The Tan-Lu fault zone(TLFZ)in East China is one of the large fault zones in East Asia and has adominant effect on local tectonics, magmatic activities, formation and distribution of mineral resources, and current earthquake activities in East China. Since TLFZ was determined through aeromagnetic anomaly inthe 1950s, geoscientists have performed a large quantity of research on its formation, evolution and activitymechanism and have scored a series of achievements. As for the origin of the TLFZ, most researchers believethat the collision and suturing of the North China Craton(NCC) and the Yangtze Craton during the IndoChinese epoch are the tectonic background of TLFZ startup(Xu J W, 1980, Xu J W and Ma G F, 1992; Yin and Nie, 1993; Xu and Zhu, 1994; Li, 1994; Wan T F and Zhu H, 1996; Lin et al., 1998; Wang X F et al., 2000;Zhang et al., 2003). Isotopic age of a series of strike-slip mylonite along the fault and lots of magmatic activitiesin the Early Cretaceous Period show a massive left-lateral displacementin the Early Cretaceous Period(Xu J W 1980; Xu et al., 1994; Wang X F et al., 2000; Zhu G et al., 2001, 2003). A series of graben basins along the faultzone in the Late Cretaceous Period-Early Tertiary Period indicate intensive extensional movement during theperiod(Ren et al., 2002; Zhang et al., 2003; Zhu G et al., 2008). Since the Late Cretaceous Period, the fault zonehas undergone compression and graben basins have generally experienced uplifts, disappearance, and extensivethrust activities(Institute of Geology, CEA, 1987; Wang X F et al., 2000; Zhu G et al., 2002). Thus it canbe seen that since the Mesozoic era TLFZ has experienced a sophisticated evolution process featuring intensiveleft-lateral displacement, extension and compression, which must leave evolutionary traces on deep-shallowstructures.

Since the 1980s, lots of deep geophysical probing work for the TLFZ like deep seismic sounding(DSS), magnetotelluric exploration, broadb and seismic array, gravity and magnetic research(Ma X Y et al., 1991; Chen H S et al., 1993;Tang X G et al., 2006; Chen et al., 2006; Zheng et al., 2008; Ye G F et al., 2009; Zhang J H et al., 2010; Li S L et al., 2011; Huang et al., 2011)have been done to reveal deep characteristics of the TLFZ, which provided for us comprehensive information about TFLZ’s deep structural features, evolution and dynamicmachnism. Those researches could be helpful, but their results only show macroscopic characteristics of thefault zone without detailed images of the inner complicated structure and tectonic patterns, which is still a gapin studying the relations between the evolutionary process of the zone and its deep-shallow structures. In thispaper, we apply the deep seismic reflection method to explore the lithospheric structure and tectonic patternsbeneath the middle-southern segment of the TLFZ, acquire the fine structure images of the fault zone from thenear-surface to the lithosphere, and determine the location, tectonic form and influence level of different faultsin the fault zone, which provides seismological evidence for us to analyze strike-slip, compression, extensionaltectonic system components and its evolutionary process.

The TLFZ starts from Guangji County of Hubei Province on the north bank of the Yangtze River, stretching for about 2400 km towards north, it crosses Lujiang City in Anhui Province, Suqian City in Jiangsu Province, Tancheng County in Sh and ong Province, Bohai Gulf, Shenyang City in Liaoning Province, and finally ends inLuobei County in Heilongjiang Province. According to tectonic features, evolutionary history, and seismicity, the TLFZ can be divided into three segments as the southern, the middle and the northern, with Bohai Gulf and Xinyi City in Jiangsu Province as dividing nodes(Xu J W and Ma G F, 1992; Institute of Geology, CEA, 1987; Wang X F et al., 2000).

