Change In Stress Orientation Research Articles

Impact of long-term depletion on horizontal wellbore stability in tight reservoirs-including changes in petrophysical and geomechanical properties

The development of tight reservoirs using a horizontal well requires caution owing to the reservoir sensitivity to stress. Predicting the horizontal wellbore instability during reservoir depletion is critical for tight reservoirs. The present study investigated the wellbore failure criteria and stress orientation changes with reservoir depletion of a horizontal well in a tight reservoir, considering rock sensitivity to stress with pressure reduction. An integrated reservoir model coupled with geomechanics was used to predict pore pressure. A mechanical earth model (MEM) was constructed to examine the stability of horizontal wells. The unconfined compressive strength (UCS) was correlated with changes in porosity due to stress changes. The MEM shows that the failure criteria around the horizontal wellbore, including breakout, loss, and breakdown pressures, change considerably with reservoir depletion. A more significant response was observed in the stress-sensitive layer, which was characterised by a higher permeability. The safe mud weight window of the studied horizontal well narrowed significantly after production for five years, whereas the stability was completely absent after ten years. The deterioration of stability with depletion is governed mainly by permeability reduction, which may cause severe pressure reduction, whereas the stability improvement due to the increase in UCS caused by porosity reduction seems marginal. Relatively small changes in the direction of the stresses around the wellbore with depletion were observed, with the stress distribution concentrated in the predominant direction as the depletion continued. This study indicates that the sensitivity of petrophysical and geomechanical properties to stress amplifies the instability in tight reservoirs, particularly around wellbores.

PLUME-GENERATED 90° STRESS CHANGE LINKED TO TRANSITION FROM RADIATING TO CIRCUMFERENTIAL DOLERITE DIKE SWARMS OF THE SIBERIAN TRAPS LARGE IGNEOUS PROVINCE AND TO EMPLACEMENT OF THE NORILSK-TALNAKH ORE DEPOSITS

Abstract A 90° change in stress orientation has been previously proposed as the trigger for the final emplacement of the world-class Norilsk-Talnakh magmatic sulfide mineralization via the migration of accumulated sulfide melts from elsewhere within the plumbing system of the Siberian Traps large igneous province (LIP). We propose that this stress change does not require and was not triggered by a distal change in plate boundary stresses, but instead can be explained both temporally and spatially by stress changes recorded in the dike swarm patterns of the Siberian Traps LIP, namely the transition from a giant radiating dike swarm (associated with mantle plume uplift) to a giant circumferential swarm (linked to flattening of the plume head). The mantle plume stress-related changes recorded by these dike swarms, rather than distal plate boundary stress changes, were therefore most likely the trigger for the emplacement of the Norilsk-Talnakh mineralization. Other LIPs that have both giant radiating and circumferential dike swarms most likely reflect similar major and rapid changes in stress orientation, indicating that mantle plume-induced stress changes revealed by dike swarms should be considered an additional tool in magmatic sulfide exploration.

Impact of faults on the remote stress state

Abstract. The impact of faults on the contemporary stress field in the upper crust has been discussed in various studies. Data and models clearly show that there is an effect, but so far, a systematic study quantifying the impact as a function of distance from the fault is lacking. In the absence of data, here we use a series of generic 3-D models to investigate which component of the stress tensor is affected at which distance from the fault. Our study concentrates on the far field, located hundreds of metres from the fault zone. The models assess various techniques to represent faults, different material properties, different boundary conditions, variable orientation, and the fault's size. The study findings indicate that most of the factors tested do not have an influence on either the stress tensor orientation or principal stress magnitudes in the far field beyond 1000 m from the fault. Only in the case of oblique faults with a low static friction coefficient of μ=0.1 can noteworthy stress perturbations be seen up to 2000 m from the fault. However, the changes that we detected are generally small and of the order of lateral stress variability due to rock property variability. Furthermore, only in the first hundreds of metres to the fault are variations large enough to be theoretically detected by borehole-based stress data when considering their inherent uncertainties. This finding agrees with robust stress magnitude measurements and stress orientation data. Thus, in areas where high-quality and high-resolution data show gradual and continuous stress tensor rotations of >20∘ observed over lateral spatial scales of 10 km or more, we infer that these rotations cannot be attributed to faults. We hypothesize that most stress orientation changes attributed to faults may originate from different sources such as density and strength contrasts.

