Crustal structure and magmatic evolution in the Pearl River Delta of the Cathaysia Block: New constraints from receiver function modeling

Journal Pre-proof Crustal structure and magmatic evolution in the Pearl River Delta of the Cathaysia Block: New constraints from receiver function modeling

Haibo Huang, Hou Xiong, Xuelin Qiu, Yuhan Li PII:

S0040-1951(20)30048-2

DOI:

https://doi.org/10.1016/j.tecto.2020.228365

Reference:

TECTO 228365

To appear in:

Tectonophysics

Received date:

6 September 2019

Revised date:

9 January 2020

Accepted date:

30 January 2020

Please cite this article as: H. Huang, H. Xiong, X. Qiu, et al., Crustal structure and magmatic evolution in the Pearl River Delta of the Cathaysia Block: New constraints from receiver function modeling, Tectonophysics(2020), https://doi.org/10.1016/ j.tecto.2020.228365

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© 2020 Published by Elsevier.

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Crustal structure and magmatic evolution in the Pearl River Delta of the Cathaysia Block: New constraints from receiver function modeling Haibo Huang1,3,4, Hou Xiong2, Xuelin Qiu1,3,4,5, and Yuhan Li1,5 1

CAS Key Laboratory of Marginal Sea Geology, South China Sea Institute of

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Oceanology, Chinese Academy of Sciences, Guangzhou, China Seismic Station of SZN (Shenzhen), China Earthquake Administration, Shenzhen,

South Marine Science and Engineering Guangdong Laboratory (Guangzhou),

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China

Innovation Academy of South China Sea Ecology and Environmental Engineering,

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Guangzhou, China

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Chinese Academy of Sciences, Guangzhou 510301, China College of Earth and Planetary Sciences, University of Chinese Academy of Science,

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Beijing, China

Corresponding author: Xuelin Qiu ([email protected])

Key Points:  Combination of receiver function modeling and H-k stacking can reveal more reliable estimates of the Vp/Vs ratio.  Average Vp/Vs ratio of 1.70-1.74 and P-wave velocity of 6.4-6.8 imply a felsic-intermediate and strongly deformed crust beneath the PRD.  Low-volume magma was transported rapidly from the mantle-derived underplating to surface through the Lianhuashan Fault zone. 1

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Abstract We utilized receiver functions to study the crustal structure beneath the Pearl River Delta in the southwestern part of the Cathaysia Block, which was characterized by widespread tectono-magmatism during the late Mesozoic. Combination of waveform modeling and H-k stacking was used to derive crustal thickness, Vp/Vs ratio

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and average P-wave velocity. The results show that the crustal thickness ranges from 26 km in the center of the Pearl River Delta to 32 km in the periphery; the Vp/Vs ratio

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is between ~1.70 and 1.74. The average P-wave velocity ranges from 6.4 to 6.8 km/s

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and shows higher value beneath the islands along the Lianhuashan Fault zone. A

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weakly positive correlation between the crustal thickness and Vp/Vs ratio is observed

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in southwestern Pearl River Delta, indicating depth-dependent extension of the

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continental crust, that is, ductile lower crust was stretched to a much higher degree in comparison with the brittle felsic upper crust. The crust in the northeastern Pearl

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River Delta may be more basaltic owing to underplating caused by the subduction of the Paleo-Pacific Plate, and thus a relatively high Vp/Vs ratio is observed there. The overall low crustal Vp/Vs ratio and the relatively higher crustal P-wave velocities beneath the Lianhuashan Fault zone may imply a felsic-intermediate composition and high metamorphic grade of the crust along the fault. The Lianhuashan Fault zone may provide a pathway for rapid magmatic transportation from the underplating beneath the crust to the surface.

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1 Introduction

The Pearl River Delta (PRD) is known as a delta composed of faulted blocks in the Cathaysia Block (Fig. 1), with the basement cut by a set of faults striking east-west, northwest and northeast (Fig.2) [Tang et al., 2011]. Late Mesozoic igneous

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rocks crop out over 39% of the Cathaysia Block, and occurred as a result of episodic tectonism between the Pacific and the Eurasian plates (Fig. 1) [Zhou and Li, 2000;

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Zhou et al., 2006]. Previous geochemical and isotopic studies showed mingling

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sources of the magmatic components from crust and mantle [Li, 2000; Zhong et al.,

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2017; Zhou et al., 2016]. However, the evolution of the multi-phase igneous rocks is

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still enigmatic, and various geodynamic mechanisms have been proposed: the

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‘Alps-type’ continental collision mode [Hsu et al., 1990], subduction and plate rollback mode [Zhou and Li, 2000] and lithosphere extension-breakup mode [Gilder

