Subhorizontal Foliation Research Articles

Structural Analysis for Qazzaz Metamorphic Core Complex, Northwestern Arabian Shield, Saudi Arabia

AbstractIdentifying deformational mechanisms and associated structures at various scales, ranging from regional‐scale structures to microscopic fabric, is crucial for the assessment of tectonic development. Thirty‐three samples were taken from the Qazzaz metamorphic core complex to estimate the finite strain for felsic and mafic minerals. These samples included gneisses rocks, monzogranite, and metavolcano–sedimentary rocks for both the Thalbah and Bayda groups. Using the Rf/j and Fry methods, the axial ratios (XZ) range about 2.20 to 7.10 and 1.90 to 9.10, respectively. For various rock units, the strain measurements show moderate to highly deformation. Most of the observed samples show shallow WNW dipping along a N to WNW trend of finite strain (X). The short axes (Z) based to be subvertical foliation related with a subhorizontal foliation. The results demonstrate that contacts generated at semi‐brittle to ductile deformation and that the strain of magnitude has the same value for different lithologic units. It concluded that nappe generation in orogens results from pure shear deformation.

Polyphase deformation of staurolite-bearing metapelite in the Chinese Altai: Recording a Devonian-Permian tectonic regime switch in the southwestern Central Asian Orogenic Belt

Recent studies revealed that the Chinese Altai may have experienced two major tectonothermal events in the Devonian and Permian but their mutual relationships remain poorly constrained. In this study, we carried out a comprehensive analysis of macro- and micro-structural features of the Ordovician schists that were subjected to Barrovian-type metamorphism in the Chonghuer area of the northwestern Chinese Altai. Field structural and microstructural analyses revealed that metamorphic foliation S1 was folded by the recumbent fold F2, forming the sub-horizontal foliation S2. The S2 fabric was later reworked by the D3 transpressional shearing along the narrow sillimanite-bearing zone to form the foliation S3. Monazite U–Pb ages obtained from the staurolite schist located close to the D3 shear zone, as well as the zircon U–Pb ages from both folded and unfolded pegmatites that cross-cut granite, indicate that the D3 deformation took place during the Permian period, approximately 257–250 Ma. The EBSD study of quartz in the matrix and staurolite porphyroblasts conducted along the NE-SW cross-section allowed us to: 1) constrain the medium-temperature D2 and high-temperature D3 regimes and, 2) reconstruct foliation intersection axis (FIA) patterns both inside and outside the D3 deformation zone. The FIA between Devonian D1 and D2 preserved in staurolite porphyroblast was initially oriented in a N–S direction, which was ultimately reoriented to a NW-SE direction after undergoing the D3 shear deformation. This result is further reinforced by the numerical stochastic modelling, which quantifies the influence of the Permian HT sinistral transpression on the reworking of the Devonian FIA fabrics. Micro-structural data of this study verifies the significant tectonic switch from the Devonian E-W contraction to the Permian NNW-SSE transpression. Quantitative EBSD-based FIA analysis proves to be an effective approach for the structural analysis of polycyclic orogenic systems.

Microstructural and strain studies of neoproterozoic rocks encapsulating hammamat molasse deposits in theWadi El-Qash area, Central Eastern Desert, Egypt

Understanding deformational processes and related structures at various scales, necessitates an estimate of microstructural and finite strain characterization of deformed rocks. In the current research, 23 collected samples of metaconglomerates, metsediments, and metavolcanics from the Wadi El-Qash region, placed in the Central Eastern Desert of Egypt, were used to quantify the finite strain of quartz, feldspar porphyroclasts, and mafic grains. Finite strain techniques were utilized to produce and analyze 46 thin sections. The metaconglomerate rocks have undergone high to moderate grades of deformation, according to the strain results. Using the Rf/φ and the Fry techniques, the axial ratios in the XZ section vary from 2.20 to 5.40 and from 2.00 to 5.30, respectively. The strain results point out high to moderate rates of deformation of the metaconglomerates, metsediments, and metavolcanic rocks. The direction of finite strain in X axis exhibits shallow WNW dipping and grouping following an N-to-WNW trend in most of the measured samples. The subvertical and subhorizontal foliation related to the short axes in Z axis. The results show that contacts developed in semi-brittle to ductile deformation and the magnitude of strain is the same for all lithologic units that are exposed in the studied area. Also, these results contradict the generally accepted belief that simple-shear deformation is responsible for nappe development in orogens.

