (blue dotted line) and the synthetics (black solid line) for stations FEN (Fengning) and MIY (Miyun) in the Yangshan region and ZHB (Zhangbei) and CHC (Chicheng) in the Taihangshan region. The synthetics are calculated from the inverted optimal velocity models. The arrivals of Ps and PpPs phases converted from multiple discontinuity of the crust-mantle transition zone are marked with different colors, and the corresponding layer velocity is marked in the same color in (b); (b) Shear-wave velocity structures of crust-mantle transition zone beneath the stations CHC, FEN, MIY, and ZHB obtained from the inverted optimal velocity models.

the dynamic factors to modify the craton. If a mantle upwelling took place, two anisotropic patterns might have occurred. The asthenosphere could have upwelled vertically and transformed the base of the thick SCLM root slowly. This could have produced uniformly weak or negligible anisotropy in the region where asthenospheric upwelling took place. The other possibility is that a mantle plume could have thermally impacted the base of SCLM and the ultrahot uprising material could then have flowed sideways. This would have produced weak splitting at the center of the upwelling plume, surrounded by a radial pattern of fast directions. The shear-wave splitting analysis shows a clean contrast in seismic anisotropy between the eastern and western parts in the Tan-Lu fault zone (Figure 4)[27], which is one of the regions suggested as an example of a mantle plume event[28,29]. The region can be divided into two zones that are separated by a ~40-km-wide transition region (between the two yellow dashed lines in Figure 4). The fast directions consistently trend NE-SW or ENE-WSW to the east of the transition region, while they trend consistently NW-SE in the west. The observed spatial variation of fast directions in the Tan-Lu Fault zone is inconsistent with both of the above-predicted anisotropy patterns from the impact of the mantle plume. Therefore, a deep mantle plume does not readily explain the abrupt change in the uppermost lithosphere in the Tan-Lu fault zone.

imaging beneath several temporal seismic array profiles revealed that the low velocity zones occur throughout the crust, probably in association with the voluminous Mesozoic-Cenozoic magmatism of the region[30,31]. However, the asymmetrical and dispersive distribution of the low velocity zone, would not favor the tectonic pattern of mantle plume. 2.4 Upper mantle convection/flow pattern Seismic tomography revealed that a subducted slab beneath the eastern NCC stagnated at the mantle transition zone (Figure 5)[32]. In contrast, seismic tomography has revealed a high-velocity root extending downward 200 km beneath the western NCC[32,33] (Figure 6). Receive function imaging provides an effective approach to construct the structure of mantle transition zone. The imaging results of the teleseismic data from dense array in the NCC revealed that the mantle transition zone appears thick in the eastern part, and becomes thinner in the central part, and slightly thicker in the western part. A receiver function inversion by restricting the 410 and

660 discontinuities to an anticorrelated topography indicated a local low-velocity anomaly zone of upper mantle beneath the central NCC (the Taihangshan region and the Fenwei rift system). The structureal features of the upper mantle beneath the NCC, including the stagnant subducted slab, the local low-velocity anomaly zone, the thick high-velocity root, and the heterogeneous anisotropic pattern, imply a possible variable mantle flow pattern beneath the NCC. From a comprehensive view of the upper mantle seismic structure beneath the NCC, we can infer the upper mantle convection/flow pattern and the interaction between the NCC lithosphere and its surrounding mantle. The westward subduction of the Pacific plate induced asthenospheric flow from beneath the eastern NCC towards the subduction zones. This in turn requires replenishment of the asthenospheric material from beneath the western NCC to the eastern NCC. This process would generate a large mantle wedge above the stagnant slab of the Pacific plate[34]. Dehydration and breakoff of

[32]

Figure 5 Vertical cross sections of P wave velocity perturbations (refer to Huang and Zhao ). The surface and seafloor topography along each profile is shown on the top of each cross section. White dots show the earthquakes that occurred within a 50-km width from each profile. Red and blue triangles denote locations of volcanoes and oceanic trenches, respectively. The velocity perturbation scale is shown at the right bottom in (a). The dashed black lines denote the 410- and 660-km discontinuities.

