crustal electrical conductivity in north central saskatchewan the north american central plains anomaly and its relation to a proterozoic plate margin seven recording magnetometers monitored time varying fields at points on a northwest southeast line 280 km long in north central saskatchewan during july 1981 the experiment was designed to test the hypothesis advanced in 1975 by alabi camfield and gough that the electrical conductivity anomaly in the north american central plains links with the wollaston domain in the exposed precambrian shield of saskatchewan from clear reversals in the phase of vertical variations it is evident that the conductor passes between two stations straddling the rottenstone la ronge magmatic belt to the immediate east of the wollaston domain enhanced horizontal variations transverse to the belt at a third intermediate station reinforce this interpretation vertical field response arrows obtained from daytime events in the period range 1 40 min clearly indicate the existence of a major conductor that extends to lower crustal depths beneath the belt to the northwest across the cree lake zone reversals in the directio

a geomagnetic induction anomaly from ims data near hudson bay and its relation to crustal electrical conductivity in central north america summary canadian geomagnetic data collected during the international magnetospheric study are used to investigate the terrestrial electrical conductivity structure of north eastern manitoba and part of the north west territories the computed transfer functions resolve a major conductor trending east west between the communities of gillam and back in manitoba regional trends in the surface geology suggest that this conductor may be linked with the north american central plains electrical conductor twodimensional modelling of the data suggests that the conductor dips to the north from a small depth beneath gillam and may extend to the lowermost cnst

moho signature from wide angle reflections preliminary results of the 1993 trans hudson orogen refraction experiment abstract in 1993 a combined crustal seismic refraction and wide angle reflection experiment was conducted over the northern saskatchewan and manitoba portion of the trans hudson orogen by lithoprobe the program included three seismic lines extending over 1900 km in length the exceptionally high quality data collected during the field experiment included seismic reflections and refraction events from the moho and upper lithospheric mantle the relatively high seismic fold and signal to noise ratio of the field observations made it possible to process the wide angle portion of the data set with standard reflection signal enhancement techniques the moho is revealed as a complex interface with significant structural relief 40 52 km that correlates in first order fashion with crustal geology and structure the moho topography is interpreted as a result of variable ductile reworking and eclogitization of the lower crust

generation of continental rifts basins and swells by lithosphere instabilities 1 continents may be affected simultaneously by rifting uplift volcanic activity and basin formation in several different locations suggesting a common driving mechanism that is intrinsic to continents we describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent we carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above dynamical behavior depends on three dimensionless numbers a rayleigh number for the unstable block a buoyancy number that scales the intrinsic density contrast to the thermal one and the aspect ratio of the block within the block instability develops in two different ways in an outer annulus and in an interior region in the outer annulus upwellings and downwellings take the form of periodically spaced radial spokes the interior region hosts the more familiar convective pattern of polygonal cells in geological conditions such instabilities should manifest themselves as linear rifts striking at a right angle to the continent ocean boundary and an array of domal uplifts volcanic swells and basins in the continental interior simple scaling laws for the dimensions and spacings of the convective structures are derived for the subcontinental lithospheric mantle these dimensions take values in the 500 1000 km range close to geological examples the large intrinsic buoyancy of archean lithospheric roots prevents this type of instability which explains why the widespread volcanic activity that currently affects western africa is confined to post archean domains

deep structure of northern mississippi embayment in september 1980 the u s geological survey conducted a seismic refraction investigation of the northern mississippi embayment during the investigation 34 shots from nine shotpoints were recorded along a series of profiles the profiles were parallel to and across an inferred precambrian rift zone which is outlined by a series of magnetic anomalies and covers an area at least 200 km 125 mi long and 70 km 45 mi wide along the northeast southwest trending axis of the rift a 0 7 to 1 1 km 2 297 to 3 609 ft thick 1 8 km sec layer representing the unconsolidated tertiary and cretaceous sediments overlies a 2 km 6 562 ft thick 5 95 km sec layer representing a paleozoic carbonate sequence beneath the carbonates is a 3 km 9 843 ft thick low velocity layer which probably consists of late precambrian clastics the presence of this low velocity layer is indicated on the seismic profiles by an abrupt cutoff of the 5 95 km sec arrivals at about 20 km 12 mi from the shotpoints this cutoff is followed by a 30 to 40 km 20 to 25 mi long zone of no arrivals beyond which are seen delayed arrivals from a 6 2 km sec crystalline basement the 11 km 6 8 mi thick 6 2 km sec basement overlies a lower crus composed of a 6 6 km sec layer whose thickness decreases to the northeast owing to its replacement by an anomalous high velocity 7 3 km sec basal layer the base of the crust ranges from a 39 km 24 mi depth at the southern end of the profile to a 46 km 29 mi depth at the northern end where the 7 3 km sec layer is 20 km 12 mi thick below the base of the crust moho the upper mantle has a velocity of 8 0 km sec the velocity structure beneath the west flank of the embayment is simpler than that of the axis 5 95 6 2 6 6 7 3 and 8 0 km sec the low velocity layer is absent and the anomalous 7 3 km sec layer is thinner than along the axial profile the profiles perpendicular to the rift show that the low velocity layer is restricted to the axial zone and that the anomalous 7 3 km sec basal crustal layer reaches maximum thickness there the tectonic model proposed to explain the origin of this embayment velocity structure includes a late precambrian mantle plume that intruded the lower crust of the northern embayment causing uplift and or crustal stretching and subsequent rifting of the axial area this was followed by erosion subsidence and subsequent deposition of sediments in the resulting trough

