Quartz Crystallographic Preferred Orientation Research Articles

Transpression or polyphase deformation along craton margins: Insights from the Archean–Proterozoic boundary near Sudbury, Canada

Continental margins vary in composition and configuration, resulting in complex structural geometries during deformation. One example is the Archean–Proterozoic boundary near Sudbury, Canada, where the Creighton-Victoria Deformation Zone (CVDZ) contains both reverse dip-slip and dextral strike-slip kinematic indicators. To resolve if these structures formed during a single transpression event or multiple overprinting events, the orientation, relative timing and style of the CVDZ are compared to those of adjacent regional structures. Regional fold axes and their axial planar foliation are subparallel to the steeply-plunging stretching lineation and steeply-dipping foliation, respectively, along the CVDZ. Phase equilibria modelling of syn-tectonic staurolite-chloritoid assemblages indicates that the regional folds and foliation formed at 480–565 °C, similar to temperatures estimated for the formation of dip-slip quartz crystallographic preferred orientations (CPO) along the CVDZ. This suggests that the CVDZ, regional folds and foliation formed during the same regional folding and reverse shearing event. Major lateral lithological changes and inflections in the orientation of the craton margin controlled the formation of structures, and evidence for regional transpression, such as strike-slip kinematic indicators, steeply-plunging stretching lineation, and obliquity of regional folds with respect to major shear zones, resulted from polyphase deformation and transcurrent reactivation of the Superior craton margin.

Practical Assessment of Quartz Crystallographic Preferred Orientation Strength

The ordering strength of crystallographic preferred orientation data can be assessed in different ways, however, the appropriateness of the methodology used to do so can depend on the geometry of the distributions. Eigenvector-based ordering evaluation methods, for example, may not be appropriate for data that comprise multiple, variably oriented distributions such as those commonly found in quartz c-axis crystallographic preferred orientations. Examination of artificial data that represent a variety of different potential c-axis distributions shows a significant correlation between the relative orientations of those distributions (i.e., the opening angle in a cross-girdled quartz c-axis pole figure) and the strengths calculated using eigen-vector based evaluation methods; larger pole figure opening angles correlate with decreasing distribution ordering strength. The same correlation does not exist when strength is evaluated using the l2 - norm of the estimated probability density function (JPF) of the same data. The direct correlations between pole figure c-axis opening angles and ordering strength noted in the artificial distributions are also demonstrated in the evaluation of real-world data, though significant complications related to heterogeneous nature, and/or deformation, of the natural specimens can partially obfuscate the relationship. Regardless, given the potential effect of geometry on eigenvector-based evaluation methods we recommend that the ordering strength of pole figure data be evaluated using JPF.

Modern style nappe stacking in the Paleoproterozoic lower crust: An example from the snowbird tectonic zone, Canadian Shield

The Intra Tantato shear zone (ITSZ), which occurs within the East Athabasca mylonite triangle in the southern portion of the ∼1.9 Ga Snowbird tectonic zone in northern Saskatchewan, Canada, is a low to high angle, curvi-planar, kilometer-thick mylonite zone hosted within granulite-facies gneiss that records a complex history with both thrust- and normal-sense motion. New field mapping, microstructural analysis, and quartz crystallographic preferred orientations (CPO) detail a composite history for the ITSZ that includes top-to-the-ENE thrusting of the hanging wall over the footwall that is overprinted by normal-sense top-to-the-WSW reactivation. Quartz CPOs further indicate constrictional deformation related to both events and quartz c-axis opening angles define deformation temperatures of ∼750 ± 50 °C for the thrusting event and ∼550 ± 50 °C for the later, normal-sense reactivation.Monazite and garnet geochronology on rocks within and around the ITSZ indicate it may have nucleated along a pre-existing 2.6–2.5 Ga sub-horizontal fabric. This appears to have occurred immediately after the ∼1.920 Ga metamorphic peak of the hanging wall and during the ∼1.905 Ga metamorphic peak of the footwall. Monazite ages from small matrix grains in interstitial positions in specimens that record reactivation kinematics indicate overprinting occurred at ∼1.860–1.830 Ga, contemporaneous with the earliest recorded activity along the nearby Grease River Shear Zone.Overall, this tectonic evolution can be attributed to a crustal thickening process that occurred in the Athabasca mylonite triangle during the Snowbird orogeny at ∼1.9 Ga and during the post-collisional exhumation processes. Moreover, these results show that the lower continental crust can actively accommodate shortening during large-scale collisional events with tectonic units piling up via nappe-stacking processes similarly to what is typically observed in the middle crust.

