crustal structure across the northern cordillera yukon territory from seismic wide angle studies omineca belt to intermontane belt a seismic refraction wide angle reflection experiment shot in 1997 in the southern yukon territory crosses the omineca belt which includes the strike slip tintina fault and terminates within the intermontane belt of the northern canadian cordillera lithospheric structure is interpreted from two dimensional forward and inverse modelling of traveltimes combined with forward amplitude modelling and from 2 5 dimensional modelling of gravity data beneath the cassiar terrane and the north america miogeocline average velocities in the upper 20 km of crust are 6 1 km s in the west beneath the accreted cache creek slide mountain and yukon tanana terranes average velocities increase to 6 3 km s in the upper crust the velocity model beneath these terranes thus correlates with more mafic accreted material and not with a subsurface extension of the cassiar terrane the tintina fault is a crustal scale structure across which significant structural differences occur a mid crustal reflector terminates

interpretation of new aeromagnetic anomaly data from the central antarctic peninsula new high resolution aeromagnetic anomaly data from the central antarctic peninsula reveal a magnetic signature typical of a magmatic arc system analysis of the new aeromagnetic map together with susceptibility data and 2 5 dimensional modelling support the interpretation of the pacific margin anomaly as reflecting a mid crustal batholithic province consisting of magnetite rich plutons examination and analysis of the magnetic anomaly map shows variations in magnetic character along the antarctic peninsula these variations in conjunction with geological and other geophysical data sets are interpreted as indicating distinct segments of continental crust of particular importance is the identification of three distinct areas of basement on the eastern side of the antarctic peninsula of which the central area 66 s to 68 appears to be gneissic basement at a relatively high crustal level to the north of 66 s the basement is formed of late palaeozoic triassic accretionary material and to the south of 68 s the basement is unknown segmentation is a common feature of arcs worldwide and previous studies of the antarctic peninsula have linked the continental segmentation to the arrival of a series of ridge crests at the subducting pacific margin this study indicates that the major segmentation of the continental crust is linked to the pre cretaceous crustal structure of the peninsula which was subsequently modified by extension during the cenozoic it is suggested that this cenozoic extension was caused by a reduction in plate convergence rates at two distinct times rather than the arrival of ridge segments at the trench as previously proposed

new geophysical data from the northern cordillera preliminary interpretations and implications for the tectonics and deep geology in this paper we analyze recently acquired geophysical data from the northern cordillera and their relation to the mapped geology a prominent gravity high 45 mgal 1 gal 1 cm s 2 coincides with a magnetic low and an aseismic region in west central yukon where the underlying geology is dominated by quartzo feldspathic rocks having moderate densities extension 15 magmatic underplating and accretion of the anomalous region onto oceanic crust are three possible explanations magnetic gravity and seismicity data all show significant differences in the physical state of the crust on either side of the tintina fault and together with geological data indicating large offset suggest it was once a major crustal scale strike slip fault the new gravity data also delineate an arcuate zone of steep gradients up to 1 4 mgal km in the miogeocline which may correlate with a west dipping proterozoic basement ramp mapped on deep seismic sections farther to the north and a transition from thin east to thick sediment cover west seismicity data show that current tectonic activity is concentrated along the pacific north america plate margin in southwestern yukon and adjacent alaska and although there is a marked decrease in activity inland of this margin notable concentrations occur along the denali fault system and in the eastern miogeocline there is a distinct absence of earthquakes in parts of the selwyn basin and in the northern yukon tanana terrane limited field studies suggest activity is confined to the upper 10 15 km of the crust

global ocean meridional overturning abstract a decade mean global ocean circulation is estimated using inverse techniques incorporating air sea fluxes of heat and freshwater recent hydrographic sections and direct current measurements this information is used to determine mass heat freshwater and other chemical transports and to constrain boundary currents and dense overflows the 18 boxes defined by these sections are divided into 45 isopycnal neutral density layers diapycnal transfers within the boxes are allowed representing advective fluxes and mixing processes air sea fluxes at the surface produce transfers between outcropping layers the model obtains a global overturning circulation consistent with the various observations revealing two global scale meridional circulation cells an upper cell with sinking in the arctic and subarctic regions and upwelling in the southern ocean and a lower cell with sinking around the antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges

