Migration of growth axial surfaces and its implications for multiphase tectono-sedimentary evolution of the Zhangwu fault depression,southern Songliao Basin,NE China

The normal fault-bend folding theory uses active axial surfaces, inactive axial surfaces, and growth axial surfaces to describe the geometric relationship between faults and deformation of a hanging wall. The dip of a growth axial surface is related to the fault slip rate and the basin sedimentation rate: higher fault slip rates result in smaller dips of growth axial surfaces, whereas higher basin sedimentation rates produce larger dips of growth axial surfaces. Moreover, the growth axial surface will be a straight line if both the fault slip and sedimentation rates remain relatively unchanged, but will become curved if both rates are variable. Therefore, the characteristics of growth axial surfaces can provide clear information on the evolution of faulting and deposition. By studying the seismic profiles of the Zhangwu fault depression of the southern Songliao Basin, we show that the migration of growth axial surfaces and unconformities can be used as indicators of basin development. Multiphase tectonic activity will not only produce unconformities but also result in migration of growth axial surfaces or inactive axial surfaces. Therefore, the normal fault-bend folding theory, particularly with regard to the evolution of growth axial surfaces, can be applied to the interpretation of geometric and kinematic evolutions of half-grabens and the exploration of related tectono-sedimentary processes.     

大巴山西北缘叠加褶皱研究

大巴山晚侏罗纪叠加褶皱可能是世界上区域规模或填图规模最典型的褶皱叠加构造之一,具有完美的干涉图像。作者在两期褶皱近乎正交的大巴山西北缘开展1/万填图和构造分析,重点研究露头尺度上的横跨褶皱的几何学、运动学特征。厘定了晚三叠世和晚侏罗世两期构造运动及其两期褶皱变形,确定地壳浅层发育的纵弯褶皱机制,在三维几何形态研究基础上,特别是根据同褶皱层间滑动线理的几何学和运动学及其相互配置关系,基于叠加褶皱力学作用方式和变形干涉图像将区内叠加褶皱划分为3类10种基本样式。研究表明晚侏罗世近南北-北北西向褶皱（F2）近垂直地跨过晚三叠世近东西-北东向褶皱（F1）,是大巴山地区最主要的定型构造,构成大巴山晚侏罗纪弧形前陆褶皱山系主体;而北西向褶皱（F3）与近南北-北北西向褶皱（F2）在其中-西部以小的角度相交,总体具非共轴的旋转应变特征,并主要表现为并置或重褶作用。     

柴达木盆地成因分析

对柴达木盆地新生代沉降机制存在不同的观点。一个合理的模型必须解释柴达木盆地的两个基本问题： 1）为什么柴达木盆地新生代沉积中心主要位于盆地中部; 2）是什么动力学过程导致盆地发生最大幅度超过15 km的基底沉降。通过对柴达木盆地主要地质特征的分析和对已有盆地模型的评述,本文发展了地壳褶皱模型,认为青藏高原北部上部地壳发生纵弯褶皱是柴达木盆地形成的主要原因。该模型不仅解释了盆地沉积中心的位置,而且揭示了柴达木盆地与周缘其它构造单元的关系。上部地壳发生强烈褶皱与下地壳侧向流动和岩石圈地幔向南俯冲的过程有关。     

一次对流层异常臭氧次峰的观测研究及其动力输送过程的分析

文中分析了 1996年 8月 1日发生在西宁 (36 .4 3°N ,10 1.4 5°E ,海拔 :2 2 96m)地区对流层异常臭氧次峰现象。观测资料揭示了高空低压槽东移是臭氧次峰的主要天气特征。三维后向轨迹计算表明 ,尽管代表臭氧次峰的气团可以追溯到中亚地区 ,但是明显的气团向下输送则发生在新疆、青海间的高空低压槽内。中尺度模拟进一步确认了对流层顶折叠和平流层向下输送是臭氧次峰出现的动力机制。臭氧次峰在对流层高度位置与准无辐散层有关     

Vertical extrusion and middle crustal spreading of omphacite granulite: a model of syn-convergent exhumation (Bohemian Massif, Czech Republic)

The exhumation of eclogite facies granulites (Omp–Plg–Grt–Qtz–Rt) in the Rychleby Mts, eastern Czech Republic, was a localised process initiated by buckling of crustal layers in a thickened orogenic root. Folding and post‐buckle flattening was followed by the main stage of exhumation that is characterized by vertical ductile extrusion. This process is documented by structural data, and the vertical ascent of rocks from a depth of c. 70 to c. 35 km is documented by metamorphic petrology. SHRIMP ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb evaporation zircon ages of 342 ± 5 and 341.4 ± 0.7 Ma date peak metamorphic conditions. The next stage of exhumation was associated with sideways flat thrusting associated with lateral viscous spreading of granulites and surrounding rocks over indenting adjacent continental crust at a depth of c. 35–30 km. This stage was associated with syntectonic intrusion of a granodiorite sill at 345–339 Ma, emplaced at a crustal depth of c. 25 km. The time required for cooling of the sill as well as for heating of the country rocks brackets this event to a maximum of 250 000 years. Therefore, similar ages of crystallization for the granodiorite magma and the peak of eclogite facies metamorphism of the granulite suggest a very short period of exhumation, limited by the analytical errors of the dating methods. Our calculations suggest that the initial exhumation rate during vertical extrusion was 3–15 mm yr^?1, followed by an exhumation rate of 24–40 mm yr^?1 during further uplift along a magma‐lubricated shear zone. The extrusion stage of exhumation was associated with a high cooling rate, which decreased during the stage of lateral spreading.     

Two conceptual models of displacement transfer and examples

The concept of displacement transfer[1] was initially utilized by Dahlstrom (1970) to explain the relation- ships of overlapping thrusts in the Canadian Rockies wherein the displacement on one thrust is transferred to another, but the total displacement is still held con- stant along trend. Displacement transfer, which may exist in compressional[2] as well as tensile environ-ments[3], is a familiar kinematic mechanism that keeps the magnitude of deformation steady along trend in the linear str…     

米仓山区盖层褶皱构造变形分析

通过１∶５万区域地质调查和构造研究，首次提出米仓山区构造模式是在印支期到喜马拉雅期秦岭造山、四川成盆的山－盆耦合作用动力学和区内岩石力学性质差异分层结构的地质背景下，经历主波长－接触应变褶皱之后，通过断展褶皱作用以先成褶皱包络面和盖层底部角度不整合面为褶皱变形面形成的一个巨型不协调背斜构造。充实并运用主波长理论公式估算了褶皱作用期间的有关参数。建立了区域构造两类四型五阶段演化模式。     

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