Strain Ellipsoids Research Articles

Relating Quartz Crystallographic Preferred Orientation Intensity to Finite Strain Magnitude in the Northern Snake Range Metamorphic Core Complex, Nevada: A New Tool for Characterizing Strain Patterns in Ductilely Sheared Rocks

AbstractDocumenting the magnitude of finite strain within ductile shear zones is critical for understanding lithospheric deformation. However, pervasive recrystallization within shear zones often destroys the deformed markers from which strain can be measured. Intensity parameters calculated from quartz crystallographic preferred orientation (CPO) distributions have been interpreted as proxies for the relative strain magnitude within shear zones, but thus far have not been calibrated to absolute strain magnitude. Here, we present equations that quantify the relationship between CPO intensity parameters (cylindricity and density norm) and finite strain magnitude, which we calculate by integrating quartz CPO analyses (n = 87) with strain ellipsoids from stretched detrital quartz clasts (n = 49) and macro‐scale ductile thinning measurements (n = 7) from the footwall of the Northern Snake Range décollement (NSRD) in Nevada. The NSRD footwall exhibits a strain gradient, with Rs(XZ) values increasing from 5.4 ± 1.4 to 282 ± 122 eastward across the range. Cylindricity increases from 0.52 to 0.83 as Rs increases from 5.4 to 23.5, and increases gradually to 0.92 at Rs values between 160 and 404. Density norm increases from 1.68 to 2.97 as Rs increases from 5.4 to 23.5, but stays approximately constant until Rs values between 160 and 404. We present equations that express average finite strain as a function of average cylindricity and density norm, which provide a broadly applicable tool for estimating the first‐order finite strain magnitude within any shear zone from which quartz CPO intensity can be measured. To demonstrate their utility, we apply our equations to published data from Himalayan shear zones and a Cordilleran core complex.

Strain Localization: Analog Modeling and Anisotropy of Magnetic Susceptibility

AbstractThe majority of the strain in Earth crust and upper mantle is localized to the high strain zones developed at ductile‐to‐brittle condition at kilometer‐to‐micrometer scale. Therefore, they represent the key to understanding the deformation evolution of the lithosphere. The finite strain pattern recorded within these zones has been therefore a subject of research in geology. The methods studying rock magnetism such as the anisotropy of magnetic susceptibility (AMS) are frequently used techniques to characterize and quantify deformation and flow record in rocks. Numerous sedimentary, subsolidus and submagmatic deformation zones exhibit typical evolution of the AMS ellipsoid across the strain gradient suggesting indirect not straightforward correlation between AMS and strain ellipsoids. To document spatiotemporal and internal fabric evolution during strain localization, pure shear, simple shear, and shear zone (SZ) analog experiments were performed using shear‐thinning thixotropic material of plaster of Paris. The experimental results closely resemble the record from natural SZs in sedimentary rock systems but also in subsolidus SZs and submagmatic mushy systems. The magnetic fabric evolution across deformation zones is interpreted to be associated with the intersection and transposition of preexisting primary fabric with shear fabrics and evolution of synkinematic subfabrics. Their development is attributed to localization of deformation at microscale due to the self‐organized slip of anisometric particles forming microshear planes reflecting the symmetry of deformation. The experimental results when confronted with the natural examples implies that the localization and partitioning of deformation is one of the most important factors for the interpretation of AMS in deformation zones.

Strain partitioning in the Kahdan shear zone: Microstructural and kinematic evidence from the SE Sanandaj-Sirjan zone, Zagros orogeny

