Pengxiang Zhou

Shaohong Xia
email: shxia@scsio.ac.cn

György Hetényi

Vadim Monteiller

Sébastien Chevrot

Jie Sun
email: sunjie@scsio.ac.cn

Seismic imaging of a mid-crustal low-velocity layer beneath the northern coast of the South China Sea and its tectonic implications

Low-velocity layers (LVLs) within the continental crust are usually closely associated with tectonic events, thus they may reflect ongoing processes or offer a chance to backtrack the evolutional history.

Mid-crustal LVLs have been reported in the northern coast of the South China Sea (SCS) by several geophysical studies, but the 3D distribution of LVLs in the whole coastal area is still poorly known.

Here, we perform a receiver function study based on 67 broadband seismological stations. We use the common conversion point method to construct structures of the Moho as well as intra-crustal interfaces. We image sharp intra-crustal negative velocity gradients broadly distributed in the eastern Cathaysia block with depth increasing from ~10 km in the NW to 12~16 km in the SE. The corresponding amplitudes of these negative interfaces show variable values from -0.03 to -0.1, with an evidential trend of strengthening eastwards. We interpret these negative gradients as top of the LVL produced by accumulation of fluids from dehydration of the paleo-Pacific subduction plate.

These mid-crustal LVLs tend to act as weak domains during continental rift and the lateral variation of the LVLs may be one possible reason explaining the along-strike variation in crustal stretching in the northern SCS rifted margin.

The presence of intra-crustal low-velocity layers (LVL) can provide insights into the styles of crustal deformation and help to reconstruct the tectonic history. For example, the well-known mid-crustal LVL in Tibet was proposed to host channel flow, playing an important role in accommodating lithospheric deformations during extension [START_REF] Nelson | Partially molten middle crust beneath southern Tibet: synthesis of project INDEPTH results[END_REF][START_REF] Beaumont | Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation[END_REF][START_REF] Bao | Two crustal low-velocity channels beneath SE Tibet revealed by joint inversion of Rayleigh wave dispersion and receiver functions[END_REF][START_REF] Sun | Anisotropic tomography beneath Northeast Tibet: evidence for regional crustal flow[END_REF], however, its geometry, connectivity and therefore importance are still debated [START_REF] Hetényi | Discontinuous low-velocity zones in southern Tibet question the viability of the channel flow model[END_REF]. The shallow-angle, long-extent and low-velocity detachments observed in Himalaya [START_REF] Schulte-Pelkum | Imaging the Indian subcontinent beneath the Himalaya[END_REF][START_REF] Caldwell | Characterizing the Main Himalayan Thrust in the Garhwal Himalaya, India with receiver function CCP stacking[END_REF][START_REF] Subedi | Imaging the Moho and the Main Himalayan Thrust in Western Nepal With Receiver Functions[END_REF], Central Andes [START_REF] Yuan | Subduction and collision processes in the Central Andes constrained by converted seismic phases[END_REF] and eastern America [START_REF] Long | Seismic imaging of mid-crustal structure beneath central and eastern North America: Possibly the elusive Grenville deformation?[END_REF][START_REF] Marzen | Constraints on Appalachian Orogenesis and Continental Rifting in the Southeastern US from Wide-Angle Seismic Data[END_REF] indicate a fundamental deformation mode in continental collision zones. In several cases, intra-crustal LVLs result from accretionary complexes that reflect a tectonic transformation from subduction to extension [START_REF] Smit | The Thor suture zone: From subduction to intraplate basin setting[END_REF]. Or, intra-crustal LVLs can indicate a component akin to upper-middle crustal materials which have 'frozen' in the present-day stable crust, that dates back to craton formation [START_REF] Zheng | New evidence from seismic imaging for subduction during assembly of the North China craton[END_REF]. Tending to form weak domains, inherited LVLs in continental crust can also impact later tectonic events, such as continental rifting. Pre-existing weak zones play an important role in controlling the location and duration of extension, rifting styles and post-rift evolution of continental margins [START_REF] Sutra | How does the continental crust thin in a hyperextended rifted margin? Insights from the Iberia margin[END_REF][START_REF] Manatschal | The role of inheritance in structuring hyperextended rift systems: Some considerations based on observations and numerical modeling[END_REF]Festa et al., 2019;[START_REF] Li | Low-viscosity crustal layer controls the crustal architecture and thermal distribution at hyper-extended margins: Modeling insight and application to the northern South China Sea margin[END_REF][START_REF] Zhao | Along-strike segmentation of the South China Sea margin imposed by inherited pre-rift basement structures[END_REF]. Furthermore, structurally inherited weaknesses can control present-day intraplate deformation and seismicity by adjusting strain localization on a local scale [START_REF] Tarayoun | Quantitative impact of structural inheritance on present-day deformation and seismicity concentration in intraplate deformation zones[END_REF].

In South China, low-velocity/high-conductivity layers have been observed in deep seismic profiles (blue lines in Figure 1) [START_REF] Liao | Explosion seismicstudy of the crustal structure in Fuzhou-Quanzhou-Shantou region[END_REF][START_REF] Xiong | Some characteristics of deep structure of the Zhangzhou geothermal field and its neighbourhood in the Fujian province[END_REF][START_REF] Yin | Crustal structure and velocity distribution from deep seismic sounding along the profile of Lianxian-Boluo-Gangkou in South China[END_REF][START_REF] Zhao | An Analysis on Deep Crustal Structure Along the Onshore-Offshore Seismic Profile Across the Binghai (Littoral) Fault Zone in Ne South China Sea[END_REF][START_REF] Zhang | Crustal structure and contact relationship revealed from deep seismic sounding data in South China[END_REF][START_REF] Zhao | Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent[END_REF], local seismic tomographic models [START_REF] Zhang | 3-D Crustal Shear-Wave Velocity Structure of the Taiwan Strait and Fujian, SE China, Revealed by Ambient Noise Tomography[END_REF] and magnetotelluric soundings [START_REF] Hu | The Lithospheric Electrical Structure of JI'AN-FUZHOU Profile in the East Part of South China[END_REF]. The observed mid-crustal LVLs are rather flat and characterized by a P wave velocity of 5.5-6.0 km/s and thicknesses of 3 to 6 km. They have been interpreted as partial melt bodies, remnants of an ancient oceanic basin, or ductile shear zones.

Through numerical simulation, [START_REF] Li | Low-viscosity crustal layer controls the crustal architecture and thermal distribution at hyper-extended margins: Modeling insight and application to the northern South China Sea margin[END_REF] argues that LVLs could determine the width of the hyper-extended crust in the northern margin of the South China Sea (SCS). The LVLs may also be responsible for the present-day seismicity and historical earthquakes by providing an unstable crustal structure [START_REF] Zou | The deep structure and earthquake mechanism of the continental margin seismic belt along the northern south china sea[END_REF][START_REF] J O U R N A L P R E -P R O O F Xu | Characteristics of the crustal structure and hypocentral tectonics in the epicentral area of Nan'ao earthquake (M7.5), the northeastern South China Sea[END_REF]). Yet, due to limited coverage of the former 2D active source J o u r n a l P r e -p r o o f seismic experiments (Figure 1), the distribution of the LVLs in the northern coast of the SCS is not clear.

Motivated by the numerous reports of mid-crustal LVLs and to address the above limitations, seismic waveforms recorded by 67 broadband seismological stations distributed in the northern coast of the SCS were used to analyze P-to-S receiver functions (RF). Taking advantage of RF's sensitivity to impedance contrasts, the goals of this research are to better constrain the spatial distribution of LVLs in the whole coastal area and discuss the nature and origin of the LVLs as well as their tectonic significance.

Tectonic overviews

Seismic stations used in this study are located in the Cathaysia block (Figure 1 and Figure 2), the convergence area of the Eurasian, Pacific and India-Australian plates. The Cathaysia and the Yangtze blocks collided and merged into the South China block (SCB) in the Neoproterozoic, forming the Jiangnan orogenic belt at their boundary (Zhao andCawood, 1999, 2012;[START_REF] Li | Grenvillian continental collision in south China: New SHRIMP U-Pb zircon results and implications for the configuration of Rodinia[END_REF][START_REF] Shu | An analysis of principal features of tectonic evolution in South China Block[END_REF].

After that, the united SCB experienced episodes of compression and extension [START_REF] Shu | An analysis of principal features of tectonic evolution in South China Block[END_REF]. At present, it shows two dominant features: 1) the widespread Mesozoic granitoids and volcanic rocks (Figure 1), with the Early Yanshanian (180-140 Ma) igneous rocks mainly distributed in the continental interior and the Late Yanshanian (140-97 Ma) magmatism exposed along the southeastern coast [START_REF] Zhou | Origin of Late Mesozoic igneous rocks in Southeastern China: implications for lithosphere subduction and underplating of mafic magmas[END_REF], 2) the thinned lithosphere and crust whose thicknesses generally decrease towards the coast [START_REF] Zhou | The structure of the crust and uppermost mantle beneath South China from ambient noise and earthquake tomography[END_REF][START_REF] Shan | Crustal and upper-mantle structure of South China from Rayleigh wave tomography[END_REF]. Such features are well explained by the flat subduction of the paleo-Pacific plate followed by slab break-off, foundering and roll-back during the Mesozoic (Li and Li, 2007;Suo et al., 2019). In early , due to the flat-slab subduction of the paleo-Pacific plate, a ~1300-km-width orogenic belt was formed from SE to NW. During the early post-orogenic stage , the slab break-off and foundering caused the mantle upwelling and underplating. In the late Mesozoic (150 Ma-), slab rollback produced seaward migrating magmatism [START_REF] Zhou | Origin of Late Mesozoic igneous rocks in Southeastern China: implications for lithosphere subduction and underplating of mafic magmas[END_REF]Li and Li, 2007).

The underplated basaltic magmas or hot mantle provided a significant amount of heat to partially melt the lower crust, generating granitic magmas and leaving behind dry mafic granulites [START_REF] Zhou | Origin of Late Mesozoic igneous rocks in Southeastern China: implications for lithosphere subduction and underplating of mafic magmas[END_REF]. However, this model is still challenged by the geophysical observations, which inferred intercontinental extension induced by the lower-crustal and mantle delamination [START_REF] He | Seismic evidence for a geosuture between the Yangtze and Cathaysia Blocks, South China[END_REF] or J o u r n a l P r e -p r o o f asthenosphere upwelling [START_REF] Deng | Lithospheric structure in the Cathaysia block (South China) and its implication for the Late Mesozoic magmatism[END_REF]. Notably, previous seismological studies mainly focused on the lithospheric and crustal structure, leaving the intra-crust poorly understood. Yet, the heterogeneous crustal interior may record much key information of the tectonic evolution.

