ndsr isotopic study of proterozoic to triassic sediments from southeastern british columbia abstract sedimentary rocks from the southeastern canadian cordillera varying in age from mid proterozoic to triassic have been analysed for their nd isotopic composition e nd t values range from 1 5 to 17 2 and nd model ages t dm nd vary from 1 00 to 2 32 ga much in excess over their stratigraphic ages the difference between the two ages ranges from 0 43 to 1 93 ga decreasing as the sediments become stratigraphically older nd isotopic compositions of sediments suggest that there are two distinct continental sources of sediments in the cordilleran miogeocline a an older source e nd 0 from 20 3 to 23 8 with apparent crustal residence ages of 2 26 2 32 ga and b a younger source average e nd 0 14 0 with the crustal residence age of about 1 8 ga the older residence ages are interpreted to be due to mixtures of materials from the archean and proterozoic crusts sediments belonging to the laib formation lower cambrian and lower clastic part of the milford group namurian appear to have derived from an archean continental segment with some admixture of sediments from the early proterozoic source the younger crustal source which seems to be the principal supplier of sediments for most of the stratigraphic units aldridge formation middle proterozoic horsethief creek group upper proterozoic hamill and lardeau groups lower cambrian nelway formation middle cambrian active formation ordovician and part of milford group is not the archean canadian shield immediately to the east recent subsurface mapping of the alberta basement beneath the phanerozoic sedimentary cover using u pb zircon dating of igneous lithologies 1 has established the extensive archean and early proterozoic crustal provinces it is proposed here that the early proterozoic province of west central alberta is the most likely source of the easterly derived sedimentary rocks of the cordilleran miogeocline with t cr ages from 1 94 to 1 50 ga on the basis of sedimentologic tectonic and isotopic evidence we suggest that the younger sediments could be to some extent recycled older sediments for the triassic slocan and ymir groups eastern part of quesnel terrane sedimentologic lithologic tectonic and isotopic evidence suggest the source was a western one with about 1 8 ga crustal residence age comparison of the crustal residence ages with stratigraphic ages shows the best fit nd evolution curve for the cordilleran miogeocline approximately coincides with the continental growth curve deduced for the north atlantic province and similar to the growth curve for gondwana during the phanerozoic this curve also passes through the north american composite the levelling off of crustal residence ages for the mid proterozoic to triassic sediments at about 1 8 ga supports the idea of negligible crustal growth since at least 1 7 ga the average crustal residence age of the modern miogeoclinal and cratonic sediments

slip tendency analysis as a tool to constrain fault reactivation a numerical approach applied to three dimensional fault models in the roer valley rift system southeast netherlands we describe a new numerical approach to constrain the three dimensional 3 d pattern of fault reactivation taking advantage of the knowledge of the tectonic stress field the ratio of the resolved shear and normal stresses slip tendency as well as the direction of the shear stress is calculated at every location on the faults modelled by triangulated surfaces although the calculated contact stresses represent only a first order approximation of the real stresses comparison of the 3 d pattern of slip tendency with the frictional resistance of the fault can provide useful constraints on the probability of fault reactivation the method was applied to 3 d geometrical fault models in the roer valley rift system southeast netherlands which is presently characterized by pronounced tectonic activity the input stress tensors were constrained by published stress indicators the analysis demonstrated that the observed fault activity could be explained within a reasonable range of frictional parameters and input stress magnitudes in addition a fairly good correlation was found between the predicted slip directions and the focal mechanisms of local earthquakes this suggests that in the study area fault models being valid in the uppermost part of the crust are suitable to constrain fault reactivation even in the deeper part of the seismogenic layer the analysis further demonstrated that fault hierarchy and the regional tectonic contexts of the fault system are important factors in fault reactivation therefore they always should be taken into account during evaluation of the calculated slip tendency and slip direction patterns

polygonal faults and their implications for hydrocarbon reservoirs in the southern qiongdongnan basin south china sea the polygonal faults were identified in qiongdongnan basin south china sea by using the technique of time coherent slice and horizon flattening of high resolution 3d seismic data these polygonal faults occur in three tiers of the upper meishan formation and the huangliu formation the faults have lengths of 150 1500 m spacings of 50 3000 m throws of 10 40 m and dips of 50 90 degrees tectonic evolution in the qiongdongnan basin can be divided into a rifted stage and a post rifted stage tectonic faults are widely distributed in the rifted sequences but are not well developed in the post rifted stage few faults in the post rifted sequences might suggest the absence of a migration pathway for hydrocarbon or other fluids however the existence of polygonal faults in the post rifted sequences can serve as the pathway and promote the hydrocarbon migration and accumulation in the qiongdongnan basin during the post rifted stage c 2010 elsevier ltd all rights reserved

detection of potential fractures and small faults using seismic attributes often it is difficult to map subtle faults and other trace to trace discontinuities hidden in 3d seismic data they may appear as minor changes in the seismic waveform that are not easily correlated using conventional interpretation of seismic cross sections to map these changes we have investigated and computed several different seismic attributes sensitive to the existence of small faults and fractures in our target reservoir the late permian unayzah sandstone of central saudi arabia in addition to the traditional geometric horizon attributes such as dip and azimuth we found that both coherence and spectral decomposition were useful in imaging seismic discontinuities

