The Late Mesozoic Structure Systems In The Northern South China Sea Margin: Geodynamics And Their Influence On The Cenozoic Structures In The Pearl River Mouth Basin

The South China Sea(SCS)is located in the junction region among the Eurasian Plate,Indo-Australia Plate and Pacific Plate,and is a Cenozoic marginal sea developed as the result of the rifting and seafloor spreading in the South China margin.Such location of the SCS make it important for reconstructing the plate tectonic evolution of the whole SE Asia from the late Mesozoic to the early Cenozoic.However,for the lack of geological evidence,how the background of SCS area has evolved from the earlier subduction of the Paleo-Pacific Plate beneath the South China margin to the Cenozoic rifting regime linked with the opening of the SCS is not yet clear,and several debated models existed in literature,hindering the reconstruction of the regional tectonic evolution.Furthermore,studied on rift basins worldwide all show that the rift structure and evolution is influenced not only by the physical processes at play during rifting but also largely by the preexisting condition of the lithosphere,especially the pre-existing structures within it.However,the majority of the existed studies on the SCS tectonic evolution have focused on the Cenozoic physical processes,such as the dynamic mechanism of the SCS opening,the break-up pattern of the lithosphere,structure of the conjugated rifted margin,the ages of the oceanic crust and the pattern of the sea-floor spreading process,the multiphase rifting process and the post-rift neotectonics et al.,while it is relatively lacking for studies on the influence of pre-existing structures on the Cenozoic rifting evolution.Using abundant 3D seismic reflection and drilling data,this paper carried out an integrated study on the pre-Cenozoic structure systems within the basement of the Zhu 1 Depression in the Pearl River Mouth Basin of the proximal domain of the northern SCS margin.Based on the study of the pre-Cenozoic structure systems and integration of geological evidence form the onshore South China margin and other areas around SCS,we discuss the late Mesozoic geodynamic evolution of the northern SCS margin.Furthermore,we study how these pre-Cenozoic systems have exerted an influence on the Cenozoic structural evolution of the Zhu 1 Depression,such as the basin structure and development of rifting faults.Base on interpretation a large amount of seismic data,we identified three Late Cretaceous structure systems for the first time in the northern SCS margin,which all developed on the basement that dominated by the Late Jurassic to the Early Cretaceous Andean-type continental arc-related granitoids.The relative chronology of these three Late Cretaceous structure systems are well determined by abundant crosscutting relationships on 3D seismic sections,and are defined as follow in order: the WNWstriking thrust system,ENE-striking extensional fault system,and 3)ENE-striking thrust system.(1)The EW-to WNW-striking thrust system mainly consist of a dense network of high-angle faults,with the development of imbricate structures,lense structures and kink structures,which reflect the strong compressive deformation of the brittle granitoidsdominated basement.The EW-to WNW-striking thrust system shows a regional doublyverging structure,consisted of a northern portion of N-verging thrusts and a southern portion of S-verging thrusts.Such large structure has a configuration of a large positive flower structure in seismic section,which is usually suggestive of a transpressive strain regime.This EW-to WNW-striking thrust system was largely crosscut by the later NEEstriking thrust system.(2)The Late Cretaceous extensional system controls the development of the Late Cretaceous rift basin,displaying an overall ENE strike while showing EW-to WNW-striking in some place as a result of reactivation of the earlier EW-to WNW-striking thrust system.This Late Cretaceous was involved in the later ENE-striking thrust system,and is sporadically distributed in the study area as a result of being uplifted and eroded.(3)The ENE-striking thrust system forms the latest,and it largely crosscut the earlier EW-to WNW-striking thrust system and also the ENE-striking rift system.Faults belonging the ENE-striking thrust system mainly dips towards the SSE,are more widely spaced in comparison to faults of the EW-to WNW-striking thrust system,and have an overall lower dip angle.These three structure systems within the basement of the Cenozoic Zhu 1 Depression,Pearl River Mouth Basin,northern SCS margin,are considered to resulted from multiphase geodynamic evolution the South China Sea area during the Late Cretaceous,which reflects a more complex transitional tectonic process form the late Mesozoic to the early Cenozoic than previously considered.