rejuvenation of continental lithosphere beneath an intracratonic basin abstract the effective elastic thickness of the lithosphere in the williston basin region has been determined for several time intervals by fitting an elastic flexure equation to the shape of the basin over time the elastic thickness increases from about 40 km just after the basin began subsiding 450 m y b p to about 80 km at the present these results agree well with the increasing elastic thickness with age predicted for a cooling 250 km thick plate taking elastic thickness to be depth to the 450 c isotherm if we assume a thermal age of the lithosphere of about 500 m y radiometric dates of basement in the area however generally yield ages of 1 7 b y churchill province and 2 5 b y superior province because basement ages are generally much greater than the inferred lithospheric thermal age in the williston basin region we suggest that lithosphere in this area was thermally rejuvenated about 500 m y b p by the same process es responsible for the thermal subsidence of the basin temperatures were not high enough in the upper crust to reset most radiometric clocks additional support for a thermal event 500 m y b p comes from fission track ages and a single rb sr age of basement rocks of about 500 m y lithospheric rejuvenation has been modelled using a one dimensional finite difference thermal model the base of a 250 km thick plate is heated from 1333 c to 1850 c approximately the solidus of basalt at that depth for approximately 100 m y with conduction only the depth to the 450 c isotherm slowly decreases then slowly increases after heating ceases rapid thinning and slow thickening of the lithosphere can be achieved only if convective thinning is simulated in the model the model with convection yields results consistent with the observed increase in lithospheric thickness as well as the observed subsidence of the basin over time

a mechanical and thermal model for the evolution of the williston basin the williston basin is a large circular intracratonic basin containing over 4 km of sediments ranging in age from cambrian to tertiary data on seven formations from over 1300 wells in montana south dakota and north dakota were used to determine the shape of the basin over time by assuming that the lithosphere behaves like an elastic plate on a fluid substrate the shape of the basin could be closely matched using a point load the match was improved by adding the equation of a plane to the equation for elastic deformation before fitting the model to the data the planar adjustment allows for the effects of regional sedimentation or erosion or eustatic sea level changes the mechanical modeling indicates that the elastic strength of the lithosphere in this area has generally increased from the ordovician to the present the geometrical center of the basin migrated less than 20 km over 450 m y the direction and magnitude of tilt of the plane removed from the data is related to tectonic events in western north america the increasing deflection of the basin over time can be explained by a simple thermal model which would explain the thickening of the lithosphere consisting of conductive cooling of a hot region in the lithosphere depths to formations in a single well differ from the subsidence predicted by the elastic model with regional tilt removed because of global sedimentation events and the development in the mesozoic of a foreland thrust basin of which the williston basin became part the mechanical and thermal models constrain possible initiating mechanisms for this basin the thermal modeling and the absence of volcanics in the stratigraphic column rule out a shallow crustal heat source neither wrench faulting nor crustal stretching were likely to have been significant in the formation of the williston basin

seismic structure of superior churchill precambrian boundary zone two reversed refraction wide angle reflection profiles obtained across the nelson front by concrust 77 group are interpreted in detail the results indicate that the moho beneath the boundary zone between the superior and churchill provinces dips down toward the west and there exists a complicated lateral seismic structure in the upper crustal and midcrustal depths the ray tracing results also indicate that there may exist a zone of negative velocity gradient along the boundary zone at the depth range of 4 13 km

inversion of the coprod2 magnetotelluric data using a diffusive to propagative mapping dpm diffusive to propagative mapping dpm transforms a diffusive electromagnetic field into a dual wave field to produce electromagnetic reflectivity images similar to non migrated seismic sections an application of dpm to the coprod2 data set gives reflectivity images showing three distinct reflectors attributed to resistivity jumps located at depths ranging from 2 to 50 km the deeper reflector may be related to a major west dipping seismic reflector detected with the cocorp seismic data

phase change and thermal subsidence of the williston basin summary a mechanism of subsidence for the williston basin is proposed and tested the mechanism combines cooling and thermal contraction of the lithosphere with a phase transformation in the lower crust the cooling and the phase boundary movement following a sudden change in the thermal boundary condition at the base of the lithosphere are calculated isostatic adjustments and sea level variations are included to determine subsidence and sediment accumulation at the basin s centre which are compared with the data the subsidence record could not be explained by temperature boundary conditions the calculations show that 1 the heat flow boundary condition explains the long duration more than 350 myrs of subsidence of the williston basin and 2 the delay of the phase change subsidence about 40myrs explains an acceleration of subsidence in the early stages of the basin s evolution when isostatic adjustments and the effect of sea level variations are included the calculated sediment accumulation history fits well the sediment record of the williston basin

