Late Quaternary Tectonic Deformation And Active Behavior Of Segmentation Of The Yilan-Yitong Fault Zone,NE China

The Yilan-Yitong Fault Zone(YYFZ), as one of the most important branch of the northern Tan-Lu fault, is the largest seismogenic structure in NE China. Different from the Weifang-Jiashan segment and Xialiaohe-Laizhouwan segment, the YYFZ has neither major earthquakes greater than M6, nor the reports of activity during the late Quaternary. The biggest event is the Ms5.8 occurred in Luobei County, Heilongjiang Province in 1963 after the earthquake observation network was established. Therefore, it is generally considered to be active in the early-middle Pleistocene.However, the latest research revealed that there exist two earthquake surface rupture zones within the Shulan basin and Fangzheng basin of the YYFZ, which may indicate more than two M7 events have occurred in the recent time, and the elapsed time is not too long. These new discoveries contradict with the general understanding of the YYFZ. The new discovery not only changes our general understanding of the YYFZ, but also generates many new scientific problems. These include:(1) is there any other active segment besides the Shulan section and the Fangzheng section? How about their geometric and kinematic features?(2) What about the active behavior of segmentation of YYFZ?(3) How about the neotectonic deformation characteristics of the YYFZ and its surrounding areas? Does it have any implications for the current stress field of the east region of China? These scientific questions are very important and fundamental for the understanding the neotectonic and active deformation of the YYFZ and neighboring area of NE China.Therefore, this dissertation manage to answer these questions through the analysis of previous data, along with interpretations of high-resolution satellite images, detailed field investigations, trenches and drill holes, seismic profiles, and the great efforts also been put into the difference existed in geometric, tectonic geomorphology, active behavior and deep geophysical filed, and then discuss the strong earthquake potential and risk of the fault, as well as the geodynamical process from the angle of regional point. Major conclusions of this research are drawn as follows:(1) YYFZ has been active since late Quaternary with the capacity of strong earthquake and tectonic background according to the field geological investigation. There exist at least 4 sections with paleo-events relic in Holocene including Shulan, Tonghe, and Shangzhi and Tangyuan segments. Also, at least 4 segments with activity relics in late Pleistocene epoch were detected including Luobei, Yilan, Yanshou and Wuchang. These findings changed our traditional viewpoint that the YYFZ’s activity in late Quaternary is weak, which have great influence and help for the improvement of the active tectonics image of NE China, and for the risk analysis of strong earthquake along the YYFZ and its surrounding areas in the future.(2) Based on the geomorphologic mapping and geometric structure research of the YYFZ, it is clear that the YYFZ could be segmented according to the differences of the geometric features. These latest sections could be delineated in a left-stepping en echelon geometrical structure in the plane, and most of which did not follow the pristine trace in the basin border but migrate into the central of the basin. All of these hint that the YYFZ’s activity has predominant newbornness, and is now under the young stage.The results show that the geometric and structure features of the YYFZ are discontinuous, and could be segmented based on the obvious difference in the strike, trend, mechanical property, lithology of fault zone, thickness of the sedimentary, breadth of the fault zone and the transverse structures along it.The difference of the depth of sedimentary in every section is also evident, which implies that the kinematic nature and active rate of different segments are distinct. The difference of breadth of YYFZ is clear. The fault breadth in transition zone at every two adjacent segment is changed. At the same time, we find that the fault breadth is proportional to the total number of transverse faults, and the bigger the fault breadth, the more the number of the transverse faults, and vice verse. More importantly, the increasing of total number of faults mainly concentrated in the boundary zone, which corresponding with the location of abrupt change of fault’s breadth.The YYFZ could be segmented into 6 independent sections based on the geometric differences: Shenyang-Changtu segment, Siping-Jilin segment, Shulan-Wuchang segment, Shangzhi-Fangzheng segment, Fangzheng-Tangyuan segment and tangyuan-Luobei segment. Also, the geometric boundary zone of segmentation could be observed obviously between every two adjacent sections. The length of each segment is 120 Km, 148 Km, 140 Km, 90 Km, 120 Km and 129 Km, separately. The length of each boundary belt is 55 Km, 23 Km, 20 Km, 14 Km, and 16 Km, respectively.(3) YYFZ could be segmented in terms of tectonic landform differences according to the geomorphologic features and river morphology parameters such as slope, river curvature and longitudinal section profile. The segmentation result in terms of difference in the bilateral landform of the YYFZ and interior valley topographic relief fits very well with the geometric segmentation. And each of them corresponding with mutation of topographic relief, which may reflect the long-term activity difference of the YYFZ.Small-scale geomorphic difference in the interior of YYFZ is obvious, which makes it possible for us to subdivide the last 4 main segments(Shulan-Wuchang segment, Shangzhi-Fangzheng segment, Fangzheng-Tangyuan segment and tangyuan-Luobei segment) into 8 sub-segments(Gangyao sub-segment, Wuchang sub-segment, Shangzhi sub-segment, Yanshou sub-segment, Tonghe sub-segment, Yilan sub-segment, Tangyuan sub-segment and Luobei sub-segment), and the length of each sub segment mentioned above is 80 Km, 51 Km, 30 Km, 55 Km, 70 Km, 30 Km, 20 Km and 104 Km, separately. The length of each boundary zone of sub-segmentation is 9Km, 5Km, 12 Km and 13 Km, respectively, and each of them also corresponding with mutation of topographic relief and fluctuation difference of terrain elevation.Microtopograph investigation proved that there existed 8 active segments in late Quaternary with significant difference of micro relief and pattern of manifestation such as scarp, linear trough and valley and pond or reservoir. We also find that the rupture length of every segment is varied ranging from 1.5Km to 70 Km, and the height of which are different ranging from 1.0m to 4.4m. The geographical location of these 8 segments did not break through the boundary zone of main sections mentioned above, and also consistent with the dividing results of 8 sub-segments.