the metamorphic internal zone of wopmay orogen early proterozoic canada 30 km of structural relief in a composite section based on plunge projection in wopmay orogen an early proterozoic volcano sedimentary marginal prism and its underlying archean structural basement deformed together during the 1885 ma calderian orogeny are exposed in oblique view as a result of regional cross folding a composite cross section displaying over 30 km of structural relief provides a framework in which to examine the geologic relationships in the metamorphic internal zone of the orogen a sole thrust separates archean basement and a thin mantle of autochthonous early proterozoic strata from the eastward transported calderian allochthon the allochthon is characterized by a transition from external thrust fold belt geometries in the east to polyphase isoclinal recumbent folds and thrusts in the west the center of the zone contains the hepburn batholith which has been been transported along with the deforming hot allochthon over the exposed basement both basement and allochthon are deformed by late large scale upright folds the metamorphic internal zone is interpreted to record the closing of a rift basin during which the basin fill was shortened thickened to over 30 km metamorphosed and thrust eastward while still hot over the relatively cool archean craton the basement presently exposed beneath the sole thrust remained relatively nondeformed during thrusting but underwent large scale folding due to continued compression during uplift of the internal zone

polylithionite from the rare metal deposits of the blachford lake alkaline complex n w t canada extensive deposits of rare lithophile elements are located within albitized and greisenized peralkaline granitb and syenite at thor lake in the blachford lake complex east of yellowknife northwest territories in canada five structural zones affected by metasomatic alteration and locally also brecciation host zoned assemblages with be nb ta ree y zr and ga mineralization polylithionite is a major phase in several units of the mineralized zones its abundance amounts to tens of thousands of tons the chemistry physical properties and structural characteristics of two polylithionite samples from widely separated locations are almost identical and are close to the typical properties of the end member lm polylithionite ti nb mg rb cs and fe3 fe2 are much higher whereas rb and cs are remarkably lower than their ranges in lepidolites from orogenic to post orogenic peraluminous granitic pegmatites however the compositional field of lm polylithionites is apparently transitional into that of lm lepidolites which partly overlaps with 2mt and 2m polytype ranges

new aeromagnetic data from the western enderby basin and consequences for antarctic india break up 1 in order to enhance our understanding of the continental break up between east india and antarctica a japanese german aerogeophysical survey was conducted off eastern dronning maud land and western enderby land antarctica the systematic survey with a line spacing of 20 km provides strong constraints on the timing of the initial break up of india antarctica compared to the conjugate margin south of sri lanka while the continent ocean transition is very well marked by pronounced positive and negative magnetic anomalies no hints on the existence of magnetic spreading anomalies north of this zone has been found in the new data set thus in contrast to most recently published models the data only support kinematic models which include a final break up of sri lanka india from our investigated area some 90 118 ma within the cretaceous normal superchron

mineralogy of the lake zone thor lake rare metals deposit n w t canada the proterozoic ca 2 1 ga blatchford lake suite hosts significant concentrations of rare metals at the core of a peralkaline granite syenite pluton after emplacement of the grace lake granite and the thor lake syenite within it bodies of nepheline syenite ijolite and urtite were intruded beneath the present day lake zone this is the largest of five zones of mineralization and lies close to the apex of the thor lake syenite a region which was then subjected to albitization microclinization and finally rare metal mineralization the underlying silica undersaturated rocks contain clots of rare metal bearing minerals including cerianite ce britholite ce thorite and calcium catapleiite interstitial to nepheline and aegirine the lake zone itself contains major quantities of zr in zircon nb in ferrocolumbite pyrochlore group minerals aeschynite group minerals and fergusonite y and ce in allanite ce monazite ce and bastnasite group minerals lesser amounts of ta y heavy

new insights into the southern margin of the archean proterozoic boundary in the north central united states based on u pb sm nd and ar ar geochronology abstract new geophysical analysis of the precambrian basement in minnesota iowa wisconsin indicates that an archean proterozoic boundary spirit lake trend previously recognized in nw iowa can be continued eastward into central wisconsin and farther east as the spirit lake tectonic zone sltz to test the age of paleoproterozoic crust south of this structure several subsurface samples of precambrian basement from the north central united states have been analyzed or re examined using modern techniques of u pb sm nd and 40ar 39ar geochronology the results fill in a major data gap for the region and show that all u pb crystallization ages for samples south of the sltz are geon 17 1700 1800 ma bedrock core samples from eastern nebraska are ca 1760 1800 ma and two samples from se south dakota immediately south of the sltz yield ages of 1762 28 ma vermillion and 1733 2 ma elk point xenoliths from impact breccia in the manson structure in north central iowa yield a similar age of ca 1705 30 ma and metagabbro from se minnesota yields an age of 1760 9 ma farther to the northeast zircons from paleoproterozoic gneiss in the basement of manitoulin island only a few km south of the superior craton in ontario also yield a geon 17 age 1714 10 ma sm nd model ages tdm for samples immediately south of the sltz fall in the range 1 9 2 2 ga indicating limited involvement of archean crust in contrast sm nd tdm ages for samples north of the sltz typically range from 2 5 to 3 0 ga for both paleoproterozoic plutons and archean gneisses ion microprobe analyses of zircons from the elk point and manson samples also show the presence of geon 16 overgrowths indicating a strong regional thermal overprint during geon 16 accretion this is supported by mid geon 16 hornblende 40ar 39ar ages for samples from se south dakota and se minnesota although no u pb ages are available for juvenile basement beneath the ca 1760 ma granite rhyolite suite of southern wisconsin south of the sltz sm nd model ages are typically ca 1 9 2 0 ga consistent with basement to the rhyolites being geon 17 in age collectively the data require that most if not all of the paleoproterozoic crust immediately south of the sltz formed during geon 17 and probably represents eastward continuation from colorado through nebraska of the yavapai crustal province in the sw united states penokean geon 18 crustal rocks are limited mainly to northern and central wisconsin east central minnesota and northern michigan these results also show that medium grade 500 c tectonothermal effects of the subsequent geon 16 mazatzal orogeny to the south continue into the north central united states both terranes probably also continue eastward into ontario canada and farther east into protolith of the grenville province

structural and tectonic interpretation of geophysical data along the eastern continental margin of india with special reference to the deep water petroliferous basins abstract the study area encompasses the eastern continental margin of india ecmi and the adjoining deep water areas of bay of bengal the region has evolved through multiple phases of tectonic activity and fed by abundant supply of sediments brought by prominent river systems of the indian shield detailed analysis of total field magnetic and satellite derived gravity data along with multi channel seismic reflection sections is carried out to decipher major tectonic features basement structure and the results have been interpreted in terms of basin configuration and play types for different deep water basins along the ecmi interpretation of various image enhanced gravity and magnetic anomaly maps suggest that in general the ene wsw trending faults dominate the structural configuration at the margin these maps also exhibit a clear density transition from the region of attenuated continental crust proto oceanic crust to oceanic crust based on which the continent ocean boundary cob has been demarcated along the margin basement depths estimated from magnetic data indicate that the values range from 1 to 12 km below sea level and deepen towards the bengal fan in the north and reveal horst graben features related to rifting a comparison of basement depths derived from seismic data indicates that in general the basement trends and depths are comparable in cauvery and krishna godavari basins whereas in the mahanadi basin basement structure over the 85 e ridge is clearly revealed in seismic data further eight multichannel seismic sections across different basins of the margin presented here reveal fault pattern rift geometries and depositional trends related to canyon fills and channel levee systems and provide a basic framework for future petroleum in this under explored frontier

abandoned paleocene spreading center in the northeastern indian ocean evidence from magnetic and seismic reflection data abstract magnetic data from the northeastern indian ocean reveal east west trending magnetic lineations 28 through 34 an abandoned spreading center asc and part of the cretaceous magnetic quiet zone cmqz which are used to reconstruct past tectonic processes involved in the evolution of the ocean offsets in the magnetic lineations outline the existence of four n s trending fracture zones 80 e indira 84 5 e and 86 e the 84 5 e fracture zone fz acts as a boundary separating oceanic crust with different magnetic isochron patterns the pairs of magnetic lineations 30 through 32n 2 between the 86 e fz and the ninetyeast ridge reveal an asc of about 65 ma age parallel to 0 5 s latitude the structure of the asc is well identified in seismic reflection data as an undulating basement topographic rise covered by up to 2 km of bengal fan sediments the spreading center might have initiated its activity along with other spreading centers of the wharton ridge and india antarctica ridge after the first major plate reorganization of the indian ocean about 95 5 ma but ceased shortly after formation of anomaly 30 about 65 ma then it jumped southward between anomalies 32n 2 and 33 the jump captured the oceanic crust of anomalies 30 through 32n 2 that initially formed on the antarctica plate but were later transferred to the indian plate the spreading centers in the vicinity of the ninetyeast ridge jumped several times southward to maintain a close proximity to the kerguelen hotspot

breakup and early seafloor spreading between india and antarctica summary we present a tectonic interpretation of the breakup and early seafloor spreading between india and antarctica based on improved coverage of potential field and seismic data off the east antarctic margin between the gunnerus ridge and the bruce rise we have identified a series of ene trending mesozoic magnetic anomalies from chron m9o 130 2 ma to m2o 124 1 ma in the enderby basin and m9o to m4o 126 7 ma in the princess elizabeth trough and davis sea basin indicating that india antarctica and india australia breakups were roughly contemporaneous we present evidence for an abandoned spreading centre south of the elan bank microcontinent the estimated timing of its extinction corresponds to the early surface expression of the kerguelen plume at the southern kerguelen plateau around 120 ma we observe an increase in spreading rate from west to east between chron m9 and m4 38 54 mm yr 1 along the antarctic margin and suggest the tectono magmatic segmentation of oceanic crust has been influenced by inherited crustal structure the kinematics of gondwanaland breakup and the proximity to the kerguelen hotspot a high amplitude e w oriented magnetic lineation named the mac robertson coast anomaly mca coinciding with a landwards step down in basement observed in seismic reflection data is tentatively interpreted as the boundary between continental transitional zone and oceanic crust the exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust which typically occurs above anomalously hot mantle together with the existence of the mca zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring steady state magmatic production of the kerguelen lip an alternative model suggests that the northward ridge jump was limited to the elan bank region whereas seafloor spreading continued in the west enderby basin and its sri lankan conjugate margin in this case the mca magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 ma