coincident vibroseis and dynamite surveys across the western flank of the trans hudson orogen abstract coincident dynamite and vibroseis reflection surveys provide comparable seismic signals along a 200 km segment of the trans hudson orogen transect on single fold field records signal amplitudes from explosive sources are consistently 40 db higher than on the corresponding vibroseis records however the vibroseis final stack exhibits higher crustal reflectivity due primarily to the difference in data acquisition fold of the two surveys within the crust distinctive seismic patterns such as dominant westerly dipping reflections on the western end of the profile and a 3 s depression in the moho beneath a crustal scale culmination are clearly visible on both sections the advantages of the dynamite source include deeper signal penetration and mapping of a number of previously unrecognized subcrustal reflections moreover the dynamite data also indicate that diffraction patterns detected at lower crustal and moho depths have large apertures and extend to greater two way traveltimes than were recorded in the vibroseis data thus proper migration of lower crustal reflections requires recording greater traveltimes than were recorded during the 1991 lithoprobe vibroseis acquisition program

evolution of proterozoic terrains beneath the williston basin a new tectonic map of the precambrian basement beneath the phanerozoic williston basin of central north america shows the archean superior craton separated from the archean churchill and wyoming cratons by a broad region of mostly proterozoic origin included in the proterozoic terrain are the vestiges of two or more volcanic island arcs and associated fore arc and back arc basins the north american central plains electrical conductor and related seismic low velocity zones are explained by the presence of hydrated oceanic type crustal material within the remnant back arc basins and adjoining terrains using geological geophysical and geochronological data from the williston basin and exposed canadian shield we derive a plausible plate tectonic model for the proterozoic evolution of this region

one and two dimensional inversion of magnetotelluric data in continental regions abstract for a preliminary inversion of magnetotelluric mt data obtained in continental regions it is common practice to assume initially that the underlying structure is one dimensional id so that one of the readily available routines for simple id inversion may be applied if the continental structure is actually 2d as is often the case such approximate inversions may lead to different conclusions depending on whether transverse electric te or transverse magnetic tm data are used and for this reason rotationally invariant impedances the berdichevsky average or the effective impedance are sometimes preferred in practice when the composite 2d model obtained by applying these inversions at different sites in an array is used as a starting model for optimizing the fit between model and observed responses it is desirable that all significant structural features are included in it with this is mind the effectiveness of the te tm and rotationally invariant impedances in yielding meaningful 1d inversions in a 2d environment is tested by studying various basic models horst resistive and conductive blocks alone and together surface conducting edge and conducting sill with the aid of a new 2d finite difference program and a recently developed automatic scheme for 1d inversion the paper concludes with the construction of a 2d structure from the real coprod2r data using as a starting model the layered models obtained from separate 1d inversions at various mt sites in the array and then optimizing the least squares fit of both the te and tm model responses to the observed responses by adjusting the parameters of the initial model the north american central plains anomaly beneath a shallow sedimentary cover is identified by this procedure in the form of two upright and adjacent conductive blocks this differs somewhat from some previously proposed models but yields a good fit to the apparent resistivity and phase responses at 20 sites

a new global crustal thickness map an up to date global compilation of crustal thickness and pn velocity values from published seismic refraction and surface wave data has been created from 297 referenced sources a total of 2508 crustal thickness and 1806 pn velocity values have been selected this represents a significant expansion of previous compilations the inclusion of surface wave data results in a more uniform coverage of the earth s surface than is possible by using only published refraction data the crustal thickness data are contoured on a global van der grinten projection this compilation currently resides in a computer data base and is available on computer compatible tape recommendations for improvement in form and content will be welcomed as periodic updates are anticipated

geological development origin and energy mineral resources of williston basin north dakota the williston basin of north dakota montana south dakota and south central canada manitoba and saskatchewan is a major producer of oil and gas lignite and potash located on the western periphery of the phanerozoic north american craton the williston basin has undergone only relatively mild tectonic distortion during phanerozoic time this distortion is largely related to movement of precambrian basement blocks sedimentary rocks of cratonic sequences sauk through tejas are present in the basin sauk cambrian lower ordovician tippecanoe ordovician silurian and kaskaskia devonian mississippian sequence rocks are largely carbonate as are the major oil and gas producing formations absaroka pennsylvanian triassic and zuni jurassic tertiary rocks have more clastic content but carbonates are locally important clastics of the zuni sequence fort union group contain abundant lignite tejas tertiary quaternary sequence rocks are not significant in the production of minerals or energy although glacial sediments cover much of the region oil exploration and development in the united states portion of the williston basin since 1972 have given impetus to restudy basin evolution and geologic controls for energy resource locations consequently oil production in north dakota for instance has jumped from a nadir of 19 million bbl in 1974 compared to a previous zenith of 27 million in 1966 to 32 million bbl in 1979 and 40 million bbl in 1980 geologic knowledge of carbonate reservoirs has expanded accordingly depositional environments throughout sauk tippecanoe and kaskaskia deposition were largely shallow marine subtidal and even basinal environments were developed in the basin center but sabkha deposits were abundant near the basin periphery evidence of subaerial weathering was commonly preserved in structurally high areas and on the basin periphery especially in upper kaskaskia rocks some pinnacle reefs were developed in kaskaskia deposition morphologically similar to the silurian pinnacle reefs of the michigan basin clastic sediments were transported into the southern part of the basin during absaroka sequence deposition a product of erosion of ancestral rocky mountain orogenic structures continental and shallow marine clastic sediments were deposited during zuni sedimentation until cretaceous deeper marine environments were established laramide orogenesis to the west provided detritus that was deposited in fluvial deltaic and marginal marine environments regressing to the east major lignite deposits are part of this postorogenic regressive rock body major structures in the basin and the basin itself may result from left lateral shear along the colorado wyoming and fromberg zones during pre phanerozoic time deeper drilling in the basin has revealed several major structures and given indications of others most structures probably resulted from renewed movement or tensing of pre phanerozoic faults meteorite impact events have been suggested as the origin for one or more structures

