Preferred Mineral Orientation Research Articles

PyDRex: Predicting crystallographic preferred orientation in peridotites under steady-state and time-dependent strain

Abstract Crystallographic preferred orientation (CPO) of peridotite minerals is frequently invoked to explain the widespread dependence of seismic wave speed on propagation direction in Earth’s mantle — a property known as seismic anisotropy. As established by rock mechanics experiments, CPO constitutes a direct signature of past and ongoing strain regimes experienced by rocks during mantle flow. Therefore, an improved understanding of CPO generation promises to yield valuable information on the rheology and corresponding deformation mechanisms activated through mantle dynamics. Simulating CPO in geodynamical models is computationally challenging and has often been restricted to steady-state mantle flows. However, within Earth’s vigorously convecting mantle the steady-state assumption is questionable, thus motivating the need to couple CPO simulations with time-evolving mantle flow models. Here, we present a new Python implementation of the D-Rex CPO model, called PyDRex, which predicts salient features of mineral grain size and orientation evolution whilst providing a well-documented, user-friendly interface that supports flexible coupling to geodynamical modelling frameworks. PyDRex also packages numerous post-processing routines for strain analysis and visualisation of grain orientation distributions. We provide a set of benchmark simulations based on previous D-Rex implementations that validate PyDRex and demonstrate sensitivities to model parameters for both steady-state and time-dependent flows. Analysis of benchmark results highlights the role of dynamic recrystallisation in controlling competing grain growth in both the softest and hardest crystallographic orientations. When employing a commonly used value for the grain boundary mobility parameter (M* = 125), we also find that transient CPO textures are generally not well resolved if crystals are represented by fewer than 5000 ‘grains’ (weighted orientation samples) — a configuration rarely employed in most previously published studies. Furthermore, kinematic corner-flow models suggest that CPO produced at mid-ocean ridges has a non-linear dependence on depth, which implies that even ostensibly simple mantle flows can result in complex distributions of seismic anisotropy. Our analyses motivate further experimental calibration of parameters controlling dynamic recrystallisation and potential improvements to the numerical treatment of subgrain nucleation.

Crystallographic Preferred Orientation of Phase D at High Pressure and Temperature: Implications for Seismic Anisotropy in the Mid‐Mantle

AbstractSeismic anisotropy has been widely observed in the lower mantle transition zone and the uppermost lower mantle near several subducting slabs, which is typically attributed to the crystallographic preferred orientation (CPO) of constituent minerals. In this study, well‐controlled uniaxial and simple shear deformation experiments were conducted on Mg‐endmember phase D and Al‐bearing phase D aggregates at 20 GPa and 500–1,000°C. Strong (0001) fabrics were observed in the uniaxially compressed samples. The CPO from all experiments, including extension and simple shear, indicated that the most active slip systems in phase D were <110>(0001) and <100>(0001), with a dominance of <110>(0001). The calculated seismic velocity suggests that the deformed phase D generates positive radial seismic anisotropy (VSH > VSV) in both subhorizontal shearing and subvertical compression, causing significant shear‐wave splitting time. Our results suggest that the observed seismic anisotropy in the mid‐mantle in several cold subduction zones can be well explained by the presence of deformed phase D.

Drilling- and coring-induced artefacts in poorly indurated clays

Macroscopic and microscopic drilling-induced deformation structures were documented in cores of Ypresian Ieper Group and Rupelian Boom Formation clays using macro-polished slabs and polarizing microscopy of thin sections. The cores were recovered in north Belgium from a depth of a few hundred metres by pushing a steel barrel with a cutting shoe into the non-indurated clays that are in a ductile to brittle transition state. The outer border of the clay cores was bent downwards due to the pushing action. The geometry of the drilling-induced fractures can be described as vertically stacked axisymmetrical conical fractures complexed by a general concave and undulating shape, and by fractures splaying from other fractures. At the microscopic scale (2D), the fractures are micro-faults that produce a breccia texture and which consist of thin planes, lines in 2D, of orientated clay minerals. The preferred orientation of these clay minerals in the fractures point to shear movement and explain the shiny appearance of these slickenside-like fractures when viewed in 3D. The micro-faults produce clay domains tilted with respect to each other. Recognizing exclusively drilling-induced structures helps in the selection of cores and core fragments for further laboratory testing, and may be of help in distinguishing them from regional faults or fracture systems in homogeneous clays. Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive

Seismic anisotropy and geodynamics of the East Japan subduction zone

Seismic anisotropy in the East Japan arc has been extensively investigated by conducting shear-wave splitting measurements, receiver-function analyses, and tomographic inversions of body-wave travel times and surface-wave dispersion data, which have provided a wealth of information on dynamic processes associated with active subduction of the Pacific plate. Measuring shear-wave splitting is popular and effective to detect seismic anisotropy, but it has poor depth resolution. This problem has been overcome by conducting 3-D anisotropic tomography, which has high-resolution in both lateral and vertical directions. Both P and S wave anisotropies are revealed in the crust, which are caused by alignment or preferred orientation of crustal minerals and stress-induced microcracks related to active faults. Trench-normal fast-velocity directions (FVDs) of azimuthal anisotropy are revealed in the back-arc mantle wedge, reflecting subduction-driven convection there. Trench-parallel FVDs appear in the forearc mantle wedge under the land area, which may reflect deformation that results in B-type olivine fabric. The forearc mantle wedge offshore may lack anisotropy, suggesting that it is stagnant and decoupled from the subducting slab and does not participate in the viscous flow, in sharp contrast with the rest of the mantle wedge. The most significant findings of the body-wave anisotropic tomography are its constraints on the slab anisotropy. The subducting Pacific slab exhibits mainly trench-parallel FVDs, which reflect shape-preferred orientation of crystals and cracks related to normal faults produced in the outer-rise area before the plate subduction, overprinting the fossil anisotropy that the Pacific plate gained when it was produced at the mid-ocean ridge. Trench-parallel intraslab fast velocity planes of anisotropy intersect the slab upper surface at high angles (∼45–90°), reflecting aligned hydrated faults in the slab. Ruptures of the hydrated faults in the upper part of the slab may cause large intraslab earthquakes (M ≥7.0) that take place frequently beneath the forearc area. Trench-normal FVDs also appear in the subslab mantle, which may reflect asthenospheric shear deformation associated with the overlying slab subduction.

The electrical conductivity of granite: The role of hydrous accessory minerals and the structure water in major minerals

The electrical conductivity of granitic rocks (granitoids) is crucial to understand the rock composition, physiochemical state and structure of the crust. However, it is not clear how the conductivity of granitoids is affected by the incorporated water in major minerals and hydrous accessory minerals. Here, we measured the electrical conductivity of two natural granites and two synthetic aggregates compositionally similar to the granites at temperatures of 573–1273 K and pressures of either ambient pressure or 1 GPa. No mineral preferred orientation was observed for the natural samples. The hydrous accessory minerals in the samples, including muscovite, biotite and amphibole, start to dehydrate at about 700 K at ambient pressure, but barely affect the bulk conductivity. The increase in conductivity of granite by more than half orders of magnitude at 1273 K and 1GPa indicated the onset of melting due to the dehydration of biotite. The conductivities of synthetic aggregates are higher than the granite samples, suggesting that the structural water in feldspars could influence, to a moderate degree, the conductivity of granite. We combined the effect of modal composition and structural water on the conductivity of granite and derived a maximal electrical conductivity of a granitoid at different geological settings. The maximal conductivity can be 0.004 ± 0.0001 S/m at 60 km under orogens, over one order of magnitude higher than a cratonic crust. The maximal conductivity provides an upper limit for interpreting the electrical anomalies in the crust.

Rock Magnetism and Magnetic Fabric Study of the Icelandite and Rhyodacite Long Volcanic Sequence at Mauna Kuwale, Wai’anae Volcano, Oahu, Hawaii, USA

