Mingxin Tao

Low slip rates and long-term preservation of geomorphic features in Central Asia.

In order to understand the dynamics of the India–Asia collision zone, it is important to know the strain distribution in Central Asia, whose determination relies on the slip rates for active faults. Many previous slip-rate estimates of faults in Central Asia were based on the assumption that offset landforms are younger than the Last Glacial Maximum (∼20 kyr ago). In contrast, here we present surface exposure ages of 40 to 170 kyr, obtained using cosmogenic nuclide dating, for a series of terraces near a thrust at the northern margin of the Tibetan Plateau. Combined with the tectonic offset, the ages imply a long-term slip rate of only about 0.35 mm yr-1 for the active thrust, an order of magnitude lower than rates obtained from the assumption that the terraces formed after the Last Glacial Maximum. Our data demonstrate that the preservation potential of geomorphic features in Central Asia is higher than commonly assumed.

Late Pleistocene/Holocene slip rate of the Zhangye thrust (Qilian Shan, China) and implications for the active growth of the northeastern Tibetan Plateau

luminescence dating, and 10 Be exposure dating. The seismically active Zhangye thrust transects late Pleistocene alluvial fan deposits and forms a prominent north facing scarp. The fault consists of two segments that differ in orientation, scarp height, and age. A series of loess-covered terraces records the uplift history of the western thrust segment. Loess accumulation on all terraces started at 8.5 ± 1.5 kyr and postdates terrace formation. Gravels from the

21Ne versus 10Be and 26Al exposure ages of fluvial terraces: the influence of crustal Ne in quartz

Abstract The accuracy of 21 Ne surface exposure ages depends critically on the correction for a trapped Ne component. Commonly, the amount of cosmogenic Ne used to calculate 21 Ne exposure ages is considered to be the Ne excess relative to a trapped component of atmospheric composition ( 21 Ne/ 20 Ne=0.00296). Here, we document a trapped Ne component in quartz samples from a series of river terraces at the northern margin of Tibet [Hetzel et al., Nature 417 (2002) 428–432], which has a non-atmospheric 21 Ne/ 20 Ne ratio varying from 0.00299 to 0.00398. Vacuum crushing of amalgamated samples, each derived from 30–80 quartz clasts, revealed that the non-atmospheric trapped component is present in fluid inclusions. It has probably been incorporated into the quartz crystals during their growth in veins at low-grade metamorphic conditions. Only if the amount of cosmogenic 21 Ne is determined relative to this trapped component are the calculated 21 Ne exposure ages consistent with the relative ages of the tectonically induced terraces and in agreement with independent 10 Be and 26 Al exposure ages of the fluvial terraces. A significant in situ production of nucleogenic 21 Ne by the reaction 18 O(α,n) 21 Ne in the Paleozoic quartz minerals is ruled out by extremely low U contents of the order of 1–5 ppb. All ages have been corrected for an inherited cosmogenic component that results from cosmic ray exposure during erosion of the host rock and transport of the clasts to the terraces. The origin of the trapped component from crustal fluids is supported by high 40 Ar/ 36 Ar ratios of 2000–6000 and slightly elevated 136 Xe/ 132 Xe ratios relative to air, which can be explained by radioactive decay of 40 K and spontaneous fission of U in the crust.

Tectonic and geochemical characteristics and reserved conditions of a mantle source gas accumulation zone in eastern China

Along both sides of the Tancheng-Lujiang Fracture Zone in eastern China, a series of mantle source gas pools constitute a massive-scale tectonic accumulation zone in NNE direction, with the mantle geochemical characteristics of high concentrations of C02 and He, high3He/4He-40Ar/36Ar ratio system and high δ13Coo2 ratios (the main frequency, -3.4%— 4.6%), showing no difference from the tectonic framework of the area. In the area, the tectonic environment is a rift formed as a result of diapiric mantle injection and crust thinning to form graben-type basins and lithospheric fractures. The mantle-derived volcanic rocks and inclusions are well-developed and a high geothermal zone (mantlesource) exists in the area. The characteristics of the three components (solid, liquid and gas) of mantle, concentrated all over the same tectonic space zone, show that the rift system is of a good tectonic environment or passage for mantle degassing and gas migration. The main types of the gas pools are volcano, fault-block, anticline, buried hill and so on, but most of them are combination traps closely related with fracture. For the mantle source gas pools, rift is an optimum tectonic region, and nearby lithospheric fracture, mantle source volcanic rocks or basement uplifts are a favourable structural location when reservoir-caprock association develops.

