Investigations on the mechanical significance of crenulation cleavage

Measurements of total, incremental and progressive strains associated with the development of small scale crenulation cleavage in some low-grade metamorphic rocks from Australia and Switzerland are applied to a discussion of the mechanical significance of the cleavage.Limits are placed on the amount of incremental and total slip or simple shear possible along the cleavage by the observation that the XY principal plane trace of bulk total crenulation strain coincides within 4° of the crenulation cleavage trace in all cases where this strain has been measured or estimated. The measurements are made on eight specimens using deformed porphyroblasts, crystal fibres in pressure-shadows around pyrite and flattened folds and include deformations with coaxial and non-coaxial histories.Further measurements derived from pressure-shadow fibres (eight specimens) show that the style and orientation of incremental deformation are essentially independent of the crenulation cleavage, except for a limit (43°) to the obliquity of the principal incremental extension axis during a given cleavage episode. The only special deformation related to the cleavage is the coaxial one. An indication of passive cleavage behaviour at high strain is shown by the progressive strain history of one specimen. Evidence for passive rotation of a transected axial plane is shown by another. A model is proposed to account for these observations, especially the conditions necessary for initiation and continued development of a new cleavage fabric.Some further applications of existing strain measurement techniques are described: of the R_f/Ø_f method to heterogeneously superposed tectonic strains and of an improved procedure of t′_α/α flattening analysis.     

Ar/Ar thermochronologic constraints on deformation, metamorphism and cooling/exhumation of a Mesozoic accretionary wedge, Otago Schist, New Zealand

Structural thickening of the Torlesse accretionary wedge via juxtaposition of arc-derived greywackes (Caples Terrane) and quartzo-feldspathic greywackes (Torlesse Terrane) at 120 Ma formed a belt of schist (Otago Schist) with distinct mica fabrics defining (i) schistosity, (ii) transposition layering and (iii) crenulation cleavage. Thirty-five ⁴⁰Ar/³⁹Ar step-heating experiments on these micas and whole rock micaceous fabrics from the Otago Schist have shown that the main metamorphism and deformation occurred between 160 and 140 Ma (recorded in the low grade flanks) through 120 Ma (shear zone deformation). This was followed either by very gradual cooling or no cooling until about 110 Ma, with some form of extensional (tectonic) exhumation and cooling of the high-grade metamorphic core between 109 and 100 Ma. Major shear zones separating the low-grade and high-grade parts of the schist define regions of separate and distinct apparent age groupings that underwent different thermo-tectonic histories. Apparent ages on the low-grade north flank (hanging wall to the Hyde-Macraes and Rise and Shine Shear Zones) range from 145 to 159 Ma (n=8), whereas on the low-grade south flank (hanging wall to the Remarkables Shear Zone or Caples Terrane) range from 144 to 156 Ma (n=5). Most of these samples show complex age spectra caused by mixing between radiogenic argon released from neocrystalline metamorphic mica and lesser detrital mica. Several of the hanging wall samples with ages of 144–147 Ma show no evidence for detrital contamination in thin section or in the form of the age spectra. Apparent ages from the high-grade metamorphic core (garnet–biotite–albite zone) range from 131 to 106 Ma (n=13) with a strong grouping 113–109 Ma (n=7) in the immediate footwall to the major Remarkables Shear Zone. Most of the age spectra from within the core of the schist belt yield complex age spectra that we interpret to be the result of prolonged residence within the argon partial retention interval for white mica (430–330 °C). Samples with apparent ages of about 110–109 Ma tend to give concordant plateaux suggesting more rapid cooling. The youngest and most disturbed age spectra come from within the ‘Alpine chlorite overprint’ zone where samples with strong development of crenulation cleavage gave ages 85–107 and 101 Ma, due to partial resetting during retrogression. The bounding Remarkables Shear zone shows resetting effects due to dynamic recrystallization with apparent ages of 127–122 Ma, whereas overprinting shear zones within the core of the schist show apparent ages of 112–109 and 106 Ma. These data when linked with extensional exhumation of high-grade rocks in other parts of New Zealand indicate that the East Gondwana margin underwent significant extension in the 110–90 Ma period.     