In order to understand further the emplacement (i.e., volcanic growth) of 22 Icelandite and 3 Rhyodacite cooling units in one of the long volcanic sequences known as Mauna Kuwale of the Wai’anae volcano (ca. 3.3 Ma), Oahu Hawaii we have conducted appropriate rock magnetic experiments described below as well as anisotropy of magnetic susceptibility (AMS) studies of such 25 units. We have undertaken rock magnetic investigations such as continuous and partial thermo-magnetic cycles of low field magnetic susceptibility versus temperature dependence, (k-T) curves experiments. We classified the k-T heating-cooling dependence of susceptibility in three groups A, B and C. Type A: yielded two components of titano-magnetite with a predominat Ti rich phase and occasionally a relevant magnetite component phase. Type B: samples are characterized by Ti poor magnetites. Magnetite dominates as the main magnetic carrier. Type C: k-T curves show one single phase of titanomagnetite, and Ti poor magnetite. The coercivity or remanence, determined by back field magnetization is always <60 mT, which suggest the predominance of magnetic components of low coercivity, like magnetite. Usually, two coercivity components are identified in the specimens. In addition we also conducted magnetic granulometry analyses on 27 specimens to determine the domain state of the flows. The ratio of hysteresis parameters (Mr/Mrs versus Hcr/Hc) show that overall samples fall in the Pseudo-Single Domain (PSD) region with high values of Mr/Mrs and very low values of Hcr/Hc. Only two samples from cooling units 17 and specially 22 show a Single Domain (SD) magnetic behavior and a sample from one unit approaches the SD-MD mixture region. We measured the magnetic susceptibility of all cooling units and we found out that in all analyzed units the magnetic susceptibility is low 13.7 ± 8.8 (10−3 SI). Magnetic anisotropy/magnetic fabric is used as a tool in rock fabric analyses to investigate the preferred orientation of magnetic minerals in rocks. Magnetic anisotropy is low on all (measured) flows from the Icelandite cooling units from 1 to 17 (mean P’ = 1.010), but becomes noticeably distinct and high in rhyodacite cooling units 23, 24 and 25 (mean P’ = 1.074). Four units show a magnetic fabric with k3 axes vertical to sub-vertical which may be denoted as normal for the horizontal to sub horizontal units. Two Icelandite cooling units display oblate shapes and two other cooling units triaxial shapes. K1 axes are horizontal but point in different directions, i.e., NE and NW. Remaining cooling units show different magnetic fabric. Units 17, 23, 24 and 25, despite important variations in anisotropy (low for units 25 and high for units 23 and 24) and shape of ellipsoid (oblate in cooling unit 23, prolate in 24 and triaxial in 25) the k3 axes show the same orientation, SW to SSW dipping around 45° and a very steady magnetic lineation azimuth NW nearly horizontal to sub horizontal. The magnetic mineralogy and magnetic fabric indicate that both the Icelandite and Rhyodacite cooling units the magmatic evolution during the shield stage of the entire Wai’anae volcano and that such growth was not affected by tectonic deformation.

Influence of Melt‐Peridotite Interactions on Deformation and Seismic Properties of the Upper Mantle Beneath a Destroyed Craton: A Case Study of the Damaping Peridotites From the North China Craton

AbstractExtensive melt‐peridotite interactions had been documented in the lithospheric mantle beneath the North China Craton (NCC), a prime example of destroyed cratons in the world. Yet the impacts of melt‐peridotite interactions on the deformation and seismic anisotropy of the NCC upper mantle remain unclear. Here we studied in detail the microstructure, crystallographic preferred orientation (CPO) of minerals, and seismic properties of 26 peridotite xenoliths from the Damaping area of the NCC. The studied samples can be classified into two groups: weakly to nonfoliated and strongly foliated. Petrographic and microstructural observations suggest that multiple melt‐peridotite interactions and at least two stages of deformation had influenced samples from both microstructural groups. Dislocation creep in response to a transpression deformation led to the [010]‐fiber type olivine CPOs in most samples. Variable degrees of annealing followed the last stage of deformation. Due to a higher degree of melt‐peridotite interactions, which had promoted nondislocation creep, and more extensive annealing, olivine and pyroxene in the strongly foliated samples developed weaker CPOs. This in turn leads to weaker maximum P wave propagation anisotropy and S wave polarization anisotropy for this microstructural group. Our data, therefore, cast light on a strong control of intensity of melt‐peridotite interactions on deformation and seismic properties of the upper mantle beneath the NCC. If foliation and lineation are vertical and horizontal, respectively, the measured SKS splitting parameters can be well explained by the “fossil” anisotropy frozen in the lithospheric mantle, with no need to invoke asthenospheric flow as a source of the anisotropy.

