crustal velocity structure of the omineca belt southeastern canadian cordillera travel time inversion and amplitude modeling of a 350 km lithoprobe seismic refraction wide angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the omineca belt of the canadian cordillera the upper crust to 12 18 km depth has velocities from 5 6 to 6 2 km s 1 and two shear zones the monashee decollement and gwillim creek shear zone are imaged by the wide angle reflections and velocity trends minor velocity differences on either side of the monashee decollement may be related to separate rock origins prominent reflections define the boundaries of a low velocity midcrustal layer from 10 15 km to 20 25 km depth with velocities less than 6 1 km s 1 the low velocities of the midcrust associated with high electrical conductivities and high heat flow may be considered as support for the hypothesis of fluids in the cordilleran crust though other possibilities such as the effect of high temperatures on rock velocities are possible in the lower crust velocities range from 6 4 6 5 km s 1 at the top of the lower crust to 6 6 6 8 km s 1 at its base the moho is very clearly defined by the refraction wide angle reflection data and has a gentle southerly dip crustal thicknesses are 35 37 km a thin crust mantle transition zone of 1 2 km thickness in which velocities vary between 7 6 and 7 7 km s 1 is consistent with coincident reflection data upper mantle velocities range from 7 9 to 8 1 km s 1 with indications from the data of upper mantle layering in comparison with neighboring regions the omineca belt has an anomalously thin crust low crustal velocities and a low velocity upper mantle similar only to the basin and range province the velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method the characteristics of a thin crust and lithosphere along with low velocities from midcrust to mantle suggests that both the basin and range and the southern canadian cordillera are currently being heated from a source within the mantle

crustal structure and tectonics of the southeastern canadian cordillera the crustal structure of the omineca ob and foreland fb belts of the southeastern canadian cordillera are interpreted from the inversion of seismic refraction reflection travel times and amplitudes and modeling of the bouguer gravity data from a 350 km east west wide angle profile the main features of the resultant velocity and density models include 1 a low average crustal velocity of 6 2 km s 2 variable upper crustal velocities 5 6 6 3 km s within the fb and purcell anticlinorium as compared to farther west in the ob 6 1 6 2 km s 3 a midcrustal 20 km depth 0 4 0 5 km s velocity increase within the ob 4 eastward crustal thickening from 35 to 42 km over 80 km distance beneath the ob fb boundary 5 the slocan lake fault slf dipping east at 15 20 to at least 35 km depth 6 decreased lower crustal velocities 6 7 6 4 km s across the slf from the ob into the fb 7 increased uppermost mantle velocities 7 9 8 0 km s from the ob into the fb and 8 an average crustal density increase of 40 kg m3 from the ob into the fb lower velocities higher conductances and increased densities within the upper crust 0 10 km depth of the purcell anticlinorium appear to be associated with middle and upper proterozoic rift related metasediments and gabbroic dikes 35 55 by volume in contrast to the mainly mesozoic felsic intrusives and metamorphic rocks that dominate farther west in the ob elevated crustal temperatures within the ob a high heat flow province are responsible for low velocity gradients to 20 km depth a more mafic lower crust beneath the ob inferred from higher velocities and the westward decrease in crustal thickness are interpreted as resulting primarily from middle and or late proterozoic extension and rifting of the craton penetration of the slf into the lower crust may be controlled by lateral strength contrasts at the edge of the craton farther west a midcrustal strength contrast a velocity boundary at 20 25 km depth may have acted as a regional detachment zone during compressional and extensional tectonic episodes

seismic structure of the central metasedimentary belt southern grenville province crust and upper mantle structure interpreted from wide angle seismic data along a 260 km profile across the central metasedimentary belt of the southern grenville province in ontario and new york state shows i relatively high average crustal and uppermost mantle velocities of 6 8 and 8 3 km s respectively ii east dipping reflectors extending to 24 km depth in the central metasedimentary belt iii weak lateral velocity variations beneath 5 km iv a mid crustal boundary at 27 km depth and v a depth to moho of 43 46 km the wide angle model is generally consistent with the vertical incidence reflectivity of an intersecting lithoprobe reflection line the mid crustal boundary correlates with a crustal detachment zone in the lithoprobe data and the depth extent of east dipping wide angle reflectors regional structure and aeromagnetic anomaly trends support the southwest continuity of grenville terranes and their boundaries from the wide angle profile to two reflection lines in lake ontario a zon