Oolite deformation in Windgällen, Canton Uri, Switzerland

Detailed analysis of the oolite deformation in Windgällen revealed that the finite-strain states vary significantly and systematically over the major recumbent structures. The strain is lowest at the fold cores and highest along the limbs, which have suffered extensions of up to 150%. These high strains are interpreted as the result of strong overfolding during the development of the nappe-like structure. The maximum ratio of the strain ellipsoids computed from the oolite shapes is 14.6:1, though individual oolites having axial ratios as high as 30 : 1 have been encountered. The intermediate axes of the strain ellipsoids throughout the area have been extended between 3 and 64% and the strain ellipsoids are of the flattening type with K being less than unity. The pre-deformation fabric of the oolites is very weak and their initial axial ratios in any two-dimensional section is generally less than 2:1. Slaty cleavage is coincident with the plane of maximum compression in the rock. The relationship of cleavage, folds and oolite deformation suggest that cleavage develops progressively during deformation and forms at the same time as the folds associated with it.     

Strain patterns and shortening in a folded thrust sheet: An example from the southern appalachians

Total strain patterns estimated across the Pulaski thrust sheet of the southwest Virginia Appalachians show an approximately homogeneous, plane strain deformation associated with folding and distortion above a subsurface décollement. Estimated strains are low (1.2 < < 2.0) with a subvertical extension. Chlorite fibers in pressure fringes on framboidal pyrite indicate that non-rotational deformation produced weak cleavage and pencil structure in mudrock. Variations in shape of pencils and fiber lengths in pressure fringes define highest strains in fold hinges and adjacent to contraction faults. Fabric transitions, delineated by distribution and intensity of cleavage, pencil structure and bedding fissility across the thrust sheet are strain dependent. Balanced cross-sections suggest 35% horizontal shortening due to regional folding and faulting within the Pulaski sheet. Strain integration techniques give 17–35% horizontal shortening associated with cleavage formation. Removal of this strain indicates that cleavage was superposed on open to tight, class-3 folds. Pre-existing thickness variations and anomalous low strains in tight folds require early folding accomodated by intergranular deformation (perhaps controlled grainboundary sliding). Suppression of cleavage formation and penetrative strain was possibly due to higher pore fluid pressure in the early stages of thrust sheet deformation. Observed variations in bedding-cleavage angle and low cleavage fans are compatible with this deformation sequence.     

The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle

A geodynamic model exists, the westward lithospheric drift model, in which the variety of overriding plate deformation, trench migration and slab dip angles is explained by the polarity of subduction zones. The model predicts overriding plate extension, a fixed trench and a steep slab dip for westward-dipping subduction zones (e.g. Mariana) and predicts overriding plate shortening, oceanward trench retreat and a gentle slab dip for east to northeastward-dipping subduction zones (e.g. Chile). This paper investigates these predictions quantitatively with a global subduction zone analysis. The results show overriding plate extension for all dip directions (azimuth α = − 180° to 180°) and overriding plate shortening for dip directions with α = − 90° to 110°. The wide scatter in data negate any obvious trend and only local mean values in overriding plate deformation rate indicate that overriding plate extension is somewhat more prevalent for west-dipping slabs. West-dipping subduction zones are never fixed, irrespective of the choice of reference frame, while east to northeast-dipping subduction zones are both retreating and advancing in five out of seven global reference frames. In addition, westward-dipping subduction zones have a range in trench-migration velocities that is twice the magnitude of that for east to northeastward-dipping slabs. Finally, there is no recognizable correlation between slab dip direction and slab dip angle. East to northeast-dipping slabs (α = 30° to 120°) have shallow (0–125 km) slab dip angles in the range 10–60° and deep (125–670 km) slab dip angles in the range 40–82°, while west-dipping slabs (α = − 60° to − 120°) have shallow slab dip angles in the range 19–50° and deep slab dip angles in the range 25–86°. Local mean deep slab dip angles are nearly identical for east and west-dipping slabs, while local mean shallow slab dip angles are lower by only 4.7–8.1° for east to northeast-dipping slabs. It is thus concluded that overall, there is no observational basis to support the three predictions made by the westward drift model, and for some sub-predictions the observational basis is very weak at most. Alternative models, which incorporate and underline the importance of slab buoyancy-driven trench migration, slab width and overriding plate motion, are better candidates to explain the complexity of subduction zones, including the variety in trench-migration velocities, overriding plate deformation and slab dip angles.     

