hypocentre determination offshore of eastern taiwan using the maximum intersection method summary the maximum intersection maxi method which derives from the master station method msm determines within a 3 d velocity model the absolute hypocentral location based on observed arrival times first the spatial node that better satisfies the arrival time differences computed at all station pairs plus or minus an error tolerance value in seconds is defined as the preliminary hypocentral solution pred second because pred depends neither on the estimate of origin time nor on the residual root mean square rms residual outliers are objectively detected and cleaned out from the original data set without any iterative process or weighting third a statistical minimization residual rms is conducted in a small domain around the pred node which results in a unique final solution the maxi method is applied to the determination of earthquake hypocentres with the proper station correction terms in the southernmost extremity of the ryukyu subduction zone where several dense seismic clusters occur near the seismogenic plate interface the location of earthquakes recorded at both the taiwanese and japanese networks is obtained for about a thousand events between 1992 and 1997 the process uses a detailed 3 d velocity model based on multiple geophysical data sources obtained in the junction area between subduction and collision east of taiwan the earthquake clustering and the significant drop in residual statistics 1 20 0 80 and 0 35 s for taiwanese catalogue msm and maxim solutions respectively indicate the accuracy of the method which can be used to routinely determine absolute hypocentre location based on observed arrival times

the metamorphic internal zone of wopmay orogen early proterozoic canada 30 km of structural relief in a composite section based on plunge projection in wopmay orogen an early proterozoic volcano sedimentary marginal prism and its underlying archean structural basement deformed together during the 1885 ma calderian orogeny are exposed in oblique view as a result of regional cross folding a composite cross section displaying over 30 km of structural relief provides a framework in which to examine the geologic relationships in the metamorphic internal zone of the orogen a sole thrust separates archean basement and a thin mantle of autochthonous early proterozoic strata from the eastward transported calderian allochthon the allochthon is characterized by a transition from external thrust fold belt geometries in the east to polyphase isoclinal recumbent folds and thrusts in the west the center of the zone contains the hepburn batholith which has been been transported along with the deforming hot allochthon over the exposed basement both basement and allochthon are deformed by late large scale upright folds the metamorphic internal zone is interpreted to record the closing of a rift basin during which the basin fill was shortened thickened to over 30 km metamorphosed and thrust eastward while still hot over the relatively cool archean craton the basement presently exposed beneath the sole thrust remained relatively nondeformed during thrusting but underwent large scale folding due to continued compression during uplift of the internal zone

locating microseismic events using borehole data constraining microseismic hypocentres in and around hydrocarbon reservoirs and their overburdens is essential for the monitoring of deformation related to hydraulic fracturing production and injection and the assessment of reservoir security for co2 and wastewater storage microseismic monitoring in hydrocarbon reservoirs can be achieved via a variety of surface and subsurface acquisition geometries in this study we use data from a single subsurface vertical array of sensors we test an existing technique that uses a 1d velocity model to constrain locations by minimizing differential s to p arrival times for individual sensors we show that small errors in either arrival time picks or the velocity model can lead to large errors in depth especially near velocity model discontinuities where events tend to cluster to address this issue we develop two methods that use all available arrival times simultaneously in the inversion thus maximizing the number of potential constraints from n 2 to n where nis the number of phase picks the first approach minimizes all available arrival time pairs whilst the second approach the equal distance time edt method defines the hypocentre as the point where the maximum number of arrival time surfaces intersect we test and compare the new location procedures with locations using differential s to p times at each individual sensor on a microseismic data set recorded by a vertical array of sensors at the ekofisk reservoir in the north sea specifically we test each procedure s sensitivity to perturbations in measured arrival times and the velocity model using monte carlo analysis in general location uncertainties increase with increasing raypath length we show that errors in velocity model estimates are the most significant source of uncertainty in source location with these experiments our tests show that hypocentres determined by the new procedures are less sensitive to erroneous measurements and velocity model uncertainties thus reducing the potential for misinterpretation of the results

impact of distance dependent location dispersion error on interpretation of microseismic event distributions along with horizontal drilling and hydraulic fracturing technology monitoring induced microseismic activity during hydraulic fracturing has played a significant role in the economic development of today s prolific tight gas and shale resource plays many microseismic fracture monitoring papers in the current literature discuss the equipment how to acquire and process microseismic data and how to determine an appropriate velocity model unfortunately for an operating company s completion engineers and geophysicists charged with interpreting these data and integrating them with other data sets only a few papers discuss the impact of artifacts and location uncertainties on the interpretation while microseismic data have been very useful in the economic development of resource plays there are a number of significant pitfalls and interpretational issues that if not understood will potentially lead to erroneous interpretations this is especially important for any real time decisions during a hydraulic

testing the ability of surface arrays to monitor microseismic activity recently there has been much interest in the use of data from surface arrays in conjunction with migration based processing methods for passive seismic monitoring in this study we use an example of this kind of data recorded whilst 18 perforation shots with a variety of positions and propellant amounts were detonated in the subsurface as the perforation shots provide signals with known source positions and origin times the analysis of these data is an invaluable opportunity to test the accuracy and ability of surface arrays to detect and locate seismic sources in the subsurface in all but one case the signals from the perforation shots are not visible in the raw or preprocessed data however clear source images are produced for 12 of the perforation shots showing that arrays of surface sensors are capable of imaging microseismic events even when the signals are not visible in individual traces we find that point source locations are within typically 45 m laterally of the true shot location however the depths are less well constrained 150 m we test the sensitivity of our imaging method to the signal to noise ratio in the data using signals embedded in realistic noise we find that the position of the imaged shot location is quite insensitive to the level of added noise the primary effect of increased noise being to defocus the source image given the migration approach the array geometry and the nature of coherent noise during the experiment signals embedded in noise with ratios 0 1 can be used to successfully image events furthermore comparison of results from data and synthetic signals embedded in noise shows that in this case prestack corrections of traveltimes to account for near surface structure will not enhance event detectability although the perforation shots have a largely isotropic radiation pattern the results presented here show the potential for the use of surface sensors in microseismic monitoring as a viable alternative to classical downhole methods

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

strain and displacement across the pinaleno mountains shear zone arizona u s a abstract the 27 km long 0 5 km thick mylonite zone of the pinaleno mountains metamorphic core complex is interpreted as a normal displacement simple shear zone on the basis of major minor and microscopic scale structures shear strains calculated as a function of the angle between the foliation and the lower boundary of the shear zone range from 2 to 19 and have a mean value of 3 5 corresponding to a mean angular shear of 74 these values are the same as those calculated independently from rotations of sheared lithologic contacts integration of the calculated shear strains yields a minimum translation estimate of 2 9 km

metamorphism in the solitude range southwestern rocky mountains british columbia comparison with adjacent omineca belt rocks and tectonometamorphic implications for the purcell thrust rocks of the solitude range british columbia have been metamorphosed from chloritoid chorite zone to kyanite zone conditions the grade of metamorphism increases southwestward toward the rocky mo