eocene oligocene paleoceanography of the deep bay of biscay benthic foraminiferal evidence abstract a major change in benthic foraminiferal assemblages occurred in the deep bay of biscay 3 km water dsdp sites 119 and site 400a between early middle eocene and earliest oligocene predominant eocene deep sea taxa nuttallides truempyi clinapertina spp abyssamina spp and associated rarer species became extinct in this interval these extinctions were followed by an increase in abundance of bathymetrically wide ranging and stratigraphically long ranging taxa globocassidulina subglobosa oridorsalis spp gyroidinoides spp and the cibicidoides ungerianus plexus the extinctions cannot be dated precisely from the stratigraphic record recovered to date in the bay of biscay however the replacement of the n truempyi dominated assemblage has been noted previously in the deep south atlantic caribbean as occurring near the middle late eocene boundary other than the decrease in abundance and extinction of n truempyi no major abundance changes are noted within the eocene at the shallower site 401 2 km water in the bay of biscay during the oligocene nuttallides umbonifera replaced the eocene species n truempyi as the predominant deep sea benthic foraminifera reaching peak abundance in the middle oligocene at sites 119 and site 400a in the modern oceans the abundance ot n umbonifera is positively correlated with increased corrosiveness of bottom water while at site 119 the abundance of nuttallides spp is negatively correlated with 13c values in benthic foraminifera as lower 13c values are often associated with older water masses large numbers of nuttallides spp are thought to reflect older and more corrosive bottom water the faunal data and oxygen and carbon isotopic data are compared with a circulation model derived from north atlantic seismic stratigraphic studies to show that old warm corrosive and sluggish eocene bottom water was replaced by younger colder less corrosive more vigorously circulating bottom water of northern origin by the early oligocene faunal and isotopic data suggest that bottom water became older and more corrosive again in the middle oligocene reflecting a reduction in circulation that can also be inferred from the seismic record in the nearby rockall plateau region

structure of a paleoproterozoic continent continent collision zone a lithoprobe seismic reflection profile across the trans hudson orogen canada abstract an 800 km reflection seismic profile across the trans hudson orogen northern saskatchewan and manitoba images crustal scale tectonic imbrication in an unprecedented picture of paleoproterozoic crustal accretion and continent continent collisional tectonism the profile is crudely symmetric about a crustal scale culmination in the western part of an accreted juvenile collage reindeer zone geologic and isotopic data suggest that this culmination is cored by microcontinental archean basement west of the culmination highly reflective juvenile crustal elements dip westward into the lower crust beneath the wathaman batholith and archean continental crust of hearne craton to the east strong reflections in the juvenile reindeer zone crust and reworked archean foreland of the thompson belt have eastward dips persisting into the middle crust and extending beneath the superior craton a continuous reflection moho well defined for 500 km in the western part of the profile shows marked relief 12 15 s including a prominent root below the crustal culmination these imaged structures give evidence of substantial crustal shortening and thickening via large scale imbrication consistent with collisional orogeny w dipping structures below the wathaman batholith and reworked hearne craton may reflect subduction polarity in this part of the orogen however geological evidence suggests that e dipping structures below superior craton are largely related to late post collisional deformation rather than to prior oceanic subduction polarity