The Mesozoic orogenic event led to the creation of NNE and ENE trending fault zones in the SCB, among which the Shaowu-Heyuan-Yangjiang Fault is one of the major fault zones (Figure 1). A large silicified quartz reef (up to 40 km of length and tens of meters thick) has developed along the Heyuan part of the fault zone and can be seen outcropping locally at several sites and visible in satellite imagery. The quartz reef is encased by a top phyllonitic seal and a bottom mylonitic zone (Tannock et al., 2020a). Another distinctive characteristic is that an abundance of hot springs are distributed along the Heyuan part of the fault zone [START_REF] Tannock | The effects of a tectonic stress regime change on crustal-scale fluid flow at the Heyuan geothermal fault system, South China[END_REF].

Following the inferred slab roll-back in the Late Cretaceous, the SCB has transformed from an active margin to a passive continental margin. Continent rifting, break up and seafloor spreading occurred at the Andean-type margin along with the formation of the SCS basin during the early and middle Cenozoic [START_REF] Taylor | Origin and history of the South China Sea basin. The tectonic and geologic evolution of Southeast Asian seas and islands[END_REF][START_REF] Bai | Crustal stretching style variations in the northern margin of the South China Sea[END_REF]. The relatively young continental lithosphere underwent extensive rifting from ~45 Ma. At ~33 Ma, continental breakup occurred in the northeast and led to the formation of the east sub-basin (ESB). Then, the spread center jumped to the south at around 25.5 Ma and the opening of the northwest sub-basin (NWSB) stopped. The spreading kept propagating southwestwards, leading to the formation of the southwest sub-basin (SWSB). In the middle Miocene (15 Ma), the seafloor spreading ceased [START_REF] Franke | Rifting, lithosphere breakup and volcanism: Comparison of magma-poor and volcanic rifted margins[END_REF][START_REF] Savva | Different expressions of rifting on the South China Sea margins[END_REF][START_REF] Li | Possible spatial distribution of the Mesozoic volcanic arc in the present-day South China Sea continental margin and its tectonic implications[END_REF]. The diachronous seafloor spreading from east to west in the Oligocene and Miocene results in the present-day V-shaped morphology of the SCS [START_REF] Pichot | Deep crustal structure of the conjugate margins of the SW South China Sea from wide-angle refraction seismic data[END_REF] (Figure 1).

Duo to the multiphase rifting and diachronous seafloor spreading, the northern SCS margin had experienced extreme crustal thinning [START_REF] Yan | A crustal structure profile across the northern continental margin of the South China Sea[END_REF][START_REF] Savva | Different expressions of rifting on the South China Sea margins[END_REF][START_REF] Lester | Rifting and magmatism in the northeastern South China Sea from wide-angle tomography and seismic reflection imaging[END_REF].

Significant along-strike variations in morphology, crustal structure and structural styles have been revealed by geophysical studies [START_REF] Franke | Rifting, lithosphere breakup and volcanism: Comparison of magma-poor and volcanic rifted margins[END_REF][START_REF] Bai | Crustal stretching style variations in the northern margin of the South China Sea[END_REF][START_REF] Zhao | Prolonged post-rift magmatism on highly extended crust of divergent continental margins (Baiyun Sag, South China Sea)[END_REF][START_REF] Yang | The structure and evolution of deepwater basins in the distal margin of the northern South China Sea and their implications for the formation of the continental margin[END_REF].

The northeast segment has a rugged topography with a seafloor characterized by local reliefs, and the northwest segment is characterized by a gentle morphology with a slightly dipping slops [START_REF] Zhao | Along-strike segmentation of the South China Sea margin imposed by inherited pre-rift basement structures[END_REF]. The east part of the SCS northern margin exhibits 400 km of extended crust, while the west part shows 800 km of extended continental crust [START_REF] Hayes | The South China sea margins: Implications for rifting contrasts[END_REF][START_REF] Pichot | Deep crustal structure of the conjugate margins of the SW South China Sea from wide-angle refraction seismic data[END_REF].

J o u r n a l P r e -p r o o f

Abrupt thinning of continental crust in front of the continent-ocean transition zone (COT) and a narrow COT have been observed in the western segment of the SCS. In contrast, over the eastern segment, the continental crust thins gently and forms a relatively wide COT [START_REF] Pichot | Deep crustal structure of the conjugate margins of the SW South China Sea from wide-angle refraction seismic data[END_REF]. To explain this lateral variability, different extensional models have been proposed, such as polyphase faulting, sequential faulting and depth-dependent stretching [START_REF] Clift | Evidence of low flexural rigidity and low viscosity lower continental crust during continental break-up in the South China Sea[END_REF][START_REF] Huismans | Rifted continental margins: The case for depth-dependent extension[END_REF][START_REF] Pichot | Deep crustal structure of the conjugate margins of the SW South China Sea from wide-angle refraction seismic data[END_REF][START_REF] Zhao | The continental extension discrepancy and J o u r n a l P r e -p r o o f Journal Pre-proof anomalous subsidence pattern in the western Qiongdongnan Basin, South China Sea[END_REF]. Such variability may also be attributed to the heterogeneous crust of the SCB because of its extremely complex evolution. However, both the heterogeneous nature of the SCB and its relationship with the extensional variability along-strike the northern SCS margin have not been well documented. [START_REF] Liao | Explosion seismicstudy of the crustal structure in Fuzhou-Quanzhou-Shantou region[END_REF][START_REF] Xiong | Some characteristics of deep structure of the Zhangzhou geothermal field and its neighbourhood in the Fujian province[END_REF]c. Yin et al., 1999;[START_REF] Zhang | Crustal structure and contact relationship revealed from deep seismic sounding data in South China[END_REF][START_REF] Zhao | Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent[END_REF]d. Zhao et al., 2004 Seismograph Network (http://www.gddzj.gov.cn/) and China Earthquake Network Center (https://www.cenc.ac.cn/). With an average spacing of about 50 km, station distribution in the coastal area is deployed denser than in the north (Figure 2). Almost all stations are located in the Cathaysia block and only a few are located in the Jiangnan orogen (Figure 1). At each station, we selected teleseismic events from the International Seismological Center (ISC) catalog with magnitude Mw>5.5 occurring at an epicentral distance between 30° and 90°. Our final data set contains 278 events for receiver function calculation (Figure 3).

P-to-S receiver function

We use the receiver function technique to image intra-crustal LVLs. By deconvolving the vertical from the horizontal components, we remove the source time function, the possible source side effects and deep-Earth propagation effects, and isolate the P-to-S conversions generated at velocity discontinuities beneath stations. We used the classical time-domain iterative deconvolution method [START_REF] Ligorría | Iterative deconvolution and receiver-function estimation[END_REF] to calculate P-to-S receiver functions. Before calculation, we first cut the teleseismic signals 30 s before and 120 s after the arrival time of the direct P phase based on the 1-D IASPEI91 velocity model [START_REF] Kennett | Traveltimes for global earthquake location and phase identification[END_REF]. To ensure the data set quality, we applied a semi-automatic quality control on the filtered original vertical-north-east (Z-N-E) waveforms following the method proposed by [START_REF] Hetényi | Crustal structure of the Pannonian basin: the AlCaPa and Tisza Terrains and the Mid-Hungarian zone[END_REF][START_REF] Hetényi | From mountain summits to roots: Crustal structure of the Eastern Alps and Bohemian Massif along longitude 13.3[END_REF]. 11'370 high-quality traces were selected from total 16'438 traces. We then rotated the Z-N-E components to a vertical-radial-transverse system (Z-R-T) based on the event back-azimuth. We downsampled the records at 20 Hz, removed the mean, applied a taper Hanning and band-pass filter at 0.2-1 Hz before RF calculation. After 150 iterations of the iterative deconvolution algorithm, the final radial and transverse RFs were obtained by a convolution of Gaussian pulse width corresponding to the signal's highest frequency. Assuming a crustal shear wave velocity of 3.6 km/s, the corresponding J o u r n a l P r e -p r o o f wavelength is 3.6 km at 1 Hz. Based on the minimum resolvable length scale of λ/4, the layer thickness larger than ~0.9 km is resolvable. An example of radial RFs of station HUJ is shown in Figure 4 and individual RFs at all stations can be found in Supporting Figure S1.

Common conversion point migration

We used the pre-stack migration method to map seismic discontinuities. Pre-stack migration allows the RF amplitudes to be mapped at the positions of P-to-S conversion along the ray path. Technically, we performed the time-to-depth migration using the widely adopted common conversion point (CCP) approach [START_REF] Zhu | Crustal structure across the San Andreas Fault, southern California from teleseismic converted waves[END_REF]. We first chose two parallel sections, P1 and P2, according to the station distribution. Then we projected stations onto the sections (95 km on both sides of P1, magenta triangles in Figure 2; 125 km on both sides of P2, blue triangles in Figure 2). Along the sections, the average station interval is 26 km for P1 and 40 km for P2 (Figures 5A and6A). Given the nature of a relatively flat Moho and no strongly spatial variations in seismic velocity in the Cathaysia block [START_REF] Zhou | The structure of the crust and uppermost mantle beneath South China from ambient noise and earthquake tomography[END_REF][START_REF] He | Seismic evidence for a geosuture between the Yangtze and Cathaysia Blocks, South China[END_REF][START_REF] Guo | Crustal thickness and Poisson's ratios of South China revealed from joint inversion of receiver function and gravity data[END_REF], we employed a 1D P and S wave velocity model derived from the deep seismic profile L-G (Figure 1) (Zhang and Zhao, 2007;[START_REF] Zhao | Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent[END_REF] for time-to-depth conversion (Table 1). To further improve the imaging, a 3D velocity model should be used in the future to take into account small-scale crustal velocity heterogeneities. We projected RF amplitudes onto the profiles with a depth interval of 1 km and a horizontal bin of 15 km, which is close to the Fresnel zone radius at the Moho. Later, images are horizontally smoothed with a 40 km Gaussian filter. The migration takes the station elevation into account.

