Quartzo-feldspathic Mylonites Research Articles

Kinematic analyses of the asymmetric boudins in the Tutak area within the Zagros hinterland fold-and-thrust belt, Iran

Exhumation of the Tutak gneiss dome occurred in a transpressional regime between the Arabian plate and the Central Iranian microcontinent. The Tutak gneiss dome is located in the eastern part of the basement-involved Zagros hinterland fold-and-thrust belt. This dome consists of exhumed middle crust including the Tutak and Sourian metamorphic complexes, which are lied on its core and edges, respectively. Integrated structural, microstructural, and flow vorticity analyses in quartzo-feldspathic mylonites and well-developed asymmetrical boudins provide significant information regarding the kinematic characteristics coeval with the Tutak gneiss dome development. Asymmetrical boudins have been used as potential shear sense markers throughout the mantle of the Tutak gneiss dome. The deformed quarzitic layers within the slate belt host domino and shear-band boudins, demonstrating both sinistral and dextral shear senses. The mean vorticity number (Wm) was estimated from the quartzo-feldspathic mylonites and asymmetric boudins. Using the hyperbolic distribution (HD) and the RXZ strain ratio/quartz c-axis fabric (RXZ/β) methods. The estimated Wms from the HD method and quartz c-axis patterns are 0.69 to 0.82 and 0.6 to 0.92, respectively, suggesting a combination of 41% to 72% simple- and 28% to 59% pure-shear components. The close match confirms that asymmetric boudins can be a good strain estimator. The estimated deformation temperature (528.33 ± 50 °C) from the opening angles (OAs) of quartz c-axes suggests amphibolite facies conditions. Our results show that the Tutak gneiss dome developed under sub-simple shear conditions, which stretched and shortened the dome parallel and orthogonal to the transpressional boundaries, respectively.

The architecture of long-lived fault zones: insights from microstructure and quartz lattice-preferred orientations in mylonites of the Median Tectonic Line, SW Japan

We combine field mapping with quartz microstructure and lattice preferred orientations (LPO) to constrain the mechanisms and spatio-temporal distribution of deformation surrounding the Median Tectonic Line (MTL), SW Japan. In the study area, the MTL occurs either as a narrow gouge zone or as a sharp contact between hanging-wall quartzofeldspathic mylonites to the north and footwall pelitic schists to the south. Along the northern margin of the MTL, there exists a broad zone of mylonitic rocks, overprinted by cataclastic deformation and a damage zone associated with brittle deformation. The mylonitic shear zone is dominated by coarse-grained protomylonite up to ~ 100 m from the MTL, where fine-grained ultramylonite becomes dominant. We observe a systematic variation in quartz LPO with distance from the MTL. In protomylonites, quartz LPOs are dominantly Y-maxima patterns, recording dislocation creep by prism<a> slip at ~ 500 °C. Closer to the MTL, we observe R- and Z-maxima, and single and crossed girdles, reflecting dislocation creep accommodated by mixed rhomb<a> and basal<a> slip, likely under cooler conditions (~ 300 °C–400 °C). Some ultramylonite samples yield weak to random LPOs, interpreted to result from the influx of fluid into the shear zone, which promoted deformation by grainsize-sensitive creep. Following cooling and uplift, deformation became brittle, resulting in the development of a narrow cataclasite zone. The cataclasite was weakened through the development of a phyllosilicate foliation. However, healing of fractures strengthened the cataclasites, resulting in the development of anastomosing cataclasite bands within the protomylonite.

