Tectonic Evolution Of Jiaolai Basin

In Early Cretaceous, eastern China and adjacent areas stretched and rifted intensively withthe lithosphere's thinning. Jiaolai basin developed within Jiaodong peninsula, and is located atthe convergence of a series of important tectonic belts which have great geological significances.The formation and evolution of this basin were probably controlled by joint process of regionalstress fields and deep dynamic forces of continental lithosphere (underplating, delamination, mantle diapir and convection) induced by the subduction of (paleo-)pacific plate beneath the Asiacontinent. Besides, the formation of Jiaolai basin probably related to orogenic extensionalcollapse because Jiaolai basin's location was also at the combination part of Huabei plate andYangtze plate. Additionally, Tan-Lu fault zone experienced an intensive strike-slip displacementin this period. So the tectonic evolution of Jiaolai basin is very complicated. For its specialtectonic location and hydrocarbon potential, Jiaolai basin's forming mechanism has attractedmany researcher's attention for a long time. But existing dynamics mechanisms cannotcompletely and reasonably interpret all kinds of geological evidence and its tectonic evolutioneither. In order to understand Jiaolai basin's basic geological condition more deeply and directhydrocarbon exploration better, we need a more reasonable tectonic revolution and thecorresponding dynamics mechanisms.Basin's dynamic mechanisms determine kinematic characteristics, whereas furthermorekinematic characteristic determine geometric characteristics, so geometric and kinematiccharacteristics must contain information of basin dynamics. For this reason, deduced dynamicmechanisms from geometric and kinematic characteristics can reasonably interpret basin'sformation and evolution. Geometric characteristics mainly include various basic tectoniccharacteristics, so we should comprehensively utilize gravity, magnetic and seismic data andsome observing/analyzing materials based on residual strata to study them. Whereas kinematiccharacteristics mainly include the features of vertical subsidence and plane strain. On the basis ofestimating eroded strata, this paper plans to utilize comprehensively various information inseismic sections (including faults' syndepositional movement characteristics, velocity spectrumetc.), drilling and logging data etc., make many balanced sections and subsidence curves ofrepresentative areas, furthermore integrate outcrop data and strata interfaces' ages to find out thekinematic characteristics of basin prototypes. When prototypes' geometric and kinematiccharacteristics are clear, their paleotectonic stress fields are almost clear too. But the stress fieldsneed to be tested with finite element simulations. Furthermore this paper combines magmatite types, its characteristics of geochemistry and spatiotemporal distribution and strata interfaces'ages to depict the process of basin formation and evolution and the corresponding dynamicmechanisms reasonably.Jiaolai basin is located at Jiaodong peninsula and is a Cretaceous residual superimposedcontinental sedimentary basin. Its south margin is connected with SuLu orogenic belt (Jiaonanuplift) through Wulian fault. Its north margin wanders on Jiaobei uplift. Its west margin istruncated by Tan-Lu fault belt and its east margin spans Haiyang and Rushan, enters Huang seaand steps eastwards until Qianliyan fault. The north boundary perhaps is a denuded one.According to the regional geophysical characteristics of eastern Shandong province, we canregard the envelope line of residual strata as Jiaolai prototype basin's north boundary. Whereasother boundaries are the same with present ones.Jiaolai basin can be divided into 7 secondary tectonic units: Zhucheng depression, Chaigouhorst, Gaomi depression, Dayetou salient, Laiyang depression, Mouping-Jimo fault zone andHaiyang depression. Furthermore the Gaomi depression can be divided into 4 independenttectonic units: Xiagezhuang sag, Pingdu sag, Lidangjia-mashan salient and Gaomi sag.The strata of Jiaolai basin can be divided into two big rock series, i.e. basement series andsedimentary cover series. The former is composed of Archaeozoic group and Proterozoic groupmetamorphic rocks. The latter is composed of Cretaceous, Paleogene and Quaternary. Laiyangformation and Qingshan formation of Lower Cretaceous and Wangshi formation of UpperCretaceous compose the main part of sedimentary cover and they are formed at Laiyang, Qingshaand Wangshi stages respectively.The Laiyang formation is mainly composed of sedimentary clastic rocks of inland lacustrinefacies and fluvial facies, interbedded by