J o u r n a l P r e -p r o o f

Table 1. 1-D Velocity Model used for time-to-depth migration (Zhang and Zhao, 2007;[START_REF] Zhao | Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent[END_REF]

Results

The Moho is visible as strong, positive interface (warm color) in Figures 5 and6, in a depth range of 25-34 km. We picked the Moho (the maximum amplitude at depth beneath each station) to map the Moho geometry in 3D, it is shallower in the western part (25-27 km) and deeper in the east (28-34 km) (Figure 7B). The shallowest Moho appears at around 110°E, while the deepest Moho is located between 110°-120°E. A slight decrease of Moho depth from continental interior to the coast can be observed, by 2-3 km. The depth variation in the Moho is consistent with previous results (26-34 km)

obtained with a single-station grid search technique [START_REF] Zhu | Moho depth variation in southern California from teleseismic receiver functions[END_REF][START_REF] Chevrot | The Poisson ratio of the Australian crust: geological and geophysical implications[END_REF][START_REF] He | Seismic evidence for a geosuture between the Yangtze and Cathaysia Blocks, South China[END_REF][START_REF] Xia | Lateral variation of crustal structure and composition in the Cathaysia block of South China and its geodynamic implications[END_REF][START_REF] Guo | Crustal thickness and Poisson's ratios of South China revealed from joint inversion of receiver function and gravity data[END_REF]. The 1D migration model has small influences on the determination of the Moho depth, probably because of the nature of a relatively flat Moho and no strongly lateral variations in speed velocity in the study area [START_REF] Zhou | The structure of the crust and uppermost mantle beneath South China from ambient noise and earthquake tomography[END_REF][START_REF] He | Seismic evidence for a geosuture between the Yangtze and Cathaysia Blocks, South China[END_REF][START_REF] Guo | Crustal thickness and Poisson's ratios of South China revealed from joint inversion of receiver function and gravity data[END_REF]. This gives support to the credibility of the intra-crustal interfaces as described below.

Besides the Moho conversion, we can observe a series of negative pulses (shown in blue) distributed between 1-2 s in Figures 5B and6B, representing a sharp negative velocity gradient with depth.

Since the study area is largely covered by igneous rocks (Figure 1), it is unlikely that the negative conversion is influenced by a shallow sedimentary layer. In addition, we don't observe strong positive conversions before the negative arrivals (Figure 4 and Figure S1). In the CCP migrated images, these negative polarity interfaces (cold color) are distributed at 9-16 km depth (Figures 5D and6D), and with amplitudes smaller compared to that of the Moho. Along P1, the negative polarity interfaces vary in a depth range of 12-16 km and rather strong amplitudes are visible between 600-800 km distance (Figures 5D). 6D) and plotted them in map view (Figure 7A). We can see that the negative polarity interfaces appear broadly in the eastern Cathaysia block, with depths shallower in the northwest (~10 km) and deeper in the southeast (~12-16 km). The deepest negative interfaces are located between longitude 111°-114°E. The corresponding amplitude map of the negative interfaces shows variable values between -0.03 to -0.1. An evidential trend is that the amplitudes generally strengthen eastwards, with the strongest amplitude appearing at around longitude 116°E (Figure 7C).

P-to-S receiver function phases can be influenced by the presence of azimuthal anisotropy or a significant dip on the discontinuity, both of which are not unexpected in the highly complex SCB with a history of multiple collisional cycles. A dipping layer will generate a zero-lag arrival on the transverse RFs that is not observed in the anisotropic case. In this study, clear zero-lag arrivals of the transverse components can be observed at station XUW, JIX, LIP and PHSG (Figure S1). However, it is difficult to assess which dipping interface of these zero-lag signals come from. The effects of a possible anisotropic layer are more complicated. For most of the stations, energy on the transverse RFs appears in the range of 1-2 s is small compared to that on the corresponding radial RFs, which indicates that anisotropy is not the dominant source of signal in this study. The above analysis points to largely isotropic velocity decrease at horizontal boundaries.

J o u r n a l P r e -p r o o f Based on previous studies [START_REF] Liao | Explosion seismicstudy of the crustal structure in Fuzhou-Quanzhou-Shantou region[END_REF][START_REF] Xiong | Some characteristics of deep structure of the Zhangzhou geothermal field and its neighbourhood in the Fujian province[END_REF][START_REF] Yin | Crustal structure and velocity distribution from deep seismic sounding along the profile of Lianxian-Boluo-Gangkou in South China[END_REF][START_REF] Zhang | Crustal structure and contact relationship revealed from deep seismic sounding data in South China[END_REF][START_REF] Zhao | Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent[END_REF], we interpret the negative velocity gradients in the mid-crust of our CCP images as the top interfaces of the low-velocity layer. They show depth variations in good agreement with different geophysical sounding velocity models, although the top of the LVLs from former results are locally deeper (gray dashed lines in Figure 8a). This may reflect differences in the ability of active-versus passive-source imaging techniques to characterize structure in the intra-crust. We couldn't present the thickness distribution of the LVLs, because the bottom interfaces of the LVLs are not observed, a phenomenon that has also been reported in some of the deep seismic profiles. For example, [START_REF] Zhao | An Analysis on Deep Crustal Structure Along the Onshore-Offshore Seismic Profile Across the Binghai (Littoral) Fault Zone in Ne South China Sea[END_REF] didn't find the reflected phases from the bottom of the LVLs (profile d in Figure 1). The reflected phases are not clear in [START_REF] Liao | Explosion seismicstudy of the crustal structure in Fuzhou-Quanzhou-Shantou region[END_REF]'s profiles (profile a in Figure 1).

The most likely reason is a spatially broad (~10 km) velocity gradient with depth, not sensed by receiver functions due to its vertical extent (a dipping interface or an anisotropic layer is not a dominant feature in this study as discussed above). The absolute amplitudes of the negative interfaces in our RF profiles are small compared to that of the Moho. This may indicate limited velocity/impedance contrast between LVLs and upper layers. In fact, the velocity profile of L-G

shows that the P-wave velocity of the LVLs is 5.8 km/s, underlying an upper layer with velocity of 6.2-6.3 km (Zhang and Zhao, 2007), and 3.5 km/s underlying 3.6-3.65 km/s for S-wave velocity [START_REF] Zhao | Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent[END_REF], thus leading to a Vp/Vs ratio of around 1.66 of the LVLs. The only common mineral with such a very low Vp/Vs ratio is quartz [START_REF] Christensen | Poisson's ratio and crustal seismology[END_REF], which suggests that the LVLs have high quartz concentrations and may be related to fluids.

Most of the previous workers attributed the LVLs in our study area to partial melting within the mid-crust. However, the relatively low Vp/Vs ratio (~1.66) does not support the presence of partial melt in the crust, as large amounts of partial melt would increase Vp/Vs [START_REF] Watanabe | Effects of water and melt on seismic velocities and their application to characterization of seismic reflectors[END_REF]. Besides, the last peak age of magmatism in south China is around 93 Ma [START_REF] Wang | Phanerozoic tectonics of the South China Block: key observations and controversies[END_REF]. It is unlikely that partially melt bodies are still preserved after such a long period of cooling. Another common interpretation of the LVLs in the orogenic zone is the remnant of crustal materials after subduction or collision, such as accretionary complexes or felsic upper-middle crust. Yet, such remnants often J o u r n a l P r e -p r o o f

show dip from mid-to lower-crust and mostly in the lower-crust [START_REF] Zheng | New evidence from seismic imaging for subduction during assembly of the North China craton[END_REF][START_REF] Smit | The Thor suture zone: From subduction to intraplate basin setting[END_REF] while the LVLs in our study area are relatively flat within the mid-crust. Therefore, we favor the interpretation of LVLs related to fluids from dehydration of the subducting slab and trapped in the mid-crust (Figure 8b). In south China, due to the flat-slab subduction of the paleo-Pacific plate, a ~1300-km-wide orogenic belt was formed in the Triassic (Li and Li, 2007;[START_REF] Liu | Age of the subducting Pacific slab beneath East Asia and its geodynamic implications[END_REF]. During subduction, fluids produced by dehydration of the slab are expected to migrate into the mantle and upper plate by channelized flow or along grain boundaries [START_REF] Jamtveit | Disequilibrium metamorphism of stressed lithosphere[END_REF]. Such transportation allows fluids to migrate at relatively large distances in a geologically rapid way and only a small volume of water can be absorbed by the mantle wedge [START_REF] John | Volcanic arcs fed by rapid pulsed fluid flow through subducting slabs[END_REF][START_REF] Wada | Effects of heterogeneous hydration in the incoming plate, slab rehydration, and mantle wedge hydration on slab-derived H2O flux in subduction zones[END_REF][START_REF] Jonas | The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation[END_REF]. It is possible that the additional fluids trapped in the mid-crust cause the observed reduction of seismic wave velocity (Figure 8b). Owing to its lower bulk modulus, fluids can reduce the compressional and shear velocity. Based on the model of [START_REF] Watanabe | Effects of water and melt on seismic velocities and their application to characterization of seismic reflectors[END_REF] on granite, a Vp/Vs of 1.66 suggests a fluid fraction in the range of ∼15 percent. Intra-crustal LVLs have long been found in western North and South America, where several low-angle and flat-slab subduction occur, and these LVLs are thought to be fluid related (Yuan et al., 2001;[START_REF] Beck | The nature of orogenic crust in the central Andes[END_REF][START_REF] Jödicke | Fluid release from the subducted Cocos plate and partial melting of the crust deduced from magnetotelluric studies in southern Mexico: Implications for the generation of volcanism and subduction dynamics[END_REF][START_REF] Wannamaker | Segmentation of plate coupling, fate of subduction fluids, and modes of arc magmatism in C ascadia, inferred from magnetotelluric resistivity[END_REF][START_REF] Vargas | Fluid Distribution in the Central Andes Subduction Zone Imaged With Magnetotellurics[END_REF]. Free water generated by dehydration of the downgoing oceanic plate can be trapped in the mid-and lower crust below an impermeable layer formed through mineral precipitation when temperature conditions are met [START_REF] Jones | MT and reflection: an essential combination[END_REF]. Very low permeability, of less than 10 -20 m 2 , allows fluids to reside for hundreds of millions of years [START_REF] Hyndman | Dipping seismic reflectors, electrically conductive zones, and trapped water in the crust over a subducting plate[END_REF].

