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Crust–upper mantle seismic velocity structure across Southeastern China
Tectonophysics 395 (2005) 137 – 157 www.elsevier.com/locate/tecto
Crust–upper mantle seismic velocity structure across Southeastern China Zhongjie Zhanga,*, Jose´ Badalb, Yinkang Lic, Yun Chena, Liqiang Yanga, Jiwen Tenga a
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China b Physics of the Earth, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain c Geological Data Center, Ministry of the Land and Resources, Heibei, 110101, China Received 6 August 2003; accepted 20 August 2004 Available online 7 December 2004
Abstract One in-line wide-angle seismic profile was conducted in 1990 in the course of the Southeastern China Continental Dynamics project aimed at the study of the contact between the Cathaysia block and the Yangtze block. This 380-km-long profile extended in NW–SE direction from Tunxi, Anhui Province, to Wenzhou, Zhejiang Province. Five in-line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line. We have used two-dimensional ray tracing to model P- and S-wave arrivals and provide constraints on the velocity structure of the upper crust, middle crust, lower crust, Moho discontinuity, and the top part of the lithospheric mantle. P-wave velocity, S-wave velocity and V P/V S ratio are mapped. The crust is 36-km thick on average, albeit it gradually thins from the northwest end to the southeast end (offshore) of the profile. The average crustal velocity is 6.26 km/s for P-waves but 3.6 km/s for S-waves. A relatively narrow low-velocity layer of about 4 km of thickness, with P- and S-wave velocities of 6.2 km/s and 3.5 km/s, respectively, marks the bottom of the middle crust at a depth of 23-km northwest and 17-km southeast. At the crust–mantle transition, the P- and S-wave velocity change quickly from 7.4 to 7.8 km/s (northwest) and 8.0 to 8.2 km/s (southeast) and from 3.9 to 4.2 km/s (northwest) and 3.9 to 4.5 km/s (southeast), respectively. This result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately. The average V P/V S ratio ranges from 1.68–1.8 for the upper crust to 1.75 for the middle and 1.75–1.85 for lower crust. With the interpretation of the wide-angle seismic data, Jiangshan–Shaoxin fault is considered as the boundary between the Yangtze and the Cathaysia block. D 2004 Elsevier B.V. All rights reserved. Keywords: Wide-angle seismic reflection/refraction; P- and S-wave velocity models; Crustal structure; Yangtze and Cathaysia blocks; Southeastern China
1. Introduction * Corresponding author. E-mail address: [email protected] (Z. Zhang). 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.08.008
Southeastern China is an important part of the tectonic framework concerning the continental mar-
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gin of eastern China and is commonly assumed to comprise the Yangtze block and the Cathaysia block, the two major tectonic units in the region. Such a distinction between regional blocks was proposed on the basis of dissimilarities in the stratigraphic records (Ji and Coney, 1985), the possible occurrence of ophiolite suites (Zhang et al., 1984), and differences in the lithology of the Precambrian basement (Granitoid Research Group, 1989; Fig. 1). Low-grade metamorphic rocks of middle to late Proterozoic ages separate both blocks (Wang and Qiao, 1984). However, the boundary between these tectonic units is poorly documented
due to the lack of detailed data as to fix its nature (suture or strike–slip). This unclear panorama becomes especially apparent when the Mesozoic tectonic models are considered. The left lateral offset model has been prompted (Xu et al., 1987; Xu and Zhu, 1993) from marker beds on the order of hundreds of kilometres and the existence of major shear zones. It has been suggested that the entire eastern part of the Chinese landmass was dominated by a sinistral shear system during the Mesozoic period. Alternatively, it has been stated that a series of fold belts within the southeast region of the Yangtze block accreted by subduction and
Fig. 1. Simplified geological setting and wide-angle seismic profile, location between Tunxi and Wenzhou in Southeastern China. Triangles and stars mark the geographical positions of stations and shot points, respectively. The major active faults and basins crossed by the long linear antenna are (1) Lin’an–Majin fault, (2) Wuxing–Jiande fault, (3) Changshan–Pujiang fault, (4) Jiangshan–Shaoxing fault, the surface boundary between the Yangtze and the Cathaysia blocks, (5) Quzhou–Tangxian fault, (6) Shangyu–Lishui fault, (7) Qingyuan–Anren fault, (8) Ningbo– Longquan fault, (9) Zhenhai–Wenzhou fault, (I) Jinwen Basin, (II) Bihu Basin, (III) Wenzhou–Pinyang hollow. Key to symbols: 1— Precambrian, 2—Paleozoic Erathem, 3—Mesozoic Erathem, 4—Cenozoic Erathem, 5—Jurassic neutral-acidic volcanic rock, 6—Cretaceous basic volcanic rock, 7—neutral-acidic lithesome, 8—basic-ultrabasic lithesome, 9—major faults, 10—deep seismic sounding profile, CB— crystalline basement, SC—sedimentary cover, FR—fold region (belt), K—cretaceous (light green), N—neogene (pale yellow), Q—quaternary (yellow), j6—basic–ultrabasic rocks (purple), s6—alkaline rocks (red), h6—basalts (bottle green). Please refer to the web version of the paper to view this figure in colour.
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closure of ocean basins in Alpine-type collisions in the early Mesozoic or later (Hsu et al., 1988). The tectonic evolution of the Southeastern China has proved to be exceedingly complex regarding the occurrence of collision or translation of various cratons during the Precambrian (Huang, 1977), Paleozoic (Zhang et al., 1984), and Mesozoic (Xu et al., 1987; Hsu et al., 1988, 1989). Their contacts are one of the unsolved problems (Wei et al., 1990), and the knowledge of the crustal structure of the region is rather poor. Actually, only the Mesozoic tectonic evolution of the region has been constrained from paleomagnetic samples, and Alpine-type mountain building is assumed to be present (Sengor, 1982; Li, 1992; Hsu et al., 1988, 1989; Gilder et al., 1996; Chen and John, 1998). There is a clear target for the study of the crustal structure beneath southeastern China: to investigate the contact between the Yangtze block and the Cathaysia block to model the crust–mantle seismic velocity structure of the region. The Southeastern China Continental Dynamics (SCCD) project was just developed with this purpose—among others— between 1984 and 1990, in particular, to study the regional crust and upper mantle. This seismological experiment, which was performed in the framework of a multidisciplinary research program supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences, involved instrumentation, and people of Chinese institutions. A wide-angle seismic profile from Fengle Reservoir near Tunxi, Anhui Province, to Dongtou Island, offshore of Wenzhou, Zhejiang Province, was conducted in 1990 as part of the SCCD project. This profile is shown in Fig. 1. It provides a good chance to study the contact between the Yangtze and southern China blocks and to determine a crustal velocity model for the area. Xiong et al. (1993) made a preliminary interpretation of the wide-angle seismic data and obtained an initial P-wave velocity model albeit no interpretation of S-wave data were advanced. In this paper, we describe the information then collected and present the results that we have obtained based on interpretation of the registered wide-angle seismic reflection/refraction P- and S-phases. We propose a new 2-D model, which allows us to constrain the seismic velocity structure of the upper lithosphere in the region.