A tale of an orbicule in the eastern Gangdese belt of southern Tibet: Petrographic, geochemical, and submagmatic structural perspectives on its formation

Owing to their intriguing appearance and unusual occurrence, as well as their significance in addressing magmatic processes, orbicules have attracted attention from geologists all over the world. We discovered an orbicule-bearing boulder in the Linzhi region, near the Eastern Himalayan Syntaxis, which is the first report of orbicules in the Gangdese magmatic belt, southern Tibet. The orbicules are composed of dioritic shells around monzodioritic cores, in a dioritic matrix. They typically have a single shell, composed of elongate and radially oriented hornblendes, with interstitial plagioclase. Zircon laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating yields an indistinguishable age of ca. 28.8 Ma for both the orbicule core and matrix, indicating approximately coeval crystallization. Zircon Lu-Hf and whole-rock Sr-Nd isotopes reveal that the original magmas of both the orbicule matrix and orbicule core were relatively juvenile (ƐHf(t) = +2.2 to +6.0 for matrix and +1.2 to +3.9 for core; ƐNd(t) = −1.68 to −1.46 for matrix and −3.65 to −2.52 for core), similar to reported Oligocene−Miocene post-collisional adakites in the Gangdese belt. Zircon and hornblende geochemistry reveals that both the orbicule matrix and cores were generated from highly oxidized, high oxygen fugacity magmas. Hornblende composition further indicates that the original magmas of the matrix and the orbicule cores were water-rich, with H2O contents greater than 4 wt%. In combination with the whole-rock geochemistry, these results suggest that the orbicules were formed in a post-collisional setting, from magma generated by partial melting of the juvenile thickened lower crust of the Lhasa terrane triggered by convective removal of the Lhasa lithospheric root. The preferred orientation of euhedral igneous minerals in the matrix and the alignment and compaction of orbicules define a magmatic to submagmatic foliation. This, together with the subsolidus deformation microstructures exhibited by minerals within the orbicules, indicates that some flow or movement of orbicules took place under magmatic to submagmatic conditions, as the subsolidus plastic orbicules were jostled and transported within a hypersolidus liquid magma.

Magnetic fabrics reveal three-dimensional flow processes within elongate magma fingers at the margin of the Shonkin Sag laccolith (MT, USA)

Unravelling magma flow in ancient sheet intrusions is critical to understanding how magma pathways develop and feed volcanic eruptions. Analyzing the shape preferred orientation of minerals in intrusive rocks can provide information on magma flow, because crystals may align parallel to the primary flow direction. Anisotropy of magnetic susceptibility (AMS) is an established method to quantify such shape preferred orientations in igneous sheet intrusions with weak or cryptic fabrics. However, use of AMS data to characterize how magma flows within the individual building blocks of sheet intrusions (i.e., magma fingers and segments), hereafter referred to as elements, has received much less attention. Here we use a high spatial resolution sampling strategy to quantify the AMS fabric of the Eocene Shonkin Sag laccolith (Montana, USA) and associated elongate magma fingers. Our results suggest that magnetic fabrics across the main laccolith reflect sub-horizontal magma flow, and inferred flow directions are consistent with an underlying NE-SW striking feeder dyke. Within the magma fingers, we interpret systematic changes in magnetic fabric shape and orientation to reflect the interaction between competing forces occurring during finger-parallel magma flow (i.e., simple shear) and horizontal and vertical inflation (i.e., pure shear flattening). For example, we highlight how local crossflow of magma between coalesced fingers increases the complexity of magma flow kinematics and related fabrics. Despite these complexities, the AMS data in coalesced magma fingers maintain their internal flow- and inflation-related fabrics, which suggests that magma flow within the fingers remains channelized after coalescence. Given that many sheet intrusions consist of amalgamated elements, our findings highlight the need to carefully consider element distribution and sample locations when interpreting magma flow based on AMS measurements.

