Interseismic slip distribution and locking characteristics of the mid-southern segment of the Tanlu fault zone

We employ the block negative dislocation model to invert the distribution of fault coupling and slip rate deficit on the different segments of the Tanlu(Tancheng-Lujiang) fault zone, according to the GPS horizontal velocity field from 1991 to 2007(the first phase) and 2013 to 2018(the second phase). By comparing the deformation characteristics results, we discuss the relationship between the deformation characteristics with the M earthquake in Japan. The results showed that the fault coupling rate of the northern section of Tancheng in the second phase reduced compared with that in the first phase. However, the results of the two phases showed that the northern section of Juxian still has a high coupling rate, a deep blocking depth, and a dextral compressive deficit, which is the enrapture section of the 1668 Tancheng earthquake. At the same time, the area strain results show that the strain rate of the central and eastern regions of the second phase is obviously enhanced compared with that of the first phase. The occurrence of the great earthquake in Japan has played a specific role in alleviating the strain accumulation in the middle and south sections of the Tanlu fault zone. The results of the maximum shear strain show that the shear strain in the middle section of the Tanlu fault zone in the second phase is weaker than that in the first phase, and the maximum shear strain in the southern section is stronger than that in the first phase. The fault coupling coefficient of the south Sihong to Jiashan section is high, and it is also the unruptured section of historical earthquakes. At the same time, small earthquakes in this area are not active and accumulate stress easily, so the future earthquake risk deserves attention.

Thermal Evolution of the Tanlu Fault Zone on the Eastern Margin of the Dabie Mountains and Its Tectonic Implications

Five samples of muscovite from mylonites of the earlier Tanlu ductile shear zone on the eastern margin of the Dabie Mountains yield 40Ar/39Ar ages ranging from 178 Ma to 196 Ma. Three of them have reliable plateau ages of 188.7±0.7 Ma, 189.7±0.6 Ma and 192.5±0.7 Ma respectively, which indicates a syn-orogenic, sinistral strike-slip thermal event. This displacement movement derived from the continent-continent collision of the North and South China blocks took place in the Early Jurassic and after uplifting of high-pressure to ultrahigh-pressure slabs to the mid-crust. It is suggested that during the collision the Tanlu fault zone was an intracontinental transform fault caused by differential subduction speeds. The 40Ar/39Ar ages of mylonite whole-rock and muscovite from the later Tanlu ductile shear zone suggest another sinistral strike-slip cooling event at 128 Ma. During this strike-slip faulting, large-scale intrusion and doming uplift occurred in the eastern part of the Dabie orogenic belt. Data of K-feldspar 40Ar/39Ar MDD and apatite fission-track analysis from metamorphic rocks indicate two high-speed cooling events experienced by the Tanlu fault zone, which took place at 90 Ma and 45-58 Ma respectively. They correspond respectively to two phases of extensional activities in the Late Cretaceous and Eogene as well as development of the Qianshan fault-controlled basin to the east of the Tanlu fault zone. The cooling times recorded by K-feldspar and apatite show that the uplifting in the Dabie orogenic belt occured earlier than that on the eastern margin occupied by the Tanlu fault zone. The above phenomena suggest that the uplifting of the Dabie orogenic belt during the Late Cretaceous to Eogene was not controlled by the Tanlu normal faulting, but as a result of the lithospheric delamination.

Meso-Cenozoic Evolution of the Tanlu Fault and Formation of Sedimentary Basins

Abstract: There are a group of large and medium-scale Meso-Cenozoic petroliferous basins along both sides of the Tanlu fault or within the fault zone, e.g., the Songliao basin, the Bohai Bay basin and the Subei-Yellow Sea basin. As shown by studies of the structural types, sedimentary formations, volcanic activities, tectonic evolution as well as the time-space relationship between the Tanlu fault zone and the basins, the formation and distribution of the basins are controlled by the movement of the Tanlu fault. This paper presents an analysis of the tectono-geometric, kinematic and geodynamic features of the basins on the basis of integrated geological-geophysical data, and an exploration into the internal relations between the fault and the basins as well as the formation mechanism and geodynamic processes of the basins.