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2. Geological setting The wide-angle seismic profile analysed in this work runs in direction NW–SE from Tunxi to Wenzhou and crosses several major active faults and basins from its northwest end to its southeast end. This can easily be seen in Fig. 1 where all these details are shown. Most of these faults are roughly parallel but perpendicular to the NW–SE seismic survey line and display the characteristics of compressive fragmentation, dynamic metamorphism, magmatism, mylonization and thrust nappe. The Jiangshan– Shaoxin fault (labelled 4 in Fig. 1) is considered to be the surface boundary between the Yangtze and the Cathaysia blocks. It separates materials of Precambrian, Paleozoic, Mesozoic, and Cenozoic ages (northernmost third of the profile) from other materials of the Jurassic and Cretaceous periods more to south. The profile also crosses several sedimentary basins, such as the Jinwei basin of Mesozoic and Cenozoic ages (Fig. 1), the Songyang basin of Cretaceous age, and the Wenping depression with Quaternary sediments. Both the lithosphere and the sediments at either side of the Jiangshan–Shaoxin fault are different. West–north from this fault, Paleozoic sediments extend between the Majing–Wuzhen fault and the Qiuchuan–Xiaoshan fault. Layers belonging to an age interval between the Sinian period and the Carboniferous–Permian periods form a series of anticlinals and synclinals at Xinanjiang (Fig. 1). Other layers of the Proterozoic period emerge at Baijishan (Fig. 1)— the boundary separating the Anhui and Zhejiang Provinces—and to northwest of the Jinwei basin. In opposite direction, neutral–acidic volcanic rock of the Late Jurassic period extends beyond the Jiangshan– Shaoxin fault, with the interruption of some Mesozoic sediment along the Lishui–Yuyao fault and other Cenozoic sediments to east of Wenzhou. Accounting now for geological aspects, the boundary between the Yangtze and Cathaysia blocks inferred in this study is also consistent with a granite belt with high eNd and low T DM values extending from Hangzhou through central Jiangxi to Guangxi (Gilder et al., 1993a, b). This natural boundary separates a steady area to northwest from an active area to southeast of China located in different geochemical zones. Thus, the Jiangshan fault is considered as a
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Neoproterozoic suture zone between the Yangtze and Cathaysia blocks. The eastern part of the belt is along the Jiangshan–Shaoxin fault and contains a series of upper Proterozoic, ultramafic, and diorite rock bodies (795~890 Ma), which are strongly mylonizated, and forms a 150-km long mylonite belt. To the northwest of the Jiangshan fault, there is a series of upper Proterozoic, low metamorphism, volcanic sedimentary rocks, northeast Jiangxi–northwest Zhejiang (Fig. 1). To the southeast of the fault, there emerges Palacozoic regional metamorphic rock of greenschist facies– amphibolite facies.
3. The SCCD profile 3.1. Design and identification of crustal phases The seismological information comes from the records obtained by wide-angle reflection/refraction profiling about 380-km long, which was performed as part of the SCCD project mentioned above. The field operations were carried out during a period of 2 months and always under control of the Institute of Geology and Geophysics (former Institute of Geophysics) of the Chinese Academy of Sciences. The profile, with azimuth nearly N30W, runs from the Fengle Reservoir near Tunxi to Dongtou near Wenzhou. Five shots were fired at different sites: Fengle, Xinanjian, Songyang, Qintian, and Dongtou (Fig. 1). Each shot consisted of a hole drilled to a depth of 20 m and loaded with a charge ranging from 1200 to 1800 kg of explosive. A total of 85 portable seismic stations were installed along the survey line: 35 single-vertical-component stations recording on magnetic tape and 50 three-component digital stations equipped with DZSS-1 seismographs. Instrumentation includes data acquisition system, digital cassette tape recorder, and playback. The time signal was provided by internal clock synchronized by radio time signal. All the stations and shots were located using 1:25,000 topographic maps with an estimated location accuracy of 12.5 m. Station spacing was of 3 km on average, and the offset range was 2 to 240 km. The seismic signals provided by digital instruments were initially sampled at a rate of 5 ms and then filtered within the 1- to 10-Hz frequency band for P-waves and within 1–6 Hz for S-waves. The analogue records provided by magnetic tape recorders
were subsequently digitized at 5 ms. The shot gathers were displayed at reduced time scale for a velocity of 6.0 km/s for P-waves and of 3.5 km/s for S-waves. Fig. 2 (upper panels) shows the registered P- and Swave record sections. Almost all the seismograms recorded along the profile exhibit a high signal-to-noise ratio. From these record sections, we are able to clearly identify Pg and Sg energy arrivals refracted above of the crystalline basement, but also, the Pm- and Smwaves reflected from the Moho. The Pn- and Sn-waves refracted from the Moho may also be recognised although their amplitudes are very weak as compared with the amplitude of the previous phases. Other P- and S-waves reflected from interfaces between the crystalline basement and the crust–mantle discontinuity might be recognised too. The amplitudes of the events P1, S1 (upper crust), P4, and S4 (lower crust) are relatively weak as compared with those of the reflections P3 and S3 from the bottom of a low-velocity layer located in the middle crust. In contrast, the events P2 and S2 coming from the top of this low-velocity channel show larger amplitudes at some offsets. 3.2. Preliminary analysis The visible offset for the event Pg is 80 km at Fengle (Fig. 2a), 60 km northwest of Xinanjiang (Fig. 2b), and 75 km southeast of this shot point. At Songyang (Fig. 2c), this crustal phase is visible as far as 100 km northwest and 80 km southeast. At Qingtian (Fig. 2d), the offset for this phase is 60–75 km at both sides, and at Dongtou (Fig. 2e), the offset is 100 km. The Pg-wave travel times reported from the shot at Dongtou show a delay when compared with the time–distance curve for offsets up to 60 km from Qingtian and 100 km from Songyang. From the shot at Qingtian, the travel times observed up to offsets of 60 km in SE direction and 75 km in NW direction towards Songyang show a similar delay. The same observation is valid for the Pg-wave travel times reported from the shot at Songyang; they show a delay for offsets up to 60 km from Xikou and 100 km to east of Qianligan. From the shot at Xinanjiang, the travel times show an abrupt delay of offsets between 40 km (Longyou, Fig. 2) and 70 km to east of Xikou. For the shot at Fengle Reservoir, the travel times are at distances of between 20 and 40 km towards Xinanjiang, which demonstrates that P-wave velocity above
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Fig. 2. Upper panels—P- and S-waves sections from five wide-angle inline shots fired at different sites: Fengle (FLS), Xinanjian (XAS), Songyang (SYS), Qintian (QTS), and Dongtou (DTS). Where the raw data are processing by band-pass filter with frequency range of 1–10 Hz for P-wave and 1–6 Hz for S-wave. These reduced sections are shown with reduction velocity of 6.0 km/s for P-wave and 3.4 km/s for S-wave. Central panels—comparison between observed (4) and computed (5) travel times. Lower panels—respective seismic velocity model organised in dipping layers and ray diagram.
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Fig. 2 (continued).
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Fig. 2 (continued).
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Fig. 2 (continued).
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Fig. 2 (continued).
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Fig. 2 (continued).
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Fig. 2 (continued).
the crystalline basement changes abruptly along the profile, which is mostly probably a result of the sedimentary basins and Quaternary sediments.
Notice the similarity of the S-phases regarding the P-phases in Fig. 2 (upper panels). We also recognise the Sg energy arrivals refracted above the crystalline
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Fig. 2 (continued).
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Fig. 2 (continued).
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Fig. 2 (continued).
basement, the Sm-phase reflected from Moho, the Snphase refracted from Moho, and other phases (S1, S2, S3, and S4) reflected from interfaces between the
crystalline basement and the Moho. The distance from the shot point to which the Sg-phase is registered is a little shorter than the offset for Pg. It is 60 km at
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Dongtou and Songyang, 90 km northwest and 75 km southeast of Qingtian, 75 km at Xinganjiang, and 70 km at Fengle Reservoir. Analogously, the Sg-phase travel times delay up to an offset of 25 km from Fengle Reservoir and of 30 km from Songyang. The same occurs at distances between 30 and 40 km from Qingtian; but these observed times change abruptly from the shot at Dongtou, which demonstrates the same features for S-waves propagating above the crystalline basement before for P-waves. From the Pm and Sm travel times, the apparent Pand S-wave velocities are deduced to be 6.8 and 3.9 km/s, respectively, for the bottom of the crust. The refracted Pn- and Sn-phases are indeed weak in amplitude, but their onsets are sufficiently clear as to be recognised. The analysis of these phases reveals a P- and S-wave velocity at the top part of the lithospheric mantle, which varies quickly from 7.8 to 8.1 km/s (offset range of 240–280 km) and from 4.1 to 4.4 km/s (offset range of 200–240 km), respectively.
