Lineation Direction Research Articles

Mapping Fault Distribution Using Lineation and Fault Fracture Density Methods in the Palu Basin and Surrounding Areas

Introduction: Research has been carried out with the title "Mapping Fault Distribution Using Lineation and Fault Fracture Density Methods in the Palu Valley and Surrounding Areas" with the aim of determining the direction of fault distribution and analyzing fault density based on lineation data. Methods: This research uses the lineation method with a total of 40 grids. Data processing uses Global Mapper 18, rockwork 17, PCI Geomatica 2014, and Arcgis 10.8 software. Result and Discussion: The results obtained show that the lineation data from the dominant rose diagram has a relatively east-west direction. Areas that have high fault density are in Lindu District, Palolo District, Nokilalaki District, Gumbasa District, Dolo District, Sigi Biromaru District, and Mantikulore District with fault density values of 5,945 - 18,950/km2. Conclusion: The direction of lineation obtained tectonically is influenced by the Palu-Koro fault which tends relatively north-south, the Graben Palolo A and Graben Palolo B faults which trend relatively southeast-northwest, and the Lore Lindu fault zone. Areas with high density are weak zones that are vulnerable to natural disasters and potential ground movements.

Paleo-Pacific plate subduction direction change (122−118 Ma): Insight from late kinematic plutons in the Wulian metamorphic core complex, Jiaodong Peninsula, eastern China

The late Mesozoic subduction of the paleo-Pacific plate (Izanagi plate) beneath Eurasia involved complex changes in the direction and angle of subduction, which have been controversial in recent decades. This study investigated Early Cretaceous late kinematic plutons of the Wulian metamorphic core complex that formed in the southern Jiaodong Peninsula. Based on geochemical and Sr-Nd isotopic analyses, we conclude that the granites in the Heshan (122 Ma) and Kuaijishan (118 Ma) plutons are I-type granites that formed in a back-arc extensional environment, and tensile fractures played an important role in the process of magma emplacement. Gravity anomaly inversion revealed the significant role of tension fractures in the uplift of the late kinematic plutons. Analysis of magnetic fabrics showed that the dominant plunging direction of magnetic lineation in the late kinematic plutons is NW-SE (127°−307°), while the plunging direction of the mineral stretching lineation in the wall rock is nearly E-W (102°−282°). We posit that the extensional stress field in the Jiaodong Peninsula underwent a significant transformation during the Early Cretaceous period, specifically from 135 Ma to 122 Ma, when it was dominated by a regional E-W or WNW-ESE extensional stress field. Later, from 122 Ma to 118 Ma, the stress field was controlled by a NW-SE extensional stress field. This variation in the regional stress field in the Jiaodong Peninsula indirectly indicates the changing subduction and slab rollback direction of the paleo-Pacific plate. From 135 Ma to 122 Ma, the paleo-Pacific plate subducted beneath Eurasia along a WNW-ESE direction. However, back-arc extension and slab rollback led to extensive extension in the Jiaodong Peninsula. Subsequently, the subduction direction of the paleo-Pacific plate shifted to the NW, with rollback occurring eastward at a high angle under the Jiaodong Peninsula from 122 Ma to 118 Ma. This shift resulted in extensive magmatism and widespread gold mineralization. The findings of this study offer new insights into the complex geodynamic processes that occurred during the Early Cretaceous period in the region.

Deformation in SW Anatolia (Turkey) Documented by Anisotropy of Magnetic Susceptibility Data

AbstractConvergence between the Eurasian and the African plates in the West Anatolian‐Aegean region results in a trench retreat due to slab roll‐back and tearing of the subducted African lithosphere. The upper plate response of this process gave way to back‐arc extension in the region. We have conducted a very detailed anisotropy of magnetic susceptibility (AMS) study on the Neogene rocks in SW Anatolia to unravel the style and magnitudes of deformation. For this purpose, from 83 sites in 11 structurally homogeneous domains, 1,680 paleomagnetic samples were analyzed. The results show that AMS fabrics are related to the tectonic deformation and that the magnetic lineation (maximum susceptibility axis, k1) is parallel to inferred maximum extension, while minimum susceptibility (k3) is typically normal to the bedding plane, corresponding to a preserved compaction associated with deposition fabric. The intermediate axis (k2) is parallel to a second extension direction and indicates that the region has been under the control of multi‐directional extension during the Neogene. Two main magnetic lineation directions are identified and represent Oligocene to middle Miocene E‐W, and late Miocene to Pliocene NW‐SE oriented extension. The magnetic lineation directions are dominantly parallel or perpendicular to the general strikes of the normal faults. The results show that the deformation in the region resembles two differentially stretched rubber sheets under the influence of SW oriented extension, exerted by the southward retreating Eastern Mediterranean subduction system.

