Orientation Of Principal Stress Axes Research Articles

Development of flanking structures in layered gneissic rock: Insights from numerical modelling

Flanking structures defined by dragged planar fabrics alongside of quartzo-feldspathic veins in gneissic rocks are very common. Varieties of mesoscale flanking structures were observed in parts of the Chhotonagpur Granite Gneissic Complex (CGGC), Eastern India. Alternate thin layering of quartzo-feldspathic and mafic minerals in gneissic rock causes mechanical anisotropy in rock during deformation. This study aims to explore specially the influence of mechanical anisotropy on the development of flanking structures. With the help of two –dimensional Finite Element modelling, we attempt to understand the effect of initial orientation of cross-cutting element (θ), kinematic vorticity number (Wk) and the anisotropy factor (δ) on the development of flanking structures. Deformed shape of cross-cutting element (RN), amount (LN) and nature (antithetic or synthetic) of offset of layers and pattern of drag (reverse or normal) are considered as important parameters for their nomenclature. As the final geometry of flanking structure does not only depend on the instantaneous deformational condition, we analyze the progressive development of flanking structures by varying the above parameters. Study reveals that mechanical anisotropy has strong influence on the geometry of flanking structures in layered gneissic rock, which depends on the initial orientation of long axis of cross-cutting element and its final position with respect to the orientation of principal stress axes and pole of anisotropy. The study also reveals that the flanking structures can be used as important tool for estimating the mechanical anisotropy in layered rock. Shape of cross-cutting element, amount of offset and the shifting of the object-tip points are the most important parameters for determining the anisotropy factor.

DEM study on the dynamic responses of a ballasted track under moving loading

This paper presents the discrete element modeling of the dynamic response of a ballasted track under moving loads. The DEM model, consisting of sleepers, ballast, and sub-ballast, has been calibrated using field and laboratory data. This model was further used to examine the dynamic responses of the ballasted track subjected to a series of moving traffic loading representing various train axle loads and speeds. The results show that the permanent settlement of the sleeper, the breakage of ballast, and the dynamic stresses in the track substructure increase with an increase in train axle load and speed. As the train moves, the magnitudes of dynamic stresses and the orientations of principal stress axes in the track change continuously, and a more pronounced principal stress rotation is observed at sleeper edges than those underneath sleepers. The capping layer is found to play a critical role in reducing train-induced stress and further alleviating the disturbance from the trains to the subgrade. The interparticle contacts and the vibration of ballast during the movement of the train including the influences of train axle load and speed on the dynamic responses of ballasted railway tracks are captured and analyzed from a micromechanical perspective.

RESULTS OF THE TECTONOPHYSICAL STUDY IN THE DZHANKHOT THRUST ZONE (NORTHWESTERN CAUCASUS)

The paper presents the results of detailed structural analysis of the Dzhankhot thrust zone which is a part of the system of overthrust nappe structures in the southern slope of the Northwestern Caucasus. The relevance of the research topic is found on the reason that despite the obvious importance of these overthrust nappes in the structure of the Greater Caucasus there is still lack of in-situ observational data on the detailed structure of thrust zones and on tectonic stress distribution pattern therein.As a result of the structural and geological studies of small disjunctive and plicative forms in the Upper Cretaceous and Paleocene sediment contact along the Dzhankhot thrust, there were identified the major structural and tectonic deformation patterns and the areal distribution of tectonic deformations depending on the distance to the thrust. There was determined the major type of stress in different parts of the thrust zone. There was shown the difference in the development of geological indicators of faults for different elements of the folded structures. In some cases, there was determined the sequence of deformations, from synsedimentary structures (clastic dikes in particular) to post-sedimentary brittle structures.The orientation of principal stress axes determined from the reconstruction data by method of cataclastic analysis of faults confirms a thrust-type deformation within the studied area near the Dzhankhot thrust. Local stress-tensor determinations are in good agreement with the location of observation points in the regional tectonic structure. Trajectories of the maximum compression axes, most of which are submeridionally and NNW-trending, change orientation to the northwest in the immediate vicinity of the Dzhankhot thrust fault zone. The identified features of tectonic structure and stressed-strained state of the Dzhankhot thrust zone allow considering this area as a natural structural and tectonophysical testing ground for both tectonic-stress-reconstruction-improvement methodologies and a study on natural deformation structures of the Greater Caucasus.

