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1.
M.P Coward 《Journal of Structural Geology》1982,4(3):247-256
The Caledonian thrust zones of Assynt show several examples of large fault-bounded structures, surge zones, up to 8 km2 in extent, which have moved further than adjacent rocks. Extensional faults can be traced into strike-slip faults and then to contractional imbricate faults. There are also zones of extensional and contractional flow as shown by strained bioturbation marks in the Cambrian Pipe Rock.Several other low-angle extensional fault zones have been recognized along the length of the Moine thrust zone, notably in the Kinlochewe district. Recognition of these extensional faults and local surge zones has solved several local problems such as the lack of continuity of the Glencoul thrust and the out-of-sequence character of some of the large low-angle faults. Though the thrust propagation direction was generally from east to west, in the transport direction, several of the eastern faults have been reactivated later and locally cut down as extensional faults. The ‘so-called’ Moine thrust shows extensional fault movement at several localities along its length.The extensional structures and the surge zones suggest that body forces have been important in driving the faults rather than just a push from the rear. The Moines and Moine thrust zone were presumably driven to the WNW by gravity spreading and thinning of the main Scottish Caledonides. 相似文献
2.
M.P Coward 《Journal of Structural Geology》1983,5(2):113-123
The paper analyses the geometry of thin-skinned thrust zones, where the thrusts shallow out at depth and of thicker-skinned fault zones where much of the crust is involved and where the thrusts are frequently observed to become steeper downwards. In the interiors of many orogenic belts the steep dip of faults is not original but due to the folding above lower decoupling zones. The energy involved in the internal deformation of hanging-wall rocks may prohibit many faults becoming more shallow upwards. Such shallowing-upwards faults may occur in more ductile rocks to maintain compatibility between zones which have experienced different deformation intensities, but displacements on the faults are unlikely to be large.Another mechanism for producing faults which steepen downwards is proposed for the major thrusts which form the southern margin to the Himalayas. These carry large thicknesses (30 to 100 km) of crustal and upper mantle rocks to the south, causing flexuring and isostatic depression of the Indian plate. The steeply dipping thrusts are not footwall ramps; these may be some distance behind the steepened zone. This thrust-induced isostatic bending of the crust has important implications when considering regional seismic interpretations as well as thrust mechanics and kinematics. 相似文献
3.
In northwest Spain thrust sheets occur in an arcuate fold belt. The fault style consists of an array of thrusts, merging downdip into a single décollement surface. Most of the thrust sheets were initiated as thrusts cutting across flat lying beds. Folds above the hanging-wall ramps and some minor structures indicate that the body of the nappes has been subjected to an inhomogeneous simple shear parallel to bedding (y = 1.15), with slip concentrated along bedding planes. This allows the rocks forming the nappe to remain unstrained. At the base of the nappes a thin zone of deformed rock exists. The thrust sheets die out laterally against an anticline-syncline couple, oblique to the thrust direction. A geometrical analysis shows that if anticline and syncline axes are oblique, the thrust sheet was emplaced with a rotational movement, which can be evaluated. As deformation progressed two sets of folds were formed: a circumferential set, following the arc, and a radial set. An arcuate trace of the thrust structures remains after unfolding the radial folds. With a rotational emplacement, the displacement vector for successive points has a progressively greater length, and forms a progressively lower angle with the thrust. The main thrust units are broken into several slices with rotational movements, so that each unit was curved as it was being emplaced, producing a first tightening of the arc. Later folding increased the arc curvature to its present shape. The palaeomagnetic data available support the above conclusions. 相似文献
4.
Abstract In order to study the thermal structure of active thrust belts, we have developed a numerical model of conductive heat transfer between thrust sheets during deformation. Our finite difference approach alternates small, instantaneous increments of displacement and isotherm translation with conductive relaxation of perturbed isotherms. In each step, conduction occurs for a length of time equal to the displacement increment divided by the thrust velocity. Computer simulations demonstrate that conductive heat transfer is significant during deformation and that temperatures in hanging-wall rocks decrease while temperatures in foot-wall rocks increase over distances of up to 10 km from the thrust surface. When the effects of internal heat production are also calculated, heating of foot-wall rocks exceeds cooling of hanging-wall rocks. Rocks located between two thrusts may experience a complicated temperature–time path of early heating followed by cooling. These models help to explain the rapid metamorphism of rocks in the Taconian thrust belt in the northern Appalachians of New England soon after deposition of the youngest sediments. 相似文献
5.
