deposition and imbrication of a 2670 2629 ma supracrustal sequence in the indin lake area southwestern slave province canada new u pb age data from the southwestern slave province demonstrate that units of the indin lake supracrustal belt form an imbricated structural stack the oldest rocks of the belt are undated mafic volcanic flows of the hewitt lake group that are crosscut by a 2670 ma felsic sill itself coeval with mafic through felsic volcanic rocks of the 2668 2671 ma leta arm group the youngest rocks of the belt are 2647 2629 ma turbidites and felsic volcanic rocks of the unconformably overlying chalco lake group tonalite orthogneiss of the adjacent cotterill gneiss complex is 2680 ma suggesting that it does not represent in situ basement to the supracrustal belt intercalation of the older hewitt lake and leta arm groups with the younger chalco lake group is interpreted to result from d1 imbrication and folding between 2629 and 2609 ma the age of a crosscutting tonalite intrusion subsequent d2 folding and regional low pressure metamorphism occurred between 2609 ma and ca 2590 ma d3 normal faulting between the

tectonic denudation of the shuswap metamorphic terrane of southeastern british columbia large scale over thrusting and regional extension are both known to have been operative within the shuswap metamorphic terrane of the southern omineca belt the monashee core complex of the northern shuswap terrane is interpreted to be a crustal scale duplex that was developed during late mesozoic shortening and thickening of the crust uplift of the monashee complex caused by basement duplexing is estimated to have been in excess of 25 km late cretaceous overthrusting of high grade metamorphic and related plutonic rocks during later stages of uplift and basement duplexing led to weakening of the crust and to gravitationally driven extension of a regional crustal welt shuswap culmination denudation and spreading of this uplifted metamorphic terrane are attributed to displacements across listric normal shear zones and faults localized along the outward dipping flanks of the shuswap culmination total extension across the shuswap culmination at the northern end of the monashee complex is estimated to be no greater than 30 this model of denudation and gravity induced spreading is extrapolated southward into the valhalla and okanagan regions where extension may be far more significant

emplacement and deformation of granites during transpression magnetic fabrics of the archean sparrow pluton slave province canada abstract the sparrow pluton is part of the prosperous suite of two mica granites that crop out within amphibolite grade meta greywackes of the southern yellowknife domain in the slave province of the canadian shield the magnetic susceptibility k and the anisotropy of magnetic susceptibility ams were used to systematically map the structural patterns in the pluton and to establish the relationship between plutonism and late archean tectonics in the region paramagnetic fe phyllosilicates biotite chlorite and very fine grained magnetite contribute to k and to the ams the magnetic foliation and the magnetic lineation are predominantly controlled by the biotite fabrics and their orientations are consistent with the regional d 2 strain field the horizontal magnetic lineations in the sparrow pluton suggest a horizontal stretching component associated with the regional d 2 event the zonation defined by k values is compatible with a fold pattern trending parallel to the regional f 2 folds and s 2 foliation and to the magnetic fabric trends in the pluton the intensities and symmetries of the ams also define map patterns that are consistent with d 2 deformation microstructural study indicates the pluton recorded d 2 strain as it crystallized and cooled from the solidus demonstrating syn d 2 emplacement the results indicate the pervasive structural patterns in the sparrow pluton are an integral part of the regional strain field and that they are kinematically consistent with a transpressive d 2 strain regime mapping the fabric patterns within syntectonic plutons provides a useful approach to the kinematic analysis of synemplacement deformation events in multiply deformed metamorphic terranes

strain patterns decollement and incipient sagducted greenstone terrains in the archaean dharwar craton south india the archaean dharwar craton is characterized by two greenstone successions the 3 ga sargur group and the 3 0 2 5 ga dharwar supergroup examples of both successions are described from the region of jayachamarajapura where they are also distinguished by different tectonic patterns the younger greenstones have undergone only minor deformation and are only slightly metamorphosed and so provide a good case study of the relative behavior of greenstones in relation to their granite gneiss country rocks a detailed structural analysis indicates two strain fields associated with two deformational episodes d1 and d2 the d1 episode produced dome and basin structures and affected merely the older greenstones and the gneisses the mapped strain field is compatible with the hypothesis that it is associated with the development of diapiric type gravitational instabilities the d2 episode affects only the younger greenstone belt which has the overall geometry of a complex syncline it is discordant over a complex of gneisses and older greenstones that was deformed during the d1 episode the base of the discordant cover sequence is tectonized and constitutes a decollement surface kinematic criteria at this surface have opposite sense and converge towards the belt axis these structural features are interpreted in terms of progressive deformation compatible with the incipient development of a sagducting trough these results are consistent with those obtained from other parts of the craton where the tectonic evolution appears to reflect mainly relative vertical displacements facilitated by the reheating of continental crust during two major archaean tectonometamorphic episodes copyright 1996 elsevier science ltd

the central slave basement complex part ii age and tectonic significance of high strain zones along the basement cover contact the basement cover high strain zone enveloping parts of the sleepy dragon complex northeast of yellowknife slave province canada has been reinvestigated integrated stratigraphic structural and geochronological data show that the high strain zone is of regional extent and is best interpreted as a decollement between crystalline ca 2 9 3 3 ga rocks of the central slave basement complex and pre 2687 ma cover rocks three temporally distinct mafic dyke swarms occur within the high strain zone the two oldest of these constrain the timing of the high strain event to between 2734 2 and 2687 1 ma at the time of decollement development the cover stratigraphy consisted of i the central slave cover group a thin pre 2734 ma succession of mafic and ultramafic volcanic rocks conglomerates fuchsitic quartzites minor rhyolites and banded iron formation and ii an overlying sequence of tholeiitic pillow basalts the central slave cover group is considered to be autochthonous whereas a variety of

antler orogeny a mediterranean type orogeny the late cenozoic thrust belts of the apennine carpathian and hellenic systems of the mediterranean region provide modern analogues for the middle paleozoic antler orogeny each of these young or active mountain belts formed in convergent systems in which thrusting occurred behind a zone of trench retreat while a region of extension developed contemporaneously within the hanging wall of the subduction systems dynamic models that have been recently developed for these tectonic systems provide a basis for interpretation for the antler belt many if not all of the geologic relations observed in the antler belt including the lack of a collided arc and subsidence of a broad extensional region behind the thrust belt are also present within these young mediterranean thrust belt systems if this interpretation is correct then the entire antler belt may have been preserved with no need for other tectonic elements to have been present west of the antler thrust belt and the havallah basin

tectonic denudation of the darwin metamorphic core complex in the andes of tierra del fuego southernmost chile implications for cordilleran orogenesis cordillera darwin in the southernmost chilean andes is a tectonically denuded metamorphic core complex similar to those widely developed in the north american cordillera detailed comparison can be drawn with the shuswap metamorphic core complex in british columbia and washington analysis of the geotectonic history of the darwin complex reinforces the view that the presence of tectonically thickened continental crust is critical for development of most if not all known core complexes and also suggests that inversion of a marginal basin during a compressional regime is one way of initiating such a crustal welt the cordillera darwin complex is unique in the andes and antarctandes this indicates that although low angle normal faulting occurs at high levels in convergent plate settings such as the central andes and the himalayas extensional denudation of deep crustal levels in active andean type orogens may require special tectonic circumstances uplift of the darwin complex began at about 70 ma in a localized extensional setting within the developing transform zone between the south american and antarctic plates this strike slip setting is analogous to some metamorphic core complexes in north america and possibly to a recently identified core complex in new zealand it is likely to be a common one for core complexes in convergent orogens

u pb zircon geochronology of the yellowknife volcanic belt nwt canada new constraints on the timing and duration of greenstone belt magmatism volcanic and sedimentary rocks in the slave province collectively termed the yellowknife supergroup have long been considered to represent laterally continuous and correlative stratigraphically coherent homoclinal sequences correlation based on the lithologic analysis of certain distinctive units has been most intensely applied to the yellowknife volcanic belt in the southern slave province u pb zircon data for seven samples collected from within the belt are reported here the distinctiveness of the townsite formation a term applied to three lenticles of rhyodacitic to dacitic tuffs breccias and pillowed flows in comparison to the overlying and underlying tholeiitic basalts kam group has led to its use as a key marker unit for stratigraphic correlations and palinspastic restorations of offsets on proterozoic faults in the area a sample from each of the three segments was collected for u pb zircon geochronology in addition numerous thinner tuffaceous beds from the kam group were also dated

