Crystallographic Preferred Orientation Data Research Articles

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.

Transformation weakening: Diffusion creep in eclogites as a result of interaction of mineral reactions and deformation

The deformation of eclogites and the driving forces for their fabric development are an important topic, potentially allowing to determine deformation rates and stresses in subduction zones, where the greatest number of large earthquakes occurs. Here, fabric studies of grain size and shape, texture, and chemical composition from two locations of Variscan and Alpine eclogites are presented. All samples show a well-developed crystallographic preferred orientation (CPO) of omphacite with a strong maximum of [001] in the lineation direction and a weaker maximum of poles to (010) normal to foliation. Garnet shows no systematic CPO. Anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde (in terms of pressure change) P,T-path. The individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal misorientations. Chemical, microstructural, and CPO data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction. Dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures. The dominant deformation is inferred to be diffusion creep, where dissolution of material took place in reacting mafic phases (plagioclase, pyroxene) and precipitation took place in the form of new eclogite facies minerals (omphacite, garnet, zoisite). This type of diffusion creep deformation represents a transformation process involving both, deformation and metamorphic reactions. It is emphasized that the weakening is directly connected to the transformation and therefore transient. The weakening facilitates diffusion creep deformation of otherwise strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. The results imply low stresses during the deformation of eclogite blocks in subduction zones. These results can be applied to other rock types, too.

Structural setting of a transpressive shear zone: insights from geological mapping, quartz petrofabric and kinematic vorticity analysis in NE Sardinia (Italy)

AbstractThe Posada–Asinara Line is a crustal-scale transpressive shear zone affecting the Variscan basement in northern Sardinia during Late Carboniferous time. We investigated a structural transect of the Posada–Asinara Line (Baronie) with the aid of geological mapping and structural analysis. N-verging F2 isoclinal folds with associated mylonitic foliation (S2) are the main deformation features developed during the Posada–Asinara Line activity (D2). The mineral assemblages and microstructures suggest that the Posada–Asinara Line was affected by a retrograde metamorphic path. This is also confirmed by quartz microstructures, where subgrain rotation recrystallization superimposes on grain boundary migration recrystallization. Crystallographic preferred orientation data, obtained using electron backscatter diffraction, allowed analysis of quartz slip systems and estimation of the deformation temperature, vorticity of flow and rheological parameters (flow stress and strain rate) during the Posada–Asinara Line activity. Quartz deformation temperatures of 400 ± 50 °C have been estimated along a transect perpendicular to the Posada–Asinara Line, in agreement with the syn-kinematic post-metamorphic peak mineral assemblages and the late microstructures of quartz. The D2 phase can be subdivided in two events: an early D2early phase, related to the metamorphic peak and low kinematic vorticity (pure shear dominated), and a late D2late phase characterized by a lower metamorphic grade and an increased kinematic vorticity (simple shear dominated). Palaeopiezometry and strain rate estimates associated with the D2late deformation event showed an intensity gradient increasing towards the core of the shear zone. The D2early deformation developed under peak temperature conditions, while the D2late event was active at shallower structural levels.

Crystallographic preferred orientations in eclogites – A review

During the last decades, processes taking place in subduction zones have been a topic of extensive debate. There are many models on a tectonic scale, but knowledge of deformation at microscopic levels is essential for an understanding of the processes involved. Crystallographic preferred orientation (CPO) investigations of minerals in eclogites yield the possibility to study deformation in the crustal part of subducted oceanic lithosphere, as well as subducted continental units containing basic intrusions, which are frequently exhumed during continental collision. From CPO data of omphacite and garnet, as well as other constituent minerals in eclogites, conclusions on stress and strain at depth in currently active subduction zones can be drawn, as the pressure-temperature and strain path of the now exhumed rocks can be unraveled. This contribution provides an overview of CPO studies of eclogites concerning slip systems, deformation, strain and other possible CPO forming mechanisms in all major constituent mineral phases in eclogites and the implications of these microstructural data on subduction zone processes are discussed. Nevertheless, there are still many open questions and future research is essential.

