Strain path partitioning within thrust sheets: microstructural and petrofabric evidence from the Moine Thrust zone at Loch Eriboll,northwest Scotland

Quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the Loch Eriboll area of the Moine Thrust zone and are used to interpret the detailed processes involved in fabric evolution. The intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude. A correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry.Two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the Moine Thrust. Within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate, mutually interfering shear bands, globular low strain detrital quartz grains whose c axes are aligned sub-parallel to the principal finite shortening direction (Z) and quartz c axis fabrics which are symmetric (both in terms of skeletal outline and intensity distribution) with respect to mylonitic foliation and lineation. Non-coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation.Tectonic models offering possible explanations for the presence of kinematic (strain path) domains within thrust sheets are considered.     

Microstructural and fabric studies from the rocks of the Moine Nappe,Eriboll, NW Scotland

Microstructures and quartz c-axis fabric diagrams from mylonites and psammitic Moine schists, collected in traverses across the lower levels of the Moine Nappe in the Eriboll area, are presented. On approaching the Moine Thrust from the Kyle of Tongue, the following microstructural sequence is encountered: interlayered coarse grained biotite psammitic and schistose tectonites being in part mylonitic with two platy slide zones, one containing biotite and the other only muscovite and chlorite and both showing quartz microstructures indicative of post-tectonic relaxation; these pass into more mylonitic rocks nearer the thrust zone which in turn passes into the main chlorite-grade mylonite belt and finally, adjacent to the Moine Thrust, into reworked lower chlorite grade mylonites. Although there is some local variation, the overall quartz c-axis fabric is an incomplete asymmetric type I girdle. The main variation is the development of type II girdles in the reworked, ultrafine grained mylonites. The extent of the mylonitization is more extensive than previously reported. Studies of folds within the mylonite belt have revealed eye structures and small-scale folds; many are sheath folds. They cannot be unequivocally correlated with large-scale recumbent folds within the Moine Nappe. The results presented indicate that mylonitization is not limited to a single phase, and raises the possibility that there may be earlier Caledonian or possibly Precambrian structural elements present in the Eriboll region Moines prior to much of the mylonitization.     

Development of quartz c-axis crossed/single girdles under simple-pure shear deformation: Results of visco-plastic self-consistent modeling

Quartz c-axis fabrics are widely used to determine the shear plane in ductile shear zones, based upon an assumption that the shear plane is perpendicular to both the central segment of quartz c-axis crossed girdle and single girdle. In this paper the development of quartz c-axis fabric under simple-pure shear deformation is simulated using the visco-plastic self-consistent (VPSC) model so as to re-examine this assumption. In the case of no or weak dynamic recrystallization, the simulated crossed girdles have a central segment perpendicular or nearly perpendicular to the maximum principal finite strain direction (X) and the XY finite strain plane, and at a variable angle relative to the imposed kinematic framework that is dependent on the modeled flow vorticity and finite strain. These crossed girdles have a symmetrical skeleton with respect to the finite strain axes, regardless of the bulk strain and the kinematic vorticity, and rotate in a way similar to the shear sense with increasing bulk strain ratio. The larger the vorticity number the more asymmetrical their legs tend to be. In the case of strong dynamic recrystallization and large bulk strain, under simple shear the crossed girdle switches into single girdles, sub-perpendicular to the shear plane, by losing the weak legs. The numerical results in our models do not confirm the above-mentioned assumption.     

Vorticity analysis in calcite tectonites: An example from the Attico-Cycladic massif (Attica,Greece)

Although calcite tectonites are widespread in nature their use to quantify flow vorticity is limited. We use new (micro-)structural, petrofabric and vorticity data to analyse the kinematics of flow in outcrop-scale calcite mylonite zones. These zones are genetically related to a crustal-scale NE-directed ductile thrust (Basal Thrust) that emplaced the Blueschist over the Basal unit during the exhumation of the Attico-Cycladic Massif. Calcite microstructures reveal that the last stage of deformation occurred at temperatures 200–300 °C achieved by mild heating, which is possibly related with the reburial of the Basal Thrust's footwall. Vorticity analyses were based on the degree of asymmetry of calcite c-axis fabrics as well as on the assumption that the orientation of the long axes of calcite neoblasts within an oblique foliation delineates the direction of instantaneous stretching axis. Both methodological approaches provide consistent estimates with a simple shear component between 55% and 82% (W_n = 0.76–0.96). The use of the stress axis (σ₁) orientation recorded by twin-c-axis-pairs to quantify vorticity generally gives significantly lower simple shear component. Comparison of our vorticity estimates with previous estimates inferred from quartz fabrics and rigid porphyroclasts reveals that exhumation-related deformation in the nappe pile was steady state.     