Microstructural, vorticity and strain analyses, combined with geological field observations are fundamental tools for retrieving information about kinematics of deformation in crustal-scale shear zones. The Kahdan shear zone is located in the SE part of the Sanandaj-Sirjan metamorphic zone within the Zagros orogen, SW Iran. The Zagros Orogen that formed due to the oblique convergence between the Afro-Arabian plate and the central Iranian plateau. Three-dimension strain analyses show that finite strain ellipsoids shape is oblate (K = 0.38–0.89). Vorticity analysis on mylonites yielded arithmetic mean maximum (Wm max) and mean minimum values (Wm min) of 0.83 and 0.72, respectively (63–47% simple shear). Higher values of Wm and Rxz estimated adjacent to the shear zone boundaries and gradually decrease toward the central part of the zone. Our yielded results show a non-unified spatial variations of vorticity and strain values in the deformed metasandstone (Late Jurassic to Early Cretaceous) of the Kahdan shear zone. Vorticity analyses and noncoaxial indicators indicate that this zone experienced general shear deformation. The horizontal shortening of ∼13–27% measured perpendicular to the shear zone boundaries. Based on yielded results, this area provides a good example of strain partitioning in a ductile shear zone.

Penetrative Strain and Partitioning of Convergence‐Related Shallow Crustal Shortening, Across Scales, in the Lesser‐ and Sub‐Himalayan Thrusts: Insights From the Eastern Himalaya, Sikkim

AbstractShortening estimates from regional balanced cross‐sections remain lower than the predicted shortening from paleomagnetism and area‐balancing methods in the Himalaya. To reconcile this shortening deficit, we evaluate the shortening contribution from the often‐ignored penetrative strain of quartz grains and integrate these results with two published regional balanced cross‐sections from Sikkim Himalaya. Penetrative strain progressively decreases from the internal to the external thrusts with changing deformation conditions. Strain increases toward the basal thrust zone within each thrust indicating strain localization. Strain ellipsoids are folded along with the thrusts indicating the strain initiated before thrusting and folding. The layer‐normal shortening (LNS) strain dominates the internal thrusts. A combination of LNS and layer‐parallel shortening (LPS) strain exist in the low‐strained external thrust sheets. Approximately 78–91 km of shortening is recorded as penetrative strain in the Lesser‐ and Sub‐Himalayan thrusts. Lateral structural variations in the thrusts are reflected in the thrust‐sheet scale shortening and in the strain ellipsoid orientations, but not in the grain‐scale shortening. Shortening is partitioned differently across scales; the duplex records the highest thrust‐sheet scale shortening, while the roof thrust toward the hinterland records the highest grain‐scale shortening. Due to the dominance of LNS strain, removing strain from thrusts will collectively decrease ∼54–59 km of base lengths, thereby reducing the total minimum shortening of the Sikkim Himalaya by ∼13%. Therefore, the penetrative strain does not contribute to the shortening deficit in the Sikkim Himalaya.

Overprint of secondary Du folding in the Sambagawa metamorphic belt, SW Japan: Implications for strain ellipsoids and Paleogene tectonics of the east‐Eurasian margin

AbstractTwo contrasting results of strain analyses, constriction and flattening, are recognized in the Sambagawa high‐pressure/temperature metamorphic belt, SW Japan. An unverified proposal to account for this situation is that the constrictional strain ellipsoids develop only in areas where there is strong overprinting by a secondary Du‐phase folding after a penetrative Ds‐phase deformation. Field studies in the Hibihara district, central Shikoku, which is located between a southern constrictional region and a northern flattening region, reveal there is a map‐scale contrast in the effect of Du: outcrop‐scale Du upright folds are common in the southern region while they are rare in the northern region. Field measurements show that overall orientation of Ds strain is characterized by E–W stretching and vertical shortening, while that of Du strain is characterized by N–S shortening and vertical extension. The shortening caused by Du in the southern high‐Du‐strain regions estimated by the fold‐curve tracing method are down to about 70%–56% and by removing this Du shortening most of the constrictional strain ellipsoids are restored back to the flattening field with only a few exceptions, suggesting that pre‐Du strain states of the Sambagawa metamorphic rocks were dominantly of the flattening type. These results support the previous ideas that invoke differences in the strength of Du overprinting for the two different types of strain ellipsoids observed in the Sambagawa belt. From a tectonic point of view, the semi‐penetrative occurrence of Du folds throughout the Sambagawa belt, which stretches about 800 km in SW Japan, suggests that the Du phase can be related to some ancient plate movement. The significant shortening subnormal to the orogen that characterizes Du may reflect a shift to orogen‐subnormal subduction of the Pacific plate beneath the Eurasian plate at around 60 Ma.