the proterozoic fort simpson structural trend in northwestern canada the fort simpson structural trend is a series of north northwest striking middle proterozoic anticlinal culminations buried beneath paleozoic and mesozoic cover in northwestern canada interpretations of seismic reflection profiles coupled with drilling data and outcrop and potential field observations show that the culminations are cored by 1 85 ga crystalline rocks and include overlying sedimentary rocks of the 1 7 1 3 ga hornby bay and dismal lakes groups formation of the structures probably occurred along west dipping thrust faults following deposition of the dismal lakes group strata and prior to deposition of the 0 8 0 7 ga mackenzie mountains supergroup strata that unconformably overlie the dismal lakes rocks this time interval coincides with major east southeast directed thrusting in the same rocks beneath the anderson plains to the north

jets and topography jet transitions and the impact on transport in the antarctic circumpolar current the southern ocean s antarctic circumpolar current acc naturally lends itself to interpretations using a zonally averaged framework yet navigation around steep and complicated bathymetric obstacles suggests that local dynamics may be far removed from those described by zonally symmetric models in this study both observational and numerical results indicate that zonal asymmetries in the form of topography impact global flow structure and transport properties the conclusions are based on a suite of more than 1 5 million virtual drifter trajectories advected using a satellite altimetry derived surface velocity field spanning 17 years the focus is on sites of cross front transport as defined by movement across selected sea surface height contours that correspond to jets along most of the acc cross front exchange is localized in the lee of bathymetric features with more than 75 of crossing events occurring in regions corresponding to only 20 of the acc s zonal extent these observations motivate a series of numerical experiments using a two layer quasigeostrophic model with simple zonally asymmetric topography which often produces transitions in the front structure along the channel significantly regimes occur where the equilibrated number of coherent jets is a function of longitude and transport barriers are not periodic jet reorganization is carried out by eddy flux divergences acting to both accelerate and decelerate the mean flow of the jets eddy kinetic energy is amplified downstream of topography due to increased baroclinicity related to topographic steering the combination of high eddy kinetic energy and recirculation features enhances particle exchange these results stress the complications in developing consistent circumpolar definitions of the acc fronts

abstract lower claiborne regional stratigraphic architecture southeast texas to east central louisiana abstract the lower claiborne section and its units discussed and illustrated here of lutetian age middle eocene figure 1 are recognized as a series of marine shelf marginal marine and deltaic strandline deposits regional closely spaced well log cross sections extending for almost 500 miles from south texas to eastern louisiana updip of the claiborne shelf margins clearly show the regional stratigraphic architecture along depositional strike from southeast central texas through east central louisiana ewing 1994 ewing and vincent 1997 the obvious internal forms when viewing the lower claiborne at such great scale are large very low angle sigmoidal clinoform sets delineated by regional flooding surfaces see figure 2 below the flooding surfaces fs are depicted as the flat to sigmoidal lines seen in that cross sectional diagram major fs s shown by heavier lines lesser fs s by thin and or dashed lines these large downlaps are roughly oriented from nw to se across coastal texas and into central louisiana demonstrating the time transgressive nature of the entire lower claiborne the downlaps proceed from oldest to youngest coming from west to east contrast with perkins and hobday 1980 and fisher 1964 basin analysis and subsurface exploration rely on the identification of genetic units or sequences because they are time bounded units which unite all of the co occurring processes and their resulting deposits genetic stratigraphic units are defined and bounded by key flooding surfaces which approximate time lines genetic sequences of galloway 1989a b these units combining progradational and transgressive elements have the advantage of being bounded by surfaces which on the shelf at least are continuous are easy to recognize have associated biozonations and are verifiably correlatable on well logs unconformities which may be detectable in up dip positions in these dynamic basin margin settings are for all practical purposes difficult to impossible to find in the down dip conformable portions of these units and for simplicity s sake are not shown on the accompanying cross section fig 2

crustal magnetization and long wavelength aeromagnetic anomalies of the minto block quebec measured susceptibilities of 3500 samples from a 500 500 km area of the minto block superior province quebec have an average value of 0 03 si equivalent to induced magnetizations of 1 5 a m charnockitic lithologies igneous orthopyroxene bearing granodiorite granite diorite that constitute large parts of the minto region yield the highest susceptibilities the variations in susceptibility show a good correlation with the variations in the observed magnetic field including those at long wavelengths 100 km suggesting that charnockitic and related granitoid rocks could be responsible for the long wavelength anomalies modeling of a magnetic profile perpendicular to the strike of mapped lithotectonic subprovinces in the minto block supports this contention charnockitic lithologies are not usually considered a possible sources of long wavelength anomalies mafic rock types are generally favored the dominant minto lithologies are more felsic but are more efficient in the production of magnetite from the available iron