Influence of principal stress orientation on stress distribution and plastic zone evolution of rock surrounding tunnels

During the excavation of tunnels, the principal stress orientation changes, with a significant impact on the stress distribution, mechanical properties, and plastic zone evolution of rocks surrounding tunnels, causing severe deformation control, and monitoring problems in the stability of tunnels. Currently, biaxial compression tests were conducted to explore the influence of principal stress orientation on mechanical properties of rocks surrounding tunnels. The analytical solution of stress and the model of plastic zone of rocks considering the principal stress orientation and the distance from the excavation boundary were established to reveal the failure mechanism of surrounding rock under different principal stress orientations. With an increase in the angle between the principal stress orientation and the long axis of tunnel, the maximum tangential stress around the tunnel gradually changed to the minimum horizontal principal stress direction, and its value gradually increased, leading to quicker failure of surrounding rocks, and reducing the strength enhancement effect of the same section size of the tunnel. However, the increase in the angle reduced the damage range and the range of the plastic zone around the tunnel and caused the plastic zone to gradually approach the bottom and roof. The maximum depth of the plastic zone remained parallel to or nearly parallel to the minimum horizontal principal stress direction. When the principal stress orientation was kept constant, the maximum depth of the plastic zone shifted to the minimum horizontal principal stress direction with an increase in the vertical principal stress.

Modelling of the impact of stress concentration on permeability in porous medium based on machine learning method

The behavior of stress-dependent permeability has been an important research topic for oil/gas production. The majority of permeability models for porous media have been proposed based on porelasticity theory. The matrix is assumed to be thoroughly separated by pores in the models and the pore compressibility is used to represent the stress-dependent behavior of permeability. However, matrix could not be separated by pores thoroughly and the impact of stress concentration around pores on pore deformation and permeability should be considered. In this study, the impact of stress concentration on permeability was illustrated by numerical simulation. In addition, the mechanism of stress-dependent behavior of permeability was analyzed. Since it is difficult to establish theoretical permeability models involving stress concentration effect caused by the complex pore structure, machine learning was applied in this paper. One of the four capable machine learning methods was selected and the corresponding machine learning model was validated through both numerical and experimental data. Moreover, the different performance of permeability prediction between the conventional model and the proposed one was discussed. The results indicate that the stress-dependent behavior of permeability results from stress concentration rather than the pore bulk modulus. Therefore, the stress-dependent permeability model with the impact of stress concentration is more accurate, compared with the numerical results and experimental data. In addition, the stress concentration increases the pore deformation and induces strong stress-dependent behavior of permeability, which is sensitive to pore shapes and related to the pore shape complexity. Specifically, the impacts of pores with different shapes on permeability are similar if the complexity index of pore shape is under 0.6 or over 0.9 and distinct for the value in between. Furthermore, the alteration in the magnitude and orientation of the stress affects stress dependency of permeability, which increases with the pore shape complexity. If the complexity index of pore shape exceeds 0.96, the alteration of permeability induced by change in stress orientation can be over 64%.

Effect of uniaxial compressive stress with different orientations on the hole mobility of wurtzite GaN heterojunction quantum well

The influence of uniaxial compressive stress with different orientations to the current channel on the physical and transport properties of the wurtzite GaN heterojunction quantum well is investigated in this work. By using the six-band stress-dependent k × p Hamiltonian, accurate two-dimensional physical pictures are given for the quantized valence subband under the uniaxial compressive stress on the (0001) transport plane. The low-field hole mobility is obtained by the Kubo–Greenwood formula, taking the scattering rates for acoustic phonon, polar optical phonon, and surface roughness into account. Using these methods, the microscopic relationship between the orientation of uniaxial compressive stress and low-field hole mobility is obtained according to the variations of valence subband dispersion and hole effective mass. Results show that for temperatures around and above room temperature, the acoustic phonon scattering is predominant. We find that the mobility gain is mostly contributed from effective mass, and there is an increasing trend under uniaxial compressive stress with all orientations due to the effective mass reduction. For the same stress value, the mobility decreases monotonically as the stress orientation changes from 0° to 90° with respect to the current channel. At room temperature, the calculated low-field hole mobility is 182 cm2/V s under 8 GPa uniaxial compressive stress parallel to the current channel, with the hole density of 5.5 × 1013 cm−2 and the effective electric field of 0.93 MV/cm.

