Compressional Stress Regime Research Articles

Lithospheric resistivity structure of the 2001 Bhuj earthquake aftershock zone

SUMMARY The Bhuj area, in the Kutch region of western India, is a unique intraplate seismic zone in the world where aftershock activity associated with a large magnitude earthquake (7.7 Mw Bhuj earthquake on 26 January 2001) has persisted over a decade and up till today. We studied the lithospheric resistivity structure of the Bhuj earthquake aftershock zone to gain more insight into the structure and processes influencing the generation of intraplate seismicity in broad and, in particular, to detect the deep origin and upward migration channels of fluids linked to the crustal seismicity in the area. A lithospheric resistivity model deduced from 2-D and 3-D inversions of long-period magnetotelluric (MT) data shows low resistive lithospheric mantle, which can be best explained by a combination of a small amount of interconnected melts and aqueous fluid in the upper mantle. The MT model also shows a subvertical modestly conductive channel, spatially coinciding with the Kutch Mainland Fault, which we interpret to transport fluids from the deep lithosphere to shallow crust. We infer that pore pressure buildup aids to achieve the critical stress conditions for rock failure in the weak zones, which are pre-stressed by the compressive stress regime generated by ongoing India–Eurasia collision. The fluidized zone in the upper mantle beneath the area perhaps provides continuous fluid supply, which is required to maintain the critical stress conditions within the seismogenic crust for continued seismicity.

Analysis of neotectonic structures in the piedmont region of Pir Panjal Range NW Himalaya by integrating geomorphic indicators coupled with geophysical transects (GPR)

Ground-penetrating radar (GPR) survey was performed at ten potential sites to delineate the blind surface ruptures, structural discontinuities/shallow subsurface active faults and palaeoseismic deformation in the dynamic range front environment of Pir Panjal. A total of 20 GPR profiles were recorded in four districts with the unshielded 100 MHz rough terrain antenna system and 250 MHz shielded antenna, and out of them, 10 profiles were processed and correlated with detailed structural and lithological map of the area. Several tectonic landforms such as strath surfaces, uplifted fluvial terraces, fault scarps, structural discontinuities, offset ridges, meanders, linear valleys and liquefaction structures mapped during field exercises were meticulously examined by GPR prospecting. Steeply dipping hyperbolas, warped and inclined radar reflections that sharply cut or offset stratigraphic reflections correspond to thrust fault strands. The processed data unearth normal and reverse faults, as well as liquefaction structures covered by recent deposits. However, their correlation with geological and structural data gave a more precise insight into the structural settings of the studied area. The different discontinuity relations specified in the Karewa deposits are mainly due to the regional compressional stress regimes and gradual anisotropic lithology prevailing in the area. The GPR data in agreement with the exposed geological units and structural models suggest that bulk of the landscapes have been evolved/sculptured under the influence of compressional and extensional tectonics along the concomitant active faults. The structural discontinuities (Faults) and the mechanical properties of Karewa deposits will be useful to infer the neotectonic activity prevailing in the area. The whole study would be helpful for the seismic hazard assessment of parts of the Kashmir Valley, NW Himalaya.

Tectonic and magmatic controls on the evolution of post-collapse volcanism. Insights from the Acoculco Caldera Complex, Puebla, México

The Acoculco Caldera Complex (ACC; 2.7–0.06 Ma) is located in the eastern part of the Trans Mexican Volcanic Belt, México. The ACC produced three major explosive eruptions: the Acoculco andesitic ignimbrite (AI; 2.7 Ma; Syn-caldera), Piedras Encimadas rhyolitic ignimbrite (1.3 Ma; Late post-caldera), and the Tecoloquillo rhyolitic ignimbrite (TI; 0.8 Ma; Late post-caldera). Faulting has been active since the Pliocene to the present; the regional stress regime developed a NW-SE normal fault system (graben-horst structures) where the ACC was built on the Rosario-Acoculco horst.We investigated the depth of the Acoculco and Tecoloquillo magmatic chambers with H2O-CO2 content in glass inclusions, magmatic processes (magma mixing), and origin of magmas with major oxides-trace-element geochemistry and mineral chemistry. Magma chambers for TI and AI magmas are isolated from each other and located approximately 6–10 km depth. We related the local stress field to the magmatic processes that modified the AI (syn-caldera) and TI (post-caldera) magmas of the ACC. The extensional regime promoted the ascent of calc-alkaline magma and stopped by a local compressive stress regime (transfer zones; Acoculco fault), generating an entrapment zone (1.5–2.5 kbar). The new ascent of calc-alkaline magmas from a deep reservoir provoked magma mixing that triggered the AI eruption and the collapse of the magma chamber roof. The collapse event destroyed the feeding conduits and created new ascent paths that favored the formation of a new plumbing system. The ascent of melt used the pre-existing structures, probably deep graben-horst NE-SW faults, allowing peralkaline magma ascent that stopped in the entrapment zone. The new alkaline and calc-alkaline magma injection caused magma mixing that catalyzed the TI explosive eruption through NE-SW trending structures forming the TI plumbing system.