seismic interpretation of crustal scale extension in the intermontane belt of the northern canadian cordillera the crustal thickness of the canadian cordillera which may have reached 50 65 km during the cretaceous is now only 32 38 km along its length in the south postorogenic extension during the eocene resulted in the formation of core complexes and crustal scale extensional shear zones whereas in the north evidence for postorogenic extension is either limited to relatively minor basin formation in a transtensional environment or has not been recognized using new images of lithoprobe seismic reflection line 2a in northern british columbia which has been previously interpreted in terms of terrane accretion i make an alternative interpretation of crustal scale postaccretion extension in the middle and upper crust as early as the late permian to early triassic the quesnellia and stikinia arc terranes were accreted to ancestral north america along a crustal ramp which is imaged in the lower crust an extensional shear zone which extends from close to the surface to 20 km depth is identified from the geometry of reflections in the overlying structural basin and a lateral change in seismic velocity the oceanic cache creek terrane which is 4 km thick in the vicinity of line 2a was preserved because it was imbricated with its overlap assemblage and dropped into the 10 km deep structural half graben which may correspond approximately to the combined extent of the whitehorse trough and the cache creek terrane in southern yukon and northern british columbia

a comment on the form of the geometrical spreading equations with some numerical examples of seismic ray tracing in inhomogeneous anisotropic media summary we point out that while the equations of seismic ray geometrical spreading given recently by norris do differ as stated from equations given by cerveny it does not imply that the latter equations are wrong the two sets of equations differ only in form and in a way which in part at least can be ascribed to different choices of hamiltonian by the two authors quantitatively though the two sets of equations are entirely equivalent we also present some numerical results of ray tracing in anisotropic models simulating a continential rift a spreading ridge and a subduction zone these three structures span a range of geological mechanisms for seismic anisotropy though definitive conclusions cannot be easily drawn when there is both anisotropy and inhomogeneity the results do indicate the magnitude of ray path travel time and amplitude variations to be expected for p waves when anisotropy is introduced

cocorp deep seismic reflection traverse of the interior of the north american cordillera washington and idaho implications for orogenic evolution deep seismic reflection data collected by the consortium for continental reflection profiling along a transect of the interior of the northwestern u s cordillera indicate the presence of crustpenetrating faults these faults developed during an orogenic cycle consisting of jurassic to paleocene accretion related crustal thickening followed by eocene crustal thinning the reflection data also suggest that the moho is a young dynamic feature which developed a relatively smooth flat geometry beneath a complexly deformed crust probably during an eocene episode of crustal extension the profiles reveal prominent west dipping reflection sequences and associated diffractions interpreted as a mesozoic thrust system of crustal dimensions that linked accretion related shortening in the western cordillera with the thrust belt in the eastern cordillera east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated eocene crustal extension and essentially flat moho reflections at 33 35 km beneath complexly deformed crust these features are consistent with a sequence of 1 late mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the north american passive continental margin along a jurassic thrust 2 propagation of crust penetrating thrusts to shallower levels in the east where they probably formed the sole of the rocky mountain thrust belt 3 eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and 4 development of a flat moho geometry during cenozoic extension and magmatism these data provide new information about the structure of the entire crust in the cordilleran interior and offer a working model both for evolution of cordilleran core complexes and possibly for other collisional orogens

eocene structural development of the valhalla complex southeastern british columbia the valhalla complex a cordilleran metamorphic core complex is a 100 km by 30 km structural culmination within the omineca belt of southeastern british columbia it comprises sheets of granitic orthogneiss ranging in age from 100 to 59 ma with intervening paragneiss of uncertain age and stratigraphic correlation the complex is roofed by the ductile valkyr shear zone and the ductile brittle slocan lake fault zone the upper plate comprises lower grade metasedimentary rocks intruded by middle jurassic plutons the valkyr shear zone and the slocan lake fault zone deform 62 and 59 ma granitic sheets in their footwalls the easterly directed valkyr shear zone is a 2 to 3 km thick zone of distributed ductile strain which is arched over the complex and is exposed around the periphery on the northern western and southern margins the shear zone was active between 59 and 54 ma under amphibolite facies conditions the juxtaposition of upper and lower plates with different structural and metamorphic histories indicates that the valkyr shear zone is a significant structure with large displacement there is evidence to support an easterly rooting direction consistent with an extensional origin its surface breakaway is suggested to be west of the valhalla complex the slocan lake fault zone on the eastern side of the complex is a gently 30 easterly dipping ductile brittle normal fault which roots to the east it was active between 54 and approximately 45 ma and truncates the valkyr shear zone timing and structural relationships indicate that the valkyr shear zone and the slocan lake fault zone are genetically related movement on the ductile valkyr shear zone arching of the complex and displacement on the slocan lake fault zone occurred as a continuum in early to middle eocene time this paper documents the presence of significant eocene ductile strain in the valhalla complex and suggests that the role of extension in this region is more profound than had been previously recognized

induced microearthquake patterns in hydrocarbon and geothermal reservoirs six case studies the injection or production of fluids can induce microseismic events in hydrocarbon and geothermal reservoirs by deploying sensors downhole data sets have been collected that consist of a few hundred to well over 10 000 induced events we find that most induced events cluster into well defined geometrical patterns in many cases we must apply high precision relative location techniques to observe these patterns at three sedimentary sites thin horizontal strands of activity are commonly found within the location patterns we believe this reflects fracture containment between stratigraphic layers of differing mechanical properties or states of stress at a massive carbonate and two crystalline sites combinations of linear and planar features indicate networks of intersecting fractures and allow us to infer positions of aseismic fractures through their influence on the location patterns in addition the fine scale seismicity patterns often evolve systematically with time at sedimentary sites migration of seismicity toward the injection point has been observed and may result from slip induced stress along fractures that initially have little resolved shear in such cases triggering events may be critical to generate high levels of seismic activity at one crystalline site the early occurrence of linear features that traverse planes of activity indicate permeable zones and possible flow paths within fractures we hope the continued development of microseismic techniques and refinement of conceptual models will further increase our understanding of fluid behavior and lead to improved resource management in fractured reservoirs

identification of degraded bone and tooth splinters from arid environments using palaeoproteomics abstract the analysis of the skeletal remains of vertebrates in archaeological contexts provides information about human animal relationship and their environment their taxonomic identification based on macroscopic observation is not always possible due to fragmentation and poor preservation in recent years proteomics has emerged as an alternative but there is clearly a lack of data in arid environment where diagenesis rapidly affects the integrity of bone proteins here we report the efficiency of three protocols for protein extraction the protocols used harsh 1 m hcl and 0 6 m hcl and soft tris edta decalcification agents and were tested on unidentified splinters from the 2000 years old site of toteng botswana the preservation of the organic phase was first estimated using attenuated total reflectance fourier transform infrared spectroscopy and a set of samples with contrasted collagen contents were selected for palaeoproteomics the extracted proteins were submitted to a bottom up proteomic approach involving trypsin digestion followed by ultra high performance liquid chromatography coupled to mass spectrometry uhplc ms our results identify tris edta buffer as the most suitable decalcification protocol for poorly preserved bones and propose a collagen content threshold of 3 weight content for successful detection of peptides this approach combined with biogeographical and chronological repartitions of mammals in africa allows refining taxonomic attributions for four out of nine splinters leading to species identification data are available via proteomexchange with identifier pxd010725

determination of s wave slowness from a linear array of borehole receivers summary locations of seismic events from a linear array of receivers may require determination of slowness vectors of arriving waves in an isotropic medium p waves are polarized along the slowness vector which enables direct determination of backazimuth i e azimuth to a source from a receiver from p wave polarization in contrast s waves usually have much larger signal to noise ratio than p waves but are polarized in a plane perpendicular to their slowness vectors which prevents direct determination of their backazimuth we have developed a novel technique to determine the slowness vector of s waves detected in a linear array of receivers in an isotropic medium we combine the s wave polarization measurement with the derivative of the s wave travel times along the array to obtain the full slowness vector and backazimuth the proposed method allows one to determine direction to sources of seismic events from a single linear array of receivers using only s waves this technique is not affected by sv waves which is shown by a test on a synthetic data set we also test the method on two real microseismic data sets from hydraulic fracturing treatments and show that it outperforms the backazimuth determination from p waves and from horizontal polarization of s waves