first scalar magnetic anomaly map from champ satellite data indicates weak lithospheric field 1 satellite magnetic anomaly maps derived by different techniques from magsat pogo data vary by more than a factor of 2 in the deduced strength of the lithospheric magnetic field here we present a first anomaly map from new champ scalar magnetic field data after subtracting a recent orsted main and external field model we remove remaining unmodeled large scale external contributions from 120 track segments by subtracting a best fitting uniform field in order to preserve n s trending features the data are not filtered along track direct integration of the spherically gridded data yields the final degree 14 65 spherical harmonic expansion of the total intensity anomaly at the mean satellite altitude of 438 km apart from enhanced long wavelength features and a smoother general appearance our initial map is strikingly similar to one of the lower strength magsat pogo maps

the moho as a magnetic boundary revisited we have re examined the question of whether it is generally true that in continental regions a relatively magnetic crust overlies a relatively non magnetic mantle whereas the original conclusion was based on limited data the present study is based on about 400 globally distributed xenolith samples while it is now clear that the moho represents a maximum velocity gradient in a complex transition from crust to mantle and while crustal magnetization is highly variable the original concept of a magnetic crust overlying a non magnetic mantle is strongly supported by the new data a large upper mantle xenolith suite of ultramafic composition is characterized by non magnetic chrome spinels and magnesian ilmenites such rocks are magnetic only where demonstrably altered the most significantly magnetic rocks within the crust are mafic types prograded to granulite grade or mafic melts of basaltic composition which crystallized deep in the crust at high pressure and temperature prograded rocks have characteristic magnetite curie points and are strongly magnetic especially the more mafic compositions while the mafic rocks formed by crystallization at depth have curie points between 400 570 c and are strongly magnetic induced magnetizations are commonly several a m and can readily account for long wavelength magnetic anomalies measured by satellite and aircraft thus there is no need to search for exotic sources for missing crustal magnetization no metal or other peculiar magnetic species have been observed by us in any of our unaltered samples mineral assemblages including magnetic components are consistent with oxidizing conditions close to the fmq buffer

petrology and palaeoenvironmental significance of authigenic iron rich clays carbonates and apatite in the claiborne group middle eocene ne texas the claiborne group eocene in north east texas consists of clayey sandstones and mudrocks both with variable proportions of dark green to brown clay pellets deposited in a marginal to open marine setting on the gulf coast margin the composition of concretionary carbonate and clay pellets from 5 localities has been investigated using a wide variety of analytical techniques our palaeontological sedimentological c s and stable isotope data support an interpretation of marginal marine facies with limited freshwater input for the claiborne group dark green mature pellets are predominantly associated with transgressive bioturbated marine sediments from the weches formation through to the youngest crockett formation there is a trend from pellets composed mostly of dark green serpentine rich mixed layer clay to those of mostly brown immature nontronite rich mixed layer clay the serpentine is intermediate between odinite and berthierine and may have been originally odinite that has undergone partial transformation to berthierine pellet maturity apparently corresponds with the degree of winnowing and sedimentation rate it does not appear to correspond with global temperature fluctuations though the serpentine rich rather than glauconite rich composition may do siderite cement mostly in the form of concretions post dates the iron rich clay pellets and pre dates calcite cement carbonate and apatite concretions show a wide range of mineral cement textures reflecting variation in sedimentary environment and early diagenetic processes based on water temperatures derived from verneriacardia bivalves a precipitation temperature of 20 c is assumed for the concretionary siderite using this temperature we surmise that the siderite precipitated from marine pore waters with intermittent meteoric water mixing

regional magnetic anomalies crustal strength and the location of the northern cordilleran fold and thrust belt the northern cordilleran fold and thrust belt in canada and alaska is at the boundary between the broad continental margin mobile belt and the stable north american craton the fold and thrust belt is marked by several significant changes in geometry cratonward extensions in the central yukon territory and northeastern alaska are separated by marginward re entrants these geometric features of the cordilleran mobile belt are controlled by relations between lithospheric strength and compressional tectonic forces developed along the continental margin regional magnetic anomalies indicate deep thermal and compositional characteristics that contribute to variations in crustal strength our detailed analysis of one such anomaly the north slope deep magnetic high helps to explain the geometry of the fold and thrust front in northern alaska this large magnetic anomaly is inferred to reflect voluminous mafic magmatism in an old devonian extensional domain the presence of massive amounts of mafic material in the lower crust implies geochemical depletion of the underlying upper mantle which serves to strengthen the lithosphere against thermal erosion by upper mantle convection we infer that deep source magnetic highs are an important indicator of strong lower crust and upper mantle this stronger lithosphere forms buttresses that play an important role in the structural development of the northern cordilleran fold and thrust belt

