Comparison of Results of Recent Seismic Profiles with Tectonic Models of the North China Craton

INTRODUCTION

For the past decade there has been a controversy over the Precambrian tectonic evolution of the North China craton, with two main models dominating the controversy, and the recent introduction of two new alternative models in the past few years. Stimulated by funding from the "North China Interior Structure Project" from the National Natural Science Foundation of China, new seismic reflection and tomographic profiles have been completed across various tectonic belts of the craton. The results of these geophysical surveys shed new light on the tectonic models of the North China craton, and show that some of the models are viable, and others are not. In this contribution, we discuss the various proposed tectonic models, then summarize the recently published geophysical profiles, then assess which tectonic models have survived the geophysical tests, and which have failed.

EARLY TECTONIC MODELS FOR THE PRECAMBRIAN EVOLUTION OF THE NORTH CHINA CRATON

One of the most popular models for the tectonic evolution of the North China craton is the one advocated by Zhao and co-workers (Zhao et al., 2010, 2005, 2001a, b, 2000, 1999a, b, 1998; Zhao, 2001). This group has used mostly U-Pb geochronology and metamorphic P-T paths to constrain the temporal and thermal evolution of rocks from different belts, and led to their definition of the North China craton being divided into two major blocks (eastern and western blocks), separated by an intervening orogen they termed the "Trans-North China orogen (TNCO). Based on their data, these workers suggested that the two blocks formed independently in the Archean, and the western margin of the eastern block was an active, Andean-style margin above an east-dipping subduction zone from the Late Archean until the two blocks collided in a continent-continent collision at circa 1.85 Ga (see Figs. 1 and 2). Polat et al. (2007) commented that in this model, the proposed Andean margin would be the longest-lived such margin in earth history, and there is no record of a large accretionary orogen as implied by the model.

Figure  1.  Tectonic divisions of the North China craton proposed by the Zhao et al. group (from Zhao et al., 2005).

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Figure  2.  Tectonic map of the North China craton, as modified from Zhao et al. (2005).

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Following the model of Kusky and Li (2003) that the northern margin of the craton experienced a continent-continent collision at circa 1.85–1.8 Ga, Zhao et al. (2005) modified their map of the North China craton to include a collision belt in the north, but only between the western block and the orogen to the north, which they termed the Khondalite belt and Yinshan block to the north (Fig. 2).

RECENT TECTONIC MODELS FOR THE EVOLUTION OF THE NORTH CHINA CRATON

Kusky et al. (2001) and Li et al. (2002) proposed an alternative tectonic model for the tectonic evolution of the North China craton, which included the collision of an arc and ophiolitic fore-arc on the western edge of the eastern block at 2.5 Ga. Kusky and Li (2003) proposed a comprehensive model for the evolution of the North China craton, which included rifting of the western edge of the eastern block at 2.7 Ga, collision of an arc (on the eastern edge of the western block) at 2.5 Ga, then a major continent-continent collision along the entire north margin of the craton at circa 1.85 Ga. This was followed by more detailed and updated analyses by Kusky et al.(2007a, b), Kusky and Santosh (2009), and Kusky and Li (2010).

In the Kusky et al. model, they also divide the craton into the eastern and western blocks, but use changes in the Archean geology to define the tectonic boundaries (e.g., Li and Kusky (2007) defined a foreland basin on the eastern block to mark the transition into the orogen), and call the intervening orogen the central orogenic belt, not the TNCO. The TNCO as defined has Mesozoic faults as its boundaries, and is supposed to delineate an orogen in which circa 1.85 Ga deformation and metamorphism is restricted to, whereas the COB is an Archean orogen, defined by transitions in structural style, sedimentation, rock types, and ages. The presence of circa 2.5 Ga accreted arc and ophiolitic fore-arc rocks in the COB has been controversial, but well-documented through structural, geochemical, and geochronological studies (Kusky and Li, 2010; Kusky et al., 2007a, b, 2004, 2001; Polat et al., 2006b, 2005).

Additional geological data suggest that the tectonic model of collision of the eastern and western blocks above an east-dipping subduction zone at circa 1.85 Ga is not viable. Granulite facies metamorphism at 1.85 Ga is not restricted to the "TNCO" as suggested by the Zhao et al.(2010, 2005, 2001a, b, 2000, 1999a, b, 1998) model, but is documented across the NCC, as predicted by the model suggesting a major continent-continent collision along the north margin of the craton at 1.85 Ga (Zhai et al., 2010; Kusky and Santosh, 2009; Kusky et al., 2007a, b). Further, it has recently been shown that in the southern TNCO, there is no record of metamorphism at circa 1.85 Ga, but only at 2.7–2.5 Ga, showing that the TNCO as defined does not exist (Liu et al., 2009). The COB is an Archean convergent belt, re-worked in the Paleoproterozoic, and the Paleoproterozoic tectonism is widespread across the NCC, as predicted by the continental collision model involving the joining of the North China craton with the Columbia supercontinent along a suture on the north margin of the craton (Kusky et al., 2007a, b), but uplift/exhumation rates are slow, as discussed recently by Zhai et al. (2010) necessitating a re-evaluation of the tectonic models of the NCC.