Structural and rheological features of the northeastern Sanyang shear zone in the eastern Jiangnan orogen: Indications for early Paleozoic orogeny in the South China Block

The N55°E−trending Sanyang shear zone is located in the eastern Jiangnan orogen, and the northeastern junction between the Cathaysia and Yangtze blocks is the key to understanding the evolution of the South China Block (SCB) for early Paleozoic crustal reworking. Most shear-sense indicators (e.g., asymmetric folds, porphyroclasts, S–C fabrics, and mica fish) indicate that the Sanyang shear zone is characterized mainly by oblique sinistral shear and locally by dextral shear. Mineral deformation behavior, quartz C-axis fabrics, and the Kruhl thermometer demonstrate that shear deformation developed under high greenschist-to low amphibolite facies conditions with temperatures ranging from 450 to 550 °C. Differential stresses deduced from recrystallized quartz grain sizes were approximately 22.4–46.5 MPa, and strain rates were on the order of 10−9 to 10−10 S−1. Three dimensional (3D) strain ellipsoid measurements and the Flinn diagram of the Sanyang shear zone indicate that all samples yield oblate and flattened strain ellipsoids, which imply a contractional regime. The kinematic vorticity (Wk) 0.716–0.999, estimated by using 3D strain ellipsoid-based methods, indicates a high simple shear contribution in the shear zone. All structural, strain, and vorticity analyses suggest that the Sanyang shear zone experienced general shear with transpression. The numerical estimates allow kinematic modeling of the Sanyang shear zone, which provides important information about the structural framework for the SCB. The development of the Sanyang shear zone was a response to the oblique convergence of the Yangtze and Cathaysia blocks during the early Paleozoic orogeny.

Numerical study of damage in particulate composites during high-strain rate loading using novel damage model

We perform large three-dimensional damage simulations of particulate composites under high-strain rate loading to address the effect of the microstructure on the overall transient mechanical behavior. We use spheres as well as oblate, prolate, and plane strain ellipsoids and show that resolving the wide range of spatial and temporal scales in complex morphologies is essential to apprehending fracture characteristics. We employ continuum damage mechanics and develop a novel damage model that properly accounts for deviatoric and volumetric energy contributions in the finite strain setting. In order to resolve the characteristic damage length-scale, we utilize a highly parallel finite element solver, PGFem3D. The high-performance parallel simulations are performed on up to 1920 processing cores. We adopt a data-driven (image-based) materials modeling approach in constructing the computational domains. We show that high-strain rates as well as microstructural details play an important role in addressing complex damage patterns and overall material response.

A kinematic approach for investigating magnetic and strain fabrics from constrictional and flattening domains of shear zones in Sikkim Himalayan fold thrust belt

We investigate the relationships among magnetic, mylonitic and strain fabrics from the Pelling-Munsiari thrust (PT), a major shear zone in the Sikkim Himalaya, that records varying mylonitic fabrics and strain geometry along its transport direction. The fabric changes from an SL-tectonite with oblate strain ellipsoids in its main exposures, to a local, transport-parallel L-tectonite with prolate strain ellipsoids, forming a synformal klippen. Magnetic fabric is related to the PT deformation, and is affected by the local folding of the klippen. The bearings of magnetic and stretching mineral lineation vary more than the orientations of the planar fabrics. Irrespective of their strain geometry, most of the fault rocks, including SL mylonites of the structurally overlying Main Central thrust, record oblate susceptibility ellipsoids. The paramagnetic phases tracked an incremental deformation stage that froze as oblate susceptibility ellipsoid in the hinterlandmost fault rocks, and was not realigned by successive higher magnitude, constrictional strain, associated with a local, lateral ramp, followed by flattening strain in the forelandmost exposure during translation of the PT. Additionally, strain partitioning between quartz, tracking the finite strain fabric, and micas primarily contributing to the magnetic fabric explain the deviations in the studied fabrics in the fault rocks.