Crustal thickness beneath the Tanlu fault zone and its tectonic significance based on two-layer H-κstacking

Tanlu fault zone(TLFZ)is the largest active fault zone in eastern China.It is characterized by complex tectonic evolution and multiple faults and marks the boundary between the North and South China blocks.An indepth understanding of the distinct crustal structures of both parts of the TLFZ will provide valuable insights into the lithospheric and crustal thinning in eastern China,extensive magmatism since the Mesozoic,and formation mechanisms of metallogenic belts along the Yangtze River.In this study,a two-layer H-κstacking approach was adopted to estimate the thicknesses of the sediment and crystalline crust as well as the corresponding vP/vS ratios based on high-quality teleseismic P-wave receiver functions recorded by permanent and temporary stations in and around the TLFZ.The geological units in the study region were delineated,especially the crustal structures beneath extensive sedimentary basins on both sides of the TLFZ.The following conclusions can be drawn:(1)The crustal thickness in and around the TLFZ greatly varies depending on the segment.In the northern segment,the crust is relatively thin beneath the eastern part of the Songliao Basin,a broad uplift of the Moho can be observed,and the Moho descends from south to north.The crust below the central and southern segments becomes thinner from west to east.The thickness of the crust is less than 30 km toward the eastern side of the boundary between the Jiangsu and Anhui provinces,that is,significantly thinner than in other areas.In terms of the vP/vS ratios,high anomalies were detected in the central-southern segments of the TLFZ,indicating the upwelling of deep mantle magma via deep faults.(2)Positive isostatic gravity anomalies were observed in the eastern part of the northern segment of the TLFZ and in the eastern part of the Suwan segment.The crustal thickness is smaller than that obtained from the Airy model of isostasy.This suggests that the lower crust in this area may have experienced intensive transformation processes,which may be related to crustal thinning(caused by crustal extension)and the strong uplift of the mantle in eastern China.The isostatic gravity anomalies between the eastern and western parts of the TLFZ indicate that the fault zone plays a dominant role in controlling the development of the deep crustal structure.(3)Significant crustal thinning was observed beneath the eastern part of the boundary between the Jiangsu and Anhui provinces in the southern segment of the TLFZ,suggesting that this area is prone to lithospheric thinning of the North China Craton.Due to the subduction,compression,and retreat of the Paleo-Pacific Plate during the Yanshanian Period as well as the dehydration of subducting oceanic crust(within subduction zones),the asthenosphere and oceanic crust in eastern China partially melted,resulting in mantle enrichment.The basic magma from the mantle is accumulated at the base of the crust,leading to magmatic underplating.In areas with weak topography toward the east of the TLFZ,magma rises to the upper crust and surface,resulting in the enrichment of multiple metal deposits in this area.

High resolution tomography of the Tanlu fault zone near Hefei with passive seismic and magnetotelluric linear array data

As the largest fault trending NNE-SSW to NE-SW in the eastern Eurasia Continent,the Tanlu fault zone(TLFZ)extends over 2,400 km within China,roughly from Wuxue,Hubei Province,to Russia.Since the Quaternary period,the TLFZ has been an earthquake-prone area in eastern China where several major earthquakes resulted by tectonic compression occurred,causing tremendous casualties and significant economic losses.Many studies on different segments of the TLFZ have been carried out in the past few decades.However,numerous key questions regarding the fault zone remain unanswered due to a lack of clear subsurface characterization and fault delineation.In this study,we present high-resolution tomographic results across the TLFZ to the east of Hefei,where one 22-km-long passive seismic array with densely spaced short-period nodes,and a 24-km-long magnetotelluric array were deployed adjacent to each other.We find the velocity and resistivity variations are highly consistent with known surface geology.Sharp property contrasts in both the seismic shear wave velocity and electrical resistivity profiles clearly delineate the Tanlu F1 fault(TLF-1)near Hefei.More interestingly,an upwelling with distinct high velocity is imaged within the Hefei Basin to the west of the TLF-1,whereas a slanted block with lowvelocity and low-resistivity seems to cut into or thrust upon the high-grade to low-grade middle-pressure rocks in the Zhangbaling uplift right below the Tanlu F2 ductile shear fault(TLF-2).The presented results show a new approach to characterize deep subsurface structure of the TLFZ beyond 2-km depths using passive data,which it is often difficult for active seismic surveys with refracted and reflected waves to image.

