Metamorphic Schistosity Research Articles

Structural and Geochronological Constraints on Devonian Suprasubduction Tectonic Switching and Permian Collisional Dynamics in the Chinese Altai, Central Asia

AbstractKinematic significance and time scales of geodynamic processes forming the Altai Orogenic Belt are addressed through structural and petrological analysis combined with zircon and monazite geochronology. The study area is composed of orogenic lower crust represented by a Devonian migmatite‐magmatite complex and orogenic middle and upper crust formed by an amphibolite‐facies Ordovician sedimentary sequence and a weakly to unmetamorphosed Devonian volcano‐sedimentary cover, respectively. The orogenic lower and middle crust were first affected by moderate thickening, which formed subhorizontal Barrovian metamorphic schistosity. This fabric was reworked by deep crustal melting and intrusion of granite sheets during horizontal extension at 400–380 Ma. Soon after, this horizontal fabric was affected by NW‐SE shortening generating crustal‐scale upright folding associated with subvertical flow of still partially molten orogenic lower crust. During this event, the orogenic lower and middle crust were tightly juxtaposed with upper crustal sedimentary rocks. The last event was related with a NE‐SW oriented convergence resulting in large‐scale folding and megafold interference pattern in the Permian at 280–273 Ma. Combined with existing regional data, our results allow proposing a Devonian tectonic switching from compression to extension and back to compression, as a response to variations of subduction dynamics between slab advance and retreat in a Pacific‐type suprasubduction system. The Permian folding was associated with the progressive northward exhumation of thermally softened crust. This tectonic evolution is in response to the indentation of the rigid Junggar arc domain into the weak Altai wedge.

Magnetic fabric overprints in multi-deformed polymetamorphic rocks of the Gemeric Unit (Western Carpathians) and its tectonic implications

In the Gemeric Unit of the Central West Carpathians, the magnetic fabric was investigated in Early Palaeozoic metasediments and metavolcanics, Permian to Early Triassic granites, and Late Palaeozoic (molasse) metasediments. In addition to Variscan greenschist facies metamorphism in the Early Palaeozoic basement, all rocks underwent Cretaceous (Alpine) low-grade metamorphism, which significantly modified their mineral composition and fabric; original magnetic minerals were destroyed at least partially and new, mostly paramagnetic, minerals were created.In the Early Palaeozoic metasediments, the magnetic fabric is entirely metamorphic/deformational in origin. In metavolcanics, the magnetic foliations are partly parallel to the Alpine metamorphic foliations developed in relatively incompetent lithologies such as tuffs and tuffites and partly to disjunctive foliations developed in competent rhyolites and its subvolcanic members. Their magnetic fabrics probably represent gradual transition from Variscan to Alpine fabrics. In the Permian granites, the magnetic foliations are near the Alpine mesoscopic disjunctive foliations crosscutting metamorphic schistosity in contact aureoles of granite. In the Late Palaeozoic (molasse) metasediments, the magnetic foliations are near the Alpine metamorphic foliations that are mostly parallel to the original bedding. On regional scale, the magnetic fabrics show curved pattern indicating complex interactions between the Gemeric and underlain Veporic units in the final stages of intendation and collision.The magnetic fabric pattern in the Gemeric Unit roughly resembles that in the Veporic Unit. This results from similar tectonic movements operating at least during Cretaceous collision in both the units. This tectonic process occurred subsequent to the closure of the Meliata ocean and was responsible for creation of the final structure of the Central Western Carpathians. Nevertheless, in contrast to the Veporic unit, where dominantly Alpine type of magnetic fabrics is present, in the Gemeric Unit the Alpine overprint had variable degrees of modification of AMS fabrics ranging from relatively weak to totally obliterated.

The structural evolution of the Dialé-Daléma basin, Kédougou-Kéniéba Inlier, eastern Senegal