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

cross double difference inversion for simultaneous velocity model update and microseismic event location microseismic monitoring is an approach for mapping hydraulic fracturing detecting the accurate locations of microseismic events relies on an accurate velocity model the 1d layered velocity model is generally obtained by model calibration from inverting perforation data however perforation shots may only illuminate the layers between the perforation shots and the recording receivers with limited raypath coverage in a downhole monitoring problem some of the microseismic events may occur outside of the depth range of these layers to derive an accurate velocity model covering all of the microseismic events and locating events at the same time we apply the cross double difference cdd method for the simultaneous inversion of a velocity model and event locations using both perforation shots and microseismic data the cdd method could provide accurate locations in both the relative and absolute sense utilizing cross traveltime differences between p and s phases over different events at the downhole monitoring scale the number of cross traveltime differences is sufficiently large to constrain events locations and velocity model as well in this study we assume that the layer thickness is known velocities of p and s wave are inverted different simultaneous inversion methods based on the geiger s dd and cdd algorithms have been compared with the same input data synthetic and field data experiments suggest that combining both perforation shots and microseismic data for the simultaneous cdd inversion of the velocity model and event locations is available for overcoming the trade offs in solutions and producing reliable results this article is protected by copyright all rights reserved

opposite vergence of nappes and crustal extension in the french italian western alps in the internal pennine zone of the french italian western alps dora maira monviso rocciavre massifs and queyras region the structural trends define an apparent dome geometry that is truncated to the east by the po basin this geometry is classically interpreted as a backthrusting system the kinematic history of the area has been investigated using structural metamorphic and geochronologic data on the basis of which two main deformation episodes have been distinguished that occurred under differing metamorphic conditions 1 the first deformation occurred under greenschist facies metamorphism it caused a regional scale foliation and an ew trending lineation that lie parallel to ductile deformation zones and thrust contacts sense of shear indicators associated with the lineation indicate contradictory top to the east and top to the west senses of shear from these it is deduced that coeval east and west vergent thrust faulting accommodated major displacements between nappes phengite ar ar data yield late eocene early oligocene ages for the greenschist facies metamorphism 2 the second deformation caused the widespread development of high angle shear bands and normal faults on opposite sides of the dora maira massif resulting in the apparent dome geometry of the massif a steep 50 80 network of west dipping normal faults is particularly well developed on the west side of the dora maira massif differences of 2 3 kbar and 100 150 c between juxtaposed monviso rocciavre and queyras nappes implies 8 km of vertical displacement along the monviso upper tectonic contact these normal faults formed in a more brittle regime only a post early oligocene age can be estimated from microstructural studies for these final movements these deformations postdate hp lt metamorphism and resulted in the juxtaposition of nappes of distinctly different origin age grade and p t t history a kinematic model is presented in which opposite vergence of nappes followed by large scale crustal extension occurred in response to uplift during the convergence between the european and south alpine plates this model may account for rapid uplift rapid unroofing and cooling of high pressure low temperature rocks thereby explaining the absence of amphibolite facies metamorphism overprint in this part of the alps that may have led to the preservation of coesite bearing rocks in the southern part of the dora maira massif

a reanalysis of the hypocentral location and related observations for the great 1906 california earthquake we determine probabilistic hypocentral locations for the foreshock and mainshock of the great 1906 california earthquake through reanalysis of arrival time observations in conjunction with modern velocity models and advanced event location techniques we obtain two additional observations for the mainshock and one for the foreshock that were not used in previous location studies using a robust likelihood function for event location we generate a usable subset of the predominantly unreliable teleseismic readings and determine new wave type identifications for some of the local and teleseismic readings our locations are much better constrained than those of earlier studies even though we do not assume that the epicenter lies on the san andreas fault as did previous authors we confirm the conclusions of earlier studies that the local and teleseismic arrival time observations can be explained by a single foreshock focus and a single mainshock focus on the san andreas fault and that there is no single unique hypocenter that satisfies all available local observations the maximum likelihood point latitude 37 78 n longitude 122 51 w for our preferred mainshock location indicates a hypocenter to the west of san francisco close to the san andreas fault zone this hypocenter has a 68 confidence error of about 8 km parallel to the san andreas fault and about 24 km perpendicular to the fault and a depth in the midcrust of about 12 7 km the closest point on the san andreas fault to this hypocenter lies about 10 km to the northwest of the widely accepted mainshock epicenter of bolt 1968 our mainshock location is consistent with the association of initial rupture of the 1906 mainshock with a dilatational right bend or step over in the submerged san andreas fault system offshore of the golden gate our foreshock location is less well constrained than our mainshock location but is consistent with the foreshock hypocenter being at the same location as the mainshock hypocenter online material visualization of 3d probabilistic hypocentral locations associated with the 1906 earthquake

alpine model for the internal metamorphic zones of the north american cordillera the tectonic significance of the internal metamorphic zones of the north american cordillera remains elusive we propose a general interpretation in which the tectonic features pervasive unidirectional transverse lineation recumbent folding eastward thrusting of the shuswap complex canada are related to a large scale intracontinental alpine type eastward shearing this shearing results from the jurassic collision between the allochthonous stikine block and the north american craton after a westward triassic subduction we extend this model to the united states cordillera despite some differences due to the late occurrence of a major extension

anatomy of a buried thrust belt activated during hydraulic fracturing abstract tectonically active fault networks are often inter connected but in the case of injection induced seismicity prior knowledge of fault architecture tends to be severely limited in most cases reactivated faults due to fluid injection are inferred after the fact by the spatial distribution of induced seismicity hypocenters such reliance on post injection seismicity impedes any pre operational risk analysis as well as development of a more holistic understanding of fault system models by combining high resolution depth migrated 3 d seismic data with a new focal depth estimation method that reduces spatial uncertainty of hypocenters this study pinpoints microearthquake fault activation within a buried thrust belt in the montney formation in western canada british columbia during hydraulic fracturing operations rupture nucleation occurred on seismically imaged thrust ramps that cut through the debolt formation a massive carbonate layer that underlies the stimulated zone high resolution seismic images reveal transverse structures interpreted as basement controlled fold hinges or tear faults that transferred displacement between thrust faults during late cretaceous paleogene compressional shortening the spatio temporal pattern of induced seismicity suggests that these transverse structures provide permeable pathways for aseismic pore pressure diffusion thus connecting distinct thrust faults and enabling earthquake triggering on a timescale of days and at distances of up to 2 km from the injection wells inferred relationships highlight how the fault system is connected including apparent stress concentrations at the intersections of transverse structures and orogen parallel thrust ramps

