| Summary: | 博士 === 臺灣大學 === 地質科學研究所 === 98 === The north- to northeast-trending Dien Bien Phu fault (DBPF) zone is one of the most conspicuous fault systems in Indochina, extending over a distance of 150 km from Yunnan, China through northwest Vietnam into Laos, and dextrally displaces the northwest-trending Song Ma suture, and Mesozoic and Paleozoic strata, acting a great geological discontinuity. Detailed mapping, compiled from topographic maps, stereographic aerial photos, ASTER satellite imageries, and field reconnaissance, along the DBPF reveals information about the fault geometry, the slip magnitude and distribution along the fault, and the relationship between river offset and fault activity. The geometry of the modern DBPF is complex, consisting of step overs and strands, and is currently dominated by left-lateral strike-slip displacement. Numerous multiple offsets along the fault can be detected and reconstructed, and the largest sinistral displacement on the DBPF is ca. 12.5 km. Since sinistral motion is likely to have initiated around 5 Ma, the most probable Pliocene to present average slip rate on the DBPF is on the order of 2.5 mm/yr. There are several basins developed along the fault zone resulted from different kinematics. The largest basin, Dien Bien Phu basin, developed by a half graben with growth fault and growth strata indicates the extensional component of displacement in the southern part of the fault. Based on the combined Global Positioning System velocity fields observed from northwest Vietnam and south China, about 2 to 3 mm/yr left-lateral slip is measured across the DBPF, indicating the left-lateral slip rate of ~10 mm/yr of the Xianshuihe-Xiaojang fault (XSHF-XJF) has been transferred to DBPF but with an abrupt decrease in magnitude. Furthermore, the northern Indochina located at the western side of DBPF represents more significant east-west extension, implying the non-rigid behavior with internal deformation. Results of this study suggest that the modern DBPF zone performs as a reactivated fault, yet different slip sense from its previous phase, and plays a role as an eastern boundary of the crustal deformation in northern Indochina. However, the present-day kinematics in northern Indochina may be dominated by east-west extension and be accommodated by internal distributed deformation different from the crustal fragment defined by XSHF-XJF zone with a clockwise rotation around the Eastern Himalayan Syntaxis.
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