Stress Field Interactions Between Overlapping Shield Volcanoes: Borehole Breakout Evidence From the Island of Hawai'i, USA

AbstractKnowledge of the in situ stress state of the Earth's crust plays a key role in understanding geological processes including plate tectonics, earthquakes, slope failure, and igneous emplacement. In this paper, we determine the in situ stress orientation from the PTA2 borehole on the island of Hawai'i, drilled into a lava flow dominated sequence between Mauna Kea and Mauna Loa. High‐resolution acoustic images were collected from the open hole interval 886 m to 1,567 m. Based on identification of 371 borehole breakouts for a total length of 310 m, the mean orientation of the minimum horizontal principal stress is N106° and remains constant across different volcanic rock fabrics. Changes in borehole breakout shape are linked to the different strength of volcanic facies and intra‐facies. The orientation of the present‐day stress field at Mauna Kea deviates from the plate forces and regional tectonic stress field. We interpret the compressive stress regime at the PTA2 site as resulting from the competing gravitational fields of the large topographic highs of Mauna Kea and Mauna Loa. Our study reveals that the mass accumulation associated with shield volcano growth imparts significant local variations to the subsurface stress state on volcanic islands consisting of overlapping shield volcanoes. The results have significant implications for stress accumulation leading to brittle failure and flank collapse, along with potentially influencing magma accumulation and ascent pathways during volcanic island evolution. This study provides the first insights into the orientation of the present‐day stress field between the major island forming shield volcanoes of Hawai'i.

Geophysical Characterisation of the Active Fault Geometry and Trends in Tectonic Deformation in the Shallow Stratigraphy along Active Faults, South Andaman, India

Abstract A comprehensive Ground Penetrating Radar (GPR) profiling with high resolution was done and complemented with total station surveys along an active faults in the South Andaman Island (near Bathubasti) to reveal the sub-surface fault geometry and trends in tectonic deformation in shallow stratigraphy. The surface manifestation of deformation along the investigated active faults is expressed in the form of explicit tectonic scarps of various levels related to persistent tectonic uplift of the area and frontal scarps expressed in younger sediments were meticulously examined with GPR profiling. The surface displacement (scarp height) was calculated by using high resolution GPR data and total station data that may be correlated to the vertical displacement component of palaeoseismic faulting. Two active fault sites were carefully chosen for the systematic profiling with high resolution GPR profiling to examine the efficacy of GPR in elucidating the complicated near surface tectonic deformation. The surface deformation of site-1 is manifested in a form of a 6 m high abrupt tectonic scarp expressed in quaternary sediments. Site-2 is piggy back basin with corresponding pop-up structures with two antithetic scarps of 10 m height each. The high resolution GPR data vividly reveals the complicated sub-surface fault geometry and tectonic deformation in the form of an inclined and distorted radar reflections upto several meters of depth in the shallow stratigraphy and upward fault termination related to the active deformation. The high resolution GPR profiling demonstrates severe near surface tectonic deformation in the form of buckling, truncation, offsetting of recent sediments and warped radar reflections suggesting the propagation of tectonic activity in a compressive stress regime whose dynamics is related to the continuous oblique subduction of Indian plate under the Burmese micro plate.

