Shear Sense Research Articles

Intermittent fracturing in the middle continental crust as evidence for transient switching of principal stress axes associated with the subduction zone earthquake cycle

AbstractIn the Neves area, eastern Alps, fractures that localized shear zones in middle continental crust above the Alpine megathrust are commonly oriented at a high angle to the inferred long-term shortening direction. Fractures show a segmentation geometry and, locally, a discernible offset, indicating movement opposite to the sense of subsequent ductile shear and implying a switch of principal stress axes σ1 and σ3 during fracturing. We propose that this repeated switch, demonstrated by overprinting relationships and different degrees of fracture reactivation, was due to sporadic co-seismic to early post-seismic rebound in the upper plate of the Alpine continental collision system. Fracturing occurred intermittently in the weak midcrustal rocks due to seismic stress release at high transient strain rates and pore-fluid pressures. Widespread transient fracturing in the hanging wall of the Alpine megathrust regionally controls the orientation of ductile shear zones in the middle crust, as well as the emplacement of magmatic dikes.

Geology and deformation of the Serbo-Macedonian massif in the northern part of the Athos Peninsula, Northern Greece: Insights from two detailed cross-sections

The Athos peninsula occupies the south-eastern part of the wider Chalkidiki peninsula in Central Macedonia, Greece. It is mainly built up by crystalline rocks belonging to the Serbo-Macedonian massif, traditionally constituting, along with the Rhodope massif, the Hellenic hinterland. According to the basic geological map of the peninsula, its northern part is mainly composed of marbles grouped into the Kerdyllion Unit, and biotite gneisses and two-mica gneisses grouped into the Vertiskos Unit of the Serbo-Macedonian massif, whereas the contact between the units is considered as a normal contact, although it has been re-evaluated as tectonic later on. Moreover, amphibolites and ultramafic rocks exist along with the previously mentioned rocks, making the geology and relationship between the two units much more complicated. Two detailed cross-sections and structural analysis permit us to revise the geology of the region concluding that the marbles, the amphibolite gneisses, formerly independent amphibolites, and the biotite gneisses belong to the Kerdyllion Unit that is strongly characterized by migmatization and anatexis, whereas the Vertiskos Unit is represented predominantly by two-mica gneisses that were not extensively, if at all, affected by these phenomena. Isoclinal folding and intense shearing with an overall top-to-the-S sense of shear resulted in the main fabric of the rocks and the mylonitic shear zone between the units. More importantly, the two-mica gneiss of the Vertiskos Unit is sandwiched between the rocks of the Kerdyllion Unit. We attribute both isoclinal folding and shearing to a Mesozoic tectonic event associated with an amphibolite facies metamorphism, leading to an Alpine reworking of the Serbo-Macedonian massif. This Alpine reworking continues during Eocene times with an ENE-WSW compression, giving rise to asymmetric to inverted folds, co-axially refolding pre-existing fabrics and structures. Our work strongly suggests that the overall structure and tectono-stratigraphy concerning the Vertiskos and Kerdyllion Units as well as the contact between them should not be based on the existence of the marbles, as traditionally followed up till now, but on the migmatization and anatexis processes that are almost absent from the rocks of the Vertiskos Unit.

Quartz textural analysis from an anastomosing shear zone system: Implications for the tectonic evolution of the Ribeira belt, Brazil

