The enigma of seismic activity beneath Tibet, a mystery that has puzzled scientists for decades, may have a surprisingly simple explanation. It all comes down to heat and the unique composition of the rocks beneath this majestic plateau.
The Tibetan Plateau, a result of the collision between India and Asia some 50 million years ago, has long been a subject of fascination and scientific inquiry. The ongoing northward movement of India continues to shape and mold the plateau, creating a complex geological landscape.
Under the surface, a clear divide is observed. Geophysicists have mapped wave speeds through the upper mantle, revealing a distinct contrast. While the southern region behaves as expected for cold, dense rock pushed by the Indian plate, the northern region presents a different picture. Seismic waves slow down significantly, raising questions about the nature of the underlying material.
Two competing models have emerged to explain this phenomenon. One suggests that a thick, intact lithosphere, comprising crust and upper mantle rock, extends beneath Tibet. The Indian lithosphere, according to this view, remains largely intact as it continues its northward journey. The rival model, however, paints a different picture. It proposes that the lithospheric mantle in northern Tibet became too thick and unstable due to the relentless collision, eventually sinking into the deeper mantle. In its place, hotter, flowing rock from the asthenosphere rose to fill the gap, creating the observed slow seismic signals.
Dr. Ajay Kumar, a geophysicist at the Indian Institute of Science Education and Research, Pune, took a rigorous approach to tackle this problem. He developed models that had to satisfy four independent datasets simultaneously: seismic wave speeds, gravity field measurements, variations in Earth's gravitational shape, and surface topography. By requiring all four datasets to align, Kumar's method left little room for error or interpretation.
The results were intriguing. Beneath southern Tibet, the findings confirmed earlier work, indicating the presence of ancient, cold rock that thickens as it moves northward. However, northern Tibet presented a different story. The lithosphere there is younger, and seismic wave speeds are strikingly low, suggesting a different composition.
Kumar's study proposes an alternative explanation for these slow seismic signals: radiogenic heating. This heat is generated by the radioactive decay of trace amounts of uranium, thorium, and potassium within the crust itself. In thickened crust, this process can produce enough heat to slow seismic waves without the need for material replacement. The key condition is that the thick crust must have been in place before the India-Asia collision began, allowing sufficient time for heat accumulation.
The implications of this study are far-reaching. If Kumar's interpretation holds true, it challenges the prevailing assumption that the northern lithosphere has been substantially removed. Instead, it suggests a lithosphere that is thermally and compositionally modified but still present. This has significant consequences for our understanding of the forces at play beneath the northern plateau and how they influence seismic activity and elevation patterns.
In my opinion, this study opens up a new avenue of exploration into the geological history of the region. The geological record can provide crucial evidence to support or refute the early thickening of the crust, offering a deeper understanding of the forces that shaped Tibet. It's an exciting development that highlights the complexity and beauty of our planet's geological processes.