two stage exhumation of ultrahigh pressure metamorphic rocks from the western dabie orogen central china abstract zircon trace element u pb age and lu hf isotope composition were determined for two eclogite samples from the xinxian ultrahigh pressure uhp unit in the western dabie orogen to constrain its exhumation processes cathodoluminescence imaging reveals that most zircons are metamorphic while some have inherited magmatic cores in one sample the magmatic cores have high th u and lu hf ratios high trace element contents and a clear negative eu anomaly consistent with their igneous genesis they yield an upper intercept age of ma and a positive ehf t value of 8 0 arguing for reworking of early mesoproterozoic crust during the middle neoproterozoic in the western dabie orogen the metamorphic zircons contain mineral inclusions of garnet omphacite rutile and quartz they are characterized by low nb ta y and heavy rare earth elements hree contents a nearly flat hree pattern and an insignificant negative eu anomaly these indicate that the metamorphic zircons formed under high pressure

thermobaric structure of a traverse across western dabieshan implications for collision tectonics between the sino korean and yangtze cratons as the best preserved high and ultrahigh pressure hp and uhp metamorphic terrane in the qinling dabieshan sulu orogen western dabieshan is divided into six lithotectonic units along a traverse across the orogen i e from north to south the nanwan balifan huwan xinxian hong an and mulanshan units in this terrane five eclogite bearing zones i v are developed the garnet and clinopyroxene in eclogites from these zones exhibit chemical zoning suggesting that the rims record general peak temperature and pressure thermobarometric study indicates that the peak p t conditions of eclogite are 550 570 c and 21 kbar for zone i 470 500 c and 14 17 kbar for zone ii 620 670 c and 26 29 kbar for zone iii 530 560 c and 20 22 kbar for zone iv and 490 510 c and 19 20 kbar for zone v the symmetrical thermobaric pattern in conjunction with structural and geochronological data demonstrates that the huwan and hong an units belong to the same hp slice overlying the uhp slice this pattern together with the mulanshan lt hp blueschist greenschist belt in the south roughly constitutes a normal metamorphic zonation however clear metamorphic gaps occur between different slices it is inferred that the lt hp hp and uhp slices were broken up from the downgoing slab during subduction and reached different depths along different geothermal gradients the successive subduction of underlying slices leads to a nearly concomitant uplift of overlying slices whereas exhumation of the deepest uhp slice was effected by underthrusting of the lower crust of the yangtze craton

introduction to special issue of canadian journal of earth sciences the alberta basement transect of lithoprobe from outcrops in northeastern alberta to exposures within structural culminations in the canadian cordillera the crystalline basement that lies buried beneath the phanerozoic alberta basin of western canada represents a significant part of the canadian shield ca 750 000 km2 the advent of modern high resolution aeromagnetic data and u pb geochronology have opened the doors for the next era of investigation of the crystalline basement of the alberta basin aeromagnetic data and geochronology provided important physical and temporal links to the exposed canadian shield which allowed its structural framework to be extrapolated into the subsurface the most recent chapter in this evolving story has been the alberta basement transects of lithoprobe a 12 year program which has used multi disciplinary geoscience studies to further elucidate the nature history and geometric characteristics of basement domains beneath the alberta basin this special issue is the second in a three issue set of publications

basement structural architecture and hydrocarbon conduit potential of polygonal faults in the cooper eromanga basin australia abstract a thorough and complete understanding of the structural geology and evolution of the cooper eromanga basin has been hampered by low resolution seismic data that becomes particularly difficult to interpret below the thick permian coal measures as a result researchers are tentative to interpret the basement fault architecture within the basin which is largely undefined to provide a better understanding of the basement fault geometry all available two dimensional seismic lines together with 12 three dimensional seismic surveys were structurally interpreted with assistance from seismic attribute analysis the upper cretaceous cadna owie formation and top permian reflectors were analysed using a common seismic attribute technique incoherency that was used to infer the presence of faults that may have otherwise been overlooked detailed basement fault maps for each seismic survey were constructed and used in conjunction with two dimensional seismic data interpretation to produce a regional basement fault map large north northeast south southwest striking sinistral strike slip faults were identified within the patchawarra trough appearing to splay from the main northeast southwest striking ridge these sinistral north northeast south southwest striking faults together with field scale southeast northwest striking dextral strike slip faults are optimally oriented to have potentially developed as a conjugated fault set under a south southeast north northwest oriented strike slip stress regime geomechanical modelling for a regionally extensive system of cretaceous polygonal faults was performed to calculate the leakage factor and dilation tendency of individual faults faults that extend into lower cretaceous oil rich reservoirs with strikes of between 060 n and 140 n and a high to near vertical dip angle were identified to most likely be acting as conduits for the tertiary migration of hydrocarbons from known lower cretaceous hydrocarbon reservoirs into shallow cretaceous sediments this research provides valuable information on the regional basement fault architecture and a more detailed exploration target for the cooper eromanga basin which were previously not available in literature

maximum horizontal stress orientations in the cooper basin australia implications for plate scale tectonics and local stress sources summary borehole breakouts and drilling induced tensile fractures ditfs were interpreted in 61 wells in the cooper basin indicating an average maximum horizontal stress orientation of 101 n a total of 890 borehole breakouts and 608 ditfs were interpreted in the cooper basin the approximately east west maximum horizontal stress orientation is consistent over much of the basin except in the patchawarra trough where maximum horizontal stress rotates to a northwest southeast orientation this rotation in maximum horizontal stress orientation is consistent with in situ stress data to the northwest of the cooper basin the stress field in the cooper basin appears to mark the apex of a major horseshoe shaped rotation in maximum horizontal stress direction across central eastern australia finite element modelling of the in situ stress field of the indo australian plate iap using a range of plate scale tectonic forces is able to match the regional maximum horizontal stress orientation over most of australia reasonably well including the mean east west maximum horizontal stress orientation in the cooper basin however plate boundary scale modelling does not adequately match the horseshoe shaped stress rotation across central eastern australia the average east west maximum horizontal stress orientation in the cooper basin indicates that stresses from tensional forces acting along the tonga kermadec subduction zone are not transmitted into the interior of the australian plate the majority of the tensional forces associated with the tonga kermadec subduction zone are most likely accommodated along the numerous spreading centres within the lau havre backarc basin a number of more localized stress anomalies have also been identified these cannot be explained by plate scale tectonic forces and are possibly a result of geological structure and or density contrasts locally perturbing the stress field

