Abstract

The Pak Lay Foldbelt in the northwest of the Lao PDR is a product of the Indosinian orogeny, resulting from the collision of Shan-Thai and Indochina along a suture now marked by the Nan-Uttaradit ophiolite zone. Current hypotheses place the timing of this collision in either the Permian or the Triassic. Recent field work and subsequent laboratory analyses suggest that, in the area to the south of Pak Lay: pre-collision sediments are as young as Middle to Upper Jurassic; arc-related volcanism continued from the Triassic into the Late Jurassic; all these sediments and volcanic rocks form an imbricate zone; and the Cretaceous Khorat Group rests with marked unconformity above the imbricate wedge. From this we conclude that the Shan-Thai-Indochina suturing occurred in the Late Jurassic.