3 d imaging using higher order fast marching traveltimes recently fast marching methods fmm beyond first order have been developed for producing rapid solutions to the eikonal equation in this paper we present imaging results for 3 d prestack kirchhoff migration using traveltimes computed using the first order and second order fmm on several 3 d prestack synthetic and real data sets the second order traveltimes produce a much better image of the structure moreover insufficiently sampled first order traveltimes can introduce consistent errors in the common reflection point gathers that affect velocity analysis first order traveltimes tend to be smaller than analytic traveltimes which in turn affects the migration velocity analysis falsely indicating that the interval velocity was too low

geology geochemistry and isotopic character of the colville igneous complex northeastern washington the colville igneous complex in northeastern washington comprises paleocene orthogneisses and leucocratic granitic rocks and eocene volcanic plutonic suites which were emplaced during early tertiary extension the colville igneous complex is divided into three components 1 65 4 ma orthogneisses of the okanogan dome or metamorphic complex and similar gneisses in the kettle and lincoln complexes 2 61 51 ma leucocratic granitic rocks and 3 53 47 ma intermediate to felsic volcanic plutonic rocks the leucocratic granites and younger volcanic plutonic rocks are divided on the basis of age geochemical and isotopic characteristics and emplacement mechanism the older leucocratic granites are silica rich corundum normative and sr and light rare earth element ree enriched and they form several large discrete compositionally zoned plutons and intrusive suites the younger eocene volcanic plutonic rocks can be divided into two suites an older compositionally restricted 68 72wt sio2 sr enriched medium k suite and a younger intermediate to felsic high k calcalkalic suite sr isotopic measurements of volcanic and plutonic rock in the colville igneous complex yield variable initial 87sr 86sr ratios sri ranging from about 0 704 to values greater than 0 710 in general the volcanic rocks are especially variable with little or no correlation between sri and geographic location the leucocratic granites however show a systematic west to east increase across the area which probably reflects an increased involvement with precambrian crust the variable nature of sri in the colville igneous complex may also be controlled by crustal thickness and or depth of melting within individual volcanic and plutonic suites there is a crude correlation of mean sri with age there is a systematic decrease in sri with increasing age which may reflect either a migration of the site s of melting or a decreased upper crustal residence time possibly related to the onset of extension the leucocratic granites may have formed by small degrees of partial melting in the lower crust perhaps facilitated by thermal blanketing of an initially overthickened crust the ascent of large bodies of leucocratic magmas may have increased the geothermal gradient which allowed for upper more radiogenic crust to be melted

age and geological setting of gold creek gneiss crystalline basement of the windermere supergroup cariboo mountains british columbia gold creek gneiss the most recently discovered body of crystalline basement in the canadian cordillera is a northwest trending granitic to dioritic orthogneiss straddling the eastern end of the canoe river valley near the southeastern end of the cariboo mountains u pb geochronometry of zircons from two samples of quartz diorite orthogneiss from two separate localities yielded similar nearly linear discordant arrays with upper intercepts of 2 08 ga the patterns of discordance in both samples preclude assigning a precise age although a date between 2 0 and 2 1 ga is geologically reasonable north of canoe river gold creek gneiss is inferred to be nonconformably overlain by quartzite and marble which in turn are inferred to be unconformably overlain by a conglomerate and diamictite bearing unit at the base of the lower kaza group of the windermere supergroup in contrast with other exposures of basement and cover in the area neither contact is marked by anomalously high strain or mylonitic rocks

a geodynamic model for extension in the shuswap core complex southeastern canadian cordillera the shuswap core complex records mesozoic and paleocene crustal shortening with superimposed eocene extension during the shortening phase the crust was thickened and slowly uplifted within at most 20 ma of the climax of compression the crustal welt was rapidly uplifted and tectonically denuded shortening of the continental crust may be balanced in the underlying lithosphere by thickening or underplating forming in either case a lithospheric root beneath the crustal welt we consider a model whereby tertiary crustal extension in the shuswap core complex and possibly also in other segments of the north american cordillera is accounted for by the detachment of this root both the time frame and the stress field resulting from the detachment are compatible with geological and geophysical observations far field stresses played an accommodating role but the potential energy required for extension of the shuswap complex was induced by lithospheric thickening

improved microseismic event location by inclusion of a priori dip particle motion a case study from ekofisk microseismic monitoring in petroleum settings provides insights into induced and naturally occurring stress changes such data are commonly acquired using an array of sensors in a borehole providing measures of arrival times and polarizations events are located using 1d velocity models p and s wave arrival times and the azimuths of p wave particle motions however in the case of all the sensors being deployed in a vertical or near vertical borehole such analysis leads to an inherent 180 ambiguity in the source location here we present a location procedure that removes this ambiguity by using the dip of the particle motion as an a priori information to constrain the initial source location the new procedure is demonstrated with a dataset acquired during hydraulic fracture stimulation where we know which side of the monitoring well the events are located using a 5 step location procedure we then reinvestigate a microseismic data set acquired in april 1997 at the ekofisk oilfield in the north sea traveltimes for 2683 candidate events are manually picked a noise weighted analytic signal polarization analysis is used to estimate the dip and azimuth of p wave particle motions a modified t test is used to statistically assess the reliability of event location as a result 1462 events are located but 627 are deemed to be statistically reliable the application of a hierarchal cluster analysis highlights coherent structures that cluster around wells and inferred faults most events cluster at a depth of roughly 3km in the ekofisk chalk formation but very little seismicity is observed from the underlying tor chalk formation which is separated from the ekofisk formation by an impermeable layer we see no evidence for seismicity in the overburden but such events may be too distant to detect the resulting picture of microseismicity at ekofisk is very different from those presented in previous studies

p t history and kinematics of the monashee decollement near revelstoke british columbia microstructural and petrofabric analyses of mylonites from the monashee decollement demonstrate that the hanging wall was displaced eastward over the footwall microstructural kinematic indicators include shear band foliation asymmetric strain shadows and s c fabrics quartz c axes locally exhibit asymmetric fabrics that are consistent with the microstructural evidence for sense of shear the kinematic evidence is reliable because multiple criteria coexist within individual specimens metamorphic assemblages from footwall monashee complex pelites at the revelstoke damsite indicate that the peak metamorphic assemblage was sillimanite k feldspar biotite almandine quartz plagioclase biotite garnet geothermometry and garnet plagioclase sillimanite quartz geobarometry set broad constraints on metamorphic temperatures but closer constraints on pressures near 650 c and 630 mpa comparison of these data with late cretaceous hornblende cooling ages from the same locality indicates that the metamorphism is at

pervasive melt migration from migmatites to leucogranite in the shuswap metamorphic core complex canada control of regional deformation abstract the shuswap metamorphic core complex exhumed in the hinterland of the canadian cordillera displays a 15 km structural section from migmatites to leucogranite it comprises a lower unit dominated by anatectic migmatites exposed in the core of domes at higher structural level the transition from a middle amphibolite facies unit to an upper crustal unit is marked by a low angle detachment associated with the emplacement of laccoliths of leucogranite the amphibolite facies middle unit is permeated by a network of granitic veins feeding the laccoliths and structurally connected to the migmatites of the lower unit this paper describes the geometric textural and structural characteristics and the distribution of the granitic fraction at the outcrop scale in the amphibolite facies unit separating the migmatites from the leucogranite this analysis constrains the mechanisms of melt migration within a plurikilometer scale section across high grade rocks of the middle crust in fertile lithologies metapelites felsic amphibolites the granitic fraction in part generated in situ forms a diffuse network of concordant veins feeding discordant veins and structural sites such as the foliation shear zones and boudin necks discordant granitic veins are oriented perpendicular and to a lesser extent parallel to the mineral and stretching lineation in refractory lithologies the granitic fraction is dominantly intrusive and displays sharp contacts with the host rock however distribution of the granitic fraction shows the same characteristics as for fertile lithologies the melt fraction migrated through a network of fractures taking advantage of mechanical weaknesses such as the foliation plane or forming dikes oriented dominantly perpendicular to the regional stretching and mineral lineations these geometric and textural characteristics suggest that granite migration in these rocks is achieved dominantly by viscous flow of the melt solid through the solid matrix driven by its buoyancy and controlled by mechanical anisotropy of the rock and local dilation created by heterogeneous deformation the relationship between the regional fabric and the distribution of the granitic fraction indicates that regional deformation incremental and finite played a major role in providing pathways for melt migration the formation of laccoliths of leucogranite at higher structural levels suggests that upward melt migration led to accumulation of granitic magmas these observations are consistent with a model of pervasive melt migration through a network of interconnected concordant and discordant granitic veins feeding laccolithic plutons extracted from the dominantly solid matrix during vertical thinning and compaction at a regional scale the presence of both transposed and intact granitic networks indicate an overlap between deformation and granite migration as a continuum or in pulses the methodology and terminology proposed in this paper provide some insights on the behavior of the granitic fraction in the middle crust this approach should be applicable to other high grade terrains exhumed in the cores of orogenic belts

