Neogene to Recent Fault Reactivation in Southern Vietnam, Implications for Modern day Extrusion of Indochina and Microblock Rotation within the Core of the Sundaland Block

Abstract:

It is currently unknown whether Indochina is situated within an extrusion or extensional tectonic regime. Therefore, we have focused our efforts in determining the age relationship between fault movement and volcanic activity in conjunction with defining a principal stress orientation (σ₁) along various faults within Vietnam. Our structural datasets and observations of cross-cutting relationships of faults with Neogene basalts suggest that faulting is active in Vietnam. Preliminary age dating of a faulted basalt outcrop yielded an ⁴⁰Ar/Ar³⁹ age of ~600 yrs, suggesting faulting is active in Vietnam. Fault slip orientations were identified through measurement of one or more lineation sets on a single fault surface and then correlated with cross-cutting relationships. For example, field observations of lineation sets in Jurassic sediments suggest an older dip-slip event followed by a younger oblique-slip event that reactivated the older faults; these younger slip orientations align with stress fields indicated by newer faults cutting basalt flows, suggesting the same regime caused both sets of later events. These observations imply a change in kinematics over time.

Our measurements of faulted surface orientations throughout south-central Vietnam indicate the presence of multiple shear zones whose strikes parallel major regional faults. A transtensional stress field was additionally identified between the Song Ba and Poko Faults, implying the adjacent Kontum and Dray Sap Blocks are moving independently and away from each other. The regional variation in stress fields and implied movements along microblock bounding faults likewise suggest independent movement of each of the microblocks. Results here thus suggest the Kontum, Dray Sap, Da Lat, Tuy Hoa, and Quang Nam microblocks are not migrating as a single coherent unit within the core of the Sundaland Block, but are instead jostling and rotating independently of each other. Findings here and future work will assist in strengthening our understanding of extrusional tectonics within Indochina and how it relates to independent microblock movement.