the rio grande fracture zone in the western south atlantic and its tectonic implications abstract a broad zone of linear mappable basement structures is observed north and northeast of the rio grande rise in the south atlantic ocean these structures lie along the same flow line as the sa o paulo ridge the florianopolis high and onshore lineaments suggesting that they all comprise the same tectonic trend the rio grande fracture zone the morphology developed along this fracture zone during the early opening of the south atlantic ocean formed a barrier to open ocean circulation during the aptian and allowed the formation of extensive evaporite deposits to the north of it

origin of the rajmahal traps and the 85 e ridge preliminary reconstructions of the trace of the crozet hotspot the 85 e ridge is a buried aseismic ridge approximately parallel to and west of the ninetyeast ridge in the northeastern indian ocean it was previously shown to be of probable volcanic origin emplaced on very young oceanic crust but no satisfactory model of emplacement of the rocks was offered we propose a model of origin of the rajmahal traps of northeastern india the 85 e ridge and afanasy nikitin seamount as the trace of the hotspot that now lies beneath the crozet islands in the southern indian ocean this reconstruction places the kerguelen hotspot which formed the ninetyeast ridge at the triple junction between greater india australia and antarctica before the breakup of eastern gondwana

sediment transport in analogue flume models compared with real world sedimentary systems a new look at scaling evolution of sedimentary systems in a flume evolution of sedimentary systems at large temporal and spatial scales cannot be scaled down to laboratory dimensions by conventional hydraulic froude scaling therefore many researchers question the validity of experiments aiming to simulate this evolution yet it has been shown that laboratory experiments yield stratigraphic responses to allocyclic forcing that are remarkably similar to those in real world prototypes hinting at scale independency with strong dependence on boundary conditions but weak dependence on the actual sediment transport dynamics this paper addresses the dilemma by contrasting sediment transport rules that apply in the laboratory with those that apply in real world geological systems it is demonstrated that the generation of two dimensional stratigraphy in a flume can be simulated numerically by the non linear diffusion equation sediment transport theory is used to demonstrate that only suspension dominated meandering rivers should be simulated with linear diffusion with increasing grain size coarse sand to gravel and shallowness of river systems the prediction of long term transport must be simulated by non linear slope dependent diffusion to allow for increasing transport rates and thus change in stratigraphic style to point out these differences in stratigraphic style three stages in infill of accommodation have been defined here i a start up stage when the system is prograding to base level e g the shelf edge with no sediment flux beyond the base level point ii a fill up stage when the system is further aggrading while progressively more sediment is bypassing base level with the progression of the infill and iii a keep up stage when more than 90 of the input is bypassing the base level and less than 10 is used for filling the accommodation by plotting the rate of change in flux for various degrees of non linearity varying the exponent in the diffusion equation it was found that the error between model and real world prototype is largest for the suspension dominated prototypes although never more than 30 and only at the beginning of the fill up stage the error reduces to only 10 for the non linear sandy gravelly and gravelly systems these results are very encouraging and open up ways to calibrate numerical models of sedimentary system evolution by such experiments

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

impact of the 90 e ridge at the indo burmese subduction zone imaged from deep seismic reflection data abstract as a result of the indo burmese active hyper oblique subduction part of the bay of bengal is presently subducting eastward below the burmese microplate the nature and structure of this basin is still poorly understood due to the lack of seismic data we have conducted two deep penetration seismic reflection surveys in the north eastern bay of bengal providing the first high resolution seismic image of the bengal basin fill and basement on basis of these data we are able to trace the 90 e ridge much more northward than previously thought i e up to 20 n along the indo burmese plate boundary we found out that the surface deformation the deep structure of the subduction zone and the geometry of the plate boundary could all be strongly influenced by the impact of a prominent asperity the 90 e ridge these effects are variable along the margin between 15 n and 18 n the ridge asperity brushes the active burmese plate boundary that strikes n10 e at this latitude all the structures framing the indo burmese wedge have a similar n10 e trend deformation at the plate boundary is mainly strike slip this is confirmed by the absence of subducted slab at depth as indicated by both seismicity and tomography the small component of shortening along this plate boundary is probably accommodated partly by the flexure of the ridge and partly within the deformed upper plate north of 19 n the ridge vanishes progressively the absence of basement topography together with the large amount of sediments provided by the brahmaputra delta facilitates the fast westward growth of the indo burmese wedge the seismicity fits a well developed subducted slab at depth in the narrow transition zone between 18 n and 19 n the 90 e ridge northern tips collides with the burmese microplate this collision could explain the rise of a subsuface flat and ramp system offshore ramree and cheduba islands and the strong uplift of the indo burmese wedge in mount victoria area

crustal evolution and sedimentation history of the bay of bengal since the cretaceous multichannel seismic reflection gravity magnetic and bathymetric investigations were made along five latitudinal profiles from the eastern shelf of india to the andaman shelf and four nw se profiles in the western basin off madras seismics on three latitudinal profiles in the bay of bengal the trend of the fracture zones the locations of the magnetic chron 34 and the cretaceous magnetic quiet zone suggest that greater india separated from antarctica after a period of transform motion in the early cretaceous that is about polarity chron m0 120 ma and drifted northwestward negative gravity anomalies are associated with basement rises including the 85 e ridge between the continental margin of india and the ninetyeast ridge magnetic reversals and northward trend of the 85 e ridge support a hotspot origin of the ridge and its emplacement most likely after the cretaceous quiet period juxtaposition of high amplitude hyperbolic reflections down faulted continental blocks buried under thick sediments and associated gravity and magnetic anomalies mark the boundary between continental and oceanic rocks at the foot of continental slope about 80 km seaward of the present continental shelf edge eight seismic sequences as thick as 8 5 km overlie the early cretaceous oceanic basement and include four unconformities lower eocene upper oligocene upper miocene and upper pleistocene which have been correlated to the major geologic tectonic events we also interpret the late cretaceous early tertiary features on the eastern flank of the buried 85 e ridge as carbonate reefs the observation of steep subduction of older cold indian plate beneath the burmese plate near the andaman islands suggests the sunda arc in this region as low to intermediate stress subduction zone

analysis of seasat l band radar imagery of the west bay indin lake fault system northwest territories seasat l band 23 5 cm synthetic aperture radar sar images of the yellowknife area in the northwestern canadian shield have been digitally processed and correlated with geologic maps of the region we specifically analyze the distribution of faults of the west bay indin lake system and conclude from cross cutting geological relationships that this fault system formed at circa 1964 pm 36 ma in response to a sinistral oblique component of extension which led to rifting of the coronation continental margin by 1961 pm 5 ma through this analysis we show that spaceborne sar is a useful tool for delineation of brittle tectonic features such as faults joints and dikes on a regional scale enabling field geologists to place their observations in a broader context when coupled with field surveys of small representative areas thousands of square kilometers can be quickly and efficiently analyzed in a statistically uniform manner broad coverage of the planet and other bodies in the solar system by sp

antarctic marine gravity field from high density satellite altimetry summary closely spaced satellite altimeter profiles 5 km collected during the geosat geodetic mission geosat gm and those planned for the extended ers 1 mission are easily converted to grids of vertical gravity gradient and gravity anomaly as profile spacing decreases it becomes increasingly difficult to perform a crossover adjustment on the original geoid height profiles without introducing large cross track gradients if one is only interested in the horizontal and vertical derivatives of the gravitational potential however adjustment of the profile is unnecessary the long wavelength radial orbit error is suppressed well below the noise level of the altimeter by simply taking the along track derivative of each profile ascending and descending slope profiles are then interpolated onto separate uniform grids these two grids are summed and differenced to form comparable grids of east and north vertical deflection using laplace s equation the vertical gravity gradient is calculated directly from the vertical deflection grids fourier analysis is required to construct gravity anomalies from the two vertical deflection grids these techniques are applied to high density 2 km profile spacing geosat gm profiles in antarctic waters 60 s to 72 s gridding and interpolation are performed using the method of projection onto convex sets where the smoothness criteria corresponds to upward continuation through 4 km of ocean the resultant gravity grids have resolution and accuracy comparable to shipboard gravity profiles after adjustment of a dc shift in the shipboard gravity profiles 5 mgal the rms difference between the ship and satellite gravity is 5 5 mgal many interesting and previously uncharted features are apparent in these new gravity maps including a propagating rift wake and a large leaky transform along the pacific antarctic rise