Seismic anisotropy and crustal deformation in the Satluj valley and adjoining region of northwest Himalaya revealed by the splitting analysis of Moho converted Ps phases

• Significant crustal anisotropy has been detected in the Satluj valley, NW Himalaya. • Fast polarization directions are parallel or sub-parallel to surface geological features . • Strength of anisotropy suggests middle and lower crust as a primary source. • Extension tectonics in Tethyan Himalaya causes E-W or NW-SE anisotropy. • Structural anisotropy is predominant in Lesser and Higher Himalaya. Seismic anisotropy in the crust beneath the Satluj valley and the adjoining region of the northwest Himalaya has been studied with the help of shear wave splitting analysis of P -to- S or Ps converted phases originating at the crust-mantle boundary. A total of 144 splitting parameters (Φ, δ t ) have been computed from 130 teleseismic earthquakes recorded by 13 broadband seismological stations spanning from the Lesser Himalaya to Tethyan Himalaya passing across the Satluj valley region. The predominant NW-SE fast polarization directions (FPDs) in the Lesser and Higher Himalaya follow the strike of surface geological features suggesting structural anisotropy. The NW-SE oriented FPDs in the Tethyan Himalaya fairly coincide with the regional extensional strain. The presence of such extensional strain within the crust might cause Lattice Preferred Orientation (LPO) of anisotropic minerals resulting in observed anisotropy. The large strength of anisotropy (δ t : 0.15–0.80 s) suggests a primary contribution of anisotropy from the middle and lower crust. These observations support the assumption that the deep crust in the study region has undergone widespread and relatively uniform strain in response to crustal shortening and E-W extension.

Seismic Anisotropy in the Lower Mantle Transition Zone Induced by Lattice Preferred Orientation of Akimotoite

AbstractSeismic anisotropy has been widely observed near the subducting slabs in the lower mantle transition zone (MTZ) and is often interpreted by the lattice preferred orientation (LPO) of constituent minerals. Akimotoite is one of the dominant minerals near the cold subducting slabs. Therefore, we conducted the well‐controlled uniaxial and shear deformation experiments on the MgSiO3 akimotoite aggregates at 21–23 GPa and 900–1300°C by using the D111‐type Kawai‐type multianvil apparatus. We observed strong LPOs and the most dominant slip system of akimotoite is suggested to be (0001). The elastic wave velocities of deformed samples were calculated to be strong azimuthal and polarization anisotropy with the velocities of horizontally polarized shear waves greater than that of vertically polarized shear waves for the horizontal mantle shearing. Our results provide important implications for the origin of observed seismic anisotropies and the mantle flow directions in the lower MTZ.

Laboratory measurements of ultrasonic wave velocities and anisotropy across a gold-hosting structure: A case study of the Thunderbox Gold Mine, Western Australia

Most lode gold deposits worldwide are associated with structures such as shear zones. Thanks to their capacity to couple resolution and depth of investigation, seismic methods can identify these indirect indicators of mineralization and help extend gold exploration targets to greater depths. Rocks from shear zones are usually seismically anisotropic. Seismic anisotropy is generally related to the intrinsic texture of the rock and the presence of cracks at depth, although the impact of the latter relative to the former fades out with increasing depth. Understanding the seismic impedance contrast between the rocks and determining seismic anisotropy in relation to the texture of the rock, and its evolution with depth (pressure) is therefore necessary to help interpret exploratory seismic surveys. To do so, we characterized in the laboratory 126 core samples representing different stages of alteration and different lithologies from the Thunderbox Gold Mine in Western Australia. Laboratory measurements consist of ultrasonic wave velocity at ambient and in situ pressure, density, mineralogy, texture analysis, and estimation of P-wave anisotropy by inverting the ultrasonic data. These experimental data were used as input in different rock physics models to calculate texture-derived velocities and to model the effects of seismic anisotropy on seismic reflectivity. Except for massive basalt samples that present a remarkable elastic impedance contrast with all lithologies and for the talc schist samples (shear zone samples) that present high seismic anisotropy values, the seismic reflectivity between the lithological units encountered at the Thunderbox Gold Mine is low. According to seismic reflectivity modelling, seismic anisotropy along the shear zone significantly affects the seismic reflectivity, with values of reflectivity reducing more with the increasing angle of seismic reflection than if the shear zone is considered as an isotropic interface. The good agreement between calculated texture-derived velocities with experimental measurements shows that the crystallographic preferred orientation of minerals of the shear zone samples is the main source of seismic anisotropy. This study seeks to improve the understanding of the seismic response across mineral deposits that are structurally controlled by shear zones.