Relationships between marginal thrusting and movement on major,internal shear zones in the Northern Highland Caledonides,Scotland

The development of the syn-metamorphic Sgurr Beag slide zone, a major ductile shear zone of initially low dip, caused at least 50 km north-western thrust displacement of part of the internal metamorphic complex of the Northern Highland Caledonides of Scotland. Initiation of the zone, and movements upon it, were earlier than formation of the marginal Moine Thrust zone. Movement on the zone followed but overlapped the peak Caledonian metamorphism and the mid to high amphibolite facies mineral assemblages, fabrics and structures produced during the development of the slide zone and those surviving from earlier events, were reworked under greenschist facies conditions during mylonitization associated with initiation of the Moine Thrust zone. Displacements on the slide zone and thrust movements were separated by emplacement of a regional suite of pegmatites and a considerable change of metamorphic grade. Thus, they may not constitute members of a progressive sequence of Caledonian thrusts formed over a short time interval. Rather, preliminary isotopic data may imply an interval of c. 25 Ma between movement on the slide zone and final, ductile translation along the Moine Thrust zone.     

Fabric development during shear deformation in the Main Central Thrust Zone, NW-Himalaya, India

Quartz microfabrics and associated microstructures have been studied on a crustal shear zone—the Main Central Thrust (MCT) of the Himalaya. Sampling has been done along six traverses across the MCT zone in the Kumaun and Garhwal sectors of the Indian Himalaya. The MCT is a moderately north-dipping shear zone formed as a result of the southward emplacement of a part of the deeply rooted crust (that now constitutes the Central Crystalline Zone of the Higher Himalaya) over the less metamorphosed sedimentary belt of the Lesser Himalaya. On the basis of quartz c- and a-axis fabric patterns, supported by the relevant microstructures within the MCT zone, two major kinematic domains have been distinguished. A noncoaxial deformation domain is indicated by the intensely deformed rocks in the vicinity of the MCT plane. This domain includes ductilely deformed and fine-grained mylonitic rocks which contain a strong stretching lineation and are composed of low-grade mineral assemblages (muscovite, chlorite and quartz). These rocks are characterized by highly asymmetric structures/microstructures and quartz c- and a-axis fabrics that indicate a top-to-the-south sense that is compatible with south-directed thrusting for the MCT zone. An apparently coaxial deformation domain, on the other hand, is indicated by the rocks occurring in a rather narrow belt fringing, and structurally above, the noncoaxial deformation domain. The rocks are highly feldspathic and coarse-grained gneisses and do not possess any common lineation trend and the effects of simple shear deformation are weak. The quartz c-axis fabrics are symmetrical with respect to foliation and lineation. Moreover, these rocks contain conjugate and mutually interfering shear bands, feldspar/quartz porphyroclasts with long axes parallel to the macrosopic foliation and the related structures/microstructures, suggesting deformation under an approximate coaxial strain path.On moving towards the MCT, the quartz c- and a-axis fabrics become progressively stronger. The c-axis fabric gradually changes from random to orthorhombic and then to monoclinic. In addition, the coaxial strain path gradually changes to the noncoaxial strain path. All this progressive evolution of quartz fabrics suggests more activation of the basal, rhomb and a slip systems at all structural levels across the MCT.     

Structures,kinematic analysis,rheological parameters and temperature-pressure estimate of the Mesozoic Xingcheng-Taili ductile shear zone in the North China Craton

