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1.
Detailed (1:60 scale) mapping of the Fort Foster Brittle Zone in the mylonitic Rye Formation of southernmost Maine has revealed the intricate internal duplex structure of a system of probable Paleozoic-age dextral strike-slip faults that have produced abundant pseudotachylyte and minor breccia. The internal configuration of this brittle zone consists of a mosaic of individual pseudotachylyte generation zones as slab-duplex structures. Individual duplex zones are up to 100 m in length and 1 m or less in width and are defined by pairs of layer-parallel slip surfaces along which frictional melts were produced. These slab-duplex structures are interpreted as zones of displacement transfer between long, overlapping, layer-parallel en échelon strike-slip fault surfaces. Contractional duplexes develop layer-parallel compressional structures that tend to shorten and thicken the fault-bounded slabs by the formation of lateral ramps and conjugate faults, kinks and asymmetric folds. Extensional duplexes develop layer-parallel stretching and thinning by the formation of oblique dextral shears, high-angle conjugate pairs and localized fault breccias. The production of pseudotachylyte by friction melting along layer-parallel fault surfaces in these exposures is attributed to rapid slip during paleoseismic events. The rupture structures developed during these events may be characteristic of fault structure and mechanics at near-focal depths in a strike-slip seismogenic zone.  相似文献   

2.
The Olinghouse fault zone is one of several NE—ENE-trending fault zones and lineaments, including the Midas Trench and the Carson—Carson Sink Lineament, which exhibit left-lateral transcurrent movement conjugate to the Walker Lane in western Nevada. The active portion of this fault zone extends for approximately 23 km, from 16 km east of Reno, Nevada, to the southern extent of Pyramid Lake. The fault can be traced for most of its length from its geomorphic expression in the hilly terrain, and it is hidden only where overlain by recent alluvial sediments. Numerous features characteristic of strike-slip faulting can be observed along the fault, including: scarps, vegetation lines, sidehill and shutter ridges, sag ponds, offset stream channels and stone stripes, enclosed rhombohedral and wedge-shaped depressions, and en-echelon fractures.A shear zone having a maximum observable width of 1.3 km is defined principally by Riedel shears and their symmetrical P-shears, with secondary definition by deformed conjugate Riedel shears. Several continuous horizontal shears, or principal displacement shears, occupy the axial portion of the shear zone. The existence of P-shears and principal displacement shears suggests evolution of movement along the fault zone analogous to the “Post-Peak” or “Pre-Residual Structure” stage.Historic activity (1869) has established the seismic potential of this zone. Maximum intensities and plots of the isoseismals indicate the 1869 Olinghouse earthquake had a magnitude of 6.7. Field study indicates the active length of the fault zone is at least 23 km and the maximum 1869 displacement was 3.65 m of left-slip. From maximum fault length and maximum fault displacement to earthquake magnitude relations, this corresponds to an earthquake of about magnitude 7.  相似文献   

3.
Fault zones commonly consist of discontinuous surfaces or are composed of different kinds of en echelon fractures (tension cracks, R or P-type fractures) which give the brittle shear zone a width. Translation along these faults and related features can occur by sliding on the elementary planes and/or by solution/deposition processes with opening of transtension zones and possible dilatation of the shear zone.In order to understand how fault planes develop and to investigate the mechanical conditions corresponding to natural configurations, displacement along an analogue fault model was examined under conditions of direct shear. The experimental fracture patterns were then compared with the natural features. The structural elements of certain faults created by testing, and showing transpression and transtension zones, were found to be similar to natural domino structures bounded by elementary shears, and could be compared with computed situations.It can be inferred that stresses are reoriented inside the shear zone; the angle between elementary fractures depends on their order of development; transpression and transtension zones occur systematically; and the shear zone undergoes dilatancy under low normal stresses.  相似文献   

