Influence of carbonate facies on fault zone architecture

Normal faults on Malta were studied to analyse fault propagation and evolution in different carbonate facies. Deformation of carbonate facies is controlled by strength, particle size and pore structure. Different deformation styles influence the damage characteristics surrounding faults, and therefore the fault zone architecture. The carbonates were divided into grain- and micrite-dominated carbonate lithofacies. Stronger grain-dominated carbonates show localised deformation, whereas weaker micrite-dominated carbonates show distributed deformation. The weaker micrite-dominated carbonates overlie stronger grain-dominated carbonates, creating a mechanical stratigraphy. A different architecture of damage, the ‘Fracture Splay Zone’ (FSZ), is produced within micrite-dominated carbonates due to this mechanical stratigraphy. Strain accumulates at the point of juxtaposition between the stronger grain-dominated carbonates in the footwall block and the weaker micrite-dominated carbonates in the hanging wall block. New slip surfaces nucleate and grow from these points, developing an asymmetric fault damage zone segment. The development of more slip surfaces within a single fault zone forms a zone of intense deformation, bound between two slip surfaces within the micrite-dominated carbonate lithofacies (i.e., the FSZ). Rather than localisation onto a single slip surface, allowing formation of a continuous fault core, the deformation will be dispersed along several slip surfaces. The dispersed deformation can create a highly permeable zone, rather than a baffle/seal, in the micrite-dominated carbonate lithofacies. The formation of a Fracture Splay Zone will therefore affect the sealing potential of the fault zone. The FSZ, by contrast, is not observed in the majority of the grain-dominated carbonates.     

Extensional deformation structures within a convergent orogen: The Val di Lima low-angle normal fault system (Northern Apennines,Italy)

A low-angle extensional fault system affecting the non metamorphic rocks of the carbonate dominated Tuscan succession is exposed in the Lima valley (Northern Apennines, Italy). This fault system affects the right-side-up limb of a kilometric-scale recumbent isoclinal anticline and is, in turn, affected by superimposed folding and late-tectonic high-angle extensional faulting.The architecture of the low-angle fault system has been investigated through detailed structural mapping and damage zone characterization. Pressure-depth conditions and paleofluid evolution of the fault system have been studied through microstructural, mineralogical, petrographic, fluid inclusion and stable isotope analyses. Our results show that the low-angle fault system was active during exhumation of the Tuscan succession at about 180°C and 5 km depth, with the involvement of low-salinity fluids. Within this temperature - depth framework, the fault zone architecture shows important differences related to the different lithologies involved in the fault system and to the role played by the fluids during deformation. In places, footwall overpressuring influenced active deformation mechanisms and favored shear strain localization.Our observations indicate that extensional structures affected the central sector of the Northern Apennines thrust wedge during the orogenic contractional history, modifying the fluid circulation through the upper crust and influencing its mechanical behavior.     

Slip line theory applied to a retaining wall–unsaturated soil interaction problem

An extension of slip line theory to unsaturated soils is presented and applied to the problem of a rigid retaining wall rotating about its toe into unsaturated soils. Suction is introduced using the effective stress concept. Soil–wall interface friction is defined carefully. The influence of suction on limiting passive earth pressures is analysed for two soils under steady state evaporation and infiltration. Suction increases the limiting passive stress at the soil–wall interface, with a dependence on the steady state flow type. The displacement of the retained soil is studied assuming the wall undergoes a rotation increment. The results show a clear difference in the displacement for evaporation and infiltration.     

A constitutive model for unsaturated soils with consideration of inter-particle bonding

The paper presents a physically-based constitutive model for unsaturated soils that considers the bonding effect of water menisci at inter-particle contacts. A bonding factor has been used to represent the magnitude of the equivalent bonding stress, defined as the bonding force per unit cross-sectional area. The average skeleton stress is employed to represent the effect of average fluid pressures within soil pores. Based on an empirical relationship between the bonding factor ζ and the ratio e/e_s (where e and e_s are void ratios at unsaturated and saturated states, respectively, at the same average skeleton stress), we propose an elasto-plastic constitutive model for isotropic stress states, and then extend this model to triaxial stress states within the framework of critical state soil mechanics. Because only one yield surface is needed in the proposed model, a relatively small number of parameters are required. Comparisons between experimental data and model results show that, in most cases, the proposed model can reasonably capture the important features of unsaturated soil behavior.     