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

cordilleria revisited with a three dimensional model for cretaceous tectonics in british columbia the cordilleria microcontinent theory for the tectonics of british columbia in the cretaceous and early tertiary and for the origin of the canadian rocky mountains and related features is reviewed new isotopic geological and paleomagnetic data support the hypothesis that the amalgamated superterranes of british columbia and alaska were not attached to the mainland of north america in the early cretaceous and were probably well south of their present relative position consideration of that evidence together with sedimentological evidence from the albian leads to a slightly earlier best estimate 115 ma for the time of initial collision in the yukon new geophysical data for southeastern british columbia and idaho lead to a comparison with the iapetus suture in northern england it is suggested that the cordilleria suture dips west from the present purcell thrust and related structures and should cut the moho a new model for regional deformation in se british columbia is proposed involving a flake

the relationship between closure pressures from fluid injection tests and the minimum principal stress in strong rocks abstract closure pressures measured during injection tests such as mini fracs are normally considered an accurate measure of the minimum in situ principal stress magnitude this paper presents stress strength and image log data from the australian cooper basin which suggests that in reservoirs with high in situ stress high tensile strength and weak geological fabrics interpreted closure pressures may be significantly greater than the minimum principal stress closure pressures interpreted from mini frac injection tests in the cooper basin suggest the minimum principal stress varies from 12 4 27 2 mpa km 0 55 1 2 psi ft to better understand the reasons for this variation in closure pressure image logs and mini frac data from 13 treatment zones and core from seven of these treatment zones were analysed the analysis revealed that treatment zones with high measured closure pressures 18 1 mpa km 0 8 psi ft high treating pressures 31 6 mpa km 1 4 psi ft and high measured hydraulic fracture complexity existed in reservoirs with high tensile rock strength 7 mpa 1015 psi and geological fabrics planes of weakness including natural fractures conversely treatment zones with lower measured closure stress 19 mpa km 0 84 psi ft and low hydraulic fracture complexity occurred in reservoirs with lower tensile strength and or no geological fabrics we suggest that closure pressures in rocks with high tensile strength and weak geological fabrics may not be representative of the minimum principal stress magnitude in the cooper basin where they are associated with hydraulic fracture complexity rather they reflect the normal stress incident on pre existing weaknesses that are exploited by hydraulic fluid during the mini frac injection

the warnie volcanic province jurassic intraplate volcanism in central australia abstract the cooper and eromanga basins of south australia and queensland are the largest onshore hydrocarbon producing region in australia igneous rocks have been documented infrequently within end of well reports over the past 34 years with a late triassic to jurassic age determined from well data however the areal extent and nature of these basaltic rocks were largely unclear here we integrate seismic well gravity and magnetic data to clarify the extent and character of igneous rocks preserved within eromanga basin stratigraphy overlying the nappamerri trough of the cooper basin we recognise mafic monogenetic volcanoes that extend into tabular basalt lava flows igneous intrusions and more locally hydrothermally altered compound lava flows the volcanic province covers 7500 km2 and is proposed to have been active between 180 160 ma we term this jurassic volcanic province the warnie volcanic province wvp after the warnie east 1 exploration well drilled in 1985 the distribution of extrusive and intrusive igneous rocks is primarily controlled by basement structure with extrusive and intrusive igneous rocks elongate in a nw se direction finally we detail how the wvp fits into the record of jurassic volcanism in eastern australia the wvp is interpreted as a product of extension and intraplate convective upwelling above the subducting pacific slab the discovery of the wvp raises the possibility of other yet unidentified volcanic provinces worldwide

an automated cross validation method to assess seismic time to depth conversion accuracy a case study on the cooper and eromanga basins australia selecting a seismic time to depth conversion method can be a subjective choice that is made by geophysicists and is particularly difficult if the accuracy of these methods is unknown this study presents an automated statistical approach for assessing seismic time to depth conversion accuracy by integrating the cross validation method with four commonly used seismic time to depth conversion methods to showcase this automated approach we use a regional dataset from the cooper and eromanga basins australia consisting of 13 three dimensional 3d seismic surveys 73 two way time surface grids and 729 wells approximately 10 000 error values predicted depth vs measured well depth and associated variables were calculated the average velocity method was the most accurate overall 7 6 m mean error however the most accurate method and the expected error changed by several metres depending on the combination and value of the most significant variables cluster analysis tested the significance of the associated variables to find that the seismic survey location potentially related to local geology i e sedimentology structural geology cementation pore pressure etc processing workflow or seismic vintage formation potentially associated with reduced signal to noise with increasing depth or the changes in lithology distance to the nearest well control and the spatial location of the predicted well relative to the existing well data envelope had the largest impact on accuracy importantly the effect of these significant variables on accuracy were found to be more important than choosing between the four methods highlighting the importance of better understanding seismic time to depth conversions which can be achieved by applying this automated cross validation method