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et al., 1996; Li, 2000]. Despite the above controversies, the Paleo-Pacific plate subduction and underplating of basaltic magmas are widely accepted as the dominant causes of the intensive tectono-magmatism in the Cathaysia Block [Li and Li, 2007; Xia and Zhao, 2014]. Geophysical surveys support the close relationship between lithospheric structures and the overlying volcanic formations. Xia and Zhao [2014] proposed, based on three-dimensional seismic tomography in the onshore-offshore area of Hong Kong in the south of the PRD, that the tilted magma conduits are connected with a localized high-velocity anomaly in the lower crust, which represents magmatic 3

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underplating. In addition, two-dimensional seismic surveys across the PRD revealed a low-velocity layer on top of the lower crust [Cao et al., 2014; Lv et al., 2018; Zhang et al., 2018], which was also observed in other places along the coastal and offshore regions of the Cathaysia Block [Huang et al., 2011; Liu et al., 2011; Zhao et al., 2007]. The low-velocity layer in the PRD is explained as partial melting in the middle crust

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and acts as a weak zone [Zhang et al., 2018]. Intensive shearing structures in the settings of orogenic and continental extension usually cause high anisotropy and

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oriented deformation of the crust-mantle minerals [Eilon et al., 2014]. This tectonic

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process also gives rise to an apparent drop in seismic velocity [Copley and McKenzie,

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2007], which is undistinguishable from the effect of the magmatism. Teleseismic

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waves are observed with direction-dependent velocities when passing through the

magmatic injections.

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deformed crust and mantle, thus providing information on the shearing structures and

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Although the localized P-wave velocities have been studied in the PRD as mentioned above, detailed S-wave structures have remained largely unexplored. In this study, we use the receiver function analysis for the Guangdong Province of China to investigate the crustal structures directly beneath the stations. The receiver function utilizes the converted S-wave from the incident teleseismic P-wave arriving at velocity interfaces beneath the recording station [Langston, 1979]. By analyzing the crustal structures estimated using receiver functions, we may infer the existence of internally deformed structures and the corresponding magmatic intrusion and/or partial melts. We also investigate the Poisson’s ratio and the P-wave velocity of the 4

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crust by searching and stacking the converted phases in the receiver functions. Finally, we provide new insights into the evolution of the Late Mesozoic magmatism and possible relationship between the fault system and the magmatic intrusions beneath the PRD.

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2 Geological and tectonic setting

The PRD has been affected by the Paleo-Pacific plate subduction, back-arc

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rifting and seafloor spreading of the South China Sea since the Mesozoic [Zhou and

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Li, 2000]. Extensive tectonic compression has formed prominent fold belts such as the

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Nanling-Wuyi orogenic zones in the northeastern PRD (Fig. 1). The subsequent

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continental extension was accompanied by upwelling of mantle materials that strongly

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modified the proto-accreted crust. Large-scale volcanism in these areas were caused by magmatic injection or underplating at the base of the stretched crust [Tao et al.,

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2017]. Isotope studies also show that the volcanic rocks are related to the lithospheric extension especially in the western PRD [Geng et al., 2006]. Asthenospheric upwelling and the corresponding episodic eruptions of the granitic magmatism has led to the widely distributed ore deposits in this area [Zhou et al., 2016]. The basement of the PRD has experienced several periods of structural movements since the Cretaceous [Yao et al., 2013; Zheng et al., 2017]. The dominant NE-striking faults have controlled the regional topography, and a large amount of Mesozoic intermediate-acid volcanic has been found along the faults. The NWstriking faults are supposed to be formed in late time, and they were more active to 5

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cause intensive neotectonics in the PRD [Yao et al., 2013]. The NE-trending Lianhuashan Fault zone is located south of the PRD (Fig. 2), and it may connect with the Zhenghe-Dapu Fault zone and the Nanao-Changle Fault zone (Fig. 1) [Li et al., 2005], both of which are related to the subduction of the paleo-Pacific plate. The boundary faults defining this fault zone have been reactivated periodically, where the

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crust is highly fractured by both dextral and sinistral strike-slip movements [Zhong et al., 2017]. Located northeast of the Lianhuashan Fault zone, Hong Kong is covered

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by acidic lava and intruded by episodes of magmas passing through the existing faults

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[Sewell et al., 2013; Xia and Zhao, 2014]. The Mesozoic magmatic suites had a

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mantle source and were contaminated by Late Archaean and Mesoproterozoic crustal

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components. A major crustal discontinuity along the axis of the Lianhuashan Fault

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Zone has promoted the passage of the magmas to the surface [Darbyshire and Sewell, 1997]. However, the volcanic suites and sediments have been altered by the later

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collision and regional metamorphism. Previous faults were also reactivated and cut by newly generated ones [Cao et al., 2014], making it difficult to image the primary fault textures. Therefore, the crustal structures needs to be explored carefully to build correlation between the volcanic and the fault distribution at depth.

