Non-axial planar S1 cleavage in the Hawick Rocks of the Galloway area,Southern Uplands,Scotland

S₁ cleavage in the Hawick Rocks of the Galloway area is non-axial planar, cutting obliquely across the F₁ folds in a predominantly clockwise sense. Individual S₁ cleavage planes within cleavage-fans in F₁ folds strike clockwise, locally anti-clockwise, of axial surfaces, and the mean plane to the S₁ cleavage-fans dips predominantly more steeply than the axial surface. F₁ folds investigated at scattered localities in Silurian and Ordovician rocks north of the Hawick Rocks are also transected by the S₁ cleavage, indicating that non-axial planar S₁ cleavage is widespread in the Southern Uplands. The S₁ cleavage is a composite fabric. Objects deformed within sandstones and tuffs indicate oblate strain. F₁ fold plunge varies from NE to SW and fold hinges locally are markedly curvilinear. Steeply plunging and locally downward-facing F₁ folds are present along the southeast margin of the Hawick Rocks. The non-axial planar S₁ cleavage relationships persist in the steeply plunging F₁ folds. Synchronous development of the non-axial planar S₁ cleavage and the variably plunging F₁ folds is proposed.     

Structural evolution of Mesozoic complexes in Western Chukotka

Detailed structural investigations were carried out in the Pevek area in order to verify the tectonic evolution of the Mesozoic thrust and fold belt in Chukotka. South-vergent F₁ folds in Triassic rocks were proved to be the earliest structures formed during the first deformation stage DI. These structures were deformed by north-vergent folds F₂ that were formed during the second deformation stage DII. North-vergent folds are the main structures of the Jurassic–Lower Cretaceous complex. The fold structures of the first two stages are deformed by shear folds F₃ finishing the stage DII. All these structures are deformed by submeridionally trending normal faults referred to the deformation stage DIII.     

Strain analysis and microstructural evolution characteristic of neoproterozoic rocks associations of Wadi El Falek,centre Eastern Desert,Egypt

The estimation of finite strain in rocks is fundamental to a meaningful understanding of deformational processes and products on all scales from microscopic fabric development to regional structural analyses. The R_f/φ and Fry methods on feldspar porphyroclasts and mafic grains from 5 granite, 1 metavolcanic, 3 metasedimentary and 1 granodiorite samples were used in Wadi El Falek region. Finite-strain data shows that a high to moderate range of deformation of the granitic to metavolcano-sedimentary samples and axial ratios in the XZ section range from 1.60 to 4.10 for the R_f/φ method and from 2.80 to 4.90 for the Fry method. Furthermore, the short axes are subvertical associated with a subhorizontal foliation. We conclude that finite strain in the deformed granite rocks is of the same order of magnitude as that from metavolcano-sedimentary rocks. Furthermore, contacts formed during intrusion of plutons with some faults in the Wadi El Falek area under brittle to semi-ductile deformation conditions. In this case, finite strain accumulated during superimposed deformation on the already assembled nappe structure. It indicates that the nappe contacts formed during the accumulation of finite strain.     

Foliations developed during slump deformation of Miocene marine sediments,Cyprus

Foliations within a Miocene slumped bioclastic sandstone unit of the Pakhna Formation, southern Cyprus, were investigated in order to assess the importance of slump strain, liquifaction and compaction in their generation. There are two approximately orthogonal sets of folds, F₁ and F₂. F₁ folds are upright to inclined slump folds formed during slope-failure translation of the sediment. The cores of upright F₁ folds have a steeply dipping macroscopic fabric defined by the axial surfaces of small tight folds in compositional layering. F₂ folds occur on steeply-dipping limbs of F₁ folds. F₂ folds are small and asymmetric with flat-lying axial surfaces, and are interpreted as compaction generated. A pervasive flat-lying microscopic fabric defined by grain and pore long axis orientation is found in both fold sets, and is probably a liquifaction fabric enhanced by compaction. A pervasive steeply-dipping microfabric parallel to the axial planes of slump folds is not present in any of the slumps investigated.     

