Structure And Tectonics Of A Late Jurassic, Arcuate Fold Belt In The Ban Don Group, Southern Vietnam

The Ban Don Group (BDG) of south-central Vietnam and NE Cambodia significantly constrains the Jurassic tectonic history of the SE Indochina plate margin. The Early to Middle Jurassic BDG comprises internally conformable marine shales and sandstones with a high volcaniclastic content, bounded below and above by major unconformities, deposited in a NW-SE trending basin. A contractional fold belt developed in the basin during the latest Middle to Late Jurassic. Folds are upright, planar, doubly plunging, and lack systematic vergence. Thrust faults are seldom observed. The Ban Don Fold Belt (BDFB) is 350 km long and 240 km wide. It forms an arcuate salient towards the foreland to the NW with nearly a 90 degrees change in fold trend orientation and an estimated average shortening of 37%. The change in fold orientation follows sedimentary facies boundaries, which parallel the depositional basin margins. The NW-trending BDG basin axis forms a line of symmetry for the facies boundaries and fold trends. An analog for the upright fold-dominated, low-taper wedge of the BDFB is the Parry Islands Fold Belt (salt detachment). However, the absence of salt in the BDFB suggests a very weak detachment was facilitated by high fluid pressures in the basal shales. Deposition and deformation of the BDG plus recently published regional data indicate the well documented Late Jurassic and Cretaceous Andean-type margin of Southeast Asia also existed during the Early and Middle Jurassic. The BDG reflects back-arc deposition with subsequent deformation driven by changes in subduction angle and/or subduction obliquity during the Jurassic. Alternatively, the arcuate geometry of the BDFB is also compatible with the collision of a small indenter.