The Dialé-Daléma group is located in the eastern part of the Kédougou-Kenieba Inlier (KKI) and is essentially composed of Paleoproterozoic rock formations. Lithostructural studies in this area, hassuggested, often controversial, deformational regimes and geodynamic evolution models. In this study, we suggest the existence of an initial tangential D1 deformation and transpressional D2 deformational phases. D1 is characterized by an initial metamorphic schistosity denoted as S1 and by F1 folds with the main shortening stress direction generally oriented in NW-SE direction. D2 in the study area is divided into two stages. The first stage designated as D2a is coaxial and compressive in nature. It is characterized by a N-S to NNE-SSW trending metamorphic schistosity termed S2a. The D2a phase evolves gradually to a transcurrent phase connoted as D2b and characterized by a NNE-SSW S2b metamorphic schistosity which is axial planar to the F2b folds with subvertical fold axes. During the D2 phase, the main shortening stress direction rotates gradually in a clockwise motion from an E-W to a NW-SE direction within a continuum of deformation. The third deformation phase D3 in the Dialé-Daléma basin, is also divided into two stages, thus D3a and D3b. D3a is transcurrent and transtensive in nature and is characterized by NE-SW sinistral shear corridors with local extensional jogs. During this deformation stage, the maximum shortening stress (σ1) direction acts in a N-S direction and rotate to a NE-SW direction, thereby creating a N-S transcurrent dextral shear corridors. D3b component of the D3 deformation is compressive in nature with a weak sinistral shear component. The D4 corresponds to a N-S extensional phase which is characterized by E-W directed normal faults. D4 in the study area denotes the final stage of the evolution of the Eburnean orogeny in the KKI.

P–T–t–D record of crustal‐scale horizontal flow and magma‐assisted doming in the SW Mongolian Altai

ABSTRACTThe Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatite–magmatite core rimmed by a lower grade metamorphic envelope of andalusite and cordierite‐bearing schists. The oldest structure in the migmatite–magmatite core is a subhorizontal migmatitic foliation S1 parallel to rare granitoid sills. This fabric is folded by upright folds F2 and transposed into a vertical migmatitic foliation S2 that is syn‐tectonic, with up to several tens of metres thick granitoid sills. Sillimanite–ilmenite–magnetite S1 inclusion trails in garnet constrain the depth of equilibration during the S1 fabric to 6–7 kbar at 710–780 °C. Reorientation of sillimanite into the S2 fabric indicates that the S1–S2 fabric transition occurred in the sillimanite stability field. The presence of cordierite, and garnet rim chemistry point to decompression to 3–4 kbar and 680–750 °C during development of the S2 steep fabric, and post‐tectonic andalusite indicates further decompression to 2–3 kbar and 600–650 °C. Widespread crystallization of post‐tectonic muscovite is explained by the release of H2O from crystallizing partial melt. In the metamorphic envelope the subhorizontal metamorphic schistosity S1 is heterogeneously affected by upright F2 folds and axial planar subvertical cleavage S2. In the north, the inclusion trails in garnet are parallel to the S1 foliation, and the garnet zoning indicates nearly isobaric heating from 2.5 to 3 kbar and 500–530 °C. Cordierite contains crenulated S1 inclusion trails and has pressure shadows related to the formation of the S2 fabric. The switch from the S1 to the S2 foliation occurred near 2.5–3 kbar and 530–570 °C; replacement of cordierite by fine‐grained muscovite and chlorite indicates further retrogression and cooling. In the south, andalusite containing crenulated inclusion trails of ilmenite and magnetite indicates heating during the D2 deformation at 3–4 kbar and 540–620 °C. Monazite from a migmatite analysed by LASS yielded elevated HREE concentrations. The grain with the best‐developed oscillatory zoning is 356 ± 1.0 [±7] Ma (207Pb‐corrected 238U/206Pb), considered to date the crystallization from melt in the cordierite stability ~680 °C and 3.5 kbar, whereas the patchy BSE‐dark domains give a date of 347 ± 4.2 [±7] Ma interpreted as recrystallization at subsolidus conditions. The earliest sub‐horizontal fabric is associated with the onset of magmatism and peak of P–T conditions in the deep crust, indicating important heat input associated with lower crustal horizontal flow. The paroxysmal metamorphic conditions are connected with collapse of the metamorphic structure, an extrusion of the hot lower crustal rocks associated with vertical magma transfer and a juxtaposition of the hot magmatite–migmatite core with supracrustal rocks. This study provides information about tectono‐thermal history and time‐scales of horizontal flow and vertical mass and heat transfer in the Altai orogen. It is shown that, similar to collisional orogens, doming of partially molten rocks assisted by syn‐orogenic magmatism can be responsible for the exhumation of orogenic lower crust in accretionary orogenic systems.