Transpressional deformation,strain partitioning and fold superimposition in the southern Chinese Altai,Central Asian Orogenic Belt

Transpressional deformation has played an important role in the late Paleozoic evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here we focus on the transpressional Irtysh Shear Zone with an aim at understanding amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped macroscopic fold structures in the southern Chinese Altai and analyzed their relationships with the development of the adjacent Irtysh Shear Zone. Structural observations from these macroscopic folds show evidence for four generations of folding and associated fabrics. The earlier fabric (S₁), is locally recognized in low strain areas, and is commonly isoclinally folded by F₂ folds that have an axial plane orientation parallel to the dominant fabric (S₂). S₂ is associated with a shallowly plunging stretching lineation (L₂), and defines ∼NW-SE tight-close upright macroscopic folds (F₃) with the doubly plunging geometry. F₃ folds are superimposed by ∼NNW-SSE gentle F₄ folds. The F₃ and F₄ folds are kinematically compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may represent strain partitioning during deformation. The sub-parallelism of F₃ fold axis with the Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation (F₃) in fold zones. The strain partitioning may have become less efficient in the later stage of transpressional deformation, so that a fraction of transcurrent components was partitioned into F₄ folds.     

Discovery of a layer of probable impact melt spherules in the Late Archaean Jeerinah Formation,Fortescue Group,Western Australia

In the high‐grade (granulite facies) metamorphic rocks at Broken Hill the foliation is deformed by two groups of folds. Group 1 folds have an axial‐plane schistosity and a sillimanite lineation parallel to their fold axes; the foliation has been transposed into the plane of the schistosity by these folds. Group 2 folds deform the schistosity and distort the sillimanite lineation so that it now lies in a plane. Both groups of folds are developed as large folds. The retrograde schist zones are zones in which new fold structures have formed. These structures deform Group 1 and Group 2 folds and are associated with the formation of a new schistosity and strain‐slip cleavage. The interface between ore and gneiss is folded about Group 1 axial planes but about axes different from those in the foliation in the gneiss. On the basis of this, the orebody could not have been parallel to the foliation prior to the first recognizable structural and metamorphic events at Broken Hill. The orebody has been deformed by Group 2 and later structures.     

Sheath fold morphology in simple shear

Sheath folds are highly non-cylindrical structures often associated with shear zones. We investigate the formation of sheath folds around a weak inclusion acting as a slip surface in simple shear by means of an analytical model. We present results for different slip surface orientations and shapes. Cross-sections perpendicular to the shear direction through the sheath fold display closed contours, so called eye-structures. The aspect ratio of the outermost closed contour is strongly dependent on the initial slip surface configuration. The center of the eye-structure is subject to change in height with respect to the upper edge of the outermost closed contour for different cross-sections perpendicular to the shear direction. This results in a large variability in layer thickness across the sheath fold length, questioning the usefulness of eye-structures as shear sense indicators. The location of the center of the eye structure is largely invariant to the initial configurations of the slip surface as well as to strain. The values of the aspect ratios of the closed contours within the eye-pattern are dependent on the strain and the cross-section location. The ratio (R′) of the aspect ratios of the outermost closed contour (R_yz) and the innermost closed contour (R_y′z′) shows values above and below 1. R′ shows dependence on the slip surface shape and orientation but not on the number of involved contours. Using R′ measurements to deduce the bulk strain type may be erroneous.     

Geological observations in high‐grade mid‐Proterozoic rocks from Else Platform,northern Prince Charles mountains region,east Antarctica

Granulite facies rocks on Else Platform in the northern Prince Charles Mountains, east Antarctica, consist of metasedimentary gneiss extensively intruded by granitic rocks. The dominant rock type is a layered garnetbiotite‐bearing gneiss intercalated with minor garnet‐cordierite‐sillimanite gneiss and calc‐silicate. Voluminous megacrystic granite intruded early during a mid‐Proterozoic (ca 1000 Ma) granulite event, M₁, widely recognized in east Antarctica. Peak metamorphic conditions for M₁ are in the range of 650–750 MPa at ～800°C and were associated with the development of a gneissic foliation, S₁ and steep east‐plunging lineation, L₁. Strain partitioning during progressive non‐coaxial deformation formed large D₂ granulite facies south‐dipping thrusts, with a steep, east‐plunging lineation. In areas of lower D₂ strain, large‐scale upright, steep east‐plunging fold structures formed synchronously with the D₂ high‐strain zones. Voluminous garnet‐bearing leucogneiss intruded at 940 ±20 Ma and was deformed in the D₂ high‐strain zones. Textural relationships in pelitic rocks show that peak‐M2 assemblages formed during increasing temperatures via reactions such as biotite + sillimanite + quartz ± plagioclase = spinel + cordierite + ilmenite + K‐feldspar + melt. In biotite‐absent rocks, re‐equilibration of deformed M₁ garnet‐sillimanite‐ilmenite assemblages occurred through decompressive reactions of the form, garnet + sillimanite + ilmenite = cordierite + spinel + quartz. Pressure/temperature estimates indicate that peak‐M₂ conditions were 500–600 MPa and 700±50°C. At about 500 Ma, north‐trending granitic dykes intruded and were deformed during D₃‐M₃ at probable upper amphibolite facies conditions. Cooling from peak D₃‐M₃ conditions was associated with the formation of narrow greenschist facies shear zones, and the intrusion of pegmatite. Cross‐cutting all features are abundant north‐south trending alkaline mafic dykes that were emplaced over the interval ca 310–145 Ma, reflecting prolonged intrusive activity. Some of the dykes are associated with steeply dipping faults that may be related to basin formation during Permian times and later extension, synchronous with the formation of the Lambert Graben in the Cretaceous.     