Strain localization and influence of grain-size reduction processes were investigated from field and microstructural observations, and from quartz textural analysis using EBSD, in rocks deformed along an anastomosing network of shear zones in the Ribeira belt in Brazil. Rocks deformed along the Lancinha shear zone (LSZ) display high percentage of recrystallized grains, in which quartz recrystallized grain size from rocks in the southern LSZ reaches a maximum of 50 μm, and in the northern LSZ a maximum of 200 μm. The dominant serrated grain boundaries and the widespread (50–85%) presence of very small (>10–50 μm) recrystallized quartz grains, suggest that BLG was the dominant recrystallization mechanism responsible for grain-size reduction in the south portion of the LSZ. In the northern LSZ the presence of large recrystallized grains with sizes close to subgrains, suggests that grain-size reduction occurred by SGR, and the presence of lobate grain boundaries point to GBM recrystallization, which led to an increase in recrystallized grain size. This suggests grain-size reduction by dynamic recrystallization as a major factor leading to weakening and strain localization. Evidence of intracrystalline deformation and the well-developed quartz crystallographic fabrics suggest that strain localization in the LSZ occurred by dislocation creep deformation mechanisms. Dislocation creep is recorded by dominant activation of basal <a> slip and secondary contribution of rhomb <a> slip, in the southeast of the LSZ, and by activation of rhomb <a> + basal <a> and prism <a> in the northeast. These slip systems together with microstructural observations suggest that plastic deformation occurred at around 400 °C in the southern region, and at around at >400–500 °C in the northern region. S–C foliation, quartz <c> axis distribution and several kinematic indicators suggest sinistral sense of shear along the LSZ, and strain partitioning during progressive deformation. The dominance of sinistral indicators observed in the LSZ are also found in parallel shear zones to the east, such as the Putunã and Serra do Azeite. The sinistral kinematics in this overall dextral transpressional orogenic system may be explained by dual sense of shear on shear zones bounding extruding blocks, likely formed during progressive orogenic evolution instead of the polyphase evolution previously suggested for this southern part of the Ribeira belt.

Petrology, geochemistry and geochronology of granites and granite gneisses in the SE Karakoram, India: Record of subduction-related and pre- to syn-kinematic magmatism in the Karakoram Fault Zone

The granites and granite gneisses from a tectono-metamorphic complex exposed along the Shyok Valley in the Karakoram region, India, forms the southern margin of Asian plate in the India-Asia collision zone. These rocks have been subjected to mineralogical, geochemical, and U-Pb zircon geochronological investigations to constrain the petrogenetic and geodynamic evolution of the Karakoram terrane. Outcrop-scale observations reveal the presence of pre- and syn-kinematic leucogranite bodies intruded within the granites and granite gneisses. The foliation-parallel deformed leucogranite sill shows the dextral shear sense in an extensive metamorphic complex that is in concordance with the Karakoram Fault (KF) in the Ladakh region, NW India. Whole-rock elemental and biotite chemistry equivocally suggest subduction-related metaluminous (I-type) calc-alkaline nature for most of the host granites and gneisses, and generation of syn-collisional peraluminous (S-type) leucogranites through crustal anatexis of pre-existing rocks. The obtained U-Pb zircon ages for the granites, granite gneisses, deformed and undeformed leucogranites vary widely from ca. 160 Ma to 15 Ma and reveal: (a) the initiation of subduction of the Neo-Tethyan oceanic crust beneath the southern Asian Plate at least ~160 Ma ago, and (b) existence of continuous or intermittent deformation along ~1000 km long lithospheric scale dextral KF during ~27−15 Ma.

Measurement of 3D-Shape Preferred Orientation (SPO) Using Synchrotron μ-CT: Applications for Estimation of Fault Motion Sense in a Fault Gouge

We propose a 3D-shape preferred orientation (SPO) measurement method of rigid grains using synchrotron micro-computational tomography (μ-CT). The method includes oriented sampling, 3D μ-CT imaging, image filtering, ellipsoid fitting, and SPO measurement. After CT imaging, all processes are computerized, and the directions of thousands of rigid grains in 3D-space can be automatically measured. This method is optimized for estimating the orientation of the silt-sized rigid grains in fault gouge, which indicates P-shear direction in a fault system. This allows us to successfully deduce fault motion sense and quantify fault movement. Because this method requires a small amount of sample, it can be applied as an alternative to study fault systems, where the shear sense indicators are not distinct in the outcrop and the fault gouge is poorly developed. We applied the newly developed 3D-SPO method for a fault system in the Yangsan fault, one of the major faults in the southeastern Korean Peninsula, and observed the P-shear direction successfully.

