Strain-slip Cleavage Research Articles

Mise En Évidence De Nouvelles Structures Géologiques Dans La Région De Brobo (Centre De La Côte d’Ivoire). Aide À La Compréhension De La Tectonique Du Paléoprotérozoïque Du Craton Ouest Africain

The lithological and structural observations of the region of Brobo (Central Côte d'Ivoire) indicate a succession of metasedimentary rocks (micaschists with cordierite, silstones, graphitic sediments, sandstones with amphibole-garnet, etc.) intermixed with volcanic rocks (rhyolite, dacite, andesite, basalt and the volcanoclastics). The whole is intruded by granites with one or two micas, sometimes porphyries, granodiorites, gabbros, and granite gneisses. Interpretations of Landsat ETM+ , RadarSat-1 and SRTM remote sensing imageries, as well as field data, revealed several lineament directions which, after field control, correspond to major faults and shear zones. These large structures show the N-S, NE-SW, NNE-SSW, E-W, NWSE, and NNW-SSE orientations. The field data also made it possible to describe several structures and to propose a preliminary geodynamic model for the setting and structuring of the formations of this region. This model suggests that the geodynamic took place in three stages: distension with a deformation of basement formations generating a gneissocity (D1), as well as deposits of sediments in the basins; followed by a NW-SE to E-W convergence generating a cleavage in the volcanogenic series (D2). This phase of deformation continues while creating, locally, a strain slip cleavage or a transposed schistosity. The third cleavage affects the volcanogenic series (fractures cleavages, D3) and ends in large corridors of ductile shear zones and associated faults.

Pan-African deformation markers in the migmatitic complexes of Parakou–Nikki (Northeast Benin)

The analysis of structural markers collected from the petrostrutural units of Parakou-Nikki, in Northeast Benin, indicates that the Pan-African remobilization or rejuvenation (600 ± 50 Ma) of the Benino-Nigerian Metacraton consits of a wide migmatization accompanied by five tectogenetic phases (Dn–Dn+4). The Dn phase is penecontemporaneous with the collision between the Benino-Nigerian shield and the oriental margin of the West-African Craton. This phase is associated with the granulitization or migmatization episode, and materialized by the Sn foliation on which the Dn+1 deformation markers (major Pan-African structuring phase in the Dahomeyide orogenic belt) are superimposed. Thus, the Sn+1 plane and the Ln+1 lineations indicate a perfect transposition of the Sn plane and a syn-Dn paragenesis remobilization in the syn-Dn+1 amphibolite facies retromorphosis. The strong resumption of the main foliation (Sn+1) corresponds to the Dn+2 deformation phase. The latter, associated with a second retromorphosis, is materialized by Pn+2 folds, with submeridian to NE–SW axes, and by a Sn+2 foliation or strain-slip cleavage. It is also the phase of emplacement of the Kandi mega-shear zone associated with the development of Cn+2 dextral shear plane very generalized over all the petrostructural units of Parakou–Nikki. At a large scale, the Dn+3 phase is expressed as Pn+3 kilometric synform and antiform structures with NE–SW to ENE–WSW trending axes. This Dn+3 phase can be considered as a transpression responsible for the development of the Cn+3 sinistral shear plane and flat shear structures displaying a westward overlapping. It ends with a fracturing episode materialized by strike-slip fault systems defining a main stress σ1 trending ESE–WNW to SE–NW. The late Dn+4 phase only corresponds to conjugated strike-slip faults which resulted from a major SE–NW to SSE–NNW compression.

Compositional Layering in Alpine Peridotites: Evidence for Pressure Solution Creep in the Mantle

Field evidence from the Josephine (southwestern Oregon) and Red Mountain (New Zealand) peridotites indicates that compositional layering in alpine-type peridotites pre-dates emplacement into the crust, and is dissimilar in origin to layering in stratiform intrusions. Field, geochemical, and textural evidence all suggest that the layering formed during anatexis and upward flow of the peridotite in the mantle. A cumulus origin for the layering is rejected as there is strong geochemical evidence to suggest that alpine-type peridotites are the residues of partial fusion in the mantle. Although the layering superficially resembles layering in stratiform intrusions, there are no other features present which suggest a magmatic origin. The orientation of the layering, however, closely resembles that of strain-slip (or crenulation) cleavage in the axial zones of old mountain belts suggesting a deformation related origin. It also appears unlikely that the layering is solely the product of a mechanical segregation accompanying deformation ("flow layering") inasmuch as evidence for prolonged high-temperature creep of the peridotites is found throughout the entire peridotite whereas the layering is not. The most likely mechanism for the origin of the layering appears to be metamorphic differentiation accompanying deformation, pressure solution creep, and anatexis of the peridotite. This suggests that pressure-solution creep may be a principal creep mechanism in areas of ascending athenosphere such as below mid-ocean ridges.

