Figure 3. (a) Stacking profile of all the receiver functions after the Pms, PmpPms, PmpSms moveout corrections with a Gaussian factor of 5.0. The solid black line indicates the error during the stacking in each interval, and it is calculated by using the standard deviation with 100 resamplings based on the 1-D bootstrap method. (b) The top panel shows the topographic relief along the profile. The middle two panels are the number of stacked receiver functions and the arrival times of the Pms, PmpPms, and PmpSms phases. The bottom two panels present the crustal thickness and Vp/Vs ratio calculated by the H–k method. The error is indicated by the differences calculated by the H–k method using the arrival times after the Pms and PmpPms moveout corrections. The original data is show in Supplementary materials (Tabel S1).
4 Dicussions
For the exploration of the gold mineralization beneath the Yuegui and Yuezhong blocks, a shorter dense seismic array was oriented in nearly the same direction to us but slightly biased to the west. From the obtained 233 teleseismic receiver functions based on 112 portable EPS-2 short-period seismometers, Li et al. (2021) deducted that a slightly uplifted Moho and the presence of a massive negative phase may be correlated with magmatism and mineralization beneath the Luoding–Yunfu nappe tectonic zone. After acquiring data from another 400 km long wide-angle seismic profile based on six active sources and approximately 54 DZSS-1 seismographs, Zhang et al. (2013) suggested that the Moho east of the Chenzhou–Linwu fault belt is obviously uplifted which can be considered as the convergent boundary bwteen the Yangtz and Cathaysia blocks, while the low P-wave velocity anomalies in this region that may have been caused by an upwelling of hot asthenosphere.. More recently, employing both active and passive dense seismic arrays with the same starting point as our array (but a southernmost geophone situated more toward the east), Deng et al. (2019) obtained more than 10,000 receiver functions from 2010 to 2015. The most stable feature was a slightly uplifted Moho towards the southeast in their stacking profile. Combined with a velocity model inverted by using surface waves, they found possible lithospheric thinning and the potential upwelling of hot material beneath the centre of Guangdong Province. In another study based on SinoProbe reflection profiling, Dong et al. (2020) found that the structure of the Zhenghe–Dapu fault could be depicted with a stack of parallel reflectors dipping 30° to the northwest. From the above results, it appears that an abnormally weak suture zone may be distributed along the coastal area of the Cathaysia block that is spatially consistent both with the metamorphic core in the Wuyi–Nanling–Yunkai domain distributed along the Zhenghe–Dapu and Gaoyao–Huilai deep faults.
The uplift of the Moho to the southeast in the coastal SCB may be connected to the passive continental margin (Bai et al., 2015). Other evidence indicates that the lithosphere-asthenosphere boundary (LAB) discontinuity also exhibits an uplift trend from the continental margin of the SCB to the northern SCS, and variation of Moho in our profile exhibits the same characteristics as previous results (Tang & Zheng, 2013). In addition, the most obvious feature in our results is the abnormal deepen of Moho beneath 23.5~22.5°N, which can be observed from the aligning profile and CCP imaging with different Gaussian factors (Figs 2, S2, and S3). Simultaneously, multiple converted waves in our results (PmpPms, PmpSms) display a similar pattern as the Pms phase (Fig 3 a). The relatively large Vp/Vs ratio (≥1.8) beneath the same area may indicate the presence of melt within the crust (Christensen, 1996). The Pms phase beneath 23.7°N shows an offset pattern, and we infer that it may be due to the extension of the deep Gaoyao–Huilai fault, which is served as a deep channel for the upwelling hot materials (Fig 4). Combined with a velocity model (Zhou et al., 2012), it can be observed that hot materials appear to accumulate beneath 23.5°N and 22.5°N (Fig S4). The consistent trend of the negative phase above the deepen Moho supports the upwelling of hot materials along the hot channel beneath approximately 23.7°N (Fig 2 a). The occurrence of the Mw 4.2 earthquake adjacent to this hot channel is evidence of the existence of such a hot channel along the deep fault (Fig 4).
According to the previously reported zircon U–Pb ages of different groups of granitic gneisses, Wang et al. (2014) proposed that the SCB was formed by the amalgamation of various fragments following the episodic closure of a series of northwestern arc-back systems along Wuyi–Yunkai, Shuangxiwu and Jiangnan at ~920 Ma, ~890 Ma, and ~830 Ma, respectively. The collision boundary between the East and West Cathaysia blocks may be an extension of the Zhenghe-Dapu fault, which is connected to the Gaoyao–Huilai fault to the southeast. Under these assumptions, a previous study confirmed that parallel reflectors dipping 30° to the northwest may represent an extension of the Zhenghe–Dapu fault (Dong et al., 2020). From our results, we support the thickened crust beneath around the Gaoyao-Huilai deep fault, while the relatively large Vp/Vs ratio above the thickened crust may be caused by the upwelling of hot materials. Furthermore, it should be mentioned that these hot materials may have formed by the slab rollback of the subducted Paleo-Pacific plate from the Late Cretaceous to late Cenozoic (Jiang et al., 2015); another possibility is that it was formed by the lateral flow of hot mantle from the Hainan plume (Zhang et al., 2020).
Taking the above considerations, we present a simple model for the variation in the Moho depth beneath the Gaoyao–Huilai and Zhenghe–Dapu faults that may represent the suture zone between the amalgamated East and West Cathaysia blocks. Moreover, the outcrops of metamorphic S-type granites indicate that the amalgamation history is heterogeneous along the two deep faults. Given the lower abundance of metamorphic rocks in the Wuyi–Nanling–Yunkai domain, we deduce that the Nanling domain may be a buffer zone (112–115°N) of the collision between the West and East Cathaysia blocks, as depicted by the pink shading in Figure 1. The Yunkai and Wuyi domains, which are depicted with red shading, may have formed through lithospheric delamination process (Gao et al., 2004) and followed by an intraplate orogeny, represent the severe collision zone of the East and West Cathaysia blocks.