SCIENCE CHINA Earth Sciences • RESEARCH PAPER •
December 2015 Vol.58 No.12: 2200–2210 doi: 10.1007/s11430-015-5162-y
Regional differences in crustal structure of the North China Craton from receiver functions WEI ZiGen1, CHU RiSheng1* & CHEN Ling2 1
State Key Laboratory of Geodesy and Earth’s Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China; 2 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 3 CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China Received April 13, 2015; accepted July 1, 2015; published online July 30, 2015
Moho depth and crustal average Poisson’s ratio for 823 stations are obtained by H- stacking of receiver functions. These, together with topography and receiver function amplitude information, were used to study the crustal structure beneath the North China Craton (NCC). The results suggest that modified and preserved crust coexist beneath the craton with generally Airy-type isostatic equilibrium. The equilibrium is relatively low in the eastern NCC and some local areas in the central and western NCC, which correlates well with regional geology and tectonic features. Major differences in the crust were observed beneath the eastern, central, and western NCC, with average Moho depths of 33, 37, and 42 km and average Poisson’s ratios of 0.268, 0.267 and 0.264, respectively. Abnormal Moho depths and Poisson’s ratios are mainly present in the rift zones, the northern and southern edges of the central NCC, and tectonic boundaries. The crust beneath Ordos retains the characteristics of typical craton. Poisson’s ratio increases roughly linearly as Moho depth decreases in all three parts of the NCC with different slopes. Receiver function amplitudes are relatively large in the northern edge of the eastern and central NCC, and small in and near the rifts. The Yanshan Mountains and southern part of the Shanxi rift show small-scale variations in the receiver-function amplitudes. These observations suggest that overall modification and thinning in the crust occurred in the eastern NCC, and local crustal modification occurred in the central and western NCC. Different crustal structures in the eastern, central, and western NCC suggest different modification processes and mechanisms. The overall destruction of the crustal structure in the eastern NCC is probably due to the westward subduction of the Pacific Plate during the Meso-Cenozoic time; the local modifications of the crust in the central and western NCC may be due to repeated reactivations at zones with a heterogeneous structure by successive thermal-tectonic events during the long-term evolution of the NCC. North China Craton, Moho depth, Poisson’s ratio, gravitational equilibrium, receiver function amplitude, regional difference in crust Citation:
Wei Z G, Chu R S, Chen L. 2015. Regional differences in crustal structure of the North China Craton from receiver functions. Science China: Earth Sciences, 58: 2200–2210, doi: 10.1007/s11430-015-5162-y
The North China Craton (NCC), on the eastern edge of the Eurasia Plate, is located at the overlapping area among the Tethys, the Paleo-Asian Ocean, and the Pacific tectonic domains. It is composed of eastern and western Archean
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© Science China Press and Springer-Verlag Berlin Heidelberg 2015
blocks along the central Trans-North China Orogen ~1.85 Ga ago (Zhao et al., 2005). The NCC is now surrounded by a series of orogenic belts generated by interactions between the NCC and adjacent blocks (Figure 1): the Paleozoic Qilian Orogenic Belt to the west (Xiao et al., 2009), the Paleozoic-Mesozoic Central Asia Orogenic Belt to the north (Davis et al., 2001) and the early Mesozoic Qinling-Dabieearth.scichina.com
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Figure 1 Map showing tectonic setting, station and earthquake distribution in the study region. Different color dots mark the station locations from different data sources. Three big dots mark typical station locations with different receiver function amplitudes (NMLCH, 207, 346; see waveform information in Figure 2). Black lines denote the boundary of the North China Craton (NCC, Zhao et al., 2005). Thick dark green line denotes the North-South Gravity Lineament (NSGL). White arrows in the upper-left inset indicate the westward subduction of the Pacific Plate and northward convergence of the Indian Plate to the Eurasia Plate. Red triangles in the lower-right inset show the projection center of earthquakes (112.5°E, 37.5°N). NEC: Northeast China; SCB: South China Block; TP: Tibetan Plateau; CAOB: Central Asia Orogenic Belt; QL-DB: Qinling-Dabie Orogen; QB: Qilian Orogenic Belt; S-L: Sulu Orogen; LA: Liaodong anteclise; YM: Yanshan Mountains; YinM: Yinshan Mountains; TM: Taihang Mountains; LM: Lüliang Mountains; LU: Luxi uplift; Y-H rift: Yinchuan-Hetao rift; S-S rift: Shaanxi-Shanxi rift; Ordos: Ordos Block; BBB: Bohai Bay Basin; SNCB: South North China Basin; AB: Alxa Block; IGCEA: Institute of Geophysics, China Earthquake Administration; IGGCAS: Institute of Geology and Geophysics, Chinese Academy of Sciences.