Origin of magmatic and tectonic fabrics in the Remal granite-gneiss, Singhbhum Craton, India

AbstractIn the Singhbhum Craton of the Indian shield, the Remal granite-gneiss preserves felsic magmatic fabrics onto which a low-temperature segregation layering has been superposed. Planar, sub-horizontal to gently dipping layers (Sign1) comprise K-feldspar megacrysts, plagioclase and quartz, with the base of each layer defined by segregations of biotite. Sign2 consists of trough cross-bedded layers composed of K-feldspar phenocrysts, plagioclase and quartz with biotite schlieren defining the base of each layer. Microstructural features such as concentrically arranged mineral inclusions in K-feldspar phenocrysts and graphic intergrowth textures testify to the magmatic origin of these fabrics, with insignificant subsequent metamorphic reconstitution. The tectonic fabric S1 has developed sub-parallel to localized greenschist-facies mylonite bands, and is defined by weakly aligned flakes of biotite. Crystallographic preferred orientations away from the mylonitized domains show a strong alignment of K-feldspar, quartz and biotite parallel to the magmatic fabric due to efficient segregation during magmatic flow. Quartz crystallographic preferred orientations within the mylonitized domains show a strong preferred orientation and dextral asymmetry. Temperature constraints from synkinematic chlorites along with estimates of deformation temperature from quartz crystallographic preferred orientations indicate that mylonitization occurred at the lower limits of quartz crystal plasticity. The results of combined thermodynamic and multiphysics modelling studies show that felsic magmas can undergo significant convective motion for a wide range of crystallinities and water contents before solidification. Additionally, segregation layering resembling a gneissosity can develop at low temperatures owing to localized mylonitization and concomitant dissolution–precipitation of biotite.

Crystallographic preferred orientation of quartz deformed at granulite conditions: the Kalinjala Shear Zone, Port Neill, South Australia

Crystallographic preferred orientations of quartz aggregates in mylonitic orthogneisses from the highest strained central region of the Kalinjala Shear Zone, Port Neill South Australia, are products of a combination of constrictional and flattening strains. These rocks were deformed under granulite facies peak metamorphic conditions, reaching pressures of ∼1.0 GPa and temperatures of ∼800 ± 50 °C. Neutron diffraction textural and fabric analysis methods reveal a dominant single c-axis maximum parallel to the Y-strain axis. This is accompanied by a symmetric orthorhombic pattern, where the c-axis fabrics and corresponding pole figure densities of {a}, {m}, {r} and {z} planes are arranged around the XY foliation plane. These fabrics from coexisting adjacent layers suggest that the activation of prism-<a> slip is characteristic for these high-temperature deformation conditions. A small asymmetry in the [c], {a} and {m} pole figures suggests a minor component of dextral shearing. The c-axis pattern is weakened where the foliation is dominated by secondary phases, such as concentrations of biotite mica. The opening angles obtained from cross-girdle c-axis patterns produce temperature estimates that are ∼100 °C lower than the peak metamorphic temperature of ∼800 ± 50 °C, but closer to temperatures related to a component of flattening strain. These c-axis patterns are consistent with other structural characteristics and can be related to the late-stage flattening strains during the Kimban Orogeny. KEY POINTS Mylonitic orthogneisses are deformed under granulite facies conditions. Quartz fabrics were measured by bulk neutron diffraction and optical methods. Mylonite c-axis fabrics can be related to the high-temperature deformation and activation of the prism-< a > slip system. Crystal preferred orientations indicate a large component of flattening strain superimposed on the dextral shearing observed in the Kalinjala Shear Zone.

