precambrian gneiss and late proterozoic sedimentation in north central british columbia u pb analyses of zircons from gneissic granite in the sifton and deserters ranges flanking the northern rocky mountain trench in north central british columbia indicate crystallization ages of 1 85 b y and 728 m y respectively augen gneiss in the sifton ranges appears to be basement rock associated with a sequence of amphibolite quartzite schist and carbonate thrust southerly over upper proterozoic strata of the ingenika group gneissic granite in the deserters range is nonconformably overlain by pebble conglomerate and pure quartzite in turn overlain by a thick sequence of amphibolite schist phyllite gritty clastic rocks carbonate quartzite and diamictite assigned to the upper proterozoic misinchinka group the deserters data establish a probable maximum age for rocks of the windermere supergroup in the northern rocky mountains the age and structural setting of these gneisses are similar to other gneisses of the omineca crystalline belt of the canadian cordillera

three dimensional structure across the tintina strike slip fault northern canadian cordillera from seismic refraction and reflection tomography accepted 2006 june 5 received 2006 june 3 in original form 2005 september 4 summary the development of the northern canadian cordillera involved major strike slip displacement of accreted terranes relative to north america along faults such as the tintina which has experienced 425 km of dextral motion since the palaeocene the snore seismic refraction wideangle reflection experiment was carried out in 1997 as one component of lithoprobe s slavenorthern cordillera lithospheric evolution snorcle transect in addition to four 2 d profiles two sets of broadside recordings were acquired to image the 3 d structure across the tintina fault tf in areas centred at about 59 5 n and 62 n simultaneous and independent refraction and reflection traveltime tomography were applied to the combined inline and broadside data set for each region to establish the range of lower crustal velocity moho depth and upper mantle velocity structure consistent with the data our preferred models are the average of the simultaneous and independent models since they represent the robust features required by the data the preferred 3 d models are generally consistent with the 2 d models obtained from the inline data in previous independent studies there are along strike variations across the tf perhaps due to the change in strike direction or the amount of motion along the fault in the north compared to the south in the lower crust the only correlation with the tf that is required by the data is a 0 1 km s 1 drop in velocity to the southwest of the fault in the northern study area the absence of a strong correlation with the tf in the lower crust is consistent with the interpreted continuity of lower crustal units across the fault in the snorcle reflection data the moho is relatively flat throughout the study area 34 35 km depth but there is broad crustal thickening of a few kilometres centred 50 km southwest of the tf in the northern and southern study areas this thickening may be the result of a period when there was a component of compression along the tf there is strong evidence for a 0 3 0 4 km s 1 drop in upper mantle velocity to the west of the tf in the south and weak evidence for 0 1 km s 1 drop to the southwest in the north the upper mantle variations in the south indicate that the tf is the boundary between cooler and or more refractory ancestral north american mantle to the east and warmer and or more fertile mantle beneath accreted north america to the west in the north the mantle appears to be more intermediate in its properties on both sides of the tf

evolution of the early paleozoic cordilleran margin of laurentia tectonic and eustatic events interpreted from sequence stratigraphy and conodont community patterns several field seasons in the canadian cordillera have allowed the measurement description and sampling of over 20 000 m of lower paleozoic strata from 26 stratigraphic sections across four platform to basin transects with the recovery of over 100 000 conodonts from more than 1200 4 5 kg samples this work was part of the lithoprobe slave northern cordillera lithospheric evolution snorcle project but is also being extended through a pan lithoprobe project to understand the tectonic and eustatic response of much of the laurentian plate through the early paleozoic based on the abundant field data the complex stratigraphic framework is interpreted in terms of sequence stratigraphy and a derived relative sea level curve using detailed conodont taxonomic and biostratigraphic results cluster analysis of conodont distributional data identified an evolving series of conodont communities through space and time these communities were partitioned across the platform to basin gradient and provide an additional sensitive indicator of relative sea level change these two independent approaches generated comparable eustatic curves for this cordilleran laurentian margin during much of the early paleozoic and identified some global eustatic events noted by earlier workers this part of laurentia was not a simple passive margin during the early paleozoic but rather was affected by four main tectonic events complicated by six principal eustatic changes some success was achieved in filtering the global and regional tectonic eustatic effects and in proposing causes for some of the events

deformation of stratiform znpb barite deposits in the northern canadian cordillera abstract the major stratiform sediment hosted zn pb barite deposits of the northern canadian cordillera occur in the palaeozoic selwyn basin kechika trough they formed in euxinic fine grained clastic dominated extensional basins that developed within the carbonate platform of the passive margin of ancestral north america during the late jurassic to mid cretaceous these basins and the stratiform ore deposits were subjected to low greenschist facies regional metamorphism and fold and thrust deformation as they were incorporated into the western part of the rocky mountains mackenzie mountains foreland fold and thrust belt this paper summarises the results of detailed field and microscopic studies on four of these deposits the tom deposit the howards pass deposit the cirque deposit and the driftpile creek deposit the dominant mineralogy of these deposits is sphalerite galena pyrite and in some cases significant barite is present low greenschist grade metamorphism and deformation is characterised by cataclastic flow and brecciation in coarse grained massive pyritic sulphides and by folding recrystallisation and cleavage development in fine grained sulphides and barite relict primary depositional diagenetic structures are found within the orebodies deformation is characterised by strong pressure solution fabrics in barite in fine grained laminated pyrite layers and in mixed sulphide carbonate beds marked local transposition is found with significant remobilisation of galena barite and pyrite the deformation mechanics of these sulphide barite deposits are illustrated using schematic deformation mechanism maps