Localisation of the brittle Bathurst fault on pre-existing fabrics: a case for structural inheritance in the northeastern Slave craton, western Nunavut, Canada

The north–northwest-striking Bathurst fault in the northeastern Slave craton displaced the 1.9 Ga Kilohigok basin and the ca. 2.02–1.96 Ga Thelon tectonic zone, and projects beneath the 1.7 Ga Thelon basin where unconformity-associated uranium deposits are spatially associated with basement faults. Here we investigate the deformation–temperature–time history of the Bathurst fault rocks using structural and microstructural observations paired with U–(Th–)Pb and 40Ar/39Ar geochronology. Highly strained hornblende-bearing granitoid rocks, the predominant rock type on the northeastern side of the Bathurst fault in the study area, show ambiguous sense of shear suggesting flattening by coaxial deformation. Quartz and feldspar microstructures suggest ductile deformation occurred at ≥500 °C. Along the main fault trace, brittle features and hydrothermal alteration overprint the pervasive ductile flattening fabric. In situ U–Th–Pb dating of synkinematic monazite suggests ductile fabric formation at ca. 1933 ± 4 Ma and ca. 1895 ± 11 Ma, and zircon from a cross-cutting dyke constrains the brittle deformation to ≤1839 ± 14 Ma. 40Ar/39Ar dating of fabric-defining minerals yield cooling ages of ca. 1920–1900 Ma and ca. 1900–1850 Ma for hornblende and muscovite, respectively, and a maximum cooling age of ca. 1840 Ma for biotite. We suggest the ca. 1933–1895 Ma ductile flattening fabric developed during orthogonal collision and indentation of the Slave craton into the Thelon tectonic zone and Rae craton. Brittle deformation on the Bathurst fault was localised parallel to the ductile flattening fabric after ca. 1840 Ma and preceded Thelon basin deposition. Brittle deformation features in Bathurst fault rocks preserve evidence for fluid–rock interaction and enhanced basement permeability, suggesting the fault is a possible conduit structure for mineralising fluids.

Modification of pre-existing folds in a shear zone: A case study from Kumbhalgarh–Ranakpur area, South Delhi Fold Belt, Rajasthan, India

Pre-existing structures (e.g., folds, foliations, lineations) are usually rotated and modified within a shear zone, depending on their attitude with respect to the kinematic framework (i.e., orientation of instantaneous stretching axes (ISA), kinematical vorticity number of the flow (Wk)) of the shear zone. In addition, new folds may also form within a shear zone, and it is not always easy to distinguish between the pre- and syn-shearing structures in the field, especially if they form on the same rock type. The present contribution describes the reorientation and shape modification (tightening) of pre-existing and syn-shearing folds in metamorphosed calcareous rocks of the Kumbhalgarh Group (part of Delhi Supergroup) due to shearing along the Ranakpur Shear Zone (RSZ), in Kumbhalgarh–Sayra–Ranakpur area of South Delhi Fold Belt (SDFB), also called South Delhi Terrane (SDT). From field-based study and measurements, it is shown that the shallow-plunging, upright second generation (DF2) folds of SDFB/SDT have been rotated to subvertical, tight folds within the RSZ. Fold shape analysis using layer thickness and limb dip of folds (i.e., Ramsay’s classification) and by Fourier transform shape analyses of fold profile sections corroborates and roughly quantifies the tightening and shape modification of pre-existing folds within the RSZ. In contrast, syn-shearing folds have formed on the foliations in calc-silicate rocks which show strongly non-cylindrical geometry with apical deflection in an oblique direction. From the available shear sense indicators like rotated porphyroclast tails and vergence of asymmetric folds, the shear sense of the RSZ is interpreted as oblique reverse (east-side-up) with subordinate sinistral (east-towards-north) shear component, which is similar to the shear sense interpreted by some earlier workers.

