Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea

Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea

TECTO-126521; No of Pages 10 Tectonophysics xxx (2015) xxx–xxx Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevi...

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TECTO-126521; No of Pages 10 Tectonophysics xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Tectonophysics journal homepage: www.elsevier.com/locate/tecto

Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea Xiaodong Wei a, Aiguo Ruan a,⁎, Minghui Zhao b, Xuelin Qiu b, Zhenli Wu a, Xiongwei Niu a a b

The Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012 China Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301 China

a r t i c l e

i n f o

Article history: Received 10 July 2014 Received in revised form 14 January 2015 Accepted 16 January 2015 Available online xxxx Keywords: South China Sea Reed Bank Wide-angle seismic OBS profile Shear-wave velocity structure

a b s t r a c t The shear wave velocity structure of a wide angle seismic profile (OBS973-2) across Reed Bank in the southern continental margin of the South China Sea (SCS) is simulated by 2-D ray-tracing method, based on its previous P-wave model. This profile is 369-km-long and consists of fifteen three-component ocean bottom seismometers (OBS). The main results are as follows.(1) The model consists of seven layers and the shear wave velocity increases from 0.7 km/s at the top of sediment layer to 4.0 km/s in the lower crust. (2) The Moho depth decreases from 20-22 km at the Reed Bank to 9-11 km at the deep oceanic basin with the shear wave velocity of 4.2 km/s below the Moho. (3) The Vp/Vs ratio decreases with depth through the sedimentary layers, attributed to increased compaction and consolidation of the rocks. (4) In the continental upper crust (at model distance 90-170 km), S-wave velocity (2.5–3.2 km/s) is relatively low and Vp/Vs ratio (1.75–1.82) is relatively high compared with the other parts of the crust, corresponding to the lower P-wave velocity in the previous P-wave model and normal faults revealed by MCS data, indicating that a strong regional extensional movement had occurred during the formation process of the SCS at the Reed Bank area. (5) The S-wave structures indicate that Reed Bank crust has different rock compositions from that in the east section of the northern margin, denying the presence of conjugate relationship of Reed Bank with Dongsha islands. According to P-wave models and other data, we inferred that Reed Bank and Macclesfield were separated from the same continental crust during the rifting and break-up process. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Based on magnetic anomaly, it is generally accepted that the Cathysia begun to rift in Eocene (40 Ma) or even earlier in the Late Cretaceous (85–65 Ma), and then following multiple episodes of seafloor spreading from late Oligocene to mid Miocene (32–17 Ma) resulted in the formation of the South China Sea (SCS) with asymmetry conjugate margins in the north and south (Barckhausen and Roeser, 2004; Barckhausen et al., 2014; Briais et al., 1993; Franke, 2013; Ru and Pigott, 1986; Taylor and Hayes, 1980, 1983; Yao, 1996). However, at present there are still various points of view on the formation mechanism and spreading anomaly model of SCS and on the ages of basin magnetic anomalies and consequently on the chronology sequences of seafloor spreading between sub-basins. Taylor and Hayes (1980, 1983) estimated the age of the oceanic crust in the SCS ranged from Late Miocene to Middle Eocene, based on heat flow measurements and basement depth observations, and identified an abandoned E–W oriented spreading center with symmetric anomalies 5D through 6C to north and south ⁎ Corresponding author. Tel.: +86 571 81963155. E-mail address: [email protected] (A. Ruan).

