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1.
Exhumed fault zones offer insights into deformation processes associated with earthquakes in unparalleled spatial resolution; however it can be difficult to differentiate seismic slip from slow or aseismic slip based on evidence in the rock record. Fifteen years ago, Cowan (1999) defined the attributes of earthquake slip that might be preserved in the rock record, and he identified pseudotachylyte as the only reliable indicator of past earthquakes found in ancient faults. This assertion was based on models of frictional heat production (Sibson, 1975, 1986) providing evidence for fast slip. Significant progress in fault rock studies has revealed a range of reaction products which can be used to detect frictional heating at peak temperatures less than the melt temperature of the rock. In addition, features formed under extreme transient stress conditions associated with the propagating tip of an earthquake rupture can now be recognized in the rock record, and are also uniquely seismic. Thus, pseudotachylyte is no longer the only indicator of fossilized earthquake ruptures.We review the criteria for seismic slip defined by Cowan (1999), and we determine that they are too narrow. Fault slip at rates in the range 10−4−101 m/s is almost certainly dynamic. This implies that features reproduced in experiments at rates as low as 10−4 m/s may be indicators of seismic slip. We conclude with a summary of the rock record of seismic slip, and lay out the current challenges in the field of earthquake geology. 相似文献
2.
断裂岩的岩石磁学研究可以揭示地震断裂作用的物理和化学环境,对于探讨地震断裂作用机制具有重要作用。本文在断裂岩岩石磁学最新文献的基础上,结合笔者及所在研究团队在龙门山断裂带获得的研究成果,综述了断裂岩的岩石磁学研究进展。大量研究发现断层泥和假玄武玻璃通常具有磁化率值或剩磁强度异常特征。顺磁性矿物在摩擦热或流体作用下形成新的铁磁性矿物是断层泥和假玄武玻璃高磁化率值或高剩磁强度的主要原因;地震断裂摩擦熔融作用中形成的单质铁是假玄武玻璃中高磁化率值或高剩磁强度异常的另一个重要原因。蠕滑断裂和出露于浅地表的断裂带中可见一些具有低磁化率值异常的断层泥,原因可能是流体作用或断裂带未经历高温摩擦热。断裂岩的岩石磁学研究为地震断裂带的应力应变、形成温度、摩擦热效应、流体作用、形成深度和氧化还原特征等提供了重要信息,可用于分析地震断裂的孕震和发震环境。综合岩石磁学测试和微米至纳米尺度的超显微学研究,并辅助地震断裂岩的摩擦实验、高温热模拟实验等研究可以更好地获得断裂岩的岩石磁学信息。 相似文献
3.
Pseudotachylytes and their host rocks from the North Range of the 1.85 Ga Sudbury impact structure have been investigated
using analytical scanning electron microscopy, electron microprobe analysis and XRF spectrometry. The results show that the
pseudotachylytes were produced in high-speed slip zones by the frictional comminution and selective melting of wall rock lithologies.
The preferential assimilation of hydrous ferromagnesian phases during frictional melting produced relatively basic melts,
leaving the more mechanically resistant quartz and, to a lesser extent, plagioclase as included mineral clasts. Three distinct
assemblages are identified within the pseudotachylytes: (a) pre-impact (>1.85 Ga) rock and mineral clasts derived from host
lithologies; (b) a syn- to immediately post-impact (1.85 Ga), rapidly cooled, quartz + sanidine + labradorite + phlogopitic
biotite matrix assemblage, formed due to crystallization from a melt at 800–900°C and (c) a post-impact (<1.85 Ga) retrograde
assemblage which overprints both clasts and matrices. Field evidence indicates that most pseudotachylyte formed in large-displacement
fault systems during gravitational collapse of the impact-generated transient cavity. The Sudbury pseudotachylytes, like endogenic
pseudotachylytes, were generated by frictional melting on fault surfaces. The difference is primarily one of scale. Large
(km) displacements occurring on impact-induced ring faults can generate immense volumes of friction melt resulting in spectacular
pseudotachylyte bodies up to 0.5 km thick and more than 10 km long.