genetic stratigraphic sequences in basin analysis ii application to northwest gulf of mexico cenozoic basin the northwest gulf of mexico cenozoic sedimentary wedge illustrates the application of genetic stratigraphic sequence analysis and documents several general conclusions 1 sequences defined by regional marine flooding are the principal genetic stratigraphic units of the basin fill continental margins are characterized by repetitive episodes of basin margin offlap punctuated by periods of transgression and marine flooding of the depositional platform 2 continental margin outbuilding is concentrated at one or more shelf edge deltaic depocenters separated by interdeltaic bights depocenters remain fixed during a depositional episode but commonly relocate during transgression and flooding 3 a distinct syndepositional structural style in prograding continental margins results in sporadic uplift of a basin fringing peripheral bulge and accentuates preservation of shelf margin facies along zones of extensional normal faulting and enhanced subsidence 4 genetic stratigraphic sequences commonly reflect an evolving interplay among two or even three variables for example early cenozoic gulf sequences are most closely related to tectonic events of the intraplate source terrane which in turn affect rate and location of sediment supply and basin margin response to loading late cenozoic sequences more closely reflect proposed eustatic cycles

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

crustal scale ramp in a middle proterozoic orogen northwest territories canada deep crustal seismic data from the fort goodhope area northwest territories canada image crustal structures associated with middle proterozoic compressional deformation these include 10 20 km wide antiforms and thrust faults that lie above a west dipping crustal scale ramp with at least 10 km of vertical relief the deformation is interpreted as being associated with structures observed in the subsurface to the east and may be partly coeval with deformation originally detected in outcrop in the rackla range of the wernecke mountains these new deep crustal profiles coupled with data to the east that delineate structures to 15 km depth reveal large scale similarity between this middle proterozoic orogen and many phanerozoic compressional orogens

the mackenzie river magnetic anomaly yukon and northwest territories canada evidence for early proterozoic magmatic arc crust at the edge of the north american craton abstract we characterize the nature of the source of the high amplitude long wavelength mackenzie river magnetic anomaly mra yukon and northwest territories canada based on magnetic field data collected at three different altitudes 300 m 3 5 km and 400 km the mra is the largest amplitude 13 nt satellite magnetic anomaly over canada within the extent of the mra source depth estimates 8 12 km from euler deconvolution of low altitude aeromagnetic data show coincidence with basement depths interpreted from reflection seismic data inversion of high altitude 3 5 km aeromagnetic data produces an average magnetization of 2 5 a m within a 15 to 35 km deep layer a value typical of magmatic arc complexes early proterozoic magmatic arc rocks have been sampled to the southeast of the mra within the fort simpson magnetic anomaly the mra is one of several broad scale magnetic highs that occur along the inboard margin of the cordillera in canada and alaska which are coincident with geometric changes in the thrust front transition from the mobile belt to stable cratonic north america the inferred early proterozoic magmatic arc complex along the western edge of the north american craton likely influenced later tectonic evolution by acting as a buttress along the inboard margin of the cordilleran fold and thrust belt

glacial water mass geometry and the distribution of δ13c of σco2 in the western atlantic ocean 1 oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa cibicidoides and planulina from 25 new piston cores gravity cores and multicores from the brazil margin the cores span water depths from about 400 to 3000 m and intersect the major water masses in this region these new data fill a critical gap in the south atlantic ocean and provide the motivation for updating the classic glacial western atlantic 13c transect of duplessy et al 1988 the distribution of 13c of co2 requires the presence of three distinct water masses in the glacial atlantic ocean a shallow 1000 m southern source water mass with an end member 13c value of about 0 3 0 5 vpdb a middepth 1500 m northern source water mass with an end member value of about 1 5 and a deep 2000 m southern source water with an end member value of less than 0 2 and perhaps as low as the 0 9 values observed in the south atlantic sector of the southern ocean ninnemann and charles 2002 the origins of the water masses are supported by the meridional gradients in benthic foraminiferal 18o a revised glacial section of deep water 13c documents the positions and gradients among these end member intermediate and deep water masses the large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation

aeromagnetic evidence for a buried early cretaceous magmatic arc northeast japan positive aeromagnetic anomalies recent drilling and models constructed from these data delineate the plutonic roots of the early cretaceous kitakami magmatic arc in northeast japan buried plutons mostly offshore produce belts of positive magnetic anomalies these anomalies and magnetotelluric data suggest that the plutons form a batholith 70 120 km wide nearly 800 km long and 10 15 km thick the batholith may mark the location of the main kitakami arc most of the exposed kitakami plutons are 2 20 km in diameter some are 3 km thick the small plutons line up along nw trending faults some may have been satellite vents that tapped into the magma supply of the main arc the batholithic roots of the main arc now compose almost half of the modern japan forearc basement steep magnetic gradients offset anomalies and basin stratigraphy portray extensive faulting of the kitakami batholith during oblique subduction in the late cretaceous and rifting in the miocene the eastern boundary of the kitakami batholith lies between 90 and 140 km west of the modern trench much closer than the 300 km distance between the active arc and trench the early cretaceous forearc basin and accretionary prism may underlie the modern forearc basin east of the batholith but clear evidence is lacking much of the early cretaceous margin including most of its forearc therefore is missing how the material was removed is unknown it could have been strike slip faulted eroded by subduction related processes or both

the three dimensional overturning circulation of the southern ocean during the woce era a box inverse model of the southern ocean during the world ocean circulation experiment is constructed to investigate the three dimensional structure of the regional overturning circulation in that era the model has many features in common with various preceding inverse studies but also contains several novel elements that make it well suited for addressing many of the significant uncertainties that surround the circulation at present the net overturning circulation of the southern ocean is found to consist of two well defined cells of similar strength the upper cell consists of a northward transport of 18 8 5 5 sv of surface mode and intermediate waters lighter than the 27 5 kg m 3 isoneutral and an equivalent southward flow in the approximate 27 5 27 9 kg m 3 neutral density range encompassing the bulk of the upper circumpolar deep water the lower cell involves the northward export of 18 6 0 9 sv of antarctic bottom water and lower circumpolar deep water denser than 28 08 kg m 3 and an opposing transport in the lighter classes of that water mass substantial structural differences between the overturning circulations of the atlantic indian and pacific basins are indicated by the model s solution overall the diagnosed southern ocean circulation shares many qualitative and some quantitative features with previous inverse estimates particularly as regards the large scale depth integrated lateral circulation and associated energy fluxes in the subtropics and in the antarctic circumpolar current and the strength of the upper overturning cell however it also suggests several significant adjustments to current views of the regional circulation most notable amongst these are the subpolar circulation of the southern ocean is more vigorous and zonally interconnected than generally thought the associated lower overturning cell is more intense than indicated by most preceding estimates contrary to common perception sub surface mixing processes play a role of comparable importance to air sea ice exchanges of buoyancy in underpinning the dianeutral closure of the southern ocean overturning even at shallow mode and intermediate water levels and the connection between north atlantic deep water formation and southern ocean upwelling is fundamentally three dimensional such that deep waters from the north atlantic must upwell dianeutrally before being returned to the permanent pycnocline of the northern oceans

isopycnal averaging and the residual mean circulation abstract the zonally averaged circulation in the atmosphere or ocean can be misleading if the averaging is performed at constant height in the ocean there is apparently anomalously large diapycnal motion forming the so called deacon cell the atmospheric equivalents are the ferrel cells there are two zonal averaging techniques commonly used to avoid these spurious cells one involves averaging at constant density and this technique has been used in both fluids the other technique which has so far been applied only in the atmosphere involves taking into account perturbation correlation terms to form the residual mean circulation using a taylor series expansion we show that these apparently dissimilar techniques are formally equivalent at leading order in perturbation amplitude the equivalence is demonstrated using output from the fram southern ocean numerical model

middle proterozoic compressional orogen in northwestern canada evidence from seismic reflection profiling drill holes potential field measurements and regional geology shows that an east verging proterozoic thin skinned thrust and fold belt is situated beneath the interior platform of northwestern canada balanced cross sections and associated palinspastic reconstructions provide constraints on the extent of the shortening in the thrust belt and suggest that at least 50 90 km of compression can be accounted for although the proterozoic stratigraphy in the subsurface is not well known the cross sections are constrained essentially by seismic reflection geometry shortening occurred on subhorizontal detachment s that lie above the autochthonous hudsonian basement seismic reflection data from the area west of the major thin skinned thrust structures show that the proterozoic strata there can be traced for up to 100 km and are characterized by broad open folds with wavelengths of 10 km or more uniform potential field anomalies and subhorizontal seismic reflections further indicate that there are no major crustal boundaries such as sutures in the area between the thrust belt and the campbell uplift near inuvik northwest territories that were associated with the thrusting the detachment s that carried the folded and thrust faulted allochthons must therefore extend for a considerable distance to the northwest of the thrust belt new deep crustal seismic reflection data from the campbell uplift however show evidence for crustalpenetrating proterozoic structures that dip northwestward and that were probably reactivated during late paleozoic compression and mesozoic extension taken together the various data thus indicate that the area from great bear lake to the campbell uplift about 400 km is underlain by a mid proterozoic decollement above which the compressional deformation took place and that the deformation extended to the north side of the present campbell uplift where steeply north dipping structures indicate that the underlying autochthonous basement is characterized by a ramp structure that was probably associated with the southern edge of a proterozoic basin