Lateral variation in Moho depth around the southern Tanlu fault zone and its adjacent area

We estimated Moho depth beneath the southern Tanlu fault zone and its adjacent area using common-conversion-point(CCP)stacking of receiver functions,which were computed from teleseismic records of the CEArray.Our estimated Moho depth matches well with 2-D profiles derived from active-source deep seismic reflection surveys,suggesting that the calculated the Moho depth map is likely accurate beyond the 2-D profiles.Overall,the estimated Moho depth map showed a high spatial correlation with tectonic provinces,i.e.,Moho topographic boundaries are in good agreement with geological boundaries.Beneath the Dabie orogenic belt and the mountainous areas in southern Anhui Province,the Moho lies relatively deep,and there is an obvious difference in Moho depth between the two sides of this segment of the Tanlu fault.We further selected four depth profiles with dense instrumentation to show Moho depth changes across different tectonic blocks in the study area.We saw two step-like changes in Moho depth beneath the Xiangfan-Guangji and Gushi-Feizhong,which run parallel along the WNW-ESE direction and delineate the southern and northern bounds of the northern Dabie orogenic belt,which is likely the suture zone between the North China Block and South China Block.Crust beneath the northeast corner of the study area is significantly thinner than other areas,which is consistent with the crustal detachment model proposed for suturing between the North and South China blocks in the region east to the Tanlu fault.

Dispersion attenuation structure of the crust around the Weifang-Jiashan segment of the Tanlu fault zone in North China

On the condition that the velocity ratio of compressive wave to shear wave is stable during the propagating process of seismic wave, this paper develops an estimation method for medium quality factor based on single station＇s travel time difference between direct S and P waves and the first period signal of direct P wave. 8 774 high SNR wave data altogether recorded by 75 stations are analyzed. The results show that： （1） under the normal regional stress field, the quality factors QmsP in the stable tectonic area are higher than that in the active tectonic area around the Weifang-Jiashan section and its surrounding area in the Tanlu fault zone; （2） in the Juxian-Tancheng section the seismic wave attenuation is relatively quick, and the media is relatively broken, suggesting no tectonic stress accumulation; （3） the Xinyi-Sihong section is currently locking and in accumulating elastic strain energy stage, which has the deep environment and conditions of strong earthquake generation similar to those of the 1668 M8.5 Tancheng strong earthquake.

Recent activity of Chihe segment of Tanlu fault zone

By means of differentiation of remote sensing image, field seismo-geological survey, analysis on drilling explora- tion materials, sampling and dating of rock samples, combined with seismicity and microscopic tectonic analysis, this paper studies the recent activity of Chihe segment of the Tanlu fault zone. The result indicates that the Chihe fault segment undergoes the deformation alternately in the mode of stick slip and creep during Late Quaternary, and its recent activity is mainly creep.

A multi-scale 3-D crust velocity model in the Hefei-Chao Lake area around the southern segment of Tanlu Fault Zone

Regional high-precision velocity models of the crust are an important foundation for examining seismic activity,seismogenic environments,and disaster distribution characteristics.The Hefei-Chao Lake area contains the main geological units of Hefei Basin,with thick sediments and the Chao Lake depression.Several major concealed faults of the southern NNE-trending Tanlu Fault Zone cross this area.To further explore the underground distribution characteristics of the faults and their tectonic evolutionary relationship with adjacent tectonic units,this study used ambient noise data recorded by a seismic array deployed in Hefei City and Chao Lake,constructing a 3-D velocity model at the depth of 1–8 km.Then a multi-scale high-resolution 3-D velocity model of this area was constructed by this new upper crustal velocity model with the previous middle and lower crustal model.The new model reveals that a high-velocity belt is highly consistent with the strike of the Tanlu Fault Zone,and a low-velocity sedimentary characteristic is consistent with the Hefei Basin and Chao Lake depression.The distribution morphology of high and low velocity bodies shows that the sedimentary pattern of Hefei-Chao Lake area is closely related to the tectonic evolution of the Tanlu Fault Zone since the Mesozoic.This study also identifies multiple low-velocity anomalies in the southeastern Hefei City.We speculate that strong ground motion during the 2009 Feidong earthquake(magnitude of 3.5)was related to amplification by the thick sediments in the Hefei Basin.We also discuss further applications of multi-scale high-resolution models of the shallow layer to strong ground motion simulations in cities and for earthquake disaster assessments.