Time relationship between emplacement, fabric development and regional deformation of the Manchi granitic pluton (western - Cameroon domain)-an integrated AMS, CPO and microstructural investigation

In this study, we integrate anisotropy of magnetic susceptibility (AMS) and microstructural data including quartz crystallographic preferred orientation (CPO) from SEM-EBSD from the Manchi Pan-African granitic pluton with the objective of deciphering the time relationship between fabric development, emplacement and regional tectonics. The Manchi pluton, located in the western Cameroon domain of the Pan-African Fold Belt of Central Africa in Cameroon, is made of hornblende-biotite granite (HBtG), biotite granite (BtG) and protomylonites of HBtG. The pluton intruded the high grade country rocks made of banded gneiss and amphibolites. AMS of the pluton is controlled dominantly by paramagnetic phases such as, Fe-bearing silicates (biotite and hornblende). This is inferred from the low Km values (<500 × 10−6 SI units) as well as from hysteresis data. The AMS ellipsoid shape is mostly oblate, implying that flattening component of the strain was important during granite emplacement. Microstructural observations reveal presence of magmatic to sub-magmatic and solid-state deformation textures developed at moderate- to low-temperatures. This is consistent with the presence of rhomb <a> and basal <a> slip systems established from quartz c-axis data through EBSD analysis. The pluton recorded top-toward SW to top-toward south sense of shear as established by the kinematic markers. This indicates a component of simple shear during tectonic evolution of the pluton. We infer that the magnetic fabrics recorded in the Manchi granitic pluton is a manifestation of the D2 regional deformation. The dominance of flattening strain and the concentric pattern of magnetic data are indicative of ballooning being an important mechanism under transpressive regime.

Progressive Development of a Distributed Ductile Shear Zone beneath the Patagonian Retroarc Fold-Thrust Belt, Chile

Abstract The southern Patagonian Andes record Late Cretaceous–Paleogene compressional inversion of the Rocas Verdes backarc basin (RVB) and development of the Patagonian fold-thrust belt (FTB). A ductile décollement formed in the middle crust and accommodated underthrusting, thickening, and tectonic burial of the continental margin (Cordillera Darwin Metamorphic Complex (CDMC)) beneath the RVB. We present new geologic mapping, quartz microstructure, and crystallographic preferred orientation (CPO) fabric analyses to document the kinematic evolution and deformation conditions of the décollement. Within the CDMC, the décollement is defined by a quartz/chlorite composite schistose foliation (S1-2) that is progressively refolded by two generations of noncylindrical, tight, and isoclinal folds (F3–F4). Strain intensifies near the top of the CDMC, forming a &amp;gt;5 km thick shear zone that is defined by a penetrative L-S tectonite (S2/L2) and progressive noncylindrical folding (F3). Younger kink folds and steeply inclined tight folds (F4) with both north- and south-­dipping axial planes (S4) overprint D2 and D3 structures. Quartz textures from D2 fabrics show subgrain rotation and grain boundary migration recrystallization equivalent to regime 3, and quartz CPO patterns indicate mixed prism &amp;lt;a&amp;gt; and [c] slip systems with c-axis opening angles indicative of deformation temperatures between ~500° and &amp;gt;650°C. Approximately 40 km toward the foreland, the shear zone thins (~1 km thick) and is defined by the L-S tectonite (S2/L2) and tightening of recumbent isoclinal folds (F3). Quartz textures and CPO patterns indicate subgrain rotation recrystallization typical of regime 2 and dominantly basal &amp;lt;a&amp;gt; slip, and c-axis opening angles are consistent with deformation temperatures between ~375° and 575°C. Deformation occurred under greenschist and amphibolite facies conditions in the foreland and hinterland, respectively, indicating that the shear zone dipped shallowly toward the hinterland. The Magallanes décollement is an example of a regional ductile shear zone that accommodated distributed middle to lower crustal thickening below a retroarc FTB.

