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1.
A combined sedimentological, shape-preferred orientation and anisotropy of magnetic susceptibility (AMS) analysis has been performed at the Arroyofrío Bed (Callovian–Oxfordian boundary level) in the locality of Moneva (Iberian Range, NE Spain). The Arroyofrío bed is a widespread iron-ooid limestone interval forming a condensed sequence. The present study has focused on the analysis of the potential presence of a preferred ooid orientation at the Arroyofrío bed. The obtained data show that ooids were originally ellipsoidal and had an imbricate disposition with respect to the bedding/lamination surface. The main ooid orientation within the bedding plane shows a NNE–SSW trend. Results of AMS analyses show a magnetic foliation parallel or slightly imbricated with respect to bedding and magnetic lineation parallel to the main ooid orientation. Magnetic mineralogy of studied samples shows that AMS is mainly controlled by magnetite with minor contributions of hematite and paramagnetic minerals (that can reach contributions of 35 %). The analyzed ooids show axial ratios between 1.4 and 2.8 (intrinsic anisotropy), while the anisotropy of their distribution shows lower anisotropies (e.g., Rs = 1.15) or very low values of the anisotropic magnetic parameters (e.g., P′ < 1.01). Sedimentary texture, matrix features, bioturbation and fossil content influenced both ooid main orientation and the magnetic fabric. Magnetic lineation and main orientation of long ooid axes are transverse to the inferred coastline in the studied area and parallel to the expected paleocurrent direction with respect to the Ejulve-Maestrazgo paleogeographic high. The direct correlation between AMS magnetic lineation and the ooid analysis permits to demonstrate that the paleocurrent imprint can be recorded by means of AMS despite the highly ferromagnetic context fabric and at coarse deposits. Obtained results support the interest and reliability of AMS to unravel paleocurrent imprints for paleogeographic reconstructions.  相似文献   

2.
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   

3.
Weakly deformed mudstone and siltstone (Middle Ordovician Knobs Formation) of the Appalachian Valley and Ridge Province, south-western Virginia, U.S.A. show strain-dependent transitions between bedding fissility, pencil structure and cleavage. Pencil structures are associated with a bulk inhomogeneous shortening deformation where minimum principal strain (Z) ranges between 9 and 26% shortening (assuming a plane strain and constant volume). Where strains are less, bedding fissility dominates. Pencil fragments are defined by intersecting fracture sets subparallel to the pre-existing bedding fissility and cleavage. Their long axes are both parallel to the bedding-cleavage intersection and to the inferred Y axis of the tectonic strain ellipsoid. Pencil development is considered to result from fracturing along both fabric anisotropies during weathering and post-tectonic stress relaxation. Pencils show variations in size and shape depending on lithology (grain size and composition), degree of initial clay preferred orientation, degree of cleavage development, type of cleavage, total bulk strain and degree of strain homogeneity. Their shape (l/w) however is a direct measure of total Y/Z strain since strain determinations from chlorite pressure-fringes on framboidal pyrite within the pencil fragments give
(YZ) = 0.913 + 0.019 (l/w)
. Pencil structure is therefore a potential strain marker in weakly deformed rocks.  相似文献   

