| The Longmenshan fault zone is located in eastern margin of Tibetan plateau and bounded on the east by Sichuan basin, it is the boundary line between strongly active Bayankala block and weaker active CHUAN-DIAN block, and it is also the middle part of the North-South seismic belt in China, the strike direction of the fault is NE-SW. It has a very important structural location. The Ms8.0 Wenchuan earthquake of 12 th May 2008 ruptured the central and the front-range faults. But the earthquake didn't rupture the back-range fault. This shows that these two faults are both active in Holocene. But until now, we don't know exactly the activity of the back-range fault.Through satellite image(Google Earth) interpretation, combining with field investigation, we preliminarily analyzed that five steps of alluvial platforms or terraces have been developed in Min river region along the Maowen-Wenchuan fault. T1 and T2 terraces are more continuous than T3, T4 and T5 terraces. Combining with the preceding work, we discuss the formation ages of the terraces and conclude, analyze and summarize existed research about terraces of Min river. We constrain the ages of T1, T2, T3, T4 and T5 surfaces to 3~10ka BP, 20 ka BP,40~45ka BP, 50~60ka BP and 80 ka BP, respectively. The surface rupture of the Wenchuan earthquake along the back-range fault were investigated. Combining with geomorphologic structural interpretation, measurements of the cross sections of the terraces that the fault cuts by differential GPS and detailed site visits including typical locations of tectonic deformation, landscape and section planes of terraces and gullies crossing the fault, we found that:(1) Fracture section exposed in the T3, T4 terraces of Min river and the gravel layer at the top of the T3 terrace is dislocated.(2) The T3~T5high terraces are all right-lateral dislocated 30~ 40 m by the fault and 20~25m fault scarps developed on the terraces.(3) The exposed layers of T1 and T2 low terraces are complete and continuous, without fracture dislocation phenomenon. The surfaces of the terraces are smooth and no fault scarp developed. The terraces have not been horizontal dislocated. Accordingly, we consider that Maowen-Wenchuan fault is determined to be active after T3 terrace formed and before T2 terrace formed, but inactive since T2 terrace formed. Its latest active era should be the middle and late time of late Pleistocene, and there is no activity since the Holocene. The slip rates of the fault before T2 terrace formed are determined. The dextral slip rate of the fault between the formation of T5 terrace and T2 terrace is bigger than 1.7 ± 0.1 mm/a, between the formation of T3 terrace and T2 terrace is about 1.8±0.4mm/a; The vertical slip rate between the formation of T3 terrace and T2 terrace is about 1.1±0.3mm/a. The structure at both ends of the Maowen- Wenchuan fault and internal segment characteristics were studied, and we preliminary divided Maowen-Wenchuan fault into three parts. Combining with the knowledge of the Central and the Front-range Faults are both quaternary active faults, the activity of Longmenshan fault zone should have shifted to the central and the front-range faults which are closer to the basin, this indicates Longmenshan Thrust Belt fits the "Piggyback Type" to some extent. The middle and lower crustal flow model should be more suitable for the structure of the longmenshan fault zone. The location of the Back-range fault down into the lower crustal flow may be closer to the CHUAN-XI plateau and away from the Sichuan basin, acceptable stress accumulation is small and not enough to trigger the fault to move. Therefore the Back-range fault is no longer active nowadays, its activity has been replaced by the Central and the Front-range fault.Through this study, we get some knowledge, but there are also some problems exist. Maybe the most important problem is the terrace age, and the reliability of age data depends on the collected dating samples and the methods of dating we choose. At present, it is generally believed that the 14 C dating data is more accurate, and the results will be more reliable if we can collect enough representative samples. Second,late artificial or erosion has a great influence on the transformation of the topography,this will change the slip rate of the fault we determined. Last but not least, we only get the late Quaternary activity of the central segment of the back-range fault. Through this study we found that the structural features of the Longmenshan fault zone is complex. If you want more in-depth and meticulous research, you need geophysics,seismic sections, drilling, seismic, GPS and other researches to support. |
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