Neogene-Recent Reactivation of Pre-Existing Faults in South-Central
Vietnam, with Implications for the Extrusion of Indochina
Abstract
Vietnam contains a complex series of 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 constrained 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 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 extrusion-driven asthenospheric flow after the collision
between India and Eurasia. We deduce 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.