A CT scan of the Earth
The Fergana Basin is a flat, fertile expanse surrounded by the immense Tian Shan and Pamir mountain ranges. On the surface, it is a region of agriculture and ancient cities. But deep beneath the soil lies evidence of a tectonic collision of astonishing scale. Using a technique called seismic tomography, geophysicists have created a three-dimensional image of the Earth's mantle below the basin. This method is like a medical CT scan, but instead of X-rays, it uses seismic waves from thousands of earthquakes. These waves travel at different speeds depending on the temperature and density of the rock they pass through. By analyzing the arrival times of these waves at seismic stations across the globe, scientists can map out colder, denser structures hidden deep within the hotter, more fluid mantle.
Beneath the Fergana Basin, this imaging technique revealed a colossal, high-velocity anomaly: a huge slab of rock about 200 kilometers below the surface. This structure is interpreted as a detached piece of oceanic lithosphere—the floor of a vanished ocean. This cold, dense slab causes seismic waves to travel faster than they do through the surrounding mantle, making it "visible" to the tomography.