Strain and vorticity analyses using rotated porphyroclasts in the Tanbour metamorphic rocks: Evidence of transpressional deformation along the Sanandaj-Sirjan metamorphic belt, SW Iran

The Tanbour Metamorphic Complex (TMC) is located within the Sanandaj-Sirjan Metamorphic Belt, SW Iran, along the northeastern margin of the Afro-Arabian plate. Detailed kinematic and strain analyses were combined with vorticity estimates on rocks from the TMC to document patterns and geometry of deformation. The strain parameter measurements including strain ratio in the XZ principal plane of strain ellipsoid (RXZ), strain ellipsoid shape (K) and strain intensity (D), highlight the deformed gneissic rocks in the TMC exhibit constrictional to approximately plane-strain shape of the strain ellipsoids. Several kinematic shear sense indicators including S/C fabrics, mantled porphyroclasts, oblique grain shape, micro-shears with bookshelf geometry and mineral fish show top-to-the-SE sense of shear. The quantitative kinematic analyses highlight that Wk varies between 0.64 and 0.78, implying a general shear flow with 44% < simple shear <58% and 42% < pure shear <57%. The exhumation of TMC is attributed to the exhumation-controlled transpressive tectonic process during oblique convergence between the Afro-Arabian continent and the Central-Iranian microcontinent which started since Upper Cretaceous.

A review and synthesis of strain studies in the Egyptian Nubian Shield: materials, methods, problems and prospects

More than seventy strain studies in the Egyptian Nubian Shield (ENS) are reviewed in this contribution. For the Hammamat basin strain studies, Rf/ϕ and Fry methods have been applied to pebble sections, mudcracks and raindrop prints. Consideration of pebble fabrics in undeformed outcrops suggests that RXZ estimates of less than 1.7 based on pebble ellipse data are doubtful. Extracted pebbles are also popular strained objects for analysis. Average Wadell sphericity and percentage of blades assist recognition of pebble strains in the Kareim, Queih, Igla and Seleimat basins. Feiran gneissic complex strains, measured using boudins and folds are much higher (X/Z > 15) than the reported strains for the Hafafit gneissic complex (X/Z usually < 2.5) that used metamorphic grain shapes and centres. The low Hafafit strains may result from progressive resetting of grain shapes and relocation of grains by diffusional effects during HT metamorphism. The Barramiya-Mubarak thrust belt provides a good example of focused study on ENS shear zones strain, with variable correlations between Rf/ϕ and Fry results, but consistently low k values. South Eastern Desert (SED) shear zone prolate strain ellipsoids record the effects of multiple deformation. The extensional shear zone overlying the Meatiq complex shows extreme strain and strongly prolate strain ellipsoids. ENS vorticity studies, applying PAR and RGN methods, have yielded higher Wm (> 0.5) for transpressional shears (Barramiya-Mubarak belt and Atalla shear zone) than for transtensional shears (Wm < 0.5). Volume strains have rarely been considered in the ENS, though evidence exists for negative (due to pressure solution) and positive (due to microveining) volume change during deformation. The ENS strain database can be improved by better documentation of sampling methods; better knowledge of primary fabrics; and by testing strain results using a range of materials. AMS-strain studies may broaden the range of lithologies suitable for strain measurement.