(4) The slip rate of different time interval in late Quaternary and evidence of strong earthquake activity were obtained based on the geomorphologic survey, samples dating, paleo-earthquake and historical earthquake investigation. The results of active behavior show that the YYFZ has different activity history, elapsed time, slip rate and active manner along the fault strike, and the segmentation based on these differences also fits well with the results of geometric and geomorphic section discussed above. In addition, the feature of cluster activity in middle-late late Pleistocene and late Holocene, respectively, along the whole fault zone could be observed.(5) The dividing result based on the geophysical detection, bouguer gravity anomalies and aeromagnetic anomalies fits well with that on the basis of surface evidence. Both the activity history, the feature of basins’ architecture, and the earthquake potential of every section are distinct.Different section has obvious control on the sedimentary and evolution process of the basins along the YYFZ, especially on the boundary faults and their architecture feature. Six basins along the YYFZ are primarily controlled by the six main segments with significant differences in sedimentary, texture and evolutionary process. In addition, these differences could also be observed in space distribution image of earthquakes’ epicenters. These results imply that each segment has difference in crust cutting depth and structure.In a word, the YYFZ could be segmented in a hierarchical way, and the segmentation results come from the differences of geometric structure, tectonic geomorphology, active behavior, seismology and geophysical field have a good consistency. Thus, the YYFZ could be segmented into 6 main sections based on the synthesized results and 8 sub-sections according to the research of micro-topography and paleo-earthquakes. Both the scale of sub-segmentation and sub boundary zone are smaller than that of main segment and main boundary zone.(6) The variation of geometric structures(abrupt change of breadth, strike bend and step over) of the fault belt and transverse structures are outstanding markers in the boundary zones of YYFZ. In addition, the variation of fault’s numbers and geomorphology also could be observed in most of boundary zones. The geometry of these segments is relatively simple, but they are separated by areas with complex geometry. Among these boundary zones, geometric mutation and transverse structure are prime signs, which could be viewed as geometric barrier of segmentation.Different from the main sections, the markers of 4 boundary zone of sub-segments are relative simple, which show as geometrical mutation and transverse structure. However, the scale of them is far smaller than that of the boundary zone of main sections.(7) Thrust structures are obviously developed in YYFZ and its surrounding areas in Cenozoic era. The thrust motion component could be observed in almost all of the basins and regional faults in NE China based on the focal mechanism solution of main earthquakes and field investigation, which imply that the main compressive force acts upon the NE~NEE trending YYFZ and DDFZ in the direction of nearly EW, whose shear component and compression component resulting in the dextral strike-slip and thrust motion on the YYFZ and DDFZ and other regional faults in NE China, respectively.The Songliao basin, locating in the west of the YYFZ, develops inversion structures during the third tectonic evolution stage. Tectonic inversion is the main deformation style of DDFZ in the Cenozoic, which coincides to the geometric shape and deformation mechanism of the “thrust-fold” structures, and showing as thrust and related fold in the section and Daqing placanticline with the width of ~20-30 km in the plane. The latest active time of the DDFZ is early middle Pleistocene. The shortening amount and rate of the DDFZ in Cenozoic era is 2.26 Km and 0.03mm/a, respectively. It is suggested that the violent earthquakes(Ms≥7.0) is impossible to be triggered.(8) Structural inversion might be developed through multi-stages. It is clear that 4 regional angular unconformities could be confirmed in both YYFZ and DDFZ where locating at the east boundary and interior of the Songliao basin, respectively. These stages include: ~65Ma、~23Ma、~5.3Ma and ~1.8Ma, which means that the pulsed strong compression deformation was occurred in NE China in Cenozoic. In addition, this structural compression inversion might be regional and large-scale because those contemporaneous regional angular unconformities almost formed in every Cenozoic basin such as Sanjiang, Fangzheng, Tangyuan, Yitong and Bohai Bay. This fact further means that regional tectonic stress field suffered great change during Cenozoic era. This geodynamic process may be controlled by the westward subduction of the West Pacific Plate and the far field effect of the India-Eurasia collision.(9) We suggest that the YYFZ has the tectonic and deep background of strong earthquakes. On the contrary, the DDFZ, which locates in the interior of the Songliao basin, only possesses the structure background of moderately-strong earthquakes(M<7.0). This means that the interior of the Songliao basin, as an independent active block, is relative stable, and the tectonic deformation and strong shocks are mainly concentrated along the boundary faults of the basin.The contents and results of this work to some extent will enrich the basic data concerning neotectonic and active tectonics in YYFZ and its surrounding areas, and could be helpful for the understanding of the earthquake activity background, as well as some scientific referable guidance for future work concerning earthquake-proof and prevention to be done in this region and surrounding areas.