Significance and dynamics of the Neoproterozoic (810 Ma) Phulad Shear Zone, Rajasthan, NW India

AbstractThe Phulad Shear Zone (PSZ) in northwestern India demarcates the boundary between Grenvillian age para‐metamorphics of the South Delhi Fold Belt (SDFB) to the east and variably deformed Neoproterozoic granitoids of the Marwar Craton to the west. In calcareous and quartzofeldspathic mylonites in and around Phulad, the shear zone is characterized by steep SE dipping mylonitic fabric and a steep oblique stretching lineation. The shear zone is developed in a ductile regime with top‐to‐the‐west reverse sense of movement coupled with pronounced flattening across the PSZ. MnNCKFMASH pressure‐temperature pseudosection analysis of closely spaced specimens of garnet‐bearing mica schists reveals that the stabilization of syn/post‐garnet porphyroblasts in the hanging wall experienced near‐isothermal decompression followed by cooling in the retrograde arm of a clockwise P‐T path. The retrieved P‐T paths are consistent with reverse motion of SDFB mica schist over the cratonic foreland. U‐Th‐Pb (total) monazite age determinations in hanging wall mica schists indicate that reverse motion on the PSZ occurred at 810 ± 6 Ma. The pre‐shearing Grenvillian age (~970 ± 9 Ma) metamorphic monazites of the SDFB occur as embayed cores, in the interiors of younger monazite grains. Intense high‐temperature deformation and the emplacement of mica‐bearing pegmatites during reverse shearing caused recrystallization of monazites by dissolution‐precipitation, mantling older monazites in the hanging wall SDFB schists. It is suggested that the PSZ may represent a terrane boundary shear zone along which the Greater India landmass accreted with the Marwar Craton at ~810 Ma.

Structural and kinematic analyses of the basement window within the hinterland fold-and-thrust belt of the Zagros orogen, Iran

AbstractThe Zagros hinterland fold-and-thrust belt is located in the central portion of the Zagros Thrust System and consists of the exhumed basement windows associated with NW-striking and NE-dipping flexural duplex structures that contain in-sequence thrusting and related folds. Mylonitic nappes of the basement were exhumed along deep-seated sole thrusts of the Zagros Thrust System. Lattice preferred orientation (LPO) c-axes of quartz show asymmetric type-1 crossed girdles that demonstrate a non-coaxial deformation under plane strain conditions. Based on the opening angles of quartz c-axis fabric skeletons, deformation temperatures vary from 425±50°C to 540±50°C, indicating amphibolite facies conditions. The estimated mean kinematic vorticity evaluated from quartz c-axis of the quartzo-feldspathic mylonites (Wm = 0.55±0.06) indicates the degree of non-coaxiality during mylonite exhumation. The estimated angle θ between the maximum instantaneous strain axis (ISA1) and the transpressional zone boundary is 17°, and the angle of oblique convergence is 57° in the M2 nappe of the basement involved. This indicates that the mylonitic nappe was formed by a combination of 62% pure shear and 38% simple shear during oblique convergence.

Using nanogranitoids and phase equilibria modeling to unravel anatexis in the crustal footwall of the Ronda peridotites (Betic Cordillera, S Spain)

Anatexis in the crustal footwall of Ronda peridotites (Betic Cordillera, S Spain) is apparently related to the hot emplacement of this mantle slab over metasedimentary rocks. In this study, we combine the analysis of melt inclusions (MI) and phase equilibria calculations on quartzo-feldspathic mylonites (former migmatites) occurring at the contact with the mantle rocks, in the region of Sierra Alpujata (Ojén unit). The goal is to better characterize anatexis in these rocks and to provide new constraints on the geodynamic evolution of the crustal footwall. Such data are important for understanding the mechanisms of crustal emplacement of the mantle slice. The quartzo-feldspathic mylonites are characterized by the mineral assemblage Qtz+Pl+Kfs+Sil+Grt+Ilm+Bt±Ap±Gr. Clusters of MI are observed both at the core and toward the rim of peritectic garnet. In each cluster, MI range from totally glassy to nanogranitoids, consisting of Qtz+Kfs+Bt+Ms+Pl aggregates. The trapped melt is leucogranitic and peraluminous with variable Na2O/K2O values and low H2O contents (≈2–4wt%). Phase equilibria modeling in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2–TiO2–O2–C (MnNCaKFMASHTOC) system with graphite-saturated fluid constrains the P–T conditions of melting at ≈6kbar, ≈820°C. MI data support the fluid-absent character of melting. The investigated MI represent the primary anatectic melts produced during prograde anatexis of the host rocks via biotite dehydration melting. Field, compositional, and textural observations indicate that mylonitic migmatites represent strongly deformed former diatexites. The comparison between the new data and some recently published information on migmatites located further from the contact with the peridotites and toward the bottom of the crustal footwall, raises some important issues which question the previously proposed geodynamic models for this region. Among them, (i) the crustal footwall at Sierra Alpujata constitutes an inverted metamorphic sequence, not only in terms of temperature but also in pressure, and (ii) the Ojén unit does not seem to represent a coherent high-P portion of a continental subduction system.