The inner structure of TLFZ is very complicated due to reasons of long evolution and activity historyas well as varied activity patterns in different segments and different times. As the main part of TLFZ, themiddle segment(from Bohai Gulf to Xinyi City, Jiangsu Province)is the most developed Quaternary activefault and one of the major earthquake active belts in China(Fang Z J et al., 1980; Wu D M et al., 1981;Institute of Geology, CEA, 1987; Li J L et al., 1994; Chao H T et al., 1997). According to the current tectonicfeatures of TLFZ, the middle segment comprises four major faults. From east to west they are Changyi-DadianFault F₁, Anqiu–Juxian Fault F₂, Yishui-Tangtou Fault F₃ and Tangwu-Gegou Fault F₄. All the four assume agraben-horst-garben structure in the Linyi–Linshu segment(the middle segment of TLFZ). In the southern partof Tancheng County, Sh and ong Province, the structure form along TLFZ is one graben, one horst(Institute ofGeology, CEA, 1987; Wang X F et al., 2000). In the Jiangsu section(i.e. south of Xinyi City), the four majorfaults are named as Shanzuokou-Sihong fault, Xinyi-Xindian fault, Mehe-Lingcheng fault and Jiji-Wangji fault.The Shanzuokou-Sihong fault and Jiji-Wangji fault are the east and west boundary of TLFZ respectively; tothe north they connect with the Changyi-Dadian fault and Tangwu-Gegou fault respectively(Wang X F et al., 2000; Li Q T, 1994; Tang J F et al., 2003; Zhang P et al., 2011).

The results from the geoscience transect across the middle-southern segment of TLFZ show that TLFZ isa steep-dip-angle deep large fault zone on the lithospheric scale, controlling hot material upwelling and magmaintrusion(Ma X Y et al., 1991; Chen H S et al., 1993). Magnetotelluric sounding results demonstrate that thedeep inner part of TLFZ is a strike-slip structure cutting through the Moho downward to the upper mantle, and its shallow part features the extensional fault superposed by graben basin(Ye G F et al., 2009; Zhang JH et al., 2010). Deep Seismic Sounding profile and receiver function results indicate that, around TLFZ, thethickness of the lithosphere is 60~80 km while that of the crust is 32~35 km, and the crustal velocity of seismicwaves varies sharply on both the eastern and western sides of TLFZ(Liu C Q et al., 1987; Chen et al., 2006;Zheng et al., 2008; Li S L et al., 2011). All previous studies suggest that the shallow part of TLFZ is not only acomplicated tectonic belt with a garben-horst structure, but also a velocity anomaly zone and a fault fracture zone cutting through the whole crust, down to the upper mantle.

Studies have shown that the seismicity of TLFZ shows obvious segmentation— weak seismicity in thenorthern and southern segment but strong seismicity in the middle segment. Historically strong earthquakesmainly occurred in the middle segment(from Bohai gulf to Xinyi City), the strongest one being the 1668Tancheng M8.5 earthquake. From Xinyi City in Jiangsu Province to Jiashan County in Anhui Province, seismicity features low temporal frequency and small intensity, lacking in M ≥ 6.5 strong earthquakes(Fang Z J et al., 1980; Wu D M et al., 1981; Institute of Geology, CEA, 1987; Lin H C et al., 1994; Huang Y et al., 2008; Zheng Y P et al., 2014).