Coalescing microstructure and fabric transitions with AMS data in deformed limestone: Implications on deformation kinematics

The microstructures of the heterogeneously deformed limestones of the Kolhan Group, Singhbhum Craton (SC), eastern India are investigated to study deformation mechanism, grain size and shape preferred orientation (SPO) development. The texture of the deformed rocks is analyzed by Electron Back-Scattered Diffraction (EBSD) in order to determine crystallographic preferred orientation (CPO) development in calcite grains. The c-axis pattern in protomylonite and mylonite define girdle patterns developed perpendicular to local shear planes showing monoclinic symmetry. The latter is interpreted in terms of a top-to-south shear, which is consistent with the observed microstructural and fabric data from the deformed rocks. The Anisotropy of Magnetic Susceptibility (AMS) studies are performed on 22 heterogeneously strained limestone specimens to document strain variation within the Kolhan rocks. The field fabric, SPO data measured from calcite and mica grains (κcal and κmi) and calcite CPO data are compared with observed variation in AMS fabric intensity (Pj) across the Kolhan basin. Integration of these data sets help to establish presence of large internal thrust planes within the Kolhan basin, which form a part of the regional foreland vergent thrust system developed along the northern margin of the SC.

Controls and implications of anisotropy across a strain gradient within granodiorite, Serifos, Greece

Microtextural analyses including electron backscatter diffraction (EBSD), anisotropy of magnetic susceptibility (AMS) and the anisotropy of anhysteretic remanent magnetization (AARM) have been applied across a shear zone within a Miocene granodioritic intrusion on Serifos island, Western Cyclades, Greece. The shear zone records progressive deformation through a ductile to brittle transition. One of the consequences of anisotropic crust is the variation in seismic wave velocity with the direction of propagation, which is largely controlled by the crystallographic preferred orientation (CPO) of anisotropic minerals such as micas and feldspars. CPO data was collected using EBSD and seismic properties were calculated using Voigt-Reuss-Hill averaging of the single minerals' elastic stiffness tensor. The magnetic fabric is used to characterize weakly defined tectonic fabric, thus complementing the CPO data. The structure of rocks exhibiting weak magmatic fabric, as defined by magnetic anisotropy and CPO, appears to reflect the elliptical shape of the pluton. The SW-directed kinematics of the Western Cyclades produced an ENE-striking, SSE dipping low-angle shear zone in the granodiorite that imparted a signature onto the magnetic data of foliated and mylonitic samples, creating a composite fabric from the magmatic emplacement fabric and the top-to-SW directed shear. As for the seismic properties, P-wave anisotropy reaches 11.8% and is consistently controlled by the CPO of plagioclase feldspar, with the direction of fast P-wave propagation subperpendicular to foliation and the direction of slow P-wave propagation subparallel to foliation. The magnetic fabric, coupled with the tectonic fabric inferred from CPO data, gives unique insight into the character of anisotropic minerals in a syntectonic intrusion. This is especially useful for weakly strained rocks where the lineation and foliation (given by the magnetic fabric) can be related to the seismic anisotropy derived from the CPO data. Given the anisotropy caused by the preferred orientation fabrics of major rock-forming minerals, the microstructural analysis of anisotropic minerals is fundamental for incorporating seismic anisotropy into large-scale geophysical models.

Microstructures and seismic properties of south Patagonian mantle xenoliths (Gobernador Gregores and Pali Aike)

