Lattice-preferred Orientation Of Olivine Research Articles

The influence of water and LPO on the initiation and evolution of mantle shear zones

We present data from the Josephine Peridotite (SW Oregon, USA) that constrain the underlying physical processes responsible for the initiation of shear localization and the evolution of ductile shear zones in Earth's mantle. Field measurements of narrow (2–60m wide) ductile shear zones in harzburgite were used to construct strain profiles, which have maximum shear strains ranging from γ=5.25 to γ>20. Measurements of pyroxene water concentrations from harzburgite samples within and immediately adjacent to the shear zones indicate that gradients in water concentration exist on a 10–100m scale, even after exhumation. Water concentration measurements are correlated with olivine lattice-preferred orientation (LPO), corroborating experimental results on the influence of water on slip system activity. Using empirical olivine flow laws and the diffusivity of water in olivine, we model initiation of a ductile shear zone through localized water weakening. We demonstrate that this mechanism can readily generate spatial perturbations in both effective viscosity and strain. However this model is not able to reproduce both the observed shear strain gradients and water concentration data from the Josephine shear zones. We evaluate other plausible localization mechanisms, which may amplify this initial strain perturbation. The most relevant at these conditions is the development of viscous anisotropy associated with the evolution of olivine LPO. Using recent experimental results, we demonstrate that progressive rotation of olivine LPO into the shear plane enhances deformation within a shear zone. We conclude that feedback between at least two microphysical processes is needed to account for observed outcrop-scale shear localization.

Natural type-C olivine fabrics in garnet peridotites in North Qaidam UHP collision belt, NW China

Water is known to change the lattice-preferred orientation (LPO) of olivine, which significantly affects seismic anisotropy in the Earth's upper mantle. Research into the LPO of olivine in the deep interior of the Earth has been limited due to inadequate specimens. We report both the water-induced LPOs of olivine and the presence of large quantities of water inside olivine, enstatite, and garnet in garnet peridotites from the North Qaidam ultrahigh-pressure (UHP) collision belt in NW China. We show that the [001] axis of olivine is aligned subparallel to the lineation and that the [100] axis is strongly aligned subnormal to the foliation. This alignment is a known feature of type-C LPO of olivine formed experimentally under water-rich conditions (≥700ppmH/Si) at high pressure and temperature. Enstatite possessed an LPO with the [001] axis aligned parallel to the lineation and the [100] axis aligned normal to the foliation. FTIR analysis of this specimen revealed that olivine contained concentrations of water up to 1130±50ppmH/Si in clean areas, whereas olivine, enstatite, and garnet contained considerably more water, i.e., 2600±100ppmH/Si, 5000±100ppmH/Si, and 21,000±200ppmH/Si, respectively, when exsolved inclusions were visible. Confocal micro-Raman spectroscopy of these exsolved inclusions revealed that they were composed of hornblende and amphiboles. Straight dislocations were also commonly observed in olivine and are characteristic of olivine that had been experimentally deformed under hydrous conditions. These observations suggest that the type-C LPO of olivine in the North Qaidam UHP belt formed under water-rich conditions.

Distribution functions for anisotropic electrical resistivities due to hydrogen diffusivity in aligned peridotite and their application to the lithosphere–asthenosphere boundary

The scaling properties of electrical anisotropy due to hydrogen diffusivity are investigated by representing the lattice-preferred-orientation (LPO) of a naturally-deformed peridotite xenolith using resistor network models. Whereas hydrogen diffusivities are approximately 40 (or 20) times faster along the a-axis of olivine single crystals than along their c-axes (or b-axes), a highly strained aggregate produces bulk electrical anisotropy factors of less than 4 (i.e. the more conductive direction can be expected to be less than 4 times more conductive than the less conductive direction) with 95% confidence. The bulk resistivities in both directions lie between those of the a-axes and the c-axes of olivine single crystals. Resistivity-depth profiles derived by assuming self-diffusion of hydrogen in a highly strained mantle indicate that for a realistic geotherm, electrical anisotropy arising due to hydrogen diffusivity within the thermal lithosphere is unlikely to be resolvable using magnetotelluric (MT) sounding, because the increase in resistivity with depth in the more resistive direction obscures the anisotropy. This is a consequence of the inherent lack of vertical resolution – i.e. inability to resolve vertical layering – of the MT method. Therefore, electrical anisotropy due to hydrogen self-diffusion in mantle minerals aligned within the lithosphere by palaeoflow is unresolvable using MT methods. However, a resolvable signature of such electrical anisotropy should be detectable, if present, in the adiabatic region below the thermal lithosphere or thermal boundary layer (TBL), where the geotherm is shallower. Since the mechanical lid or plate is generally assumed to be thinner than the TBL, this raises the question whether any anisotropic signature detected by MT can be attributed to LPO of olivine as a consequence of plate motion. If so, what does this tell us about heat transfer and strain dissipation in the enigmatic region between the mechanical lid and the TBL?

