Fault on–off versus coseismic fluids reaction

The fault activation （fault on） interrupts the enduring fault locking （fault off） and marks the end of a seismic cycle in which the brittle-ductile transition （BDT） acts as a sort of switch. We suggest that the fluid flow rates differ during the different periods of the seismic cycle （interseismic, pre-seismic, coseismic and post-seismic） and in particular as a function of the tectonic style. Regional examples indicate that tectonic-related fluids anomalies depend on the stage of the tectonic cycle and the tectonic style. Although it is difficult to model an increasing permeability with depth and several BDT transitions plus independent acquicludes may occur in the crust, we devised the simplest numerical model of a fault constantly shearing in the ductile deeper crust while being locked in the brittle shallow layer, with variable homogeneous permeabilities. The results indicate different behaviors in the three main tectonic settings. In tensional tectonics, a stretched band antithetic to the normal fault forms above the BDT during the interseismic period. Fractures close and fluids are expellecl during the coseismic stage. The mechanism reverses in compressional tectonics. During the interseismic stage, an over-compressed band forms above the BDT. The band dilates while rebounding in the coseismic stage and attracts fluids locally. At the tip lines along strike-slip faults, two couples of subvertical bancls show different behavior, one in dilationJcompression and one in compressionJdilation. This deformation pattern inverts during the coseismic stage. Sometimes a pre-seismic stage in which fluids start moving may be observed and could potentially become a precursor.     

http://www.sciencedirect.com/science/article/pii/S1674987114000024

We propose that the brittle-ductile transition(BDT) controls the seismic cycle.In particular,the movements detected by space geodesy record the steady state deformation in the ductile lower crust,whereas the stick-slip behavior of the brittle upper crust is constrained by its larger friction.GPS data allow analyzing the strain rate along active plate boundaries.In all tectonic settings,we propose that earthquakes primarily occur along active fault segments characterized by relative minima of strain rate,segments which are locked or slowly creeping.We discuss regional examples where large earthquakes happened in areas of relative low strain rate.Regardless the tectonic style,the interseismic stress and strain pattern inverts during the coseismic stage.Where a dilated band formed during the interseismic stage,this will be shortened at the coseismic stage,and vice-versa what was previously shortened,it will be dilated.The interseismic energy accumulation and the coseismic expenditure rather depend on the tectonic setting(extensional,contractional,or strike-slip).The gravitational potential energy dominates along normal faults,whereas the elastic energy prevails for thrust earthquakes and performs work against the gravity force.The energy budget in strike-slip tectonic setting is also primarily due elastic energy.Therefore,precursors may be different as a function of the tectonic setting.In this model,with a given displacement,the magnitude of an earthquake results from the coseismic slip of the deformed volume above the BDT rather than only on the fault length,and it also depends on the fault kinematics.     

全球地震震源深度统计分析与地震层分布

地震数据可以反演地球内部的构造信息,能够查明地球内部层圈结构及地震发生的机制,对揭示地球结构,进行地震预测及防震减灾具有重要意义。与此同时,对地震数据的统计分析可以获得地壳变形及其深度层次的相关信息。本文利用美国地质调查局（USGS）记录的1900～2018年近3 000 000条全球地震数据进行分析,立足地震震源计算方法,排除不确定性数据,使用震源深度频次分析及高斯分解得到地震层和地震集中区深度的推定。以国际地震中心（ISC）的1970～2016年近2 000 000条数据及中国国家地震科学数据共享中心2009～2018年近300 000条数据进行比较。结果显示,地球标准椭球体深度10 km左右普遍存在一个全球性的地震集中区,与地壳中脆韧性过渡带一致。其上的地壳是全球绝大多数地震发震的深度范围,推定其为地震层;与此同时,局限在洋壳俯冲带中,约35 km处出现了另一个地震集中区,认为是地球岩石圈深度内不容忽视的界面。研究表明,在地球表层（40 km以内）,层圈结构对地震有较大的控制作用;而对有洋壳俯冲区域的地震三维结构图成图显示有“贝尼奥夫带”形态,表明俯冲机制能将具有弹性力学性质的刚性块体带至地球较深部,并孕育中、深源地震。因此,地震震源分布可以指示地震层,即刚性块体,在岩石圈中的分布,地震震源深度集中区是岩石圈和中、上地幔最主要的变形和能量释放区。     

