The Structural Styles And Evolvement Law In Southern Depressions Of Dayangshu Basin In Inner Mongolia

Dayangshu basin is located in the Arong Banner and Oroqen Banner, east of Inner Mongolia, whose geographic coordinate is 123°21'～125°38'E, 47°55'～50°59'N. It is the bigger one of the Mesozoic and Cenozoic faulted basins, 15460km2, around the Daqing exploration area. In geotectonic, this basin is located in Inner Mongolia—Xing'anling folds system, north of the Xing'anling fold belt. Its western boundary is Daxing'anling ultracrustal fault belt whose trend is NNE and the eastern boundary is the western border of Songliao basin, crustal fault belt whose trend is NNE. The Hailar-Jiayin crustal fault and Boktu-NamoheJiayin crustal fault, whose trends are EW, cross the Dayangshu basin. In the Dayangshu basin, there are the Early Cretaceous basic volcanics and Late Cretaceous-Paleogene basalt and Yanshan epoch little intrusions. The Early Cretaceous strata distribute in the west of Dayangshu basin. Late Cretaceous - Paleogene basalt distribute in east and south of this basin. There are Lower Cretaceous Longjiang group, Jiufengshan group and Ganhe group volcanics in the research area.1 The basic structural characteristicsThe generation of the Dayangshu basin is combination of mantle spot and levorotation stress, and the activity of the later fault leads to the eruption. There are many faults in the Dayangshu basin, whose trends are NNE or NW and basic distribution characteristics are more faults in south than north. The faults, whose trends are NNE, control the sedimentary of caprock and the whole trend of this basin. They are the faults, controlling the basin, boundary of the basin. They generated in the early time then were leaped by the faults whose trends are NW, distribute in segment. The faults, whose trends are NW, are generated in the later time. Based on the gravity and magnetic data, the fault's characteristics and the controlling on structural evolvement of the basin and strata, we divided the Dayangshu basin into three first-grade structural units, two depressions and one uplift, which are the western depression, the middle uplift, and the eastern depression. The basic characteristics are zoning in east to west and blocking in south to north, distribution in south to north.2 The characteristics of seismic stratigraphy in southern depressionsBased on the analysis of seismic profiles and drilling data in the southern depressions of Dayangshu basin, we have get the corresponding characteristics between the seven seismic reflecting interfaces, T4-01, T4-02, T4-03, T4-1, T4-2, T4-3, and T5, and the seven geological interfaces: T4-01is equivalent to the basement of the fourth segment of Ganhe group, T4-02 is equivalent to the basement of third segment of Ganhe group, T4-03 is equivalent to the basement of the second segment of Ganhe group, T4-1 is equivalent to the basement of the first segment of Ganhe group(the top of the upper Jiufengshan group), T4-2 is equivalent to the basement of the upper Jiufengshan group, T4-3 is equivalent to the basement of the lower Jiufengshan group, and T5 is equivalent to the basement of the Longjiang group(basement of the basin). Besides these, log data also response to the geological interface.3 Structural styles in southern depressionsThe main structural styles in southern depressions of Dayangshu basin are the basic structural styles in the extensional rift basin. There are strike-slip structural styles in the depressions. Besides these two main styles, there are also some early compressions and later inversions in the basin, and forming some complicated complex structural styles.4 History of tectonic evolution on the typical seismic profiles in the southern depressionsIn the geological profiles, we can know that there are Longjiang group, Jiufengshan group, and Ganhe group strata in general. There is an angle unconformity interface between Ganhe group and basement which is metamorphic rock system. The thickness of the Ganhe group is great, and the maximum is 2000m. The strata are in reverse tilted faulted-block structural style whose dip is west. The faults are controlling the generation of the faulted basin in the Ganhe group. During the beginning of the Jiufengshan group, the faults which is controlling the faulted basin, was going on movement and generated the thick sediment. In the middle of the Jiufengshan group, the uplift was happened, the southern depressions underwent the compaction from east and west. There are also many fault propagate folds in southern depressions, and underwent erosion in part then generated the complex compaction structural assembly and unconformity interfaces. In the west, these are break up of some fault propagate folds, corresponding the violent uplift. In the front limb syncline of the fault propagate folds; there is some sedimentation whose sediment is from the front limb anticline. Going with the compaction, there is strike-slip which is identified easily from the profile of tectonic evolution history before the first segment of Ganhe group. Latterly, the extension and sedimentation was gong on and the fault transferred toward to east which is controlling the generation of the faulted basin. After the extension of Jiufengshan group, Ganhe group was going on and the active fault is high angle normal fault which has the characteristic of strike-slip, little slip, lower rate of movement, and less difference between the depth of the footwall and hanging wall. During the second segment of Ganhe group, some new normal faults generated. The third and fourth segments of Ganhe group strata are suffered from the destruction of the latter uplift. We can't resume them, but we can infer that the latter violent compaction must be happened after the Ganhe group.Based on the profile of nowadays, we know that Ganhe group suffered from the erosion widely even though the Jiufengshan group, which means the latter uplift is very violent. Based on the lithology, during the formation of Longjiang group, the research area walked into the initial rift period, which is the important period for the faults which is controlling the faulted basin. In this period, the volcanic facies built up the sedimentation. During the period of the Jiufengshan group, uplift partly, the intermediate volcanic and shallow lake sedimentation is the main elements for the lithology. The lithology of the first segment of Ganhe group is basalt-sedimentary rock-andesite-basalt, begin with basalt and end with basalt. The second segment of Ganhe group is composed of basic volcanic, basalt, andesite-basalt, tuff, rhyolite-tuff and so on (Daqing oil field, 2005).5 The relationship between Dayangshu basin and Songliao basinAccording to the structural styles, fill sequence of sediment, subsidence history and generation of structure between Dayangshu basin and Songliao basin during the Early Cretaceous, we can infer that they have the same background on tectonic, similar eruption on magma activity and difference on time of activity, which is caused by the migration of the structural activities in the later time. The later structural activity and difference on geography cause the difference on location between the two basins.Based on the stress background in local, the two basins are the results of the same extensional stress whose trends are west to east. They have the same trend, NNE, which gave us the irrefutable evidence and they are both faulted basin formed in the Early Cretaceous. It is the stress that originated from the extension stress which is generated by the breakup of the Pangea. This stress divided the Songliao basin into several pieces of faulted depression which are undertook sedimentation and form the faulted basins around Songliao basin. During this period, Dayangshu basin is just faulted depression of Songliao basin.Based on the analysis of the data, the depression process of Songliao basin is caused by the Jurassic extensional stress. During this period, Songliao Plate was divided into several pieces and becoming thin. In order to equipoise the loss of the lithosphere's mass, the mantle should uplift. This model is consistent with the traditional views, but we should pay attention to the fact that the mantle uplift resulted in rift of the lithosphere which the formation of the Dayangshu basin can illuminate. If the rift of lithosphere resulted in mantle uplift, the Dayangshu basin will in the compress background which borders on Daxing'anling fold belts, and the Dayangshu basin won't be formed.Above all, the Songliao basin and Dayangshu basin is the different depression in the same basin or different part of the same depression in Early Cretaceous. This different location caused that the strata above Later Cretaceous were denudate widely in Dayangshu basin, which is different with Songliao basin-the strata of sag were absent.