niobium enriched basalts from the wabigoon subprovince canada evidence for adakitic metasomatism above an archean subduction zone abstract late archean niobium enriched basalts from the central sturgeon lake assemblage and neepawa group of the western wabigoon subprovince have mantle normalized nb la between 0 8 and 1 3 and zr y between 4 and 7 they are compositionally similar to basalts attributed to adakite metasomatism of mantle wedge regions in cenozoic subduction zones sajona et al j petrol 37 1996 693 726 in detail their sc ree systematics suggest the archean basalts were generated above the garnet stability field an association with adakite like volcanic rocks an absence of komatiites and the arc like attributes of their host sequences suggest a subduction related origin for the basalts if current models of adakite and niobium enriched basalt genesis are valid then additional examples of these rocks should be relatively common in other archean greenstone belts

subsurface energetics of the gulf stream near the charleston bump abstract the energy budgets of the eddies and the mean flow in the gulf stream near a topographic feature known as the charleston bump are computed first we consider these results in the context of the amplification hypothesis for the development of gulf stream meanders according to this hypothesis the finite amplitude gulf stream fluctuations observed offshore of onslow bay are the result of the destabilizing effect of the bump on the stream the present dataset was obtained both immediately upstream and downstream of the bump and the results of our analysis suggest 1 immediately south of the charleston bump the eddies perform net work on the gulf stream at a rate of 1 02 66 10 2 ergs cm 3 s 1 by transporting momentum offshore 2 the net work performed by the eddies south of the bump is not used locally to accelerate the mean rather it is exported to the rest of the ocean at a rate of 1 58 1 39 10 2 ergs cm 3 s 1 3 in spite of the net work performed by the eddies south of the bu

subduction and volcanism in the iberia north africa collision zone from tomographic images of the upper mantle abstract new tomographic images of the upper mantle beneath the westernmost mediterranean suggest that the evolution of the region experienced two subduction related episodes first subduction of oceanic and or extended continental lithosphere now located mainly beneath the betics at depths greater than 400 km took place on a nw se oriented subduction zone this was followed by a slab tear process that initiated in the east and propagated to the west leading to westward slab rollback and possibly lower crustal delamination the current position of the slab tear is located approximately at 4 w and to the west of this location the subducted lithosphere is still attached to the surface along the gibraltar arc our new p wave velocity model is able to image the attached subducted lithosphere as a narrow high velocity body extending to shallow depths coinciding with the region of maximum curvature of the gibraltar arc the occurrence of intermediate depth earthquakes and anomalously thick crust this thick crust has a large influence in the measured teleseismic travel time residuals and therefore in the obtained p wave tomographic model we show that removing the effects of the thick crust significantly improves the shallow images of the slab and therefore the interpretations based on the seismic structure

neoarchaean volcanic rocks central hearne supracrustal belt western churchill province canada geochemical and isotopic evidence supporting intra oceanic supra subduction zone extension abstract the kaminak segment of the central hearne supracrustal belt chsb western churchill province canada comprises a diverse sequence of neoarchaean volcanic and less abundant metasedimentary rocks that were emplaced as two assemblages between 2695 2711 ma and 2681 2686 ma respectively these are intruded by uncommon pre tectonic diorites and tonalites ca 2691 ma voluminous syn tectonic tonalites and granodiorites ca 2679 2686 ma and rare post tectonic potassic monzogranite and syenite ca 2659 2666 ma metasedimentary rocks include turbidites epiclastic tuffs minor iron formation oxide and rare volcanic conglomerates volcanic rocks of assemblage i include abundant pillowed and massive basalts to andesites with less common silicic lavas tuffs and volcaniclastic debris flow deposits assemblage ii contains voluminous silicic tuff and volcaniclastic debris flow deposits but fewer basaltic to andesitic flows the critical diagnostic feature of the chsb is the stratigraphic intercalation of compositionally diverse basaltic andesitic and felsic volcanic rocks throughout both assemblages mapping u pb geochronology and lithogeochemistry suggest that an initial morb like basaltic plain containing widespread intercalations of dacite to rhyolite was replaced at ca 2688 ma by a relatively short lived dacite to rhyolite dominated magmatic environment characterized by localized felsic volcanic centres and a bloom of 2686 2679 ma tonalitic to granodioritic plutons basaltic to andesitic rocks are dominated by iron rich tholeiites although the proportion of calc alkaline rocks increases with silica content felsic volcanic rocks all exhibit calc alkaline affinities the wide range in chemistry of the basaltic to andesitic rocks of both volcanic assemblages implies diverse mantle sources capable of generating voluminous morb with less common arc neb oib and rare babb like rocks similarly the variable composition of the felsic volcanic rocks indicates both anatexis of eclogitic to garnetiferous mafic crust and also extensive fractionation of mafic precursors in crustal magma chambers two geochemically distinct arc like mafic suites were generated through contamination of primary mantle derived magmas by juvenile ca 2700 ma silicic crust either in their mantle source or through assimilation upon ascent ndt isotopic data are comparable to contemporaneous depleted mantle with only local evidence for incorporation of older 2700 ma crust the chsb may have formed via tectono magmatic processes comparable to those of the eocene infant arcs of the sw pacific whereby the formation of a thick sequence of coeval intercalated compositionally diverse mantle and crustal derived rocks are generated in an extensional supra subduction setting the cessation of supra subduction zone extension at ca 2688 ma was followed by the short lived development of felsic volcanic edifices incipient arc the extrusion of mafic to felsic magmas and the concomitant intrusion of voluminous syn kinematic tonalitic plutons this accompanied a major change in the tectono magmatic setting accompanying and presumably following the termination of extensional supra subduction zone processes

transition of subduction related magmatism from slab melting to dehydration at 2 5 ga abstract the archean plate tectonics and subduction related magmatism have been concerned for several decades here we investigate the archean plate subduction from a new perspective using the secular records of global scale non ttg tonalite trondhjemite granodiorite felsic rocks with arc like geochemical affinities the statistical results show that the la yb and sm yb of non ttg arc felsic rocks evolved at high values in the archean and decreased sharply at 2 5 ga this abrupt transition indicates a significant change of the petrogenesis during the archean the oceanic crust subducted into the mantle along a high geothermal gradient and reached the conditions of partial melting with garnet stable in the residue resulting in high la yb and sm yb in the arc felsic rocks as a consequence of the secular cooling of the mantle the geothermal gradient of a post archean subducting cold oceanic crust intersected with the stability limits of hydrous minerals before it reached the wet solidus of basalts so that the dehydration occurred preferentially and resulted in a drier crust with a much higher temperature of solidus as such the post archean arc felsic rocks were mostly originated from melting of the hydrated mantle wedge overlying the subducted oceanic slab the magma source of the arc felsic rocks thereafter became much shallower corresponding to the low la yb and sm yb of those rocks as a consequence of the less stable garnet in the residue we thus conclude that the subduction related magmatism changed from slab melting to dehydration model at 2 5 ga