proterozoic craton to basin crustal transition in western canada and its influence on the evolution of the cordillera deep seismic reflection data have imaged a crustal scale west facing ramp or ramp system in the subsurface of western canada in northwestern canada the ramp is within proterozoic crust east of the cordillera and is unconformably overlain by paleozoic sedimentary rocks indicating that it was formed during the proterozoic in this region similar structures are visible within the cordillera in southern canada and the northwestern united states along a south projection of the ramp observed in the north in the monashee complex of british columbia and in the priest river complex in northern washington reflections are visible that dip westward from the surface to near the base of the crust and are structurally discordant with underlying more horizontal reflections thus outlining footwall ramps we propose that the proterozoic crustal ramp in the north and the footwall ramps in the cordillera probably coincide with a proterozoic crustal transition from thick craton on the east to thin crust on the west thi

precambrian crust beneath the mesozoic northern canadian cordillera discovered by lithoprobe seismic reflection profiling 1 the cordillera in northern canada is underlain by westward tapering layers that can be followed from outcrops of proterozoic strata in the foreland belt to the lowermost crust of the orogenic interior a distance of as much as 500 km across strike they are interpreted as stratified proterozoic rocks including 1 8 0 7 ga supracrustal rocks and their basement the layering was discovered on two new deep seismic reflection profiles in the yukon line 3 650 km and northern british columbia line 2 1245 km in two segments that were acquired as part of the lithoprobe slave northern cordillera lithospheric evolution snorcle transect in the mackenzie mountains of the eastern yukon the layering in line 3 is visible between 5 0 and 12 0 s 15 to 36 km depth it is followed southwestward for nearly 650 km 500 km across strike and thins to less than 1 0 s 3 0 3 5 km thickness near the moho at the yukon alaska international boundary in the northern rocky mountains of british columbia the upper part of the layering on line 2 correlates with outcrops of proterozoic 1 76 1 0 ga strata in the muskwa anticlinorium at this location the layering is at least 15 km thick and is followed westward then southward into the middle and lower crust for 700 km 300 km across strike it disappears as a thin taper at the base of the crust 150 km east of the coast of the alaskan panhandle the only significant disruption in the layering occurs at the tintina fault zone a late to postorogenic strike slip fault with up to 800 km of displacement which appears as a vertical zone of little reflectivity that disrupts the continuity of the deep layering on both profiles 300 km apart the base of the layered reflection zone coincides with the moho which exhibits variable character and undulates in a series of broad arches with widths of 150 km in general the mantle appears to have few reflections however at the southwest end of line 3 near the alaska british columbia border a reflection dips eastward from 14 0 s to 21 0 s 45 to 73 km depth beneath exposed eocene magmatic rocks it is interpreted as a relict subduction surface of the kula plate our interpretation of proterozoic layered rocks beneath most of the northern cordillera suggests a much different crustal structure than previously considered 1 ancient north american crust comprising up to 25 km of metamorphosed proterozoic to paleozoic sediments plus 5 10 km of pre 1 8 ga crystalline basement projects westward beneath most of the northern canadian cordillera 2 the lateral 500 km by at least 1000 km and vertical up to 25 km extent of the proterozoic layers and their internal deformation are consistent with a long lived margin for northwestern north america with alternating episodes of extension and contraction 3 the detachments that carry deformed rocks of the mackenzie mountains and northern rocky mountains are largely confined to the upper crustal region above the layering 4 accreted terranes include thin klippen that were thrust over north american pericratonic strata e g yukon tanana and terranes such as nisling and stikinia that thicken westward as the underlying proterozoic layers taper and disappear 5 the ages of exposed rocks are not necessarily indicative of the ages of underlying crust a frequent observation in lithoprobe interpretations so that estimates of crustal growth based on surface geology may not be representative

