The revelation of the crustal geometry of the western Qilian Mountains, NE Tibetan Plateau

(Press-News.org) As the largest orogenic plateau on earth, the Qinghai-Tibet Plateau was caused by a complex crustal deformation process during the continuous collision and compression process between the Indian and Eurasian continents starting at least 60-50 Ma ago. The formation of the Qinghai-Tibet Plateau records the collision of the two continents and the deformation process and mechanism within the continents. Therefore, Qinghai-Tibet Plateau is considered as a natural ideal laboratory for the study of continent-continent collision and dynamics. At present, the continuous collision between Eurasia and Indian continents is still ongoing, resulting in the Qinghai-Tibet Plateau is still continuing to expand outward. The western section of Qilian Mountains on the northeast margin of The Qinghai-Tibet Plateau, as the northeast boundary of the Plateau, was uplifted and became part of the present Qinghai-Tibet Plateau during the Middle Miocene, according to the latest chronology results. Therefore, as one of the youngest parts of the Qinghai-Tibet Plateau, the western Qilian Mountains is one of the key areas to test various proposed models of the formation of the Qinghai-Tibet Plateau.

The crustal deformation mechanism in the northeast margin of Qinghai-Tibet Plateau has been proposed by many predecessors. However, with the deepening of the research, more and more evidence has been revealed, and the previously proposed crustal deformation mechanisms have been unable to fully explain many new evidence. The differences in resolution of various means of studying the interior of the earth's crust undoubtedly add to the divergence. Therefore, earth scientists call for more precise methods to reveal the crustal structure in the northeast margin of the Qinghai-Tibet Plateau. The deep seismic reflection profiling is one of the internationally recognized methods for revealing high precision crustal structural image. Therefore, using the crustal structural image revealed by the deep seismic reflection profile to study the crustal deformation pattern in the northeast margin of Qinghai-Tibet Plateau in this paper will undoubtedly provide very important scientific significance and reference value for the study of this area.

The researchers reprocessed the high-resolution deep seismic reflection data, which were originally collected in the 1990s, for a transect across the NE margin of the western Qilian Mountains and the Hexi Corridor. The reprocessed seismic image has a higher signal-to-noise ratio compared with the first published result, which imaged the southward dipping north Qilian Mountains fault (NQSF) and a southerly dipping fault extending downward into the lower crust, which was named the north border thrust (NBT). In addition to these results, the reprocessed image more clearly delineates the geometry of the crust beneath the junction between the western Qilian Mountains and Hexi Corridor, yielding a better understanding of the processes responsible for the outward growth of the Tibetan Plateau.

The reprocessed seismic profile across the junction of the north margin of the western Qilian Mountains and the Hexi Corridor reveals the decoupled crustal deformation that is partitioned by the intra-crustal decollement layer at a depth of 14?24 km. The crustal deformation above the decollment is mainly characterized by a series of southward-dipping thrust faults downward ended at the decollement layer. Crustal-scale duplexing presents in the crustal beneath the decollement layer. The imbricate Moho structure beneath the study region implies that the Asian lithospheric mantle is being underthrust beneath the northeastern margin of the Tibetan Plateau. Integrating the results with previous geological and geophysical observations, the researchers propose an evolutionary model regarding the outward growth across the western Qilian Mountains, northeastern margin of the Tibetan Plateau (Figure 1).

This result enriches the crustal structure research on the northeast margin of Qinghai-Tibet Plateau. It is not only of great significance to the study of crustal deformation mechanism in the northeast margin of the Qinghai-Tibet Plateau, but also of great reference value to the understanding of crustal deformation mechanism in the Qinghai-Tibet Plateau.

INFORMATION:

This research was funded by the National Natural Science Foundation of China (Nos. 41904083, 41430213, 41590863, and 41774114).

See the article: Huang X, Gao R, Li W, Xiong X. 2020. Seismic reflection evidence of crustal duplexing and lithospheric underthrusting beneath the western Qilian Mountains, northeastern margin of the Tibetan Plateau. Science China Earth Sciences, https://doi.org/10.1007/s11430-020-9677-y