The NE to ENE trending Mesozoic Xingcheng-Taili ductile shear zone of the northeastern North China Craton was shaped by three phases of deformation. Deformation phase D₁ is characterized by a steep, generally E–W striking gneissosity. It was then overprinted by deformation phase D₂ with NE-sinistral shear with K-feldspar porphyroclasts forming a subhorizontal low-angle stretching lineation on a steep foliation. During deformation phase D₃, lateral motion accommodated by ENE sinistral strike-slip shear zones dominated. Associated fabrics developed at upper greenschist metamorphic facies conditions and show the deformation characteristics of middle- to shallow crustal levels. In some parts, the older structures have been in turn overprinted by late-stage sinistral D₃ shearing. Finite strain and kinematic vorticity in all deformed granitic rocks indicate a prolate ellipsoid (L-S tectonites) near plane strain. Simple shear-dominated general shear during D₃ deformation is probably of general significance. The quartz c-axis textures indicate prism-gliding with a dominant rhomb <a> slip and basal <a> slip system formed mainly at low-middle temperatures. Mineral deformation behavior, quartz c-axis textures, quartz grain size and the Kruhl thermometer demonstrate that the ductile shear zone developed under greenschist facies metamorphic conditions at deformation temperatures ranging from 400 to 500 °C. Dislocation creep is the main deformation mechanism at a shallow crustal level. Fractal analysis showed that the boundaries of recrystallized quartz grains had statistically self-similarities. Differential stresses deduced from dynamically recrystallized quartz grain size are at around 20–39 MPa, and strain rates in the order of 10⁻¹² to 10⁻¹⁴ s⁻¹. This indicates deformation of granitic rocks in the Xingcheng-Taili ductile shear zone at low strain rates, which is consistent with most other ductile shear zones. Hornblende-plagioclase thermometer and white mica barometer indicate metamorphic conditions of medium pressures at around ca. 3–5 kbar and temperatures of 400–500 °C within greenschist facies conditions. The main D₃ deformation of the ENE-trending sinistral strike-slip ductile shearing is related to the roll-back of the subducting Pacific plate beneath the North China Craton.     

Heterogeneous constrictional deformation in a ductile shear zone resulting from the transposition of a lineation-parallel fold

We use new (micro-)structural, petrofabric, strain and vorticity data to analyze the deformation path in a mesoscopic quartz mylonite zone. The mylonite zone resulted from the complete transposition of a stretching lineation-parallel isoclinal fold. Symmetric cleft-girdle quartz c-axis fabrics were recorded in the middle domain, which occupies the inner limbs of the precursor isoclinal fold, while asymmetric cleft- and crossed-girdle fabrics were observed in the upper and lower domains that represent the outer limbs. Constrictional strain, with increasing k values towards the middle domain, is inferred from petrofabric and 3D strain data. Oblique grain shape fabrics yield vorticity estimates of 0.72–0.90 in the zone. However, in the middle domain, pure shear dominated deformation is suggested by orthorhombic crystallographic fabrics. Strain rate is constant throughout the zone; a strain decrease towards the zone center implies that deformation ceased earlier in the middle domain. The data indicates that fold transposition and subsequent mylonitization started as pure-shear-dominated constrictional deformation and progressively changed to simple-shear-dominated, plane strain. During this flow path the asymmetric quartz c-axis fabrics likely developed by depopulation of cleft-girdle maxima rather than from the synthetic rotation of fabric maxima itself.     

Quartz fabrics in shear-zone mylonites: Evidence for a major imprint due to late strain increments

Geometrical relations between quartz C-axis fabrics, textures, microstructures and macroscopic structural elements (foliation, lineation, folds…) in mylonitic shear zones suggest that the C-axis fabric mostly reflects the late-stage deformation history. Three examples of mylonitic thrust zones are presented: the Eastern Alps, where the direction of shearing inferred from the quartz fabric results from a late deformation oblique to the overall thrusting; the Caledonides nappes and the Himalayan Main Central Thrust zone, where, through a similar reasoning, the fabrics would also reflect late strain increments though the direction of shearing deduced from quartz fabric remains parallel to the overall thrusting direction. Hence, the sense of shear and the shear strain component deduced from the orientation of C-axis girdles relative to the finite strain ellipsoid axes are not simply related nor representative of the entire deformation history.     

Thermal structure and tectonic evolution of the Scandian orogenic wedge,Scottish Caledonides: integrating geothermometry,deformation temperatures and conceptual kinematic‐thermal models