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4. Interpretation: crust–mantle model We first carry out inversion for 1-D crustal structure under the assumption of a horizontally layered medium. Then we construct a 2-D velocity model of the crust by fitting the observations using ray-tracing and theoretical travel times. In Fig. 2 (central panels) and for every shot point, we show comparisons between observed and computed travel times for Pand S-waves. Also in Fig. 2 (lower panels), we show ray paths computed from the final velocity model, which is constrained by the travel times of the identified P- and S-waves. On the basis of the excellent fit between our velocity model (Fig. 2) and the observed data, we are able to give the following overall results. The average thickness of the crust is about 36 km, albeit it gradually thins from northwest to southeast as the Moho depth decreases. The average P-wave velocity for the crust is 6.26 km/s; this average value becomes 3.6 km/s for S-wave velocity. A relatively low velocity layer of
Fig. 3. Overall P-wave (upper part) and S-wave (lower part) velocity models for southeastern China showing an overall view of the upper crust, middle crust, lower crust, Moho discontinuity, and lithospheric mantle. Isolines for seismic velocity values given in kilometres per second emphasize different structures. Traces of major active faults (Fig. 1) on a part of the map of the study area have been drawn to provide a comprehensible view of the velocity patterns. Please refer to the web version of the paper to view this figure in colour.
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about 4 km of thickness, with P- and S-wave velocity of 6.2 and 3.5 km/s, respectively, marks the bottom of the middle crust at a depth of 23 km northwest and 17 km southeast. The V p/V s takes the mean values of 1.73 for the upper crust although higher mean values of 1.76 are obtained for the middle and lower crust. Fig. 3 shows the P-wave velocity and S-wave velocity models obtained for the southeastern China. The crust–uppermost mantle model that we propose, which is similar to that proposed for other parts of the region (Wei et al., 1990), consists of nonhorizontal layers in which P- and S-velocity change both vertically and horizontally. In the following, we refer to the seismic velocity structure of the upper, middle, and lower crust. The uncertainties are inferred to be less than 3% for crustal P-wave velocity and 10 % for the crustal thickness estimated from the conventional wide-angle seismic profiling (Mooney, 1989). In this study, the uncertainty in shear wave velocity is larger than P-wave velocity by the fact that there are 50 three-component records, namely, the resolution is better for P-wave than S-wave. 4.1. Upper crust The reflections P1 and S1 place the bottom of the upper crust down from 10 km (southeast end of the sounding) to 13 km (northwest end). A sedimentary cover extends over this structure along the profile with variable thickness of 4 km at Xinanjian and Jingwei, 3 km at Songyang, and only 2 km at Dongtou (Fig. 3). The P and S seismic velocities corresponding to the shallow materials are, respectively, 5.2 and 3.4 km/s at Fengle Reservoir, 5.6 and 3.2 km/s at Xinanjiang, 5.4
and 3.3 km/s at Jingwei basin and Songyang, 5.7 and 3.3 km/s at Qingtian, and lastly 5.6 and 3.2 km/s at Dongtou (Fig. 3). A good correlation between the velocity models and the geological outcrop is generally observed; relatively high velocities coincide with zones where basement is near the surface, and low-velocity zones are clearly associated with the presence of sedimentary basins or volcanic material. The frontier fault systems (2–5 in Fig. 1) are closely related to the observed low velocities (Fig. 3). High V p value is observed in the upper crust between Fengle Reservoir and Xinanjiang (Fig. 4), corresponding to the presence of Precambrian rocks dipping westwards and overthrusting the Fengle basin, which are marked as outcrops in the surface geology maps (Petroleum Geology Group for Investigating Oil and Gas Areas in Jiansu, Zhejiang and Anhui Provinces, 1992). Such a conspicuous feature between Fengle and Xinanjiang may have its origin in the uplifted Precambrian rocks of the lower crust with a rather dominant felsic composition, which is consistent with some materials, such as stishovite and mantlederived basalt founded in a drilled hole (Petroleum Geology Group for Investigating Oil and Gas Areas in Jiansu, Zhejiang and Anhui Provinces, 1992). Anomalously low V p value for acid rocks is found near Longyou basin (Fig. 3), which may indicate the presence of high-pressure porous rock in the basin. 4.2. Middle crust The middle crust consists of two layers that give rise to the branches P2, S2, P3, S3, marked as
Fig. 4. Mapping of the V P/V S ratio constraining the seismic velocity structure of the lithosphere in the tested region. As in Fig. 3, a partial view of the surface of the investigated area has been enclosed to understand the patterns. Please refer to the web version of the paper to view this figure in colour.
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reflections on the record sections (Fig. 2, upper panels). There is a gap between the first energy arrivals relative to the events P1 and P3 emphasising the existence of a low-velocity layer beneath the upper crust. We infer that the event P3 is a reflection from the bottom of a low-velocity layer representing the bottom of the middle crust. The event P2 comes from the top of this channel. Therefore, a thin low-velocity layer seems to be sandwiched between the reflectors P2 and P3 overlaying the lower crust. Values of 6.2 km/s for P-velocity and of 3.6 km/s for S-velocity characterise the upper half of the middle crust. The lower half—the narrow low-velocity channel—which is only 4-km thick exhibits velocity values lower than the previous ones: 6.0 km/s for P-velocity and 3.4–3.5 km/s for Svelocity (Fig. 3). The middle crust is about 8-km thick and has a minimum depth of 17 km to the southwest at Dongtou and a maximum of 23 km to the northwest at Fengle. A smooth lateral variation in P-wave velocity and Poison ratio, above all in S-wave velocity, is a feature of the middle crust. Simultaneous gravity and geomagnetic (Li, 1992) analyses led to proposal of a model where the lowvelocity layer is strongly weathered by the presence of partially molten rock. Such a proposal suggesting partial melting is also consistent with our results on the basis of low V p and V s values (Fig. 3). 4.3. Lower crust The lower crust consists of two layers with a very irregular contact that give rise to the branches P4, S4, Pm, Sm, marked as reflections on the record sections (Fig. 2, upper panels). As can be seen in Fig. 3, Pwave velocity increases quickly to 6.2–6.4 km/s for the materials immediately below the middle crust and then takes values of 6.6–6.7 km/s in the upper half of the lower crust. S-wave velocity jumps to 3.6 km/s underneath the middle crust and then takes values from 3.6 to 3.75 km/s all over the upper layer of the lower crust. For the deepest crustal layer, these velocities increase for both P- and S-waves, taking values between 6.8 and 7.4 km/s in the first case and from 3.8 to 3.9 km/s in the second one. The lower crust, which is about 11-km thick at Fengle and 14 km at Dongtou, exhibits obvious lateral variations in velocity as a result of the variable thickening and imbrications of its two layers (Fig. 3).
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4.4. Moho discontinuity The reflection amplitudes observed from the crust– mantle boundary are sufficiently large to suggest that there is no significant partial melt in the deep crust. There are, however, multiple refracted Pn and Sn phases clustered after the Pm and Sm arrivals. We interpret that this fact is due to irregular scattering from the Moho; that is, the crust–mantle boundary is not a simple discontinuity in velocity but a transition zone enhanced by a strong velocity gradient (Zhang et al., 2000). In effect, we observe on 2–3 km a sharp variation in P-wave velocity from 7.4 to 7.8 km/s to the northwest and from 7.4 to 8.0–8.2 km/s to the southeast (Fig. 3, upper part). Analogously, changes in velocity from 3.90 to 4.50 km/s to the northwest and from 3.90 to 4.70 km/s to the southeast (Fig. 3, lower part) seem to be characteristic for the S-wave Moho. The top of the lithospheric mantle features a sharp lateral variation in P- and S-velocity, which is approximately beneath Songyang. The crust is gradually thinner from northwest to southeast due to the irregular topography of the Moho. The maximum depth of the crust is 38 km at Fengle and 34 km at Dongtou (Fig. 3). These results are consistent with those obtained in the course of oil and gas exploration work in the study area (Petroleum Geology Group, 1992). A clear uplift of the Moho is observed between Qingtian and Wenzhou, suggesting mirrorimage symmetry between the sedimentary basin and the consolidated crust (Zhang et al., in press).