The Sklené garnet peridotite: petrology, geochemistry, and structure of a mantle-derived boudin in Moldanubian granulite

*Corresponding author The Moldanubian Sklene garnet peridotite, which preserves mineralogical, geochemical, and structural evidence for a protracted mantle evolution, is a small boudin (~1 m) enclosed by granulite of the Gfohl Assemblage. The peridotite has a porphyroclastic texture, in which large garnet and smaller olivine and pyroxene porphyroclasts (Stage 1) reside in a fine-grained, recrystallized matrix of olivine, pyroxene, and pargasitic amphibole (Stage 2). Garnet is surrounded by kelyphite, consisting of an outer zone of pargasite and an inner zone of spinel and pyroxene (Stage 3). Lattice-preferred orientations (LPO) for olivine, orthopyroxene, and clinopyroxene were measured by electron backscatter diffraction. The LPO of olivine (mostly Stage 2) has an [010] maximum parallel to the Z fabric direction and [100] and [001] girdles in the X–Y fabric plane. Both orthopyroxene and clinopyroxene have [010] maxima parallel to Z and [001] girdles in the X–Y plane. The Sklene peridotite is slightly depleted in incompatible major and minor elements relative to primitive mantle. Whole-rock REE contents are 0.3 to 0.8 times that of primitive mantle, with a convex upward pattern, and Rb, Th, U, Pb, and Sr show positive anomalies in an extended element plot. Among the highly siderophile elements, Os, Ir, Ru, and Pt are unfractionated, but Pd and Re are significantly depleted. A Sm-Nd garnet-whole rock-clinopyroxene isochron yields a cooling age of 338 ± 13 Ma, and e Nd and ( 87 Sr/ 86 Sr) i values for clinopyroxene (at 335 Ma) are +4.5 and 0.7035, respectively. By comparison with other Moldanubian garnet peridotites in the Czech Republic, the Sklene peridotite likely represents a fragment of subcontinental lithospheric mantle. It experienced early partial fusion and depletion of incompatible elements at moderate pressure in the spinel stability field, followed by transport to high-pressure conditions at 1230 oC and 50 kbar, where the Stage 1 garnet-bearing assemblage was stabilized. Subsequently, the depleted peridotite was metasomatized by transient, primitive basaltic melts, with the addition of Rb, Th, U, Pb, and Sr. Dynamic recrystallization, possibly with a progressive transition to grain size sensitive creep, occurred during Stage 2 at ~915 oC and 24 kbar in a flattening-dominated, transpressive deformation regime. This event occurred prior to incorporation of the peridotite in granulite and may reflect the early stages of convergence between Brunia and Moldanubia. The static recrystallization and local growth of spinel within kelyphite at 900 oC and 17–21 kbar (Stage 3) probably took place after entrainment in granulite and was likely isofacial with granulite peak metamorphism.

맨틀상부에서 암석의 변형 및 광물의 격자선호방향(LPO) 형성과 지진파 비등방성과의 연계성

감람석은 맨틀 상부의 주요 광물이고, 탄성적으로 매우 비균질하다. 맨틀암석이 고압고온에서 변형될 때, 이러한 감람석이 배열되어 격자선호방향(LPO, Lattice Preferred Orientation)이 형성된다. 감람석의 격자선호방향은 압력, 응력, 그리고 감람석 내부의 물(OH)의 양에 따라 크게 영향을 받을 수 있다. 이 논문에서는 물, 응력, 그리고 압력이 어떻게 감람석의 격자선호방향(LPO)의 형성에 영향을 주는지에 대해 그동안 발표된 논문들을 리뷰했고, 자연 맨틀암석에서 발견된 감람석의 격자선호방향들을 요약했으며, 감람석의 격자선호방향과 맨틀상부에서 나타나는 지진파의 비등방성과의 연계성에 대해 토의하였다. 그리고 최근에 맨틀암석에서 발견된 사방휘석의 네 가지 격자선호방향들을 기술하였다. Olivine is a dominant mineral in the upper mantle and is elastically very anisotropic. When olivine is deformed under stress at high pressure and high temperature, lattice preferred orientation (LPO) is formed. It is known that the LPO of olivine is affected by water, stress, and pressure. In this paper, I reviewed the papers dealing with the effects of water, stress, and pressure on the LPO of olivine, summarized the papers on the LPOs of olivine in natural mantle rocks, and discussed its implications for seismic anisotropy in the upper mantle. In addition, I also described four types of LPOs of orthopyroxene recently found in natural spinel lherzolite.