imaging changes in morphology geometry and physical properties of the subducting pacific plate along the izu bonin mariana arc abstract recent studies on slab structure in the northwest pacific have imaged variable geometries of the subducted oceanic lithosphere along the plate margin tomographic images show portions of the subducted pacific plate penetrate vertically into the lower mantle below the mariana arc whereas below the izu bonin arc the slab appears to be lain down horizontally on top of the 670 km discontinuity using new technology to grid and visualize regional seismic tomography data even more detail about the structure of the subducting plates can be extracted from the three dimensional images we investigate the morphology and geometry of the subducting pacific plate as it changes between horizontal to nearly vertical using p wave tomographic images the most profound result from these new models is the presence of a distinct change of seismic property in the downgoing pacific slab beneath the izu bonin arc at a depth of 350 400 km the position of this anomaly corresponds to a region north and west of the ogasawara plateau we propose the change in morphology and physical property of the slab is related to the distortion of the pacific plate geometry as its shape converts from near horizontal to vertical together with the subduction of the ogasawara plateau

continental subduction and exhumation of high pressure rocks insights from thermo mechanical laboratory modelling abstract thermo mechanical physical modelling of continental subduction is performed to investigate the exhumation of deeply subducted continental crust the model consists of two lithospheric plates made of new temperature sensitive analogue materials the lithosphere is underlain by liquid asthenosphere the continental lithosphere contains three layers the weak sedimentary layer the crust made of a stronger material and of a still stronger lithospheric mantle the whole model is subjected to a constant vertical thermal gradient causing the strength reduction with depth in each lithospheric layer subduction is driven by both push force and pull force during subduction the subducting lithosphere is heating and the strength of its layers reduces the weakening continental crust reaches maximal depth of about 120 km and cannot subduct deeper because its frontal part starts to flow up the subducted crust undergoes complex deformation including indicated upward ductile flow of the most deeply subducted portions and localised failure of the subducted upper crust at about 50 km depth this failure results in the formation of the first crustal slice which rises up between the plates under the buoyancy force this process is accompanied by the delamination of the crustal and mantle layers of the subducting lithosphere the delamination front propagates upwards into the interplate zone resulting in the formation of two other crustal slices that also rise up between the plates average equivalent exhumation rate of the crustal material during delamination is about 1 cm year the crust asthenosphere boundary near the interplate zone is uplifted the subducted mantle layer then breaks off removing the pull force and thereby stopping the delamination and increasing horizontal compression of the lithosphere the latter produces shortening of the formed orogen and the growth of relief the modelling reveals an interesting burial exhumation evolution of the sedimentary cover during initial stages of continental subduction the sediments of the continental margin are dragged to the overriding plate base and are partially accreted at the deep part of the interplate zone at 60 70 km depth these sediments remain there until the beginning of delamination during which the pressure between the subducted crust and the overriding plate increases this results in squeezing the underplated sediments out part of them is extruded upwards along the interplate zone to about 30 km depth at an equivalent rate of 5 10 cm year

north american central plains conductivity anomaly goes east the north american central plains electrical conductivity nacp anomaly has been known for almost two decades and its influence on palaeotectonic models of the north american continent has been significant magnetometer array studies have located the structure in a gross sense but there has not been until now a comprehensive magnetotelluric mt survey of it in this letter we report on the preliminary interpretation of data from thirty five mt sites recorded along a 400 km east west profile just north of the us canadian border we show that the anomaly at this latitude is 75 km farther east of the location previously mapped by the array studies modelling indicates that the conductive body has its top surface at about 10 km depth and appears to be anticlinal in shape the profile also extends sufficiently east that it traverses another previously known electrical structure referred to herein as the tobe anomaly we consider briefly other geophysical and geological data for the region and tentatively infer that the nacp structure is terminated to the east by the interpreted southern extension of the tabbernor fault fold system and that the tobe anomaly is an expression of the known southern extension of the thompson nickel belt both beneath phanerozoic sediments of the williston basin

smnd upb and rbsr geochronology and lithostructural relationships in the southwestern rae province constraints on crustal assembly in the western canadian shield the rae province is an area of archean crust bounded by the early proterozoic thelon orogen to the northwest and mylonites of the snowbird tectonic zone to the southeast the southern part of the province in saskatchewan is a highgrade complex of ultramafic mafic and felsic orthogneisses with interlayered supracrustal units the oldest rocks in the area were derived from layered mafic plutonic complexes and supracrustal units the earliest deformation d1 resulted in development of a layer parallel foliation preserved in small lensoid pods of anorthosite gabbro and pyroxenite a pyroxenite yields a sm nd tdm age of 3 5 ga and is isotopically similar to a mafic granulite metabasalt associated with early supracrustal units these rocks were intruded by a sequence of mafic and tonalitic orthogneisses that yield a sm nd isochron age of 3 32 2 87 and tdm ages of 3 2 3 1 ga the mafic tonalitic gneisses were subsequently migmatized producing anatectic net veins of tonalitic to granitic pegmatite these were deformed during d2 deformation the terrane was then intruded by minor granodioritic to tonalitic plutonic sheets and a swarm of mafic dikes that were deformed by the last major period of inhomogeneous deformation d3 which ended at 2 0 ga the clearwater complex a large 24 8 km long anorthosite massif intruded the gneiss complex following d3 and yields a sm nd mineral wholerock isochron age of 1 85 ga sm nd rb sr and u pb shrimp data record multiple thermotectonic events and provide constraints on crustal evolution of the rae province u pb shrimp data for metamorphic zircons separated from an early anorthosite associated with units that give pre 3 2 ga nd model ages show considerable variation in th u ratio zircons yield virtually concordant data that record archean 2 9 ga and several early proterozoic between 2 3 and 1 85 thermotectonic events sm nd data are generally poorly correlated possibly as a result either of slight remobilization of ree s during granulite facies metamorphism or that rocks with distinct origins were compared rb sr whole rock and mineral data show substantial re equilibration in mafic tonalitic gneisses at 2 8 ga and in supracrustal units at 2 3 ga also a discordant granitic dike and biotite separated from a metapelite yield rb sr tdm ages of 1 9 ga these metamorphic ages are interpreted to reflect thermotectonism associated with formation and assembly of juvenile early proterozoic crust in the talston thelon plutonic zone to the west and with terminal collision in the trans hudson orogen to the east