Quantitative finite strain and kinematic analyses of Dahomeyide orogenic belt (Benino-Nigerian shield) around Igarra, southwestern Nigeria

The Igarra area (including Sebe-Ogbe and Otuo) in Igarra schist belt, southwestern Nigeria exposes a good section of the Dahomeyide orogenic belt (Benino-Nigerian shield). The area provides an opportunity to investigate quantitative finite strain, types of strain and strain ellipsoid, kinematic vorticity number (Wk) and proportions of pure and simple shear components from metaconglomerate outcrops. Measurements of the long, short and intermediate axes of deformed clasts in the metaconglomerates were carried out. The angular relationships between a constant reference line and the long axes of some of the clasts were determined. Combined strain in two dimensions (2-D) and three dimensions (3-D) analyses involving the Rf/ɸ (Phi), centre to centre (Fry) and Flinn methods were performed on the data with the aid of Ellipsefit software to generate Flinn, the tear-drop shaped diagrams and finite strain ellipsoids. The kinematic vorticity numbers were estimated in order to quantify the amount of shear. The results indicated that the rocks in the Dahomeyide orogenic belt around Igarra have ellipticity in initial state (Ri) and true strain (Rs) of 3.6 and 4.4, respectively. The Ri and Rs for Otuo are 2.43 and 3.35, respectively while the values for Sebe-Ogbe are 1.96 and 3.16, respectively. The results of centre to centre method showed strain ellipses with different orientations from near horizontal to vertical, while the Flinn diagram showed that the rocks in the three areas have both constrictional and flattening strains corresponding to prolate and oblate strain ellipsoids, respectively which are characteristic of L-S tectonites. The kinematic vorticity number (WK or Wm) for the locations are 0.97 (Igarra), 0.79 (Sebe-Ogbe) and 0.97 (Otuo). These probably suggest homogeneous subsimple shear transpressional deformation involving approximately 42% pure shear and 58% simple shear for Sebe-Ogbe and 17% pure shear and 83% simple shear for both Igarra and Otuo. The state of strain of the rocks at Igarra, Otuo and Sebe-Ogbe was relatively high and increased towards the south from Igarra (north) to Sebe-Ogbe (south) where there is highest strain probably due to clast support nature of the metaconglomerates where the clasts acted as single rigid body, and influence of contact metamorphism. The implication of this is that the rocks at Igarra have undergone less deformation than those at Otuo which in turn have undergone less deformation than those at Sebe-Ogbe.

Active fault segmentation in Northern Tunisia

Active shortening structures in Northern Tunisia have developed by tectonic inversion since the Pliocene, after Late Miocene extensional collapse of the whole region. Restored Plio-Quaternary deformation observed on reflection seismic lines indicates deformation rates around 0.6–0.8 mm/yr in the studied segments and larger amounts of shortening to the West of Northern Tunisia (16%) than to the East (7%), which suggests tectonic inversion started earlier to the West and later propagated eastwards, reaching Northeastern Tunisia in the Late Pliocene. This shortening is registered on striated pebbles in Quaternary alluvial terraces and fault-slip data giving two populations of strain ellipsoids with N–S and WNW-ESE maximum shortening. Morphometric analysis in combination with field fault segmentation mapping show that topographic uplift and drainage rejuvenation occurs in relation to 20–30 km long ENE-WSW reverse fault segments and related antiforms that are offset and linked by E-W to WNW-ESE dextral and NE-SW-oriented sinistral faults. The largest fully linked fault system is the Alia-Thibar fault. This 130 km long fault zone shows an helicoidal geometry with five different fault segments, including reverse, dextral, sinistral and oblique faults. Due to the young age of tectonic inversion, after late Miocene extensional collapse of the region, the present relief of Northern Tunisia is characteristic of a young thrust and fold belt, with dominating axial valleys along synforms and an incipient transverse drainage development propagating from West to East.

Pure shear-dominated transpression and vertical extrusion in a strike-slip fault splay from the Itapirapuã Shear Zone, Ribeira Belt, Brazil