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the subducted oceanic lithosphere would result in mantle contamination and local disturbance of the mantle convection pattern, making the mantle flow fast and unstable. The interaction between the unsteady mantle flow and the weakened part of the ancient SCLM would intensify the thermal erosion process, while the interaction between the unsteady mantle flow and the western thick cratonic root would result in turbulent convection. Based on seismic observations, we speculate that the mantle convection and flow system beneath the NCC (Figure 7) resulted from stable mantle convection beneath the western NCC combined with fast and unstable mantle flow beneath the eastern NCC.

3 Model and dynamic mechanism of the NCC destruction Although the nature of SCLM beneath the NCC has been documented by studies of petrology, geochemistry and seismology, the mechanism for destroying the NCC is quite controversial. According to previous studies, two main models have been suggested: delamination and

thermal erosion. The delamination model states that gravitational instability, derived from a thickened lower crust that has converted to eclogite, can result in the cratonic lithosphere (particularly the lower crust) to rapidly founder into the underlying asthenospheric mantle where they undergo partial melting. The uprising asthenospheric mantle would replace the volume that was formerly occupied by the sunken SCLM and lower crust. This would result in a present SCLM that has been reju- venated. In the mode of thermal erosion, however, the lowermost SCLM was warmed conductively by heat transported from an upwelling asthenosphere. This weakened and softened lower SCLM would be prone to removal by lateral shear stresses produced by horizontal flow in the asthenosphere. Such removal would then enhance the conduction of heat into the overlying SCLM, softening the latter further and allowing a higher degree of mechanical removal. In addition to the conventional thermomechanical erosion, the role of chemical erosion in lithospheric thinning also plays a role, as it can modify the rheology of the mechanical boundary layer and generate positive feedback between the chemical and


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Figure 6 Lateral P wave velocity perturbations at different depths as indicated on the left corner (refer to Li et al. ). The blue and red represent fast and slow perturbation, respectively. The perturbation scale is shown on the right upper corner. The significant structures have labeled as number.

Figure 7 Schematic diagram of upper mantle flow model beneath the North China Craton. The top plane shows the topography map. The structure of the stagnant slab (dark blue), cratonic lithosphere (green) and the mantle convection pattern are shown in the profiles. Black arrows mark the stable convection, and red and blue arrows mark local disturbed flow. The sketch map beneath the Tan-Lu Fault zone and Taihangshan region marked the destruction mode of thermal erosion, the sketch map beneath the Yanshan region marked the destruction mode of delamination.

mechanical erosion. Several other scenarios of destruction have also been suggested, including peridotite-melt interaction, mechanical extension, and magma extraction. According to the geological records, several tectonic events occurred in and around the NCC, including southward subduction of the Paleo-Asian ocean in the Carboniferous, collision of the NCC with the South China Block in the Triassic, the subduction of the Pacific plate in the Mesozoic-Cenozoic. Also included are the dispersal of supercontinent Gondwanaland, the involvement of mantle plume, and the India-Eurasia collision. All of these have been suggested as the geodynamic factors in causing the destabilization of the NCC lithosphere. In order to provide more precise insights into geodynamics of the changes, it is necessary to thoroughly study the state and structure of the crust and the upper mantle of the whole NCC and its adjacent regions. The destruction of the lithosphere beneath the NCC is characterized by large-scale, long-term magmatism, heterogeneous and variable tectono-magmatic events. These imply that a deep dynamic process should control the interaction between the cratonic lithosphere and the underlying/surrounding mantle. According to the seismological studies, a convection/flow system was produced by the big mantle wedge convection and unstable mantle flow, as well as the interaction between the sub3362

