Research

Cenozoic mountain building in eastern China and its correlation with reorganization of the Asian climate regime

The Cenozoic Asian climate system experienced a transformation from a zonal pattern to a monsoon-dominant pattern around the Paleogene-Neogene boundary. A series of dynamic mechanisms, such as uplift of the Tibetan Plateau, retreat of the Paratethys Sea, expansion of the South China Sea, and decreasing atmospheric CO2 content, has been suggested to be responsible for the transformation of the Asian climate pattern. However, the role of topo- graphic growth in eastern China has been rarely considered. As the natural divides of geogra- phy, climate, and biology, the two most distinct sets of topographic relief in eastern China, the Qinling and Taihang Mountains, play an important role in shaping the Asian climate pattern. We report low-temperature thermochronology data from the Qinling and Taihang Mountains and use age-elevation relationships and thermal history modeling to show that both mountain ranges experienced a phase of rapid exhumation during the late Oligocene and early Miocene. The building of the Qinling and Taihang Mountains around the Oligocene-Miocene bound- ary temporally and spatially coincided with the reorganization of the Cenozoic Asian climate regime, suggesting that the mountain building in eastern China acted as a possible driving mechanism for the alleged reorganization of the Cenozoic Asian climate regime.

East Tacheng (Qoqek) Fault Zone: Late Quaternary Tectonics and Slip Rate of a Left‐Lateral Strike‐Slip Fault Zone North of the Tian Shan

Although the Tacheng Basin may play an important role in accommodating crustal shortening north of the Tian Shan and west of the Junggar Basin based on observations of several conspicuous surface ruptures in satellite imagery, little is known about its late Quaternary tectonic activity. We analyze the tectonic geomorphology of the East Tacheng fault zone, including the subparallel NE-SW trending East Tacheng and Wuerkashier faults, using field observations, interpretations of satellite imagery, and construction of decimeter-scale digital elevation models from unmanned aerial vehicle surveys. Geomorphological features such as displaced stream channels and terrace risers, negligible vertical offset, opposite facing directions of a single fault scarp, and linear fault traces suggest that the East Tacheng fault is a nearly pure left-lateral subvertical strike-slip fault. The late Quaternary horizontal slip rate is estimated to be 0.4–2.0 mm/year, likely ∼0.7 mm/year. Paleoearthquake studies in trenches reveal that the most recent earthquake occurred between 3.1 and 0.5 ka. The Wuerkashier fault, which shows pristine surface ruptures along a total length of ∼67 km, is also a nearly pure left-lateral strike-slip fault. The most recent earthquake along this fault is dated to be 7.9–0.5 ka. Combining GPS velocity and active fault kinematics in the northern Tian Shan and western Junggar, we suggest that the western Junggar is controlled by regional N-S crustal shortening related to the India-Eurasia collision, and the reactivated Paleozoic fault zones in the western Junggar play a key role in transferring deformation northeastward into the Altai and Mongolia.

Miocene Range Growth Along the Altyn Tagh Fault: Insights From Apatite Fission Track and (U‐Th)/He Thermochronometry in the Western Danghenan Shan, China

The left‐lateral strike‐slip Altyn Tagh fault that defines the northern margin of the Tibetan Plateau plays a crucial role in accommodating the Cenozoic deformation related to the growth of plateau. However, the slip history along the fault remains highly debated. Here we report new 14–16 Ma apatite fission track (AFT) and 9–11 Ma apatite (U‐Th)/He (AHe) data in the western Danghenan Shan, north Tibet. Age‐elevation relationships and AFT/AHe age differences suggest a period of rapid exhumation with an average rate of 0.1–0.3 km/Ma from 16 to 9 Ma for this area. Thermal history modeling indicates that this was preceded by accelerated exhumation between the late Oligocene and middle Miocene (~15 Ma). A northward increase in AFT ages and asymmetric topography across the western Danghenan Shan indicate that the uplift and exhumation are mainly controlled by the thrust fault along the southern flank of the western Danghenan Shan. As the thrust fault is a branch of the Altyn Tagh fault, the rapid exhumation probably represents onset of the ransition along the Altyn Tagh fault from left‐slip motion to crustal shortening in the Dangnenan Shan region. Our findings show that the middle Miocene deformation is not only recorded in the middle and northern Qilian Shan but also in the southwestern portion of the Qilian Shan, which favors a synchronous middle‐Miocene deformation model for the entire Qilian Shan.