Magnetic fabric study across the Ailao Shan–Red River shear zone

The anisotropy of magnetic susceptibility (AMS) of 351 specimens from 51 sites across the Ailao Shan–Red River shear zone (ASRR) was measured to determine its magnetic fabric. Rocks range westward from core schistose gneiss, through low-grade schist, to Triassic sediment. Magnetic ellipticity analysis shows that 41 of 51 sites have an oblate compressional fabric and the other 10 sites have a prolate fabric. P_J value drops by 22.4% in the low-grade schist and by 27.4% in the Triassic sediment on average with respect to the gneiss, suggesting a rapid decrease of deformational intensity. The directions of principal susceptibilities are closely related to the deformation of the Ailao Shan–Red River shear zone. The susceptibility plane always coincides with the schistosity or cleavage plane. Most of the maximum susceptibility axes trend NW–SE. In the shear zone, the maximum susceptibility axes (K_max) are parallel to the lineation within the foliation plane. With increasing distance from the shear zone, there is a trend that they become parallel to the down-dip of reverse faults or cleavage. This indicates changes in deformation mode, inside and outside the shear zone. Within the shear zone, horizontal movement is dominant. Outside, shortening prevails. The overall minimum magnetic axes align NE–SW with subhorizontal to low dip angles, suggesting that the dominant shortening is NE–SW directed. Caution should be exercised when AMS is used to determine shear sense in strong shear zones because the angle between the minimum susceptibility axis (K_min) and pole of foliation is small, and also because the attitude of foliation varies from place to place. They result in unreliable or even wrong shear sense. Another important result is the axial ratio of magnetic susceptibility ellipsoid along the study section. With these data, it is possible to establish an axial ratio relationship between the finite strain ellipsoid and magnetic susceptibility ellipsoid for quantitative calculation of offset.     

Evolution of a calcite marble shear zone complex on Thassos Island, Greece: microstructural and textural fabrics and their kinematic significance

The deformation history of a monophase calcite marble shear zone complex on Thassos Island, Northern Greece, is reconstructed by detailed geometric studies of the textural and microstructural patterns relative to a fixed reference system (shear zone boundary, SZB). Strain localization within the massive marble complex is linked to decreasing P–T conditions during the exhumation process of the metamorphic core complex. Solvus thermometry indicates that temperatures of 300–350°C prevailed during part of the shear zone deformation history. The coarse-grained marble protolith outside the shear zone is characterized by symmetrically oriented twin sets due to early coaxial deformation. A component of heterogeneous non-coaxial deformation is first recorded within the adjacent protomylonite. Enhanced strain weakening by dynamic recrystallization promoted strong localization of plastic deformation in the ultramylonite of the calcite shear zone, where high strain was accommodated by non-coaxial flow. This study demonstrates that both a pure shear and a simple shear strain path can result in similar crystallographic preferred orientations (single c-axis maximum perpendicular to the SZB) by different dominant deformation mechanisms. Separated a-axis pole figures (+a- and −a-axis) show different density distributions with orthorhombic texture symmetry in the protolith marble and monoclinic symmetry in the ultramylonite marble consistently with the observed grain fabric symmetry.     