The Itapirapuã Shear Zone is a major NE-trending dextral transcurrent zone from the southern Ribeira Belt (Brazil), which was active during the Neoproterozoic Brasiliano-Pan African Orogeny, related with West Gondwana assembly. Macroscopic sigmoid-shaped blocks in its northernmost splay termination present a NE-trending subvertical tectonic foliation and subvertical stretching lineation suggesting a transpressional tectonics. This paper investigates this splay termination using integration of data from geological mapping, thermodynamic modeling, microtectonics, quartz c-axis fabrics, kinematic vorticity and finite strain analysis. Distinct rock assemblages are placed in tectonic contacts along the splay termination, including Statherian sillimanite-biotite paragneiss and granitic orthogneiss (Apiaí-Mirim Complex), Statherian phyllites (Bairro dos Prestes Formation), Calymmian chlorite-epidote-actinolite metabasites (Fazenda Velha Suite), Ediacaran phyllites, metalimestone and quartzite (Itaiacoca Group) and Ediacaran granite (Três Córregos Suite). Isochemical phase diagrams constrain synkinematic metamorphic conditions of 490–510 °C and 11–12.5 kbar (chlorite-biotite-phengite phyllite), 520–560 °C and 9–10 kbar (biotite-phengite phyllite) and 680–690 °C and 5–6 kbar (sillimanite-biotite paragneiss), with isothermal decompression paths. Microstructural evidence suggests that increase in metamorphic temperatures were accompanied by changes in the dominant deformation mechanism in quartz aggregates from subgrain rotation to grain boundary migration recrystallization. Significant contribution of dissolution-precipitation creep indicates water-abundant deformational conditions. Strain ellipsoids are oblate-shaped and present NE-trending subvertical XY planes and subvertical X-axes in agreement with field tectonic fabric. Quartz c-axis fabrics in XZ sections of the strain ellipsoid are dominated by symmetrical crossed girdles, while the fabrics present dextral asymmetry in horizontal YZ sections, consistent with macroscopic structural deflection, indicating a subvertical vorticity axis. Vorticity results indicate 48–59% of pure shear contribution. The data indicate that strain in the Itapirapuã Shear Zone northern termination was accommodated by a pure shear-dominated transpression with vertical extrusion at mid-crustal conditions, which contrasts with a transtensional deformation in the southeastern block.

Strain pattern and vorticity analysis of deformed conglomerates in the Heneshk area within the Sanandaj–Sirjan Metamorphic Belt, Zagros Mountains, Iran

Strain measurements have been carried out, using the Rf/ϕ method on a Jurassic deformed conglomerate, in the Sanandaj–Sirjan Metamorphic Belt, Zagros Mountain of Iran. Estimated finite strain ellipsoids range from general flattening to near plane strain (Flinn parameter K = 0.6–0.9) with strain ratios in the X/Z plane with Rxz = 5.4. Quantitative kinematic analyses were performed on the conglomerates using the rigid clast method. Estimated kinematic vorticity numbers (Wk) varied between 0.43 and 0.63, implying a general shear flow (30% < simple shear < 42% and 58% < pure shear < 70%, i.e., pure shear-dominated non-coaxial flow). Combining strain and vorticity data, we estimated a horizontal shortening of ~ 42–55% measured normal to the walls of the deformation zone. We discuss this pure shear-dominated deformation in terms of convergence between the Afro-Arabian continent and the Central-Iranian microcontinents.

Strain and vorticity analysis in the Zagros suture zone (W Iran): Implications for Neo-Tethys post-collision events

Shear zone development in the Zagros orogenic belt is a result of the oblique collision of the Arabian plate with the Iranian plateau. To distinguish areas deformed during the collisional phase from those deformed by post-collisional events, the coaxiality of deformation and the shape of the strain ellipsoid (SES) have been analyzed. The variation of strain intensity in each shear zone has been evaluated by calculating the strain ratio in the principal strain planes, the θ′ angle, and the kinematic vorticity number (Wk). In the post-collisional shear zones, Wk varies from 0.80 to 0.99 and θ′ from 16.7° to 44.01°; the strain component is mostly simple shear, though in some shear zones it is sub-simple shear. Outside of the shear zones, Wk is extremely low, varying from 0.00 to 0.26, and θ′ values are under 5°; a coaxial deformation regime is dominant and the strain component is pure shear. In most of the shear zones, SES is constricted and prolate, indicating that deformation occurred under transtensional conditions. However, in some regions, SES is flattened and oblate, as is common in transpressional zones. Shear zones that have evolved under dual deformation conditions suggest that transtensional events can be superimposed on transpressional structures.