mapping permeable subsurface fracture networks a case study on the cooper basin australia abstract the spatial distribution of permeable fracture networks is constrained in the subsurface cooper basin australia through the integration of most positive curvature k analysis of five three dimensional 3d seismic surveys wellbore fracture data and geomechanical modelling the k provides the likely distribution of subsurface extensional fractures based on the stress redistribution along the outer arc of an anticline these results are reprocessed into the semblance similarity attribute to improve the signal noise ratio prior to being extracted along the gas rich and low permeability patchawarra formation the subsurface fracture distribution maps show the geometry and density of extensional fractures that have been shown to facilitate fluid migration in this province particularly those striking se nw and e w as these are properly oriented to undergo tensile reactivation the density of these permeable se nw and e w striking fracture sets is predicted to significantly increase along e w elongate anticlines and may contribute to improved hydrocarbon recovery in this low permeability stratigraphic interval we show that wellbore fracture data n 917 in the patchawarra formation presents a close relationship to k results and can be used to generate high density structural data in this basin with the methodology applicable to other subsurface and offshore provinces

precollision tilt of crustal blocks in rifted island arcs structural evidence from the kohistan arc 1 we describe results from field and microstructural investigations in the unusually large chilas plutonic complex which was emplaced within the mesozoic kohistan island arc northern pakistan magmatic and deformational fabrics of the chilas gabbronorite are concordant with magmatic fabrics of associated gabbros diorites and tonalites and show that this reportedly huge intrusion is composed of juxtaposed plutonic bodies with predominantly ssw dipping flow structures over the northeastern part of the complex which turn to sw ne striking subvertical attitudes to the west evidence for multiple intrusions includes concordant and discordant intrusive and or mylonitic contacts between gabbronorites and associated rocks owing to structural and kinematic consistency we attribute transitions from magmatic to more localized mylonitic fabrics to higher finite strain and or strain rate produced from drag of magma against its margin we discuss this structural pattern and conclude that the chilas gabbronorite emplaced incrementally into generally south dipping listric faults that dominated arc extension about 85 ma in addition several sets of hornblende pegmatite indicate that brittle deformation occurred in the presence of residual melt during consolidation of the main body the latest pegmatite set is composed of variably striking vertical extensional veins that show no relationship to the magmatic fabric we argue that the original attitude of these veins along with that of the enclosing chilas complex has been mostly preserved we hypothesize that the northward dip of the southern side of the kohistan is inherited from late cretaceous rotation of crustal blocks in the hanging wall of the listric faults

3d seismic analysis investigating the relationship between stratigraphic architecture and structural activity in the intra cratonic cooper and eromanga basins australia abstract this research uses four three dimensional 3d seismic surveys located in australia s largest onshore hydrocarbon province the intra cratonic cooper and eromanga basins to present an approach that extracts important structural and stratigraphic information from geophysical data that can then be used to define the tectonostratigraphic evolution of subsurface provinces the methodology consists of 1 analysing isopach maps 2 cross section interpretation of stratigraphic features erosional surfaces and faults and 3 constraining the evolution of fault activity most faults within this province are basement involved with high dip angles the primary fault set is ne sw striking with secondary sets striking n s e w and nw se these high angle faults most likely developed as normal faults before being reactivated by five of the six major tectonic events field scale nw se strike slip faults are prolific and can often be overlooked due to the low seismic resolution a close relationship between on lapping features and present day structural highs was found during each of the major structural events particularly within hydrocarbon rich permian stratigraphy inferring that present day structures were present throughout basin development and intermittently reactivated significant stratal package thinning and a high presence of on lapping features were associated with regional basement involved faults particularly along the gidgealpa merrimelia innamincka and murteree nappacoongee ridges initial structural trap development occurred during the early permian but was most significant during the late triassic hydrocarbon accumulations were unaffected by structural growth after the critical moment in the petroleum system 90 ma as the final period of fault activity was during the late cretaceous this research constrains the tectonostratigraphic evolution of the intra cratonic cooper and eromanga basins while detailing an approach that extracts and analyses important structural and stratigraphic information from geophysical data where outcrop is not accessible

seismic reflection and potential field studies of the vulcan structure western canada a paleoproterozoic pyrenees the vulcan structure is a major tectonic boundary between the medicine hat and loverna blocks archean crustal domains that are buried beneath the western canada sedimentary basin it is characterized by prominent east trending gravity and magnetic anomalies more than 350 km in length which cut across the potential field fabric of southern alberta at a high angle three decades ago the vulcan structure was the target of one of the first deep crustal seismic profiles and it has since been variously interpreted as a failed proterozoic rift an intraplate collisional zone and or a largely amagmatic proterozoic suture several recent lithoprobe seismic reflection profiles coupled with new coincident high resolution gravity data regional gravity data and updated aeromag netic coverage provide the basis for a revised model of the crustal architecture in this region we interpret the vulcan structure to be a relatively narrow 40 70 km axial zone of a continental collisional belt the seismic images imply crustal delamination and south directed subduction of the lower crust of the loverna block the axial gravity low and corresponding magnetic anomaly are interpreted to originate from a large granitic pluton while prominent gravity highs on the flanks of the structure are modeled as thrust slices of lower crustal and or mantle material brought up during the collision cross cutting relationships inferred from aeromagnetic data indicate that the timing of collision postdated formation of archean fabrics in the medicine hat block but predated terminal collision in the adjacent transhudson orogen the geographic extent inferred net shortening and tectonic setting of the vulcan structure appear to resemble the modern pyrenees belt although the deep structure appears to be more akin to the scandinavian caledonides either scenario is consistent with an interpretation of the vulcan structure as the proterozoic collisional suture between the wyoming and hearne provinces of the laurentian craton