geochemistry and geodynamic origin of the mesoarchean ujarassuit and ivisaartoq greenstone belts sw greenland abstract the ivisaartoq ca 3075 ma and ujarassuit ca 3070 ma greenstone belts are the largest mesoarchean supracrustal lithotectonic assemblages in the nuuk region sw greenland both greenstone belts underwent polyphase deformation and amphibolite facies metamorphism and were in due course variously dismembered pillow lavas pillow breccia magmatic layering and relic sedimentary structures are well preserved in the ivisaartoq belt volcanic rocks include basalts and minor andesites and picrites boninite like rocks occur in the ujarassuit belt metasedimentary rocks in the ujarassuit belt occur as thin layers 0 5 1 0 m of biotite schists and in the ivisaartoq belt as 500 m thick biotite schists intercalated with minor quartzitic gneisses there is no field evidence indicating that the ivisaartoq and ujarassuit supracrustal rocks were deposited on older continental basement and their volcanic rocks do not exhibit any geochemical trends indicating contamination by archean upper continental crust four groups of amphibolites metavolcanic rocks were recognized in the ujarassuit greenstone belt on the basis of their ree and hfse characteristics 1 group 1 is characterized by near flat ree patterns la smcn 0 77 1 14 la ybcn 0 84 1 24 and moderate negative nb anomalies nb nb 0 60 0 79 2 group 2 displays lree depleted patterns la smcn 0 53 1 02 la ybcn 0 32 0 61 and pronounced negative nb anomalies nb nb 0 32 0 67 3 group 3 consists of lree depleted patterns la smcn 0 69 0 84 la ybcn 0 55 0 91 and absence of significant nb anomalies nb nb 0 92 1 15 and 4 group 4 has concave upward ree patterns la smcn 1 64 2 42 gd ybcn 0 57 1 01 and large negative nb anomalies nb nb 0 28 0 42 the depletion of lree in group 2 amphibolites appears to have resulted from mobility of these elements during metamorphism group 1 3 and 4 amphibolites retain their near primary geochemical signatures and their trace element patterns are comparable to those of phanerozoic oceanic island arc tholeiites iat normal mid ocean ridge basalts n morb and boninites respectively the trace element patterns of the least altered meta ultramafic rocks la ybcn 2 48 1 35 nb nb 0 31 0 60 are comparable to those of modern subduction related picrites amphibolites with an andesitic composition show large negative nb anomalies nb nb 0 21 0 55 enriched lree patterns la ybcn 6 29 15 64 and fractionated hree gd ybcn 2 61 3 12 implying deep melting in equilibrium with residual garnet in the source biotite schists and quartzitic gneisses have low chemical indexes of alteration values cia 46 to 62 and trace element characteristics indicating volcaniclastic sedimentary protoliths derived from poorly weathered felsic to mafic source rocks collectively the geochemical features of metavolcanic and metasedimentary rocks suggest that the ivisaartoq and ujarassuit greenstone belts represent dismembered fragments of mesoarchean supra subduction zone oceanic crust formed in an arc forearc backarc tectonic setting

absolute plate velocities from seismic anisotropy importance of correlated errors the errors in plate motion azimuths inferred from shear wave splitting beneath any one tectonic plate are shown to be correlated with the errors of other azimuths from the same plate to account for these correlations we adopt a two tier analysis first find the pole of rotation and confidence limits for each plate individually second solve for the best fit to these poles while constraining relative plate angular velocities to consistency with the morvel relative plate angular velocities our preferred set of angular velocities sks morvel is determined from the poles from eight plates weighted proportionally to the root mean square velocity of each plate sks morvel indicates that eight plates amur antarctica caribbean eurasia lwandle somalia sundaland and yangtze have angular velocities that differ insignificantly from zero the net rotation of the lithosphere is 0 25 0 11 ma 1 95 confidence limits right handed about 57 1 s 68 6 e the within plate dispersion of seismic anisotropy for oceanic lithosphere 19 2 differs insignificantly from that for continental lithosphere 21 6 the between plate dispersion however is significantly smaller for oceanic lithosphere 7 4 than for continental lithosphere 14 7 two of the slowest moving plates antarctica vrms 4 mm a 1 29 and eurasia vrms 3 mm a 1 33 have two of the largest within plate dispersions which may indicate that a plate must move faster than 5 mm a 1 to result in seismic anisotropy useful for estimating plate motion the tendency of observed azimuths on the arabia plate to be counterclockwise of plate motion may provide information about the direction and amplitude of superposed asthenospheric flow or about anisotropy in the lithospheric mantle

active surface deformation and sub lithospheric processes in the western mediterranean constrained by numerical models we present the results of dynamic modeling of the western mediterranean that accounts for observed global positioning system gps surface deformation of the alboran sea and surrounding rif and betic mountains as the result of the combined effects of relative motion of the eurasian and nubian plates low strength in the alboran sea region and sub lithospheric processes occurring beneath the external rif domain assuming that the lithosphere behaves elastically over the short time period of the gps observations an elastic plate model is considered in our study including an area of weak lithosphere factor of 10 centered on the alboran sea and in which lateral boundary conditions consist of the nubia eurasia oblique convergence sub crustal processes are modeled by application of a horizontal traction on a small area patch at the base of the elastic plate our modeling studies demonstrate the need for sub crustal or sub lithospheric southwestward directed forcing to account for observed southwestward motion of the rif and betic domains based on the location orientation and small area of the traction patch we hypothesize that forcing is associated with delamination and rollback of the subducted african continental lithospheric mantle beneath the external rif zone due to the pull of the oceanic part of the western mediterranean slab a dynamic process that may be similar to that where the over riding plate is driven toward the subduction zone during slab rollback