Modelling of Elastic Stress Field of the Earth’s Crust in Central Asia Based on the Orientations of Principal Stress Axes

Modelling of Elastic Stress Field of the Earth’s Crust in Central Asia Based on the Orientations of Principal Stress Axes

Effects of Initial Direction and Subsequent Rotation of Principal Stresses on Liquefaction Potential of Loose Sand

AbstractThe effects of the initial orientation of principal stress axes and subsequent rotation of principal stresses on liquefaction susceptibility of sands were investigated. Monotonic and cyclic...

The focal depths of the 2008 Panzhihua earthquake sequence and the stress field in the source region

Focal depths of the 2008 MS6.1 Panzhihua earthquake sequence and tectonic stress field in the source area are investigated. Source depths of 24 earthquakes in Panzhihua earthquake sequence with a magnitude M≥3.0 were determined using the seismic depth phase sPL; additionally, the focal depths of 232 earthquakes were measured by fitting the three-component waveforms of the P and S waves. The source depth of the main shock is ~12 km. The majority of the aftershocks with magnitude M≥3.0 occurred in the brittle upper crust at the depths range of 12–18 km. Further, the Source mechanisms of the 232 events around the Panzhihua earthquake source area were determined, and the results show that the earthquakes have predominantly strike-slip mechanisms in the Dianzhong Block, but display complexity of the focal mechanisms outside and near the boundary of the Dianzhong block. The 232 earthquake mechanisms from this study are combined with the solutions from the Global Centroid Moment Tensor (GCMT) catalog to derive 2D stress field. The inversion results show that the Dianzhong block is predominantly under a strike slip faulting regime and the direction of the maximum principal compression σ1 is northwest-southeast (NW-SE)-trending. The distribution is coincide with GPS velocity field. However, orientations of principal stress axes as well as the faulting types change outside and near the Dianzhong block. The results show that the tectonic stress field in the study area is predominantly controlled by the southeast (SE)-trending horizontal movement and clockwise rotation of the Dianzhong block as a result of the eastward movement of eastern Tibetan meeting the old and rigid South China block (SCB). The Panzhihua earthquake ruptured at ~12 km depth where the tectonic stress regime is under the SE-direction horizontal compression and the NE-direction horizontal extension.

A km-scale “triaxial experiment” reveals the extreme mechanical weakness and anisotropy of mica-schists (Grandes Rousses Massif, France)

The development of Andersonian faults is predicted, according to theory and experiments, for brittle/frictional deformation occurring in a homogeneous medium. In contrast, in an anisotropic medium it is possible to observe fault nucleation and propagation that is non-Andersonian in geometry and kinematics. Here, we consider post-metamorphic brittle/frictional deformation in the mechanically anisotropic mylonitic mica-schists of the Grandes Rousse Massif (France). The role of the mylonitic foliation (and of any other source of mechanical anisotropy) in brittle/frictional deformation is a function of orientation and friction angle. According to the relative orientation of principal stress axes and foliation, a foliation characterized by a certain coefficient of friction will be utilized or not for the nucleation and propagation of brittle/frictional fractures and faults. If the foliation is not utilized, the rock behaves as if it was isotropic, and Andersonian geometry and kinematics can be observed. If the foliation is utilized, the deviatoric stress magnitude is buffered and Andersonian faults/fractures cannot develop. In a narrow transition regime, both Andersonian and non-Andersonian structures can be observed. We apply stress inversion and slip tendency analysis to determine the critical angle for failure of the metamorphic foliation of the Grandes Rousses schists, defined as the limit angle between the foliation and principal stress axes for which the foliation was brittlely reactivated. This approach allows defining the ratio of the coefficient of internal friction for failure along the mylonitic foliation to the isotropic coefficient of friction. Thus, the study area can be seen as a km-scale triaxial experiment that allows measuring the degree of mechanical anisotropy of the mylonitic mica-schists. In this way, we infer a coefficient of friction μweak=0.14 for brittle-frictional failure of the foliation, or 20% of the isotropic coefficient of internal friction.

