Side-stepping of axial surface traces in superposed folding

During the refolding of an early non-isoclinal fold (say,F ₁) we may find an offset or side-stepping of the axial surfaces of the later folds (say,F ₂). The offsets can be seen in both type 2 and type 3 interference patterns. An analysis of the shear fold model shows that there is a maximum limit for the magnitude of side-stepping. The side-stepping is larger for larger interlimb angles ofF ₁. It decreases with progressive tightening ofF ₂. By recognizing such side-stepping we can predict on which side the F₁ hinge should lie even if the hinge is unexposed or lies outside the domain of observation. The general rule for the sense of side-stepping is the same for shear folds, flexural slip folds and buckling folds. However, the side-stepping in buckling folds should be used with caution, sinceF ₂ folds on buckled single-layers may show an offset whose sense is opposite to that predicted by the general rule.     

The effect of fault-bend folding on seismic velocity in the marginal ridge of accretionary prisms

Fluid venting in accretionary prisms, which feeds chemosynthetic biological communities, occurs mostly on the marginal thrust ridge. New seismic data for the marginal ridge of the Cascadia prism show significantly lower velocity than that in the adjacent oceanic basin and place important constraints on the interpretations of why fluid venting occurs mostly on the marginal ridge. We employed a finite-element method to analyze a typical fault-bend folding model to explain the phenomenon. The fault in the model is simulated by contact elements. The elements are characterized not only by finite sliding along a slide line, but also by elastoplastic deformation.We present the results of a stress analysis which show that the marginal ridge is under subhorizontal extension and the frontal thrust is under compression. This state of stress favors the growth of tensile cracks in the marginal ridge, facilitates fluid flow and reduces seismic velocities therein; on the other hand, it may close fluid pathways along the frontal thrust and divert fluid flow to the marginal ridge.On leave from Peking University, Beijing, 100871, China     

Chhotanagpur granite gneiss in relation to the schistose rocks around Murhu,Ranchi district,Bihar: a structural approach

Structural analysis of the Chhotanagpur gneiss and the adjoining schistose rocks of the Singhbhum Group indicates perfect conformity in their structures on macroscopic, mesoscopic and microscopic scales. This precludes the possibility of the gneissic rocks having intruded into the deformed and metamorphozed schistose rocks. The observed features can be best explained by considering the gneissic rocks as the basement and the schistose rocks as the cover, both deformed and metamorphozed together. However, this does not exclude the possibility of the gneissic rocks being reactivated and intruding elsewhere.     

Satellite-based Observational Study of the Tibetan Plateau Vortex:Features of Deep Convective Cloud Tops

In this study, an east-moving Tibetan Plateau vortex (TPV) is analyzed by using the ERA-5 reanalysis and multi-source satellite data, including FengYun-2E, Aqua/MODIS and CALIPSO. The objective is to demonstrate: (i) the usefulness of multi-spectral satellite observations in understanding the evolution of a TPV and the associated rainfall, and (ii) the potential significance of cloud-top quantitative information in improving Southwest China weather forecasts. Results in this study show that the heavy rainfall is caused by the coupling of an east-moving TPV and some low-level weather systems [a Plateau shear line and a Southwest Vortex (SWV)], wherein the TPV is a key component. During the TPV's life cycle, the rainfall and vortex intensity maintain a significant positive correlation with the convective cloud-top fraction and height within a 2.5° radius away from its center. Moreover, its growth is found to be quite sensitive to the cloud phases and particle sizes. In the mature stage when the TPV is coupled with an SWV, an increase of small ice crystal particles and appearance of ring- and U/V-shaped cold cloud-top structures can be seen as the signature of a stronger convection and rainfall enhancement within the TPV. A tropopause folding caused by ageostrophic flows at the upper level may be a key factor in the formation of ring-shaped and U/V-shaped cloud-top structures. Based on these results, we believe that the supplementary quantitative information of an east-moving TPV cloud top collected by multi-spectral satellite observations could help to improve Southwest China short-range/nowcasting weather forecasts.     

Tectonic geomorphology and hydrocarbon induced topography of the Mid-Channel Anticline, Santa Barbara Basin, California

The geomorphology of the western sector of the Mid-Channel Anticline (MCA), Santa Barbara, southern California suggests the actively growing fold is laterally propagating to the west. The presence of fold scarps and cross faults that segment the structure suggests that buried faults that are producing the folding are present at shallow depths. The summit area of the anticline at the Last Glacial Maximum (22 to 19 ka) was probably a small late Pleistocene island. Evidence for presence of the island includes what appears to be terrestrial erosion and is supported by assumption of sea level change and rates of uplift and subsidence.Pockmarks and domes ranging in diameter from  10 to 100 m, and several meters deep are present along the crest and flanks of the MCA. These features appear to be the result of hydrocarbon emission. Their formation has significantly modified the surface features, producing simple to complex erosional and/or constructional topography. A large pockmark near the anticline crest dated by two calibrated AMS radiocarbon dates of 25.3 and 36.9 ka continues to emit hydrocarbon gases. We term the topography produced by hydrocarbon emission as Hydrocarbon Induced Topography (HIT).     