In recent years, there were some M < 5 earthquakes in regions from Xinyi City to Jiashan County and its surrounding areas along TLFZ, most of which occurred on the eastern or western sides of TLFZ, or nearthe intersections or joints of the fault zone and NWW or NE trending faults; only a few occurred inside TLFZ(Fig. 1). Focal mechanism analysis of regional small earthquakes is conducive to the assessment of the stress state and seismicity prediction. There was often stress axis deflection in regional mid-small earthquakes before strongearthquakes(Gupta, 1975; Li Q Z et al., 1980; Diao G L et al., 1994, 2005). Studies on the focal mechanism ofthe research area show that the focal mechanism solutions near TLFZ are NNE trending, nearly in parallel withTLFZ, while on the eastern and western sides of TLFZ the focal mechanisms make a large angle with the TLFZ(Wu D M et al., 1981; Liu D W et al., 2006). Assuming that those earthquakes occur in the existing fault plane, its seismogenic structure trend and dip are not consistent with those of TLFZ. Researches on paleoearthquakes and historical strong earthquakes indicate that strong earthquakes in different parts of Weifang-Jiashan segmentof TLFZ periodically repeat. In Bohai-Xinyi segment, there are three Holocene paleoearthquakes in additionto the 1668 Tancheng M8.5 earthquake, with a recurrence interval of about 3500 years(Lin W F et al., 1987;Gao W M et al., 1991). In Xinyi-Sihong segment, there are also three paleoearthquake events, and the strongearthquake recurrence interval is about 4000 years(Li J L et al., 1994; Chao H T et al., 1997).

(Fig.1)

Fig.1 Geological structure and location of deep seismic reflection profile in the research region Red circles denote modern Earthquakes; Blue circles show historical earthquakes; Red dotted lines are buried faults.

Through tectonic geomorphology, Quaternary geology, regional survey and research on paleoearthquaketraces, seismologists and geologists found out that active faults in Weifang-Jiashan segment are the most developed ones along TLFZ; Historically strong earthquakes often occurred at the intersections of TLFZ and NW and NE trending active faults; the seismicity is closely related to late Quaternary fault activity, the geotectonicunit of individual segment and its deep structural features. Although the historical strong earthquakes happened mainly in the middle segment of TLFZ which is the most developed Quaternary active fault with a largerfault slip rate, taking many other factors such as the paleoearthquake recurrence interval, fault developmentlevel, regional stress field change and current seismicity into consideration, some scholars believe that thereis high-risk segment of strong earthquakes’ occurrence at the structural intersections, where there is a lack ofhistorical strong earthquake in the past or they occurred quite a long time ago, for example, the Jiangsu-Anhuisegment of TLFZ(Fang Z J et al., 1980; Gao W M et al., 1991; Li J L et al., 1994; Chao H T et al., 1997;Zheng Y P et al., 2014). Therefore, studies on the deep structure of the mid-southern segment of TLFZ notonly can provide seismological evidence for us to underst and the deep dynamic process and deep-shallow faultstructure, but also can be of great significance to earthquake risk assessment in this area.

Deep seismic reflection profile is located in the vicinity of Suqian City of Jiangsu Province, which is at themiddle-southern segment of TLFZ. The profile is in near EW direction and 100 km long(Fig. 1). The profilestarts from the east of Shiji Town, Siyang City in Jiangsu Province(E118.66°; N33.76°), and ends at the northof Yugou Town, Lingbi County in Anhui Province(E117.60°; N33.89°).

Deep seismic reflection data were acquired by using the spread geometry with 720 recording channels, 25m station spacing and 150 m shot spacing. This spread geometry provided 60-fold Common mid-point(CMP)coverage for the data processing. In consideration of the weaker reflectivity from deeper crystalline rocks, thesurvey used dynamite sources with varied shot sizes to ensure adequate imaging of both the crust and thelithospheric mantle. The 24 kg shots with 150 m spacing were performed in single wells at a depth of 25~30m. The 200~300 kg shots with 750 m spacing were executed in four wells at a depth of 50 m which leads to12 CMP stacks and then guarantees the signal-to-noise ratio of deeper reflection signals. The French SERCELSN408UL seismograph and a linear array of 12-geophones with 10 Hz natural frequency were used to gatherseismic signals. The length of the seismic record was designed for 30 s at a 2 ms sampling rate.

To acquire high-quality raw data, we applied field playback and quality monitoring to raw common-shotgather. Also, GRISYS seismic reflection data processing system was applied to daily primary process to generatethe pre-stacked section which will be used to guide the data acquisition. We managed to acquire high-qualityraw data through above-mentioned methods and techniques.

