Research On Studies Of Tectonic Evolution And Formation Mechanism In The Qingdong Sag During Cenozoic

The Qingdong sag in the southeast of the Bohai Bay basin in eastern North China Craton is an Cenozoic rift-type to depression-type basin controlled by the western branch of the Tan-Lu fault zone. It has long, complex and intensive tectonic activities, well-developed faultsand complex structural features due to its suffering complicated structural evolution, and also presents, which make it more difficult to determine hydrocarbon traps and selection of exploration areas. Therefore, it is of great theoretical and practical significance to analyze its Cenozoic structural features, evolution history and the formation mechanism. With the Cenozoic structural evolution as a main clue, characteristics of sedimentation and structures in the sag are studied in detail.Based on analysis of regional geophysics materials, structural features and boundary faults of the basin have been recognized. The studies find that the basin has a dual texture which consists of the Paleogene rift and Neogene-Quaternary depressions on its profile. It is surrounded by uplifts on the plane. Boundaries between the basin and uplifts are large faults and overlap lines. Controlled by strong activities of the western branch of the Tan-Lu fault zone, the basin framework of Paleogene shows NNE-SSW trend in general, with faults at the eastern margin and overlap at the western margin on its E-W profiles. Secondary structure units of the basin can be divided into the western slop belt, central sub-sag belt and the Tan-Lu fault fault-slope belt from west to east, which are all NNE-trending. The sag is controlled by series of E-W and NW-NWW faults on the NS profiles, forming the tectonic framework of division in the N-S direction accompanied by alternating uplifts and depressions.Based on the analysis of Cenozoic sedimentary characteristics in the Qingdong sag, we can find that abundant sediments of the sag were supplied by surrounding uplifts during the rifting period. The sedimentary sequences vary obviously from boundaries to centre of the basin. Generally, the sedimentary distribution underwent a change process of N(N)E and N(N)Wâ†'nearly EWâ†'N(N)E from the early to late stage during Paleogene in the interior of the basin, Meanwhile, the depositional area and thickness continued to decrease. The depocenter and subsidence centers mainly focused on the vicinity of the Tan-Lu fault zone in the sub-sag belt at the end of Paleogene. The basin deposition is uniform and sediment thicknesses have little variations during Neogene and Quaternary.After quantitative analysis on subsidence history of the Qingdong sag, the paper reveals that the sag subsidence differ greatly in space and time during Cenozoic, which can be classified into initial subsidence and rapid subsidence stages during Paleogene and the slow subsidence from Neogene to Quaternary. Their subsidence curves indicate a typical extensional basin of Paleogene. The subsidence mainly concentrated in the southern areas during deposition of the Kongdian Formation, then migrated to the northeastern areas gradually during deposition of the upper Sha-4and Sha-2members. Finally, the subsidence center focused on the vicinity of the Tan-Lu fault zone in the northern basin during deposition of the Sha-1member and Dongying Formation, meanwhile, the southern areas were be under the regional uplifting relatively. According to the sedimentary and subsidence evolution rules of the whole Bohai Bay basin during Paleogene, the paper proposes that it is the tectonic migration that leads to the basin uplifting gradually during the Oligocene together with basin shrinking and the wide absence of the top Paleogene strata.There are three sets of predominant fault systems developed during Paleogene in the Qingdong sag, including the NNE-trending fault system (including NNE, NE and NEE) represented by the west branch of Tan-Lu fault zone, NW-trending (including NNW,NW and NWW) and E-W trending fault systems. From analyses of many geophysical data and regional tectonic evolution, it is proposed that the NNE-and NW-striking faults belong to basement faults, and result from the Tan-Lu sinistral faulting in Late Jurassic and foreland deformation in the Indosinian period respectively.By analysis of kinematic characteristics of Paleogene for faults with different orientations in the Qingdong sag, it is found that the reactivated NNE and NW basement faults show dextral and sinistral normal faulting respectively. Majority of the basement faults show intensive reactivity in the earlier stage, and became weak or dying in the later stage except for some large-scaled faults which have strong and sustained activities. A large number of E-W faults formed during the Paleogene rifting are newly-formed normal faults without strike-slip components, and they show increase in quantities and activity intensity gradually from the earlier to later stage and predominated in the basin finally.According to the distribution pattern and activity features of the Paleogene faults and comparing with previous results about regional stress fields, it is proposed that the basin was under N-S extension during the Paleogene basin formation period. The NNE and NW basement faults reactivated by a way of oblique extension. Reactivity methods of the basement faults can be classed into three types, i.e. the direct reactivity, formation of new en echelon faults and direct reactivity plus propagating into E-W faults. The direct reactivity faults mainly appear in some basement faults close to orthogonal extension while the deriving en echelon faults usually in the major basement with larger strike-slip components, such as the western branch of the Tan-Lu fault zone.By analysis of Neogene structural characteristics in the Qingdong sag and Bohai region, it is found that the fault framework formed during this period is similar to that of the underlying Paleogene strata, but activity intensity, mainly controlled by lithospheric thermal subsidence, became weaker. Active faults of Neogene in the basin were reactivity of pre-existing ones mostly. Newly-formed faults of Neogene were few and often small-scale. The Tan-Lu fault zone showed dextral normal faulting while the NW-and E-W striking faults showed sinistral normal faulting and normal faulting respectively. Geometry and kinematics of the three sets of faults indicate that the region was under a weaker, N-S extension setting during Neogene.Comprehensive studies show the Qingdong sag experienced seven phasesof evolution, i.e. the early rifting stage during deposition of the Kongdian Formation and lower sha-4member, basin inversion and uplifting stage at the end of deposition of the lower sha-4member, intensive rifting stage from deposition of the upper sha-4to sha-2members, weak rifting stage during deposition of the sha-1member and Dongying Formation, basin inversion and uplifting stage at the end of Paleogene, weak extension stage during the Neogene and regional compression stage since the latest Neogene. Geodynamics for the basin evolution might come from changes in oceanic plate subduction in the Pacific Ocean as well as far-field influence of the India-Eurasiancollision in different periods. It is the destruction deep processes of the eastern North China Craton that lead to the structural migration in the Qingdong sag and its surrounding areas during Paleogene-Neogene.