Oligo-Miocene midcrustal subhorizontal shear zone in Indochina

New structural observations of Oligo-Miocene deformation in north Vietnam along the Red River Shear Zone suggest that left-lateral strike-slip shear was restricted to the upper and middle crust above a horizontal shear zone. Left-lateral shear deformation is associated with low-pressure-low-temperature parageneses. High-temperature deformation is restricted to zones where the foliation has a low dip in the core of the Dai Nui Con Voi antiformal dome. These observations complete those made earlier in the Bu Khang extensional dome farther south. Above the horizontal shear zone left-lateral transpression was active during the first stage and changed to transtension some 33 Myr ago. Purely extensional metamorphic domes (Bu Khang) or transtensional domes (Dai Nui Con Voi) were exhumed during this younger stage. Our observations plead for caution when interpreting strike-slip structures at lithospheric scale.