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1.
The Mondoñedo thrust sheet has been studied to investigate the complex dynamic relationships that may be involved in the development of low- and medium-P metamorphic domains. This unit underwent an initial medium-P event during the initial stages of Variscan convergence, related to crustal thickening. Subsequently, the thrust sheet evolved to a low-P baric type of metamorphism, related to syn-convergence thinning and exhumation. Its footwall, cropping out in two tectonic windows, registered a different evolution, with a low-P history that evolved from low- to high-T under a high geothermal gradient. Several different P–T paths of the Mondoñedo thrust sheet and its relative autochthon are traced and interpreted according to the structural evolution of the area. Following the initial crustal thickening, two main syn-convergence extensional shear zones developed. One of them occurs in the hangingwall, whereas the other affects the footwall unit. Both extensional shear zones were contemporaneous with ductile thrusting in the inner parts of the thrust sheet, and their activity is viewed as a consequence of the need for gravitational re-equilibration within the orogenic wedge.The most commonly accepted models of tectonothermal evolution in regions of thickened continental crust assume that low-P metamorphism is essentially a late phenomenon, and is linked to late-orogenic tectonic activity. In the Mondoñedo thrust sheet, our conclusions indicate that low-P metamorphism may also develop during convergence, and that this may occur in at least two cases. One is tectonic denudation of an allochthonous unit during its emplacement, and the other, thinning and extension at the footwall unit of an advancing thrust sheet. As a consequence, the low-P evolution may show different characteristics in different units of an orogenic nappe pile.  相似文献   

2.
The Gran Sasso chain in Central Italy is made up of an imbricate stack of eight thrust sheets, which were emplaced over the Upper Miocene—Lower Pliocene Laga Flysch. The thrust sheets are numbered from 1 to 8 in order of their decreasing elevation in the tectonic stack, and their basal thrusts are numbered from T1 to T8, accordingly. On the basis of their different deformation features, the major thrust faults fall into three groups: (1) thrust faults marked by thick belts of incoherent gouges and breccia zones (T1, T2, T3); (2) thrust faults characterized by a sharp plane which truncates folds that had developed in the footwall rocks (T5, T6); and (3) thrust faults truncating folds developed in both the hangingwall and footwall units, and bordered by foliated fault rocks (T7). The deformation features observed for the different faults seem to vary because of two combined factors: (1) lithologic changes in the footwall and hangingwall units separated by the thrust faults; and (2) increasing amounts of deformation in the deepest portions of the imbricate stack. The upper thrust sheets (from 1 to 6) are characterized by massive calcareous and dolomitic rocks, they maintain a homoclinal setting and are truncated up-section by the cataclastic thrust faults. The lowermost thrust sheets (7 and 8) are characterized by a multilayer with competence contrasts, which undergoes shear-induced folding prior to the final emplacement of the thrust sheets. Bedding and axial planes of folds rotate progressively towards the T5, T6, T7 and T8 thrust boundaries, and are subsequently truncated by propagation of the brittle thrust faults. The maximum deformation is observed along the T7 thrust fault, consistent with horizontal displacement that increases progressively from the uppermost to the lowermost thrust sheet in the tectonic stack. The axial planes of the folds developed in the hangingwall and footwall units are parallel to the T7 thrust fault, and foliated fault rocks have developed. Field data and petrographic analysis indicate that cleavage fabrics in the fault rocks form by a combination of cataclasis, cataclastic flow and pressure-solution slip, associated with pervasive shearing along subtly distributed slip zones parallel to the T7 thrust fault. The development of such fabrics at upper crustal levels creates easy-slip conditions in progressively thinner domains, which are regions of localized flow during the thrust sheet emplacement.  相似文献   

3.
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4.
In the Saint-Barthélemy Massif, French Pyrenees, a ductile thrust zone developed in gneisses during retrogression from lower amphibolite facies conditions to the upper greenschist facies. The last major structures formed in the zone are isolated shear bands, divided into three types.Anastomosing, inhomogenous ultramylonitic shear bands (Type I) are subparallel to the mylonitic foliation in the gneiss (Sg). Most of these bands developed by ductile deformation processes only.Planar, homogeneous ultramylonite bands (Type II) are usually oblique to Sg. They generated as pseudotachylyte bands by brittle fracturing and underwent strong subsequent ductile deformation.Type III shear bands are planar and oblique to Sg. They consist of pseudotachylyte, weakly affected by ductile deformation.Type I, II and III bands seem to represent progressively younger structures on a local scale, linked to falling P-T conditions. The systematic variation in orientation of the different shear bands with respect to Sg is interpreted as being due to a different response of brittle and ductile structures to the orientation of the kinematic frame and the rock anisotropy.  相似文献   