seismic structure of the crust and upper mantle in the peace river arch region canada four intersecting 300 km long reversed refraction lines within northwestern alberta and northeastern british columbia have been interpreted for crustal and upper mantle seismic velocity structure in the peace river arch pra region of the western canada sedimentary basin the data have been modeled using a two dimensional ray trace forward modeling approach based on asymptotic ray theory to match travel times and amplitudes of first and coherent later arrivals in addition an inversion of first arrival travel times along a fan shot profile has been performed to constrain crustal thickness northwest of the arch in a region not sampled by the reversed profiles the major features of the structural models are 1 weak to moderate lateral variations in crustal structure with no evidence of significant layering or thick low velocity zones within the crust 2 an average sub basement rms crustal velocity of 6 6 km s average upper mantle velocity of 8 25 km s and average crustal thickness of 40 km 3 regional variations in structure which appear related to the dominant n s trending cratonic structure including crustal thickness upper crustal and upper mantle velocities and pmp character and 4 subtle variations in structure that may be associated with the e w trending devonian axis of the pra including a shallowing of high lower crustal velocities thickening of the crust and an anisotropic pmp character beneath the arch and a weak trend in the dip of intracrustal reflectors away from the center of the pra region evidence from the refraction and other geophysical data for the presence of a local crustal expression of the pra is weak but suggests a thermal as opposed to flexural origin for its anomalous vertical movements

geoelectric structure of the northeastern williston basin and underlying precambrian lithosphereearth science sector ess contribution 20080509 magnetotelluric mt measurements were made in southern manitoba canada as part of the portable observatories for lithosphere analysis and research investigating seismicity polaris project to image the northeastern part of the williston basin and underlying precambrian lithosphere data collected at 21 sites along a 400 km east west profile at 49 5 n and a 300 km north south profile at 100 w were analyzed using robust spectral analysis tensor decomposition and two dimensional inversion the resulting resistivity models allow subdivision of the williston basin into three layers an upper layer of 1 5 m corresponding to mesozoic and upper paleozoic rocks a 20 50 m layer corresponding to lower paleozoic carbonate rocks and a 2 3 m layer corresponding to the ordovician winnipeg formation deeper penetrating mt responses interpreted with other mt data reveal a region in the westernmost superior craton with a southwest northeast geoelectric fabric that is oblique to subprovince boundaries the

an initial analysis of the earth s crust under the williston basin 979 cocrust experiment four 300 km long refraction profiles from western manitoba and eastern saskatchewan were shot over the canadian portion of the williston basin on the basis of three distinct seismic velocity intervals 5 95 6 50 6 60 6 75 and 7 00 7 61 km s the crust in the nelson front and the churchill tectonic province can be vertically separated into three distinct zones the crust thickens from east to west reaching a maximum of more than 50 km in the central portion of the basin the upper mantle velocity increases from 7 84 km s in the east to 8 47 km s in central saskatchewan along the eastern flank of the basin in the transition zone between the churchill and superior tectonic provinces extensive structural variations are interpreted as major faults which appear to disturb the entire crust this thick high velocity crust beneath the williston basin indicates that it is not of extensional origin

crustal refraction surveys across the trans hudson orogen williston basin of south central canada eight reversed seismic refraction profiles have been recorded across the phanerozoic williston sedimentary basin that overlies the margins of the superior and wyoming archean cratons and the proterozoic trans hudson orogen these data have been modeled using two dimensional ray tracing techniques to match the times and amplitudes of primary and coherent secondary arrivals within the southern extension of the superior craton margin thompson belt a high velocity 6 4 km s lid overlies a narrow region of high electrical conductivity and a zone of westerly dipping reflections is nearly coincident with a weak velocity discontinuity that seems to delineate the western edge thompson fault of the craton margin complex zones of low velocity material and the north american central plains electrical conductivity anomaly are concluded to lie within the southern extension of the reindeer south indian lakes terrain a component of the trans hudson orogen a high velocity 7 0 km s lower crustal layer which represents a zone of complexity separating normal crust from the upper mantle extends throughout the region the crust is relatively thick with depths to moho varying smoothly between 41 and 48 km there is no evidence in the relatively low resolution seismic refraction data for abrupt changes in the level of the moho but there may be a minor 3 4 km thinning of the crust near the center of the williston basin