in the eastern sub-basin and an older spreading center with a center anomaly 7 in the northwest of Macclesfield Bank. Briais et al. (1993) reinterpreted the youngest anomalies at the spreading axis identified by Taylor and Hayes (1983) to be 5C, and dated the anomalies in the triangular southwestern sub-basin as 6B through 5C much different from that of Yao (1996) and Ru and Piggott (1986). Barckhausen and Roeser (2004) and Barckhausen et al. (2014) presented a model largely agree with that of Briais et al. (1993) in the older part of the spreading history including the ridge jump at 25 Ma, but for the younger part, their model involves faster spreading rates and reinterpreted the end of the seafloor spreading at anomaly 20.5 Ma. For marginal conjugation relationship, Yao (1996) suggested Reed Bank and Dongsha block are two pieces from the same rigid continental block. Differently, Barckhausen and Roeser (2004) postulated Reed Bank broke-up from Macclesfield at around 25 Ma. For the boundary separating the eastern sub-basin from the southwestern sub-basin, some authors postulated the existence of a major N–S fault zone (around 116°E), often called Zhongnan Fault, through nearly the whole oceanic basin (Li, 1997; Ru and Piggott, 1986; Taylor and Hayes, 1983; Yao, 1996), while Barckhausen and Roeser (2004) and Barckhausen et al. (2014) suggested a transform fault with a shorter length exists only between

http://dx.doi.org/10.1016/j.tecto.2015.01.006 0040-1951/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: Wei, X., et al., Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.01.006

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Macclesfield Bank and Reed Bank, traced either along the eastern edges of Macclesfield and Reed Banks, or from the eastern edge of Macclesfield Bank to the western edge of Reed Bank. On the other hand, more detailed and reliable seismic crustal structures of SCS, especially the deep part of the crust and Moho, have been gradually revealed by wide-angle seismic profiles of OBS, most of which were completed in recent years, and provide these questions mentioned above with some useful information that cannot be inferred from magnetic anomalies or gravity models. It has shown that the middle and eastern part of the northern margin of SCS is of volcanic type with a high velocity layer in the lower crust, the thick high velocity body beneath Dongsha islands in particular, that was interpreted as magma underplating or intruded mantle (Wang et al. 2006; Wei et al., 2011a, 2011b; Yan et al., 2001; Zhao et al., 2010). While no high velocity layer in the lower crust has been found in the OBS seismic models in the western part of the northern margin of SCS (Qiu et al., 2001; Wu et al., 2011). On the southern margin of SCS, it has shown that the middle and eastern parts of Nansha Block have similar tectonic characters with crust thickness of 20 km in continent and 5–6 km in basin without high velocity layer in the lower crust (Pichot et al., 2014; Qiu et al., 2011). Based on the similarity of crustal structure and Moho depth variation, Ruan et al. (2011) supposed Reed Bank and Macclesfield to be a pair of conjugate blocks. Nevertheless, we think there are still two problems in these seismic models. The high velocity layer in the lower crust, found in the northeastern part of the northern margin of SCS, is mainly identified by ray-tracing technique and inversion method but lacks substantial corresponding signals in the OBS seismic sections. The postulated conjugate relationship between Reed Bank and Macclesfield has not been analyzed from the aspect of rock characteristics or from detailed variation of velocity. For these problems, our previous studies (Wei

et al., 2011b; Zhao et al., 2010) have simulated the shear-wave velocity structure of Dongsha area and obtained a low Vp/Vs ratio (1.73–1.78) for the high velocity layer in the lower crust revealed by P-wave model (Wang et al. 2006; Wei et al., 2011a). But for crustal structures in the southern margin of SCS, no shear-wave velocity model has been built from OBS data. In this paper, we present a shear-wave velocity model from a wideangle seismic profile (OBS973-2) crossing Reed Bank in the southern continental margin of SCS (Fig. 1), simulated by 2-D ray-tracing method and based on its previous P-wave model (Ruan et al., 2011) (Fig. 2), and compared it with that of northern margin for understanding the marginal conjugation relationship of SCS. 2. OBS data and P-wave velocity model 2.1. OBS data In May of 2009, R/V “Shiyan 2” completed a wide-angle seismic profile (OBS973-2) in the southern margin of SCS that extends 369 km long in NW-SE direction across the northeastern Reed Bank to the central basin (Fig. 1). Seventeen OBS (three components and one hydrophone) were deployed along this profile in 20 km interval (OBS1 and OBS10 were lost). OBS data were sampled in 4 ms interval. An air gun array of 4 × 24.5 l shot at a pressure of 110 kg/cm2 every 120 s, giving an average shooting interval of 280 m approximately. Bathymetry measurement was also done simultaneously. The processing of OBS data includes correction of shooting time, localization of shooting coordinates, correction of OBS coordinates and time drift, and finally micro-adjust of OBS position by theoretic simulation of direct water wave (Zhao et al., 2010). We use filtering frequency band of 3–15 Hz.