Geology of the older Precambrian rocks of the Grand Canyon Part I. Petrology and structure of the Vishnu Complex

Abstract Rocks comprising the older Precambrian of the Grand Canyon are subdivided into the Vishnu Complex, the Trinity and Elves Chasm orthogneisses and the Zoroaster Plutonic Complex. Lithologies of the Vishnu Complex include metamorphosed sandstones, shales, impure carbonate sediments and basic volcanic rocks. The metasandstones and shales greatly predominate. Two major metamorphic/deformational events and one minor event are recorded in the structures of the Vishnu Complex. A penetrative schistosity and associated upright macroscopic folds mark the first deformation (D1). This was accompanied by lower amphibolite facies metamorphism. The second metamorphic episode (D2) to a large extent overprinted and obliterated earlier features, and produced most of the minerals and structures seen today in the Vishnu Complex. S2 foliation ranges from strain-slip cleavage to penetrative schistosity; L2 lineation is marked by crenulations and mineral alignment; F2 folds are open to isoclinal and steeply plunging. Mineral assemblages of the second metamorphism are of mid- to upper-amphibolite facies and indicate maximum temperatures of approximately 650°C at pressures of 3–4 Kb. The age of this event is in the range of 1650–1700 M.a. A third, relatively weak, metamorphic deformational event (D3) produced local shear zones and in these areas caused retrogressive metamorphism of high-grade rocks to the greenschist facies.

Geologie et pétrographie des roches sédimentaires et volcaniques kétilidiennes (Protérozoique inférieur) de la baignoire d'Arsuk, Groenland méridional

The Arsuk ø area is situated along the north-western border of the Early Proterozoic (> 1750 m.y.) mobile belt of South Greenland. Around Arsuk ø reactivated Archaean (> 2500 m.y.) basement is represented by gneiss, amphibolites and migmatites belonging to several lithological series. In the Arsuk basin Early Proterozoic (Ketilidian) supracrustals consist of a group of sedimentary rocks which is overlain by a group of volcanic rocks. The sedimentary Ikerasârssuk Group, with a thickness between 1000 and 1500 m, consists of semi-pelites and pelites with several zones of pyrite-bearing graphite schists and dolomitic limestones. There are also numerous sills of basic rocks which have the same age as the overlying group of volcanic rocks. In some localities the basal member of the group consists of feldspathic quartzites. The volcanic Arsuk Group, the upper part of which is eroded away, has a measured thickness of 4200 m. It consists of pillow lavas, basic massive lavas, volcanic breccias, lapillis and tuffites. There are also some ultrabasic rocks and thin horizons of pyrite-bearing graphite schists with chert. These supracrustal rocks underwent intense deformation at the close of the Early Proterozoic. Three phases can be recognised. The first phase produced N-S to NNE-SSW recumbent folds and the regional schistosity. Refolding during the second phase resulted in folds with E-W to ESE-trending axial planes and a strain slip cleavage. The last phase produced N-S trending structures. The grade of metamorphism during the first phase of deformation corresponds to greenschist facies. In the supracrustals close to the basement recrystallisation in amphibolite facies took place between the first and third phases of folding. This shows the existence of a gradient towards still higher grade metamorphic conditions in the underlying Archaean basement undergoing thorough reconstitution at the end of the Early Proterozoic. As a result of the deformation the stratigraphical unconformity between the Early Proterozoic (Ketilidian) supracrustals and the Archaean basement has been destroyed. During the Gardar period (Middle Proterozoic: > 950 m.y.) and again during the Mesozoic faulting and dyking occurred.

Relation of Cleavage and Metamorphism in the Macduff Slates

Synopsis Field and textural evidence from a coastal traverse immediately west of Macduff indicates that the main phase of regional metamorphism separates in time the movements which produced the slaty cleavage and folds at Macduff from those which produced the strain-slip cleavage and monoclines on the steep western limb of the Boyndie Syncline. The slaty cleavage of the Macduff Slates is, therefore, thought to be equivalent to the early movements in the rest of the Dalradian; an important conclusion in the light of the recent discovery of Lower Ordovician fossils in the Macduff Slates.