the hsi bright reflector further evidence for extensive magmatism in the precambrian of western canada abstract seismic reflection lines acquired in 1995 as part of lithoprobe canada s national geoscientific research program revealed unusually bright reflectors of extensive lateral scale in the archaean medicine hat block of southern alberta the herein named head smashed in hsi reflector is imaged over an area of at least 6000 km 2 in the upper half of the crust the reflector appears as a mostly horizontal to subhorizontal feature although continuous it has a complex and laterally variable internal structure comparison with a reflector of known impedance contrast in the overlying western canadian sedimentary basin indicates a high impedance structure to cause the reflections dense one dimensional modelling is consistent with the reflector being a multilayered structure with layer thicknesses in the range of tens to hundreds of metres and lateral scales of discrete reflecting elements between tens of metres and hundreds of metres polarity constraints overall geometry and reflection character strongly suggest that mafic ultramafic horizontal sheets are the cause of hsi reflectivity the hsi is the third major sequence of strong crustal reflectivity imaged by lithoprobe deep seismic data in the precambrian crust of western canada all three features are very similar in overall geometry reflector strength and character and have been interpreted as horizontal sheetlike intrusions small scale variations of the reflectivity signatures suggest a pronounced rheological heterogeneity in the upper crustal rocks within the host tectonic domains

tectonic entrapment and its role in the evolution of continental lithosphere an example from the precambrian of western canada new geophysical data acquired over the buried crystalline basement of western canada provide constraints on the history of tectonic assembly of the western canadian shield in the interval 1 75 1 85 ga specifically these data provide new perspectives on the evolution of an archean continental fragment heame province that was trapped in a tectonic vise between coeval orogenic belts that dipped beneath the hearne province the trans hudson orogen developed along the southeastern margin of the hearne province as a series of ocean floor oceanic arc and arc marginal basins were telescoped and thrust obliquely beneath the hearne along the northwest edge of the hearne collapse and subduction of a narrow marginal basin now marked by the subsurface extension of the snowbird tectonic zone led to formation of magmatic arc and collision of older proterozoic terranes the hearne province itself is characterized by regional granulite grade metamorphism and evidence of extensive and pervasive partial melting of the crust the internal character of the hearne province seen on crustal seismic reflection profiles is that of a crustal scale structural fan with reflection fabrics that verge toward the bounding orogens the deformation of the hearne is predominantly of paleoproterozoic age and constitutes a thorough reworking of this formerly archean crustal domain over a distance of more than 600 km across strike entrapment and thermal weakening of the hearne resulted from mechanical coupling of inferred buoyant subduction collision zones and removal of or modification of archean lithospheric mantle that may have originally formed the keel to the hearne long period magnetotelluric profiles show that anomalously conductive mantle lithosphere underlies the present day hearne province which is attributed to metasomatic modification of the subcontinental lithosphere following collisional thickening and delamination convective removal of thickened lithosphere in the proterozoic tectonic entrapment as illustrated by the proterozoic structural and thermal evolution of the hearne province crust and subcontinental mantle may be an example of the lithospheric consequences of opposing collisional polarity during assembly of continents

crustal history of the rae and hearne provinces southwestern canadian shield saskatchewan constraints from geochronologic and isotopic data abstract the cree lake zone hearne province and western granulite domain rae province in northern saskatchewan are contrasting parts of the variably reworked upper plate western hinterland of the paleoproterozoic trans hudson orogen their junction is defined by the virgin river shear zone a part of the more extensive snowbird tectonic zone subsurface extension of the 2 0 ga thelon orogen lies west of the western granulite domain we present u pb zircon ages sm nd and rb sr isotopic data from the cree lake zone and western granulite domain and from the junction granite which intrudes the virgin river shear zone data from the western granulite domain indicate that prevalent granulite facies metamorphism occurred between 2 3 and 2 0 ga but is imposed on archean protoliths 2 8 3 0 ga old low amphibolite facies retrogression which increases toward the virgin river shear zone probably postdates 2 1 ga data from the cree lake zone also yield archean protolith ages as old as 3 0 ga and some rocks yield interpreted high grade metamorphic overprint ages of 2 3 2 0 ga however two samples yield data indicating hudsonian high grade metamorphic overprint 1 87 to 1 81 ga the junction granite has a u pb zircon age of 1 82 0 30 ga but relict zircons and sm nd data show that it was derived by melting of archean crust high grade mid crustal thermal resetting in the western granulite domain may be coeval with the 1 9 2 0 ga thelon orogen although the data do not preclude an earlier event or events similar 2 3 2 1 ga reworking extends into the cree lake zone but high grade hudsonian 1 87 1 84 ga reworking is also evident in parts of this zone well to the west of the wollaston domain and may extend entirely across the zone to the virgin river shear zone relative importance of the thelon and hudsonian overprints in cree lake zone is unknown data from the junction granite and western granulite domain suggest that all recorded high strain events in the virgin river shear zone postdate 2 3 ga and are thus unrelated to deep crustal archean granulite facies mylonites in the tantato black lake segment of the snowbird tectonic zone north of the athabasca basin