4.
The Pyramid Lake fault zone is within the Honey Lake—Walker Lake segment of the Walker Lane, a NW-trending zone of right-slip transcurrent faulting, which extends for more than 600 km from Las Vegas, Nevada, to beyond Honey Lake, California. Multiscale, multiformat analysis of Landsat imagery and large-scale (1: 12,000) lowsun angle aerial photography, delineated both regional and site-specific evidence for faults in Late Cenozoic sedimentary deposits southwest of Pyramid Lake. The fault zone is coincident with a portion of a distinct NW-trending topographic discontinuity on the Landsat mosaic of Nevada. The zone exhibits numerous geomorphic features characteristic of strike-slip fault zones, including: recent scarps, offset stream channels, linear gullies, elongate troughs and depressions, sag ponds, vegetation alignments, transcurrent buckles, and rhombohedral and wedge-shaped enclosed depressions. These features are conspicuously developed in Late Pleistocene and Holocene sedimentary deposits and landforms.The Pyramid Lake shear zone has a maximum observable width of 5 km, defined by Riedel and conjugate Riedel shears with maximum observable lenghts of 10 and 3 km, respectively. P-shears have formed symmetrical to the Riedel shears and the principal displacement shears, or continuous horizontal shears, isolate elongate lenses of essentially passive material; most of the shears are inclined at an angle of approximately 4° to the principal direction of displacement. This suggests that the shear zone is in an early “PreResidual Structure” stage of evolution, with the principal deformation mechanism of direct shear replacing the kinematic restraints inherent in the strain field.Historic seismic activity includes microseismic events and may include the earthquake of about 1850 reported for the Pyramid Lake area with an estimated Richter magnitude of 7.0. Based on worldwide relations of earthquake magnitude to length of the zone of surface rupture, the Pyramid Lake fault zone is inferred to be capable of generating a 7.0–7.5-magnitude event for a maximum observable length of approximately 6 km and a 6.75–7.25-magnitude event for a half length of approximately 30 km.  相似文献   

5.
The paper presents the results of electromagnetic radiation (EMR) measurements in the Feuerberg tunnel in southwest Germany. EMR is associated with small scale fracturing processes. The measured numbers of EMR impulses are shown to be proportional to shear stresses. From the correlation of EMR and shear stresses along the long axis of the tunnel, orientations and magnitudes of the horizontal principal stresses are determined. The major horizontal principal stress is 3.6±0.3 MPa and has an azimuth of 143±6°. The minor principal horizontal stress is 2.1±0.3 MPa. Zones in the tunnel are located where low shear stresses occur because vertical overburden and horizontal stresses are equal. In these zones also minimum radiation was detected. A possible stress accumulation close to a fault is suggested by higher EMR values in a part of the tunnel. Orientations and magnitudes of the horizontal principal stresses, which are derived from the measurements of EMR, correlate well with conventional stress measurements. It is suggested that the cross-section measuring method described in the study is used to determine regional stress fields as well as to investigate endangered zones with high stresses in underground facilities, which may be critical with regard to stability.  相似文献   

6.
A 150 μm thick fused layer of rock has been produced by rotating two metadolerite core faces against each other at 3000 r.p.m. under an axial load of 330 kg for 11 s using friction welding apparatus. Scanning electron microscopy and electron microprobe analysis reveal that the melt layer comprises sub-angular to rounded porphyroclasts of clinopyroxene, feldspar and ilmenite (>20 μm diameter), derived from the host metadolerite, set within a silicate glass matrix. Thermal calculations confirm that melting occurred at the rock interface and that mean surface temperatures in excess of 1400°C were attained. The fused layer shows many textural similarities with pseudotachylyte described from fault zones. Morphologically, the fused layer consists of a series of stacks of porphyroclasts welded together by melt to form ‘build-ups’ oriented at right angles to the friction surface. There is also evidence of gouging, ploughing and plucking, as well as transfer and adhesion of material having occurred between the rock faces. The mean surface velocity attained by the metadolerite (0.24 m s−1) and duration of the experiment are comparable with velocities and rise times of typical single jerk earthquakes occurring during stick-slip seismic faulting within brittle crust (i.e. slip rates of 0.1-0.5 m s−1 for, say, 1–10s). In these respects the experiment successfully simulated frictional fusion on a fault plane in the absence of an intergranular fluid. Power dissipation during the experiment was about MW m−2, comparable only to very low values for earthquakes (e.g. 1–100 MW m−2 for displacement rates of 0.1-0.5 m s−1 at shear stresses of 100–1000 bars). This indicates that melting on fault planes during earthquakes should be commonplace. Field evidence, however, does not support this contention. Either pseudotachylyte is not being recognized in exhumed ancient seismic fault zones or melting only occurs under very special circumstances.  相似文献   