European continuous active tectonic strain–stress map

This paper shows a new continuous strain–stress map for Europe obtained from the direct inversion of earthquake focal mechanisms calculated from the centroid tensor method. A total of 1608 focal mechanisms have been selected with several quality criteria from different catalogues (CMT Harvard, ETH, Med-Net, I.G.N. and I.A.G.) from 1973 to the present day. Values for the maximum horizontal shortening direction and brittle strain–stress regime defined by the k′ ratio (e_y/e_z, horizontal maximum/vertical strain) have been calculated following in Europe and Pannonian Basin the slip model of tri-axial deformation. The individual results including Dey and the shape of the active brittle strain ellipsoid have been interpolated to a final 15′ regular grid taking into account the relationship between the tectonic horizontal strain–stress value and the vertical load. Both continuous strain regime and maximum horizontal shortening (Dey) maps show a good correlation with the primary tectonic forces generated along the plate boundaries, plate kinematics and also some local perturbations related with main crustal heterogeneities and topography, as well as significant spatial variations in integrated crustal strength.     

Spatial correlation of deformation and mineral reaction in experimentally deformed plagioclase–olivine aggregates

Shear deformation of hot pressed plagioclase–olivine aggregates was studied in the presence and absence of mineral reaction. Experiments were performed at 900 °C, 1500 MPa, and a constant shear strain rate of 5×10⁻⁵ s⁻¹ in a solid medium apparatus. Whether the mineral reaction between plagioclase and olivine takes place or not is controlled by choosing the appropriate plagioclase composition; labradorite (An₆₀) does not react, anorthite (An₉₂) does. Labradorite–olivine aggregates deformed without reaction are very strong and show strain hardening throughout the experiment. Syndeformational reaction between olivine and anorthite causes a pronounced strain weakening. The reaction produces fine-grained opx–cpx–spinel aggregates, which accommodate a large fraction of the finite strain. Deformation and reaction are localised within a 0.5-mm-wide sample. Three representative samples were analysed for their fabric anisotropy R* and shape-preferred orientation α* (fabric angle with the shear plane) using the autocorrelation function (ACF). Fabric anisotropy can be calibrated to quantify strain variations across the sheared samples. In the deformed and reacted anorthite–olivine aggregate, there is a strong correlation between reaction progress and strain; regions of large shear strain correspond to regions of maximum reaction progress. Within the sample, the derived strain rate variations range up to almost one order of magnitude.     

Development of fluid conduits in the auriferous shear zones of the Hutti Gold Mine, India: evidence for spatially and temporally heterogeneous fluid flow

The gold mineralization of the Hutti Mine is hosted by nine parallel, N–S trending, steeply dipping, 2–10 m wide shear zones, that transect Archaean amphibolites. The shear zones were formed after peak metamorphism during retrograde ductile D₂ shearing in the lower amphibolite facies. They were reactivated in the lower to mid greenschist facies by brittle–ductile D₃ shearing and intense quartz veining. The development of a S₂–S₃ crenulation cleavage facilitates the discrimination between the two deformation events and contemporaneous alteration and gold mineralization. Ductile D₂ shearing is associated with a pervasively developed distal chlorite–sericite alteration assemblage in the outer parts of the shear zones and the proximal biotite–plagioclase alteration in the center of the shear zones. D₃ is characterized by development of the inner chlorite-K-feldspar alteration, which forms a centimeter-scale alteration halo surrounding the laminated quartz veins and replaces earlier biotite along S₃. The average size of the laminated vein systems is 30–50 m along strike as well as down-dip and 2–6 m in width.Mass balance calculations suggest strong metasomatic changes for the proximal biotite–plagioclase alteration yielding mass and volume increase of ca. 16% and 12%, respectively. The calculated mass and volume changes of the distal chlorite–sericite alteration (ca. 11%, ca. 8%) are lower. The decrease in δ¹⁸O values of the whole rock from around 7.5‰ for the host rocks to 6–7‰ for the distal chlorite–sericite and the proximal biotite–plagioclase alteration and around 5‰ for the inner chlorite-K-feldspar alteration suggests hydrothermal alteration during two-stage deformation and fluid flow.The ductile D₂ deformation in the lower amphibolite facies has provided grain scale porosities by microfracturing. The pervasive, steady-state fluid flow resulted in a disseminated style of gold–sulfide mineralization and a penetrative alteration of the host rocks. Alternating ductile and brittle D₃ deformation during lower to mid greenschist facies conditions followed the fault-valve process. Ductile creep in the shear zones resulted in a low permeability environment leading to fluid pressure build-up. Strongly episodic fluid advection and mass transfer was controlled by repeated seismic fracturing during the formation of laminated quartz(-gold) veins. The limitation of quartz veins to the extent of earlier shear zones indicate the importance of pre-existing anisotropies for fault-valve action and economic gold mineralization.     