Received: 15 March 1996 / Accepted: 15 June 1996 相似文献
4.
Fault generated melt, pseudotachylyte, is an established indicator of palaeoseismic faulting. The existing consensus that frictionally induced melting occurs within the classic seismogenic zone contrast the contention over how pseudotachylyte forms within the ductile regime. Central to this issue is whether all pseudotachylyte originates as pressure-dependent frictional melt along slip surfaces, or if pressure-independent processes have roles in its formation. Propagation of high-velocity slip into deeper crustal levels provides a satisfactory explanation for pseudotachylyte at depth, but does not of itself rationalize earthquake nucleation outside the classic seismogenic zone. Pseudotachylyte from the Minas Fault Zone, Nova Scotia, Canada is used to demonstrate the formation and preservation of fault-related melt under lower crustal conditions. Microstructures retain evidence of intense dislocation glide with minimal climb, and ductile disaggregation of the host; the latter are consistent with intracrystalline deformation in the Peierls stress-controlled glide regime. It remains unclear whether the crystal plasticity serves only as a precursory stage to rupture and high-velocity slip or is itself responsible for both instability and the thermal transient. There are similarities between accelerating plastic slip leading to rupture and aseismic creep bursts (tremor) that emphasize the mechanistic complexity of deep faulting, and the need to extend consideration beyond that of a simple brittle-ductile response. The occurrence of tremor bursts fall within the depth range of “paradoxical” pseudotachylyte and provides a circumstantial link between active tectonics and the geologic record that merits examination. 相似文献
5.
Pseudotachylyte in the Cima di Gratera ophiolite, Alpine Corsica, is distributed in the peridotite unit and in the overlying metagabbro unit and was formed under blueschist to eclogite metamorphic facies conditions, corresponding to a 60–90 km depth range. Peridotite pseudotachylyte is clustered in fault zones either beneath the tectonic contact with overlying metagabbros or at short distance from it. Fault zones are either parallel to the contact or make an angle of 55° to it. Displacement sense criteria associated with fault veins indicate top-to-the-west or top-to-the-northwest reverse senses. Cataclasite flanking most veins was formed before or coevally with frictional melting and likely mechanically weakened the peridotite, facilitating subsequent seismic rupture. In the basal part of the metagabbro unit, post-mylonitization pseudotachylyte can be distinguished from pre-mylonitization pseudotachylyte formed earlier. In the equant metagabbro above the mylonitic sole, only one episode of pseudotachylyte formation can be identified. Kinematics associated with metagabbro pseudotachylyte remain unknown. The geometry and kinematics of the pseudotachylyte veins from the peridotite unit and to a lesser extent from the metagabbro unit are similar to modern seismic ruptures of the upper parts of the Wadati-Benioff zones such as in the Pacific plate beneath NE Japan. 相似文献
6.