coronation loop resurrected oscillatory apparent polar wander of orosirian 2 05 1 8 ga paleomagnetic poles from slave craton abstract the coronation loop is a 110 arcuate sweep of 15 paleomagnetic poles with ages of ca 1950 1850 ma derived from contemporaneous basins on the western coronation southern great slave and eastern kilohigok margins of the slave craton in the northwestern canadian shield although the paleomagnetic results are either demonstrated as primary or most parsimoniously interpreted as such it is likely they were subsequently rotated shortly after deposition during conjugate transcurrent faulting along the conjugate mcdonald great slave and bathurst kilohigok strike slip fault systems no rotation is expected of poles from the epicratonic coronation margin previous analyses have debated the amounts of local rotations in the other basins with one end member view that the spread in paleomagnetic poles is entirely due to local rotations here we propose that relative to the principal axis of compression for conjugate faulting the far field bathurst and mcdonald fault systems have rotated equally and oppositely 12 to widen an original 60 geometry to the present day 84 angle we rotate pre 1840 ma poles from the immediate 50 km environs of these faults 12 ccw bathurst and cw mcdonald this simple structural retrodeformation brings time correlative poles from the kilohigok and great slave basins into agreement as independent support for this modest amount of local rotations paleocurrent measurements from the great slave basin from sections interspersed among paleomagnetic sampling localities and elsewhere indicate that large vertical axis rotations have not taken place our restoration of the coronation loop to slave s cratonic reference frame slightly increases its angular sweep and implies rapid polar wandering that is irreconcilable with plate tectonic rates of motion we interpret the coronation loop as a signal of rapid oscillatory true polar wander tpw which suggests that archean paleoproterozoic supercratons modulated mantle topology in much the same manner as recent supercontinents rodinia and pangea

structure and evolution of the afanasy nikitin seamount buried hills and 85 e ridge in the northeastern indian ocean abstract geophysical data of the afanasy nikitin seamount ans partly buried hills and 85 e ridge in the northeastern indian ocean were studied together with published seismic refraction results to understand genesis and evolution of the structures the ans joins the 85 e ridge through isolated buried hills and intervening subsurface structures and together they form a linear ridge system the gravity signature of the ridge system changes from negative to positive towards south of 5 n which seems to coincide with termination thinning of pre collision continental sediments in the bay of bengal thick pile of bengal fan sediments had great impact on underneath pre collision sediments as well as on basement rocks the process resulted in attaining higher velocities up to 6 6 and 7 1 km s respectively gravity model studies suggest that structures of the ridge system are compensated in different modes the ans is underlain by an 8 km thick deep crustal body of magmatic rocks while beneath other structures oceanic crust is down flexed up to 2 5 km the presence of metasediments more dense than volcanic rocks and flexure of the lithosphere would explain the negative gravity anomaly over the 85 e ridge whereas lack of metasediments and magmatic rocks at depth would explain the compensated positive anomaly over the ans the width of the 85 e ridge the wavelength 190 km and amplitude 2 5 km of the flexed oceanic crust and the intersection of the ridge with the mesozoic fracture zones suggest that the ridge was formed in intraplate position when the lithosphere underneath was approximately 35 myr old initial emplacement of the ans was coeval with the formation of oceanic lithosphere at 80 73 ma the hotspot forming the 85 e ridge had reactivated the ans during the paleocene and brought it to the sea surface then it underwent erosion and subsidence processes the deformation activity had converged the north and south parts of the ans at late miocene and again at late pleistocene and northward buried hills at early pliocene

mesozoic anomalies in the bay of bengal abstract the analysis of 8200 line km of total magnetic intensity data in the bay of bengal northeastern indian ocean revealed the presence of approximately n30 e trending seafloor spreading type magnetic anomalies these anomalies resemble the mesozoic anomaly series m11 m0 reported elsewhere the oldest anomaly m11 132 5 ma identified close to the east coast of india is followed by the younger series of mesozoic anomalies towards the offshore some of the anomalies are offset by 60 80 km the configuration of the offsets of the isochrons allowed us to propose approximately n120 e trending oceanic fracture zones the mesozoic crust of 132 5 118 ma is estimated to evolve with an average half spreading rate of 3 5 cm yr except for the ocean floor between the m9 and m4 magnetic isochrons this part of the crust appears to be affected by the buried subsurface 85 e ridge

great slave lake shear zone nw canada mylonitic record of early proterozoic continental convergence collision and indentation abstract great slave lake shear zone in the nw canadian shield is an excellent example of the kinematic magmatic and thermal evolution of a crustal scale shear zone associated with a collisional continental boundary it is a 25 km wide corridor of granulite to lower greenschist facies mylonites and cataclastic fault rocks developed in the deep seated parts of an early proterozoic 2 0 1 9 ga magmatic arc constructed on the upper rae plate at the contact between the archean slave and rae continents the rocks of great slave lake shear zone the thelon magmatic arc and the taltson magmatic zone are all components of the same magmatic arc but their geological histories reflect different aspects of the continental interaction

geodynamic implications of moving indian ocean hotspots abstract the reconstructed positions of many of the indian ocean intraplate volcanic features are inconsistent with the present positions of the hotspots to which they have been ascribed the discrepancy can be explained by the past motion of the hotspots themselves and in the errors introduced in an absolute plate model based on fixed hotspots previous models for the motion of hotspots in the indian ocean e g steinberger and o connell in the history and dynamics of global plate motions 2000 pp 377 398 and geophys j int 132 1998 412 434 antretter et al earth planet sci lett 203 2002 635 650 have had success in matching paleolatitudes of the kerguelen hotspot but not the marion or reunion hotspots we calculate the motion of the indian ocean hotspots using a method described in steinberger and o connell op cit employing a plate motion model revised using identification of mesozoic magnetic anomalies in the enderby basin gaina et al in the evolution and dynamics of the australian plate in press we find a motion of 7 south for the kerguelen plume similar to previous models antretter et al op cit this motion alleviates the discrepancies in the position of the kerguelen plume and its purported involvement in the formation of the ninetyeast ridge rajmahal traps bunbury basalts and assorted volcanics of the western australian margin we find a motion of 5 north and 7 south for the reunion and marion plumes respectively the motion of the reunion hotspot is consistent with paleolatitude estimates the apparent fixity of the marion hotspot torsvik et al earth planet sci lett 164 1998 221 232 is not well constrained but may be due to the combination of hotspot motion and true polar wander acting in opposing directions we present revised absolute finite rotations for the indian plate for the last 65 ma based on the motion of these hotspots which place india farther north in the past than for fixed hotspot models

implications of middle eocene epizonal plutonism for the unroofing history of the bitterroot metamorphic core complex idaho montana petrologic and geochronologic limits on the progression of east directed extensional unroofing of the bitterroot metamorphic core complex provide a means to compare rates of denudation across the footwall during the earlystages of extensional denudation u pb and 4 0 ar 3 9 ar data from epizonal plutons in the northwestern and southwestern bitterroot metamorphic core complex demonstrate that much of the western footwall was exhumed to shallow levels 1 5 km by ca 49 ma while eastern footwall rocks remained at high temperatures following an episode of isothermal decompression u pb ages of epizonal plutons along the western margins of the footwall suggest that as much as 13 km of unroofing occurred between anatectic melting of belt supergroup metasedimentary rocks in the western footwall no later than 56 ma and emplacement of the adjacent epizonal lolo hot springs batholith at ca 49 ma in the northwestern bitterroot range tectonic denudation during this interval removed roughly the same amount of overburden from eastern footwall rocks suggesting that the initial stages of unroofing may have been accommodated by a planar moderately east dipping 30 50 shear zone 4 0 ar 3 9 ar cooling data show that cooling in the western footwall was synchronous with unroofing in contrast we suggest that a significant delay in cooling of eastern footwall rocks may have resulted from a slight increase in geothermal gradient following this unroofing stage exposure of a west dipping mylonitic shear zone along the western limb of the bitterroot complex may provide a key for interpreting the postmylonitic structural evolution of the core complex if this shear zone can be correlated to the bitterroot mylonite zone to the east then strong evidence for flexural uplift consistent with postmylonitic passage of a rolling hinge emerges

early proterozoic geomagnetic field in western laurentia implications for paleolatitudes local rotations and stratigraphy abstract the east west trending post orogenic subvertical cleaver dykes from the great bear magmatic arc are dated by the u pb baddeleyite method to be 1740 5 4 ma they have a mean direction of magnetization of d i 136 5 and 57 4 k 64 95 4 5 based on samples from 17 diabase dykes a mean location 65 7 n 118 03 w and a corresponding paleopole at 19 4 n 83 3 w a95 6 1 studies of baked contacts country rocks distant from dykes and later intrusions that cut them show that the dykes became magnetized soon after intrusion the paleopole is not significantly different from that obtained from the 1759 1 ma post orogenic jan lake granite from the hudsonian orogen in southeast saskatchewan these paleopoles fall among a cluster previously determined from post orogenic overprints in a wide variety of rocks and geological settings across laurentia their mean is 20 8 n 94 5 w a95 5 2 collectively they record a widespread remagnetization episode following the hudsonian orogeny during which the polarity of the field remained constant this single polarity interval the cleaver superchron has a mean age of approximately 1747 ma and its duration is at least 13 million years but is probably much longer with further refinement it could become a global statigraphical marker there are 18 paleopoles from red beds and igneous rocks which record the paleofield in western laurentia mainly over an extended interval we estimate to be at least 100 million years between about 1960 and 1830 ma during the hudsonian orogeny over this interval western laurentia remained at a low and essentially constant latitude reversals of the geomagnetic field occurred opposed magnetizations being of approximately equal frequency there are numerous magnetochrons which could in future assist in regional stratigraphic correlations on the basis of six studies we estimate that the reference paleopole for this interval is at 1 1 s 71 7 w a95 12 1 n 6 paleopoles from the other 12 studies are spread over a 100 arc indicating large variable rotations about local vertical axes which probably resulted from transcurrent faulting caused by indentation of the bordering archean slave and superior cratons during the hudsonian orogeny these reference paleopoles show that during the hudsonian orogeny western laurentia was in paleolatitudes of 15 30 during post hudsonian time ca 1750 ma paleolatitudes were about 20 higher paleowind directions inferred by others indicate that laurentia during these intervals was in the northern hemisphere