Regression models for estimating coseismic landslide displacement

Newmark's sliding-block model is widely used to estimate coseismic slope performance. Early efforts to develop simple regression models to estimate Newmark displacement were based on analysis of the small number of strong-motion records then available. The current availability of a much larger set of strong-motion records dictates that these regression equations be updated. Regression equations were generated using data derived from a collection of 2270 strong-motion records from 30 worldwide earthquakes. The regression equations predict Newmark displacement in terms of (1) critical acceleration ratio, (2) critical acceleration ratio and earthquake magnitude, (3) Arias intensity and critical acceleration, and (4) Arias intensity and critical acceleration ratio. These equations are well constrained and fit the data well (71% < R² < 88%), but they have standard deviations of about 0.5 log units, such that the range defined by the mean ± one standard deviation spans about an order of magnitude. These regression models, therefore, are not recommended for use in site-specific design, but rather for regional-scale seismic landslide hazard mapping or for rapid preliminary screening of sites.     

Structure of a normal seismogenic fault zone in carbonates: The Vado di Corno Fault,Campo Imperatore,Central Apennines (Italy)

The Vado di Corno Fault Zone (VCFZ) is an active extensional fault cutting through carbonates in the Italian Central Apennines. The fault zone was exhumed from ∼2 km depth and accommodated a normal throw of ∼2 km since Early-Pleistocene. In the studied area, the master fault of the VCFZ dips N210/54° and juxtaposes Quaternary colluvial deposits in the hangingwall with cataclastic dolostones in the footwall. Detailed mapping of the fault zone rocks within the ∼300 m thick footwall-block evidenced the presence of five main structural units (Low Strain Damage Zone, High Strain Damage Zone, Breccia Unit, Cataclastic Unit 1 and Cataclastic Unit 2). The Breccia Unit results from the Pleistocene extensional reactivation of a pre-existing Pliocene thrust. The Cataclastic Unit 1 forms a ∼40 m thick band lining the master fault and recording in-situ shattering due to the propagation of multiple seismic ruptures. Seismic faulting is suggested also by the occurrence of mirror-like slip surfaces, highly localized sheared calcite-bearing veins and fluidized cataclasites. The VCFZ architecture compares well with seismological studies of the L'Aquila 2009 seismic sequence (mainshock M_W 6.1), which imaged the reactivation of shallow-seated low-angle normal faults (Breccia Unit) cut by major high-angle normal faults (Cataclastic Units).     

Tracing fluids in basins to refine mineral exploration strategies

To trace the fluid history of sedimentary basins requires integration of relatively diverse sub-disciplines including sedimentology, stratigraphy, tectonics, structural geology, petrography, geochemistry and geophysics. These aspects of basin evolution are interrelated and thus all are required to understand the source, character and distribution of fluids associated with ore deposits hosted in basins. Fluids are strategically associated with both the formation and preservation of almost every type of economic ore deposit in basins. As such, knowledge of the geochemical and physical characteristics, timing, origin, reactivity and flow histories of fluids are basic to formulating effective exploration strategies.     