(1)The northern SCS margin have been in the Andean-type continental arc background that related to the low-angle subduction of the Paleo-Pacific Plate from the Late Jurassic to the Early Cretaceous.A sharp change of plate motion of the Paleo-Pacific Plate resulted in the intense oblique convergence between the Paleo-Pacific Plate and the Eurasia Plate around 100 Ma at the Early-Late Cretaceous boundary.As a result,a left-lateral transpressional event that took place along the SE South China margin,which led to the development of a large-scale Riedel shear system,including the coexistent development of many known NE-striking sinistral shear zones(the R Shear)and NW-striking dextral shear faults(the R' Shear)in the coastal area,and the EW-WNW-striking thrusts or reverse faults within the basement of the Zhu 1 Depression in this paper(the deformation perpendicular to the maximum shortening direction).(2)The slab rolled back and high-angle subduction of the Paleo-Pacific Plate led to broad back-arc extension in the northern SCS margin and the South China margin in the early Late Cretaceous(~100 Ma to 72 Ma),with the development ENE-trending rift basins.This extensional stage is considered to be related to the rifting process prior to the opening of the Proto-SCS.(3)The ENE-striking thrust system in the northern SCS margin is suggested to have developed in response to process of ridge push related to the sea floor spreading of the Proto-South China Sea in the late stage of the Late Cretaceous(~72 Ma to 66 Ma).The pre-Cenozoic structures have significant control on the Cenozoic basin structure and evolution in the northern rifted margin of the SCS,and is the key factor responsible for the spatial variation of the rift structure.Specifically,in the Zhu 1 Depression,the EW-to WNW-striking thrust system and the ENE-striking thrust system are two significant pre-existing structures,the locally diverse reactivation of which is the essential reason for the development various rift structures,including the coexistent development of two non-collinear rift structures(EW-to WNW-striking and ENE-striking),and highand low angle rifting faults.The combination mode and vertical crosscut relationship of the two pre-existing structures play an important role in controlling the combination and superposition of the Cenozoic rift structures.Furthermore,the influence of the two preexisting structure systems run through the Cenozoic structural evolution process including the multiphase rifting stage and the post-rift neotectonic periods.The two noncollinear pre-existing structure systems undergone multiphse and selective reactivation in the context of multiphase extensional evolution due to the constant clockwise rotation of the extension direction.(1)During the first rifting phase(Wenchang stage),in the context of NNW-SSE extension,both the two pre-existing structure systems were reactivated and resulted in the contemporaneous development of both ENE and EW-to WNW striking rifting structures,and some complex “Z” shape and “V” shape bounding faults,along with their associated depocenters.However,in the context of NNW-SSE extension,the ENE-striking pre-existing struture system was in a more extension-orthogonal trend,and was generally more prone to be reactivated.As a result,the activity of the ENE-striking bounding faults generally started earlier and is much intense relative to the EW-to WNWstriking bounding faults,and they were more likely to control larger,deeper thick depocenters with the development of more third-order sequences,such as the EP17 and XJ33 Sub-sag.(2)During the syn-rift 2 Enping stage,the extension direction has rotated clockwise into N-S,the EW-to WNW thrust system(especially the EW-striking faults)become more perpendicular to the extension direction,while the ENE-striking faults were no longer the most optimally oriented.As a result,more pre-existing faults of the EW-to WNW-striking struture system were reactivated at this stage,and the activity of the EWto WNW-striking normal faults developed during the Wenchang stage increased greatly,such as the HZ13,HZ13 S,XJ24,and HZ08 bounding faults,which controlled the development of the major depocenters.In contrast,many of the major syn-rift 1 ENEstriking normal faults sharply decrease their activity or became either inactive,such as the XJ33,PY4 and XJ23 bounding faults.(3)The extensional direction underwent a further clockwise rotation in the post-rift stage to NNE-SSW,which became approximate orthogonal to the WNW-striking faults while high-angle oblique to the ENE-striking faults.As a result,the syn-rift the ENE-striking normal faults generally become inactive and buried,while most of the EW-to WNW-striking faults(especially the WNW-striking faults)have remained active.Besides,some of the post-rift faults directly reactivated preexisting WNW-striking structures that were un-reactivated during the syn-rift stage,especially in the high portion,such as the northern margin of the Dongsha Uplift.