According to the complex seismic wave field in the raw data, we lay more considerations on keepingeffective reflector and improving the signal/noise ratio of data during the data processing. The seismic datawere processed using conventional common midpoint reflection methods mainly including surface consistentamplitude compensation, tomographic static correction, time-varying b and -pass filter and 2D filter, surfaceconsistent deconvolution, time-varying spectrum whitening, velocity analysis and dynamic correction, severaliterations of the residual static correction and post-stack denoising.

Proper velocity determination is the key to better reflection stack profile image and reflection interfacedepth’s calculation. To obtain velocity distribution from near surface to the lowest lithosphere, we used refraction first-arrival traveltimes of the deep seismic reflection records as well as non-linear first-arrival traveltimetomography(Zhang and Toksöz, 1998)to image the near surface velocity structure(Fig. 2a). We also appliedit to tomographic static corrections and near-surface structure analysis. For reflection events above Two-WayTraveltime(TWT)5~6 s, its reflection events are mostly hyperbolic curves, so we applied velocity spectrumanalysis to calculate stack-velocity along the profile, then applied dynamic correction to those reflections. Fordeeper reflections below TWT 6 s, its reflection events are nearly flat(Fig. 2b)due to smaller normal moveoutamong adjacent traces. To obtain more precise stack-velocity of deeper reflections, we not only applied reflectionvelocity scanning to calculate stack-velocity, but also referred to the results from the Yancheng-Baotou seismicwide-angle reflection/refraction profile accomplished by Geophysical Exploration Center of CEA in 2012 and lithospheric P-wave velocity structure of the seismic wide-angle reflection/refraction profile provided by Li S Let al.(2011). To calculate the depth of different reflection interfaces in the profile, we obtained average velocitydistribution of the lithosphere along the profile(Fig. 2c), using stack-velocity smoothing, two-way traveltime(TWT)of the reflection event from different interfaces and DIX’s formula.

(Fig.2)

Fig.2 The near-surface velocity structure (a), the common-shot gather (b) and the average velocity distribution image of the lithosphere (c)

According to the near-surface velocity structure along the profile(Fig. 2a), the complicated shallow geological structure in the study area features clear vertically layered horizontal blocks and velocity gradientstructures in different layers. Above depth 200~250 m, the near-surface velocities of most sections are lowerthan 3000 m·s^-1, which indicates a small thickness of unconsolidated sediments along the profile. Below depth500 m, the velocity varies significantly with alternating high and low velocities in the horizontal direction, whichshows that the burial depth of bedrocks is characterized by alternating sinking and rising, and there may befaults in places of abrupt velocity variation.

Average velocity of the lithosphere along the profile(Fig. 2c)shows that P-wave average velocity graduallyincreases from shallow to deep in the depth direction, but in the profile horizontal direction it varies slightly.Where the two-way traveltime is less than 2.5 s, the average velocity of seismic waves is lower than 4500 m·s^-1.Near the Moho(TWT 10.2~11.0 s), the average velocity increases to 6000~6100 m·s^-1. Where TWT is 19~20s(corresponding to the west-dipping LAB reflection in Fig. 2b), the average velocity is 7500~7600 m·s^-1, whileit is higher than 8000 m·s^-1 where TWT is over 22~23 s.

We’ve produced a clear image of fine lithospheric structure(Fig. 3)where TWT is above 25 s(the depthis around 100 km). The lithosphere in this area is a block structure bounded by TLFZ along the profile, and where TWT is 10.2~11.8 s(the depth is about 31 km to 36 km), it is divided by the Moho strong reflection intotwo parts in the depth direction—the crust above the Moho and lithospheric mantle below the Moho. Featuresof the reflection structure are as follows.