Rising fluids released from the subducted oceanic slab are thought to be silica saturated and the deposition of silica with decreasing temperature and pressure is expected to form quartz [START_REF] Hyndman | Cascadia subducting plate fluids channelled to fore-arc mantle corner: ETS and silica deposition[END_REF], some of which is presently exposed as quartz veins, such as in New Zealand [START_REF] Breeding | Slab-derived fluids and quartz-vein formation in an accretionary prism, Otago Schist[END_REF] and Alaska [START_REF] Fisher | The role of silica redistribution in the evolution of slip instabilities along subduction interfaces: Constraints from the Kodiak accretionary complex, Alaska[END_REF]. In south China, a more than 40-km-long and 75-m-width giant quartz reef is exhumed where stations LTK, XFJ, HYJ and XIG are located (Figures 1, 2, 5 and 8c). Microstructural analyses revealed that this quartz reef was formed at mid-crustal conditions, for which significant volumes of metamorphic and meteoric fluids were required (Tannock et al., 2020a). The changes in morphology and grain size are controlled by the transformation of stress regime since the Mesozoic, and the present-day hot springs are still fed by J o u r n a l P r e -p r o o f deep fluid sources via a fracture network [START_REF] Tannock | The effects of a tectonic stress regime change on crustal-scale fluid flow at the Heyuan geothermal fault system, South China[END_REF].

We acknowledge that there may be other interpretations. For example, a sub-horizontal or shallowly dipping crustal detachment extending over hundreds of kilometers cannot be ruled out. However, if there is such an architecture due to the paleo-Pacific plate subducting beneath the Cathaysia block, the dipping would be northwestwards, opposite to our findings. Our 1D velocity model used in the migration may lead to slightly distorted dip estimates. However, observations from active source imaging revealed an even more flat LVL (Figure 8a) [START_REF] Liao | Explosion seismicstudy of the crustal structure in Fuzhou-Quanzhou-Shantou region[END_REF][START_REF] Xiong | Some characteristics of deep structure of the Zhangzhou geothermal field and its neighbourhood in the Fujian province[END_REF][START_REF] Yin | Crustal structure and velocity distribution from deep seismic sounding along the profile of Lianxian-Boluo-Gangkou in South China[END_REF][START_REF] Zhao | An Analysis on Deep Crustal Structure Along the Onshore-Offshore Seismic Profile Across the Binghai (Littoral) Fault Zone in Ne South China Sea[END_REF][START_REF] Zhang | Crustal structure and contact relationship revealed from deep seismic sounding data in South China[END_REF][START_REF] Zhao | Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent[END_REF].

Spatial variation of the Low-velocity layer

The presence of the mid-crustal LVLs shows a longitudinal difference. That is, the LVLs mainly J o u r n a l P r e -p r o o f distributed in the eastern Cathaysia block spanning over 500 km, with increasing strength eastwards within the mapped volume (Figure 7C). Also, the Moho is about 3-4 km deeper in the eastern Cathaysia block than in the west. The Moho depth exhibits a slight decrease towards the coast. On the contrary, the depth of the LVLs becomes shallower towards the continental interior (Figure 7).

Such distribution may be determined by inherited conditions: 1) the lateral variations in the degree of hydration of the incoming plate, 2) variations in permeability structure of the subduction oceanic plate, and 3) variations in permeability of the upper plate. Heterogeneity of the incoming plate hydration are controlled by variations in density and the maximum fault throw (vertical component of fault displacement) of bending-related faults [START_REF] Boneh | Intermediate-J o u r n a l P r e -p r o o f Journal Pre-proof depth earthquakes controlled by incoming plate hydration along bending-related faults[END_REF], and the permeability structure of the downgoing slab ultimately determines the release of fluids [START_REF] Delph | Fluid controls on the heterogeneous seismic characteristics of the Cascadia margin[END_REF]. The difference in rock porosity or permeability of the upper plate controlled by fluid and rock composition can also lead to the observed lateral variation of the LVLs [START_REF] Hyndman | Dipping seismic reflectors, electrically conductive zones, and trapped water in the crust over a subducting plate[END_REF][START_REF] Vargas | Fluid Distribution in the Central Andes Subduction Zone Imaged With Magnetotellurics[END_REF], even though we cannot give direct evidence.

The depth variation of the LVLs in this study is compared to these reported in western North and South America. The LVL in the northern Cascadia subduction zone, labeled as 'E' layer, shows landwards dipping following the isothermal temperature of 400-500°C [START_REF] Hyndman | Dipping seismic reflectors, electrically conductive zones, and trapped water in the crust over a subducting plate[END_REF]. However, most of the other LVLs exhibit irregular variations in depth [START_REF] Beck | The nature of orogenic crust in the central Andes[END_REF][START_REF] Jödicke | Fluid release from the subducted Cocos plate and partial melting of the crust deduced from magnetotelluric studies in southern Mexico: Implications for the generation of volcanism and subduction dynamics[END_REF], similar to our results. To be noticed, the observed LVLs may have different causes because clear depth variation of the LVLs can be seen in Figure 7A and Figure 9C. The amplitude variation of the LVL-is independent of RF ray sampling as shown by both the profile (Figure 5 and 6) and map (Figure 7) views. Thus, it may be a reflection of water content, with larger values concentrated in the east. Such variation may be also determined by the inherited conditions as discussed above. The spatial presence, depth variation and the amplitude variation of the LVLs can be reworked by subsequent processes, such as asthenosphere upwelling caused by slab break-off and full-scale foundering in the continental interior, as well as slab roll-back along the southeastern coast (Li and Li, 2007;Li et al., 2007). The additional reheating or re-dehydrating by these post-orogenesis mechanisms is expected to alter the LVLs following an NW-SE orientation, however, we didn't find a very clear trend.

Implications for continental rift

J o u r n a l P r e -p r o o f

A fluid-filled layer will inevitably have a large influence on crustal rheology. Laboratory experiments showed that even small concentrations (of a few tens of parts per million) of water contained in hydrous minerals can decrease strength dramatically [START_REF] Mackwell | High-temperature deformation of dry diabase with application to tectonics on Venus[END_REF]. The spatial distribution of the LVLs emphasizes the lateral difference in rheological structure in the Cathaysia block (Figure 7 and 9), with the eastern segment expected to be less viscous and mechanically weaker. In a fault zone near station NAP (Figure 2), swarm-type seismicity has been observed and mainly occurs in a mid-crustal low-velocity zone (LVZ). The LVZ is relatively continuous at about 13-15 km depth [START_REF] Xia | Seismogenic structure in the source zone of the 1918 M7. 5 NanAo earthquake in the northern South China Sea[END_REF]. High pore pressure due to the trapped fluids or fluids induced strain localization [START_REF] Getsinger | Influence of water on rheology and strain localization in the lower continental crust[END_REF] may be responsible for this mid-crustal seismicity layer. In the late Mesozoic, the SCB has transformed from an active margin to a passive continental margin. Continent rifting, break up and seafloor spreading occurred at the Andean-type margin along with the formation of the SCS basin. Situated in the transition between the tectonically complex SCB and the newly formed SCS (Figure 1), the coastal area may largely record the pre-rift or early-stage rift structures inherited from subduction and orogenesis (for instance, the LVLs), shortly before the onset of continental failure and seafloor spreading. To the south, longitudinal contrast in crustal structures, stretching and faulting along-strike the northern SCS rifted margin have been reported by geological and geophysical observations [START_REF] Bai | Full-fit reconstruction of the South China Sea conjugate margins[END_REF][START_REF] Zhao | Along-strike segmentation of the South China Sea margin imposed by inherited pre-rift basement structures[END_REF]. While the east part of the northern SCS margin (NSM) exhibits relatively wide necking or distal domain, the west part shows a wide proximal domain and a narrow necking zone [START_REF] Yang | The structure and evolution of deepwater basins in the distal margin of the northern South China Sea and their implications for the formation of the continental margin[END_REF], which indicates a reducing degree of crustal stretching from east to west on the lower continental slope (Figure 9a). The lateral variation of the presence of the LVLs is spatially consistent with the along-strike variation in crustal stretching along the NSM, suggesting that they may be related.

To connect observations onshore and offshore, we chose three seismic profiles including OBS200 [START_REF] Wang | Seismic imaging of the transitional crust across the northeastern margin of the South China Sea[END_REF], OBS1993 [START_REF] Yan | A crustal structure profile across the northern continental margin of the South China Sea[END_REF] and ESP-W1985 [START_REF] Nissen | Deep penetration seismic soundings across the northern margin of the South China Sea[END_REF], from eastern, central and western SCS northern margin; correspondingly, three RF profiles were produced from our results as the extensions of the above offshore profiles, namely P3, P4 and P5 (Figure 9a). Over the eastern segment, a clear mid-crustal LVL onshore and a wide COT offshore are shown (Figure 9b and 9e). While in the western segment, LVL is invisible and a COT-like thinned crust is narrow (Figure 9c the NWSB to the southwest sub-basin (SWSB) [START_REF] Li | Possible spatial distribution of the Mesozoic volcanic arc in the present-day South China Sea continental margin and its tectonic implications[END_REF][START_REF] Zhao | The continental extension discrepancy and J o u r n a l P r e -p r o o f Journal Pre-proof anomalous subsidence pattern in the western Qiongdongnan Basin, South China Sea[END_REF]. The presence of several rigid blocks in the western SCS, e.g. the Xisha block (XSB), Macclesfield Bank (MB) (Figure 9a), which are inherited from the SCB [START_REF] Yan | Petrology and geochemistry of Mesozoic granitic rocks from the Nansha micro-block, the South China Sea: Constraints on the basement nature[END_REF] and deformed weakly during the rifting phase [START_REF] Ding | Conjugate margin pattern of the Southwest Sub-basin, South China Sea: insights from deformation structures in the continent-ocean transition zone[END_REF][START_REF] Lü | The role of rifting in the development of the continental margins of the southwest subbasin, South China Sea: Insights from an OBS experiment[END_REF], may be the most direct evidence. Actually, the fact that crust in the western NSM is more rigid than that in the east has been noticed long ago [START_REF] Clift | Evidence of low flexural rigidity and low viscosity lower continental crust during continental break-up in the South China Sea[END_REF] and such phenomenon is suspected to be partly caused by the heterogenous nature of the SCB crust [START_REF] Pichot | Deep crustal structure of the conjugate margins of the SW South China Sea from wide-angle refraction seismic data[END_REF]. Here, we provide a direct observation of crustal heterogeneity from the perspective of crustal hydration, and emphasize the importance of inheritance in controlling the along-strike variations of rifting styles in the northern SCS margin. To be noticed, there are other hypotheses explaining this along-strike variation. For example, the V-shape geometry of propagating rift systems may also be responsible for such variation.