New insights on the Neogene tectonic evolution of the Aksu Basin (SE Turkey) from the Anisotropy of Magnetic Susceptibility (AMS) and paleostress data

The junction between the Aegean and Cyprus arc in the southern Turkey forms a triangular-shaped morphological structure so-called Isparta Angle (IA). In Neogene time, the inner part of the Isparta Angle became overlain by sedimentary basins including Manavgat, Köprüçay and Aksu, characterized by marine clastics and carbonates.Aksu Basin, one of these marine basins, provides geological records about the Neogene crustal deformation and stress field in the Eastern Mediterranean region. Therefore, we kinematically assess if and when tectonic stress fields affected the Aksu basin in early Miocene to recent times. For this purpose, Anisotropy of Magnetic Susceptibility (AMS) data (AMS) of cored samples and fault-slip data from the brittle mesoscopic faults were collected. The ~490 oriented samples for Anisotropy of Magnetic Susceptibility measurements were analyzed and two different maximum magnetic lineation's directions are recognized; 1) N–S magnetic lineation in Pliocene and 2) from N–S to NW-SE magnetic lineation in the Miocene. The results inferred from more than 1000 fault-slip data collected from 83 different sites combined with AMS results demonstrate that the Aksu Basin developed under four different tectonic phases; 1) ~E-W extensional phase (related to basin formation), 2) ~N–S compressional (Lycian) phase, 3) ~(N)E-(S)W compressional (Aksu) phase and 4) N–S extensional (Recent) phase.

Transformation weakening: Diffusion creep in eclogites as a result of interaction of mineral reactions and deformation

The deformation of eclogites and the driving forces for their fabric development are an important topic, potentially allowing to determine deformation rates and stresses in subduction zones, where the greatest number of large earthquakes occurs. Here, fabric studies of grain size and shape, texture, and chemical composition from two locations of Variscan and Alpine eclogites are presented. All samples show a well-developed crystallographic preferred orientation (CPO) of omphacite with a strong maximum of [001] in the lineation direction and a weaker maximum of poles to (010) normal to foliation. Garnet shows no systematic CPO. Anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde (in terms of pressure change) P,T-path. The individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal misorientations. Chemical, microstructural, and CPO data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction. Dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures. The dominant deformation is inferred to be diffusion creep, where dissolution of material took place in reacting mafic phases (plagioclase, pyroxene) and precipitation took place in the form of new eclogite facies minerals (omphacite, garnet, zoisite). This type of diffusion creep deformation represents a transformation process involving both, deformation and metamorphic reactions. It is emphasized that the weakening is directly connected to the transformation and therefore transient. The weakening facilitates diffusion creep deformation of otherwise strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. The results imply low stresses during the deformation of eclogite blocks in subduction zones. These results can be applied to other rock types, too.

Kinematics, deformation partitioning and late Variscan magmatism in the Agly massif, Eastern Pyrenees, France

In order to constrain the finite deformation pattern of the Variscan basement of the Agly massif, a detailed structural analysis over the whole Agly massif was performed. Our investigation combined geological mapping, reappraisal of published and unpublished data completed with our own structural work. Results are provided in the form of new tectonic maps and series of regional cross-sections through the Agly massif. At variance from previous studies, we identified three deformation fabrics named D1, D2 and D3. The D1 deformation is only relictual and characterized by a broadly northwest-southeast striking and eastward dipping foliation without any clear mineral and stretching lineation direction. D1 might be attributed to thickening of the Variscan crust in a possible orogenic plateau edge position. The D2 deformation is a heterogeneous non-coaxial deformation, affecting the whole massif, that produced a shallowly dipping S2 foliation, and an anastomosed network of C2 shear zones that accommodated vertical thinning and N20 directed extension. D2 is coeval with LP-HT metamorphism and plutonism at ca. 315–295 Ma. D2 corresponds to the extensional collapse of the partially molten orogenic crust in a global dextral strike-slip at the scale of the whole Variscan belt. The D2 fabrics are folded and steepened along a D3 east-west trending corridor, called Tournefort Deformation Zone (TDZ), where the Saint-Arnac and Tournefort intrusives and surrounding rocks share the same NE-SW to E-W subvertical S3 foliation. Along the D3 corridor, the asymmetrical schistosity pattern and kinematic criteria suggest a D3 dextral kinematics. The D3 deformation is a record of E-W striking dextral shearing that facilitated and localized the ascent and emplacement of the diorite and granitic sheet-shaped plutons. D3 outlasted D2 and turned compressional-dominated in response to the closure of the Ibero-Armorican arc in a transpressional regime. The progressive switch from D2 thinning to D3 transpression is attributed to the lessening of gravitational forces at an advanced stage of extensional collapse that became overcome by ongoing compressional tectonic forces at the southern edge of the Variscan orogenic plateau.