The influence of deformation history on the interpretation of seismic anisotropy

Seismic anisotropy in the mantle is primarily caused by the lattice‐preferred orientation (LPO) of olivine, which forms and evolves in response to progressive plastic deformation. The style of LPO and its rate of evolution depend on the effects of physiochemical conditions on the microphysics of deformation and recovery. Here, we integrate results from laboratory experiments and naturally deformed shear zones to evaluate how variability in the evolution of olivine LPO modifies existing strategies for interpreting seismic anisotropy. We concentrate on two particular effects. First, we show that olivine A‐ and E‐type LPOs evolve with opposite senses of rotation, producing a wide spectrum of angular relationships between the orientation of fast seismic wave propagation and the direction of flow. Second, we show that the presence of a pre‐existing LPO introduces a lag between changes in deformation kinematics and alignment of the seismic fast direction with the shear direction. The combination of these two factors affects the inference of deformation kinematics from seismic anisotropy and the interpretation of shear wave splitting parameters.

Ductile deformation and rheology of sub-continental mantle in a hot collisional orogeny: Example from the Bohemian Massif

Fabric patterns of strongly serpentinized peridotite were determined using eigenvector analysis and eigenvalue classification of lattice preferred orientation of olivine and orthopyroxene. This approach has been applied to a rootless fold-shaped body of mylonitized spinel to garnet peridotite surrounded by fine-grained and partially retrogressed ky–kfs granulite. The EBSD data show either axial [010] or [100](0kl) pattern, both characteristic for ‘dry’ slip systems. The former pattern occurs predominantly along the inner margin and southern limb while the latter is mainly developed in the hinge of the fold shaped body. Foliations and lineations deciphered from the LPO data suggest that the [100](0kl) pattern reflects constrictional deformation (prolate strain ellipsoid) in the hinge of the peridotite fold while the axial [010] pattern reflects pure flattening (oblate strain ellipsoid) inherited from the period of emplacement of the peridotite sheet in the crust. Similarity in finite strain pattern of peridotite and surrounding granulites indicates their common thermal and mechanical evolution during folding. The petrology and structural data result in a model of burial of peridotite below thickened crustal root, its exhumation and folding. The burial stage is associated with prograde metamorphism resulting in a coarse-grained microstructure and development of spinel and garnet zones. The emplacement of peridotite into lower crustal granulites occurred along a shear zone associated with grain size reduction in both peridotite and granulite and rapid cooling of mylonitized peridotite to the ambient temperatures of lower crust. Further ascent to mid-crustal levels occurred within the vertical granulite channel. Final fold shape of the peridotite developed during subsequent indentation of the weak vertically anisotropic crust by the adjacent continental promontory. The degree of mechanical coupling between folded peridotite and granulite in mid-crustal levels is estimated using comparison of studied microstructures with experimental data.

Fabric development in the mantle section of a paleotransform fault and its effect on ophiolite obduction, New Caledonia

The Bogota Peninsula shear zone has been interpreted as a paleotransform fault in the mantle section of the New Caledonia ophiolite. New, detailed field measurements document the rotation of foliation, lineation, and pyroxenite dikes across a 50-km-wide region. Deformation intensity recorded by folding and boudinage of dikes increases toward a central, 3-km-wide mylonitic zone. We used several additional methods to characterize fabric patterns across the shear zone. The shape-preferred orientation of orthopyroxene grains, computed from outcrop tracings, closely parallels field fabrics, with increased alignment and flattening near the center of the shear zone. The lattice-preferred orientations of olivine are consistent with high-temperature fabrics; the a axes within the mylonitic core were used to constrain the orientation of shear zone boundaries. Seismic anisotropy calculations, based on the lattice-preferred orientation of olivine, indicate 5%–11% shear-wave anisotropies, with increased values in the center of the shear zone. The magnetic silicate fabric in the rocks, determined from anisotropy of magnetic susceptibility techniques, broadly matches field fabrics but provides less consistent information across the shear zone than other fabric methods. This suite of field and laboratory data provides a unique and detailed view of strain and fabric patterns across a shear zone in oceanic mantle lithosphere. Because the primary mantle fabrics seem to be related to the present distribution of ophiolitic rocks in New Caledonia, we propose that ophiolite obduction and Neogene extension may have been controlled by preexisting fabrics and structures in the oceanic lithosphere.