Microstructures and quartz c-axis fabric diagrams from mylonites and psammitic Moine schists, collected in traverses across the lower levels of the Moine Nappe in the Eriboll area, are presented. On approaching the Moine Thrust from the Kyle of Tongue, the following microstructural sequence is encountered: interlayered coarse grained biotite psammitic and schistose tectonites being in part mylonitic with two platy slide zones, one containing biotite and the other only muscovite and chlorite and both showing quartz microstructures indicative of post-tectonic relaxation; these pass into more mylonitic rocks nearer the thrust zone which in turn passes into the main chlorite-grade mylonite belt and finally, adjacent to the Moine Thrust, into reworked lower chlorite grade mylonites. Although there is some local variation, the overall quartz c-axis fabric is an incomplete asymmetric type I girdle. The main variation is the development of type II girdles in the reworked, ultrafine grained mylonites. The extent of the mylonitization is more extensive than previously reported. Studies of folds within the mylonite belt have revealed eye structures and small-scale folds; many are sheath folds. They cannot be unequivocally correlated with large-scale recumbent folds within the Moine Nappe. The results presented indicate that mylonitization is not limited to a single phase, and raises the possibility that there may be earlier Caledonian or possibly Precambrian structural elements present in the Eriboll region Moines prior to much of the mylonitization. 相似文献
6.
The development of the syn-metamorphic Sgurr Beag slide zone, a major ductile shear zone of initially low dip, caused at least 50 km north-western thrust displacement of part of the internal metamorphic complex of the Northern Highland Caledonides of Scotland. Initiation of the zone, and movements upon it, were earlier than formation of the marginal Moine Thrust zone. Movement on the zone followed but overlapped the peak Caledonian metamorphism and the mid to high amphibolite facies mineral assemblages, fabrics and structures produced during the development of the slide zone and those surviving from earlier events, were reworked under greenschist facies conditions during mylonitization associated with initiation of the Moine Thrust zone. Displacements on the slide zone and thrust movements were separated by emplacement of a regional suite of pegmatites and a considerable change of metamorphic grade. Thus, they may not constitute members of a progressive sequence of Caledonian thrusts formed over a short time interval. Rather, preliminary isotopic data may imply an interval of c. 25 Ma between movement on the slide zone and final, ductile translation along the Moine Thrust zone. 相似文献
7.
R.J. Knipe 《Journal of Structural Geology》1985,7(1):1-10
The inter-relationships between the exact footwall geometry and the rheology of thrust sheets are investigated. Deviations in the thrust fault surface from an ideal plane will induce a local heterogeneous deformation. The resulting deformation processes depend upon the rate of thrust sheet displacement, the geometry of the feature causing heterogeneous flow, the deformation conditions and the lithologies involved. Two classes of features are particularly important in causing heterogeneous deformation in thrust sheets. The first features are small perturbations on bedding planes which may be inherited sedimentary structures or produced during layer-parallel shortening; the second class of features are ramps, where the thrust sheet climbs up the stratigraphic section. Displacement over these features causes repeated, cyclic straining in the hanging-wall during movement. The strain rates associated with deformation at perturbations, ramps of different geometries and different displacement rates are estimated and used to discuss the influence of footwall geometry on the structural evolution of a thrust sheet. Particular attention is given to the range of fault rocks and deformation microstructures preserved after movement over a footwall with a complex geometry. Perturbations are suggested to be important in the localization of ramps, either because they create ‘sticking points’ near the fault tip during propagation or because they induce eventual failure in the hanging-wall after the movement over a number of these features raises the accumulated damage to a critical level. Analysis of the influence of the exact geometry of ramps on deformation processes during displacement leads to two important conclusions. Firstly, the exact geometry of ramps (i.e. the maximum dip angle and the straining distance from a flat to this maximum angle) may be used to estimate a maximum displacement rate of the thrust sheet. Secondly, the listric geometry of ramps may be an equilibrium shape adjusted to the displacement rate and the rheology of the hanging-wall. Adjustments towards the final geometry may involve the generation of shortcuts on either hanging- or footwall which reduce the imposed deformation rate in the hanging-wall during displacement. 相似文献
8.
