Improved Geometric Model of Extensional Fault–bend Folding

Extensional fault–bend folds, also called rollovers, are one of the most common structures in extensional settings. Numerous studies have shown that oblique simple shear is the most appropriate mechanism for quantitative modeling of geometric relations between normal faults and the strata in their hanging walls. However, the oblique simple shear has a rather serious issue derived from the shear direction, particularly above convex bends. We use geometric and experimental methods to study the deformation of extensional fault–bend folds on convex bends. The results indicate that whether the fault bends are concave or convex, the shear direction of the hanging wall dips toward the main fault. On this basis, we improve the previous geometric model by changing the shear direction above the convex bends. To illustrate basin history, our model highlights the importance of the outer limit of folding instead of the growth axial. Moreover, we propose a new expression for the expansion index that is applicable to the condition of no deposition on the footwall. This model is validated by modeling a natural structure of the East China Sea Basin.     

Advances and Overview of the Study on Paleo-earthquake Events: A Review of Seismites

The distribution and formation mechanisms of typical identified seismites are analyzed based on various factors from plate tectonic positions, types of sedimentary basins and properties of seismogenic faults to focal mechanisms. Especially, structural styles, reserved positions, activity times, formation mechanisms and dynamics of soft-sediment deformation structures triggered by seismic activity are systematically analyzed. According to the genetic types of seismites, we propose 5 categories, including liquefied deformation, thixotropic deformation, hydroplastic deformation, superimposed gravity driving deformation and brittle deformation. Further, based on the main genetic types, composition of sediments and deformation styles, we draw up 35 secondary classifications. To determine paleo-seismic sequences in different times, activities of seismogenic faults, high-resolution tectonic events in one main tectonic movement and paleo-tectonic settings, and to understand the inducing mechanisms of paleo-earthquakes and ecologic environment evolution, researching on seismites are of great significance. Combing multiple approaches to identify the paleoearthquake records, simulating experiments on various soft-sediment deformation structures triggered by different magnitudes of shocking, dating precisely on paleo-seismic events, impacting on paleogeography and biological environment and on energy and resources domain are the frontiers of paleoseismic research.     

The impact of sulfate reduction on the distribution of methylmercury in reservoirs in the city of Guiyang

In this paper, the relationship between sulfate reduction potential and mercury methylation potential was studied in the Aha, Baihua and Hongfeng reservoirs from Guiyang City. The methylmercury (MeHg) concentrations of lake water in the Aha Reservoir were greatly elevated as compared to those of the Hongfeng and Baihua reservoirs, which was correlated with its distinctly high SRB abundance, SO42-, and S2- concentrations. Among the three reservoirs, however, the highest MeHg was observed in in the top several centimenters of pore water profile in the Hongfeng Reservoir where the lowest S2- in pore water occurred. The distributions of MeHg in lake water and pore water showed the highest methylation potential occurred at water-sediment surface for the Aha Reservoir and the in the top several centimenters of sediments for the Hongfeng Reservoir. It is guessed that the highest mercury methylation only occurs at the sites with certain sulfide concentrations.     

Iron Isotopic Fractionation and Origin of Chromitites in the Paleo-Moho Transition Zone of the Kop Ophiolite, NE Turkey

The Kop ophiolite in NE Turkey is a fragment of Neo-Tethyan forearc.It can be mainly divided into a paleo-Moho transition zone(MTZ)in the North and a harzburgitic mantle sequence in the South.Dunites are predominant in the MTZ of the Kop ophiolite,and they are locally interlayered with chromitites and enclose minor bodies of harzburgites near the petrological Moho boundary.Large Fe isotopic variations were observed for magnesiochromite(-0.14‰to 0.06‰)and olivine(-0.12‰to 0.14‰)from the MTZ chromitites,dunites and harzburgites.In individual dunite samples,magnesiochromite usually has lighter Fe isotopic compositions than olivine,which was probably caused by subsolidus Mg-Fe exchange between the two mineral phases.Both magnesiochromite and olivine display an increasing trend ofδ56Fe along a profile from chromitite todunite.This trend reflects continuous fractional crystallization in a magma chamber,which resulted in heavier Fe isotopes concentrated in the evolved magmas.In each cumulative cycle of chromitite and dunite,dunite was formed from relatively evolved melts after massive precipitation of magnesiochromite.Mixing of more primitive and evolved melts in the magma chamber was a potential mechanism for triggering the crystallization of magnesiochromite,generating chromitite layers in the cumulate pile.Before mixing happened,the primitive melts had reacted with mantle harzburgites during their ascendance;whereas the evolved melts may lie on the olivine-chromite cotectic near the liquidus field of pyroxene.Variable degrees of magma mixing and differentiation are expected to generate melts with differentδ56Fe values,accounting for the Fe isotopic variations of the Kop MTZ.     