J o u r n a l P r e -p r o o f

Figure 1 |

 1 Figure 1 | General tectonic framework of the study area and its surrounding regions. Left panel: Gray thick dashed lines denote the borders of the Yangtze and the Cathaysia block. Black thin dashed lines indicate fault zones. Blue straight lines represent wide-angle deep seismic profiles in the study area with the extent of the LVLs indicated by white dashed lines. a.[START_REF] Liao | Explosion seismicstudy of the crustal structure in Fuzhou-Quanzhou-Shantou region[END_REF][START_REF] Xiong | Some characteristics of deep structure of the Zhangzhou geothermal field and its neighbourhood in the Fujian province[END_REF] c. Yin et al., 1999;[START_REF] Zhang | Crustal structure and contact relationship revealed from deep seismic sounding data in South China[END_REF][START_REF] Zhao | Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent[END_REF] d. Zhao et al., 2004. Black box indicates the location of the giant quartz reef (modified after[START_REF] Guo | Crustal thickness and Poisson's ratios of South China revealed from joint inversion of receiver function and gravity data[END_REF]. Right panel: Tectonics of the surrounding region. NSMnorthern SCS margin; ESBeast sub-basin; NWSBnorthwest sub-basin; SWSBsouthwest sub-basin; QDN -Qiongdongnan basin; XSB -Xisha block; MB -Macclesfield Bank; RB -Reed Bank.

  Figure 1 | General tectonic framework of the study area and its surrounding regions. Left panel: Gray thick dashed lines denote the borders of the Yangtze and the Cathaysia block. Black thin dashed lines indicate fault zones. Blue straight lines represent wide-angle deep seismic profiles in the study area with the extent of the LVLs indicated by white dashed lines. a.[START_REF] Liao | Explosion seismicstudy of the crustal structure in Fuzhou-Quanzhou-Shantou region[END_REF][START_REF] Xiong | Some characteristics of deep structure of the Zhangzhou geothermal field and its neighbourhood in the Fujian province[END_REF] c. Yin et al., 1999;[START_REF] Zhang | Crustal structure and contact relationship revealed from deep seismic sounding data in South China[END_REF][START_REF] Zhao | Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent[END_REF] d. Zhao et al., 2004. Black box indicates the location of the giant quartz reef (modified after[START_REF] Guo | Crustal thickness and Poisson's ratios of South China revealed from joint inversion of receiver function and gravity data[END_REF]. Right panel: Tectonics of the surrounding region. NSMnorthern SCS margin; ESBeast sub-basin; NWSBnorthwest sub-basin; SWSBsouthwest sub-basin; QDN -Qiongdongnan basin; XSB -Xisha block; MB -Macclesfield Bank; RB -Reed Bank.

Figure 2 |Figure 3 |

 23 Figure 2 | Map of seismic stations (triangles) used in the analysis. Magenta triangles belong to section P1 and blue belong to P2. Gray lines mark the faults. Black crosses represent earthquakes (up to ML 5.2) in the past ten years provided by China Earthquake Data Center (http://data.earthquake.cn/). The yellow arrow point to the location of the giant quartz reef. Stations with red names show direct evidence for a mid-crustal LVL. J o u r n a l P r e -p r o o f

Figure 4 |

 4 Figure 4 | An example of radial receiver functions of station HUJ arranged as a function of back-azimuth using the time-domain iterative deconvolution in the frequency band of 0.2-1 Hz. The Moho and top of the LVL (LVL-) are labeled.

  Other negative amplitude patches in the crust (e.g. 5 km depth at 300-400 km distance, and lower-crustal variable depth at 600-800 km) have smaller amplitudes and we consider them as secondary with respect to the main mid-crustal LVL. The CCP image of P2 J o u r n a l P r e -p r o o f (Figures 6D) shows two groups of blue patches along 100-200 km and 500-800 km, lying relatively flat at around 10 km depth. We picked the depths of these continuous negative interfaces at the local minima beneath each station (crosses in Figures5D and

Figure 5 |Figure 6 |

 56 Figure 5 | Receiver function images of section P1. From top to bottom panels: A. Topographic profile, with triangles indicating station locations. The black arrow indicates the location of a giant quartz reef. B. Single-station stacked radial receiver function traces. Red pulses correspond to positive velocity gradients; blue pulses correspond to negative velocity gradients with depth. C. Individual receiver function traces plotted along profile representing raypaths of individual phase arrivals. D. Common conversion point migrated image with amplitudes indicated by color bar at right. Black dots represent the estimated Moho depth and crosses indicate the inferred top interfaces of the LVLs. J o u r n a l P r e -p r o o f

Figure 7 |

 7 Figure 7 | A. Topography of the LV L-. LVL-the top of the low-velocity layer. Gray thick dashed lines denote the borders of the Yangtze and the Cathaysia block. Black thin dashed lines indicate the fault zones. F1: Shaowu-Heyuan-Yangjiang Fault zone. F2: Zhenghe-Dapu Fault zone. B. Topography of the Moho. C. Amplitude map of the LV L-. D. Normalized RF ray piercing points density map.

Figure 8 |

 8 Figure8| (a) CCP image of the crustal structure along profile P1 (Figure2) and P4 (Figure9). Gray lines indicate the top of the LVLs by former studies[START_REF] Liao | Explosion seismicstudy of the crustal structure in Fuzhou-Quanzhou-Shantou region[END_REF][START_REF] Zhang | Crustal structure and contact relationship revealed from deep seismic sounding data in South China[END_REF][START_REF] Zhao | Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent[END_REF]. The Quartz reef is marked by an oval. (b) Interpretative diagram showing the migration of dehydration fluids coming from the Paleo-pacific slab and the accumulation of fluids in the mid-crust. (c) Cross-sectional view of the giant quartz reef. CP -Cataclasite-Phyllonite. (modified from Tannock et al., 2020a).

  and 9f). The central part exhibits transitional characteristics (Figure9d and 9g). According J o u r n a l P r e -p r o o f to the numerical analysis by[START_REF] Li | Low-viscosity crustal layer controls the crustal architecture and thermal distribution at hyper-extended margins: Modeling insight and application to the northern South China Sea margin[END_REF], the rheologically weak LVLs in the mid-crust could control the width of the highly thinned crust (thickness smaller than 15 km) and the crustal thermal distribution by accommodating deformation during extension. They therefore proposed that thicker LVLs result in wider range of ultra-thinned crust in the eastern NSM, while thinner LVLs are responsible for narrower range of ultra-thinned crust in the western NSM. Here, we supplement observations and explanations on the variability of LVL signal strength. The west segment of the NSM might experience more complex evolution because of the limited distribution of LVLs, for instance, the unresolved cessation of seafloor spreading in the northwest sub-basin (NWSB), the failed hyper-extension rift in the Qiongdongnan basin (QDN), the jump of the spreading ridge from

Figure 9 |

 9 Figure 9 | Comparison of onshore and offshore crustal structures. (a) Onshore and offshore sections represented by black lines. Magenta, blue and red triangles indicate seismic stations for section P3, P4 and P5. Major extensional domains are shown in the northern SCS margin according to[START_REF] Yang | The structure and evolution of deepwater basins in the distal margin of the northern South China Sea and their implications for the formation of the continental margin[END_REF]. (b -d) CCP images of section P3, P4 and P5. (e -g) Velocity structures along profile OBS2001[START_REF] Wang | Seismic imaging of the transitional crust across the northeastern margin of the South China Sea[END_REF]; profile OBS1993[START_REF] Yan | A crustal structure profile across the northern continental margin of the South China Sea[END_REF] and transect ESP-W[START_REF] Nissen | Deep penetration seismic soundings across the northern margin of the South China Sea[END_REF]. HN -Hainan Island; ESBeast sub-basin; NWSBnorthwest sub-basin; QDN -Qiongdongnan basin; XSB -Xisha block; MB -Macclesfield Bank; COTcontinent-ocean transition; OBoceanic basin.

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8

 12345678 Figure1

Figure 9

 9 Figure9

  . The Vp/Vs ratio considered in the three layers are 1.73, 1.71 and 1.74 respectively.

		0-10km	10-20km	20-35km
	P (km/s)	5.92	6.07	6.65
	S (km/s)	3.42	3.55	3.82

Acknowledgements

We thank the Data Management Centre of China National Seismic Network at Institute of Geophysics, China Earthquake Administration (doi:10.11998/SeisDmc/SN, http://www.seisdmc.ac.cn) for providing waveform data for this study. We thank Lisa Tannock for sharing their unpublished manuscript. We thank Xusong Yang and Shiba Subedi for their technical help. This work is supported by the National Natural Science Foundation of China (Grant No. U1701641), Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0204), K.C. Wong Education Foundation (GJTD-2018-13), the Research Foundation of Guangdong Province of China (2019BT2H594), the Rising Star Foundation of the South China Sea Institute of Oceanology (NHXX2017DZ0101). The authors gratefully acknowledge financial support from China Scholarship Council (NO. 201804910670).