imaging and regional distribution of basalt flows in the faeroe shetland basin we demonstrate that the use of long offset seismic data allows wide angle reflections and diving waves to be recorded and that these can be used in conjunction with prestack depth migrations to constrain and to image the base of the basalt flows and the underlying structure in the faeroe shetland basin crustal velocity models are built first by inverting the traveltimes of the recorded reflections and diving waves using ray tracing methods finer details of the velocity structure can then be refined by analysis of the amplitudes and waveforms of the arrivals we show that prestack depth migration of selected wide angle arrivals of known origin such as the base basalt reflection using the crustal velocity model allows us to build a composite image of the structure down to the pre rift basement this has the advantage that the wide angle first arriving energy must be primary and not from one of the many multiples or mode converted phases that plague near offset seismic data this allows us to tag these primary arrivals with confidence and then to identify the same arrivals on higher resolution prestack migrations that include data from all offsets examples are drawn from the faeroe shetland basin with a series of regional maps of the entire area showing the basalt depths and the thickness of the basalt flows and underlying sediment down to the top of the pre rift basement the maps show how the basalts thin to the southeast away from their presumed source west of the present faeroe islands and also show the extent to which the structure of the pre rift basement controls the considerable variations in sediment thickness between the basement and the cap formed by the overlying basalt flows

a practical approach to surface multiple attenuation multiply reflected events in marine seismic data can be divided into two classes surface multiples are events that have at least one downward reflection from the water surface and internal multiples on the other hand have all of their downward reflections at the water bottom or below surface multiples are usually stronger more broadband and more of a problem than internal multiples because the reflection coefficient at the water surface is much larger than the reflection coefficients found in the subsurface

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

an archean metamorphic core complex in the southern slave province basement cover structural relations between the sleepy dragon complex and the yellowknife supergroup archean rocks in the fenton lake brown lake area southern slave province are subdivided into two lithotectonic domains a supracrustal domain which consists mainly of the archean yellowknife supergroup and a gneiss granite domain the latter is composed of gneissic and metaigneous rocks of the sleepy dragon complex determined to be basement to the yellowknife supergroup and granite plutons including the 2641 3 5 ma suse lake granite and the 2583 5 1 ma morose granite volcanic rocks of the cameron river belt and greywacke mudstone turbiditic metasedimentary rocks of the burwash formation constitute the supracrustal domain a late archean amphibolite to greenschist facies ductile to local brittle high strain zone separates the domains kinematic indicators demonstrate that the zone experienced two kinematically opposed episodes of displacement the older episode involved pre to synthermal peak thrusting of the supracrustal rocks over the gneiss granite domain thrusting is kinematically an

single bubble marine source offers new perspectives for lithospheric exploration abstract we present deep penetration seismic time sections obtained in the last two years with the single bubble pulse generating method using marine seismic airguns whole crustal penetration and resolution have been obtained on the ionian quasi oceanic basin and its margin on the continental crust of the aegean sea on the continental margins and oceanic basin of the gulf of biscay we show that this method is more efficient than others in radiating the lower frequencies returned by deep structure without loosing resolution hence it allows deep crustal imaging from relatively low power academic ships and if the method is applied to powerful industrial vessels it may open the subcrustal lithosphere to imaging

crustal structure of edoras bank continental margin and mantle thermal anomalies beneath the north atlantic we discuss the structure of the continent ocean transition of the edoras bank area of the ne atlantic margin and the influence of the iceland mantle plume during continental breakup edoras bank lies close to the present day limit of influence of the iceland plume sequences of seaward dipping reflectors imaged on multichannel seismic profiles are well developed over much of the margin they form aerially extensive dipping sheets with individual reflectors imaged on margin parallel profiles as laterally continuous and subhorizontal features we interpret them as basalt flows extruded from linear fissure vents close to sea level at the time of continental breakup a crustal velocity model for the margin was developed by travel time modeling of wide angle refractions and reflections recorded on digital ocean bottom hydrophones and the structure confirmed by gravity modeling oceanic crust formed immediately after breakup is 10 km thick and exhibits classic layer 2 and layer 3 seismic velocities the crustal thickness increases to the se reaching 17 km beneath the continental fragment of edoras bank the continent ocean transition is dominated in the lower crust by a lens up to 8 km thick of high velocity material 7 2 7 5 km s 1 which exhibits a very low velocity gradient it extends over a 60 km wide region beneath the seaward dipping reflectors the high velocities and densities of the lower crust are probably caused by igneous rock with a high mgo content that underplated the margin or heavily intruded pre existing crustal material during continental breakup both the addition of large quantities of melt to the margin and the subsidence curves inferred from postrift sediments sampled by deep sea drilling project dsdp boreholes on the margin indicate the presence of abnormally hot mantle beneath the area at the time of continental breakup comparison of the structure and subsidence history of the edoras bank margin with other areas of the nw european margin allows us to deduce the spatial extent and temporal variation of the thermal anomalies caused by the iceland mantle plume during and following continental breakup we find that at the time of breakup a thin layer of anomalously hot asthenospheric mantle 150 c hotter than normal lay beneath the 2000 km long rift extending from the lofoten margin off norway in the north to the edoras bank margin in the south over a period of 5 10 m y following the onset of rifting the mantle thermal anomaly decayed rapidly to only 50 c or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism

geology and u pb geochronology of the neoarchean snare river terrane tracking evolving tectonic regimes and crustal growth mechanisms u pb zircon crystallization ages determined by isotope dilution thermal ionization mass spectrometry id tims and laser ablation microprobe inductively coupled plasma mass spectrometry lam icp ms for 13 intrusive units in the neoarchean snare river terrane srt provide tight constraints on the timing of crust formation and orogenic evolution seven metaluminous plutons were emplaced over 80 ma from ca 2674 to 2589 ma whereas six peraluminous bodies were emplaced in a 15 ma interval from ca 2598 to 2585 ma a detrital zircon study yielded an age spectrum with peaks correlative with known magmatic events in the slave province with the ca 2635 ma age of the youngest detrital zircon population providing a maximum estimate for the onset of sedimentation this age contrasts with evidence for pre 2635 ma sedimentation elsewhere in the srt indicating that sedimentation was protracted and diachronous evolution of the srt can be subdivided into four stages i 2674 2635 ma formation of a metal

physical and chemical controls on the composition of waters in sedimentary basins abstract the composition of subsurface waters is determined not only by diagenetic reactions but also by physical processes of fluid convection and hydrodynamic dispersion the south west louisiana gulf coast provides an instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional siliciclastic sequence most formation waters here have compositions totally unlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition dissolved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout most of the upper 3 km of the sedimentary section the simultaneous systematic variation in dissolved na k mg ca and alkalinity with chloride in these waters supports the hypothesis that metastable thermodynamic buffering by silicate carbonate mineral assemblages is a first order control on fluid compositions the chemical potential of chloride or alternatively total anionic charge appears to be a master variable which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region this variable is controlled largely by physical processes of advection and dispersion in the upper 3 km of the section and by dehydration reactions in deeper mudstone dominated sediments where the composition of the fluid is largely rock buffered the ultimate origin of the fluid and its pathway of chemical evolution may be obscured at least in terms of major solute composition non buffered components such as cl and br or isotopic compositions are more likely to retain information on original end member fluid compositions and reaction pathways