Structural evolution and Microstructural analysis for al Faydh area, southern Arabian shield, Saudi Arabia

Al Faydh area is a major significant tectonic structure in southwestern part of Arabian shield, and it was estimated using finite strain analysis and microstructural investigation. Our research focuses structural features of the Al Faydh area in order to obtain field identification and better comprehend the tectonic setting for studied region. The Rf/φ and Fry techniques were used the most of quartz and feldspar for felsic minerals and biotite and hornblende for mafic minerals. The axial ratios with the mean average about 1.09–1.83 (using Rf/φ method) and 1.13 to 1.95 (using the Fry method) in the XZ section for the collected samples of metavolcanic, metasedimentary and meta-granite (gneisses). The finite strain direction displays clustering with slight plunging along a W–E trend along the long axes. The Z axes are related the sub-vertical and sub-horizontal foliation. The majority results suggest oblate strain symmetry and there is noticeable rise in strain magnitudes toward the tectonic contacts. The volume change was related the accumulation of finite strain in the different lithologies during a polyphase deformation regime. Applying the finite strain and microstructural analysis data from Al Faydh region concluded that the structural contacts for different lithologies were generated during faults, plutonic intrusion and fracture zones. Also, the deformation in the Al Faydh region are formed under brittle to semi-ductile deformational with simple shearing.

Mantle flow under the Central Alps: Constraints from shear-wave splitting for non-vertically-incident SKS waves

Shear-wave splitting measurements map the orientation of geodynamically induced fabrics in the upper mantle. However, due to the limited depth resolution of shear-wave splitting investigations from core-mantle refracted phases, it has so far been difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To improve this differentiation, we take advantage of backazimuthal variations of the fast orientation ϕ and delay time Δt that is expected for an olivine-mantle block with vertical b-axis as compared to vertical c-axis. This suggests that shear-wave splitting observations can in principle provide constraints that have so far not been recognized. The two end member cases can occur depending on the type of deformation, for example, a subhorizontal foliation in the case of a Simple Asthenospheric Flow and a subvertical foliation when considering transcurrent/transpressional circumstances during Vertical Coherent Deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Fast orientations ϕ show a spatially coherent and relatively simple mountain-chain-parallel pattern without large azimuthal variations per station, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the entire study area, our procedure indicates b-up olivine and thus favors an asthenospheric source of the anisotropy, with a horizontal flow plane of deformation. We also test the influence of position relative to the (European) slab under the Central Alps, distinguishing northern and southern subarea based on vertically-integrated travel times calculated from a tomographic model. Differences in the statistical distribution of splitting parameters ϕ and Δt, and in the backazimuthal variation of their variation δϕ and δΔt, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of planar Poiseuille flow around the Alps, the origin in the southern subarea remains uncertain, but it is likely to be affected by the slab itself.

Petrology and kinematic vorticity constraints of the Al Amar region, Northern Ar-Rayn terrane, Eastern Arabian shield, Saudi Arabia

The Neoproterozoic basement rocks in Ar Rayn terrane set as parts in the Eastern Arabian Shield. It focus the petrological and kinematic vorticity for high deformed volcanic and granitoids rocks in the Al-Amar locality. The volcanic rocks divide into two main series, the eastern sequence cycle represents dacite, rhyodacite, rhyolite and ignimbrites, and the western sequence cycle characterizes andesite and pyroclastics. Furthermore, the granitoids rocks content monzodiorite, tonalite, granodiorite and alkali-feldspar granite. For assessing the proportions of pure and simple shear contributions in penetrative deformed rocks, the kinematic vorticity number (Wm) to the mean appreciation by applying porphyroclast, rotating in a flowing matrix is crucial. In the Ar Rayn terrane of Arabian shield, kinematic vorticity numbers for high deformed metavolcanics and granitoids gneisses vary from 0.4 to 0.6. The deformation in the studied area varied from simple shear, according to vorticity and strain studies. Nappe stacking getting ready during the thrusting activity only through the Al Amar –Idsas fault, according to the results, most likely due to brittle imbrication. A penetrative sub-horizontal foliation formed sub-parallel to the nappe contacts with the beneath laying nappes, showing that the ductile strain was superimposed on the construction of the nappe under high pressure. The Ar Rayn terranewas created sub-horizontal foliation by the buildup of ductile strain during under plating, which was induced by a vertical shortening component. This foliation was formed in the majority of cases during the thrusting nappes upon each other, implying that nappe stacking was related to vertical shortening due to a massive scale by the active Al Amar –Ideas fault.