pre breakup continental extension in east gondwanaland and the early opening of the eastern indian ocean abstract extension of continental crust by up to 360 km on a north northeast azimuth in the great australian bight occurred prior to the middle cretaceous 96 ma onset of seafloor spreading between australia and antarctica this large amount of continental extension is constrained to the late jurassic 160 ma middle cretaceous interval about a pole estimated to lie about 40 southeastward of southeastern australia using bathymetric data combined with seismic and magnetic determinations of the continent ocean boundaries off australia india and antarctica we determine a revised fit of east gondwanaland prior to continental extension and at various stages thereafter in the first stage from 160 to 132 5 ma m11 india australia rotated from antarctica with continental crustal extension between australia and antarctica and largely transform motion between the coromandel coast margin of india and the kron prins olav kyst margin of antarctica in the second stage from 132 5 to 96 ma india rotated northwestward away from australia antarctica producing m10 and younger magnetic anomalies along the western margin of australia continental extension was greatest between australia and antarctica during this stage the newly determined 132 5 96 ma rotation between india and australia provides an improved fit between the orientation of observed m10 m0 magnetic anomalies and the southward decrease in spreading rate off the western australian margin together with bathymetry the new rotation parameters lead to identification of the abandoned 96 ma spreading ridge along the lost dutchmen and dirck hartog ridges the naturaliste fracture zone is found to lie nearly parallel to the early cretaceous transform direction between india and antarctica suggesting that slow seafloor spreading required between australia and antarctica to accommodate the continental extension further east occurred between naturaliste fracture zone and the m4 m0 anomalies adjacent to the naturaliste plateau the revised fit of india in east gondwanaland reduces the eastward extent of greater india to 95 e but does not change its extent as far north as the cape range fracture zone the new pole determined for the early cretaceous rotation between india and australia antarctica lies within 15 of the southern tip of india significantly closer than in previous determinations thereby reducing the amount of seafloor spreading generated between africa and antarctica by the india antarctica rotation and thus offering a possible solution to the perceived spreading rate problems between m11 and m0 in the somali mozambique and antarctic basins

do fracture zones define continental margin segmentation evidence from the french guiana margin abstract plate reconstructions suggest that the french guiana margin in the west equatorial atlantic is a highly segmented margin with both rift and transform style features we describe here the results of modelling coincident multi channel and wide angle seismic gravity and magnetic data acquired along two transects of this margin the resulting models not only highlight the degree of structural segmentation but also demonstrate the effect of trans tension on margin evolution as a whole the margin is characterised by 35 37 km thick pre rift continental crust which is separated from unusually thin oceanic crust 3 4 km by thinned continental and or transitional regions to the north the margin exhibits a 320 km wide zone of thinned continental crust adjacent to a narrow ocean continent transition and is interpreted as a transform margin where the wide zone of thinned crust is a result of profile orientation being highly oblique to the direction of rifting approximately 240 km to the south the margin is characterised by a 70 km wide zone of thinned continental crust which is wider than typical for transform and narrower than typical for rifted margins this crustal structure is interpreted to reflect a leaky transform formed by trans tensional extension these observations suggest that fracture zone influenced geometry of equatorial atlantic rifting did not produce a well defined margin crustal structure but instead resulted in margin segments which display characteristics of both rift and transform tectonic processes the associated abundance of fracture zones has likely also affected the post rift evolution of the margin and provided topographic basement highs which acted as sediment dams to the northwards flux of sediment from the amazon

thin oceanic crust below bay of bengal inferred from 3 d gravity interpretation abstract the bay of bengal bob lithosphere evolved due to rifting between india and antarctica and subsequent seafloor spreading process since the early cretaceous is known for the presence of thick bengal fan sediments covering the entire bob with maximum sediment accumulation of 22 km in the offshore bangladesh three dimensional crustal structure of the bob is investigated by integrating the sediment isopach map seismic reflection refraction and wide angle reflection data with the 3 d gravity modelling large amount of sediment seismic velocities have been used to construct depth varying density contrast with respect to the underlying crustal layer due to deeper burial effect by a quadratic density function 3 d gravity effect of the sedimentary layer computed for the derived density distribution shows large variations ranging from 5 mgal over the ninetyeast ridge near the equator to as low as 210 mgal at the apex of bengal fan in the bangladesh shelf region the residual crustal bouguer anomaly map obtained after subtracting the effect of water and sediment layers and normalised for 6 km uniform crust display crustal thickness variations with depth to moho in the bob region varying from 11 km in the south to as much as 35 km in the bangladesh shelf with moho depressed locally below both 85 e and ninetyeast ridges the total crust map obtained after subtracting the water and sediment layers from the moho depth map shows an unusually thin oceanic crust of 2 4 km in the deep basinal areas of bob and 10 12 km of thick crust below 85 e and ninetyeast ridges due to the widespread nature of observed thin crust as well as the presence of plume traces that have emplaced 85 e and ninetyeast ridges we infer its formation during plume spreading interactions in the eastern indian ocean further higher flexural rigidity of lithosphere and cold mantle in the bob region indicates that huge amount of bengal fan sediments were emplaced over the unusually thin oceanic crust and a strong mantle similar to that observed in the amazon fan

paleoproterozoic stark formation athapuscow basin northwest canada record of cratonic scale salinity crisis abstract the paleoproterozoic stark formation a thick breccia unit formed during the transition from marine to non marine foreland basin fill in the athapuscow basin northwest territories canada four types of breccia occur in the stark formation 1 bedded breccias at the base of the unit overlying the basinal facies of the underlying pethei group are interpreted as debris flows 2 massive chaotic breccias occur in the lower part of the stark formation throughout the basin and these are interpreted to be evaporite collapse breccias 3 siltstone and mudstone breccia occurs in the upper part of the stark formation and is also interpreted as evaporite collapse breccia though much less evaporite was involved in its formation than the underlying chaotic breccia and 4 laminar accretionary breccia consisting of laminar carbonate clasts commonly with flat bottoms and convex up upper surface geometries these clasts occur in lenticular pods and are interpreted to form in caves following dissolution of evaporites or carbonates the widespread presence and abundance of evaporite pseudomorphs especially halite in all breccia types the stratigraphic restriction of the stark formation and irregular filling above this unit indicates that all but the bedded breccia formed by the dissolution and collapse of evaporites chaotic breccia directly above underlying pethei group shallow water and deep water carbonates indicates the evaporates formed across the platform and adjacent deep basin foundered megaclasts up to 1 5 km long and 40 m thick in basinal settings suggest cumulative evaporite thicknesses on the order of a few tens to a few hundreds of meters carbonate clasts in all the breccias contain abundant wave ripples planar laminations stromatolites and ooids these structures along with a paucity of subaerial exposure features indicates nearly all carbonate deposition in the stark was subaqueous thus by inference the evaporites in the stark formation were also partially subaqueous displacive halite pseudomorphs are common throughout all facies in the stark formation silicified and dolomite filled halite pseudomorphs are common along the contact between the evaporite collapse breccia and underlying carbonate platform pethei group and surrounding large clasts in basinal settings silicified hopper crystal casts within deep water rhythmites of the uppermost pethei group indicate precipitation of halite over deep waters gypsum pseudomorphs are exceedingly rare and anhydrite pseudomorphs were not observed the dominance of halite features in this basin and two penecontemporaneous basins kilihigok basin and wopmay orogen surrounding the slave craton suggests that relative to the present day seawater at 1 9 to 1 8 ga was oversaturated with respect to calcium carbonate had higher hco3 concentration and possibly was depleted in so42 the dearth of extensive thick bedded marine gypsum in the archean and early paleoproterozoic can be explained by low atmospheric oxygen content prior to 2 ga in conjunction with increased carbonate saturation before 1 6 ga

iron formations a global record of neoarchaean to palaeoproterozoic environmental history abstract iron formations if represent an iron rich rock type that typifies many archaean and proterozoic supracrustal successions and are chemical archives of precambrian seawater chemistry and post depositional iron cycling given that if accumulated on the seafloor for over two billion years of earth s early history changes in their chemical mineralogical and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving earth perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes the supply of nutrients to the oceans and ultimately diversification of the biosphere and complex life forms many of the key recent innovations in understanding if genesis are linked to geobiology since biologically assisted fe ii oxidation either directly through photoferrotrophy or indirectly through oxygenic photosynthesis provides a process for if deposition from mineral precursors the abundance and isotope composition of fe ii bearing minerals in if additionally suggests microbial fe iii reduction a metabolism that is deeply rooted in the archaea and bacteria linkages among geobiology hydrothermal systems and deposition of if have been traditionally overlooked but now form a coherent model for this unique rock type this paper reviews the defining features of if and their distribution through the neoarchaean and palaeoproterozoic this paper is an update of previous reviews by bekker et al 2010 2014 that will improve the quantitative framework we use to interpret if deposition in this work we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans

rb sr isochron ages magmatic 87sr 86sr initial ratios and oxygen isotope geochemistry of the proterozoic lava flows and intrusions of the east arm of great slave lake northwest territories canada two rb sr isochrons from the oldest wilson island group and one of the youngest pearson formation proterozoic volcanic units in the east arm of great slave lake give dates and initial ratios of 1810 19 ma 0 7051 0 0008 and 1809 30 ma 0 7089 0 0004 respectively these dates restrict the great slave supergroup entirely to the aphebian the pearson formation date is interpreted as magmatic but it is considered to be rapidly followed by a significant metamorphic and tectonic event within the area both the above suites and the volcanic formations of intermediate age have undergone metamorphism up to and including epidote amphibolite facies a study of remnant clinopyroxene grains from these formations has indicated general increases in 18o of whole rocks during epidote actinolite facies metamorphism of 0 4 2 0 the tholeiitic volcanic rocks and intrusions all indicated 18o values typical of modern continental tholeiites 5 9 7 0 the highest 18o came from those suites that were suspecte

