The Cenozoic activities of Yangjiang-Yitongdong Fault: insights from analysis of the tectonic characteristics and evolution processes in western Zhujiang(Pearl) River Mouth Basin

The Yangjiang-Yitongdong Fault (YJF) is an important NW-trending regional fault, which divides the Zhujiang (Pearl) River Mouth Basin (ZRMB) into western and eastern segments. In Cenozoic, the northern continental margin of the South China Sea (SCS) underwent continental rifting, breakup, seafloor spreading and thermal subsidence processes, and the Cenozoic activities of YJF is one part of this series of complex processes. Two long NW-trending multichannel seismic profiles located on both sides of the YJF extending from the continental shelf to Continent-Ocean Boundary (COB) were used to study the tectonic and sedimentary characteristics of western ZRMB. Using the 2D-Move software and back-stripping method, we constructed the balance cross-section model and calculated the fault activity rate. Through the comprehensive consideration of tectonic position, tectonic evolution history, featured structure, and stress analysis, we deduced the activity history of the YJF in Cenozoic. The results showed that the YJF can be divided into two segments by the central uplift belt. From 65 Ma to 32 Ma, the YJF was in sinistral motion as a whole, inherited the preexisting sinistral motion of Mesozoic YJF, in which, the southern part of YJF was mainly in extension activity, controlling the formation and evolution of Yunkai Low Uplift, coupled with slight sinistral motion. From 32 Ma to 23.8 Ma, the sinistral motion in northern part of YJF continued, while the sinistral motion in southern part began to stop or shifted to a slightly dextral motion. After 23.8 Ma, the dextral motion in southern part of YJF continued, while the sinistral motion in northern part of YJF gradually stopped, or shifted to the slightly dextral motion. The shift of the YJF strike-slip direction may be related to the magmatic underplating in continent-ocean transition, southeastern ZRMB. According to the analysis of tectonic activity intensity and rift sedimentary structure, the activities of YJF in Cenozoic played a regulating role in the rift extension process of ZRMB.

Crustal S-wave velocity structure across the northeastern South China Sea continental margin: implications for lithology and mantle exhumation

The northeastern margin of the South China Sea (SCS), developed from continental rifting and breakup, is usually thought of as a non-volcanic margin. However, post-spreading volcanism is massive and lower crustal high-velocity anomalies are widespread, which complicate the nature of the margin here. To better understand crustal seismic velocities, lithology, and geophysical properties, we present an S-wave velocity (VS) model and a VP/VS model for the northeastern margin by using an existing P-wave velocity (VP) model as the starting model for 2-D kinematic S-wave forward ray tracing. The Mesozoic sedimentary sequence has lower VP/VS ratios than the Cenozoic sequence;in between is a main interface of P-S conversion. Two isolated high-velocity zones (HVZ) are found in the lower crust of the continental slope, showing S-wave velocities of 4.0–4.2 km/s and VP/VS ratios of 1.73–1.78. These values indicate a mafic composition, most likely of amphibolite facies. Also, a VP/VS versus VP plot indicates a magnesium-rich gabbro facies from post-spreading mantle melting at temperatures higher than normal. A third high-velocity zone (VP : 7.0–7.8 km/s;VP/VS: 1.85–1.96), 70-km wide and 4-km thick in the continent-ocean transition zone, is most likely to be a consequence of serpentinization of upwelled upper mantle. Seismic velocity structures and also gravity anomalies indicate that mantle upwelling/ serpentinization could be the most severe in the northeasternmost continent-ocean boundary of the SCS. Empirical relationships between seismic velocity and degree of serpentinization suggest that serpentinite content decreases with depth, from 43% in the lower crust to 37% into the mantle.