Several other models have been proposed for the tectonic evolution of the North China craton. Zhai et al. (2010, and references therein) suggested that there are more numerous blocks in the North China craton than depicted in either the Zhao et al. or Kusky et al. models (although the Kusky et al. model accounts for older blocks, see Fig. 5), and noted that the circa 1.85– 1.8 Ga high grade metamorphism is not restricted to the TNCO (or COB) belt as predicted by the Zhao et al. model, but instead is recorded across many parts of the craton. Further, recent studies by Liu et al. (2009) have shown that in parts of the southern TNCO there is no record of circa 1.8–1.85 Ga metamorphism, but only shows metamorphism at circa 2.7–2.5 Ga. Faure et al. (2007) and Trap et al. (2009) examined the structural and temporal evolution of the central orogenic belt (TNCO) and concluded that there were four main deformation events, including one at circa 2.1 Ga, and one at circa 1.9 Ga, predating the regional metamorphism at circa 1.8 Ga. Trap et al. (2009) also recognized an earlier deformation event, but have no constraints on whether it is Archean or Paleoproterozoic. The Faure et al. (2007) model is consistent with the Kusky et al. model, in that after the collision of the arc terrane with the eastern block at 2.5 Ga, there was still a basin behind the arc to close, and these deformation events may correlate with those events. The observations of Faure et al. (2007) could also be fit into the Zhao et al. model, in that if there were an ocean basin open for 900 Ma between the eastern and western blocks, one would expect different accretionary events, and a large accretionary orogen to form, perhaps similar in scale to the Makran or southern Alaskan orogens. The problem is that no such accretionary orogen is recognized in the central orogenic belt.

Figure  3.  Tectonic model for the evolution of the North China craton proposed by Zhao et al. (2001b), and followed in essence thereafter. The essential point is that they propose that an ocean was open between the eastern and western blocks from circa 2.7 or 2.5 Ga, with eastward directed subduction beneath the eastern block, until the two blocks collided at circa 1.85 Ga.

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Figure  4.  Tectonic subdivisions of the North China craton proposed by Kusky et al. (2007a).

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Figure  5.  Model for Precambrian evolution of North China craton proposed by Kusky et al.(2007b). Note that an arc in the COB (with ophiolitic fore-arc) is about to collide with the eastern block at circa 2.5 Ga, and that there was likely a basin behind this arc, separating the COB from the western block. IMNHO. Inner Mongolia-northern Hebei orogen.

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GEOPHYSICAL TESTS OF MODELS

Zheng et al. (2009) reported the results of recent seismic imaging across the central orogenic belt (TNCO) of the North China craton (Fig. 6). Their data, from radial receiver functions from a dense array of 52 temporary seismic stations, were used to image the structure of the crust and upper mantle along a transect from the eastern block, across the central orogenic belt, and well into the western block. The data of Zheng et al. (2009) include 3 183 receiver functions, stacked for each station, yielding clear Ps phases from the Moho, and strong heterogeneities in the crust and upper mantle. To clearly image crustal features, Zheng et al. (2009) used a technique including waveform inversion and common conversion point (CCP) stacking of receiver functions. The resulting CCP images show some salient points about the deep crustal structure. First, the structure beneath the eastern block is characterized by undulating interfaces, whereas beneath the western block most interfaces are flat-lying. Zheng et al. (2009) also identified two west-dipping low-velocity zones (L1 and L2) beneath the central orogenic belt, extending through the crust to the Moho (Fig. 6). Their results revealed the presence of a west dipping paleosubduction zone beneath the COB, as predicted by the Kusky et al. group, but in stark contrast to the prediction of the Zhao et al. group models. The seismic imaging results of Zheng et al. (2009) also show a second paleosubduction zone, also dipping west, on the west side of the central orogenic belt, showing that another ocean basin closed there. This is in agreement with the two episodes of possible subduction-related deformation predicted by the Faure et al. (2007) model, and is acceptable in the Kusky et al.(2007a, b) model, in that there was still a basin to close behind the arc that collided with the eastern margin of the eastern block at 2.5 Ga. The seismic results are not compatible with the models of the Zhao et al. group, which require long-lived eastward dipping subduction beneath the eastern block. Further, the seismic imaging of Zheng et al. (2009) shows that location of the paleosubduction zones do not correspond with the boundaries of the TNCO proposed by Zhao et al. (2005), which are younger, Mesozoic faults.

