The deep Earth
Beneath the dusty plains of Uzbekistan's Navoi Region, the Earth's mantle is flowing. This deep movement is invisible at the surface, but it leaves a detectable trace in the seismic waves from distant earthquakes. The North Nurata Fault, part of a fault system extending over 1000 kilometers, is a major tectonic boundary. This zone is a suture, the remains of a Paleozoic ocean basin that closed when ancient continents collided. The immense pressures and temperatures at this boundary have created a unique geological laboratory for studying the behavior of the deep Earth.
When seismic shear waves (S-waves) from an earthquake travel through the upper mantle and hit this region, they split into two separate waves. One wave travels faster than the other. This phenomenon, called seismic anisotropy, shows that the rock in the mantle is not uniform in all directions. The cause is the alignment of mineral crystals, primarily olivine, which makes up over 60% of the upper mantle. Under intense pressure and directed stress, these crystals physically rotate and align, creating a kind of grain in the rock fabric. This grain dictates the "fast" and "slow" directions for seismic waves passing through.
Mapping the mantle
Seismologists measure the time delay between the fast and slow waves to understand the structure deep below. Around the Nurata Fault, the velocity difference can be as high as 10%. The orientation of the fast wave reveals the direction of the mineral alignment, which in turn is a proxy for the direction of mantle flow. The process is similar to how logs align in a river, indicating the water's direction. By analyzing data from multiple seismograph stations, scientists can map the flow of the asthenosphere—the ductile upper layer of the mantle—beneath the rigid lithospheric plate.
These measurements in the Tian Shan region, which includes the Nurata Fault, show a multi-layered anisotropy. The upper layer of anisotropy, within the lithosphere, seems to be "fossilized," reflecting ancient tectonic events like the formation of the mountain belt. Deeper down, in the asthenosphere (around 100-200 km deep), the anisotropy shows the current mantle flow. This flow is a consequence of large-scale geodynamic processes, primarily the ongoing collision of the Indian and Eurasian plates that has pushed up the Himalayas and the Tian Shan mountains. The Nurata Fault is caused by these immense continental forces.