proterozoic tectonic accretion and growth of western laurentia results from lithoprobe studies in northern alberta the western canadian shield of northern alberta is composed of a series of continental slivers that were accreted to the margin of the archean rae hinterland ca 1 9 2 0 ga preserving a unique record of continental evolution for the interval 2 1 2 3 ga this part of laurentia owes its preservation to the accretionary style of tectonic assembly south of the great slave lake shear zone which contrasts with indentation escape processes that dominate the paleoproterozoic record farther north the buffalo head and chinchaga domains form the central core of this region comprising a collage of ca 2325 2045 ma crustal elements formed on an archean microcontinental edifice and similar age crust is preserved as basement to the taltson magmatic zone the distribution of magmatic ages and inferred collision and subduction zone polarity are used to indicate closure of intervening oceanic basins that led to magmatism and emplacement of continental margin arc and collisional belts that formed from ca 1998 to1900

winagami reflection sequence seismic evidence for postcollisional magmatism in the proterozoic of western canada a recent lithoprobe seismic reflection transect in the peace river arch region of the western canada sedimentary basin traverses continental crust that was assembled in a series of magmatic and collisional events during the interval 2 0 1 9 ga the seismic data reveal panels of dipping reflectivity at tectonic domain boundaries but they are otherwise dominated by prominent subhorizontal reflections that occur between 3 5 and 18 5 km depth over an area of at least 120 000 km 2 these reflections here named the winagami reflection sequence overlap several tectonic domains and are comparable in geometry depth and areal distribution to sills and discordant intrusive sheets imaged by previous seismic profiling in the baltic shield and the basin and range province of the western united states as many as five distinct reflections are present some of which can be traced for more than 100 km contrasting reflection character in adjacent tectonic domains suggests an intrusive geometry controlled partly by the mechanical properties of the host rocks seismic modeling indicates that observed waveforms are consistent with reflections from tabular mafic bodies 70 m thick the winagami reflection sequence crosscuts the collisional fabric ca 1890 ma but the sequence is offset by a brittle fault with an estimated age of 1760 ma these timing constraints suggest that the winagami reflection sequence is a previously unknown component of a regional magmatic event emplaced during a period of brittle indentation of the western canadian shield by the slave province

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

crust and upper mantle velocity structure of the intermontane belt southern canadian cordillera as part of the lithoprobe southern cordillera transect seismic refraction data were recorded along a 330 km long strike profile in the intermontane belt an iterative combination of two dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle p wave velocity structure this region is characterized by i a thin near surface layer with large variations in velocity between 2 8 and 5 4 km s and low velocity regions that correlate well with surface expressions of tertiary sedimentary and volcanic rocks ii an upper and middle crust with low average velocity gradient possibly a weak low velocity zone and lateral velocity variations between 6 0 and 6 4 km s iii a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth approximately 8 km thick with average velocities of 6 5 and 6 7 km s at top and base iv a depth to moho as defined by wide angle reflections that aver

gravity patterns and precambrian structure in the north american central plains bouguer gravity patterns revealed in horizontal gradient images are used to locate and examine the tectonic nature of precambrian structural boundaries buried beneath the north american central plains the southern boundary of the superior province west of the midcontinent gravity high mgh interpreted as a collisional suture is relocated as much as 200 km south of previous geophysically defined positions close to a position defined recently by using drill hole information east of the high this boundary is thought to trace a 900 km arc subparallel to the mgh the western boundary of the superior province is regarded as a suture containing significant components of transcurrent faulting the eastern boundary of the wyoming province another suture is traced northward into the churchill province near the northwestern margin of a 1900 1600 ma magmatic belt this boundary and proterozoic terrane to the east are transected by previously unrecognized faults hundreds of kilometres long gravity patterns also suggest that the north american central plains conductivity anomaly may not be a continuous suture related conductor