Numerical studies on the effect of temperature on the unsaturated hydraulic response of geotextiles

A series of coupled thermo-hydraulic simulations were performed on a soil–geotextile column to understand the effect of temperature on suction distribution throughout the soil column and on the hydraulic performance of the geotextile as a drainage/capillary barrier layer. Two different constant temperatures of 0 °C and 38 °C and a temperature gradient of 4 °C along the column were modeled. Changing the temperature from 0 °C to 38 °C did not have a significant effect on the suction head distribution in the soil–geotextile column. The temperature gradient resulted in appreciable thermal vapor flow and changes in suction head and hydraulic conductivity of the geotextile. During drainage, the temperature gradient and lower temperature at the top of the column increased suction in the geotextile and its ability to function as a capillary barrier. During capillary rise, the temperature gradient and lower temperature at the top of the column decreased the suction in the geotextile and its ability to function as a capillary barrier. Changing the direction of the thermal gradient reversed the water vapor flow direction and its effect on the suction in the geotextile. A temperature gradient did not have a noticeable effect on the suction head of the geotextile when positive pore pressure was developed in the geotextile and adjacent soil during drainage.     

Semi-analytical solution to one-dimensional consolidation for viscoelastic unsaturated soils

This paper presents a semi-analytical solution to one-dimensional consolidation of viscoelastic unsaturated soils with a finite thickness under oedometric conditions and subjected to a sudden loading. The solution is obtained by using Lee’s correspondence principle based on the semi-analytical solution to one-dimensional consolidation of elastic unsaturated soils. The boundary contains the top surface permeable to water and air and the bottom impermeable to water and air. A typical example is given to show the evolution of excess pore-air and pore-water pressures as well as the total degree of consolidation of the soil layer with time for different ratios of air–water permeability coefficient, elastic modulus and viscoelastic coefficient. The one-dimensional consolidation behavior of viscoelastic unsaturated soil is discussed according to the semi-analytical solution. These results contribute to a better understanding of the consolidation behavior of viscoelastic unsaturated soils.     

藏北商旭造山型金矿床的发现及意义

西藏双湖县商旭金矿床位于班公湖-怒江缝合带中段。矿体赋存于中-下侏罗统木嘎岗日群浅变质复理石中,受北西西向断裂构造控制,呈透镜体状、脉状,或者似层状产出;矿石矿物以自然金为主,矿石类型为石英脉型和蚀变岩型,围岩蚀变特征明显。该矿床主要受地层及构造因素控制,矿区的构造、围岩蚀变、矿物、地球化学异常等可作为找矿标志。野外地质调查和矿床对比研究显示,商旭金矿床在矿物共生组合、蚀变类型、构造类型等方面,与典型的造山型金矿床相似,其成因类型属于造山型金矿床。商旭金矿床的发现为班公湖-怒江成矿带金等贵金属矿产的地质找矿工作开辟了新的方向。     

西秦岭岷-礼成矿带岩浆活动与金矿成矿模式探讨

岷-礼成矿带发现较多大型金矿,同时在岩体内发现斑岩型钼矿。金矿床矿石中除有低温矿物外,还有黄铜矿、白钨矿等中-高温矿物,白钨矿在寨上金矿中是载金矿物之一;钼矿中也有黄铜矿、白钨矿矿物;金、钼、钨矿化分带有一定空间关系;金矿床地球化学、稳定同位素、放射性同位素和流体包裹体的研究表明成矿物质,成矿流体有深源特征;中川岩体群的岩石学、年代学、岩石化学、微量元素、稀土元素特征表明岩体为复式岩体,具有壳幔混源特征;据此建立成矿带内受印支-燕山期岩浆活动控制的金矿成矿模式。     