5. Discussion 5.1. Crustal Illumination of this experiment By considering the seismic energy propagating in the crust or the ray trajectories from different reflectors and from the Moho, we infer that the different markers are correctly illuminated. In effect, as Fig. 5 shows, most of the portions of the probed crust are well illuminated by seismic rays from two to three times or more for offsets 35–45 km (segment A), 100–130 km (segment B), 130–160 km (segment C), 230–250 km (segment D), and 315–340 km (segment E). Seismic rays cover other portions only one time. Regarding those portions of
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Fig. 5. Zones of the probed crust illuminated by ray coverage. Entire numbers from 1 to 5 indicate the times that the seismic rays cross a portion of lithosphere. Capital letters (A, B, etc.) indicate the zones illuminated at least twice by seismic rays.
the crust with a reliable coverage, the average P- and S-wave velocities found in each case are, respectively, 6.4 and 3.5 km/s for segment A, 6.2 and 3.5 km/s for segment B, 6.5 and 3.5 km/s for segment C, 6.3 and 3.6 km/s for segment D, and 6.3 and 3.5 km/ s for segment E. For both P- and S-waves, Fig. 6 shows the velocity–depth functions, which corre-
spond to the well-illuminated portions, as well as the V p/V s ratio for every crustal column. 5.2. Upper lithosphere There is a remarkable lateral variation in relation with the Pm and Sm reflections. The travel times of P-
Fig. 6. P-wave (dashed line) and S-wave (continuous line) velocity–depth curves and variation of the V P/V S ratio with depth defining the seismic velocity structure for the columns of crustal lithosphere mentioned in Fig. 5.
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Fig. 6 (continued).
phases reflecting from the Moho advance from 80 to 160 km northwestward of Dontou, from 80 to 130 km northwestward of Qingtian, from 140 to 200 km southeastward of Fengle Reservoir (Fig. 2, upper panels). We attribute these advances to a thinning of the crust going from the northwest end to the southeast end of the profile. Most of the Pm-phase arrivals and the Pn-phase arrivals coming from Qingtian exhibit a large amount of irregular scattering (Fig. 2, upper panels). This is interpreted as taking place at the major boundary between the continental crust and the oceanic crust beneath the eastern part of the profile. An abrupt change to a simpler character of the Pn arrivals from the shot of Qingtian occurs under the Jiangshan– Shaoxin fault, such as it is seen at surface, and this position may be the continent–ocean contact. The V P/V S ratio is 1.74 for the whole crust. These values are within the (normal as compared with) worldwide average values for the crust (Christensen
and Mooney, 1995). However, they range in a wide interval at different crustal depths (Fig. 4), as they range from 1.70 to 1.76 for the upper crust and from 1.76 to 1.80 for the middle and lower crust. These results are due to changes in P- and S-velocity and reflect the heterogeneity of the crustal structure. 5.3. Cathaysia block/Yangtze block boundary The Jiangshan–Shaoxin fault is considered to be the natural boundary between the Cathaysia block and the Yangtze block (Huang, 1977; Li, 1992). According to the wide-angle seismic profiling results (emerging from the wide-angle seismic profile interpretation), the fault located at 150 km to the south of Xikou marks a significant change in the crustal properties. The subcrustal P- and S-velocities under the vertical of Songyang change, respectively, from 7.8 to 8.2 km/s (Fig. 3, upper part) and from 4.2 to 4.5 km/s (Fig. 3, lower part).
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The crustal structure to the northwest of the fault is more complex than to the southeast. The boundary is confirmed from magnetic field and gravity measurements. The magnetic anomaly is stable to the southeast of the fault, but it changes suddenly to the northwest of the fault (Teng and Yan, 2004). The Bouguer anomaly also seems to aim at the Jiangshan–Shaoxin fault as the boundary between the Cathaysia block and the Yangtze block (Wang and Zhou, 1993). A high heat flow of 75 mW/m2 (Hu et al., 1993) has been found in Jingwei basin (Fig. 1) to the west of the boundary.
6. Conclusion A detailed analysis of the NW–SE wide-angle seismic profile from Fengle to Dongtou performed in 1990 in the course of a seismic experiment in the southeastern China has allowed us to achieve a quantitative model of the crust–upper mantle structure in the region in terms of P- and S-wave velocity. A relatively thin sedimentary layer covers the whole structure with a variable thickness down to 4 km at Xinanjian and decreasing to only 2 km at Dongtou. The results permit to distinguish upper, middle, and lower crust. The upper crust shows a higher degree of lateral variation in P- and S-wave velocity than the deepest one, the lateral changes in velocity map some major active faults, in particular the Jiangshan–Shaoxin fault, which is presented as the boundary between the Cathaysia block and the Yangtze block. One of the most conspicuous structural features is a comparatively thin middle crust made of two layers, the lower half containing a narrow, prolonged, lowvelocity channel overlying the lower crust, which has very little lateral variation in velocity, suggesting the presence of partially molten rock. The observed velocity gradient at the bottom of the crust permits to interpret the crust–mantle boundary like a finite transition zone where the seismic energy is scattered. In the vertical of Songyang, the middle point of the profile approximately, the lithospheric mantle shows a clear lateral increase in P- and S-velocity southeastward. The Jiangshan–Shaoxin fault is concluded as the boundary between the Yangtze and the Cathaysia block.
Acknowledgments We are indebted to Prof. Shaobai Xiong who supplied us with original information for this study and also for his constructive comments and guidance during the preparation of the draft that led to significant improvement of the manuscript. The authors would like to thank the Institute of Geology and Geophysics, Chinese Academy of Sciences (CAS), for the permission to publish this work, which is part of the aims stated in the Collaboration Agreement between the Institute of Geology and Geophysics and the University of Zaragoza, Spain. Valuable comments and suggestions from Dr. Kissling, Furlong, Horscroft and another anonymous reviewer are gratefully acknowledged. The Bureau of Resources and Environment, CAS, Grants KZCX2109 and KZ951-B1-407, and the National Natural Science Foundation of China, Grant 49825108, supported this work. References Chen, J.F., John, B.M., 1998. Crustal evolution of southeastern China: Nd and Sr isotopic evidence. Tectonophysics 284, 101 – 133. Christensen, N.I., Mooney, W.D., 1995. Seismic velocity structure and composition of the continental crust: a global view. J. Geophys. Res. 100, 9761 – 9788. Gilder, S.A., Coe, R.S., Wu, H.R., Kuang, G.D., Zhao, X.X., Wu, Q., Tang, X.Z., 1993. Cretaceous and Tertiary paleomagnetic results from southeast China and their tectonic implications. Earth Planet. Sci. Lett. 117, 637 – 652. Gilder, S.A., Zhao, X.X., Coe, R.S., Wu, H.R., Kuang, G.D., 1993. Discordance of Jurassic paleomagnetic data from south China and their tectonic implications. Earth Planet. Sci. Lett. 119, 259 – 269. Gilder, S.A., Gill, J., Coe, R.S., 1996. Isotopic and paleo constraints on the Mesozoic tectonic evolution of south China. J. Geophys. Res. 101, 16137 – 16154. Granitoid Research Group of the Nanling Project, 1989. Geology of Granitoids of Naling Region and their Petrogenesis and Mineralization. Geol. Publ. House, Beijing. 471 pp. Hsu, K.J., Sun, S., Li, J.L., Chen, H.H., Peng, H.P., Sengor, A.M.C., 1988. Mesozoic overthrust tectonics in south China. Geology 16, 418 – 421. Hsu, K.J., Sun, S., Li, J.L., Chen, H.H., Peng, H.P., Sengor, A.M.C., 1989. Reply to Gupta on "Mesozoic overthrust tectonic in south China". Geology 17, 386 – 387. Hu, S., Wong, J., He, L., 1993. Geotherm and temperature fields in Zhejiang Province. In: Li, J. (Ed.), Lithospheric Structure and Evolution in Southeastern China. Meteorogical Press, pp. 257 – 264 (315 pp.).