Declaration of competing interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:


T Perez

D Pilloud

J Ghanbaja

P Miska

T Belmonte

T Gries

Tunable morphologies of ultrathin ZnO nanostructures synthesized by a plasma afterglow-assisted oxidation process and their photocatalytic properties

Keywords: Microwave plasma afterglow, ultrathin ZnO nanostructures, photocatalysis

The fast growth of ultrathin zinc oxide nanostructures is reported in this study, by using a flowing microwave plasma oxidation method: zinc films coated by a thin copper buffer layer are oxidized in low-pressure afterglow. This work highlights the versatility of plasma afterglow treatments and demonstrates that the morphology of ZnO nanostructures (nanowires, nanoribbons and nanocombs) can easily be controlled with good reproducibility by adjusting experimental parameters such as the working pressure during the oxidation process. The rapid growth of ultrathin nanostructures (∼1 min) is due to the afterglow area which is a very oxidizing medium since it contains high concentrations of oxygen atoms and excited oxygen molecules. The surface morphology and structural and optical properties of the as-synthesized nanostructures are studied by means of SEM, HRTEM and UV-visible absorption, respectively.

Ultrathin ZnO nanowires and nanoribbons with high aspect ratios exhibit excellent photocatalytic activity for the degradation of a model organic dye: methyl orange (MO), by comparison with pure ZnO thin films without nanostructures and thick ZnO nanowires synthesized by thermal oxidation. These results are supported by photocurrent measurements. The enhancement in photocatalytic performance could be attributed to higher surface area and higher transfer efficiency of photoinduced charge carriers to the surface of ultrathin nanowires and nanoribbons. The reusability of ZnO nanowire samples is also investigated by recording the photoactivity after several cycles of photodegradation.

Introduction

The use of metallic oxide semiconductors as photocatalysts has been widely studied due to its potential applications in the removal of hazardous pollutants and in energy fuel production such as water splitting [START_REF] Ibhadon | Heterogeneous photocatalysis: recent advances and applications[END_REF][START_REF] Hisatomi | Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting[END_REF]. Among the various semiconductors, ZnO has emerged as one of the most promising and most widely investigated photocatalysts with TiO2 due to its excellent oxidation ability, its physical, chemical and thermal stability, its nontoxicity, its abundance in nature and its low cost. Photocatalysis is a promising technique for the degradation of inorganic and organic pollutants in air and water. Especially, ZnO seems to be an ideal photocatalyst for practical applications on the wastewater treatments. For more details, the reviews of Kumar and Rao [START_REF] Kumar | Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications[END_REF] and Lee et al [START_REF] Lee | Recent developments of zinc oxide based photocatalyst in water treatment technology: a review[END_REF] give an overview on recent developments of zinc oxide-based photocatalysts in water treatment technology.

The photodegradation efficiency can be improved through a better design of the ZnO structure. The surface area and thus the morphology are factors of paramount importance which affect directly the photocatalytic efficiency of metallic oxide semiconductors [START_REF] Ahmed | Morphological evolution of ZnO nanostructures and their aspect ratio-induced enhancement in photocatalytic properties[END_REF][START_REF] Habba | Enhanced photocatalytic activity of ZnO nanostructure for water purification[END_REF][START_REF] Han | Synthesis and photocatalytic application of oriented hierarchical ZnO flower-rod architectures[END_REF][START_REF] Iwu | Enhanced quality, growth kinetics, and photocatalysis of ZnO nanowalls prepared by chemical bath deposition[END_REF]. As a consequence, ZnO nanopowders with different morphologies have been extensively studied as photocatalysts in suspension, which can provide high enhancement of photocatalytic activity [START_REF] Kansal | Studies on photodegradation of two commercial dyes in aqueous phase using different photocatalysts[END_REF][START_REF] Lu | ZnO hierarchical micro/ nanoarchitectures: solvothermal synthesis and structurally enhanced photocatalytic performance[END_REF]. However, the use of ZnO nanostructures with high aspect ratio (nanowires, nanoribbons, etc.) grown on substrates offers as main advantage to overcome the difficult and expensive process for the separation and recycling of ZnO powders [START_REF] Chang | Low temperature growth of ZnO nanowire arrays with enhanced high performance photocatalytic activity and reusability[END_REF]. Furthermore, these nanostructures offer a larger surface-to-volume ratio by comparison with nanoparticles deposited on a flat substrate and hence are expected to show a higher photocatalytic activity [START_REF] Zhang | Synthesis, characterization, and applications of ZnO nanowires[END_REF]. The challenge in the widespread use of ZnO as photocatalyst is to limit the recombination of photogenerated charge carriers, which is typically faster than the production rate of reactive oxidation species. This recombination step lowers the quantum yield and causes energy wasting. Thereby, different strategies are employed to promote charge separation such as doping with metallic and non-metallic elements, coupling with semiconductors or modifying with noble metals. Improved segregation efficiency for photogenerated electron and hole pairs can also be obtained with ultrathin ZnO nano-objects: since the mean free path of charge carriers is also in nanometric scale, the photogenerated electrons and holes can reach the surface of the nanostructured material and are readily available for degradation reactions [START_REF] Wan | Enhanced photocatalytic activity of ZnO nanotetrapods[END_REF][START_REF] Murphy | Controlling the aspect ratio of inorganic nanorods and nanowires[END_REF]. This leads to a reduction of electron-hole recombination possibilities and thus to an enhancement of the photocatalytic activity.

Only few studies have been reported on the synthesis of ultrathin one dimensional ZnO nanostructures with most of them achieved using chemical solutions [START_REF] Fang | Synthesis of ultrathin ZnO nanofibers aligned on a zinc substrate[END_REF][START_REF] Gokarna | Highly crystalline urchin-like structures made of ultra-thin zinc oxide nanowires[END_REF][START_REF] Gu | Quantum confinement in ZnO nanorods[END_REF]. ZnO nanostructures can also be synthesized through the thermal oxidation of Zn foils [START_REF] Ghoshal | Direct synthesis of ZnO nanowire arrays on Zn foil by a simple thermal evaporation process[END_REF][START_REF] Zhao | Large-scale synthesis of bicrystalline ZnO nanowire arrays by thermal oxidation of zinc film: growth mechanism and high-performance field emission[END_REF].

However, this simple and cost-effective approach leads to nanowires with large diameters (typically between 20 and 200 nm) and requires relatively long treatment durations (∼2 h). In this work, we propose another strategy to synthesize ultrathin ZnO nanostructures with high growth rates, by using flowing microwave plasma afterglow-assisted oxidation treatments.

Plasma afterglow-assisted oxidation processes enable the synthesis of a large variety of metallic oxide nanostructures, even at atmospheric pressure [START_REF] Altaweel | Controlled growth of copper oxide nanostructures by atmospheric pressure micro-afterglow[END_REF][START_REF] Imam | Nanostructures design by plasma afterglow-assisted oxidation of ironcopper thin films[END_REF][START_REF] Saa | Synthesis of RuO2 nanowires by alkali-assisted oxidation of ruthenium in plasma afterglow at atmospheric pressure[END_REF]. Indeed, afterglow processes benefit from a higher reactivity of the oxidizing gas phase at moderate temperature compared to thermal heating processes, because of the presence of active oxygen species like oxygen atoms or metastable states of O2. In previous work [START_REF] Altaweel | Fast synthesis of ultrathin ZnO nanowires by oxidation of Cu/Zn stacks in lowpressure afterglow[END_REF], the growth of ultrathin nanowires with high aspect ratios was reported for optimal experimental conditions (treatment duration, furnace temperature, oxygen concentration, etc). In the present study, different morphologies of nanostructures (nanowires, nanoribbons or nanocombs) are synthesized by this process and these morphologies can be easily controlled with good reproducibility by adjusting experimental conditions, especially the total pressure in the reactor. Furthermore, the photocatalytic activity of these nanostructures is systematically investigated by measuring the degradation of a commonly-used organic dye in aqueous solution: methyl orange (MO). The photocatalytic activity of these ultrathin ZnO nanostructures will be also compared with the properties of flat ZnO thin films and thick ZnO nanowires.

Experimental procedure

Synthesis of ZnO nanostructures

ZnO nanostructures are synthesized by oxidation of Cu/Zn stacks (50 nm Cu/950 nm Zn) formerly deposited by DC magnetron sputtering (80W) on soda lime glass (SLG) substrates and on fluorine-doped tin oxide coated glass slide (FTO, 7 Ω sq -1 ). The experimental procedure was described in detail elsewhere [START_REF] Altaweel | Fast synthesis of ultrathin ZnO nanowires by oxidation of Cu/Zn stacks in lowpressure afterglow[END_REF]. Thicknesses are set by adjusting the sputtering time. The distance between the sample and the targets (50 mm in diameter, 3 mm thick and >99.99% purity for Zn target and >99.95% purity for Cu target) is 100 mm. The base pressure in the sputtering chamber is 5 × 10 -4 Pa and thin films are deposited at a pressure of 0.6 Pa in a 10 vol% H2-90 vol% Ar mixture. These films are oxidized by using a flowing microwave afterglow at low pressure (figure 1). A surface wave-driven plasma is ignited in a fused silica tube (inner diameter5mm) by exposure to 2.45 GHz microwaves launched with a Surfatron. The microwave power absorbed by the plasma is kept constant at 200 W. An Ar-2 vol% O2 mixture, flowing at 560 standard cubic centimetres per minute (sccm), is used as plasma gas. The samples are located in a quartz reactor tube (internal diameter of 28 mm) downstream the plasma glow region, at a distance of 60 cm from the plasma gap. The treatment starts when the plasma is turned on (t  0) concurrently with the injection of the reactive gas. The temperature of the sample during oxidation is controlled by an additional furnace (set at 500 °C).

Plasma and thin film characterizations

The surface morphology is imaged with a high-resolution scanning electron microscope (SEM) Philips XL-FEG 30 equipped with a TLD detector (through the lens detector).

The morphology, size distribution, composition and crystallographic features of the assynthesized nanostructures are characterized by high-resolution TEM (HRTEM).

Nanostructures are taken off from the samples by scratching the surface upon a holey carbon nickel grid. HRTEM investigations are carried out using a JEM-ARM 200F Cold FEG TEM/STEM operating at 200 kV and equipped with a spherical aberration (Cs) probe and image correctors (point resolution 0.12 nm in TEM mode). Transmittance measurements are done with a Cary 5000 UV-vis-NIR spectrophotometer in the ultraviolet, visible and near-infrared ranges.