(Fig.3)

Fig.3 Geological structure (a) and interpretation results of deep seismic reflection profile (b)

The deep seismic reflection profile extends from the west to the east, crosses the eastern margin of theNorth China Block, TLFZ and the southwestern segment of the Sulu orogen belt(Fig. 1). The plane position ofTLFZ is located between profile distance 34 km and 68 km. Deep seismic reflection profile(Fig. 3)shows manygroups of clear reflections from crustal sub-interface(R₁~R₄), crust-mantle transitional zone and the Moho, which are all bounded by TLFZ with different reflection structures on its two sides that will be presented infour aspects as follows:

First of all, the thickness and morphology of the crust-mantle transition zone, consisting of the crustalbottom reflection and Moho strong reflection(underlined by blue dotted line), feature different reflection structures bordered by TLFZ. In the eastern margin of the North China Block(i.e. west of the profile distance68 km), the crust-mantle transition zone comprises a series of near-parallel west-dipping strong reflections, thevertical duration time of which is 1.8~2.0 s and the thickness of the transition zone is 5~6 km accordingly. Onthe east side of TLFZ and under the western margin of Sulu orogen belt(i.e. east of the profile distance 45 km), the crust-mantle transition zone features “wedge-like” reflection structure, the thickness of which is graduallydecreasing from the east to the west ~6.0 km to 6.5 km on the eastern end but ~2.5 km to 3.0 km at the profiledistance 45 km. Compared with the western part of the profile, the crust-mantle transition zone in the east ofthe profile comprises weak and short west-dipping reflections, and the bottom interface of the transition zoneis clearly undulating. Between profile distance 45 km and 68 km which in beneath TLFZ, the reflection of thecrust-mantle transition zone uplifts clearly and its thickness is apparently thinner than that of both sides, and the depth of Moho interface is shallower.

Secondly, the Moho configulation and the crustal thickness are different between TLFZ and blocks on bothsides along the profile. The Moho is deep in the west and shallow in the east. The Moho interface exhibits awavy variation and the crustal thickness is 31~34 km on the east side of profile distance 45 km. Beneath TLFZ and between profile distance 45 km and 70 km, the crust-mantle transition zone and Moho interface show alarger rise, the depth of the Moho is 28~30 km. Beneath the eastern margin of the North China Block(i.e. thewest part of the profile distance 68 km), the Moho is west-dipping and the crustal thickness is 33~36 km. Allthese phenomena mentioned above suggest that TLFZ is not only the boundary zone between the North China and Yangtze Blocks, but a belt of abrupt crustal thickness change.

Third, crustal reflection distribution and reflective structural features of TLFZ are different from those ofblocks on both its sides. Beneath the western edge of Sulu orogen belt and the eastern margin of the NorthChina Block, there are 2 groups of laterally continuous reflections with strong reflection energy in the crust, i.e., the R₃ and R₄ reflection with different reflection interface occurrences on the two sides of TLFZ. To the east ofTLFZ, R₃ and R₄ are both east-dipping in the profile, while on the west of TLFZ, R₃ and R₄ reflection appearsto be west-dipping. Within TLFZ, many groups of crustal reflections are arch arcs with unequal length laterally, the reflection energy being alternately strong and weak and the morphology being crossing, interrupting and overlapping, which evinces the complexity of TLFZ’s crustal structure.

