Structure and shearing conditions in the Day Nui Con Voi massif:: Implications for the evolution of the Red River shear zone in northern Vietnam

[1] The Day Nui Con Voi massif bears a record of the Red River shear zone (RRSZ) activity in North Vietnam. It forms a large-scale antiformal "core complex''-type structure, bounded by the Song Hong and Song Chay faults. The kinematics of both faults are identical and reflect transtensional shear initiated under upper amphibolite facies conditions and propagated into greenschist facies. Microfabric analysis establishes that both extensional and strike-slip shearing initiated between 700 and 500 degrees C. The RRSZ evolved from a single, subvertical fault, which, due to strike-perpendicular extension, underwent progressive dilation. The created space was "intruded'' by already metamorphosed and deformed ductile middle crust in the form of a gneissic "dome.'' Both strike-slip and extensional shearing were accommodated in the limbs of the antiform, while its core was uplifted from midcrustal level bearing only a minor record of sinistral shear.