Su-Lu Orogen to the south and east (Li et al., 1993). At present, the NCC is subject to westward subduction of the Pacific Plate to the east and India-Eurasia collision to the west (Ren et al., 2002).The NCC was free from tectonic activity for >1.5 Ga after its final formation and then experienced significant thermo-tectonic reactivation and destruction in the Late Mesozoic and Cenozoic. The eastern NCC underwent severe lithospheric thinning and destruction and formed widespread thick sedimentary basins with elevations generally 500 m (Zhai et al., 2003; Chen et al., 2009; Jiang et al., 2013; Wang et al., 2014). The unique tectonic evolution and lithospheric structures make the NCC a natural laboratory to study cratonic formation, evolution and destruction. Crust, the top layer of the lithosphere, can preserve continental tectonic evolution information for a long time owing to its relatively low temperature and pressure. Therefore, crustal structures can provide important constraints on the tectonic evolution and deep mechanisms of destruction for the NCC. Especially, Moho depth and Poisson’s ratio are two key parameters that characterize crustal structure and rock composition. The correlation between Moho depth and
Poisson’s ratio is usually related to crustal tectonic setting (Ji et al., 2009) or formation age (Chevrot et al., 2000). In addition, the correlation between topography and Moho depth can be used to study the isostatic equilibrium of the crust. Consequently, a comparative study of Moho depth, Poisson’s ratio, and topography will provide valuable constraints on regional crustal structure and tectonic evolution of the NCC. Receiver function analysis is usually used to study Moho depth and Poisson’s ratio because of its strong sensitivity to seismic discontinuity. Recently, many researchers obtained Moho depth and Poisson’s ratio beneath the NCC by H- stacking (Zhu et al., 2000) of receiver functions (Xu et al., 2005; Liu et al., 2009; Wang et al., 2009; Zhang et al., 2009; Li et al., 2010; Liu et al., 2011; Ge et al., 2011; Pan et al., 2011; Wei et al., 2011a, 2011b, 2012, 2013; Ren et al., 2012; Hong et al., 2013; Gao et al., 2014; Wang et al., 2014). These studies revealed a thicker crust beneath the western NCC, obvious differences in crust between basins and mountains in the Capital area, and striking crustal structural differences between the southern and northern parts of the Ordos. These results provide valuable observations for understanding the crustal structural features and their tectonic evolution of the NCC. However, these studies are mainly confined to linear profiles or areas with strong lateral crustal
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variation. In other instances, these studies have lower resolution and lack systematic comparative study of the crust beneath the eastern, central, and western NCC. Moreover, Moho depth and Poisson’s ratio for the same stations often differ among studies because of data and parameter selection in the H- stacking method, which remain insufficient for complete understanding of crustal structure and Phanerozoic tectonic evolution processes of the NCC. Therefore, it is necessary to study fine structures of the crust and systematically analyze its lateral variations beneath different parts from multiple perspectives using uniform processing flow and by increasing number of observations. Recent large-scale deployment of dense seismic stations in the NCC provides numerous observations for detailed study of the crust. We applied the H- stacking method (Zhu et al., 2000) to analyze teleseismic waveforms from 538 stations, which were collected from multiple dense temporary arrays and relatively evenly distributed permanent stations. These H- stacking results, along with previous results from 285 temporary stations, provide detailed Moho depth and crustal average Poisson’s ratio beneath the NCC. Moho depth was further analyzed to determine correlation with different topography. Based on the aforementioned observations and receiver function amplitude information, we primarily focus on regional comparisons of the crustal structure beneath the eastern, central, and western NCC to study crustal modifications and tectonic evolution of the NCC in the Mesozoic-Cenozoic time.
1 Data and method 1.1
Data
The Moho depth and Poisson’s ratio results obtained in this study are derived from two different data sources. The first data source is from receiver functions of 8 linear temporary arrays of the North China Interior Structure Project and the Destruction of the North China Craton projects (http://www. seislab.cn/) operated by the Institute of Geology and Geophysics, Chinese Academy of Sciences from 2000 to 2010, and permanent stations deployed by the China Earthquake Administration (Zheng et al., 2009). The temporary arrays traverse different tectonic units of the NCC with an average spacing of 10–20 km. They were operated for 12 to 18 months. Permanent stations are distributed relatively evenly in the study region and recorded data from 2007 to 2010. Some Moho depth and Poisson’s ratio results from these stations were previously published (Wei et al., 2011a, 2011b, 2012, 2013). The second data source is previously published results of Moho depth and Poisson’s ratio near the Capital area and the Ordos (Wang et al., 2011; Wang et al., 2014; Qi et al., 2015). Except for scarcity of data in the Bohai Bay Basin and South North China Basin because of significant interference to Moho phases from thick loose sediments, we obtained good spatial coverage of Moho
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depth and Poisson’s ratio, which enables us to investigate crustal structures beneath the NCC in details. 1.2
Method
We used a time-domain maximum entropy deconvolution method (Wu et al., 1998) to extract receiver functions from teleseismic waveforms with magnitude ≥5.5 and an epicentral distance ranging from 30° to 90° (Figure 1). The obtained radial receiver functions were then filtered from 0.03 to 0.5 Hz to suppress high-frequency noises and preserve resolutions. After rigorous manual selection for filtered receiver functions, we used the H- stacking method (Zhu et al., 2000) to estimate Moho depth and average vp/vs ratio utilizing the converted Ps and multiple PpPs and PpSs+PsPs phases together. The method defines an H- domain stack function S(H, ) in an assumed 1D singlelayer crustal model:
S ( H , ) w p s r (t p s ) w p p ps r (t p p p s ) w p p ss ps ps r (t p p ss ps ps ),
(1)
where H is the crustal thickness; is the average vp/vs; r(t) is the radial receiver function; tPs, tPpPs, and tPpSs+PsPs are the predicted arrival times of different phases, wPs, wPpPs, and wPpSs+PsPs are the weighting factors whose summation equals 1. In the H- domain, the optimal H and parameters correspond to the maximum stacking amplitude of S(H, ). The Poisson’s ratio () under each station can be estimated from vp/vs by the following equation:
1 1 [(v p / vs )2 1]1 . 2
(2)
Several key parameters may affect the estimates of H and
. In the study region, H generally increases by 0.5–0.8 km with a 0.1 km/s increase in average crustal P-wave velocity v p ; whereas in most stations, varies by no more than 0.01 when changing v p from 6.0 to 6.8 km/s in the H- stacking. Referring to previous studies from both seismic exploration (Li et al., 2006) and earthquake data (Zheng et al., 2009), we chose 6.35 and 6.15 km/s as v p values to estimate H and for stations with elevations >100 m and