Influence of loading and heating processes on elastic and geomechanical properties of eclogites and granulites

Increased knowledge of the elastic and geomechnical properties of rocks is important for numerous engineering and geoscience applications (e.g. petroleum geoscience, underground waste repositories, geothermal energy, earthquake studies, and hydrocarbon exploration). To assess the effect of pressure and temperature on seismic velocities and their anisotropy, laboratory experiments were conducted on metamorphic rocks. P- (Vp) and S-wave (Vs) velocities were determined on cubic samples of granulites and eclogites with an edge length of 43 mm in a triaxial multianvil apparatus using the ultrasonic pulse emission technique in dependence of changes in pressure and temperature. At successive isotropic pressure states up to 600 MPa and temperatures up to 600 °C, measurements were performed related to the sample coordinates given by the three principal fabric directions (x, y, z) representing the foliation (xy-plane), the normal to the foliation (z-direction), and the lineation direction (x-direction). Progressive volumetric strain was logged by the discrete piston displacements. Cumulative errors in Vp and Vs are estimated to be <1%. Microcrack closure significantly contributes to the increase in seismic velocities and decrease in anisotropies for pressures up to 200–250 MPa. Characteristic P-wave anisotropies of about 10% are obtained for eclogite and 3–4% in a strongly retrogressed eclogite as well as granulites. The wave velocities were used to calculate the geomechanical properties (e.g. density, Poisson's ratio, volumetric strain, and elastic moduli) at different pressure and temperature conditions. These results contribute to the reliable estimate of geomechanical properties of rocks.

Nature of Flow Patterns of Rajahmundry Lava, Gowripatnam Area, West Godavari, India:Insights from AMS Studies

Anisotropy of magnetic susceptibility (AMS) data for a single basaltic lava flow, herein named the Gowripatnam lava flow from the Rajahmundry Traps, are evaluated for determining precisely the mechanism of lava flow. At Rajahmundry, lava flows are found on both banks of the Godavari River and sandwich intertrappean sedimentary layers in between. The ones on the west bank of the river are studied here. This study has an implication as the mechanism of lava flow or nature of lava flow patterns of Rajamundry Trap basalt is still a debatable issue. AMS directions is a powerful tool for investigating the source and direction of lava. Its application to single lava flow from Rajahmundry indicates maximum direction of susceptibility axes in almost all possible directions, indicating radial flowage from radial vent source(s). Two AMS directions (towards north-west – 14%, and north – 7%), however, have the maximum number of petals. This indicates that at least there were palaeo-flowage patterns towards these directions. Incidentally the Godavari lineament strikes NW–SE and the flow dip is due south. However, the two dominant magnetic lineation directions are due north. Hence, the possibility of the earlier suggested river piracy model for lava flow can be ruled out. The fissure eruption near Rajahmundry is pointed out as the then prevailing lava flow mechanism. This is further supported by an overall random distribution of maximum susceptibility axes apart from two dominant flow directions.

The Effect of Lineation on Anisotropy in Dry and Saturated Himalayan Schistose Rock Under Brazilian Test Conditions

The paper presents experimental observations of the anisotropic effect of dry and saturated lineated schistose rock on deformational behaviour and fracture patterns under indirect tensile loading conditions. Dry mylonitic quartz–mica schist (M-QMS) shows strength minima in between lineation orientation of 20°–30° with loading direction, whereas dry crenulated quartz–mica schist (C-QMS) shows strength minima in between 45° and 60°. Water saturation significantly reduces the failure strength and shifts the strength minima towards a higher angle. In addition, the water saturation effect induces plasticity along the lineation direction. P-wave velocity is found to be maximum along lineation direction, which continuously decreases to a minimum value across the lineation. Three types of fracture patterns, namely layer activation, central and non-central, are observed for both the rock types. For both the rock types, tensional fractures occur when the lineation is inclined at 0° and 90° to the loading direction, whereas shear and mixed-mode fractures occur at all other orientations. Fracture through the layer activation is prominent in saturated specimens compared to the dry specimens for β angle range of 15°–60° and 15°–45° in the case of M-QMS and C-QMS, respectively.