Fourth, above depth of about 800 m, the deep seismic reflection profile(Fig. 3) and near-surface velocitystructure(Fig. 2a)show that the structural form of ridges alternates with depressions. On the whole, sedimentary thickness and near-surface velocity structure of TLFZ between profile distance 34 and 68km is differentfrom those of blocks on both its sides. To the west of TLFZ, the depth of the reflection R1 is about 200 m, and velocity is higher than 4000 m·s^-1 below R1. Within the fault zone(i.e. between the profile distance 34 and 68km), the near-surface velocity is considerably lower than those of its eastern or western sides, and thesedimentary thickness distribution is thicker in the center and thinner at each end; its thickest part is near theprofile distance 55 km, Lingcheng Town. To the east part of TLFZ, the depth of R1 is shallower, the near-surfacevelocity is higher and the sedimentary thickness is thinner. Between profile distance 12 and 27 km, the profileexhibits a 15 km-wide sedimentary depression. Geological references of this area show that the thickness ofQ+N strata at the site of TLFZ is 200~400 m, being unconformity with underlying Cretaceous strata; in theuplifts, Q+N strata becomes thinner and lacking in Cretaceous sediments(Li Q T, 1994; Zhang P et al., 2011).The reflection R1 and R2 in the shallow part of the profile clearly manifest lateral heterogeneity of near-surfacesediments and strata offsets caused by fractures. Integrating the distribution characteristics of R1 and R2 onthe deep seismic reflection profile, the near-surface velocity distribution(Fig. 2a) and geological data in thisarea, we interpret R1 as the bottom boundary of Q+N, and R2 as a reflection from the Cretaceous strata.

The deepest set of interface reflection revealed by deep seismic reflection profile is marked as LAB. TheLAB reflection is west-dipping to the east of profile distance 55 km; its two-way travel time is 19.5~21.5 s, its corresponding interface depth is 75~86 km, and the interface dip-angle is about 11.3°. To the west ofprofile distance 68 km, the LAB reflection is nearly horizontal, and its depth is about 75 km. Previous deepgeophysical study results demonstrate that TLFZ is a weak tectonic zone between the North China and theYangtze Blocks, which has developed a wide range of lower-velocity layers in the crust; the lithospheric thicknessbeing 60~80 km, it is the thinnest area in the lithosphere of the North China Craton(Chen et al., 2006, 2008;Zheng et al., 2008; Zhu R X et al., 2011; Li S L et al., 2011). Xiangshui of Jiangsu Province-M and ula of InnerMongolia Geoscience Transect crossing Linyi City in Sh and ong Province, shows that the lithospheric thicknessnear TLFZ is 70~80 km(Ma X Y et al., 1991). Geoscience Transect HQ-13 goes across TLFZ through the areanear Wuhe-Jiashan to the south of the deep seismic reflection profile, whose results show that the lithospherethickness beneath the fault zone thinned to 90~100 km(Chen H S et al., 1993; Wang X F et al., 2000). Ourdeep seismic reflection profile results show that the lithospheric thickness along the profile is 75~86 km, thickerthan that of the Yishu segment of the fault zone demonstrated by Ma X Y et al.(1991), Chen et al.(2006) and Li S L et al.(2011), but thinner than that of Geoscience Transect HQ-13. It indicates that the lithosphereof Weifang–Jiashan section in TLFZ is gradually thinning from the south to the north.

In the lithospheric mantle between the Moho and the LAB, deep seismic reflection profile also displays somereflection events with drastic changes on interface occurrence, reflector morphology and reflection amplitude(i.e. RA, RB, and R_U), among which the reflection RA and RB are reflected as overlapping arch arcs in theprofile, and their presence can be seen only between TWT 11 s and 15 s beneath the TLFZ. Studies above showRA and RB is a reflection event with local significance, of which phase group morphology and reflection energydiffer a lot from Moho strong reflection and LAB reflection. We infer that it may be lateral residues of the hot materials in the asthenosphere upwelling along the fault zone. Reflection R_U of the upper mantle exhibitsdifferent interface occurrence and reflection energy on the eastern and western sides of the TLFZ. To the westof profile distance 68 km(i.e. to the west of the TLFZ), R_U shows strong energy, pitching from the west to theeast with a depth of 54.5~58.5 km. Between profile distance 25 and 50 km, R_U is slightly west-dipping, and itsdepth is about 66.5 km. To the east of profile distance 25 km, the presence and morphology of R_U are barelyvisible in the profile.