Reappraisal of Variscan tectonics in the southern French Massif Central

The comprehensive kinematic analysis of the Variscan fold nappes and associated wildflysch in the Southeastern Montagne Noire shows that the nappes were thrust northeastwardly along the Cévennes fault as a result of strike-slip-dominated sinistral transpression between 340 and 323 Ma. Macro- and map-scale recumbent folding and nappe stacking were achieved by ENE-directed, strike-parallel thrust shearing. The recumbent folds evolved into sheath folds by interplay of continued strike-parallel thrust shearing and strike-normal shortening manifested by north-verging folds that grew during wildflysch sedimentation. These results call for a tectonic reassessment of the southern side of the Variscides. The foreland basin was decoupled from the hinterland along orogen-scale sinistral fault(s) trending at a high angle to the orogenic trend, which interacted with a variety of hinterland-ward thrust systems. The shift from sinistral transpression to dextral transtension along the Cévennes fault after 317 Ma accompanied the closure of the Iberian-Armorican syntax by accommodating northeastward escape and gravitational collapse of the northern limb of the syntax. These results illustrate the impact of the shift from plate convergence-controlled to escape-controlled kinematics on shear sense reversal and lower crust exhumation along long-lived strike-slip fault systems.

Evolution of structures and hydrothermal alteration in a Palaeoproterozoic supracrustal belt: Constraining paired deformation–fluid flow events in an Fe and Cu–Au prospective terrain in northern Sweden

Abstract. An approximately 90 km long Palaeoproterozoic supracrustal belt in the northwestern Norrbotten ore province (northernmost Sweden) was investigated to characterize its structural components, assess hydrothermal alteration–structural geology correlations, and constrain a paired deformation–fluid flow evolution for the belt. New geological mapping of five key areas (Eustiljåkk, Ekströmsberg, Tjårrojåkka, Kaitum West, and Fjällåsen–Allavaara) indicates two major compressional events (D1 and D2) have affected the belt, with each associated with hydrothermal alteration types typical for iron oxide–apatite and iron oxide Cu–Au systems in the region. Early D1 generated a regionally distributed, penetrative S1 foliation and oblique reverse shear zones that show a southwest-block-up sense of shear that formed in response to NE–SW crustal shortening. Peak regional metamorphism at epidote–amphibolite facies broadly overlaps with this D1 event. Based on overprinting relationships, D1 is associated with regional scapolite ± albite, magnetite + amphibole, and late calcite alteration of mafic rock types. These hydrothermal mineral associations linked to D1 structures may form part of a regionally pervasive evolving fluid flow event but are separated in this study by crosscutting relationships. During D2 deformation, folding of S0–S1 structures generated F2 folds with steeply plunging fold axes in low-strain areas. NNW-trending D1 shear zones experienced reverse dip-slip reactivation and strike-slip-dominated movements along steep, E–W-trending D2 shear zones, producing brittle-plastic structures. Hydrothermal alteration linked to D2 structures is a predominantly potassic–ferroan association comprising K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulfides. Locally, syn- or post-tectonic calcite is the main alteration mineral in D2 shear zones that intersect mafic rocks. Our results highlight the importance of combining structural geology with the study of hydrothermal alterations at regional to belt scales to understand the temporal–spatial relationship between mineralized systems. Based on the mapping results and microstructural investigations as well as a review of earlier tectonic models presented for adjacent areas, we suggest a new structural model for this part of the northern Fennoscandian Shield. The new model emphasizes the importance of reactivation of early structures, and the model harmonizes with tectonic models presented by earlier workers based mainly on petrology of the northern Norrbotten area.

Subduction Reversal in a Divergent Double Subduction Zone Drives the Exhumation of Southern Qiangtang Blueschist‐Bearing Mélange, Central Tibet

Abstract(Ultra) high‐pressure (HP) rocks can be exhumed rapidly by subduction reversal or divergent plate motion. Recent studies show that subduction reversal can in particular occur in a divergent double subduction zone when the slab pull of one slab exceeds that of the other, shorter one, which then experiences a net upward pull. This recent hypothesis, first proposed for Triassic HP‐rocks exposed in the central Qiangtang mélange belt in central Tibet, can explain the exhumation of (ultra) HP rocks through upward slab movement. However, this model lacks the support of kinematic evidence. In this study, based on the recognition of multiple deformational phases, we analyze the kinematics of the HP‐bearing mélange in central Qiangtang. Based on new 40Ar‐39Ar geochronology data and those collected from the literature, we present a temporal framework for the new observations. We recognize a switch in sense of shear between the prograde (D1) and exhumation (D2‐3) paths. The change of shear sense reflects the reversal from downward to upward movement of the oceanic slab below. Early D2 represents the early exhumation stage that caused retrograde metamorphism from eclogite to blueschist facies. No magmatism occurred during this period. Continued exhumation from blueschist facies to greenschist facies resulted in D2‐D3 structures. Voluminous igneous activity occurred during this stage. We suggest that subduction reversal in a divergent double subduction zone can best explain the kinematic evolution and temporal framework above. This exhumation model may provide a new perspective on the exhumation mechanism for other HP rocks around the world.

