A Study on the Activity of Maduo-Gande Fault
|School||China Seismological Bureau, Institute of Earthquake Prediction|
|Keywords||Maduo-Gande Fault Active Tectonics Active Rate Bayan Har Block Qinghai-Tibet Plateau|
Long term of research on the earth science shows that the surface of the Earth (crust and upper mantle) is constantly moving. The history of these movements is the history of the formation and evolution of Earth’s tectonic pattern. The research of various tectonic activities has been listed as one of the themes of geology research and a lot of results are achieved. Currently, most of scholars consider the active tectonics as the tectonics which active since Late Pleistocene (about 100 to 120 thousand years ago) and will still be active within a period of time in the future, and, an independent direction of research has been developed gradually. As there is a very close relationship between the active tectonics and human survival and development, the research of active tectonics has been increasingly given attention by scholars in earth science and engineering study area home and abroad with the economic development and social progress. Because it’s not only provide the basic data for the study of modern crustal movement and geodynamical characteristics, even play an important role in modern urban planning, earthquake prediction and disaster prevention, project stability and seismic safety evaluation and some other relative fields.Active tectonics is widely distributed in China, showing a very complex status and with the obvious regional characteristics (Deng Qidong, 1996). Qinghai-Tibet Plateau is one of the most intense areas of modern crustal movement and tectonic activities. Within and around the plateau, there are distributed a large number of active faults which control the activities of the strong earthquake of these area. According to statistics, within Qinghai-Tibet Plateau, above MS8.0 earthquake occurred 10 times, MS7.0~MS7.9 magnitude earthquake 78 times, from MS6.0 to MS6.9 earthquake more than 250 times. No matter the magnitude or the number size of earthquake is far more than other areas of our country. In addition, tectonic activities within the Qinghai-Tibet Plateau also play an important role in the formation and occurrence of earthquake in their surrounding areas. It is believed that the cause of 2008 Wenchuan MS8.0 earthquake is closely related to the SE movement of Bayan Har block which lies in the Qinghai-Tibet Plateau.Bayan Har block is a strip terrane in the middle-eastern part of Qinghai-Tibet Plateau. Its southern and northern borders are controlled by the Ganzi-Yushu-Xianshui River fault zone and the East Kunlun fault zone, and the eastern boundary consist of the middle-southern part of the Longmen Shan fault zone and the Minjiang fault zone. These boundary faults have occurred many times of above MS7.0 destructive earthquakes in the history. For example, over the past decade at the South, North, East boundary occurred a number of strong earthquakes above MS7.0 (2010 Yushu MS7.1 earthquake, 2001 Kunlun Mountain MS8.1 earthquake and 2008 Wenchuan MS8.0 earthquake et al.). Predecessors have done some detailed research and discussion on the activities of these main controlling boundary faults and on the rapid uplift process of the eastern Qinghai-Tibet Plateau.Except the active faults in the boundary of Bayan Har block, there are still some active faults inside the block which are not thoroughly researched. For example, the newly discovered Longriba fault which lies in the eastern of the block, the east extension of the East Kunlun fault zone-Tazang fault, the Kunlunshankouxi-Dari fault and Maduo-Gande fault (studied in this paper)which are lies in the central of Bayan Har block. There used to be some strong earthquake occurred around these fault zones in the history, in which the biggest one is the 1947 MS7.8 earthquake near Dari.Based on the considerations above and combined with available materials and relevant research results that the faults within the block probably had strong activities since the late Quaternary and may still active nowadays. So, these active faults should be given adequate attention and concern, and a gradual development of quantitative research should be given too. This paper chose the Maduo-Gande fault as the aim of study, based on the interpretation of remote sensing, combined the field investigation, considered the quantitative study of active faults as the core, integrated use of remote sensing geology, seismogeology, geomorphology, geometry, mathematical physics and so on, combined with trench excavation and geological dating, carried out on a comprehensive study of fault activity.The main research contents and results of this paper are as follows:1. Using the remote sensing geological methods to have a preliminary study about the faults overall. Processed and gave integrated interpretation of the TM/ETM/SRTM remote sensing