On the geometric relationship between deformation microstructures in zircon and the kinematic framework of the shear zone

We present novel microstructural analyses of zircon from a variety of strained rocks. For the first time, multiple plastically deformed zircon crystals were analyzed in a kinematic context of the respective host shear zones. Our aim was to derive how the orientation of zircon grains in a shear zone affects their deformation, based on careful in situ observations. For sampling, we selected zircon-bearing rocks that were deformed by simple shear. Samples covered a range of P–T conditions and lithologies, including various meta-igneous and meta-sedimentary gneisses.Microstructural analyses of zircon crystals in situ with scanning electron backscatter diffraction mapping show strong geometrical relationships between orientations of: (i) the long axes of plastically deformed zircon crystals, (ii) the crystallographic orientation of misorientation axes in plastically deformed zircon crystals and (iii) the foliation and lineation directions of the respective samples. We assume that zircon crystals did not experience post-deformation rigid body rotation, and thus the true geometric link can be observed. The relationships are the following: (a) plastically deformed zircon crystals usually have long axes parallel to the mylonitic foliation plane; (b) crystals with <c> axes oriented at an angle>15° to the foliation plane are undeformed or fractured.Zircon crystals that have <c> axes aligned parallel or normal to the stretching lineation within the foliation plane develop misorientation and rotation axes parallel to the [001] crystallographic direction. Zircon grains with the <c> axis aligned at 30–60° to the lineation within the foliation plane often develop either two low Miller indices misorientation axes or one high Miller indices misorientation axis. Host phases have a significant influence on deformation mechanisms. In a relatively soft matrix, zircon is more likely to develop low Miller indices misorientation axes than in a relatively strong matrix.These relationships are independent of zircon's grain size and shape, and reflect the strong geometric control of the macroscopic kinematic rotation axis on the crystal-plastic deformation behavior of zircon and on the geometry of its slip systems. We describe previously unknown rheological and crystallographic properties of zircon, which suggest a tool for deriving an orientation of the plastically deformed zircon crystals with respect to the associated foliation and stretching lineation. Additionally, relationships between zircon deformation microstructures and the macroscopic kinematic framework have implications for zircon geochronology. If deformation events result in zircon distortion and, as a consequence, partial or complete resetting of the zircon isotopic system, the age of deformation can be accurately dated.

Petrofabric and seismic properties of lithospheric mantle xenoliths from the Calatrava volcanic field (Central Spain)

The microstructural and petrofabric study of peridotite xenoliths from the El Aprisco (Neogene Calatrava Volcanic Field) has provided new information on deformation mechanisms, ambient conditions and seismic properties of the central Iberian subcontinental mantle. Olivine, orthopyroxene, clinopyroxene, amphibole and spinel constitute the mineral assemblage in equilibrium. Their microstructure indicates that they accommodated crystal-plastic deformation under high water fugacity conditions. Crystallographic preferred orientation patterns of key minerals were determined with the EBSD technique. The xenoliths exhibit B, C and A olivine fabrics. B-type fabrics, involving the (010)[001] slip system, may develop in domains where deformation occurs under comparatively lower temperature, higher water-content and faster strain rates. They are interpreted here as the result of deformation in a suprasubduction mantle setting triggered by changing conditions imposed by a cooler subducting slab that incorporated fluids into the system. Xenoliths with olivine C-type fabrics involve activation of the dominant (100)[001] slip system, denote intracrystalline slip at higher temperatures and water-contents. They are here interpreted to sample lithospheric mantle domains where the impact of those new conditions was not so strong. Finally, the A-type fabrics, characteristic of the (010)[100] slip system, are frequent in the mantle under moderate to high temperature. These fabrics are considered here as characteristic of the mantle prior to subduction. The olivine fabrics constrain heterogeneous seismic properties. Propagation orientation of P waves (8.27–8.51km/s) coincides with olivine [100] axis concentrations, whereas the fastest S1 waves (5.13–5.22km/s) propagate parallel to [010] axis minima. The maximum shear wave birefringence (VS1–VS2=0.17–0.37km/s) is close to the direction of the macroscopic lineation. Heterogeneity of calculated seismic properties would concur with preservation of fossil flow perturbations, reworking and metasomatism in the subcontinental lithospheric mantle of central Spain, likely related to changes in the ambient conditions close to paleosubductions.