Magnetic fabrics of the Douglas Till of the Superior lobe: exploring bed-deformation kinematics

The Superior lobe of the Laurentide ice sheet, during its last advance into northwestern Wisconsin, may have moved and transported sediment primarily through deep, widespread shear of its bed to high strains (>100). To study the kinematics of this deformation, we measured the anisotropy of magnetic susceptibility of 3750 intact samples of its basal till—the Douglas member of the Miller Creek formation—collected from eight profiles at 0.2 m depth increments. A benchmark for interpretations was provided by results of ring-shear experiments conducted on the Douglas Till, which provided fabric characteristics for the case of horizontal simple shear. Orientations of maximum susceptibility (k₁) were controlled by preferred orientations of silt-sized magnetite grains and were similar to sand-grain orientations measured in thin sections. Strengths of fabrics formed by orientations of k₁ indicate that most of the till sampled was deformed but to variable strains. Deformation averaged over the 20 km spanned by the study approximated horizontal simple shear. However, large differences in k₁ fabric azimuths (>45°) over lateral distances of meters to tens of meters indicate highly heterogeneous bed deformation, perhaps focused in anastomosing zones with associated divergent and convergent till shear. This interpretation is supported by orientations of principal susceptibilities that imply, in many cases, either that shear planes were steeply dipping, particularly transverse to the shearing direction, or that there were major components of pure shear. Variations in k₁ fabric azimuth with depth indicate that most of the till thickness did not shear simultaneously; rather, till accumulated at the bed as shear direction changed in response to temporally shifting zones of shallow deformation (<1 m). This heterogeneous, temporally variable deformation of the bed differs from many applications of the bed-deformation model but is consistent with subglacial measurements at modern glaciers.     

Transpressive Structures in the Ghadir Shear Belt,Eastern Desert,Egypt: Evidence for Partitioning of Oblique Convergence in the Arabian‐Nubian Shield during Gondwana Agglutination

Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt. The Ghadir Shear Belt is a 35 km-long, NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within this shear belt, strain is highly partitioned into shortening, oblique, extensional and strike-slip structures at multiple scales. Moreover, strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains. In the East Ghadir and Ambaut shear belts, the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated. These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones. The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones. The earlier fabric(S1), is locally recognized in low strain areas and SW-ward thrusts. S2 is associated with a shallowly plunging stretching lineation(L2), and defines ～NW-SE major upright macroscopic folds in the East Ghadir shear belt. F2 folds are superimposed by ～NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt. The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones. Dextral ENEstriking shear zones were subsequently active at ca. 595 Ma, coeval with sinistral shearing along NW-to NNW-striking shear zones. The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt. Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments. Upright folds, fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning. The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana.     

The strain history of a dalradian slide: Using pebbles with low fluctuations in axis orientation

The strain associated with the Horn Head Slide, a major tectonic break in the Dalradian rocks of NW Ireland, is recorded by pebbles in an adjacent quartzite horizon. Mean X/Y ratios of the deformed pebble shapes in excess of 8.0 are seen closest to the slide and the field of three-dimensional shapes lies along the K = 1 line. The usual methods of separating initial shape ratio (R_i) and tectonic strain ratio (R_s) from the deformed shape ratio (R_f) of ellipsoidal markers are based on measurements of variation in fluctuation (e.g., the (R_f/φ technique). However, due to the high X/Y strains in this situation and since the pebbles initially lay parallel to bedding and to a principal plane of the finite strain ellipsoid, fluctuation is generally very low. Thus, except for the least deformed X/Y data, the R_f/φ technique is inapplicable and other methods are used. For X/Y data with mean (R_f > 4.0: R_s is calculated as the harmonic mean of R_f; maximum R_i values only are obtained from the range of R_f data. For all Y/Z and X/Z data: R_i is calculated from the logarithmic range (ω_log) of R_f; R_s is simply obtained from the geometric mean of R_f modified by R_i. It is concluded from this that a varying prolate tectonic strain (K - 1.5) reaching X/Y values in excess of 8.00 was coaxially superimposed on an initial oblate shape fabric to produce the present field of deformed pebbles in the quartzite near the slide.     