Relation between finite strain geometry and quartz petrofabrics in a folded conglomerate in the Norwegian Caledonides

A relationship between crystallographic preferred orientation (CPO) and strain geometry (strain-ellipsoid shape) is often assumed, but not well documented. In this work, we use the EBSD method to study quartz CPO and microstructure in relation to strain geometry in a folded conglomerate in the SW Scandinavian Caledonides, where strain varies from strongly flattening (long limbs) to constrictional (hinges and inverted short limb). Our work shows that strain was accommodated by dynamic recrystallisation through subgrain rotation, bulging and Dauphine twinning. Differential stress during deformation is estimated at ∼125 MPa from recrystallised grain size. Deformed and recrystallised grains show similar patterns of crystallographic orientation, despite a significant difference in grain size. We find no correlation between strain geometry and CPO, which we explain by means of a flow perturbation fold model. In this model, the variation in strain geometry is a result of progressive layer rotation during shearing. A correlation may be expected for steady-state deformations with no folding of layers with strain markers. However, flow perturbation may be a common cause of complex 3D strain patterns in sheared rocks. Hence, estimating strain geometry from CPO alone is inherently difficult and should be avoided unless a simple and steady deformation history can be assumed.

Greenstone‐Up Shear Sense at the Margin of the Mt Edgar Dome, East Pilbara Terrane: Implications for Dome and Keel Formation in the Early Earth

AbstractThe Paleoarchean Mt Edgar dome in the East Pilbara Terrane has long been studied as an archetypal dome within Archean dome‐and‐keel terranes, but the history of its formation is debated. Kinematic data presented in this study provide new insights into the late‐stage development of the Mt Edgar dome and East Pilbara Terrane. Quartz crystallographic preferred orientation (CPO), optical microstructures, and field structures all indicate that the granite‐greenstone contact of the Mt Edgar dome experienced reverse (greenstone‐up, dome‐down) sense of shear after the formation of the dominant schistosity. This reverse sense of shear is observed at localities along the entire extent of the sheared margin that rings most of the Mt Edgar dome, but is best documented along the southwest margin in the Warrawoona Greenstone Belt. Additionally, quartz CPO data from a dome triple junction outside of the sheared margin dominantly indicate a constrictional strain geometry, consistent with the previous interpretation that this area represents a zone of vertical foundering in a buoyancy‐instability driven system. However, buoyancy‐instability models do not necessarily predict the occurrence of greenstone‐up sense of shear preserved in solid‐state fabrics along the dome margin. Several geologic explanations are considered, including dome expansion or post‐doming deformation. The data are most consistent with explanations that directly relate to dome formation, especially when considered in tandem with recently published structural data from within the Mt Edgar dome. These kinematic data suggest that late dome development occurred in a near‐static crustal environment rather than an extensional or contractional setting.

Late Paleozoic deformation and tectonic significance of the South Central Tianshan Shear Zone, Kawabulake area, East Tianshan, NW China: Constraints from quartz fabrics and geochronologic data

The South Central Tianshan Shear Zone occurs along the southern margin of the Central Tianshan Block (CTB), NW China. This shear zone contains important information about the history of the amalgamation of various plates in the southwestern margin of the Central Asian Orogenic Belt. This study focuses on the deformation of the South Central Tianshan Shear Zone within the Kawabulake area in East Tianshan and provides insights into the late Paleozoic tectonic history of this region. Field observations indicate that the shear zone records at least two ductile deformation events (D1 and D2), where D1 is a sinistral strike-slip deformation event and D2 is dextral strike-slip. The shear zone contains four giant tectonic lenses that were formed during the D2 event. Microstructures and quartz crystallographic preferred orientation patterns indicate that D1 and D2 both occurred under low temperature conditions. We constrained the timing of deformation using mica 40Ar/39Ar and zircon U–Pb approaches. The timing of D1 deformation is constrained in the range of 400–290 Ma and the early and late stages of D2 deformation occurred at ca. 290 and 276–270 Ma, respectively. Regional comparisons indicate that D1 resulted from the clockwise rotation of the Tarim Craton and the oblique subduction of the South Tianshan Ocean crust beneath the CTB, the early stage of D2 was the result of the oblique collision between the Tarim Craton and the CTB, and the late stage of D2 records the convergence of the Tarim Craton and plates north of the CTB.