magmatic evolution of the southern great bear continental arc northwestern canadian shield geochronological constraints the calc alkaline great bear continental arc in the wopmay orogen developed after a collision ca 1890 ma of the archean slave craton with the paleoproterozoic hottah terrane to the west u pb zircon dating of three volcanic and six intrusive rocks from the southern part of the arc shows four stages of development i intrusion of a few small sodic leucogranite plutons at 1873 2 ma into a previously folded metasedimentary sequence ii abundant calc alkaline felsic volcanism and subvolcanic intrusions during the period 1870 1866 ma bracketed by five ages iii intrusion of large calc alkaline granitic plutons including the marian river batholith dated by zircon and titanite from two samples at 1866 2 3 ma and iv emplacement of the potassic faber lake rapakivi granite at 1856 2 3 ma the arc was developed on the hottah terrane due to easterly subduction of an oceanic plate under the amalgamated slave craton hottah terrane the oldest exposed rocks in the southern part of the arc are remnants

stratigraphy and structure of the early proterozoic wilson island group east arm thrust fold belt n w t investigation of the stratigraphy and structure of the 1 9 ga wilson island group has led to sedimentological and tectonic interpretations that are consistent with models for an early proterozoic collision between the slave and rae provinces the lower part of the 8 km thick wilson island group consists of alluvial clastic rocks and a bimodal volcanic suite these are overlain by mixed siliciclastic and carbonate rocks that are inferred as representing a combination of alluvial and shallow marine sedimentation finally an upward fining succession of feldspathic sandstone ironstone and mudstone records progradation of an alluvial system followed by a major transgression soft sediment deformation structures intrabasinal coarse detritus and abrupt vertical facies changes collectively indicate that the early stages of deposition were tectonically controlled regional penetrative foliations and a steeply plunging stretching lineation reflect deformation concomitant with greenschist facies to lower amphibo

organic remains in late palaeoproterozoic granular iron formations and implications for the origin of granules toward the end of the palaeoproterozoic era over 109 billion tonnes of banded bif and granular gif iron formations were deposited on continental platforms granules in iron formations are typically sub spherical structures 0 2 to 10 mm in size whereas concretions are 10 mm both types of spheroids are preserved throughout the sedimentological record their formation has typically been interpreted to originate from reworked fe rich sediments in high energy wave agitated shallow marine environments new evidence from six different late palaeoproterozoic granular iron formations gif however suggests that some granules are the result of diagenetic reactions in addition to other features driven by microbial processes and mechanical movements characteristic coarse grain interiors and septarian type cracks inside granules akin to those features in decimetre to meter size concretions are interpreted as desiccation features from hydrated diagenetic environments where sulphate and or ferric iron were reduced while organic matter om was oxidised inside granules those granules derived from sulphate reduction preserve diagenetic pyrite rims whereas those formed via ferric iron reduction preserve diagenetic magnetite along their rims other diagenetic minerals including apatite mixed with om and various carbonate phases are commonly preserved within granules combined with systematically 13c depleted carbonate these diagenetic mineral assemblages point to the oxidative decay of om as a major process involved in the formation of granules spheroidal equidistant haematite laminations surround some granules and contain apatite associated with carbonate om and ferric ferrous silicates and oxides that further suggest these structures were not shaped by wave action along sediment water interfaces but rather by chemical wave fronts and biomineralisation our results demonstrate that the formation mechanisms of gif also involve microbial activity and chemically oscillating reactions as such granules have excellent potential to be considered as promising biosignatures for studying precambrian biogeochemistry as well as astrobiology

depositional facies and detrital composition of the paleoproterozoic et then group n w t canada sedimentary response to intracratonic indentation paleoproterozoic intracratonic compression across the slave province resulted in eastward indentation of the wedge shaped slave craton into the thelon orogen indentation was accommodated by shortening and crustal thickening at the apex of the slave wedge bounded to the south by the right lateral mcdonald fault and to the north by the left lateral bathurst fault the paleoproterozoic nonmarine et then group in the east arm of great slave lake records the history of this indentation the et then group consists of two formations the older murky formation is an alluvial fan conglomerate deposited unconformably over older paleoproterozoic sedimentary and igneous rocks and archean metamorphic and igneous rocks clast composition indicates that the murky formation was derived from these older rocks eroded during initial translation and uplift on the mcdonald fault system the younger preble formation conformably overlies the murky formation and is a braided fluvial sandstone the modal framework composition

fracture zones in the north atlantic morphology and a model a morphology is presented for the typical cross section of an inactive fracture zone in the atlantic inactive fracture zones typically consist of an asymmetric valley to the young side of the fracture zone plane and a high wall or scarp to the old side large offset fracture zones may be accompanied by a marginal valley on the other side of the high wall this topography is superimposed on the depth age step due to lithospheric cooling a model is developed which accounts for this morphology the model relates the inactive fracture zone morphology to the topography found at present day intersections of the spreading axis with transform faults the asymmetry of the median valley near fracture zones plays an important role in explaining the typical fracture zone morphology the existence of a median valley is related to the viscous delay of the upwelling mantle material at the spreading axis its asymmetry near fracture zones can be accounted for by modelling the viscous drag exerted by the lithosphere on the asthenosphere if the viscosity is low as under reykjanes ridge and the east pacific rise no median valley develops and a different morphology may be expected the occurrence of marginal valleys is interpreted as the result of lithospheric warping when the graben walls in the transform domain which are caused by tension due to horizontal thermal contraction are excessively high

short lived 1 9 ga continental margin and its destruction wopmay orogen northwest canada a 1 9 ga continental margin in the northwest of the canadian shield evolved in less than 15 m y from a zone of crustal stretching through a phase of passive margin subsidence to attempted subduction beneath an exotic microcontinent reversal of subduction polarity and generation of a major episutural magmatic arc occurred within 15 m y following microcontinental accretion terminal collision occurred 90 m y at most after initial rifting certain differences in detail between this collided margin and those of phanerozoic age may be related to subduction of newly rifted lithosphere rapid evolution of the orogen the contracted passive margin phase in particular is consistent with but does not prove more rapid recycling of oceanic plates at that time

temporal distribution of granitoid plutonic rocks in the archean slave province northwest canadian shield granitoid rocks in the slave province consist of 4 0 2 8 ga granitoid gneisses that predate the 2 71 2 65 ga volcanic and turbiditic rocks of the yellowknife supergroup and 2 70 2 58 ga granitoid plutons that intrude the yellowknife supergroup u pb zircon ages and nd and pb isotopic data indicate that the older granitoids are restricted to the western part of the slave province granitoid gneisses in the eastern slave province previously suspected to predate the yellowknife supergroup are similar in age to the volcanic rocks in this paper the results of a detailed geochronological study of plutonic rocks of the contwoyto lake nose lake area of the northcentral slave province are reviewed and integrated with the available age data base for plutonic rocks elsewhere in the slave province the data indicate that the timing of the later 2 7 ga plutonism is bimodal consisting of two distinct magmatic periods separated by approximately 20 ma of apparent magmatic quiescence the first period was synvolc

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

accretion history of the trans hudson orogen in manitoba and saskatchewan from paleomagnetism lithoprobe s sponsorship has led to the acquisition of paleomagnetic data from 20 units throughout the paleoproterozoic trans hudson orogen tho of saskatchewan and manitoba essentially the first such data for the region discussed summarily in this paper they provide an initial framework for the tho s tectonic evolution they show that the archean hearne and superior cratons were at subtropical and subpolar paleolatitudes respectively at 1875 ma with the lynn lake laronge arc midway between them in the manikewan ocean by 1855 ma this ocean was still 5500 700 km wide and its seafloor was subducting northwestward under the hearne craton and pericratonic peter lake domain margin with the coeval development of an andean type continental magmatic arc the wathaman chipewyan batholith between 1855 and 1810 ma coalescing apparent polar wander path apwp segments record closing of the ocean at a rate of 12 cm a trapping and accreting the several separate intervening terranes flin flon hanson lake lynn lake laronge rottenstone and presumably also other tho terranes from 1815 to 1775 ma the assembled terranes drifted as a coherent craton yielding a stillstand and hairpin in the apwp

thermomechanical evolution of oceanic fracture zones a fracture zone fz model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere as the lithosphere cools by conduction thermal and mechanical boundary layers develop and increase in thickness as age 1 2 surface expressions of this development such as topography and deflection of the vertical i e gravity field are most apparent along major fz s because of the sharp age contrast a simple model including the effects of lateral heat transport but with no elastic layer predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the fz ages observations however show that both the fz topography and the deflection of the vertical remain sharp as the fz evolves these two observations as well as the observed asymmetry in deflection of the vertical profiles across the udintsev romanche and mendocino fz s are explained by including a continuous elastic layer in the model the asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the fz modeling also shows that the evolution of the fz topography is extremely sensitive to the initial thermal structure near the ridge transform intersection model geoid steps and their development with age are used to access techniques for measuring geoid offsets across fz s reasonable step estimation techniques will underestimate the overall step amplitude by up to 50 this implies that abnormally thin thermal boundary layers derived from studies of geoid height versus age are not required by the data

two paleoproterozoic statherian siliciclastic metasedimentary sequences in central wisconsin the end of the penokean orogeny and cratonic stabilization of the southern lake superior region abstract statherian siliciclastic sedimentary rocks in the southern lake superior region are key to understanding the evolution and stabilization of paleoproterozoic crust in the midcontinent deposited during an important period of significant crustal growth controversy exists regarding the conditions of sandstone deposition and the relations between magmatism and sedimentation in central wisconsin the hamilton mounds inlier although restricted in areal extent is significant in that it reveals the existence of two separate statherian units an 1800 1760 ma immature meta arkose and a post 1760 ma baraboo interval supermature quartzite the meta arkose is crosscut by 1762 ma granite and contains abundant penokean age detrital monazite grains the younger quartzite contains detrital monazite as young as 1750 ma and was deposited unconformably upon these older units all units experienced 1630 ma low grade recrystallization and isotopic disturbance associated with the mazatzal orogeny in addition the supermature quartzite experienced k metasomatism and brecciation at 1470 ma ascribed to regional scale hydrothermal alteration related to the influence of the nearby 1470 ma wolf river batholith collectively these data suggest an early statherian episode of arkosic deposition and magmatism during yavapai orogenesis followed by a longer interval of deposition of supermature quartz arenite associated with crustal stabilization the existence of two separate statherian units reconciles apparent conflicts in depositional models and provides a greater understanding of the paleoproterozoic evolution of the midcontinent