crustal structure and evolution of the trans hudson orogen results from seismic reflection profiling a 400 km long deep seismic reflection transect across northeastern montana and northern north dakota reveals the crustal scale structural fabric of the early proterozoic trans hudson orogen beneath the williston basin comparison with deep seismic reflection data across the canadian portion of the same orogen 700 km to the north reveals first order similarities in crustal architecture but documents significant along strike variation in orogenic evolution both transects display a broad crustal scale antiform axial to the orogen in the north geologic data suggest that this antiform is cored by an archean microcontinent in the south west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the archean wyoming craton within the wyoming craton the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the early proterozoic trans hudson orogen as interpreted from potential field and drill core data on the basis of subsurface geochronologic data from the crystalline basement and by analogy with the glennie domain within the exposed trans hudson orogen in canada we suggest that the southern antiform is cored by an archean crustal fragment that was caught up in the terminal collision of the wyoming and superior cratons during hudsonian orogeny the eastern side of the trans hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections we interpret the easterly dip of these reflections as evidence that the superior province was thrust westward over the interludes of the orogen during terminal collision although juvenile early proterozoic terranes characterize the exposed segment of the trans hudson orogen in canada limited drill core information within the dakota segment of the orogen shows a predominance of granulitic archean age crust this difference in basement lithologies along strike within the orogen may indicate the following either juvenile crust comparable to that exposed in the northern trans hudson was never present in the south or it was removed by progressive over thickening erosion and or faulting postorogenic extensional collapse may be responsible for preservation of juvenile terranes in the north

deep probe imaging the roots of western north america analysis of the lithoprobe deep probe and southern alberta refraction experiment data sets focusing on the region between deep probe shots 43 and 55 has resulted in a continental scale velocity structural model of the lithosphere of platformal western laurentia reaching depths of 150 km three major lithospheric blocks were investigated i the hearne province a typical continental archean cratonic province lying beneath the western canada sedimentary basin ii the wyoming province an even older block of phanerozoic modified archean crust with an enigmatic lower lithosphere and iii the yavapai mazatzal province proterozoic terranes underlying the colorado plateau and southern rocky mountains in this study the northern two of these regions are investigated with a modified ray theoretical traveltime inversion routine that respects the spherical geometry of the earth the resulting crustal velocity structure combined with supporting geological and geophysical data reveals that the medicine hat block mhb lying between the hearne and wyoming provinces is a third independent archean crustal block the subcrustal lithosphere along the profile is homogeneous in velocity structure but two significant northward dipping reflectors are apparent and interpreted as relic subduction zones associated with sutures between the three archean blocks the hearne crust is typical of an archean shield or platform both in its thickness of 34 50 km and its seismic velocity structure the crust of the archean mhb and wyoming province which ranges in thickness from 49 to 60 km includes a 10 30 km thick high velocity layer interpreted to be proterozoic in age such a feature is unexpected beneath archean crustal provinces but if the region is considered to be the remanent marginal portion of a larger archean continent then the interpreted proterozoic underplating and lack of an archean lithospheric root can be explained the variable topography along the reflective upper and lower boundaries of this layer especially within the mhb suggests considerable variability in its emplacement and subsequent tectonic history

kisseynew metasedimentary gneiss belt trans hudson orogen canada back arc origin and collisional inversion the kisseynew metasedimentary gneiss belt initially developed in a back arc setting the ages of detrital zircons and crosscutting plutons constrain turbidite sedimentation to 1 855 1 841 ga postdating early deformation and metamorphism associated with arc arc and arc continent collisions affecting much of the trans hudson orogen these turbidites are interpreted to have been deposited in a back arc basin behind a retreating subduction boundary the kisseynew domain and its southern flank are marked by 1 84 1 83 ga magmatic rocks that may be related to subduction of back arc basin oceanic crust collapse of the kisseynew basin by ductile fold and thrust deformation began during 1 84 1 83 ga magmatism and continued through peak metamorphism at ca 1 820 1 805 ga from authors