Deformation history of the Hampden Synform in the Eastern Fold Belt of the Mt Isa terrane

The moderately metamorphosed and deformed rocks exposed in the Hampden Synform, Eastern Fold Belt, in the Mt Isa terrane, underwent complex multiple deformations during the early Mesoproterozoic Isan Orogeny (ca 1590–1500 Ma). The earliest deformation elements preserved in the Hampden Synform are first‐generation tight to isoclinal folds and an associated axial‐planar slaty cleavage. Preservation of recumbent first‐generation folds in the hinge zones of second‐generation folds, and the approximately northeast‐southwest orientation of restored L¹ ₀ intersection lineation suggest recumbent folding occurred during east‐west to northwest‐southeast shortening. First‐generation folds are refolded by north‐south‐oriented upright non‐cylindrical tight to isoclinal second‐generation folds. A differentiated axial‐planar cleavage to the second‐generation fold is the dominant fabric in the study area. This fabric crenulates an earlier fabric in the hinge zones of second‐generation folds, but forms a composite cleavage on the fold limbs. Two weakly developed steeply dipping crenulation cleavages overprint the dominant composite cleavage at a relatively high angle (>45°). These deformations appear to have had little regional effect. The composite cleavage is also overprinted by a subhorizontal crenulation cleavage inferred to have developed during vertical shortening associated with late‐orogenic pluton emplacement. We interpret the sequence of deformation events in the Hampden Synform to reflect the progression from thin‐skinned crustal shortening during the development of first‐generation structures to thick‐skinned crustal shortening during subsequent events. The Hampden Synform is interpreted to occur within a progressively deformed thrust slice located in the hangingwall of the Overhang Shear.     

Aspects of the structural and late thermal evolution of the Redbank Thrust system,central Australia: Constraints from the Speares Metamorphics

We present new data on the field geology and late thermal evolution of the Redbank Thrust system in the Arunta Block of central Australia. Geochronological and field data from the Speares Metamorphics are also used to relate the thermal evolution of the Redbank Thrust system to the structural evolution of the region. We show that several stages in the evolution might be discerned. An originally sedimentary sequence was intruded by mafic intrusions and then deformed during partial melting to form the principal foliation observed in the region (D1). This sequence was then folded during D2 into upright folds with north‐ to northeast‐plunging fold axes. These events are likely to correlate with the Strangways and/or Argilke and Chewings Orogenies known from previous studies. Subsequently, the Redbank Thrust was initiated during D3. This event is recognised by deflection of the host rocks into the shear zone and might therefore have been associated with a component of strike‐slip motion. It occurred probably at or before 1500–1400 Ma. Subsequent north‐over‐south thrust motion in the Redbank Thrust formed the intense mylonitic fabric and folded the mylonitic fabric during D4 into asymmetric folds with shallow fold axes. New ⁴⁰Ar/³⁹Ar K‐feldspar ages from three samples collected from variably deformed branches of the Redbank Thrust and undeformed rocks in the Speares Metamorphics suggest that most parts of the Redbank Thrust system cooled relatively slowly after metamorphism and deformation in the Mesoproterozoic so that the D4 thrusting might have been very long‐lived. Minimum ages of the K‐feldspar age spectra show that the entire region cooled below 200°C by approximately 300 Ma. Apatite fission track ages from nine samples show that cooling through the apatite partial annealing zone occurred during Cretaceous time (ca 150–70 Ma) and modelled cooling histories are consistent with the cooling rates obtained from the K‐feldspar data. They indicate that final exhumation of the Redbank Thrust system occurred probably in response to erosion, possibly driven by rifting around the margins of Australia.     