Refining the image of the San Andreas Fault near Parkfield, California using a finite difference travel time computation technique

The Parkfield Area Seismic Observatory (PASO) was a dense, telemetered seismic array that operated for nearly 2 years in a 15 km aperture centered on the San Andreas Fault Observatory at Depth (SAFOD) drill site. The main objective of this deployment was to refine the locations of earthquakes that will serve as potential targets for SAFOD drilling and in the process develop a high (for passive seismological techniques) resolution image of the fault zone structure. A challenging aspect of the analysis of this data set was the known existence of large (20–25%) contrasts in seismic wavespeed across the San Andreas Fault. The resultant distortion of raypaths could challenge the applicability of approximate ray tracing techniques. In order to test the sensitivity of our hypocenter locations and tomographic image to the particular ray tracing and inversion technique employed, we compare an initial determination of locations and structure developed using a coarse grid and an approximate ray tracer [Thurber, C., Roecker, S., Roberts, K., Gold, M., Powell, M.L. , and Rittger, K., 2003. Earthquake locations and three-dimensional fault zone structure along the creeping section of the San Andreas fault near Parkfield, CA: Preparing for SAFOD, Geophys. Res. Lett., 30 3, 10.1029/2002GL016004.] with one derived from a relatively fine grid and an application of a finite difference algorithm [Hole, J.A., and Zelt, B.C., 1995. 3-D finite-difference reflection traveltimes, Geophys. J. Int., 121, 2, 427–434.]. In both cases, we inverted arrival-time data from about 686 local earthquakes and 23 shots simultaneously for earthquake locations and three-dimensional Vp and Vp/Vs structure. Included are data from an active source seismic experiment around the SAFOD site as well as from a vertical array of geophones installed in the 2-km-deep SAFOD pilot hole, drilled in summer 2002. Our results show that the main features of the original analysis are robust: hypocenters are located beneath the trace of the fault in the vicinity of the drill site and the positions of major contrasts in wavespeed are largely the same. At the same time, we determine that shear wave speeds in the upper 2 km of the fault zone are significantly lower than previously estimated, and our estimate of the depth of the main part of the seismogenic zone decreases in places by  100 m. Tests using “virtual earthquakes” (borehole receiver gathers of picks for surface shots) indicate that our event locations near the borehole currently are accurate to about a few tens of meters horizontally and vertically.     

Carbon–oxygen isotope and trace element constraints on how fluids percolate faulted limestones from the San Andreas Fault system: partitioning of fluid sources and pathways

Understanding the way fluids flow in fault zones is of prime importance to develop correct models of earthquake mechanics, especially in the case of the abnormally weak San Andreas Fault (SAF) system. Because fluid flow can leave detectable signatures in rocks, geochemistry is essential to bring light on this topic. The present detailed study combines, for the first time, C–O isotope analyses with a comprehensive trace element data set to examine the geometry of fluid flow within a significant fault system hosted by a carbonate sequence, from a single locality across the Little Pine Fault–SAF system. Such a fault zone contains veins, deformation zones, and their host rocks. Stable isotope geochemistry is used to establish a relative scale of integrated fluid–rock ratios. Carbonate δ¹⁸O varies between 28‰ and 15‰ and δ¹³C between 5‰ and −7‰. From highest to lowest delta values, thus from least to most infiltrated, are the host rocks, the vein fillings, and the deformation zone fillings, respectively. Infiltration increases toward fault core. The fluids are H₂O–CO₂ mixtures. Two fluid sources, one internal and the other external, are found. The external fluid is inferred to come essentially from metamorphism of the Franciscan formation underneath. The internal (local) fluid is provided by a 30% volume reduction of the host limestones resulting from pressure solution and pore size reduction. Most trace elements, including the lanthanides, show enrichment at the 100-m scale in host carbonate rocks as fluid–rock ratios increase toward the fault core. In contrast, the same trace element concentrations are low, relative to host rocks, in veins and deformation zone carbonate fillings, and this difference in concentration increases as fluid–rock ratio increases toward the fault core. We suggest that the fluid trace elements are scavenged by complexation with organic matter in the host rocks. Elemental complexation is especially illustrated by large fractionation of Y–Ho and Nb–Ta geochemical pairs. Complexation associated with external fluid flow has a significant effect on trace element enrichment (up to 700% relative enrichment) while concentration by pressure solution associated with volume decrease of host rocks has a more limited effect (up to 40% relative enrichment). Our observations from the millimeter to the kilometer scale call for the partitioning of fluid sources and pathways, and for a mixed focused–pervasive fluid flow mechanism. The fluid mainly flows within veins and deformation zones and, simultaneously, within at least 10 cm from these channels, part of the fluid flows pervasively in the host rock, which controls the fluid composition. Scavenging of the fluid rare earth elements (REE) by host rocks is responsible for the formation of REE-depleted vein and deformation zone carbonate fillings. Fluid flow is not only restricted to veins or deformation zones as commonly believed. An important part of fluid flow takes place in host rocks near fault zones. Hence, the nature of the lithologies hosting fault zones must be considered in order to take into account the role of fluids in the seismic cycle.     

