Meso‐Cenozoic tectonic history of the Altai: New insights from apatite U‐Pb and fission track thermochronology for the Fuyun area (Xinjiang, China)

He Z., Glorie S., Boutsalis C., Jolivet M., Nixon A., De Grave J.

AbstractThe Qilian Shan is situated along the northeastern margin of the Tibetan Plateau, and has undergone multiple episodes of tectonic deformation since the Paleozoic. Strikingly, tectonic deformation in the Qilian Shan and adjacent areas encompasses newly formed and inherited lithospheric and crustal structures. This study presents the Mesozoic‐Cenozoic post‐orogenic thermo‐tectonic histories of these active tectonic units utilizing an integrated approach combining apatite U‐Pb and fission track thermochronometers. The apatite U‐Pb dates are dominantly Paleozoic, mainly recording post‐magmatic cooling of the sampled granitoid rocks. The apatite fission track data record Late Triassic to Cretaceous cooling ages (central ages between ∼223 and ∼88 Ma), and most of the measured mean track lengths are between ∼13 and ∼12 μm. The apatite fission track results and thermal history models revealed that the Qilian Shan, Longshou Shan and Beida Shan experienced various moderate to rapid basement cooling episodes during the Late Triassic to Early Cretaceous, which we interpret as a far‐field response to a series of distant tectonic events including the Paleo‐Asian Ocean's closure, Qiangtang collision, Tethys‐deformation and Lhasa collision. During this period, these fault‐bounded tectonic units displayed different exhumation patterns, with the North Qilian Shan exhibiting a late Early Cretaceous rapid denudation while others preserved uplifted apatite partial annealing zones.

Li S., Yuan W., Zhao Z., Zhang A., Dong G., Li X., Sun W.

This study presents new fission track data from 40 apatite and 40 zircon samples in the Southern Altai Mountains (SAMs), revealing apatite fission track (AFT) ages of 110 ± 8 Ma to 54 ± 4 Ma and zircon fission track (ZFT) ages of 234 ± 24 Ma to 86 ± 7 Ma. The exhumation rates derived from three thermochronological methods range from 0.01 to 0.1 km/Ma (Age-Elevation method), 0.01 to 0.14 km/Ma (Half-Space thermal model), and 0.027 to 0.075 km/Ma (Age2exhume model). Thermal history modeling using HeFTy software reveals similar thermal histories on both sides of the Kangbutiebao Fault, with a notable cooling event and higher exhumation rates to the northeast. The Late Cretaceous (100–75 Ma) rapid cooling is associated with tectonic reactivation, likely linked to the collapse of the Mongol–Okhotsk Orogen and slab rollback in the southern Tethys Ocean. In the Late Cenozoic (10–0 Ma), cooling and uplift reflect the influence of tectonic stresses from the India–Eurasia collision, which also drove the reactivation of the Kangbutiebao Fault. These findings suggest a complex interplay of tectonic processes driving exhumation in the SAMs from the Late Jurassic to the Early Paleogene.

Khukhuudei U., Kusky T., Windley B.F., Otgonbayar O., Wang L., Nie J., Wenjiao X., Zhang L., Song X.

Xie Y., Wang F., Liu S.

Characterization of the spatial and temporal variability of stable isotopes in surface water is essential for interpreting hydrological processes. In this study, we collected the water samples of river water, groundwater, and reservoir water in the Burqin River Basin of the Altay Mountains, China in 2021, and characterized the oxygen and hydrogen isotope variations in different water bodies via instrumental analytics and modeling. Results showed significant seasonal variations in stable isotope ratios of oxygen and hydrogen (δ18O and δ2H, respectively) and significant differences in δ18O and δ2H among different water bodies. Higher δ18O and δ2H values were mainly found in river water, while groundwater and reservoir water had lower isotope ratios. River water and groundwater showed different δ18O–δ2H relationships with the local meteoric water line, implying that river water and groundwater are controlled by evaporative enrichment and multi-source recharge processes. The evaporative enrichment experienced by reservoir water was less significant and largely influenced by topography, recharge sources, local moisture cycling, and anthropogenic factors. Higher deuterium excess (d-excess) value of 14.34‰ for river water probably represented the isotopic signature of combined contributions from direct precipitation, snow and glacial meltwater, and groundwater recharge. The average annual d-excess values of groundwater (10.60‰) and reservoir water (11.49‰) were similar to the value of global precipitation (10.00‰). The findings contribute to understanding the hydroclimatic information reflected in the month-by-month variations in stable isotopes in different water bodies and provide a reference for the study of hydrological processes and climate change in the Altay Mountains, China.

Wang Y., Yin J., Thomson S.N., Chen W., Cai K., Dong Z., Tan F.

Based on a compilation of AFT, AHe ages and apatite MTLs from previous studies, the following conclusions can be made regarding the spatial and temporal distribution of exhumation in Altai-Sayan region.

Chu X., Shen P., Bai Y., Feng H., Luo Y., Li C.