A paleomagnetic and rock magnetic study of the Manicouagan impact structure: Implications for crater formation and geodynamo effects

AbstractWe report rock magnetic and paleomagnetic data from the ~214 Ma Manicouagan (Canada) impact crater based on 25 widely distributed sites of impact melt and basement rocks collected at the surface as well as from boreholes drilled to depths ≤1.5 km. Titanomagnetite and titanohematite carry the magnetic remanence in impact melts above 320 m elevation and in most basement rocks. Impact melts below 320 m contain solely titanomagnetite. Magnetic susceptibility and saturation magnetization, proxies for titanomagnetite concentration, increase more than tenfold toward the base of the thickest impact melt that underwent fractional crystallization. The titanomagnetite‐enriched zone partially contributes to a 2000 nT magnetic anomaly in the crater's center. Stepwise demagnetization reveals a single, normal polarity magnetization component in all samples regardless of the magnetic phases present. Coeval lock‐in remanence times for titanomagnetite and titanohematite indicate that the titanohematite formed &gt;570°C during oxi‐exsolution. The average paleomagnetic direction and intensity coincide well with 214 Ma reference values. We find no evidence for an aberration of the geomagnetic field over the several thousands of years it took to cool a 481 m thick portion of the impact melt body. Hence, the energy released by the Manicouagan impact that created one of the 10 largest known craters on Earth provoked no measurable disturbance of the geodynamo. Magnetic anisotropy of clast‐free impact melts define magnetic lineation directions that are, in places, radially oriented with respect to the crater's center. Centrifugal flow of the melt within the evolving transient crater probably generated the fabric.

Applying the anisotropy of magnetic susceptibility technique to the study of the tectonic evolution of the West Spitsbergen Fold-and-Thrust Belt

We demonstrate the use of the anisotropy of magnetic susceptibility (AMS) method to determine the orientation of the principal tectonic strain directions developed during the formation of the West Spitsbergen Fold-and-Thrust Belt (WSFTB). The AMS measurements and extensive rock-magnetic studies of the Lower Triassic rocks reported here were focused on the recognition of the magnetic fabric, the identification of ferromagnetic minerals and an estimation of the influence of ferro- and paramagnetic minerals on magnetic susceptibility. At most sites, the paramagnetic minerals controlled the magnetic susceptibility, and at only one site the impact of ferromagnetic minerals was higher. The AMS technique documented the presence of different types of magnetic fabrics within the sampled sites. At two sites, a normal (Kmin perpendicular to the bedding) magnetic fabric of sedimentary origin was detected. This was associated with a good clustering of the maximum AMS axes imposed by tectonic strain. The Kmax magnetic lineation directions obtained here parallel the general NNW–SSE trend of the WSFTB fold axial traces and thrust fronts. The two other investigated sites possessed mixed and inverted fabrics, the latter of which appear to reflect the presence of iron-bearing carbonates. Keywords: AMS; Eurekan orogeny; tectonic strain; rock-magnetism; Hornsund–Sorkapp area; magnetic fabric. (Published: 16 December 2016) Citation: Polar Research 2016, 35, 31683,http://dx.doi.org/10.3402/polar.v35.31683

Impact‐related noncoaxial deformation in the Pułtusk H chondrite inferred from petrofabric analysis

AbstractPetrofabrics in chondrites have the potential to yield important information on the impact evolution of chondritic parent asteroids, but studies involving chondritic petrofabrics are scarce. We undertook an analysis of the Pułtusk H chondrite regolith breccia. Measurements of anisotropy of magnetic susceptibility and quantitative tomographic examination of metal grains are presented here and the results are compared with petrographic observations. The major fabric elements are in Pułtusk shear fractures cutting the light‐colored chondritic clasts as well as brittly and semibrittly deformed, cataclased fragments in dark matrix of regolith breccia. Cataclasis is accompanied by rotation of silicate grains and frictional melting. Fabric of metal grains in chondrite is well defined and coherently oriented over the breccia, both in the clasts and in the cataclastic matrix. Metal grains have prolate shapes and they are arranged into foliation plane and lineation direction, both of which are spatially related and kinematically compatible to shear‐dominated deformational features. We argue that the fabric of Pułtusk was formed in response to impact‐related noncoaxial shear strain. Deformation promoted brittle cataclastic processes and shearing of silicates, and, simultaneously, allowed for ductile metal to develop foliation and lineation. We suggest that plastic flow is the most probable mechanism for the deformation of metal grains in the shear‐dominated strain field. The process led also to the formation of large metal nodules and bands in the dark matrix of breccia.