The regional significance of the Caledonian structures of the Sandland Peninsula, West Finnmark, northern Norway

A detailed analysis of the mesoscopic structures of the Sandland peninsula is used as the key to the synthesis of the structural history of the Loppen district. Three fold episodes are recognized. F 1 has folded the meta-sediments around the westerly-facing Haettekjeilen syncline, the steep, partly inverted easterly limb of which forms the Sandland peninsula and Silda, whilst the flat-lying westerly limb forms Loppa, the Andsnes peninsula, the Olderfjord meta-sediments and outcrops in the Kvaenangan Fjord. On Skjervoy, south of the Kvaenangan Fjord, the succession lies on the inverted limb of a westerly facing anticline, the axial trace of which must pass between Skjervoy and the Kvaenangan Gabbro. These large westerly facing F 1folds contrast with the large easterly facing F 1 folds reported from Soroy by Ramsay & Sturt (1963). Between the two areas a major F 1anticlinal rise with a n–s trend is envisaged passing to the south of Soroy and through the Loppen district where it follows the outcrop of the oldest meta-sediments exposed. These are intruded by a series of layered basic and ultrabasic bodies. Three sets of F 2 folds are described: major warps with vertical planes and a conjugate pair developed around easterly and westerly dipping axial planes. All have the same axial trend but develop on different parts of the F 1fold system. The suggested maximum stress responsible was nearly horizontal in an e–w direction, very similar to that responsible for F 1 and therefore unlikely to have modified greatly the regional orientation of the F 1structures. Later ( F 3) strain-slip cleavage and mild warps are associated with minor faults, shear zones and joints on near vertical planes with trends of 340°, 280° and 70°.

Mid-Mesozoic Multiphase Folding Along the Border of the Shuswap Metamorphic Complex

Research Article| May 01, 1968 Mid-Mesozoic Multiphase Folding Along the Border of the Shuswap Metamorphic Complex DONALD W HYNDMAN DONALD W HYNDMAN Department of Geology, University of Montana, Missoula, Montana Search for other works by this author on: GSW Google Scholar Author and Article Information DONALD W HYNDMAN Department of Geology, University of Montana, Missoula, Montana Publisher: Geological Society of America Received: 27 Jun 1966 Revision Received: 16 Jan 1967 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1968, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1968) 79 (5): 575–588. https://doi.org/10.1130/0016-7606(1968)79[575:MMFATB]2.0.CO;2 Article history Received: 27 Jun 1966 Revision Received: 16 Jan 1967 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DONALD W HYNDMAN; Mid-Mesozoic Multiphase Folding Along the Border of the Shuswap Metamorphic Complex. GSA Bulletin 1968;; 79 (5): 575–588. doi: https://doi.org/10.1130/0016-7606(1968)79[575:MMFATB]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The high-grade regional metamorphism of the extensive Shuswap metamorphic complex of southeastern British Columbia has been considered by most geologists to be Precambrian in age. On the basis of structural analysis, it is now apparent that low-grade Triassic rocks of the nearby Slocan Group have undergone the same three generations of folding as the Shuswap, beginning with that accompanying regional metamorphism. Intense isoclinal recumbent folding of the first generation accompanied regional metamorphism and development of a schistosity parallel to the axial surfaces of these folds and a penetrative lineation parallel to their axes. Following regional metamorphism, a second deformation, accompanying formation of nearly upright macroscopic folds, formed a strain-slip cleavage by crenulation of the schistosity. Subsequent intrusion of granitic plutons developed mesoscopic folds having coplanar axes in any single domain. This third deformation refolded the nearly constant eastward plunge of earlier folds and lineations into small cones. Deformation accompanying regional metamorphism of the Shuswap complex must have occurred since the Triassic and probably during the Jurassic period. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Discussion

Dr A. S impson commented that the author’s observations emphasized the importance of polyphase folding, and the accompanying superimposition of cleavages, in the deformed Lower Palaeozoic strata south of the Highland boundary fault. It was perhaps premature to attempt a tectonic correlation with the polyphase structural sequence of the Irish sea region to the east. There was, however, a close geometrical similarity between the acute F 1 folds, with their steep axial-plane S 1 , in Murrisk and the Caledonian F 1 folds developed in the Isle of Man, in North Wales, and in the Skiddaw Slates of the Lake District. The Caledonian F 2 folds at Murrisk possessed steep to vertical axial planes. This steep disposition afforded a contrast with the F 2 folds observed in the vicinity of the Irish Sea; the latter were usually gently inclined. It was interesting to note that in both central Murrisk and in the Manx Slate Series of the Isle of Man a second metamorphism had resulted in the static overgrowth of a varied selection of porphyroblasts during the F 2 – F 3 static interval. The speaker asked the author if the effect of the static post- F 2 metamorphism was uniform throughout his area or whether the occurrence of the later minerals showed any localization that might reflect F 2 structural control? In the Irish Sea region a steep strain-slip cleavage ( S 3 ). trending to the north-western quarter and possessing a steep north-easterly dip, was widely associated with F 3 cross-folding. Had the author observed post- F 2 structures of this type in Murrisk? Had the author