The terminology of structures in thrust belts 总被引：1，自引：0，他引：1
Robert W.H Butler 《Journal of Structural Geology》1982,4(3):239-245
A review of structures and geometric relationships recognized in thrust belts is presented. A thrust is defined as any contractional fault, a corollary being that thrusts must cut up-section in their transport direction. ‘Flats’ are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ‘ramps’ are those portions of thrusts which cut across datum surfaces. Ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust. Lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike-slip movement.An array of thrusts which diverge in their transport direction may form by either of two propagation models. These are termed ‘piggy-back’ propagation, which is foreland-directed, and ‘overstep’ propagation which is opposed to the thrust transport direction. An array of thrust surfaces is termed an ‘imbricate stack’ and should these surfaces anastamose upwards a ‘duplex’ will result; the fault-bounded blocks are termed ‘horses’. A duplex is bounded by a higher, ‘roof’ thrust and a lower, ‘floor’ thrust. The intersection of any two thrust planes is termed a ‘branch line’.Thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut. A further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one.The movement of thrust sheets over corrugated surfaces, or the local development of thrust structures beneath, will fold higher thrust sheets. These folds are termed ‘culminations’ and their limbs are termed ‘culmination walls’. Accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust. These accommodation surfaces are termed ‘hangingwall detachments’ and they need not root down into the active thrust. This category of detachment includes dip-slip ‘hangingwall drop faults’ which are developed by differential uplift of duplex roofs, and ‘out-of-the-syncline’ thrusts which develop from overtightened fold hinges. Back thrusts, as well as forming as hangingwall detachments, may also form due to layer-parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp. 相似文献
9.
火山成因块状硫化物矿床研究进展 总被引：3，自引：0，他引：3
火山成因块状硫化物（VMS）矿床可形成于太古宙至现代各个地质时期.现代海底热液成矿作用是赋存于海相火山岩系中的古代VMS矿床成矿作用的再现.VMS矿床可形成于多种构造环境,但均与拉张背景有关.按照构造环境和容矿岩系将VMS矿床分为黑矿型、塞浦路斯型、别子型和沙利文型.VMS矿床的热液蚀变由下盘蚀变带和上盘蚀变带两个结构单元组成.下盘蚀变一般有两种类型：（1）不整合的蚀变岩筒,直接产在块状硫化物带的下方;（2）半整合或层控的上盘蚀变带,并可以在区域范围内展布.与下盘蚀变类似,上盘蚀变也具有蚀变岩筒和半整合的蚀变带两种类型.成矿金属主要有两种可能的来源：（1）在岩浆侵入体和浅位岩浆房之上被加热的循环海水对含矿火山岩系及下伏基底物质的淋滤;（2）深部岩浆房挥发分通过释气作用直接释放.下渗的海水是成矿流体的主要来源,同时流体包裹体和稳定同位素资料显示岩浆流体对成矿流体系统亦有较大贡献.VMS矿床具有明显的金属分带现象,其金属序列组合从下至上依次为Fe→Fe-Cu→Cu-Pb-Zn→Pb-Zn-Ba,这种金属分带的演化反应了流体的演化和块状硫化物丘的生长机制.根据成矿流体温度变化而不断演化的4阶段成矿模式可以很好的解释经典丘堆式矿床的形成过程. 相似文献
10.
