Hongshan Gao

Rates and kinematics of active shortening along the eastern Qilian Shan, China, inferred from deformed fluvial terraces

In the eastern Qilian Shan, a flight of fluvial terraces developed along the Jinta River valley are deformed across the Nanying anticline. Four individual fluvial terraces are preserved at different elevations above the river, and higher terrace treads are draped by systematically thicker aeolian loess. Optically stimulated luminescence dating of deposits at the base of the loess provides constraints on the timing of surface abandonment; terraces were abandoned at 69 +/- 4 ka B.P. (T4), 57 +/- 4 ka B.P. (T3), and between 34 +/- 3 ka B.P. (T2), respectively. Differential GPS measurement of the terrace profile across the anticline allows reconstruction of subsurface fault geometry; we model terrace deformation above a listric thrust fault with a tip line at 2.2 +/- 0.1 km depth and whose dip shallows systematically to 23 +/- 3 degrees at depth of 5.8 +/- 1.1 km. Combining terrace ages with this model of fault geometry, we estimate a shortening rate of 0.8 +/- 0.2 mm/a across the Nanying fold and a shortening rate of similar to 0.1 mm/a across the mountain front fault since similar to 70 ka B.P. This rate suggests that the frontal fault system along the eastern Qilian Shan accomplishes crustal shortening at rates of approximately 0.9 +/- 0.3 mm/a during late Pleistocene time.

Planation surface extraction and quantitative analysis based on high-resolution digital elevation models

Lanzhou Univ, Minist Educ, Key Lab Western Chinas Environm Syst, Lanzhou 730000, Peoples R China

Fluvial terraces and their implications for Weihe River valley evolution in the Sanyangchuan Basin

Fluvial terraces are important geomorphic markers for modern valley development. When coupled with numeric ages, terraces can provide abundant information about tectonic, climatic, paleohydrological and the paleoenvironmental changes. On the basis of the paleomagnetic, electron spin resonance (ESR) and optically stimulated luminescence (OSL) dating, in addition to an investigation of local loess-paleosol sequences, we confirmed that 13 fluvial terraces were formed, and then preserved, along the course of the Upper Weihe River in the Sanyangchuan Basin over the past 1.2 Ma. Analyses of the characteristics and genesis of these terraces indicate that they resulted from the response of this particular river system to climate change over an orbital scale. These changes can further be placed within the context of local and regional tectonic uplift, and represent an alternation between lateral migration and vertical incision, dependent upon the predominance of climatic and tectonic controls during different periods. Most of the terraces are strikingly similar in that they have several meters of paleosols which have developed directly on top of fluvial deposits located on the terrace treads, suggesting that the abandonment of terraces due to river incision occurred during the transitions from glacial to interglacial climates. The temporal and spatial differences in the distribution patterns of terraces located on either side of the river valley indicate that a tectonic inversion occurred in Sanyangchuan Basin at ~0.62 Ma, and that this was characterized by a transition from overall uplift to depression induced by fault activity. Synthesized studies of the Basin’s terraces indicate that formation of the modern valley of the Upper Weihe River may have begun in the late Early Pleistocene between 1.4–1.2 Ma.

Identifying the degree of luminescence signal bleaching in fluvial sediments from the Inner Mongolian reaches of the Yellow River

Abstract The partial bleaching of the luminescence signal prior to deposition results in age overestimation, and can be a problem in delineating fluvial evolution within an OSL chronological framework. The Inner Mongolian reaches of the Yellow River are characterised by a high sediment load and complex sources of sediments. To test the incomplete bleaching occurring in this type of environment, the residual doses and the luminescence signal characteristics of different particle size fractions from 14 modern fluvial sediment samples were investigated. Furthermore, 26 OSL ages derived from drilling cores were compared with 11 radiocarbon ages. Our results show that the residual equivalent doses principally range between 0.16 and 0.49 Gy for silt grains, and between 0.35 and 3.72 Gy for sand grains of modern samples. This suggests that medium-grained quartz has been well bleached prior to deposition, and is preferable to coarse-grained quartz when dating fluvial sediments in this region. The results also show that the De values of coarse-grained fractions display a stronger correlation with distance downstream. In addition, a comparison of OSL and radiocarbon ages from drilling cores establishes further confidence that any initial bleaching of these sediments was sufficient. As a result, we believe that the studied fluvial samples were well bleached prior to deposition.

Quantitative analysis of planation surfaces of the upper Yangtze River in the Sichuan-Yunnan Region, Southwest China

Identification of the planation surfaces (PSs) is key for utilizing them as a reference in studying the long-term geomorphological evolution of the Upper Yangtze River Basin in the Sichuan-Yunnan region, Southwest China. Using a combined method of DEM-based fuzzy logic and topographic and river profiles analysis and based on a comprehensive analysis of four morphometric parameters: slope, curvature, terrain ruggedness index, and relative height, we established the relevant fuzzy membership functions, and then calculated the membership degree (MD) of the study area. Results show that patches with a MD >80% and an area >0.4 km2 correspond well to the results of Google Earth and field investigation, representing the PS remnants. They consist of 1764 patches with an altitude, area, mean slope, and relief of mostly 2000–2500 m above sea level (asl), 0–10 km2, 4°–9°, 0–500 m, respectively, covering 9.2% of the study area’s landscape, dipping to southeast, decreasing progressively from northwest to southeast in altitude, and with no clear relation between each patch’s altitude and slope, or relief. All these results indicate that they are remnants of once regionally continuous PSs which were deformed by both the lower crust flow and the faults in upper crust, and dissected by the network of Upper Yangtze River. Additionally, topographic and river profiles analysis show that three PSs (PS1–PS3) well developed along the main valleys in the Yongren-Huili region, indicating several phases of uplift then planation during the Late Cenozoic era. Based on the incision amount deduced from projection of relict river profiles on PSs, together with erosion rates, breakup times of the PS1, PS2, and PS3 were estimated to be 3.47 Ma, 2.19 Ma, and 1.45 Ma, respectively, indicating appearance of modern Upper Yangtze River valley started between the Pliocene to early Pleistocene.

A warm-return event during the transition of last interglacial-glacial cycle

Lanzhou Univ, Minist Educ, Key Lab Western Chinas Environm Syst, Lanzhou 730000, Peoples R China