thrusting magmatic intraplating and metamorphic core complex development in the archaean belleterre angliers greenstone belt superior province quebec canada abstract the belleterre angliers greenstone belt in the pontiac subprovince is the youngest and southernmost of the greenstone belts in the archaean superior province of the canadian shield the greenstone belt has been disrupted into three fragments from north to south the baby belleterre and lac des bois groups respectively the belleterre angliers greenstone belt has sheared contacts with younger pontiac metasediments and older tonalites the pontiac metasediments dip beneath the baby group and are tectonically interlayered with the tonalites thus the belleterre angliers belt is interpreted to be a thrust sheet 6 km thick in the north thinner in the south above a mid crustal duplex consisting of metasedimentary and tonalite imbricates the thrust sheet was transported from the north but age and geochemical constraints rule out derivation from the abitibi subprovince it probably roots in a band of ultramafic rocks that mark a major south vergent thrust in the northern pontiac subprovince after emplacement the belleterre angliers sheet and the adjacent pontiac metasediments were deformed by a system of linked extensional shears and transfer faults that border dome or antiformal areas in a style similar to phanerozoic metamorphic core complexes the orientation of shear zones and foliations indicates that the regional far field stresses were compressional both before and after extensional shearing since the system of extensional shears is intruded by a suite of monzodiorite granodiorite syenodiorite and syenite plutons with high mg number it is concluded that crustal inflation caused by the injection intraplating of large volumes of magma perturbed the regional compressive stress field and facilitated extensional faulting

extensional structures during deposition of the 3460 ma warrawoona group in the eastern pilbara craton western australia abstract structures in the granite greenstone terrain of the eastern pilbara were mapped in both the coongan belt greenstones and shaw batholith granitoids important structures that are coeval with an early magmatic event in both the granitoids and the greenstones were identified in the coongan greenstones a brittle extensional fault array is syn kinematic with felsic volcanics of the duffer formation dated at 3452 16 ma pidgeon 1978a in the underlying shaw batholith a major ductile shear zone was mapped at the contact with the coongan belt although the foliation in the shear zone forms part of a domal structure the stretching lineations constantly plunge to the ene the sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the e w trending part and dextral movement on the n s trending part a wedge shaped granitoid intruded into the shear zone during deformation this granitoid is part of the north shaw suite which has been dated at 3467 6 ma mcnaughton et al 1988 the structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels the structural kinematic magmatic metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry in turn this implies that the domal geometry of the shaw batholith was initiated at mid crustal levels at a very early stage namely during deposition of the warrawoona group in the evolution of the pilbara craton as extension occurred during intrusion and extrusion of large volumes of magma the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry the initial domal shape of the shaw batholith has been enhanced during later deformation phases which caused steepening of n s trending structures

a new look at the stratigraphy of the yellowknife supergroup at yellowknife n w t implications for the age of gold bearing shear zones and archean basin evolution based on recent detailed mapping a revised stratigraphic column is proposed for the rocks of the archean yellowknife supergroup in the yellowknife greenstone belt the mafic volcanic rocks of the kam formation previously thought to represent the oldest supracrustal rocks of the belt overlap remnants of an earlier volcanic sedimentary sequence here referred to as the octopus formation as its enormous thickness makes it too unwieldy to be described as a single formation the kam formation is raised to group status and subdivided into four formations it is proposed that the kam group should replace the beaulieu group in the yellowknife area the chan formation at the base of the kam group consists of multiple gabbroic intrusions that were emplaced into a carapace of pillowed flows the intrusions locally resemble sheeted mafic dyke complexes in phanerozoic ophiolites thought to represent evidence for sea floor spreading the crestaurum formation which overlies the chan formation is characterized b

seismic characterization of basalt flows from the faroes margin and the faroe shetland basin high quality seismic profiles across the faroes volcanic continental margin and the adjacent continental shelf show contrasting styles of basalt emplacement across two major structural boundaries at the rifted continental margin the basalts form prominent seaward dipping reflector sequences up to 5 km thick the same basalts after they had travelled in a landward direction across the elevated subaerial faroes shelf produced prominent landward dipping foresets where they flowed across the paleo coastline into the deep water of the faroe shetland basin in both cases we are able to image details of the internal structure of the basalts as a result of using a low frequency airgun source capable of penetrating through the basalt sequence together with long offset recordings from ocean bottom seismometers and a 12 km long deep towed hydrophone streamer

geophysical response of flood basalts from analysis of wire line logs ocean drilling program site 642 voring volcanic margin analysis of wire line logs at ocean drilling program site 642 the deepest scientific hole yet drilled in an offshore large igneous province lip reveals the main features of the extrusive complex constructed during formation of a volcanic margin a characteristic cyclic log response images tholeiitic lavas and thin tuffs corresponding to the seaward dipping reflectors in the seismic record flows are 0 6 18 5 m thick while most sediments are 2000 m focused resistivity kicks the large flow scale variations in physical properties show that representative core measurements must be densely sampled both on flow and sequence scales site 642 log responses appears typical for the north atlantic volcanic province and other lips but differs from logs in deep ocean crustal holes principal differences in log responses reflect mode of crustal emplacement and alteration hence site 642 forms a reference hole for extrusive constructions on oceanic lips

products of 2 65 2 58 ga orogenesis in the slave province correlated with slave northern cordillera lithospheric evolution snorcle seismic reflection patterns reflection patterns along lithoprobe slave northern cordillera lithospheric evolution snorcle seismic reflection line 1 in the southwestern slave province are interpreted as products of tectonic wedging during late archean lithospheric convergence the interpretation is aided by application of new seismic imaging techniques by correlation of upper crustal reflection patterns to known geology and by a comparison of lithospheric reflection patterns to similar convergent zones elsewhere in the yellowknife area reflection patterns consist of 1 east dipping reflections at 12 14 s that project into the upper mantle 2 a wedge shaped body in the lower crust with an east dipping reflection fabric that is truncated on the west by 3 a series of west dipping reflections that outline thrust and fold structures in the upper crust the similarity of these reflection patterns to those of the proterozoic fort simpson hottah collision zone 300 km to the west provides support for the interpretation that reflection patterns beneath the slave province are also products of collisional tectonics rocks within the slave province preserve evidence of a 2 65 2 58 ga pan slave orogenic event in which the 2 9 ga central slave basement collided with the 2 7 ga juvenile eastern slave province their suture is interpreted to be a west dipping surface at 4 5 s 12 15 km beneath yellowknife and to project to the surface east of the profile in the lower crust and upper mantle east dipping reflections are interpreted to delineate a coeval subduction zone and accretionary wedge the upper crustal thrust and fold structures are likely linked to gold bearing shear zones at yellowknife these results provide tantalizing evidence that processes similar to those of modern convergent zones were operational at 2 65 2 58 ga

neodymium isotopic evidence for the tectonic assembly of late archean crust in the slave province northwest canada the ca 2 7 2 5 ga slave province is a granitegreenstone terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks the nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km east west transect at 65 n across the structural grain of the province initial nd values reveal distinctly different crustal sources in the eastern compared to the western parts of the province as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust supracrustal sequences ca 2 71 2 65 ga from the central and eastern parts of the province have positive nd 1 values 0 3 to 3 6 consistent with juvenile sources and formation remote from significantly older crust syn to late deformation ca 2 63 2 60 ga mantle derived diorites and related tonalites type i from the central and eastern parts of the province have similar initial nd values 0 1 to 2 7 in contrast samples from the westernmost plutons which intrude exposed pre 3 1 ga crust have much lower nd 1 values 1 0 to4 6 suggesting contamination of these magmas by older crust the nd 1 values of post deformation granites s s type ii also vary systematically across the province values for granites west of longitude 110 30 w range from 0 2 to 5 3 those to the east range from 0 6 to 3 7 these data suggest mixed crustal sources dominated by mid to early archean material nd 2 6 to 17 at 2 6 ga for the western granitoid rocks and juvenile sources for the eastern granites the nd isotopic data are consistent with the geology of the province in that exposures of mid to early archean crustal rocks predating the principal 2 7 2 5 ga orogenic event are restricted to the western part of the province the asymmetric pattern defined by the nd isotopic data indicates the presence of distinct crustal rocks beneath the slave province similar isotopic variations observed across phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin this process may also have been an important mechanism in the cratonization of the slave province