Shear zones in the southern Eyre Peninsula, South Australia: Quartz c-axis fabrics in granulite facies mylonitic orthogneisses and relationship to mafic dykes

The south eastern portion of the Eyre Peninsula, South Australia, is dominated by a suite of granites and orthogneisses deformed in steep-dipping shear zones during the Kimban Orogeny (1730–1690 Ma). The largest is the transpressional Kalinjala Shear Zone with shortening strains of ≥90% and shear strains of γ = ≥20, with other near-parallel secondary shear zones displaying similar kinematics. These ductile shear zones are dominated by mylonitic orthogneisses, with shearing commonly nucleating on, or adjacent to, precursor mafic dykes. The mafic dykes are highly sheared, folded and boudinaged with a newly formed matrix assemblage (orthopyroxene + clinopyroxene + garnet + magnetite + quartz + K-feldspar) that indicates the peak P/T metamorphic conditions of 0.7–1.0 GPa and 750–850 °C. Partial melting, developed as leucosomes, is associated with this metamorphism. Microstructural relationships between quartz crystallographic preferred orientation (CPO) and shape preferred orientations (SPO) analyses of these polymineralic rocks are best interpreted in terms of a granulite facies isothermal temperature regime of ~750 °C. The quartz c-axis fabrics, obtained using an optical fabric analyser, reveal a range of CPOs from weak girdles normal to the shear plane, to maxima parallel to the Y-strain axis. There is an overall symmetric orthorhombic pattern, with c-axis fabrics oriented with respect to the XY plane of flattening. There is a minor component of non-coaxial deformation in some shear zones. These fabrics indicate the increased activation of predominantly the prism-<a > slip system in quartz within the shear zones, which is characteristic for these high-temperature and high-strain deformation conditions.

Exhumation of plutons controlled by boundary faults: Insights from the kinematics, microfabric, and geochronology of the Taibai shear zone, Qinling Orogen, China

Abstract The Taibai granitic plutons lie between the Taibai ductile shear zone to the north and the Shangdan suture to the south. The deformation mechanism of the ductile shearing is crucial to understanding the exhumation processes of the multiple plutons that formed after the Late Mesozoic period. Geological investigations, microstructures, and kinematic vorticity calculations indicate that the Taibai shear zone deformed in response to pure shear-dominated (54%–65%) transpression and top-to-NW shear sense as a result of NE–SW oblique contractional tectonics. The quartz crystallographic preferred orientations of the prism &amp;lt;a&amp;gt; slip system, the grain boundary migration, and sub-grain rotation dynamic recrystallization of quartz—combined with the plagioclase–hornblende thermometer—constrain the main deformation temperatures to a range of 400–650 °C, which suggests amphibolite to greenschist facies conditions. In addition, it is extremely likely that the mylonites experienced late-stage, lower temperature deformation as demonstrated by the sporadic bulging recrystallization, the quartz basal &amp;lt;a&amp;gt; slip system, and the two-feldspar geothermometer calculation. The samples collected from the weakly deformed mylonitic granite pluton and the undeformed quartz-feldspathic dike that intruded into the mylonites yield zircon U–Pb ages of 129 ± 1 Ma and 115 ± 1 Ma, respectively. This information, with the lower intercept ages of ca. 120 Ma obtained from the mylonite samples, suggests that the ductile shearing probably occurred from ca. 129 Ma to 115 Ma. Combined with the regional geological data, these findings suggest that the Taibai shear zone and the Shangdan suture accommodated the oblique upward extrusion of the Taibai plutons during Early Cretaceous time.