Regional stress field around the Taigu fault zone in Shanxi Province, China

A comprehensive study on regional stress field around the Taigu fault zone in Shanxi Province, China, was performed in this study. To get a better understanding of the present-day stress status in this area, 31 focal mechanisms of M L ≥3 earthquakes since 1965 were compiled, and the best stress tensor was then inverted based on the database. Additionally, magnetic fabrics along the Taigu fault zone were investigated to get an indication of the regional stress field in the past. Our results show that the present-day stress field around the Taigu fault zone is characterized by astable NW-SE extension with a strike-slip component, consistent with the geological surveys and recent GPS data. Results from magnetic fabrics indicate that the orientations of principal stress axes from magnetic fabrics of sedimentary rocks in Neogene coincide to the orientations of principal stress axes from focal mechanisms. The south segment of the Taigu fault displays more complicated magnetic fabrics and more activity of moderate earthquakes. It is connected with the Mianshan west fault and intersects with NW-SE striking Fenyang fault and the north fault of the Lingshi uplift at the south edge of Taiyuan basin. This may be the area needing more attention in terms of seismic risk along the Taigu fault.

States of local stresses in the Sea of Marmara through the analysis of large numbers of small earthquakes

We invert the present day states of stresses for five apparent earthquake clusters in the Northern branch of the North Anatolian Fault in the Sea of Marmara. As the center of the Sea of Marmara is prone to a devastating earthquake within a seismic gap between these selected clusters, sensitive analyses of the understanding of the stress and strain characteristics of the region are all-important. We use high quality P and S phases, and P-wave first motion polarities from 398 earthquakes with ML≥1.5 using at least 10 P-wave first motion polarities (FMPs), and a maximum of 1 inconsistent station, obtained from a total of 105 seismic stations, including 5 continuous OBSs. We report here on large numbers of simultaneously determined individual fault plane solutions (FPSs), and orientations of principal stress axes, which previously have not been determined with any confidence from the basins of the Sea of Marmara and prominent fault branches.We find NE–SW trending transtensional stress structures, predominantly in the earthquake clusters of the Eastern Tekirdağ Basin, Eastern Çınarcık Basin, Yalova and Gemlik areas. We infer that a dextral strike-slip deformation exist in the Eastern Ganos Offshore cluster. Furthermore, we analyze FPSs of four ML≥4.0 earthquakes, occurred in seismically quiet regions after 1999 Izmit earthquake. Stress tensor solutions from a cluster of small events that we have obtained, correlate with FPSs of these moderate size events as a demonstration of the effectiveness of the small earthquakes in the derivation of states of local stresses. Consequently, our analyses of seismicity and large numbers of FPSs using the densest seismic network of Turkey contribute to better understanding of the present states of the stresses and seismotectonics of the Sea of Marmara.

Moment tensor solutions for the Iberian-Maghreb region during the IberArray deployment (2009–2013)

We perform regional moment tensor inversion for 84 earthquakes that occurred in the Iberian-Maghreb region during the second and third leg of IberArray deployment (2009–2013). During this period around 300 seismic broadband stations were operating in the area, reducing the interstation spacing to ~50km over extended areas. We use the established processing sequence of the IAG moment tensor catalogue, increasing to 309 solutions with this update. New moment tensor solutions present magnitudes ranging from Mw 3.2 to 6.3 and source depths from 2 to 620km. Most solutions correspond to Northern Algeria, where a compressive deformation pattern is consolidated. The Betic-Rif sector shows a progression of faulting styles from mainly shear faulting in the east via predominantly extension in the central sector to reverse and strike-slip faulting in the west. At the SW Iberia margin, the predominance of strike-slip and reverse faulting agrees with the expected transpressive character of the Eurasian-Nubia plate boundary. New strike-slip and oblique reverse solutions in the Trans-Alboran Shear Zone reflect its left-lateral regime. The most significant improvement corresponds to the Atlas Mountains and the surroundings of the Gibraltar Arc with scarce previous solutions. Reverse and strike-slip faulting solutions in the Atlas System display the accommodation of plate convergence by shortening in the belt. At the Gibraltar Arc, several new solutions were obtained at lower crustal and subcrustal depths. These mechanisms show substantial heterogeneity, covering the full range of faulting styles with highly variable orientations of principal stress axes, including opposite strike slip faulting solutions at short distance. The observations are not straightforward to explain by a simple geodynamic scenario and suggest the interplay of different processes, among them plate convergence in old oceanic lithospheric with large brittle thickness at the SW Iberia margin, as well as delamination of thickened continental lithosphere beneath the Betic-Rif arc.