Structural Analysis of the Luowei Orefield in Xidamingshan, Guangxi, China

AbstractMany types of hydrothermal deposits (e.g. W, Bi, Pb, Zn, Ag) are confined by faults and hidden granodiorite in the Luowei Orefield in Xidamingshan, Guangxi, China. The orebodies in the Luowei W–Bi deposit are predominantly layered and distributed along bedding in sandstones of the Cambrian Xiaoneichong Formation. The orebodies in the Lujing Pb–Zn deposit are controlled mainly by west‐south‐west (WSW)‐trending faults, and those in the Fenghuangshan Ag deposit are controlled mainly by west‐north‐west (WNW)‐trending faults, which were reverse faults during mineralization and were later reactivated as sinistral strike‐slip faults. The Luowei fault was formed postmineralization and resulted in sinistral displacement of the subsurface granodiorite and the Cambrian strata. A tectonomagmatic mineralization model of the Luowei Orefield is proposed, and the following conclusions were made. (i) Under a regional N–S compressive stress regime, WSW‐ and WNW‐trending reverse faults and N–S‐trending tensional fractures were formed. (ii) Magma intruded along the tensional fractures. Under the force of magmatic thermodynamics, mineralizing fluid migrated along bedding planes in sandstones and formed W–Bi orebodies at favorable sites. Some fluid migrated along WSW‐ and WNW‐trending faults to sites farther from the magma source, forming vein‐type Pb–Zn and Ag orebodies. (iii) After mineralization, under ~E–W compression, a NW‐trending left‐lateral slip fault was formed, cutting the subsurface granodiorite and orebodies. Concurrently, sinistral shear slip occurred on WNW‐trending ore‐controlling faults. However, the small displacement on these faults did not change the overall distributions of the rock mass and orebodies.

Implications of the pore pressure and in situ stress for the coalbed methane exploration in the southern Junggar Basin, China

The southern Junggar Basin hosts abundant coalbed methane (CBM) resources and is expected to be an important CBM production base in China. The pore pressure and in situ stress are key factors that influence the exploration and development of CBM; however, these factors are poorly understood in this area. Based on extended leak-off test data, mathematical models were established for pore pressure and in situ stress prediction using regression analysis. Subsequently, the correlations between the pore pressure, in situ stress, and permeability were analyzed, and the stress ratios were investigated. Finally, the forming mechanism of stress zonation in the study area was discussed. The results show that CBM reservoirs are generally underpressured and partly overpressured at depths greater than 800m in the study area. The study area is characterized by notable stress zoning. The stress regime varies from reverse to strike-slip to normal faulting with increasing depth. Significantly positive correlations exist between the pore pressure, maximum principal stress, minimum principal stress, vertical stress, and principal stress difference. The minimum horizontal stress is approximately 70% of the vertical stress in the study area. A mud weight window, varying from the pore pressure to the most likely breakdown pressure, is suggested. At depths greater than 600m, a relatively high principal stress difference is observed, which indicates that deep coals have a relatively high mechanical strength and their permeability can be enhanced by hydraulic fracturing. The depth-dependent stress zonation is mainly due to the influence of thrusting on the in situ stress, which diminishes with increasing depth. The compressional stress regime in the shallow area benefits the sealing of faults and the preservation of CBM resources in the study area. The deep CBM resources have a development potential. The outcomes of this study contribute to the CBM exploration and development in the study area.

First MnO2-based electrorheological fluids: high response at low filler concentration

Nanoparticles of MnO2 with different shapes (needle-like α-MnO2 and spherical δ-MnO2) were proposed as promising fillers for electrorheological fluids (ERFs). A thorough study of the dielectric properties of α-MnO2 and δ-MnO2 suspensions in silicone oil enabled estimation of the values of their dielectric permittivity and dielectric relaxation times. The study of MnO2-filled materials with various filler contents, and under various electric field strengths, in shear and uniaxial tensile and compressive stress regimes, revealed that needle-like α-MnO2 nanoparticles, in comparison with spherical δ-MnO2 particles, lead to better mechanical properties of the fluids. The most intriguing property of MnO2-based electrorheological fluids is their high electrorheological response at low filler content: ERF containing only 4.5 vol.% of α-MnO2 in silicone oil demonstrated a 18 kPa tensile strength in a 5 kV electric field.