data in the study area with the RSI ENVI 4.3 remote sensing image processing software, and Global Mapper v8.03 professional graphics software. Combined with 1:100 thousand topographic maps and 1:60 thousand aerial photographs and other mapping information, we gave a preliminary analysis of the topographic and geomorphologic features and the distribution characteristics of the faults in the study area, and had an initial understanding about the nature of the faults.2. A detailed field survey work was given and an earthquake surface rupture zone was found in Gande segment on the basis of remote sensing interpretation. A large number of earthquake traces were found and also the distribution characterics of rupture zone and its displacement via continuous tracking through the field. According to the field investigation, the rupture zone has the lenth of about 50km. We got a maximum horizontal displacement of 7.6m and a maximum vertical displacement of 4m. Due to the fault activities, a series of linear arranged pass landforms formed. And the phenomena of various types of dislocation are also very rich, along the fault zone, there are neat fault triangles, ridges (beam) twisted, gullies dislocation, fault scarps, fault springs, pressure ridges, fault troughs, head/tail missing gullies, sag-ponds, et al. Through the observation and analysis of various types of geological and geomorphological features along the fault, we believe that the Maduo-Gande fault has obvious sinistral strike-slip feature, and both thrust characteristic.3. We measured a variety of micro-geomorphic phenomena by TOPCON Hiper Ga/Gb differential GPS accurately along the fault. After indoor processing and calculating of GPS data, we obtained the displacement of about 100 points. Include the horizontal displacement of gullies and vertical displacement of fault scarps. All kinds of gullies along the fault zone generally dislocated, and the displacement from the smallest of 2m or so to the largest of 850m or so; new and old alluvial fan to retain a high degree of fault scarps around the minimum 1m, maximum up to 10m or so.4. Through field investigation and trench excavation to obtain faults occurrence and other basic elements. And through the observing and drawing section of the faults, we obtained its characteristic of activity with intuitive understanding. More than 20 OSL dating samples are made in the trench and other geological body. The dating results show that from the mid-Late Pleistocene (Q23) to the Holocene (Q4), which is correspond to the preliminary judge while field investigation. Combines both indoor and field work, using conventional methods of displacement of geomorphology unit, and the establishment of theoretical model, we figured out the active rates of Gande segment in different time scales.5. Horizontal active rates(1) In F2-2 segment, using the gully dislocation on the Holocene alluvial fan in Muriwama west and the dating data, we got a slip rate of 3.38±0.08mm/a since Holocene (Q4).(2) In F2-1 segment, using the gully dislocation on the old alluvial fan in front of Suoheluo basin west piedmont and the dating data, we got a slip rate of 8.39±0.09mm/a since the late period of late Pleistocene(Q33). In F2-2 segment, using the synchronous dislocation of 6 big gully between Muriwama and Anbeidongwu, combined with the date of the gully, we got a slip rate of 8.13±0.15mm/a～8.41±0.06mm/a since the middle period of late Pleistocene(Q23). On the principles of gully headward erosion, we established the relationship model between the gully displacement and the fault activities. Then, figure out the slip rate of 7.2mm/a since the middle period of late Pleistocene(Q23).(3) In F1 segment, using the total displacement of the rock between each side of the fault and the age of strata, we got a slip rate of 3.48mm/a～5.23mm/a since Quaternary. Using the theoretical and statistical relationships of strike-slip faults between its displacement and its rupture width, we got a slip rate of 4mm/a～6mm/a since Quaternary. Then, take the average, we got the mean slip rate about 4.68 mm/a.6. Vertical active rates(1) In F2-1 segment, using the height of two gullies’terrace in Murixiuma and Anmuchangyasheng, combined with the dating results, we got a respectively regional uplift rate of 0.39±0.1mm/a and 0.37±0.05mm/a since mid-Holocene(Q24) and early-Holocene (Q14).(2) In F2 and F3 segment, using the differential GPS to measure the scarps exist on the different topography in fault zone, then, we got the average vertical active rates of about 0.33mm/a since the late period of late Pleistocene(Q33) and about 0.78mm/a since Holocene (Q4).by using the diffusion equation of scarp evolution.(3) Using DEM data to analysis the terrain profiles across the faults, we obtained the cumulative vertical displacement between each side of fault F2. Then, combined with strata age, we got a vertical active rate of 0.15mm/a～0.22mm/a since Quaternary.