Research Of Basin Marginal Active Normal Faults Through Multi-scale Tectonic Geomorphology

Located in114-116.5°E,39-41°N, the northwest bounded Beijing area is a basin-range zone,consisting of a series of rift basin units bounded by NE-striking active normal fault systems. Therift basins include the Huaizhuo basin, Yuguang basin, Yanfan basin, Yangyuan basin, Lingqiubasin, Huai’an basin and Laiyuan basin. The Yuguang basin lies in the southwestern part of thisarea.The Yuguang basin is a half-graben basin tectonically located in the basin-range tectonic zonenorthwest Beijing, whose southern boundary is controlled by a normal fault zone known as theYuguang Basin South Margin Fault (YBSMF). YBSMF is about120km long, with a NE70°general strike. Since its formation, YBSMF has undergone a long evolutionary process, causingthe uplift of mountainous areas in the south and deposition in the basin in the north. YBSMF is aHolocene active fault where five earthquakes have occurred since the historical record, includingthe231BC and the1618events of Ms=6.5, the1581Ms=5.75event, the1911Ms=5.5event andthe1956Ms=4.5event.YBSMF consists of several secondary faults, which take on zigzag shape along the boundarybetween the basin in the north and mountainous area in the south. According to fault geometry,fault activity and the difference in faulted landforms, YBSMF can be divided into5segments,namely, the Shangbaiyang segment (S1), the Tangshankou segment (S2), the Beikou segment (S3),the Songzhikou segment (S4) and the Shanghupen segment (S5) from west to east. Since theHolocene, fault movements were mainly concentrated on S2, S3and S4, whereas the activity ofS1and S5became very weak.Previous studies about YBSMF focused mainly on segmentation, tectonic characteristics andfault propagation at the Jiugongkou segment. Under the support of “Yuguang Basin1:50000Geological Mapping Project”, a sub-subject of “Active Fault Seismic Hazard Assessment Projectof China’s Key Areas for Surveillance and Protection”, the author had an opportunity to take partin the scientific research of YBSMF for a long time.During the interpretation of planation surfaces, great amounts of deformed planation surfaceswere founded around the range-front fault zone, mainly including the Tangxian surface and theDianziliang surface. However, previous research about these deformed planation surfaces is quitelimited. The spatial distribution characteristics and deformation features of these surfaces on thescale of the whole fault zone are not clear enough.Through remote sensing interpretation and field geologic investigation, various kinds ofrange-front tectonic geomorphology, especially a lot of typical fault linear scarps or traces arefound. These range-front tectonic landforms are different in their evolutionary process, geometricfeatures and the activity of the fault. There is little research about these tectonic landforms, except the only study about the propagation of fault scarps at the Jiugongkou segment. In a word, on thescale of local fault segments, the geometry, activity and the evolution pattern of fault segmentswhich control and form the tectonic landforms are all not very clear and deserve further research.Besides, on the scale of individual fault scarps, such as the fault linear scarps in the piedmontfans around the Yixing township, the study about ancient earthquakes of these scarps is quitelimited. The times of ancient earthquakes, the reoccurrence of ancient earthquakes and fault sliprates at these sites are all not known and also deserve further research.Under the research background above, based on the latest data collected from practical work,I make a research about the YBSMF through tectonic geomorphology on different scales. Aimingat the issues which are unstudied or not-well studied by the previous researchers, I make asupplement or creative research and obtain some new conclusions or viewpoints.The scale includes the time scale and the spatial scale. The time scale means the duration ofthe tectonic evolution process of the geomorphology, while the spatial scale means the range orarea of the spatial distribution of the geomorphology. Moreover, the spatial scale includes theplanar scale and the altitude scale. In this thesis, the large-scale tectonic geomorphology is mainlythe deformed planation surfaces, including the Tanxian surface and the Dianziliang surface; thesmall-scale tectonic geomorphology is mainly the range-front tectonic landforms controlled by thelatest activities of the range-front active faults, mainly including all kinds of typical fault linearscarps or traces; the micro-scale tectonic geomorphology is mainly the localmicro-geomorphology within an individual scarps, such as the fault linear scarps in the piedmontfans around the Yixing township in the study area.1. Study of deformed planation surfaces.The study of deformed planation mainly focuses on the deformation characteristics of thesurfaces, aiming to find out the ways of the block tectonic movement in a half-graben systemwhich includes the pattern, direction, amplitude of the block movement.Through the analysis of the spatial distribution characteristics of the deformed Tangxiansufaces and the Tangxian broad valleys, I find that the range-front faults are all located within theareas of the deformed Tangxian surfaces and the Tangxian broad valleys, and this indicates that theformation of the deformed Tangxian surfaces and the Tangxian broad valleys is affected orcontrolled by the activities of the range-front fault. During the formation of the Tangxian surfacesand broad valleys, the basin marginal fault was active, and the main river system evolved alongthe strike of the marginal fault first. Then, as the further broadening and lateral erosion of the riversystem, the Tangxian surfaces and valleys formed gradually, which finally caused spatialdistribution of the Tangxian surfaces and valleys along the strike of the range-front fault.As for the distribution direction of the Tangxian surfaces faulted by the range-front fault,some are nearly vertical to the fault strike, while some are nearly parallel to the fault strike. Forthe Tangxian surfaces which are distributed parallel to the fault strike, when they were offset bythe range-front fault, one half of the surface dropped down and was finally buried by the sedimentlayers, while another half of the surface was uplifted and finally became the present piedmont stepsurfaces. Through the staking of the profiles of these deformed Tangxian surfaces at differentlocations in the research area, the ways and patterns of the tilting movement of the block arerecovered. The tilting movement includes two components, namely the clockwise rotatingcomponent and the vertical uplifting component. As the block tilts gradually, the uplifting altitudeof fault scarp apex increases gradually, and the length of the exposed fault scarp increases gradually