Organized melt, seismic anisotropy, and plate boundary lubrication

Based on observations in both the laboratory and the Earth, we develop the hypothesis that plate boundaries are lubricated by networks of melt‐rich shear zones. Such lubrication would serve to reduce effective strength and focus deformation at plates boundaries. This idea emerges from two sets of observations: (1) stress‐driven melt segregation and organization in experimentally deformed mantle rocks and (2) seismic anisotropy patterns as observed at three divergent plate boundaries (the Ethiopian Rift, the Reykjanes Ridge, and the East Pacific Rise). In all three tectonic settings, the magnitude of anisotropy is greatest at the probable locations of the lithosphere‐asthenosphere boundary within the plate boundary (“marginal LAB”). Seismic anisotropy in the upper mantle is controlled by the lattice preferred orientation (LPO) of predominant olivine and the alignment of melt structures. The observed patterns of anisotropy are controlled by the dip angle of the marginal LAB. When steeply dipping, shear wave splitting in vertically traveling waves (e.g., SKS phases) is most sensitive to the alignment of melt, and surface waves should reveal faster Rayleigh wave velocities than Love wave velocities (VSV > VSH). When shallowly dipping, shear wave splitting in vertically traveling body waves is controlled by olivine LPO, and surface waves show faster Love wave velocities than Rayleigh wave velocities (VSV < VSH). The formation of melt‐rich networks by stress‐driven segregation should be most effective where strain rates are highest. These melt‐lubricated shear zones will reduce effective viscosity relative to the direct extrapolation of viscosity values derived from laboratory creep experiments on homogenous samples. A composite model of anisotropic seismic properties is developed to test the hypothesis that melt segregates along the LAB, incorporating olivine fabrics with oriented and segregated melt over a range of length scales. This model is applied to observations from the three example plate boundaries, leaving the reader to speculate on the implications for interpretation of anisotropy patterns at other geodynamic settings.

Lattice preferred orientation, water content, and seismic anisotropy of orthopyroxene

Lattice preferred orientation (LPO) and seismic anisotropy of orthopyroxene (enstatite) in mantle xenoliths from Spitsbergen, Svalbard, near the Arctic, are studied. LPOs of enstatite were determined using electron backscattered diffraction (EBSD). We found four types of LPOs of orthopyroxene and defined them as type-AC, -AB, -BC, and -ABC. Type-AC LPO of orthopyroxene is defined as (100) plane aligned subparallel to foliation and [001] axis aligned subparallel to lineation. Type-AB LPO is defined as (100) plane aligned subparallel to foliation and [010] axis aligned subparallel to lineation. Type-BC LPO is defined as (010) plane aligned subparallel to foliation and [001] axis aligned subparallel to lineation. Type-ABC LPO is defined as both (100) and (010) planes aligned subparallel to foliation with a girdle distribution of both [100] and [010] axes normal to lineation and [001] axis aligned subparallel to lineation. We report for the first time the type-AB, -BC, and -ABC LPO of orthopyroxene. We found that the LPO pattern has a correlation with the content of orthopyroxene in the specimen. Nicolet 6700 FTIR (Fourier transformation infrared) study of enstatite showed that type-AC LPO was observed mostly in the samples of enstatite with low water content. It is found that the strength of the LPO of enstatite decreases with increasing water content and has a correlation with the strength of the LPO of olivine: the stronger the LPO of enstatite, the stronger the LPO of olivine. Seismic anisotropy of enstatite was smaller than that of olivine in the same specimen.

