Stratigraphic, structural and tectonic contexts of Permo-Triassic igneous activity in the Khao Khwang fold and thrust belt, central Thailand

Permo-Triassic igenous activity along the N-S Loei Fold Belt (LFB), on the western margin of the Indochina Terrane, is generally not well-described in detail due to poor, discontinuous natural outcrops, although geochemically and geochronologically a Late Permian-Late Triassic volcanic arc setting has been determined. At the southern end of the trend, the Khao Khwang Fold and Thrust Belt (KKFTB) displays a dramatic increase in the level of detail available regarding the stratigraphic and structural relationships with igneous activity due to widespread quarry activity. This study describes these outcrop relationships, in particular: 1) the stratigraphic context of Early Permian (arc) volcanic activity interbedded with Early Permian deepwater Alum Shales, 2) Late Permian volcanic activity following on from deposition of the Early-Late Permian Saraburi Group. 3) Intrusion of sills and dykes into the Saraburi Group prior to the onset of deformation in the Early Triassic (pre-kinematic). 4) Intrusion of sills, dykes and plutions in the Middle-Late Triassic after the main folding and thrusting episode of the KKFTB (post-kinematic). Post-kinematic intrusion geometries are strongly influenced by fold and thrust geometries in the Permian carbonates, due to the strong control by bedding and thrust fault strength anisotropies. The KKFTB differs from the LFB in the Early Permian timing and large volume of Late Triassic igneous activity, the E-W orientation and Early-Middle Triassic age of folds and thrusts. This is attributed to a phase of Permian-age northwards directed subduction below the KKFTB, when a small ocean between North and South Indochina closed, and preceded latest Permian-Triassic eastwards subduction commencing on the western margin of a united (N and S) Indochina.