evolution of an archean basement complex and its autochthonous cover southern slave province canada abstract the sleepy dragon complex sdc is one of several antiformal exposures of a 4 0 2 85 ga basement block underlying the west central region of the archean slave province northwestern canada this basement block the central slave basement complex csbc is overlain by an autochthonous to locally parautochthonous dominantly quartz rich clastic sequence the 2 85 2 80 ga central slave cover group together these units comprised a fundamental building block during neoarchean growth and assembly of the slave craton the well exposed patterson lake morose lake area represents a type locality for study of the csbc central slave cover group and overlying rocks of the yellowknife supergroup field mapping and u pb geochronology employing both thermal ionisation mass spectrometry tims and sensitive high resolution ion microprobe shrimp document a prolonged crustal history oldest intact basement units consist of foliated to gneissic tonalite dated in two places at 2955 12 ma and 2944 9 ma indirect evidence for a ca 3150 ma basement component is obtained both from xenocrystic zircon in tonalite and detrital zircons from the overlying central slave cover group which here consists of the patterson lake formation a granite boulder from this formation is dated at 2934 3 ma and may be locally derived a sheared mafic volcanic unit along the basement cover contact contains 2942 3 ma metamorphic titanite which provides a minimum deposition age this volcanic unit is therefore also a part of the basement complex younger events in the sdc mainly reflect tonalite trondhjemite granodiorite plutonism during construction of the overlying cameron river and beaulieu river greenstone belts of the yellowknife supergroup a unit of k feldspar megacrystic granodiorite is dated at 2726 3 ma and tonalite and granodiorite bodies located toward the centre of the sdc have primary crystallization ages of 2683 3 2 ma 2677 2 ma and 2672 7 6 ma the core of the southern sdc is occupied by a late tectonic to post tectonic granite pluton previously dated at 2586 2 ma an intravolcanic unconformity along the southern margin of the sdc developed in part on 2 7 ga granodiorite is constrained by existing u pb data to have formed sometime between ca 2685 2660 ma an abundance of synvolcanic plutons of similar age and the presence of a 3 km thick mafic volcanic sequence overlying a less dense magmatically active felsic crust suggest that gravitational instability and magmatic diapirism may have played important roles in doming and uplift of the sdc prior to regional d 1 deformation the younger toward centre zonation of plutonic rocks in the sdc is documented in other domal granite gneiss complexes of archean age e g pilbara craton western australia where it is attributed to upward and outward displacement of older crust during repeated magma channeling into a growing mid crustal dome in the west central slave craton such a process might account for the regional presence of local unconformities of apparently similar age the domal basement plutonic complexes would have been further amplified after 2660 ma during the development of regional f 1 and f 2 fold belts

the rise and fall of the faroe shetland basin evidence from seismic mapping of the balder formation seismic mapping of the faroe shetland basin from three dimensional surveys provides striking evidence for uplift of the southern faroe shetland basin in the late palaeocene and rapid subsidence in the early eocene the seismic reflector at the base of the balder formation follows a regional unconformity surface which records erosion of a major branching drainage network into the underlying palaeocene section following a major base level fall the topography of this surface was subsequently in filled following marine transgression and deep water conditions returned to the basin in the early eocene we attribute base level changes to the influence of the proto iceland mantle plume beneath the lithosphere providing support until continental break up west of the faroe islands

complex evolution of folds and cleavages in archean rocks yellowknife n w t overturned folds and later cleavages in a metasedimentary terrain with marginal volcanics near yellowknife form a complex succession in which structures repeatedly trend eastward and northward early major east west folds are overturned away from a southern granodioritic complex and terminate westward against a major syncline with limbs overturned inward trending north south along the western granitoid boundary of the terrain later smaller folds limbs 1 km which die out away from granitoid plutons include a progression from northerly to easterly trending structures several sets of cleavage foliations mostly steeply inclined transgress the folds an east west set succeeded by a regionally dominant set striking northwest to north and a later north south set confined to near the western margin of the terrain the easterly and northerly near orthogonal trends followed by successive structures could reflect the influence of fault bounded crustal blocks during emplacement of fault controlled grani

secular trends in the thermal evolution of metamorphic terrains the thermal evolution of convergent orogens is sensitive to the deformation history of the mantle part of the lithosphere if the dense mantle lithosphere thickens as a consequence of convergent deformation and remains attached during subsequent denudation of the thickened crust then high pressure low to intermediate temperature glaucophane schist assemblages will form during both thickening and denudation the actual form of the p t paths for a given deformation depending primarily on the initial thermal state of the lithosphere and particularly the ratio of the thickness of the crust and mantle lithosphere however if mantle lithosphere is thinned during convergence by processes such as thermal boundary layer detachment then any early formed high pressure low to intermediate temperature glaucophane schist assemblages will be obliterated during subsequent high temperature overprints since the stability of the thermal boundary layer during convergent deformations is controlled by the rate of convergence and the convective vigour of the subjacent mantle any change in the convective vigour of the mantle through geologic time in response to for example changing global heat production may therefore be reflected in the thermal evolution of metamorphic terrains formed during convergent deformations models of the thermal evolution of deforming lithosphere appropriate to an earth in which the effect of declining global heat production and convective vigour is mediated by the process of thermal boundary layer detachment suggest that the preservation of high pressure low to intermediate temperature glaucophane schist assemblages in convergent orogens has become increasingly probable through geological time a suggestion in accord with the observed secular trend in the character of metamorphic terrains preserved in the geological record

crustal shortening and thickening in neoarchean granite greenstone belts a case study from the link between the 2 7 ga elu and hope bay belts northeast slave craton canada abstract the elu link between the 2 7 ga hope bay and elu belts in the northeast bathurst block of the slave craton comprises supracrustal and intrusive rocks variably deformed by three tectono metamorphic events d1 d3 the geometry of d1 structures formed during prograde metamorphism is uncertain because of subsequent overprint d2 occurred in two stages predating d2a and postdating d2b peak metamorphism d1 and d2a were thrusting events inferred from peak metamorphic pressures of 6 7 kbar 670 mpa retained by a garnet orthogneiss the latter is diagnostic of thrust tectonism in archean granite greenstone belts with no characteristic thrust faults unlike d2a d2b was a vertical general flattening event prevailing during the formation of magmatic domes and interdomal folds that form the main strain patterns of the belts this was followed by the formation of buckled f3 folds associated with d3 vertical constriction the switch from thrust to vertical tectonics during peak metamorphism and subsequent deformation resulted in intense recrystallization that explains the poor preservation and scarcity of early formed shears including thrust zones a tectonic process combining d1 d2a thrust stacking sagduction and vertical stretching during d2b and d3 is suggested to explain crustal thickening in the elu link and terrains of similar ages

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

characterization of volcanic units using detailed velocity analysis in the atlantic margin west of shetlands united kingdom the offshore faroe shetlands basin northwest of scotland and south of the faroe islands figure 1 has seen renewed exploration interest following development of schiehallion foinaven and loyal fields and opening of the first licensing round of the faroe islands figure 1 significant basalt wells rock property studies and key velocity inversion profiles in area of study interpretation of volcanic data in the area is very challenging and many local oil and gas prospects involve clastic objectives above or below volcanic zones high uncertainty about the thickness and morphology of these zones results in high uncertainties in structural geometry reservoir definition and hydrocarbon migration and charge risk only a few wells have penetrated these volcanic zones to date sonic logs from these wells and regional rock property trends for tertiary aged clastic sediments suggest that significant rms root mean square velocity changes occur at or near these volcanic boundaries boundaries not easily identified with conventional 2 d or 3 d data and interpretation techniques this case study discusses how interpretation was improved via a focus on velocity coherence analysis data were reprocessed using various demultiple and filtering techniques specifically designed to improve velocity coherence analysis emphasis was placed on identification of bona fide velocity coherences and associated inflection points within stacking velocities that might indicate bulk changes to true interval velocities the geology of the late cretaceous and early tertiary in the vicinity of the faroe islands is dominated by incipient continental break up and associated rifting between greenland and northwest europe this break up and its association with a major mantle plume has created one of the world s major basalt provinces the faroe islands are predominantly made of these basaltic lavas intercalated with tuff and relatively minor sedimentary rocks faroese lavas are broadly equivalent in age to those encountered in the british isles greenland and offshore norway and have