Repeated reactivation of the Gavilgarh-Tan Shear Zone, Central India: Implications for the tectonic survival of deep-seated intra-continental fault zones

Continental crust accommodates crustal deformation through repeated reactivation of old shear/fault zones that constitute domains of long-term structural weakness. Reactivation is often triggered by mechanical and chemical (fluid-assisted) changes of the host rocks within the fault core region. However, faults can also reactivate in fluid-poor condition if they are oriented favourably with respect to the prevalent stress regime. An example comes from the Gavilgarh-Tan shear/fault zone (GTSZ) in the central Indian craton which shows clear evidences of repeated fault reactivation movements widely separated in time (in Neoproterozoic, Ordovician, Permo-Triassic, Late Cretaceous and Holocene). Presence of pseudotachylyte along some reactivated fault strands indicates seismogenic fault movements in the past. Kinematic signatures of the shear zone with contrasting slip senses indicate repeated ‘geometric reactivation’ of the GTSZ in the pre-Quaternary as well as one episode of ‘kinematic reactivation’ in the Quaternary period. Under the present NNE-SSW directed compressive stress regime of the Indian craton, the GTSZ is prone to further seismogenic fault movements.

Open shear fractures at depth in anticlines: insights from the Kuqa foreland thrust belt, Tarim Basin

Shear fractures that close under compressive stress regime were described by previous workers. Here, we show the existence of hinge-parallel shear-opening mode fractures at a certain depth within an anticline structure, through core observations, Formation Micro Imager (FMI) log interpretations and production data. Studied reservoir is a tight sandstone body of Cretaceous age, which is emplaced in KS2 Anticline, with depth ranging between 6000 and 8100 m. We explore the mechanism of hinge-parallel shear fractures by stereographic technique and backtilting, as well as show field evidences for opening displacements. These shear fractures mainly consist of vertical tension set, moderate-dipping shear set, steeply dipping shear set and sub-horizontal shear set. Crosscutting relationships and surface textures suggest that although the moderate-dipping shear set is older and maybe sealed by cements, it may have been opened by local stress due to outer-arc extension through folding. Calcite bridges with rhombohedral structures may support fracture walls helping them to remain open in case of compressive stress. Our work documents shear fracture openings in the anticline at depth, and fluid flow in subsurface reservoirs.

Neogene sedimentation and tectonics in the Collón Curá basin (Patagonian Andes of Argentina)

We recall here recent developments concerning the filling and paleogeography of the Neogene Collón Curá basin in the foothill of the North Patagonian Andes. To better constrain its evolution, we bring two new 40Ar/39Ar on biotite datings of two ignimbrites that allow bracketing of the filling between 19.04 ± 0.72 and 5.06 ± 0.69 Ma. Like the other intramontane basins of the North Patagonian foothill, the Collón Curá basin initiated around 18 Ma at the time of the Quéchua phase. This corresponds in the area neither to an increase of the convergence rate of the Nazca and South America plates, nor to a significant change in the angle of convergence. We propose to look at this profound alteration of the regional stress regime in the light of the recent results of analog and numerical modeling of subduction. Folding of the slab above the mantle discontinuity at 660 km depth leads to the development of geometries that we retrieve in seismic tomography images in the region. Modeling shows that the overriding plate is coupled to this folding in depth. It suffers from changes in stress (from extensive to compressive stress regime) and topography. It is assumed that a more compressive stress regime will favor the reactivation of border faults of the basin and a regional uplift, hence agradation in basins. On the contrary, subsidence associated with a decrease in shortening will favor the backward erosion that will cannibalize these basins. The life duration of basins will be modulated by efficiency of earth surface processes proper to each watershed (catchment area, river slopes, lithology …). This internal cyclicity of subduction due to slab folding is freed from the kinematics of convergence. It is now necessary to look for the expression of this process in the sequential filling of the basins.

Sill complexes in the Karoo LIP: Emplacement controls and regional implications

Field and sub-surface data from the Victoria West sill complex in the Karoo Large Igneous Province (ca. 180 Ma) of South Africa are used to constrain the emplacement controls of the regional-scale sill complexes in the central Karoo basin. Cross-cutting relationships point to the presence of five distinct and successively emplaced saucer-shaped sills. Growth of the sill complex was achieved through magmatic underaccretion of magma batches below earlier sills and associated uplift of the overlying strata. The magmatic underaccretion suggests that earlier sills were fully crystallized during the emplacement of later magma pulses and that the rigid (high E) dolerites, in particular, acted as stress barriers that impeded further upward propagation of steep feeder sheets. The resulting nested structure of sills-in-sills within a confined area of less than 2000 km2 also suggests the reutilization of the same or similar feeder system even after full crystallization thereof.The emplacement controls of sills in the central Karoo through stress barriers implies that sill emplacement occurred under very low deviatoric stresses or in a mildly compressional stress regime prior to the break-up of Gondwana. The swap from earlier (184-180 Ma), mainly sill complexes to later (182-174 Ma) dykes and dyke swarms is indicative of a switch in the stress field during the early stages of Gondwana break-up. We speculate that loading, thermal subsidence and lithospheric flexure associated with the emplacement of the earlier, stacked and voluminous sill complexes in the Karoo basins may have determined the formation of the large Karoo dyke swarms, particularly when coinciding with deeper crustal structures. The original and inherited basin geometry and lithospheric structure is pivotal in the development of later Karoo magmatism.