3 Data

We used teleseismic data recorded at 17 permanent seismic stations operated by the China Earthquake Administration (Fig. 2). Teleseismic waveforms from more than 160 events with magnitudes ≥ 6.0 and with epicentral distances of 30° to 90° were 6

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chosen to estimate receiver functions beneath the stations. The earthquakes mainly occurred along the Kurile and Aleutian island arcs, New Hebrides and Tonga-Kermadec trenches, Indonesia and the Tibet continental collision zone (Fig. 3). The seismograms were manually checked to extract the data with high signal-to-noise ratio. The traces were windowed to include 30 s before and 100 s after the direct

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P-wave arrivals, and then horizontal components were rotated into radial and transverse components. We computed receiver functions using the source equalization

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procedure with a water-level parameter of 0.001 and a Gaussian width of 2.5 [Ammon,

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1991; Langston, 1979]. By introducing a water-level parameter, very small or zero

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values of the denominator in the frequency domain can be filled to avoid corrupted

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deconvolution. The final receiver functions were further checked based on the noise

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level of the signals before the direct P-wave arrivals as well as the shape of the following converted phases. Overall, almost every station has recorded 10 to 50

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receiver functions (Fig. 4). The station SZN, as it has accumulated over ten years of data, has 86 high-resolution receiver functions with almost uniform epicentral distances (Fig. 5). In the following section, we focus on this station to illustrate the modeling details of the receiver functions. Fig. 5 shows the 86 radial and tangential receiver functions for station SZN ordered by back azimuth, which can be grouped into four quadrants including northeast (back azimuth: ~30°-54°), southeast (~110°-170°), southwest (~186°-206°) and northwest (~280°-343°). In all quadrants, the strongest positive phases arrive at 3-4 s after the direct P phases and are advanced by up to 0.5 s in the range of 7

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280°-305°. Beyond the coherent phases at 3-4 s, a series of positive and negative arrivals are either due to interfering multiples of the previous phases or produced by Ps conversions from deeper discontinuities. The azimuth-dependent signals are found in both radial and tangential receiver functions. For example, clear phases at ~2 s can be seen in the northeast quadrant but disappear or change its polarity in the other

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quadrants (Fig.5), indicating presence of anisotropy and/or dipping layers beneath the

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station.

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4 Methodology and results

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4.1 Receiver function analysis and waveform modeling

Receiver functions were isolated from three-component seismograms that record

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converted S-waves when a teleseismic P-wave propagates through the structures

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beneath the seismic station (Fig. 6a). The amplitudes and the relative traveltime between the incident P-wave and the converted S-wave arrivals depend on the ray parameter and the velocity contrasts across the deep boundaries. To determine the velocity structures, we used the time-domain linearized inversion program developed by Ammon et al. [1990], which assumes that the crust/mantle consists of thin, horizontal and isotropic layers of constant thickness. This method requires that the initial velocity model approximates the true structures, and a smoothness constraint is implemented to minimize the roughness of the inverted model. The efficacy of the inversion procedure is demonstrated by synthetic tests from a series of “true” models with variable S-wave velocities (Fig. 6b and Fig. 6c). The P-wave velocity and the 8

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boundary depths were kept constant, while the Vp/Vs ratio was varied from 1.64 to 2.01 for the models. For each noise-free synthetic receiver function, the same starting model with a Vp/Vs ratio of 1.73 was used for the inversion, and only the layer S-wave velocities were inverted. The results suggest that the final inverted models are able to reproduce the “true” models. The depths of the velocity boundaries can be recovered effectively, even though poor waveform fitting appears when the Vp/Vs ratio of the

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starting model is very different from the “true” one. The inversion method can thus be

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applied when we have very little prior information of the Vp/Vs ratio beneath the

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station.

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To enhance the signal-to-noise ratio, the radial and tangential receiver functions

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are usually stacked in narrow bounds of the back azimuths (~10°-20°) and epicentral

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distances (~5°-10°). Fig. 5 shows the stacked receiver functions of station SZN from different back azimuths. The ±1 standard deviation bounds were calculated to assess

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coherence of the individual waveforms. In this study, the starting model for inversion has 2-km thick layers with constant P-wave velocity of 6.0 km/s and Vp/Vs value of 1.73, respectively, within the crust. The Moho depth was initially set with the same value of 40 km for all stations to avoid any prior information. The model dependence was broadly explored by applying a perturbation defined by a cubic polynomial in depth and scaled to the maximum velocity perturbation of 0.9 km/s [Ammon et al., 1990]. The perturbed initial model was then inverted to minimize the misfit between the observed and synthetic receiver functions. We performed all the inversions with a smoothness parameter of 0.1. For each stacked receiver function, a total of 1200 9

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models were generated after perturbation and inversion, and the accepted models were evaluated based on goodness of the fitting and by visual inspection. The maximum fitting degree of all results was found for each receiver function, firstly. Then, the models with the fitting degree higher than 80% and simultaneously approaching 91% of the maximum value were accepted for further analysis. The accepted models were

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then grouped, and for different averaged models with equivalent misfit, the ones with large number of samples and/or sharing common characteristics with adjacent stations

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were kept.

