Neogene-Recent Reactivation of Pre-Existing Faults in South-Central Vietnam, With Implications for the Extrusion of Indochina

Abstract

Vietnam contains complex faults coupled with a diffuse igneous province that has been active since the mid-Miocene. However, existing fault maps demonstrate little consensus over the location of Neogene basalt flows and relative ages of mapped faults, which complicates interpretations of tectonic model for the evolution of Indochina. This paper identifies discrete tectonic blocks within Vietnam and aims to define the Neogene-Recent tectonic setting and kinematics of south-central Vietnam by analyzing the orientation, kinematics, and relative ages of faults across each block. Fault ages and relative timing are estimated using cross-cutting relationships with dated basalt flows and between slickenside sets. Remote sensing results show distinct fault trends within individual blocks that are locally related to the orientations of the basement-involved block-bounding faults. Faults observed in the field indicate an early phase of dip-slip motion and a later phase of strike-slip motion, recording the rotation of blocks within a stress field. Faulting after the change in motion of the Red River Fault Zone at ∼16 Ma is inferred, as faults cross-cut basalt flows as young as ∼0.6 Ma. Strike-slip motion on block-bounding faults is consistent with rotation and continuous extrusion of each block within south-central Vietnam. The rotation of the blocks is attributed to the “continuum rubble” behavior of small crustal blocks influenced by upper mantle flow after the collision between India and Eurasia. We infer a robust lithospheric-asthenospheric coupling in the extrusion model, which holds implications for other regions experiencing extrusion even in the absence of a free surface.