The Changning-Menglian suture zone: a segment of the major Cathaysian-Gondwana divide in Southeast Asia

In southwest China, the major Cathaysian-Gondwana divide (the Palaeo-Tethyan suture) is very well delineated by a narrow north-south zone of oceanic siliceous sedimentary rocks and dismembered ophiolite complexes including probable remains of reef-capped oceanic islands. The location of this zone, the Changning-Menglian suture, is not well appreciated in the English language literature, probably because of the failure to recognise the approximately 600 × 400 km Simao terrane which lies to the east of the suture. Many analyses mistakenly include the Simao element in the Sibumasu terrane thus placing a terrane of distinct Cathaysian affinities on the Gondwana side of the major palaeogeographic divide. A measure of the size and time span of Palaeo-Tethys is contained in the Changning-Menglian suture zone sedimentary rocks; deep marine cherts are dated between Early Devonian and Middle Permian, and limestones, interpreted as caps to seamounts, extend the age range of the ocean to Late Permian. These data agree with recent views of Cathaysian elements rifting from Gondwana in the Silurian or Devonian followed by a major rifting event on the north Gondwana margin early in the Permian. Subduction created an active continental margin on the western edge of the Simao terrane throughout much of the Triassic and the closure of this branch of Palaeo-Tethys is marked by the cessation of this activity in the early Late Triassic.