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The NNE-SSW-trending Dien Bien Phu fault, which dextrally displaces the NW-SE-trending Song Ma suture, in northwestern Vietnam, is widely considered one of the most seismically active faults in Indochina. In order to better understand the fault's activity, this study reports new 40Ar/39Ar geochronological data for mica schists and mylonites from several areas along the Dien Bien Phu fault, showing 40Ar/39Ar ages of 194-212 Ma for mica schists and 158-198 Ma for mylonites. These ages suggest that after the collision of the South China and Indochina blocks, dextral shearing deformation initiated along the Dien Bien Phu fault in the Jurassic, significantly earlier than previously thought. In light of the relevant tectonic events in Indochina, the successive suturing/collision of the Indochina, Simao, and Sibumasu blocks may have been responsible for the initiation of dextral shearing along the Dien Bien Phu fault, which in turn resulted in offset of the Song Ma suture. [ABSTRACT FROM AUTHOR]