seismicity and active tectonics in the alboran sea western mediterranean constraints from an offshore onshore seismological network and swath bathymetry data seismicity and tectonic structure of the alboran sea were derived from a large amphibious seismological network deployed in the offshore basins and onshore in spain and morocco an area where the convergence between the african and eurasian plates causes distributed deformation crustal structure derived from local earthquake data suggests that the alboran sea is underlain by thinned continental crust with a mean thickness of about 20 km during the 5 months of offshore network operation a total of 229 local earthquakes were located within the alboran sea and neighboring areas earthquakes were generally crustal events and in the offshore domain most of them occurred at crustal levels of 2 to 15 km depth earthquakes in the alboran sea are poorly related to large scale tectonic features and form a 20 to 40 km wide nne ssw trending belt of seismicity between adra spain and al hoceima morocco supporting the case for a major left lateral shear zone across the alboran sea such a shear zone is in accord with high resolution bathymetric data and seismic reflection imaging indicating a number of small active fault zones some of which offset the seafloor rather than supporting a well defined discrete plate boundary fault moreover a number of large faults known to be active as evidenced from bathymetry seismic reflection and paleoseismic data such as the yusuf and carboneras faults were seismically inactive earthquakes below the western alboran basin occurred at 70 to 110 km depth and hence reflected intermediate depth seismicity related to subducted lithosphere

chemical processes affecting the mobility of major minor and trace elements during weathering of granitic rocks abstract the behaviour of 38 major and trace elements as well as changes in the mineralogy have been examined in 10 weathering profiles developing on some portuguese granitic rocks element mobilities are calculated from geochemical data normalized with respect to ti in the fresh parent rock chemical elements are divided into two groups immobile and mobile on the basis of their geochemical distribution during weathering elements that are immobile during weathering are zr hf fe al th nb sc and the ree very mobile are ca na p k sr ba rb mg and si mobile elements are derived mainly from leachable minerals such as feldspars micas and apatites whereas immobile elements are either concentrated in resistate phases or strongly adsorbed by secondary minerals the geochemical behaviour of mn cr v fe and ce is very dependent on redox conditions redox transformations of these elements can be used to set limits on the oxidation state of a weathering suite the ree are mobilized or fractionated during late stages of weathering but not during moderate stages of weathering this fractionation is caused by selective leaching of rocks composed of both stable and unstable minerals containing ree

the gibraltar subduction a decade of new geophysical data the gibraltar arc spans a complex portion of the africa eurasia plate boundary marked by slow oblique convergence and intermediate and deep focus seismicity the seemingly contradictory observations of a young extensional marine basin surrounded by an arcuate fold and thrust belt have led to competing geodynamic models delamination and subduction geophysical data acquired in the past decade provide a test for these models and support a narrow east dipping subduction zone seismic refraction studies indicate oceanic crust below the western gulf of cadiz tomography of the upper mantle reveals a steep east dipping high p wave velocity body beneath gibraltar the anisotropic mantle fabric from sks splitting shows arc parallel fast directions consistent with toroidal flow around a narrow westward retreating subducting slab the accompanying wsw advance of the rif betic mountain belt has constructed a thick pile of deformed sediments an accretionary wedge characterized by west vergent thrust anticlines bathymetric swath mapping images an asymmetric embayment at the deformation front where a 2 km high basement ridge has collided subduction has slowed significantly since 5 ma but deformation of recent sediments and abundant mud volcanoes suggest ongoing activity in the accretionary wedge three possible origins for this deformation are discussed gravitational spreading overall nw se convergence between africa and iberia and finally a wsw tectonic push from slow but ongoing roll back subduction in the absence of arc volcanism and shallow dipping thrust type earthquakes evidence in favor of present day subduction can only be indirect and remains the object of debate continued activity of the subduction offers a possible explanation for great m 8 5 earthquakes known to affect the area like the famous 1755 great lisbon earthquake recent gps studies show sw motion of stations in n morocco at velocities of 3 6 mm yr indicating the presence of an independent block a rif betic alboran microplate situated between iberia and africa

puzzling features of western mediterranean tectonics explained by slab dragging the recent tectonic evolution of the western mediterranean region is enigmatic the causes for the closure of the moroccan marine gateway prior to the messinian salinity crisis for the ongoing shortening of the moroccan rif and for the origin of the seismogenic trans alboran shear zone and eastern betics extension are unclear these puzzling tectonic features cannot be fully explained by subduction of the east dipping gibraltar slab in the context of the regional relative plate motion frame here we use a combination of geological and geodetic data as well as three dimensional numerical modelling of subduction to show that these unusual tectonic features could be the consequence of slab dragging the north to north eastward dragging of the gibraltar slab by the absolute motion of the african plate comparison of our model results to patterns of deformation in the western mediterranean constrained by geological and geodetic data confirm that slab dragging provides a plausible mechanism for the observed deformation our results imply that the impact of absolute plate motion on subduction is identifiable from crustal observations identifying such signatures elsewhere may improve the mantle reference frame and provide insights on subduction evolution and associated crustal deformation

along strike translation of a fossil slab abstract the isabella anomaly is a high seismic velocity anomaly beneath the southern central valley of california that has previously been interpreted to represent a lithospheric drip or delaminated sierra nevada root however recent work suggests that the anomaly is a remnant slab left over from cenozoic subduction attached to the pacific plate underneath the edge of the north american plate this hypothesis requires the slab to translate hundreds of kilometers along strike while remaining intact and attached to the pacific plate despite drag from the surrounding asthenosphere and overriding lithosphere we use 3 d finite element models to simulate this scenario and calculate the viscosity ratio between the slab and the surrounding asthenosphere required for the slab to translate undeformed the required viscosity ratio increases with increasing downdip extent of the slab and decreases with increasing slab dip for geometries approximating that of our proposed slab the required ratio ranges from 10 2 to 10 4 given the thermal and petrologic history of the slab mantle flow laws predict viscosity contrasts greater than or equal to these requirements as such we conclude that along strike translation of a remnant slab is feasible and serves as a possible explanation of the isabella anomaly the significance of this finding extends beyond our general understanding of subduction dynamics in that the presence of such a slab could have implications for the water budget of the san andreas fault and its role in aseismic slip