4.
Transpression occurs in response to oblique convergence across a deformation zone in intraplate regions and plate boundaries. The Korean Peninsula is located at an intraplate region of the eastern Eurasian Plate and has been deformed under the ENE–WSW maximum horizontal compression since the late Pliocene. In this study, we analyzed short-term instrumental seismic (focal mechanism) and long-term paleoseismic (Quaternary fault outcrop) data to decipher the neotectonic crustal deformation pattern in the southeastern Korean Peninsula. Available (paleo-)seismic data acquired from an NNE–SSW trending deformation zone between the Yangsan and Ulleung fault zones indicate spatial partitioning of crustal deformation by NNW–SSE to NNE–SSW striking reverse faults and NNE–SSW striking strike-slip faults, supporting a strike-slip partitioned transpression model. The instantaneous and finite neotectonic strains, estimated from the focal mechanism and Quaternary outcrop data, respectively, show discrepancies in their axes, which can be attributed to the switching between extensional and intermediate axes of finite strain during the accumulation of wrench-dominated transpression. Notably, some major faults, including the Yangsan and Ulsan fault zones, are relatively misoriented to slip under the current stress condition but, paradoxically, have more (paleo-)seismic records indicating their role in accommodating the neotectonic transpressional strain. We propose that fluids, heat flow, and lithospheric structure are potential factors affecting the reactivation of the relatively misoriented major faults. Our findings provide insights into the accommodation pattern of strain associated with the neotectonic crustal extrusion in an intraplate region of the eastern Eurasian Plate in response to the collision of the Indian Plate and the subduction of the Pacific/Philippine Sea Plates.  相似文献   

5.
Neoproterozoic rocks, Oligocene to Neogene sediments and Tertiary Red Sea rift-related volcanics (Harrat) are three dominant major groups exposed in the Jeddah tectonic terrane in Western Arabia. The basement complex comprises amphibolites, schists, and older and younger granites unconformably overlain by a post-amalgamation volcanosedimentary sequence (Fatima Group) exhibiting post-accretionary thrusting and thrust-related structures. The older granites and/or the amphibolites and schists display mylonitization and shearing in some outcrops, and the observed kinematic indicators indicate dextral monoclinic symmetry along the impressive Wadi Fatima Shear Zone. Finite strain analysis of the mylonitized lithologies is used to interpret the deformation history of the Wadi Fatima Shear Zone. The measured finite strain data demonstrate that the amphibolites, schists, and older granites are mildly to moderately deformed, where XZ (axial ratios in XZ direction) vary from 2.76 to 4.22 and from 2.04 to 3.90 for the Rf/φ and Fry method respectively. The shortening axes (Z) have subvertical attitude and are associated with subhorizontal foliation. The data show oblate strain ellipsoids in the different rocks in the studied area and indication bulk flattening strain. We assume that the different rock types have similar deformation behavior. In the deformed granite, the strain data are identical in magnitude with those obtained in the Fatima Group volcanosedimentary sequence. Finite strain accumulated without any significant volume change contemporaneously with syn-accretionary transpressive structures. It is concluded that a simple-shear deformation with constant-volume plane strain exists, where displacement is strictly parallel to the shear plane. Furthermore, the contacts between various lithological units in the Wadi Fatima Shear Zone were formed under brittle to semi-ductile deformation conditions.  相似文献   

6.
The tectonic effects of the Thulean mantle plume on the opening of the North Atlantic Ocean is still poorly understood. An analysis of the brittle deformation affecting the Late Cretaceous Chalk and Lower Tertiary igneous formations cropping out in Ulster (Northern Ireland), part of the Thulean Province, leads to the recognition of two tectonic phases. Each of these phases is characterized by different stress regimes with similar trends of the horizontal maximum principal stress σH. The first phase, syn-magmatic and dominated by NE–SW to ENE–WSW extension, occurred during the Palaeocene. It is followed by a second post-magmatic phase, characterized initially by a probably Eocene strike-slip to compressional palaeo-stress regime with σ1 (=σH) trending NE–SW to NNE–SSW associated with the partial reactivation (as reverse faults) of normal faults formed during the first phase NE–SW extension. This episode is postdated by an Oligocene extension, with σH (=σ2) still striking NNE–SSW/NE–SW, which reactivated Eocene strike-slip faults as nearly vertical dip-slip normal faults. This Palaeogene tectonic evolution is consistent with the tectonic evolution of similar age in western Scotland and in the Faeroe Islands. In particular, the post-magmatic NE–SW compression is here related to the ‘Faeroe compressive event’, which is related to the earliest stages of drift of the Greenland plate.  相似文献   