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

lithospheric structure of the trans hudson orogen from seismic refraction wide angle reflection studies the trans hudson orogen tho is the world s largest paleoproterozoic orogenic belt data from three refraction profiles are used to investigate its lithospheric structure in saskatchewan and manitoba r1 crosses the orogen from the hearne craton on the west to the superior craton on the east r2 and r3 are along the orogen p wave velocity structural models are generated using a ray based technique on line r1 higher crustal velocities in its eastern part coincide with rocks of the flin flon namew gneiss complex depth to moho is in the 40 45 km range and equates to that from the reflection data including a small crustal root below the sask minicontinent along lines r2 and r3 depth to moho varies from about 40 km up to 55 km at the north end of r2 and south end of r3 in general variations in crustal velocity and depth to moho do not correlate with the location and extent of geological domains they appear to reflect the complex deformation and metamorphic history of the crustal rocks mantle velo

the north american central plains conductivity anomaly and its correlation with gravity magnetic seismic and heat flow data in saskatchewan canada abstract the north american central plains conductivity anomaly nacp lies virtually in its entirety within the trans hudson orogen accordingly should these two features prove to be contemporaneous then the geometrical relationship between these two is of foremost importance to any evolutionary tectonic model proposed to explain the collision of the superior and churchill provinces in the hudsonian and a model that does not include a mechanism for the generation of this anomaly is obviously untenable in order to map better the trend of the nacp in the province of saskatchewan canada two magnetotelluric mt profiles were conducted over the nacp previously defined by magnetometer arrays and profiles data from these two profiles along with an earlier mt profile just north of the u s canadian border suggest that the nacp is not a continuous feature but rather that it exhibits a definite break at latitude 51 n other geophysical evidence examined herein is concordant with this characteristic if a second mt anomaly mapped to the northwest of this break is in fact a manifestation of the same geological structure then one possible interpretation is of a major nw se trending sinistral fault in the deep crust previously undetected with a movement of some 100 150 km along strike the mt data from the southernmost profile after correction for static shift are modelled in a 2d manner and it is shown that the nacp is consistent with an arcuate structure in vertical section of high conductivity 2 s m 1 beginning at a depth of 10 km centred on 103 w dipping down to the west reaching possibly the base of the crust it is also shown that such an arcuate shape in section can explain an observed gravity high of 40 mgal such high conductivities cannot be explained in terms of connected fluids as it would require implausibly high porosities 12 20 a comprehensive interpretation of all the geophysical data would have to account for the observed characteristics of the anomalous region which are high electrical conductivity positive density contrast no magnetization and high heat flow in the basement in terms of a single causative body one possible explanation is presented in terms of a zone of lithospheric weakness along which was emplaced low density differentia from a mantle derived body an alternative explanation could be the phase transformation of obducted material into eclogite or serpentinite however difficulties exist with both of these explanations and further data are required

joint inversion of rayleigh wave phase velocity and ellipticity using usarray constraining velocity and density structure in the upper crust rayleigh wave ellipticity or h v ratio observed on the surface is particularly sensitive to shallow earth structure in this study we jointly invert measurements of rayleigh wave h v ratio and phase velocity between 24 100 and 8 100 sec period respectively for crust and upper mantle structure beneath more than 1000 usarray stations covering the western united states upper crustal structure in particular is better constrained by the joint inversion compared to inversions based on phase velocities alone in addition to imaging vs structure we show that the joint inversion can be used to constrain vp vs and density in the upper crust new images of uppermost crustal structure 3 km depth are in excellent agreement with known surface features with pronounced low vs low density and high vp vs anomalies imaged in the locations of several major sedimentary basins including the williston powder river green river denver and san juan basins these results demonstrate not only the consistency of broadband h v ratios and phase velocity measurements but also that their complementary sensitivities have the potential to resolve density and vp vs variations

mantle involvement in lithospheric collision seismic evidence from the trans hudson orogen western canada three seismic refraction wide angle reflection profiles were recorded over the trans hudson orogen the profiles display high quality coherent seismic signals associated with the moho and lithospheric mantle to depths in excess of 160 km as outlined by wide angle reflections depth to moho varies between 40 54 km with a number of well defined structural reliefs immediately beneath the moho in the central part of the orogen an anomalous high velocity zone 8 45 km s with lateral dimensions of 100 km north south by 130 km east west and maximum thickness of 50 km was delineated by mantle refraction arrivals this zone may represent a locally preserved remnant of older mantle from micro continental collisions mantle discontinuities with some structural relief are interpreted at average depths of 75 and 158 km based on wide angle reflections at offsets beyond 400 km they bound a layered zone with variable velocities probably representing imbricated sequences of peridotite and eclogite we postulate that these unusual features of the upper mantle are the result of lithospheric convergence of bounding archean cratons and closure of the intervening ocean basin