Rock Magnetic Signature of Heterogeneities Across an Intraplate Basal Contact: An Example From the Northern Apennines

AbstractHeterogeneities in the magnetic signature along intraplate shear zones complicate their correlation with the physical processes that are involved in the geodynamic evolution of megathrusts. Isolating the preferred orientation of different magnetic minerals may provide insights into faulting processes, tectonics, and strain partitioning. Studies of exhumed analogs are fundamental to constrain variations in the magnetic properties with respect to a geodynamic context of intraplate shear zones. This study uses a combined statistical and magnetic approach to separate the contribution of coexisting petrofabrics to better interpret the heterogeneities in the magnetic signature. The main results indicate that there is a strong dependency among the variation in the magnetic properties and their anisotropy with the distance from the thrust faults and strain localization within the shear zone. Close to the main thrust, we observed a shear‐related fabric, indicating a high degree of non‐coaxial strain. Away from the thrust faults, the degree of anisotropy and the ellipsoids' oblateness gradually diminishes and subfabrics related to previous tectonic events or less intense deformation become dominant. Our observations also indicate strain decoupling across the basal thrust with dominant vertical uniaxial strain within the footwall. In contrast characterization of the anisotropy of magnetic remanence provides significant information on the variable response of different subpopulations of ferromagnetic minerals to shearing, indicating different degree of reworking or independent deformation processes.

An automatized XKS-splitting procedure for large data sets: Extension package for SplitRacer and application to the USArray

Recent technological advances have led to community wide use of large-scale seismic experiments which produce seismic data on previously impossible scales. Standard processing procedures thus require automatization to facilitate a fast and objective analysis of the data. Among these, XKS-splitting is an important tool to derive first insights into the Earth's deformation regimes at depth by studying seismic anisotropy. Most often, shear-wave splitting is interpreted to represent crystallographic preferred orientation (CPO) of mantle minerals like olivine as dominating feature and can thus be used as a proxy of mantle flow processes. Here, we introduce an addition to the MATLAB®-based SplitRacer tool box (Reiss and Rümpker 2017) which automatizes the entire XKS-splitting procedure. This is achieved by the automatization of 1) choosing a time window based on spectral analyses and 2) categorization of results based on three different XKS-splitting methods (energy minimization, rotation correlation and splitting intensity). This provides effective and objective results for splitting as well as null-measurement results. This extension allows to use SplitRacer without a graphical interface and introduces a bootstrapping statistics as error estimate of the single layer joint splitting method. The procedures are designed to allow a fast and more objective analysis of a vast amount of data, as produced by recent seismic deployments (e.g. USArray, AlpArray). We test this automatization by applying the analysis to the USArray data set, which has approximately 1900 stations with between two to fifteen years of data. We can reproduce the general pattern of the results from former studies with the more objective automatic analysis. Based on a joint-splitting approach, we approximate the splitting effect at individual stations by a single anisotropic layer. As we include null-measurements as well as a larger data set as previous studies, we can provide improved statistical evidence for these effective splitting parameters.

The onset of the right-lateral strike-slip setting recorded in magnetic fabrics of A-type granite plutons of the Ribeira belt (SE Brazil)

Strike-slip shear zones formed during continental collisions accommodate the convergence and indentation of rigid cratonic blocks. In high-temperature settings of deeply eroded orogens, such as the Neoproterozoic Ribeira belt in southeastern Brazil, the shear deformation is associated with syntectonic granitic magmatism. Anisotropy of magnetic susceptibility (AMS) and zircon U-Pb results from late A-type granites were used to investigate the timing of the strike-slip deformation. Some of these plutons show reddish colors, local rapakive textures and magnetic susceptibilities that are typically lower than older batholiths. Rock magnetic data and SEM analysis indicate that AMS of the A-type plutons is dominated by coarse magnetite and locally (titano)hematite, the latter formed by oxidizing hydrothermal fluids in the final stages of magmatic crystallization. The shape preferred orientation of mafic minerals and feldspars measured in the reddish granite facies , however, indicate that AMS reproduced the magmatic fabric defined by the silicates. Magnetic fabric orientations vary from concordant (Ouro Verde and Capão Bonito plutons) to partially concordant (Sguário pluton) with respect to the host rock shear zones, and define the transition from concentric magnetic lineations and foliations experienced by older I-type batholiths. These A-type plutons constrain the onset of the ductile transpressional deformation at ca. 595 Ma. The magmatism and shear deformation persisted up to 550 Ma in the western Ribeira belt when more brittle transtensional conditions prevailed. These results are consistent with the convergence and indentation of the Congo-São Francisco craton that partitioned the deformation across the orogen and promoted the belt lateral escape mediated by dextral shear zones.