Intensification of the East Asian Monsoon in Southern China at about 300–400 kaBP Inferred from Eolian Deposits in the Middle-lower Reaches of the Yangtze River

In the East Asian monsoon region,eolian deposits widely distributed in the middle-lower reaches of the Yantgze River are among the best materials available for studies on Quaternary climate change in the subtropical zone of Southern China.Typical eolian deposits in this region include upper Xiashu Loess(XL) and underlying Vermiculated Red Soil(VRS) layers.In this paper,chronological and paleoclimatic studies are conducted on an eolian deposit sequence near Jiujiang(JJ) city in northern Jiangxi province.A magnetostratigraphic study,combined with optically stimulated luminescence(OSL) dating,is conducted on the JJ section and provides further evidence that eolian deposits in the middle-lower reaches of the Yangtze River have been formed since the late Early Pleistocene,and that the boundary age between the XL and VRS layers is about 300-400 kaBP.In grain-size records of the JJ section,the median grain-size and content of the 30 μm size fraction increase sharply after 300-400 kaBP,representing an East Asian winter monsoon intensification event.Further pollen analysis reveals differing pollen assemblages before and after 300-400 kaBP:there is an evident increase in plants adapted to grow in a warm humid environment after 300-400 kaBP,implying an increase in precipitation caused by intensification of the East Asian summer monsoon.Global ice volume and uplift of the Tibet Plateau(TP) are regarded as crucial factors influencing variations of the East Asian monsoon on a long-term scale.The deep-sea δ~(18)O record,which reflects variations in global ice volume,shows no obvious change after 300-400 kaBP.Moreover,the influence of global ice volume changes on the East Asian summer and winter monsoons is inverse;the global ice volume increase(decrease) implies a strengthened(weakened) winter monsoon and weakened(strengthened) summer monsoon.We therefore interpret the coupled intensifications of the East Asian summer and winter monsoons at about 300-400 kaBP to the uplift of the TP in the Middle Pleistocene.This climate event is also documented in eolian deposits from the southern margin of the Chinese Loess Plateau(CLP) and from the desert-loess transitional belt.However,it is not recorded in the loess-paleosol sequences from the central part of the CLP,thereby indicating differing climate responses to TP uplift in different regions,which requires further study.     

Permian High Ba-Sr Granitoids: Geochemistry, Age and Tectonic Implications of Erlangshan Pluton, Urad Zhongqi, Inner Mongolia

Abstract: Erlangshan Pluton from Urad Zhongqi, central Inner Mongolia, is located in the middle segment of the northern margin of the North China Plate. The rocks consist mainly of diorites with gneissic structure. Petrochemical characteristics reveal that the diorites belong to metaluminous, high-potassium calc-alkaline series, with chemical signatures of I-type granites. They are characterized by low SiO2 contents (56.63%–58.53%) and A/CNK (0.90–0.96), high Al2O3 contents (17.30%–17.96%) and Na2O/K2O ratios (1.20–1.70), enrichment in large ion lithophile elements (LILE, e.g., Ba=556–915 ppm, Sr=463–595 ppm), and relative depletion in high field strength elements (HFSE, e.g., Nb, Ta, Ti) in primitive mantle-normalized spidergram, and right-declined rare earth element patterns with slightly negative Eu anomalies (d?Eu=0.72–0.90). They have Sr/Y ratios (20–25) evidently less than Kebu Pluton (49–75) to its east. Sensitive high resolution ion micro-probe U-Pb zircon dating of the diorites has yielded an intrusive age of 270±8?Ma. This leads us to conclude that Erlangshan diorites were formed by mixing between the middle or lower crustal-derived magma and minor mantle-derived mafic magma, followed by fractional crystallization, which was trigged by crustal extension and fault activity in post-collisional setting.     