carrier phase ambiguity resolution for the global positioning system applied to geodetic baselines up to 2000 km the global positioning system gps carrier phase data are biased by an integer number of cycles a successful strategy has been developed and demonstrated for resolving these integer ambiguities for geodetic baselines of up to 2000 km in length resulting in a factor of 3 improvement in baseline accuracy and giving centimeter level agreement with coordinates inferred by very long baseline interferometry in the western united states for this experiment a method using pseudorange data is shown to be more reliable than one using ionospheric constraints for baselines longer than 200 km an automated algorithm exploits the correlations between the many phase biases of a gps receiver network to enable the resolution of ambiguities for very long baselines a method called bias optimizing has been developed which unlike traditional bias fixing does not require an arbitrary confidence test bias optimizing is expected to be preferable to bias fixing for poorly configured networks in order to enable ambiguity resolution for long baselines it is recommended that future gps networks have a wide spectrum of baseline lengths ranging from 1000 km and that gps receivers be used which can acquire dual frequency p code data

seismic strain rates in regions of distributed continental deformation we compare the rates and configurations of seismic deformation determined from moment tensor mechanisms of earthquakes in five continental regions with the rates and configurations expected if the seismicity reflected the relative plate motions in the regions the five regions the aegean iran california western new guinea and central asia are characterized by high levels of seismicity distributed over hundreds to thousands of kilometers from the accepted plate boundaries the earthquake mechanisms are compiled primarily from the catalog of centroid moment tensor solutions which is nearly complete over the interval 1977 1987 for earthquakes greater than ms 5 the thickness of the seismogenic layer in each region is constrained by the results of modeling broadband body wave phases and of previous earthquake studies in the aegean california and western new guinea the observed configurations of deformation agree with a simple model of distributed accommodation of relative plate motion in iran and central asia the moment tensors indicate that there is appreciable strain parallel to the strikes of the belts we augment our 10 year estimates of seismic deformation with the available historical moment and magnitude estimates for the five regions and find that moderate and large earthquakes account for between 25 and 70 of the deformation of the seismogenic layer

trans hudson orogen and williston basin in montana and north dakota new cocorp deep profiling results cocorp consortium for continental reflection profiling deep reflection profiling across the williston basin and underlying trans hudson orogen shows the following 1 there is no evidence for a precursor rift basin beneath the axis of the williston basin with the exception of small scale structures e g nesson and cedar creek anticlines the basement surface beneath the williston basin is smooth and has a dish shaped geometry 2 crustal penetrating reflections within the eastern trans hudson orogen dip eastward in apparent contradiction to the conventional view that the superior province underthrusts the eastern side of the orogen 3 basement reflections beneath the center of the orogen define a broad crustal scale antiform the axis of which coincides approximately with the depocenter of the overlying williston basin midcrustal reflections defining the crest and western flank of this antiform also coincide generally with the location and shape of the source of the north american central plains conductivity anomaly 4 crustal penetrating reflections beneath the western side of the orogen dip westward supporting the current view that subduction polarity was to the west beneath the wyoming province 5 the eastern edge of the wyoming province is marked by east dipping upper crustal reflections and subhorizontal lower crustal reflections that are truncated by the west dipping reflectivity of the trans hudson orogen

united states crustal thickness abstract the thickness of the crust the thickness of the basal intermediate or lower crustal layer and the average velocity at the top of the mantle p n have been mapped using all available deep penetrating seismic refraction profiles in the conterminous united states and surrounding border areas these profiles are indexed to their literature data sources the more significant long wavelength anomalies on the three maps are briefly discussed and analyzed an attempt to use bouguer gravity to validate mantle structure was inconclusive

structure of the lithospheric mantle beneath the trans hudson orogen canada abstract refraction and wide angle reflection seismic data collected in northern saskatchewan and manitoba in 1993 by lithoprobe provide new glimpses of the deep structure of the lithospheric mantle beneath the trans hudson orogen velocity models obtained through ray tracing modelling techniques revealed the presence of complicated mantle structures down to a depth of 160 km beneath the orogen like the crust the lithospheric mantle appears to show a seismically transparent upper and a highly reflective lower part suggesting that the rheological environment responsible for development of reflections in the lithospheric mantle and of that in the crust are similar this appears to be consistent with the observation that the strong and brittle upper 30 km of the mantle contain remnants of former tectonic processes in a form of velocity anisotropy and the lower more ductile zone seems highly reflective and isotropic the suspected beginning of the lithosphere asthenosphere transition zone at a depth of 160 km imaged as gradual disappearance of mantle reflections coincides well with models established using other geophysical methods

receiver function studies in the trans hudson orogen saskatchewan teleseismic events were recorded on an array of three component broadband and short period seismographs in the trans hudson orogen over a period of almost 3 years receiver functions calculated from these data were used in a forward modelling study to derive the local shear wave velocity structure beneath 20 stations station ffc flin flon located within the flin flon belt was one of two stations situated on bedrock and produced the highest quality receiver functions the ffc velocity model correlates well with an interpretation of nearby reflection data and features a distinct lower crust interpreted as archean basement which cores the entire reindeer zone both radial and tangential receiver functions at ffc show strong evidence for a northeasterly dipping shallow boundary within rocks of the flin flon belt with a dip angle of 20 total crustal thickness is 37 km eighteen stations are situated on phanerozoic sedimentary cover ranging in thickness from 0 4 km to greater than 2 km receiver function