In the Caledonides of northwest Scotland, two independent geothermometers (Fe‐Mg exchange and quartz c‐axis fabric opening angle) are used to characterize the thermal structure of the lower part of the Scandian (435–420 Ma) orogenic wedge within the Moine, Ben Hope and Naver‐Sgurr Beag thrust sheets. Traced from west (foreland) to east (hinterland), Fe‐Mg exchange thermometry yields peak or near‐peak temperatures ranging from 484 ± 50 °C to 524 ± 50 °C in the immediate hangingwall of the Moine thrust to 601 ± 50 °C in the immediate hangingwall of the Ben Hope thrust, to 630 ± 50 °C in the Naver thrust sheet. Preserved metamorphic facies and textural relationships are consistent with thermometric estimates. Deformation temperatures calculated from quartz c‐axis fabric opening angles across two similar orogen‐perpendicular transects also yield systematic increases (Glen Golly – Ben Klibreck, 520–630 °C; Ullapool‐Contin, 465–632 °C) traced towards the Naver and Sgurr Beag thrusts. In addition, deformation temperatures show a pronounced increase along the leading edge of the Moine thrust sheet moving south towards the Assynt window, which is interpreted to reflect deeper exhumation of the thrust plane above the Assynt footwall imbricate stack. Because temperatures calculated from metamorphic assemblages are within error of the quartz fabric‐derived deformation temperatures that are of demonstrably Scandian age, the metamorphic sequence between the Moine and Naver‐Sgurr Beag thrusts is interpreted to have developed during the Scandian orogeny. Integration of our results with previous 2D thermal‐mechanical studies allows development of new conceptual thermal‐kinematic models of Scandian orogenesis that may be broadly applicable to other collisional systems. Furthermore, it highlights the critical nature of coupling between orogen kinematic and thermal evolution.     

Quartz microfabric of the Laxfordian Canisp Shear Zone,NW Scotland

The Canisp Shear Zone transects layered Lewisian gneisses near Lochinver, NW Scotland. It is a vertical ductile shear zone with a dextral shear sense, formed during Laxfordian amphibolite facies metamorphism, transposing the layering to new foliation and linear structures. Minerals in the layered gneisses show little or no shape fabric, while a strong shape fabric defines the foliation. For quartz, this shape fabric is accompanied by development of a preferred crystal orientation with fabric patterns reflecting the geometry of the shear deformation. The quartz fabric shows a pole-free area around the lineation with the c-axes concentrated in an asymmetric cross-girdle or a point maximum perpendicular to the shear plane, and a monoclinic symmetry consistent with the shear sense.     

Surge zones in the Moine thrust zone of NW Scotland

The Caledonian thrust zones of Assynt show several examples of large fault-bounded structures, surge zones, up to 8 km² in extent, which have moved further than adjacent rocks. Extensional faults can be traced into strike-slip faults and then to contractional imbricate faults. There are also zones of extensional and contractional flow as shown by strained bioturbation marks in the Cambrian Pipe Rock.Several other low-angle extensional fault zones have been recognized along the length of the Moine thrust zone, notably in the Kinlochewe district. Recognition of these extensional faults and local surge zones has solved several local problems such as the lack of continuity of the Glencoul thrust and the out-of-sequence character of some of the large low-angle faults. Though the thrust propagation direction was generally from east to west, in the transport direction, several of the eastern faults have been reactivated later and locally cut down as extensional faults. The ‘so-called’ Moine thrust shows extensional fault movement at several localities along its length.The extensional structures and the surge zones suggest that body forces have been important in driving the faults rather than just a push from the rear. The Moines and Moine thrust zone were presumably driven to the WNW by gravity spreading and thinning of the main Scottish Caledonides.     

The two intracrustal boundary thrusts of the Himalaya

The series of four different, steeply inclined thrusts which sharply sever the youthful autochthonous Cenozoic sedimentary zone, including the Siwalik, from the mature old Lesser Himalayan subprovince is collectively known as the Main Boundary Thrust (MBT). In the proximity of this trust in northwestern and eastern sectors, the parautochtonous Lesser Himalayan sedimentary formations are pushed up and their narrow frontal parts split into imbricate sheets with attendant repetition and inversion of lithostratigraphic units. The superficially steeper thrust plane seems to flatten out at depth. The MBT is tectonically and seismically very active at the present time.The Main Central Thrust (MCT), inclined 30° to 45° northwards, constitutes the real boundary between the Lesser and Great Himalaya. Marking an abrubt change in the style and orientation of structures and in the grade of metamorphism from lower amphibolitefacies of the Lesser Himalayan to higher metamorphic facies of the Great Himalayan, the redefined Main Central Thrust lies at a higher level as that originally recognized by A. Heim and A. Gansser. They had recognized this thrust as the contact of the mesozonal metamorphics against the underlying sedimentaries or epimetamorphics. It has now been redesignated as the Munsiari Thrust in Kumaun. It extends northwest in Himachal as the Jutogh Thrust and farther in Kashmir as the Panjal Thrust. In the eastern Himalaya the equivalents of the Munsiari Thrust are known as the Paro Thrust and the Bomdila Thrust. The upper thrust surface in Nepal is recognized as the Main Central Thrust by French and Japanese workers. The easterly extension of the MCT is known as the Khumbu Thrust in eastern Nepal, the Darjeeling Thrust in the Darjeeling-Sikkim region, the Thimpu Thrust in Bhutan and the Sela Thrust in western Arunachal. Significantly, hot springs occur in close proximity to this thrust in Kumaun, Nepal and Bhutan. There are reasons to believe that movement is taking place along the MCT, although seismically it is less active than the MBT.     

