| The Pearl River Mouth Basin is a Cenozoic rift basin developed in the northern continental margin of the South China Sea.The Zhu?Depression is an important hydrocarbon-bearing depression on its northern margin.The Cenozoic Zhu ?Depression is located at the intersection of the tectonic stress of the three plates of the Eurasian Plate,the Pacific Plate,and the Indian Plate.It is influenced by the subduction of the Pacific Plate and the Indian-Eurasian plate collision and impact,because of which it has a unique tectonic stress field environment and Complex evolutionary history.The typical dual structure is "breaking up and falling down" and the evolutionary history of "Land first sea last",with the development of the Paleogene Wenchang Period(Zhuqiong Movement I,Tg-T80),Enping Period(Zhuqiong Movement II,T80-T70)Terrestrial rifting lake basin sediments and the late Paleogene-present(T70-T0)marine-continental interaction-marine depression deposits.The Eocene Zhu Qiong Movement I(abbreviated as I rift)acted as the first large-scale rift after the transition from the active continental margin to the passive continental margin in the Pearl River Mouth Basin,and was the most important and most intense rifting period in the early Cenozoic era.It is also the main source rock of the Pearl River depression.Under the background of tension and tectonic stress mechanism,the rifting depression with intracontinental rifting developed in the I curtain rift of the Zhuoqiong Movement.Compared with typical rifting basins,it has unique structure and structural characteristics.Within the limited area,there are a large number of complex fault systems with different strikes,and many different rifts(depressions/depressions)with different types of structures,characteristics and dynamic properties who spreading spatially.Evolutionary characteristics have long puzzled relevant researchers.In this paper,through theoretical investigation and research,it was found that in addition to being controlled by regional stress factors,the tectonic evolution of the Zhu?Depression has a direct genetic link with the basement properties of the rifting structure(basal pre-existing structure and substrate material Homogeneity).The formation of complex fracture system and rifting structure of Zhu?Depression are mainly controlled by the regional stress and the basement properties.The Pearl River Mouth Basin on the northern continental margin of the South China Sea develops on the bases of Caledonian and Indosinian and Yanshan fold belts.It has a complex composition and distribution of basement lithologies;in the former rifting basin,it undergoes multi-period tectonic movement,and extensively develops a large number of different strikes and properties.During the initial development of rifting,complex basement lithology and pre-existing structures with different properties may play an important role in controlling the development and distribution of faults and the formation and evolution of rifts.They are jointly controlled by regional extensional stresses.This may be the root cause of the development of a complex rift in the Pearl River depression.The study takes the Zhu?Depression of the Pearl River Mouth Basin as an object,and uses drilling,seismic,gravity,and magnetic data as the basis to comprehensively apply seismic profile analysis,seismic attribute clustering under the constraints of drilling,weight,and wavelet multi-scale decomposition of magnetic data.The development and distribution of the basement lithology and pre-existing structures(pre-existing faults)in the Zhu ? Depression were carefully characterized and systematically analyzed from depressions and even depressions.In combination with regional stress,we studied the differences in basal properties(basal pre-existing structure and lithological heterogeneity)and the combined control of regional stretching stress on rift structure,structure and evolution from a finer and smaller scale,and explored these complex fractures.The formation mechanism and development model of the trap have achieved the following understanding:In the Zhu ? depression,a total of NE,NEE,NWW,and EW faults were developed.The NE-trending and NEE-trending boundary fractures,distributed on the north and south margins of the Zhu?depression,are composed by a series of large normal faults.The boundary fractures integrally contral the distribution and evolution of Zhu?depression.The NWW-trending fractures are mainly,distributed in Huizhou sag and Lufeng sag,are skewed by boundary fractures.Large-scale NWW-trending fractures control the development of subsags in the depression.The EW-trending fractures are small contemporaneous fractures in the depressions and subsags.In the area,there is east-west differentiation in the distribution,attitudes,and kinematic characteristics among