甘肃西部昌马地区金矿成矿条件及找矿标志

通过对区内金矿特征及地质背景的分析，认为鹰嘴山、寒山、车路沟一带的金矿分别为受脆韧性剪切带改造的蚀变岩型金矿、受脆韧性剪切带控制的蚀变岩型金矿和受断裂破碎带控制的金矿，它们分别形成于加里东晚期、华力西中期和华力西晚期--印支期．指出：发育于寒武纪火山岩和加里东期基性超基性岩中的韧性剪切带，是寻找受脆韧性剪切带改造的蚀变岩型金矿的有利地段：寒山一带的脆韧性剪切带被近EW向断裂叠加的部位，是受脆韧性剪切带控制的蚀变岩型金矿成生的有利地段；发育于车路沟花岗闪长岩体外接触带的NNW向断裂，是寻找受断裂破碎带控制的金矿的有利地段．     

1995年日本兵库县南部地震时淡路岛上出现的地震断层的形态特征

1995年日本兵库县南部地震时在淡路岛上出现的地表地震断层主要由三条地表破裂带组成、野岛地震断层、松帆地震层和楠本地震断层。野岛地震断层从淡路岛北端的淡路町住西南延伸到一宫町的尾崎,长达18km,其北段沿着早期存在的野岛断层分布,而南段则作为新断层出现,野岛地震断层的断层的北段主要由一些相互平行或次平行的右列剪切断层和许多左列和性裂隙组成,其南段则是由集中在十多米宽的大量不连续的地表破裂带所组成。野岛地震断层一般走向N30°～60°E倾向SE,地貌错位和断层擦痕均显示出此断层为一具有逆断层性质的右旋走滑断层,沿一些主要露头测定的北段水平位移量一般为100～200cm,垂直位移量为5O～100cm;而南段的水平、垂直位移量均只有几厘米至20cm。最大位移量在平林断层崖测得;水平180cm、垂直l30cm、实际位移量2l5cm。松帆地震断层走向N40°～60°W,沿着淡路岛北端部的海岸线分布,长达约1km。楠本地震断层沿早期存在的楠本断层出现,分布于淡路岛东北部的海岸边上,走向N35°～6O°W倾向NW 。根据地表地震断层的形态及地貌错位特征,野岛地震断层可被分为四条断层段,并在形态上呈现右列。地质和地貌证据以及余震分布的特征清楚地表明这4条断层段的几何形态和分布特征是受早期存在的地质构造所控制的,同时也说明了地震断层的破裂过程具有拉分(pulling-apart)和推隆(pushing－up)的过程,这两个过程分别产生了张性裂隙、拉分盆地、逆断层和挤压隆起等构造。     