brittle deformation in the columbia river fault zone near revelstoke southeastern british columbia the brittle columbia river fault zone forms part of the eastern margin of the shuswap metamorphic complex and for much of its 230 km length it coincides with the eastern exposure of the ductile monashee decollement the eocene brittle fracturing and displacement are broadly parallel with but cut and disrupt the middle jurassic ductile mylonite zone excavations for a hydroelectric dam and a highway have facilitated a detailed analysis of fracture patterns at several localities along the fractured zone regionally the brittle fault zone strikes 350 locally 310 030 and dips shallowly eastward over most of the studied part of the zone the distributions of subsidiary fractures and displacement vectors demonstrate a normal dip slip displacement consistent with subhorizontal east west extension within the fractured zone irrespective of the local trend of the zone at revelstoke damsite where the zone trends 030 for 2 km motion was towards the southeast possibly indicating a localized clockwise rota

cross double difference inversion for microseismic event location using data from a single monitoring well abstractthe double difference dd location method has long been applied for locating a cluster of earthquakes with data recorded at surface seismic stations this method has also been used for locating microseismic events with multiple monitoring wells during hydraulic fracturing we first extended the approach for locating a cluster of microseismic events with data recorded from a single well to do this we have reduced the 3d location problem to 2d by projecting all of the events onto a vertical xz plane with a vertical z axis and a horizontal x axis representing the distance to the monitoring well considering the symmetric character of the 1d velocity model and the vertical monitoring well we then performed a 2d location inversion and projected the results back to 3d using the event azimuths which were determined from a separate analysis of the initial p wave polarizations however although the dd method could determine relative locations of events reasonably well it yielded poor absolute loc

shuswap terrane of british columbia a mesozoic core complex the shuswap terrane of the northwest cordillera owes its distinctive characteristics to deformation and high grade regional metamorphism that occurred primarily during the mesozoic within the terrane the monashee complex the overlying selkirk allochthon and the intervening monashee decollement record strain and metamorphism that occurred in middle to late jurassic time during and subsequent to the accretion of a western allochthonous terrane the selkirk allochthon moved eastward across the monashee complex after emplacement of the western allochthonous terrane but before late mesozoic to early cenozoic telescoping of the rocky mountain belt in this latter event the monashee complex and overlying allochthonous slices were transported eastward relative to the north american craton on a sole fault that developed during listric thrusting of the rocky mountain foreland synmetamorphic to late metamorphic elongation lineations and associated fabrics of the mylonitic rocks of the monashee decollement zone were generated by shear strain during the middle jurassic emplacement of the selkirk allochthon and are not related to any upper crustal extension that occurred in the tertiary uplift normal faulting brittle reactivation of the mylonitic decollement zone widespread resetting of k ar and rb sr mineral dates and arching of the terrane are events that culminated in the eocene

timescales of migmatization melt crystallization and cooling in a cordilleran gneiss dome valhalla complex southeastern british columbia 1 integration of in situ and conventional ion microprobe u th pb zircon and monazite geochronology with 40ar 39ar hornblende biotite and muscovite thermochronology provides a temporal record of metamorphism partial melting fluids and cooling of exhumed midcrustal to lower crustal rocks the valhalla complex southeastern british columbia is one of a series of elongate gneiss domes within the omineca belt of the canadian cordillera we have performed u th pb and 40ar 39ar age measurements on migmatitic gneisses from a range of structural levels and settings within the valhalla complex in an effort to better document the timing of migmatization and exhumation stromatic migmatites and leucosomes crystallized in boudin necks yield concordant u pb zircon ages that cluster near 60 ma and th pb monazite ages of circa 60 50 ma in situ analyses of monazite in samples that contain 60 50 ma however the combined data also suggest that melt crystallization circa 60 ma in the valhalla complex occurred rapidly and before the other domes within the omineca belt cooling at 49 ma throughout the omineca is associated with regional extension and exhumation of migmatites in the domes the presence of abundant migmatites and the similarity in timing of melt crystallization and cooling in domes along the 400 km long omineca belt is consistent with localization of exhumation at the boundary between the flowing crust of the hinterland and the rigid colder crust of the rocky mountain foreland

the relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rocks several hundred microearthquakes with local magnitudes ranging between 4 and 2 were recorded during hydraulic stimulation experiments at the hot dry rock demonstration site fenton hill new mexico these events are probably caused by shear failure induced by high pore fluid pressures since the event locations seem to cluster in a narrow band near the hydraulic fracture we were able to use micro seismic techniques to locate the hydraulic fracture and monitor its growth we calculate the minimum pore pressure increase necessary to induce rock failure using a simple model of slippage on preexisting fractures by comparing a pore pressure distribution calculated using a one dimensional diffusion model with the distribution of event foci we demonstrate that high pore pressures are a probable cause for these microearthquakes stress drops for these microearthquakes calculated from spectral parameters increased by a factor of 3 during the course of the experiment the stress drop increase correlates with a migration of seismic activity away from the injection well bore which suggests relatively low in situ shear stresses near the well bore caused by strain release in this region during previous pressurizations

comparison of surface and borehole locations of induced seismicity monitoring of induced microseismic events has become an important tool in hydraulic fracture diagnostics and understanding fractured reservoirs in general we compare microseismic event and their uncertainties using data sets obtained with surface and downhole arrays of receivers we first model the uncertainties to understand the effect of different acquisition geometries on location accuracy for a vertical array of receivers in a single monitoring borehole we find that the largest part of the final location uncertainty is related to estimation of the backazimuth this is followed by uncertainty in the vertical position and radial distance from the receivers for surface monitoring the largest uncertainty lies in the vertical position due to the use of only a single phase usually p wave in the estimation of the event location in surface monitoring results lateral positions are estimated robustly and are not sensitive to the velocity model in this case study we compare event location solutions from two catalogues of microseismic events one from a downhole array and the second from a surface array of 1c geophone our results show that origin time can be reliably used to find matching events between the downhole and surface catalogues the locations of the corresponding events display a systematic shift consistent with a poorly calibrated velocity model for downhole dataset for this case study locations derived from surface monitoring have less scatter in both vertical and horizontal directions

