Maodu Yan

Late Cenozoic deformation and uplift of the NE Tibetan Plateau: Evidence from high-resolution magnetostratigraphy of the Guide Basin, Qinghai Province, China

The Cenozoic intramontane Gonghe–Guide Basin in Qinghai Province, China, is tectonically controlled by the sinistral strike-slip framework of the Kunlun and Altyn Tagh–South Qilian faults in the northeastern Tibetan Plateau. The basin is filled with thick Cenozoic clastic sedimentary formations, which provide important evidence of the deformation of this part of the plateau, although they have long lacked good age constraints. Detailed magnetostratigraphic and paleontologic investigations of five sections in the Guide Basin and their lithologic and sedimentary characteristics allow us to divide a formerly undifferentiated unit (the Guide Group) into six formations (where ages are now magnetostratigraphically well established, they are given in parentheses): the Amigang (1.8–2.6 Ma), Ganjia (2.6–3.6 Ma), and Herjia formations (3.6 to ca. 7.0–7.8 Ma), and the older Miocene Ashigong, Garang, and Guidemen formations. These rocks document a generally upward coarsening sequence, characterized by increasing accumulation rates. Increasing gravel content and sizes of its components, changes of bedding dips and source rock types, and marginal growth faults collectively reflect accelerated deformation and uplift of the NE Tibetan Plateau after 8 Ma, punctuated by a sharp increase in sedimentation rate at ca. 3.2 Ma that reflects the boulder conglomerates of the Ganjia formation. Interestingly, much of the vergence of the compressional deformation in the basin is to the south, accommodated by a sequence of six thrusts (F1–F6), which become active one by one progressively later toward the south, undoubtedly contributing to the uplift of this part of the plateau. F1 likely initiated the Guide Basin due to crustal flexure in the Oligocene, F2 was active in the early Miocene, F4 and F5 at ca. 3.6 Ma, and F6 was active in the early Pleistocene. The detailed late Miocene and younger magnetostratigraphy allows us to place much improved time constraints on the deformation and, hence, uplift of northeastern Tibet, which, when compared with ages for events on other parts of the plateau, provides important boundary conditions for the geodynamical evolution of Tibet.

Paleomagnetic dating of the Jiuquan Gravel in the Hexi Corridor: Implication on mid-Pleistocene uplift of the Qinghai-Tibetan Plateau

The sediments in the foreland basins around the Qinghai-Tibetan Plateau preserved crucial information to reveal its tectonic history. In the Hexi Corridor, north periphery of the Qinghai-Tibetan Plateau, the angular unconformity between the Jiuquan Gravel and the Yumen Conglomerate has been well known to represent an intensive tectonic event of the plateau. However, its age is poorly constrained. Our paleomagnetic dating at the Laojunmiao section in the Jiuxi Basin show that the bottom of the Jiuquan Gravel reaches 0.84 MaBP, the top of the Yumen Conglomerate is about 0.93 MaBP. This result clearly demonstrates that the northern Qinghai-Tibetan Plateau experienced an intensive movement at mid-Pleistocene.

Correction to “Northeastward growth and uplift of the Tibetan Plateau: Magnetostratigraphic insights from the Guide Basin”

Figure 7. Magnetostratigraphic jackknife analysis [from Tauxe and Gallet, 1991] for the studied sections. The plot indicates the relationship between average percent of polarity zones retained and the percentage of sampling sites deleted [Tauxe, 1998]. The slope of the line gives the magnetostratigraphic jackknife parameter J. See text for details. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B8, 2400, doi:10.1029/2003JB002624, 2003

After the Dust Settles: Why is the Blake Event Imperfectly Recorded in Chinese Loess?

The aeolian deposits in China are known to provide a continuous and fairly complete magnetostratigraphy in sections with alternating horizons of loess and paleosols that correlate with cold-dry and warm-humid periods, respectively. These deposits have the potential, therefore, to provide a precise timing of the Blake Event relative to Pleistocene climatic shifts. However, paleomagnetic studies in loess/paleosol deposits in China have shown that the Blake Event is imperfectly recorded, appearing in different stratigraphic positions or being completely absent. We sampled four loess/paleosol successions in the western Chinese Loess Plateau to investigate the inconsistent recording of the Blake Event. Disappointingly, the reverse Blake Event is completely masked by the dominantly normal Brunhes Chron in these four sections. Rock-magnetic results indicate the enhanced presence of superparamagnetic magnetite, suggesting that new magnetic grain growth occurred long after deposition also in the loess. We argue that this caused the remanent magnetization acquisition process to have been rather protracted throughout the interval of 130-100 ka. Together, paleomagnetic evidence and rock-magnetic observations are compatible with a model of long-delayed magnetization acquisition, similar to that which earlier had already been surmised from the misplaced (too old) position of the Matuyama-Brunhes boundary in loess layer L8. Delayed magnetization acquisition is most likely the result of pedogenesis in paleosols as well as loess, as suggested by the absence of marked contrasts between the normally-distinct loess and paleosol horizons in our studied sections. Similarly to widespread remagnetizations in orogenic belts, the enhanced presence of secondary SP magnetite in loess seems to be a fingerprint for protracted magnetization.

Magnetostratigraphy, fence diagrams and basin analysis

Abstract Correlation of lithostratigraphic sections is widely used to examine the nature of lateral facies changes within or between basins. It can provide significant clues for regional environmental and palaeogeographic reconstructions. There are problems associated with lithostratigraphic correlation; diachronous deposition of similar lithological units may not be recognized. We report here an attempt to determine lateral facies changes in coeval sedimentary sections, through three-dimensional magnetostratigraphic correlations in the Guide Basin, an intramontane basin in the northeastern part of the Tibetan Plateau. The method is successful for correlating lateral facies and helps to identify sediment sources in the basin.