Mid-Miocene uplift of the northern Qilian Shan as a result of the northward growth of the northern Tibetan Plateau

The northern Tibetan Plateau, north of the Qaidam Basin and south of the Hexi Corridor (China), consists of a series of WNW- to NW-trending elongated mountain ranges. Deciphering the time-space deformation pattern of these ranges is central to understanding the mechanism of plateau formation and to the controversial issue of whether Tibet has undergone progressive northward growth or synchronous growth since the India-Eurasia collision. Here, we report new constraints on the timing of accelerated uplift of the Tuolai Shan, one of the elongated mountain ranges in the northern Tibetan Plateau. New apatite fission-track data from an elevation transect in the Tuolai Shan provide a definitive tie to rapid cooling that began at 17–15 Ma. We attribute this rapid cooling to accelerated exhumation resulting from thrusting in the hanging wall of the Haiyuan fault in response to progressive northward growth of the plateau. Combining these fission-track data and the published geologic, sedimentological, and thermochronologic data from the northern Qilian Shan and Hexi Corridor, we propose a progressively north-northeastward growth model for the northernmost part of Tibet, suggesting that deformation in the inner Qilian Shan occurred synchronously in the middle Miocene, and subsequently, increasingly further north.

Neogene Expansion of the Qilian Shan, North Tibet: Implications for the Dynamic Evolution of the Tibetan Plateau

The Qilian Shan, at the northeastern frontier of the Tibetan Plateau, is a key area for studying the expansion mechanism of the Tibetan Plateau. Although previous hermochronology and paleomagnetic studies indicate Neogene northward expansion of the northern Qilian Shan, there is a distinct temporal gap in knowledge relative to the tectonic history of the southern Qilian Shan. This has hindered a complete understanding of the Cenozoic deformation pattern of the entire Qilian Shan. To study the growth history of the southern Qilian Shan, apatite fission track (AFT) data have been acquired from Zongwulong Shan and the Huaitoutala section. AFT thermal history modeling from the former shows a rapid cooling episode occurred at ~18–11 Ma, which is interpreted as marking the onset of intensive exhumation in the southern Qilian Shan. Within the uaitoutala section, detrital grain up‐section shows progressively decreasing peak AFT ages followed by an age increase from midsection, implying that a sediment‐recycling event occurred at approximately 7 ± 2 Ma. Together with a shift in paleocurrent directions, this change marks the onset of Late Miocene deformation of the northern Qaidam Basin. Combined with previous studies on the deformation time of the Qilian Shan, our findings suggest that both the northern and southern Qilian Shan region grew outward synchronously in opposite directions during the Neogene. This resulted in the formation of a flower structure, which had an important impact on the deformation pattern of north Tibet. The synchronous utward expansion may have been triggered by the removal of mantle beneath north Tibet.

Constraints of new apatite fission-track ages on the tectonic pattern and geomorphic development of the northern margin of the Tibetan Plateau

The northern margin of the Qilian Shan is the northernmost edge of the Tibetan Plateau. The deformation timing and geomorphic development along the northern margin of the Qilian Shan are critical to understanding the dynamics of plateau growth. Although previous studies have suggested that thrusting along the middle and westernmost sections of the northern margin of the Qilian Shan began in the middle Miocene, the>600 km mountain range front may have experienced differential thrusting and exhumation history. We collected 11 apatite fission-track samples along an elevation transect to the south of Jiayuguan to unravel the cooling history and erosion variation of the northern margin of western section of the Qilian Shan. The apatite fission-track ages, fission-track lengths, and thermal modeling suggest a period of rapid exhumation initiated at ∼15–10 Ma along the northern margin of the Qilian Shan. The new fission-track ages can be divided into three domains with different mean ages. As the distribution of the fission-track ages is tightly confined by tectonic structures among them, we infer that tectonics plays the first-order control on erosion variations. Meanwhile, the strong linkage between erosion rates and glacier distribution may indicate that glacial buzzsaw exerts an important role in mountain building in the northern Qilian Shan. Additionally, combining this result with published data, we suggest that the northern margin of the Qilian Shan experienced a phase of synchronous deformation in the middle Miocene.