Koktokay No. 3 pegmatite is a globally renowned rare metal deposit. Recently, rare metal-enriched leucogranite was identified within this deposit. The leucogranite is locally cut by No. 3 pegmatite and scattered as fragments among the pegmatite in some outcrops. In this study, we aimed to investigate the emplacement depth, thermal history, and evolution process of leucogranite based on quartz chemical composition and apatite thermochronology. Li, Al, P, Ge are the higher content elements in leucogranite quartz, and Ti is the lower content element (approximately 38.7, 178, 23.9, 2.7, and 2.0 ppm, respectively). The Al, Ti, and Li contents decreased from the core to the rim of the quartz. The leucogranite quartz compositions have a high Ge/Ti ratio, similar to that of the quartz in the Li-Cs-Ta pegmatites, particularly the Koktokay No. 3 pegmatite. This similarity indicates that the leucogranite was formed by a highly evolved melt. Using laser ablation-inductively coupled plasma-mass spectrometry, the U–Pb age of leucogranite apatite was determined as 189.4 ± 6.8 Ma, which is substantially younger than the previous columbite U–Pb age of No. 3 pegmatite, suggesting that the magmatic-thermal activity of No. 3 pegmatite had interfered with the apatite U–Pb closure system. The apatite low-temperature thermochronology results showed that the thermal history of leucogranite can be divided into two cooling stages: rapid cooling stage (180–155 Ma, 9.6–10.6 ℃/m.y.) and slow cooling stage (from 155 Ma to the present day, 0.8–1.0 ℃/m.y.). According to the thermal history, the exhumation and erosion of the Koktokay No. 3 rare-metal deposit is at least 5–6 km long. Leucogranite formation pressure is approximately 2.5 kbar (1.8–3.7 kbar), and the emplacement depth approximately 9.5 km (7–14 km).

Yang X., Tian X., Wan B., Yuan H., Zhao L., Xiao W.

AbstractCompressive stress generated at collision fronts can propagate over long distances, inducing deformation within the continent's interior. Nevertheless, the factors governing the partitioning of intracontinental deformation remain enigmatic. The Altai Mountains serve as a type‐example of ongoing intracontinental deformation. Here, we investigate the crustal architecture of the Chinese Altai Mountains, using receiver functions obtained from newly deployed dense seismic nodal arrays. The new seismic results reveal distinct crustal features, including (a) a negative polarity discontinuity beneath Chinese Altai Mountains, suggesting a low‐velocity layer; (b) a north‐dipping mid‐crustal structure beneath the suture zone between East Junggar and Chinese Altai, indicating underthrusting of East Junggar's lower crust beneath the Chinese Altai Mountains; (c) a double Moho structure beneath East Junggar, revealing a high‐velocity lower crustal layer. In conjunction with constraints from previous multi‐disciplinary regional studies, the double Moho structures are interpreted as mafic restite from Late Paleozoic magma underplating. The addition of mafic materials can significantly enhance the rheological strength of East Junggar's crust, causing it to function as an indenter that thrust beneath the Chinese Altai Mountains during the subsequent convergence process. As a consequence, significant deformation occurs in the Chinese Altai region, resulting in the emergence of decollements, as evident by the negative polarity discontinuity. The presence of pre‐existing decollements makes the Altai Mountains region more susceptible to deformation, thereby facilitating the concentration of intracontinental deformation. These findings illuminate the evolution history of the Chinese Altai Mountains and highlight the great impacts of ancient tectonics on intracontinental deformation partitioning.

Wang F., Luo M., He Z., Wang Y., Zheng B., Zhang Z., Hu X., Zhu W.

Abstract The Beishan orogen, a significant component of the southern Altaids, presents an opportunity for investigating the intracontinental deformation and exhumation history of the Altaids during the Mesozoic era. Although previous studies indicated that the Beishan orogen has experienced multiple reactivation since the late Mesozoic, the precise extent of these events remains poorly constrained. Here, we provide a comprehensive synthesis of field observations and apatite fission track (AFT) thermochronological dating throughout the Beishan orogen. Detailed field observations confirmed four major E-W trending thrusts in our study area. Based on the youngest truncated strata associated with the thrusts and previous dating results from neighboring regions, we propose that these thrust sheets likely developed in the late Middle Jurassic. AFT dating results from seven pre-Mesozoic granitoid samples and associated with thermal history modeling demonstrate that the Beishan orogen experienced a rapid basement cooling during the mid-Cretaceous (~115–80 Ma). Moreover, a compilation of previously published and newly gained AFT data reveals a comparable mid-Cretaceous cooling event in other parts of Central Asia, such as Qilian Shan, Eastern Tianshan, and Altai-Sayan. This observation suggests that the mid-Cretaceous cooling event is more likely to be regional rather than localized. This mid-Cretaceous cooling pulse is interpreted as a tectonic exhumation controlled by boundary faults and related to the rotation of the Junggar and Tarim basins. These processes are linked to distant plate-margin events along the Eurasian continent.