利用成像测井资料分析汶川地震断裂带科学钻探3号孔(WFSD-3)裂缝特征

WFSD-3孔是汶川地震断裂带科学钻探主要钻孔之一,全井段(终孔深度1 502.30 m)实施了连续取心(累计取心进尺1 548.44 m)和测井作业。采集到的成像测井资料包含丰富的原位地质特征信息,对裂缝、破碎带识别和构造应力场分析具有重要作用。利用该钻孔的电阻率成像测井和超声成像测井资料,结合岩心资料进行了裂缝特征分析。结果表明,WFSD-3孔岩层高角度斜交缝最为发育,其次是低角度斜交缝,垂直缝和水平缝极少;25～200 m和900～1 000 m深度范围内裂缝尤为发育;不同深度的裂缝倾向存在明显差异:410 m之上主要分布于260°～290°,410～730m集中于330°～360°,730～960 m主要分布于210°～240°,960～1 185 m与410 m之上基本一致,1 410～1 450 m与前述各深度段不同,集中于180°～200°;裂缝与破碎带、层理密切相关,宏观分布受构造控制。     

Fault plane processes and mesoscopic structure of a strong-type seismogenic fault in tonalites (Adamello batholith, Southern Alps)

The Gole Larghe Fault is an exhumed paleoseismic fault crosscutting the Adamello tonalites (Italian Southern Alps). Ambient conditions of faulting were 9–11 km in depth and 250–300 °C. In the study area the fault accommodates 1100 m of dextral strike-slip over a fault thickness of 550 m. Displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons (faults_1–2–3). Fault displacement is in the range of few centimeters (faults₃) to a maximum of a few tens of meters in major faults₁. Faults_1–2 nucleated on pre-existing joints, whereas faults₃ are newly generated fractures produced during slip along faults_1–2. Each fault within the Gole Larghe Fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production. Pseudotachylytes are usually generated at the host rock/cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins. Pseudotachylytes and cataclasites have a similar chemical composition which is enriched in Loss On Ignition, K, Rb, Ba, U and Fe³⁺ compared to host rock.We envision two models for the evolution of the Gole Larghe Fault. In both models synkinematic fluid–rock interaction along a fault causes fault hardening by precipitation of abundant K-feldspar+epidote (and minor chlorite) in the cataclasite matrix conducive to final production of pseudotachylyte. In the first model, induration occurs progressively by differential precipitation related to fabric evolution in cataclasites. In the second model, induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir. Whatever the model, the Gole Larghe Fault represents a strong fault, where hardening processes resulted in a low displacement/fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement/fault thickness ratios.     

断层内部结构及其对封闭性的影响

先前的研究多考虑断层封堵和开启的2种极端状态,近来的研究认为,在多数情况下断层处于2种之间的状态,只有在静止期具有封闭能力的断层,才有可能对油气起封堵作用。分析断层对流体运移的影响,需要分析断层在演化过程中的内部结构特征。断层可以划分出破碎带、诱导裂缝带和围岩3部分,断层岩和伴生裂缝构成破碎带的主体部分。常见的断层岩包括断层角砾岩、断层泥和部分碎裂岩,它们充填在断层裂缝空间中,断层内部结构受断层形成时的构造应力性质、断层活动强度和围岩岩性因素的控制。从动态角度看,随着断距增加,断层活动伴随着裂缝的发育和岩石的破碎混杂,可用泥质源岩层厚度和断距的比值来划分不同的发育阶段。断层活动期为油气运移通道,在静止时表现出差异性的封闭,通常用断层渗透率和排替压力2个参数来定量评价断层的封闭程度。断层岩渗透率主要受断距、泥质含量、埋深等因素的控制;断层排替压力的预测方法有2种:一种是从断层岩成岩角度分析的"等效埋深法",另一种是分析实测排替压力与主控地质因素的"拟合法"。通过简化的断层模型,建立了渗透率、排替压力与主控因素的预测关系。和储层类似,流体在断层中的运移遵循多孔介质的渗流特征。利用断层两侧的流体压力和油气柱高度并不能直接评价封闭性能,还必须考虑油气充注史和流体压力变化历史。     