Anisotropic stratification beneath Africa from joint inversion of SKS and P receiver functions

The analysis of rock anisotropy revealed by seismic waves provides fundamental constraints on stress‐strain field in the lithosphere and asthenosphere. Nevertheless, the anisotropic models resolved for the crust and the upper mantle using seismic waves sometimes show substantial discrepancies depending on the type of data analyzed. In particular, at several permanent stations located in Africa, previous studies revealed that the observations of SKS splitting are accounted for by models with a single and homogeneous anisotropic layer whereas 3‐D tomographic models derived from surface waves exhibit clear anisotropic stratification. Here we tackle the issue of depth‐dependent anisotropy by performing joint inversion of receiver functions (RF) and SKS waveforms at four permanent broadband stations along the East African Rift System (EARS) and also on the Congo Craton. For three out of the four stations studied, stratified models allow for the best fit of the data. The vertical variations in the anisotropic pattern show interesting correlations with changes in the thermomechanical state of the mantle associated with the lithosphere‐asthenosphere transition and with the presence of hot mantle beneath the Afar region and beneath the EARS branches that surround the Tanzanian Craton. Our interpretation is consistent with the conclusion of earlier studies that suggest that beneath individual stations, multiple sources of anisotropy, chiefly olivine lattice preferred orientation and melt pocket shape preferred orientation in our case, exist at different depths. Our study further emphasizes that multiple layers of anisotropy must often be considered to obtain realistic models of the crust and upper mantle.

Melt‐induced seismic anisotropy and magma assisted rifting in Ethiopia: Evidence from surface waves

The East African rift in Ethiopia is unique worldwide because it captures the final stages of transition from continental rifting to seafloor spreading. A recent study there has shown that magma intrusion plays an important role during the final stages of continental breakup, but the mechanism by which it is incorporated into the extending plate remains ambiguous: wide‐angle seismic data and complementary geophysical tools such as gravity analysis are not strongly sensitive to the geometry of subsurface melt intrusions. Studies of shear wave splitting in near‐vertical SKS phases beneath the transitional Main Ethiopian Rift (MER) provide strong and consistent evidence for a rift‐parallel fast anisotropic direction. However, it is difficult to discriminate between oriented melt pocket (OMP) and lattice preferred orientation (LPO) causes of anisotropy based on SKS study alone. The speeds of horizontally propagating Love (SH) and Rayleigh (SV) waves vary in similar fashions with azimuth for LPO‐ and OMP‐induced anisotropy, but their relative change is distinctive for each mechanism. This diagnostic is exploited by studying the propagation of surface waves from a suite of azimuths across the MER. Anisotropy is roughly perpendicular to the absolute plate motion direction, thus ruling out anisotropy due to the slowly moving African Plate. Instead, three mechanisms for anisotropy act beneath the MER: periodic thin layering of seismically fast and slow material in the uppermost ∼10 km, OMP between ∼20–75 km depth, and olivine LPO in the upper mantle beneath. The results are explained best by a model in which low aspect ratio melt inclusions (dykes and veins) are being intruded into an extending plate during late stage breakup. The observations from Ethiopia join a growing body of evidence from rifts and passive margins worldwide that shows magma intrusion plays an important role in accommodating extension without marked crustal thinning.

Fabric development in cm-scale shear zones in ultramafic rocks, Red Hills, New Zealand

The Red Hills ultramafic massif, the South Island, New Zealand, contains a suite of cm-scale shear zones that are composed of dunite, pyroxenite, and olivine websterite. Offset on these shear zones was measured using displacement of cross-cutting dikes or distinct compositional foliation layers in the host rock, and is interpreted to occur by dominantly simple shear. Shear zones contain microstructures similar to the host rock, including coarse grain sizes (> 1 mm) and dominantly polygonal grains. The olivine lattice preferred orientation (LPO) in the host rocks is consistent with the (010)[100] slip system, active at high-temperature, dry upper mantle conditions. Within the shear zones, the olivine LPO, when plotted relative to the shear zone foliation and lineation, suggests (010)[001] slip. The shear zone LPO is typically more poorly clustered than in host rock samples and double maxima are observed in some samples, with the second maxima suggesting (010)[100] slip. When olivine LPO from both host rock and shear zone samples is plotted within the same (geographic) reference frame, it is apparent that the shear zone LPO retains evidence for the dominant LPO of the host rocks. The LPO in the shear zone rocks reflects the pre-existing fabric; the poorly clustered data and double maxima reflect the changes that developed during shear zone deformation. These rocks demonstrate the influence of pre-existing LPO on textures formed during deformation and the need to consider the LPO of deformed rocks within different reference frames.

