Indosinian NW-trending shear zones within the Truong Son belt (Vietnam) 40Ar 39Ar Triassic ages and Cretaceous to Cenozoic overprints

Abstract

Several NW-trending ductile and partly mylonitic shear zones cross the Truong Son belt of Central Vietnam, along the Song Ma and Song Ca valleys and, north of the Kontum block, from Da Nang to Aluoi and Khe Sanh. High-grade metamorphic rocks of amphibolite facies, showing a retrograde evolution and consisting of ortho- and paragneisses, metavolcanics, amphibolites, marbles and quartzites are exposed along these structures. They display a homogeneous deformation pattern characterized by a generally steeply dipping foliation and a near-horizontal to gently plunging mineral stretching lineation, indicating a strike-slip tectonic regime of deformation. Along the southernmost fault zone at least, various and consistent kinematic indicators, including SC structures, asymmetric tails of porphyroclasts, prove that the strain and metamorphism have been generated by a phase of dominantly non-coaxial deformation with a dextral sense of shear. 40Ar 39Ar dating, applied on the high-grade metamorphic rocks minerals occurring along these zones, provide plateau cooling ages, closely around 245 Ma, establishing that this event took place in the lowermost Triassic as an early phase of the Indosinian orogeny. The existence of Indosinian movements in Vietnam, as they have been defined by previous authors during the early century, is now accurately confirmed and this is the first insight in the occurrence of ductile strike-slip tectonics of Indosinian age along NW-SE fault zones. Well-expressed in Central Vietnam is a Cretaceous thermal and deformational overprint, marked by epimetamorphism, which took place between 90 and 120 Ma, as attested by low-temperature degassing ages. This Cretaceous event is not found further north in the Song Ma zone where younger ages appear as a result of the influence of Cenozoic shear movements along the Red River fault which displays 20–30 Ma ages. On the northern flank of Song Ca, in the Bu Khang-Phu Hoat core complex, comparable Oligocene-Lower Miocene ages (20–35 Ma), yielded by biotite and phlogopite, reflect a strong overprinting and attest to a rapid uplift of the basement.