Structural Setting and Kinematic Analysis of the Halaban Region, Eastern Arabian Shield, Saudi Arabia

AbstractThe vorticity analysis technique was applied to measure the different lithological units, such as schist, metagranite and metavolcano‐sedimentary rocks, which are present in the Halaban region. This work aims to interpret the relationship between the different lithologies and the tectonic setting, in order to elucidate the nature of kinematic analysis in the Halaban region. The kinematic analyses were applied to feldspar porphyroclasts, quartz and hornblende for twenty‐six samples. The kinematic vorticity number (Wm) for deformed rocks in the study area ranged from ∼0.6 to 0.9. The direction of the long axes for finite strain data (X axes) revealed a WNW trend with shallow dipping. The direction of the short axes for finite strain data (Z axes) were represented by vertical with associated horizontal foliation. The results of the kinematic vorticity and strain analyses are characterized by simple shear with different degrees of deformation in the Halaban region. Furthermore, our finite strain data shows no significant volume change during deformation. The subhorizontal foliation was synchronized with thrusting and deformation. Furthermore, throughout the overlying nappes, the same attitudes of tectonic contacts are observable, the nappes in the orogens being formed from simple shear deformation.

Structural, metamorphic and geochronological constraints on Palaeozoic multi-stage geodynamic evolution of the Altai accretionary wedge system (Hovd Zone, western Mongolia)

A polyphase tectono-metamorphic evolution of the Mongolian tract of the Central Asian Orogenic Belt (CAOB) is inferred from a structural, petrological and geochronological study in the Hovd Zone at the NE periphery of the Altai Domain. Four tectonic events are recorded here. A relict sub-vertical fabric (S1) is associated with the growth of garnet cores and staurolite, typical of MP–MT Barrovian-type metamorphism (M1). This fabric was isoclinally folded and overprinted by sub-horizontal foliation (S2) associated with growth of garnet rims, sillimanite and cordierite, typical of LP–HT metamorphism (M2). The P–T path inferred from petrology and thermodynamic modelling shows that the peak conditions reached 590–610 °C and c. 5.8 kbar (M1) and were followed by decompression to 4–5 kbar associated with heating to 630–690 °C (M2). Subsequently, the studied region was strongly reworked by NW–SE trending upright folds (F3) locally associated with development of muscovite- and chlorite-bearing cleavage (S3). The region is weakly affected by post-metamorphic D4 overprint. Monazite in-situ electron-microprobe dating was complemented by LA–ICP-MS U–Pb dating, revealing three distinct texturally defined age groups at c. 455, 435 and 385 Ma. The ages at c. 455 Ma are related with monazite included in garnet cores, the age group at c. 435 Ma with monazite in garnet rims and in S2 foliation, while the ages of c. 385 Ma come mostly from monazite in chlorite–muscovite-bearing bands. Based on the monazite textural position and REE patterns, these ages are correlated with the tectono-metamorphic events D1–D3, reflecting short-lived Late Ordovician shortening, prolonged Middle Silurian–Early Devonian extension and Late Devonian–Early Carboniferous compression, respectively. Alternation of shortening and extension periods is interpreted as being caused by tectonic switching, i.e. repeated alternation of advancing and retreating subduction regimes. The latest Permian event resulted from the collision of the Mongolian orogenic collage with the Tarim and North China cratons in the south and caused rotation of previous fabrics to the current configuration.