present day stress of the central persian gulf implications for drilling and well performance abstract the present day state of stress in the persian gulf is poorly understood but has significant impacts on well drilling and performance the upper permian to lower triassic formation of kangan dalan persian gulf exhibits a complex structural context in the neighborhood of the oman mountains and the zagros orogenies this formation is divided into four reservoir layers k1 to k4 where three main lithologies limestone dolomite and anhydrite are alternating we conduct an analysis of the present day stress and natural fractures at the wellbore using full bore fmi logs leak off test and density logs for this purpose borehole breakout and tensile fracture data are used to determine orientation of s h furthermore density log leak off test and kirsch equation for tensile fracture formation in the wellbores are used to calculate the magnitude of s v s h and s h respectively vertical stress s v gradient at 3100 m depth approximates 20 mpa km 2 9 psi m indicating a bulk density of 2 04 g cm 3 a total of 131 drilling induced tensile fractures and 21 breakouts with an overall length of 262 m are observed in two wells indicating a mean maximum horizontal stress s h orientation of n53 18 45 for drilling induced tensile fracture ditf data and n50 10 79 for breakout data the mean orientation of s h rotates counterclockwise with depth from k2 n70 4 2 to k4 n40 5 1 reservoirs noticed correlation between these data and stress orientations from earthquake focal mechanism solution first of all indicates that the stresses are linked to the resistance forces generated by the arabia eurasia collision at the zagros orogeny and secondly confirms the reliability of focal mechanism solution data near continental collision zones in the kangan dalan formation the nw se main open fracture direction is found as a common regional direction which is sub perpendicular to the present day maximum horizontal stress minimum horizontal stress s h gradient in reservoir sections is estimated to be equal to 17 mpa km 2 5 psi m the concluded strike slip stress regime s h s v s h in the study area is consistent with the compressive regime in the zagros thrust fold belt the present day stress in the kangan dalan formation has implications for wellbore stability lost circulation and well inflow performance relationship ipr wells are more unstable if deviated toward the s v direction whereas well productivity and mud loss increase in wells deviated toward s h which conveys the idea of a strike slip faulting effect tends to keep the natural fractures open in that direction

spherical earth gravity and magnetic anomaly analysis by equivalent point source inversion abstract to facilitate geologic interpretation of satellite elevation potential field data analysis techniques are developed and verified in the spherical domain that are commensurate with conventional flat earth methods of potential field interpretation a powerful approach to the spherical earth problem relates potential field anomalies to a distribution of equivalent point sources by least squares matrix inversion linear transformations of the equivalent source field lead to corresponding geoidal anomalies pseudo anomalies vector anomaly components spatial derivatives continuations and differential magnetic pole reductions a number of examples using 1 averaged surface free air gravity anomalies and pogo satellite magnetometer data for the united states mexico and central america illustrate the capabilities of the method

mapping of cleats and fractures as an indicator of in situ stress orientation jharia coalfield india abstract cleats and fractures in coal of barakar formation in jharia coalfield india have been mapped from the coal exposures around 21 opencast and 2 underground coal mines macro cleat distribution observed from this mapping is similar to that of well developed coalbed methane cbm basins the opencast coal mines are distributed in se nw direction along the boundary of the sickle shaped jharia coal basin permeability of cbm through coal seams is the most critical property deciding the viability of any cbm field it is largely controlled by the degree of cleat development this paper discusses cleat orientation and other structural features observed in the outcrops of 14 major coal seams in jharia coalfield and its directional relationship with the in situ stress sh orientation pattern cleat orientation is found to vary laterally within same seam at the different locations distributed from the south east part to the north western part of jharia coalfield face cleat orientation of the 21 opencast coal mines varies from n45 e to n45 w change of orientation of cleat observed around the mining areas is related the orientation of fault systems and orientation of igneous intrusions occurring within the coal bearing packet maximum in situ horizontal compressive stress sh direction is found to be parallel to the face cleat orientation in normal faulted basin like jharia coalfield an overall nne ssw and nw se orientation of the sh direction is predicted in the coal mines from cleat orientation mapping sh orientation is locally modified at places close to the tectonic features and igneous intrusives finite element stress modeling is carried out near moonidih area cleat orientation predicted stress direction matches well with the finite element stress results

separation of lithospheric external and core components of the south polar geomagnetic field at satellite altitudes we present a new approach to producing scalar magsat magnetic anomaly maps based on correlation coefficient filtering and the use of almost all of the available orbits our method differs from earlier techniques with respect to the following 1 passes are selected based on their variance properties rather than planetary indices such as kp 2 the core field model is least squares fit to individual passes and subsequently removed instead of subtracting the model directly this technique replaces band pass filtering or polynomial trend removal methods 3 each selected pass is sorted geographically and by local time placed into one of four different altitude bands and correlation coefficient filtered with the two adjacent passes the filtering is the second step toward isolating the static lithospheric signal from the more dynamic external field signals 4 least squares collocation is used to grid the correlated passes subsequently the dawn and dusk maps are also correlation filtered providing another step toward removal of external fields 5 the four resultant total field maps are continued to a common altitude and again correlation filtered for the static lithospheric anomalies 6 the filtered results are then averaged together to provide a new total field map of the lithosphere south of 40 s latitude our total field map differs from previous efforts over the crustal blocks of west antarctica we obtained a positive anomaly over edward vii peninsula extending into the byrd subglacial basin and obtained a negative anomaly over the ellsworth mountains and parts of the byrd subglacial basin also a positive anomaly extending from the ross sea to offshore wilkes land is present in our map however this feature is absent in other maps positive anomalies marking the weddell sea in previous efforts are not present in our map prominent external field anomalies in the quadrant offshore of wilkes land are present in all previous efforts however these signatures are reduced in our total field map this map significantly advances efforts for the identification and interpretation of magnetic anomalies of the south polar lithosphere

