Underthrusting of the Tarim Lithosphere Beneath the Western Kunlun Range, Insights From Seismic Profiling Evidence

Abstract

Uplift of the western Kunlun range results from the Indian indentation into Eurasia and induced underthrusting of the Tarim lithosphere. This underthrusting keeps pace with migration of the western Kunlun foreland basin. The basin-scale sequences provide a decipherable record of the migration. To quantify the underthrusting, we analyzed a ∼323.2-km-long seismic profile crossing the middle segment of the western Kunlun foreland. We traced seismic reflectors to investigate the architecture of the foreland sequences and assigned ages of reflectors by correlating them with the dated outcropping sections. The foreland sequences consist of four layers. The lowermost Layer 1 starts to lap on the preforeland unit at ∼40 Ma, indicating that the Cenozoic western Kunlun initiated slightly prior to ∼40 Ma. Layer 2 shows a large sharp step northward to exceed the forebulge at ∼25 Ma, suggesting that the modern geomorphology of the Tarim Basin took shape. The southwestern boundary of Layer 4 migrates northeastwards by a fold row relative to that of Layer 3. The gradual retreat of reflector terminations from ∼40 to ∼30 Ma suggests that the Tarim lithosphere is viscoelastic. The linear fitting of the forelandward envelop of reflector termination sites since ∼30 Ma indicates that the Tarim lithosphere underthrusted at an average rate of ∼8.18 mm/yr. This rate comprises the shortening rate of ∼3.8–5.7 mm/yr and the western Kunlun latitudinal propagation rate of ∼4.4–2.5 mm/yr. The abrupt increase in sedimentation rate since ∼11 Ma suggests that western Kunlun had reached its current elevation.