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4.2 S-wave velocity profiles beneath the stations

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The S-wave velocity profile beneath station SZN generally shows three-layer

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crustal structures in all back azimuths (Fig. 7). A low-velocity layer with thickness of ~2 km and S-wave velocity of ~3.3 km/s is located at the crustal top. The upper crust

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shows increasing velocity from ~3.3 km/s on top to ~3.8 km/s at the depths of ~10-13 km. The lower crust has a relative constant velocity of ~3.8-4.0 km/s, but showing localized, back-azimuth dependent variations. For instance, strong velocity contrasts are located at ~22 km depth on the southeast and northwest profiles, and they are correlated to the positive phases at ~2.5 s of the receiver function (Fig. 7). The Moho can be marked by the main velocity boundaries at the depths of ~30-31 km; the velocity contrast across the Moho was recovered mainly by fitting the amplitudes of the Pms and the corresponding multiple phases [Julià, 2010]. Much stronger Pms and especially the multiples can be observed in the receiver functions from the southeast 10

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quadrant (Fig. 7), where the velocity contrast is much higher. In comparison, a much smoother transitional zone of the S-wave velocity at the crustal bottom results in weak amplitudes especially for the multiple phases [Julià, 2010]. This case is obvious for the receiver functions from the azimuth quadrants of northeast and southwest at station SZN, and from azimuth quadrants of northeast and northwest at station HGK

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(Fig. 8). S-wave velocity below the Moho is ~4.5 km/s in average, and a velocity drop is observed especially at the depths of ~40-45 km beneath some stations (Fig. 7-9).

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The inverted S-wave velocity profiles for different back azimuths are plotted

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together to check the lateral heterogeneity of the structures beneath all stations (Fig.

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9). Overall, identical velocity distributions are observed in the crustal structures with

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different back azimuths for most stations, except for some wild variations within

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small thickness ranges that could be attributed to overfitting of noise. However, significant disparities still exist in the upper and middle part of the velocity profiles

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beneath stations XNH, SCD, TIS and HGK. For instance, high velocity anomalies exist right beneath the topmost thin layers in the azimuth quadrants of southeast and southwest beneath station HGK, and they are required for fitting the negative arrivals after the direct P phases (Fig. 8). The structures beneath the Moho show much more variations, which could be mainly related to the fitting of the interferences from the crustal multiple phases.

4.3 Crustal thickness and Vp/Vs ratios

Crustal thickness (H) and Vp/Vs ratio (k) of the Cathaysia Block have been studied 11

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based on receiver functions from events recorded during 2001-2008 [Huang et al., 2014]. However, the predicted average crustal P-wave velocity (Vp) for the stacking influences the results both for H and k (Fig. 10), which inevitably brings bias in interpreting rock properties. We attempt to reassess the previous results as we have accumulated many more high-resolution receiver functions for stations SZN and HGK.

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In addition, since Moho depth can be directly found from the 1-D S-wave velocity profiles, a priori information on H can be provided to search k values. Especially, the

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tests in Fig.6 show that the Moho depth is pretty well constrained by the linearized

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inversion, even if the assumed k deviates from the true value. As the receiver function

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is more sensitive to the velocity contrast between layers than absolute velocity values,

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the average Vp and k beneath stations are not always easily recovered from the S-wave

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velocity profiles. Given this condition, we tried to use the H-k stacking procedure of Zhu et al. [2006] by considering different crustal Vp ranging from 6.0 km/s to 6.9 km/s

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(Fig. 10). For each Vp value, H and k were searched to find the maximum stacking amplitude of the Pms and the multiples phases. The final average Vp and k were estimated when the H found in H-k analysis becomes similar to the value manually picked from the S-wave velocity profiles (Fig. 10). All of the assumed Vp values produce the same maximum stacking amplitude. The results show that the H values from the S-wave velocity profiles are generally greater than those from Huang et al. [2014] by ~1-2 km (Table 1 and Fig. 11a-e), indicating a relatively higher average Vp beneath stations than the hypothetical

value in Huang et al. [2014] (6.2 km/s). However, fluctuations of the H values among 12

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stations are fairly consistent between these two studies (Fig. 11d). The final average Vp shows significant variations and is at or above 6.4 km/s at all stations except station ZHH (Fig. 11a-b and Fig. 12a). In general, the average Vp increases from northwest to southeast and has very high values beneath the islands at both ends of the Lianhuashan Fault zone (Fig. 12a). Crustal thickness decreases toward the center (Fig.