abstract reservoirs in the abo formation southeastern nm a fractured play within the perturbed stress field at the termination of wrench faults production of natural gas from the low pemreability arkoses and sandstones of the permian abo formation southeastern new mexico exceeds matrix deliverability capacity suggesting that the reservoirs are fractured this suggestion is corroborated by the fact that the abo formation is highly fractured in the relatively undeformed outcrops far distant from the reservoirs but indicating that the strata are susceptible to fracturing natural fractures are also present in the few available cores thus the presence of significant fracturing is inferred in the subsurface although the abo formation was deposited over much of southeastern new mexico abo sandstones are most productive where the strata are associated with a series of sub parallel northeast trending deep seated right lateral wrench faults called the pecos buckles the surface strata along these buckles consist of poorly exposed evaporite and carbonate facies complicating structural interpretations however well exposed sandstones along similar wrench faults in the central sahara have numerous irregular fractures that are commonly formed in the vicinity of such wrench faults strata are fractured due to volume constraints at irregularities during lateral offset along the faults by analogy reservoirs in the abo formation were fractured during lateral offset along the pecos buckles most of the abo reservoirs are located near the northeastern termination of these wrench faults therefore fracturing may also have been a response to stresses associated with offsets along fault splays at the ends of the pecos buckles iti troduction

getting more for less the unusual efficiency of fluid flow in fractures although it is recognized that the opening of fractures changes with changing fracture spacing it is a common practice to consider only the effects of loading and rock properties and to assume that opening of all fractures is independent of fracture spacing thus one would expect that the more fractures contained in a rock mass the greater the fluid flow rate using the cubic law and the finite element method we have investigated the volumetric flow rate through a number of equally spaced fractures in a fractured rock layer as a function of the ratio of fracture spacing to layer thickness under a remote extension results show that there is an optimum value for the ratio of fracture spacing to layer thickness that yields the maximum flow rate this value is independent of the fluid properties the head gradient the applied extension the internal fluid pressure and the overburden stress

stress induced borehole elongation a comparison between the four arm dipmeter and the borehole televiewer in the auburn geothermal well the nature and origin of borehole elongation recorded by the four arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in auburn new york a preferred orientation n10 w s10 e 10 and a less prominant e w orientation of borehole elongation was observed on two runs of the dipmeter comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures obtained from hydrofracturing tests and televiewer data show that the n10 w s10 e borehole elongations 1 are axisymmetric 2 are aligned with the minimum horizontal stress sh and 3 are not associated with natural fractures intersecting the well these elongations are interpreted as stress induced well bore breakouts the e w elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts this asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter this study demonstrates that the dipmeter can be used to determine the orientation of sh by mapping breakouts confirming the results of earlier less detailed studies and provides a firm basis for mapping regional stress patterns using existing dipmeter data

regional magnetic anomaly constraints on continental breakup continental lithosphere magnetic anomalies mapped by the magsat satellite are related to tectonic features associated with regional compositional variations of the crust and upper mantle and crustal thickness and thermal perturbations these continental scale anomaly patterns when corrected for varying observation elevation and the global change in the direction and intensity of the geomagnetic field show remarkable correlation of regional lithospheric magnetic sources across rifted continental margins when plotted on a reconstruction of pangea accordingly these anomalies provide new and fundamental constraints on the geologic evolution and dynamics of the continents and oceans

lithospheric structure in northwestern canada from lithoprobe seismic refraction and related studies a synthesis the snorcle refraction wide angle reflection r war experiment snore 97 included four individual lines along the three transect corridors a combination of snore 97 results with those from earlier studies permits generation of a 2000 km long lithospheric velocity model that extends from the archean slave craton to the present pacific basin using this model and coincident near vertical incidence nvi reflection data and geological information an interpreted cross section that exemplifies 4 ga of lithospheric development is generated the velocity structural models correlate well with the reflection sections and provide additional structural compositional and thermal constraints geological structures and some faults are defined in the upper crust at a larger scale the seismic data identify a variety of orogenic styles ranging from thin to thick skinned accretion in the cordillera and crustal scale tectonic wedging associated with both paleoproterozoic and mesozoic collisions models of poisson

cryogenian of yukon abstract cryogenian strata of the windermere supergroup were deposited in tectonically active basins throughout the north american cordillera from alaska to mexico the windermere supergroup of yukon canada hosts key geochronological constraints on the start of the cryogenian period and the onset of the sturtian snowball earth glaciation at ca 717 ma a 57 million year duration for the sturtian glaciation has been suggested by correlations of the glacigenic rapitan group in yukon with equivalent strata in the mackenzie mountains of northwest territories where it is overlain by organic rich argillaceous carbonate of the ca 660 ma twitya formation however syn depositional faulting and large lateral facies variation complicate these stratigraphic correlations here we describe cryogenian strata of the northern wernecke mountains yukon where correlations with the rapitan and hay creek groups of the mackenzie mountains have been previously established these units are then traced south across cryogenian structures to the southern wernecke mountains and west through the hart river inlier to the coal creek and tatonduk inliers of the ogilvie mountains in order to construct a new regional stratigraphic framework the coal creek inlier preserves volcanic rocks in the mount harper group and the eagle creek formation of the rapitan group for which we provide new u pb ca idtims zircon ages we further formalize the glacigenic cryogenian eagle creek formation together the new correlations and geochronological data allow us to more precisely integrate geological data from northwest canada with the rest of the cordillera and beyond particularly these data refine the onset of the sturtian glaciation in northwestern canada to between 716 9 0 4 and 717 4 0 2 ma and demonstrate that syn sedimentary tectonism resulted in unconformities throughout northwest laurentia between the end of the sturtian glaciation at 660 ma and the end of the marinoan glaciation at 635 ma these unconformities confound correlations throughout the cordillera by commonly preserving only one of the two cryogenian glacial deposits