マントル科学研究の最近の動向−特に再肥沃化，レオロジー，オフィオライト問題：第５回国際レルゾライト会議報告

We introduce major scientific topics on mantle-derived material sciences, “refertilization”, “rheology” and “ophiolite problems”, discussed in the Fifth International Orogenic Lherzolite Conference held at Oregon, USA, in 2008. Structural and geochemical relationships between harzburgite and lherzolite in the most orogenic peridotite massifs suggest that clinopyroxene-rich lherzolites do not represent a pristine mantle, but are formed via “refertilization”, addition of basaltic components from infiltrated melts to a refractory harzburgitic-dunitic protolith which is a residue of melt extraction from the pristine mantle. Refertilization has been recognized as “impregnation” which was used to explain the origin of plagioclase-bearing peridotites. On the other hand, “mantle metasomatism” and/or “melt-mantle interactions” caused by infiltrated melts/fluids have been also documented in many peridotite massifs and mantle xenoliths to explain the observed chemical diversities of peridotites. We emphasized here that “refertilization” and other commonly used models such as mantle metasomatism and melt-mantle interaction do not provide enough constraints for better understanding magmatic processes in the upper mantle. They represent end member processes of more general open processes, “open-system reaction model”, which can constrain chemical compositions and rate of influx fluids/melts, and rate of separation of the formed melt on the basis of geochemical observation on peridotites. We need further investigation on physical mechanisms that controls rate of influx and separation of fluids/melts, involved heat transfer, the origin of involved fluids/melts, and kinetic processes of reactions between peridotites and influx fluids/melts. Relationships between rock deformation and lattice preferred orientation (LPO) of mantle minerals have been now growing interests because of their relevance to seismic anisotropy in the mantle. Natural observations on LPO of olivine as a function of shear strain observed in peridotite massifs are apparently in agreement with the experimental observations. In natural systems, a pre-existing LPO, however, could result in higher strain for LPO aligning with the shear direction during deformation than in experimental systems. Ultramafic pseudotachylytes, product of solidification from melts caused by shear-induced melting during earthquakes at mantle depth, are now reported in several peridotite massifs. Further studies on ultramafic pseudotachylytes will provide new insights into earthquake nucleation at mantle depth. The origin of ophiolite, in mid-ocean ridge, supra-subduction zone, or back-arc setting, is still controversial. Recent studies on the Oman ophiolite by Japanese scientific groups revealed that the northern part of the Oman ophiolite was formed at a fast-spreading ridge and was later modified by subduction-related magmatism. Tectonic switching from a mid-oceanic ridge to an island arc has been observed in many ophiolites, and is therefore essential in the formation and/or exhumation of ophiolites. These studies will provide information on evolution of crust-mantle sections of island-arc, where direct sampling is difficult.

沈み込み帯での地震波異方性とかんらん石の格子選択配向

Seismic anisotropy is a powerful tool for detecting the geometry and style of deformation in the Earth's interior, as it primarily reflects the deformation-induced preferred orientation of anisotropic crystals. Although seismic anisotropy in the oceanic upper mantle is generally parallel to the plate motion, the propagation direction of the fast shear-wave is oriented subparallel to the trench axis in several subduction zones. In this study, the distribution of seismic anisotropy in the mantle wedge is inferred from deformation mechanism: a lattice-preferred orientation and seismic anisotropy are generated by deformation via dislocation creep, but not by diffusion creep or frictional sliding. Based on the thermal structure beneath northeast Japan, deformation throughout most of the mantle wedge is inferred to be controlled by diffusion creep, and the region of dislocation creep is limited to a thin layer of 10-20 km thickness within a region of relatively high stress and low temperature located above the subducting plate and beneath the island arc crust. The variation of the seismic anisotropy in subduction systems is probably due to the change of active slip-system in olivine as a consequence of chemical and physical properties in the mantle wedge.

Strain coupling between upper mantle and lower crust: natural example from the Běstvina granulite body, Bohemian Massif