either, while the slope of the fault scarps decreases gradually. Through the correlationanalysis and regression analysis of the parameters in the block tilting movement, I estimate thatthe length of the tilting radius which controls the deformation of the Tangxian surface is about1300-1400m.Besides, the uplifting altitude of the Tangxian surface changes along the strike of therange-front fault, and the geometric shape of the uplifting altitude curve is identical to thegeometry of the fault. On one hand, the flat segment of the uplifting altitude curve corresponds tothe fault segment with a flat geometry; on the other hand, the curved segment of the upliftingaltitude curve corresponds to the fault segment with a curved geometry. There are probably tworeasons for this. One reason is due to the relative position of the fault planar trace and the tiltingaxis planar trace. If assuming that the fault planar trace is irregular and curved, while the tiltingaxis planar trace is relatively flat, then when the whole block tilts and rotates around the flat tiltingaxis, the uplifting altitude curve corresponding to the flat fault segment is also flat, while theuplifting altitude curve corresponding to the curved and irregular fault segment is also curved. Atthe same time, the uplifting altitude of the Tangxian surface increases towards the range-front fault,while decreases backwards the range-front fault.Another reason for the correspondence of the geometric shape of the uplifting altitude curveand the fault segment is probably due to the variation and heterogeneity of the tectonic tilting itself.As for the fault segment with stable tilting, the uplifting altitude curve is relatively flat, and thefault surface can be regarded as an inclined3D plane. The intersecting line of the fault surface andthe horizontal plane is namely the fault, so the fault segment is also flat. As for the fault segmentwith variable tilting, the uplifting altitude curve is irregular and curved, and if the fault segment isflat, then the fault surface becomes an irregular3D surface which is unstable and unsuitable forthe growth and propagation of the fault. If the fault segment is a little curved, then the fault surfacealso becomes a curved and smooth3D surface which is stable and suitable for the growth andpropagation of the fault. In general, fault growth and propagation are primary and in the first place,so the fault surface prefers to become the curved and smooth3D surface, and this finally causesthe curved or irregular geometry shape of fault segments.Besides, through the synthetic analysis of the deformed Tangxian and Dianziliang surface,this work finds that the degree of tilting attenuates as the distance to the range-front faultincreases.2. Study of range-front tectonic geomorphologyThe study of range-front tectonic geomorphology mainly focuses on the linear scarps in thefault zone. I studied the geomorphic features, geometry and fault activity of these scarps. Throughthe comparative analysis and summary of all these characteristics of the fault scarps, I attempt tofind out the distribution pattern and tectonic evolution process of the latest active faults.Through field geological investigation, high-resolution satellite image interpretation, DEM3D analysis, trenching engineering, and Ground Penetrating Radar (GPR) survey, I discovervarious kinds of fault scarps at many locations along YBSMF. Fault scarps tend to form and growat two locations, namely the fault overlapping zone and the irregular fault segments.First, I note that the irregularity of fault geometry is a primary factor which determineswhether the fault scarps form and grow or not. The fault scarp leads to the “cutting off” andelimination of the irregularity of fault geometry, such as the cutting off of the uneven orunsmoothed segments, and the linkage of the discontinuous segments along the strike. The elimination of the irregularity makes the fault geometry smooth and continuous, and reduces theroughness of the fault surface, which contributes to the sliding movement of the half-graben blockinside the basin along the fault surface.Actually, the irregularity of fault geometry makes the fault plane rough. This roughnessenhances friction on the fault slip surface, and thus hinders the slipping movement of thehalf-graben block in the basin along the fault surface. On the other hand, under the field of gravity,the half-graben block has a natural and persistent tendency to slip down along the fault surface.Therefore, in order to overcome this hindrance to slip down, fault scarps tend to propagate andgrow at or around the irregular segment to eliminate these irregularities from the original majorfault plane and make both the fault geometry and fault plane much smoother than before. Lessroughness of the fault plane will contribute to the slipping movement of the half-graben block.Inconclusion, it is the irregularity of fault geometry that determines the propagation and growth offault scarps, which in turn will finally eliminate these irregularities and make the fault planesmoother.Second, the maximum possible slip displacement of a fault segment influences the activeduration of fault scarps. The duration at the central segments with a large slip displacement islonger than that at the end segments with a smaller slippage value.3. Study of micro-geomorphology controlled by active faults and ancient earthquakesThe study of micro-geomorphology controlled by active faults focuses on topographycharacteristics, influencing factors and evolution patterns, while the study of its ancientearthquakes focuses on the times of ancient earthquakes, the reoccurrence of ancient earthquakesand slip rates of the fault scarps. The actual study object in this part is obvious fault linear scarpsin the piedmont fans near the Yixing township in the research area.I identify ancient earthquakes through the evidence from deformation and deposition in thetrench, such as the faulted formation, the faulted basement formation, the fault fracture zone,seismic colluvial wedges and sedimentary sequences. Based on these analysis, I identify2events,the first event is about16.66±1.39ka to10.62±0.87ka, and the second event is about9.20±0.79kato7.85±0.14ka, probably close to9.20±0.79, and the reoccurrence of ancient earthquakes is about2.7-7.5ka. Besides, the vertical slip of each event is about3.5m, and the corresponding magnitudeis estimated to be about7.9.According to the vertical slip of each event and the estimated magnitude, I further estimatethat the rupture length is about49km, and it is the fault segment forming through the combinationof S2and S3that causes the two events and the formation of the fault scarps in the piedmont fansnear the Yixing township.Finally, based on the synthetic analysis of all the three aspects above, this thesis summarizesthe tectonic evolutionary process of the range-front active fault of YBSMF.