Multiple phases of structural deformation in the evolution of the Klaeng fault zone, Eastern Thailand, constrained by zircon U–Pb ages and Hf isotopic compositions

The Klaeng fault zone comprises mid–late Palaeozoic clastic sedimentary rocks, mylonitic and low-to-medium-grade metamorphic rocks, granites, and Mesozoic redbed sandstones. Eight sheared granites from this zone were analysed using zircon U–Pb dating, confirming ages of 448 Ma (Late Ordovician), 219–217 Ma (Late Triassic), and 46.38 Ma (middle Eocene). This study identifies four deformation events (D₁–D₄) based on our granite ages and structural analyses. D₁ is characterised by tight folds resulting from E–W compression during the Triassic Indosinian orogeny. D₂ is defined by open folds and gentle dips in the redbeds due to Late Cretaceous E–W transpressional shortening. D₃ demonstrates top-to-E or sinistral ductile shears observed in the Ordovician and Eocene granites, which occurred during the mid-Eocene. D₄ exhibits dextral brittle shears that overprinted the earlier deformations, concurrent with the Oligocene exhumation of some granites. D₃ and D₄ reflect intra-continental strike-slip faulting related to the early collision of India with Eurasia. Zircon Hf isotopes indicate that the Ordovician and Triassic granites in the western margin of the fault zone derived from an ancient, enriched mantle or crust (εHf_(T) − 24.3 to −3.5), suggesting a basement associated with the Gondwanan Sibumasu block. In contrast, the Eocene granite in the eastern margin originated from a mixed source of depleted mantle and enriched crust (εHf_(T) −10.2 to + 6.8), indicating a basement related to the Sukhothai arc. Therefore, the Klaeng fault zone broadly lies along the collisional boundary between the two terranes, representing the Palaeotethys suture in Eastern Thailand.