The Canadian Cordillera is separable into two major northerly trending tectonic units—the Pacific Orogen and the Columbian Orogen, with the latter further separated into the Omineca Crystalline Belt and the easterly Rocky Mountain Fold and Thrust Belt. Synkinematic metamorphism of Jurassic age within the Omineca Belt is thought to be associated with accretion of westerly terranes of the Pacific Orogen—more specifically the Quesnellia terrane—that was thrust easterly over the Omineca Belt towards the craton. Mylonitic rocks mark the margin between these two belts and this margin is well-exposed near Crooked Lake, central British Columbia.Structural analysis across the zone of convergence between these two terranes indicates that the cratonic basement and the accreted cover sequences have several phases of deformation and metamorphism in common. The initial common phase of deformation, wherein convergence is accomplished, is characterized by easterly verging folds that are superposed by a second common phase having westerly verging folds that deform the zone of convergence and control the present regional map pattern. A final common phase of deformation produced easterly verging folds.Change in vergence direction is interpreted as resulting from change in direction of transport related to subduction process: first obduction of Quesnellia onto the Omineca craton, followed later by easterly subduction of an oceanic Quesnellia below the craton.All evidence of transport direction(s) points to convergence occurring at very high angles to the zone of convergence. There appears to be no evidence of transport parallel with the strike of the zone. If transport has taken place parallel to the strike of the zone, then this transport occurred before convergence or evidence of this motion has been destroyed during the convergence. 相似文献
11.
This study investigates the tectonic evolution of the Omalos transverse zone, which served as a crustal-scale oblique ramp in the External Hellenides thrust belt on Crete island. The Omalos oblique ramp developed above an inherited Mesozoic fault zone that strikes NE–SW, oblique to the regional SSW-directed tectonic transport. During the Early Miocene–Pleistocene evolution of the thrust belt, the oblique ramp was repeatedly reactivated localizing deformation above the inherited structure. Geological and structural mapping combined with kinematic analysis of ductile and brittle structures suggest that the Omalos oblique ramp generated a local kinematic field, which deviated significantly from the regional kinematic pattern in the thrust belt. The most conspicuous feature in the tectonic evolution of the oblique ramp is a change from a ductile wrench-dominated to a brittle, primarily reverse faulting regime across the brittle–ductile transition, followed by brittle wrench deformation after the final exhumation of high-pressure (HP) rocks. Deflections of transport and compression orientations from the regional pattern are attributed to buttressing against basement-cover offsets produced by the pre-existing fault zone, to oblique ramping, and to transfer faulting, respectively. Our findings are potentially applicable to other examples of crustal-scale oblique thrust ramps in various tectonic settings. 相似文献
12.
Quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the Loch Eriboll area of the Moine Thrust zone and are used to interpret the detailed processes involved in fabric evolution. The intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude. A correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry.Two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the Moine Thrust. Within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate, mutually interfering shear bands, globular low strain detrital quartz grains whose c axes are aligned sub-parallel to the principal finite shortening direction (Z) and quartz c axis fabrics which are symmetric (both in terms of skeletal outline and intensity distribution) with respect to mylonitic foliation and lineation. Non-coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation.Tectonic models offering possible explanations for the presence of kinematic (strain path) domains within thrust sheets are considered. 相似文献
13.
We investigate the internal deformation of orogenic wedges growing by frontal accretion with a two-dimensional numerical model. Our models are limited to crustal deformation and assume a horizontal detachment as observed for various natural orogens (e.g. Alaska and Costa Rica). The model wedges develop as a result of convergence of a brittle sediment layer in front of a strong backstop. We find that our reference model develops in-sequence forward-thrusts which propagate upward from the basal detachment. For this reference model we investigate the sensitivity of shear zone activity to surface processes and strain softening. Model results show that diffusive or slope dependent erosion enhances material transport across the wedge and slows down forward propagation of the deformation front. Frictional strain softening focuses deformation into narrow shear zones and enhances displacement along them. This has also been postulated for natural thrusts such as the Glarus thrust in the Swiss Alps and the Moine thrust in the Scottish Caledonides. A second series of models investigates the effects of regularly spaced weak inclusions within the sediment layer which simulate remnants of previous deformation phases. These inclusions facilitate and focus internal deformation, influence the thrust dip and thrust vergence and enable thrust reactivation in the internal part of the wedge. Our results show that inactive thrusts in the internal part of the wedges may be reactivated in models with diffusive surface processes, strain softening or weak inclusions. Thrust reactivation occurs as models seek to maintain their critical taper angle. First order characteristics of our numerical models agree well with natural orogenic wedges and results from other numerical and analogue models. 相似文献
14.
