The Scar of a Lost Ocean
The mountains of Bolikhamxay Province contain dramatic evidence of a continental collision that occurred during the Triassic Period. This geologic feature, known as the Truong Son Orogenic Belt, is the suture zone where the Indochina and South China continental blocks collided. The collision in the Indosinian Orogeny, took place over millions of years, beginning around 255 million years ago and concluding around 202 million years ago. Before this collision, an ancient ocean, a branch of the Paleo-Tethys, separated the two landmasses. As the tectonic plates converged, this ocean basin was consumed through subduction.
During the final stages of the collision, sections of the dense oceanic crust and the underlying upper mantle were not subducted. Instead, they were scraped off and thrust up onto the continental crust in a process called obduction. These stranded slabs of deep-earth rock are known as ophiolites. The ophiolites found in the Truong Son belt are remnants of the vanished Paleo-Tethys Ocean floor. They show rocks that are normally found at depths of 30 to 200 kilometers below the Earth's surface. The entire mountain belt is characterized by a series of northwest-trending strike-slip faults, showing the shearing forces involved as the continents scraped past one another.