tectonic model and three dimensional fracture network analysis of monte alpi southern apennines abstract the monte alpi southern italy is well known as an outcropping analogue of the val d agri subsurface units which host a number of oil fields of economic importance this paper presents an example of a detailed study which integrates stratigraphy structural geology tectonic analysis and fracture network study the new structural model presented is based on a detailed 1 12 500 field survey of the area whose results are illustrated in a comprehensive outcrop map geological map and structural cross sections and stereographic projections and palaeostress inversion calculations of a mesostructural data set the monte alpi unit is characterised by a 2000 m thick sequence of jurassic cretaceous platform carbonates conformably covered by middle upper miocene calcarenites this sequence is unconformably covered by upper messinian terrigenous clastics mainly along a number of major growth faults our detailed mapping revealed that the monte alpi unit is tectonically overlain by the lagonegro basinal unit thin siliceous carbonates and remnants of pelagic pelitic and carbonate deposits all highly deformed and by the monte la spina unit cataclastic mesozoic carbonates and dolomites this solves earlier raised questions concerning the palaeogeographic provenance of the carbonates outcropping in the area which can thus without any doubt be correlated with the external carbonate domains known from the subsurface the present day structure shows a structural inversion along the major late miocene growth faults which acted as sinistral transpressive faults the latter faults are part of a regional set dissecting the southern apennines active up till at least middle pleistocene times a fracture study was performed on the mesozoic carbonates of the area in order to collect data regarding length distribution orientation density and spatial heterogeneity of the fracture network to be used for considerations regarding exploration and development of the val d agri reservoirs the data were collected on 37 horizontal strata parallel and vertical outcrop surfaces through photographic restitution and digitising of fracture lineament traces and measurements of orientations of all visible fractures furthermore three major outcropping rock walls along the flanks of monte alpi and santa croce were photographed and fracture and fault zone lineaments were digitised all data were inserted and calibrated in a three dimensional 3d geographic information systems and computer aided design giscad environment consisting of a database and a 3d geographical framework comprising topographic map digital terrain model dtm geological map aerial photograph lineament map and a 3d reconstruction of major fault surfaces making use of the potential of the 3d working environment outcrop surfaces and rock walls were reconstructed at their correct 3d position and orientation data analysis comprised stereographic contour diagrams and rosette diagrams of 1800 fracture orientations in order to recognise main fracture sets three corrections were performed on these data length weighting terzaghi correction removal of sample orientation bias and tilt removal from a comparison between orientation data on various scales combined with field evidence it can be deduced that fault and fracture sets are part of the same structural pattern and that fracture sets were generated prior to plio pleistocene tectonics and passively rotated fracture and fault length distribution was analysed by the generation of diagrams which plot cumulative frequency versus length in log log space normalising for area of observation these diagrams composed of about 30 000 measurements and constituting the most complete data set presented in the literature up till now show that small scale and large scale features form part of one scale invariant system with fractal properties no scale gap is present in our data set which comprises rock wall data in the subseismic scale range it appears that the 3d fracture network can best be described by a fractal dimension of 2 98 and a coefficient of 0 25 the outcome of the study places important constraints on fracture network simulation exercises in hydrocarbon reservoir simulation studies which use numerical input parameters in order to make predictions of the tectonic texture present in the subsurface

faulting fracturing and in situ stress prediction in the ahnet basin algeria a finite element approach abstract many low efficiency hydrocarbon reservoirs are productive largely because effective reservoir permeability is controlled by faults and natural fractures accurate and low cost information on basic fault and fracture properties orientation in particular is critical in reducing well costs and increasing well recoveries this paper describes how we used an advanced numerical modelling technique the finite element method fem to compute site specific in situ stresses and rock deformation and to predict fracture attributes as a function of material properties structural position and tectonic stress presented are the numerical results of two dimensional plane strain end member fem models of a hydrocarbon bearing fault propagation fold structure interpretation of the modelling results remains qualitative because of the intrinsic limitations of numerical modelling however it still allows comparisons with the little available geological and geophysical data in all models the weak mechanical strength and flow properties of a thick shale layer the main seal leads to a decoupling of the structural deformation of the shallower sediments from the underlying sediments and basement and results in flexural slip across the shale layer all models predict rock fracturing to initiate at the surface and to expand with depth under increasing horizontal tectonic compression the stress regime for the formation of new fractures changes from compressional to shear with depth if pre existing fractures exist only sub horizontal fractures are predicted to open thus defining the principal orientation of effective reservoir permeability in models that do not include a blind thrust fault in the basement flexural amplification of the initial fold structure generates additional fracturing in the crest of the anticline controlled by the material properties of the rocks the folding induced fracturing expands laterally along the stratigraphic boundaries under enhanced tectonic loading models incorporating a blind thrust fault correctly predict the formation of secondary syn and anti thetic mesoscale faults in the basement and sediments of the hanging wall some of these faults cut reservoir and or seal layers and thus may influence effective reservoir permeability and affect seal integrity the predicted faults divide the sediments across the anticline in several compartments with different stress levels and different rock failure and proximity to failure these numerical model outcomes can assist classic interpretation of seismic and well bore data in search of fractured and overpressured hydrocarbon reservoirs

age and stratigraphic tectonic significance of proterozoic diabase sheets mackenzie mountains northwestern canada diabase dikes and sills were intruded during a late proterozoic phase of extensional tectonics in the northern canadian cordillera this event is strongly expressed in the depositional pattern of formations immediately below and above the base of the windermere supergroup rb sr isochron dates of 766 24 and 769 27 ma have been obtained for two of the these diabase sheets which intruded the tsezotene formation of the mackenzie mountains supergroup the emplacement of the diabase sills and dikes may be related to either of two volcanic events in the region one documented the other inferred in either case emplacement of the diabase at approximately 770 ma is close to the boundary between the mackenzie mountains supergroup and the overlying windermere supergroup precedes a regional glaciation shezal tillite and reflects a strong accentuation of the west facing cordilleran miogeocline