Paleocene to Miocene migmatization and kinematics of the deformation at the northern boundary of the Xolapa Complex: implications for the Chortís Block-southern Mexico connection

ABSTRACT The Xolapa Complex of southern Mexico preserves evidence of its connection with the Chortís Block of Central America. Orthogneisses yielded U-Pb protolith ages from 172 to 169 Ma and paramigmatites are dated at 217 Ma. El Salitre and the Amatlán granitoids show crystallization ages of 65 and 59–57 Ma, respectively. In central Xolapa, diatexites have ages from 38 to 31 Ma, and granites were dated from 30 to 29 Ma. In the southeastern Xolapa, crystallization ages of 45 Ma and 23 to 21 Ma were obtained from migmatites and anatectic granites, respectively. Previous data indicate three diachronous stages of crustal melting along the Xolapa block. Structural data reveal that the Xolapa Complex is tectonically juxtaposed against the Acatlán Complex through ductile zones. Two mylonitic foliations are predominant in the northern boundary of the Xolapa Complex: (i) a sub-vertical NW-SE foliation with subhorizontal or low-plunging stretching lineation and left-lateral kinematics; and (ii) a subhorizontal NE-SW foliation with normal plunging stretching lineation and top-to-NE kinematics. The first mylonitic event related to the sinistral eastward motion of the Chortís Block occurred between 58 and 54–50 Ma. The second event took place in central Xolapa after or synchronous with the 38–31 Ma emplacement of diatexites. This event is related to an extensional phase that uplifts the Xolapa Complex with concerning the Acatlán Complex. In the southeastern region, deformation occurred after or synchronous with the emplacement of anatectic granites dated at 23–22 Ma. Our geochronological and structural data delineated an eastward decreasing age trend of migmatization, whereas the mylonitic deformation reported through the Xolapa Complex is consistent with a diachronous migration and detachment of the Chortís Block relative to the Acatlán Complex from Paleocene to early Miocene times.

Late Paleozoic tectonic transition from subduction to collision in the Chinese Altai and Tianshan (Central Asia): New geochronological constraints

The evolution of the largest accretionary orogen in the world, the Central Asian Orogenic Belt (CAOB), involved a prolonged accretion history since the Neoproterozoic, followed by a collisional phase in response to the closure of the Paleo-Asian Ocean in the latest Paleozoic. The exact process for the tectonic transition from subduction to collision is still poorly constrained. Here we address this issue by investigating the late Paleozoic tectonic evolution of the Chinese Altai and Tianshan orogens in the western CAOB. We provide new geochronological data from two areas of the Chinese Altai and Tianshan orogens, which allow us to link polyphase deformation with orogenic processes. In the Fuyun area of the Chinese Altai Orogen, we conducted monazite U-Pb dating on four samples that show pervasive foliations with the originally sub-horizontal orientation (DS2/DQ2). The monazite U-Pb ages cluster at ∼284 to 281 Ma, which interpreted to represent the time of sub-horizontal foliations (DS2/DQ2) that may result from orogen-parallel extension related to the collision of the Chinese Altai Orogen with the East Junggar Terrane. Farther south, in the Gangou area of the Chinese Tianshan Orogen, we obtained a muscovite 40Ar/39Ar plateau age of 256.6 ± 0.6 Ma for a mica schist from the dextral South Central Tianshan Shear Zone. This age confirms the Permian activity of dextral strike-slip deformation (DCT4) in the Chinese Tianshan Orogen. In contrast, three mylonitic schist/granitoid samples from the dextral Main Tianshan Shear Zone are characterized by 40Ar/39Ar plateau ages of 353.9 ± 1.9 Ma (biotite), 353.9 ± 1.5 Ma (biotite) and 352.1 ± 0.7 Ma (muscovite). We interpret these early Carboniferous ages to either represent a pre-Permian dextral shearing event, or to record an early Carboniferous tectono-thermal event with recrystallized micas not reset during the Permian strike-slip deformation (DCT4). An additional 40Ar/39Ar plateau age of 280.9 ± 0.5 Ma (hornblende) from a mafic dike (dolerite) that crosscuts macroscopic folds (DST2) in the southern Chinese Tianshan Orogen, provides a minimum time constraint for these folds. This age supports the simultaneous folding deformation (DST2) with dextral shearing (DCT4) in the Chinese Tianshan Orogen. Combined with a comprehensive synthesis of available geological and geochronological data, we argue that orogen-parallel extension and transpressional tectonics might have played a significant role in the late Paleozoic arc/continental amalgamation of the western CAOB.