compressional tectonic inversion of the algero balearic basin latemost miocene to present oblique convergence at the palomares margin western mediterranean interpretation of new multichannel seismic reflection profiles indicates that the palomares margin was formed by crustal scale extension and coeval magmatic accretion during middle to late miocene opening of the algero balearic basin the margin formed at the transition between thinned continental crust intruded by arc volcanism and back arc oceanic crust deformation produced during the later positive inversion of the margin offshore and onshore is partitioned between n50 e striking reverse faults and associated folds like the sierra cabrera and abubacer anticlines and n10 20 e sinistral strike slip faults like palomares and terreros faults parametric subbottom profiles and multibeam bathymetry offshore structural analysis available gps geodetic displacement data and earthquake focal mechanisms jointly indicate that tectonic inversion of the palomares margin is currently active the palomares margin shows a structural pattern comparable to the north maghrebian margins where africa eurasia plate convergence is accommodated by ne sw reverse faults nnw sse sinistral faults and wnw ese dextral ones contractive structures at this margin contribute to the general inversion of the western mediterranean since 7 ma coeval to inversion at the algerian margin shortening at the alboran ridge and al idrisi faults occurred later since 5 ma indicating a westward propagation of the compressional inversion of the western mediterranean

slab rupture and delamination under the betics and rif constrained from receiver functions abstract we map the lithospheric structure under the westernmost mediterranean convergent setting interpreting p receiver functions obtained from a dense seismic network no orogenic root occurs under the eastern and great part of the central betics however the subducted south iberian continental lithosphere is found beneath the western betics where the iberian moho reaches depths of approximately 65 km dipping gently towards the se meanwhile at the rif strong crustal and lithospheric thickness contrasts occur across the nekor nw se sinistral fault that overlies the region of present slab tearing east of the nekor fault there is no orogenic root and the crust has been thinned to approximately 22 km whilst to the west the crust reaches 55 km thickness and the maghrebian continental lithosphere is attached to the lithospheric slab imaged by tomography under the alboran basin these data suggest that subduction rollback under the alboran and algerian basins together with continental slab tearing or detachment producing edge delamination under the betics and rif have been the main tectonic mechanisms driving extension magmatism and regional uplift in the westernmost mediterranean since the late miocene until present the surface expression of edge delamination and slab tearing is marked by regional uplift denudation of hp rocks in elongated core complex type domes late miocene volcanism in the eastern betics and rif and by large ne sw strike slip transfer faults like the alpujarras crevillente torcal or nekor faults that accommodate strong gradients in crustal displacements the iberian slab is still attached to the oceanic slab imaged under the alboran basin at the western betics where intermediate depth seismicity recent dextral strike slip faulting and folding could reflect slab tearing meanwhile active faulting and differential gps measured displacements would mark slab tearing beneath the rif coinciding with the trace of the sinistral nekor fault

monazite u pb and th pb geochronology by ion microprobe with an application to in situ dating of an archean metasedimentary rock abstract monazite grains from nine different precambrian rock samples and having concordant isotope dilution u pb and th pb ages were used to develop and evaluate techniques for u pb th pb and pb pb isotopic dating using an ion microprobe operating at high mass resolution r 5500 1 an unidentified isobar at 203 960 amu complicated the determination of 204 pb in the monazites with higher th but it appeared to have been eliminated when using a slight energy offset the 207 pb 206 pb ratios of individual spots were determined in 10 min analyses to 0 3 0 5 1 of the true values using a 20 m wide o 2 primary beam whereas the uncertainty in the mean of several spots on a single generation of monazite was typically 0 1 0 4 2 instrumental bias in the measured pb uo and pb tho values of individual analyses was monitored using linear relationships between 206 pb uo vs uo 2 uo and 208 pb tho vs uo 2 uo the bias appears to vary somewhat with monazite th content requiring the use of compositionally matched standards individual spot measurements of the 206 pb 238 u and 208 pb 232 th ratios in unknown monazites of low to moderate th can be determined to 2 1 the dating technique was applied to in situ analysis of small monazite inclusions hosted in an amphibolite grade metapelite of archean origin both the contextual setting and the morphology of the monazite grains were found to be important factors in interpreting the timing of growth of the host metamorphic mineral e g garnet staurolite plagioclase euhedral monazite grains armoured in low cao garnet and matrix plagioclase yielded an age of 2548 17 ma interpreted as dating an early episode of low pressure metamorphism partially resorbed monazite within a high cao garnet rim yielded ages of ca 2500 ma but are interpreted as inherited from the first event rather than syn genetic with respect to the growth of this garnet an age of 1755 30 ma was obtained for anhedral monazite associated with late chlorite filled fractures apparently reflecting post tectonic retrogression

tethys atlantic interaction along the iberia africa plate boundary the betic rif orogenic system abstract initial se dipping slow subduction of the ligurian tethys lithosphere beneath africa from late cretaceous to middle oligocene twisting to a later faster e dipping subduction of the subcrustal lithosphere is proposed as an efficient geodynamic mechanism to structure the arcuate betic rif orogenic system this new subduction related geodynamic scenario is supported by a kinematic model constrained by well dated plate reconstructions tectonic sedimentary and metamorphic data sets the slow initial se dipping subduction of the ligurian tethys realm beneath the malaguide upper plate unit is sufficient to subduct alpujarride and nevado filabride rocks to few tens of kilometers of depth in middle eocene times the shift from se to e dipping subduction during latest oligocene early miocene was possibly caused by both the inherited geometry of the highly segmented ligurian tethys domain and by the fast roll back of the subducted lithospheric slab the early miocene rather synchronous multiple crustal and subcrustal processes comprising the collision along the betic front the exhumation of the hp lt metamorphic complexes the opening of the alboran basin its flooring by hp alpujarride rocks and subsequent ht imprint can be explained by the fast nw and w directed roll back of the ligurian tethys subcrustal lithospheric slab the w retreat of the ligurian tethys lithosphere in middle late miocene times could partly explain the initiation of its lateral tear and consequent subcrustal processes from latest miocene onward the betic rif system evolved under both the northerly push of africa resulting in tightening at crustal and subcrustal levels and by the distinct current dynamics of the steep lithospheric slab the sw directed scape of the rif fold belt is one of the most striking evidences linked to the recent evolution of the squeezed betic rif system between africa and iberia