Heterogeneous strain regime at the west of the Ogasawara Plateau in the Western Pacific Ocean from inversion of earthquake focal mechanisms

The Ogasawara Plateau has a major control on the geology of the region between the Izu-Bonin and Mariana subduction zones. This region is important for understanding active collisional zones. We attempt to illustrate the stress field in the Wadati-Benioff zone of this region through seismic estimations. 49 events (M ≥ 3.5) are chosen for waveform inversion by the ISOLA software to distinguish the stress field. According to available focal mechanism solutions, the heterogeneous stress field parameters are determined for several depth ranges. Based on the epicentral distribution and seismic stress field, the subduction system at the west of the Ogasawara Plateau can be divided into three segments separated by 26.5°N and 24°N. At depths of 0–50 km in the outer-trench slope, the slab is characterized by a varying stress filed. The best-fit stress model with horizontal σ3 and vertical σ1 is related to the plate bending and preexisting faults. For the central segment, the solutions present a compressional stress regime in the outer-trench slope without preexisting faults, induced by the resistance of the buoyant plateau. The solutions for the depth range of 0–50 km in the forearc region, σ1 coincides with the plate convergence and σ3 dips steeply to the west in the northern and central segments, controlled by the friction effect and convergence of the plates. However, this depth range in the southern segment shows an extensional stress range, associated with subduction of older slab segment, relatively low rate of plate convergence and the morphological adjustment of the slab between different segments. At depths of 50–200 km, the inhomogeneous stress field exhibits a complicated extensional stress system with strike-slip focal mechanisms. At depths of 200–400 km, the seismicity presents a pronounced seismic gap and the stress field cannot be determined. The seismic gap may be attributed to the missing or tear of the subducting slab, which is caused by the subduction of the buoyant Ogasawara Plateau. The mechanism solutions are characterized by an inhomogeneous strain regime below 400 km, driven by gravity and strong contortion of the slab and resistance to penetration of the 660-km discontinuity. In contrast, there are little earthquakes below 400 km in the southern region of the Ogasawara Plateau.

Source parameters, focal mechanisms and stress tensor inversion from moderate earthquakes and its relationship with subduction Zone

Nine earthquakes occured between 2005 and 2009, with magnitudes between 4.9 and 5.5, at the zone that correspond to the triple junction of the Cocos, Rivera and North America plates were relocated. Records from RESCO seismic stations were used together with records from SSN seismic network. Source parameters were estimated: source dimension, seismic moment, stress drop, average displacement and source time.The seismic moment tensor inversion was used to estimate focal mechanism and seismic moment to compare then whit short and long period estimation of these parameters.A stress tensor inversion was also performed in order to evaluate stress state of the area of study. The results indicate a compressive stress regime, which is characteristic of a subduction zone. It is interesting however, that plunge of ?1 goes from a moderate angle in the northwest to sub-horizontal in the southeast. The existence of normal and strike-slip mechanisms is also interesting in a subduction zone. Their existence was interpreted in terms of the interaction of Cocos and Rivera plates in this tectonically complicate zone.

An experimental and theoretical stress-strain-damage correlation procedure for constitutive modelling of granite