measurements and relationships of diagenetic alteration of bone from three archaeological sites four diagenetic parameters have been chosen to represent the state of diagenesis of bone buried on archaeological sites they are histological preservation protein content crystallinity and porosity how these parameters are measured is described and results from populations of bones from three different sites are presented the results show the extent and variation in the degree of change both within a site and between sites in particular the correlations between diagenetic parameters are examined which give clues about the processes which cause alteration the value of porosity determinations both at the intercrystalline level and at coarser levels in revealing the degree of diagenetic change in bone and in underlying the dynamics of the interaction between buried bone and the surrounding water is stressed the data also indicate but are too restricted to prove the following microbiological attack is generally complete within less than 500 years substantial levels of protein may remain in bone after maximal micromorphological alteration loss of protein appears to be independent of other diagenetic change the correlated lose of microporosity with increase of crystallinity suggests these changes may arise from the dissolution perhaps with subsequent recrystallization of the smallest hydroxyapatite crystallites

the 1998 valhall microseismic data set an integrated study of relocated sources seismic multiplets and s wave splitting werelocate303microseismiceventsrecordedin1998bysensorsinasingleboreholeinthenorthseavalhalloilfield asemiautomated array analysis method repicks the p and s wave arrival times and p wave polarizations which are needed to locate these events the relocated sources are confined predominantly toa50 m thickzonejustabovethereservoir andlocationuncertaintiesarehalfthoseofpreviousefforts multipletanalysisidentifies 40 multiplet groups which include 208 of the 303 events the largest group contains 24 events and five groups contain 10 or more events within each multiplet group we further improve arrival time picking through crosscorrelation which enhances the relative accuracy of the relocated events and reveals that more than 99 of the seismic activity lies spatially in three distinct clusters the spatial distribution of events and waveform similarities reveal two faultlike structures that match well with north northwest south southeast trending fault planes interpreted from 3d surface seismic data most waveform differencesbetweenmultipletgroupslocatedonthesefaultscanbeattributed to s wave phase content and polarity or p to s amplitude ratio the range in p to s amplitude ratios observed on the faults is explained best in terms of varying source mechanisms wealsofindacorrelationbetweenmultipletgroupsandtemporal variations in seismic anisotropy as revealed by s wave splitting analysis we explain these findings in the context of a cyclic rechargeanddissipationofcap rockstressesinresponsetoproduction driven compaction of the underlying oil reservoir the cyclic nature of this mechanism drives the short term variations in seismic anisotropy and the reactivation of microseismic source mechanismsovertime

kinematics of compressional and extensional ductile shearing deformation in a metamorphic core complex of the northeastern basin and range abstract analysis of shear criteria enables the kinematics of two main ductile shearing events d 1 and d 2 to be established in the raft river grouse creek and albion metamorphic core complex the first event d 1 is a nne thrusting and corresponds to mesozoic shortening a well developed non coaxial ductile deformation d 2 of cenozoic age is marked by the occurrence of opposing eastward in raft river and westward shear criteria in albion grouse creek these characterize an arch structure where the shear strain increases outwards in the axial zone of the complex d 2 seems coaxial cenozoic extension is considered to be related to gravitational instability induced by mesozoic overthickening of the crust involving uplift erosion and abnormal heating brittle extension occurs in the upper part of the uplifted domain it is transformed laterally above undeformed basement towards stretched domains of the middle and lower crust through the ductile shear zones localized at the precambrian paleozoic interface this extension of the middle and lower crust occurred in the vicinity of the root zones of the former mesozoic thrusts which may thus have been reactivated as ductile normal faults during cenozoic extension

reservoir characterization using surface microseismic monitoring microseismic monitoring of reservoir processes can be performed using surface or near surface arrays we review the published technical basis for the use of the arrays and the historical development of the method beginning with locating earthquakes through geothermal exploration to the growing field of hydraulic fracture monitoring practical considerations for the array deployment and data processing are presented the road ahead for the technology includes a move toward life of field buried arrays as well as opportunities for extended interpretation of the data particularly inversion for source mechanism estimation and measurement of anisotropy in the monitored subsurface

rapid three dimensional hypocentral determination using a master station method a master station ms method is presented in this paper to rapidly determine hypocenters in three dimensional 3 d heterogeneous velocities an equal differential time edt surface is defined as the collection of all spatial points that satisfy the time difference between two arrivals which can be two picks at two stations or two different phase picks at one station the edt surface is independent of the origin time and will contain the hypocenter for an event with j arrivals there are j 1 independent edt surfaces the ms method determines the hypocenter that satisfies two types of constraints to be traversed by most edt surfaces and to yield minimum travel time residual statistics the statistics include both the residual variance and the amplitude of the origin time error the combined use of the edt surfaces and residual statistics allows for a unique determination of the hypocenter and origin time using different types of phase arrivals in principle only three arrivals are minimally required to constrain a unique solution if three different stations are used for a 3 d velocity model the edt surfaces and the residual statistics can be computed efficiently using a reference file created by moser s 1991 ray tracing method an illustration of the ms method is given for 27 small events that occurred in southern california using a p wave velocity model modified from that of magistrale et al 1992 the average misfit between the bulletin hypocenters and the new solutions is 3 8 km if a 3 d velocity model is accurate the ms method can be a viable means of hypocenter determination

multi genetic origin of the continental moho insights from lithoprobe this paper reviews four hypotheses for the origin of the continental crust mantle boundary and discusses seismic parameters with which these hypotheses might be tested the relict moho hypothesis posits that the oceanic moho is preserved during continental assembly the magmatic underplating hypothesis posits formation of a new moho by episodic emplacement of sill like intrusive bodies the metamorphic or metasomatic front hypothesis posits that the moho is overprinted by a phase transformation and the regional decollement hypothesis posits that the moho behaves as a structural detachment these hypotheses are not mutually exclusive and examples from the canadian lithoprobe program suggest that all four may be applicable in different regions of north america comparison of seismic images from a fossil subduction zone with modern subduction at cascadia suggests that serpentinization of the forearc mantle a previously unrecognized mechanism for overprinting and erasing the reflection moho may have occurred in the proterozoic

kinematic interpretation of mylonitic rocks in part of the columbia river fault zone shuswap terrane british columbia mylonitic rocks of the columbia river fault zone have been examined in a 600 m thick section 20 km north of revelstoke british columbia at this locality coarse grained granite with well preserved igneous texture lies in the hanging wall of the fault zone adjacent to the boundary of the mylonite zone the granite is foliated and mineral segregation gives rise to well defined compositional layering the foliation sf is defined by oriented mineral grains and flattened porphyroclasts in areas of intense strain compositional layering sc is parallel to sf at the boundary of the mylonite zone and in weakly deformed areas within the mylonite zone sc and sf are inclined at angles up to 30 sf is taken to be a plane of flattening and sc appears to parallel a plane of shear after correction for effects of late stage flexural slip folding the angular relationship of sc to sf may be used to determine sense of shear in the mylonitc zone the hanging wall rocks have been displaced toward the east relative to

metamorphic core complexes windows into the mechanics and rheology of the crust metamorphic core complexes are products of normal fault displacements sufficient to exhume rocks from below the brittle ductile transition these faults detachments may initiate within the brittle crust at steep angles but they sole into the ductile middle crust and during displacement rotate to gentler dips due to hanging wall extension the exhumed footwall commonly adopts an arched or domed geometry owing to flexural isostatic readjustment and may be overlain by strongly extended upper crustal rocks that slipped on gently dipping low friction shallow segments of the detachment metamorphic rocks exhumed beneath the detachment record progressively increasing flow stress strain localization and strain rate with decreasing temperature providing a window into physical conditions and deformational processes in the mid crust the metamorphic and deformational history of the footwall rocks may reflect tectonic processes that predate formation of the detachment fault in addition to those accompanying exhumation these processes may include diapiric emplacement of gneiss domes or exhumation in a subduction channel and may not be directly related to formation of the core complex factors favouring core complex formation are high gravitational potential energy of the extending crust weak rheology and a change in the tectonic boundary conditions such as a cessation or slowing of plate convergence

