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.
Rocks from the Mantle
The specific mantle rocks exposed in this region are primarily peridotites and their altered form, serpentinites. Peridotite is a dense, coarse-grained igneous rock, composed mainly of the minerals olivine and pyroxene. It is the dominant rock type of Earth's upper mantle. When exposed to lower pressures and temperatures near the surface, peridotite undergoes a process called serpentinization. This hydration reaction alters the original minerals into serpentine group minerals, which have a greasy luster and a slick, snakeskin-like texture—the origin of the name "ophiolite" from the Greek word for snake.
The unaltered peridotite has a high magnesium content and is typically dark green to black. As serpentinization progresses, the rock's volume can increase significantly, causing further fracturing and deformation. This process also changes the rock's color, often producing mottled green, yellowish, or even reddish hues. These rocks are a geological curiosity. They are a direct sample of the planet's interior. Analysis of ophiolites like those in Bolikhamxay helps geologists study the composition of the mantle and the processes that occur at mid-ocean ridges where new oceanic crust is formed.