crustal insights from gravity and aeromagnetic analysis central north slope alaska aeromagnetic and gravity data are processed and interpreted to reveal deep and shallow information about the crustal structure of the central north slope alaska regional aeromagnetic anomalies primarily reflect deep crustal features regional gravity anomalies are more complex and require detailed analysis we constrain our geophysical models with seismic data and interpretations along two transects including the trans alaska crustal transect combined geophysical analysis reveals a remarkable heterogeneity of the pre mississippian basement in the central north slope pre mississippian basement consists of two distinct geophysical domains to the southwest the basement is dense and highly magnetic this basement is likely mafic and mechanically strong possibly acting as a buttress to basement involvement in brooks range thrusting to the northeast the central north slope basement consists of lower density moderately magnetic rocks with several discrete regions intrusions of more magnetic rocks a conjugate set of geophysical trends northwest southeast and southwest northeast may be a factor in the crustal response to tectonic compression in this domain high resolution gravity and aeromagnetic data where available reflect details of shallow fault and fold structure the maps and profile models in this report should provide useful guidelines and complementary information for regional structural studies particularly in combination with detailed seismic reflection interpretations future challenges include collection of high resolution gravity and aeromagnetic data for the entire north slope as well as additional deep crustal information from seismic drilling and other complementary methods

inversion of the proterozoic wernecke basin during tectonic development of the racklan orogen northwest canada new deep seismic reflection data coupled with regional stratigraphic correlations drill hole information and potential field data are interpreted to provide images of middle proterozoic wernecke supergroup meta sedimentary layers that were uplifted during tectonic development of the ca 0 9 1 3 ga racklan orogen in canada s western northwest territories the reflection data are located at the eastern front of the mackenzie mountains portion of the canadian cordillera and on the western flank of the fort simpson structural trend that is a prominent proterozoic structure in the subsurface throughout the region along three parallel profiles layers that are correlated with thick wernecke supergroup sedimentary rocks produce prominent reflections between about 3 0 and 9 0 s about 7 5 and 23 km that were arched prior to deposition of younger proterozoic probably mackenzie mountains supergroup and phanerozoic sedimentary rocks the strata are considered to be wernecke basin sedimentary rocks that were

weakly magnetic crust in the canadian cordillera abstract current models of continental crust favor an increase in magnetization with depth here we report a counter example from the canadian cordillera where almost a full thickness of non magnetic continental crust is suggested by joint interpretation of magnetic and seismic data the magnetic field over the cordillera is characterized by complex short wavelength 100 km portion of the cordilleran field is subdued and mainly featureless and suggests a lack of magnetic sources at greater depths seismic reflection and refraction data from three major transects in the yukon and british columbia canada support this interpretation and indicate that sedimentary like formations make up the majority of the crust the dominance of shallow upper crustal magnetization in the canadian cordillera contrasts with the generally held view that the lower continental crust is the primary source for long wavelength magnetic anomalies sources for these anomalies are often assumed to be located in the lower crust when surface magnetizations are insufficient to produce such anomalies or no correlation exists between the magnetic field and the mapped surface geology the canadian cordillera appears to be an example of a non magnetic lower crust overlain by a more magnetic upper crust that is however not magnetized strongly enough to produce significant long wavelength magnetic anomalies

sensitivity of the southern ocean overturning circulation to surface buoyancy forcing 1 the sensitivity of the southern ocean overturning to altered surface buoyancy forcing is investigated in a series of eddy permitting idealised simulations the modelled response indicates that heat and freshwater fluxes in the southern hemisphere mid latitudes may play a significant role in setting the strength of the overturning circulation enhanced buoyancy fluxes act to increase the meridional overturning up to a limit approaching the wind driven ekman transport the sensitivity of the overturning to surface buoyancy forcing is strongly dependent on the relative locations of the wind stress profile buoyancy forcing and upwelling region the numerical simulations provide support for the hypothesis that changes in upwelling during deglaciations may have been driven by changes in heat and freshwater fluxes instead of or in addition to changes in wind stress

potential field modeling of a proterozoic half graben near blackwater lake northwest territories canada and its implications for the fort simpson magnetic anomaly a wedge of proterozoic strata in the blackwater lake area of the northwest territories canada is preserved below the sub cambrian unconformity in a half graben associated with a large normal fault reflection seismic data define the shape of the wedge but neither it nor drilling information is adequate for identifying the strata preserved within the half graben although these data permit either tweed lake assemblage density d 2730 2780 kg m3 or mackenzie shaler assemblage d 2580 kg m3 as the half graben fill gravity and to a lesser extent magnetic modeling show tweed lake assemblage 1270 ma to be the more likely candidate inclusion of any appreciable amount of mackenzie shaler assemblage in the half graben produces too low a gravity response and is therefore rejected the character of the fort simpson magnetic anomaly fsa differs north and south of the liard line n61 north of the line and south of great bear lake the fsa consists of a collection of strong short wavelength featu