Emplacement of the La Peña alkaline igneous complex,Mendoza, Argentina (33° S): Implications for the early Miocene tectonic regime in the retroarc of the Andes

The La Peña alkaline complex (LPC) of Miocene age (18–19 Ma) lies on the eastern front of the Precordillera (32°41ʹ34ʺS, 68°59ʹ48″W, 1400–2900 m a.s.l.), 30 km northwest of Mendoza city, Argentina. It is a subcircular massif of 19 km² and 5 km in diameter, intruded in the metasedimentary sequence of the Villavicencio Formation of Silurian-Devonian age. It is the result of integration of multiple pulses derived from one or more deep magma chambers, which form a suite of silicate rocks grouped into: a clinopyroxenite body, a central syenite facies with a large breccia zone at the contact with the clinopyroxenite, bodies of malignite, trachyte and syenite porphyry necks, and a system of radial and annular dikes of different compositions. Its subcircular geometry and dike system distribution are frequent features of intraplate plutons or plutons emplaced in post-orogenic settings. These morphostructural features characterize numerous alkaline complexes worldwide and denote the importance of magmatic pressures that cause doming with radial and annular fracturing, in a brittle country rock. However, in the LPC, the attitude of the internal fabric of plutonic and subvolcanic units and the preferential layout of dikes match the NW–SE extensional fractures widely distributed in the host rock. This feature indicates a strong tectonic control linked to the structure that facilitate space for emplacement, corresponding to the brittle shear zone parallel to the N–S stratigraphy of the country rock. Shearing produced a system of discontinuities, with a K fractal fracture pattern, given by the combination of Riedel (R), anti-Riedel (R′), (P) and extensional (T) fracture systems, responsible for the control of melt migration by the opening of various fracture branches, but particularly through the NW–SE (T) fractures. Five different pulses would have ascent, (1) an initial one from which cumulate clinopyroxenite was formed, (2) a phase of mafic composition represented by dikes cross-cutting the clinopyroxenite, (3) a malignite facies that causes a small breccia in the clinopyroxenite, (4) a central syenite facies that develops breccias at the contact with the clinopyroxenite and, finally, (5) porphyry necks and a system of radial dikes intruding all units. At the moment of the emplacement different mechanisms would have acted, they summarized in: 1) opening of discontinuities synchronous to the magma circulation as the principal mechanism for formation of dikes and conduits; 2) stoping processes, that play an important role in the development of the breccia zone and enabling an efficient transference of material during the emplacement of the syenitic magma and 3) shear-related deformation (regional stress), affected the internal fabric of the facies, causing intracrystalline deformation and submagmatic flow, which is very evident in the central syenite intrusive. The kinematic analysis of shear planes allows proposing that emplacement of the LPC took place in a transtensive regime, which would have occurred in the back-arc of the Andes orogen, during a long period spanning from Miocene to the present, of the compressive deformation responsible, westward and at the same latitude, for the development of the Aconcagua fold and thrust belt.     

柴达木盆地西部昆北断阶带基底花岗岩锆石U-Pb年龄、地球化学特征及其地质意义

柴达木盆地西部基底分布有大量的有花岗岩类岩石。通过对柴达木盆地西部昆北断阶带钻遇的基底花岗岩样品开展详细的岩石学、锆石激光探针等离子体质谱U-Pb同位素年代学及岩石地球化学研究表明,锆石U-Pb同位素年龄为467~450Ma,显示基底花岗岩的结晶年龄为中-晚奥陶世,属于加里东期岩浆侵入旋回。详细的岩石地球化学分析表明,昆北断阶带基底花岗岩属过铝高钾钙碱性系列,其稀土元素配分模式为具有Eu负异常的轻稀土元素富集型,昆北断阶带中南部基底花岗岩属上地壳物质熔融,同碰撞环境下形成的花岗岩。综合区域上的研究成果,昆北断阶带及其以西地区存在中奥陶世-早志留世的加里东期构造-岩浆事件,这对探讨柴达木盆地西部基底花岗岩成因类型及岩浆演化具有重要的意义。