Geochemical Characteristics of Wuyang Siliceous Rocks in the Southern Margin of North China Craton and its Constraint on the Formation Environment of BIF of Tieshanmiao Formation

Precambrian banded iron formation(BIF) is one of the most important mineral resources in China, mostly abundant in the North China Craton(NCC) with relatively less common in South China. Since the BIF and siliceous rocks both originated from chemical deposition, the syngenetic BIF and Siliceous rocks can help evaluate their environment of formation. We examine here the mineralogy and geochemistry of siliceous rocks associated with the Tieshanmiao Formation BIF, aiming to decipher the conditions of formation of both BIF and Siliceous rocks in the Wuyang area in the NCC. Analysis of the geochemical characteristics of whole rock shows that the Si O2 content of the siliceous rock ranges from 90.11% to 94.85% and is relatively high overall. Trace element contents of Ba and U are also high, the Ba/Sr ratio ranges from 3.89 to 25.28 and the U/Th ratio ranges from 0.09 to 0.20. Finally, the ΣREE value of rare earth elements ranges from 57.03 ppm to 152.59 ppm, and these indexes all indicate that siliceous rock resulted from hydrothermal deposition. Plots of Al2 O3-Si O2, Si O2/(K2 O+Na2 O)-Mn O2/Ti O2 and Mn-10×(Cu+Co+Ni)-Fe in discrimination diagrams also verify this interpretation. However, both the Mg O content, ranging from 0.16 to 0.32, and the Fe/Ti ratio, ranging from 2.50 to 9.72, suggest that terrigenous material was added during the depositional process. Major and trace element parameters of siliceous rock, such as the Al/(A1+Fe+Mn) ratio(from 0.81 to 0.93), Mn O/Ti O2(from 0.00 to 0.17), Al/(Al+Fe)(from 0.82 to 0.93), Sc/Th ratio(from 0.21 to 0.50), U/Th(from 0.09 to 0.20),(La/Yb)N(from 0.83 to 3.04), and the(La/Ce)N(from 0.01 to 0.02) all imply that the siliceous rock formed in a continental margin. In addition, the Sr/Ba ratio from 0.08 to 0.26, the δCe value from 0.31 to 0.90, and the δEu value from 0.14 to 0.58, all indicate that the siliceous rock was formed at a relatively deeper water depth and under weak hydrodynamic conditions. Siliceous rock and BIF formed in the same geological setting, with the Si O2/(K2 O+Na2 O) ratio of siliceous rock ranging from 28.61 to 47.43, the Si O2/Al2 O3 ratio from 16.53 to 32.37, and the Si O2/Mg O ratio from 287.28 to 592.81, which are all in agreement with chemical deposition associated with volcanic eruptions. The Al2 O3/Ti O2 ratio from 37.82 to 50.30 indicates that the magma source of siliceous rock was of slightly intermediate composition. During the Late Archean in the Wuyang area, the high concentration and high purity Si O2 quickly precipitated from hydrothermal fluids to finally result in the accumulation of siliceous rock in a marginal sea, while the input corresponding to iron formation components was deposited to form iron formation layers, and limestone was only the product formed during the deposition intervals of siliceous rock and iron formations. In this study, the synsedimentary siliceous rocks of BIF act as a new way to provide direct evidence to understand the formation environment of BIF due to its high geochemical stability.     

An Evaluation of Phase Behaviors of the Oil-Gas-Water System in the Displacement of Crude Oil with CO2

CO₂ dissolution into an aqueous phase and water evaporation into a gaseous phase takes place during CO₂ injection into an oil reservoir. This study aims to evaluate the phase behaviors of the oil-gas-water system in the displacement of crude oil by CO₂. The composition of the JL oilfield in the northeast of China is taken as an example. The flash calculation of the oil-gas-water system was performed, based on the method presented by Li and Nghiem. The research re...