Variations in Forearc Stress and Changes in Principle Stress Orientations Caused by the 2004–2005 Megathrust Earthquakes in Sumatra, Indonesia

Coseismic changes in principal stress orientation in the northern Sumatra subduction zone due to two giant megathrust earthquakes there in 2004 and 2005 are estimated to investigate the in-situ stress. The two megathrust earthquakes, the 2004 Sumatra-Andaman and the 2005 Nias-Simeulue events, are both among the 11 largest earthquakes ever recorded. Previous studies have shown that these giant earthquakes perturbed the stress field in the Sumatra subduction zone enough to alter the principal stress directions there, and here we investigate whether these changes can be used to better understand spatial variations in stress along the subduction zone. We used 330 previously published focal mechanisms to estimate pre- and post-mainshock principal stress orientations in 3 outer forearc segments and assessed whether orientation differences were resolved and what they imply about the pre- and post-mainshock stress fields. Our results agree with previous studies in establishing that coseismic changes in stress orientation in the forearc are resolvable, and consistent with a low level of stress in the outer Sumatran forearc before the earthquake, with almost all the shear stress on the megathrust relieved in the 2004 and 2005 earthquakes. In this study, we reveal that both the stress orientations and coseismic changes in them exhibit along-strike variations, with a decrease in both the pre-mainshock stress and stress drop found in the rupture area of 2005 relative to that of the 2004 earthquake. The forearc segment between the 2004 and 2005 rupture areas, which coincides with a well-known megathrust rupture barrier beneath the island of Simeulue is observed to have a characteristic signature, with lower shear stress relative to the pre-mainshock stress field and higher shear stress relative to the post-mainshock stress field in the adjacent segments.

Stress behavior in the near fracture region between adjacent horizontal wells during multistage fracturing using a coupled stress-displacement to hydraulic diffusivity model

This work presents results for the application of finite element simulation to determine changes in stress orientation and magnitude during a multistage fracturing job in horizontal wellbores. A technique that couples stress-displacement with hydraulic diffusivity was used to account for effective in-situ stress and to estimate pore pressure distribution in the near fracture region. The results captured in this study aid understanding of propagation of induced fractures in a multistage fracturing job and in a multiple horizontal wellbore scenario. This study is a valid and reliable estimation of effective stresses in the near fracture area because effective stresses control strain in a saturated poroelastic material. Additionally, reorientation of in-situ stress in the same near region to the fracture was confirmed. The local reorientation of in-situ stresses will consequently cause the reorientation of subsequent fractures which will stop growing transverse to the horizontal wellbore. As a result, induced fractures do not propagate as expected, creating a non-efficient drainage pattern in the reservoir.

Uncertainties in the estimation of in situ stresses: effects of heterogeneity and thermal perturbation

In situ stress directions and magnitudes in subsurface are commonly estimated using two different methods: (1) borehole break-out—drilling-induced fracture interpretation complemented by extended leak-off tests; (2) stress estimation from acoustic measurements using advanced wireline tools. Both methods use stress perturbations around boreholes to estimate far field stresses employing Kirsch’s equations, which are only valid for boreholes aligned with one of the principal stresses, linear elastic deformation and homogeneous isotropic materials. Furthermore, the original stress state may have been altered by drilling-mud-circulation induced temperature changes. Using heuristic models including heterogeneity, this study investigates potential errors in the estimation of far-field stress due to (a) the generalisation of Kirsch’s equations to heterogeneous media and (b) plausible temperature perturbations. First, errors due to uncertainties in measuring wave slowness are analysed for an idealised homogeneous material. Second, errors due to application of Kirsch’s equations are investigated considering potential effects of frictional interfaces between layers, pore pressure in the rock matrix and thermal perturbations induced by drilling for example cases. To analyse stress in these complex scenarios finite element analysis was used, revealing strong effects of lithological and thermal variations. Stress magnitude was amplified by stiff layers and was attenuated by soft ones. At layer interfaces, substantial changes in stress orientation occurred. Kirsch’s equations for the considered cases resulted in errors in far field stresses as large as 44% in the magnitude and 90° in orientations. An uncertainty propagation analysis indicated a high accuracy of acoustic estimates for homogenous materials. However, a dramatic impact of small-scale heterogeneity may not be resolved by the logging process.