Structural assemblies and age of deformation in the western sector of the Anyui-Chukotka Fold System, Northeastern Asia

Indications of intense deformation in the Anyui-Chukotka Fold System and the South Anyui Suture Zone have been noted for a long time [3, 5, 19, 36]. The character and age of the deformation, however, remain a matter of debate. Using structural paragenetic and deformational kinematic analyses, we establish three deformation stages in the Anyui-Chukotka Fold System. The structural assembly comprising open folds and NW-trending axial-plane cleavage was formed during the stage of regional compression (D1) related to the collision of the Chukotka-Arctic Alaska microcontinent with Eurasia. The assembly of the second stage in the Alyarmaut Rise is distinguished by isoclinal folds F2, gently dipping metamorphic schistosity, and pervasive cleavage in combination with folded quartz veins and lenses. Planar structural elements of the second stage are disturbed by low-amplitude normal and reverse faults and kink folds of stage D3. The U-Pb (SHRIMP-RG) and 40/39Ar methods were used for determination of the isotopic age of the deformations. The Aptian-Albian zircon age (117–108 Ma) has been established for six postcollision granitic plutons of the Anyui-Chukotka Fold System and the South Anyui Suture. Syncollision deformation completed 125–117 Ma ago. The extensional tectonic stage D2 accompanied by emplacement of the Lyupveem pluton occurred 120–105 Ma ago. The 40/39Ar age of the biotite from the metamorphic rocks marks the age of syndeformation metamorphism (109–103 Ma). The lower limit of brittle failure and deformation D3 is estimated at 105 Ma.

Emplacement of an Archean gneiss dome, northern Ontario, Canada: Inflation inferred from magnetic fabrics

Previous field structural studies of the Ash Bay gneiss dome revealed an upward convex foliation with an integral east‐west lineation defined by a quartzo‐felspathic matrix. Unfortunately, the amphibolite facies gneisses lack traditional indicators of microfabric evolution, such as kinematic indicators, so that it is difficult to relate the foliation's orientation distribution to an emplacement mechanism for the dome. Fortunately, anisotropy of magnetic susceptibility isolates a distinct microfabric because of the preferred orientation of high‐susceptibility, accessory magnetite. The magnetite subfabric developed late in the metamorphic crystallization history, and its magnetic foliation and magnetic lineation are similarly oriented to the S‐L fabric of the quartz‐feldspar matrix. However, the magnetite foliation dips less steeply and forms a less convex domal surface than the field schistosity. The metamorphic schistosity and the magnetite foliation thus isolate the domal deflection at different stages, confirming the inflation and upward motion of the Ash Bay diapir during the closing stages of amphibolite facies metamorphism.

P-Wave anisotropy of rocks from the seismic area in Western Bohemia

P-Wave velocity of metamorphic and magmatic rocks from the West Bohemian Saxothuringian (e.g. phyllites, quartzites, metabasites, mica schists, orthogneisses, granites and young volcanics) was measured using an acoustic method. The specimens were saturated with water in order to avoid the influence of open microcracks and porous space. Anisotropy coefficients were calculated from measurements in three perpendicular directions. Phycodes phyllitic slates as well as phyllites of the Frauenbach group from the region near the towns Luby and Kraslice exhibit relatively high V P up to 6800 m/s and extremely high anisotropy in the interval 25–49%. The lowest V P, under 4744 m/s, was found in light and phyllitic quartzites exhibiting, their anisotropy up to 29%. The high anisotropy of phyllites and quartzites is caused by an arrangement of phyllosilicates defining the metamorphic schistosity S 2 and controlling directional changes of V P. In the case of mica-schists of the Svatava crystalline complex, significant roles may be played also by other foliations. The relatively lower anisotropy of mica-schists from the vicinity of the town of Olovı́ compared to that of phyllites is partly due to this feature, partly due to lesser amount of mafic minerals. The anisotropy found by laboratory measurements has helped to find the relation between the geological structure and elastic wave propagation in the seismic area under study.