experimental high pressure granulites some applications to natural mafic xenolith suites and archean granulite terranes high pressure granulite assemblages have been produced as residues in partial melting experiments on a natural alkalic basalt amphibolite between pressures of 12 and 18 kbar in particular at 18 kbar partial melting of the hornblende plagioclase quartz assemblage under fluid absent conditions produces garnet clinopyroxene new albitic plagioclase melt seismic velocities v p are estimated from the modal data for the experimental assemblages and range from 6 90 km s for hornblende bearing residues to 7 62 for the dominantly garnet clinopyroxene residues these values are typical for rock types in the lowermost crust transitional to mantle the experimental results help place additional pressure temperature a h 2 o constraints on the source region for the natural high pressure granulites the experimental residue assemblage formed at 18 kbar has been described in natural xenolith suites from the delegate pipes in australia where the pipes intrude upper ordovician and lower devonian continental crust and in the bearpaw mountains montana where the xenolith bearing magmas intrude older crust of archean age the combination of these data shows that the xenoliths may indeed represent lowermost continental crust and furthermore helps interpret the nature of the crust mantle boundary in these areas the delegate pipes xenoliths suggest that the crust mantle boundary may be the site for partial melting and assimilation whereas the bearpaw mountains samples indicate that magmatic underplating may have been a major process in generating thick continental archean crust the experimental data and bearpaw mountains xenoliths suggest that the large range of rock types found in archean granulite terranes may not be representative of the lowermost continental crust

precambrian basement geology of north and south dakota combined analysis of drill hole gravity and magnetic data indicates that the buried precambrian basement rocks of the dakotas can be divided into several lithotectonic terranes eastern north dakota and northeastern south dakota are underlain by archean gneiss except for the black hills region of south dakota where archean rocks are also exposed the western third of both dakotas is underlain mainly by early proterozoic gneiss and metasedimentary rocks part of this region is underlain by archean crust with an early proterozoic tectonic overprint a broad transition zone of strongly overprinted archean crust occurs between the proterozoic rocks to the west and the archean rocks to the east south central south dakota is underlain by an early proterozoic batholith early proterozoic felsic volcanic rocks occur in southeast south dakota the bootheel portion of south dakota contains a diverse assemblage of basement rocks that are partly archean in age churchill province rocks of the trans hudson foldbel

origin and epeirogenic history of the williston basin evidence from fission track analysis of apatite we have tested the hypothesis that the lithosphere beneath the williston basin was heated uplifted and eroded prior to basin subsidence this was accomplished by fission track dating of apatite from samples of basement rock beneath and adjacent to the basin apatite fission track ages of basement rocks beneath the basin range from about 4 ma near the center to 554 ma near the eastern edge combined with stratigraphic evidence the ages suggest that at least 3 km of basement were eroded prior to subsidence of the williston basin rates of apparent uplift and erosion were on the order of 0 10 to 0 15 km m y apatite fission track ages of basement outcrop samples on the eastern flank of the basin range from about 572 ma to 441 ma these ages suggest that erosion of the flank occurred from the early cambrian to the late ordovician we conclude that timing of uplift of the basement prior to subsidence of the williston basin is consistent with the thermal origin hypothesis of basin formation

the 1982 cocrust seismic experiment across the ottawa bonnechere graben and grenville front in ontario and quebec summary during the summer of 1982 the canadian consortium for crustal reconnaissance using seismic techniques cocrust conducted a major long range seismic refraction and wide angle reflection experiment across the grenville province of the canadian shield three seismic lines each approximately 300 km in length were located i along the ottawa bonnechere graben ii perpendicular to the graben and iii perpendicular to the crenville front geological evidence indicates that the graben may have originated from a failed arm of the st lawrence rift system and the grenville front marks the boundary between the grenville province and the much older superior province other geological features of the grenville province that were traversed by the profiles were the central metasedimentary belt and the central gneiss belt analysis of the data involved conventional traveltime procedures coupled with the use of synthetic seismogram analysis using programs that were written to handle laterally heterogeneous structures results from the survey indicate a variation in near surface seismic velocity