initial scalar magnetic anomaly map from magsat a scalar magnetic anomaly map between 50 geographic latitude was derived from magsat data acquired during the november 1979 to june 1980 mission lifetime separation of anomaly fields from core and external fields was accomplished utilizing techniques developed for pogo data mayhew 1979 langel et al 1980 comparison with a similar map from pogo data shows that the basic anomaly patterns are essentially reproducible except in a few regions such as the atlantic and pacific at latitudes south of 15 increased resolution due to a lower measurement altitude is evident in the magsat map

proterozoic mantle under quesnellia variably reset rb sr mineral isochrons in ultramafic nodules carried up in cenozoic volcanic vents of the southern omineca belt handpicked acid washed diopside orthopyroxene and olivine separates from 14 lherzolite nodules from four localities in southern british columbia have been analyzed for 87rb 86sr and 87sr 86sr the host lavas of late cenozoic to pleistocene age are nepheline normative alkali basalt and basanite with sr and nd isotopic ratios 0 7025 0 7029 0 51310 0 51326 close to those of mid ocean ridge basalt the present day sr isotopic compositions of nodule diopsides 9 239 ppm sr are 0 7021 0 7041 similar to those of the host rocks orthopyroxene and olivine however have 87rb 86sr ratios of 0 1 to 1 0 and much more radiogenic sr 87sr 86sr commonly 0 708 0 720 and in several cases the three minerals form linear arrays in 87rb 86sr versus 87sr 86sr plots if interpreted as mineral isochrons these give dates from 99 to 1989 ma many cluster around 440 770 ma four nodules from two localities west kettle river and big timothy mountain give dates of 1201 1989 ma maximum dates for the other localities

lithospheric magnetic property contrasts within the south american plate derived from damped least squares inversion of satellite magnetic data abstract a ridge regression or damped least squares method has been used in a large scale equivalent source inversion of south american magsat data the optimum damping parameter is selected based on percent increase in the sum of squared residual of the observed and the calculated anomaly and on the variance and the range of the solution for a series of damping parameters solutions obtained by the damping parameter selected in this manner are numerically stable and geologically meaningful consequently these solutions are used in various applications such as transformation into effective magnetic susceptibility variation altitude normalization differential reduction to pole and downward continuation of the data set the spacing of equivalent sources and the source observation distance are shown to introduce significant errors in the estimation of the physical property variation from equivalent source solutions for the large source spacing 1 commonly in use the correction for these geometric factors is essential in estimating the physical property variation from the equivalent source solutions a continental and an oceanic profile have been analyzed from the south american maps derived from the above techniques the continental profile transects a variety of geologic tectonic features including shields aulacogens basins and fold belts near the north coast of south america along the profile there is a good correlation between the radially polarized magsat magnetic anomalies and other available geologic and geophysical information in addition the variation in effective susceptibility obtained from the damped equivalent source inversion is consistent with our present knowledge of continental lithospheric susceptibilities expected from the geologic tectonic provinces along the profile the oceanic profile extends from the trailing margin of south america through the cretaceous normal polarity rocks of the south atlantic ocean the variations in the magnetic properties along this profile can be broadly correlated with the depth anomaly variation indicating a possible genetic relationship two conceptual models requiring undepleted mantle intrusions under the oceanic uplifts appear possible in the first model the high magnetization can be accounted for by a weakly magnetic upper mantle below the uplifts in the second model very highly magnetic rocks are required in the crust of these uplifted regions to account for the observed anomaly values

influence of syn sedimentary faults on orogenic structure examples from the neoproterozoic mesozoic east siberian passive margin abstract the east margin of the siberian craton is a typical passive margin with a thick succession of sedimentary rocks ranging in age from mesoproterozoic to tertiary several zones with distinct structural styles are recognized and reflect an eastward migrating depocenter mesozoic orogeny was preceded by several mesoproterozoic to paleozoic tectonic events in the south verkhoyansk the most intense pre mesozoic event 1000 950 ma rifting affected the margin of the siberian craton and formed half graben basins bounded by listric normal faults neoproterozoic compressional structures occurred locally whereas extensional structures related to latest neoproterozoic early paleozoic rifting events have yet to be identified devonian rifting is recognized throughout the eastern margin of the siberian craton and is represented by numerous normal faults and local half graben basins estimated shortening associated with mesozoic compression shows that the inner parts of ancient rifts are now hidden beneath late paleozoic mesozoic siliciclastics of the verkhoyansk complex and that only the outer parts are exposed in frontal ranges of the verkhoyansk thrust and fold belt mesoproterozoic to paleozoic structures had various impacts on the mesozoic compressional structures rifting at 1000 950 ma formed extensional detachment and normal faults that were reactivated as thrusts characteristic of the verkhoyansk foreland younger neoproterozoic compressional structures do not display any evidence for mesozoic reactivation several initially east dipping late devonian normal faults were passively rotated during mesozoic orogenesis and are now recognized as west dipping thrusts but without significant reactivation displacement along fault surfaces

crustal structure interpreted from magnetic anomalies this review discusses publications during the last quadrennium 1987 1990 that used aeromagnetic data marine magnetic data satellite magnetic data and rock magnetic and petrologic data to provide information on the sources of magnetic anomalies the publications reviewed reflect increased integration of rock magnetic property and petrologic studies with magnetic anomaly interpretation studies particularly in deep crustal magnetization exploration for hydrocarbons and inversion of marine magnetic anomalies interpretations of aeromagnetic data featuring image display techniques and using the horizontal gradient method for locating magnetization boundaries became standard

multicomponent avo analysis vacuum field new mexico shear wave amplitude variation with offset avo analysis can be used to map changes in density shear wave velocity and fracturing at reservoir scale by allowing the influence of each factor to be separately extracted from the observed seismic response weighted least squares inversion of the anisotropic reflection coefficients was implemented to find the shear wave splitting coefficient and velocity contrast parameters a time lapse nine component 4 d seismic survey acquired over vacuum field in lea county new mexico was used to test our methodology of shear wave avo analysis and to compare the results with well production and azimuthal p wave avo analysis weighted least squares shear wave avo stacks of the splitting parameter were found to be excellent predictors of well fluid production performance implying a strong link between seismically inferred fracturing and reservoir scale permeability of the san andres dolomites at vacuum field analysis of the shear wave velocity contrast indicated the p