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

sediment volume and mass beneath the bay of bengal rates of sediment accumulation and the amount of sedimentary fill in depocenters lying downstream of erosion in the himalayas and tibet can provide some insight into tectonics and geological history the objective of this paper is to put on record the best estimates which are possible with existing data of the volume and mass of sediments sedimentary rock and metasedimentary rock beneath the sea floor of the bay of bengal the sedimentary section in the bay of bengal is divided into two parts 1 eocene through holocene sediments and sedimentary rocks which post date the initial india asia collision volume 12 5 106 km3 mass 2 88 1016 t this is most of the bengal fan including its eastern lobe the nicobar fan plus some of the outer bengal delta 2 early cretaceous through paleocene pre collision sedimentary and metasedimentary rocks volume 4 36 106 km3 mass 1 13 to 1 18 1016 t these are interpreted as continental rise and pelagic deposits

early cretaceous fracture zones in the bay of bengal and their tectonic implications constraints from multi channel seismic reflection and potential field data abstract due to the lack of clearly discernible magnetic anomaly identifications and fracture zones in the bay of bengal bob the early cretaceous plate reconstruction history of eastern gondwanaland remains an unresolved issue with regard to the separation of the east coast of india from the conjugate enderby basin of antarctica based on the correlation of oceanic basement information obtained from a vast amount of deep reflection seismic data we identified 16 nw se trending fracture zones fz1 fz16 in the bob region the trends of these fracture zones at the ecmi indicate two distinctly different sets one set in the direction n25 w to n35 w in the southern part fz1 fz7 and the other set in n50 w fz8 fz15 direction along the northern part these two sets of fracture zones further support the earlier inferences about the segmented nature of the ecmi into southern sheared oblique rifted margin and the northern normal rifted margin while the southern part of the ecmi had formed under slow asymmetric spreading and mixed transform setting with its conjugate western enderby basin the northern part of the ecmi formed after the breakup of the elan bank from india at m2 considering this scenario we infer the presence of late mesozoic younger to m4 anomalies along the ecmi and the subdued nature of magnetic anomalies in the western bob can be explained by the obliquity of fracture zones with respect to the coast as observed in many marginal smooth zones of the world oceans the disposition of the 85 e ridge with respect to the pre evolved bob crust and early cretaceous volcanic episodes in the region together indicate an emplacement of the ridge from 105 ma onwards as a result of a hotspot source related to the kerguelen super plume during its intense eruption activity

u pb zircon ages from athapuscow aulacogen east arm of great slave lake n w t canada athapuscow aulacogen is an early proterozoic intracratonic basin located in the east arm of great slave lake between the slave and northwest churchill provinces athapuscow aulacogen comprises three stratigraphic sequences the wilson island group the great slave supergroup and the et then group new u pb zircon ages provide constraints on the development of the aulacogen the blachford lake intrusive suite consists of an older alkaline phase hearne channel granite dated at 2175 7 ma and a younger peralkaline phase thor lake syenite dated at 2094 10 ma confirming the suggestion that the two phases may not be related a felsite from the wilson island group has an age of 1928 11 ma the wilson island group is intruded by epizonal granites butte island intrusive suite one of which has an age of 1895 8 ma the wilson island group and the butte island instrusive suite are entirely allochthonous with respect to the slave craton rocks of the great slave supergroup overlie mylonitized wilson isl

paleomagnetism and rock magnetism of the ca 1 87 ga pearson formation northwest territories canada a test of vertical axis rotation within the great slave basin abstract a geometrically quantitative plate kinematic model based on paleomagnetism for the initial assembly of laurentia has taken form in the past few decades within this framework there remains but one problematic interval of data predominantly from the slave craton which is the 1 96 1 87 ga coronation apparent polar wander path apwp the coronation apwp shows large 110 back and forth oscillations that are difficult to explain in terms of plate motion nonetheless poles from the coronation apwp have been incorporated in various paleogeographic reconstructions of laurentia and the supercontinent nuna pointing to the importance of testing its veracity in this study we conducted a detailed paleomagnetic and rock magnetic study of the ca 1 87 ga pearson formation east arm of great slave lake northwest territories canada our results show that pearson formation yields a characteristic remanent magnetization carried by single domain or small pseudo single domain magnetite the age of the magnetization is constrained to be older than paleoproterozoic deformation and is interpreted as primary paleomagnetic declinations reveal a one to one correlation with local structural attitudes indicating that some small blocks in the fold belt likely experienced significant 60 vertical axis rotations presumably related to large dextral displacements along the mcdonald fault system alternative explanations such as true polar wander or a non dipole magnetic field are considered less parsimonious for the data presented here it is suspected that some existing christie bay group poles the stark and tochatwi formations which were sampled in areas with anomalous structural attitudes and differ from time equivalent poles obtained from areas of the slave craton far from major transcurrent faults may similarly suffer from vertical axis rotation we suggest further study before using possibly rotated christie bay group poles for paleogeographic reconstructions

constraints on paleoproterozoic atmospheric oxygen levels the oxygenation of earth s surface environment dramatically altered key biological and geochemical cycles and ultimately ushered in the rise of an ecologically diverse biosphere however atmospheric oxygen partial pressures p o 2 estimates for large swaths of the precambrian remain intensely debated here we evaluate and explore the use of carbonate cerium ce anomalies ce ce as a quantitative atmospheric p o 2 proxy and provide estimates of proterozoic p o 2 using marine carbonates from a unique precambrian carbonate succession the paleoproterozoic pethei group in contrast to most previous work we measure ce ce on marine carbonate precipitates that formed in situ across a depth gradient building on previous detailed sedimentology and stratigraphy to constrain the paleo depth of each sample measuring ce ce across a full platform to basin depth gradient we found only minor depleted ce anomalies restricted to the platform and upper slope facies we combine these results with a ce oxidation model to provide a quantitative constraint on atmospheric p o 2 1 87 billion years ago ga our results suggest paleoproterozoic atmospheric oxygen concentrations were low near 0 1 of the present atmospheric level this work provides another crucial line of empirical evidence that atmospheric oxygen levels returned to low concentrations following the lomagundi event and remained low enough for large portions of the proterozoic to have impacted the ecology of the earliest complex organisms

structure and tectonics of 85 e ridge from analysis of geopotential data abstract the bay of bengal bob sedimentary basin is one of the largest offshore sedimentary basins of the world having the thickest accumulation of sediments which mask the underlying crust and it is the geopotential data that offers the unique opportunity of seeing below the thick sediments satellite derived free air gravity fag data marine magnetic data bathymetry available seismic profiles and isopach maps over the bay of bengal are utilized in the present paper to throw light on the nature and tectonics of the bob with particular emphasis on the 85 e ridge combined analysis of gravity and magnetic data along six ew profiles of length varying from 325 to 440 km is undertaken incorporating seafloor spreading anomalies in the model of the several theories put forth for the evolution of the 85 e ridge the present study supports the sagging of the crust and rules out the hotspot trace and magmatic under plating theories the crustal model derived from the magnetic profile analysis suggests that the 85 e ridge does not extend to the north of 15 n latitude and this is supported by the euler solutions of the fag and the long wavelength filtered fag however the intermediate wavelength filtered fag suggests that the 85 e ridge extends up to the ocean continent boundary thereby suggesting that the 85 e ridge thins out dramatically to the north of 15 n latitude analysis of gravity data along 17 n latitude profile predicts that in the northern segment the 85 e ridge is a geo morphological feature within the sediments above the basement whereas to the south of 15 n latitude it is a feature within the oceanic crust below the basement further the reverse magnetization associated with the seafloor spreading anomalies lying on the 85 e ridge belongs either to the albian period or the anomaly a34 a33 the 85 e ridge would be younger than these seafloor spreading anomalies and would have formed due to horizontal compressional forces of the lithosphere preceding development of the subduction zone at the andaman trench