seismic structure of the continental crust based on rock velocity measurements from the kapuskasing uplift the michipicoten greenstone belt mgb wawa gneiss terrane wgt and kapuskasing structural zone ksz in ontario canada are regarded as a 25 km thick partial cross section through the crust of the superior province we measured compressional wave velocities vp at confining pressures up to 600 mpa for representative rock samples from these terranes vp for mgb is variable 6 1 7 km s and depends on composition and anisotropy compressional wave velocities for low anisotropy quartzofeldspathic gneisses and intrusive rocks from the wgt and the ksz show little variation over a wide range of silica content but mafic gneisses and anorthositic rocks show a wide velocity range over a narrower silica range a trend corroborated by data for other high grade rocks a model of the continental crust based on the laboratory data shows three megalayers with differing seismic characteristics the shallowest levels mgb are seismically heterogeneous with high average velocities 6 6 km s at 100 mpa because of the dominance of metabasalts the balance of the upper crust wgt is seismically homogeneous as it is dominated by a variety of isotropic quartzofeldspathic rocks with average velocities of 6 35 km s at 100 mpa and 6 6 km s at 600 mpa the uppermost lower crust ksz is layered seismically heterogeneous and transversely isotropic the average velocity at 600 mpa is 6 84 km s the boundary between wgt and ksz is marked by a velocity increase of between 0 2 and 0 3 km s an increase similar to that commonly associated with midcrustal discontinuities i e conrad in shield areas here this boundary corresponds to the amphibolite granulite facies transition and to a change from a tonalitic middle crust to a lithologically heterogeneous uppermost lower crust refraction results show an anomalous high velocity zone in the upper crust under the ksz that on the basis of laboratory velocity data can be correlated with ksz lithologies these data indicate that ksz rocks can be traced to the west to depths as great as 20 km levels deeper than 20 km not exposed at the surface in the ksz are characterized by velocities greater than 7 0 km s and may be dominated by mafic and anorthositic lithologies or by more garnet rich rocks the velocity model of the superior province crust based on geological reconstruction and laboratory velocity data is in general agreement with the velocity structure to depths of about 25 km determined for the crust below the mgb from refraction experiments the velocity structure of this model is also generally similar to the velocity structure of shield areas

a gap in the north american central plains conductivity anomaly abstract the palaeotectonic models of the north american continent have been deeply influenced by the concept of a north american central plains conductivity anomaly nacpca which was postulated as a result of large area wide spaced array studies using the magnetic deep sounding mds method a recent closely spaced magnetotelluric mt survey across this proposed anomaly near the canadian u s boundary by jones and savage 1986 failed to detect the nacpca at the location suggested by the mds however this survey did verify a conductivity anomaly discovered by rankin and kao 1979 and a new anomaly 75 km east of nacpca our present study verifies this latter anomaly and quantitative results are presented the striking isotropy in apparent resistivity does not prevent the interpretation of anomalous resistivity distributions this is discussed in the text the failure of the mds array studies to detect our anomaly is readily explainable by the wide station spacing however the failure of the mt survey to detect the nacpca leads us to the conclusion that it may not be a continuous feature

the subsidence of the williston basin a detailed comparison between the pre cretaceous stratigraphic record in the williston basin and the available information about phanerozoic sea level changes indicates that most stratigraphic breaks in the basin reflect eustatic control the basin appears to have been subsiding continuously and steadily about the same centre for most of the phanerozoic above a thick 55 km continental crust deposition kept up with subsidence and most of the sedimentary record was laid down close to sea level erosional breaks in the stratigraphic record may represent periods of removal of strata during episodes of low sea level a possible explanation of this essentially steady nonexponential subsidence record is that a mafic subcrustal body is undergoing transformation to eclogite

novel type of crust produced during continental rifting a normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments the magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs the sediments are themselves metamorphosed by the heat flux from the underlying mantle a metasedimentary crust is formed in direct contact with an abnormal mantle vp 7 5 km s l produced by partial serpentinization

least squares inversion of spatial seismic refraction data a method is demonstrated for recording and inverting in line and broadside seismic refraction and reflection data to obtain three dimensional structure and velocity information this technique of spatial seismic refraction recording produces a superabundance of common ray intersections at the target horizon the full power of the linear least squares inversion method may be used in the overconstrained case a large number of recording instruments are required to perform the experiment efficiently but this has the advantage of reducing the number of sources and difficult source corrections a damped least squares inversion method has been found to give rapid convergence even in the case of complex faulted models using noisy data the spatial seismic refraction recording method has been applied in a suboptimal experiment over the williston basin in southern saskatchewan the crustal structure consists of several normally faulted blocks defined by aeromagnetic anomalies over an area notable for its thick crust local seismicity and a linear conductive anomaly

the complexity of the crust from refraction wide angle reflection data thepmp wide angle reflected signal from the moho shows a wide variation in its characteristics when cornpared from region to region if the earth s crust is simple and homogeneous and the moho is a sharp discontinuity supercriticalpmp wavelets are large and isolated in the 100 km to 300 km distance range when compared to the preceding signals on the seismic traces if the crust itself has numerous short reflectors and the moho transition zone is thick or badly disrupted from past tectonic disturbances thenpmp is often poorly defined and difficult to identify in the coda of earlier arriving signals these signals are poorly correlated because the reflectors themselves are in general not only discontinuous but tend to be distributed at various depths within the crust the effect of the vertical velocity gradient in the crust is to make the reflective field for downgoing waves much greater than for upgoing waves the large variations in reflection coefficients wiht angle of incidence has the effect of making the coda generated by a reflective machanism distance dependent the reflected signals are also contaminated with scattered signals from smaller scale heterogeneities which may be distributed more uniformly within the crust