locating oceanic earthquakes the influence of regional models and location criteria summary earthquakes in oceanic areas are normally located using traveltime tables which are representative of continental paths since most seismic stations lie on continents it should therefore be possible to improve such locations by employing a set of traveltimes more appropriate to paths from oceanic events to continental stations a comparison has therefore been made between locations for a number of oceanic events using the recent iasp91 global traveltimes and the times for the pac91 model derived from observations of events in the pacific although there were often significant differences in the location estimates for the two models these were often no larger than the shifts induced by changing the misfit criterion used for determining the location for events in purely oceanic regions such as tonga and the marianas with little nearby continent the results from the pac91 model either provided a significantly better fit to the data or produced depth estimates in close accord with independent constraints e g centroid moment tensor locations in these cases the use of a specific set of oceanic traveltimes can be recommended however for marginal zones and island arcs the situation is less clear and it is probably best to employ the global traveltime set with the use of additional phases to improve depth estimates

errors in hypocenter location picking model and magnitude dependence abstract the location procedures of seismic events are influenced by two major classes of errors the error in picking individual seismic phases and modeling error due to the departure of the real earth from the reference model used in the location both classes of error influence the estimate of location and it is difficult to separate them the role of picking errors can be assessed by a nonlinear analysis using a monte carlo procedure arrivals times are perturbed with random numbers drawn from a normal distribution and the event is relocated using these perturbed arrival times by repeating the procedure many times a cluster of locations is obtained which can be used to investigate the effects of picking errors on the hypocenter this analysis is insensitive to velocity model errors as these are fixed for a given combination of stations and phases some care must be exercised when analysing multidimensional distributions in two dimensional slices because of a projection effect the modeling error due to the influence of lateral heterogeneity in the earth is examined by comparing the locations of the same event using different combinations of phases and network geometries which reinforces the need to use arrivals other than p for accurate depth resolution the sensitivity of p arrivals to changes in depth are swamped by model errors and inclusion of depth sensitive phases such as pp is highly recommended the effect of picking errors on location is found to be much smaller than the mislocation caused by neglecting lateral heterogeneity when only p arrivals are used consequently the monte carlo analysis which is primarily aimed at picking errors only is most appropriate when multiple phases have been used to more accurately constrain the hypocenter especially for the depth component altering the type of phase data used in the location plays a similar role in changing the network geometry in that both are mechanisms that influence the nature of the constraint on the hypocenter by relocating events with network geometries corresponding to the different magnitudes it is found that the location of the event can be affected significantly by the magnitude and when using robust statistics to describe earthquake residuals the mislocation can occur in a systematic manner the effect is marked in regions with significant lateral variations in seismic velocities for example low magnitude events in the flores sea are found to be dragged toward australia as a result of the fast paths to australian stations relative to the iasp 91 reference velocity model

multiply deformed terrains problems of correlation abstract the best tools available for carrying out a structural analysis in a complex area are overprinting style and patterns of orientation unfortunately all have limitations which are reviewed briefly and discussed in areas of continuous unfaulted outcrop the deformational history can be established purely on the basis of overprinting but even in such areas there can be problems since not all overprinting relationships are unambiguous most fold interference patterns are reliable overprinting relationships based on foliations however are commonly unreliable and various examples are cited and discussed even in areas where the sequence of deformational events can be established their temporal significance is unclear since the methods applied only determine local relative timing and absolute relationships must vary in space and time across an orogen

structural evolution of the thor odin gneiss dome abstract the thor odin dome is a large structural culmination along the eastern margin of the shuswap terrain previous workers have presented divergent models for the origin of the dome some have proposed that it is a product of diapiric uprise of material from the lower crust alternatively the dome is interpreted to have formed from the interference of later folding this study presents the results of a detailed analysis of new data on the structure pattern of strain variation and metamorphic fabric of the dome these data together with new geochronological and metamorphic phase assemblage information provide the basis for a revised interpretation of the evolution of the thor odin gneiss dome the pre doming evolution of the thor odin gneiss dome was characterized by the formation of large scale nappe structures and the imbrication of archean basement rocks with the cambrian and younger cover rock sequence the first period of deformation phase one consisted of large scale infolding of the cover rock sequence into the basement rocks the pingston fold in the core of the dome is a product of this event the second period of deformation phase two was marked by the forcing of wedges of basement into the cores of northerly directed nappes these structures had no observable effect on the central core zone which is here termed the autochthonous core gneiss domain the third period of deformation phase three was co axial with phase two and consisted of imbrication and refolding of the upper levels of the stack of phase two nappes the actual domal shape was formed by fold interference of phases four five and six which were upright chevron style flexural slip folds with northwest southwest and northerly trending axial planes the doming events occurred at upper crustal levels probably during the tertiary analysis of the strain pattern suggests that little of the total deformational strain is related to these dome forming deformations this outline of the events that formed the dome is consistent with the new geochronological results and interpretation of metamorphic phase assemblages cordierite gedrite sillimanite assemblages previously interpreted as indicating 1000 mpa pressure probably formed under reduced water fugacities and 500 700 mpa total pressure the thor odin gneiss dome thus provides no evidence for lower crustal upwelling related either to dome formation or to the formation of the rocky mountain thrust belt

hydraulic stimulation of natural fractures as revealed by induced microearthquakes carthage cotton valley gas field east texas we produced a high resolution microseismic image of a hydraulic fracture stimulation in the carthage cotton valley gas field of east texas we improved the precision of microseismic event locations four fold over initial locations by manually repicking the traveltimes in a spatial sequence allowing us to visually correlate waveforms of adjacent sources the new locations show vertical containment within individual targeted sands suggesting little or no hydraulic communication between the discrete perforation intervals simultaneously treated within an 80 m section treatment i e fracture zone lengths inferred from event locations are about 200 m greater at the shallow perforation intervals than at the deeper intervals the highest quality locations indicate fracture zone widths as narrow as 6 m similarity of adjacent source waveforms along with systematic changes of phase amplitude ratios and polarities indicate fairly uniform source mechanisms fracture plane orientation and sense of slip over

characterization of fractures and faults a multi component passive microseismic study from the ekofisk reservoir fractures and faults within a reservoir can provide important pathways for the movement of reservoir fluids understanding the character and properties of these features on a range of length scales can be vital for the efficient exploitation of natural resources whether it be enhanced oil and gas recovery the safe storage of co2 o r better exploitation of geothermal heat the monitoring of microseismicity within a reservoir illuminates active faults but these events can be also used to characterize fracture networks through measurements of seismic anisotropy in this study we use microseismic data acquired over an 18 day period in april 1997 at the ekofisk oil field in the north sea using the analysis of seismic multiplets we delineate a number of sub vertical tectonic faults which are consistent with previous core data analysis and seismic reflection work we use shear wave splitting measurements which are indicative of fracture induced seismic anisotropy to infer the orientation of aligned aseismic fracture sets within the reservoir the estimated fracture dip and strike from the shear wave splitting analysis are consistent with the active tectonic fractures characterized by the multiplets but this analysis also illuminates spatial variations in fracture properties such monitoring on a longer term and with multiple wells is a promising tool for better understanding fracture and fault controlled flow within reservoirs