Structural evolution within the Luning–Fencemaker fold-thrust belt, Nevada: progression from back-arc basin closure to intra-arc shortening

The Luning–Fencemaker fold-thrust belt (LFTB) of central Nevada reflects major Mesozoic shortening in the western US Cordillera, and involved contractional deformation in Triassic and lower Jurassic back-arc basinal strata. Structural analyses in the Santa Rosa Range, in the northern LFTB, provide new insight into the evolution of this belt. Four phases of deformation are recognized in the Santa Rosa Range. D₁ involved tight to isoclinal folding, cleavage development under low-grade metamorphic conditions, and reverse faulting. This deformation phase reflects NW–SE shortening of 55–70% in the Early and/or Middle Jurassic. D₂ structures are similar in orientation to D₁ but involved much less overall strain and are well developed only to the southeast. D₂ appears to be related to thrusting along the eastern margin of the LFTB in the Middle and/or Late Jurassic. D₃ deformation reflects very minor shortening (<5%) in a subvertical direction, and is tentatively interpreted to reflect stresses generated during initial intrusion of mid-Cretaceous plutons in the area. D₄ deformation demonstrably occurred synchronously with emplacement of Cretaceous granitoids dated at 102 Ma (U–Pb zircon) based on syntectonic relations between D₄ structures and thermal metamorphism associated with intrusion, and an upgrade in D₄ strain in the thermally softened metamorphic aureoles of the intrusions. This last phase of deformation reflects minor regional NE–SW shortening, coupled with localized strain associated with pluton emplacement.Formation of the LFTB structural province was accomplished during the D₁ and D₂ phases of deformation, and most shortening occurred during the D₁ event. This Jurassic deformation led to structural closure of the back-arc basin by top-to-the-SE tectonic transport and development of a largely ductile fold-thrust belt. Subsequent deformation (D₃ and D₄) is >50 m.y. younger and unrelated to development of the LFTB. The younger deformation reflects a combination of minor regional shortening, interpreted to be related to the Cretaceous Sevier orogeny, plus localized shortening related to emplacement of Cretaceous intrusions.     

Deformation-induced polymorphic transformation: experimental deformation of kyanite, andalusite, and sillimanite

Torsion experiments were performed on the Al₂SiO₅ polymorphs in the sillimanite stability field to determine basic rheological characteristics and the effect of deformation on polymorphic transformation. The experiments resulted in extensive transformation of andalusite and kyanite to sillimanite. No transformation occurred during the hot-press (no deformation) stage of sample preparation, which was carried out at similar P–T conditions and duration as the torsion experiments. Experiments were conducted on fine-grained (< 15 µm) aggregates of natural andalusite, kyanite and sillimanite at 1250 °C, 300 MPa, and a constant shear strain rate of 2 × 10^− 4/s to a maximum shear strain of 400%. Electron back-scattered diffraction (EBSD) analysis of the experiments revealed development of lattice-preferred orientations, with alignment of sillimanite and andalusite [001] slightly oblique to the shear plane. The kyanite experiment could not be analyzed using EBSD because of near complete transformation to sillimanite. Very little strain ( 30%) is required to produce widespread transformation in kyanite and andalusite. Polymorphic transformation in andalusite and kyanite experiments occurred primarily along 500 µm wide shear bands oriented slightly oblique and antithetic to the shear plane and dominated by sub-µm (100–150 nm) fibrolitic sillimanite. Shear bands are observed across the entire strain field preserved in the torsion samples. Scanning transmission electron microscope imaging shows evidence for transformation away from shear bands; e.g. fibrolitic rims on relict andalusite or kyanite. Relict grains typically have an asymmetry that is consistent with shear direction. These experimental results show that sillimanite is by far the weakest of the polymorphs, but no distinction can yet be made on the relative strengths of kyanite and andalusite. These observations also suggest that attaining high bulk strain energy in strong materials such as the Al₂SiO₅ polymorphs is not necessary for triggering transformation. Strain energy is concentrated along grain boundaries, and transformation occurs by a dynamic recrystallization type process. These experiments also illustrate the importance of grain-size sensitive creep at high strains in a system with simultaneous reaction and deformation.     