7.
Alpine deformation of Austroalpine units south of the Tauern window is dominated by two kinematic regimes. Prior to intrusion of the main Periadriatic plutons at ~30 Ma, the shear sense was sinistral in the current orientation, with a minor north-side-up component. Sinistral shearing locally overprints contact metamorphic porphyroblasts and early Periadriatic dykes. Direct Rb-Sr dating of microsampled synkinematic muscovite gave ages in the range 33-30 Ma, whereas pseudotachylyte locally crosscutting the mylonitic foliation gave an interpreted 40Ar-39Ar age of ~46 Ma. The transition from sinistral to dextral (transpressive) kinematics related to the Periadriatic fault occurred rapidly, between solidification of the earlier dykes and of the main plutons. Subsequent brittle-ductile to brittle faults are compatible with N-S to NNW-SSE shortening and orogen-parallel extension. Antithetic Riedel shears are distinguished from the previous sinistral fabric by their fine-grained quartz microstructures, with local pseudotachylyte formation. One such pseudotachylyte from Speikboden gave a 40Ar-39Ar age of 20 Ma, consistent with pseudotachylyte ages related to the Periadriatic fault. The magnitude of dextral offset on the Periadriatic fault cannot be directly estimated. However, the jump in zircon and apatite fission-track ages establishes that the relative vertical displacement was ~4-5 km since 24 Ma, and that movement continued until at least 13 Ma.  相似文献   

8.
3500 m以深页岩气资源量占整个川南地区总资源量的比例高达86.5%,该区深层页岩气藏构造复杂,压裂形成复杂缝网的难度大,有必要通过数值模拟研究深层页岩气复杂缝网主控因素,对实现川南地区深层页岩气的效益开发具有重要意义。在对川南地区页岩气气田某井的岩芯进行细观尺度下的观察并构建二维裂缝模型的基础上,利用位移间断边界元法(DDM)模拟深层页岩水力压裂过程中水力裂缝与天然裂缝相互作用的物理力学过程,研究主应力、应力差和压裂液排量对裂缝扩展的影响。结果表明：在高应力差条件下缝网的复杂程度和总长度急剧降低,缝网的平均宽度增大,且平均宽度随排量增加而增大的能力变得有限。在高应力差条件下提升压裂液排量,缝网长度的增加以产生新生裂缝为主,同时提升排量对于激活天然裂缝有一定的提升作用,但是效果有限。相比于拉张裂缝,剪切裂缝的形成受主应力和压裂液排量的影响更显著,在高应力差条件下缝网中剪切裂缝的长度急剧降低。随着压裂液的注入,在较低应力差和相同压裂液注入量的情况下,低排量工况下的裂缝长度逐渐大于高排量工况下的裂缝长度。在应力差较高的情况下裂缝扩展的速率较低,同时会使提升排量而形成更多新生裂缝的能力变得...  相似文献   

9.
A suite of migmatites in uppermost amphibolite facies schists of the Koettlitz Group exposed in the Taylor Valley, Antarctica, provides direct evidence of the behaviour of partially molten rock during syn-anatectic deformation. The geometry of the migmatites is directly related to their position relative to the hinge of a kilometre-scale antiform. Migmatitic rocks on the fold limbs are characterized by extensional shears and fractures, filled with leucosome material, that intersect the pervasive foliation and millimetre-thick stromatic leucosomes. Vein- and dyke-like leucosomes become more common and thicker from the limb to the hinge region of the antiform. Rocks characterized by high leucosome-to-rock ratios near the antiform hinge are xenolithic in appearance. Major parasitic folds within the hinge contain leucogranite 'microplutons' up to 50 m across beneath refractory 'cap-rock' layers.
Angular boudinage structures in schists surrounded by leucosomes indicate a relatively low yield strength in the leucosome, which is compatible with a molten rather than solid leucosome. Leucogranite-bearing extensional shears and fractures indicate that repeated extensional fracturing and shearing promoted by high fluid (melt) pressure is an important mechanism of melt segregation. Dilation in the hinges of developing folds aids the migration of melt into fold hinges and the development of 10–50-m-wide 'microplutons' of xenolith-rich leucogranite.
Lack of vapour-absent melting and consequent low melt-to-rock ratios allowed the Koettlitz Group to maintain its structural coherency on a kilometre scale. Consequently, leucosome 'microplutons' did not exceed 50 m in width, and therefore observed leucosomes have not contributed to the development of adjacent plutonic-scale granitoids.  相似文献   