Fig. 1. (a) The distribution of deep seismic experiments in the SCS. The black solid lines represent deep seismic profiles; the yellow box shows the study area. (b) The red line is the profile OBS973-2, the white dots indicate OBS positions along the profile OBS973-2, the red dots indicate the lost OBSs, and the black line stands for multi-channel seismic profile NH973-2.

Please cite this article as: Wei, X., et al., Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.01.006

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Fig. 2. The P-wave velocity mode of the profile 973-2 (Ruan et al., 2011).

Stations OBS2–OBS8 were located at oceanic basin and transition zone with water depth 2000–4000 m, and recorded many seismic signals from deep part and Moho because of quiet environment at deep seabed and the shallowness of Moho discontinuity in this segment. For example, in the seismic record section of OBS3 (Fig. 3a), Pg (refraction wave from crust) is very clear with the offset of approximate 10–35 km in both sides, PmP (reflection wave of Moho) appears near the offset of 30 km

and Pn (head wave of Moho) is also clear with the largest offset of 60 km. Stations OBS9–OBS17 were located on the slope of Reed Bank (water depth of 370–2000 m) with a few of volcanoes and ruptures. Although the buried depth of Moho is deep in this segment, amount of signals such as PmP and Pn were also recognized in the OBS record sections. The converted S-wave signals of three-component OBS are primarily concentrated in the two orthogonal horizontal components.

Fig. 3. The vertical (a) and radical (b) component of OBS3 station, plotted with 8 km/s reduction velocity and 3–15 Hz band-pass filtering. The single-trace waveform of trace 1153 in the vertical (c) and radical (d) component, and particle's moving trajectories of the 1153rd trace in the vertical component (3.9–4.2 s) (e) and the radical component (5.6–6.2 s) (f) are presented, respectively.

Please cite this article as: Wei, X., et al., Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.01.006

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Therefore, the polarization angle that is automatically measured is used to rotate the horizontal components into radical (in-line) and transverse (cross-line) components, shown as Fig. 3b for the radical component of OBS3. 2.2. P-wave velocity model The P-wave velocity model is shown as Fig. 2 (Ruan et al., 2011). The sediment layers are much thinner in general or even absent in some parts and divided into three layers with velocities of 1.8–2.0 km/s, 2.0–2.7 km/s and 3.5–4.0 km/s, respectively. On Reed Bank there are some small volcanoes on the upper crust and P wave velocity increases downward from 5.5–6.4 km/s in the upper crust (9–10 km thick) to 6.6– 7.0 km/s in the lower crust (11 km thick). In the transition zone and oceanic basin P wave velocity increases downward from 5.9–6.1 km/s in the upper crust (4–5 km thick) to 6.6–6.9 km/s in the lower crust (2–4 km thick). The transition zone and oceanic basin have a relatively thinner crust showing some tectonic effects of stretching. The velocity structure of the model indicates that its continental crust is a standard one but its oceanic upper crust is of relatively higher velocity than that of standard one. The seaward rising depth of Moho is 20–22 km in Reed Bank and 9–11 km in the oceanic basin.

Table 1 Numbers of picks, picking error, RMS misfit, χ2 value, conversion mode, and conversion interface (P-S interface) for phases identified along the profile OBS973-2 in the S-wave modeling. OBS stations

Mode

Phases

No. of picks

Uncertainty of picks

RMS misfit(s)