multiple dikes in lower kam group yellowknife greenstone belt evidence for archean sea floor spreading a mafic intrusive extrusive complex at the base of the kam group in the archean yellowknife supergroup at yellowknife northwest territories canada grades from gabbros through a zone of multiple and sheeted dikes into massive and pillowed flows with thin beds of interflow sediments although rock types and primary structures resemble those in upper parts of phanerozoic ophiolites the kam group lacks an ultramafic base and is much thicker than typical ophiolites because similar mafic sections occur in certain atypical ophiolites and in plateau areas of modern ocean basins however the kam group is viewed as a remnant of archean oceanic crust rather than as a product of intracontinental rift volcanism the dikes are analogous to sheeted dikes of ophiolites and are suggestive of sea floor spreading in an archean marginal basin setting

groundwater upwelling near bay st elaine salt dome in southeastern louisiana as inferred from fluid property variations fluid pressures temperatures and salinities derived from 62 well logs were mapped in three vertical cross sections across the bay st elaine salt dome in southeastern louisiana to qualitatively determine groundwater flow patterns around the dome near hydrostatic conditions prevail in the sand rich shallow section from depths of 0 to 2 5 km however fully enclosed within the hydropressured section are lenses of slightly overpressured sediments which slope up toward the dome hydraulic heads may be as much as 500 m above ground level within these enclosed lenses in some strongly overpressured sediments deeper than 2 5 km hydraulic heads are as much as 2500 m above ground level pore water salinities increase with depth within the hydrostatically pressured section fr m 0 wt at the surface to maximum values of 10 12 wt at the top of the overpressured section within the overpressured section salinities decrease to as little as 6 wt with increasing depth immediately adjacent to the northeastern flank of the dome is a steeply dipping lobe of relatively fresh waters 6 wt sodium chloride that separates the flank of the dome from saltier 8 10 wt waters away from the dome this low salinity water mass appears to be connected to similar low salinity waters in the overpressured section and is postulated to be due to pore water upwelling from a depth of 4 km to a depth of about 1 5 km

post tectonic uplift of the cordillera darwin orogenic core complex evidence from fission track geochronology and closing temperature time relationships the cordillera darwin orogenic core complex is a structural and topographic culmination in the southern andes which underwent mid cretaceous penetrative deformation and metamorphism associated with the deformation and incipient obduction of a partly ophiolitic back are basin rocas verdes block along its southern boundary whereas structural evidence suggests uplift of the rocas verdes terrane relative to the cordillera darwin during this event metamorphic assemblages reflect a greater net uplift of the opposite sense new fission track ages together with existing k ar and rb sr ages were used to estimate cooling and uplift histories and their relationships to foreland sedimentation and present day topographic relief the results suggest that whereas initiation of uplift in the cordillera darwin and rocas verdes blocks may have accompanied mid cretaceous deformation the major rapid uplift around 0 5 1 5 mm y occurred after the deformation also uplift occurred earlier in the rocas verdes block than in the cordillera darwin block the latter having experienced a greater net uplift since the mid cretaceous the major post tectonic uplift of the cordillera darwin occurred at a slower rate 0 05 0 20 mm y and coincides with the deposition of thick flysch and conglomerates in the foreland basin

seismic structure of basalt flows from surface seismic data borehole measurements and synthetic seismogram modeling we use the wide angle wavefield to constrain estimates of the seismic velocity and thickness of basalt flows overlying sediments wide angle means the seismic wavefield recorded at offsets beyond the emergence of the direct wave this wide angle wavefield contains arrivals that are returned from within and below the basalt flows including the diving wave through the basalts as the first arrival and p wave reflections from the base of the basalts and from subbasalt structures the velocity structure of basalt flows can be determined to first order from traveltime information by ray tracing the basalt turning rays and the wide angle base basalt reflection this can be refined by using the amplitude variation with offset avo of the basalt diving wave synthetic seismogram models with varying flow thicknesses and velocity gradients demonstrate the sensitivity to the velocity structure of the basalt diving wave and of reflections from the base of the basalt layer and below the diving wave amplitudes of the

tectonomagmatic evolution of cenozoic extension in the north american cordillera the spatial and temporal distributions of cenozoic extension and magmatism in the cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt 100 km pre extension and followed the onset of intermediate and silicic magmatism by no more than a few million years extension began in early eocene time in southern british columbia northern washington idaho and montana farther s extension began at about the eocene oligocene boundary in the great basin and slightly later in the mojave sonora desert region the intervening area at the latitude of las vegas remained quiescent until mid miocene time compositional and isotopic characteristics of most pre miocene magmas are consistent with their containing major components of melted continental crust in mid miocene time two major changes occurred widening of the area of extension and the widespread appearance of basaltic magmas the area affected by extension from southwestern montana to the lake mead region widened to several hundred kilometres by this time extension in southern british columbia northern washington and northern montana had ceased probably before the end of the eocene and extension s of lake mead except in the gulf of california had waned regions affected by the broader belt of extension during late miocene pliocene and quaternary time experienced basaltic magmatism which began along a central rift zone in the northern part of the region and which had within a few million years spread to include most of the region later basaltic activity has tended to concentrate in restricted zones especially near the margins of the extended area we recognize a correlation between the amount of earlier crustal thickening and cenozoic extension and between the length of time after shortening but before extension and the degree to which a given region was intruded by late cretaceous plutons the localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal mechanical model developed in a companion paper sonder et al this raises the possibility that stresses inherent in the north american plate dominated over plate interaction forces as controls of the cenozoic tectonomagmatic evolution of the north american cordillera especially in its earlier stages

seismic response and construction of seaward dipping wedges of flood basalts voring volcanic margin geological and geophysical data from the 900 m of volcanic basement drilled at ocean drilling program site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins the main part of the drilled section corresponding to a prominent seaward dipping reflector sequence consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments the basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity pore aspect ratio distribution and alteration stacks of laterally continuous basalt flows appear to have thin layer transverse isotropic properties for typical wavelengths in multichannel seismic data vertical seismic profiling and average sonic log velocities are similar 3 77 km s and 3 88 km s respectively while comparable refraction velocities are 10 20 higher synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces though the most continuous reflectors are related to thick flows seismic models based on the characteristic velocity and density basalt flow distributions and site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows the base of the dipping reflector sequence reflector k correlates with flows in the lower part of upper series basalts while the transition from basaltic to underlying dacitic andesitic lavas correlates with a locally defined reflector a model for the breakup related volcanism includes 1 prebreakup dacitic andesitic volcanism 2 early breakup basaltic volcanism infilling the prebreakup relief 3 main breakup stage with intense focused volcanism large subsidence and lava pounding and 4 late breakup volcanism during a period of decreased subsidence and local off axis activity

structural development of angular volcanic belts in the archean slave province narrow belts of metavolcanics lying between metasediments and granitoid rocks in the southwestern part of the archean slave province display a distinctive angular map pattern steep homoclinal segm

enhancing subbasalt reflections using parabolic τ p transformation potential hydrocarbon bearing sediment structures in the faroe shetland basin are often overlain by basaltic sequences of stacked flows up to several kilometers thick with conventional seismic acquisition and processing methods it is difficult to image the internal intrabasalt and underlying subbasalt structures scattering from the highly reflective top of the basalt and short period multiples from intrabasalt boundaries may completely mask reflections from the base of the basalt and below high attenuation of seismic energy in the basaltic sequence due to its complex internal structure generally causes a weak subbasalt signal long period multiples such as those generated between the sea bottom and the top basalt boundary may obscure subbasalt reflections and energy reflected from the basement

transition from back arc to foreland basin development in the southernmost andes stratigraphic record from the ultima esperanza district chile during the mesozoic evolution of the southernmost andes back arc basin formation in an extensional setting was followed by foreland basin development in a compressional setting a stratigraphic record of this tectonic transition is described from jurassic and cretaceous rock units exposed in the patagonian fold thrust belt at 51 s latitude initiation of an extensional deep marine trough in late jurassic time is documented by interstratified rhyolites and marine mudstones in the tobifera formation development of the early cretaceous rocas verdes back arc basin is recorded by zapata formation submarine slope deposits which draped the west facing passively subsiding cratonic margin of the basin the onset of andean compressional orogenesis and creation of the magallanes foreland basin is marked by the influx of coarse grained sandstone turbidites of the albian cenomanian punta barrosa formation during late cretaceous time the foreland basin configuration consisted of a narrow foredeep trough bounded by a gently sloping foreland ramp on the craton to the east this geometry reflects greater flexural subsidence of the stretched and thermally weakened passive back arc basin margin beneath the load of the obducted rocas verdes basin floor changes in depositional regime and sediment dispersal patterns in the latest cretaceous together with an eastward shift in the magallanes basin depocenter record cratonward migration of deformation in the patagonian fold thrust belt loading of the thicker cratonic lithosphere caused subsidence of the foreland ramp to form a wide flexural basin