the spi lake formation of the central hearne domain western churchill province canada an axial intracratonic continental tholeiite trough above the cogenetic kaminak dyke swarm the spi group is a package of plagioclase porphyritic amygdaloidal basalt flows and sandstone spi lake formation conformably overlain by coarse conglomerate and sandstone old boot formation these unconformably overlie the kaminak lake segment of the central hearne supracrustal belt western churchill province the spi group developed in a small isolated basin 8 km2 and is older than the paleoproterozoic hurwitz group our data demonstrate that the basalts of the spi lake formation are genetically related to the ca 2450 ma kaminak dyke swarm a speculation previously promoted by some workers based on field characteristics alone they are continental tholeiites with large ion lithophile and light rare earth element enriched multielement profiles with prominent high field strength element troughs the 143nd 144nd compositions overlap the contemporaneous chondritic bulk earth data all of these are features we infer to have resulted from mixing of melts from dominantly depleted mid ocean ridge basal

evidence for slab rollback in westernmost mediterranean from improved upper mantle imaging this work is funded by the picasso project nsf grant ear 0808939 the deployment and data processing for spanish stations was funded by consolider ingenio 2010 project topo iberia csd2006 00041 as well as alert es cgl2010 19803 c03 02

trace element geochemistry of ore associated and barren felsic metavolcanic rocks in the superior province canada reply archaean felsic metavolcanic rocks in the superior province of the canadian shield may be divided into three major groups on the basis of trace element abundances and ratios 1 fi felsic metavolcanic rocks are dacites and rhyodacites characterized by steep ree patterns with weakly negative to moderately positive eu anomalies high zr y low abundances of high field strength elements e g hree y zr hf and high abundances of sr examples occur in the bowman subgroup and skead group in the abitibi belt in the kakagi lake lake of the woods shoal lake and sturgeon lake areas of the wabigoon belt and in the confederation lake area of the uchi belt none of these horizons as known hosts base metal sulphide deposits 2 fii felsic metavolcanic rocks are rhyodacites and rhyolites characterized by gently sloping ree patterns with variable eu anomalies moderate zr y and intermediate abundances of hfs elements and sr examples occur in the misema subgroup of the abitibi belt in the wabigoon lake a

age of felsic volcanism and the role of ancient continental crust in the evolution of the neoarchean rio das velhas greenstone belt quadrilatero ferrifero brazil u pb zircon dating of volcaniclastic graywackes abstract a felsic eruptive event within the rio das velhas greenstone belt is mainly represented by volcaniclastic and resedimented volcaniclastic rocks besides rare lava flows of dacitic composition the volcaniclastic rocks comprise four lithofacies 1 monomictic and polymictic breccias 2 conglomerate graywacke 3 graywacke sandstone and 4 graywacke argillite shrimp and id tims u pb zircon dating of three graywacke samples dated the eruptive event at 2792 11 2773 7 and 2751 9 ma indicating a range of about 40 ma for the felsic magmatism this time range may also encompass the whole magmatic activity within the belt because of the association of the felsic rocks with the more extensive basaltic flows volcaniclastic graywackes are characterized by an abundance of detritic xenocrystic zircon grains displaying a polymodal age spectrum ranging from 2 8 to 3 4 ga older zircon grains must be derived from magma contamination and or post eruptive sedimentary reworking and mixing with felsic detritus from other sources it may be concluded that felsic magmas were in contact with older granitoid crust and that this basement was also exposed as source for detritic material in the quadrilatero ferrifero the ancient complexes of ttg gneisses and migmatites make up most of the exposed archean terrains and its present juxtaposition with the supracrustal assemblage suggests an authochthonous to parauthocthonous development for the rio das velhas greenstone belt in a continental or near continental tectonic setting

dual sources of ensimatic magmas hearne domain western churchill province nunavut canada neorchean infant arc processes abstract in the henik segment of the central hearne supracrustal belt neoarchean supracrustal rocks accumulated in deep water depositional systems that were well removed from a continental influence repeated in time and space these systems consisted of mafic lava plain slope and apron subenvironments with local emergent to near emergent felsic volcanic centres volcanic and plutonic rocks are subalkaline and range in composition from basalt to rhyolite consistent with field relationships the volcanic rocks are juvenile and with predominantly positive values that are independent of compositional variation display little evidence of contamination by continental crust however despite primary interlayering on scales of 10s of metres mafic and felsic rocks display markedly different rare earth element ree characteristics the mafic volcanic rocks are morb like and display a tholeiitic magma evolution trend flat ree patterns and positive values suggesting that they were derived from partial melting of depleted mantle in marked contrast the felsic rocks display steeply sloping ree patterns with significantly lower heavy ree abundances negative nb anomalies and positive zr and hf anomalies suggesting derivation from partial melting of slightly older basaltic crust this duality of magma sources supports a generalized model for the central hearne supracrustal belt that draws analogies from eocene intraoceanic infant arc processes described from the southwestern pacific ocean rather than from those normally associated with fully developed phanerozoic subduction

whole rock and nd isotopic geochemistry of neoarchaean granitoids and their bearing on the evolution of the central hearne supracrustal belt western churchill province canada abstract the central hearne supracrustal belt chsb hearne domain western churchill province forms a large ca 30 000 km2 greenschist grade neoarchaean 2711 2660 ma terrane of predominant metavolcanic and less common metasedimentary rocks intruded by three groups of plutonic rocks pre tectonic group 1 plutons 2690 ma include rare gabbro dominant diorite to tonalite and rare granodiorite and granite these intrude and incorporate angular inclusions of cogenetic volcanic rocks and have variably developed foliations syn tectonic group 2 plutons 2690 2679 ma comprise rare gabbro and diorite predominant tonalite and granodiorite and rare granite and exhibit n s trending ductiley deformed supracrustal schlieren rich intrusive margins post tectonic group 3 plutons are rare and comprise either potassic biotite monzogranite or alkalic nepheline syenite and rare carbonatite abundant tonalite to granodiorite biotite hornblende bearing mineralogies and metaluminous and generally low medium k2o compositions indicate that most rocks are i or m type granitoids molecular na ca k variations and afm indices indicate transitional calc alkaline trondjhemitic trends with both tholeitic and calc alkaline affinities rare earth and incompatible element variations suggest that most granitoids exhibit volcanic arc or tonalite trondhjemite granodiorite ttg like abundances with multi element patterns varying gradationally from la poor lan ybn 12 nd isotopic values overlap with contemporaneous depleted mantle indicating that the granitoids represent melts derived from predominantly juvenile mantle or crust la poor rocks likely formed through low p anatexis of amphibolitic crust with plagioclase amphibole clinopyroxene present whereas high la rocks were generated via high p partial melting of a garnet clinopyroxene bearing protolith plausibly a subducted slab granitoid evolution from early dominant low p tholeiitic and calc alkaline melts to later predominant high p high al2o3 ttg melts reflects a change in the tectonomagmatic setting at ca 2690 ma construction of proto arc crust from ca 2711 2690 ma and extension of the leading edge of normal ca 40 km archaean oceanic crust in response to lithospheric processes analogous to those of the eocene sw pacific ocean resulted in asthenospheric upwelling intrusion of mantle derived melts into the lower crust and their subsequent ascent and fractionation partial melting at the base of the extended thick oceanic crust likely yielded sparse high la melts at this time at ca 2690 ma a change from an extensional to a shortening regime resulted in initiation of subduction of adjacent oceanic lithosphere partial melting of the eclogitic downgoing slab and generation of voluminous high p la rich granitoid magmas this yielded less abundant low p la poor melts emplaced into the infant arc crust during and following tectonism the complete sequence was intruded by late potassic granites derived from tonalitic lower crust and alkalic magmas that originated in the lithospheric mantle