acidic～intermediate-basic volcanic rocks. From bottomto top, this formation can be divided into 6 members: Xiaoxianzhuang member, Zhifengzhuangmember, Maershan member, Shuinan member, Longwangzhuang member and Qugezhuangmember. The Qingshan formation is composed of polygenic volcanic rocks and volcaniclasticrocks. The volcanic rocks in the Qingshan formation resulted from central volcanic eruption anda series of central volcanic apparatus distributed along Mouping-Jimo-Wulian fault belt. Mainlithology of the Wangshi formation is red clastic rocks of fluvial facies interbedded by variegatedclastic rocks and a little marl of shallow lake facies as well as basic volcanic rocks.At the beginning of Laiyang stage, the salient and depocenters extending in NW-SEdirection and depocenters extending in NE-SW direction in Jiaolai basin not only indicated thatthe basin was probably in the condition of plane bidirectional extension, but also indicated thebasin forming was controlled by ubiquitous basement faults in NW-SE direction. The existenceof Yishu rift system at the Qingshan stage and the normal faults' notable controlling effect onstrata depth at the Wangshi stage suggested extension stress fields were very common in studyareas.The magmation was very active in Jiaolai basin and its adjacent areas. This resulted in allkinds of magmatites occurred well in study areas including plutonic intrusive rocks～volcanicrocks and acidic～ultrabasic magmatic rocks. The geochemical characteristics of intrusive rocks in the Laiyang formation indicate the strong underplating of mantle-derived magma at this stage.Underplating is regarded as one of processes that can cause crust stretching. The volcanic rockscan be divided into 6 eruption cycles: Laiyang cycle in the Laiyang formation, Houkuang cycle, Bamoudi cycle, Shiqianzhuang cycle and Fanggezhuang cycle in the Qing shan formation andShijiatun cycle in the Wangshi formation.The rock geochemical data show that the Laiyang～Fanggezhuang volcano eruption cyclesbelong to the mesozoic "Shoshonite Province" in eastern China. These cycles developed due toenriched lithospheric mantle's partial melting process under tensile background. And the negativeEu anomalies of the intermediate-acidic volcanic rocks in these cycles become stronger graduallyfrom early to late. The Shijiatun cycle's basic volcanic rocks show a sudden change of theirmagma source from lithospheric mantle to asthenosphere. The above characteristics suggestthat Jiaolai basin was dominated by extensional stress field at its three geological stages and itslithosphere was thinned continuously.The various tectonic geometric characteristics of Jiaolai basin, such as the status of mohodepth(uplifting) and the configuration patterns of faults, also indicate Jiaolai basin has undergonenotable extension.The syndepositonal movements of normal fault groups with different running directions inJiaolai basin occurred simultaneously at its three geological stages. This shows the status of planeextention. The characteristics displayed by some balanced cross sections in different directionsindicate that the three prototypes of Jiaolai basin were all under the control of bidirectionalextentional stress field. Several subsidence curves of representative areas show it is not acontinuous rifting process from the Laiyang stage to Wangshi stage but a superimposing processof three rifting prototypes. At the beginning of basin forming, the Zhucheng and Gaomidepression had more characteristics that suggested their initiative rifting, whereas the Laiyangdepression had more passive rifting characteristics.There are good correlations among the characteristics of syndepositional fault movements, horizontal extension and vertical subsidence. This also strengthens the reliability of each aspect.The syndepositional fault movements in the Zhucheng depression mainly occurred at the Laiyangand Wangshi stages, whereas there was almost no syndepositional movements at the Qingshanstage. Correspondingly, extentional velocities of seismic sections in three different directions hada law of big→small→big from the Laiyang to Wangshi stage; Tectonic subsidence velocity wasthe smallest at the Qingshan stage but relatively big at the Laiyang and Wangshi stages. Mostsyndepositional faults in Gaomi depression kept to move at the three stages. Extentionalvelocities of seismic sections in three different directions in Gaomi depression had a law of big→small→biggest from the Laiyang to Wangshi stage and correspondingly tectonic subsidencevelocity was small at the Qingshan stage but relatively big at the Laiyang and Wangshi stages.The faults within the Laiyang depression had more notable syndepositional