postcollision extension in arc continent collision zones eastern indonesia postcollisional extension superimposed on arc continent collision complexes is a common feature in the orogenic belts of eastern indonesia in the southern banda arc the arc continent collision event is essentially a pliocene and younger feature but already an element of extension is being superimposed on the compressional structures of the collision complex it is thus likely that the extension commences within a very short period 5 m y after the initiation of collision related compressional deformation a similar history of fold belt development immediately followed by extension can be inferred for the lengguru fold belt and the adjacent wandamen wondiwoi terranes of irian jaya and for the gulf of bone in sulawesi it is suggested that the extension results from decoupling of the subducting oceanic lithosphere from the unsubductable continental lithosphere and that the superimposition of extension is a virtually unavoidable consequence of the arc continent collision

basement controlled structural fronts forming an apparent major refold pattern in the yellowknife domain slave province trend lines of tight folds f0 and f1 in archean metasediments curve across the yellowknife supracrustal domain to form an apparent major refold pattern hinge traces marking pronounced changes in trend extend up to 50 km more locally f1 folds are discontinuous and markedly discordant in trend suggesting that the major hinge traces rather than simply following axial surfaces denote structural fronts where fold sets with distinctive initial trends have interacted cleavages grouped as s2 and s3 each with subsets postdate the f1 folds a regional s3 cleavage strikes parallel to major hinge traces whereas discontinuous s3 subsets are oblique some terminating along well defined cleavage fronts for example one subset replaces another along a front generally parallel to a hinge trace or fold front but offset 1 2 km the development of the apparent refold pattern and the parallelism of various generations of structural fronts and cleavages can be explained by the response of cover rocks to recur

u pb zircon ages and field relationships of archean basement and proterozoic intrusions south central wopmay orogen nwt implications for tectonic assignments new field and u pb zircon data from the south central wopmay orogen south of 65 n establish crystallization ages of basement and plutonic phases and bring to the forefront questions on the architecture of the orogen the complex and extensive 3300 2575 ma basement domain was derived from the adjacent slave craton paleoproterozoic supracrustal rocks dominated by an extensive pelitic psammitic sequence with basal arenite conglomerate and carbonate unconformably overlie this basement pre to post kinematic paleoproterozoic plutons intrude both basement and overlying strata and crystallized at 1877 1867 1862 and 1858 1850 ma the first pulse of plutonism constrains an early generation of metamorphism and deformation to younger than 1877 ma and is corroborated by metamorphic zircon in archean basement the interval from 1867 to 1862 ma brackets ductile deformation in granite and granodiorite intrusions although corresponding structure in the metasedimentary strata is equivocal post kinematic ca 1

accretion of the archean slave province continental rift models have long been applied to the archean slave province of northwestern canada a reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones regions composed of different rock suites e g orthogneisses vs mafic volcanics are separated by high strain zones recording large displacements it is proposed that the high strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis from west to east these include the anton terrane interpreted as an archean microcontinent the sleepy dragon terrane possibly an exhumed more eastern part of the anton terrane the contwoyto terrane a westward verging fold and thrust belt containing tectonic slivers of greenstone volcanics and the hackett river volcanic terrane interpreted as an archean island arc the contwoyto and hackett river terranes represent a paired accretionary prism and island arc system that formed above an east dipping subduction zone these collided with the anton microcontinent producing a basement nappe expressed as the sleepy dragon terrane during the main accretion event within the slave province the whole tectonic assemblage was intruded by late kinematic to postkinematic granitoids

role of aquitards in hydrogeochemical systems a synopsis abstract aquitards exert significant influence on the hydrogeochemistry of aquifer systems this influence is manifested somewhat differently depending on the relative position of aquitards within a system in the deeper regimes they are influential in the origin and distribution of brines and the development of geopressured zones in intermediate regimes they form multi layered aquifer systems and provide a source of reactive minerals and exchangeable ions in shallow regimes aquitards can influence the topography and drainage patterns this influences the relationship between the water table and the potentiometric surface of confined aquifers controls the rates of infiltration and discharge and controls whether the geochemical system is open or closed to exchange of carbon dioxide gas in coastal areas aquitards can determine the depth of the saltwater freshwater interface its distance from the shoreline and the position of the mixing zone that causes geochemical alteration of minerals and development of porosity

use of low frequencies for sub basalt imaging many prospective passive ocean margins are covered by large areas of basalts these basalts are often extremely heterogeneous and scatter the seismic energy of the conventional seismic reflection system so that it becomes difficult to obtain information on deeper reflectors since high frequencies are scattered more than low frequencies we argue that the acquisition system for sub basalt targets should be modified to emphasize the low frequencies using much larger airguns and towing the source and receivers at about 20 m depth in the summer of 2001 we obtained seismic reflection data over basalt in the northeast atlantic using a system modified to enhance the low frequency energy these new data show deep reflections that are not visible on lines shot in the same places with a conventional system

crust mantle coupling revisited the archean slave craton nwt canada abstract lithospheric domains in the slave province inferred from regional variations in chemistry of mantle derived garnets are parallel with northeasterly structural trends in neoarchean surface rocks and were used previously to infer late neoarchean crust mantle coupling and argue against the presence of diamondiferous mesoarchean lithosphere under the western slave craton prior to its late neoarchean orogenic climax a review of more recent evidence does not support this view but concludes that a mesoarchean lithospheric root was coupled to mesoarchean crust of the central slave superterrane csst prior to accretion of adjacent juvenile neoarchean terranes a significant part of this root survived the neoarchean break up of the csst neoarchean terrane accretion and a complex sequence of post accretion deformation granitoid plutonism and metamorphism it was least modified in the ultra depleted harzburgitic layer udl under the central part of the contwoyto terrane where it is trapped above the deepest part of an underplated paleoproterozoic slab which stabilized the remnants of the archean root and caused downward younging of the lithosphere upper mantle domains recognized on the basis of the distribution of peridotitic garnets are not primary neoarchean features but were either inherited from the original mesoarchean root or resulted from neoarchean and paleoproterozoic tectonic erosion and or metasomatism of this root the slave province resembles other diamondiferous cratons in that its diamond endowment consists of both mesoarchean and paleoproterozoic stones

gas exsolution during fluid migration and its relation to overpressure and petroleum accumulation abstract petroleum systems in the tertiary sections of the gulf of mexico and niger delta among other basins are characterized by the close association between petroleum accumulation and over pressure although overpressure is generated mainly by compaction disequilibrium of the low permeability sediments exsolution of gases from water and petroleum during migration over great vertical distance may create overpressure or modify the existing overpressure in this paper the author combined relative permeability data and petroleum phase behavior modelling to explain and predict the presence of an overpressure and petroleum baffle due to gas exsolution generalized isoperm diagrams for oil gas and water phases in sandy and silty shaly sediments were created based on published data aqueous solubility of methane and carbon dioxide as a function of temperature pressure and salinity is summarized based on literature data phase diagrams of petroleum with a wide range of composition api gravity and gor were produced using phase behavior software with calibration using field data these tools were integrated to model overpressure and petroleum phase variations in two oil gas fields in the gulf of mexico the pressure gradients in these fields are likely modified by gas exsolution from the petroleum which migrated and accumulated within and below the pressure transition zone ptz petroleum trapped within ptz tends to be fractionated whereas that accumulated below ptz tends to be single phase with high api gravity these results provide a model which explains both the effect of gas exsolution on ptz and the occurrence of different types of petroleum in these fields these tools may be integrated with geological and seismic data to assess more accurately the economic value of petroleum accumulations in petroleum systems governed by overpressure and petroleum phase separation and fractionation