paleoproterozoic 1 90 1 86 ga arc volcanism in the flin flon belt trans hudson orogen canada geochemical and isotopic nd sr data are reported on paleoproterozoic 1904 1864 ma maficintermediate 1870 ma collision and subsequent intra arc deformation

the relative motion between africa and eurasia as derived from itrf2000 and gps data 1 studies of intra and inter plate deformation typically need a model describing the motions of the stable part of the tectonic plates for reference purposes we have developed deos2k a model for the current motion of seven major tectonic plates derived from space geodetic observations this paper focuses on relative motion between africa and eurasia in the past this motion has been poorly established because of poor data coverage for africa deos2k is based on itrf2000 altamimi et al 2002 and new african gps observations it is an improvement over the nuvel 1a model for predicting the present day relative motions of these two plates deos2k predicts in northeastern africa that africa eurasia relative motion is about 40 smaller in magnitude than nuvel 1a and trends more to the northwest this is consistent with independent local geodetic observations a similar shift in orientation clockwise is observed at the western tip of the plate boundary

tectonic evolution of the hengshan wutai fuping complexes and its implication for the trans north china orogen abstract the hengshan wutai fuping complexes comprise the central segment of the trans north china orogen tnco in the north china craton ncc the tectonic evolution of these complexes is regarded as representative for the entire tnco although the timing of the events has been controvesial on the basis of new nd isotope and structural data and a re appraisal of published geochemical and geochronological data a refined model for the tectonic evolution in the neoarchean to paleoproterozoic has been proposed a major ocean named the proto north china ocean developed between the western and eastern blocks prior to 2565 ma northwest ward intra oceanic subduction resulted in the formation of a proto arc as represented by the 2565 2540 ma wutai granitoids the proto arc was rifted and a back arc basin propagated and extended to form an intra oceanic arc basin system at about 2540 2525 ma the main products during the development of this system were a morb arc back arc basalt adakite association in the wutai greenstone belt and synchronous granitoids in the wutai complex the main subduction accretion process involving multiple subduction zones occurred at 2525 2475 ma resulting in closure of the wutai back arc basin and the formation of diverse arc magmas although the tectonic history between 2 5 and 1 8 ga remains uncertain final collision between the eastern and western blocks and the buildup of the tnco most likely took place at 1 9 1 8 ga the tectonic evolution of the tnco is characterized by subduction accretion processes and the development of arc basin systems thus it is more likely a cordilleran type orogen than a simple continent continent collisional orogen

geochemistry zircon u pb geochronology and lu hf isotopes of metavolcanics from eastern hebei reveal neoarchean subduction tectonics in the north china craton abstract the neoarchean metavolcanic rocks of saheqiao occur within the central domain of the precambrian terrane in eastern hebei within the north china craton these rocks consist mainly of garnet two pyroxene granulites garnet clinopyroxene amphibolites amphibole plagioclase gneisses and biotite plagioclase gneisses these metavolcanic rocks are exposed as several kilometers scale of massive blocks or several tens of centimeters scale of enclaves within precambrian granitoid gneisses based on whole rock geochemistry we classify these intermediate mafic volcanic rocks into four groups group 1 exhibits low sio2 yb and nb ta ratios and high ni and cr and shows n morb like normal mid ocean ridge basalts geochemical features suggesting that the magmatic precursors of these rocks were derived through partial melting of depleted mantle compared to group 1 group 2 shows slightly right declined ree and nearly flat multi element patterns with variable negative nb anomalies matching with primitive arc basaltic rocks formed by partial melting of the mantle wedge group 3 exhibits chemical compositions of basaltic and andesitic rocks and is characterized by pronounced lree enrichment but hree depletion and shows distinctly negative nb and ta anomalies and weakly negative zr and hf anomalies these rocks may compare with island arc volcanic rocks and were probably generated by partial melting of the mantle wedge modified by dehydration of subducted oceanic slab the magmatic precursors of the andesites were derived from the crystallization differentiation of the basaltic magma group 4 is characterized by clearly right declined ree patterns and slightly positive or no nb ti anomalies which are analogous to nb enriched basalts neb and originated from melting of hfse enriched mantle sources la icp ms u pb dating of magmatic zircons from five samples from the three groups reveals that group 1 n morb like rocks formed at 2525 7 ma group 2 primitive arc basaltic rocks at 2518 15 ma and group 3 typical arc volcanic rocks at 2523 6 ma to 2614 18 ma zircons from all the dated samples also preserve signatures of 2450 2370 ma metamorphisms or thermal events the zircon lu hf data from the three dated samples in group 3 show ehf t values ranging from 1 4 to 9 0 and tdm values of 2627 ma to 2965 ma suggesting a significant episode of 2 6 2 9 ga crustal growth in this region the 2525 ma n morb like basaltic rocks are younger than the arc related rocks 2614 2518 ma suggesting that the morb like basaltic rocks were generated in a back arc environment on the basis of the rock association of back arc morb basalts group 1 arc related andesites basalts groups 2 and 3 and neb like rocks group 4 we suggest that the neoarchean volcanic rocks of the eastern hebei region formed in a subduction related setting within a convergent plate margin