7.
A revised interpretation of a number of faults across the hinge and western limb of a large-scale anticlinal flexure in the Mount Isa district has been made in terms of the faults following earlier-formed be joints. Such joints often develop in weakly or moderately folded competent sediments, as a result of either tensile stresses that were active at a late stage during folding or the influence of residual stresses generated during tectonic uplift. The joints are oriented such that on a stereographic projection their poles plot parallel to the a axis of a fabric cross and at 90° to the fold axis (b). bc joints are thus approximately normal to bedding and contain the fold axis, and hence they fan around the axial plane of the fold containing them. Across the hinge and western limb of a steeply N-plunging large-scale F2 flexure in the Mount Isa district, a number of faults at high angles to bedding fan about the axial plane. Making use of the fold geometry and local bedding orientation it is possible to predict the orientation of ideal bc fractures at locations within the fold. These predictions fit well with the observed fault pattern. The movement on the faults, although apparently complex, appears consistent with continued shortening perpendicular to an axial-plane cleavage during the D2 deformation or as part of a later D2 deformation.  相似文献   

8.
《Journal of Structural Geology》2001,23(6-7):1141-1150
Emplacement of the Proterozoic Gunnison annular complex, Colorado, involved brittle failure and both subhorizontal sheeting and steeply-dipping dyking. The annular complex consists of a central diorite body (1730 Ma), surrounded by a ring of metamorphosed supracrustal rocks, in turn ringed by tonalite and granodiorite (1721 Ma). The older central diorite was emplaced as sills parallel to bedding, prior to regional deformation of the Gunnison volcanic arc, ∼1730–1710 m.y. ago. This central body was deformed during regional shortening, into an upright bowl with inward-dipping walls. The surrounding country rocks were folded and locally transposed against the central body, forming an arcuate foliation, conforming roughly to the shape of the body. This foliation and bedding acted as mechanical planes of weakness, which localized the syn-deformational emplacement of the outer ring intrusions, as multiple magma sheets parallel to steeply dipping bedding and foliation. Portions of the outer rings were also injected along concentric, inward-dipping, shear fractures generated under high magma pressure, a process similar to cone sheet emplacement in volcanic ring-dyke complexes. Multiple mechanisms were therefore involved in the emplacement of this sheeted annular plutonic complex, which otherwise superficially resembles a subvolcanic ring-dyke complex.  相似文献   

9.
A ‘soft’ carbon-based high-volatile bituminous (Ro max=0.68%) coal and a ‘hard’ carbon-based Pennsylvania anthracite (Ro max=5.27%) were deformed in the steady state at high temperatures and pressures in a series of coaxial and simple shear deformation experiments designed to constrain the role of shear strain and strain energy in the graphitization process. Tests were carried out in a Griggs-t type solid (NaCl) medium apparatus at T=400–900°C, constant displacement rates of 10-5−10-6 s−1, at confining pressures of 0.6 GPa (coaxial) or 0.8 and 1.0 GPa (simple shear). Coaxial samples were shortened up to 50%, whereas shear strains up to 4.9 were attained in simple shear tests. Experiments lasted up to 118 h. Deformed, high-volatile bituminous coal was extensively coked and no correlation between strain and Ro max, bireflectance or coal texture was observed in any samples. With increasing temperature, Ro max and bireflectance increase in highly anisotropic, coarse mosaic units, but remain essentially constant in the fine granular mosaic, which becomes more abundant at higher temperatures. Graphite-like reflectances are observed locally only in highly reactive macerals and in pyrolytic carbon veins. The degree of molecular ordering attained in deformed bituminous coal samples appears to be determined by the heating-pressurization path rather than by subsequent deformation.Graphitization did not occur in coaxially deformed anthracite. Nonetheless, dramatic molecular ordering occurs at T>700°C, with average bireflectance values increasing from 1.68% at 700°C to 6.36% at 900°C. Anisotropy is greatest in zones of high strain at all temperatures. In anthracite samples deformed in simple shear over the 600–900°C range at 1.0 GPa, the average Ro max values increase up to 11.9%, whereas average bireflectance values increase up to 10.7%. Bireflectance increases with progressive bedding rotation and, thus, with increasing shear strain. Graphitization occurs in several anthracite samples deformed in simple shear at 900°C. X-ray diffraction and transmission electron microscopy of highly anisotropic material in one sample confirms the presence of graphite with d002=0.3363 nm. These data strongly suggest that shear strain, through its tendency to align basic structural units, is the factor responsible for the natural transformation of anthracite to graphite at temperatures far below the 2200°C required in hydrostatic heating experiments at ambient pressure.  相似文献   