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12b), which may correspond to continental extension and/or asthenospheric upwelling in the basin. Although distribution of the H and k as a function of the average Vp

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suggests no distinct correlation between each other (Fig. 11a-b), we recognize

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synchronous abnormal high and low values of the H and Vp respectively at stations

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HGK and ZHH. The Poisson’s ratio (υ) is calculated based on k and varies in a range

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of 0.23-0.26 (Fig. 11c). The low-intermediate value is consistent with the high-quartz

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contents in the granitic basement mixed by melting from the lower crust in this area but excludes a prominent or large-scale magmatic contamination of the crust from the

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deep mantle. Poisson’s ratio beneath station HGK is ~0.25, which further commits the high average Vp to the crustal nature rather than mantle source of the high-speed materials. The H and k values show zonal distribution and can be separated approximately into two areas taking 114°E as a boundary (Fig. 12b-c). The k values are in the range of 1.72-1.75 on the east side of the boundary, while on the west side most of the values are below 1.72. The higher k values on the east side are in accordance with the results in the coastal area of the Fujian province [Huang et al., 2014], showing an increasing trend from southwest to northeast.

5 Discussion 13

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5.1

Evolution of the continental crust beneath the PRD

According to Huang et al. [2014], the H and k (or Poisson’s ratio ‘υ’) in South China exhibit a variety of correlations in different tectonic divisions, depending on the structural evolution and interactions between shallow and deep lithospheric materials. The tectonic process and the corresponding crustal evolution may have contributed to

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the relations between H and υ as shown in Fig. 11e. The 1-D S-wave velocity profiles

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of stations GZH, HUZ, ZHQ, SHW, DOG and DGD clearly have layered crustal

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structures with different velocity gradients separated by boundaries at ~10~13 km in

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all azimuth quadrants (Fig. 9). Also, these layered structures are observed in the

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velocity profiles from some azimuth quadrants of other stations e.g. the southwest quadrant in station SZN. These stations generally distribute in the northeast and

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peripheral region of the PRD (Fig. 2), where the basement could be attributed to a

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typical or less stretched continental crust. To the southwest and the center of the PRD with decreased crustal thicknesses (Fig.2 and Fig.12), low-velocity layers are observed starting from depths of ~10 km (Fig.9), which is consistent with the previous seismic studies [Lv et al., 2018; Zhang et al., 2018]. These layers can be considered as a possible presence of ductile anisotropy under continental extension [Copley and McKenzie, 2007; Zhou and Li, 2000] or ongoing magmatism caused by high terrestrial heat flow. The decreased k values (1.70~1.72) in these areas do not support the latter hypothesis as crustal melting could lead to a much higher k of more 2.0 [Ji et al., 2009]. Zhang et al. [2002] also proposed that deep-rooted faults and fluid activity actually controlled distribution of the wide-spread hot springs in the 14

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Cathaysia Block, which was considered as related to melting at great depths. Therefore, the low-velocity layers would probably be a result of the ductile shearing under intensive crustal extension and deep-rooted fault activities in the middle-lower crust. The oriented crustal deformation, e.g., rock fracture and inclined Moho, could also account for the azimuthally dependent receiver functions and crustal profiles, and

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it needs to make further study. According to the seismic surveys and compilations of laboratory velocity data of

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the crustal rocks [Christensen and Mooney, 1995; Ranalli and Murphy, 1987], the

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typical continental crust has layered structures including the felsic upper crust and the

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middle-lower crust with more intermediate-mafic components. Correlation between H

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and υ can thus provide a useful constraint on the tectonic evolution of the continental

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crust. In this study and previous results, the H and υ tend to be positively correlated (Fig. 11e and Fig.12). The station SZN shows a pair of H and υ values more consistent

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with this correlation than the results in Huang et al. [2014]. As suggested by Ji et al. [2009], greater thinning of the ductile lower crust than the upper crust causes decreased Poisson’s ratio and H value, therefore H and υ are positively correlated. This type of lower-crust thinning has been related to ductile flowing in foldbelts [Gerbault et al., 2005] and passive margins [Clerc et al., 2015]. Since the Mesozoic, the crust beneath the PRD has been compressed by the Pacific plate and was further stretched in an extensional tectonic setting. Due to the crustal thickening and magmatic activity, viscosity of the lower crust could be reduced to levels at which flow can occur [Afonso and Ranalli, 2004; McKenzie et al., 2000]. However, 15

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influences from localized volcanism and fault activities cannot be excluded, as there are still some abnormal values such as the station HGK.