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or

deep seismic profile across a proterozoic collision zone surprises at depth collisions between continents generate the world s most striking mountains the deep structure of these collision zones inferred from seismic reflection profiles1 3 can be equally dramatic if less well understood improved knowledge of the structure of ancient collisional belts now known to have developed through plate tectonic processes4 5 can provide important new insights into the mechanisms of crustal growth although many such belts have been partly destroyed by severe post collisional deformation6 7 or magmatism8 some have been preserved in the stable interiors of continents9 one example is the early proterozoic trans hudson orogen9 10 in western canada here we present seismic reflection images across the entire orogen which reveal a broadly symmetric structure with reflections linked to island arc rocks dipping beneath both bounding archaean cratons the observed reflection geometries imply a doubling of crustal thickness during the collision along crust penetrating faults the presence of a significant amount of archaean crust at depth the extent of which could not have been determined from surface mapping suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11 12

source of the northeastern idaho batholith isotopic evidence for a paleoproterozoic terrane in the northwestern u s the northeastern portion of the idaho batholith nib intruded proterozoic rocks of the belt purcell supergroup between 50 and 90 ma whole rock sm nd isotopic analyses of batholithic rocks yield depleted mantle model ages t dm between 1 72 and 1 93 ga and values of epsilon nd between 17 7 and 21 2 similar to associated metamorphic rocks and within the range for belt purcell sedimentary rocks premagmatic zircons from one sample of the nib were analyzed individually using the shrimp ion microprobe and yielded a single age population at 1 74 ga this apparently single aged source contrasts with the range of ages reported for zircons from sedimentary rocks of the belt purcell supergroup and suggests that the batholith was not the product of melting belt purcell sediments nor was it significantly contaminated with these sediments the source of the batholith however appears to be of appropriate age and composition to be a major contributor of sediment to the belt basin in addition the

rapid warming and ostracods mass extinction at the lower toarcian jurassic of central spain abstract here we present the results of the study of two lower toarcian carbonate sections located in the iberian range of central spain analyses of stable isotope on belemnite calcite allowed calculation of seawater palaeotemperature variations which were compared with the stratigraphical distribution of ostracods these organisms are particularly sensitive to ratios of temperature and salinity variations and hence are good indicators of climate changes from a cooling interval with seawater temperatures of 13 2 c recorded at the pliensbachian toarcian transition seawater temperature began to rise in the lowermost toarcian tenuicostatum zone reaching average temperatures between 14 6 c and 16 3 c during the time of deposition of this zone coinciding with this seawater warming up to 85 of the ostracods species progressively disappeared during a period of approximately 300 kyr marking the extinction interval the extinction boundary located around the tenuicostatum serpentinum zonal boundary coincides with a marked increase in temperature in the serpentinum zone on which average seawater temperatures of 22 c have been calculated warming continued through part of the middle toarcian bifrons zone reaching average temperatures of 24 7 c readjustment of the ostracod population allowed recovery of these faunas in the upper serpentinum zone although the extinction of a major ostracod group the healdioids was also recorded the correlation between mass extinction and warming infers a causal relationship comparison of the results with the records of stable isotopes in belemnites and in bulk carbonates as well as toc and facies analysis suggests that the anoxia linked to the early toarcian oceanic anoxic event was not the main responsible for the ostracod mass extinction

crustal velocity structure from sarex the southern alberta refraction experiment lithoprobe s southern alberta refraction experiment sarex extends 800 km from east central alberta to central montana it was designed to investigate crustal velocity structure of the archean domains underlying the western canada sedimentary basin from north to south sarex crosses the loverna domain of the hearne province the vulcan structure the medicine hat block previously considered part of the hearne province the great falls tectonic zone and the northern wyoming province ten shot points along the profile in canada were recorded on 521 seismographs deployed at 1 km intervals to extend the line an additional 140 seismographs were deployed at intervals of 1 25 2 50 km in montana data interpretation used an iterative application of damped least squares inversion of traveltime picks and forward modeling results show different velocity structures for the major blocks loverna medicine hat and wyoming indicating that each is distinct wavy undulations in the velocity structure of the lov