10.
Pseudotachylytes occur associated with mylonite and ultramylonite in the Mahanadi shear zone (MSZ) in the Eastern Ghats Mobile Belt (EGMB). The MSZ is about 200 km long curvilinear high strain zone trending WNW-ESE in its eastern part that splays out in the west. In Kantilo-Ganian segment of MSZ in northern EGMB, an interbanded sequence of granulite facies lithoassemblage has undergone ductile shearing. Kinematic studies of mylonite and ultramylonite indicate MSZ to be a NE-dipping, extensional type ductile shear zone. Non-coaxial metamorphic growth of garnet and presence of truncated sillimanite-fish in ultramylonite suggest high temperature regime during shearing. Pseudotachylytes in MSZ occur as millimetre thick layers to decimetre thick zones containing fragments of mylonite, ultramylonite and lithic clasts. Pseudotachylyte generation veins are mostly sub-parallel to C-planes and the injection veins cross-cut at high angle to these. The presence of an isotropic glassy matrix, injection features, corroded grains and dendritic microlites can be evidences for the existence of a melt phase. The composition of pseudotachylyte matrix (by EPMA) indicates silica deficiency with higher normative hypersthene, plagioclase and lower quartz compared with average whole rock composition for host. Absence of overprinting of mylonitic fabric on pseudotachylytes indicates their formation by brittle failure without passing through a plastic deformation and thus a two stage development for mylonite-ultramylonite and pseudotachylyte generation is suggested.  相似文献   

11.
Many of the major lineaments in southern Africa are major ductile shear zones with large displacement, occurring within, though often bounding orogenic belts. An example is the boundary to the Limpopo belt in Botswana and Zimbabwe. However, some of these shear zones only record slight displacement when considered on a crustal scale; they are merely planes recording differential movement on much larger, flat to gently dipping, shear zones where the boundary to the orogenic belt is a low-angle thrust zone. These different types of shear zones are clearly shown in the Pan-African belt of Zambia where large ENE-trending lineaments have been recorded. Recent work has shown the northern group of shears to be large lateral ramps; for example, the rocks of the copper belt are part of an ENE-verging thrust package, the southern boundary of which is a major, oblique to lateral ramp. In southern Zambia shears are more analogous to major transform faults; they form as tear faults separating zones of different thrust vergence. A possible plate tectonic model is given for this part of Africa, showing the different relative plate movement vectors estimated from the geometry of the Pan-African shear zones.  相似文献   

12.
闽—粤东南沿海大陆边缘韧性剪切带的基本特征   总被引：2，自引：0，他引：2  
东南沿海大陆边缘剪切带是西太平洋活动大陆边缘构造带的组成部分,它是一条具有多次活动的左旋韧性平移剪切带。在本剪切带中可以观察到3种类型的构造:（1）规模巨大的呈北北东—北东向展布的糜棱岩带以及山拉伸线理组成的线状构造带。它们在平面上有明显的从断目两侧向中心递进变形特征;（2）呈北东走向陡倾角的糜棱叶理（Sa）、应变滑劈理或破劈理（Sb）、小型剪切带（Sc）;（3）由西到东断层作用样式和断层岩具有明显的递进变化特点。西部（浅部）断层作用以脆性剪切滑动为主,其断层岩则由假玄武玻璃及镜面糜棱岩组成;中部断层作用以跪—韧性剪切为主,为断层泥—碎裂岩—超碎裂岩;东部(深部,以韧性剪切作用为主,其断层岩为暖棱岩—花岗糜桂片麻岩—眼球状糜棱岩系列。以上特点表明在本剪切带内透入性和非透入性变形之间有着密切的关系,反映出在变形过程中具有由韧性变形逐渐向脆性变形的发展趋势。  相似文献   