Retrogressed lawsonite blueschists from the NW Iberian Massif: P–T–t constraints from thermodynamic modelling and 40Ar/39Ar geochronology

Blueschist facies terranes in the Variscan Ibero-Armorican Arc are restricted to scarce and relatively small areas. One of these examples is the Ceán Unit, which is the westernmost exposure of the middle allochthonous sheet of the Variscan belt in the Malpica–Tui Complex (NW Iberian Massif). The Ceán Unit is a highly condensed metamorphic succession with a lower part in the blueschist facies and an upper part without HP relicts. It comprises variable proportions of glaucophane–chloritoid-bearing metapelites and mafic rocks with abundant well-preserved pseudomorphs after euhedral lawsonite. Both lithologies show systematic changes in texture and mineral composition that are spatially related depending on deformation. The metamorphic evolution of the metabasic rocks has been constrained in the P–T space through pseudosection approach and is characterised by H2O-undersaturated prograde evolution induced by the crystallisation of lawsonite. Peak conditions in the blueschist/LT-eclogite facies have been constrained at ca. 2.2 GPa and 560 °C. Exhumation-related metamorphism is characterised by a nearly isothermal decompression from the lawsonite-bearing fields to fields with stable albite at P ≈ 1 GPa. This lead to the pseudomorphism of lawsonite in the early-decompression stages, and a subsequent amphibolite–greenschist facies overprint at P < 0.8 GPa and T ≈ 440–480 °C. The preservation of the lawsonite crystal shape despite complete retrogression indicates that pseudomorphism occurred as a static process and that particular levels of the blueschist host rock were not affected by penetrative deformation during exhumation. 40Ar/39Ar step heating of phengitic muscovite from the pelitic schists interbedded with the lawsonite pseudomorph-bearing metabasic rocks yield plateau ages of ca. 363 ± 2 and 354 ± 1 Ma. The older age is interpreted as the age of the peak blueschist facies metamorphism. The age of 355 Ma is interpreted as a cooling age and is inferred to represent some point relatively close to peak conditions at the onset of the isothermal decompression. 40Ar/39Ar dating of muscovite from the quartzo-feldspathic mylonites of the Bembibre–Ceán detachment, at the base of the Ceán Unit, yields an age of ca. 337 ± 3 Ma, interpreted as the age of the post-nappe extensional tectonics. Similar data obtained from the blueschists of Ile de Groix (Armorican Massif; Bosse et al. in Chem Geol 220:21–45, 2005) support the equivalence of the Ceán Unit and the Upper Unit of Ile de Groix along the Ibero-Armorican Arc and suggest that these units share a blueschist facies event constrained at ca. 360–370 Ma, that is inferred to represent the Late Devonian–Early Carboniferous subduction of the northern margin of Gonwana beneath Laurussia.