duted slab and the lithospheric mantle as illustrated in Figure 7. The SCLM would be weakened and softened by the dehydration of the subducted slab. Such a local mantle convection/flow system would cause the increase of melt/fluid concentrations, and heating and decompression, which would further facilitate partial melting and melt-rock reactions[18,35]. As a result, the Yanshan region and Taihang Mountains/Tan-Lu Fault zone in the NCC were destroyed via the delamination and the thermal erosion, respectively. Imaging of the contrasting structures on the two sides of the Tan-Lu Fault zone provides solid evidence that the trans-lithospheric weakened zone operated as an asthenospheric upwelling channel and thus facilitated the NCC reactivation by thermo-erosion. The thick crustmantle transition zone beneath the Taihangshan region also favors reactivation by the thermal erosion. However, a sharp crust-mantle boundary revealed by the seismic image in the Yanshan belt could be attributed to the delamination. The different tectonic settings caused by the previous tectonism from place to place might be responsible for the variation in the mode of destruction of the NCC. All of these, either the delamination or the thermo-erosion, among the others, is possible ways of thinning the SCLM. Although the southward subduction of the PaleoAsian Ocean in the late Carboniferous and the collision between the NCC and the South China Block in the Tri-


4 Plate tectonic system in the Paleoproterozoic Understanding when and why plate tectonics work on the Earth is one of the unresolved important questions in earth sciences. Finding solid evidence for the operation of early plate tectonics requires a clear understanding of its distinctive characteristics preserved within the rocks. Meanwhile, it is crucial to find and identify the imprint of plate tectonics in the ancient rock records, such as polar wander paths, subduction-related magmatism, eclogitized oceanic crust, seafloor spreading-related ophiolites, and “frozen-in” structural information in the crust[37,38]. As one of the oldest continent blocks in the world, the NCC is a promising region that may provide evidence for Precambrian plate tectonism with geodynamic features similar to those at present. Available geochronological data, especially the Nd and Hf model ages, reveal that the age of 4.0 Ga is considered to represent the most primitive continental crust age for the NCC. The major crustal growth of the NCC took place from 3.0 to 2.5 Ga, with a peak at 2.8―2.9 Ga. Seismological study also reveals a flat crustal fabric beneath the Ordos basin, with a three-layer crustal structure and flat-lying interfaces. This indicates a stable cratonic regime in the western NCC. The Ordos Block has remained stable since cratonic amalgamation, which raises the possibility of finding information about the early plate tectonic evolution even though the eastern NCC experienced significant tectonic rejuvenation in the Late Mesozoic


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and Cenozoic. In the last two decades, detailed field investigation in the Lüliang Mountain-Hengshan Mountain-Wutai Mountain and the southern Inner Mongolia, and available petrologic, geochemical and geochronologic data were integrated to provide insights into the geodynamic evolution scheme of the Ordos Block. It was suggested that the eastern NCC was separated from the western NCC by an ancient ocean during the late Archean and early Paleoproterozoic. The subduction of oceanic lithosphere led to the development of an active continental margin, island arc and back-arc basin. The closure of the paleo-ocean led to the continent-continent collision, and the assembly of the NCC. Several collisional models have been suggested, including the westward subduction of the oceanic lithosphere beneath the western block, the eastward subduction beneath the eastern block, and the amalgamation of the NCC through two stages of continental collisions and subduction[39−41]. We reconstruct the crustal structures beneath the NCC using the teleseismic data from two profiles of temporary array observations cross the NCC with E-W trending (Figure 8). The image from northern profile exhibits the intriguing characteristic of two low-velocity zones extending 200―300 km in the crust of the western and the central NCC (L1 and L2 shown in Figure 8). Combining with the geological and geochemical results, we speculate that the L1 unit represents a remnant of upper-middle crust associated with the westward subduction beneath the western block during the assembly of the NCC, the L2 unit is possibly a slab-like remnant attached to the bottom of the original lower crust during the last stage of this subduction[42]. The final amalgamation of the NCC at ca.1.85 Ga may indicate that plate tectonism had operated with geodynamic features close to the present at least since the late Paleoproterozoic. To support this conclusion, a similar remnant of subduction slab should be found in different locations of the western NCC. Therefore, we deployed another temporary seismic array along the Hebi-Qingyang profile trending E-W, which is nearly parallel to the northern Lijin-Etuoke profile (Figure 8(c)). We also found a westward-dipping low-velocity zone, similar to that in the northern profile, from the upper crust to the Moho (L1 in Figure 8(b)) in the southern profile. The coherence of the crustal structural characteristics between the two observed profiles ca. 400 km apart documents the