fractures with different directions.The trunk fractures of Western Enping sag and Xijiang sag,mainly composed of NE-oriented low Angle boundary normal faults,parallel to the rift axis,have large-scale rifts and single strike.In the eastern sag(Huizhou sag and Lufeng sag),NEE directional and NWW directional faults coexisted and high-angle faults are generally dominant.The strike of faults are skewed by NE-trending rift axis;The NEE-trending boundary fractures development more apparent to the multiple stage characteristic,with "Z" word or "L" type distribution in plane and multiple fractures with echelon arrangement.The interior structure of Zhu?depression is extremely complex due to control of diverse fractures.Taking the depression as the basic structural unit,the I-Screw of the Zhu?Depression developed different strikes and structural pattern rifts in the same period,and.According to the development characteristics of the rifting structure,the structure styles of the depressions in the Zhu?Depression are divided into three types: single-type,semi-mantle,tandem compound,and non-symmetrical mantle.According to the size of the main fault,further It is divided into high-angle single-type semi-mantle,low-angle single-type semi-mantle,high-angle tandem compound hemispherical mantle,low-angle tandem compound hemispherical mantle and asymmetrical mantle five sub-categories.Different types of rifts have regional developmental characteristics of east-west differentiation: Western sag generally develops simple low-angle single-type semi-mantle,eastern Huizhou sag is dominated by high-angle rifts,and a large number of different strikes and structural patterns are at high angles.The rifts coexist,and there are significant differences between the rift structure style and the inside and the inside of the sag: There are widely distributed high-angle tandem compound hemispheric mantles in the south-north marginal depression belt,and high-angle single-type hemispherical mantles and asymmetrical growth in the central depression belt.Through the seismic profile recognition and magnetic anomaly data analysis,a large number of basal fractures were identified.According to the nature of pre-existing faults,the base is devided into two layers of different nature verticall.The lower structural layer is the Mesozoic Yanshanian left-handed torsional fold overthrust structure system,which is widely distributed in the whole area of the Zhu?Depression.NE compression-shear fracture system and NW-trending shear-fractured fault system are developed.The NE direction is on the section.The hedged pre-existing low-angle thrust fault system and the NWW-oriented pre-existing high-angle shear-tension fault system are dominant.And the upper structural layer is an extensional or torsional extensional tectonic system developed in the late-late Cretaceous passive-periphery environment.NE and NWW are the two mainly pre-existing high-angle normal faults,and control the development of small rifts in Wenchang period.They are locally developed in the study area and are mainly distributed in the eastern depression.Horizontally,NE-trending pre-existing fractures in the west of Zhu ?depression,mainly composed of pre-existing low-angle reverse faults,are mainly distributed beside the boundary of the rifts.NE-trending pre-existing fractures is mainly high-angel normal faults.The NW-NWW-trending pre-existing fractures develop in the whole study area.For early stage,left-lateral slip faults developed during Yanshan tectonic movement,then during late Cretaceous period,pre-existing fractures activated and the normal faults formed.There is a counterclockwise rotation of about 20° from west to east faults.In addition to the pre-existing rifts,complex lithologic composition also exists in.Zhu?depression.Moderately acidic intrusive rocks are widely developed in the region,mainly consisting of granite,granodiorite and quartz monzonite,accounting for more than 80% of the study area.Granite rocks of different age zoned along the south boundary of the depression from south to north and divided along the boundary of Panyu low uplift and Dongsha uplift.The rock intrusion has the distribution characteristics of new rocks in the east and north,old rocks in the west and south,different times of large rock contact boundary between granite rocks..In Mesozoic sedimentary rocks mainly include Mesozoic Marine sediment and late Cretaceous sediment.The distribution of volcanic rocks is relatively limited.The volcanic rocks generally located near deep faults and is distributed in bands or braids,which is related to the associated magmatic action in the Late Cretaceous rift.However,the metamorphic rock bases were partially revealed in part of the Enping sag,mainly composed of Late Jurassic shallow metamorphosed fine-silty sandstones and metamorphic