Late Cretaceous to recent tectonic evolution of the North German Basin and the transition zone to the Baltic Shield/southwest Baltic Sea

In this study we investigate the Late Cretaceous to recent tectonic evolution of the southwestern Baltic Sea based on a dense grid of seismic reflection profiles. This area covers the Baltic Sea sector of the salt influenced North German Basin and its transition to the salt free Baltic Shield across the Tornquist Zone. The Upper Cretaceous to recent structural evolution is discussed by means of individual seismic sections and derived high-resolution time-structure maps of the main horizons, i.e., the Upper Cretaceous, Tertiary and Pleistocene. The Upper Cretaceous and Tertiary layers reveal numerous significant faults throughout the study area. Several of these faults propagate upwards across the unconsolidated Pleistocene sediments and occasionally penetrate the surface. The salt influenced North German Basin reveals three major fault trends: NW-SE, N-S and NNE-SSW. Several of these faults are located directly above basement (sub-salt) faults and salt pillows. The majority of these faults are trending N-S to NNE-SSW and parallel the direction of the Glückstadt Graben faults. In the salt free Tornquist Zone, we identify two major shallow fault trends, which are NW-SE and NE-SW. The majority of these faults are located above basement faults, following the direction of the Tornquist Zone. We conclude that generally basement tectonics controls activation and trends of shallow faults. If salt is present, the ductile salt layer causes a lateral shift between the sub- and supra-salt faults. Major plate reorganisation related to the Africa-Iberia-Europe convergence and the subsequent Alpine Orogeny caused reactivation of pre-existing faults and vertical salt movement in the Late Cretaceous. The change of stress orientation from NE-SW to a NW-SE during Neogene caused another phase of fault and salt tectonic reactivation. We explain that the ice-sheet loading and/or present-day stress field may have acted in combination, causing the recent tectonics and upward extension of the faults.

고분해능 시추공 변형률계 활용을 통한 지진 연구 가능성

고분해능 시추공 변형률계로부터 측정된 변형 신호를 이용하여 지진에 의한 응력 변화 해석의 가능성에 대해 고찰하였다. 응력 변화 분석을 위해 2010년 4월 4일 발생한 El Mayor-Cucapah 지진(M_w=7.2)에 의한 변형 신호가 기록된 자료를 이용하여 주압축응력 방향과 크기를 산정하였다. 분석 결과는 지진 전후의 응력 변화 양상에 대한 차이점과 지진 발생과 동시에 특징적인 응력 변화 양상을 보였다. 지진 발생 전후 응력 방향 변화는 지진 해로 규명되는 최대주응력 방향에서의 응력 증가와 연관성이 있는 것으로 해석된다. 지진 발생 시 응력 크기 변화는 주방향에서 10⁻³-10⁻² MPa 수준의 응력 급상승, 하강으로 나타났다. 산정된 응력 크기 변화량에 대한 검증을 위해 지진과 관련된 매개변수를 이용하여 응력 전파를 모사할 수 있는 쿨롱 응력 전파 모델링을 실시하였다. 시추공 측정 결과로부터 계산된 전단응력 변화량은 (0.3-0.8) × 10⁻²MPa의 증가가 있었으며 이는 쿨롱 응력 전파 모델을 통해 얻어진 전단응력 증가량 (0.1-0.6) × 10⁻²MPa과 상당히 유사함을 확인하였다. 따라서 시추공 변형률계 자료가 설치 지역에서 작용하는 주응력의 영향과 지진에 의해 변화하는 응력 변화를 충분히 반영하고 있음을 시사하며 시추공 변형률계 자료로부터 얻어지는 유의미한 응력 정보는 지진 관련 연구에서 활용될 수 있을 것으로 기대한다. This work investigates whether stress changes induced by an earthquake can be estimated using the deformation measured by high-resolution borehole strainmeters. We estimate the changes in the orientation and magnitude of the principal compression stresses using borehole strainmeter data recorded before and after the M7.2 El Mayor-Cucapah earthquake on April 4, 2010. Clear differences in the stress orientations and magnitudes are apparent before and after the event. The change in stress orientation appears related to subtle increases of stress in the tectonic maximum principal orientation, which is in agreement with the earthquake focal mechanism solution. The sudden stress drop at the onset of the earthquake was 10⁻³-10⁻² MPa in the principal orientations. The Coulomb stress transfer model, which can estimate stress transfer, predicts a shear stress increase of (0.1-0.6) × 10⁻² MPa at the strainmeter site, which is in line with the measured data (0.3-0.8) × 10⁻² MPa. Overall, our results suggest that borehole strainmeter data reflect the subtle stress changes associated with earthquake occurrence, and that such data can be utilized for earthquake-related research.