slip tendency analysis fault reactivation potential and induced seismicity in a deep geothermal reservoir abstract a slip tendency analysis is used to assess the reactivation potential of shear and dilational fractures in a deep geothermal reservoir in the northeast german basin based on the notion that slip on faults is controlled by the ratio of shear to normal stress acting on the plane of weakness in the in situ stress field the reservoir rocks composed of lower permian sandstones and volcanics were stimulated by hydraulic fracturing a surprisingly low microseismic activity was recorded with moment magnitudes mw ranging from 1 0 to 1 8 the slip tendency analysis suggests a critically stressed reservoir exists in the sandstones whereas the volcanic rocks are less stressed rock failure first occurs with an additional pore pressure of 20 mpa presumed failure planes form a conjugate set and strike nw and ne slip failure is more likely than tensional failure in the volcanic rocks because high normal stresses prevent tensional failure these results from slip tendency analysis are supported by the spatial distribution of recorded microseismicity source characteristics indicate slip rather than extension along presumed ne striking failure planes this suggests that slip tendency analysis is an appropriate method that can be used to understand reservoir behavior under modified stress conditions

geodynamics double dip the indian plate moved north unusually quickly during the late cretaceous numerical simulations suggest that this rapid migration was caused by the pull of two coupled narrowing subduction zones

source terranes for proterozoic sedimentary rocks in southern british columbia nd isotopic and petrographic evidence quartz arenites of the fort steele formation the lowest exposed stratigraphic unit of the purcell supergroup have nd crustal residence times tcr of 2 61 2 63 ga these overlap the low end of the tcr range of the polymetamorphic kenoran hudsonian basement of southern alberta and southwestern saskatchewan the tcr values of overlying fine grained purcell rocks average 2 0 ga near lake windermere the basal toby conglomerates of the windermere supergroup apparently derived from underlying purcell strata have tcr values of 1 98 and 2 03 ga stratigraphically higher argillite and arkoses range from 2 35 to 2 54 ga this higher range indicates contributions from a source terrane different from the main body of the purcell supergroup our interpretation of events in the east kootenay area is that early purcell sedimentation fort steele formation was derived from reworking of the crystalline basement of the alberta shelf as the beltian sea transgressed as the basin deepened it was filled by fine grained

the s type ladybird leucogranite suite of southeastern british columbia geochemical and isotopic evidence for a genetic link with migmatite formation in the north american basement gneisses of the monashee complex abstract the 62 52 ma ladybird granite lbg suite is a peraluminous leucocratic s type quartz monzonitic to granitic suite which occurs as batholiths stocks dikes sills and pegmatite veins predominantly in the high grade rocks of the shuswap complex in southeastern british columbia the emplacement of the lbg was synchronous with the production of abundant migmatites within thor odin dome of the monashee complex an exposure of north american basement exhumed from depths of ca 26 33 km by eocene extensional faults the lbg and the leucosome in migmatites from thor odin dome have similar major and trace element patterns and are both characterized by zircons which have inherited precambrian cores whole rock nd isotope compositions show a range of values for the lbg with end 55 ma values from 5 0 to 17 2 the end 55 ma for the leucosome samples range from 9 5 to 23 6 overlapping with those of the granitic suite these data support the interpretation of a genetic link between formation of the lbg suite and melting of north american basement rocks such as those exposed in the core of thor odin dome the leucosome samples have lower high field strength element hfse concentrations and positive eu anomalies whereas the lbg samples have higher hfse concentrations and negative eu anomalies the similar trace element characteristics suggest that the leucosome from the migmatites and the lbg are related whereby most of the leucosome samples are cumulates and the lbg samples represent evolved or residual melts the initial 87sr 86sr isotope values for both the lbg and leucosome samples have a large range however the initial sr isotopic ratios for the lbg suite are lower than those of the leucosome samples with 87sr 86sr 55 ma ranging from 0 70603 to 0 73688 and 0 74256 to 0 76593 respectively this isotopic discrepancy suggests either a isotopic disequilibrium during partial melting in the mid to lower crust where the leucosome formed b the distribution of sr during partial melting was controlled by different melt producing reactions and or c isotopic heterogeneity in the source rocks at least part of the lbg suite likely formed via melting of north american basement rocks that were dominantly of sedimentary origin melting of the proterozoic supracrustal metasedimentary rocks overlying north american basement may also have contributed to the formation of the different phases of the suite found at the regional scale however the abundant leucosomes in the basement rocks of thor odin dome may mark the paths along which anatectic melt migrated in the structurally overlying ladybird granites of the south fosthall pluton

