Upper Plate Responses and Driving Mechanisms of the 'Tethys One-Way Train'

In the evolutionary history of the Tethys tectonic realm, numerous continental fragments progressively split from the southern hemisphere's Gondwana continent and “unidirectionally” converged and assembled with the northern hemisphere's Eurasian continent, ultimately shifting the center of the Earth's continental masses from the southern hemisphere in the late Paleozoic to the present northern hemisphere. Previous studies have vividly summarized this seemingly unidirectional process of plate fragmentation and reassembly as the “Tethys one-way train.” As a “welcoming ceremony” for this train's arrival, the upper plates' lithosphere, such as in the Tibetan Plateau and Anatolia from different geological eras, records unique tectonic-magmatic responses. For example, tectonic-magmatic activity may first appear in the interior, thousands of kilometers away from the convergence boundary, then expand from the inside out. This can also develop into a “piston-like” cycle of transformations: crustal compression + magmatic quiescence → crustal extension + magmatic peak → crustal compression + magmatic quiescence → and so on. Addressing these typical geological phenomena of the Tethys tectonic realm and combining the tectonic background revealed by plate reconstruction with the contemporaneous multiple episodes of block assembly, we employ forward numerical simulation to interpret the deep driving processes and mechanisms behind these phenomena. By utilizing geological, geochemical, and geophysical observations to constrain model results, we propose that the abrupt changes in the lower-plate movement characteristics (such as subduction angle and rate) caused by the subduction of high-buoyancy blocks significantly control the rapid transition of tectonic-magmatic patterns in regions like Tibet and Anatolia. The multiple episodes of block assembly can explain the accordion-like tectonic-magmatic cycles of the active continental margins. Given that the high-buoyancy blocks require continuous northward driving forces during their journey from Gondwana's fragmentation to their convergence with the Eurasian continent, we further calculated the temperature distribution in today's upper mantle using previous global seismic wave attenuation models to establish a forward geodynamic model, exploring the deep driving mechanisms of the convergence process in the Tethys tectonic realm. The modeling results indicate that the current temperature structure of the upper mantle, with colder northern regions and warmer southern regions, can create sufficiently large lateral mantle density contrasts and trigger the initiation of oceanic plate subduction towards the low-temperature areas, essentially starting the engine of the express train. Subsequently, the demise of secondary ocean basins during convergence often accompanies the subduction and rebound of high-buoyancy blocks, which rapidly returning fragments strongly collide with the rear oceanic plates, triggering a new round of oceanic subduction and further cooling the northern hemisphere's upper mantle, thereby giving the convergence process a chain reaction characteristic. Therefore, although the continental blocks fragmented from Gondwana may be seen as “passengers” of the one-way train, they have played a significant role in both the welcoming ceremony and the sustainable operation of the train.