5.
The timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are important for understanding the growth process of continental crust. In this study, the petrology and geochronology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to understand the relationship between igneous activity and metamorphism during crustal evolution in the Central Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressive changes in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimanite and cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones) occur in the central part of the Tseel area, where granitoids are abundant. To the north and south of these granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in the pelitic gneisses varies systematically across the mineral zones, from grossular‐rich garnet in the garnet zone to zoned garnet with grossular‐rich cores and pyrope‐rich rims in the staurolite zone, and pyrope‐rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individual garnet crystals reveal two main stages of metamorphism: (i) a high‐P and low‐T stage (as recorded by garnet in the garnet zone and garnet cores in the staurolite zone) at 520–580 °C and 4.5–7 kbar in the kyanite stability field and (ii) a low‐P and high‐T stage (garnet rims in the staurolite zone and garnet in the sillimanite and cordierite zones) at 570–680 °C and 3.0–6.0 kbar in the sillimanite stability field. The earlier high‐P metamorphism resulted in the growth of kyanite in quartz veins within the staurolite and sillimanite zones. The U–Pb zircon ages of pelitic gneisses and granitoids reveal that (i) the protolith (igneous) age of the pelitic gneisses is c. 510 Ma; (ii) the low‐P and high‐T metamorphism occurred at 377 ± 30 Ma; and (iii) this metamorphic stage was coeval with granitoid intrusion at 385 ± 7 Ma. The age of the earlier low‐T and high‐P metamorphism is not clearly recorded in the zircon, but probably corresponds to small age peaks at 450–400 Ma. The low‐P and high‐T metamorphism continued for c. 100 Ma, which is longer than the active period of a single granitoid body. These findings indicate that an elevation of geotherm and a transition from high‐P and low‐T to low‐P and high‐T metamorphism occurred, associated with continuous emplacement of several granitoids, during the crustal evolution in the Devonian CAOB.  相似文献   

6.
In Kameng Valley of Arunachal Pradesh, the crystalline rocks of Se La Group of Higher Himalaya are thrust over the Lesser Himalayan rocks of Dirang Formation, Bomdila Group along the Main Central Thrust and exhibit well preserved structures on macro- to microscopic scales. Detailed analysis of structures reveals that the rocks of the area have suffered four phases of deformation D1, D2, D3 and D4. These structures have been grouped into (i) early structures (ii) structures related to progressive ductile thrusting and (iii) late structures. The early structures which developed before thrusting formed during D1 and D2 phases of deformation, synchronous to F1 and F2 phases of folding respectively. The structures related to progressive ductile shearing developed during D3 phase of deformation, when the emplacement of the crystalline rocks took place over the rocks of Dirang Formation along the Main Central Thrust. Different asymmetric structures/kinematic indicators developed during this ductile/brittle-ductile regime suggest top-to-SSW sense of movement of the crystalline rocks of the area. D4 is attributed to brittle deformation. Based on satellite data two new thrusts, i.e. Tawang and Se La thrusts have been identified parallel to Main Central Thrust, which are suggestive of imbricate thrusting. Strain analysis from the quartz grains of the gneissic rocks reveals constriction type of strain ellipsoid where k value is higher near the MCT, gradually decreases towards the north. Further, the dynamic analysis carried out on the mesoscopic ductile and brittle-ductile shear zones suggest a NNE-SSW horizontal compression corresponding to the direction of northward movement of Indian Plate.  相似文献   

7.
Fault-slip data are used to reconstruct varying tectonic regimes associated with transverse fold development along the eastern and southern margins of the Jaca basin, southern Pyrenees, Spain. The Spanish Pyrenean foreland consists of thrust sheets and leading-edge décollement folds which developed within piggyback basins. Guara Formation limestones on the margins of the Jaca basin were deposited synchronously with deformation and are exposed in the External Sierra. Within the transverse folds, principal shortening axes determined from P and T dihedra plots of fault-slip data show a shift from steep shortening in stratigraphically older beds to NNE–SSW horizontal shortening in younger beds. Older strata are characterized by extensional faults interpreted to result from halotectonic (salt tectonics) deformation, whereas younger strata are characterized by contraction and strike-slip faults interpreted to result from thrust sheet emplacement. The interpretation of the timing for the shortening axes in the younger strata is supported by the observation that these axes are parallel to shortening axes determined from finite strain analysis, calcite twins, and regional thrusting directions determined from fault-related folds and slickenlines. This study shows that fault population analysis in syntectonic strata provides an opportunity to constrain kinematic evolution during orogeny.  相似文献   