In this study, new theoretical and experimental stress-strain-damage correlation procedures for hard rock constitutive models are proposed. The damage-plasticity modelling framework is first supplied with the unified yield-failure criterion to describe the failure behaviour of granite under triaxial compression. This unified criterion allows the initial yield surface to evolve to a final failure surface through the utilisation of an appropriate damage evolution law. This evolution automatically captures the quasi-brittle behaviour of rocks under shearing at low confining pressure and ductile response under high confining pressure, as well as the transition from quasi-brittle to ductile reactions. In this theoretical sense, damage and plasticity are tightly coupled to govern the behaviour of rocks under different confining pressures without requiring any separate formulations for softening or hardening. Next, an innovative experimental correlation procedure is proposed to better link the experimental damage measure to stress states throughout triaxial loading. By obtaining full stress, strain and acoustic emission damage results from testing it was possible to construct a series of evolving yield surfaces from experiment. These surfaces, coupled with experimental damage evolution with respect to accumulated plastic strain, provide a comprehensive data set to a constitutive model for calibration. The results of numerical simulation show that this new method to incorporate coupled stress-strain-damage evolution characteristics directly from experiment removes the need for trial and error curve fitting. Also, by maintaining a closer link to detailed experimental results, the model is easier to calibrate and can be relied upon to predict the damage and stress states for compressive stress regimes. Finally, it is shown that the theoretical and experimental procedures can capture the key behaviours of granite under a range of confining pressures.

Growth and provenance of a Paleozoic subduction complex in the Broken River Province, Mossman Orogen: evidence from detrital zircon ages

A Paleozoic subduction complex dominates the Mossman Orogen developed at the northern extremity of the Tasmanides, eastern Australia. Its southern part, displayed in the Broken River Province, is characterised by dismembered ocean-plate stratigraphy in which turbidite-dominated packages and widespread tectonic mélange development are characteristic. The Broken River complex is characterised by formations with quartzose sandstone alternating with those largely formed of sandstone of more labile character. The two compositional groups are considered to reflect separate, age-significant sedimentary regimes, but their ages have hitherto been poorly constrained. With the use of 1082 concordant detrital zircon ages from 13 samples we provide age control for the complex and track its sedimentary provenance. Of quartzose units, the Tribute Hills Arenite and Pelican Range Formation are late Cambrian–Early Ordovician, and the Wairuna Formation is Middle to Late Ordovician, in age. The more labile units (Greenvale, Perry Creek and Kangaroo Hills formations) are collectively of late Silurian–mid-Devonian age. Development of the complex spanned some 130 Myr. Continent-derived sediment involved in accretion of much the complex, from mid-Ordovician to mid-Devonian, was largely sourced from a nearby magmatic arc of late Cambrian–Devonian age, now represented by granitoid plutons of the Macrossan and Pama igneous associations. An older far-field Pacific-Gondwana sediment source is characteristic of early-phase (late Cambrian–Early Ordovician) accretion, in common with sedimentary units of this age generally developed in the Tasmanides. We consider the complex to have grown largely by underplating that positioned younger components beneath those that are older, with out-of-sequence thrust interleaving of these components occurring late in the accretionary history. A Late Devonian contractional folding and cleavage development (Tabberabberan orogenesis) is uniformly expressed across the entire complex and reflects an abrupt change in plate engagement with imposition of a compressional stress regime.

Structural controls on granitoid-hosted gold mineralization and paleostress history of the Edikan gold deposits, Kumasi Basin, southwestern Ghana

The > 9 Moz total aggregate gold endowment at the Edikan mine, Kumasi Basin, Ghana, is contained within a cluster of orogenic gold deposits located along the Akropong fault zone. The granitoid-hosted orebodies at Edikan (e.g., AG2, AG3, Fobinso, Esuajah), essentially an interconnected mesh of gold-bearing quartz veins, formed during deformation event D3Edk, which postdates the penetrative regional D2Edk deformation. The gold-bearing quartz veins developed in, and adjacent to, N-S- and NW-SE-trending, low-angle thrust faults that crosscut lithological contacts and earlier formed, steeply dipping D2Edk faults. Our paleostress analysis shows that the D3Edk deformation, during which the mineralized fault system developed, was characterized by a WNW-ESE “hybrid” compression that evolved to a strike-slip regime. This progressive deformation is best described with the following stress regimes: WNW-ESE transpression-pure compression (T1) associated with low-angle thrusting, subsequent transpression-strike-slip (T2), and later strike-slip-transtension (T3) associated with steeply dipping strike-slip faulting. The bulk of the granitoid-hosted gold mineralization at Edikan is associated with two principal sets of gold-bearing quartz veins, including low-angle fault-fill veins controlled by thrusts and shallow dipping oblique-extension veins that developed during T1. The activation of the reverse and sinistral strike-slip faults led to the development of restraining jogs characterized by abundant shallow and steeply dipping gold-quartz veins with moderately NE-plunging ore shoots. The geometry of the mineralized fault-fracture meshes is consistent with fault-valve behavior in a horizontal compressive stress regime under sustained conditions of supralithostatic fluid pressures at low differential stress.