A Middle Crustal Channel of Radial Anisotropy Beneath the Northeastern Basin and Range

AbstractA challenge in interpreting the origins of seismic anisotropy in deformed continental crust is that composition and rheology vary with depth. We investigated anisotropy in the northeastern Basin and Range where prior studies found prevalent depth‐averaged positive radial anisotropy (VSH > VSV). This study focuses on depth‐dependence of anisotropy and potentially distinct structures beneath three metamorphic core complexes (MCCs). Rayleigh and Love wave dispersion were measured using ambient noise interferometry, and Bayesian Markov chain Monte Carlo inversions for VS structure were tested with several (an)isotropic parameterizations. Acceptable data fits with minimal introduction of anisotropy are achieved by models with anisotropy concentrated in the middle crust. The peak magnitude of anisotropy from the mean of the posterior distributions ranges from 3.5–5% and is concentrated at 8–20 km depth. Synthetic tests with one uniform layer of anisotropy best reproduce the regional mean results with 9% anisotropy at 6–22 km depth. Both magnitudes are plausible based on exhumed middle crustal rocks. The three MCCs exhibit ~5% higher isotropic upper crustal VS, likely due to their anomalous levels of exhumation, but no distinctive (an)isotropic structures at deeper depths. Regionally pervasive middle crustal positive radial anisotropy is interpreted as a result of subhorizontal foliation of mica‐bearing rocks deformed near the top of the ductile deformation regime. Decreasing mica content with depth and more broadly distributed deformation at lower stress levels may explain diminished lower crustal anisotropy. Absence of distinctive deep crustal VS beneath the MCCs suggests overprinting by ductile deformation since the middle Miocene.

Crustal Deformation in Southern California Constrained by Radial Anisotropy From Ambient Noise Adjoint Tomography

AbstractWe build a new radially anisotropic shear wave velocity model of Southern California based on ambient noise adjoint tomography to investigate crustal deformation associated with Cenozoic evolution of the Pacific‐North American plate boundary. Pervasive positive radial anisotropy (4%) is observed in the crust east of the San Andreas Fault (SAF), attributed to subhorizontal alignment of mica/amphibole foliation planes resulting from significant crustal extension. Substantial negative anisotropy (6%) is revealed in the middle/lower crust west of the SAF, where high shear wave speeds are also observed. The negative anisotropy could result from steeply dipping amphibole schists in a shear zone developed during Laramide flat slab subduction. Alternatively, it could be caused by the crystal preferred orientation (CPO) of plagioclase, whose fast axis aligns orthogonally to a presumed subhorizontal foliation. The latter new mechanism highlights potentially complex CPO patterns resulting from different lithospheric mineralogy, as suggested by laboratory experiments on xenoliths from the region.

Finite strain analysis for deformed Neoproterozoic rock associations of the Al Amar fault zone, Eastern Arabian Shield, Saudi Arabia

The Al Amar area has played important role to effect the Al Amar–Idsas fault zone, which consists of a sequence of volcanic rocks and sedimentary detrital. Also, it is characterized by a difference in facies with intruded granitoid bodies. The present work has used the finite strain techniques to investigate the relationship of the different lithologies and nappe contacts, which recorded in the Al Amar area. Furthermore, the microstructural analysis has investigated the Abt schist, deformed granitoids, and metavolcanic-sedimentary rocks, which represented the Al Amar area. The axial ratio results average from 1.55 to 2.87 for the Rf/φ method and 1.43 to 7.51 for the Fry method in the XZ sections for metamorphic rocks. In the finite strain data, the long axes direction (X) displays grouping along a NW–SE trend with shallow plunging. The direction of short axes (Z) displays subvertical and accompanied with a subhorizontal foliation. Our strain measurement reveals that strain symmetry shows an oblate shape (flattening) and the strain magnitudes do not display any valuable increasing towards the tectonic nappe contacts. Also, it is indicated that the volume change in the different rock units was unassociated with the accumulation of finite strain. In the investigated area, the subhorizontal foliation in the different lithologies was parallel to thrusting. Furthermore, it shows mainly the similar attitudes for tectonic contact nappes. In the present work, it is suggested that the ductile strain is accumulated during thrusting and involves as vertical shortening component with the subhorizontal foliation. The field observations with finite stain data are not sufficient to decide that the tectonic contact in the orogeny is formed as the simple-shear deformation.