10.
Preferred orientations of phyllosilicate grains in the shaley rocks of the Labrador Trough were studied using a pole-figure goniometer. Transmitted X-rays permit determination of the basal planes of chlorite and muscovite. From their preferred orientation, strain is calculated according to March's theory. By choosing samples in successively more strongly deformed domains, a strain history can be recognized which started with compaction by loss of pore volume under an overburden and was later followed by an early homogeneous tectonic shortening parallel to bedding and at a right angle to the fold axis. Buckling occurred next, with strain in the limbs becoming different from that in the hinges; the limbs were lengthened parallel to bedding and nearly normal to the fold axis. Continuing appression, finally, led to pervasive shortening normal to the axial plane with concurrent stretching both updip along the axial plane and along the fold axis. This latter stretching may have occurred when variable-plunge basin and dome structures of the Labrador Trough were formed.  相似文献   

11.
The strain associated with the Horn Head Slide, a major tectonic break in the Dalradian rocks of NW Ireland, is recorded by pebbles in an adjacent quartzite horizon. Mean X/Y ratios of the deformed pebble shapes in excess of 8.0 are seen closest to the slide and the field of three-dimensional shapes lies along the K = 1 line. The usual methods of separating initial shape ratio (Ri) and tectonic strain ratio (Rs) from the deformed shape ratio (Rf) of ellipsoidal markers are based on measurements of variation in fluctuation (e.g., the (Rf/φ technique). However, due to the high X/Y strains in this situation and since the pebbles initially lay parallel to bedding and to a principal plane of the finite strain ellipsoid, fluctuation is generally very low. Thus, except for the least deformed X/Y data, the Rf/φ technique is inapplicable and other methods are used. For X/Y data with mean (Rf > 4.0: Rs is calculated as the harmonic mean of Rf; maximum Ri values only are obtained from the range of Rf data. For all Y/Z and X/Z data: Ri is calculated from the logarithmic range (ωlog) of Rf; Rs is simply obtained from the geometric mean of Rf modified by Ri. It is concluded from this that a varying prolate tectonic strain (K - 1.5) reaching X/Y values in excess of 8.00 was coaxially superimposed on an initial oblate shape fabric to produce the present field of deformed pebbles in the quartzite near the slide.  相似文献   