Secondary biogenic coalbed gas in some coal fields of China

The secondary biogenic coalbed gas, a new genetic and energy source type of coalbed gas in China, has been found in Xinji, Liyazhuang and Enhong areas. The essential characteristics of this type of gas are: (i) the major component of the gas is methane, with C1/C1–5 value higher than 0.99, indicating that the gas is part of dry gas; (ii) the δ13C1 value is in the range of −61.7‰ to −47.9‰, mostly lower than −55‰, which is much lower than the estimated δ13C1 value of thermogenic methane according to the thermal evolution degree of the coal rocks (with Ro value from 0.87% to 1.43%), showing the characteristics of the secondary biogenic gas; (iii) the δD value of methane ranges from −244‰ to −196‰; (iv) δ13C2 value ranges from −26.7‰ to −15.9‰ and δ13C3 value ranges from −10.8‰ to −25.3‰, indicating that the heavier hydrocarbons have a thermogenic origin; (v) the content of CO2 is very low, and δ13CCO2 value changes greatly, reflecting a characteristic of secondary change; (vi) δ15N2 value ranges mainly from −1‰ to + 1‰, indicating N2 derived significantly from air. The negative linear correlation between the contents of N2 and CH4 reflects the activity of bacteria bearing surface water infiltrating into coal beds. The comprehensive tracing indices show that the coalbed gas in the studied areas is the mixed gas of primarily secondary biogenic gas and a part of remnant thermogenic gas. The uplift of coal beds and the development of faults in the studied areas create favorable conditions for the formation of the secondary biogenic gas.

Evolution of the CBM reservoir-forming dynamic system with mixed secondary biogenic and thermogenic gases in the Huainan Coalfield, China

The secondary biogenic gas is an important original type of the coalbed methane (CBM) in China. Based on the analyses of sedimentary and burial history of the Permian coal-bearing strata, combined with thermal history and gas generation process of coals, the CBM reservoirforming dynamic system with mixed secondary biogenic and thermogenic gases in the Huainan Coalfield is subdivided into four evolutionary stages as follows: (i) shallowly-buried peat and early biogenic gas stage; (ii) deeply buried coal seams and thermogenic gas stage; (iii) exhumation of coal-bearing strata and adsorbed gas lost stage; and (iv) re-buried coal-bearing strata and secondary biogenic gas supplement stage. The Huainan CBM reservoir-forming model has the features of the basin-centered gas accumulation. The evolution of the reservoir-forming dynamic system proves that the thermogenic gas is not the main gas source for the Huainan CBM reservoir. Only the secondary biogenic gases as an additional source replenish into the coal bed after basin-uplift, erosional unroofing and subsequent scattering of thermogenic gases. Then this kind of mixed CBM reservoirs can be formed under suitable conditions.

Geochemistry on mantle-derived volatiles in natural gases from eastern China oil/gas provinces (I)——A novel helium resource——commercial accumulation of mantle-derived helium in the sedimentary crust

Commercial accumulation of mantle-derived helium in the sedimentary shell is discussed. Generally speaking, a commercial helium pool is formed by accumulated4He that comes from uranium and thorium via α-decay; therefore, it has a very low3He/4He value in the magnitude of 10-8. The helium concentration in some gas wells of eastern China oil/gas provinces is about or over 0.05% —0. 1%, consequently forming commercial helium wells (pools), such as the Wangjinta Gas Pool in Songliao Basin, Huangqiao Gas Pool in North Jiangsu Basin and some gas wells in Sanshui Basin. Studies have proved that when the3He/4He value of a helium gas pool is about 3.7 × 10-6 -7.2×10-6 namely mantle-derived helium in its total helium concentration accounts for 33.5%—65.4%, it is a crust-mantle dual-source or dominantly mantle-derived helium gas pool, which is a novel helium resource and its formation is mainly related to the distribution of megafractures.

Geochemistry on mantle-derived volatiles in natural gases from eastern China oil/gas provinces (I )——Helium, argon and hydrocarbons in mantle volatiles

Rescarches on helium, argon, carbon dioxide and methane are very significant in studies of mantle substance characteristics and mantle evolution. A < -shaped pattern of the isotope composition distribution of helium and argon sourced from the mantle and the crust, abundance distribution, isotopic composition and reservoir formation of carbon dioxide, and mantle-sourced methane are discussed.Rescarches on helium, argon, carbon dioxide and methane are very significant in studies of mantle substance characteristics and mantle evolution. A < -shaped pattern of the isotope composition distribution of helium and argon sourced from the mantle and the crust, abundance distribution, isotopic composition and reservoir formation of carbon dioxide, and mantle-sourced methane are discussed.