Z. Zhang et al. / Tectonophysics 395 (2005) 137–157 Huang, C., 1977. Basic features of the tectonic structure of China. Int. Geol. Rev. 5, 289 – 302. Ji, X., Coney, P.J., 1985. Accreted terranes in China. In: Howell, D.G. (Ed.), Tectonostratigraphic Terranes of the Circum-Pacific Region. Circum-Pac. on Energy Miner. Resour., Earth Sci. Ser., vol. 1, pp. 349 – 361 (Houston, TX). Li, J., 1992. Lithospheric Structure and Evolution in the Southeastern China. Science and Tectonology Press of China. 315 pp. Mooney, W.D., 1989. Seismic methods for determining earthquake source parameters and lithospheric structure. In: Pakiser, L.C., Mooney, W.D. (Eds.), Geophysical Framework of the Continental Unites States, Mem. Geol. Soc. Am., vol. 172, pp. 11 – 34. Petroleum Geology Group for Investigating Oil and Gas Areas in Jiansu, Zhejiang and Anhui Provinces, 1992. Oil and gas zone, Jiangsu, Zhangjiang and Anhui Petroleum Geology Commission. Oil and Gas Prospers, Series of Petroleum Geology in China, vol. 8. Oil Industry Press, pp. 25 – 26 (in Chinese). Sengor, A.M.C., 1982. Edward Suess’ relations to the pre-1950 schools of thought in global tectonics. Ged. Rundsch., vol. 71, 381 – 420. Teng, J.W., Yan, Y.F., 2004. Abnomal geomagnetic field response at intraplate tectonic boundary in continent and continental margin in southeastern China. Geotecton. Metallogen. 28, 105 – 117. Wang, H.Z., Qiao, X.F., 1984. Peterozoic stratigraphy and tectonic framework of China. Geol. Mag. 121, 599 – 614. Wang, Q., Zhou, W., 1993. Gravity measurements between Tunxi and Wenzhou and its lithosphere structure. In: Li, J. (Ed.),
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Lithospheric Structure and Evolution in Southeastern China. Meteorogical Press, pp. 234 – 241 (315 pp.). Wei, S., Teng, J., Wang, Q., Zhu, Z., 1990. Continental Margin Lithosphere Structure and Geo-Dynamics of the Eastern China. Chinese Sciences Press. 188 pp. (in Chinese). Xiong, S., Lai, M., Liu, H., Yu, K., 1993. Lithosphere structure and velocity distribution from Tunxi to Wenzhou. In: Li, Jidian. (Ed.), Lithosphere Structure and Geological Evolution of Southeastern China. Meteorological Press, pp. 267 – 277 (315 pp.). Xu, J.W., Zhu, G., 1993. Basic characteristics and tectonic evolution of the Tancheng–Lujiang fault zone. In: Xu, J.W. (Ed.), The Tancheng–Lujiang Wrench Fault System. John Wiley, New York, pp. 1 – 49. Xu, J.W., Zhu, G., Tong, W.X., Cui, K.R., Liu, Q., 1987. Formation and evolution of the Tancheng–Lujiang wrench fault system: a major shear system to the northwest of the Pacific Ocean. Tectonophysics 134, 273 – 310. Zhang, Z.M., Liou, J.G., Coleman, R.G., 1984. An outline of the plate tectonics of China. Geol. Soc. Amer. Bull. 95, 295 – 312. Zhang, Z., Wang, G., Teng, J., Klemperer, S., 2000. CDP mapping to obtain the fine structure of crust and upper mantle from seismic sounding data: an example in the southeastern China. Phys. Earth Planet. Inter. 122, 133 – 146. Zhang, Z., Yang, L., Teng, J., Badal, J., 2004. Mirror-image symmetry between sedimentary basins and the consolidated crust in China. JGR Solid Earth (submitted).
Crust–upper mantle seismic velocity structure across Southeastern China Zhongjie Zhanga,*, Jose´ Badalb, Yinkang Lic, Yun Chena, Liqiang Yanga, Jiwen Tenga a
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China b Physics of the Earth, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain c Geological Data Center, Ministry of the Land and Resources, Heibei, 110101, China Received 6 August 2003; accepted 20 August 2004 Available online 7 December 2004
Abstract One in-line wide-angle seismic profile was conducted in 1990 in the course of the Southeastern China Continental Dynamics project aimed at the study of the contact between the Cathaysia block and the Yangtze block. This 380-km-long profile extended in NW–SE direction from Tunxi, Anhui Province, to Wenzhou, Zhejiang Province. Five in-line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line. We have used two-dimensional ray tracing to model P- and S-wave arrivals and provide constraints on the velocity structure of the upper crust, middle crust, lower crust, Moho discontinuity, and the top part of the lithospheric mantle. P-wave velocity, S-wave velocity and V P/V S ratio are mapped. The crust is 36-km thick on average, albeit it gradually thins from the northwest end to the southeast end (offshore) of the profile. The average crustal velocity is 6.26 km/s for P-waves but 3.6 km/s for S-waves. A relatively narrow low-velocity layer of about 4 km of thickness, with P- and S-wave velocities of 6.2 km/s and 3.5 km/s, respectively, marks the bottom of the middle crust at a depth of 23-km northwest and 17-km southeast. At the crust–mantle transition, the P- and S-wave velocity change quickly from 7.4 to 7.8 km/s (northwest) and 8.0 to 8.2 km/s (southeast) and from 3.9 to 4.2 km/s (northwest) and 3.9 to 4.5 km/s (southeast), respectively. This result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately. The average V P/V S ratio ranges from 1.68–1.8 for the upper crust to 1.75 for the middle and 1.75–1.85 for lower crust. With the interpretation of the wide-angle seismic data, Jiangshan–Shaoxin fault is considered as the boundary between the Yangtze and the Cathaysia block. D 2004 Elsevier B.V. All rights reserved. Keywords: Wide-angle seismic reflection/refraction; P- and S-wave velocity models; Crustal structure; Yangtze and Cathaysia blocks; Southeastern China
1. Introduction * Corresponding author. E-mail address: [email protected] (Z. Zhang). 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.08.008
Southeastern China is an important part of the tectonic framework concerning the continental mar-
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gin of eastern China and is commonly assumed to comprise the Yangtze block and the Cathaysia block, the two major tectonic units in the region. Such a distinction between regional blocks was proposed on the basis of dissimilarities in the stratigraphic records (Ji and Coney, 1985), the possible occurrence of ophiolite suites (Zhang et al., 1984), and differences in the lithology of the Precambrian basement (Granitoid Research Group, 1989; Fig. 1). Low-grade metamorphic rocks of middle to late Proterozoic ages separate both blocks (Wang and Qiao, 1984). However, the boundary between these tectonic units is poorly documented
due to the lack of detailed data as to fix its nature (suture or strike–slip). This unclear panorama becomes especially apparent when the Mesozoic tectonic models are considered. The left lateral offset model has been prompted (Xu et al., 1987; Xu and Zhu, 1993) from marker beds on the order of hundreds of kilometres and the existence of major shear zones. It has been suggested that the entire eastern part of the Chinese landmass was dominated by a sinistral shear system during the Mesozoic period. Alternatively, it has been stated that a series of fold belts within the southeast region of the Yangtze block accreted by subduction and
Fig. 1. Simplified geological setting and wide-angle seismic profile, location between Tunxi and Wenzhou in Southeastern China. Triangles and stars mark the geographical positions of stations and shot points, respectively. The major active faults and basins crossed by the long linear antenna are (1) Lin’an–Majin fault, (2) Wuxing–Jiande fault, (3) Changshan–Pujiang fault, (4) Jiangshan–Shaoxing fault, the surface boundary between the Yangtze and the Cathaysia blocks, (5) Quzhou–Tangxian fault, (6) Shangyu–Lishui fault, (7) Qingyuan–Anren fault, (8) Ningbo– Longquan fault, (9) Zhenhai–Wenzhou fault, (I) Jinwen Basin, (II) Bihu Basin, (III) Wenzhou–Pinyang hollow. Key to symbols: 1— Precambrian, 2—Paleozoic Erathem, 3—Mesozoic Erathem, 4—Cenozoic Erathem, 5—Jurassic neutral-acidic volcanic rock, 6—Cretaceous basic volcanic rock, 7—neutral-acidic lithesome, 8—basic-ultrabasic lithesome, 9—major faults, 10—deep seismic sounding profile, CB— crystalline basement, SC—sedimentary cover, FR—fold region (belt), K—cretaceous (light green), N—neogene (pale yellow), Q—quaternary (yellow), j6—basic–ultrabasic rocks (purple), s6—alkaline rocks (red), h6—basalts (bottle green). Please refer to the web version of the paper to view this figure in colour.