Determining heterogeneous deformation for granitic rocks in the northern thrust in Wadi Mubarak belt,Eastern Desert,Egypt

Finite-strain was studied in the mylonitic granitic and metasedimentary rocks in the northern thrust in Wadi Mubarak belt to show a relationship to nappe contacts between the old granitic and metavolcano-sedimentary rocks and to shed light on the heterogeneous deformation for the northern thrust in Wadi Mubarak belt. We used the R_f/ϕ and Fry methods on feldspar porphyroclasts, quartz and mafic grains from 7 old granitic and 7 metasedimentary samples in the northern thrust in Wadi Mubarak belt. The finite-strain data shows that old granitic rocks were moderate to highly deformed and axial ratios in the XZ section range from 3.05 to 7.10 for granitic and metasedimentary rocks. The long axes (X) of the finite-strain ellipsoids trend W/WNW and E/ENE in the northern thrust in Wadi Mubarak belt. Furthermore, the short axes (Z) are subvertical associated with a subhorizontal foliation. The value of strain magnitudes mainly constants towards the tectonic contacts between the mylonitic granite and metavolcano-sedimentary rocks. The data indicate oblate strain symmetry (flattening strain) in the mylonitic granite rocks. It is suggested that the accumulation of finite strain was formed before or/and during nappe contacts. The penetrative subhorizontal foliation is subparallel to the tectonic contacts with the overlying nappes and foliation was formed during nappe thrusting.     

Relationships between strain and quartz crystallographic fabrics in the Roche Maurice quartzites of Plougastel,western Brittany

An integrated microstructural and petrofabric study of the plastically deformed and partially recrystallized Roche Maurice quartzites of Plougastel, western Brittany, has revealed a clear correlation between the pattern of c-axis fabrics displayed by detrital quartz grains and the symmetry of the calculated strain ellipsoid. In specimens with flattening (k = 0) strains, c axes lie on a small circle girdle (opening angle 28–42°) centred about the principal finite shortening direction (Z). For specimens that exhibit approximate plane strain (k = 1), cross-girdle c-axis fabrics consisting of a small circle girdle centred about Z and connected through the intermediate principal extension direction (Y) were detected.Within individual specimens c-axis fabrics of syntectonically recrystallized new quartz grains within the matrix are similar to those of detrital quartz grains. c axes of new grains located within the relatively undeformed sections of the host detrital grains are commonly orientated at angles between 10 and 40° to the host c axis and are, in addition, statistically orientated at a higher angle to Z than their host c axes. These relationships are interpreted as indicating that both host grain control and the local strain (and/or stress) field may have influenced the process of recrystallization; the relative influence of these factors is, however, unknown.Microstructural and petrofabric studies indicate that the Roche Maurice quartzites have been subjected to essentially coaxial strain histories. The role of syntectonic recrystallization in facilitating continued plastic deformation in quartzites subjected to such strain histories is considered.     

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.     

Progressive development of structures in a ductile shear zone

In the Singhbhum Shear Zone of eastern India successive generations of folds grew in response to a progressive ductile shearing. During this deformation a mylonitic foliation was initiated and was repeatedly transposed. The majority of fold hinges were formed in an arcuate manner at low angles to the Y-axis in an E-W trending subhorizontal position and major segments of the fold hinges were then rotated towards the down-dip northerly plunging X-axis. The striping and intersection lineations were rotated in the same manner. The down-dip mylonitic lineation is a composite structure represented by rotated early lineations and newly superimposed stretching lineations. The consistent asymmetry of the folds, the angular relations between C and S surfaces and the evidence of two-dimensional boudinage indicate that the deformation was non-coaxial, but with a flattening type of strain with λ₁ ⪢ λ₂. The degree of non-coaxiality varied both in space and time. From the progressive development of mesoscopic structures it is concluded that the 2–3 km wide belt of ductile shear gave rise to successive anastomosing shear zones of mesoscopic scale. When a new set of shear lenses was superimposed on already sheared rocks, the preexisting foliation generally lay at a low angle to the lenses. No new folds developed where the acute angle was sympathetic to the sense of shear displacements. Where the acute angle was counter to the sense of shear, the pre-existing foliation, lying in the instantaneous shortening field, was deformed into a set of asymmetric folds.     

The effect of sample size on geological strain estimation from passively deformed clastic sedimentary rocks

The role of sample size in the estimation of geological strain, both finite strain (R_s) and that of the orientation of the finite strain ellipse (φ_s), is investigated for clastic sedimentary rocks. This study looks at four strain methods, the Robin method, the linearization method, the Mulchrone and Meere method and the mean radial length method that are initially tested using simulated strained data sets and subsequently by applying the methods to real data. It is found that the optimum strain analysis sample size for a clastic sedimentary rock is primarily dependant on the intensity of strain suffered by that rock because of the error behavior associated with R_s estimates. An iterative process is therefore recommended starting with a minimum sample size of 150, which can be maintained or reduced based on the initial R_s estimates.     

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.