The Alpine overprints of the magnetic fabrics in the basement and cover rocks of the Veporic Unit (Western Carpathians, Slovakia)

In the Veporic Unit building up the central areas of the Western Carpathians, the magnetic fabrics are more or less coplanar and coaxial in metamorphic (Early Palaeozoic in age), granitic (Early to Late Palaeozoic in age) and sedimentary cover rocks (Permian/Triassic in age). The magnetic fabrics in metamorphic and granitoid basement rocks, as well as in sedimentary (-volcanic) cover rocks, appear to reflect a very wide P– T conditions of Cretaceous (originally Variscan) basement reactivation connected with a prograde metamorphism of the sedimentary cover rocks in lower greenschist (in the North-Veporic area) or the upper greenschist to lower amphibolite facies (in the South-Veporic area) during collisional burial in a collisional wedge. In addition, there are deformation zones where the magnetic fabrics formed due to a ductile deformation associated with extensional exhumation along (sinistral) lateral strike slip and normal faults. The late open folds did not principally change the former magnetic fabrics. The magnetic fabrics in sedimentary cover rocks of the North-Veporic area are either related to still recognizable bedding planes (in competent layers) with parallel superimposed metamorphic schistosity, or they are conformable to subparallel or oblique metamorphic cleavages. In the South-Veporic area, they are undoubtedly related to the metamorphic–mylonitic fabrics of the basement granitoids and their metamorphic mantle rocks. The magnetic fabrics in granitoid rocks of the South-Veporic area (S of the Pohorelá sinistral strike-slip fault) are influenced with micromechanisms of dominated low-temperature ductile deformation and partial dynamic recrystallization. We ascribe the observed coaxial magnetic fabrics in metamorphic, granitic and sedimentary rocks to an Alpine collision (110–95 Ma; P max∼8–9 kbar, T max∼530 °C) and extension tectonometamorphic events (95–80 Ma; up to P∼4–5 kbar, T∼300 °C) of Cretaceous age. The magnetic fabrics seem not to be strongly dependent on temperatures until the medium part of the amphibolite facies (as in this example), but they are strongly following simultaneous deformational low-temperature to medium-temperature meso- and microfabrics.

Sillimanite-bearing shear zones in syntectonic leucogranite: fluid-assisted brittle–ductile deformation under amphibolite facies conditions

In the Imja Kola valley (eastern Nepal), Miocene peraluminous leucogranite dykes were emplaced within transtensional shear zones synchronously with regional amphibolite–facies metamorphism in the metamorphic host rocks. Within the dykes the magmatic foliations, parallel to the metamorphic schistosity, are cross-cut by thin (millimetre to centimetre size) shear zones, which are made up of prismatic and/or fibrolitic sillimanite with a minor amount of white mica. These structures have a heterogeneous distribution and form networks of strongly deformed and foliated layers wrapping around poorly deformed granitic pods. The textural and strain features are consistent with development of sillimanite shear zones as a result of heterogeneous deformation and fluid action, which allowed the replacement of strained magmatic minerals such as feldspar and biotite by means of incongruent pressure solution. The described example illustrates that nucleation and development of heterogeneous shear zones under amphibolite facies conditions, is also strongly favoured by brittle fracturing and fluid action at the grain-scale along pre-existing planes of weakness. The growth of cracks and fractures can be regarded as a sequence of short lived brittle events, due to work hardening, which punctuate a history of plastic deformation.

Magnetic fabric in folds of the easternmost Rheno-Hercynian Zone

The magnetic fabric in folds was investigated in the easternmost Rheno-Hercynian Zone, the Nízký Jeseník Mts. In their eastern areas, the rocks show signs of only weak achimetamorphism and very gentle ductile deformation; SE vergent buckle folds of long wavelength are developed whose magnetic fabric can be easily unfolded geometrically. In the central areas, spaced cleavage and NW vergent buckle folds can be found; the folds can be unfolded mostly only partially. In the western areas, NW vergent cleavage folds and very well developed slaty cleavage occur. The magnetic fabric in the folds is homogeneous, the folds cannot be unfolded at all. The cleavage is transformed into metamorphic schistosity at the western border of the area.

Development of magnetic and elastic anisotropies in slates during progressive deformation

Magnetic and elastic anisotropies were investigated in rocks of the Nízký Jeseník Mountains (northeast Bohemian Massif) ranging in lithology from almost unmetamorphosed sediments, through slate, to phyllite, and showing a range of structural styles from sedimentary, through spaced and slaty cleavage, to metamorphic schistosity. In unmetamorphosed and undeformed sedimentary rocks, both the anisotropies display close relationships to the sedimentary fabric. During the development of the spaced and slaty cleavage they are gradually re-oriented into the attitudes of the deformational fabrics, and in the rocks with metamorphic schistosity they are fully related to the deformational fabric elements, which can be oriented in a very different way from the original sedimentary structures. The magnetic anisotropy is mostly due to the preferred orientation of phyllosilicates generated during very weak regional metamorphism, and subordinately due to the preferred orientation of magnetite. The elastic anisotropy is probably controlled by the preferred orientation of phyllosilicates and by the existence of oriented systems of microcracks.