χ2

P-S interface

OBS2

PPS

OBS3

PSS PPS

PPSb PPSg PmSb PPSnb PSSucb PPSb PmSuc PPSnm PSSuc PSSlc PPSb PmSb PPSnb PSSuc PPSb PmSuc PPSnm PSSuc PSSlc PPSb PmSuc PPSnuc PSSuc PPSb PmSb PPSnb PSSuc PPSb PmSb PPSnb PSSucb PPSb PmSb PPSnuc PSSuc PPSb PmSuc PSSuc PPSb PmSb PSSuc PPSb PmSm PSSuc PSSlc PPSb PmSuc PSSuc PPSb PmSuc PSSuc PPSb PSSuc PPSb PmSuc PSSuc

77 50 10 14 55 85 54 31 44 18 48 25 38 32 44 99 17 42 28 66 9 21 38 43 39 74 58 93 85 13 37 97 40 26 33 133 24 32 72 64 59 96 38 26 11 98 14 35 127 26 38 103 17 48 13 9

0.08 0.10 0.07 0.16 0.16 0.10 0.12 0.18 0.18 0.18 0.16 0.12 0.12 0.12 0.14 0.14 0.10 0.18 0.16 0.16 0.10 0.20 0.20 0.10 0.12 0.20 0.10 0.14 0.18 0.08 0.14 0.16 0.16 0.12 0.20 0.12 1.14 0.20 0.12 0.12 0.20 0.16 0.20 0.20 0.20 0.10 0.14 0.18 0.20 0.18 0.14 0.12 0.12 0.12 0.10 0.08

0.163 0.104 0.07 0.186 0.193 0.107 0.135 0.193 0.198 0.183 0.170 0.136 0.163 0.128 0.154 0.145 0.090 0.197 0.175 0.192 0.082 0.202 0.255 0.166 0.165 0.215 0.106 0.163 0.183 0.095 0.173 0.176 0.163 0.121 0.194 0.130 0.147 0.208 0.162 0.158 0.221 0.184 0.246 0.231 0.142 0.115 0.181 0.193 0.202 0.193 0.148 0.171 0.123 0168 0.123 0.83

1.366 1.020 1.245 1.568 1.808 1.087 1.233 1.164 1.174 0.993 1.158 1.337 1.911 1.153 1.891 1.287 0.594 1.984 1.128 1.695 0.880 1.665 1.931 1.953 1.941 1.86 1.147 1.379 1.116 1.123 1.852 1.954 1.102 1.146 1.421 1.164 1.042 1.056 1.822 1.744 1.396 1.243 1.565 1.432 0.762 1.117 1.807 1.135 1.009 1.124 1.018 1.607 1.008 1.541 1.256 1.106

Basement Basement Basement Basement Basement Basement Upper crust Moho Seafloor Seafloor Basement Basement Basement Seafloor Basement Upper crust Moho Seafloor Seafloor Basement Upper crust Upper crust Seafloor Basement Basement Basement Seafloor Basement Basement Basement Basement Basement Basement Upper crust Seafloor Basement Upper crust Seafloor Basement Basement Seafloor Basement Moho Seafloor Seafloor Basement Upper crust Seafloor Basement Upper crust Seafloor Basement Seafloor Basement Upper crust Seafloor