12.
Analysis of strain in Jurassic argillites forming part of the folded and thrusted sedimentary succession of the Lagonegro basin (southern Italian Apennines) has been carried out using ellipsoid-shaped reduction spots as strain markers. Most of the determined finite strain ellipsoids are of oblate type and show a peculiar distribution of the maximum extension direction (X), with maxima either subparallel or subperpendicular to the local fold axes. Using the strain matrix method, two different deformation histories have been considered to assist the interpretation of the observed finite strain pattern. A first deformation history involved vertical compaction followed by horizontal shortening (occurring by a combination of true tectonic strain and volume loss), whereby all strain is coaxial and there is no change in the intermediate axis of the strain ellipsoid. By this type of deformation sequence, which produces a deformation path where total strain moves from the oblate to the prolate strain field and back to the oblate field, prolate strain ellipsoids can be generated and may be recorded where tectonic deformation has not been large enough to reverse pretectonic compaction. This type of deformation history may be of local importance within the study area (i.e. it may characterize some fold hinge regions) and, more generally, is probably of limited occurrence in deformed pelitic rocks. A second deformation sequence considered the superposition of pre-tectonic compaction and tectonic strain consisting of initial layer-parallel shortening followed by layer-parallel shear (related to flexural folding). Also in this instance, volume change during tectonic deformation and tectonic plane strain have been assumed. For geologically reasonable amounts of volume loss due to compaction and of initial layer-parallel shortening, this type of deformation history is capable of producing a deformation path entirely lying within the oblate strain field, but still characterized by a changeover, during deformation, of the maximum extension axis (X) from a position parallel to the fold axis to one perpendicular to it. This type of deformation sequence may explain the main strain features observed in the study area, where most of the measured finite strain ellipsoids, determined from the limb regions of flexural folds, display an oblate shape, irrespective of the orientation of their maximum extension direction (X) with respect to the local structural trends. More generally, this type of deformation history provides a mechanism to account for the predominance of oblate strains in deformed pelitic rocks.  相似文献   

13.
In the eastern Indian shield, a dextral strike-slip system juxtaposed the Archaean Singhbhum Province against the Proterozoic Eastern Ghats Belt at ∼490–470 Ma. Two WNW–ESE trending strands of the strike-slip system enclose a multiply deformed (D1 to D3) intervening domain called the Rengali Province, with D3 representing dextral shearing. In a granulite lens within the province, an early fabric (Sgr) was deformed by an amphibolite facies D1–D2 deformation continuum in the late Archaean time, forming cylindrical folds. In the surrounding quartzofeldspathic gneisses, quartzites and mica schists of the province, superimposition of syn-D3 shortening on D1-D2 folds generated complex non-cylindrical geometries; the granulites escaped D3 strain. Microstructures in the province-bounding shear zones confirm that D3 deformation was associated with mylonitization, dynamic recrystallization and greenschist facies metamorphism. In the quartzites, syn-D3 folds can be correlated with rotation of D1–D2 structures through the shortening zone of bounding dextral shears. Since the province-bounding shears form a step-over zone, the structural complexity within the Rengali Province arises from superposition of syn-D3 shortening structures on initially asympathetically oriented inherited cylindrical D1-D2 folds. Hydrous fluid channeling causing greenschist facies metamorphism and quartz vein emplacement accompanied D3 as the step-over zone was dilational in nature.  相似文献   

14.
A 3000 m Jurassic-Cretaceous-Palaeogene succession dominated by carbonates is deformed by NNE trending open folds of Palaeogene age. Conjugate wrench faults and a system of normal faults extend the fold belt axially and probably evolved during anticlockwise rotation in a transpressive regime related to the oblique convergence of the African and Arabian plates across the Lebanese segment of the Dead Sea transform fault. Three sets and four systems of conjugate mesoscopic fractures, symmetrically orientated with reference to bedding and the plunge of the fold in which they are contained, resulted in minor axial elongation. Pressure solution on surfaces striking parallel to the fold belt locally achieved up to 50% shortening.The N30°E vertical Yammouné Fault Zone, which connects with the principal rift faults to the north and south, is accompanied by mesostructures which indicate that displacements were dominantly left-lateral and that the 1–2 km Zone is younger than the folds, possibly of Neogene age.  相似文献   