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closure of ocean basins in Alpine-type collisions in the early Mesozoic or later (Hsu et al., 1988). The tectonic evolution of the Southeastern China has proved to be exceedingly complex regarding the occurrence of collision or translation of various cratons during the Precambrian (Huang, 1977), Paleozoic (Zhang et al., 1984), and Mesozoic (Xu et al., 1987; Hsu et al., 1988, 1989). Their contacts are one of the unsolved problems (Wei et al., 1990), and the knowledge of the crustal structure of the region is rather poor. Actually, only the Mesozoic tectonic evolution of the region has been constrained from paleomagnetic samples, and Alpine-type mountain building is assumed to be present (Sengor, 1982; Li, 1992; Hsu et al., 1988, 1989; Gilder et al., 1996; Chen and John, 1998). There is a clear target for the study of the crustal structure beneath southeastern China: to investigate the contact between the Yangtze block and the Cathaysia block to model the crust–mantle seismic velocity structure of the region. The Southeastern China Continental Dynamics (SCCD) project was just developed with this purpose—among others— between 1984 and 1990, in particular, to study the regional crust and upper mantle. This seismological experiment, which was performed in the framework of a multidisciplinary research program supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences, involved instrumentation, and people of Chinese institutions. A wide-angle seismic profile from Fengle Reservoir near Tunxi, Anhui Province, to Dongtou Island, offshore of Wenzhou, Zhejiang Province, was conducted in 1990 as part of the SCCD project. This profile is shown in Fig. 1. It provides a good chance to study the contact between the Yangtze and southern China blocks and to determine a crustal velocity model for the area. Xiong et al. (1993) made a preliminary interpretation of the wide-angle seismic data and obtained an initial P-wave velocity model albeit no interpretation of S-wave data were advanced. In this paper, we describe the information then collected and present the results that we have obtained based on interpretation of the registered wide-angle seismic reflection/refraction P- and S-phases. We propose a new 2-D model, which allows us to constrain the seismic velocity structure of the upper lithosphere in the region.
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2. Geological setting The wide-angle seismic profile analysed in this work runs in direction NW–SE from Tunxi to Wenzhou and crosses several major active faults and basins from its northwest end to its southeast end. This can easily be seen in Fig. 1 where all these details are shown. Most of these faults are roughly parallel but perpendicular to the NW–SE seismic survey line and display the characteristics of compressive fragmentation, dynamic metamorphism, magmatism, mylonization and thrust nappe. The Jiangshan– Shaoxin fault (labelled 4 in Fig. 1) is considered to be the surface boundary between the Yangtze and the Cathaysia blocks. It separates materials of Precambrian, Paleozoic, Mesozoic, and Cenozoic ages (northernmost third of the profile) from other materials of the Jurassic and Cretaceous periods more to south. The profile also crosses several sedimentary basins, such as the Jinwei basin of Mesozoic and Cenozoic ages (Fig. 1), the Songyang basin of Cretaceous age, and the Wenping depression with Quaternary sediments. Both the lithosphere and the sediments at either side of the Jiangshan–Shaoxin fault are different. West–north from this fault, Paleozoic sediments extend between the Majing–Wuzhen fault and the Qiuchuan–Xiaoshan fault. Layers belonging to an age interval between the Sinian period and the Carboniferous–Permian periods form a series of anticlinals and synclinals at Xinanjiang (Fig. 1). Other layers of the Proterozoic period emerge at Baijishan (Fig. 1)— the boundary separating the Anhui and Zhejiang Provinces—and to northwest of the Jinwei basin. In opposite direction, neutral–acidic volcanic rock of the Late Jurassic period extends beyond the Jiangshan– Shaoxin fault, with the interruption of some Mesozoic sediment along the Lishui–Yuyao fault and other Cenozoic sediments to east of Wenzhou. Accounting now for geological aspects, the boundary between the Yangtze and Cathaysia blocks inferred in this study is also consistent with a granite belt with high eNd and low T DM values extending from Hangzhou through central Jiangxi to Guangxi (Gilder et al., 1993a, b). This natural boundary separates a steady area to northwest from an active area to southeast of China located in different geochemical zones. Thus, the Jiangshan fault is considered as a
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Neoproterozoic suture zone between the Yangtze and Cathaysia blocks. The eastern part of the belt is along the Jiangshan–Shaoxin fault and contains a series of upper Proterozoic, ultramafic, and diorite rock bodies (795~890 Ma), which are strongly mylonizated, and forms a 150-km long mylonite belt. To the northwest of the Jiangshan fault, there is a series of upper Proterozoic, low metamorphism, volcanic sedimentary rocks, northeast Jiangxi–northwest Zhejiang (Fig. 1). To the southeast of the fault, there emerges Palacozoic regional metamorphic rock of greenschist facies– amphibolite facies.
3. The SCCD profile 3.1. Design and identification of crustal phases The seismological information comes from the records obtained by wide-angle reflection/refraction profiling about 380-km long, which was performed as part of the SCCD project mentioned above. The field operations were carried out during a period of 2 months and always under control of the Institute of Geology and Geophysics (former Institute of Geophysics) of the Chinese Academy of Sciences. The profile, with azimuth nearly N30W, runs from the Fengle Reservoir near Tunxi to Dongtou near Wenzhou. Five shots were fired at different sites: Fengle, Xinanjian, Songyang, Qintian, and Dongtou (Fig. 1). Each shot consisted of a hole drilled to a depth of 20 m and loaded with a charge ranging from 1200 to 1800 kg of explosive. A total of 85 portable seismic stations were installed along the survey line: 35 single-vertical-component stations recording on magnetic tape and 50 three-component digital stations equipped with DZSS-1 seismographs. Instrumentation includes data acquisition system, digital cassette tape recorder, and playback. The time signal was provided by internal clock synchronized by radio time signal. All the stations and shots were located using 1:25,000 topographic maps with an estimated location accuracy of 12.5 m. Station spacing was of 3 km on average, and the offset range was 2 to 240 km. The seismic signals provided by digital instruments were initially sampled at a rate of 5 ms and then filtered within the 1- to 10-Hz frequency band for P-waves and within 1–6 Hz for S-waves. The analogue records provided by magnetic tape recorders
were subsequently digitized at 5 ms. The shot gathers were displayed at reduced time scale for a velocity of 6.0 km/s for P-waves and of 3.5 km/s for S-waves. Fig. 2 (upper panels) shows the registered P- and Swave record sections. Almost all the seismograms recorded along the profile exhibit a high signal-to-noise ratio. From these record sections, we are able to clearly identify Pg and Sg energy arrivals refracted above of the crystalline basement, but also, the Pm- and Smwaves reflected from the Moho. The Pn- and Sn-waves refracted from the Moho may also be recognised although their amplitudes are very weak as compared with the amplitude of the previous phases. Other P- and S-waves reflected from interfaces between the crystalline basement and the crust–mantle discontinuity might be recognised too. The amplitudes of the events P1, S1 (upper crust), P4, and S4 (lower crust) are relatively weak as compared with those of the reflections P3 and S3 from the bottom of a low-velocity layer located in the middle crust. In contrast, the events P2 and S2 coming from the top of this low-velocity channel show larger amplitudes at some offsets. 3.2. Preliminary analysis The visible offset for the event Pg is 80 km at Fengle (Fig. 2a), 60 km northwest of Xinanjiang (Fig. 2b), and 75 km southeast of this shot point. At Songyang (Fig. 2c), this crustal phase is visible as far as 100 km northwest and 80 km southeast. At Qingtian (Fig. 2d), the offset for this phase is 60–75 km at both sides, and at Dongtou (Fig. 2e), the offset is 100 km. The Pg-wave travel times reported from the shot at Dongtou show a delay when compared with the time–distance curve for offsets up to 60 km from Qingtian and 100 km from Songyang. From the shot at Qingtian, the travel times observed up to offsets of 60 km in SE direction and 75 km in NW direction towards Songyang show a similar delay. The same observation is valid for the Pg-wave travel times reported from the shot at Songyang; they show a delay for offsets up to 60 km from Xikou and 100 km to east of Qianligan. From the shot at Xinanjiang, the travel times show an abrupt delay of offsets between 40 km (Longyou, Fig. 2) and 70 km to east of Xikou. For the shot at Fengle Reservoir, the travel times are at distances of between 20 and 40 km towards Xinanjiang, which demonstrates that P-wave velocity above
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Fig. 2. Upper panels—P- and S-waves sections from five wide-angle inline shots fired at different sites: Fengle (FLS), Xinanjian (XAS), Songyang (SYS), Qintian (QTS), and Dongtou (DTS). Where the raw data are processing by band-pass filter with frequency range of 1–10 Hz for P-wave and 1–6 Hz for S-wave. These reduced sections are shown with reduction velocity of 6.0 km/s for P-wave and 3.4 km/s for S-wave. Central panels—comparison between observed (4) and computed (5) travel times. Lower panels—respective seismic velocity model organised in dipping layers and ray diagram.