Structural analysis of sheath folds and geochronology in the Cuonadong Dome, southern Tibet, China: New constraints on the timing of the South Tibetan detachment system and its relationship to North Himalayan Gneiss Domes

AbstractStructural analysis and new zircon U/Pb geochronology were performed in the Cuonadong Dome in order to test the hypothesis that the top‐to‐North deformation in the North Himalayan Gneiss Domes (NHGD) has been correlated with deformation along the South Tibetan detachment system (STDS). Structural analysis suggests that the X‐axes of sheath folds at the different flanks of the dome are consistent, and show average a trend and plunge of 350° and 18°. Many shear sense indicators are recorded in the ductile shear zone during the top‐to‐north deformation, including the augen structure, σ‐type and δ‐type rotating porphyroclast, S–C fabric and pressure shadow structure. The development of the sheath folds first provide a direct‐field structural evidence that represents a deeper ductile manifestation of the STDS, and further suggests that the top‐to‐north deformation in the NHGD have been correlated with deformation along the STDS. The emplacement of the synkinematic granites at ca. 32 Ma exposed at the core of the sheath folds is also interpreted as the initiation of slip along the STDS.

Brittle vs. ductile strain during the synorogenic exhumation of HP-LT rocks: An example from the Lungro-Verbicaro Unit mylonites (northern Calabria, Italy)

We present a study on the mylonites exposed in the Cirella area along the Tyrrhenian coast of northern Calabria (southern Italy), in order to understand the role of ductile and brittle deformation on the exhumation process of these rocks. The analyzed shear zone is located in the footwall of a regional tectonic contact between metaophiolites (Ligurian Complex) on the top, and the continental Lungro-Verbicaro Unit (Adria plate) at the bottom. The latter consists of a metasedimentary succession with Triassic shallow-water carbonates at the base of it, evolving upward to Jurassic-lower Miocene slope to deep basin carbonates and foredeep siliciclastic rocks. Both oceanic and continental successions are characterized by high-pressure and low-temperature (HP-LT) metamorphism. The analyzed mylonites show different degrees of strain, ranging from protomylonites to ultramylonites, the latter characterized by well-marked foliation and stretching lineations. Several micro- and mesoscale progressive deformation structures, including two fold sets, S-C’ structures are present in these rocks, and a late oblique foliation, indicating a NW sense of shear. A successive shortening stage produced thrust faults and related folds, indicating a tectonic transport toward SE. We associate the mylonitic deformation with the activity of the overlying detachment fault, which is responsible for the synorogenic exhumation of the Lungro-Verbicaro Unit during the Miocene time. However, the ductile strain, recorded by the studied mylonites, accounted for a small displacement during the exhumation of this tectonic slice. In fact, the calculated displacement achieved by the Lungro-Verbicaro Unit, during exhumation, is much larger than the one estimated considering only the ductile zones activity. This evidence suggests that, synchronously with ductile strain, brittle deformation occurred along the detachment fault located at the top of the thrust sheet. In addition, a severe erosion due to the tectonic exhumation of the HP-LT rocks (in the late Miocene) produced a large amount of clastic sediment that filled the extensional basins widespread in northern Calabria.