Mica-controlled anisotropy within mid-to-upper crustal mylonites: an EBSD study of mica fabrics in the Alpine Fault Zone, New Zealand

AbstractThe lattice preferred orientation (LPO) of both muscovite and biotite were measured by electron backscatter diffraction (EBSD) and these data, together with the LPOs of the other main constituent minerals, were used to produce models of the seismic velocity anisotropy of the Alpine Fault Zone. Numerical experiments examine the effects of varying modal percentages of mica within the fault rocks. These models suggest that when the mica modal proportions approach 20% in quartzofeldspathic mylonites the intrinsic seismic anisotropy of the studied fault zone is dominated by mica, with the direction of the fastest P and S wave velocities strongly dependent on the mica LPOs. The LPOs show that micas produce three distinct patterns within mylonitic fault zones: C-fabric, S-fabric and a composite S–C fabric. The asymmetry of the LPOs can be used as kinematic indicators for the deformation within mylonites. Kinematic data from the micas matches the kinematic interpretation of quartz LPOs and field data. The modelling of velocities and velocity anisotropies from sample LPOs is consistent with geophysical data from the crust under the Southern Alps. The Alpine Fault mylonites and parallel Alpine schists have intrinsic P-wave velocity anisotropies of 12% and S-wave anisotropies of 10%.

Constraints from diffusion profiles on the duration of high-strain deformation in thickened crust

Diffusion profiles in garnet are used to determine the duration of movement on shear zones. In the Musgrave Block, central Australia, Grenville age granulites were overprinted at depths of ~40 km by high-strain zones during the intracratonic Petermann orogeny (ca. 550 Ma ago). In these zones, quartzofeldspathic mylonites contain garnet from the early granulite facies event that developed compositional gradients during the high-pressure overprint. The diffusion of Ca into garnet is linked with the release of Ca in plagioclase (the only calcic mineral in the rock) during neocrystallization of sodic feldspar during high-strain deformation. Available diffusion data on garnet directly define the duration of high-strain deformation at eclogite facies. Our results indicate that the integrated time scales of movement in individual high-strain zones were short lived (between 0.07 Ma and 1.4 Ma), and that wide shear zones were active longer than narrow ones. In addition, we find that relict K-feldspar in one of the narrow eclogite facies shear zones yields a 40 Ar- 39 Ar age that is partially reset, indicating that the thermal event was also short lived. These short time scales support the hypothesis that shear heating was the dominant source of heat for the eclogite facies overprint.

Ductility of garnet as an indicator of extremely high temperature deformation: Reply

Abstract Discoidal shaped garnets (Alm63 Prp33 Grs3 Sps1) occur in the foliation plane of lineated quartzofeldspathic mylonites of the Morin shear zone located along the eastern boundary of the Morin anorthosite complex (Grenville Province, Quebec). The flattened shape of garnets records a component of coaxial strain (X = Y ⪢ Z), different from the dominant rotational shear strain (X >Y >Z) in this mylonite. TEM observations indicate dislocation slip and recovery to be the main mechanism for the deformation of garnet. Extrapolation of available experimentally-determined flow laws to natural strain-rates shows an inversion of flow strength between quartz ± feldspar and garnet: garnet, much stronger than quartz and feldspar at temperatures below 700°C, becomes weaker than quartz and feldspar above 900°C. The coaxial ductile strain recorded by the garnets could be produced by a local, uncharacterized deformation, under extremely high temperature conditions at the roof of the rising anorthositic diapir (∼1155 Ma). Subsequent, lower temperature rotational deformation is related to strike-slip movement along the Morin shear zone (∼1020 Ma).

Quartz deformation in a very fine grained quartzo-feldspathic mylonite: a lack of evidence for dominant grain boundary sliding deformation

Microstructural and microtextural investigations by transmission electron microscopy (TEM) have been performed on very fine grained quartzo-feldspathic mylonites from the Red Bank Deformed Zone, Central Australia. The study of the quartz reveals a mean grain size (2.3 ± 0.8μm) and microstructural features indicative of dislocation creep processes within the very fine grained quartz. Analysis of local crystallographic preferred orientation (CPO) by TEM, showed a strong CPO for both the c- and a-axes. The crystallographic relationships between adjacent grains and subgrains are consistent with dominant 〈 c + a〉 and 〈 a〉 slip. Microstructural and microtextural studies also show that the recrystallization mechanism included local grain boundary migration and the formation of high angle grain boundaries from low angle (sub)grain boundaries by progressive misorientation. We found no abnormal grain rotations as would be expected to result from grain boundary sliding processes. This suggests that grain boundary sliding was not significant and the dominant deformation mechanism was dislocation creep.