Electron density measurements are carried out by using a 26.5 GHz microwave interferometer (Miwitron MWI 2650). This technique based on the determination of the refractive index of the plasma permits measurements of electron density at moderate pressure [START_REF] Grotjohn | Electron density in moderate pressure diamond deposition discharges Diamond[END_REF][START_REF] Derkaoui | Determining electron temperature and electron density in moderate pressure H2/CH4 microwave plasma[END_REF]. For optical emission spectroscopy (OES) measurements, the afterglow light is collected by an optical fibre connected to a 550 nm focal length monochromator (Jobin-Yvon TRIAX 550) equipped with a 1800 grooves mm -1 grating. The spectrometer is coupled with a Horiba Jobin-Yvon i-Spectrum Two iCCD detector.

Photocatalytic measurements

The photocatalytic activity of as-prepared samples (2 cm×1 cm) is evaluated by the degradation of MO dye (200 ml, 20 mg l -1 ) under light irradiation from a Xenon lamp (Newport Xe 300 W ozone free) with an air mass filter (AM 1.5 G) to better match the total solar spectrum. Before illumination, the reactor is magnetically stirred in the dark for 30 min to reach the adsorption-desorption equilibrium between the photocatalyst and the dye. The irradiance reaching the photoactive surface is measured with a PM100D Thorlabs radiometer equipped with a 200-1100 nm sensor and is set equal to 100 mW cm -2 . The concentration of MO is monitored by recording variations in the characteristic absorption band (464 nm) using an Ocean Optics S2000 spectrometer.

Photoelectrochemical measurements of as-prepared samples are performed by a SP150 BioLogic potentiostat connected to a standard three electrode cell: Pt wire as the counter electrode, a saturated Ag/AgCl electrode as reference electrode and FTO glass coated with nanostructured ZnO films (1 cm × 1 cm) as the working electrode. 0.1M Na2SO4 aqueous solution is used as electrolyte. The photocurrent response is recorded at a constant potential of +0.30 V versus the reference electrode.

Results and discussion

Plasma diagnostics

Figure 2(a) is a plot of the electron density estimated by interferometry as a function of distance from the Surfatron launcher exit (in the small-diameter quartz tube). The spatial distribution of electron density was assumed to be constant, radially. Therefore, this assumption gives the average value of the density across the line-of-sight path of the millimetre wave. The detection limit of microwave interferometer in the present configuration is approximately 1.0 × 10 11 cm -3 . As generally observed in Surface Wave Discharges (SWD), the electron density decreases linearly with the distance in the plasma column up to a critical density for surface wave propagation (ne ∼ 3 × 10 11 cm -3 ) [START_REF] Moisan | Contribution of surfacewave (SW) sustained plasma columns to the modeling of RF and microwave discharge with new insight into some of their features. A survey of other type of SW discharges Plasma Sources[END_REF]. This position corresponds to the transition from plasma to the early afterglow (at about 10.5 cm from Surfatron in our conditions).

The light emitted from the afterglow is weak and only visible in the dark with the naked eye. Three main contributions of oxygen are observed by OES: OH(A 2 Σ + , v '  0 X 2 Π, v ''  0) at 306 nm, the green line of atomic oxygen O( 1 S 1 D) at 557.7 nm and the atmospheric band of O2(b 1 Σg + X 3 Σ g -) at 762 nm (figure 2(b)). An argon line appears at 763.5 nm that is removed to determine the rotational temperature of the gas according to data available in [START_REF] Touzeau | Spectroscopic temperature-measurements in oxygen discharges[END_REF]. The rotational temperature of O2(b) state is generally considered as a good approximation of the kinetic temperature of O2 molecules in the ground state. A temperature of 330 ± 30 K is obtained, confirming the relatively low temperature of the afterglow. In a previous work, the absolute density of oxygen atoms was determined by NO titration method using an Ar-1.4 vol% NO gas mixture [START_REF] Mafra | Treatment of hexatriacontane by Ar-O2 remote plasma: formation of active species Plasma Process[END_REF]. This technique was performed at the end of the small fused silica tube, as a function of the total pressure. This two-step method, described in reactor with geometry close to that of this study [START_REF] Kutasi | Active species downstream of an Ar-O2 surface-wave microwave discharge for biomedicine, surface treatment and nanostructuring Plasma Sources[END_REF][START_REF] Vesel | Etching of blood proteins in the early and late flowing afterglow of oxygen plasma Plasma Process[END_REF].

Morphology and crystallographic structure of ZnO nanostructures

Figure 3 depicts SEM micrographs of nanostructures grown for 1 min of flowing afterglow oxidation as a function of the working pressure. By contrast with thermal oxidation process which requires relatively long treatment duration to obtain nanowires, the rapid growth of nanostructures in this study is attributed to the non-equilibrium behaviour of the afterglow, which provides directly active species on the sample surface. In these experimental conditions, the afterglow contains a concentration of oxygen atoms of about 10 15 cm -3 and a high concentration of excited oxygen molecules like O2(a 1 Δg), O2(X 3 Σg -) and to a lesser extent, O2(b 1 Σg + ) [START_REF] Kutasi | Active species downstream of an Ar-O2 surface-wave microwave discharge for biomedicine, surface treatment and nanostructuring Plasma Sources[END_REF]. As described in a previous work [START_REF] Altaweel | Fast synthesis of ultrathin ZnO nanowires by oxidation of Cu/Zn stacks in lowpressure afterglow[END_REF], copper thin film plays also a key role in the growth mechanism: this buffer layer prevents film evaporation and allows treatments at temperatures that are higher than the melting temperature of zinc (m. p.  419.5 °C) mainly due to the formation of CuZn5 intermetallic by interdiffusion, which melts at a higher temperature. The growth mechanism of ZnO nanowires seems to be driven by diffusion at grain boundaries, Zn having higher oxygen affinity than Cu. As generally observed in thermal oxidation at moderate temperature, zinc oxidation involves Zn ions outward diffusion from Zn-ZnO interface to the surface through grain boundaries or defects, creating sites on surface from which ZnO nanowires grow [START_REF] Zhao | Large-scale synthesis of bicrystalline ZnO nanowire arrays by thermal oxidation of zinc film: growth mechanism and high-performance field emission[END_REF]. The surface morphology is clearly highly dependent on the pressure. Ultrathin nanowires with fairly high surface density are synthesized at 200 Pa.

The average length of nanowires is estimated to be 750 nm with a mean diameter of 6 nm [START_REF] Altaweel | Fast synthesis of ultrathin ZnO nanowires by oxidation of Cu/Zn stacks in lowpressure afterglow[END_REF]. The growth of nanoribbons is observed at 1000 Pa. Nanoribbons are longer (average length ∼1500 nm) with a mean width of approximately 60 nm and an assessed thickness of about 5 nm (confirmed by TEM measurements). As the total pressure is increased to 2000 Pa during afterglow oxidation treatment, a large number of long ZnO nanocombs are observed among nanoribbons. ZnO nanocombs with dense nanowire branches grown on one side of nanoribbons contain very thin, straight, aligned and evenly spaced branches (∼20 wires μm -1 ).

The evolution of morphology with the working pressure could be mainly attributed to density profiles of active species which depend on experimental conditions including pressure, as observed by Kutasi et al. They especially observed a large increase in the main active oxygen species densities (O( 3 P), O2(a) and O2(b)) with pressure in the late afterglow (see figure 8 in [START_REF] Kutasi | Active species downstream of an Ar-O2 surface-wave microwave discharge for biomedicine, surface treatment and nanostructuring Plasma Sources[END_REF]), whereas their lifetimes are relatively independent of the value of pressure. Therefore, higher active oxygen species densities seem to promote the growth of nanoribbons and nanocombs.

Nanoribbons exhibit high aspect ratios with a non-uniform width distribution within a nanoribbon (figures 4(a) and (b)). Figures 4(c) and (d) show HRTEM micrographs taken from an individual nanoribbon, including its fast Fourier transform pattern. As in the case of nanowires [START_REF] Altaweel | Fast synthesis of ultrathin ZnO nanowires by oxidation of Cu/Zn stacks in lowpressure afterglow[END_REF], the values of measured angles and distances between atomic planes reveal that all the tested nanoribbons can be indexed by the ZnO wurtzite structure (space group: P63mc(186), hexagonal, a  b  3.2494 Å, c  5.2038 Å) [START_REF] Kihara | Anharmonic thermal vibrations in ZnO Can[END_REF]. Again, ZnO nanoribbons have also high-quality, single-crystalline structure with no preferential growth direction after analysis of a large set of nanoribbons randomly taken in different areas on the surface of the sample.

Copper is present in nanostructures only as traces, with concentrations typically inferior to 1 at% (EDS analyses). In both cases, the growth mechanism is based on the outward diffusion of zinc ions through grain boundaries and defects in overlayers underneath nanostructures [START_REF] Altaweel | Fast synthesis of ultrathin ZnO nanowires by oxidation of Cu/Zn stacks in lowpressure afterglow[END_REF].

Photocatalytic activity of ZnO nanostructures

Methyl orange (MO) is selected as a model organic pollutant to evaluate the photocatalytic activity performance of ZnO nanostructures with different morphologies. In order to evaluate the influence of ultrathin nanostructures on MO degradation, the photocatalytic activities of ZnO nanowires and ZnO nanoribbons are compared with pure ZnO thin films without nanostructures and ZnO nanowires with larger diameters (mean diameter of about 60 nm). Pure ZnO thin films and thick nanowires have been synthesized by thermal oxidation for 3 h in air at 300 °C and 400 °C respectively, from 1 μm thick Zn thin films formerly deposited by DC magnetron sputtering [START_REF] Ren | Catalyst-free synthesis of ZnO nanowire arrays on zinc substrate by low temperature thermal oxidation[END_REF]. The results indicate that ultrathin ZnO nanowires and nanoribbons exhibit similar trends with a good photocatalytic activity in aiding the degradation of MO, approximately 92% after irradiation for 420 min. By comparison, the photolysis process, pure ZnO without nanostructures and thick ZnO nanowires (mean diameter of about 60 nm) under identical conditions present degradation of MO of 18%, 33%

and 62%, respectively. The photodegradation kinetics of MO could be fitted to a pseudofirst order reaction and the rate equation is expressed as: ln (C / C0)  -kt. The apparent reaction rate constant k is calculated to be 1.5 × 10 -3 min -1 (R 2  0.9830) 2.5 × 10 -3 min -1 (R 2  0.9837), 7.0 × 10 -3 min -1 (R 2  0.9961) and 6.5 × 10 -3 min -1 (R 2  0.9957) for pure ZnO thin film without nanostructures, thick ZnO nanowires (∅ ∼ 60 nm) synthesized by thermal oxidation, ultrathin ZnO nanowires (∅ ∼ 6 nm) and ZnO nanoribbons synthesized by plasma afterglow oxidation process, respectively. The corresponding squares of linear correlation coefficients (R 2 ) confirm a pseudo-first order reaction in degradation kinetics.