The β1 Triad-Related Configurations in a Mg-RE Alloy

As the building block of a honeycomb precipitate network, the triadic arrangement of β1 strengthening precipitates is technologically important for the potential development of highly tuneable nanostructure in rare-earth (RE) containing Mg alloys. In this work, we provide systematic experimental observations of those impinged β1 variants in a Mg-Nd alloy, which are directly related to the characteristic triadic configuration. It is found that the isolated α-Mg crystal in a β1 triad has a 10.5 deg rotation when it has a perfect equilateral-triangular shape. This rotation angle decreases to ~ 9.4 deg when the isolated α-Mg crystal exhibits a non-equilateral shape. In this case, one or two of its lateral interfaces include a step with a height of 0.56 nm to reduce the lattice mismatch. Furthermore, the pre-stage prior to the formation of β1 triadic configuration is revealed for the first time. It originates from the implement of two β1 variants with the same sense of shear. The presence of such two variants, in the absence of the third variant, leads to a rotation of the α-Mg crystal in the small area close to the approaching ends of these two variants. A low-angle symmetrical tilt boundary around the [0001]α rotation axis is formed between the rotated and unrotated α-Mg crystals. Most of the tilt boundaries observed in this work have a tilt angle of ~ 4.7 deg. The atomic structure of such a tilt boundary is constructed using a crystallographic model and validated by molecular dynamics simulation. The unique distribution of Nd-rich solute clusters along the tilt boundary is qualitatively discussed based on crystallographic analysis.

Mechanical control on patterns of overburden deformation induced by dyke-fed sandstone intrusions: Insights from numerical experiments

Conical sandstone intrusions, as a distinct type of hydrocarbon reservoir, remain poorly understood regarding their emplacement mechanics. Here, we report a numerical modelling study of conical sandstone intrusions using the two-dimensional discrete element method. We built numerical models that contain bonded elastic particles with predefined mechanical properties in an open box as the overburden. A thin tube filled with unbonded particles, and connected to the upper box, was used to model the feeder dyke. The dynamic behavior of the assembly was enabled by the displacement of a driving wall that defines the lower boundary of the feeder dyke. The results show that the model composed of soft materials produced a pair of conical, opening-mode fractures in the host sediments as the result of tensile stress concentrations in the fracture tip zones. The overburden deformation was largely localised within the sediments adjacent to the sandbody, and without the formation of a forced fold and significant uplift of the surface. In contrast, the model composed of stiffer materials produced conical fractures that have closed lower segments and opening upper segments with a reverse sense of shear. The intrusion also caused a forced fold in the overburden, with a vertical opening-mode fracture generated in the fold hinge. The modelling results demonstrate that dyke-fed sand intrusions can significantly distort the local stress field and the overburden can be subjected to fracturing and/or folding due to differential compaction during gradual inflation of the intrusive sandbody. Importantly, deformation patterns of the overburden in response to sandstone intrusion largely depend on mechanical properties of the host sediments.

Contribution to crustal strain accumulation of minor faults: a case study across the Niigata\u2013Kobe Tectonic Zone, Japan

Recent global navigation satellite system (GNSS) data for the Japanese Islands have revealed a high-strain-rate region suggesting the existence of a region of broad-scale crustal deformation. The Niigata–Kobe Tectonic Zone (NKTZ), which is the high-strain-rate zone in central Japan, shows a short-term dextral strain rate of ~ 12 mm/year. The total slip rate of the Quaternary fault zones in the NKTZ has been estimated as ~ 6.7 mm/year, accounting for just over half the short-term strain rate of the zone. However, this slip rate underestimates the total slip rate on faults within the NKTZ owing to possible distributed deformation on minor faults. This study quantifies the slip rate attributable to these other faults in the southeastern-central NKTZ and reveals the unique deformation structure across the high-strain-rate zone, which comprises a Quaternary fault core, a Quaternary fault damage zone, an incipient brittle shear zone (active background), and an inactive background. The spatial characteristics of the incipient brittle shear zone can be explained in terms of fault density, which increases toward the central NKTZ. Minor faults located > 500 m from major Quaternary faults but within the NKTZ have sense of shear consistent with that of the major faults. In contrast, minor faults outside of the NKTZ show sense of shear that differ from the dextral displacement of the high-strain-rate zone and do not contribute to the slip rate of the zone. The total slip rate of minor faults in the southeastern-central NKTZ is estimated to be 0.46–2.88 mm/year (roughly equal to a major Quaternary fault in the zone), which implies 4–24% of crustal strain is stored in the active background.