Magnetic anisotropy as a tracer of crystal accumulation and transport, Middle Banded Series, Stillwater Complex, Montana

Fabric studies of layered mafic intrusions have led to improved understanding of the mechanical processes operating in large magma chambers, including crystal accumulation and crystal mush deformation. Such studies, however, are typically limited by a tradeoff between breadth (number of sites studied, characteristic of field-focused work) and sensitivity (ability to discern subtle fabric elements, characteristic of laboratory fabric analyses). Magnetic anisotropy, if analyzed in a systematic way and supported by single-crystal and petrofabric measurements, permits relatively rapid characterization of magmatic fabrics for large numbers of samples. Here we present the results of a study of remanence and susceptibility anisotropy from three transects through the Middle Banded Series of the Stillwater Complex, Montana. All three transects exhibit a magnetic foliation that increases with stratigraphic height up to the top of Olivine Bearing Zone III, consistent with crystal mush compaction. Perhaps more importantly, each transect is characterized by a subtle lineation in the anisotropy of magnetic susceptibility with a consistent direction within that transect. The magnetic lineation directions, which generally coincide with crystallographic preferred orientations of silicate minerals, likely record a pre-compaction fabric. Lineation directions differ from one transect to another, implying that the process generating the lineation – either slumping of a semiconsolidated crystal mush or magma transport – acted on length scales of at most a few km. These results demonstrate the sensitivity of magnetic anisotropy to petrofabric in mafic rocks.

Late Pan-African granite emplacement during regional deformation, evidence from magnetic fabric and structural studies in the Hammamat–Atalla area, Central Eastern Desert of Egypt

Field investigations, microstructural observations, and magnetic fabric analyses revealed a polyphase, late Pan-African deformational evolution in the Um Sheqila–Um Had (595Ma) composite pluton and in the Hammamat and Atalla areas of the Central Eastern Desert of Egypt in Ediacaran times. Major stages are early shortening (NNW–SSE), subsequent strike-slip (NW–SE shear zones), and late shortening (NW–SE). Strain studies on pebbles and xenoliths together with AMS data show a predominance of shallow, NW–SE trending X axes or magnetic lineations, associated with steep, NW–SE striking magnetic foliations. Magnetic fabrics and microstructures indicate a tectonic fabric in the Um Sheqila–Um Had granitoid plutons, which is dominated by steep NW–SE striking foliations and shallow NW–SE trending lineations, similar to those in the high-angle Atalla Shear Zone. There is a change of lineation directions from ESE–WNW at Um Sheqila (oldest) to NW–SE to Um Had II (youngest). This pattern may indicate an influence of strike-slip and is also consistent with NE–SW compression. This holds also true for the asymmetry of the contact aureole, which is extended towards NW, parallel with the trend of the magnetic lineation. The character and orientation of the deformation pattern in the Um Sheqila–Um Had plutons and the Atalla Shear Zone is thus similar to the pattern of the late shortening phase. The intrusion of the Um Sheqila–Um Had granitoid rocks, therefore, took place before the late shortening stage, but postdates early deformation, which, according to published data, was associated with lithospheric thinning in the Central Eastern Desert. Therefore, these Pan-African plutons do not represent the earliest post-deformational intrusions but a late stage of syn-deformational magmatic activity. At a regional scale, this deformation with steep foliations and shallow lineations may also be related with lateral escape tectonics. The pluton emplacement, the importance of transcurrent shear zones, and the low lithospheric thickness in the area are not consistent with tectonic elements at the Pan-African orogenic margin but imply a more internal position for the Wadi Hammamat area.