15.
Field studies in the Palaeoproterozoïc Daléma basin, Kédougou-Kéniéba Inlier, reveal that the main tectonic feature comprises alternating large shear zones relatively well-separated by weakly deformed surrounding rock domains. Analysis of the various structures in relation to this major D2 phase of Eburnean deformation indicates partitioning of sinistral transpressive deformation between domains of dominant transcurrent and dominant compressive deformation. Foliation is mostly oblique to subvertical and trending 0–30° N, but locally is subhorizontal in some thrust-motion shear zones. Foliation planes of shear zones contain a superimposed subhorizontal stretching lineation which in places cross-cuts a steeply plunging stretching lineation which is clearly expressed in the metasedimentary rocks of weakly deformed surrounding domains. In the weakly deformed domains, the subhorizontal lineation is absent, whereas the oblique to subvertical lineation is more fully developed. Finite strain analyses of samples from surrounding both weakly deformed and shearing domains, using finite strain ratio and the Fry method, indicate flattened ellipsoid fabrics. However, the orientation of the long axis (X) of the finite strain ellipsoid is horizontal in the shear zones and oblique within the weakly deformed domains. Exceptionally, samples from some thrust zones indicate a finite strain ellipsoid in triaxial constriction fabrics with a subhorizontal long axis (X). In addition, the analysis of the strain orientation starting from semi-ductile and brittle structures indicates that a WNE–ESE (130° N to 110° N) orientation of strain shortening axis occurred during the Eburnean D2 deformation.  相似文献   

16.
Strain analysis based on initially uniformly oriented elliptical particles in an oolitic limestone (Blegi oolite) was used to study the homogeneity of the state of strain on various scales, kinematics of folding and deformation mechanisms. A computer (reduced means) method for strain analysis is presented which is based on deforming a population of ellipses with shape and orientation properties of measured undeformed ooids. The strain values obtained with this method are within an accuracy of about 10% (in terms of axial ratios) and are in good agreement with the ones obtained with existing graphical methods. The state of strain is homogeneous on the scale of a thin section, handspecimen and outcrop, provided that regions around relatively strong fossils and regions of marked variations in lithology are avoided. Whole rock strains and strains as indicated by ooids alone are similar. Strain patterns in folds in limestones embedded in sandstones, shales and marl are compatible with bending accompanied simultaneously with a shortening perpendicular to the axial surface. The shortening may be attributed to the shear strains related to fold asymmetry and overthrusting. Strains on the outer arcs of a competent dolomite layer compare well with theoretical and experimental fold models; strain patterns include complex contact strains and change along the fold hinge line across a transverse fault which was active during the folding process. Strains parallel to the hinge line are more or less uniform but do not necessarily represent a plane strain state. Volume change took place during deformation. It was accomplished by pressure solution processes, the pressure solved material being partly redeposited. Pressure solution accounts for only a relatively small fraction of the bulk finite strain and was accompanied by plastic flow. Intracrystalline deformation together with grain boundary sliding and/or grain boundary migration went hand in hand with recrystallization (noteably grain growth).  相似文献   

17.
The current contribution presents aspects of the structural style and fault kinematics of the Rus Formation that expose at Jabal Hafit, Al Ain, United Arab Emirates. Although the major structure of Jabal Hafit is an anticlinal fold, fractures (joints and faults) are the prominent structure of the study area. The fractures can be interpreted as the distributed effect of deep-seated basement fault reactivation or to be as reactivation of deep-seated basement faults. These fractures were created during two main tectonic stress regimes. The first is a WNW–ESE S Hmax strike-slip stress regime, responsible for producing E–W to ESE–WNW joints and E–W dextral strike-slip and NNE–SSW reverse faults. This stress is interpreted to be post-Early Eocene in age and related to the second phase of thrusting in the Oman Mountains in the Miocene. The second stress regime is a NNE–SSW S Hmax transtensional (strike-slip extensive) stress regime that was responsible for N–S to NNE–SSW striking joints and NE–SW sinistral strike-slip and N–S normal faults. This regime is interpreted to be post-Middle Eocene in age. This stress was the response to the collision of the Arabian–Eurasian Plates which began during the Late Eocene and continues to the present day.  相似文献   