Figure  6.  Common conversion point (CCP) receiver function image of crust and uppermost mantle along western block–Trans-North China orogen (TNCO) profile based on the inverted velocity model (from Zheng et al., 2009). Blue represents positive (brown represents negative) amplitude of receiver function annotated in the right color bar, indicating velocity increase (or decrease) downward. Intracrustal interfaces and Moho identified in common conversion point (CCP) image are compared with those derived from the best-fitting models by waveform inversion. Dots in CCP image mark velocity discontinuities in the best-fitting models, including upper-middle (orange) and middle-lower (red) crustal interfaces, Moho (white), the interfaces with negative velocity gradient above L1 and L2 layers (blue), and the bottom interfaces of L1 and L2 (green). (b) Shear-wave velocity structure of crust and uppermost mantle compiled from inverted velocity model along east-west profile (A-B in Fig. 3 above). L1 is a westward-dipping low-velocity zone beneath stations 274–296 that separates TNCO and western block, and L2 is a horizontal low-velocity zone in the lower crust beneath TNCO and the western block. (c) Schematic diagram of ancient subduction model, which is remarkably similar in geometry for the circa 2.5 Ga collision between the eastern and western blocks proposed by Kusky et al.(2009, 2007a, b, 2001), and Kusky and Li (2003), but opposite in polarity to the model proposed by Zhao et al.(2005, 2001). (d) Topographic map of the study region and locations of stations. Triangles represent broadband seismic stations used in the study by Zheng et al. (2009). Selected station numbers are labeled on top of plots in (a) and (b). In (a), (b), and (d), red arrow marks boundary location between western block and TNCO identified by this study, and blue arrow marks boundary previously envisaged by Zhao et al. (2001). EB. Eastern block; WB. western block.

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CONCLUSIONS

In accordance with the geological and geophysical data summarized above, we present a new tectonic model for the evolution of the North China craton that is consistent with the structural, geochronological, PTt, sedimentological, and new geophysical data for the craton (Fig. 7).

Figure  7.  Precambrian tectonic evolution of the central orogenic belt of the North China craton. Note the gradual change in orientation of cross sections from E-W at 2.5 Ga, to NS by 1.8 Ga, to illustrate the main tectonic elements active at each time. DWO. Dongwanzi ophiolite; ZOM. Zunhua ophiolitic mélange; HS. Hengshan; WA. Wutai arc; ZSB. Zunhua structural belt.

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Parts of the North China craton had a long history prior to the Late Archean, with the amalgamation of several different crustal blocks to form the present eastern and western blocks (e.g., Zhai et al., 2010). By 2.7 Ga, the eastern block had rifted from whatever continental block it was adjacent to prior to then, forming komatiites (Cheng et al., 2007; Polat et al., 2006a), and a passive margin sequence on the western edge of the eastern block. From 2.55 to 2.5 Ga (Fig. 7) an arc terrain, now preserved in Wutaishan (Polat et al., 2006b, 2005) and Zunhua structural belt (Kusky et al., 2007a, b, 2004) with a fore-arc ophiolite belt on its leading edge (Kusky et al., 2001) collided with the passive margin on the western edge of the eastern block, above a west-dipping subduction zone, emplacing ophiolites including the Dongwanzi and Zunhua belts (Kusky et al., 2004). From 2.4 to 2.3 Ga, the ocean basin behind the collided arc began closing, either by westward-dipping, or double-sided subduction, with the collision of the western block with the arccollision modified western margin of the eastern block by 2.1 Ga.

The final major event in the Precambrian tectonic evolution of the North China craton is the pan-craton circa 1.85 Ga high-grade metamorphic event. Kusky et al.(2007a, b), Kusky and Li (2003) and Kusky and Santosh (2009) have related this to collision of the northern margin of the craton with the Columbia supercontinent. This continent-continent collision caused the widespread metamorphic overprinting of all older events, including the well-studied rock in the Wutaishan-Hengshan areas (Zhao et al., 2005, 2001a, b, 2000, 1999a, b, 1998; Zhao, 2001), and the HP granulite belts that extend across and overprint older rocks of the COB. It remains enigmatic, however, how such a large area affected by granulite facies metamorphism experienced such a slow post-orogenic uplift and exhumation history (Zhai et al., 2010, 2005), an observation not explained by any of the current tectonic models for the North China craton.

ACKNOWLEDGMENTS

Funds were provided by the National Natural Science Foundation of China (Nos. 91014002, 40821061), and the Ministry of Education of China (No. B07039).