Evidence for two stages of compressive deformation in a buried basin of Mercury

The surface of Mercury shows abundant compressive tectonic landforms, including lobate scarps, wrinkle ridges and high-relief ridges, which are different manifestations of thrust faults, and long-wavelength topography variations, which could be the expression of large scale folding. These landforms probably relate to planetary cooling, although other factors such as mantle convection, tidal despinning or true polar wander could affect the distribution, expression and orientation patterns of compressive features. In this work we show that an area of smooth plains including a buried ∼500-km-diameter impact basin experienced two different stages of deformation. The younger deformation stage is characterized by a set of NW–SE oriented wrinkle ridges affecting the smooth plains and having the same approximate orientation as the wrinkle ridges and lobate scarps deforming the surrounding terrains. The older set of tectonic structures consists of NE–SW oriented, closely spaced, subparallel, quasi-rectilinear and low-relief ridges, partially buried by the smooth plains material and crossed by the wrinkle ridges. Therefore, our results suggest that several events occurred between both deformation stages: at least one stage of basin filling; a change in stress orientation, an increasing in the wavelength and amplitude of deformation, and maybe an increasing of the thickness of the deformed layer. Our observations imply a complex stress history for compressive deformation, maybe influenced by the internal and/or orbital/rotational history of Mercury, and are illustrative of the complexity of tectonic history likely to have affected many or most other regions of the planet.

Strike-slip motion of a mega-splay fault system in the Nankai oblique subduction zone

We evaluated the influence of the trench-parallel component of plate motion on the active fault system within the Nankai accretionary wedge from reflection seismic profiles, high-resolution seafloor bathymetry, and deep-towed sub-bottom profiles. Our study demonstrated that a large portion of the trench-parallel component of oblique plate subduction is released by strike-slip motion along a fault located just landward of and merging down-dip with a mega-splay fault. The shallow portion of the splay fault system, forming a flower structure, seems to accommodate dominant strike-slip motion, while most of the dip-slip motion could propagate to the trenchward décollement. Numerous fractures developed around the strike-slip fault release overpressured pore fluid trapped beneath the mega-splay fault. The well-developed fractures could be related to the change in stress orientation within the accretionary wedge. Therefore, the strike-slip fault located at the boundary between the inner and outer wedges is a key structure controlling the stress state (including pore pressure) within the accretionary prism. In addition, the strike-slip motion contributes to enhancing the continuous mega-splay fault system (outer ridge), which extends for approximately 200 km parallel to the Nankai Trough.

Mapping crustal stress and strain in southwest British Columbia

This paper investigates the orientation and sources of stress in the forearc of the Cascadia subduction zone in southwest British Columbia, using Bayesian inversion results from focal mechanism data and comparing results with GPS derived short-term strain rates. The subduction margin in this region includes a change in orientation from N-S in Washington State to NW-SE in British Columbia. Over 1000 focal mechanisms from North American crustal earthquakes have been calculated to identify the dominant style of faulting, and ∼600 were inverted to estimate the principal stress orientations and the stress ratio. Our results indicate the maximum horizontal compressive stress orientation changes with distance to the trench, from approximately margin-normal along the coast to approximately margin-parallel 100-150 km inland from the coast. Comparing stress orientations with GPS data, we relate the margin-normal stress direction to subduction-related strain rates due to the locked interface between the North American and Juan de Fuca plates just west of Vancouver Island. Further from the margin the plates are coupled less strongly, and the margin-parallel maximum horizontal compressive stress in the North American Plate relates to the northward push of the Oregon Block, which is also observed in the horizontal shortening direction of the residual strain rates, after the subduction component is removed.