The subcontinental lithospheric mantle evolves through time due to tectonic events and processes as static recrystallization and melt percolation. To constrain the extent of these processes in the South Patagonian subcontinental mantle lithosphere we performed a microstructural and Electron Backscattered Diffraction (EBSD) study of a suite of 35 peridotite xenoliths brought to the surface by Plio-Pleistocene alkaline volcanic rocks from Gobernador Gregores and Pali Aike. All samples show a well-developed olivine and pyroxene crystallographic preferred orientation (CPO), consistent with deformation by dislocation creep with dominant activation of [100]{0kl} in olivine. The coarse granular or tabular textures and the low density of intracrystalline deformation features indicate that deformation was followed by annealing under static conditions. The xenoliths also show microstructural evidence of multiple episodes of reactive melt percolation. Neither annealing nor melt percolation erased the olivine CPO, which has [010]-fiber, [100]-fiber and orthorhombic patterns in Pali Aike xenoliths and essentially [010]-fiber and orthorhombic patterns in Gobernador Gregores xenoliths. Seismic properties calculated based on the CPO and modal compositions are, however, rather homogeneous, with fast S-wave polarization and P-wave propagation parallel to the [100] olivine axis. The variation in the olivine CPO solely changes the minimum S-wave birefringence direction, which is normal to the foliation for axial-[010] olivine CPO. Average samples for the two localities, obtained by adding up the individual samples CPO data in a common reference frame, show, however, a 'normal' upper mantle anisotropy with a maximum S-wave birrenfringence of ca. 5% at high angle to the both the maximum [010] and [100] axes concentrations, that is in the foliation, but normal to the lineation, and a minimum birefringence at low angle to the [100] maximum, that is parallel to the lineation.

Kinematic and temporal relationships between parallel fold hinge lines and stretching lineations: A microstructural and crystallographic preferred orientation approach

Folds with hinge lines subparallel to stretching lineation are common features in shear zones and represent extreme cases of fold modification. However, their microstructures and in particular their kinematic significance remain poorly understood. Here we describe the meso- and micro-scale structures and crystallographic preferred orientation (CPO) of quartz and muscovite from a metric-scale shear-modified fold from Plattjen, Saas Fee, Western Internal Alps. The studied fold is of class-2 and the samples collected around the fold have essentially the same microstructures. Quartz [c]-axes plotted in the tectonic coordinate system exhibit an apparent reverse obliquity on opposing limbs. Muscovite fabrics are controlled essentially by the orientation of {001} planes. If the CPO data are plotted in a common reference frame (e.g. either fold or geographical), all samples exhibit essentially the same patterns for both minerals. In particular, relative to the fold axis and axial surface, the reversal of quartz CPO obliquity on opposing limbs is no longer evident. The small obliquity (~ 15°) between the intersection lineation of the two planes of {001} of muscovite and the fold axis results in the apparent reversal of shear sense on opposite limbs when viewing the CPO related to the macroscopic fabric. The homogeneity of the microstructures and CPOs of quartz and muscovite across the fold support the interpretation that the fold developed during simultaneous activation of heterogeneous and homogeneous simple shear, with the former responsible for fold development and the latter responsible for fold rotation and the overwhelming of the heterogeneous deformation. These processes led to the observed parallelism between fold hinge line and local stretching lineation.

Microstructural analysis of anhydrite rocks from the Triassic Evaporites, Umbria-Marche Apennines, Central Italy: An insight into deformation mechanisms and possible slip systems

Anhydrite-rich layers within foreland fold and thrust belts are frequently the weakest horizon of the sequence. Characterizing the microstructural textures of anhydrite is therefore important for interpreting the larger scale deformation history of these rocks. The microstructure of naturally deformed anhydrite from the Triassic Evaporites of the Umbria-Marche Apennines, Italy has been analyzed using electron backscatter diffraction (EBSD). This technique has been used to measure the misorientation across distorted grains and the crystallographic preferred orientation (CPO) of the samples. The reference frame of the standard space group Cmcm was used to represent the structure of anhydrite during this analysis which is different to that used in previous studies. Some grains show significant intracrystalline distortions (4 –18° misorientation). Lines of measurements across distortions showed a dispersion of crystallographic data around a clear misorientation axis [010]. These data are combined with the trace orientations of distortions to confirm that the (100)[001] slip systems previously recognized also best explains grain distortions. These interpretations are based on, and consistent with, a tilt boundary model containing edge dislocations. CPO data are also interpreted as a likely (100)[001] slip system, as has previously been recognized. A high degree of twinning within many of the anhydrite grains apparently has little effect on the crystallographic textures observed.

