A Study On The Characteristics Of Meso-Cenozoic Tectonics And Present Seismogenic Structure Of The Southern North China Craton

The North China Craton (NCC) is one of the most active continental blocks in the world, as indicated by the frequent occurrence of devastating earthquakes. The natural hazards have enormous impacts on both the economy of China and the quality of life in the region. The study on the mechanism of the hazards requires a complete knowledge of geodynamics, lithology, tectonophysics, geochemistry, seismonogy and geology as well as synthetic analysis of them.This thesis focuses on the evolution of Meso-Cenozoic sedimentary and tectonics, craton destruction, crustal thinning and seismogenic faults of the southern area of the North China Craton (SANCC) and summarizes the progress and problems in these studies of the extensional tectonics represented by metamorphic core complexes and fault depressions. Some important subjects, which merit the further study and their significance in the study of the destruction and lithospheric thinning of the NCC, were discussed.Research has been carried out in three ways, fault investigation, sedimentary analysis and numerical simulation in SANCC. Some important results have been obtained and are presented as follows.1. Characteristics of surface structures and buried structures and structural system(1) The paleo-Taihangshan faultThe paleo-Taihangshan fault is a bounding fault between The Taihangshan high uplift area and range-front uplift area (hilly terrain), which extends towards NE near the western Xintai and turns into NNE or nearly NS gradually from Cejing, passing by Wu'an, then to southern Cixian. Outcrops of the fault can be seen time and again. The fault is the main detechment fault of Zanhuang metamorphic core complexes. With large fault throw, the hanging wall was extended intensively and formed the topography with alternating trenches and ridges.(2) The Taihang range-frontal fault (including the Yuanshi fault, the Handan fault and the Tangxi fault)The Handan fault is a major bounding fault between The Taihangshang uplift area and North China depression area in SANCC. Near the faut, gravity anomaly belt and crustal deformation transition belt can be noticed. Low gravity anomalies and positive vertical deformation appear in its western area, while the values are just on the contrary in its eastern area. According to the survey of broad band array and small earthquakes relocation and interpretation of oil seismic profiles, The Handan fault does not control the sedimentary of the Handan depression, which is a listric nomal fault with dip 70 degrees and fault throw up to 4000~5000m and is composed of sevaral sub-faults.There maybe a basal fault extending to the detachment fault (Paleo-Taihangshan fault) under the Handan fault.(3) The Cixian-Daming faultThe Cixian-Daming fault is the bounding fault between The Linqing depression and Neihuang uplift. The fault can be divided into three segments, the Daming-Linzhang segment, the Cixian-Fengfeng segment and the Nanshancun-Chakou segment, which are characteristic of buried faults and moderate earthquakes, buried faults and surface fractures, respectively.(4) The Qumo faultThe south-dipping Qumo fault is the north boundary fault of the Handan sub-depression, with length 60km, strike 290 degrees and dip 70-80 degrees. The fault extends to the Permian and Triassic formation with dislocation 1200m. The arc-like fault merges into the East Handan fault with reducing throw. The 1708 (M5.5) earthquake occurred near the convergence area with East Handan fault.(5) The West Zishan faultThe West Zishan fault begins west of Xintai city, extends along the west of Zishan mountains, vanishing near Boyan county, with length 70km, strike NNE, west-dipping. According to the survey of broad band array and small earthquakes relocation, there is a fault extending to deep crust.(6) Fault systemThe fault system of the Handan sub-depression can be divided into three units, which are surface fault unit (0~2km), middle fault unit (0.1~9km) and lower fault unit (10~12km). The lower fault unit is mainly composed of the paleo-Taihangshan fault, and many faults of the middle faults unit merge into the lower fault unit-detachment plane.All in above, the faults trending NE developed later relatively than the faults of NWW strike. But the Taihangshan fault was cut by the Qumo fault because of the intensive activity of Qumo fault. There are three transition zones in SANCC, which are the Longyao transition zone, Qumo transition zone and Maling transition zone from north to south.2. Evolution of Meso-Cenozoic tectonicsThe Indosinian movement in the early Mesozoic occurred