The geological interpretation of potassium-argon ages of metamorphic rocks from the Scottish Caledonides

Synopsis The K-Ar age pattern in the Scottish Caledonides is interpreted in terms of a cooling history. Eighty-four new K-Ar ages are reported and these are systematically related to be retentivities of the materials dated and to the structural position of the rocks sampled. Dalradian slates near the Highland Border with a flow-cleavage associated with the initial (F 1 ) deformation yield whole-rock ages averaging 505±6 m.y. This is a minimum age for F 1 in the Scottish Dalradian. Post-500 m.y. Dalradian mica ages reflect cooling times following the main (post-F 2 , pre-F 3 ) metamorphism, but a lower limit of 471±18 m.y. for this metamorphism is obtained from whole-rock analyses of high-level Dalradian slates, among the first rocks to cool after the metamorphic peak. The F 3 deformation is dated from peripheral slates with F 3 strain-slip cleavage of the Middle Dalradian and Ballapel successions, which yield whole-rock ages averaging 448±13 m.y. and 443±14 m.y. respectively. Uplift associated with F 3 accelerated cooling of the main mass of Dalradian and Moine schists and gneisses and these yield mica averages of 451±17 m.y. (muscovite) and 440±13 m.y. (biotite). Intrusion of Newer Granites occurred between 425 and 390 m.y. ago, and localised high temperatures associated with this activity are evidenced in the age pattern obtained from the metamorphic rocks.

The structure of the south-eastern part of the morar antiform, Inverness-shire

Four phases of folding are recognised within the Moine Schists of the south-eastern part of the Morar Antiform. The first recognisable fold phase has given rise to a very large isoclinal fold. Few minor structures can be related to this fold. The second phase of folding was intense on both a major and a minor scale, and has resulted in the formation of generally tight, similar type folds. This second phase was probably associated with the development of a tectonic break located at a pelitic horizon in the Moine succession. The third fold phase is characterised by generally small scale, open folds and by a strong micro-folding of earlier planar elements which, in places, gives rise to a strain slip cleavage. The fourth phase is evidenced by major and minor folds of varying style. The minor folds are, more commonly, open and step-shaped. The mutual interference of the various sets of fold structures is discussed briefly. The detailed structural analysis of this part of the Morar area, combined with stratigraphical considerations ( Powell, 1964), suggests that it is possible to regard the main outcrop of Lewisian rocks, within the core of the southern part of the Morar Antiform, as the core of a large, first phase, isoclinal fold (see also Ramsay & Spring, 1962, and Lambert & Poole, 1964). It further suggests that the isoclinal fold, and thus its Lewisian core, may root in the west, perhaps on the western limb of the Morar Antiform.

Experiments on preferred orientation of platy minerals

Polycrystalline aggregates of phlogopite, talc, and brucite have been grown hydrothermally from their constituent oxides at 300–600° C, 3–5 kb, and compressed 10–30% in short-term experiments (typically 30 minutes). Under hydrostatic conditions, approximately random orientation of crystals results. When the specimen is strained at high temperature, either during or after growth of the minerals, a preferred orientation of basal planes normal to the axis of compression results. Since a similar result is obtained by straining at room temperature after growth of the minerals, the mechanism of orientation is probably mainly mechanical rotation after formation. Microscope examination showed that the preferred orientation is most marked in coarser grains of the aggregates. A second kind of foliation is defined in some specimens by closely spaced, narrow domains within which coarse grains are slightly rotated. These domains occur in conjugate sets symetrically oriented at about 45° to the axis of compression. They are interpreted as shear domains and are geometrically similar to incipient strain-slip cleavage in foliated rocks. The experiments may represent likely behavior in geological situations where the temperature or time scale precludes recrystallization during deformation, but they are probably not directly revelant to cases of axial-plane cleavage where reorientation through an influence of stress or strain during recrystallization is believed to have occurred. No unequivocal indication of the latter process was obtained in the experiments.