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Fig. 2 (continued).
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Fig. 2 (continued).
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Fig. 2 (continued).
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Fig. 2 (continued).
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Fig. 2 (continued).
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Fig. 2 (continued).
the crystalline basement changes abruptly along the profile, which is mostly probably a result of the sedimentary basins and Quaternary sediments.
Notice the similarity of the S-phases regarding the P-phases in Fig. 2 (upper panels). We also recognise the Sg energy arrivals refracted above the crystalline
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Fig. 2 (continued).
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Fig. 2 (continued).
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Fig. 2 (continued).
basement, the Sm-phase reflected from Moho, the Snphase refracted from Moho, and other phases (S1, S2, S3, and S4) reflected from interfaces between the
crystalline basement and the Moho. The distance from the shot point to which the Sg-phase is registered is a little shorter than the offset for Pg. It is 60 km at
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Dongtou and Songyang, 90 km northwest and 75 km southeast of Qingtian, 75 km at Xinganjiang, and 70 km at Fengle Reservoir. Analogously, the Sg-phase travel times delay up to an offset of 25 km from Fengle Reservoir and of 30 km from Songyang. The same occurs at distances between 30 and 40 km from Qingtian; but these observed times change abruptly from the shot at Dongtou, which demonstrates the same features for S-waves propagating above the crystalline basement before for P-waves. From the Pm and Sm travel times, the apparent Pand S-wave velocities are deduced to be 6.8 and 3.9 km/s, respectively, for the bottom of the crust. The refracted Pn- and Sn-phases are indeed weak in amplitude, but their onsets are sufficiently clear as to be recognised. The analysis of these phases reveals a P- and S-wave velocity at the top part of the lithospheric mantle, which varies quickly from 7.8 to 8.1 km/s (offset range of 240–280 km) and from 4.1 to 4.4 km/s (offset range of 200–240 km), respectively.
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4. Interpretation: crust–mantle model We first carry out inversion for 1-D crustal structure under the assumption of a horizontally layered medium. Then we construct a 2-D velocity model of the crust by fitting the observations using ray-tracing and theoretical travel times. In Fig. 2 (central panels) and for every shot point, we show comparisons between observed and computed travel times for Pand S-waves. Also in Fig. 2 (lower panels), we show ray paths computed from the final velocity model, which is constrained by the travel times of the identified P- and S-waves. On the basis of the excellent fit between our velocity model (Fig. 2) and the observed data, we are able to give the following overall results. The average thickness of the crust is about 36 km, albeit it gradually thins from northwest to southeast as the Moho depth decreases. The average P-wave velocity for the crust is 6.26 km/s; this average value becomes 3.6 km/s for S-wave velocity. A relatively low velocity layer of
Fig. 3. Overall P-wave (upper part) and S-wave (lower part) velocity models for southeastern China showing an overall view of the upper crust, middle crust, lower crust, Moho discontinuity, and lithospheric mantle. Isolines for seismic velocity values given in kilometres per second emphasize different structures. Traces of major active faults (Fig. 1) on a part of the map of the study area have been drawn to provide a comprehensible view of the velocity patterns. Please refer to the web version of the paper to view this figure in colour.
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about 4 km of thickness, with P- and S-wave velocity of 6.2 and 3.5 km/s, respectively, marks the bottom of the middle crust at a depth of 23 km northwest and 17 km southeast. The V p/V s takes the mean values of 1.73 for the upper crust although higher mean values of 1.76 are obtained for the middle and lower crust. Fig. 3 shows the P-wave velocity and S-wave velocity models obtained for the southeastern China. The crust–uppermost mantle model that we propose, which is similar to that proposed for other parts of the region (Wei et al., 1990), consists of nonhorizontal layers in which P- and S-velocity change both vertically and horizontally. In the following, we refer to the seismic velocity structure of the upper, middle, and lower crust. The uncertainties are inferred to be less than 3% for crustal P-wave velocity and 10 % for the crustal thickness estimated from the conventional wide-angle seismic profiling (Mooney, 1989). In this study, the uncertainty in shear wave velocity is larger than P-wave velocity by the fact that there are 50 three-component records, namely, the resolution is better for P-wave than S-wave. 4.1. Upper crust The reflections P1 and S1 place the bottom of the upper crust down from 10 km (southeast end of the sounding) to 13 km (northwest end). A sedimentary cover extends over this structure along the profile with variable thickness of 4 km at Xinanjian and Jingwei, 3 km at Songyang, and only 2 km at Dongtou (Fig. 3). The P and S seismic velocities corresponding to the shallow materials are, respectively, 5.2 and 3.4 km/s at Fengle Reservoir, 5.6 and 3.2 km/s at Xinanjiang, 5.4
and 3.3 km/s at Jingwei basin and Songyang, 5.7 and 3.3 km/s at Qingtian, and lastly 5.6 and 3.2 km/s at Dongtou (Fig. 3). A good correlation between the velocity models and the geological outcrop is generally observed; relatively high velocities coincide with zones where basement is near the surface, and low-velocity zones are clearly associated with the presence of sedimentary basins or volcanic material. The frontier fault systems (2–5 in Fig. 1) are closely related to the observed low velocities (Fig. 3). High V p value is observed in the upper crust between Fengle Reservoir and Xinanjiang (Fig. 4), corresponding to the presence of Precambrian rocks dipping westwards and overthrusting the Fengle basin, which are marked as outcrops in the surface geology maps (Petroleum Geology Group for Investigating Oil and Gas Areas in Jiansu, Zhejiang and Anhui Provinces, 1992). Such a conspicuous feature between Fengle and Xinanjiang may have its origin in the uplifted Precambrian rocks of the lower crust with a rather dominant felsic composition, which is consistent with some materials, such as stishovite and mantlederived basalt founded in a drilled hole (Petroleum Geology Group for Investigating Oil and Gas Areas in Jiansu, Zhejiang and Anhui Provinces, 1992). Anomalously low V p value for acid rocks is found near Longyou basin (Fig. 3), which may indicate the presence of high-pressure porous rock in the basin. 4.2. Middle crust The middle crust consists of two layers that give rise to the branches P2, S2, P3, S3, marked as
Fig. 4. Mapping of the V P/V S ratio constraining the seismic velocity structure of the lithosphere in the tested region. As in Fig. 3, a partial view of the surface of the investigated area has been enclosed to understand the patterns. Please refer to the web version of the paper to view this figure in colour.