the bell lake group and anton complex a basement cover sequence beneath the archean yellowknife greenstone belt revealed and implicated in greenstone belt formation u pb zircon and monazite geochronology indicates the presence of a 2 93 ga basement anton complex and 2 8 ga cover sequence bell lake group beneath the 2 70 to 2 72 ga kam group of the yellowknife greenstone belt in the southern slave province the bell lake group comprises remnants of a quartzite rhyolite banded iron formation succession the ages of detrital zircons from the quartzite unit constrain its deposition to be younger than 2 92 and range up to 3 7 ga u pb ages for gneisses beneath the bell lake group are in excess of 2 93 ga and they are locally overlain by the quartzite the contact between the tholeiitic to calc alkaline kam group and the bell lake group is poorly exposed and equivocal approximately 6 km higher in the section the ranney chert a felsic volcaniclastic layer separates 2 70 2 72 ga tholeiitic to calc alkaline upper kam group rocks from a lower tholeiitic section containing sheeted dike complexes zircons from the ranney chert yield pb pb ages ranging from 2 72

use of hydraulic head and hydraulic gradient to characterize geopressured sediments and direction of fluid migration in louisiana gulf coast abstract abstract the characterization of geopressured sediments in the gulf coast is commonly made on the basis of increases in pressure gradient or geostatic ratio that is in situ fluid pressure divided by total depth fluids and sediments having geostatic ratios in excess of 0 47 psi ft are usually considered geopressured those with geostatic ratios greater than 0 70 psi ft hard geopressured an alternative and more informative way of characterizing geopressured sediments is by use of the parameter hydraulic head hydraulic head can be used not only to distinguish between hydropressured and geopressured sediments but also to provide potentially quantitative information on the direction and rate of subsurface fluid flow hydraulic head h feet is defined as h z pg where z is depth below a reference datum p is measured or calculated fluid pressure g is acceleration due to gravity and is fluid density in the absence of osmotic effects the hydraulic gradient dh dl dimensionless where l is path length can be used in conjunction with hydraulic conductivity in darcy s law to predict the direction and rate of fluid migration variations in calculated hydraulic head have been correlated along several regional cross sections through the tertiary sedimentary sequence of south louisiana four distinct hydrodynamic zones can be distinguished with depth in the region studied near surface to 7000 feet in depth hydropressured fresh to saline waters having hydraulic heads near zero and vertical hydraulic gradients of 0 01 or less 7000 feet to 10 000 14 000 feet in depth a transition zone of weakly geopressured sediments with hydraulic heads ranging from 0 to 3000 feet above sea level hydraulic gradients range from 0 1 to 4 and show many reversals with depth many commerically important hydrocarbon plays occur within this zone 10 000 14 000 feet to 12 000 18 000 feet the hard geopressured zone in which hydraulic head progressively increases with depth from values of 3000 to 16 000 feet of head above sea level gradients of 4 to 5 are typical but locally can exceed 25 there are few reversals in gradient with depth depths below zone 3 suggestion based on sparse data that hydraulic gradients begin to decrease with depth in contrast to the hard geopressured zone where hydraulic flow appears to consist of a broad generally upward moving front hydraulic flow in the transition zone appears to be more strongly focused through preferred channel ways

evidence for archean ocean opening and closing in the southern slave province archean mafic volcanic and plutonic rocks of the cameron and beaulieu river greenstone belts outcrop in the southern slave structural province of the canadian shield lower portions of the greenstone belts have ophiolitic characteristics but young felsic units may represent superimposed arc magmatism the greenstone belts are separated from older circa 2 7 3 2 ga quartzo feldspathic gneisses of the sleepy dragon complex by a regional decollement zone patterson lake structural complex consisting of deformed mafic volcanic rocks thin slivers of gneiss ultramafic rocks and banded iron formation mixed in a highly deformed phyllonitic and phacoidally cleaved metapelitic matrix high synmetamorphic strains are indicated by abundant intrafolial folds transposed layering rare sheath folds and by the parallelism of elongated primary features and metamorphic mineral lineations the patterson lake structural complex is similar to melanges of phanerozoic mountain belts adjacent quartzo feldspathic gneisses are strongly mylonitized the melange and deeper mylonites formed when the greenstone belts were emplaced as thrust sheets wildflysch shed from tips of emergent thrusts was in part incorporated into the basal melange as the allochthon overrode its own detritus continued compression involved the basement in the deformation uplifting the sleepy dragon complex in a structural culmination mantled by the imbricated greenstone stack late granodioritic to granitic intrusions formed regional thermal aureoles which overprint earlier metamorphic fabrics

fluid rock interactions in the salt dome environment an introduction and review abstract the salt dome environment in the gulf of mexico coast u s a provides one of the most diverse and long lived records of fluid rock interactions in an active sedimentary basin dominated by saline formation waters the geochemical record most of which is isotopic appears to span a temperature range between earth surface temperatures and lower greenschist facies most mineral forming processes in the salt dome cap rocks appear to involve mixtures of warm saline formation fluids from deep basin sources and cool dilute meteoric waters papers in this issue of chemical geology document some of the current research on fluid rock interactions in the salt dome environment these papers highlight the diversity of opinion about salt dome processes offering clues for further research the present paper reviews some of the key and more current literature on salt dome geochemistry fluid convection around salt domes and fluid rock interactions within salt and within the salt dome environment

structural patterns and tectonic evolution of supracrustal domains in the archean slave province canada a new plate tectonic model accounts for lithological relations and regional structural patterns in late archean supracrustal domains of the slave province multiple dykes and pillowed mafic flows most common in the western part of the province suggest sea floor spreading the mafic volcanics lying in narrow homoclinal belts stratigraphically below more extensive turbidites are viewed as megaxenolithic remnants of oceanic crust preserved on the periphery of granitoid plutons and blocks of sialic crust closure of an oceanic basin was marked by emplacement of the granitoid plutons and coeval felsic volcanics the latter predominating over mafic volcanics in northeastern domains the felsic calc alkaline magmas may have risen from a shallow dipping subduction zone westerly verging folds westerly convex fold arcs and inclinations of later foliations particularly in lower level rocks of higher metamorphic grade are in accord with underthrusting to the east the zone of underthrusting shifted progressi

laser ablation icp ms u pb and 40 ar 39 ar age constraints on neoarchean to paleoproterozoic magmatic and tectono metamorphic evolution of the link between hope bay and elu greenstone belts northeast slave craton nwt canada abstract the elu link connecting the neoarchean hope bay and elu granite greenstone belts in the bathurst block of the northeast slave craton consists of volcano sedimentary and gabbro granite rocks laser ablation icp ms zircon and titanite u pb dating was combined with mineral 40 ar 39 ar age data to date the elu link rocks and d 1 d 3 tectono metamorphic events as well as characterize the response to the thelon orogeny in the area the volcanic rocks are correlated with the ca 2716 ma flake lake suite in the adjacent hope bay belt whereas the sedimentary units have a minimum depositional age of 2684 11 ma nearly coinciding with the onset of d 1 deformation most gabbro granite bodies were emplaced between 2651 14 ma and 2577 13 ma coeval with the d 2 deformation whose accompanying metamorphism culminated at 2632 7 ma older ages up to 3042 22 ma retained by either the gabbro granite or their hosts are indicative of crustal contamination a few gabbro granites were also emplaced during the 2577 13 ma to 2494 21 ma d 3 deformation event however the 2494 21 ma intrusions are unusual and mark the archean paleoproterozoic transition the d 3 deformation was followed by erosional exhumation and localized thermal pulses associated with the emplacement of diabase dikes at 2228 8 ma 2128 11 ma and 802 75 ma the thelon orogeny imprint inferred from the 2054 11 ma to 1919 5 ma biotite ages corresponds to nearly isobaric low temperature