Co-seismic Coulomb stress changes on the northern Tanlu fault zone caused by the Tohoku-Oki M_(W)9.0 earthquake

Based on the spherical earth dislocation theory and a fault slip model of the Tohoku-Oki M_(W)9.0 earthquake,the co-seismic Coulomb failure stress changes(ΔCFS)on the northern Tanlu fault zone at depths of 0–40 km are calculated.By comparing two sets of results from the spherical earth dislocation theory and the semi-infinite space one,the effect of earth curvature on the calculation results is analyzed quantitatively.First,we systematically summarize previous researches related to the northern Tanlu fault zone,divide the fault zone as detailed as possible,give the geometric parameters of each segment,and establish a segmented structural model of the northern Tanlu fault zone.Second,we calculate the Coulomb stress changes on the northern Tanlu fault zone by using the spherical earth dislocation theory.The result shows the Coulomb stress changes are no more than 0.003 MPa,which proves the great earthquake did not significantly change the stress state of the fault zone.Finally,we quantitatively analyze the disparities between the results of semi-infinite space dislocation theory and the spherical earth one.The average disparity between them is about 7.7%on the northern Tanlu fault zone and is 16.8%on the Fangzheng graben,the maximum disparity on this graben reaches up to 25.5%.It indicates that the effect of earth curvature can not be ignored.So it’s necessary to use the spherical earth dislocation theory instead of the semi-infinite space one to study the Coulomb stress change in the far field.

Tensile Fractures and in situ Stress Measurement Data Constraints on Cretaceous-Present Tectonic Stress Field Evolution of the Tanlu Fault Zone in Shandong Province,North China Craton

Tectonic stress fields are the key drivers of tectonic events and the evolution of regional structures.The tectonic stress field evolution of the Tanlu fault zone in Shandong Province,located in the east of the North China Craton(NCC),may have preserved records of the NCC’s tectonic history.Borehole television survey and hydraulic fracturing were conducted to analyze the paleo and present tectonic stress fields.Three groups of tensile fractures were identified via borehole television,their azimuths being NNW-SSE,NW-SE and NE-SW,representing multiple stages of tectonic events.Hydraulic fracturing data indicates that the study region is experiencing NEE-SWW-oriented compression and nearly-N-Soriented extension,in accordance with strike-slip and compression.Since the Cretaceous,the orientation of the extensional stress has evolved counterclockwise and sequentially from nearly-NW-SE-oriented to NE-SW-oriented and even nearly N-S-oriented,the stress state having transitioned from strike-slip-extension to strike-slip-compression,in association with the rotating and oblique subduction of the Pacific Plate beneath the NCC,with the participation of the Indian Plate.

A recent paleoearthquake on Qingfengling seismic fault of Tanlu fault zone

Introduction The Tanlu fault zone lies in the eastern China, which is an important huge active fault with a long history. It has experienced a complex generation and evolution process and affects significantly the regional structure, paleogeography, magma activity, minerogenesis and earthquake activity in the area. With a length of 2 400 km, the fault zone consists of 2-4 or more parallel faults of 10-40 km in width, cutting through different geotectonic elements in the eastern China （FANG et al, 1986）. On July 25 in 1668, an extraordinarily large earthquake of M=8.5 occurred on the Changyi-Dadian fault （F1） that is an embranchment of Tanlu fault zone, resulting in a surface rupture with a total length of 130 km （LI et al, 1994; CHAO et al, 1995）. The paleoseismic study reveals that 3 events with a magnitude equal to 8 occurred on the Changyi-Dadian fault. The recent event occurred 3 500 a ago and the reoccurrence interval is about 3 500 a （LIN and GAO, 1987）. During the Tancheng earthquake （on July 25, 1668）, the Anqiu-Juxian fault was not ruptured, which was a Late Pleistocene active fault （ZHENG et al, 1988; GAO et al, 1988; CHAO et al, 1994） and was doubted as the seismogenic fault of the M=7.0 Anqiu earthquake occurred in 70 BC by certain geologists （CHAO et al, 1994）.

Bouguer gravity study of middle section of Tanlu fault

Although Tanlu fault is one of the most important tectonic fault zones and active earthquake belts in eastern China, little is known about its deep structure. In this study, we use the existing Bouguer gravity data to study the middle section of the Tanlu fault zone, which is also known as the Yishu fault zone. Our gravity inversion results indicate that the Moho has an abrupt offset in depth at the Tanlu fault zone and it has a relatively smooth variation away from the fault zone. The crustal structures on both sides are different from each other. Sediment is thin on the west side with an average thickness of less than 5 km, while it is as thick as 6 km on the east side. The thinnest sediment （3-4 km） is at the fault zone. Moho depth increases from 33 to 34 km on east side and from 36 to 38 km on west side. Tanlu fault zone is shown as a wide zone of linear gradient in the Bouguer gravity anomaly.