Strain Slip Cleavage in the Ordovician Sediments of Central Victoria

AbstractAt a late stage of folding, strain slip cleavage was imposed on the pelitic sediments of the Ordovician greywacke-slate association in Central Victoria. The nature of the cleavage surfaces was controlled by the type of sediment in which they were developed. In the coarser siltstones, and in the interlaminated zone of oscillatory graded beds, they are second order shear zones. In the finer siltstones and slates they are thin continuous shear domains of the first order.

A Note on Cleavages in Crenulated Rocks

AbstractPublished descriptions and new field observations in Ireland and Canada confirm that strain-slip cleavage should not be classified with fracture cleavage. The termcrenulation cleavageis proposed to replace the generic one of strain-slip cleavage. Crenulation cleavage differs essentially from slaty and fracture cleavage in that it only develops in laminated rocks and is consequently nearly always a secondary structure. Intense metamorphism converts crenulation cleavage into a new schistosity or cleavage with characteristics similar to slaty cleavage. Two fundamentally different types are recognized and it is suggested that crenulation cleavage is a valuable indicator of metamorphic conditions during the various phases of deformation.

THE AUREOLE OF THE MAIN DONEGAL GRANITE

The Main Donegal Granite, intruded into Dalradian metasediments a n d metadolerites under conditions that combined high temperature and strong directed pressure, has produced a wide aureole of contact schists that are like many regionally metamorphosed rocks. The aureole rocks were primarily of low metamorphic grade and, towards the granite contact, intense new structures and high-grade mineral assemblages have replaced the regional characters. Metadolerites occur throughout the aureole and can be used for metamorphic correlation of the diversified metasedimentary contact schists. The regional metadolerites are assemblages of albite, zoisite, actinolite and quartz, with relict pyroxene ; ophitic textures are preserved. Towards the granite contact, these rocks pass into coarse plagioclase-amphibolites with strong foliation and lineation. Mineralogical and structural transformations in the aureole are interconnected. Calc-pelites, outside the aureole showing calcite, quartz and phlogopitic mica, develop tremolite within the aureole with diopside at the immediate contact. Pelitic rocks develop kyanite, staurolite, garnet, andalusite, chloritoid, plagioclase, muscovite, biotite, with fibrolite near the contact. Structural effects are recorded farther out than mineralogical reconstruction. Small open folds appear and become tightened and steeper towards the granite; strain-slip cleavage first appearing in the outer aureole becomes the powerful schistosity of the inner aureole. Foliation, lineation, mullioning and boudinage of the rocks of the inner aureole are shared by the marginal parts of the granite. Certain minerals, such as staurolite, kyanite, plagioclase and garnet, formed during the early stages of deformation, were rotated and ruptured during a later stage in which other minerals continued to grow to make the sehistosity. A final episode is marked by great cataclastic ruptural belts that form most of the external edge of the aureole. The conditions in the aureole began with a brittle state, passed to plastic deformation and returned to brittleness. In the rafts enclosed in the granite, stability is reached by the formation of granoblastic hornfels-like rocks of simple character. Pelites give sillimanite- or andalusitegneisses, metadolerites give granoblastic hornblende-andesine-quartz rocks. The question of the controls of aluminium silicate formation is discussed. Fibrolite is considered to be metasomatic. Kyanite and andalusite are not replacements of one another and are coeval with the staurolite, garnet and plagioclase. Andalusite tends to occur in the outer aureole, kyanite in the inner. This suggests that andalusite forms at lower temperature and stress, kyanite at higher temperature and stress. But their development is not that of simple isochemical zoning, since andalusite seems to occur in rocks of special composition––pelites rich in divalent cations, Fe and Mg, the concentration of which may control the species of aluminium silicate formed. The mineral composition, the texture and structure of the aureole rocks agree with the interpretation of the emplacement of the Main Donegal Granite previously put forward by the authors.

The tectonic pattern in the Dalradian of the Craignish-Kilmelfort district, Argyllshire

Summary The Dalradian rocks of the Craignish-Kilmelfort district form a part of the easterly dipping western limb of the Loch Awe syncline. The succession comprises phyllites, quartzites, limestones and grits which have been intruded by a large number of basic sills and sheets; this sequence is now in a low-grade metamorphic condition. Folding was initiated about a NNE.–SSW. axis, at first by flexure along the pre-existing bedding surfaces and later by slip along curved planes of slaty cleavage. Continuation of these movements, about the same axis, resulted in the development of a sequence of strain-slip and fracture cleavages; the final strain-slip cleavage is associated with steep E.–W. axial planes. The style and attitude of the minor structures are discussed in relation to the major structure.