identifying fracture orientation in a mature carbonate platform reservoir the permian guadalupian san andres reservoir at keystone field winkler county texas is divided into three major stratigraphic units and twelve flow units on the basis of an analysis of multiple shoaling upward cycles of shallow water marine to tidal flat carbonate facies these rocks are now thoroughly dolomitized and cemented with anhydrite and gypsum the distribution of original oil in place was mapped both laterally and vertically most of the resource is in the upper five flow units and the original oil in place map of these upper flow units indicates that the highest concentration of hydrocarbons is in the center of the study area porosities in this reservoir are nearly 10 and permeabilities are generally less than 1 md despite these low matrix permeabilities recently drilled wells initially produced at rates as high as 120 bbl of oil per day although these rates declined an average of 75 in the first 6 months primary recovery from this reservoir is only 8 of original oil in place subvertical fractures in this reservoir are visible in cores and on a microimage log early floodwater breakthrough occurred without increased oil production in a pilot waterflood these production characteristics combined with direct observations of fractures indicate that productivity is dependent on fracture permeability horizontal boreholes perpendicular to the strike of effective fractures and within the part of the reservoir that contains highest remaining oil will maximize primary recovery borehole breakouts and regional stress measurements suggest that the direction of principal horizontal compressive stress is northeast southwest and natural fractures that strike in this direction are most likely to be open and capable of transmitting fluids

karst landforms revealed at various scales using lidar and uav in semi arid brazil consideration on karstification processes and methodological constraints abstract this paper analyzes different types of karst landforms and their relationships with fracture systems sedimentary bedding and fluvial processes we mapped karst features in the cretaceous carbonates of the jandaira formation in the potiguar basin brazil we used high resolution digital elevation models acquired using lidar and aerial orthophotographs acquired using an unmanned aerial vehicle uav we grouped and described karst evolution according to scale and degree of karstification these degrees of karst evolution are coeval fractures are opened by dissolution forming vertical fluid conduits whereas coeval dissolution occurs along horizontal layers this conduit system acts as pathways for water flow the enlargement of conduits contributes to the collapse of blocks in sinkholes and expansion of caves during an intermediate degree of karstification propagation of dissolution can cause the coalescence of sinkholes and the capture of small streams fluvial processes dominate karst dissolution at an advanced degree of karstification comparisons with previously published ground penetrating radar gpr borehole and seismic surveys in sedimentary basins indicate that these structures can be partially preserved during burial

improved inversion of geopotential field anomalies for lithospheric investigations instabilities and the large matrices which are common to inversions of regional magnetic and gravity anomalies often complicate the use of efficient least squares matrix procedures inversion stability profoundly affects anomaly analysis and hence it must be considered in any application wildly varying or unstable solutions are the products of errors in the anomaly observations and the integrated effects of observation spacing source spacing elevation differences between sources and observations geographic coordinate attributes geomagnetic field attitudes and other factors which influence the conditioning of inversion solution instabilities caused by ill posed parameters can be efficiently minimized by ridge regression with a damping factor large enough to stabilize the inversion but small enough to produce an analytically useful solution an effective choice for the damping factor is facilitated by plotting damping factors against residuals between observed and modeled anomalies and by then comp

pyrite deformation in stratiform lead zinc deposits of the canadian cordillera pyrite textures in five stratiform lead zinc deposits from lower to upper greenschist facies environment of the canadian cordillera are described and discussed in terms of deposition early diagenesis deformation metamorphism and hydrothermal alteration processes overgrowth is an important process during both diagenesis and deformation diagenetic and deformational overgrowths can be distinguished diffusive mass transfer involving pressure solution and oriented overgrowth of pyrite is the main deformation mechanism in pyrite deposits at low metamorphic grades although diffusive mass transfer favours fine grained mineral aggregates its effect on coarse pyrite grains has also been identified ore minerals dissolved by pressure solution may be transported with the assistance of pore fluids within fractures and grain boundaries over distances significantly greater than the scale of individual grains to give a range of pressure solution overgrowth textures the textural modification of pyritic ores from the early stages of diagnesis through metamorphism and deformation to post deformation thermal annealing has important implications for the distribution of trace elements and isotopic compositions in pyritic ores

geopotential field anomaly continuation with multi altitude observations abstract conventional gravity and magnetic anomaly continuation invokes the standard poisson boundary condition of a zero anomaly at an infinite vertical distance from the observation surface this simple continuation is limited however where multiple altitude slices of the anomaly field have been observed increasingly areas are becoming available which are constrained by multiple boundary conditions from surface airborne and satellite surveys this paper describes the implementation of continuation with multi altitude boundary conditions in cartesian and spherical coordinates and investigates the advantages and limitations of these applications continuations by eps equivalent point source inversion and the ft fourier transform as well as by scha spherical cap harmonic analysis are considered these methods were selected because they are especially well suited for analyzing multi altitude data over finite patches of the earth such as those covered by the admap database in general continuations constrained by multi altitude data surfaces are invariably superior to those constrained by a single altitude data surface due to anomaly measurement errors and the non uniqueness of continuation

