Large river offsets and Plio-Quaternary dextral slip rate on the Red Riverfault (Yunnan, China)

Using multispectral SPOT images and 1/100,000 topographic data, we present an improved map of the active Red River fault zone between Midu (Yunnan, Ch ina) and Hanoi (Vietnam). The fault zone is composed of parallel strands, o ne of which, the Yuanjiang fault was previously undetected. There also appe ars to be a component of extension all along the fault zone. Such extension increases toward the SE, from Yunnan to the south China sea coast, and the vector describing the motion of south China relative to Indochina points w ithin the N45 degrees -135 degreesE quadrant. We attempt to assess the Plio -Quaternary dextral slip rate on the Red River fault (RRF) by restoring lar ge river offsets and searching for the largest, plausible one. Across much of Yunnan, the fault is perpendicular to local catchments that drain into t he Red River. From precise mapping of the river courses on SPOT satellite i mages and on 1/100,000 topographic maps, numerous multiple offsets along th e fault can be detected and reconstructed. The lack of correlation between the apparent offsets and the lengths of the rivers upstream from the fault suggests either that the drainage system was in large part established prio r to the onset of dextral slip along the fault or that frequent captures ha ve occurred. We thus try to find the best fit between series of river chann els upstream and downstream from the fault by progressively restoring the d extral displacement in increments of 500 m, up to an offset of 50 km. For e ach increment we measure the misfits (root mean squares, RMS) between the u pstream and downstream channels. The best fit and smallest RMS are obtained for an offset of 25 +/- 0.5 km that we interpret to represent the dearest, large right-lateral displacement recorded in the geomorphology along the a ctive Red River fault. Since dextral motion is likely to have started aroun d 5 Myr, the most probable average Plio-Quaternary slip rate on the fault i s of order of 5 mm/yr. We attribute the apparent lack of seismic activity o n a large stretch of the fault to millennial recurrence times between great earthquakes. Our study shows that relatively small drainage systems can ke ep a good record of fairly large cumulative fault offsets.