movements at theQinshan stage. Correspondingly, extentional velocities of seismic sections in two differentdirections had a law of small→big→small from the Laiyang to Wangshi stage; Tectonicsubsidence velocity is big at the Qingshan stage but relatively small at the Laiyang and Wangshi stages.The azimuthes of principal stress 1 of the three plane bidirectional extensional stress fieldwere 30°-210°, 143°-323°and 3.5°-183.5°respectively. The principal stress 3 should beperpendicular to principal stress 1, so their azimuths were 120°-300°, 53°-233°and 93.5°-273.5°respectively. These azimuthes can be regarded as the directions of regional tectonic stressesimposed on the whole Jiaolai basin, so they can be used to build initial models for stress fieldsimulations.According to the distributing characteristics of sedimentary facies (or lithofacies) andmajor syndepositional faults, the simulation models for the Laiyang～Qingshan stages can bedivided into 16, 12 and 9 areas respectively. Further, 954, 764 and 758 finite elements and 2723, 2179 and 2101 nodes are obtained respectively. Altogether, two element types and five materialsare defined in the three simulation models.Results of finite element simulations based on Ansys system suggest the principal stressdirections obtained by geological analyses only need small corrections. The Laiyang stage'sinitial direction need a 10°clockwise rotation, the Qingshan stage's need no rotation and theWangshi stage's need a 13.5°anticlockwise rotation. Moreover, imposing loads with the status of40: 15, 60: 30 and 60: 50 respectively, we can get ideal simulation results when compared with theones of geological analyses. This further confirms that the three prototypes of Jiaolai basin wereall dominated by plane bidirectional extentional regional stresses.The bidirectional extentional regional stresses obtained from geological analyses andconfirmed by finite element simulations indicate possible deep dynamic processes, because thiskind of stress can be produced by uplitling of lithosphere mantle or asthenosphere.Judging from ubiquitous tiny angular or parallel unconformity among the three formationsof Jiaolai basin and the fact that sedimentoiogical cease after the Wangshi stage, we can knowcompressive tectonic reversions occurred at the end of each prototype. Three sets of conjugatedshear joints were corresponding with the three tectonic reversions. The characteristics of thesejoints indicate they were generated due to regional compressive tectonic stresses whose pitchangles were tiny or horizontal and their directions were SSE-NNW, SEE-NWW and NE-SWrespectively. These compressive stresses also produced some gentle folds. But some other foldswere possibly generated by strike-slip movements of large faults, especially the Yishu fault belt, this reflected their reconstructing influences on basin.Based on the data, evidence and analyses of this paper and former studies, we can concludethat the formation of Jiaolai basin probably had a closer relationship with the regional stress fieldand the deep lithospheric dynamics induced by westward subduction of the (paleo-) pacific platebeneath the Asia continent. Because the sudden change of subduction direction and velocity ofthe Izanagi plate 140 Ma ago, the induced magma underplating probably made Jiaolai basin comeinto being. At the end of the Laiyang stage, a possible pulsative acceleration made the Laiyangrift prototype inverted because of compressive stress. At the same time, this tectonic inversioncaused the prelude of the Qingshan rift prototype. After this pulsative acceleration, lower crustwas delaminated perhaps because of the subsequent continuing subduction, so the Qingshan rift prototype was generated. Almost 100 Ma ago, the subduction pacific plate began to speed up andmade the Qingsha rift prototype inverted because of the possible induced compressive stress field.The subsequent continuing fast subduction of pacific plate in high angle perhaps caused thedelamination of lithosphere root and made the forming of the Wangshi prototype. At the end ofthe Wangshi stage, the Wangshi prototype began to be in tectonic inversion possibly because ofthe joint process of the compressive stress from NE to SW generated by the forming of hotcentral mantle plume under Japan sea at that time and the withstanding caused by long distancegentle compressive stress field that came from SW direction. After the Wangshi stage, with thegradual northern movement of the pacific plate, the extensional activities caused by high anglesubduction of the pacific plate moved norwards too and stepped out of study area, so the Jiaolaibasin hasn't developed rift prototype again since then.