Refraction of the principal stress trajectories and the stress jumps on faults and contact surfaces: Part 1. Non-constrained regular trajectories

Refraction of the principal stress trajectories and the stress jumps on faults and contact surfaces: Part 1. Non-constrained regular trajectories

Modelling of structural and lithological controls on mobility of fluids and gold in orogenic belts, New Zealand

AbstractNumerical models have been run to evaluate the key parameters that affect fluid flow and gold mineralization at a range of scales, from the full thickness of the crust to the mineral grain scale. These models are constrained with real examples of orogenic gold in southern New Zealand. Large scale modelling shows that differences in crustal strength and thickness affect the locus, scale, and rate of crustal fluid flow and hence influence gold deposition. The most vigorous hydrothermal activity and gold mineralization occurs in narrow zones with maximum uplift close to a major crustal boundary. Relatively strong middle crust develops deformation-driven permeability and fluid flow in a broad diffuse zone. Deformation of randomly inhomogeneous rocks under mid-crustal conditions results in horizontal fabric and horizontal shear zones, and these can control fluid flow locally. The addition of graphite to flat shear zones causes an increase in deformation-induced permeability, and this can encourage further graphite deposition in a feedback effect. Fluid flow in these flat shears is very slow (mm/year) compared to fluid flow in fracture-controlled permeability at shallow crustal levels. Rock strength inhomogeneities in mid-crustal shear zones can result in localized decrease in differential stress, facilitating the switching of orientations of principal stress axes and the formation of steeply-dipping quartz veins.

Temporal and spatial variation characteristics of present stress field in the northern area of North China

From focal mechanism solutions of the earthquakes in the northern part of North China during the period of 2002∼2006, the directions of principal stress axes in 4 stress sub-districts are analyzed using a grid test method. The characteristic of present crustal stress directions is discussed. Based on this result and on the focal mechanism solutions calculated for some events in the period of 1977∼1998, in combination with some other study results, the temporal variation of present crustal stress directions in the northern part of North China is investigated. The results confirm that the direction of crustal principal stress in some regions had somewhat rotated after the 1976 Tangshan M7.8 earthquake. The mean P axes of the focal mechanism solutions rotated clockwise not only in Tangshan sub-district, but also in Beijing and Xingtai sub-districts after the Tangshan earthquake. In Beijing and Xingtai sub-districts the orientations of principal stress axes in the period of 2002∼2006 are consistent with that before the Tangshan earthquake, implying that the stress orientations has rotated back to the state before the Tangshan earthquake in these two sub-districts. The directions of the mean P axes are nearly E-W in Tangshan sub-district since the M7.8 earthquake. The present stress field in the sub-district northwest to Beijing, or in the western part of the Zhangjiakou-Bohai fault zone, is relatively stable during the time period concerned in this study. Because of the limitation of data, this paper only states a possible variation of stress field in the northern part of North China in the recent decades.

A present-day tectonic stress map for eastern Asia region

Based on the data of earthquake centroid moment tensor (CMT) solution, P-wave first motion focal mechanism solution and deep hole breakouts, a present-day tectonic stress map for eastern Asia region is compiled. The original stress data are smoothed for every 200 km × 200 km area by taking the average of all stress indicators within each sub-region. The stress map shows the spatial distribution of the orientation of principal stress axes and the stress regimes. An earthquake focal mechanism map for the eastern Asia is also given. The maps of orientation of principal stress axes show that, apart from the strong influence of the collision between the Indian Ocean plate and Eurasian plate, the present-day tectonic stress in eastern Asia is significantly affected by the back-arc extension of the subduction zones. The joint effect of the continental collision at the Himalaya arc and back-arc extension in the Burma arc region may be responsible for the remarkable rotation of the principal stress orientations in southeastern part of the Tibet plateau. The joint action of the collision between the Philippine Sea plate and Eurasian plate at Taiwan Island and the back-arc extension of the Ryukyu arc affect the stress field in eastern part of China. There are no strong earthquakes in the present day in the vast back-arc region of the Java trench subduction zone. The back-arc extension there may create a condition favorable to the southward flow of the lithosphere material in southeastern Asia. In the inner part of the Tibet plateau region, roughly demarcated by the Kunlun mountain, the northern and northeastern part is a broad intracontinental compressive zone, while the southern and southwestern part is generally in a normal-faulting stress state.