Large volumes of pseudotachylyte (an intrusive, fault-related rock interpreted to form by a combination of cataclasis and melting) occur in Tertiary normal faults and accommodation zones along 400 km of the East Greenland volcanic rifted margin. Analysis of representative pseudotachylyte samples reveals a wide range of mesoscopic and microscopic textures, mineralogies, and chemistries in the aphanitic pseudotachylyte matrix. Three distinct types of pseudotachylyte (referred to as angular, rounded and glassy) are identified based on these characteristics. Angular pseudotachylyte (found primarily in dike-like reservoir zones) is characterized by angular grains visible on all scales, with micron-scale fragments of mica and amphibole. Its matrix is enriched in Fe2O3, MgO, and TiO2 relative to the host rock, with minor increases in CaO, K2O, and small decreases in Na2O. Rounded pseudotachylyte is found in reservoir zones, injection veins (pseudotachylyte-filled extension fractures), and fault veins (small faults with pseudotachylyte along their surfaces). It is characterized by smooth-surfaced, compacted grains on microscopic scales, and encloses rounded, interpenetrative lithic clasts on outcrop scale. Its matrix is enriched in Fe2O3, MgO, TiO2, and Al2O3 relative to the host rock, with minor depletion in Na2O and K2O. Glassy pseudotachylyte is found primarily along fault surfaces. Its matrix is characterized by isotropic, conchoidally fractured material containing microscopic, strain-free amphibole phenocrysts, and is enriched in TiO2, Al2O3, K2O, Fe2O3, MgO, CaO, and Na2O relative to the host rock. These observations suggest that angular pseudotachylyte was produced by cataclasis, with enrichment in metallic oxides resulting from preferential crushing of mechanically weak amphibole and mica minerals found in the gneissic host rock. Cataclasis and concomitant frictional heating resulted in the textural and chemical modification of angular pseudotachylyte by sintering or melting, producing rounded and glassy pseudotachylyte, respectively. Compositional and textural observations constrain the temperatures reached during frictional heating (700–900°C) which in turn delimit the amount of frictional heat imparted to the pseudotachylytes during slip. Our results suggest that the East Greenland pseudotachylytes formed during small seismic events along faults at shallow crustal levels. Consistent relative ages and widespread occurrence of pseudotachylyte-bearing faults in East Greenland suggest that widespread microseismicity accompanied the early development of this volcanic rifted margin. 相似文献
7.
The redox conditions during frictional melting provide information on the physical and chemical conditions during seismic slip in the crust. Here we examine frictional melts from five localities by analyzing host rocks and corresponding pseudotachylytes using Mössbauer spectroscopy. The faults examined are located at South Mountain, Arizona; Fort Foster, Maine (two localities); Long Ridge fault, North Carolina; and the Homestake shear zone, Colorado. The main iron-bearing phases in the pseudotachylytes are phyllosilicates (biotite, muscovite and clays) and iron oxides (magnetite and hematite) and minor pyrite. The ferrous/ferric ratios of the phyllosilicates in the host rocks are the same as those in the pseudotachylytes, except for the hematite-bearing pseudotachylyte from the Long Ridge fault, which is more oxidized. The magnetites in the host rocks and the corresponding pseudotachylytes have different ferric and ferrous iron distributions, which is attributed to different cation chemistry, rather than redox conditions. With the exception of the South Mountain locality, the ferric/ferrous ratios of the micas are interpreted to record the primary redox state of the pseudotachylyte melt as the calculated oxygen fugacities are consistent with magnetite and hematite equilibria. Pyrite-bearing pseudotachylytes plot ~0–1 log10 units above the fayalite-magnetite-quartz (FMQ) buffer. Magnetite-bearing pseudotachylytes plot ~2–4 log10 units above the FMQ buffer, and hematite-bearing pseudotachylytes plot 3.5 log10 units above the hematite-magnetite (HM) buffer. Hematite-bearing pseudotachylytes, together with previous oxygen isotope data, are inferred to represent melting in the presence of externally derived pressurized water. Other localities are inferred to represent melting under rock-buffered, closed system, conditions. If the localities studied are representative of seismogenic faulting, the calculated oxygen fugacities indicate that, in the system C–O–H–S, H2O and CO2 should be the dominant fluid species. This is the first detailed study of the redox state of pseudotachylytes. 相似文献
8.