The deep seismic reflection profile includes comprehensive information about underground structure and tectonics; variations of both reflection energy and reflection phase features along the profile correlate closelywith the composition of underground structure and tectonics. Fig. 3 shows the interpretation results of the deepseismic reflection profile, from which we see that TLFZ is a fault system cutting the whole crust down to thelithosphere. What’s more, TLFZ has different structural characteristics in the crust and lithosphere.

Within the lithospheric mantle below the Moho, TLFZ is a zone of weak reflection energy featuring nearlyvertical dips, a wide-top narrow-bottom shape and different crustal thickness and lithospheric thickness on itstwo sides, which suggests that TLFZ is not only a fault zone cutting the lithosphere but also a rapid changebelt of the crust and lithospheric thickness. Variations of both reflection energy and reflection event featuresfrom the Moho, LAB and RU indicate that the material composition and structure of the lithospheric mantleon both sides of the fault zone may be different. Burial depth variations of the Moho, LAB reflection and upper mantle reflection RU suggest that there is a vertical drop in the Moho, RU reflection interface and thelithospheric bottom on both sides of the fault zone, and the drop gradually increases. Further supported byinformation of flexural uplift and abrupt change in the crustal bottom below the fault zone and the Moho, webelieve that TLFZ has experienced intensive and successive extension and compression besides large strike-slipcomponents in the process of formation and evolution. In addition, the deep seismic reflection profile shows thatthe crust-mantle transition zone in the western block of the fault zone and the Moho are of eastward obduction;thereby in the eastern block of the fault zone, the low-angle westward subduction structure of the crust-mantletransition zone, the Moho and the lithospheric bottom may indicate that the eastern block subducts beneaththe western block. However, because those structures are cut by high-angle Tan-Lu strike-slip fault zone in thelater period and the length of the deep seismic reflection profile is too short, we could not confirm whether thereis such a tectonic phenomenon by the profile.

In the crust above the Moho, the TLFZ is composed of a number of complex fault zones, whose faultplanes are nearly vertical, smooth and wavy, branching out from the deep to the shallow to develop a typical“flower-like” structure. The “flower-like” structure is a special structural form that the strike-slip fault systemexpressed in the profile domain; its structural pattern characteristics differ from those of listric or “spade-like”normal faults, showing strike-slip effects of TLFZ. In addition, we can see some stratum fold with unequallateral length between the main faults in TLFZ, and the uplifted antiform reflection coming from many groupsof crustal interface cut by faults indicates the nature of compressional tectonics, which shows that TLFZ featuresa large thrust displacement component in the course of left-lateral strike-slip. Cretaceous fault basin developedin the shallow part of the profile and its border-controlling normal faults suggest that TLFZ has experiencedmore intensive extensional rift movement in the process of its formation and evolution.

In summary, the intra-crustal structural configuration of the TLFZ is more sophisticated than that in thelithospheric mantle. Within the lithospheric mantle, TLFZ is a fault zone consisting of lithospheric deep faultsF_L1 and F_L2. This tectonic zone goes down to the asthenosphere and up to the crust with a wider top and narrower bottom. Its width is 6~8 km in the lithospheric mantle and about 15 km around the Moho. The deepfault F_L1 extends up to the depth of 15~18 km, controlling the deformation and the lower crustal structure onthe western side of TLFZ, and the deep fault F_L2 extends up to link with the “flower-like” structure consistingof many faults. Previous studies believed that TLFZ in the south of Suqian City, Jiangsu Province, consists of four major faults F₁–F₄(Institute of Geology, CEA, 1987; Li Q T et al., 1994; Zhang P et al., 2011). Inthe deep seismic reflection profile, these four faults in shallow parts of the crust cut near-surface sediments asnormal faults, controlling the formation of Cretaceous basin and graben-horst structures; they go down to cutmultiple groups of crustal interfaces to form a fault system in the bottom of the lower crust and then link withthe deep lithospheric fault zone. In addition to the four main faults, the deep seismic reflection profile alsoreveals two intra-crustal buried faults(i.e. F₅ and F₆)to the east of TLFZ’s eastern boundary fault F1. Thesetwo faults cut the intra-crustal R₃ and R₄ reflector, control the crustal structure and deformation with TWTbeing 2 s(depth of about 4.5 km) and combine with the fault F₁–F₄ into a tectonic system at the depth 28~30km. We believe that faults F₅ and F₆ are also part of TLFZ, which do not extend up to the surface.