A refined approach for quantitative kinematic vorticity number estimation using microstructures

This study proposes a refined approach for quantification of the vorticity of kinematic flow in natural shear zones. This approach, here called C′-c method, is based on estimation of the angle between the flow apophyses A1 and A2 by performing two independent measurements. The tilting angle of a quartz crystallographic preferred orientation (β) and the orientation of the C′ shear bands (θ). Using these two angles it is possible to quantify the sectional vorticity number using the following formula: Wn = Cos[2(θ-β)]. Data obtained by applying the C′-c method on known shear zones well match the vorticity numbers estimated with other approaches on the same structures. In this study, we applied the C′-c method to the Abloviak Shear Zone (Torngat Orogen) and the Ben Hope Thrust (Caledonides of NW Scotland) obtaining the first vorticity data on these structures. Our data show that the C′-c method can increase the potential range of rocks in which it is possible to estimate kinematic vorticity. Our results also show that the C′-c method can be negatively affected by microscale kinematic flow partitioning, highlighting the importance of a detailed microstructure characterization of the studied rocks before any estimate of kinematic vorticity is attempted.

The effect of hydrous mineral content on competitive strain localization mechanisms in felsic granulites

This study investigates the grain-scale mechanisms that facilitated strain localization in the lower crust within the East Athabasca Mylonite Triangle of northern Saskatchewan, Canada. There, two distinct lithotectonic units, the Upper Deck and Northwestern Domain, are juxtaposed across a km-scale shear zone that has been previously interpreted to record thrust and/or normal-sense displacement. Our study reports new quartz c-axis opening angle, Ti-in-biotite and Ti-in-quartz geothermometry and amphibole-plagioclase geothermobarometry within syn-kinematic phases indicating that both units were deformed and metamorphosed at upper amphibolite to granulite facies (~725 °C and ~0.8 GPa). While sharing similar conditions, the dominant mechanisms of deformation across the two units are strongly linked to modal proportion of syn-kinematic hydrated mineral phases. Detailed microstructural and crystallographic preferred orientation (CPO) analyses reveals that specimens poor in hydrated minerals (<10%) deformed mainly by dislocation creep and dynamic recrystallization allowing it to develop a strong quartz CPO, while in specimens rich in hydrated minerals (>10%) pressure solution and (re)precipitation processes enhanced phase mixing and promoted grain boundary sliding resulting in weakly developed or absent quartz CPO. These results indicate a strong mineralogical and fluid control on deformation and, therefore, which strain softening process (es) may dominate in lower crustal shear zones.

Spatio-temporal challenges in dating orogen-scale shear zones: The case of the Himalayan Main Central thrust

The spatial extent and dynamic evolution of orogen-scale shear zones renders the determination of the timing and duration of ductile deformation and displacement challenging. The Himalayan Main Central thrust (MCT) is one of the most widely studied thrust-sense shear zones in the world, particularly regarding its timing, and yet ‘when was the MCT active’ remains an outstanding question. New and existing pressure-temperature-deformation-time (P-T -d-t) data from three well-exposed structural sections of the MCT in western Nepal, which together represent > 100 km in the direction of tectonic transport, are compared to examine how spatio-temporal variations in recorded shear complicate the answer to this question. Metamorphic P-T data, quartz microstructures and quartz crystallographic preferred orientation suggest that all three MCT transects underwent similar shear conditions and kinematic behaviour. However, each shear-zone transect reached peak T conditions during different windows of time. Furthermore, 40Ar/39Ar thermochronology on white mica reveals contrasting timing for the end of ductile deformation. When combined with published monazite and zircon dates, our data indicate that the record of ductile deformation on the MCT is diachronous in the transport direction and a single shear zone transect is thus not representative of the timing and duration of deformation on the MCT even for a narrow along-strike position. This study represents the first attempt to track ductile deformation in the direction of thrust propagation at this spatial extent on an orogen-scale thrust system. These results clearly point to the need to study the temporal evolution of shear zones over significant distances in the direction of tectonic transport, acknowledge potential gaps in the petrochronological record, and incorporate P-T -d data when using shear zone timing and duration results to make regional conclusions.