Seismic potential of Southern Italy

To improve estimates of the long-term average seismic potential of the slowly straining South Central Mediterranean plate boundary zone, we integrate constraints on tectonic style and deformation rates from geodetic and geologic data with the traditional constraints from seismicity catalogs. We express seismic potential (long-term average earthquake recurrence rates as a function of magnitude) in the form of truncated Gutenberg–Richter distributions for seven seismotectonic source zones. Seismic coupling seems to be large or even complete in most zones. An exception is the southern Tyrrhenian thrust zone, where most of the African–European convergence is accommodated. Here aseismic deformation is estimated to range from at least 25% along the western part to almost 100% aseismic slip around the Aeolian Islands. Even so, seismic potential of this zone has previously been significantly underestimated, due to the low levels of recorded past seismicity. By contrast, the series of 19 M6–7 earthquakes that hit Calabria in the 18th and 19th century released tectonic strain rates accumulated over time spans up to several times the catalog duration, and seismic potential is revised downward. The southern Tyrrhenian thrust zone and the extensional Calabrian faults, as well as the northeastern Sicilian transtensional zone between them (which includes the Messina Straits, where a destructive M7 event occurred in 1908), all have a similar seismic potential with minimum recurrence times of M ≥ 6.5 of 150–220 years. This potential is lower than that of the Southern Apennines (M ≥ 6.5 recurring every 60 to 140 years), but higher than that of southeastern Sicily (minimum M ≥ 6.5 recurrence times of 400 years). The high seismicity levels recorded in southeastern Sicily indicate some clustering and are most compatible with a tectonic scenario where the Ionian deforms internally, and motions at the Calabrian Trench are small. The estimated seismic potential for the Calabrian Trench and Central and Western Sicily are the lowest (minimum M ≥ 6.5 recurrence times of 550–800 years). Most zones are probably capable of generating earthquakes up to magnitudes 7–7.5, with the exception of Central and Western Sicily where maximum events sizes most likely do not exceed 7.     

Strain analysis of a Northern Apennine shear zone using deformed marble breccias

The Alpi Apuane region of the Northern Apennines appears to have been deformed within a large-scale, low-angle shear zone with an overthrust sense of movement. The presence of mineral stretching lineations, folds progressively rotated into the X strain direction, and schistosities which intersect the nappe boundaries at small angles suggest that a component of shear strain occurred during the deformation. The strain ratios and orientations on two-dimensional sections have been determined from deformed marble breccias, reduction spots, and oncalites. Data from three or more non-perpendicular, non-principal sections have been combined to determine the finite strain ellipsoids at 33 sites within the shear zone.The finite strains have been separated into components of simple shear (γ), longitudinal strain (λ), and volume change (Δ). Algebraic expressions have been derived and graphs constructed which enable components of γ, γ and Δ, and γ and λ to be determined directly from a knowledge of strain ratio (R) within the shear zone and the angle (θ) between the principal strain direction and the shear zone boundary. The Alpi Apuane data indicate that neither simple shear alone, nor simple shear with volume change can satisfactorily explain the observed strains. Consideration of simple shear plus longitudinal strain leads to a general relationship in which the value of shear increases, and the values of longitudinal strain change along a SW-NE profile across the zone. Integration of the resulting shear strain-distance curves gives a minimum displacement of 4 km within the shear zone. Combination of the finite strains with the total time of deformation known from K/Ar studies leads to average strain rates from 1.4 to 9.6 × 10⁻¹⁵ sec⁻¹.A characteristic flat-ramp-flat geometry initially formed the boundaries of what was later to develop into the overthrust shear zone, and deformation of the underlying crystalline basement is believed to have occurred by ductile shearing. Estimates of 21% crustal shortening for the region suggest that the crustal thickness prior to deformation was approximately 20 km in this part of the Northern Apennines.     