The stratigraphy,structure and regional significance of the Moine Rocks of Mull,Argyllshire, W. Scotland

Two right-way-up Moine lithostratigraphic units—the Shiaba (older) and Assapol (younger) Groups—are distinguished in the Ross of Mull. These were intruded in the west by the post-tectonic, Ross-of-Mull granite complex at 414 ± 3 Ma. Apparently undisturbed inclusions of Moine metasediments within the granite permit the boundary between the lithostratigraphic units to be followed westward almost to the supposed trace of the Moine thrust in the Sound of Iona. The Shiaba and Assapol Groups which have a transitional, albeit attenuated, stratigraphic contact are correlated with the Morar and Glenfinnan Divisions of Inverness-shire, the Sgurr Beag ductile thrust (slide) normally found at the Morar/Glenfinnan Division boundary being absent. This implies that the stratigraphic relationship between these two divisions, which is elsewhere obscured by the thrust, is uniquely preserved on Mull. Within a local, D1-D4 sequence, D2 and D3 structures are dominant. Originally subhorizontal D2 folds are intensely curvilinear on all scales about an originally flat-lying, NNW-SSE trending stretching lineation (L2). In sections parallel to L2, D2 minor folds display a constant sense of vergence throughout the Ross-of-Mull Moine Rocks, overturning generally NW-NNW. The present day structure is dominated by the almost upright, SSW-plunging D3 Assapol synform which overprints all earlier structures. Tentative correlation of the deformation sequence with that of Inverness-shire, suggests that the D1-D2 structures of Mull, with accompanying moderate-to-high-grade metamorphism, may be Precambrian, while the D3 Assapol synform may be Caledonian. The presence of migmatites of kyanite grade means that metamorphic grade, established during MS1-MP2, is anomalously high for Moine rocks lying close to the supposed Caldonian front. This suggests that they may lie within one of the higher Caledonian thrust nappes of the North Highland Moine—possibly the Knoydart nappe, where metamorphic grade is comparable. The greenschist facies metamorphism and single phase of deformation affecting the ‘Torridonian’ rocks of Iona presents a significant contrast to the Moine rocks of the Ross of Mull. A major fault in the Sound of Iona is implied, but the Moine thrust itself probably does not outcrop.     

Sense of shear in high-temperature movement zones from the fabric asymmetry of plagioclase feldspars

A new method for determining the sense of shear in plagioclase-bearing tectonites from the (010) orientation of plagioclase feldspar is presented. The method is based on the asymmetry of the (010) plane with respect to the structural frame (foliation and lineation) and the dominant activity of the (010) slip plane in the high-temperature plasticity of plagioclase feldspar. Using examples from the Zabargad gneisses (Red Sea) the method is applied to plagioclases of An25–An45 and compared with other methods of shear-sense determination (quartz c-axis fabrics and microstructural criteria).     

Kinematic analyses of orogen-parallel L-tectonites from Pelling-Munsiari thrust of Sikkim Himalayan fold thrust belt: Insights from multiple,incremental strain markers

Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB) change from SL tectonites to local, transport-parallel L-tectonites that are exposed in discontinuous klippen south of the PT zone. By estimating the incremental kinematic vorticity number (W_k) from quartz c-axes fabric, oblique fabric, and subgrains, we reconstruct a first-order, kinematic path of these L-tectonites. Quartz c-axes fabric suggests that the deformation initiated as pure-shear dominated (∼56–96%) that progressively became simple-shear dominated (∼29–54%), as recorded by the oblique fabric and subgrains in the L-tectonites. These rocks record a non-steady deformation where the kinematic vorticity varied spatially and temporally within the klippen.The L-tectonites record ∼30% greater pure-shear than the PT fault rocks outside the klippen, and the greatest pure-shear dominated flow among the published vorticity data from major fault rocks of the Himalayan FTB. The relative decrease in the transport-parallel simple-shear component within the klippen, and associated relative increase of transport-perpendicular, pure-shear component, support the presence of a sub-PT lateral ramp in the Sikkim Himalayan FTB. This study demonstrates the influence of structural architecture for fault systems for controlling spatial and temporal variations of deformation fabrics and kinematic path of deforming thrust wedges.     