Stress orientation and fracturing during three-dimensional buckling: Numerical simulation and application to chocolate-tablet structures in folded turbidites, SW Portugal

Two orthogonal sets of veins, both orthogonal to bedding, form chocolate tablet structures on the limbs of folded quartzwackes of Carboniferous turbidites in SW Portugal. Structural observations suggest that (1) mode 1 fractures transverse to the fold axes formed while fold amplitudes were small and limbs were under layer-subparallel compression and (2) mode 1 fractures parallel to the fold axes formed while fold amplitudes were large and limbs were brought to be under layer-subparallel tension. We performed two- and three-dimensional numerical simulations investigating the evolution of stress orientations during viscous folding to test whether and how these two successive sets of fractures were related to folding. We employed ellipses and ellipsoids for the visualization and quantification of the local stress field. The numerical simulations show a change in the orientation of the local σ 1 direction by almost 90° with respect to the bedding plane in the fold limbs. The coeval σ 3 direction rotates from parallel to the fold axis at low fold amplitudes to orthogonal to the fold axis at high fold amplitudes. The stress orientation changes faster in multilayers than in single-layers. The numerical simulations are consistent with observation and provide a mechanical interpretation for the formation of the chocolate tablet structures through consecutive sets of fractures on rotating limbs of folded competent layers.

Stress orientation to 5 km depth in the basement below Basel (Switzerland) from borehole failure analysis

A vertical profile of maximum horizontal principal stress, SHmax, orientation to 5 km depth was obtained beneath the Swiss city of Basel from observations of wellbore failure derived from ultrasonic televiewer images obtained in two 1 km distant near-vertical boreholes: a 2755 m exploration well (OT2) imaged from 2550 m to 2753 m across the granitic basement-sediment interface at 2649 m; and a 5 km deep borehole (BS1) imaged entirely within the granite from 2569 m to 4992 m. Stress-related wellbore failure in the form of breakouts or drilling-induced tension fractures (DITFs) occurs throughout the depth range of the logs with breakouts predominant. Within the granite, DITFs are intermittently present, and breakouts more or less continuously present over all but the uppermost 100 m where they are sparse. The mean SHmax orientations from DITFs is 151 ± 13° whereas breakouts yield 143 ± 14°, the combined value weighted for frequency of occurrence being N144°E ± 14°. No marked depth dependence in mean SHmax orientation averaged over several hundred meters depth intervals is evident. This mean SHmax orientation for the granite is consistent with the results of the inversion of populations of focal mechanism solutions of earthquakes occurring between depths of 10–15 km within regions immediately to the north and south of Basel, and with the T-axis of events occurring within the reservoir (Deichmann and Ernst, this volume). DITFs and breakouts identified in OT2 above and below the sediment-basement interface suggest that a change in SHmax orientation to N115°E ± 12° within the Rotliegendes sandstone occurs near its interface with the basement. The origin of the 20–30° change is uncertain, as is its lateral extent. The logs do not extend higher than 80 m above the interface, and so the data do not define whether a further change in stress orientation occurs at the evaporites. Near-surface measurements taken within 50 km of Basel suggest a mean orientation of N–S, albeit with large variability, as do the orientation of hydrofractures at depths up to 850 m within and above the evaporite layers and an active salt diapir, also within 50 km of Basel. Thus, the available evidence supports the notion that the orientation of SHmax above the evaporites is on average more N–S oriented and thus differs from the NW–SE inferred for the basement from the BS1/OS2 wellbore failure data and the earthquake data. Changes in stress orientation with depth can have significant practical consequences for the development of an EGS reservoir, and serve to emphasise the importance of obtaining estimates from within the target rock mass.

Coupled Models of Brittle-plastic Deformation and Fluid Flow: Approaches, Methods, and Application to Mesoproterozoic Mineralisation at Mount Isa, Australia

Rock deformation has an important effect on the spatial distribution and temporal evolution of permeability in the Earth’s crust. Hydromechanical coupling is of fundamental significance to natural fluid–rock interaction in porous and fractured hydrothermal systems, and in the assessment and production of hydrocarbon resources and geothermal energy. Shearing and fracturing of rocks can lead to the creation or destruction of permeability when fractures or faults form, or when existing structures are reactivated. Changes in stress orientation or fluid pressure can drive rock failure and create dilating fault zones that have the potential to focus fluid flow, or to breach seals above overpressured fluid compartments. Here, numerical models of deformation and fluid flow related to Mesoproterozoic copper mineralisation at Mount Isa, Australia, are presented that show how changes in deformation geometry in multiply deformed geological architectures relate to changes in dilation patterns, fluid pathways and flow geometry. Coupled numerical simulations of deformation and fluid flow can be useful tools to better understand structural control on fluid flow in hydrothermal mineral systems.