13.
Typical pull-apart structures were created in scaled clay experiments with a pure strike-slip geometry (Riedel type experiments). A clay slab represents the sedimentary cover above a strike-slip fault in the rigid basement. At an early stage of the development of the deformation zone, synthetic shear fractures (Riedel shears) within the clay slab display dilatational behaviour. With increasing basal displacement the Riedel shears rotate and open further, developing into long, narrow and deep troughs. The shear displacement and the low angle with the prescribed principal basal fault set them apart from tension gashes. At a more evolved stage, synthetic segments (Y-shears) parallel to the basal principal fault develop and accommodate progressive strike-slip deformation. The Y-shears connect the tips of adjacent troughs developed from the earlier Riedel shears, resulting in the typical rhomb-shaped structures characteristic for pull-apart basins. The Strait of Sicily rift zone, with major strike-slip systems being active from the Miocene to the Present, comprises pull-apart basins at different length scales, for which the structural record suggests development by a mechanism similar to that observed in our experiments.  相似文献   

14.
Surface fractures in the overburdened sedimentary rocks, formed above a deep-seated basement fault, often provide important information about the kinematics of the underlying master fault. It has already been established that these surface fractures dynamically evolve and link one another with progressive displacement on the master fault below. In the present study, two different series of riedel-type experiments were carried out with clay analogue models under different boundary conditions viz., (i) heterogeneous simple shear of the cover rocks above a buried strike slip fault (wrench system) and (ii) heterogeneous simple shear with a component of shear-normal compression of the overburden package above a basement fault (transpressional system), to observe the initiation and linkage of surface fractures with varying T′ (where T′ = thickness of the overburden normalized with respect to the width of the master fault). In the wrench system, Riedel (R) shears were linked by principal displacement (Y) shears at early stages (shear strain of 0.8 to 1) in thin (2 < T′ < 18) models and finally (at a minimum shear strain of 1.4) gave rise to a through-going fault parallel to the basement fault without development of any other fracture. Conjugate Riedel (R′) shears develop only within the thicker (T′ > 18) clay models at a minimum shear strain of 0.7. With increasing deformation (at a minimum shear strain of 1.2) two R′ shears were joined by an R shear and finally opened up to make a sigmoidal vein with an asymmetry antithetic to the major fault-movement sense. Under transpression, the results were similar to that of heterogeneous simple shear for layers 2 < T′ < 15. In layers of intermediate thickness (15 < T′ < 25) early formed high angle R shears were cross cut by low angle R shears (at a minimum shear strain of 0.5 and shortening of 0.028) and “Riedel-within-Riedel” shears were formed within thick (T′ > 25) models (at minimum shear strain of 0.7 and shortening of 0.1), with marked angularity of secondary fault zone with the master fault at depth.  相似文献   

15.
Vein structures are typically the earliest expression of brittle deformation within sediments. These mud-filled veins, which characteristically occur regularly spaced within bed-parallel arrays, form in sediments that possess a strong interlocking particle framework. Downslope creep has been proposed to explain the origins of vein structures, however, a recent suggestion that they are generated by the passage of earthquake shear waves through sediments explains aspects of their morphology, and their dominant occurrence at active convergent margins. Their coexistence with less disruptive “ghost veins” in Peru margin sediments, and their almost normal attitude to bedding, however, suggests that vein structures were formed by processes more complex than downslope creep, or seismically induced shearing alone.

Experimental earthquake simulation was undertaken by laterally shaking a box containing crushed diatomite. Fractures were induced almost normal to the horizontal shaking direction, and to a lesser extent as antithetic Riedel shears, both of which closely resembled vein structures. The fracturing process during shaking may be viewed as a progressive fragmentation of the diatomite, in which new fractures form half-way between pre-existing ones. Thus fracture spacings are progressively halved. Shear zones oriented at a low angle to the shaking direction were also generated, combining with the high-angle fractures to form structures very similar to those observed in Peru margin sediments. When shaken, fines added to the diatomite segregated into planar zones that resembled ghost veins, half-way between fractures. The alternating pattern of fractures and fines indicated that a standing pressure wave had been created within the box during shaking. The fractures were created by alternating compression and extension at the antinodes, while the fines concentrated in zones of minimum grain movement around the nodal planes. This suggests that vein structures are initiated by the combined action of shear and pressure waves within a sediment. The strain waves may be seismic in origin, or may also form in downslope movement system.  相似文献   


