| Qilian mountain range in the northern Tibetan Plateau is the most front of Tibetan Plateau deformation and extrusion. A plurality of large active reverse faults which show strong activity since late Pleistocene develop in the piedmont of Qilian mountain, such as Yumu Shan fault, Fodongmiao-Hongyazi fault, Jinta Nanshan north-margin fault and Dachaidan fault.Combining with the achievements of predecessors, we carried out detailed geological survey, trench excavation, paleosearthquake analysis and old trenches reinterpretation about Dachaidan fault, Yumu Shan fault, Fodongmiao-Hongyazi fault, Jinta Nanshan north-margin fault and achieved key techniques in paleoearthquake study of reverse faults in the piedmont of Qilian mountain, relating to the landform expressing style of fault, trench excavation site, trench style, deformation types on the trench wall, paleoearthquake analysis, paleoearthquake symbols, age-dating samples.1. Generally, the faults develop on the diluvial fan in the piedmont of mountain range or spread along the mountain edge. Distinct linear characteristic can be recognised by means of satellite image interpretation.2. Two factors should be considered about trench excavation:(1)Trench site should be selected where the late Pleistocene and Holocene stratum developed, so that the latest fault activity can be recorded. Overall, the first and second terraces or proluvial platform with fine sediment are the best choices.(2)If the fault dislocate multilevel terraces, the trench should be laid on the lower terrace.If multilevel scarps developed, large trench will be used to penetrate all the scarps so as to expose complete paleoearthquake sequence.3. For trench excavating, generally single slot may be the best choice in gobi surroundings in the northwest of China, with the width of 3-4m.The model of single slot in the lower part and bulldozer trench in the upper part is proved slightly ineffective because of the special depositional environment and sediment, which will add difficulties greatly for the analysis and interpretation of trench profiles.4. With scientific research for the faults in the study area, the conclusion indicates deformation types include simple thrust, simple pressure ridge, multilevel fault scarp deformation, bending dislocation and relevant combination deformation. For the paleoearthquake recognition of fault dislocation, it is a good method that is the relationship between displacement and cover of the fault, stratum and colluvial wedge. It is of great importance that the fault and displacement may be invisible or tailing-out. In fact, it is difficult to recognize colluvial wedges especially if it is not typical. For simple pressure ridge, there are important paleoearthquake indicators for the identifying of paleoraethquake. The relationship between displacement and cover of the fault, the fold degree of faulted stratum, the displacement difference of marker strata, and the thin-long colluvial wedge like strata in front of the scarps. For bending dislocation deformation style, formation will be bent and deformed similar to displacement deformation but orientation is not standard. Compared with coarse gravel stratum, the bending phenomenon is more obvious with fine formation.Compression and thickening phenomenon occurred in the footwall of the fault. In a paleoearquake event, winding deformation is greater in the lower layer and the bending deformation is greater in higher stratum so that the overall displacement is basically balanced.5. In Qilian mountain Gobi environmental conditions, it is difficult to find conventional carbon chip used in the 14 C dating and studys shows that actual dating results of OSLsample is not ideal. In the practice, it is an important discovery that peat or sand sandy soil rich in organic can also be used for 14 C dating. The fine thin layer development particularly need to be paid attention to. Under most conditions, 14 C dating and OSL dating will be applicated together. |
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