tectonic interpretation of west verging folds in the selkirk allochthon of the southern canadian cordillera the selkirk allochthon a composite tectonic slice composed of north american paleocontinental margin deposits and more distal possibly marginal basin suspect terrane was displaced eastward toward the craton in the late jurassic and late cretaceous the carnes nappe a major west verging recumbent anticline within the selkirk allochthon is considered the southern continuation of scrip nappe which in the monashee mountains has an inverted limb length of 50 km the west verging nappe and associated structures are interpreted as having originated in the early to middle jurassic during accretion of western allochthonous terranes and prior to eastward displacement of the selkirk allochthon the reversal from westward vergence away from the north american craton to eastward vergence is considered as marking a fundamental change in the evolution of the orogenic belt and may reflect a transition from underthrusting of western allochthonous terranes on blind shear zones to east directed breakthrough thrusts

the role of the shuswap metamorphic complex in cordilleran tectonism a review the shuswap metamorphic complex consists of three parts each with unique stratigraphy and orogenic evolution separated by major faults of diverse nature and having in common only a post late mesozoic tectonic history the first part the monashee complex is a possible extension of the precambrian shield that contains limited evidence of mesozoic orogenesis and that was rapidly uplifted during the cretaceous to paleogene the monashee decollement a warped mylonite zone interpreted as a regional thrust fault active through the middle jurassic separates this complex from the second part which contains rocks correlative with hadrynian to late paleozoic strata of the pericratonic prism the third part the okanagan complex straddling the 49th parallel from the okanagan valley to kootenay lake contains the probable exhumed roots of a mesozoic magmatic arc built upon possible north american continental and transitional crust and includes late paleozoic and early mesozoic suspect terranes the columbian or

petroleum reservoir characterization using downhole microseismic monitoring imaging of microseismic data is the process by which we use information about the source locations timing and mechanisms of the induced seismic events to make inferences about the structure of a petroleum reservoir or the changes that accompany injections into or production from the reservoir a few key projects were instrumental in the development of downhole microseismic imaging most recent microseismic projects involve imaging hydraulic fracture stimulations which has grown into a widespread fracture diagnostic technology this growth in the application of the technology is attributed to the success of imaging the fracture complexity of the barnett shale in the fort worth basin texas and the commercial value of the information obtained to improvecompletions and ultimately production in the field the use of commercial imaging in the barnett is traced back to earlier investigations to prove the technology with the cotton valley imaging project and earlier experiments at the m site in the piceance

extension in high grade terranes of the southern omineca belt british columbia evidence from paleomagnetism paleomagnetic observations are used to construct a map of structural tilts in metamorphic and plutonic rocks of the southern omineca belt in southeastern british columbia during eocene extension midcrustal rocks were exhumed in footwalls of low to moderate angle normal faults that dip east and west in the eastern and western parts of the study area respectively tilts averaging 30 to 40 are consistent over large areas and generally antithetic to the major faults in the central part of the study area however the eastern half of the coryell pluton is untilted and its western half in the hanging wall of the east dipping kettle river fault is tilted down to the east in the opposite sense to that expected north of the pluton these eastward tilted rocks are juxtaposed against westward tilted rocks to explain the anomalous tilts a major west dipping eocene normal fault arrow lake fault is inferred and a revised kinematic history for the area proposed observations of magnetic polarity 86 normal laboratory unblocking temperatures and isotopic age data suggest that magnetizations in the west were acquired during chron 23n and in the east in chron 23n and or chron 24n unless extension postdated the acquisition of magnetization tilts would not be observed thus it is argued that extension in the west occurred after 50 6 ma and in the east between 51 6 possibly 53 3 and 50 6 ma these time intervals do not correspond to any known global plate reorganization event indicating that the last phase of eocene extension in the southern omineca belt was governed by regional rather than global processes

thermochronological constraints on the eocene exhumation of the grand forks complex british columbia based on 40ar 39ar and apatite fission track geochronology the grand forks complex gfc is a metamorphic core complex within the composite shuswap complex in the southern omineca belt of the canadian cordillera it is juxtaposed against the surrounding lo

preservation of bone collagen sulphur isotopic compositions in an early holocene river bank archaeological site abstract this study investigates the reliability of the sulphur isotopic compositions 34 s of collagen in archaeological bones from an early holocene river bank site noyen sur seine france the chemical composition c n s of whole bones compared to those of bones from cave sites suggests that contamination with sulphur is higher in the bones sampled from river bank deposits compared to those from caves especially those that occur well above the water table sulphur content in fresh bone collagen suggests specific values for different mammal taxa while sulphur content in reptile bones may not always be higher than those of mammals in the early holocene bones from noyen sur seine the collagen has chemical characteristics within the overall range observed in modern bone collagen however co variation between some diagenetic indicators such as sulphur content and n s in whole bone s in collagen and s yield and 34 s values of collagen from the same species or ecological groups indicate that some diagenetic alteration may have influenced the collagen excluding samples possibly affected by this alteration a difference in 34 s is measured between freshwater and terrestrial fauna we recommend further work on the collagen sulphur contents in different species moreover whole bone chemical compositions may help to screen samples for sulphur isotopic analyses of collagen that are to be used for palaeodietary reconstructions

a comparison of passive seismic monitoring of fracture stimulation from water and co2 injection hydraulic fracturing is used to create pathways for fluid migrationandtostimulateproduction usually wateristheinjected fluid although alternative fluids such as carbon dioxide co2 have been used recently the amount of fracturing that co2 can induce is also of interest for the security of carbon capture and storage hydraulic fracturing is usually monitored using passive seismic arrays detecting microseismic events generated by the fracturing it is of interest to compare the amount of seismicity that co2 injection can generate in comparison with water with this in mind we have analyzed a passive seismic data set monitoring the injection of water and supercritical co2 under very similar conditions allowing us to make a direct comparison between the fluids we examined event locations and event magnitudes and we used shear wave splitting to image the fractures that are generated for both fluids the event locations map the formation of fractures moving away from the injection well with normalsparalleltotheminimumprincipalstress theeventsduring water injection are limited to the injection depth while during co2 injection activity migrates above the injection depth eventmagnitudesaresimilarinbothcases andlargereventmagnitudes appear to correlate with higher injection pressures shear wave splitting suggests that water injection generates more fractures though the data quality is not good enough to make a robust conclusion about this the comparability between water and co2 injection means that lessons can be learned from theabundantexperienceofconventionalwaterinjection

effective media for transversely isotropic models based on homogenization theory with applications to borehole sonic logs sonic log records including measurements of wave speeds in boreholes provide critical input to the geological geophysical and petrophysical studies of a region under exploration 1d background models are routinely built based on sonic log records for applications such as seismic imaging of hydrocarbon reservoirs and microseismic source inversions smoothing or upscaling techniques are required to produce models in coarser scales than the very fine layers in the raw log data in this paper we follow the recently popular homogenization theory derive its application to the special case of 1d ti models for both p sv and sh waves and show that it is consistent with the backus averaging technique commonly used to upscale 1d fine layered models we examine a study case of sonic log data from a well in the horn river basin in northeastern british columbia a region known for its tight shale gas deposit we demonstrate the computational accuracy and efficiency gained by proper upscaling procedures for spectral element simulations of seismic wave propagation and discuss the effect of control parameters on wavefield recovery