A stress inversion procedure for polyphase calcite twin and fault/slip data sets

In rocks that are polydeformed an approach which separates faults prior to stress inversion is more appropriate. The traditional stress inversion approach involving the concept of the best-fit stress tensor, e.g. a tensor which minimises the misfit between calculated and measured fault-striae data, often risks computing artificial stress tensors that are some form of average of mixed sets of real stress tensors. A new approach is proposed in which fault data are pre-processed to group the faults on the basis of their response to all possible orientations and magnitudes of applied stress. A computer method is described which utilises cluster analysis based on the right-dihedra method to divide dynamically-mixed fault populations to monophase subsets. This division is based on the ranked similarity coefficients of each fault pair from the raw data set. The data clusters form dynamically-homogeneous subsets, which are used for the composite right-dihedra solution. This solution is re-computed for the reduced stress tensor defined by the orientation of principal stress axes and the ratio of their magnitudes.

Relation between the Gondwanide Orogen and contemporaneous intracratonic deformation

A causal relationship between collisional processes in the Permo-Triassic Gondwanide Orogen and intracratonic deformation of a compressional, strike-slip and extensional nature, was suggested in the literature. In this paper available data are reviewed to test this model. It appears that the balance of evidence favours a Gondwanide Orogeny produced in a compressional back-arc setting, rather than by major collision. A possible cause for the deformation under relatively low-grade metamorphic circumstances is flat slab subduction. An important dextral strike-slip component may result from oblique subduction. The contemporaneous intracratonic deformation with similar orientation of principal stress axes may be related to the same flat slab subduction processes and/or orogen parallel shear, but was probably not a direct consequence of collision.

Intracrustal subduction and gravity currents in the deep crust: Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit Gneiss Complex, Washington

Isotopic and thermobarometric (pressure-temperature, P-T) data on a gneissic quartz diorite, combined with previous U-Pb ages and P-T data on wall-rock metapelites, define a precise P-T time series for a portion of the Skagit Gneiss Complex, a deep crustal crystalline complex within a zone of intracontinental collision. Concordant U-Pb and Sm-Nd ages on zircon indicate crystallization at 68 Ma, which preceded metamorphism of nearby pelitic wall rocks (garnet core assemblages) and the pluton (plagioclase-hornblende coronas on garnet) at ≈800–900 MPa, 700–800 °C. Following nearly isothermal decompression and sillimanite-cordierite metamorphism of wall rocks (garnet rims plus matrix) at 300–500 MPa, 650–700 °C, a Sm-Nd mineral isochron including two garnet separates records an initial phase of cooling of the complex at 60 Ma. Slow cooling (≈10 °C/m.y.) extends from approximately 60 Ma to 50 Ma. Hornblende and biotite Ar-Ar ages of 47 Ma and 45 Ma, respectively, define a second unroofing event; cooling rates during this period were approximately 100 °C/m.y. Late Cretaceous deep burial followed by two distinct unroofing episodes is also observed in some Cordilleran metamorphic core complexes and may suggest a common origin. We propose that regional shortening in the Cordilleran interior is accommodated by a process of intracrustal subduction, i.e., the upper crust is forced downward into a crustal asthenosphere, resulting in a cold, buoyant root. Heating and weakening of the root cause the formation of gravity currents, effective viscosity being in the range of 10^(16)–10^(19) Pa s, spreading outward below nonfluid upper crust. Spreading of the gravity current, which does not necessarily contribute to crustal thinning, is driven by the small density contrast between the root and its mid-crustal surroundings. Hence unroofing is neither a response to overthick crust nor controlled by the orientation of principal stress axes in the upper crust and upper mantle. The root then cools slowly until final unroofing to near-surface levels, which may occur via either extension or shortening with consequent erosion.