16.
In-situ deformation experiments were performed on partially molten analog materials (norcamphor in the presence of a benzamide–norcamphor melt) undergoing pure shearing at a constant melt fraction of 0.13. Melt in the samples induces a strain-dependent transition from purely dislocation creep to dislocation creep associated with minor intergranular fracturing and grain boundary sliding (GBS). Intergranular fractures drain the melt from initially isotropic melt pockets to grain boundaries. Along such boundaries, grain-boundary migration recrystallization is inhibited, while GBS occurs. Intergranular melt pockets occur along grain boundaries oriented subparallel to the shortening direction, but melt must have migrated parallel to the elongation direction of the samples, as indicated by melt accumulations at both extruding ends of the sample. Intergranular melt pockets parallel to the elongation direction were only rarely observed, because melt was rapidly expelled from these sites. Nevertheless, these grain boundaries are the pathways of melt segregation in the samples.  相似文献   

17.
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   

18.
This study relates the anatectic generation of in situ granites to deformation in the Spanish Central System. Regionally, the principal rocks of the anatectic complexes in central Iberia are migmatites, and interlayered with them are anatectic granites. The second phase of the Hercynian orogeny is characterized by subhorizontal structures (e.g. shears, folding) due to a contractional deformation, followed by extensional collapse of the orogen. Melting and the generation of in situ granites occurred in fertile rocks, but was concentrated in subhorizontal structures formed during the contractional deformation. Accumulation of volatile elements, such as boron, in shear structures may have triggered the anatexis, and enabled local migration of the melt fraction. This process, plus the existence of a fertile source, are determining factors in the generation of different melt-fraction granitoids.  相似文献   

19.
Small-scale structures along strike-slip fault zones in limestones exposed around the Bristol Channel, U.K., suggest that pressure solution plays a key role during fault nucleation and growth. Incipient shear zones consist of enéchelon veins. The first generation of solution seams form due to bending of the intact rock (bridge) between overlapping veins. As the bridge rotates, slip occurs along the seams, linking the veins, causing cm-scale calcite-filled pull-apart structures to form and allowing fault displacement to increase. A second generation of solution seams forms at the tip of the sliding seams. As displacement increases further, causing larger rotation, slip also can occur along these second-generation solution seams, producing the third generation of solution seams as well as tail cracks (pinnate veins) at their tips. These three generations of solution seams all contribute to the formation of individual fault segments. Fourth and fifth generations of solution seams occur within larger-scale contractional oversteps between side-stepping fault segments. The oversteps are breached by slip along these localized solution seams, eventually leading to the formation of a distinct through-going fault with several metres of displacement.The initial enéchelon veins, solution seams of various generations and tail cracks progressively fragment the fault-zone material as fault slip accumulates. Slip planes nucleate on these pre-existing discontinuities, principally along the clay-enriched, weaker solution seams. This can be observed at a variety of scales and suggests that Mode II shear fracturing does not occur as a primary fracture mechanism, but only as a macroscopic phenomenon following Mode I (veins and tail cracks) and anti-mode I (solution seams) deformation. It appears that solution seams can play a similar role to microcracks in localizing a through-going slip plane. This micromechanical model of faulting may be applicable to some other faults and shear zones in host rocks which are prone to pressure solution.  相似文献   

20.
新疆西南天山金矿床主要类型、特征及成矿作用   总被引：7，自引：1，他引：6  
文章在总结前人研究成果的基础上,综合论述了西南天山金矿的成矿地质背景、金矿床的时空分布和基本特征。根据矿床地质特征和控矿因素,将西南天山的金矿划分为与剪切带有关的金矿床、与侵入岩有关的金矿床(包括斑岩型)、石英-重晶石脉型金矿床、与火山岩有关的金矿床和矽卡岩型金矿床5类,其中与剪切带有关的金矿床是最重要的矿床类型。探讨了西南天山金矿的成矿时代、成矿物质和成矿流体来源,以及成矿地球动力学机制。提出与剪切带有关的金矿床成矿物质主要来源于岩浆和海相碳酸盐岩,成矿流体主要来源于岩浆水或主要来自大气降水,混合少量岩浆水。石英-重晶石脉型金矿床成矿物质来自容矿地层,成矿流体主要来源于沉积建造水。与剪切带有关的金矿、与侵入岩有关的金矿、石英重晶石脉型金矿和矽卡岩型金矿成矿时代主要集中在二叠纪—三叠纪,形成于后碰撞构造演化阶段。斑岩型和浅成低温热液型金矿床形成于岛弧挤压环境。  相似文献   

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