Strain Analysis and Microstructural Investigation of the Jabal Tays Ophiolite Complex, Eastern Arabian Shield, Saudi Arabia

The present study deals with the Jabal Tays ophiolite area, which was affected by Al Amar-Idsas fault. It is one of the most important tectonic features in the Eastern Arabian Shield. This study aims to accomplish through field investigation, finite strain and microstructural analysis for understanding the nature of tectonic evolution and the deformation in the Jabal Tays area. Field and microscopic investigations indicate that mafic-ultramafic rocks at Jabal Tays include the large amount of serpentinite, subordinate amounts of gabbro intruded by mafic dikes, melange, serpentinite schist, and listwaenite. The serpentinite makes up the bulk of Jabal Tays, is variably sheared and typically consists of relatively massive serpentinite cut by shear zones marked by serpentinite schist. Based on finite strain and microstructural analysis, mafic-ultramafic rocks in the Jabal Tays area are generally confined to brittle-ductile shear zones, which they are sub-parallel to the prevailing NW or NNW structural trends. It is confined mainly to the metamorphic terrains, especially to zones intruded by mafic volcanic rocks. It is concluded that the contacts between the Abt schist and metavolcano-sedimentary rocks were formed during progressive thrusting under brittle to semi-ductile deformation conditions by simple shear that also involved a component of vertical shortening, causing subhorizontal foliation in the Jabal Tays area.

Critical-wedge theory and the Mesozoic accretionary wedge of New Zealand

We apply principles of critical-wedge theory to explain the Jurassic and Early Cretaceous tectonic development of the Mesozoic accretionary wedge of New Zealand, one of the best-exposed subduction complexes on Earth, by summarizing factors that can lead to drastic changes in wedge balance. The wedge has a Carboniferous to Cretaceous (∼330-110 Ma) stratigraphic record, with an early Cretaceous (∼145-130 Ma) hiatus in clastic deposition across the Esk Head Melange near the front of the wedge. In the rear of the wedge, the age of moderately high-pressure metamorphism of the Otago Schist, which represents the forearc-high of the wedge, occurred near the end of this stratigraphic hiatus. The Otago Schist has a subhorizontal foliation indicative of underplating as the mode of accretion. The outer (frontal) part of the wedge has a subvertical foliation indicative of frontal accretion. We propose that in the Jurassic and earliest Cretaceous the wedge grew by frontal accretion. By ∼135 Ma the wedge became too long and was subcritical. This resulted in tectonic erosion of the front of the wedge. The tectonically eroded material was underthrust/underplated beneath the rear of the wedge to form the subhorizontally foliated Otago Schist. Colliding seamounts and/or oceanic plateaux (preserved in the Esk Head Melange) may have aided wedge instability. The proposed wedge instability and increased horizontal shortening was largely coeval with major changes in the chemistry of subduction-related arc magmatism and a phase of shortening and high-pressure metamorphism in the arc. Our model provides a holistic and coherent interpretation explaining the major tectonic features of the long-lived New Zealand part of the east Gondwana subduction zone.

Microstructural Study and Strain Analysis of Deformed Neoproterozoic Lithologies in the Um Junud Area, Northern Nubian Shield

Estimation of finite strain and microstructural analysis of deformed rocks are keys to better understanding deformational processes and related structures in a variety of scales started from microscopic fabric development to regional-scale structures. In the present work, we carried out the quantitative calculation of strain using the Rf/φ and Fry methods for quartz, feldspar and mafic grains (e.g. biotite and hornblende) from twenty two collected samples for granitic gneiss, amphibolite and hornblende schist samples from the Um Junud area situated in south Eastern Desert of Egypt. Forty four thin sections were prepared and measured by using finite strain methods. The strain data indicate high to moderate ranges of deformation of the amphibolite to granitic rocks. The axial ratios in the XZ section range from 1.74 to 4.37 and 1.50 to 4.46 for the Rf/φ and the Fry methods respectively. The finite strain direction for the long axes displays clustering along N to WNW trend, and shallow WNW plunging in the majority of the studied samples. The short axes are found to be subvertical associated with a subhorizontal foliation. It is concluded that finite strain is of the same order of magnitude for various lithologic units outcropping in the area, and that contacts were formed under semi-brittle to semi-ductile deformation conditions. Thus, the finite strain accumulated during superimposed deformation on a previously nappe structure assemblage, which pointed out that these contacts were created during the accumulation of finite strain. This result is inconsistent with the generally believed that nappe creation in orogens carried out by simple shear deformation.