subsurface fracture analysis and determination of in situ stress direction using fmi logs an example from the santonian carbonates ilam formation in the abadan plain iran abstract the relationship between the present day stress field and natural fractures can have significant implications for subsurface fluid flow in particular fractures that are aligned in orientations favourable for reactivation by either shear or tensile failure in the in situ stress field often exhibit higher hydraulic conductivities the ilam formation of southwestern iran is an important hydrocarbon reservoir containing numerous natural fractures however little is known about the state of stress in this region or any of iran s petroleum provinces we conducted analysis of the present day maximum horizontal stress orientation and the density orientation and hydraulic conductivity of natural fractures in the ilam carbonates using high resolution formation micro imager resistivity logs in two wells a total of 51 breakouts with an overall length of 215 m were observed in the two wells indicating a maximum horizontal stress orientation of 68 n 7 6 in well a and 58 n 6 3 in well b furthermore the wellbore derived stress orientations determined herein are consistent with those inferred from nearby earthquake focal mechanism solutions indicating that stresses in the sedimentary cover are linked to the resistance forces generated by arabia eurasia collision furthermore the correlation between stress orientations estimated from earthquake focal mechanism solutions and breakouts indicates that focal mechanism solution data which is often considered to be unreliable for stress field analysis near transform margins may provide reliable information on the stress orientation near continental collision zones the image log data also reveals three sets of open and presumably hydraulically conductive fractures with strikes of i 160 170 n ii 110 140 n and iii 070 080 n fracture set iii is consistent with being formed and open in the present day stress field however fracture sets i and ii strike at a high angle to the present day maximum horizontal stress and are interpreted herein to be the result of either pre or syn folding related forces the observation that different sets of open fractures in the field can be either sensitive or insensitive to the present day stress is critical for improving hydrocarbon recovery

magsat scalar anomaly map of south america a scalar magnetic anomaly map for south america and adjacent marine areas was prepared from magsat data preparation of the map posed problems notably in separating external field and crustal anomalies and in reducing data to a common altitude external fields are manifested in a long wavelength ring current effect a medium wavelength equatorial electrojet and short wavelength noise the ring current is corrected through use of a standard ring current equation augmented by further wavelength filtering because the electrojet is confined primarily to dusk profiles its effect is minimized by drawing the data set from dawn profiles only the noise is reduced by selecting profiles from quiet periods k 2 the optimum filter cutoff level is determined by a statistical regression technique to be approximately 50 degrees wavelength when wavelengths longer than this are rejected resultant profiles from redundant satellite orbits correlate well with differences due primarily to satellite altitude the

lithospheric structure beneath the archaean slave province and proterozoic wopmay orogen northwestern canada from a lithoprobe refraction wide angle reflection survey the slave geological province a relatively small craton in nw canada that includes the oldest rocks in the world is one of five cratons that form the archaean continental core of north america the palaeoproterozoic wopmay orogen lying between the craton and the phanerozoic northern canadian cordillera represents crustal growth through addition of a series of north south oriented magmatic arcs and accreted terranes as part of lithoprobe s multidisciplinary studies line 11 of the snore 97 seismic refraction wide angle reflection survey investigated the structure of the lithosphere in this region interpretation of crustal and mantle arrivals from 13 record sections followed an iterative procedure of traveltime inversion and forward modelling results show that crustal thickness at 32 35 km remains relatively constant from the archaean regions in the east to proterozoic domains in the west contrary to average values of crustal thicknesses from global compilations that suggest thicker proterozoic crust at the westernmost end of the line where crustal extension and basin formation are inferred the crust thins to 30 km upper crustal velocities to about 12 km depth lie in the range of 5 8 6 2 km s 1 the middle crust of thickness 8 10 km has velocities between 6 2 and 6 6 km s 1 average velocities for the lower crust differentiated as a 10 km thick layer range between 6 6 and 6 9 km s 1 with a higher velocity region 7 1 km s 1 in the transition zone between the fort simpson magmatic arc and hottah accreted terrane in the wopmay orogen where a palaeoproterozoic subduction zone has been inferred from coincident multichannel reflection data in the same area anomalously low upper mantle velocities 7 5 km s 1 have been found indicating that the effects of collision subduction and accompanying physical changes in the rocks comprising the mantle lithospheric wedge above the underthrusting structure may have been retained for 1800 ma data from most record sections with offset distances greater than 250 km show continuous wide angle reflections from upper mantle depths of about 75 km and almost 100 km although poorly constrained velocities in this depth range vary from 8 4 to 8 8 km s 1 interpretation of these phases and mantle reflections from equivalent depths in coincident near vertical incidence multichannel data relates them to subduction and underthrusting of oceanic crust and subsequent phase changes that occurred during the assembly of the craton and the evolution of the palaeoproterozoic orogen

preferential rifting of continents a source of displaced terranes lithospheric rifting while prevalent in the continents rarely occurs in oceanic regions to explain this preferential rifting of continents we compare the total strength of different lithospheres by integrating the limits of lithospheric stress with depth comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of 3 also a thickened crust can halve the total strength of normal continental lithosphere because the weakest area acts as a stress guide any rifting close to an ocean continent boundary would prefer a continental pathway this results in the formation of small continental fragments or microplates that once accreted back to a continent during subduction are seen as displaced terranes in addition the large crustal thicknesses associated with suture zones would ironically make such areas likely locations for future rifting episodes this results in the tendency described as the wilson cycle of new oceans to open along the suture where a former ocean had closed

using shear waves to predict lateral variability in vertical fracture intensity during the last five years the use of shear wave splitting in geophysical exploration has made the transition from research project to very promising field technique the measurement of the azimuth of the fast shear wave and analysis of the s wave splitting or birefringence are now commonplace furthermore a very timely practical application is suggested by the recent proposal that reflectivity on post rotation s1 and s2 data sets should indicate lateral variation of anisotropy this could be of great value because the determination of lateral variation in fracture intensity in a vertically fractured sedimentary layer is necessary to insure high efficiency in a horizontal drilling program designed to drain the fractures