magnetic susceptibility and relation to initial 87sr 86sr for granitoids of the central sierra nevada california measurement of the magnetic susceptibility of more than 6000 samples of granitic rock from the mariposa 1 by 2 quadrangle which crosses the central part of the sierra nevada batholith between 37 and 38 n latitude shows that magnetic susceptibility values are above 10 2 si units in the east and central parts of the batholith and drop abruptly to less than 10 3 si units in the western foothills in a narrow transitional zone intermediate values 10 3 to 10 2 prevail magnetic susceptibility appears to decrease slightly westward within the zones of both high and low values magnetic susceptibility in plutonic rocks is chiefly a function of the abundance of magnetite which depends in turn on the total iron content of the rocks and their oxidation ratio lower magnetic susceptibilities of felsic members of sierran intrusive suites and of some felsic rock units relative to adjacent mafic rock units commonly reflect differences in total iron content but the differences of magnetic susceptibility that define the regional pattern generally are much larger and are determined chiefly by the oxidation ratios of the rocks the relatively unaltered condition of the samples and restriction of 18o to the range of 7 to 10 3 indicate that neither hydrothermal fluids nor subsolidus alteration were important in modifying oxidation ratios correlations of magnetic susceptibility with initial 87sr 86sr suggest that oxidation ratios have been inherited from the source regions for the magmas from which the rocks crystallized reduction of fe3 to fe2 by organic carbon or other reducing substances may also have affected magnetic susceptibility

abyssal atlantic circulation during the last glacial maximum constraining the ratio between transport and vertical mixing transport to vertical diffusivity in the deep atlantic our calculations suggest that the ratio was at least twice as large at the lgm we speculate that the primary cause was reduced mixing between northern and southern component waters associated with movement of this water mass boundary away from the zone of intense mixing near the seafloor the shallower water mass boundary yields an order of magnitude increase in the volume of southern component water suggesting its residence time may have increased substantially our analysis supports the idea that an expanded volume of antarctic bottom water and limited vertical mixing enhanced the abyssal ocean s ability to trap carbon during glacial times

a multibasin residual mean model for the global overturning circulation the ocean s overturning circulation is inherently three dimensional yet modern quantitative estimates of the overturning typically represent the subsurface circulation as a two dimensional two cell streamfunction that varies with latitude and depth only this approach suppresses information about zonal mass and tracer transport in this article the authors extend earlier zonally averaged overturning theory to explore the dynamics of a figure eight circulation that cycles through multiple basins a three dimensional residual mean model of the overturning circulation is derived and then simplified to a multibasin isopycnal box model to explore how stratification and diabatic water mass transformations in each basin depend on the basin widths and on deep and bottom water formation in both hemispheres the idealization to multiple two dimensional basins permits zonal mass transport along isopycnals in a southern ocean like channel while retaining the dynamical framework of residual mean theory the model qualitatively reproduces the deeper isopycnal surfaces in the pacific basin relative to the atlantic this supports a transfer of antarctic bottom water from the atlantic sector to the pacific sector via the southern ocean which subsequently upwells in the northern pacific basin a solution for the full isopycnal structure in the southern ocean reproduces observed stratification differences between atlantic and pacific basins and provides a scaling for the diffusive boundary layer in which the zonal mass transport occurs these results are consistent with observational indications that north atlantic deep water is preferentially transformed into antarctic bottom water which undermines the importance of an adiabatic upper overturning cell in the modern ocean

the intracratonic paleoproterozoic forward orogeny and implications for regional correlations northwest territories canada reflection seismic data from the colville hills and anderson plain document an intracratonic compressional event herein named the forward orogeny which affected strata considered equivalent to the 1663 ma hornby bay group outcropping on coppermine homocline forward orogeny structures were peneplaned and unconformably overlain by strata considered equivalent to the 1267 ma dismal lakes group a comparable tectonic history is recorded in the exposed rocks of coppermine homocline to the east structural orientations indicate a general northwest southeast direction of maximum compression during an early phase and a more west east direction in a later phase regional sequence a of g m young and co workers is subdivided into preorogenic sequence a1 hornby bay and equivalents and postorogenic sequences a2 dismal lakes and equivalents and a3 coppermine river group and equivalents the forward orogeny dated as approximately 1663 ma the age of the syntectonic kaertok formation on coppermine homocline