Deformation partitioning during updoming of the Sonnblick area in the Tauern Window (Eastern Alps, Austria)

The Sonnblick Dome forms a NE-vergent dome structure cored by basement gneisses within the southeastern Tauern Window of the Eastern Alps (Austria). A succession of ductile and brittle deformation stages documents doming and exhumation subsequent to the thermal peak of metamorphism. Contrasting deformation geometries within internal parts and along the margins of the dome are explained in terms of deformation partitioning. Subhorizontal shortening is documented by subvertical en-echelon extensional gashes within central parts of the dome. Subhorizontal as well as subvertical flattening is also documented by fold structures. During dextral transpression strike-slip is accommodated along the NE margin (Sonnblick Lamella, Möll Valley Fault) and the southern margin (Moser Fault) of the Sonnblick Dome, while vertical thickening occurred within the interior of the dome. Crustal thickening triggered unroofing and extension parallel to the dome axis which is documented by ductile low-angle normal faults in the uppermost structural sections of the dome. This normal fault system contributed to footwall uplift and exhumation of the dome structure. The Sonnblick Lamella, associated with the dextral Möll Valley Fault, forms a stretching fault where deformation is concentrated along a potential zone of weakness. This fault is interpreted to represent the transition from vertical thickening within the dome to vertical thinning at the dome margins. During upbending, the dome structure passed the isotropic stress surface that is characterized by equality of σ 1 and σ 2. This is documented in a perturbation of the orientations of principal stress axes σ 1 and σ 2, while σ 3 remains constant in orientation. Transpression contributed substantially to updoming and to rapid, nearly isothermal, exhumation subsequent to the thermal peak of metamorphism.

Inversion of field data in fault tectonics to obtain the regional stress-II. Using conjugate fault sets within heterogeneous families for computing palaeostress axes

SUMMARY In a family of faults that have been neither reactivated nor reoriented, each fault whose direction and sense of movement are known can be assigned Pθ and Tθ axes. If the rocks are isotropic and the orientation of each fault is controlled only by the orientations of the active principal stress axes and the apparent angle of internal friction φ that controls the angle 2θ between conjugate faults, Pθ and Tθ are parallel to the maximum and minimum principal stress axes, respectively. In most cases, however, this relationship does not hold accurately. Defining probability density regions around Pθ and Tθ that take into account variations of fault geometry and using the mathematical process of convolution, one can estimate the principal stress axes associated with any family of faults, whether the family be homogeneous or heterogeneous. The probability regions around Pθ and Tθ are in the form of Gaussian functions. The orientations of principal stress axes determined using different values for the constant C and reasonable values for φ (and therefore θ), are in good agreement with other numerical methods. The only major requirement of the method is the presence of conjugate fault systems within the fault slip data set studied.

Selected geomorphic method applied to neotectonic studies in the Northern Carpathians

A number of geomorphic techniques have been applied to study morphotectonic pattern of the Northern Carpathians. Apart from classical ones, a wide range of morphometric and statistical methods has also been used. Among these, the construction of the base-level, goniobase and isolong maps require most serious consideration. Neotectonic implications can also be drawn from careful analyses of geometries of the valley network. The highest bifurcation ratios are usually confined to areas showing most intensive Quaternary uplift. The same applies to increased drainage density measures and the ratio of first-order valleys to the total number of valleys. Parameters of longitudinal river bed profiles are sometimes indicative of tectonic tendencies. The North Carpathian rivers reveal an eastward increase in order, length and relief, being accompanied by a decrease in the mean river bed gradient. The highest concavity measures express the river profile maturity while the position of inflexion point delimits sectors modelled by erosional and accumuiationai processes. Another opportunity for morphotectonic reasoning is provided by the studies of orientation of principal stress axes, inferred from the drainage pattern. All these techniques should be verified by detailed geological mapping, aided by interpretation of aerial photographs and radar images, as well as by geophysical soundings.