paleomagnetism of the late neoproterozoic blueflower and risky formations of the northern cordillera canada paleomagnetic analysis of the risky and underlying blueflower formations at the top of the neoproterozoic windermere supergroup in the northern cordillera of canada revealed several magnetic components of geological significance rock strata of the blueflower locality in the mackenzie mountains yielded no primary remanence due to extensive pyritization and weathering however dolomitic rocks of the risky locality in the eastern wernecke mountains 250 km to the north yielded a probable primary remanence the remanence has a direction at d i 055 64 n 11 samples from 4 sites k 18 95 11 and pole at 007 e 29 s p m 14 18 and indicates a paleolatitude of 46 14 s comparison between poles from the risky and the overlying backbone ranges and ingta formations suggests a profound post risky unconformity three secondary components are also present component b in both the blueflower and risky units appears to be a thermal overprint possibly related to vendian rifting its respective direction in each unit is d i 062 77 n 4 sites k 143 95 8 and d i 064 56 n 6 sites k 21 95 15 yielding poles of 110 e 64 s p m 13 14 and 142 e 44 s p m 15 21 component c only found in the risky is attributed to late cambrian to middle ordovician extension and magmatism component d which predominates in both formations relates to late mesozoic compression and to weathering

paleoproterozoic subduction in northwestern canada from near vertical and wide angle seismic reflection data near vertical incidence and refraction wide angle reflection seismic data recorded as part of lithoprobe studies in the paleoproterozoic archean domains of canada s northwest territories show remarkable reflections from within the upper mantle a parallel pair of reflectors imaged by the near vertical data can be traced from moho levels 33 km down to 70 km depth in a previous study the reflectors were interpreted as the top and bottom of an 1 8 ga subducted oceanic crust beneath the hottah terrane further inboard where the seismic line changes its direction from east west to nearly north south another pair of reflectors extends subhorizontally for about 100 km at 70 km depth before dipping downward the subhorizontal reflectors were not correlated with the dipping slab instead they were interpreted as a separate feature however they roughly coincide with a horizontal interface modeled from wide angle data by an earlier study considering the crooked line acquisition geometry we re exami

lower paleozoic stratigraphic and biostratigraphic correlations in the canadian cordillera implications for the tectonic evolution of the laurentian margin the ancient laurentian margin rifted in the latest neoproterozoic to early cambrian but appears not to have developed as a simple passive margin through a long post rift drift phase stratigraphic and conodont biostratigraphic information from four platform to basin transects across the margin has advanced our knowledge of the early paleozoic evolution of the margin in northeastern british columbia two northern transects span the macdonald platform to kechika trough and ospika embayment and a third transect spans the parautochthonous cassiar terrane in the southern rocky mountains new conodont biostratigraphic data for the ordovician succession of the bow platform is correlated to coeval basinal facies of the white river trough in total from 26 stratigraphic sections over 25 km of strata were measured and 1200 conodont samples were collected that yielded over 100 000 conodont elements key zonal species were used for regional correlation of uppermost cambrian to middle devonian strata along the cordillera the biostratigraphy temporally constrains at least two periods of renewed extension along the margin in the latest cambrian and late early ordovician alkalic volcanics associated with abrupt facies changes across the ancient shelf break intervals of slope debris breccia deposits and distal turbidite flows suggest the margin was characterized by intervals of volcanism basin foundering and platform flooding siliciclastics in the succession were sourced by a reactivation of tectonic highs such as the peace river arch prominent hiatuses punctuate the succession including unconformities of early late ordovician sub llandovery possibly early to middle silurian and early devonian ages

conodont biostratigraphy of the lower to middle devonian deserters formation new road river group northeastern british columbia a new lower to middle devonian basinal unit of the road river group herein formally named the deserters formation contains argillaceous crinoidal limestone and black shale deposited in a linear sub basin of the ospika embayment southern kechika trough the abrupt lateral facies changes in the region facies thickness and occurrence of volcanics indicate a period of extensional tectonism a total of 53 4 5 kg each samples from the deserters formation yielded 7766 conodont elements assigned to 14 genera representing 33 species the formation ranges from the lochkovian eleanorae zone or lower part of the delta zone of the cordilleran region to eifelian australis zone the temporal constraints established by conodont biostratigraphy allow correlation to the grizzly bear formation a regionally restricted lower to middle devonian unit in the selwyn basin to the north

proterozoic palaeomagne tism and single continent plate tectonics summary proterozoic 2700 570 ma palaeomagnetic poles from the major shields conform to a single apparent polar wander path on a contiguous reconstruction which brings into alignment both with each other and with the long axis of the primary continental crust tectonic and magmatic features ranging from late archaean 2900 ma to late proterozoic 1100 ma age this discovery implies that the continental lithosphere behaved on a megatectonic scale as a single coherent plate during this era subject to internal deformation but not breaking up in a brittle fashion until late precambrian times peripheral parts of the supercontinent were redistributed over a short interval at ca 1100 ma but the continental crust was not dismembered and widely dispersed until 570 530 ma this latter episode correlates with the proterozoic phanerozoic transition and defines the onset of multi continent plate tectonics the proterozoic apw record comprises nine large loops with periods ranging from 270 to 110ma and averaging 200ma between 2150 and 1000 ma the loops had a similar signature and their resultant effect was to return the continental lithosphere repeatedly to a stable position with its centre at the geographic pole probably by the goldreich toomre mechanism this periodicity correlates with theoretical and experimental studies of mantle convection predicting avalanche type behaviour due to build up of instability in the surface boundary layer with cycles of 50 200 ma the consolidation of the continental lithosphere defines several phases in the history of mantle convection 1 between 3500 and 2800 ma the sialic scum resulting from differentiation of a thickened and subducting lithosphere was accumulated into the primary body by a large scale mantle convection system 2 this operated simultaneously with a subcontinental lithosphere small scale 50 100 km convection system showing a transition from bimodal to cylindrical roll mode represented by the later greenstone belts 2900 2200 ma and straight belts ca 2800 2400 ma 3 a transition to larger scale episodic subcontinental convection is represented by the proterozoic mobile belts 2200 1100 ma 4 a modification of the major suboceanic lithosphere convection is suggested by reorganization of the peripheral shields at 1100 ma and 5 a subdivision of this major system resulted in continental dispersal during lower cambrian times