Ductility of garnet as an indicator of extremely high temperature deformation

Discoidal shaped garnets (Alm63 Prp33 Grs3 Sps1) occur in the foliation plane of lineated quartzofeldspathic mylonites of the Morin shear zone located along the eastern boundary of the Morin anorthosite complex (Grenville Province, Quebec). The flattened shape of garnets records a component of coaxial strain ( X = Y ⪢ Z), different from the dominant rotational shear strain ( X > Y > Z) in this mylonite. TEM observations indicate dislocation slip and recovery to be the main mechanism for the deformation of garnet. Extrapolation of available experimentally-determined flow laws to natural strain-rates shows an inversion of flow strength between quartz ± feldspar and garnet: garnet, much stronger than quartz and feldspar at temperatures below 700°C, becomes weaker than quartz and feldspar above 900°C. The coaxial ductile strain recorded by the garnets could be produced by a local, uncharacterized deformation, under extremely high temperature conditions at the roof of the rising anorthositic diapir (∼1155 Ma). Subsequent, lower temperature rotational deformation is related to strike-slip movement along the Morin shear zone (∼1020 Ma).

Marble mylonites in the Bancroft shear zone, Ontario, Canada: microstructures and deformation mechanisms

Mylonitization of medium-grade marbles in the Bancroft shear zone, Ontario, Canada, is characterized by decreasing grain-size of both calcite and graphite, and a variety of textures. Calcite grain-sizes vary from several millimeters in the protolith, to 50–200 μm in mylonite, to <30 μm in ultramylonite. Corresponding calcite grain shapes are equant in the protolith, elongate in protomylonite (first-developed dimensional preferred orientation), equant in coarse mylonite, elongate in fine mylonite (second-developed dimensional preferred orientation) and generally equant in ultramylonite, which suggests that external energy (applied stress) that tends to elongate grains competed with internal energy sources (e.g. distortional strain) that favor equant shapes. Graphite grain-size changes from several millimeters to centimeters in the protolith to submicroscopic in ultramylonite. In the mylonitic stages, graphite is present as dark bands, while in the ultramylonitic stage it is preserved as a fine coating on calcite grains. Based on textural evidence, twinning (exponential creep; regime I), dynamic recrystallization (power law creep; regime II) and possibly grain boundary sliding superplasticity (regime III) are considered the dominant deformation mechanisms with increasing intensity of mylonitization; their activity is largely controlled by calcite grain-size. Calcite grain-size reduction occurred predominantly by the process of rotation recrystallization during the early stages of mylonitization, as indicated by the occurrence of core and mantle or mortar structures, and by the grain-size of subgrains and recrystallized grains. Grain elongation in S- C structures indicates the activity of migration recrystallization; these structures are not the result of flattening of originally equant grains. Differential stress estimates in coarse mylonites and ultramylonites, based on recrystallized grain-size, are 2–5 and 14–38 MPa, respectively. Initial grain-size reduction of graphite occurred by progressive separation along basal planes, analogous to mica fish formation in quartzo-feldspathic mylonites. Calcite-graphite thermometry on mylonitic and ultramylonitic samples shows that the metamorphic conditions during mylonitization were 475 ± 50°C, which, combined with a differential stress value of 26 MPa, gives a strain rate of 1.2 x 10 −10 s −1 based on constitutive equations; corresponding displacement rates are <38 mm yr −1.