8.
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   

9.
The geometry and age relations of syntectonic veins within calcareous rocks of one imbricate sheet within a thrust belt in the external French Alps, are described.The earliest veins developed during the main ductile deformation by cleavage-parallel extension. The majority of the syntectonic veins developed towards the end of the deformation, and after the formation of second folds. They include a conjugate set of normal shears, an abundant set of upright extension veins, and en echelon sets.The dominantly simple shear strain making up the main ductile phase of deformation occurred by a mechanism of grain to grain pressure solution. The stretching lineation records the overall direction of thrust sheet movement. A change in the microchemical mechanism of pressure solution is thought to have caused the change from first to second phase deformation as recorded by slaty cleavage and crenulation folds in the field. From the shear and vein geometries, directions of principal stress have been inferred. The directions rotated throughout the deformation, the maximum principal stress being inclined to bedding during simple shear strain, becoming normal to bedding during the phase of abundant vein growth, and becoming vertical at the very end of the deformation.  相似文献   

10.
Strongly deformed volcaniclastic metasediments and ophiolitic slices hosting the Sukari gold mineralization display evidence of a complex structural evolution involving three main ductile deformational events (D1–D3). D1 produced ENE-trending folds associated with NNW-propagating thrust slices and intrusion of the Sukari granite (689 ± 3 Ma). D2 formed a moderately to steeply dipping, NNW-trending S2 foliation curved to NE and developed arcuate structure constituting the Kurdeman shear zone (≤ 595 Ma) and East Sukari imbricate thrust belt. Major NE-trending F2 folds, NW-dipping high-angle thrusts, shallow and steeply plunging mineral lineation and shear indicators recorded both subhorizontal and subvertical transport direction during D2. D3 (560–540 Ma) formed NNE-trending S3 crenulation cleavage, tight F3 folds, Sukari Thrust and West Sukari imbricate thrust. The system of NW-trending sinistral Kurdeman shear zone (lateral ramps and tear faults) and imbricate thrusts (frontal ramps) forming the actuate structure developed during SE-directed thrusting, whereas the prevailing pattern of NNE-trending dextral Sukari shear zone and imbricate thrusts forming Sukari thrust duplex developed during NE-directed tectonic shearing. Sukari granite intruded in different pluses between 689 and 540 Ma and associated with at least four phases of quartz veins with different geometry and orientation. Structural analysis of the shear fabrics indicates that the geometry of the mineralized quartz veins and alteration patterns are controlled by the regional NNW- and NE-trending conjugate zones of transpression. Gold-bearing quartz veins are located within NNW-oriented sinistral shear zones in Kurdeman gold mine area, within steeply dipping NW- and SE dipping thrusts and NE- and NS-oriented dextral and sinistral shear zones around Sukari mine area, and along E-dipping backthrusts and NW-SE and N-S fractures in Sukari granite. The high grade of gold mineralization in Sukari is mainly controlled by SE-dipping back-thrusts branched from the major NW-dipping Sukari Thrust. The gold mineralization in Sukari gold mine and neighboring areas in the Central Eastern Desert of Egypt is mainly controlled by the conjugate shear zones of the Najd Fault System and related to E-W directed shortening associated with oblique convergence between East and West Gondwana.  相似文献   