5.2 Crustal composition and magmatism evolution in the PRD

Magmatic addition is an inevitable consequence of the lithosphere thinning and

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plate subduction, and it has also affected the tectonic evolution of the crust in the PRD. Petrologic and geochemistry analyses of the volcanic samples have demonstrated that

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basaltic underplating is widespread in the Cathaysia Block since the Late Mesozoic

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[Zhou and Li, 2000]. The average crustal Vp of 6.46 km/s is very close to the global

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mean of cratons summarized by Christensen and Mooney [1995], and also agrees with

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seismic surveys [Lv et al., 2018; Xia et al., 2010]. Higher values are only observed

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close to or beneath the islands along the Lianhuashan Fault zone e.g. stations HGK, SCD and DGD, and they are supposed to be related to intrusion of deep-seated molten

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materials, which is also ubiquitously observed in isotopic studies [Musacchio et al., 1997]. This agrees with the localized high-velocity anomaly in the lower crust of Hong Kong [Xia and Zhao, 2014], where the solidified basaltic underplating is supposed to feed the volcanic intrusion and eruptions. Isotope signatures of the Late Mesozoic granites also indicate that the mantle-derived basaltic magmas have interacted with the whole crust beneath Hong Kong [Fletcher et al., 1997]. In general, magmatic underplating and ascending of the basic magmas usually results in higher velocities and Poisson’s ratios for the crust [Gallacher and Bastow, 2012]. Nevertheless, the lower k values or Poisson’s ratio (~0.24 in average) presented in this 16

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study are more akin to the measurements from the Mesozoic-Cenozoic granitic regions of active or recent tectonics worldwide [Christensen and Mooney, 1995; Zandt and Ammon, 1995], showing that the crust of the PRD is dominated by felsic composition. If the magmatic addition had occurred widely during orogenesis, a delamination-type process must have removed the mafic component or little new crust

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was added by mantle-derived magmatism [Zandt and Ammon, 1995]. Trace-element geochemistry of the mafic dikes in the northern PRD shows characteristics of

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within-plate basalts, which formed with little crustal contamination [Li, 2000]. Thus,

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another possibility is that the basaltic magmas are low in volume and have

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experienced only small amounts of crustal fractionation because of rapid

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transportation from the crustal bottom to surface. A similar case has been found along

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central West Africa where intra-plate volcanism is also dominant [De Plaen et al., 2014; Gallacher and Bastow, 2012]. The difference is that clear evidence is found for

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magmatic underplating/lower-crustal intrusions at the crustal bottom in the Hong Kong area, and the lower-crustal melts are the main sources of extruded volcanoes. According to high precision U-Pb zircon ages [Sewell et al., 2013], there are four episodes of the “flare-ups” to form the plutons in Hong Kong. The extremely high average crustal Vp here is mainly contributed by the high velocity layer on top of the 1-D S-wave velocity profiles from the south (Fig. 8; southeast and southwest quadrants). We suggest that this high velocity layer represents the mafic-rich volcanic pile accumulated on top of the crust after multiple rapid eruptions. The Lianhuashan Fault zone has experienced multi-phase metamorphism since 17

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the Mesozoic and acted as a large-scale shear zone during subduction of the paleo-Pacific Plate. It is also related to a NE-trending gravity anomaly along the coast region as shown in Fig. 2b. Compositional characteristics of primary biotite in the Lianhuashan tungsten deposit indicate that the ore-forming materials were derived mainly from the mantle [Tan, 1985]. It has been found that the large Late-Mesozoic

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calderas of Hong Kong are mostly located within the Lianhuashan Fault zone [Xia and Zhao, 2014]. Seismic surveys reveal that this fault zone is divided into different

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segments by NW-striking faults [Zhong et al., 2017]. We thus suggest that the highly

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fractured crust along the Lianhuashan Fault zone has provided a “high-speed” channel

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for migration of the materials from the molten lower crust and underplating magmas

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(Fig. 13). The underplated mantle-derived basalt has also provided the heat to produce

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6 Conclusions

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the granitic magmas to form the islands along the Lianhuashan Fault zone.