thermally induced brittle deformation in oceanic lithosphere and the spacing of fracture zones brittle deformation of oceanic lithosphere due to thermal stress is explored with a numerical model with an emphasis on the spacing of fracture zones brittle deformation is represented by localized plastic strain within a material having an elasto visco plastic rheology with strain softening we show that crustal thickness creep strength and the rule governing plastic flow control the formation of cracks the spacing of primary crack decreases with crustal thickness as long as it is smaller than a threshold value creep strength shifts the threshold such that crust with strong creep strength develops primary cracks regardless of crustal thicknesses while only a thin crust can have primary cracks if its creep strength is low for a thin crust the spacing of primary cracks is inversely proportional to the creep strength suggesting that creep strength might independently contribute to the degree of brittle deformation through finite versus zero dilatation in plastic strain associated and non associated flow rule results in nearly vertical and v shaped cracks respectively changes in the tectonic environment of a ridge system can be reflected in variation in crustal thickness and thus related to brittle deformation the fracture zone free reykjanes ridge is known to have a uniformly thick crust the australian antarctic discordance has multiple fracture zones and thin crust these syntheses are consistent with enhanced brittle deformation of oceanic lithosphere when the crust is thin and vice versa

structure and origin of the 85 e ridge the submerged 85 e ridge in the bay of bengal trends approximately n s between 19 n and 6 n latitudes off the southeast coast of sri lanka it takes an arcuate shape and seems to terminate with the northward extension of the afanasy nikitin seamounts situated around 2 s latitude the ridge is characterized by positive magnetic 100 400 nt and negative free air gravity 60 mgal anomalies with variable widths of 100 180 km magnetic model studies revealed that the rocks of the 85 e ridge are magnetized with reversed polarity the well defined geophysical anomalies and lack of magnetic polarity reversals together with the deep burial nature of the ridge may not favor a hotspot origin two alternative processes for the ridge emplacement have been suggested ridge emplacement may be 1 due to shearing of the lithosphere caused by stretching and compressional forces associated at the time of major plate reorganization immediately after the evolution of the early cretaceous crust in the bay of bengal more precisely at m0 isochron or during the middle albian reversals within the cretaceous long normal polarity k t superchron epoch when the earth s magnetic polarity changed from normal to reversed polarity and or 2 due to sagging followed by deformation produced by the buckling instability of the oceanic plate caused by horizontal compressional forces on the passive continental margin however more marine geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their associated model further the rajmahal traps normally polarized and the 85 e ridge reversely polarized appear to be associated with two different episodes of eruption that might have been triggered by the kerguelen mantle plume the 85 e ridge seems to extend into the onshore west bengal basin as a subsurface ridge and merges with the reported nne ssw trending zone of strong geophysical anomalies east of rajmahal traps up to 25 n latitude

evidence from the northern red sea on the transition from continental to oceanic rifting abstract the northern red sea is an active rift in the last stages of continental rifting which is beginning to undergo the transition to oceanic seafloor spreading the rift can be divided into two distinct regions the marginal areas consist of a series of bathymetric terraces and as indicated by geophysical data are underlain by a series of rotated fault blocks the axial depression which forms the axis of deep water is a continuous region 10 to 25 km wide near the center of the sea which is characterized by high heat flow a large gravity minimum and very deformed sediments both regions are divided along strike into segments roughly 60 km long by cross trending transfer zones the presently active tectonics are characterized by the concentration of extension and deformation in the axial depression and by the emplacement of a number of large intrusions the intrusions occur at distinct sites located at the intersection of transfer zones with the faults bounding the rotated blocks in the marginal areas and at deeps located in the axial depression halfway between transfer zones although extension is becoming concentrated in the axial depression geophysical data imply that at least the upper few kilometers of the crust in that region consists of non magnetic continental type material and that this ca apace has been ruptured by large intrusions only at the deeps a wide range of crustal seismic velocities has been observed in the northern red sea including velocities normally associated with oceanic crust the velocity variations appear spatially related to corresponding large variations in basement rock type observed on shore in the eastern desert of egypt the northern red sea is in an intermediate stage between a continental rift valley and a young ocean basin and delineation of its structure and tectonics allows a model to be developed for the evolution of a rifted continental margin rifting starts in a series of linked half grabens 50 60 km long and formed over detachment surfaces which probably penetrate no deeper than the mid or lower crust this style of rifting can accommodate at least the amcunt of extension represented by the gulf of suez 35 km at some greater amount of extension the rift acquires a more symmetric shape probably through the development of antithetically faulted blocks in the hanging wall extension in this stage is still distributed throughout the rift and is accommodated in the upper crust by rotation of fault blocks at the stage represented by the northern red sea extension becomes concentrated in an axial depression and begins to be partially accommodated by large intrusions which form deeps the position of the deeps is controlled by the transfer zones established in the earliest stages of rifting with further extension the deeps serve as nucleation points that develop into small discrete cells of seafloor spreading these cells grow and coalesce to form a continuous mid ocean ridge spreading center such as is found in the southern red sea

u pb geochronology of the bokan mountain peralkaline granite southeastern alaska the bokan mountain arfvedsonite aegirine granite is the only peralkaline acid intrusion actually known on the margin of the canadian cordillera it is the end product of a peralkaline magmatic evolution and is characterized by genetically associated concentrations of u and th the u pb method on zircons has been used to date two samples one from a barren peralkaline granite and one from a low grade u th mineralized albitite a typological study has revealed two genetically different populations of zircons in the barren granite the upper intercept age of 171 5 ma obtained on a concordia diagram dates the emplacement of the peralkaline granite acid wash experiments on zircons have allowed us to remove important quantities of common lead this point is discussed as well as the abnormally low pb content of the feldspar extracted from the barren granite a jurassic period of magmatic activity must be integrated into the geological history of prince of wales island peralkaline magmatism may have occurred w

chronostratigraphy of the hottah terrane and great bear magmatic zone of wopmay orogen canada and exploration of a terrane translation model the paleoproterozoic hottah terrane is the westernmost exposed bedrock of the canadian shield and a critical component for understanding the evolution of the wopmay orogen thirteen new high precision u pb zircon crystallization ages are presented and support field observations of a volcano plutonic continuum from hottah terrane through to the end of the great bear magmatism from 1950 to 1850 ma the new crystallization ages new geochemical data and newly published detrital zircon u pb data are used to challenge hitherto accepted models for the evolution of the hottah terrane as an exotic arc and microcontinent that arrived over a west dipping subduction zone and collided with the slave craton at ca 1 88 ga although the hottah terrane does have a tectonic history that is distinct from that of the neighbouring slave craton it shares a temporal history with a number of domains to the south and east domains that were tied to the slave craton by ca 1 97 ga it is interpreted herein that hottah terra

delineation of the 85 e ridge and its structure in the mahanadi offshore basin eastern continental margin of india ecmi from seismic reflection imaging abstract the passive eastern continental margin of india ecmi evolved during the break up of india and east antarctica in the early cretaceous the 85 e ridge is a prominent linear aseismic feature extending from the afanasy nikitin seamounts northward to the mahanadi basin along the ecmi earlier workers have interpreted the ridge to be a prominent hot spot trail in the absence of conclusive data the extension of the ridge towards its northern extremity below the thick bengal fan sediments was a matter of postulation in the present study interpretation of high resolution 2 d reflection data from the mahanadi offshore basin located in the northern part of the ridge unequivocally indicates continuation of the ridge across the continent ocean boundary into the slope and shelf tracts of the ecmi its morphology and internal architecture suggest a volcanic plume related origin that can be correlated with the activity of the kerguelen hot spot in the nascent indian ocean in the continental region the plume related volcanic activity appears to have obliterated all seismic features typical of continental crust the deeper oceanic crust over which the hot spot plume erupted shows the presence of linear ns aligned basement highs corresponding with the ridge underlain by a depressed moho discontinuity in the deep oceanic basin the ridge influences the sediment dispersal pattern from the early cretaceous early part of late cretaceous times till the end of oligocene which is an important aspect for understanding the hydrocarbon potential of the basin

controls on the distribution of gallium in the nechalacho ree deposit nwt canada despite the numerous industrial and scientific applications of gallium its behavior in nature and the processes that concentrate it to potentially economic levels are poorly understood although the main supply of this metal is as a by product of the mining of bauxite it is also concentrated by magmatic hydrothermal processes in peralkaline igneous systems here we report the results of a study of the distribution of gallium and the controls on this distribution in the nechalacho rare metal deposit northwest territories canada which has been shown to contain significant reserves of this critical metal electron microprobe analyses and x ray element maps of gallium bearing minerals were used to determine the mineralogical distribution of gallium in the nechalacho intrusive suite elevated gallium concentrations were identified in albite biotite orthoclase chlorite and allanite of these aluminum bearing minerals the most important hosts of gallium are albite biotite and orthoclase ferric iron bearing minerals including magnetite and aegirine which were considered potential candidates for gallium sequestration contain relatively low concentrations of the metal this behavior of gallium at least from a magmatic perspective is consistent with its predicted partitioning between phenocrysts and melt however there is also evidence that gallium was redistributed by hydrothermal fluids chloritization of biotite resulted in the enrichment of gallium in the secondary mineral chlorite and the development of secondary albite albitization led to a depletion of gallium in primary albite on the basis of these results we argue that the overall distribution of gallium within the nechalacho deposit was controlled by magmatic crystal fractionation whereas hydrothermal processes led to local remobilization of the metal during fractional crystallization gallium was moderately compatible in minerals such as albite and biotite whereas it bordered between compatibility and incompatibility in minerals such as magnetite and orthoclase and was incompatible in aegirine this resulted in a relatively constant bulk gallium concentration in the nechalacho deposit although locally gallium was remobilized hydrothermally particularly within the most altered parts of the intrusion notably the albitite

