Late Cenozoic tectonics of the Red River Fault Zone, Vietnam, in the light of geomorphic studies

The main episode of Pliocene–Quaternary tectonic activity in the Red River Fault Zone in Vietnam was related to dextral strike-slip motions while uplift of the SW and NE sides of the fault zone was limited. In the Vietnamese segment of this zone, traces of pre-Pliocene mature relief were preserved to a much smaller extent compared to those in Yunnan Province, China. The main factors controlling headward erosion of tributary streams were lithologic contrasts within the solid bedrock. Tectonic-geomorphic studies indicate that the amount of Quaternary dextral offset of the Red River Fault Zone in Vietnam, calculated from offset and deflection of tributary valleys of the Red River, ranges between some 400 m and 5.3 km, averaging 1.1–2 km. The corresponding rates of slip are 0.43–1.1 mm/yr for individual displaced segments and 5.5–7.8 mm/yr for the inferred cumulative displacement. Fluvial terraces in the medial segment were mainly shaped in the Late Pleistocene and Holocene, except the highest one (40–50 m) which was probably formed in Mid-Pleistocene time. Taking into account the amount of dextral displacement of terrace steps and related alluvial fans, the most realistic estimation of Mid-Pleistocene through Holocene slip rate is 0.9–3.9 mm/yr. The axis of maximum horizontal compression associated with dextral slip of the fault zone was aligned NNW–SSE to N–S, and the fault motion in the Quaternary resulted mainly from aseismic creep.