Structural Evolution During Cyclic Glacier Surges: 2. Numerical Modeling

The mesoscale structures of a glacier express the history of flow, temperature, and stress. Thus, in principle, numerical ice dynamics models have sufficient physics to examine the formation and transport of these structures. In this study we use a vertically integrated thermomechanical ice dynamics model to simulate the temporally evolving patterns of surficial moraine, stratification, foliation, and folding of glacier ice, and the density and orientation of traces of former crevasses. The modeled glaciers are simplified versions of Trapridge Glacier in northwest Canada that allow diagnostic modeling of influences on glacier structure and help to clarify the physics and numerics. In the model, surges occur every 50 years in response to a prescribed cyclic change in bed friction. Medial moraine patterns are simulated by tracking the englacial and supraglacial trajectory of debris injected at fixed points in the accumulation region. Stratification is assumed to be associated with isochronal surfaces, and vertical foliation is explained in terms of horizontal flattening of strain ellipsoids. Crevasses form when and where the intensity of tensile stress exceeds a prescribed threshold; crack damage is cumulative so that crevasse traces observed at sampling sites are a superposition of the damage accumulated en route. Folding is parameterized but not resolved. By evaluating the deformation gradient tensor along ice particle trajectories and applying the polar decomposition to this tensor, we isolate the cumulative effects of rotation and stretching by ice flow and calculate strain ellipsoids as well as other practical indicators of deformation.

Structures, strain analyses, and 40Ar/39Ar ages of blueschist‐bearing Heilongjiang Complex (NE China): Implications for the Mesozoic tectonic evolution of NE China

The high‐pressure Heilongjiang Complex belongs to a Jurassic accretionary complex in Yilan area, NE China. The complex is located in the junction of three orogenic systems in the NE Asia: the E‐W‐striking Central Asian Orogenic Belt (CAOB), the N‐S‐striking Paleo‐Pacific system, and the ENE‐WSW‐striking Mongol‐Okhotsk system. Thus, a detailed tectono‐metamorphic analysis coupled with accurate radiometric dating of the Heilongjiang Complex provided in this study represent important evidence for understanding the complex interactions between the three orogenic systems in the Late‐Palaeozoic to Mesozoic. New structural and microstructural data combined with the strain analysis highlight the presence of three distinct deformation events affecting the high‐pressure Heilongjiang Complex. The first deformation event D1 is related to development of blueschist facies prolate to plane strain fabrics that were subsequently reworked by the blueschist‐amphibolite facies event D2 that transposed all previous structures by the main west‐dipping schistosity. This planar fabric was folded by upright F3 folds and heterogeneously reworked by greenschist facies E‐W‐striking vertical cleavage during large scale N‐S shortening. Each deformation event was related to specific strain fabrics and metamorphic conditions highlighting dynamic evolution of the subduction complex during burial, exhumation, and final shortening. The first D1 event is related to westward directed prograde constrictional flow and progressive dismembering of competent mafic, ultramafic layers, and quartzites in highly stretched meta‐sedimentary matrix. This mélange was folded by recumbent detachment folds resulting in emplacement of the exhumed unit progressively reaching amphibolite facies conditions atop of W‐WSW‐dipping channel. This event is associated with dominant flattening implying increasing mechanical coupling between the colliding Songliao and Jiamusi blocks. Subsequently, the accreted subduction complex was reworked during N‐S‐directed horizontal shortening. This event was associated with an important post‐buckle flattening of fold limbs and rotation of competent blocks with inherited D1 and D2 strain ellipsoids along F3 fold axes. Our new 40Ar‐39Ar plateau ages on muscovite and phengite (173–177 Ma), together with previous radiometric data, are interpreted as reflecting exhumation of subduction channel during the blueschist‐amphibolite facies event while the N‐S shortening (D3) probably operated in Late Jurassic as indicated by some previously published cooling ages (~140–160 Ma). The westward‐oriented subduction and exhumation fabrics of the high‐pressure Heilongjiang accretionary complex followed by N‐S‐directed upright folding are interpreted to reflect transition from E‐W Pacific subduction regime to N‐S‐directed shortening related to the closure of the Mongol‐Okhotsk Ocean.