断层面摩擦强度是评价煤炭开采中应力扰动诱发断层滑动危险性的依据。依托晋城矿区成庄井田,采用理论分析和数值模拟计算方法,分析了断层面摩擦强度对深部地应力的约束机制,研究了成庄井田F13断层及其在不同摩擦强度条件下对回采工作面顶板稳定性、超前支承压力分布和断层滑动的影响规律。研究结果表明:地壳深部最大与最小主应力比值受断层面摩擦强度的限制,当其达到临界方向断层的摩擦强度极限时,断层就会发生滑动;断层破碎带的存在导致初始应力场扰动,形成断层带低应力区及高应力集中区,在回采过程中将直接影响煤层顶板移动变形和采动应力分布;断层面摩擦强度较小时,工作面开采至断层附近顶板下沉量及断层上下盘错动位移较大,支承压力峰值由大变小明显,断层面上剪应力与正应力的比值易达到断层面的摩擦系数,断层滑动的危险性较大。 相似文献
9.
A. R. Allen 《Journal of Structural Geology》1979,1(3):231-243
Spherulitic pseudotachylytes from the Arunta Block formed by frictional fusion of mylonitic parent rocks during high-level reactivation of a previously ductile fault zone. Fusion occurred preferentially in mica-rich domains due to release of water through disruption of the mica lattice by frictional sliding. This generated selective localised melting of mica during frictional heating, with the production of initial pseudotachylyte melts enriched in water and ferromagnesian components. Subsequent fusion of adjacent salic phases, promoted by the high water content of the existing melt, would tend to shift the trend of later melts towards a total melt composition. Therefore, under conditions of frictional sliding, fusion appears to be favoured in crystalline quartzofeldspathic rocks possessing both a high shear strength, and a significant water content locked up in the lattices of hydrous minerals, principally biotite.The presence or pre-existence of glass in many pseudotachylytes demands that they cooled in a near-surface environment, i.e. at depths of less than about 5 km. Thus glassy pseudotachylytes must postdate associated mylonite series rocks, generally forming subsequent to exhumation of the mylonites to a higher level in the crust. Some non-glassy pseudotachylytes, however, may possibly form towards the end of movements in a ductile regime, as strain hardening sets in. 相似文献
10.
Can pseudotachylytes be used to infer earthquake source parameters? An example of limitations in the study of exhumed faults 总被引：1，自引：2，他引：1
Tectonic pseudotachylytes might be used to constrain earthquake source parameters, such as dynamic shear stress resistance, average dynamic friction and slip-weakening distance. Estimation of dynamic shear stress resistance and dynamic friction from field studies is based on the assumption that the volume of melt produced during coseismic slip is proportional to the frictional work converted to heat on the fault surface. Conditions conducive to a realistic estimate of dynamic shear resistance are: (i) the presence of large outcrop exposures that allow for estimation of the volume of pseudotachylyte; (ii) the presence of structural markers offset by faults in order to relate the displacement accommodated by the fault with the volume of melt produced; (iii) data that provide an estimate of the initial melt temperature; and (iv) determination of host-rock temperature and pressure conditions that may have existed during seismic faulting. An independent indication that steady-state friction in the presence of melts might be achieved during coseismic slip arises from the dependence of the fractal dimension of the fault profile (intersection of the fault surface with the outcrop surface) with displacement. This relation could also indicate the slip-weakening distance (Hirose, T., Shimamoto, T., 2003. Fractal dimension of molten surfaces as a possible parameter to infer the slip-weakening distance of faults from natural pseudotachylytes. Journal of Structural Geology 25, 1569–1574).The above conditions are all satisfied in the case of the Gole Larghe Fault Zone, which consists of hundreds subparallel strike-slip faults that cut tonalites of the Adamello batholith (Italy). The thickness of pseudotachylyte-bearing faults increases with displacement. From displacement/thickness ratios and energy balance calculations, we determined the dynamic shear resistance for several pseudotachylyte-bearing faults. In the same faults, the fractal dimension of the fault profile increases from 1.0 to 1.16 with displacement. This was also observed in experiments where steady-state friction in the presence of melt was achieved (Hirose, T., Shimamoto, T., 2003. Fractal dimension of molten surfaces as a possible parameter to infer the slip-weakening distance of faults from natural pseudotachylytes. Journal of Structural Geology 25, 1569–1574). However, we will show that the estimate of the dynamic shear stress resistance, average dynamic friction and slip-weakening distance in the studied faults is limited by the uncertainties to attribute the measured displacement to a single seismic rupture. Since many pseudotachylytes in the upper seismogenic crust overprint preexisting cataclasites, it is suggested that future field and experimental work should be addressed to determine microstructural indicators (i.e. evolution of cataclastic fabric with displacement) within cataclasites, which might constrain the contribution of the cataclastic, pre-pseudotachylyte displacement to the total displacement accommodated by the fault. 相似文献
11.