Immature and mature transform zones near a hot spot: The South Iceland Seismic Zone and the Tjörnes Fracture Zone (Iceland)

We describe and compare the two transform zones that connect the Icelandic rift segments and the mid-Atlantic Ridge close to the Icelandic hot spot, in terms of geometry of faulting and stress fields. The E–W trending South Iceland Seismic Zone is a diffuse shear zone with a Riedel fault pattern including N0°–N20°E trending right-lateral and N60°–N70°E trending left-lateral faults. The dominant stress field in this zone is characterised by NW–SE extension, in general agreement with left-lateral transform motion. The Tjörnes Fracture Zone includes three major lineaments at different stages of development. The most developed, the Húsavík–Flatey Fault, presents a relatively simple geometry with a major fault that trends ESE–WNW. The stress pattern is however complex, with two dominant directions of extension, E–W and NE–SW on average. Both these extensions are compatible with the right-lateral transform motion and reveal different behaviours in terms of coupling. Transform motion has unambiguous fault expression along a mature zone, a situation close to that of the Tjörnes Fracture Zone. In contrast, transform motion along the immature South Iceland Seismic Zone is expressed through a more complicate structural pattern. At the early stage of the transform process, relatively simple stress patterns prevail, with a single dominant stress field, whereas, when the transform zone is mature, moderate and low coupling situations may alternate, as a function of volcanic–tectonic crises and induce changes in stress orientation.

Cleavage fronts and fans as reflections of orogen stress and kinematics in Taiwan

Recent observations of cleavage patterns, strain histories, and kinematics across the Taiwan mountain belt depict systematic orogen-scale variations with respect to the synorogenic divide and suggest that the pattern of cleavage development is a predictable consequence of orogen stresses and kinematics. In Taiwan, continental crust within the collision is accreted in the prowedge facing Asia, but is advected eastward into the east-verging retrowedge, where the most deeply exhumed rocks are exposed. Wedge mechanics predict a reversal in the direction of plunge of the principal compressive stress at the topographic divide between the opposing wedges. The observation of a single cleavage in western Taiwan suggests that the cleavage in the prowedge remains stable with respect to the stress orientation. In contrast, the existence of a second crenulation cleavage in the retrowedge is evidence for an abrupt change in stress orientation and unstable buckling of preexisting prowedge fabrics. Advection of a fabric across a topographic divide in a doubly vergent wedge provides an explanation for the occurrence of cleavage fronts and fans in natural systems such as Taiwan.

Evidence for reactivation of Eocene joints and pre-Eocene foliation planes in the Okanagan core-complex, British Columbia, Canada

We studied the brittle deformational history of the Okanagan metamorphic core complex, and its hanging wall carapace, in the Canadian Cordillera, in southern British Columbia by an extensive structural investigation at mesoscopic scale. The deformational history reveals that the transition from ductile to brittle deformation occurred during the early-middle Eocene and was associated with cooling due to tectonic and/or extensive erosional unroofing of the Okanagan portion of the Sushwap metamorphic core complex. Our study is based on mesostructures because in this region macro-structures are rarely exposed, although their existence can be deduced from field relationships. Measured fault systems that are either perpendicular to bedding, or which were sub-vertical to inferred paleohorizontal and with very small dispersion of strike are interpreted as faulted joints, i.e. joints that were reactivated by subsequent shear due to a relative change in stress orientation. The faulted joints are similar to observed open-mode I fracture system that exhibit no evidence for subsequent reactivation. In this study we suggest that fault sets characterized by consistent attitude with a very small dispersion can be interpreted as faulted joints using only their stereographic projection pattern relative to the inferred paleohorizontal. The first brittle deformation was a pervasive open-mode fracturing in the hanging wall carapace of both Eocene sedimentary and volcanic rocks and Eocene and older igneous rocks characterized by a N-S trend, with a very small dispersion, which formed either perpendicular to bedding or was sub-vertical with respect to the inferred paleohorizontal. These fractures are of similar trend to a middle Eocene dyke swarm in part of the study area and beyond. The Okanagan core complex metamorphic rocks exhibit brittle fractures that are interpreted as metamorphic foliation planes which were reactivated as faults in response to the same E-W extension recorded by the joints and dykes. Subsequent shear deformation, associated with normal dip-slip motion along the open-mode fractures, is explained by either regional tilting of the area in the presence of a similar stress orientation accompanying continued extension, or as the result of a minor reorientation of the stress field. Later deformations, suggested by strike-slip and reverse motions indicate significant changes in the stress field orientation.