Role of fluids in the metamorphism of the Alpine Fault Zone,New Zealand

Models of fluid/rock interaction in and adjacent to the Alpine Fault in the Hokitika area, South Island, New Zealand, were investigated using hydrogen and other stable isotope studies, together with field and petrographic observations. All analysed samples from the study area have similar whole‐rock δD values (δD_WR = ?56 to ?30‰, average = ?45‰, n = 20), irrespective of rock type, degree of chloritization, location along the fault, or across‐strike distance from the fault in the garnet zone. The green, chlorite‐rich fault rocks, which probably formed from Australian Plate precursors, record nearly isothermal fluid/rock interaction with a schist‐derived metamorphic fluid at high temperatures near 450–500°C (δD of water in equilibrium with the green fault rocks (δD_{H2O, green}) ≈ ?18‰; δD of water in equilibrium with the greyschists and greyschist‐derived mylonites (δD_{H2O, grey}) ≈ ?19‰ at 500°C; δD_{H2O, green} ≈ ?17‰; δD_{H2O, grey} ≈ ?14‰ at 450°C). There is no indication of an influx of a meteoric or mantle‐derived fluid in the Alpine Fault Zone in the study area. The Alpine Fault Zone at the surface shows little evidence of late‐stage retrogression or veining, which might be attributed to down‐temperature fluid flow. It is probable that prograde metamorphism in the root zone of the Southern Alps releases metamorphic fluids that at some region rise vertically rather than following the trace of the Alpine Fault up to the surface, owing to the combined effects of the fault, the disturbed isotherms under the Southern Alps, and the brittle–ductile transition. Such fluids could mix with meteoric fluids to deposit quartz‐rich, possibly gold‐bearing veins in the region c. 5–10 km back from the fault trace. These results and interpretations are consistent with interpretations of magnetotelluric data obtained in the South Island GeopHysical Transects (SIGHT) programme.     

示波器故障维修

本文介绍了示波器的基本工作原理 ,以ST— 1 0型示波器为例阐述了该仪器常见故障的维修方法     

The origin of high magnetic remanence in fault pseudotachylites: Theoretical considerations and implication for coseismic electrical currents

Several examples of fault-related pseudotachylites display a significantly higher initial magnetic susceptibility than their granitic host rock (10:1 to 20:1). These higher values are attributed to the presence of fine magnetic particles formed during melt quenching. The hysteresis properties of the particles indicate a single domain (SD) to pseudo single domain (PSD) magnetic grain size. The Curie temperature (Tc) of the magnetic particles is close to 580 °C.The natural remanent magnetization (NRM) of these pseudotachylites is also significantly higher than that of the host rock (up to 300:1). Such anomalously high remanence cannot be explained by a magnetization acquired in the Earth's magnetic field, regardless of pseudotachylite age.Ground lightning and other strong electric pulses can cause anomalously high NRM intensities. A ground lightning explanation seems unlikely to explain the systematically high NRM intensities, particularly in the case of recently exposed samples that have been collected from active quarries. Alternatively, high NRM intensities could be explained by earthquake lightning (EQL), a seismic phenomenon occasionally reported in connection with large magnitude earthquakes (M > 6.0).The coseismic electrical properties of the pseudotachylite vein–host rock system are characterized by (1) a core of molten material (high conductivity), (2) vapor-rich margins of thermally and mechanically fractured host rocks (low conductivity) and (3) moderately fractured to undeformed host rock (normal conductivity). Such a core conductor bordered by insulating margins is potentially responsible for the propagation of EQL pulses.The coseismic thermal history of pseudotachylite veins has been modeled in 2-D using conductive heat transfer equations. It shows that EQL can be recorded only during a brief time interval (less than 1 min) for a given vein thickness and host-rock temperatures. If the vein is too thick or if the host rock is too hot, the pseudotachylite remains above Tc after the electric pulse has lapsed.     