11.
Many Precambrian granulite-facies metamorphic complexes contain so-called straight gneisses, which are massive rocks with a clearly pronounced blastomylonitic texture, lineation, and gneissosity. These rocks occur exclusively in high-temperature ductile shear zones, which can develop either during the primary exhumation of rock complexes or during the overprinting by high-temperature dynamometamorphism. The main criterion for distinguishing between these two types of straight gneisses is the configuration of their P-T trajectories, which are recorded in the mineral assemblages in these rocks and their host gneisses. Ductile shear zones developed in Archean granulite gneisses simultaneously with their exhumation, and, hence, their P-T trajectories are segments of decompression and/or isobaric cooling paths. Straight gneisses in Proterozoic polymetamorphic complexes commonly compose high-temperature ductile shear zones overprinted on Archean granulite complexes, and the P-T paths of these rocks are Z-shaped. This means that, at a constant pressure in the middle part of the continental crust, the T min of the older P-T trajectory corresponded to T max of the younger trajectory, and often T max–T min > 100°C. Such ductile shear zones commonly have a strike-slip morphology and can be easily seen in aerial photographs and discerned during structural geological surveying. These zones can overprint older gneisses without any notable thermal effect on the latter. Relations of this type were identified in the granulite complexes of Limpopo in South Africa, Sharyzhalgai in the southwestern Baikal area, and Lapland in the Kola Peninsula. The results of our research propose a solution for the well-known problem of the significant discrepancies between the isotopic ages in high-temperature-high-pressure complexes and the partial or complete distortion of radiogenic isotopic systems under the effect of a newly inflowing metamorphic fluid. The application of geochronologic techniques to these situations is senseless, and only P-T trajectories provide insight into the actual age relations between the discrete tectono-metamorphic stages. It is thus expedient to conduct not only structural studies of metamorphic complexes but also their detailed petrological examination and the calculation of their P-T paths before geochronologic dating.  相似文献   

12.
Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt. The Ghadir Shear Belt is a 35 km-long, NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within this shear belt, strain is highly partitioned into shortening, oblique, extensional and strike-slip structures at multiple scales. Moreover, strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains. In the East Ghadir and Ambaut shear belts, the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated. These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones. The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones. The earlier fabric(S1), is locally recognized in low strain areas and SW-ward thrusts. S2 is associated with a shallowly plunging stretching lineation(L2), and defines ～NW-SE major upright macroscopic folds in the East Ghadir shear belt. F2 folds are superimposed by ～NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt. The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones. Dextral ENEstriking shear zones were subsequently active at ca. 595 Ma, coeval with sinistral shearing along NW-to NNW-striking shear zones. The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt. Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments. Upright folds, fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning. The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana.  相似文献   

13.
Three-dimensional analysis of irrotational, longitudinal, finite strain was carried out on samples from a crescentic sheet which intruded and was deformed with a host gneiss unit of probable Helikian age. Analytical methods were compared using deformed feldspar grains representing four ideal degrees of strain intensity observed in the porphyritic sheet. The polar plot and Rf/φ, Rs methods proved most reliable and sensitive.Data derived from fabric and strain analysis at 38 sites in the units suggest a two-stage deformational sequence. The first stage produced recumbent, isoclinal, similar (class 2) folds with northwest-trending hinge surface traces. This fold form was modified during the second stage to produce an overall type 2 fold interference pattern. The second stage produced upright, open buckle folds as well as the resultant strain fabric currently observed. Strain analysis confirms the general fold geometries of the model, and also documents competency contrasts between the matrix and feldspar grains with increased strain intensity and magnitude. Deformation of feldspar grains in the sheet involved modification of a fabric of low strain magnitude (?s = 0.3) and a k value near unity to magnitudes of ?s = 2.6 and k = 0.6. Matrix strain intensities and magnitudes are consistently higher than those of the feldspar markers in the sheet. This variation is related to competency differences between the matrix and the feldspar grains. Fabric anisotropy accounts for the strain gradient observed between the sheet and gneiss.  相似文献   

14.
Finite-strain was studied in the mylonitic granitic and metasedimentary rocks in the northern thrust in Wadi Mubarak belt to show a relationship to nappe contacts between the old granitic and metavolcano-sedimentary rocks and to shed light on the heterogeneous deformation for the northern thrust in Wadi Mubarak belt. We used the Rf/ϕ and Fry methods on feldspar porphyroclasts, quartz and mafic grains from 7 old granitic and 7 metasedimentary samples in the northern thrust in Wadi Mubarak belt. The finite-strain data shows that old granitic rocks were moderate to highly deformed and axial ratios in the XZ section range from 3.05 to 7.10 for granitic and metasedimentary rocks. The long axes (X) of the finite-strain ellipsoids trend W/WNW and E/ENE in the northern thrust in Wadi Mubarak belt. Furthermore, the short axes (Z) are subvertical associated with a subhorizontal foliation. The value of strain magnitudes mainly constants towards the tectonic contacts between the mylonitic granite and metavolcano-sedimentary rocks. The data indicate oblate strain symmetry (flattening strain) in the mylonitic granite rocks. It is suggested that the accumulation of finite strain was formed before or/and during nappe contacts. The penetrative subhorizontal foliation is subparallel to the tectonic contacts with the overlying nappes and foliation was formed during nappe thrusting.  相似文献   