CPO and kinematic analysis of the Bitou S-tectonites (Central Cameroon shear zone): AMS and EBSD investigations

The development of foliation is not always associated with mineral stretching lineation in deformed rocks. Sometimes, S-tectonites only display foliation with no mineral stretching lineation, and it becomes a real challenge to perform kinematic analysis, i.e., to define the XZ section of the ellipsoid. In this study, we present in a portion of the Central Cameroon Shear Zone (CCSZ) (the Bitou biotite gneiss mylonite), the usefulness of anisotropy of magnetic susceptibility (AMS) studies to identify the three principal axes of the AMS ellipsoid (K1 ≤ K2 ≤ K3), equivalents respectively to the principal axes of the strain ellipsoid (X ≤ Y ≤ Z). The K1K3 plane of the AMS ellipsoid is equivalent to the XZ section of the strain ellipsoid. The fabrics developed in the studied mylonitised biotite gneiss strike ENE–WSW to E–W with steep dips for the mylonitic and magnetic foliations and moderate plunges for the magnetic lineation. The rock is paramagnetic. The AMS ellipsoids are mostly of oblate shape, while the quartz c-axis pattern is typical of non-coaxial flow. This implies that deformation partitioning took place during mylonitisation. Quartz crystallographic preferred orientation (CPO) measured using electron backscatter diffraction reveals the activation of prism <a> slip, implying that the mylonitisation occurs under moderate temperature conditions (450°C < T < 550°C). Microstructures observed in the K1K3 section of the AMS ellipsoid and CPO of the quartz c-axis indicate sinistral top-to-SW sense of shear. These results support the shear senses of movement that earlier studies in the CCSZ have emphasised and are assumed to be related to the early syn-D2 and D3 events of the Pan-African tectonic dated at ca. 613–585 Ma.

Deformation conditions during syn-convergent extension along the Cordillera Blanca shear zone, Peru

AbstractStrain localization across the brittle-ductile transition is a fundamental process in accommodating tectonic movement in the mid-crust. The tectonically active Cordillera Blanca shear zone (CBSZ), a ∼200-km-long normal-sense shear zone situated within the footwall of a discrete syn-convergent extensional fault in the Peruvian Andes, is an excellent field laboratory to explore this transition. Field and microscopic observations indicate consistent top-down-to-the-southwest sense of shear and a sequence of tectonites ranging from undeformed granodiorite through mylonite and ultimately fault breccia along the detachment.Using microstructural analysis, two-feldspar and Ti-in-quartz (TitaniQ) thermometry, recrystallized quartz paleopiezometry, and analysis of quartz crystallographic preferred orientations, we evaluate the deformation conditions and mechanisms in quartz and feldspar across the CBSZ. Deformation temperatures derived from asymmetric strain-induced myrmekite in a subset of tectonite samples are 410 ± 30 to 470 ± 36 °C, consistent with TitaniQ temperatures of 450 ± 60 to 490 ± 33 °C and temperatures &gt;400 °C estimated from microstructural criteria. Brittle fabrics overprint ductile fabrics within ∼150 m of the detachment that indicate that deformation continued to lower-temperature (∼280–400 °C) and/or higher-strain-rate conditions prior to the onset of pervasive brittle deformation. Initial deformation occurred via high-temperature fracturing and dissolution-precipitation in feldspar. Continued subsolidus deformation resulted in either layering of mylonites into monophase quartz and fine-grained polyphase domains oriented subparallel to macroscopic foliation or the interconnection of recrystallized quartz networks oriented obliquely to macroscopic foliation. The transition to quartz-controlled rheology occurred at temperatures near ∼500 °C and at a differential stress of ∼16.5 MPa. Deformation within the CBSZ occurred predominantly above ∼400 °C and at stresses up to ∼71.4 MPa prior to the onset of brittle deformation.