widespreadedly in the whole China and formed the tectonic regime of uplift in the east and depression in the west which extend in NE-NNE. The uplift in the east continued intensively with the left-lateral shearing and compressing in the Yanshan movement. With the leteral compressing and deep heating, the thickness of lithosphere and continental crust incresed sharply. Under the tectonic background, the Zanhuang metamorphic core complexes were formed, which indicated the deep heating and deep crustal faults.According to the tectonic evolution history, the sedimentary sequences in SANCC experienced marine and land facies(Permian), fluvial and lacustrine facies (Triassic, Jurassic and Cretaceous), fault depression (Paleogene), depression (Neogene) in turn. The sedimentary sequences and tectonic evolution history reflected the influence of plate movement and deep heating.3. NCC destruction and lithosphere thinningThrough studies on the sedimentary sequences, this thesis determines the mechanism of transition from thrusting to extension, regional extensional kinematics and process of extension by taking metamorphic core complexes as the window. The NCC destruction can be divided into three stages including pre-destruction stage(257~161Ma), destruction developing stage(161~65Ma) and destruction stage(65~0Ma). Based on the chacteristics of sedimentary sequences, the weak sedimentary process responsed to the deep intensive heating events in late Jurassic-Cretaceous, while intensive sedimentary process responsed to the deep weak heating events in the Paleogene.As the sedimentary process has less relationship to the heating events in the craton basin, two kinds of manners of NCC destruction can be estimated: first, mantle active uplift and crust passive extention, second, crust active extention and mantle passive uplift. This study has improved the research on the extensional tectonics of the NCC and helped to make clear the crustal response to the lithospheric thinning and deep dynamics of destruction of the NCC.4. Interpretation of gravity anomalies along the Taihangshan mountainsBased on the survey of broad band array and small earthquakes relocation, the crustal interfaces are all nearly horizontal and no deep crustal faults exist to the east of Handan fault. Combined with the neutral and alkaline magma along the west of the Zishan fault, this work can draw a conclusion that there is a deep crust fault under the mountains. So the gravity anomalies is the result of double actions of the deep crust fault and the Moho boundary depression.5. Crust structure in the Handan sub-depression and surrouding areasThe crust structure in the Handan sub-depression and its surrouding areas was given by broadband array observations. The crustal interfaces changed smoothly, and the Moho boundary is high in the north and low in the south. A low velocity anomal belt can be found under the West Zishan fault by small earthquakes relocation, concentrating in the depth range 10~20km. The crust structure can be divided into 4 structural units including the upper crust, the middle crust, the lower crust and the Moho boundary. 6. Relationshiop between earthquakes and seismgenic structure of SANCCIn SANCC, two great earthquakes have occurred since 1830, i. e. 1830 (M7 1/2)and 1966 (M7.2) earthquakes which are located in Cixian and Xintai seismotectonic province, respectively. These two provinces lie in the Tangshan-Hejian-Cixian earthquake belt and has a seismic gap between them. The Qumo weakly active fault just passes through the seismic gap.In Cixian seismotectonic province, the small earthquakes are located in a depth range 10~20km which is abundance of low velocity bodies. The crystalline basement and early Paleozoic brittle layers of NCC maybe a stress accumulating area because of the existence of low velocity bodies. Therefore it would produce grand earthquakes when the strain accumulation reaches the critical value.All in above, the mechanism of the earthquakes occurred in Craton is: the existence of deep crust fault serves as the channel of molten lava, which forms the low velocity bodies under the crystalline basement. When the accumulation of stress reaches critical point of the crystalline basement, earthquakes will occur.7.Seismogenic stucture modelThe seismogenic structure model has been obtained based on the previous studies and the data from the project of active fault surveying. In the same while, earthquake prediction has been attempted by finite element simulation. The prediction shows that the dangerous region maybe in the western hanging wall of the Qumo fault and the earthquake occurrence period maybe in the next 100 years, with magnitude probably M6.0~M7.0.