PSS OBS4

PPS

OBS5

PSS PPS

PSS OBS6

PPS

OBS7

PSS PPS

OBS8

PSS PPS

OBS9

PSS PPS

OBS11

PSS PPS

OBS12

PSS PPS

OBS13

PSS PPS

3. Identification of S-wave phases and modeling S-wave velocity structure 3.1. Identification of S-wave phases To identify and pick the converted S-wave arrivals are critical to constructing S-wave velocity and Vp/Vs ratios. Air gun shooting in water produces only P-wave, the S-wave signals observed by OBS, when incident angle is large enough, are P-to-S converted waves through some physical discontinuities beneath seafloor such as sediment basement or Moho that have prominent velocity and density contrast on two sides (Digrances et al., 1998). There are two P-to-S conversion modes named PPS and PSS respectively based on the downward versus upward directions associated with the incident wave conversion. The PPS arrivals have been P-to-S converted on the way up to the OBS, the PSS arrivals have been P-to-S converted on the way down (Fig. 4) and the latter has more S-wave signals but slower apparent velocity (Kodaira et al., 1996; Mjelde et al., 2007). Identification of S-waves was based on their kinematics and dynamic features such as travel time (Fig. 3a–b), apparent velocity and particle motion (Fig. 3e–f). Here we have identified 2054 PPS mode converted S-wave phases and 612 PSS mode converted S-wave phases in fifteen OBS's record sections (Table 1) by using the same scheme adopted by Zhao et al. (2010). Figs. 5–7 give some examples of the identified converted S-wave phases from OBS record sections. Various kinds of converted S-wave phases of PPS mode were identified at station OBS5 with maximum offset of 63 km in southeast and 87 km in northwest (Fig. 5), respectively. PPSb is converted from Pg at the basement in PPS mode and then propagate upward to station; PmSuc is converted from PmP at upper crust in PPS mode in the range of offset of 50–70 km in northwest and 30–50 km in southeast; PPSnm is converted from Pn at Moho discontinuity in PPS

PSS OBS14

PPS

OBS15

PSS PPS

OBS16 OBS17

PSS PPS PSS PPS PSS

Fig. 4. Sketch describing two types of P-S conversion modes of PPS and PSS. Solid lines represent P wave ray, and dotted lines represent S wave ray.

Please cite this article as: Wei, X., et al., Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.01.006

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Fig. 5. (a) Radical component data of OBS5, plotted with 8 km/s reduction velocity and 3–15 Hz band-pass filtering. (b) Comparison of interpreted PPS and PSS modes (hatched lines) and model-derived travel-time curves (solid lines) for OBS5. (c) Model-derived ray paths for the calculated PPS arrivals. (d) Same as (c) for the PSS arrivals, solid lines denote P-waves, and dotted lines denote S-wave in (c) and (d).

mode with the maximum offset of 87 km in northwest; PSSuc and PSSlc are converted at seafloor from Pg through upper and lower crust respectively in PSS mode. The identified converted S-wave phases at station OBS3 include PPSb, PmSuc, PPSnm in PPS mode and PSSuc, PSSlc in PSS mode (Fig. 6). The identified converted S-wave phases at station OBS15 include PPSb and PmSuc in PPS mode and PSSuc in PSS mode (Fig. 7). All of these S-wave phases are re-determined and assigned to suitable interfaces during inversion. 3.2. Simulation approach The previous P-wave velocity model (Fig. 2) has enabled refinement of interfaces and velocities extending from several sedimentary layers down to Moho, it consists of sea water, sediment (three layers), upper crust, lower crust and upper mantle (Ruan et al., 2011). S-wave simulation involves determining the location of P-to-S conversion for each arrival to estimate S-wave velocity (expressed as the Poisson's ratio in the RayInvr software) and Vp/Vs ratios (Zelt and Smith, 1992) for different layers and blocks, using the same modeling software

as for P-waves. During the simulation, the P-wave velocity structure was used as the initial model, and the interface were kept constant as the conversion interface. Initially, we assumed a mean Poisson's ratio of 0.25 (i.e., Vp/Vs = 1.732) and tried to identify potential S-wave phase locations and conversion interfaces. We then identified and picked S-wave seismic phases on the seismograms, and tested the new Vp/Vs ratios and conversion interfaces by using 2-D forward raytracing method (Zelt and Smith, 1992), during this stage, the conversion interface and Poisson's ratio were changed by trail-and-error ray-tracing to improve agreement between calculated and observed travel time. The modeling exercise was done from sediment to mantel and one station by one station by repeatedly using forward simulation, and finally all arrivals of fifteen stations were calculated together. 3.3. S-wave structure, Vp/Vs ratios, and implications for lithology 3.3.1. Oceanic basin (model distance of 190–370 km) The resulting 2-D S-wave velocity structure (Fig. 8a) and Vp/Vs structure (Fig. 8b) show that the S-wave velocities increase with

Please cite this article as: Wei, X., et al., Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.01.006