the mesoarchean emergence of modern style subduction abstract well preserved volcanic sequences span the paleoarchean to neoarchean evolution of the pilbara craton in northwestern australia this region provides the best physical evidence bearing on the stage of earth s history when modern style tectonic processes began paleoarchean assemblages in the eastern nucleus of the craton the 3 51 3 24 ga pilbara supergroup show few features that can reasonably be interpreted as evidence for modern style subduction processes incompatible trace element enriched felsic volcanic horizons show geochemical evidence for the interaction between mafic magmas and crust but this evidence on its own can equally well be interpreted in terms of either a subduction enriched mantle source or local and limited assimilation of felsic crust into the voluminous tholeiitic magmas that dominate the pilbara supergroup viewed in context within the thick autochthonous and consistently upward younging pilbara supergroup these felsic units are most likely related to the same plume dominated processes that formed the basalts that dominate the supergroup it is very unlikely that modern style plate tectonic processes played any role in the paleoarchean evolution of the pilbara craton although some form of non uniformitarian e g flat subduction process may have operated in stark contrast the mesoarchean units of the west pilbara terrane and the late tectonic basins that cover that boundary between the west and east pilbara terranes show clear evidence for modern style convergent margin processes igneous rocks in this belt which flanks the old eastern cratonic nuclei have enriched geochemical signatures that cannot be accounted for by crustal contamination this region is also characterised by a linear magmatic and structural fabric by the presence of lithologically and geochronologically exotic belts and by the presence of a broad belt of isotopically more juvenile crust the collective strength of these arguments provides compelling evidence that a modern style oceanic arc fringed the east pilbara terrane at 3 12 ga and accreted to that terrane by 2 97 ga these assemblages mark the minimum age for the birth of modern style plate subduction process

the ferguson lake deposit an example of ni cu co pge mineralization emplaced in a back arc basin setting the world s largest ni cu platinum group element pge deposits are dominantly hosted by ultramafic rocks within continental extensional settings e g raglan voisey s bay resulting in a focus

temporal evolution of the neoarchean central hearne supracrustal belt rapid generation of juvenile crust in a suprasubduction zone setting abstract new u pb zircon ages are reported for 17 volcanic plutonic and sedimentary rocks from the neoarchean central hearne supracrustal belt in the western churchill province of the canadian shield the central hearne supracrustal belt historically referred to as the rankin ennadai greenstone belt is a major granite greenstone terrain with a strike length of over 700 km this study focused on the central and eastern segments of the belt in the kaminak lake area represents the first detailed chronological study of this extensive greenstone belt the geochronological data indicate rapid formation of crust between 2711 and 2667 ma rocks of the oldest assemblage i formed between 2711 and 2691 ma and consist of mixed tholeiitic and calc alkaline mafic and felsic submarine to subaerial volcanic rocks minor plutonic rocks group 1 are associated with this interval a younger assemblage ii of volcanic rock with significant calc alkaline plutonic rocks group 2 developed between 2686 and 2679 ma a period of deformation d1 locally separates the two assemblages major regional penetrative deformation and metamorphism developed during the latter stages of the plutonic event at ca 2680 ma an extensive belt of metasedimentary rocks and associated iron formation was deposited after 2681 ma based on ages of detrital zircon the youngest events include intrusion of 2666 4 1 1 ma post deformational granite group 3 2659 ma carbonatite can j earth sci 29 1992 896 and deposition of post 2660 ma conglomerates and arenites of possible timiskiming type there is no indication of contributions from mesoarchean crust either in inherited grains in igneous rocks or as detrital grains in sedimentary rocks although the evolutionary sequence has parallels to greenstone belts such as the abitibi belt in terms of the rock types and relative timing of events there are some differences we envision development of the belt in an extensional oceanic suprasubduction environment

the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland the mesoarchean ca 3075 ma ivisaartoq greenstone belt contains well preserved primary magmatic structures such as pillow lavas volcanic breccias and clinopyroxene cumulate layers picrites despite the isoclinal folding and amphibolite facies metamorphism the belt also includes variably deformed gabbroic to dioritic dykes and sills actinolite schists and serpentinites the ivisaartoq rocks underwent at least two stages of post magmatic metamorphic alteration including seafloor hydrothermal alteration and syn to post tectonic calc silicate metasomatism between 3075 and 2961 ma these alteration processes resulted in the mobilization of many major and trace elements the trace element characteristics of the least altered rocks are consistent with a supra subduction zone geodynamic setting and shallow mantle sources on the basis of geological similarities between the ivisaartoq greenstone belt and phanerozoic forearc ophiolites and intra oceanic island arcs we suggest that the ivisaartoq greenstone belt represents a relic of dismembered mesoarchean supra subduction zone oceanic crust this crust might originally have been composed of a lower layer of leucogabbros and anorthosites and an upper layer of pillow lavas picritic flows gabbroic to dioritic dykes and sills and dunitic to wehrlitic sills the sm nd and u pb isotope systems have been disturbed in strongly altered actinolite schists in addition the u pb isotope system in pillow basalts appears to have been partially open during seafloor hydrothermal alteration gabbros and diorites have the least disturbed pb isotopic compositions in contrast the sm nd isotope system appears to have remained relatively undisturbed in picrites pillow lavas gabbros and diorites as a group picrites have more depleted initial nd isotopic signatures end 4 23 to 4 97 than pillow lavas gabbros and diorites end 0 30 to 3 04 consistent with a variably depleted heterogeneous mantle source in some areas gabbros include up to 15 cm long white inclusions xenoliths these inclusions are composed primarily 90 of ca rich plagioclase and are interpreted as anorthositic cumulates brought to the surface by upwelling gabbroic magmas the anorthositic cumulates have significantly higher initial end 4 8 to 6 0 values than the surrounding gabbroic matrix 2 3 to 2 8 consistent with different mantle sources for the two rock types

surface deformation and slab mantle interaction during banda arc subduction rollback the reason for the spectacular curvature of the banda subduction zone is debated tomographic images and plate reconstructions reveal subduction of a single slab the ancient geometry of the australian plate as well as the interaction between the slab and the mantle caused the deformation of the slab the spectacularly curved banda arc comprises young oceanic crust1 2 enclosed by a volcanic inner arc outer arc islands and a trough parallel to the australian continental margin3 4 5 strong seismic activity in the upper mantle defines a folded surface6 7 for which there are two contrasting explanations deformation of a single slab5 8 or two separate slabs subducting from the north and south6 9 here we combine seismic tomography with the plate tectonic evolution of the region to infer that the banda arc results from subduction of a single slab our palaeogeographic reconstruction shows that a jurassic embayment which consisted of dense oceanic lithosphere enclosed by continental crust once existed within the australian plate banda subduction began about 15 million years ago when active java subduction tore eastwards into the embayment the present morphology of the subducting slab is only partially controlled by the shape of the embayment as the australian plate moved northward at a high speed of about 7 cm yr 1 the banda oceanic slab rolled back towards the south southeast accompanied by active delamination separating the crust from the denser mantle increasing resistance of the mantle to plate motion progressively folded the slab and caused strong deformation of the crust the banda arc represents an outstanding example of large scale deformation of the earth s crust in response to coupling between the crust slab and surrounding mantle