15.
This paper presents the results of a comprehensive experimental study of the formation of granitoid melts at the expense of olivine-normative amphibolites. It was shown that trondhjemite-tonalite and granite-granodiorite melts can be generated by incongruent melting reactions at pressures of 5–25 kbar at T = 800–1000°C. The compositions of coexisting phases and phase reactions were investigated in detail. It was found that interaction between these hydrous melts and the overlying peridotite material results in the metasomatic alteration of peridotites and formation of andesite melts. The granitization of amphibolite was explored. Infiltration granitization was experimentally reproduced for the first time at T = 750°C and P f = 5 kbar. Fluid percolation through amphibolite produced a column of feldspathized and debasified rocks and granite melt completely replacing amphibolite in the proximal zone. Another extreme type of granitization occurring in amphibolite at the contact with granite melt was investigated at T = 800–950°C and P f = 7 kbar. The diffusion of silica and alkalis resulted in the metasomatic alteration of amphibolite and formation of granitic droplets and lenses with the development of migmatite-like zones, which significantly differ in composition and structure from the zones of infiltration granitization. All the models addressed in this paper (derivation of granitoid series, interaction of granitoid melts with peridotites, and infiltration and diffusion granitization) provide insight into the mechanism of formation of many natural objects.  相似文献   

16.
会泽地区的南红玛瑙矿主要产于上三叠统飞仙关组的中上部砾岩层、上二叠统宣威组底部砾岩层以及晚二叠世峨眉山组杏仁状玄武岩中。玛瑙新鲜面多为红色、浅红色及灰白色;呈椭球状或扁球状分布于砾岩之中,分布不均匀,局部富集;玛瑙砾石大小不一,玛瑙粒径多集中在0.5~5 cm,偶见大于6 cm。会泽地区的南红玛瑙矿形成可分为三个阶段：1）晚二叠世时期,玄武质火山岩（P3e）中的含硅质热液在晚期充填于玄武岩气孔或裂隙中,经过（快速）冷却形成隐晶质石英或晶质石英集合体——玛瑙,即原生玛瑙;2）晚二叠世晚期的华力西运动和印支运动早期的地壳继续隆升,使得含玛瑙玄武岩遭受风化-剥蚀作用,两次剥离出来的玛瑙单体经流水搬运作用及河湖相沉积作用,形成宣威组（P3x）、飞仙关组（T1f）含玛瑙砾石层——沉积砾岩型玛瑙矿产;3）近-现代地质作用,使得二叠纪玄武岩气孔中原生玛瑙和宣威组、飞仙关组沉积砾岩层中的次生玛瑙砾石,再次受到风化-剥蚀作用,玛瑙砾石经流水搬运至现代河流或湖泊中,形成次生玛瑙砂矿。  相似文献   

17.
Numerical simulations have been carried out in the general three-body problem with equal masses with zero initial velocities, to investigate the distribution of the decay times T based on a representative sample of initial conditions. The distribution has a power-law character on long time scales, f(T) ∝ T ?α , with α = 1.74. Over small times T < 30T cr (T cr is the mean crossing time for a component of the triple system), a series of local maxima separated by about 1.0T cr is observed in the decay-time distribution. These local peaks correspond to zones of decay after one or a few triple encounters. Figures showing the arrangement of these zones in the domain of the initial conditions are presented.  相似文献   