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Fig. 6. (a) Radical component data of OBS3, plotted with 8 km/s reduction velocity and 3–15 Hz band-pass filtering. (b) Comparison of interpreted PPS and PSS modes (hatched lines) and model-derived travel-time curves (solid lines) for OBS3. (c) Model-derived ray paths for the calculated PPS arrivals. (d) Same as (c) for the PSS arrivals, solid lines denote P-waves, and dotted lines denote S-wave in (c) and (d).

depth, similar to the P-wave velocities. The velocities in sediment layers vary downward from 0.7 km/s to 2.0 km/s and Vp/Vs ratios are estimated at 2.18–4.10, this relatively high Vp/Vs ratio may indicate the presence of unconsolidated sediment and high porosity. The S-wave velocity in the upper crust is 3.2–3.6 km/s and Vp/Vs ratios are estimated at 1.82–1.85; the S-wave velocity in the lower crust varies from 3.6 km/s to 3.9 km/s and Vp/Vs ratios are estimated at 1.76–1.81.

3.3.2. Reed Bank (model distance of 0–190 km) The velocities in sediment layers are also increasing downward from 0.8 km/s to 2.1 km/s with the Vp/Vs ratio varies from 2.3 to 3.8, this value may indicate the presence of high porosity rock such as carbonate that has been identified at the slope of Reed Bank (Franke, 2013). The Swave velocity in the upper crust is 2.5–3.6 km/s with the average Vp/Vs ratio of 1.75 except the abnormal zone (1.78–1.82), indicating the continental crust is of granodiorite.

The velocity in the lower crust is 3.6–4.0 km/s and Vp/Vs ratios are estimated at 1.76–1.80, which is similar to the western side of the Jan Mayen ridge, North Atlantic, where the Vp/Vs ratios is 1.75–1.80, suggests an intermediate composition crust and non-volcanic margin (Mjelde et al., 2007). 3.4. Uncertainties of the S-wave model Utilizing the P-wave model geometry as a starting model has reduced the uncertainty in the final S-wave velocity model since the quality of the P-wave data is generally better than that of the Swave data, and the interfaces in the P-wave model are generally well constrained (Kodaira et al., 1996; Mjelde et al., 2007). We then identified and picked S-wave seismic phases on the seismograms, and calculated and estimated their associated uncertainties generally to be within 80–200 ms. We obtain the limit range of Poisson's ratio (± 0.01–0.04) by keeping the agreement between

Please cite this article as: Wei, X., et al., Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.01.006

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Fig. 7. (a) Radical component data of OBS15, plotted with 8 km/s reduction velocity and 3–15 Hz band-pass filtering. (b) Comparison of interpreted PPS and PSS modes (hatched lines) and model-derived travel-time curves (solid lines) for OBS15. (c) Model-derived ray paths for the calculated PPS and PSS arrivals, solid lines denote P-waves, and dotted lines denote S-wave in (c).

calculated and observed travel times with the picking uncertainty. Based on the p-wave model, variations of ± 0.01–0.04 in Poisson's ratio yield uncertainties of ± 0.01–0.2 km/s and ± 0.03–0.16 in calculated S-wave velocity and Vp/Vs ratios, respectively. Fig. 9 shows the variations of ± 0.05 in Poisson's ratio lead to the uncertainties of 200 ms between calculated and observed travel times. We also used the chi-squared (χ2) value to weigh the mismatch between the observed and calculated arrival times (Zelt and Smith, 1992) and calculated rays coverage to estimate the reliability of the model. After S-wave modeling, the fitting errors (root mean square misfits, RMS) are less than 190 ms and χ2 values are less than 2 of most Swave phases (Table 1). The ray coverage for the main S-wave crustal layers and upper mantle in the final model of S-wave crustal structure and Vp/Vs ratios (Fig. 8c) shows that S-wave velocity is well covered within the crust and most interfaces are covered 10–40 times, indicating high reliability and resolution of the model.

4. Discussion 4.1. The crustal characters of Reed Bank and vicinity area The S-wave velocity model and Vp/Vs ratios derived from the OBS data together with the P-wave velocity model yield valuable insights into the crustal structure of Reed Bank along profile OBS973-2. The Swave velocity in the lower crust is 3.6–4.0 km/s, Vp/Vs ratios are estimated at 1.75–1.80, and no high velocity layer is found, implying a non-volcanic crust without obvious magma underplating during rifting process.

