Record of the Indosinian Orogeny from conglomerates and detrital zircon U–Pb ages of the western Indochina Block, central Thailand

Abstract

Middle Triassic to Early Jurassic conglomerates and associated sedimentary rocks that record the Indosinian Orogeny are exposed in the western Indochina Block, central Thailand. To elucidate the tectonic and sedimentary history of the Indosinian Orogeny, we studied the conglomerates and analyzed detrital zircon U–Pb ages of samples from the conglomerates and associated sedimentary rocks. On the basis of provenance analysis and estimation of depositional ages, the three recognized events (I, II, and III) of the Indosinian Orogeny are identified. The Indosinian I Event is represented by Type-1 limestone conglomerates, which were derived from collapse of the Permian carbonate platform. This event took place during the Norian (middle Late Triassic) and was triggered by collision between the Indochina and South China blocks. The Indosinian II Event is represented by Type-2 conglomerates, which are composed of various types of clasts, as well as basaltic sandstones. These sedimentary rocks were derived from the uplift and collapse of the Sukhothai Arc, the Nan Back-arc Basin, the Phetchabun Volcanic Belt, and basement rocks of the Indochina Block. Event Ⅱ occurred as a result of the closure of the Paleo-Tethys and the commencement of collision between the Sibumasu Block, the Sukhothai Arc, and the Indochina Block during the Rhaetian (latest Triassic). The Indosinian III Event is represented by Type-3 conglomerates, which comprise volcaniclastic rocks and volcanic breccias composed of andesite and dacite. Event Ⅲ occurred after the Toarcian (late Early Jurassic), associated with post-collision magmatism that occurred between the Sibumasu and Indochina blocks. Our study clarifies the timing and nature of Indosinian Orogeny I, II, and III events in the western Indochina Block and reveals that the Indosinian Orogeny involved Paleo-Tethyan subduction and micro-continental collision tectonics and associated processes during the Middle Triassic to Early Jurassic.