It is believed that the collision and suturing of the North China Craton and the Yangtze Craton duringthe Indo-Chinese epoch are the tectonic background of TLFZ startup, and the fault zone has experienced acomplicated evolution including left-lateral displacement, stretch and compression since the Mesozoic era. Thetectonic evolutionary diversity certainly would have left significant imprints on the deep-shallow structures and could be imaged by geophysical methods. Our deep seismic reflection profile clearly displays the finelithospheric structure of TLFZ and blocks on both its sides, major tectonic traces and sophisticated tectonicpattern combination and deep-shallow faults’ geometric configuration during the process of the fault zoneevolution.

The fault zone structure and its geometric morphology have been closely connected with the dynamicsprocess. The dip-angle of strike-slips is always near-vertical, and the “flower-like” structure is a special tectonicpattern that strike-slips embodied along the profile, of which the structural pattern characteristics differ fromthose of listric normal faults resulting from extensional tectonics and low-angle thrust faults in compressionsurroundings. The deep seismic reflection profile shows that TLFZ is not a simple listric normal fault orlow-angle thrust fault, but a large “flower-like” structure consisting of many faults, with the structural planechanging with the depth, fault plane being nearly vertical, and main faults combining extension, compression and twists together. It indicates that TLFZ is a complex fault zone, with large strike-slip and some thrustor normal fault components. From shallow part to deep, this fault zone cuts near-surface sediments, uppercrust and lower crust, the Moho interface and the lithospheric mantle in turn, and it transforms from brittle, brittle-ductile to ductile with increasing depth. Therefore, it is a deep large fault zone of the lithospheric scale.

TLFZ is the boundary zone between the North China and Yangtze Blocks. The crustal and lithosphericthickness on its two sides are different. On the eastern margin of the North China Block(the west of TLFZ), the Moho is west-dipping, and the lithospheric bottom interface is nearly horizontal, and the crustal thicknessis 33~36 km while that of the lithosphere is about 75 km. To the east of TLFZ, the Moho appears wavy and the lithospheric bottom interface is west-dipping, and the crustal thickness there is 31~34 km while that ofthe lithosphere is 75~86 km. Beneath TLFZ, the Moho rises about 4~6 km, and the lithospheric thicknessvaries by about 10 km. All the phenomena mentioned above suggest that TLFZ is not only a large faultzone incising the lithosphere, but also a belt of abrupt change in the crustal and lithospheric thickness. Thefault zone provides deep geodynamic sources for high temperature high pressure(HTHP)material from theasthenosphere to upwell to the lithosphere. Magmatic under-plating or thermal erosion from the asthenospherematerial upwelling leads to lithospheric extension and thinning, which probably causes changes in lithosphericstructure and composition. Through the comparison between inner and outer mantle xenoliths of TLFZ, (Zheng J P et al.2000, 2006)found that mantle replacement significantly strengthens in the fault zone. The oldlithospheric mantle is completely replaced by new lithospheric materials while there is still some residue of theancient lithospheric mantle in NCC away from TLFZ, which shows that the fault zone is a good channel fornew asthenosphere material upwelling, mantle transformation and the replacement effect. The results of thispaper and other studies indicate that TLFZ is a deep large fault zone on the lithospheric scale, a weak tectonic zone and the channel for the upper mantle’s hot materials’ upwelling, which plays an important role in thelithospheric thinning and destruction of NCC.

This work was supported by the National Natural Science Foundation of China(41374100, 91214205).