Quantifying Intrinsic and Extrinsic Contributions to Radial Anisotropy in Tomographic Models

AbstractSeismic anisotropy in the Earth's mantle inferred from seismic observations is usually interpreted in terms of intrinsic anisotropy due to crystallographic preferred orientation (CPO) of minerals, or extrinsic anisotropy due to shape preferred orientation (SPO). The coexistence of both contributions confuses the origins of seismic anisotropy observed in tomographic models. It is thus essential to discriminate CPO from SPO. Homogenization/upscaling theory provides means to achieve this goal. It enables computing the effective elastic properties of a heterogeneous medium, as seen by long‐period waves. In this work, we investigate the effects of upscaling an intrinsically anisotropic and heterogeneous mantle. We show analytically in 1‐D that the observed radial anisotropy parameter is approximately the product of the intrinsic and the extrinsic components: This law is verified numerically in the case of a homogenized 2‐D marble cake model of the mantle in the presence of CPO obtained from a micro‐mechanical model of olivine deformation. Our numerical findings predict that for wavelengths smaller than the scale of deformation patterns, tomography may overestimate intrinsic anisotropy due to significant extrinsic anisotropy. At longer wavelengths, intrinsic anisotropy is always underestimated due to spatial averaging. Therefore, we show that it is imperative to homogenize a CPO model first before drawing comparisons with tomographic models. As a demonstration, we use our composite law with a homogenized CPO model of a plate‐driven flow underneath a mid‐ocean ridge, to estimate the SPO contribution to an existing tomographic model of radial anisotropy.

Conjugate Fault Deformation Revealed by Aftershocks of the 2013 Mw6.6 Lushan Earthquake and Seismic Anisotropy Tomography

AbstractSeismic anisotropy is sensitive to the alignment of minerals, fluid‐filled cracks, and fractures caused by fault deformation, thus providing a signature of fault deformation, especially in the aseismic layer. In this study, we use seismic traveltime anisotropy tomography to study the spatial distribution of azimuthal anisotropy in the region of the 2013 Mw 6.6 Lushan earthquake. Our analysis reveals both stress‐induced and structure‐controlled seismic anisotropy mechanisms. The distribution of seismicity and anisotropy clearly delineates conjugate faults in the seismogenic zone between depths of 8 and 15 km. Two near‐vertical stripes or zones of strong seismic anisotropy reveal the continuation of these faults into the diagenetic or overlying aseismic crust. The anisotropic corridors associated with the conjugate faults are interpreted in terms of the crystal preferred orientation of fault rock minerals, which may be enhanced by shear‐band compaction. Our results demonstrate how seismic anisotropy can provide new insights into fault deformation.

Field Occurrence, Petrography and Structural Characteristics of the Basement Rocks in the Northern Part of Kushaka and Birnin Gwari Schist Belts, Northwestern Nigeria

