Large river offsets and Plio-Quaternary dextral slip rate on the Red River fault (Yunnan, China)
A. Replumaz,1 R. Lacassin, P. Tapponnier and P. H. Leloup
Laboratoire de Tectonique, CNRS UMR 7578, Institut de Physique
du Globe de Paris, Paris, France.
Abstract. 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, China) and Hanoi (Vietnam).
The fault zone is composed of parallel strands, one of which,
the Yuanjiang fault was previously undetected. There also appears
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 within the N45°-135°E quadrant.
We attempt to assess the Plio-Quaternary dextral slip rate on
the Red River fault (RRF) by restoring large river offsets and
searching for the largest, plausible one. Across much of Yunnan,
the fault is perpendicular to local catchments that drain into
the Red River. From precise mapping of the river courses on SPOT
satellite images and on 1/100,000 topographic maps, numerous multiple
offsets along the 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 prior to the onset
of dextral slip along the fault or that frequent captures have
occurred. We thus try to find the best fit between series of river
channels upstream and downstream from the fault by progressively
restoring the dextral displacement in increments of 500 m, up
to an offset of 50 km. For each increment we measure the misfits
(root mean squares, RMS) between the upstream 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 clearest, large
right-lateral displacement recorded in the geomorphology along
the active Red River fault. Since dextral motion is likely to
have started around 5 Myr, the most probable average Plio-Quaternary
slip rate on the fault is of order of 5 mm/yr. We attribute the
apparent lack of seismic activity on a large stretch of the fault
to millennial recurrence times between great earthquakes. Our
study shows that relatively small drainage systems can keep a
good record of fairly large cumulative fault offsets.
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