Discriminating Tectonic and Magmatic Fabrics in the Remal Granite Gneiss: Implications for Terrane Amalgamation Processes in Southeastern Singhbhum, India

The Remal granite gneiss body in the southeastern part of the Singhbhum Craton, near its contact with the Rengali Province, preserves two orthogonal penetrative foliations. The subhorizontal foliation defines cross and trough bedding structures, and is characterized by (i) systematic grain-size variation between the layers, (ii) the presence of feldspar laths and (iii) graphic intergrowth textures, confirming that it is primary and of igneous origin (Sign). Sign has a non-planar geometry and shows a spread in orientations, while a later foliation of tectonic origin is defined by alternating biotite-rich and biotite poor bands that define a consistently oriented gneissic foliation (S1). S1 gneissic layering is associated with the alignment of biotite flakes and myrmekite formation, and operated under low grade metamorphic conditions. S1 shows a remarkable similarity in orientation with the shear fabric along the amalgamation front of the Rengali Province with the Eastern Ghats Province further to the south, indicating that strain related to this regional strike-slip event can be identified even in southeastern Singhbhum.

Effect of Foliation and Fluid Viscosity on Hydraulic Fracturing Tests in Mica Schists Investigated Using Distinct Element Modeling and Field Data

Several hydraulic fracturing tests were performed in boreholes located in central Hungary in order to determine the in-situ stress for a geological site investigation. At a depth of about 540 m, the observed pressure versus time curves in mica schist with low dip angle foliation shows atypical pressure versus time results. After each pressurization cycle, the fracture breakdown pressure in the first fracturing cycle is lower than the refracturing or reopening pressure in the subsequent pressurizations. It is assumed that the viscosity of the drilling mud and observed foliation of the mica schist have a significant influence on the pressure values. In order to study this problem, numerical modeling was performed using the distinct element code particle flow code, which has been proven to be a valuable tool to investigate rock engineering problems such as hydraulic fracturing. The two-dimensional version of the code applied in this study can simulate hydro-mechanically coupled fluid flow in crystalline rock with low porosity and pre-existing fractures. In this study, the effect of foliation angle and fluid viscosity on the peak pressure is tested. The atypical characteristics of the pressure behaviour are interpreted so that mud with higher viscosity penetrates the sub-horizontal foliation plane, blocks the plane of weakness and makes the partly opened fracture tight and increase the pore pressure which decreases slowly with time. We see this viscous blocking effect as one explanation for the observed increase in fracture reopening pressure in subsequent pressurization cycles.

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.

The control of deformation partitioning and strain localization on porphyroblast behaviour in rocks and experiments

Multiple generations of sub-vertical and sub-horizontal foliations preserved as inclusion trails in garnet in mylonitic rocks from the hanging wall of the Main Central Thrust in the Himalayas indicate that these porphyroblasts did not rotate during thrusting. This result is predicated by (i) a consistent succession of 5 changes in FIA trend (foliation inflection/intersection axes in porphyroblasts) for samples where the orientation changes from porphyroblast cores to rims; (ii) sub-vertical and sub-horizontal foliations occur as inclusion trails around each of the 5 FIAs in the succession, which would not be the case if the garnet porphyroblasts rotated during subsequent phases of deformation as tectonism continued; (iii) a change in inclusion trail asymmetry immediately prior to the commencement of mylonitzation indicates top to the south thrusting only if the porphyroblasts had not rotated as they grew; (iv) the latter asymmetry matches truncated crenulation relics preserved within the mylonitic matrix foliation that indicate top to the south thrusting as the latter foliation formed. Partitioning of deformation into shortening and shearing components stops rotation of porphyroblasts during their growth and during following periods of ductile tectonism. This can be replicated via computer modelling by duplicating the crenulation-hinge-like coaxial environment in which porphyroblasts nucleate and grow before the strain intensifies. This was done using Drucker-Prager constitutive models with temperature-dependent strain softening behaviour and resulted in no porphyroblast rotation when followed by non-coaxial deformation no matter how intense. Furthermore, strain localization in the model containing competent objects of variable size, shape and orientation, produced no rotation during deformation involving components of shortening and shearing. These approaches to modelling mechanically resolve the sub-vertical/sub-horizontal foliations defined by inclusion trails and consistent FIA trend successions obtained from the Main Central Thrust rocks as well as in orogens elsewhere.