Abstract Thailand forms part of the core of SE Asia and, together with the rest of Sundaland, is considered to have formed from continental fragments derived from the break-up of the northern margin of Gondwana, a major Precambrian continent, during the Palaeozoic. Fragments of this continent rifted away and became separated from Gondwana by the development of new oceanic crust, and moved northwards leaving expanding oceans behind them. At the same time oceanic crust lying to the north of these fragments was subducted beneath the southern margin of Asia until, during the Triassic, the fragments collided with and were accreted to Asia. Collision zones between continental fragments are marked by accretionary complexes, belts of serpentinized and highly disrupted oceanic mantle, basaltic crust and ocean-floor sediments forming ophiolites together with thrust and imbricated continental margin sediments; these include mélanges, composed of a variety of blocks of igneous and sedimentary rocks in a fine-grained clay or serpentinous matrix.