Pseudotachylytes are generally considered to be remnent products of past earthquakes, conveying significant information, which provide improved insight into the fault behavior and their mechanical properties. Pseudotachylytes and cataclastic rocks are exposed in the Pengguan complex along the Yingxiu-Beichuan fault zone, which ruptured during the 2008 Wenchuan earthquake, at the eastern margin of the Tibetan Plateau in Sichuan province, China. From outcrop investigations, pseudotachylyte veins are gray, dark-gray, brown and black, with thicknesses ranging from several mm to ∼10 cm. Microstructural observations, via optical microscope and SEM, show that distinct textures, such as spherulites, microlites, honeycomb-like vesicle structures, flow textures, irregularly shaped and deep embayed clasts, are present in the pseudotachylytes, suggesting that the pseudotachylytes are indeed melt-origin. Different colored veins and their overprinting relations observed at both macroscopic and microscopic scales, demonstrate that seismic faulting events and the associated generations of pseudotachylyte occurred repeatedly along the Yingxiu-Beichuan fault zone. Friction melt lubrication, thermal pressurization and mechanical lubrication, all can exist during one fault slip motion, indicating that multiple fault weakening mechanisms may work during a single coseismic event. 相似文献
12.
Although the Indus-Tsangpo Suture(ITS) is the most spectacular thrust system of continent-continent collision in the world, fundamental questions about its strength evolution and deformation behavior transition remain unanswered. Here we reported, for the first time, frictional melting-induced pseudotachylytes in the intensively deformed felsic rocks along the ITS zone in southern Tibet. This study reveals that pseudotachylytes induced profound weakness of the boundary fault between Indian and Asian plates. The intrinsically low strength of the foliated microlites crystallized from frictional melt or glass(i.e., pseudotachylyte) at seismogenic depths compared with the surrounding coarse-grained quartzofeldspathic rocks in the brittle and semi-brittle regime is sufficient to explain the localization of shear strain, the development of ductile shear zones embedded in strong wall rocks, and the transition from the strong to weak fault behaviors without invoking the presence of high fluid pressure or low friction coefficient metasomatic materials(e.g., smectite or lizardite) within the faults. 相似文献
13.
In the North Cascade Mountains, Washington, rocks that underwent friction melting commonly contained sulfide minerals, mostly pyrrhotite and pyrite. During pseudotachylyte melt formation, sulfides melted to form immiscible sulfide droplets present in five distinct textural settings. The largest droplets formed through melting of lithic clasts, whereas micron-scale sulfide droplets are common in many of the pseudotachylytes veins.Microprobe analysis indicates that nearly all droplets are pyrrhotite. The disappearance of pyrite indicates that melt temperatures must have exceeded 750 °C, but other indications suggest that the melt temperature must have been much higher. The extremely common presence of pyrrhotite droplets suggests that pyrrhotite from the protolith melted, requiring a minimum melt temperature of 1200 °C. In some samples, evidence for fluid-rich bubbles, and possible silicate spherules indicates three coexisting immiscible phases within the silicate melt. The presence of sulfide droplets appears to be common, especially in relatively low oxygen-fugacity melts that formed at shallow crustal levels. This can provide a good textural marker of melting and therefore of pseudotachylyte formation, especially where other indications of melting (i.e., high temperature microlites, vesicles, etc.) are lacking, and illustrates the extreme temperatures possible along frictionally sliding surfaces during seismic events. 相似文献
14.