A study on neogene volcanic landforms of the middle Tanlu fault belt

The west block of the middle belt of TanLu fault belt is one of the areas where volcanic landforms are mainly distributed. This paper divides the volcanic landforms into four grades according to formation, morphology, and ingredient of matter. Lava cones are divided into two types based on the force of top-support and the fluid form of magma: the arc-projecting type and spring-spilling type. Furthermore, the courses of development of volcanic landforms are divided into three stages according to the form and strength of volcanic activities.

Active fault survey on the Tanlu fault zone in Laizhou Bay

Shallow-depth acoustic reflection profiling survey has been conducted on the Tanlu fault zone in Laizhou Bay. It is found that the Tanlu fault zone is obviously active during the late Quaternary and it is still the dominating structure in this region. The Tanlu fault zone consists of two branches. The KL3 fault of the western branch is composed of several high angle normal faults which had been active during the period from the latest Pleistocene to early Holo- cene, dissected by a series of northeast or approximate east-west trending fault which leaped sediment of the late Pleistocene. The Longkou fault of the eastern branch consists of two right-laterally stepped segments. Late Quaternary offsets and growth strata developed along the Tanlu fault zone verify that the fault zone retained active in the latest Pleistocene to the early Holocene. The Anqiu-Juxian fault that passes through the middle of Shandong and corresponds to the Longkou fault is composed of a series of right-laterally stepped segments. The active faults along the eastern branch of the Tanlu fault zone from the Laizhou bay to the north of Anqiu make up a dextral simple shear deformation zone which is characterized by right-lateral strike-slip movement with dip-slip component during the late Quaternary.

New Activity of the Tanlu Fault Zone in the South of Huaihe to the Nvshanhu Segment since the Late Quaternary

Taking the Huaihe to the Nvshanhu segment of the Tanlu( Tancheng-Lujiang) fault zone as the main research target to explore whether there has been new activity since the late Quaternary,and based on the interpretation of remote sensing images and repeated surface investigations,we excavated trenches at the sections where the tectonic landform is significant,identified and recorded the deformation patterns of the fault and analyzed the activity behavior. Samples of new activity and deformation were collected and oriented slices were ground based on the samples ' original state to make the micro structural analysis and demonstration. All of the above research shows very clear linear tectonic geomorphology along the fault,three trenches across the fault zone all revealed new deformation traces since late Quaternary. The latest stratum dislocated by the fault is the late Quaternary and Holocene. The main slip mode is stick slip,as represented typically by fault scarps,wedge accumulation,the faults and the filled cracks and so on. In general,it shows the characteristics of brittle high-speed deformation and belongs to the prehistoric earthquake ruins. The above understanding was confirmed partially by microscopic analysis. In addition,the similarities and differences and the possible reasons for the characteristics of the latest activities of the Tancheng-Lujiang fault zone in the north and south of the Huaihe River regions are also discussed in this paper.

Fault detection by reflected surface waves based on ambient noise interferometry

Detecting subsurface fault structure is important for evaluating potential earthquake risks associated with active faults.In this study,we propose a new method to detect faults using reflected surface waves observed in ambient noise cross correlation functions.Ambient noise tomography using direct surface waves obtained from ambient noise interferometry has been widely used to characterize active fault zones.In cases where a strong velocity contrast exists across the fault interface,fault-reflected surface waves are expected.We test this idea using a linear array deployed in the Suqian segment of Tanlu fault zone in Eastern China.The fault-reflected surface waves can be clearly seen in the cross-correlation functions of the ambient noise data,and the spatial position of the fault on the surface is close to the stations where the reflected signals first appear.Potentially reflected surface waves could also be used to infer the dip angle,fault zone thickness and the degree of velocity contrast across the fault by comparing synthetic and observed waveforms.