Moreover, it is shown that the upper crust S-wave velocity of continental Reed Bank is 2.5–3.6 km/s with the average Vp/Vs ratio of 1.75 except the abnormal zone (1.78–1.82). The relatively low S-wave velocity accompanied by the striking high Vp/Vs ratio may indicate the presence of faults in the upper crust (Ding and Li, 2011), in agreement with the low velocity zone in the 1D P-wave model simulated by Niu et al. (2014). This velocity abnormal zone corresponds to a series of half-garbed structure and some deep faults cutting through sediment base even the lower crust revealed by a multi-channel seismic profile parallel close to the OBS line (Ding and Li, 2011). It indicates that Reed Bank area has been being under tectonic extension during rifting period until to present. 4.2. The conjugation relationship between Reed Bank and Macclesfield Massif The southern margin of the SCS has been postulated to separate gradually in Cenozoic from the South China continent and moved southward to the present position during the spreading and formation of the SCS basin, based on some lithology and geophysical data. The drill of Sampaguita-1 in Reed Bank met marine sand and shale rock of 600 m of the Early Cretaceous (Taylor and Hayes, 1983). Kudrass et al. (1986) suggested the continental crystalline basement of Nansha islands are similar with that of Macclesfield Block and Xisha islands based on dredged rock samples in SCS. The further analysis of these dredged rock samples showed that the Mesozoic rocks were from the South China and belonged to the southern continental margin before seafloor spreading (Sales et al., 1997). Based on the geological and geophysical

Please cite this article as: Wei, X., et al., Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.01.006

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Fig. 8. (a) S-wave velocity model of profile OBS973-2. (b) Vp/Vs ratio along profile OBS973-2. (c) Ray density and distribution of profile OBS973-2 (calculation grid: 1 km × 0.5 km).

data of the Sino-German joint survey of SCS in 1989, Li (1997) indicated that the Cenozoic structure evolution of SCS developed and rifted from the north to the south, and Macclesfield Massif, Xisha islands, Nansha islands and the northern Palawan had good cognate relationships with one another and experienced the same magmatism–metamorphic events from the Late Jurassic to the Early Cretaceous. Physical simulation showed that Reed Bank is a kind of rigid block with weak deformation in Cenozoic similar to that of Macclesfield Massif and Xisha Block (Sun et al., 2009). However, the concrete conjugate points or boundaries in the northern margin with the southern margin have not been clear, and the geometric similarity of the two margins is disharmonic with the magnetic anomalies distribution of SCS. The most popular viewpoint from Yao (1996) suggests that Reed Bank is conjugate with Dongsha Block mainly

based geophysical field especially the magnetic anomaly, assuming the existence of magnetic anomaly 13–18 (42–35 Ma) in N-E direction around the south of Zhongnan seamount chain and using a large fault in N–S direction (named Zhongnan fault) to separate the southwestern sub-basin from the eastern sub-basin with magnetic anomaly 5d-11 (32–17 Ma) in EW direction. But based on the basin magnetic anomaly (Barckhausen and Roeser, 2004) and magnetic crystalline basement (Hao et al., 2011), some authors postulated that Reed Bank broke-up from Macclesfield around 25 Ma. Here we intend to discuss the conjugate relationship of SCS margins mainly through comparison of crustal velocities derived from OBS profiles in the northern margin with that in the southern margin. In our previous study, the comparison of P-wave velocity models between profile OBS973-2 and profile OBS2006-1 in the west section of the

Fig. 9. The comparison of calculated and observed travel times, the yellow ditched lines represent observed travel times, and the solid lines represent calculated travel times.