tectonics of the bay of bengal new insights from satellite gravity and ship borne geophysical data abstract recently released satellite derived free air gravity anomalies and the existing ship board geophysical data provide new insights into the tectonics of the bay of bengal with respect to the structure and regional extension of the buried 85 e ridge and the tectonics of the eastern continental margin of india ecmi the 85 e ridge can be visualized extending inland via the mahanadi basin volcanics to the rajmahal traps a large volcanic province in eastern india encompassing the rajmahal and sylhet traps and volcanics in the bengal and mahanadi basins almost on the scale of the deccan volcanic province along the west coast can be envisaged taking into account the occurrences of intrusive and extrusive rocks around the age of 117 ma the 85 e ridge represents the deep ocean volcanic trace of this magmatic activity towards the south the ridge continues in an arcuate manner to the afanasy nikitin seamount at equatorial latitudes in the central indian ocean gravity models of the ridge are indicative of hotspot related crustal underplating processes beneath the ridge the ecmi can be divided into a southern transform and northern rifted segments on the basis of gravity and bathymetry data which bear similarities with the conjugate east antarctica margin

u pb geochronology of the wolf river batholith north central wisconsin evidence for successive magmatism between 1484ma and 1468ma abstract single grain zircon u pb age determinations were completed on individual phases of the mesoproterozoic anorogenic wolf river batholith wrb of wisconsin and the nearby wausau syenite complex in order to a define the age more precisely and accurately b determine whether there is a resolvable age difference within units of the wrb and c determine whether the wausau syenite complex is coeval with any or all parts of the wrb u pb results for many individual zircon grains from the wrb plot near concordia and yield ages ranging from 1468 4 ma to 1484 2 ma plutons of the northeastern lobe of the wrb peshtigo monzonite hager porphyry belongia granite and high falls granite have a weighted average crystallization age of 1470 3 1 2 ma which is younger than the main complex plutons wolf river granite wiborgite porphyry red river granite and waupaca granite which yields a weighted average crystallization age of 1475 6 1 7 ma excluding one sample of wolf river granite from hogarty which yields an age of 1484 2 ma two samples from the main complex also contain zircons with younger apparent ages of 1470 1471 ma these younger grains may reflect late stage alteration of some zircons coeval with emplacement of the northeastern lobe the wausau syenite and the nine mile granite of the nearby wausau syenite complex 1522 ma and 1506 ma respectively are significantly older than the wrb and definitely not coeval or comagmatic the syenite complex may be a precursor to the larger wrb magmatism sm nd t dm ages for most wrb samples indicate crust formation ages from 1 8 ga to 1 9 ga compatible with formation of the magmas from late paleoproterozoic penokean crust one unit which locally intrudes archean gneiss has a small component of that older rock involved in its evolution the two wausau complex units have younger tdm ages and appear to have incorporated slightly greater amounts of juvenile constituents

morphology and tectonics of mahanadi basin northeastern continental margin of india from geophysical studies abstract the bathymetry total intensity magnetic and satellite free air gravity data over the eastern continental margin of india between the latitudes 18 30 n and 20 n within the water depths of 25 2300 m between paradip and gopalpur were analyzed and interpreted onshore aeromagnetic and bouguer gravity data over the adjacent coastal region are also supplemented in the present analysis the basement configuration inferred from the bathymetry magnetic and gravity data resembles a series of coast parallel structural highs and depressions and their shearing pattern in the mahanadi offshore basin the chilika offshore lineament and dhamara offshore lineament constitute the southern and northern boundaries of the offshore basin respectively in this sector the inferred continent ocean boundary cob around 170 km from the coast approximately mimics the 2000 m isobath the width of the inferred offshore basin between the chilika offshore lineament and dhamara offshore lineament is around 240 km and it extends approximately up to 170 km from coast to offshore perhaps up to the cob the inferred tectonics of the northeastern continental margin of india suggests pull apart and sheared transform mechanisms during the breakup of india from antarctica the model studies suggest that the northern part of the 85 e ridge abuts the coast at chilika lake the shape of 2000 m isobath over the northeastern continental margin of india closely resembles to that of 2000 m isobath off the lambert graben of east antarctica this inference appears to further support the theory that the eastern continental margin of india and east antarctica are closely aligned in the pre breakup tectonic setting of the eastern gondwanaland

geology market and supply chain of niobium and tantalum a review tantalum ta and niobium nb are essential metals in modern society their use in corrosion prevention micro electronics specialty alloys and high strength low alloy hsla steel earns them a strategic designation in most industrialised countries the ta market is unstable due in part to historic influx of conflict columbite tantalite concentrate or coltan that caused ta mines in australia and canada to be placed on care and maintenance more recently the growing appetite of modern society for consumer goods made of conflict free minerals or metals has put pressure on suppliers pegmatites rare element enriched granites related placer deposits and weathered crusts overlying carbonatite and peralkaline complexes account for the majority of ta production several carbonatite related deposits e g upper fir and crevier canada are being considered for potential co production of ta and nb pyrochlore nb ta columbite tantalite nb ta wodginite ta nb and sn and microlite ta and nb are the main ore minerals approximately 40 of ta used in 2012 came from ta mines 30 from recycling 20 from tin slag refining and 10 from secondary mine concentrates due to rapid industrialisation and increased use of nb in steel making in countries such as china and india demand for nb is rising weathered crusts overlying carbonatite complexes in brazil and one hard rock carbonatite deposit in canada account for about 92 and 7 of nb world mine production respectively since the bulk of the production is geographically and politically restricted to a single country security of supply is considered at risk other prospective resources of nb beside carbonatites and associated weathered crusts are peralkaline complexes e g nechalacho where nb is considered as a potential co product of ree and zirconium economically significant deposits of ta and nb contain pyrochlore columbite tantalite fersmite loparite and struverite assuming continued elasticity of ta and nb prices and that the law of the supply and demand applies new sources of these metals can be developed in the long term there is no need to worry about ta and nb availability temporary disruptions in ta and nb supply are possible and could be difficult to cope with so new sources of supply may be developed to diversify geographic sources of supply for strategic reasons

early proterozoic collisional orogeny along the northern thelon tectonic zone northwest territories canada evidence from the foreland the subsidence history of kilohigok basin 1 9 ga n w t canada reflects the opening and closing of an ocean basin along the present thelon tectonic zone the basal sedimentary sequence represents a passive margin shelf that faced se toward the orogen following passive margin subsidence tectonic loading occurred resulting in rapid subsidence of the outer shelf margin and simultaneous uplift and erosion of the platform interior a flexural moat formed subparallel to the thelon tectonic zone the shelf was subsequently drowned and successively buried by deep water flysch shallow marine shelf sediments and fluvial molasse forming a foreland basin tapered to the nw direct structural evidence for the tectonic load is provided by the leading edge of a thin skinned thrust fold belt exposed in oblique cross section within the se extremity of the basin upright folds at high structural levels can be traced downward to recumbent tight folds and thrusts near the basement cover contact documenting increasing shear strain with depth overall northwest vergence and about 50 shortening is indicated by fold thrust geometry and finite stretching lineations above a brittle ductile decollement that roots within the thelon tectonic zone

structural lineaments from the magnetic anomaly maps of the eastern continental margin of india ecmi and nw bengal fan abstract three major structural lineaments are inferred from the magnetic and bathymetry data collected over the eastern continental margin of india ecmi and nw bengal fan covering the area between 10 55 n to 20 15 n and 80 e to 88 e analysis of magnetic data associated with these trends suggests that 1 trend i located at the foot of the continental slope around 3000 m water depth represents the ocean continent boundary ocb of ecmi 2 trend ii represents the northern extension of 85 e ridge abutting the continental shelf off chilka lake and 3 trend iii located over the continental shelf slope between visakhapatnam and paradip represents a folded ridges and depressions nature of the continental basement the study also reveals the occurrence of dyke intrusions within the continental crust landward of ocb which might be related to the rift phase volcanism along the ecmi

geoid and gravity anomaly data of conjugate regions of bay of bengal and enderby basin new constraints on breakup and early spreading history between india and antarctica 1 timing of breakup of the indian continent from eastern gondwanaland and evolution of the lithosphere in the bay of bengal still remain as ambiguous issues geoid and free air gravity data of bay of bengal and enderby basin are integrated with shipborne geophysical data to investigate the early evolution of the eastern indian ocean geoid and gravity data of the bay of bengal reveal five n36 w fracture zones fzs and five isolated ne sw structural rises between the eastern continental margin of india ecmi and the 85 e ridge 86 e fz the fzs meet the 86 e fz at an angle of 39 the rises are associated with low gravity and geoid anomalies and are oriented nearly orthogonal to the fzs trend the geoid and gravity data of the western enderby basin reveal a major kerguelen fz and five n4 e fzs the fzs discretely converge to the kerguelen fz at an angle of 37 we interpret the fzs identified in bay of bengal and western enderby basin as conjugate fzs that trace the early cretaceous rifting of south ecmi from enderby land structural rises between the fzs of bay of bengal may either represent fossil ridge segments possibly have extinct during the early evolution of the bay of bengal lithosphere or may have formed later by the volcanic activity accreted the 85 e ridge two different gravity signatures short wavelength high amplitude negative gravity anomaly and relatively broader low amplitude negative gravity anomaly are observed on south and north segments of the ecmi respectively the location of continent ocean boundary cob is at relatively far distance 100 200 km from the coastline on north ecmi than that 50 100 km on the south segment on the basis of geoid gravity and seismic character and orientation of conjugate fzs in bay of bengal and western enderby basin we believe that transform motion occurred between south ecmi and enderby land at the time of breakup which might have facilitated the rifting process in the north between combined north ecmi elan bank and macrobertson land and in the south between southwest sri lanka and gunnerus ridge region of east antarctica approximately during the period between the anomalies m1 and m0 and soon after detachment of the elan bank from north ecmi the rifting process possibly had reorganized in order to establish the process along the entire eastern margins of india and sri lanka