3 d seismic discontinuity for faults and stratigraphic features the coherence cube seismic data are usually acquired and processed for imaging reflections this paper describes a method of processing seismic data for imaging discontinuities e g faults and stratigraphic features one application of this nontraditional process is a 3 d volume or cube of coherence coefficients within which faults are revealed as numerically separated surfaces figure 1 compares a traditional 3 d reflection amplitude time slice with the results of the new method to our knowledge this is the first published method of revealing fault surfaces within a 3 d volume for which no fault reflections have been recorded

striding athabasca mylonite zone implications for the archean and early proterozoic tectonics of the western canadian shield study of the northern saskatchewan district of mackenzie segment of the snowbird tectonic zone suggests that fragments of relatively stiff mid archean crust possibly arc related have controlled the localization shape and complex kinematics of the multistage striding athabasca mylonite zone during the archean as well as the geometry of the early proterozoic rifted margin of the western churchill continent by the late archean the striding athabasca mylonite zone was located in the interior of the western churchill continent well removed from the contemporaneous plate margins except for the alberta segment the snowbird tectonic zone was not the site of an early proterozoic plate margin we suggest that the geometry of the archean early proterozoic boundary in the western canadian shield represents a jagged continental margin composed of a pair of reentrants defined by rifted and transform segments these segments were inherited from early proterozoic breakup and controlled by the archean structure

nd evidence for extensive archean basement in the western churchill province canada to determine the extent of reworked archean crust in the western churchill province we have examined sm nd crustal residence ages of basement cores in southern alberta and southwestern saskatchewan along with crustal residence ages calculated for other sm nd data available from the churchill province the deep drill hole samples from the interior platform give sm nd crustal residence ages that average 2 8 ga granulites from northeastern alberta and composite gneisses from northern saskatchewan also provide archean crustal residence ages these data demonstrate that the presence of reworked archean crust in the western churchill province is more widespread than has been confirmed previously we suggest that sm nd crustal residence ages are valuable estimates of crust formation times especially in orogenic areas where other isotope systems have been disturbed

geophysical anomalies and quartz microstructures eastern warburton basin north east south australia tectonic or impact shock metamorphic origin abstract the eastern warburton basin northeast south australia features major geophysical anomalies including a magnetic high of near 200 nt centred on a 25 km wide magnetic low 30 000 km large eastern warburton basin the qz pe include multiple intersecting planar to curved sub planar elements with relic lamellae less than 2 m wide with spacing of 4 5 m qz pe are commonly re deformed displaying bent and wavy patterns accompanied with fluid inclusions u stage measurements of a total of 243 planar sets in 157 quartz grains indicate dominance of 10 12 10 13 and subsidiary 11 22 22 41 m 10 11 and x 51 61 planes transmission electron microscopy tem analysis displays relic narrow 1 m wide lamellae and relic non sub grain boundaries where crystal segments maintain optical continuity extensive sericite alteration of feldspar suggests hydrothermal alteration to a depth of 500 m below the unconformity which overlies the qz pe bearing warburton basin terrain the data are discussed in terms of a tectonic metamorphic deformation and b impact shock metamorphism producing planar deformation features qz pdf deformed qz pe are compared to re deformed qz pdfs in the sudbury vredefort manicouagan and charlevoix impact structures a 4 5 km uplift of the big lake granite suite during 298 295 ma is consistent with missing of upper ordovician to devonian strata and possible impact rebound the occurrence of circular seismic tomography anomalies below the east warburton basin the poolowana basin and the woodleigh impact structure signifies a potential diagnostic nature of circular tomographic anomalies

zircon u pb and hf evidence for coupled subduction of oceanic and continental crust during the carboniferous in the huwan shear zone western dabie orogen central china both oceanic and continental hp rocks are juxtaposed in the huwan shear zone in the western dabie orogen and thus provide a window for testing the buoyancy driven exhumation of dense oceanic hp rocks the hp metamorphic age of the continental rocks in this zone has not been well constrained and hence it is not known if they are of the same age as the exhumation of the hp oceanic rocks in situ laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icp ms u pb trace element and hf isotope analyses were made on zircon in a granitic gneiss and two eclogites from the huwan shear zone u pb age and trace element analysis of residual magmatic zircon in an eclogite constrain its protolith formation at 411 4 ma the zircon in this sample displays ehf t values of 6 1 to 14 4 the positive ehf t values up to 14 4 suggest that the protolith was derived from a relatively depleted mantle source most likely palaeotethyan oceanic crust a granitic gneiss and the other eclogite yield protolith u pb ages of 738 6 and 700 14 ma respectively which are both the neoproterozoic basement rocks of the yangtze block the zircon in the granitic gneiss has low ehf t values of 14 2 to 10 5 and old tdm2 ages of 2528 2298 ma suggesting reworking of palaeoproterozoic crust during the neoproterozoic the zircon in the eclogite has ehf t values of 1 0 to 7 4 and tdm1 ages of 1294 966 ma implying prompt reworking of juvenile crust during its protolith formation metamorphic zircon in both eclogite samples displays low th u ratios trace element concentrations relatively flat heavy rare earth element patterns weak negative eu anomalies and low 176lu 177hf ratios all these features suggest that the metamorphic zircon formed in the presence of garnet but in the absence of feldspar and thus under eclogite facies conditions the metamorphic zircon yields u pb ages of 310 3 and 306 7 ma therefore both the oceanic and continental type eclogites share the same episode of carboniferous eclogite facies metamorphism this suggests that high pressure continental type metamorphic rocks might have played a key role in the exhumation and preservation of oceanic type eclogites through buoyancy driven uplift