Reconstructing Deformation History by Using Microstructural and Petrographic Analysis of Sorobo, Konso Area, Southern Ethiopia

The deformation history of Sorobo, Konso area (southern Ethiopia) within the Mozambique Belt, is described using microstructural and petrographic analysis. The area is characterized by folds and metamorphic fabrics that trend between NNE and NNW and consist of high-grade, amphibolite- to granulite-facies rocks. It is affected by five deformational phases (D1 to D5) and two metamorphic events (M1 & M2). During shortening deformation events (D1 to D3) the development of gneissosity (D1), the formation of tight to isoclinal and recumbent folds (D2) and the superposition of secondary (upright) fold (D3) on earlier recumbent fold resulting in type-3-fold interference pattern are formed respectively. During fourth deformation phase (D4) most of the rock units of the area are affected by shearing or shows east and west vergence. The area is affected by both sinistral and a dextral sense of shearing but dextral shear-sense appears dominant. Brittle type of deformation phase (D5) was developed and resulting in different types of faults varies in orientation. Peak progressive metamorphism of the granulite facies (M1) and retrogression (M2) are also associated with the deformational phase. An equal-area plot and rose diagram shows the orientation of all fabric, which indicates multiphase deformation. Geological structures in the study areas like faults, joints & fissures, shear zones and folds are important depositional sites for different hydrothermal economic mineral deposits.

Chapter 11 Reworking of older (1.8 Ga) continental crust by Mesoproterozoic (1.5–1.4 Ga) orogeny, Blekinge–Bornholm orogen, southeastern Sweden

Abstract The Blekinge–Bornholm orogen in southeastern Sweden consists of calc-alkaline to alkali–calcic intrusive rocks, rhyolites and dacites (1.8 Ga) that were structurally reworked under amphibolite facies conditions, affected by migmatization at mid-crustal levels at c. 1.44 Ga and intruded at c. 1.47–1.43 Ga by ferroan alkali–calcic plutons. This Mesoproterozoic orogen is bordered westwards by the Sveconorwegian orogen and northwards, along the boundary with well-preserved 1.8 Ga magmatic rocks in the Svecokarelian orogen, by a stitching c. 1.45 Ga pluton and steeply dipping ductile zones with a south-side-up, dip-slip shear component. A variably developed gneissic fabric (S 1 ) dips gently to moderately northwards and is affected by asymmetrical F 2 folds with a southerly vergence. Ductile high-strain zones with top-to-the south shear sense are suggested to correspond at depth to anomalously reflective zones along seismic profile BABEL line A. Open folding of the gneissosity around gently, north-plunging fold axes (F 3 ) completed the ductile deformational evolution. Uncertainty remains about the timing of the amphibolite facies ductile fabric and the D 2 folding, which is either late-stage Svecokarelian ( c. 1.77–1.75 Ga) or Hallandian ( c. 1.47–1.43 Ga). Non-collisional, accretionary orogenic systems are suggested to have operated during both time periods, radical reorganization of the subduction trend accompanying the Mesoproterozoic event.

Microstructural analyses of the Najd Fault System in Midyan Terrane, NW Arabian Shield, Saudi Arabia