Fingerprinting Patterns of SSR Markers Constructed for 33 Loquat[Eriobotrya japonica (Thunb. ) Lindl. ]Germplasms

[ Objective ] This study aimed to explore the ways of using SSR technology to construct the fingerprinting maps of the loquat. [ Method ] Fingerprinting patterns of the SSR markers for 33 loquat varieties in national loquat germplasm resource garden were constructed by specifically amplifying palymorphism from 7 pairs of primers screened from 51 pairs of primers of the relatives Mains and Pyrus. [ Result ] The results showed that the uses of 7 primers, namely, Hi01d05F, Hil2iO4F, CH02B10F, Hi03a03, Hi03e04, Hi04a05 and Hi02c07 could distinguish 33 loquat germplasms from one another completely. [ Conclusion] This study provides an essential criterion for classification and identification of the loquat resources.

Crustal velocity structure of the Northwest Sub-basin of the South China Sea based on seismic data reprocessing

The deep crustal structure of the Northwest Sub-basin(NWSB)of the South China Sea(SCS)is of great importance for understanding the tectonic nature of the continent-ocean transition(COT)and magmatism in this oceanic basin.The 2-D wide-angle reflection/refraction seismic profile OBS2006-2 is almost parallel to the extinct spreading ridge(ESR)of the NWSB.In addition to the original data,we added the data of two reprocessed OBS stations,and carried out seismic phase re-picking and travel-time imaging to obtain the crustal velocity structure along this profile.Resolution tests demonstrate that the newly acquired velocity structure is more reliable than the prior interpretation.The depth of the Moho(23.5–11.8 km)and crustal thickness(20.5–6.5 km)systematically changes from continental crust of the Xisha Block to the oceanic crust within the NWSB.The COT zone has a width of^20 km and the depth of the Moho decreases from 15.0 to 11.0 km,corresponding to a^4 km decrease in crustal thickness(6–10 km).A high velocity layer(HVL,7.2–7.4 km s–1)exists at the bottom of the crust at the location where the sharp lateral transition of the continental crust to the oceanic crust occurs.Age dating shows that the Doublepeak Seamount was formed at^23 Ma,after the cessation of the NWSB seafloor spreading(~32–25 Ma).The crust beneath the Double-peak Seamount is oceanic with a thickness of 9 km.We infer that this oceanic crust was formed by magmatic upwelling and decompression melting along a pre-existing zone of weakness.

Fault-controlled regional magmatism and mineral deposition in central Cathaysia——Evidence from ambient noise tomography

Guangdong Province in the central Cathaysian Block has two world-class metallogenic belts, namely, the Nanling and Southeastern Coastal Metallogenic Belts(NLMB and SCMB), which are spatially coincide with the major regional Ganjiang and Zhenghe-Dapu Fault Zones(GJFZ and ZDFZ). However, what roles the faults played in mineral deposition and magmatism is unclear. Using ambient noise tomography, we obtain a 3-D whole-crust shear wave velocity model. By combining available regional geophysical models, we characterize the architecture of the regional shallow lithosphere and infer its possible tectonic connection to magmatic sources, pathways and surface deposition. The results show that the study area is loosely divided by the two major faults, the GJFZ and ZDFZ, into distinct velocity domains. In the north high Vand low V/Vcrust in the NLMB imply crustal remelting, which leads to the general felsic composition. In the coastal area, the lower crustal high Vanomaly is attributed to upwelling melts associated with Cretaceous magmatic activity. Between mineral belts, a swath of crustal lowvelocity zones extend into the uppermost mantle, manifesting partial melting related to upwelling magmas that may hint at a deep origin of magma from subcrustal lithosphere and likely feed surface mineral deposits through major faults. Secondary NW-trending faults coincide with low velocities and facilitated magmatic migration. A correlation between coastward extension of low velocities and younging of the Jurassic and Cretaceous magmatism is suggestive of a combined effect of slab rollback and a change in the direction of the Paleo-Pacific subduction system. We speculate a regional fault-control model in the central Cathaysian Block for the spatial-temporal evolution of regional deformation and magmatism during the middle Mesozoic.