Please cite this article as: Wei, X., et al., Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.01.006

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Fig. 10. The relations of Vp, Vs, and its lithology.

northern margin showed much more similarity (Ruan et al., 2011). Moreover, the comparison of Vs and Vp/Vs in the lower continental crust between profile OBS2001 (Zhao et al., 2010) and profile OBS2006-3 (Wei et al, 2011b), both located at the east section of the northern margin (Dongsha islands), also showed much more similarity with mafic composition, which is obviously different from the intermediate composition inferred from the profile OBS973-2 (Fig. 10). Therefore we doubt the existence of conjugate relationship of Reed bank with Dongsha islands. In contrast, Reed Bank and Macclesfield have similar P velocity structures (Ruan et al, 2011), we are in agreement with the point of view that Reed Bank and Macclesfield separate from the same Continental Plate during the rifting and break-up process (Braitenberg et al., 2006). 4.3. Oceanic crust composition The upper crust with S-wave velocity of 3.2 to 3.6 km/s and Vp/Vs ratios of 1.82–1.85 can be interpreted in terms of gabbroic composition. We also infer the lower crust with S-wave velocity of 3.6 to 3.9 km/s and Vp/Vs ratios of 1.76–1.81 to be gabbroic composition, since hightemperature melt would increase Mg content at the expense of Fe (White and McKenzie, 1992) and reduce the Vp/Vs ratio (Christensen, 1996), which is consistent to the existence of high thermal anomaly in the oceanic basins of the SCS (He et al., 1998; Shi et al., 2003). 5. Conclusions From the S-wave velocity model and Vp/Vs ratio along Reed Bank and vicinity oceanic basin obtained by 2D ray-tracing approach, we draw the following conclusions: (1) In the deep oceanic basin, the velocities in sediment layers vary downward from 0.7 km/s to 2.0 km/s and Vp/Vs ratios are estimated at 2.18–4.10. The relatively high Vp/Vs may indicate the presence of unconsolidated sediment. The velocity in the upper crust is 3.2–3.6 km/s and Vp/Vs ratios are estimated at 1.82–1.85. The velocity in the lower crust is 3.6–3.9 km/s and Vp/Vs ratios are estimated at 1.76–1.81. (2) In the shelf and continental of Reed bank, the Vp/Vs ratio decreases with depth through the sedimentary layers, attributed

to increased compaction and consolidation of the rocks. The upper continental crust has relatively high Vp/Vs ratio (1.75– 1.82) compared with the other parts, in agreement with the lower P-wave velocity in the previous P-wave model and normal faults revealed by MCS data, indicating a strong regional extensional movement had occurred during the formation process of the SCS at the Reed Bank area. (3) The S-wave structures indicate Reed bank crust has different rock compositions from that in the northeastern section of the northern margin, denying the presence of conjugate relationship of Reed bank with Dongsha islands. According to P-wave models and other data, we inferred that Reed Bank and Macclesfield were separated from the same Continental Plate during the rifting and break-up process. Acknowledgments We are grateful to the scientists who joined cruise and the crew of the R/V “Shiyan 2”. The seismic experiment and our research are supported by the National Natural Science Foundation of China (grants 41406052, 41176046, 91228205 and 41276049), the National Basic Research program of China (grant2012CB417301) and Scientific Research Fund of the Second Institute of Oceanography, State Oceanic Administration (JG1413). We used the RayInvr code (Zelt and Smith, 1992) for seismic inversion. Some figures were plotted using GMT (Wessel and Smith, 1995). And we really appreciate Hans Thybo and two anonymous reviewers for their constructive comments. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.tecto.2015.01.006. References Barckhausen, U., Roeser, H.A., 2004. Seafloor spreading anomalies in the South China Sea revisited. Continent-Ocean Interactions Within East Asian Marginal Seas, AGU Chapman Conference, San Diego, CA, ETATS-UNIS (11/2002). 149, pp. 121–125. Barckhausen, U., Engels, M., Franke, D., Ladage, S., Pubellier, M., 2014. Evolution of the South China Sea: revised ages for breakup and seafloor spreading. Mar. Pet. Geol. 58 (B), 599–611.

Please cite this article as: Wei, X., et al., Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.01.006

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Please cite this article as: Wei, X., et al., Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.01.006