18.
罗良  贾东  李一泉  邓飞  孙圣思 《地质学报》2008,82(6):850-856
磁组构是一种灵敏的应变指示计。单一方向应力作用下,在平行层缩短的初始阶段,磁线理与地层走向是一致的。然而,在构造叠加背景下弱变形的沉积岩地区,另一个（多个）不同方向的应力使得已经产生定向排列的磁性矿物发生旋转,表现为磁线理和与地层走向斜交。川西北盆地在新生代是一个典型的构造叠加区域,来自龙门山和米仓山的变形在此相互作用。本文在川西北盆地分3条剖面在18个采样点中采集了172个样品进行了磁组构研究。研究区内观察到3种弱变形的磁组构类型：沉积磁组构、初始变形磁组构和铅笔状磁组构。由于应变的叠加,由盆地内部向造山带前缘没有出现应变由弱到强的变化趋势,同时磁线理的方向也不一致。由盆地向造山带,来自米仓山的变形逐渐增强,磁线理从与地层走向一致转变成与地层走向斜交。  相似文献   

19.
Recent field campaign in the southern Menderes Massif in southwestern Turkey revealed that the so-called ‘core of the massif’ comprises two distinct types of granitoid rocks: an orthogneiss (traditionally known as augen gneisses) and leucocratic metagranite, where the latter is intrusive into the former and the structurally overlying ‘cover’ schists. These differ from one another in intensity of deformation, degree of metamorphism and kinematics. The orthogneiss display penetrative top-to-the-N–NNE fabrics formed under upper-amphibolite facies conditions during the Eocene main Menderes metamorphism (MMM), whereas foliation and stretching lineation exists in the leucocratic metagranites but are not strongly developed. The leucocratic metagranites show evidence of syn- to post-emplacement deformation in a series of weakly developed top-to-the-S–SSW fabrics formed under lower greenschist-facies (?) conditions. Leucocratic metagranite bodies occur all along the augen gneiss–schist contact in the southern Menderes Massif; they are emplaced as sheet-like bodies into country rocks (previously deformed and metamorphosed during a top-to-the-N–NNE Alpine orogeny) along a ductile extensional shear zone, located between orthogneisses and metasediments, which was possibly active during emplacement. The data presently available indicate that emplacement and associated ductile extensional deformation occurred during Late Oligocene–Early Miocene time. These results confirm previous contentions that there are Tertiary granites in this part of the Menderes Massif.  相似文献   

20.
The Dating rocks and Darjeeling gneisses, which constitute the Sikkim dome in eastern Himalaya, as well as the Gondwana and Buxa rocks of ‘Rangit Window’, disclose strikingly similar sequences of deformation and metamorphism. The structures in all the rocks belong to two generations. The structures of early generation are long-limbed, tight near-isoclinal folds which are often intrafolial and rootless. These intrafolial folds are associated with co-planar tight folds with variably oriented axes and sheath folds with arcuate hinges. Penetrative axial plane cleavage and mineral lineation are related structures; transposition of bedding is remarkable. This early phase of deformation (D 1) is accompanied by constructive metamorphism. The structures of later generation are open, asymmetrical or polyclinal; a crenulation cleavage or discrete fracture may occur. The structures of early generation are distorted by folds of later generation and recrystallized minerals are cataclastically deformed. Recrystallization is meagre or absent during the later phase of deformation (D 2). The present discussion is on structures of early generation and strain environment during theD 1 phase of deformation. The concentration of intrafolial folds in the vicinity of ductile shear zones and decollement or detachment surface (often described as ‘thrust’) may be considered in this context. The rocks of Darjeeling-Sikkim Himalaya display minor structures other than intrafolial folds and variably oriented co-planar folds. The state of finite strain in the rocks, as observed from features like flattened grains and pebbles, ptygmatic folds and boudinaged folds indicate combination of flattening and constrictional type strain. The significance of the intrafolial folds in the same rocks is discussed to probe the environment of strain during progressive deformation (D 1).  相似文献   

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