quartzites.The stretch of the I-screen region in the riftbed of the Pearl River Mouth Basin is mainly affected by the backward retreat of the SSE in the subduction zone of the Pacific plate and the SSE-SN drag of the ancient South China Sea plate.In the Late Cretaceous-Middle Eocene,the subduction of the Pacific Plate took a major turning point.The direction of the plate subduction shifted from NE to NNW.The subduction rate quickly decreased and the subduction angle increased,causing the subduction zone SSE to retreat toward the ocean.With the withdrawal of the East Asian continental margin subduction zone,Borneo turned counterclockwise and the ancient South Ocean shell began to dive southward along the Lupar line.The downward migration of the SSE in the continental marginal subduction zone and the SSE-SN drag of the ancient South China Sea plate resulted in the weakening of the horizontal compressive stress transfer between the Pacific and Eurasian plates,resulting in a stress field in the northern continental margin of the South China Sea in the late Late Cretaceous.The transformation of the left-hand pressure twisting to the right-handed torsion is under a stretching environment,forming a rift-like structure that stretches the NNW-SSE towards the region.The Anderson model was used to select contemporaneous faults that were far from the pre-existing structure and did not break through the basement in the depression.Through its orientation,the basic direction of the main extensional stress in different regions of the rift I was determined,and the minimum principal stress in the Zhu?Depression(?3)was found.The direction from west to east from SSE of Enping depression gradually transitions(~155°)to the near SN direction(~175°)of the Huizhou depression in the east,and there is a clockwise rotation of about 15-20°.The base attribute and regional stress are two core elements that control the development of rifts.In different regions and evolutionary stages,they have different degrees of influence on faults and rift structures because of their geometric,structural,and control differences.The different combinations between the two have further exacerbated the differentiation of the concave-convex mechanism,and led to a variety of oblique stretching modes such as “restricted”,“master-controlled” and “joint-controlled”.In the area,there is east-west differentiation in the diagonal extension of the Zhujiang-Jiao depression,and the main depression is mainly in the west depression.The NE obliquely intersects the pre-existing low-angle reverse fault and the NNW in the direction of the extensional stress.The active faults are strongly activated,forming a single large-scale boundary fault that controls the development and evolution of the depression as a whole,enabling the depression(depression)to develop a simple low-angle simple-type half-graben.In the east of the sag,under the “joint control type” diagonal extension,the pre-existing faults with different strikes and high angles of the nature and the interface between the rock body and the NNW-SN extensional stress are skewed and collectively activated,forming a complex fracture system and many different High angle rift to the trend.At the same time due to the difference between the pre-existing rifts with different strikes and nature and rock interface activity,new fractures have different development scale.The NE-trending faults are more active,easy to form large-scale sag-controlling faults in the process of rifting,controling the distribution of sag boundary and first-order tectonic zone.The activation level of NWW-trending fracturesis is a bit weak,mainly controling a series of NWW-trending second-order structures,hindering the vertical extension of NE-trending structures,making the zonation,frequent changes of structure strikes and segmentation of the eastern depression.Besides,the differential distribution of pre-existing faults and rock mass interfaces in different regions resulted in different fault propagation-connection patterns during the activation process.Pre-existing rifts with different strikes coexist in the boundary of the depression.The pre-existing rifts and the Lithological boundary activated at the same time,forming new faults,controling different sedimentary centers and complex “high angle tandem compound half graben”formed.Inside the depression,a large number of flat NWW-trending pre-existing strike-slip fault activated,forming a series of faults,which are“simple-type half-graben”and“asymmetric graben”.The joint restriction of differential base properties and tectonic stress in Zhu?depression,which lead to the formation of various rifts with different strikes and types,is the fundamental cause of the development of complex rifts in the depression. |
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