In this study, we processed teleseismic waveforms recorded at 17 permanent seismic stations in the PRD. Receiver function method was used to model the shear-wave velocity structures beneath the stations. Crustal thicknesses and Vp/Vs ratio were also calculated based on a modified H-k stacking procedure to estimate the rock properties of the crust. The crustal structures are related to the widespread tectono-magmatism and fault activities in the study area. The summaries of our conclusions are as follows. (1) Internally deformed structures and velocity boundaries beneath the PRD can 18

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be imaged closely by the receiver function modeling. Combination of the waveform modeling and H-k stacking is able to obtain more reliable estimates of the Vp/Vs ratio and average P-wave velocity. (2) Crustal thicknesses and Poisson’s ratios in the PRD were affected by multi-phase faults and granite/volcanism, driven by subduction of the Paleo-Pacific

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plate and the continental extension. The crust thickness ranges from 26 km to 32 km with the lowest value in the center of the PRD. The Poisson’s ratio is, in general,

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lower than 0.26, indicating felsic-granite dominated components of the crust in this

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region. Low volume, high-pressure magmas have experienced rapid migration from

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the basaltic underplating beneath the crust to the surface, resulting in low-grade

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crustal fractionation from mantle-derived magmas. The Lianhuashan Fault zone has

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highly fractured crust extending to the Moho, and thus provides a pathway for the rapid migration of the magmatic melt. Higher average Vp is measured along the

crust.

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Lianhuashan Fault zone, representing volcanic intrusion originated from the lower

(3) Correlation between the crustal thickness and Poisson’s ratio is a measure of both depth-dependent extension and magmatic addition. Weaker and flowing lower crust was stretched to a much higher degree than the brittle felsic upper crust. The low-velocity layer in the middle-lower crust may indicate ductile shearing due to flow of the middle-lower crust, inducing anisotropy. Continental extension mainly developed in the southwest of the PRD where much thinner crust was observed. The thicker crust with an increased Poisson’s ratio in northeast of the PRD is possibly 19

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related to later-stage Cretaceous volcanism, indicating a greater extent of lithospheric

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thinning and increase in mantle-derived magmas.

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Acknowledgments Teleseismic waveforms in this study were provided by China Earthquake Networks Center. We would like to thank Lupei Zhu and Charles J. Ammon for providing free program codes for receiver function modeling and stacking. We thank

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Sung-Joon Chang for improving the language of this manuscript. This study is funded by Natural Science Foundation of China (41676045, 41676042, 41676063 and

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41676043), Earthquake monitoring-prediction-scientific research project of CEA

Science

and

Engineering

Guangdong

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Marine

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(CEA-JC/3JH-173901), Key Special Project for Introduced Talents Team of Southern Laboratory

(Guangzhou)

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(GML2019ZD0204) and Guangdong NSF research team project (2017A030312002).

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All original figures are produced using the Generic Mapping Tools (GMT) software [Wessel and Smith, 1995]. The data discussed in the manuscript are freely available at

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the link http://dx.doi.org/10.17632/t8896tjjgp.1 (DOI: 10.17632/t8896tjjgp.1).

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Longitude

Latitude

Vp (km/s)

H (km)

∂H

k

∂κ

DGD

114.303

22.0602

6.60

29.739

1.7363

1.719

0.0707

DOG

113.723

22.8750

6.40

27.766

1.1429

1.713

0.0531

GZH

113.652

23.6525

6.50

32.015

1.3561

1.719

0.0623

HGK

114.142

22.2776

6.80

31.328

1.3210

1.724

0.0543

HUD

113.227

23.5164

6.40

27.973

1.1541

1.690

0.0556

HUZ

114.424

23.2424

6.50

1.0740

1.723

0.0493

SCD

112.796

21.7112

6.60

28.191

1.5596

1.709

0.0705

SHW

115.370

22.7916

6.40

28.950

1.1628

1.738

0.0552

SLG

113.347

23.0911

6.40

28.030

1.1658

1.740

0.0566

SZN

114.133

22.5330

6.40

29.951

1.1297

1.747

0.0560

TIS

112.883

22.2667

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6.50

29.039

1.2209

1.713

0.0494

XFJ

114.657

23.7385

6.40

30.975

1.3744

1.748

0.0609

XNH

113.034

22.5697

6.60

29.165

1.2708

1.697

0.0471

ZHH

113.566

22.2706

6.20

27.031

1.2402

1.720

0.0659

ZHQ

112.539

23.1783

6.40

28.075

1.3093

1.703

0.0555

ZHS

113.360

22.4873

6.40

28.039

1.5466

1.700

0.0587

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Table 1. Locations of seismic stations and corresponding H-k stacking results.

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30.089

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Fig. 1 Tectonic background and distribution of the Mesozoic granitoids in the Cathaysia

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Block. Inbox denotes the study area. PRD: Pearl River Delta. TXNB: Taixinan Basin. Two major faults are numbered, in which the fault “1” is Zhenghe-Dapu Fault zone and the fault “2”

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is Nanao-Changle Fault zone. Distribution of the Mesozoic granites is referring to Li [2000], and the Late Mesozoic volcanic arcs are referring to Li et al. [2018].