Field, studies and geological mapping on a scale of 1:50000 were carried out to determine the lithologic framework and structural features of the Basement Complex rocks in northern parts of the Kushaka and Birnin Gwari schist belts (Kushaka Sheet 122). The area is underlain predominantly by five main rock types mainly (i) Migmatite-Gneiss-Quartzite suite comprising dioritic, granodioritic and granitic gneisses with fissile and ferruginous quartzites and banded iron formations (BIF); (ii) Kushaka graphite and sulphur bearing biotite and muscovite quartz schist inter-banded in places with iron formations; (iii) Birnin Gwari biotite-staurolite quartz schist; iv) the Kushaka Gneiss Complex composed of basalts (which is being reported for the first time), staurolite and muscovite gneisses and banded iron formations (BIF), and (v) syn-tectonic and late-orogenic biotite-hornblende syenite (BHS) and biotite-hornblende granite (BHG) in the Kushaka schist belt and biotite muscovite granite (BMG) in the Birnin Gwari schist belt area. Petrographic studies have revealed that essential minerals are quartz, K-feldspars (orthoclase, microcline), plagioclase, pyroxene, epidote, hornblende, biotite and muscovite while the accessory minerals are titanite, zircon, apatite, iron oxide (magnetite and hematite). With pyroxenes occurring in the dioritic and granodioritic rocks, metamorphism may have locally reached grannulite facies. Imprints of Pan-African thermo-tectonic events have shown observable migmatization as the first thermo-tectonic event resulting in plastic deformation D 1 and regional S 1 foliation, demostrated by presence of tight isoclinals fold, compositional banding and N – S preferred orientation of mafic minerals. The D 2 deformation is co-axial with D 1 and resulted in the formation of decimeter sized F 2 isoclinal folds, B 2 boudins and eye ball structures that are parallel to S 1 plane schistocity. Strike-slip faults with dextral sense of movements were mapped in a number of places. D 3 deformation is concentrated in the Kushaka Gneiss Complex with near circular deep fractures south of the Kalangai fault. Here granitization and fragmentation of proto-mylonitic staurolite resulted in brittle deformation and F 3 open fold that refolded or transposed the earlier tight isoclinal F 2 folds. The D 4 deformation resulted in N-S and NW-SE quartz veins and pegmatite dykes which serve as channels for epigenetic gold-sulphide and rare metal bearing ore fluids. Keywords: Basement Complex, Pan-African, metamorphism, deformation, Kushaka, Birnin Gwari, Nigeria DOI: 10.7176/JNSR/12-12-02 Publication date: June 30 th 2021

Localized Anisotropy in the Mantle Transition Zone Due to Flow Through Slab Gaps

AbstractMeasurement of anisotropy advances our understanding of mantle dynamics by linking remote seismic observations to local deformation state through constraints from mineral physics. The Pacific Northwest records the largest depth‐integrated anisotropic signals across the western United States but the depths contributing to the total signal are unclear. We used the amplitudes of orthogonally polarized P‐to‐S converted phases from the mantle transition zone boundaries to identify anisotropy within the ∼400–700 km deep layer. Significant anisotropy is found near slab gaps imaged by prior tomography. Focusing of mantle flow through slab gaps may lead to locally elevated stress that enhances lattice preferred orientation of anisotropic minerals within the transition zone, such as wadsleyite.

Assessment of structural, magnetic, and P-wave velocity anisotropy of two biotite gneisses from X-ray and neutron tomography

Anisotropy of the physical properties of rocks is directly related to their internal structure, which is often exhibited by lineation and foliation fabrics as anisotropy markers. Conventional methods of diffraction measurements only provide a part of the structural information that is related to the crystallographic preferred orientation of minerals. In this work, we show the contribution of imaging methods such as X-ray and neutron tomography in studying rock structure that exhibits anisotropic character through lineation and foliation fabrics. In our tomography studies, spherical samples of two different biotite gneisses were tested. Virtual 3D structure models of samples were obtained by both neutron and X-ray tomography, which depict the spatial distribution of mica minerals (biotite and muscovite) - the major carriers of rock anisotropy in the studied samples. An application of scanning method to the 3D data of mica's spatial distribution allowed us to reveal the presence, orientation and strength of foliation and lineation fabrics. We have also performed grain shape analysis using the approximation of the individual elements of segmented mica phase by the equivalent (Legendre's) ellipsoids. As a result, we determined the shape preferred orientations of mica grains and calculated corresponding shape orientation distribution functions. The comparison of structural properties obtained from tomography studies with experimentally determined P-wave, and magnetic anisotropy demonstrated their mutual correlation, providing a basis for the quantitative interpretation of anisotropic rock fabric in relation to the physical properties of rock. The effective elastic and magnetic properties were calculated based on the shape orientation distribution functions determined from tomography data. The good agreement between calculated and measured effective properties have shown the potential of X-ray and neutron tomography for the prediction of magnetic and seismic anisotropy using a 3D model of a rock sample.