anton terrane revisited late archean exhumation of a moderate pressure granulite terrane in the western slave province the anton terrane of the western slave province has been interpreted as a remnant of a meso archean 3500 2800 ma microcontinent the largest coherent entity of the terrane is a previously unrecognized 50 by 250 km granulite complex herein termed the snare domain the granulite facies mineral assemblages give the domain a characteristic magnetic signature that is distinct from areas of exposed meso archean basement in the slave province the domain consists of interlayered 3100 2624 ma orthogneisses and paragneisses and 2600 2585 ma granitoid rocks its structure was largely established during 2610 2590 ma orogeny throughout the province that culminated in ca 2590 ma granulite facies metamorphism pressure temperature p t determinations yield peak conditions of 775 875 c at 6 7 kbar shortly thereafter 2 4 kbar of decompression occurred before the domain cooled below 450 c decompression and retrogression textures are widespread but are most pervasive adjacent to late extensional detachment faults u pb zircon age data suggest that the snare domain was partially exhumed from mid crustal levels ca 2585 2580 ma we attribute exhumation to tectonic denudation of an overthickened crustal welt in the western slave province localization of the crustal welt in this part of the slave province may be related to the presence there of meso archean crust

the 1991 1996 natmap slave province project introduction the 11 papers contributed to this issue on the geology of the slave province are summarized and where appropriate links between data from the various contributions are emphasized in the discussion we provide a synthesis of the present knowledge base on pre yellowknife supergroup basement rocks throughout the slave province all available age data on mesoarchean and paleoarchean basement rocks are compiled and in a first attempt to seek regional correlations 10 events of crustal growth and or reworking are recognized spanning well over 1 billion years of earth s history the spatial distributions of these events across the craton are discussed finally we pose a series of important questions that remain and that may help guide future research

hydrogeology of sedimentary basins abstract hydrogeologic environments in sedimentary basins are as variable as are the different types of basins important hydrologic characteristics can be used to distinguish the different types of basin 1 the topographic setting as determined by the geologic and structural history of the basin 2 permeability distribution within the basin and 3 potential energy distributions and flow mechanisms these parameters control residence times of waters rates and directions of saline groundwater flow and the origin and chemical composition of the saline waters the gulf coast and palo duro basins texas exemplify two end member types of sedimentary basins the gulf coast basin is a relatively young tertiary age basin which is presently compacting fluid movement is from the overpressured undercompacted sediments up the structural dip or up fault zones into the hydrostatic section natural fluid pressures are either hydrostatic or overpressured the palo duro is an older paleozoic age basin that has been tectonically uplifted fluid flow is gravity driven from topographically high recharge areas to discharge in topographically low areas fluid pressures are subhydrostatic fluids discharge more easily than they are recharged not all flow is derived by a simple recharge discharge model brines may flow from other basins into the palo duro basin and waters may discharge from the palo duro basin into other basins areal differences in the chemical composition of the basin brines may be the result of different origins

late archean tectono magmatic evolution of the central slave province northwest territories an early tectono stratigraphic and late plutonic assemblage are distinguished in the central part of the archean slave province contwoyto lake area on the basis of their relationship to deformation and metamorphic episodes and by geochemical characteristics the older assemblage which predates the onset of intense deformation and the thermal peak of metamorphism comprises 2 68 2 65 ga calc alkaline volcanic and plutonic rocks as well as extensive turbidites the younger assemblage comprises ca 2 61 2 58 ga plutonic units that were emplaced during and after deformation d d3 and the thermal peak of metamorphism compositions of the late plutonic units vary systematically with time of intrusion from an early calc alkaline diorite tonalite suite to late peraluminous granites the early assemblage is interpreted to represent remnants of an island arc or arcs overlain by turbidites and accreted during d1 and d2 the later assemblage is interpreted as a product of late to post accretion suprasu

laser 40 ar 39 ar thermochronology of archean rocks in yellowknife domain southwestern slave province insights into the cooling history of an archean granite greenstone terrane laser 40 ar 39 ar ages for hornblende muscovite and biotite from archean plutonic rocks of yellowknife domain southwestern slave province complement u pb ages from minerals with higher closure temperatures zircon monazite titanite together the data indicate that the plutonic suites of different ages defeat prosperous morose coring three major thermal culminations in the domain have distinct archean cooling histories until below approximately 250 c biotite closure the cooling trajectories for successive suites appear to document a decrease in cooling rate over the course of orogeny possibly reflecting the build up of heat within the orogen coupled with a transition to lower rates of exhumation and or erosion greater structural relief over the sleepy dragon basement culmination may be responsible for slower cooling of the morose granite situated in its core at or below the closure temperature of biotite the cooling curves converge with one another documenting a transition from localized to regional cooling the fact that cooling was locality specific until at least the biotite closure temperature suggests that just as intrusion and related peak metamorphism were diachronous across the region so was cooling to approximately 250 c uniform regional isotherms thus were not established until 200 ma after the youngest archean plutonism the variable disturbance of biotite ar spectra but generally not muscovite or hornblende indicates that the paleoproterozoic thermal disturbance of archean rocks in yellowknife domain did not exceed 200 300 c

a synmetamorphic lateral fluid flow model for gold mineralization in the archean southern kalgoorlie and norseman terranes western australia gold deposits are widely distributed in the archean southern kalgoorlie and norseman greenstone terranes eastern goldfields superterrane western australia which have collectively produced more than 1 850 metric tons t of gold gold deposits occur in a variety of host rocks and structural settings but all are associated with quartz veins and or shear zones that formed during the latest stages of regional compression available data indicate a similar h 2 o co 2 ch 4 ore fluid at each deposit alteration assemblages vary with metamorphic grade in a manner which indicates that late tectonic regional metamorphic gradients were still in place when gold was deposited from hydrothermal fluids these late tectonic thermal gradients were related to differential uplift of deep hot crust at the margin of the greenstone belt accompanied by intrusion of voluminous monzogranite magma at this time the western margin of the greenstone belt was a zone of high heat flow as were intrusive centers in the northern part of the southern kalgoorlie terrane these spatial and temporal relationships in combination with the apparent absence of a viable local source for the ore fluid at most deposits suggest that gold was deposited from a small number of large synmetamorphic hydrothermal systems published thermal models predict up temperature lateral fluid flow toward zones of high heat flow under these circumstances a model involving up temperature lateral flow of metamorphic fluids overcomes some of the problems associated with deeply sourced and conventional metamorphic models for the ore fluids and is consistent with most of the features of gold deposits in the southern kalgoorlie and norseman terranes potassium metasomatism and radiogenic isotope signatures associated with gold mineralization are ascribed to a metamorphic fluid that was modified by exchange with biotite bearing felsic rocks granitoids felsic volcanic and volcaniclastic rocks in the course of lateral flow toward zones of high heat flow

timing of plutonism deformation and metamorphism in the yellowknife domain slave province canada u pb zircon titanite and monazite ages for plutonic rocks are used to constrain the timing of post 2660 ma deformational and metamorphic events in the yellowknife domain southern slave province diorite and tonalite plutons of the defeat suite yield ages of 2628 3 and 2624 3 ma older than previous less precise estimates for this suite of ca 2618 2620 ma the defeat suite plutons crosscut and postdate upright f1 folds in turbidites of the ca 2680 2660 ma burwash formation d1 deformation is therefore bracketed between 2660 and 2630 ma the d1 event significantly predates deposition of polymict conglomerates of the jackson lake formation a second folding event f2 represented by northwest to north trending steeply plunging folds of bedding and d1 structures affected the area after intrusion of the hidden lake pluton at 2606 2 ma and synchronously with intrusion of the 2596 2 ma sparrow lake pluton a member of the two mica granite prosperous suite the time of initiation and the duratio

the central slave basement complex part i its structural topology and autochthonous cover new field and geochronological data are used to define the distribution of mesoarchean basement rocks in the south central slave province this distribution reflects a single contiguous basement terrane that we propose to call the central slave basement complex it shows a structural topology that is internally consistent and compatible with known regional folding and faulting events a sample of a proposed basement gneiss below the courageous lake greenstone belt central slave province has been dated by u pb methods and yields an age of 3325 8 ma consistent with the new basement distribution this sample also contains 2723 3 ma metamorphic zircon and ca 2680 ma titanite the central slave basement complex is overlain by a thin discontinuous but distinctive cover sequence that includes minor volcanic rocks clastic sedimentary rocks and banded iron formation all previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the