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assic would affect the stability of the SCLM of the NCC, the inherent properties of the SCLM were not changed. In a view of the interaction between the SCLM and adjacent mantle flow, it is suggested that the subduction of the Pacific plate made the mantle flow underneath the eastern Asian continent fast and unstable (Figure 7). The variation in the size of mantle wedge, and the shift in the unstable flow region followed by the movement of the subducted slab and the stagnant slab, would raise the instability of the mantle flow, thus facilitating the asthenospheric upwelling along weakened channel and the thermal erosion. Therefore, the Pacific plate subduction played a pivotal role in the wholesale destruction of the cratonic roots of the NCC. This is concordant with the significant episode of igneous activity at 130―120 Ma in the eastern NCC[36].

Figure 8 The shear-wave velocity structure of the crust and uppermost mantle compiled from the inverted velocity model along (a) the [42] northern profile (Lijin-Ertuoke profile) (refer to Zheng et al. ), and (b) the southern profile (Hebi-Qingyang profile). The seismic stations (triangle) shows in (c). L1 is a westward-dipping low velocity zone, and L2 is a horizontal low velocity zone in the lower crust. Some station numbers are labeled on the top of the plot. LM, Lüliang Mountain, TM, Taihang Mountains, TNCO, Trans-North China Orogen, QS, Qinshui basin.

Figure 9 Schematic diagram of ancient plate tectonic model for the collision-subduction between the eastern and western blocks during the [42] assembly of the North China Craton (refer to Zheng et al. ). TNCO, Trans-North China Orogen, EB, eastern block, WB, wastern block.

western-dipping subduction during the NCC assembly. It provides strong evidence that the plate tectonic system had operated before 1.85 Ga. We have constructed a tectonic model of the ancient plate evolution in Figure 9 to explain the amalgamation of the eastern and western blocks, the subduction of ancient ocean crust, and the residues of the ancient continental crust in the NCC[42]. However, we expect some differences in the fossil slab configuration and the present slab structures. One source of difference may be that conditions were different in ancient and modern subduction zones, combined with the effects of subsequent tectonism after assembly 3364

of the NCC. Different environments in the early Earth could have led to different processes of subduction in the Precambrian from that in the Phanerozoic. Moreover, subsequent tectonic activity and the Mesozoic-Cenozoic reactivation of the NCC may have modified the architecture of subduction zones. Paleomagnetic studies indicated that the magnetic field at 2.5 Ga is similar to that of the present-day[43,44]. For example, the ancient field exhibited reversals and similar paleosecular variation. Moreover, the paleointensity recorded by these ancient rocks is comparable to modern values. This strongly indicates that the Earth’s


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GEOPHYSICS

1

during the early Paleoproterozoic; and (4) as a typical reactivated craton, the NCC is the best natural laboratory to investigate the tectonic event of cratonic destruction. Our multidisciplinary research was supported by the Key Program “NCC destruction” of NSFC. This program has being carried out to investigate the geodynamics of cratonic destruction, and the development of a plate tectonic system in the early Earth by placing the NCC destruction into the temporal and spatial framework of the Earth’s evolution. It would be of great help to acquire multidisciplinary understanding from the observations, and extend the study from cratonic destruction to the formation and evolution of the craton, and from the NCC to the eastern Chinese continent, even the Earth.

REVIEW

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