dynamics of exhumation and deformation of hp uhp orogens in double subduction collision systems numerical modeling and implications for the western dabie orogen abstract the dynamics of formation and exhumation of high pressure hp and ultra high pressure uhp metamorphic orogens in double subduction collision zones remain enigmatic here we employ two dimensional thermo mechanical numerical models to gain insights on the exhumation of hp uhp metamorphic rocks as well as their deformation during the collision of a micro continent with pro and retro continental margins along two subduction zones a three stage collisional process with different convergence velocities is tested in the initial collisional stage a fold and thrust belt and locally rootless superimposed folds are developed in the micro continent and subduction channel respectively in the second exhumation stage of hp uhp rocks a faster convergence model results in upwelling of the asthenosphere which further leads to a detachment between the crust and lithospheric mantle of the micro continent a slower convergence model results in rapid exhumation of hp uhp rocks along the north subduction channel and a typical piggy back thrusting structure in the micro continent a non convergence model produces a slab tear off leading to the rebound of residual lithosphere of the micro continent in the third and final stage a series of back and ramp thrusts are formed in the micro continent with the pro continent re subducted based on an analogy of our numerical results with the western dabie orogen wdo we suggest that 1 slab tear off results in a rebound of residual lithosphere which controls the two stage syn collisional exhumation process of hp uhp rocks in the wdo and 2 in contrast to the single subduction collision system the exhumation range of the partially molten rocks with lower viscosity and density is restricted to a specific region of the micro continent by the mianlue and shangdan subduction zones which generated the complex deformation features in the wdo

triassic high pressure metamorphism in the huwan shear zone tracking the initial subduction of continental crust in the whole dabie orogen abstract la mc icpms trace element u pb and hf isotope analyses were performed on zircons from continental derived retrograde high pressure metamorphic rocks which were attested by the occurrence of hp mineral inclusions in metamorphic zircons from the huwan shear zone in the western dabie orogen zircons in a schist sample are mostly detrital and gave u pb ages from 1212 to 2312 ma suggesting that the detritus were derived from multiple sources or suffered partial pb loss during subsequent metamorphism magmatic zircons in an amphibolite sample yielded an upper intercept u pb age of 1806 63 ma which was interpreted as its protolith formation age the zircons in the amphibolite show a weighted e hf t value of 16 3 and a weighted hf model age of 3455 ma suggesting that the protolith was derived from reworking of ancient continental materials metamorphic zircons in a quartz rich leucosome sample and the amphibolite sample gave weighted mean 206 pb 238 u ages of 241 1 and 243 4 ma respectively these metamorphic zircons contain mineral inclusions of garnet and omphacite and are characterized by relatively flat hree patterns with slight negative eu anomalies this indicates that they formed in the presence of garnet but the absence of plagioclase and thus under eclogite facies conditions the u pb age of ca 243 ma is consistent with eclogite facies metamorphic ages of the juvenile oceanic crust in the western dabie orogen suggesting that the initial continental subduction for high pressure metamorphism of the whole dabie orogen can be traced back to ca 243 ma in addition the western dabie hp uhp orogenic belt might have suffered multi slice subduction and exhumation during the triassic

tectonothermics of the north american great plains basement abstract heat flow and heat generation data from the superior churchill and wyoming provinces of western north america contribute to our understanding of the nature of the crust beneath the sedimentary cover of the great plains a major structural boundary defined by gravity gradient data thomas et al 1987 splits the terrains between the cratons into two distinct geothermal provinces the wyoming and churchill provinces are characterised by higher heat flow than the superior province and the eastern part of the intervening hudsonian terrain the proterozoic mobile belt pmb heat flow in the pmb is typical of that of the superior craton this suggests on the basis of a model by ballard and pollack 1987 that the pmb is underlain by archaean cratonic lithosphere the similarity in heat flow between the pmb and the superior craton suggests that the crust of the pmb is akin to that of the superior craton rather than to that of the more radiogenic wyoming and churchill cratons geochronologic gravity aeromagnetic and geothermal data suggest that the structural boundary defining the western boundary of the pmb is also the eastern boundary of the wyoming craton geothermal data alone suggest that the wyoming craton might extend as far north as the cratonic segment of the churchill province or that the wyoming and churchill provinces may gradually merge together

constraining stress magnitudes using petroleum exploration data in the cooper eromanga basins australia abstract the magnitude of the in situ stresses in the cooper eromanga basins have been determined using an extensive petroleum exploration database from over 40 years of drilling the magnitude of the vertical stress sv was calculated based on density and velocity checkshot data in 24 wells upper and lower bound values of the vertical stress magnitude are approximated by sv 14 39 z 1 12 and sv 11 67 z 1 15 functions respectively where z is depth in km and sv is in mpa leak off test data from the two basins constrain the lower bound estimate for the minimum horizontal stress shmin magnitude to 15 5 mpa km closure pressures from a large number of minifrac tests indicate considerable scatter in the minimum horizontal stress magnitude with values approaching the magnitude of the vertical stress in some areas the magnitude of the maximum horizontal stress shmax was constrained by the frictional limits to stress beyond which faulting occurs and by the presence of drilling induced tensile fractures in some wells the maximum horizontal stress magnitude can only be loosely constrained regionally using frictional limits due to the variability of both the minimum horizontal stress and vertical stress estimates however the maximum horizontal stress and thus the full stress tensor can be better constrained at individual well locations as demonstrated in bulyeroo 1 and dullingari north 8 where the necessary data i e image logs minifrac tests and density logs are available the stress magnitudes determined indicate a predominantly strike slip fault stress regime shmax sv shmin at a depth of between 1 and 3 km in the cooper eromanga basins however some areas of the basin are transitional between strike slip and reverse fault stress regimes shmax sv shmin large differential stresses in the cooper eromanga basins indicate a high upper crustal strength for the region consistent with other intraplate regions we propose that the in situ stress field in the cooper eromanga basins is a direct result of the complex interaction of tectonic stresses from the convergent plate boundaries surrounding the indo australian plate that are transmitted into the center of the plate through a high strength upper crust

anomalously fast convergence of india and eurasia caused by double subduction prior to collision with eurasia the indian plate rapidly accelerated northwards numerical simulations show that the combined pull of two slabs in two parallel north dipping subduction systems could have caused this pulse of rapid movement