These rate constant values confirm that the degradation rate constant is higher for ZnO catalysts with ultrathin nanostructures. The surface area plays a significant role on the photocatalytic activity. Thus, nanostructured materials offer generally enhanced photodegradation kinetics due to their extremely large surface to volume ratios. Moreover, the photocatalytic activity depends also on the effectiveness of photogenerated charge carrier separation: photoinduced electrons and holes have strong oxidation and reduction activities leading to MO degradation. The main limitation is due to the high recombination rate of these charge carriers. In this work, ultrathin nanowires and nanoribbons provide at least one dimension extremely low (<10 nm). In this case, charge carriers need to diffuse only over a short distance to reach active sites on the surface and react with water molecules before recombination. High crystallinity can also enhance the charge diffusion and thus decrease the possibility of charge recombination within nanostructures. On the other hand, the presence of a small amount of copper in the sublayer (i.e. underneath the nanostructures) increases crystal defects, which could serve to trap electrons or holes and then restrain their recombination in the film [START_REF] Xu | Enhanced photocatalytic activity of hierarchical ZnO nanoplatenanowire architecture as environmentally safe and facilely recyclable photocatalyst[END_REF].

These results are confirmed by photoelectrochemical measurements, because the photocurrent is linked to the efficient generation, separation and transportation of photogenerated electron/hole pairs during light irradiation. As it can be seen in figure 6, all ZnO-coated electrodes exhibit fast and reversible photoresponses with a good reproducibility over many cycles. It is important to note that the growth mechanism of nanostructures is not affected by the nature of the substrate: morphologies, structures and compositions of nanostructures are similar on FTO substrates and on SLG substrates. Ultrathin ZnO nanowires and nanoribbons show the highest photocurrent (respectively 12 and 2.5 times higher than ZnO thin films and thick ZnO nanowires synthesized by thermal oxidation), which confirms the reduction of charge carrier recombination.

However, electron-hole pairs are generated when semiconductors are irradiated by photons with energy equal to or greater than its bandgap. In order to evaluate the bandgap of ZnO nanostructures with copper in the sub-layer, the absorption spectrum of ZnO nanowire sample is investigated through UV-visible spectrometry (figure 7). Absorbance corresponding to SLG substrate is systematically subtracted. A strong absorption peak is observed in the UV region with an absorption edge at about 375 nm (∼3.3 eV). The optical bandgap energy (Eg) can be calculated from the Tauc's formula, which is for direct allowed transition [START_REF] Viezbicke | Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system[END_REF]: (αhν) 2  A(hν -Eg), 2 where h is Planck's constant, ν is the photon frequency, α is the absorption coefficient and A is a constant, respectively. The inset of figure 7 shows the Tauc's plot for ZnO nanowires. The optical bandgap is determined by the interception between the tangent to the linear portion of the curve and the horizontal axis. are separately added into the MO solution. The trapping experiments are carried out under light irradiation using the same experimental conditions. The degradation efficiencies decrease from 92% without scavengers to 77%, 74%, 55% and 25% in the presence of DMSO, AO, t-BuOH and BQ, respectively, which is consistent with the results of Chen et al [START_REF] Chen | Preparation of ZnO photocatalyst for the efficient and rapid photocatalytic degradation of azo dyes[END_REF] and Huang et al [START_REF] Huang | One-step pyrolytic synthesis of ZnO nanorods with enhanced photocatalytic activity and high photostability under visible light and UV light irradiation[END_REF]. Thus, the superoxide O2•-radical is the main reactive species involved in the photocatalytic degradation of MO with ultrathin ZnO nanowire coated electrode under light irradiation.

Photocatalytic stability is another important limitation in the development of photocatalysts for organic dye degradation, mainly due to photocorrosion in the case of ZnO [START_REF] Lee | Recent developments of zinc oxide based photocatalyst in water treatment technology: a review[END_REF][START_REF] Cao | Photocatalytic activity and photocorrosion of atomic layer deposited ZnO ultrathin films for the degradation of methylene blue[END_REF]. In order to estimate the photostability of ultrathin ZnO nanowires, MO degradation is observed in repeated runs in similar experimental conditions. Five photodegradation cycles of MO solution are conducted (figure 9), which reveal only a small reduction in MO degradation after each cycle (mean value of 2.7%). Nanostructured ZnO sample continues to show considerable photocatalytic activity even after five cycles. Therefore, ZnO nanowires exhibit good stability during the photocatalytic test, possibilities of reusability and thus could be a potential candidate for practical applications.

Conclusion

Ultrathin The possibility of reusability of ZnO nanowire samples is confirmed by a high photocatalytic activity even after several cycles of photodegradation. Thus, ultrathin ZnO nanostructures, synthesized by microwave plasma afterglow oxidation process could be considered as promising candidates to degrade organic pollutants in water.

[ 29 ]

 29 , leads, for example, to an oxygen atom concentration [O]  1.1 ± 0.3 × 10 15 cm -3 at a pressure of 200 Pa. These results are consistent with the calculated values predicted by Kutasi et al. by using self-consistent theoretical models developed to investigate the early and remote flowing afterglows of a surface-wave Ar-O2 microwave discharge generated in a

Figure 5 (

 5 figure 5(b).Figure 5(c) shows the relative concentration (C/C0) of MO as a function of time

Figure 5 (

 5 figure 5(b). Figure5(c) shows the relative concentration (C/C0) of MO as a function of time under light irradiation, where C is the concentration of MO at irradiation time (t) and C0 is the concentration of the dye before sample immersion. As a control, the absorbance peak of MO solution is also monitored with photocatalysts in the dark and without photocatalysts under light illumination. In the latter case, MO solution is irradiated to estimate the degree of degradation of MO only by photolysis and ensure the action of photocatalytic reaction on MO by all the catalysts. It can be observed that the MO dye concentration after magnetic stirring in the dark for 30 min only slightly decreases in presence of ZnO, indicating that the adsorption of MO on the surface is limited after the adsorption-desorption equilibrium is reached.

  photocatalytic activity compared with pure ZnO thin films without nanostructures and thick ZnO nanowires synthesized by thermal oxidation. The enhancement in photocatalytic performance could be attributed to high surface area. Moreover, these nano-objects provide at least one extremely low dimension (<10 nm). As a consequence, charge carriers need to diffuse only over a short distance to reach active sites on the surface and then can react with water molecules before their recombination. However, the results indicate that the photoinduced electrons and holes are generated almost exclusively under UV light irradiation.

Figure 1 :

 1 Figure 1: Experimental set-up of the flowing afterglow oxidation process.

Figure 2 :

 2 Figure 2: (a) Evolution of electron density in the plasma column as a function of the distance from the Surfatron determined by microwave interferometry. (b) Experimental (black) and simulated (red) rotational spectrum of the atmospheric band of molecular oxygen O2 (b 1 Σg + → X 3 Σg -) recorded in the afterglow at a distance of 40 cm from the Surfatron. (560 sccm, Ar-2 vol% O2, 200 Pa).

Figure 3 :

 3 Figure 3: SEM micrographs of ZnO synthesized in flowing microwave afterglow oxidation process as a function of working pressure: (a) nanowires at 200 Pa, (b) nanoribbons at 1000 Pa and (c) nanocombs at 2000 Pa. (Afterglow oxidation duration  1 min).

Figure 4 :

 4 Figure 4: (a), (b) General views of by TEM. (c), (d) HRTEM micrographs of an individual nanoribbon at different magnifications and the corresponding FFT pattern indexed by ZnO wurtzite structure.

Figure 5 :

 5 Figure 5: (a) Absorption spectra showing temporal evolution of MO degradation upon light irradiation in the presence of ZnO nanowires as photocatalyst and (b) corresponding pictures of MO solution at different exposure durations. (c) MO degradation rate without photocatalyst and with pure ZnO thin film, thick ZnO nanowires (∅ ∼ 60 nm), ultrathin ZnO nanowires (∅ ∼ 6 nm) and ultrathin ZnO nanoribbons, respectively.

Figure 6 :

 6 Figure 6: Transient photocurrent density responses of ZnO photocatalyst electrodes under light on-off cycles.

Figure 7 .

 7 Figure 7. UV-vis absorption spectrum of ultrathin ZnO nanowires sample. The inset shows the corresponding plot of (αhν) 2 versus (hν).

Figure 8 :

 8 Figure 8: Influence of scavengers on the photodegradation of MO in the presence of ultrathin ZnO nanowire coated electrode under light irradiation.

Figure 9 :

 9 Figure 9: Recycling test in photocatalytic degradation of MO dye in water over ultrathin ZnO nanowires under light irradiation.

  The results indicate a slight decrease in the optical bandgap value (∼3.03 eV) by comparison with the accepted literature value of 3.27 eV for ZnO thin films[START_REF] Viezbicke | Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system[END_REF]. Similar optical bandgap values are obtained for ZnO nanoribbons and ZnO nanocombs (2.98 eV and 3.06 eV, respectively). While the accuracy of this technique may be debatable, it seems to mean that ZnO nanostructures can mainly be activated under UV light to generate electrons and holes for the MO photodegradation, even in presence of copper in the sub-layer.

	To understand the mechanism of dye degradation, different scavengers are used to
	investigate the main active species during the photocatalytic degradation of MO over ultrathin
	ZnO nanowire coated electrode (figure 8). p-benzoquinone (BQ, O2•-radical scavenger,
	0.1mM), ammonium oxalate monohydrate (AO, hole scavenger, 10mM), dimethyl sulfoxide
	(DMSO, electron scavenger, 10mM) and tert-butanol (t-BuOH, •OH radical scavenger, 10mM)

Acknowledgments

The authors wish to thank the 'Réseau des Plasmas Froids' of CNRS/MI (France) for providing the microwave interferometer (Miwitron MWI 2650).