Orientation,composition, and entrapment conditions of fluid inclusions in the footwall of the northern Snake Range detachment,Nevada

Footwall rocks of the northern Snake Range detachment fault (Hampton and Hendry's Creeks) offer exposures of quartzite mylonites (sub-horizontal foliation) that were permeated by surface fluids. An S–C–C′ mylonitic fabric is defined by dynamically recrystallized quartz and mica. Electron backscatter diffraction analyses indicate a strong preferred orientation of quartz that is overprinted by two sets of sub-vertical, ESE and NNE striking fractures. Analyses of sets of three perpendicular thin sections indicate that fluid inclusions (FIs) are arranged according to macroscopic fracture patterns. FIs associated with NNE and ESE-striking fractures coevally trapped unmixed CO₂ and H₂O-rich fluids at conditions near the critical CO₂–H₂O solvus, giving minimum trapping conditions of T = 175–200 °C and ∼100 MPa H₂O-rich FIs trapped along ESE-trending microcracks in single crystals of quartz may have been trapped at conditions as low as 150 °C and 50 MPa indicating the latest microfracturing and annealing of quartz in an overall extensional system. Results suggest that the upper crust was thin (4–8 km) during FI trapping and had an elevated geotherm (>50 °C/km). Footwall rocks that have been exhumed through the brittle-ductile transition in such extensional systems experience both brittle and crystal-plastic deformation that may allow for circulation of meteoric fluids and grain-scale fluid–rock interactions.     

Constraints from high-pressure veins in eclogites on the composition of hydrous fluids in subduction zones

Hydrous high-pressure veins formed during dehydration of eclogites in two paleo-subduction zones (Trescolmen locality in the Adula nappe, central Alps and Münchberg Gneiss Massif, Variscan fold belt, Germany) constrain the major and trace element composition of solutes in fluids liberated during dehydration of eclogites. Similar initial isotopic compositions of veins and host eclogites at the time of metamorphism indicate that the fluids were derived predominantly from the host rocks. Quartz, kyanite, paragonite, phengite, zoisite and omphacite are the dominant minerals in the veins. The major element compositions of the veins are in agreement with experimental evidence indicating that the composition of solutes in such fluids is dominated by SiO₂ and Al₂O₃. Relative to N-MORB, the veins show enrichments of Cs, Rb, Ba, Pb, and K, comparable or slightly lower abundances of Sr, U, and Th, and very low abundances of Nd, Sm, Zr, Nb, Ti and Y. The differential fractionation of highly incompatible elements such as K, U and Th in the veins, as well as the presence of hydrous minerals in the eclogites rule out partial melting as a cause for vein formation. These results confirm previous suggestions that fluids derived from subducted basalt may have low abundances of high field strength elements, rare earth elements and Y. Variable vein-eclogite enrichment factors of incompatible alkalis and to a lesser extent Pb appear to reflect mineralogical controls (phengite, epidote-group minerals) on partitioning of these elements during dehydration of eclogite in subduction zones. However, abundance variations of incompatible elements in minerals from eclogites suggest that the composition of fluids released from eclogites at temperatures <700°C may not reflect true equilibrium partitioning during dehydration. Simple models for the trace elements U and Th indicate the relative importance of the basaltic and sedimentary portions of subducted oceanic crust in producing the characteristic chemical signatures of these elements in convergent plate margin volcanism.     

Fluids migration and dynamics of a blocks-and-faults system

We explore the impact of fluids migrating through a fault network on the dynamics of lithosphere, both on slow movements and seismicity. For that purpose fluids in the fault zones are incorporated into modelling of blocks-and-faults systems, which takes into account driving forces and the system's geometry. Simulations have been performed for two-dimensional models: an idealised “brick wall” structure, and a coarse image of Sinai Subplate. Migrating fluids originating in different locations are considered, as well as fluids trapped in closed pockets. Basic features of the modelled and observed seismicity are in good accord, as shown by comparison with the earthquake catalog compiled by Geophysical Institute of Israel.