南黄海陆架沉积物中褶皱的形态及成因

更新世以来南黄海陆架具有构造活动性强、沉积厚度大(100~300m)、褶皱变形保存完整等特点,是研究未固结沉积物中褶皱变形的理想“实验室”.高分辨率单道地震剖面揭示了南黄海陆架沉积物中具有不同几何形态的三种褶皱:(1)断层相关褶皱(包括断层扩展褶皱和断层牵引褶皱);(2)横向弯曲褶皱;(3)复合作用褶皱.对褶皱的成因和几何形态的研究表明,控制未固结沉积物中褶皱形成的因素是基底断层及其形成的断块,而不是区域应力场.大量孔隙水可能对沉积物的几何形变具有重要的影响作用.沉积物中断层扩展褶皱和牵引褶皱在形变程度上具有连续性,说明两者在成因上存在联系.断层扩展褶皱可能引发的地震活动和变形带在海洋工程地质评估中应受到重视.     

论多层叠置独立含煤层气系统——以贵州织金-纳雍煤田水公河向斜为例

基于贵州织金—纳雍煤田水公河向斜的实测资料,采用地质分析等方法,探讨了煤层气地质条件在垂向上的非均质分布规律,提出和初步论证了“多层叠置独立含煤层气系统”的学术观点。研究表明,上二叠统龙潭组单一煤层甲烷平均含量及相邻主煤层之间含气量梯度均呈波动式变化,煤层埋深—压力系数关系在垂向上分为截然不同的两套系统,层序地层格架中二级层序与含气量梯度的独立分段高度吻合。由此揭示,含煤地层地下流体在不同主煤层之间总体上缺乏交换,导致不同煤层群之间的煤层气系统相对独立。笔者等认为,龙潭组层序地层格架特点奠定了该类系统形成的物性基础,含煤地层与上覆、下伏含水层之间缺乏水力联系而构成了该类系统产生的水文地质基础,即多层叠置独立含气系统是沉积—水文—构造条件耦合控气作用的产物。笔者等也指出,在以三角洲—潮坪—潟湖沉积体系为主的多煤层含煤地层中,该类含气系统可能具有普遍意义。     

中国中始新世—早更新世构造事件与应力场

中始新世—渐新世（52—23．3Ma）的华北构造期是以太平洋板块朝NWW方向位移为主要特征,使我国大陆受到近东西向的挤压,造成一系列近南北向的褶皱、逆掩断层和许多走向近东西的正断层、单断箕状盆地。此构造事件的发生可能与始新世末期北美、加勒比海和东太平洋的大量微玻璃陨石的坠落、冲击有关。中新世--早更新世（23．3⁰．7Ma）的喜马拉雅构造期是以印度—澳大利亚板块与菲律宾海板块向北推移为主要特征,造成喜马拉雅山和日本列岛南部的俯冲带,使我国西部发育走向近东西的褶皱、逆掩断层系,而在东部地区则形成许多走向近南北的深切地幔的正断层系.并使南海与日本海再次张开。出现洋壳。喜马拉雅构造事件可能与印度洋、南亚、澳大利亚附近地区的微玻璃陨石群的冲击有关。     

Relationship between the Altyn Tagh strike-slip fault and the Qaidam Basin：New insights from superposed buckle folding in Hongsanhan

ABSTRACT

The relationship between the sinistral strike-slip Altyn Tagh Fault (ATF) and the internal tectonic deformation of the Qaidam Basin remains a controversial issue. Uncovering the relationship between the structures along the southern slope of the ATF is a viable solution to this problem. The ‘snake-like’ Hongsanhan superposed fold is located on the southern slope of the ATF. The Hongsanhan structure is controlled by the superposition of two generations of folds. The first-generation NW–SE-trending f₁ fold, which developed during the Oligocene, constitutes the principal component of the present-day Hongsanhan fold, while the second-generation NE–SW-trending f₂ folds formed during the middle Miocene. The structure of the Hongsanhan superposed fold proves that the large-scale sinistral strike-slip movements along the ATF began during the Oligocene. In addition, a flower structure associated with the ATF continuously extends into the Qaidam Basin.