18.
Temperature and fluid content are critical parameters that control rock rheology and strain localization in the continental crust. Here, we determine by thermodynamic modelling the of localized ductile shearing during cooling of three different granitoid plutons: the Rieserferner and the Adamello plutons in the Italian Alps, and the Lake Edison pluton in the Sierra Nevada—USA. Shear zones exploited precursor joints, associated veins and alteration zones. and P−T phase diagram sections were computed with Perple_X in the system MnO−Na2O−CaO −K2O−FeO−MgO−Al2O3−SiO2−H2O−Fe2O3. The phase diagram sections show that the nucleation of the brittle precursors (joints, veins) occurred at T» 450°C at fluid-saturated conditions. Localized ductile shearing likely occurred at temperature ranging between 420 and 460°C evolving from initially fluid-saturated to fluid-undersaturated conditions in a closed system. In this temperature range, granitoid rocks are potentially subject to a series of retrograde metamorphic reactions replacing the load-bearing feldspars with weaker phyllosilicates. Metamorphic reactions occurred in spatial association with the precursory structures, leading to localized shearing. Decreasing temperature and fluid-undersaturated conditions likely hampered progressive strain accommodation in shear zones by slowing down metamorphic reactions, thermally activated dislocation creep processes, fluid-mediated deformation mechanisms and weakening mechanisms. Polyphase granitoid ultramylonite and mylonitic quartz veins have been affected differently by the fluid-undersaturated conditions of the system, as consequence of different dominant deformation mechanisms and syn-kinematic paragenesis during localized shearing. Localized ductile shearing in cooling plutons effectively occurs in a limited temperature range (420–460°C) in which the strain accommodation capacity of the shear zone is controlled by the negative feedback between the cooling rate, the kinetics of metamorphic reactions and deformation mechanisms, and the consumption of the limited amount of available fluids.  相似文献   

19.
Folds and thrust faults formed by layer-parallel shortening coaxial with extensional structures such as normal dip-slip faults and ductile necking structures with orthorhombic fabric symmetry are usual, but little-recognised structures formed within normal dip-slip shear zones bounding rifts. They are generated because of the shear distribution in a zone of progressive deformation and may be later extended and disrupted depending on which part of the strain ellipsoid they may be located. We here describe folds and thrust faults from the southern margin of the Ala?ehir Rift in western Turkey as an opportunity to discuss the properties of pure extension–related structures formed by layer-parallel shortening. Such structures are more commonly generated during the early stages of rifting, when deformation rates are slow and the shear zones broader than those forming later in the life of a rift when strain rates are usually higher. Such structures have commonly been mistaken for witnesses documenting regional episodes of shortening rather than as integral parts of the extensional structures forming rifts. Not all layer-parallel shortening-related structures therefore indicate regional shortening. We plead that hasty statements concerning the meaning of geological structures at all scales be avoided before a thorough understanding of bulk strains that have affected a region are properly understood.  相似文献   

20.
The amphibolite facies Puolankajärvi Formation (PjF) occupies the western margin of the Early Proterozoic Kainuu Schist Belt (KSB) of northern Finland. The lower and middle parts of the PjF consist of turbiditic psammites and pelites and tempestitic semipelites. This report concentrates on the pelitic lithologies which include quartz–two-mica–plagioclase schists with variable amounts of garnet, staurolite, andalusite and biotite porphyroblasts as well as sillimanite and cordierite segregations. The KSB forms a major north–south-trending synclinorium between two Archaean blocks. It contains both autochthonous and allochthonous units and is cut by faults and shear zones. The PjF lies on the western side of the KSB and is probably allochthonous. The formation has undergone six major deformation phases (D1, D2, D3a, D3b, D4 and D5). During D3a-D5 the maximum principal stress (σ1) changed in a clockwise direction from south-west to north-east. Between D2 and D3 the intermediate principal stress (σ2) changed from horizontal to vertical and the interval between D2 and D3 marks a transition from thrust to strike-slip tectonics. Relict structures in the porphyroblasts indicate the following mineral growth–deformation evolution in the PjF. (1) Throughout the PjF there was a successive crystallization of garnet (syn-D1), poryphyroblastic biotite (inter-D3/4) and staurolite (inter-D3/4) during the pre-D4 stage. (2) A syn-D4-inter-D4/5 crystallization of kyanite, sillimanite (fibrolite), porphyroblastic tourmaline, magnetite, rutile, cordierite and muscovite–biotite–plagioclase pseudomorphs after staurolite was most localized at and near D4 shear zones. (3) A syn- to post-D5 generation of andalusite, ilmenohematite and sheet silicates after staurolite and after cordierite occurred near D5 faults. The evolution outlined here permits the relative dating of the PjF parageneses, which is used in the second part of the study (Tuisku & Laajoki, 1990), and, together with the knowledge of the pressure–temperature conditions during various growth events, makes it possible to compile pressure–temperature–deformation paths for the PjF.  相似文献   

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