Vitrinite anisotropy under differential stress and high confining pressure and temperature: preliminary observations

The orientation of the optical indicating surface of vitrinite in reflected light has been determined following deformation at 350 and 500°C, confining pressures of 500 and 800 MPa and a strain rate of 10⁻⁵ s⁻¹. High temperature and large strain have facilitated reorientation of the indicating surface, increase in anisotropy (bireflectance) and an increase in maximum vitrinite reflectance. In a specimen deformed at 500°C and 23% axial strain the maximum vitrinite reflectance has been reoriented more than 70° from close to parallel to σ₁ in the undeformed state to perpendicular to σ₁ following deformation. Orientation of the optical indicating surface of some of the deformed specimens suggests the orientation of the maximum reflectance is a composite product of the original orientation of the indicating surface and an orientation produced during deformation.     

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.     

Very high strains recorded in mylonites along the Alpine Fault, New Zealand: implications for the deep structure of plate boundary faults

Oblique displacement on the Alpine Fault, which forms the principal structure along the Australian–Pacific plate boundary in South Island, New Zealand, has resulted in exhumation of a kilometre-wide mylonite zone in the hanging wall adjacent to the current brittle fault trace. The mylonites formed under amphibolite facies conditions at depths of ca. 25 km and have been uplifted during the past 5 Ma. A suite of 65–70 Ma pegmatite veins in the hanging wall Alpine schists has been progressively deformed within the mylonite zone and sheared out over a strike length of ca. 100 km. Measurements of the thickness distribution of the pegmatite veins within the non-mylonitised schists and at three localities within the progressively strained mylonites have been used to estimate strain values within the mylonites. The thicknesses approximate a log-normal distribution, with a mean value that is progressively reduced through the protomylonites, mylonites and ultramylonites. By assuming that the thickness distribution currently observed in the schists was the same for the pegmatites within the mylonites before strain, a model of deformation incorporating simple shear and simultaneous pure shear is used to strain the undeformed veins until a fit is obtained with the strained distributions. Shear strains calculated range from 12 to 22 for the protomylonites, 120 to 200 for the mylonites and 180 to 300 for the ultramylonites, corresponding to pure shear values of 1–3 in each case. These values are compatible with the strains predicted if most of the surface displacement on the fault over the past 5 Ma were accommodated within a 1–2-km-wide mylonite zone through the middle and lower crusts. The results suggest that processes such as erosional focussing of deformation and thermal weakening may cause intense strain localisation within the lower crust, with plate boundary deformation restricted to narrow zones rather than becoming increasingly distributed over a widening shear zone with depth.     