Quartz CPO and kinematic analysis in deformed rocks devoid of visible stretching lineations: An integrated AMS and EBSD investigation

Kinematic analysis of deformed rocks requires study of structures in a section parallel to the stretching lineation and perpendicular to the foliation, i.e., XZ section of the strain ellipsoid. However, presence of stretching lineation is more of an exception rather than a norm in naturally deformed rocks. This raises challenges for kinematic studies. In this study the authors advocate use of Anisotropy of Magnetic Susceptibility (AMS) to identify the three principal axes of AMS ellipsoid (K1>K2>K3), which are equated with principal axes of the strain ellipsoid (X > Y > Z). This helps identify X direction of strain ellipsoid in deformed rocks that lack visible stretching lineations. It is proposed that a section prepared parallel to the K1K3 plane of a deformed rock can be treated as equivalent to XZ section of strain ellipsoid, thus making it possible to perform kinematic studies in a rock that did not have stretching lineation. Use of this method is demonstrated on massive metavolcanic rocks of Hutti region (Dharwar Craton, South India), which is replete with quartz veins that contain gold. Crystallographic Preferred Orientation (CPO) of quartz veins is measured using SEM-EBSD studies of thin sections prepared parallel to K1K3 plane of the host rock. Obtained data help recognize the presence of down-dip sense of movement as well as strain partitioning within veins, which are aspects that were not recognized in earlier studies. It is concluded that integration of AMS and SEM-EBSD studies will play an important role in kinematic studies in future.

Kinematic evolution of the Mbakop Pan–African granitoids (western Cameroon domain): An integrated AMS and EBSD approach

This study integrates anisotropy of magnetic susceptibility, microstructural and crystallographic preferred orientation (CPO) data from the Mbakop granitic pluton (MGP; Pan-African age) in order to decipher its kinematic evolution. The MGP lies close to NE-SW branch of Central Cameroon Shear Zone (CCSZ) and is emplaced in gneissic basement. High mean magnetic susceptibility and presence of multi–domain magnetite are recorded. Quartz CPO measured using Electron Backscatter diffraction reveals dominance of rhomb <a>, prism <a> and prism <c> slip in different samples, which is consistent with microstructures developed under upper greenschist/amphibolite facies conditions. Quartz CPO along with other kinematic indicators (feldspar porphyroclasts/mineral fish) indicate non-coaxial deformation was important during tectonic evolution of the MGP. Contrasting sense of shear is recorded implying multi-stage mylonitization in the Western Cameroon Domain. Top-towards-south sense of shear is related to regional D2 deformation (613–585 Ma), while top-towards-north is related to D3 (585–540 Ma). The magnetic fabric in MGP records D3. The obliquity between mean orientation of magnetic foliation (N12°E) and the NE-SW CCSZ branch (N48°E) gives kinematic vorticity number of 0.95. This indicates dominantly simple shear with a minor pure shear component. It is concluded that regional transpression was important during MGP emplacement.

Determining the reference frame for kinematic analysis in S-tectonites using AMS

In this paper a methodology based on anisotropy of magnetic susceptibility (AMS) measurements is presented to identify XZ section of the strain ellipsoid in order to perform kinematic analysis in S-tectonites, which are rocks that lack stretching lineation on the foliation plane. The similarity between orientations of three principal axes of the AMS ellipsoid (K 1 >K 2 >K 3 ) and strain ellipsoid (X>Y>Z) is demonstrated by discussing the case of a mylonite (LS-tectonite) that contains both foliation and stretching lineation. Subsequently, the case of S-tectonite from the peninsular gneiss (south India) is discussed, where the K 1 K 3 plane of AMS ellipsoid is equated with XZ section of strain ellipsoid. Quartz crystallographic preferred orientations are investigated using SEM-EBSD studies in thin section prepared parallel to the K 1 K 3 plane. Quartz c-axis patterns characteristic of rhomb slip and indicating top-to-325° sense of shear are recorded. This supports the movement sense inferred by earlier researchers from northern parts of the sampled locality. Thus the usefulness of AMS in determining the reference frame for kinematic studies in S-tectonites is documented.