《Journal of Structural Geology》2003,25(10):1569-1574
High-velocity friction experiments on gabbro and monzodiorite, using a rotary-shear high-velocity friction apparatus, have revealed that frictional melting and progressive growth of a molten layer along a fault cause slip weakening, eventually reaching a nearly steady-state. The melting surface at the host rock/molten layer interface is initially very flat, but it becomes more complex and rounded in shape towards the steady state owing to the selective melting of minerals with lower melting points and the Gibbs–Thomson effect. This change in the melting-surface topography can be quantitatively expressed by the fractal dimension D, as determined by the divider method, from about 1.0 near the peak friction to around 1.1 near the steady-state friction. The ultimate fractal dimension at steady-state friction tends to decrease with increasing heat production rate presumably due to more rapid and uniform melting. A systematic correlation of D with mechanical behavior of the fault during frictional melting may provide a way of estimating slip-weakening distance and heat production rate at steady-state friction by measuring D for natural pseudotachylytes on slip surfaces with different displacements. The weakening distance is of vital significance in relation to fault instability and the heat production rate is related to the fault strength. The experimental studies point to ways to estimate these difficult quantities for natural faults. 相似文献
15.
ESR detection of seismic frictional heating events in the Nojima fault drill core samples, Japan 总被引：3，自引：0，他引：3
We have analyzed the Nojima fault NIED 1800 m drill core samples by ESR (Electron Spin Resonance) to detect seismic frictional heating events, especially during the 1995 Kobe Earthquake. Dark gray fault gouge with foliation > 10 cm away from the fault plane at about 1140 m in depth, which was produced by ancient fault movements, has a FMR (ferrimagnetic resonance) signal. Heating experiments show that this FMR signal is derived from ferrimagnetic trivalent ion oxides (γ-Fe2O3: maghemite) with imperfect crystallinity, which is produced by thermal dehydration of γ-FeOOH (lepidocrocite) or Fe(OH)3 (limonite). The existence of the FMR signal means that dry heating such as frictional heating once occurred, and that the frictional heat temperature along the dark gray fault gouge may have risen to over 350 °C during ancient seismic fault slip. In order to detect frictional heating events in fault zones, the increase of the FMR signal and the color change of fault gouge into dark gray or black are important indexes. On the other hand, no FMR signal is detected from the fault gouges just on two fault planes at about 1140 m and 1300 m in depth, which are considered to be possible main fault planes in the 1995 Kobe Earthquake. These two fault planes may not have played an important role of fault slip in the Earthquake. 相似文献
16.
The anisotropy of magnetic susceptibility (AMS) is widely used to analyze magmatic flow in intrusive igneous bodies including plutons, sills and dikes. This method, owing its success to the rapid nature of measurements, provides a proxy for the orientation of markers with shape anisotropy that flow and align in a viscous medium. AMS specimens typically are 25 mm diameter right cylinders or 20 mm on-a-side cubes, representing a volume deemed statistically representative. Here, we present new AMS results, based on significantly smaller cubic specimens, which are 3.5 mm on a side, hence∼250 times volumetrically smaller than conventional specimens. We show that, in the case of frictional melts, which inherently have an extremely small grain size, this small volume is in most cases sufficient to characterize the pseudotachylyte fabric, particularly when magnetite is present. Further, we demonstrate that the mini-AMS method provides new opportunities to investigate the details of frictional melt flow in these coseismic miniature melt bodies. This new method offers significant potential to investigate frictional melt flow in pseudotachylyte veins including contributions to the lubrication of faults at shallow to moderate depths. 相似文献
17.