AbstractStrain patterns within mantle rocks and surrounding coarse‐grained felsic granulites from the Kutná Hora Crystalline Complex in the Variscan Bohemian Massif have been studied in order to assess their strain coupling. The studied rock association occurs within low‐strain domains surrounded by fine‐grained granulite and migmatite. The Doubrava peridotite contains closely spaced and steeply dipping layers of garnet clinopyroxenite, which are parallel to the NE–SW‐striking, high‐temperature foliation in nearby granulites, while the Úhrov peridotite lacks such layering. The Spačice eclogite is not associated with peridotite and shows upright folds of alternating coarse‐ and fine‐grained varieties bearing NE–SW‐striking axial planes. Electron back‐scattered diffraction measurements revealed full strain coupling between clinopyroxenites and coarse‐grained granulites in the S1 fabric that is superposed on the S0 fabric preserved in peridotites. The B‐type olivine lattice preferred orientation (LPO) characterizes the S0 fabric in peridotites and its reworking is strongly controlled by the presence of macroscopic clinopyroxenite layering. The S1 in clinopyroxenites and coarse‐grained granulites is associated with the LS‐type clinopyroxene LPO and prism <c> slip in quartz respectively. While the S1 fabric in these rock types is accompanied invariably by a sub‐vertical stretching lineation, the S1 fabric developed in reworked Úhrov peridotite is associated with strongly planar axial (010) type of olivine LPO. The peridotites with the S0 fabric are interpreted to be relicts of a fore‐arc mantle wedge hydrated to a various extent above the Saxothuringian subduction zone. The prograde metamorphism recorded in peridotites and eclogites occurred presumably during mantle wedge flow and was reaching UHP conditions. Strain coupling in the S1 fabric between clinopyroxenites and granulites at Doubrava and upright folding of eclogites at Spačice document a link between tectonic and magmatic processes during orogenic thickening, coeval with intrusions of the arc‐related calcalkaline suites of the Central Bohemian Plutonic Complex (c. 360–345 Ma). Juxtaposition of peridotites and granulites could be explained by a rheological heterogeneity connected to the development of clinopyroxenite layering in the upper mantle and a previously published model of a lithospheric‐scale transpressional arc system. It invokes vertical shearing along NE–SW trending, sub‐vertical foliations in the upper mantle that could have led to an emplacement of mantle bodies into the granulitized, orogenic root in the sub‐arc region. Clearly, such a transpressional arc system could represent an important pathway for an emplacement of deep‐seated rocks in the orogenic lower crust.

Microstructural and Rheological Evolution of a Mantle Shear Zone

We conducted a microstructural study of a high-strain mantle shear zone from the Josephine Peridotite, SW Oregon, USA. The goal of this study is to understand how microstructural evolution at large strains leads to transitions in rheological behavior. The shear zone we investigated exhibits higher strain and greater localization than previously studied shear zones in the Josephine Peridotite. The margins of the shear zone have a homogeneous microstructure, characterized by moderately strong olivine fabrics, fairly weak orthopyroxene fabrics, and grain sizes of 2^3 mm. The highly deformed samples from the center of the shear zone display two distinct microstructural domainsca relatively coarse-grained domain (� 550m) that contains only olivine and a finer-grained domain (� 250m) that contains both olivine and orthopyroxene. The coarse-grained domain has a strong E-type olivine lattice-preferred orientation (LPO). Within the fine-grained domain the olivine LPO is also E-type, but significantly weaker.The E-type fabrics are rotated slightly past the shear plane, providing the first field-based confirmation of similar experimental observations. The presence of E-type fabrics, which form in the presence of moderate quantities of water, also highlights the potential importance of water to shear zone evolution.The orthopyroxene in the fine-grained domains has no LPO, suggesting that a transition to grain-size sensitive deformation occurred. The microstructural transition in orthopyroxene may have resulted in a marked weakening of the rock, suggesting that orthopyroxene plays a critical role in shear localization. These samples provide a crucial microstructural link between moderately localized shear zones and highly deformed ultramylonites.

Fabric kinematics of the ultrahigh-pressure metamorphic rocks from the main borehole of the Chinese Continental Scientific Drilling Project: Implications for continental subduction and exhumation