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Fig. 2. Distribution of the seismic stations around the PRD (a) and the free-air gravity anomaly (b). Solid lines denote the faults in the study area. Thick doted lines are the boarding faults near the Lianhuashan Fault zone. The green rectangles are the stations with low velocity layers, while the blue ones are those with clear upper-crust/lower crust layers. PRE: Pearl River Estuary; LTF: Littoral Fault Zone; PRMB: Pearl River Mouth Basin.

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Fig. 3 Distribution of the teleseismic events recorded by station SZN (see Fig 2 for location). Red stars denote the earthquake epicenters. Yellow boxes indicate the regions where the

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receiver functions were stacked for inversion modeling. Dash circle represents the epicentral distance at 30°.

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Fig. 4 Receiver functions for several stations with the number of recorded functions great than

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15. Top figure shows the tangential components of the receiver functions, while the bottom

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figure shows the radial components. Red and cyan colors respectively denote the positive and negative amplitudes. At the base of each plane, the red, blue, yellow and green horizontal bar denote the quadrants from NE, SE, SW and NW, respectively.

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Fig. 5. Stacking results of the radial (left) and tangential (right) receiver functions recorded at station SZN. Thick solid line on top of each figure is the stacked receiver functions, while the dash ones denote the ± 1 standard deviation bounds. The numbers between the left and the right panels are arranged in the format of “back-azimuth/distance” both in unit of degree.

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Fig. 5 Continue

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Fig. 6 Ray diagram of the P-to-S converted phases in the receiver function for a single layer

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over Moho (a). (b). The initial model

with a constant Vp/Vs ratio of 1.73 for inversion is

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denoted by the dashed line. The “true” models with variable Vp/Vs ratios and constant Vp are denoted by black solid lines, and the inverted models are denoted by red lines. The numbers beneath the figure are Vp/Vs values for the “true” models. (c). The “true” (black solid lines) and inverted (red lines) waveforms corresponding to the models in Fig.6b. The numbers on the right are Vp/Vs values for the “true” models.

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Fig. 7. Inversion results of the receiver functions of station SZN. (Top) The fitting of the receiver functions, in which the solid lines are stacked real data while the dashed ones are synthetic receiver functions. The arrows indicate the Pms phases, and the shaded box shows the time when the multiple phases show up. (Bottom) The 1-D shear wave velocity profiles from different azimuth quadrants. The Moho interfaces are indicated by arrows and labeled by the depth values.

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Fig. 8. Inversion results of the receiver functions in station HGK. (Top) The fitting of the receiver functions, in which the solid lines are stacked real data while the dashed ones are synthetic receiver functions. The arrows indicate the Pms phases, and the shaded box shows the time when the multiple phases show up. (Bottom) The 1-D shear wave velocity profiles from different azimuth quadrants. The Moho interfaces are indicated by arrows and labeled by the depth values. 38

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Fig. 9. S-wave velocity profiles beneath all stations. In which, the red line is the model from NE, the blue line from SE, the purple line from SW and the green line from NW. The black lines represent the averaged profiles from all quadrants. The Moho interfaces are indicated by solid arrows and labeled by the depth values. The upper boundary of the low velocity layer is indicated by dotted arrow, while the boundary between the upper and lower crust is indicated by dashed arrow. 39

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Fig. 10. Synthetic receiver functions and the stacking results using the H-k method [Zhu et al.,

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2006] using different average Vp. (a) Synthetic receiver functions in a range of ray parameters,

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in which red color denotes positive amplitudes. (b) S-wave velocity profile composing of five

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layers for generating synthetic receiver functions, in which the crustal thickness, Vp/Vs ratio and average Vp are shown. (c) Distribution of the H (blue squares and line) and Vp/Vs (red

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squares and line) uncertainty errors. (d) Stacking results of the H and Vp/Vs. The colors and symbols are identical to those in (c). The stars denote the real values of the H and Vp/Vs.

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Fig. 11 The H-k stacking results and the trade-off relations between H and Poisson’s ratio. (a) Correlations between the average Vp and k; (b) Correlations between average Vp and H; (c) and (d) The values of Poisson’s ratio and H beneath each station, respectively. (e) Trade-off relations between H and Poisson’s ratio. The red circles and squares are results from Huang et al. [2014], while the blue ones are from this study. 41

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Fig. 12. Averaged P-wave velocities (a), crustal thicknesses (b) and Poisson’s ratios (c) beneath all of the seismic stations. The labeled squares denote the seismic stations.

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Fig. 13. 3D schematic diagram showing magmatic activity resulting from subduction of the

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paleo-Pacific plate during the Late Mesozoic. Solid and dash lines denote active faults

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providing conduit for magmatic upwelling.

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Haibo Huang: Conceptualization, Methodology, Writing – Original Draft, Software. Hou Xiong: Data curation, Investigation. Xuelin Qiu: Writing – Review & Editing, Supervision.

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Yuhan Li: Investigation.

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Graphical abstract

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