State of strain in mylonites from the western Blue Ridge province, southern Appalachians: the role of volume loss

This study examines the state of finite strain in quartzo-feldspathic mylonites from two localities in the western Blue Ridge province of the southern Appalachians. Mineral shape fabric and whole-rock chemical and modal data suggest that the mylonites deformed by inhomogeneous shortening normal to the foliation and progressive shear parallel to the foliation. The deformation involved substantial bulk-rock volume loss. Strain was partitioned between solution transfer processes in feldspar and dislocation creep processes in quartz and micas at temperatures of between 300 and 450°C. Initially feldspars underwent grain-size reduction by cataclasis but at advanced stages of mylonitization alkali feldspar grains display subrounded oblate shapes (0 < k < 0.5) which are attributed to dissolution processes normal to the foliation. Quartz deformed by crystal-plastic processes and quartz grains and ribbons display a range of k values (0.2 < k < 0.9) reflecting heterogeneous deformation. Whole-rock mylonite samples are enriched by a factor of 3 relative to their gneissic protolith in ‘immobile’ trace elements such as Ti, Zr, Y, P and V, and the REE which is attributed to the concentration of residual phases such as zircon, apatite, epidote and ilmenite observed in the mylonites. The trace element enrichments are interpreted as due to large (>60%) bulk volume loss. Density changes were minor. On the basis of feldspar grain shapes and modal and whole-rock chemical data, volume loss was largely accommodated by fluid infiltration and incongruent dissolution of alkali feldspar (producing muscovite) and involved the loss of alkalis and silica to the fluid phase. The mylonites may have nucleated as solution zones which subsequently underwent strain softening and displacement parallel to the zone boundaries.

Initiation of cataclastic flow in a mylonite zone

At the mylonite-cataclasite transition in the homogeneous quartzo-feldspathic mylonites of the Great Slave Lake shear zone, northwest Canadian Shield, arrays of distributed macroscopic fractures evolved into zones of cataclastic flow. Fracture arrays occupy elliptical areas (metres by 10s of centimetres) aligned in the mylonite foliation plane. Fracture segments rotated either forwards or backwards in response to deformation. Where fractures rotated backwards, they remained planar and cataclasite did not develop. In the case of forward-rotation, only the central part of each fracture rotated. The rotated fracture segments are notably shorter and more closely spaced (1–2 mm) than those which have not rotated and the fracture array has a sigmoidal geometry. Cataclasite developed by the further deformation of such sigmoidal, forward-rotated fracture arrays. Back-rotated fractures were initially planar, regularly spaced (2–10+ cm) and oriented at 70° to the mylonite foliation; they correspond to extensional faults. Forward-rotated fractures were initially planar, regularly spaced (2 cm) and oriented perpendicular to the mylonite foliation; they do not correspond to classical arrays of extensional or contractional faults. Both the initial orientation of the fractures and their subsequent rotational behaviour were constrained by the ability/inability of the wallrock to extend and by the ability/inability of the fractures to dilate. Within the forward-rotated fracture arrays, cataclasite was produced after the rotated fracture segments had locked. It is proposed that a second perpendicular fracture set cut the earlier sigmoidal fractures resulting in stubby fragments of mylonite. The angular fragments acted as stress raisers within the otherwise homogeneous mylonite. This allowed the transition from distributed macroscopic fracture to impingement-induced localized microfracture and reduction of the fragment size with further strain. When a sufficient volume of interconnected fine material had formed, significant displacements could be accommodated by cataclastic flow, leading eventually to the development of thick (1–100s m) zones of cataclasite. Models of cataclasis developed for porous and carbonate media predict initial grain-scale micro-fracture, slip and fragment rotation leading to frictional wear. The observations made in the Great Slave Lake shear zone suggest that such models may not be directly applicable to the initiation of cataclastic flow developed within non-porous, homogeneous, quartzo-feldspathic mylonite belts in crustal-scale fault zones, at the elevated confining pressures typical of the transition from the plastic to the brittle deformation regimes.

Frictional constraints on thrust, wrench and normal faults

IT is generally true that in ancient fault zones, quartzofeldspathic mylonites which indicate partial plastic yield throughout a volume of rock1, are most extensively developed in association with thrust faults. They are less widespread along wrench faults (unless the finite displacement is very large) and are comparatively rare along normal faults2. The release of seismic energy at shallow depths follows the same general pattern. More than 90% is released in areas of compressive plate interaction, mostly where underthrusting is the dominant mechanism, whereas only 6% is derived from the midocean ridges, where tensional rifting is operative3.