The 5158-m-deep main borehole of the Chinese Continental Scientific Drilling Project (CCSD-MH) penetrated granitic gneisses, paragneisses, eclogites, retrograde eclogites, amphibolites and ultramafic rocks in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. The CCSD-MH consists of four petro-structural units separated by three SE-dipping ductile shear zones DFa (835–1280 m), DFb (2010–2280 m) and DFc (2920–3225 m), which are correspondent with the regional shear zones in the northern Sulu UHP supracrustal zone. Using the electron backscatter diffraction (EBSD) technique, we investigated the lattice-preferred orientations (LPOs) of omphacite, diopside and quartz in core samples from the CCSD-MH. Omphacite from eclogites and diopside from garnet pyroxenites display very strong LPOs, which are characterized by the maximum concentration of [001]-axes parallel to the lineation and (010)-poles normal to the foliation. Quartz in para- and granitic mylonites/gneisses from the shear zones DFa, DFb and DFc developed multiple slip systems. 40Ar/ 39Ar dating of biotite in para- and granitic gneisses from the CCSD-MH yields 223–202 Ma, which constrains the formation ages of the quartz high-temperature prism slip systems {m}<a> and {m}[c]. The asymmetric LPOs of omphacite, diopside, olivine and quartz with respect to the structural frame reveal three deformation phases in the Sulu terrane. In the Middle Triassic, the northward subduction of the Yangtze plate to depths > 100 km produced a top-to-the-south shear sense in LPOs of omphacite, diopside and olivine, and a nearly N–S-striking foliation and a subhorizontal N–S-trending lineation in eclogites and ultramafic rocks. In the Late Triassic, the UHP rocks were exhumed to the lower crust and quartz developed high-temperature slip systems with a top-to-the-NW shear sense, which is consistent with the regional SE-dipping foliation and SE-plunging lineation in the ductile shear zones. In the Cretaceous the UHP rocks were exhumed to the middle crust when the migmatization and granitic intrusion formed a NE-striking antiform structure. As a result, the activation of quartz low-temperature basal slip (0001)<a> is characterized by a top-to-the-SE shear sense in the south, but a top-to-the-NW shear sense in the north.

Deformation fabrics of olivine in Val Malenco peridotite found in Italy and implications for the seismic anisotropy in the upper mantle

An exhumed huge body of ultramafic rocks is preserved in Val Malenco, Italy. In this study, petrofabrics and deformation microstructures of olivine in the Val Malenco peridotite were examined, and new observations of lattice preferred orientation (LPO) of olivine such as type-B and type-E LPO in the peridotite are reported. Three types, depending on the grain size, of olivine fabrics were observed: type-A fabric exhibiting the (010)[100] slip system in the large relic grain area, and type-B and type-E fabrics exhibiting the (010)[001] and (001)[100] slip systems, respectively, in the widely distributed recrystallized-grain area. The slips occurred mainly on (010)[001], and the (001)[100] slip system has only rarely been reported to be the dominant slip system of olivine found in natural rocks. Further, seismic velocities and seismic anisotropies were calculated for the three types of fabrics found. The seismic anisotropies in the upper mantle can be attributed to the type-B, type-E, or type-A LPOs of olivine. Moreover, the anomaly in seismic low velocity in the mantle wedge can be attributed to the serpentinization as well as the mixed fabrics of the type-A, type-B, and type-E.

Seismic anisotropy in the wedge above the Philippine Sea slab beneath Kanto and southwest Japan derived from shear wave splitting

We conduct shear wave splitting measurements on waveform data from the Hi-net and the broadband F-net seismic stations in Kanto and SW Japan generated by shallow and intermediate-depth earthquakes occurring in the subducting Philippine Sea and Pacific slabs. We obtain 1115 shear wave splitting parameter pairs. The results are divided into those from the shallow (depth < 50 km) and the deep (depth > 50 km) events. The deep events beneath Kanto are further divided into PHS1 and PHS2 (upper and lower planes of the double seismic zone in the Philippine Sea slab, respectively), PAC1 and PAC2 (western and eastern Pacific slab, respectively) events. The results from the shallow events represent the crustal anisotropy, and their fast directions are more or less aligned in the σ Hmax directions, implying that the anisotropy is produced by the alignment of the vertical cracks in the crust induced by the compressive stresses. In Kanto, Kii Peninsula and Kyushu regions, the results from the deep events suggest a contribution from the mantle wedge anisotropy. Events from all groups beneath Kanto show NW, NE and EW fast directions. This complex pattern seems to be produced by the corner flows induced by both the WNW PAC plate subduction and the oblique NNW PHS slab subduction with the associated olivine lattice-preferred orientations (LPOs), and the anisotropy frozen in the PHS slab. The deep events beneath Kii Peninsula show NE and NW fast directions and may be produced by the corner flow produced by the NNW PHS slab subduction with the associated olivine LPOs. The NE directions might also be produced by the segregated melts in the thin layers parallel to the PHS slab subduction. The deep events beneath N Kyushu show NNW fast directions, which may result from the southeastward flow in the upper mantle inferred from the stresses in the upper plate. Results from the deep events beneath middle-south Kyushu show dominantly E–W fast directions, in both the fore- and back-arcs. They may be produced by the corner flow of the westward PHS slab subduction with the olivine LPOs. Because the source regions with multiple fast directions are not resolved in this study, further detailed analyses of shear wave splitting are necessary for a better understanding of the stress state, the induced mantle flow, and the melt-segregation processes.