The Role of Diagnostic Fracture Injection Testing to Improve Reservoir Evaluation and Stress Characterisation in Compressive Stress Regimes

The diagnostic fracture injection test, commonly known as a DFIT, is frequently used in conventional and unconventional reservoirs (e.g., tight gas, shale gas, tight coals) to calibrate the hydraulic fracture treatment. In a normal stress regime, a single test can calibrate the in-situ stress profile and provide parameters such as reservoir pressure and transmissibility. However, in strike-slip regimes a single test cannot adequately derive strain values to develop an accurate stress profile as compared to multiple, precise, well-designed multi-DFIT program. Thus, if more consideration were given to the design process and stepwise implementation, a more robust stress profile and definitive reservoir characterization can result from implementation of DFITs in low permeability, unconventional gas reservoirs. This presentation will define the workflow of a multi-DFIT program and the governing equations for stress profiling to allow practitioners to incorporate DFIT data with other available data to derive accurate geomechanical parameters. Further, a well-defined program can provide insight for hydraulic fracturing modelling and key information regarding natural fracturing and transmissibility for reservoir modelling. For unconventional, non-normal, tectonically-stressed reservoirs this includes defining minimum and maximum horizontal strains as well as intermediate stress values acting on known natural fracturing azimuths. Overall, a comprehensive set of recommendations and references are made for the practical application of DFITs to illustrate the overall benefit for the well design processes.

Stress reconstruction and lithosphere dynamics along the Sumatra subduction margin

Gravity modeling and stress reconstructions over the Sumatra subduction margin between Indian-Australian and Asian plates provide insights into the geodynamic setting and kinematics of the area. Based on ridge-trench interaction, plate obliquity, plate age and seismicity, the area was divided into two sectors (northern and southern) for gravity modeling along two profiles. The stress data with magnitude Mw ≥ 5.0 and focal depth ≤ 100 km for the subducting plate were compiled for six zones (I–VI), and were isolated under different time-domains for high-resolution study. The reconstructed gravity models were used to isolate the events for subducting (oceanic/Indian-Australian) and overriding (continental/Asian) plates. The data sets for the subducting lithosphere were used for stress inversion to determine the predominant stress regimes for pre-earthquake and post-earthquake phases of the Sumatra earthquake of December 2004. The results were interpreted in terms of operative intra-plate stress fields, and for reconstruction of kinematics and dynamics along the Sumatra subduction margin. Analysis shows both sparse and intense seismic activities identified over different time-segments in the six zones under pre- and post-December 2004 phases with respect to the incidence of Mw 9.2 off Sumatra mega-earthquake of 2004. The stress regime of the 2004 mega-event and subsequent release of strain energy through cyclic changes of the stress obliquities in the northern part of the study area account for heterogeneous coupling caused by rough topography of the subducting Indian-Australian plate. Reduction of compressive stress regime in the deeper segment of the subducting plate in the northern sector triggered twin great strike-slip dominated earthquakes in 2012 in the floor of the northeastern Indian Ocean. The weak zones of a spreading ridge subducting below the Asian plate possibly suffered strain localization and weakening. The less transmission of co- and post-seismic strain energy from northern to southern part of the margin was possibly prohibited near the zone of ridge trench interaction. The trench-orthogonal tearing (slab window) at the edge of the subducting plate in this area cannot be ruled out. The slab window, caused by the subduction of the young buoyant ridge and the trench-ward movement of the upper plate, has transformed the central segment into compressional regime.

Illumination of deformation by bending stresses and slab pull within the Southern Hikurangi Double Benioff Zone

ABSTRACTDouble Benioff Zones (DBZ) are ubiquitous in subduction systems worldwide, but the stress systems that give rise to them are not well known. We characterise stress orientations in the upper and lower bands of a DBZ in the dipping subducted Pacific plate beneath the southern Hikurangi margin, New Zealand. Stress orientations were calculated from focal mechanisms with 10 or more polarity picks, using a Bayesian stress inversion technique. The stress orientations in the upper band of seismicity are consistent with a down-dip extensional stress regime, whereas stress orientations within the lower band of seismicity are consistent with a down-dip compressional stress regime. This demonstrates that bending stresses are dominant within the Pacific plate beneath the southern North Island, possibly due to subduction beneath thickened mantle lithosphere of the Australian plate.