A 230-million-year-old collision
The hills around Xam Neua contain dramatic evidence of a continental collision that closed an ancient ocean. These rocks belong to the Song Ma Suture Zone, a geological seam where the Indochina and South China tectonic plates collided during the Triassic period. This collision squeezed shut the Paleo-Tethys Ocean, a body of water that once separated the two landmasses. The rocks found here are remnants of that ocean's floor and the continental crust that was dragged down with it.
Geological analysis dates the peak of this tectonic event to around 230 million years ago. The process involved the subduction of oceanic crust, which was then followed by continental crust being pulled to depths far below the surface. The rocks underwent what is known as ultrahigh-pressure (UHP) metamorphism, a transformation that occurs at extreme depths and pressures. The conditions are estimated to have reached 21–28 kilobars of pressure and temperatures between 600–680°C, corresponding to depths of over 60 kilometers. Over millions of years, tectonic forces and erosion have brought these deep-earth rocks back to the surface, exposing them in the mountainous terrain of Houaphan province.