Deformation Pattern and Vein Development in Delhi Supergroup Rocks, Northeastern Part of Delhi-Aravalli Belt: Implication of Late Stage Dilation

Abstract: The Delhi Supergroup rocks of Mesoproterozoic age in the north-eastern part of Delhi-Aravalli belt are poly-deformed. These rocks show meso and micro-scale stretched quartzo-feldspathic veins in the gneissic rocks in association with marble and amphibolites in contrast to those commonly described from the low grade metamorphic rocks in the northeastern part of the Delhi-Aravalli belt. The textural features of the veins which show stretched crystals in gneisses indicate that these developed in phases episodically in the direction of mineral lineation. The veins occupy tensional cracks without shear component and developed when the fluid pressure was high enough as they form in amphibolite facies of metamorphism.

The process and mechanisms of metamorphism and deformation of hornblende—An example from the Hengshan Mountains, Shanxi Province

Hornblende’s plastic and superplastic deformation mechanisms were studied on the basis of some new information obtained from amphibolite samples in the Hengshan Mountains (Hengshan, Shanxi, China). For this purpose, the samples were measured and analyzed by optical microscope (OM), electron probe microanalysis (EPMA), transmission electron microscope (TEM) and electron backscattered diffraction (EBSD) respectively. Because localized strong strain is more than 1000%, it is presumed that structural superplastic deformation was developed during the ductile shearing process. It is calculated that deformation occurred at 650–679°C and 0.770–0.914 GPa, based on the study of plagioclase-hornblende geothermometry and geobarometry. TEM images show that new hornblende grains developed few dislocations and hornblende crystals exhibit straight grain boundaries. EBSD of strongly deformed hornblendes reveals a fabric dominated by a {100} 〈001〉 lattice preferred orientation (LPO), indicative of new grain recrystallization along the lineation direction (X axis). Thus superplastic deformation of hornblendes from the Hengshan Mountains was developed by mainly solid-state diffusive mass transfer and grain boundary sliding.

The Shir-Kuh pluton (Central Iran): Magnetic fabric evidences for the coalescence of magma batches during emplacement

The ∼136Ma, NW–SE elongate Shir-Kuh pluton is one of the most poorly understood geological feature of Central Iran. It is composed of peraluminous rocks, corresponding to ilmenite-bearing S-type granites compositionally ranging from granodiorites to leucogranites. These rocks show a continuum in their chemistry attributed to progressive differentiation. This allows using the anisotropy of magnetic susceptibility technique to tempt establishing the relative chronology between emplacements of magma batches in the pluton. The rather low susceptibility magnitudes (Km<400μSI) depict a dominant paramagnetic behavior of the pluton. The magnetic fabrics data (magnetic lineation and foliation maps, K, P and T parameters), complemented by field and microstructural observations, reveal that two feeder zones at least, as characterized by areas having steep lineations ascribed to magma flow likely issued from the base of the brittle crust, served as conduits for the magmas. The early Cretaceous age of the pluton, the orientations of the feeding zones, the overall lineation directions throughout the pluton, as well as the S-type nature of the magmas call for a dextral transpressive regime which might have been active in the back-arc region located above the subducting eastern branch of the Neo-Tethys.

Direct evidence for dynamic magma supply fossilized in the lower oceanic crust of the Troodos ophiolite

[1] Temporal and spatial variabilities of mantle upwelling and melt supply in mid-ocean ridges (MORs) have long been documented. Such variabilities span a range of scales and have a profound effect on the structure as well as the composition of the oceanic crust. Previous seismic and gravity studies have suggested that the lower oceanic crust plays a major role in accommodating these changes in melt supply. Here we report the first direct evidence for a sharp transition from coherent sub-horizontal to near vertical magma flows frozen in the lower oceanic crust of the Troodos ophiolite at the segment edge near a fossil ridge-transform intersection. We constrain the preferred petrofabric lineation directions at 13 gabbroic sites using anisotropy of magnetic susceptibility (AMS) verified by electron backscatter diffraction. Pre-emplacement accretion-related rotations were corrected using magnetic remanence directions. We identify two provinces of nearly uniform susceptibility directions (principal axes) and attribute them to two magmatic episodes. A more focused mantle upwelling and melting episode near the segment midpoint may have resulted in lower crustal lateral magma flows along the fossil segment-edge, whereas uniform mantle upwelling and melt supply along the entire axis may have resulted in vertical magma flows at the segment-edge. Overall, our data verify the vital role of the lower oceanic crust in accommodating changes in mantle upwelling and melt supply beneath MORs.