The >200 km long Moonlight Fault Zone (MFZ) in southern New Zealand was an Oligocene basin-bounding normal fault zone that reactivated in the Miocene as a high-angle reverse fault (present dip angle 65°–75°). Regional exhumation in the last c. 5 Ma has resulted in deep exposures of the MFZ that present an opportunity to study the structure and deformation processes that were active in a basin-scale reverse fault at basement depths. Syn-rift sediments are preserved only as thin fault-bound slivers. The hanging wall and footwall of the MFZ are mainly greenschist facies quartzofeldspathic schists that have a steeply-dipping (55°–75°) foliation subparallel to the main fault trace. In more fissile lithologies (e.g. greyschists), hanging-wall deformation occurred by the development of foliation-parallel breccia layers up to a few centimetres thick. Greyschists in the footwall deformed mainly by folding and formation of tabular, foliation-parallel breccias up to 1 m wide. Where the hanging-wall contains more competent lithologies (e.g. greenschist facies metabasite) it is laced with networks of pseudotachylyte that formed parallel to the host rock foliation in a damage zone extending up to 500 m from the main fault trace. The fault core contains an up to 20 m thick sequence of breccias, cataclasites and foliated cataclasites preserving evidence for the progressive development of interconnected networks of (partly authigenic) chlorite and muscovite. Deformation in the fault core occurred by cataclasis of quartz and albite, frictional sliding of chlorite and muscovite grains, and dissolution-precipitation. Combined with published friction and permeability data, our observations suggest that: 1) host rock lithology and anisotropy were the primary controls on the structure of the MFZ at basement depths and 2) high-angle reverse slip was facilitated by the low frictional strength of fault core materials. Restriction of pseudotachylyte networks to the hanging-wall of the MFZ further suggests that the wide, phyllosilicate-rich fault core acted as an efficient hydrological barrier, resulting in a relatively hydrous footwall and fault core but a relatively dry hanging-wall. 相似文献
18.
近六十多年来,关于造山带高压变质作用发生的原因一直存在争议。当前的主流观点认为高压变质岩形成于俯冲带深部以后折返到地壳。虽然早就认识到蓝片岩相变质作用总是与逆冲断裂相伴并且变质压力朝着冲断面增高,而标志地震的榴辉岩相假玄武玻璃也已经发现了二十多年,但是一般认为断裂或剪切带的作用只是引进流体促进深部岩石的变质反应。几年前,苏文辉等人(Su et al.,2006)通过实验演示了石英经过非晶化可以在柯石英稳定的压力下迅速转变为柯石英,提出无需深俯冲,浅源地震即可形成柯石英。本文介绍笔者等人对中国苏鲁超高压变质带仰口榴辉岩的部分工作结果,从地质观察角度为这一论点提供证据。例如,粒间柯石英仅出现在榴辉岩相角砾岩的角砾而非胶结物中,它必须在相当于地震的时间尺度快速降压冷却才能得以保存。变辉长岩中的榴辉岩相碎裂岩脉也记录了应力导致的瞬时高压和高温。角砾岩和碎裂岩中包含大量高压矿物的石榴石和针状蓝晶石以及局部出现的"显微花岗岩"都是类似地震熔体淬火的结果。它们指示震后高压矿物没有再生长。而如果地震发生在深部,流体浸入后矿物应当在高压下持续结晶,从而消除淬火结构。碎裂岩脉中星散的铬铁矿微粒极可能是附近超镁铁岩通过断裂迸溅进入基性岩的,是地震中不同岩石机械混合的证据;环绕它们的富铬榴辉岩矿物记录着一次无可争议的瞬时高压结晶事件。碎裂岩脉相对围岩更缺乏流体的事实反映应力而非流体在高压相变中的关键作用。榴辉岩相角砾岩和碎裂岩脉形成的时间尺度太小,来不及完成俯冲和折返过程。它们更可能是辉长岩在地壳原地因地震波引起的高压发生了榴辉岩化作用。目前的地震力学模型不能导出高压相变所需要的压力,只是因为建立模型时没有考虑涉及高压相变的地震。需要强调的是,假说或理论需要观察事实的检验而不是相反。已有资料显示,瞬时熔融和非晶化可能是震击高压相变的路径。 相似文献
19.