Finite strain of the Tertiary rocks and their relation to tectonic deformation at Al-Tib Anticline in Missan governorate, Southeastern part of Iraq

This paper presents the finite strain results from seven oriented samples data on Tertiary sandstone of Muqdadiya Formation and (400) samples of pebbles and conglomerate of Bai –Hassan Formation at the southwestern limb of Al-Tib Anticline in the Southeastern part of Iraq. Measurement and analysis of finite strain are carried out including these rocks at fluvio- lacustrine environment. The present study followed Fry method. The computed strain was, in the form of ellipses, within three prepared perpendicular planes in a single sample and Center to Center method was used to determine the strain ratio of the these samples. The strain in the studied area is low, this is mainly due to the sampled rocks underwent brittle deformation during folding. The calculated axial ratios (Rs), (three dimensional orientations of the strain axes) of strained rocks are shown which is equivalent to the log Flinn diagram normally used for plotting strain ellipsoids and according to the stereographic projection method, the long axes of this ellipsoid are sub parallel to measured Al-Tib Anticline axis and the short axes are approximately perpendicular to the axis of Al- Tib Anticline in the study area. This orientation of these axes is related to the movement of the Arabian plate.

Finite Strain Analysis of the Wadi Fatima Shear Zone in Western Arabia, Saudi Arabia

Neoproterozoic rocks, Oligocene to Neogene sediments and Tertiary Red Sea rift-related volcanics (Harrat) are three dominant major groups exposed in the Jeddah tectonic terrane in Western Arabia. The basement complex comprises amphibolites, schists, and older and younger granites unconformably overlain by a post-amalgamation volcanosedimentary sequence (Fatima Group) exhibiting post-accretionary thrusting and thrust-related structures. The older granites and/or the amphibolites and schists display mylonitization and shearing in some outcrops, and the observed kinematic indicators indicate dextral monoclinic symmetry along the impressive Wadi Fatima Shear Zone. Finite strain analysis of the mylonitized lithologies is used to interpret the deformation history of the Wadi Fatima Shear Zone. The measured finite strain data demonstrate that the amphibolites, schists, and older granites are mildly to moderately deformed, where XZ (axial ratios in XZ direction) vary from 2.76 to 4.22 and from 2.04 to 3.90 for the Rf/φ and Fry method respectively. The shortening axes (Z) have subvertical attitude and are associated with subhorizontal foliation. The data show oblate strain ellipsoids in the different rocks in the studied area and indication bulk flattening strain. We assume that the different rock types have similar deformation behavior. In the deformed granite, the strain data are identical in magnitude with those obtained in the Fatima Group volcanosedimentary sequence. Finite strain accumulated without any significant volume change contemporaneously with syn-accretionary transpressive structures. It is concluded that a simple-shear deformation with constant-volume plane strain exists, where displacement is strictly parallel to the shear plane. Furthermore, the contacts between various lithological units in the Wadi Fatima Shear Zone were formed under brittle to semi-ductile deformation conditions.

A strain state in synthetic diamond crystals by the data of electron backscatter diffraction method

The authors put forward a procedure of determination of strain tensor components through the analysis of distribution of intensity of back-scattered electrons in Kikuchi patterns. The strain state has been studied in local areas of a synthetic diamond crystal produced by the temperature gradient method in the Fe–Al–C system through growing onto a diamond single crystal synthesized in the Ni–Mn–C system. Characteristic surfaces of the strain tensors and strain ellipsoids have been plotted; special features of strain distribution in the crystal have been analyzed. Diagonal tensor components have been determined from the changes of distributions of intensity of individual bands, the other components have been found from the displacements of axes of zones relative to their positions in the standard Kikuchi pattern.