This article discusses the microstructural indicators exposed within the Precambrian Ajjaj–Qazaz–Hanabiq (AQH) shear zones in order to determine the shear sense. The AQH shear zones are located in the southern sector of the Midyan Terrane in the northwestern part of the Arabian Shield. Structural and microfabric analyses along these shear zones indicate a top-to-the-NW sense of shear. Exposures in this area comprise elongated ridges of ophiolitic rocks, arc metavolcanics and sporadic arc-related Neoproterozoic intrusions. They are unconformably overlain by post-amalgamation molasses-type sediments and were finally intruded by late to post-tectonic intrusions. Structural field relations along with microfabric analyses and overprinting relationships provide evidence of the sense of movements and phases of deformation. There were at least four phases of deformation. The oldest (Hijaz) D1 structures are rare and are represented by foliation striking WNW to E–W and by F1 interfolial folding with an axial plane striking ENE–WSW and moderately plunging fold axes trending WSW. D2 microstructures are also scarce and represented by foliation and (F2) folding directed in NNW to N–S trends parallel to the Nabitah suture. However, they are commonly deformed and overprinted by the Najd Fault System that formed during the D3 deformational phase. The D3 deformational phase is prominent, and it developed during the propagation of the NW transpressional shearing of the Najd Fault System. These shear zones could be identified in this region with the help of the following microstructures: sigmoidal structures, mantled porphyroclastic structures, asymmetric boudins, tension gashes, fish-shaped structures, oblique foliation, bookshelf sliding structures, slickenlines, C–S tectonites and fracturing along the AQH shear zones. Shear sense analysis of microstructural markers at the microscopic scale, supported by field observations and map-scale structures, implies that both the Ajjaj and Qazaz shear zones display top-to-the-NW or top-to-the-WNW slip. The shear sense indicators along the Hanabiq shear zone show top-to-the-NNE slip. These inconsistencies in their shear sense and their orientations were caused by the exhumation of high-grade gneisses, which distorted the shear trend to the north in the Hanabiq shear zone. All the previous structures are overprinted by the brittle D4 deformational phase in the form of microfaulting and fracturing.

Mesoscopic to microcropic structures associated with the Wanni Complex/Highland Complex Boundary Shear Zone in Sri Lanka

In this study, we report mesoscopic and microscopic structures in sheared rocks associated with the Wanni Complex/Highland Complex boundary shear zone (WHBSZ) which represents a deep part of a granulite-facies, crustal-scale ductile shear zone in the Proterozoic basement of Sri Lanka. Normal khondalitic mylonites, spinel-bearing khondalitic mylonites, semipelitic mylonites and metagranitoid mylonites (striped gneisses) found along the WHBSZ show mesoscopic to microscopic scale shear sense indicators such as S-C' fabrics, rotated garnet porphyroblasts, δ-type and σ-type asymmetric objects, asymmetric folds and sheath folds. These asymmetric mesoscopic to microscopic structures indicate that the dominant deformation along the WHBSZ is non-coaxial ductile flow and that the WC has been emplaced from NNW (present orientation) on top of the HC. The presence of mylonites and ultramylonites along the WHBSZ suggests severe grain size reduction during the shearing. All the mylonites studied along the boundary shear zone have developed a mylonitic foliation, defined by quartz plates and ribbons, elongated and recrystallized fine-grained aggregates of feldspar, and a very strong stretching lineation mostly defined by c-axis of sillimanite and stretched quartz and elongated aggregates of other minerals. All the mesoscopic and microscopic structures suggest that the WHBSZ has formed during the second phase of D2 non-coaxial deformation accompanying high-temperature (HT) to ultrahigh temperature (UHT) granulite facies meta-morphism related to the first collision between the WC and the HC. This study emphasizes that careful study of granulite facies mylonites may provide reliable shear sense indicators for understanding the nature and kinematics of deep crustal shear zones.

Crystal clustering in magmas: Insights from HP–HT experiments

We have studied by high-resolution X-ray Computed Tomography the effect of crystal clustering on the Shape-Preferred Orientation (SPO) development in synthetic magmas experimentally deformed at 300 MPa and 475–550°C. A fully connected solid network that could potentially induce the onset of yield strength is not achieved in these suspensions containing 16 vol% of crystals. The alumina grain population exhibits a glomeroporphyritic texture made of isolated grains (59.6%) and clusters of touching grains (40.4%). The SPO of the sub-population of isolated grains exhibits foliation and lineation, which are closely parallel to the plane and direction of shear, respectively. The SPO of clustered grains has little influence overall shape fabric. The angular relationships between the average foliation and cluster elongation provide a good indicator of the shear sense. Finally, we highlight the strain partitioning between nearly non-deformed large clusters acting as rigid glomerocrysts and highly sheared zones in low concentrated suspensions.