I.McDermid J.C.Aitchison Badengzhu A.M.Davis Liu Jianbing Luo Hui Wu Hiyun S.V.Ziabrev WT ”BX 《地学前缘》2000,(Z1)
ZEDONG TERRANE, A MID CRETACEOUS INTRA-OCEANIC ARC, SOUTH TIBET 相似文献
15.
J.R. Thigpen R.D. Law C.L. Loehn R. A. Strachan R.J. Tracy G.E. Lloyd B.L. Roth S.J. Brown 《Journal of Metamorphic Geology》2013,31(8):813-842
In the Caledonides of northwest Scotland, two independent geothermometers (Fe‐Mg exchange and quartz c‐axis fabric opening angle) are used to characterize the thermal structure of the lower part of the Scandian (435–420 Ma) orogenic wedge within the Moine, Ben Hope and Naver‐Sgurr Beag thrust sheets. Traced from west (foreland) to east (hinterland), Fe‐Mg exchange thermometry yields peak or near‐peak temperatures ranging from 484 ± 50 °C to 524 ± 50 °C in the immediate hangingwall of the Moine thrust to 601 ± 50 °C in the immediate hangingwall of the Ben Hope thrust, to 630 ± 50 °C in the Naver thrust sheet. Preserved metamorphic facies and textural relationships are consistent with thermometric estimates. Deformation temperatures calculated from quartz c‐axis fabric opening angles across two similar orogen‐perpendicular transects also yield systematic increases (Glen Golly – Ben Klibreck, 520–630 °C; Ullapool‐Contin, 465–632 °C) traced towards the Naver and Sgurr Beag thrusts. In addition, deformation temperatures show a pronounced increase along the leading edge of the Moine thrust sheet moving south towards the Assynt window, which is interpreted to reflect deeper exhumation of the thrust plane above the Assynt footwall imbricate stack. Because temperatures calculated from metamorphic assemblages are within error of the quartz fabric‐derived deformation temperatures that are of demonstrably Scandian age, the metamorphic sequence between the Moine and Naver‐Sgurr Beag thrusts is interpreted to have developed during the Scandian orogeny. Integration of our results with previous 2D thermal‐mechanical studies allows development of new conceptual thermal‐kinematic models of Scandian orogenesis that may be broadly applicable to other collisional systems. Furthermore, it highlights the critical nature of coupling between orogen kinematic and thermal evolution. 相似文献
16.
Evidence for an inverted metamorphic gradient associated with a Precambrian suture, southern Wyoming
E.M. DUEBENDORFER 《Journal of Metamorphic Geology》1988,6(1):41-63
Abstract An inverted metamorphic gradient associated with the northern mylonite zone of the Cheyenne belt, a deeply eroded Precambrian suture in southern Wyoming, has been documented within metasedimentary rocks of the Early Proterozoic Snowy Pass Supergroup. Metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite, garnet, and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone. A minimum temperature increase of approximately 100° C over a km-wide zone is required for this transition. Parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the Snowy Pass Supergroup. Field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite-grade rocks over rocks of the Snowy Pass Supergroup along the northern mylonite zone. Mineral equilibria and garnet-biotite geothermometry on synkinematic mineral assemblages within the Snowy Pass metasedimentary rocks indicate deformation at minimum temperatures of 480° C and pressures of 350–400 MPa (3°5–4°0 kbar). This implies tectonic burial or upper plate thickness of 13–15 km. The narrow character of metamorphic zonation and microtextures within the Snowy Pass Supergroup which indicate late synkine-matic growth of garnet and staurolite, preclude rotation of pre-existing isograds by folding as a mechanism for development of the inverted gradient. Conductive transport of heat from the upper into the lower plate across the originally low-angle thrust is insufficient to produce the necessary temperatures in the lower plate. Shear heating is considered insufficient to produce the observed metamorphic transition unless high shear stresses are postulated. Up-dip advection of metamorphic fluids is a feasible, but unproven, mechanism for heat transport. The possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present. 相似文献
17.