Anisotropic seismic properties of the upper mantle beneath the Torre Alfina area (Northern Apennines, Central Italy)

Central Italy is an active tectonic area that has been recently studied by several regional mantle, Pn and SKS, studies which revealed the presence of a strong regional anisotropy. In this paper, we present the first petrophysical results on the only mantle xenoliths from Central Italy, which place new constraints on the upper mantle structures of this region. The Torre Alfina mantle xenoliths are very small in size, from few millimetres to about 1.5 cm. They are mainly dunites and harzburgites, with subordinate lherzolites and wehrlites. Since olivine and spinel are always present, they should have crystallised in the spinel-bearing lherzolite field. Their mineralogical composition is ol+spl±opx±cpx. Both olivines and pyroxenes are present as porphyroclasts and as neoblasts. The xenoliths show different degrees of recrystallization. Geothermobarometry on these xenoliths give a temperature range of 1040±40 °C and a pressure estimate of about 1.5 GPa, corresponding to 50 to 60 km depth. Previous seismic studies have estimated the Moho to be at 20 to 25 km in this region, hence the xenoliths come from a hot mantle, probably asthenospheric, below a lithosphere of about 25 to 40 km in thickness below the Moho. We measure the crystallographic preferred orientation (CPO) of olivines and pyroxenes using a SEM and the Electron Back Scattered Diffraction (EBSD) technique. The CPO shows all three axes of olivine are tightly clustered: [100] axis is typically more tightly clustered than [010] and [001] is the most widely distributed axis. The fabric strength expressed by the integral J index, varies from 4.5 to 25.9, and decreases with the degree of recrystallization. We use CPO data to calculate anisotropic seismic properties of the xenoliths. They are very homogenous and probably statistically representative of the mantle below the Torre Alfina area. Vp ranges from 8.4 to 9.1 km/s, Vs 1 from 4.8 to 5.0 km/s. The seismic anisotropy is more variable; AVp ranges from 9.8% to 19.3% and AVs from 7.3% to 13.4%. The majority of the xenoliths display an orthorhombic seismic symmetry, but xenoliths with a transverse isotropic behaviour have also been observed. We consider four geodynamic models for the source region of the xenoliths (extension, shear, upwelling, slab tilted), defined by different orientations of the structural reference frame, and we calculated for each model the variation of the seismic properties with temperature, pressure and volume fraction of orthopyroxene. After comparing this variation of calculated seismic parameters with seismic observations from the region, we form the hypothesis that the xenoliths come from either an extensional tectonic zone (lineation X and foliation plane XY horizontal) or transcurrent shear zone (lineation X horizontal and foliation plane XY vertical) and that the mantle beneath Torre Alfina is composed by 70% olivine and 30% orthopyroxene forming an anisotropic layer of about 160 or 110 km in thickness, respectively.

Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes

Caledonian eclogite facies shear zones developed from Grenvillian garnet granulite facies anorthosites and gabbros in the Bergen Arcs of western Norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation (CPO) in lower continental crust. Field relations on the island of Holsnøy show that the eclogites formed locally from granulite facies rocks by progressive development of: (1) eclogite adjacent to fractures; (2) eclogite in discrete shear zones (> 2 m thick); (3) eclogite breccia consisting of >80% well‐foliated eclogite that wraps around rotated granulite blocks; and (4) anastomosing, subparallel, eclogite facies shear zones 30–100 m thick continuous over distances > 1 km within the granulite terrane. These shear zones deformed under eclogite facies conditions at an estimated temperature of 670 ± 50°C and a minimum pressure of 1460 MPa, which corresponds to depths of >55 km in the continental crust. Detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite/garnet‐rich and kyanite/zoisite‐rich layers. A consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite. The CPO of omphacite, determined from five‐axis universal stage measurements, shows a strong b‐axis maximum normal to foliation, and a c‐axis girdle within the foliation plane with weak maxima parallel to the lineation direction. These patterns are consistent with deformation of omphacite by slip parallel to [001] and suggest glide along (010). The lineation and CPO data reveal a consistent sense of shear zone movement, although the displacement was small. Localized faulting of high‐grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust. Field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth. Deformation of granulite and partial conversion to eclogite, such as is exposed on Holsnøy Island, may be an orogenic‐scale process in the lowermost crust of collisional orogens.