The division of tectonic units and tectonic evolution in Laos and its adjacent regions

Abstract: Structural research on the Sanjiang-Indosinian region of southeast Tibet has aroused much interest among geologists all over the world. However, Laos, which is located in north-central Indochina peninsula, remains insufficiently studied in comparison with neighboring China, Thailand and Vietnam. Combined with the latest project research results as well as the previous research data, the authors preliminarily made division of tectonic units and summarized tectonic evolution of Laos and adjacent regions through the regional comparative analysis and the study of the extension trends of individual tectonic units and their interrelations. On the basis of the temporal-spatial distribution nature of the regional structure-rocks, the authors divided the study area into 7 third grade structural units, i.e., Jinghong-Sukhothai arc block, Nam-Uttaradit suture, Simao-Phitsanulok block, Dien bien phu-Loei suture, Vientiane-Kontum block, Sepon-Tam Ky suture, and Truongson block. The study area had multiple tectonic natures during different geological stages. On the whole, the study area has experienced three important tectonic evolution stages, i.e., Pre-Tethys, Tethys, and Meso-Cenozoic intracontinental stages. During the Pre-Tethys evolution which lasted to Early Paleozoic, mainly rigid Kontom and Truongson block came into being, which had affinity to the Yangtze-South China block. From the Late Paleozoic-Early Mesozoic, the study area became a part of the Tethyan domain with the opening of Paleo-Tethys, which showed an ocean-land tectonic framework that led to the formation and evolution of Dien bien phu-Loei Ocean, Sepon-Tam Ky Ocean, Nam-Uttaradit arc-back Ocean and Song Ma Ocean. Late Mesozoic-Cenozoic was a period during which modern geological structure framework began to form, and there existed intraplate extension, strike-slipping, crustal mass isostatic adjustment, basin formation and coeval alkaline magmatic activities.