This paper analyses a major shear zone from the Iberian Hercynian belt which forms the basal thrust of the Mondoñedo Nappe. The shear zone developed by ductile deformation under amphibolite facies metamorphic conditions and later by brittle-ductile deformation in greenschists facies. Folds in the shear zone are asymmetric, very tight, 1C or similar class and frequently developing sheath geometries. The sheath folds originated by non-coaxial flow superimposed on earlier irregularities. The fabric of quartzitic rocks in the shear zone changes from bottom to top from ultramylonites through blastomylonitic rocks to non-mylonitic tectonites. c-axis fabrics vary across the shear zone, but show a dominant monoclinic symmetry. The blastomylonitic rocks include the fabrics representing the highest temperatures. The main foliation of the schists results from flattening of an earlier foliation, recording occasional microfolds. The use of different kinematic criteria has allowed an analysis of their validity as well as an assessment of movement direction towards the foreland of the orogen. 相似文献
18.
内蒙古苏尼特左旗交其尔推覆构造带的发现及其地质意义 总被引：5，自引：0，他引：5
内蒙古苏尼特左旗中部交其尔一带发现一条近ＥＷ向延伸的推覆构造带。上元古界－下古生界浅变质岩以低角度向北推覆于晚古生代花岗岩基之上．推覆构造上盘的浅变质岩中发育有一套轴面向南倾的同斜倒转褶皱系，并伴有一系列向南倾的叠瓦状逆冲断面。下盘花岗岩中发育有较宽的糜棱岩带．沿推覆构造带分布有一系列的飞来峰及构造窗。这一构造带的发现，为纬向构造带的存在提供了有力证据，并显示了晚古生代末期以来地壳沿经线方向的强烈缩短。 相似文献
19.
20.
Garnet morphology distribution in the northern part of the Moine Supergroup,Scottish Caledonides 
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下载免费PDF全文 Garnet is a versatile and useful indicator mineral exploited in numerous geological studies. Despite its utility in providing thermobarometry and geochronology constraints, many difficulties remain in making meaningful interpretations of such data. In this paper, we characterize garnet grains from over 140 garnet‐bearing metasedimentary rock samples collected from the northern part of the Moine Supergroup (Scottish Caledonides). Large, euhedral garnet grains are interpreted to be indicative of prograde metamorphic growth during the most recent (Scandian, c. 430 Ma) phase of orogenesis. Anhedral garnet is largely restricted to the relatively low‐grade (greenschist – lower amphibolite facies) Moine thrust sheet, with an abrupt change in morphology and grain size when traced across the overlying Ben Hope and Sgurr Beag thrusts into the higher grade, more hinterland‐positioned thrust sheets. Our results suggest that caution should be exercised in using anhedral garnet in the Moine thrust sheet to estimate peak P–T conditions associated with low temperature (< ~500 °C) Scandian metamorphism, because in at least some cases garnet growth may have occurred during an earlier metamorphic event. However, chemical and isotopic data from this structurally lower anhedral garnet may still be useful for deconvolving a possible polymetamorphic history for this thrust sheet. In the immediately overlying Ben Hope thrust sheet, garnet has prograde euhedral (Scandian?) rims, indicating that the Ben Hope thrust must represent a significant thermal and/or chemical break. Inclusion distribution and mineral assemblages in garnet have been used to gain further insight on garnet growth conditions and to distinguish garnet that likely contains multiple generations of growth. Although our results are specific to the Caledonides of northern Scotland, this work highlights the general necessity of a comprehensive petrographic assessment in advance of interpreting large suites of garnet‐derived thermodynamic and geochronological data. 相似文献