RECENT DEFORMATION ALONG THE RED RIVER FAULT AS CONSTRAINED BY RIVER INCISION INTO A LOW RELIEF RELICT LANDSCAPE, YUNNAN, CHINA

On the basis of field mapping, and digital elevation model and longitudinal profile analyses of 102 tributaries to the Red River, Yunnan, China, we have identified and characterized a low relief relict landscape, constrained river incision parameters and related these to the regional structural setting. The Red River flows southeasterly between the Yangzi craton to the northeast and the Ailao Shan range to the southwest along the active trace of the Red River fault. The low relief landscape is extensively developed on the Yangzi craton and is preserved locally on the Ailao Shan range where it is characterized by a thick grus cover. The Red River has partially incised and dissected this landscape. In general, tributary profiles on both sides of the river display an upper concave section with low steepness index that corresponds to the part of the tributary that flows over the relict landscape. Downstream of the upper section are from one to three knickpoints which can be correlated to a limited extent. The lower longitudinal profile sections are characterized by high steepness indices, but with systematic variations along the length of the river. Steepness indices were used to identify areas of higher uplift rate, and upper knickpoint elevations were used to constrain the elevation of the relict landscape. Our findings are: (1) the elevation of the relict landscape increases gently and smoothly from southeast to northwest; (2) there is a significant component of normal displacement (600-800 m) along the Red River fault north of the point at which the fault changes strike abruptly from northwest to more northerly; (3) there is no evidence for a normal component to displacement on the Red River fault southeast of the major bend, but there may be a small amount of shortening along structures on the Yangzi craton side of the Red River. This differs from previous structural interpretations in which normal displacement is argued to extend to the SE of the bend along the Red River fault (Allen et al. 1984, Leloup et al. 1995). A full understanding of the geomorphic and tectonic history of the Red River fault, as a feature of first order tectonic significance in Asia, is crucial to any tectonic model of the region.