Kinematics of late Quaternary slip along the Yabrai fault: Implications for Cenozoic tectonics across the Gobi Alashan block, China

The Yabrai range-front fault accommodates deformation within the middle Gobi Alashan block between the Tibetan Plateau and the Ordos block. As such, it provides the pportunity to examine the transition between contractional deformation associated with the growth of the Tibetan Plateau and extensional deformation across North China. Geomorphic mapping of the active fault trace and trench investigations reveal that the Yabrai range-front fault is composed of three segments of varying fault strike, but for which the sense of motion, scarp height, and slip history appear to be kinematically compatible along the fault. Displaced Holocene and late Pleistocene alluvial deposits indicate that the southwestern segment is characterized by oblique-normal displacement with a minor sinistral component, whereas the middle segment appears to exhibit nearly dip-slip normal displacement. In contrast, slip along the northeastern segment appears to be primarily sinistral strike-slip with a minor reverse component. Geomorphically fresh fault scarps are developed within late Pleistocene–Holocene alluvial fans and terraces along the southwestern and northeastern segments, whereas the middle segment of the fault defines the bedrock-alluvial contact along the range front. The 10Be exposure ages of displaced alluvial fans along the southwestern segment yield a throw rate of ~0.1 mm/yr over late leistocene time. Lateral slip rates along the northeastern fault segment range between 0.23 ± 0.02 and 0.78 ± 0.12 mm/yr. Regionally, the orientation and sense of motion along the Yabrai range-front fault are consistent with NE-SW shortening, and we suggest that recent activity along this fault system reflects incipient deformation of the foreland at the ortheastern margin of the Tibetan Plateau.

Late Quaternary strike-slip along the Taohuala Shan-Ayouqi fault zone and its tectonic implications in the Hexi Corridor and the southern Gobi Alashan, China

The Hexi Corridor and the southern Gobi Alashan are composed of discontinuous a set of active faults with various strikes and slip motions that are located to the north of the northern Tibetan Plateau. Despite growing understanding of the geometry and kinematics of these active faults, the late Quaternary deformation pattern in the Hexi Corridor and the southern Gobi Alashan remains controversial. The active E-W trending Taohuala Shan-Ayouqi fault zone is located in the southern Gobi Alashan. Study of the geometry and nature of slip along this fault zone holds crucial value for better understanding the regional deformation pattern. Field investigations combined with high-resolution imagery show that the Taohuala Shan fault and the E-W trending faults within the Ayouqi fault zone (F2 and F5) are left-lateral strike-slip faults, whereas the NW or WNW-trending faults within the Ayouqi fault zone (F1 and F3) are reverse faults. We collected Optically Stimulated Luminescence (OSL) and cosmogenic exposure age dating samples from offset alluvial fan surfaces, and estimated a vertical slip rate of 0.1–0.3 mm/yr, and a strike-slip rate of 0.14–0.93 mm/yr for the Taohuala Shan fault. Strata revealed in a trench excavated across the major fault (F5) in the Ayouqi fault zone and OSL dating results indicate that the most recent earthquake occurred between ca. 11.05 ± 0.52 ka and ca. 4.06 ± 0.29 ka. The geometry and kinematics of the Taohuala Shan-Ayouqi fault zone enable us to build a deformation pattern for the entire Hexi Corridor and the southern Gobi Alashan, which suggest that this region experiences northeastward oblique extrusion of the northern Tibetan Plateau. These left-lateral strike-slip faults in the region are driven by oblique compression but not associated with the northeastward extension of the Altyn Tagh fault.