Neogene-Recent Reactivation of Jurassic-age Faults in Southern Vietnam, with Implications for the Extrusion of Indochina

Onshore Vietnam contains a complex series of faults coupled with a diffuse igneous province that has been active since the mid-Miocene. However, there are several conflicting fault maps in the literature and no consensus concerning the relative age of mapped faults and Neogene basalt flows, which becomes problematic when trying to use structural data to distinguish between competing tectonic models for the SE Asia region. This paper aims to define the Neogene-Recent tectonic setting and kinematics of the Da Lat block of the onshore Vietnam region, by analyzing the orientation, kinematics and ages of faults across a sub-region of the block. Fault ages can be constrained by the cross-cutting relationships with dated basalt flows. Results from remote sensing show a strong NE-SW fault trend for southern Vietnam, with additional, minor N-S, E-W and NW-SE trends. Fault orientations observed in the field fall into this NE-SW trending class, and are sub-vertical. In the basalt flows (with eruption ages < 5 Ma) these faults have oblique lineations with a strong strike-slip component. In Jurassic sediments, these faults show two sets of lineations: an older, dip-slip set, and a younger, oblique-slip set. We postulate that Jurassic-age dip-slip faults have been reactivated as strike-slip faults post 5 Ma. Strike-slip motion on NE-SW oriented faults is consistent with rotation and extrusion of the Kontum and Da Lat blocks. Rotation of the blocks is consistent with continuum rubble behavior of small crustal blocks under the influence of extrusion-driven asthenospheric flow.