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reflections on the record sections (Fig. 2, upper panels). There is a gap between the first energy arrivals relative to the events P1 and P3 emphasising the existence of a low-velocity layer beneath the upper crust. We infer that the event P3 is a reflection from the bottom of a low-velocity layer representing the bottom of the middle crust. The event P2 comes from the top of this channel. Therefore, a thin low-velocity layer seems to be sandwiched between the reflectors P2 and P3 overlaying the lower crust. Values of 6.2 km/s for P-velocity and of 3.6 km/s for S-velocity characterise the upper half of the middle crust. The lower half—the narrow low-velocity channel—which is only 4-km thick exhibits velocity values lower than the previous ones: 6.0 km/s for P-velocity and 3.4–3.5 km/s for Svelocity (Fig. 3). The middle crust is about 8-km thick and has a minimum depth of 17 km to the southwest at Dongtou and a maximum of 23 km to the northwest at Fengle. A smooth lateral variation in P-wave velocity and Poison ratio, above all in S-wave velocity, is a feature of the middle crust. Simultaneous gravity and geomagnetic (Li, 1992) analyses led to proposal of a model where the lowvelocity layer is strongly weathered by the presence of partially molten rock. Such a proposal suggesting partial melting is also consistent with our results on the basis of low V p and V s values (Fig. 3). 4.3. Lower crust The lower crust consists of two layers with a very irregular contact that give rise to the branches P4, S4, Pm, Sm, marked as reflections on the record sections (Fig. 2, upper panels). As can be seen in Fig. 3, Pwave velocity increases quickly to 6.2–6.4 km/s for the materials immediately below the middle crust and then takes values of 6.6–6.7 km/s in the upper half of the lower crust. S-wave velocity jumps to 3.6 km/s underneath the middle crust and then takes values from 3.6 to 3.75 km/s all over the upper layer of the lower crust. For the deepest crustal layer, these velocities increase for both P- and S-waves, taking values between 6.8 and 7.4 km/s in the first case and from 3.8 to 3.9 km/s in the second one. The lower crust, which is about 11-km thick at Fengle and 14 km at Dongtou, exhibits obvious lateral variations in velocity as a result of the variable thickening and imbrications of its two layers (Fig. 3).
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4.4. Moho discontinuity The reflection amplitudes observed from the crust– mantle boundary are sufficiently large to suggest that there is no significant partial melt in the deep crust. There are, however, multiple refracted Pn and Sn phases clustered after the Pm and Sm arrivals. We interpret that this fact is due to irregular scattering from the Moho; that is, the crust–mantle boundary is not a simple discontinuity in velocity but a transition zone enhanced by a strong velocity gradient (Zhang et al., 2000). In effect, we observe on 2–3 km a sharp variation in P-wave velocity from 7.4 to 7.8 km/s to the northwest and from 7.4 to 8.0–8.2 km/s to the southeast (Fig. 3, upper part). Analogously, changes in velocity from 3.90 to 4.50 km/s to the northwest and from 3.90 to 4.70 km/s to the southeast (Fig. 3, lower part) seem to be characteristic for the S-wave Moho. The top of the lithospheric mantle features a sharp lateral variation in P- and S-velocity, which is approximately beneath Songyang. The crust is gradually thinner from northwest to southeast due to the irregular topography of the Moho. The maximum depth of the crust is 38 km at Fengle and 34 km at Dongtou (Fig. 3). These results are consistent with those obtained in the course of oil and gas exploration work in the study area (Petroleum Geology Group, 1992). A clear uplift of the Moho is observed between Qingtian and Wenzhou, suggesting mirrorimage symmetry between the sedimentary basin and the consolidated crust (Zhang et al., in press).
5. Discussion 5.1. Crustal Illumination of this experiment By considering the seismic energy propagating in the crust or the ray trajectories from different reflectors and from the Moho, we infer that the different markers are correctly illuminated. In effect, as Fig. 5 shows, most of the portions of the probed crust are well illuminated by seismic rays from two to three times or more for offsets 35–45 km (segment A), 100–130 km (segment B), 130–160 km (segment C), 230–250 km (segment D), and 315–340 km (segment E). Seismic rays cover other portions only one time. Regarding those portions of
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Fig. 5. Zones of the probed crust illuminated by ray coverage. Entire numbers from 1 to 5 indicate the times that the seismic rays cross a portion of lithosphere. Capital letters (A, B, etc.) indicate the zones illuminated at least twice by seismic rays.
the crust with a reliable coverage, the average P- and S-wave velocities found in each case are, respectively, 6.4 and 3.5 km/s for segment A, 6.2 and 3.5 km/s for segment B, 6.5 and 3.5 km/s for segment C, 6.3 and 3.6 km/s for segment D, and 6.3 and 3.5 km/ s for segment E. For both P- and S-waves, Fig. 6 shows the velocity–depth functions, which corre-
spond to the well-illuminated portions, as well as the V p/V s ratio for every crustal column. 5.2. Upper lithosphere There is a remarkable lateral variation in relation with the Pm and Sm reflections. The travel times of P-
Fig. 6. P-wave (dashed line) and S-wave (continuous line) velocity–depth curves and variation of the V P/V S ratio with depth defining the seismic velocity structure for the columns of crustal lithosphere mentioned in Fig. 5.
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Fig. 6 (continued).
phases reflecting from the Moho advance from 80 to 160 km northwestward of Dontou, from 80 to 130 km northwestward of Qingtian, from 140 to 200 km southeastward of Fengle Reservoir (Fig. 2, upper panels). We attribute these advances to a thinning of the crust going from the northwest end to the southeast end of the profile. Most of the Pm-phase arrivals and the Pn-phase arrivals coming from Qingtian exhibit a large amount of irregular scattering (Fig. 2, upper panels). This is interpreted as taking place at the major boundary between the continental crust and the oceanic crust beneath the eastern part of the profile. An abrupt change to a simpler character of the Pn arrivals from the shot of Qingtian occurs under the Jiangshan– Shaoxin fault, such as it is seen at surface, and this position may be the continent–ocean contact. The V P/V S ratio is 1.74 for the whole crust. These values are within the (normal as compared with) worldwide average values for the crust (Christensen
and Mooney, 1995). However, they range in a wide interval at different crustal depths (Fig. 4), as they range from 1.70 to 1.76 for the upper crust and from 1.76 to 1.80 for the middle and lower crust. These results are due to changes in P- and S-velocity and reflect the heterogeneity of the crustal structure. 5.3. Cathaysia block/Yangtze block boundary The Jiangshan–Shaoxin fault is considered to be the natural boundary between the Cathaysia block and the Yangtze block (Huang, 1977; Li, 1992). According to the wide-angle seismic profiling results (emerging from the wide-angle seismic profile interpretation), the fault located at 150 km to the south of Xikou marks a significant change in the crustal properties. The subcrustal P- and S-velocities under the vertical of Songyang change, respectively, from 7.8 to 8.2 km/s (Fig. 3, upper part) and from 4.2 to 4.5 km/s (Fig. 3, lower part).
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The crustal structure to the northwest of the fault is more complex than to the southeast. The boundary is confirmed from magnetic field and gravity measurements. The magnetic anomaly is stable to the southeast of the fault, but it changes suddenly to the northwest of the fault (Teng and Yan, 2004). The Bouguer anomaly also seems to aim at the Jiangshan–Shaoxin fault as the boundary between the Cathaysia block and the Yangtze block (Wang and Zhou, 1993). A high heat flow of 75 mW/m2 (Hu et al., 1993) has been found in Jingwei basin (Fig. 1) to the west of the boundary.
6. Conclusion A detailed analysis of the NW–SE wide-angle seismic profile from Fengle to Dongtou performed in 1990 in the course of a seismic experiment in the southeastern China has allowed us to achieve a quantitative model of the crust–upper mantle structure in the region in terms of P- and S-wave velocity. A relatively thin sedimentary layer covers the whole structure with a variable thickness down to 4 km at Xinanjian and decreasing to only 2 km at Dongtou. The results permit to distinguish upper, middle, and lower crust. The upper crust shows a higher degree of lateral variation in P- and S-wave velocity than the deepest one, the lateral changes in velocity map some major active faults, in particular the Jiangshan–Shaoxin fault, which is presented as the boundary between the Cathaysia block and the Yangtze block. One of the most conspicuous structural features is a comparatively thin middle crust made of two layers, the lower half containing a narrow, prolonged, lowvelocity channel overlying the lower crust, which has very little lateral variation in velocity, suggesting the presence of partially molten rock. The observed velocity gradient at the bottom of the crust permits to interpret the crust–mantle boundary like a finite transition zone where the seismic energy is scattered. In the vertical of Songyang, the middle point of the profile approximately, the lithospheric mantle shows a clear lateral increase in P- and S-velocity southeastward. The Jiangshan–Shaoxin fault is concluded as the boundary between the Yangtze and the Cathaysia block.
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