A collision of continents
The mountainous landscape of Mae Hong Son province is the direct result of a continental collision that took place during the Late Triassic period. This event, part of the larger Indosinian Orogeny, involved two major landmasses: the Shan-Thai terrane to the west and the Indochina terrane to the east. For millions of years, these two continental blocks were separated by the Paleo-Tethys Ocean. As tectonic plates shifted, the ocean began to close. The Shan-Thai block, which had drifted north after breaking away from the supercontinent Gondwana, eventually slammed into the Indochina block.
The immense pressure of this collision folded and faulted the Earth's crust, thrusting rocks that once formed the deep ocean floor upwards to create the mountains seen today. The line of this ancient impact is known as a suture zone. In Thailand, this zone is primarily identified as the Nan-Uttaradit Suture. The rocks throughout this region are the physical proof of the vanished Paleo-Tethys Ocean and the tectonic forces that assembled mainland Southeast Asia. The entire process of collision and mountain-building significantly shaped the geology of the peninsula.
Evidence in the rocks
Geologists identify the history of this region by studying specific rock types that are tectonic markers. Among the most significant are ophiolites, which are fragments of oceanic crust and the underlying upper mantle. Finding ophiolites on land shows a former ocean basin that has been destroyed by a continental collision.
Alongside ophiolites, the area contains extensive deposits of chert, a hard, dense sedimentary rock. This chert formed from the silica-rich skeletons of countless microscopic marine organisms called radiolarians, which rained down on the deep ocean floor over millions of years. Well-preserved radiolarian fossils found within these chert beds, dating from the Permian to the Triassic, allow geologists to precisely date the rocks and confirm their deep-ocean origin. The presence of these deep-sea sediments, now found high in the mountains, confirms the massive scale of the tectonic uplift that occurred.