Influence of self-similar stresses on scenario earthquake construction:An example along the Tanlu Fault

As a famous deep and large fault in eastern China,the Tanlu Fault passes through Anhui,Jiangsu,and Shandong and into northeastern China.It is important to improve the understanding of seismic hazard assessments in areas near faults.We start a scenario earthquake simulation in the M7.5 earthquake potential area of the Xinyi-Sihong segment of the Tanlu Fault.The fault rupture length and width are constrained according to the scaling law of large intraplate earthquakes,the background normal stress is depth dependent,and the initial shear stresses are determined using trial and error by matching the earthquake magnitude.Considering the 120 km rupture length of the M7.5 earthquake,we compare the horizontal uniform stress model and self-similar stress perturbation model.Our findings reveal that the seismic source time function of the horizontal uniform stress model is similar to that of the Haskell model and that of the self-similar stress perturbation model is more similar to that of a real earthquake case.We compare the dynamic rupture simulation and ground motion results under four different stress conditions and find that the shorter the characteristic length of the self-similar function is,the rougher the initial stress.For the M7.5earthquake with an epicenter in the vicinity of Suqian,the Xinyi-Tancheng segment,which is located in the IX-intensity zone north of the epicenter,vibrates more strongly on the northern side than on the southern side due to the influence of the lowvelocity zone and the peak slip rate.The response spectra analysis at stations in the study area is useful for improving the earthquake resistance capability.

Characteristics of geological structures in Shiling and Zhuanshanhu areas of Yehe Uplift,NE China

The basin marginal fault system is the key to understand the formation and evolution of Songliao Basin.In order to investigate the influence of marginal fault system on the structural evolution of Songliao Basin,a comprehensive study was conducted on Shiling Town and Zhuanshanhu area of Yehe Town in Siping City of Jilin Province,where is the southeastern margin of the Songliao Basin and there are a series of well-exposed fault,fold and intrusive bodies belonging to the main marginal fault system of the Songliao Basin known as the Jiamusi-Yitong(Jia-Yi)fault zone.Through profile measurement and field investigation,samples with various lithologies and distinctive features were collected.Detailed field and laboratory works include component and microstructure analysis of these samples,rock-rock contact analysis,main strike measurement and statistics analysis.These data reveal the structural characteristics of the fold,fault and intrusive bodies in the study area.The research results show that the folds are distributed in the Mesozoic strata near the main fault of the eastern branch of the Jia-Yi fault zone,and the folded strata involve the Cretaceous Denglouku and Quantou formations.In addition,the section is dominated by high-angle strikeslip thrust faults.Light-colored veins and dark-colored veins are extensively distributed in the exposed granites.Statistical analysis of joint and fault attitudes in the study area reveals a right-lateral strike-sliping along the main fault.The large-scale right-lateral strike-slip and thrust fault system in Shiling Town occurred in right-lateral transpressive stage in Late Cretaceous.Based on the results above,tectonic evolution sequence in Shiling section of the Jia-Yi fault zone during the Mesozoic can be divided into five stages:Middle Jurassic left-lateral ductile strike-slip stage,Late Jurassic compression stage,Early Cretaceous tension stage,Early Cretaceous extension stage and Late Cretaceous right-lateral transpressive stage.These may have important constraint on understanding the Mesozoic evolution of the Songliao Basin.

燕山东部柳江地区构造属性新解与郯庐断裂系活动

传统意义上的柳江盆地位于燕山东段,完整地出露了从前寒武纪到中生界的华北地台盖层沉积地层,地层序列总体上呈中间新,两侧老的特点。前人认为柳江盆地为一轴向N10°E的不对称向斜。新的野外调查显示,柳江地区西侧陡立的产状是断裂作用形成的构造面理产状,为北北东向断裂构造的一部分,可以清楚识别的原始层面产状实际上与东部地层相似,为250°西倾25°～30°倾角。因此,将柳江地区东侧的原始层面产状与西侧的构造面理产状分别对应同一向斜两翼的传统观点是不恰当的。所谓柳江盆地实际上为单斜构造,是地壳断块在北北东向断裂的作用下西侧上升东侧下降的掀斜作用结果,过去对这一地区关于盆地和向斜构造属性的界定应该予以纠正。柳江地区在古生代和中生代时期是华北盆地的一部分,在新生代以来又同时接受剥蚀,一直不曾是独立的盆地单元。由于柳江断陷的沉降与西侧响山隆起相耦合,在两者边界起调节作用的北北东向断裂系具有左旋走滑兼具垂直运动分量的运动性质,是郯庐断裂系的组成部分,北北东向断裂系活动的时限与响山隆起的时限相对应,根据裂变径迹年龄分析结果,活动时代为白垩世末—古新世。