The origin of large or great thrust-type earthquakes along subducting plate boundaries

Seismicity, deformation, state of stress, and abundance of fluids along subducting plate boundaries are reviewed, and the origin of large or great thrust-type earthquakes is discussed based on the recent experimental results on the slip behavior of halite and serpentine gouges. Shallow subducting plate boundaries above 20–25 km in depth are characterized by low seismicity, low tectonic stress, inter-plate decoupling, ductile deformation associated with the formation of metamorphic schistosity (except at very shallow depths), metamorphism suggesting solution processes on massive scale, and presence of abundant H 2O. It is argued that these unique features are due to pressure-solution processes, to high fluid pressure, to low strength and stable behavior of clayey sediments under wet environments, and/or to the deformation of soft, unconsolidated sediments at very shallow depths. The low seismicity in this zone is in marked contrast with major strike-slip faults along which large earthquakes occur at depths shallower than 15–20 km. It is emphasized that these unique features are expected only for restricted regions where there is constant supply of H 2O due to progressive metamorphism or where fluids in the rocks are trapped and cannot escape to the surface. Large or great thrust-type earthquakes in subduction zones initiate at depths of 30–50 km, below the shallow decoupled zone. In this focal depth range, the supply of H 2O during progressive metamorphism perhaps diminishes downwards, the overriding and subducting plates are coupled and stick to each other during much of the inter-seismic period, and the resistance to slip (or shear stress) is presumably high. It is suggested that these earthquakes begin to occur at a depth where the plate-boundary zone becomes fairly dry. Deformation at these depths appears to be predominantly ductile, so that the earthquakes cannot be regarded simply as a brittle phenomenon. (1) Creep instability i.e., instability associated with plastic deformation, and (2) dehydration-induced instability are the most likely mechanisms for initiating the earthquakes, and both have some experimental support. Stick-slip of halite gouge while undergoing ductile deformation primarily by intracrystalline gliding is described and discussed as a supporting evidence for (1). Shear resistance of halite gouge increases with increasing confining pressure in stick-slip regimes. Hence the observed stick-slip may be a semi-brittle phenomenon with respect to the pressure dependence of the shear resistance, although the deformation texture cannot be distinguished from that formed by pressure-insensitive flow. Serpentine gouge exhibits violent stick-slip upon its decomposition under dry, not wet, environments, supporting the mechanism (2) above. Exact mechanisms which lead to the unstable fault motion are poorly understood as yet, but stick-slip of both halite and serpentine gouges is recognized only when the slip-rate dependence of friction is negative i.e., lower friction at faster slip rate, consistent with the theoretical prediction of Rice and Ruina (1983). There is a possibility that the thrust-type earthquakes can be explained essentially within the framework of fault constitutive laws developed by Dieterich (1979) and Ruina (1983).

Structural elements and deformational history of rocks in eastern Massachusetts

Late Precambrian stratified rocks of the Boston platform were intruded by late Precambrian granitic magma. Later they were regionally metamorphosed, folded, mylonitized, and faulted during the Acadian orogeny or an earlier event. Still later, rocks of the Boston basin were folded and faulted during the Alleghenian disturbance. Rocks along an extensive fracture system were deformed intermittently by faulting from possibly middle Devonian to Permian time. The platform rocks were deformed during three periods of deformation. The first period began early in the regional metamorphic event when folds and metamorphic schistosity (S 1 ) formed in response to ductile deformation. Possibly some cataclastic foliation (S x ) formed during this period. The second period of deformation began near the end of regional metamorphism. First, minor folds developed; later, regional north- to northeast-trending folds formed. The schistosity (S 1 ) was folded, a slip cleavage (S 2 ) developed, and cataclastic foliation (S x ) associated with deep-seated faulting also formed in the rocks during the second period. The third period began near the end of Paleozoic time, when rocks of the Boston basin were folded and faulted. A cleavage (S 3 ) formed in some rocks during this time. The Bloody Bluff is the principal fault in a complex zone that strikes northeast across a large part of eastern Massachusetts. This zone has an irregular but prominent belt of cataclastic rock associated with it. Regional relations and aeromagnetic data suggest that the Bloody Bluff fault may be part of a fracture system that trends across the Gulf of Maine into Canada.