Dynamic recrystallization of wet synthetic polycrystalline halite: dependence of grain size distribution on flow stress, temperature and strain

It is often observed that dynamic recrystallization results in a recrystallized grain size distribution with a mean grain size that is inversely related to the flow stress. However, it is still open to discussion if theoretical models that underpin recrystallized grain size–stress relations offer a satisfactorily microphysical basis. The temperature dependence of recrystallized grain size, predicted by most of these models, is rarely observed, possibly because it is usually not systematically investigated. In this study, samples of wet halite containing >10 ppm water (by weight) were deformed in axial compression at 50 MPa confining pressure. The evolution of the recrystallized grain size distribution with strain was investigated using experiments achieving natural strains of 0.07, 0.12 and 0.25 at a strain rate of 5×10⁻⁷ s⁻¹ and a temperature of 125 °C. The stress and temperature dependence of recrystallized grain size was systematically investigated using experiments achieving fixed strains of 0.29–0.46 (and one to a strain of 0.68) at constant strain rates of 5×10⁻⁷–1×10⁻⁴ s⁻¹ and temperatures of 75–240 °C, yielding stresses of 7–22 MPa. The microstructures and full grain size distributions of all samples were analyzed. The results showed that deformation occurred by a combination of dislocation creep and solution-precipitation creep. Dynamic recrystallization occurred in all samples and was dominated by fluid assisted grain boundary migration. During deformation, grain boundary migration results in a competition between grain growth due to the removal of grains with high internal strain energy and grain size reduction due to grain dissection (i.e. moving boundaries that crosscut or consume parts of neighbouring grains). At steady state, grain growth and grain size reduction processes balance, yielding constant flow stress and recrystallized grain size that is inversely related to stress and temperature. Evaluation of the recrystallized grain size data against the different models for the development of mean steady state recrystallized grain size revealed that the data are best described by a model based on the hypothesis that recrystallized grain size organizes itself in the boundary between the (grain size sensitive) solution-precipitation and (grain size insensitive) dislocation creep fields. Application of a piezometer, calibrated using the recrystallized grain size data, to natural halite rock revealed that paleostresses can vary significantly with temperature (up to a factor of 2.5 for T=50–200 °C) and that the existing temperature independent recrystallized grain size–stress piezometer may significantly underestimate flow stresses in natural halite rock.     

The rheology of plasticine

Plasticine rheology has been investigated in plane strain compression and stress relaxation tests. Constant strain rate tests indicate Reiner-Rivlin fluid characteristics or alternatively the deformation properties of plasticine may be described by a constitutive flow law of the type e = K σ^N where N= 6–9. This relationship was found for the three types of plasticine studied. Relaxation data indicate complex behaviour with probable structure changes during deformation. Under plane strain conditions at 25°C inhomogeneous flow characterized by ductile shears often occurs at high strains in all three types of plasticine. These effects were not apparent during 45°C tests. The application of plasticine analogues to simulate rock structures is discussed.     

Reaction-induced weakening of plagioclase–olivine composites

The localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions. In order to study the effects of solid–solid mineral reactions on plastic deformation of rocks, we have investigated the shear deformation of plagioclase–olivine composites during the reaction plagioclase + olivine → orthopyroxene + clinopyroxene + spinel (± garnet). Microstructures of plagioclase–olivine composites were studied after shear deformation experiments in a Griggs apparatus. Experiments were performed on anorthite–forsterite (An–Fo) and labradorite–forsterite (Lab–Fo) composites at 900 °C, confining pressures between 1000–1600 MPa and with constant shear strain rates of 5 × 10⁻⁵ s⁻¹.In absence of reaction, Lab–Fo composites are stronger than pure olivine and labradorite end-members that deform with a high temperature plasticity mechanism. Lab–Fo composites strain–harden due to the inhibition of extensive recrystallisation by interphase boundaries.In An–Fo composites, the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine-grained (size < 0.5 μm) polyphase reaction products. Interconnecting layers of reaction products accommodate most of the applied strain by grain size sensitive creep. Recovery processes are pronounced during syndeformational reaction: original anorthite and olivine dynamically recrystallise by subgrain rotation and bulging recrystallisation. Presumably, the dynamic recrystallisation is caused by reduced stress conditions and partitioning of strain and strain rates between the new reaction products and the relict An–Fo grains. The results of our experiments are in good agreement with natural observations of shear localisation in the lower crust and upper mantle, and imply that anhydrous mineral reactions can be important causes for localisation of deformation.