Qingquan Meng

Oligocene slow and Miocene–Quaternary rapid deformation and uplift of the Yumu Shan and North Qilian Shan: evidence from high-resolution magnetostratigraphy and tectonosedimentology

Abstract Most existing tectonic models suggest Pliocene–Quaternary deformation and uplift of the NE Tibetan Plateau in response to the collision of India with Asia. Within the NE Tibetan Plateau, growth of the terranes was suggested to progress northeastward with the Yumu Shan (mountain) at the northeasternmost corner of the Qilian Shan (mountains) being uplifted only since about 1 Ma ago. Here we present a detailed palaeomagnetic dating and tectonosedimentological measurement of Cenozoic sediments in the eastern Jiuquan Basin related to the deformation and uplift of the North Qilian Shan and Yumu Shan. The results show that the eastern Jiuquan Basin is a Cenozoic foreland basin and received sediments at about 27.8 Ma at the latest. Eight subsequent tectonic events at about 27.8, 24.6, 13.7–13, 9.8–9.6, 5.1–3.6, 2.8–2.6, 0.8 and 0.1 Ma demonstrate the development of the foreland basin in response to Oligocene–Quaternary uplift of the North Qilian Shan and subsequent propagation of thrust–fold system owing to collision of India with Asia. The Yumu Shan is the late phase of deformation front in the thrust–fold system and commenced rapid uplift at about 9.8–9.6 Ma at the latest. A rigid block-floating model is proposed to interpret the mechanism of this deformation and uplift history.

Paleomagnetic Constraints on the Middle Miocene-Early Pliocene Stratigraphy in the Xining Basin, NE Tibetan Plateau, and the Geologic Implications

The Xining Basin lies in the transitional zone between the arid Asian interior and the East Asian monsoon region. The continuous Cenozoic sediments in the basin provide a unique archive recording the uplift of the Tibetan Plateau and its environmental effects on central Asian aridification and Asian monsoon evolution. However, sediments deposited since the middle Middle Miocene have not been precisely dated, hindering our ability to address these issues. Here, we dated a 336-m-thick section containing many Late Miocene fossil mammals from the eastern basin. High-resolution paleomagnetism revealed 16 normal and 16 reversed zones that correlate well with chrons 3n to 5Ar.1r of the Geomagnetic Polarity Time Scale, constraining the section to ∼12.7-4.8 Ma. The changes in lithofacies from floodplain to braided river at ∼8.6 Ma and to thick alluvial fan at ∼6.3 Ma with predominantly southerly paleocurrent directions occur simultaneously with an increase in the sedimentation rates, representing two periods of rapid uplift in the eastern Qilian Shan to the north. Our results provide a robust oldest age constraint (<4.8 Ma) on the ancient Huang Shui and Yellow River terraces on the basin sediments and eolian deposits on the terraces. The presence of well-developed floodplain paleosols, shallow lake marls and many large mammals in the lower section suggests that the monsoon climate in the late Middle Miocene to Late Miocene was more humid and stronger than that at present.

Sedimentary Conditions of Evaporites in the Late Jurassic Xiali Formation, Qiangtang Basin: Evidence from Geochemistry Records

The Qiangtang Basin (QB), located in the central Tibetan Plateau, is a Jurassic marine basin and one of the most important prospective salt resource belts in China. In recent decades, many outcrops of gypsiferous bed have been found in the Jurassic marine strata in the basin. Salt springs with abnormally high sodium (Na+) contents had been identified in the Late Jurassic Xiali Formation (Fm.) in the basin in the last years. However, to date, no potash or halite deposits have been identified in the QB. Gypsum outcrops and salt springs are very important signs in the investigation of halite and potash deposits. Therefore, the Xiali Fm. is a potentially valuable layer to evaluate for the possible presence of halite and potash deposits in the basin. However, few studies have explored the formation conditions of evaporites in the unit. Here, we present detailed geochemical records from the Yanshiping section related to the study of the formation conditions of evaporites in the Xiali Fm. of the QB. Climate proxies based on the obviously increased anion concentrations of SO42- and Cl- and the significant correlation coefficients of Ca2+-SO42- (R = 0.985) and Na+-Cl- (R = 0.8974) reveal that the upper member of the Xiali Fm. (the upper Xiali Fm.) formed under an arid climate and evolved into the sulfate phase or early chloride phase. Provenance proxies based on the obviously increased K+ and Na+ ion concentrations and the significant correlation coefficient of Na+-Cl- (R = 0.8974) suggest that the upper Xiali Fm. featured optimal provenance conditions for the possible formation of halite deposits. The regression and the semi-closed tidal flat environment in the upper Xiali Fm. were favorable for the formation of potash and halite deposits. The low Mg2+ /Ca2+ values (mean value = 1.82) and significant Na+-Cl- correlation coefficient (R = 0.8974) also suggest that the upper Xiali Fm. is the layer most likely to contain potential halite deposits. In addition, the macroscopic correlations of tectonism, provenance, paleoclimate, saliferous strata and sedimentary environment between the QB and the adjoining Amu Darya Basin in Central Asia reveal that the two basins shared similar geologic settings that were favorable for the formation of evaporites during the Late Jurassic. Therefore, the upper Xiali Fm. is a valuable layer to explore for halite deposit and may be potentially valuable in the future exploration for potash deposits in the QB.