The simulation of fabric development during plastic deformation and its application to quartzite: the influence of deformation history

The effect of deformation history on the development of crystallographic preferred orientation in quartzities has been simulated using a computer program based on the Taylor-Bishop-Hill analysis. Model quartzities with different combinations of glide systems have been subjected to various coaxial and non-coaxial deformation histories. It is possible to obtain information from the fabrics that develop during simple histories; for example, the location of the axis of extension is generally associated with a pole free area on a c-axis plot, and progressive axial shortening, plane strain and axial shortening produce characteristic fabrics. In progressive simple shear the fabric skeleton becomes asymmetric relative to the sense of shear and a-axes preferentially align in the flow plane parallel to the flow direction. However, this example illustrates that the fabric orientation and characteristics are controlled by the kinematic framework and bear only an indirect relationship to the finite strain accumulated to that point in the history.The imprint of the closing stages of deformation limits to some degree the use of crystallographic fabrics as a tool for structural geologists, but in favourable circumstances data can be obtained concerning characteristics of the deformation history, on the scale of the hand-specimen, for the last part of this history.     

Construction of crossed girdles by superposing four subfabrics,each with a single maximum

Commonly, basal glide is the predominant deformation mechanism of quartz in tectonites. Therefore, local deformation is probably mostly progressive simple shear rotating the sheared domains as well as deforming them. If a tectonite body is constrained to be deformed irrotationally and approximately homogeneously throughout, it is necessarily traversed by closely spaced material surfaces that are approximately plane and orthogonal originally, and stay so through time. These surfaces act as internal boundaries and enforce cancellation of the rigid-body rotations of, in the general case, four distinct families of domains, with slip planes and directions mutually mirror-symmetric. The overall symmetry of the fabric is orthorhombic, with the mirror planes coinciding with the principal planes of strain. Certain grains with basal planes in favorable orientation for one of the four ideal simple shears could initiate the deformation, and because of the need for compatibility, entrain neighboring grains into a similar strain, making the surroundings of an initiating grain a shear zone. Compatibility also requires thec-axes of grains in a domain to be rotated progressively toward the direction of maximum shortening. If the original orientation of crystallographic axes was random, domains of one family thus acquire a fabric with a single maximum, and the four resulting fabrics with single maxima combine to form crossed-girdle patterns. Depending on the orientation of the average shear planes and slip directions in the four families, the crossed girdles can be of different types; most fabric types that have been observed in quartz tectonites can be obtained by superposition. Crossed-girdle fabrics with low symmetry result from non-coaxial strain histories.     

Domainal and fabric heterogeneities in the Cap de Creus quartz mylonites

A characteristic domainal configuration is reported for both micro-structures and c-axis fabrics in the Cap de Creus pure quartz mylonites as displayed in 50 samples from the centres of different shear zones. Three types of domains are found a, b and c. Each domain has a distinct c-axis orientation pattern. These three fabric elements, also labelled a, b and c make up the total fabric. c-axis fabrics are symmetric or asymmetric with respect to the main mylonitic foliation depending on the presence or absence of the b domain and its fabric element. The boundaries of the domains are parallel to the main mylonitic foliation. Two domain types, a and b display an internal foliation defined by preferred grain boundary alignment parallel to the direction of optical orientation within the domain. The internal foliations are oblique to the main mylonitic foliation in two different senses giving the sample a herring-bone appearance. These internal foliations are shown to be related to extensional crenulations. Domains are not produced by host-controlled recrystallization. The fabric elements and corresponding domains are the expression of kinematic heterogeneities on the scale of the thin section.     

Skolithos pipes as strain markers in mylonites

It is argued that mylonite zones result from translatory movements between rock masses and that the deformation mechanism is one of simple shear. Evidence is adduced to show that the mylonite zones in the Moine Thrust Belt of northwestern Scotland were developed in association with the inverted limbs of early Caledonian folds which trend parallel to the thrust front. On this basis a method is developed for the determination of shear strain from parameters which can be measured in mylonites which contain deformed Skolithos worm burrows. Very large strains are indicated (γ - 10). Some general implications are discussed.