Bhuban Mohan Behera V Thirukumaran Aishwaraya Soni Prasanta Kumar Mishra Tapas Kumar Biswal 《Journal of Earth System Science》2017,126(4):46
Gangavalli (Brittle) Shear Zone (Fault) near Attur, Tamil Nadu exposes nearly 50 km long and 1–3 km wide NNE–SSW trending linear belt of cataclasites and pseudotachylyte produced on charnockites of the Southern Granulite Terrane. Pseudotachylytes, as well as the country rock, bear the evidence of conjugate strike slip shearing along NNE–SSW and NW–SE directions, suggesting an N–S compression. The Gangavalli Shear Zone represents the NNE–SSW fault of the conjugate system along which a right lateral shear has produced seismic slip motion giving rise to cataclasites and pseudotachylytes. Pseudotachylytes occur as veins of varying width extending from hairline fracture fills to tens of meters in length. They carry quartz as well as feldspar clasts with sizes of few mm in diameter; the clast sizes show a modified Power law distribution with finer ones (<1000 \({\upmu }\)m\(^{2})\) deviating from linearity. The shape of the clasts shows a high degree of roundness (>0.4) due to thermal decrepitation. In a large instance, devitrification has occurred producing albitic microlites that suggest the temperature of the pseudotachylyte melt was >1000\(^{\circ }\hbox {C}\). Thus, pseudotachylyte veins act as a proxy to understand the genetic process involved in the evolution of the shear zone and its tectonic settings. 相似文献
20.
Summary The structural and chemical characteristics of pseudotachylytes generated during seismic events along a Pan-African fault zone in Kenya document an evolution consisting of two principal steps. In the first stage, crushing of the host rock during the onset of frictional sliding led to preferential disruption of biotite and hornblende, due to their low fracture toughness and low shear yield strength. The products of this first stage are preserved as thin cataclasite zones along the margins of the pseudotachylyte veins. Melting of the crushed host rock occurred during the second stage, due to the heat generated by frictional sliding, grain size reduction, and the release of water from biotite and hornblende. The chemical and mineralogical composition of the cataclasite and the increasing temperature during seismic slip were the main factors that controlled the composition of two chemically distinct pseudotachylyte melts. During rapid cooling, amphibole microlites (melt 1) and plagioclase microlites (melt 2) crystallized from the two pseudotachylyte melts.
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Die strukturelle und chemische Entwicklung von Pseudotachylyten während seismischer Ereignisse
Zusammenfassung Die strukturellen und chemischen Eigenschaften von Pseudotachylyten, die durch seismische Ereignisse entlang einer Pan-Afrikanischen Störungszone in Kenia erzeugt wurden, dokumentieren eine zweistufige Entwicklung. Im ersten Stadium, zu Beginn des Reibungsgleitens, führte die mechanische Zerkleinerung des Ausgangsgesteins zu einem bevorzugten Zerbrechen von Biotit und Hornblende, aufgrund ihrer geringen Bruch- und Scherfestigkeit. Die Produkte dieses ersten Stadiums sind in Form dünner Kataklasitzonen an den Rändern der Pseudotachylitgänge erhalten. Während des zweiten Stadiums kam es aufgrund der Reibungswärme, der Kornverkleinerung und dem bei der Zerstörung von Biotit und Hornblende freigesetzten Wasser zum Aufschmelzen des zermahlenen Gesteins. Die chemische und mineralogische Zusammensetzung der Kataklasite und die zunehmende Temperatur während des seismischen Gleitens waren die wesentlichen Faktoren, die die Zusammensetzung zweier chemisch unterschiedlicher Schmelzen kontrollierten. Während der schnellen Abkühlung kristallisierten Amphibol-Mikrolithe (Schmelze 1) und Plagioklas-Mikrolithe (Schmelze 2) aus den beiden Pseudotachylit-Schmelzen.
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