the red lake greenstone belt superior province evidence of plume related magmatism at 3 ga and evidence of an older enriched source abstract the red lake greenstone belt of the superior province contains a number of volcano sedimentary assemblages that range in age from 2 99 ga to 2 74 ga the oldest of these is the 2 99 2 96 ga balmer assemblage which is part of the north caribou terrane one of the oldest portions of the superior province the balmer assemblage dominates the red lake greenstone belt and is significant in that it contains evidence for plume generated magmatism the assemblage is dominated by mafic volcanic flows with intercalated komatiitic lavas mafic and ultramafic sills and shallow water clastic and chemical sediments the basalts have 5 12 mgo and mantle normalized signatures which range from slightly light rare earth element ree depleted through essentially flat to light ree enriched some lavas display slight negative nb and ti anomalies indicating selective crustal contamination the komatiites have 12 14 mgo are the al undeplated type and are strongly depleted in light ree they have e nd values of ca 2 the geochemistry combined with sm nd isotope data indicates that the komatiites came from a long term enriched source that was probably generated at ca 3 4 ga that source underwent a melting event that depleted it in light ree prior to komatiite generation the presence of komatiites from a long term enriched source and the strong contrast in the ree profiles of the basalts and the komatiites are consistent with a plume model for the generation of the balmer assemblage the komatiites having originated in the plume core and the basalts from entrained upper mantle or mantle melted by the plume

seismic observations of large scale deformation at the bottom of fast moving plates abstract we present a new tomographic model of azimuthal anisotropy in the upper mantle dr2012 and discuss in details the geodynamical causes of this anisotropy our model improves upon dkp2005 seismic model debayle et al 2005 through a larger dataset expanded by a factor 3 7 and a new approach which allows us to better extract fundamental and higher mode information our results confirm that on average azimuthal anisotropy is only significant in the uppermost 200 250 km of the upper mantle where it decreases regularly with depth we do not see a significant difference in the amplitude of anisotropy beneath fast oceanic plates slow oceanic plates or continents the anisotropy projected onto the direction of present plate motion shows a very specific relation with the plate velocity it peaks in the asthenosphere around 150 km depth it is very weak for plate velocities smaller than 3 cm yr 1 increases significantly between 3 and 5 cm yr 1 and saturates for plate velocities larger than 5 cm yr 1 plate scale present day deformation is remarkably well and uniformly recorded beneath the fastest moving plates india coco nazca australia philippine sea and pacific plates beneath slower plates plate motion parallel anisotropy is only observed locally which suggests that the mantle flow below these plates is not controlled by the lithospheric motion a minimum plate velocity of around 4 cm yr 1 is necessary for a plate to organize the flow in its underlying asthenosphere the correlation of oceanic anisotropy with the actual plate motion in the shallow lithosphere is very weak a better correlation is obtained with the fossil accretion velocity recorded by the gradient of local seafloor age the transition between frozen in and active anisotropy occurs across the typical age isotherm that defines the bottom of the thermal lithosphere around 1100 c under fast continents mostly under australia and india the present day velocity orients also the anisotropy in a depth range around 150 200 km depth which is not deeper than what is observed under oceans

characterising the southern part of the hearne province a forgotten part of canada s shield revisited abstract the hearne craton comprises the southeastern part of the western churchill craton the central part of the craton has been well studied in nunavut and northeastern saskatchewan in comparison with the southern part of the craton which is mainly exposed south of the athabasca basin in saskatchewan mapping and analytical results indicate that the southern hearne craton is dominated by a felsic orthogneiss complex derived mainly from tonalites with crystallisation ages in the range 2 81 2 78 ga and ca 2 73 ga sm nd isotopic analysis suggests that these evolved plutons mean nd 3 3 were derived from a mesoarchean to paleoarchean source other components of the orthogneisses include leucotonalite intrusions dated at ca 2 664 ga and regionally extensive k feldspar phyric granites emplaced between 2 63 and 2 60 ga both of these suites have sm nd isotopic signatures similar to the older tonalites a suite of granodiorite and quartz diorite one of which yielded a crystallisation age of 2 654 ga has unique sm nd isotopic results with nd values near chur the felsic orthogneiss complex is unconformably overlain by the black birch lake assemblage which contains dominantly orthquartzites mafic volcanic rocks and pelites detrital zircon signatures in the orthoquartzites are dominated by archean sources and indicate that the supracrustal rocks were deposited after 2 60 ga the central hearne province extends into northeastern corner of saskatchewan it contains oceanic mafic volcanic rocks of the ennadai group that were intruded by juvenile tonalites at 2 71 2 70 ga the age and isotopic character of these tonalites contrasts markedly with those in the felsic orthogneiss complex of the southern hearne province a comparison of plutonic suites and supracrustal assemblages of the central and southern hearne provinces suggests that there is heterogeneity between the two entities analysis of geophysical data coupled with existing geological maps suggest that an important tectonic boundary separates them the current age relationships in the central and southern parts of the hearne province imply that the two were juxtaposed after 2 60 ga however the time of suturing remains equivocal

bridging onshore and offshore present day kinematics of central and eastern mediterranean implications for crustal dynamics and mantle flow we present a new kinematic and strain model of an area encompassing the calabrian and hellenic subduction zones western anatolia and the balkans using haines and holt s 1993 method we derive continuous velocity and strain rate fields by interpolating geodetic velocities including recent gps data in the balkans relative motion between stable eurasia and the western aegean sea is gradually accommodated by distributed n s extension from southern balkans to the eastern corinth gulf so that the westward propagation of the north anatolian fault naf throughout continental greece or peloponnesus is not required we thus propose that the naf terminates in north aegean and that n s extension localized in the corinth gulf and distributed in southern balkans is due to the retreat of the hellenic slab the motion of the hyblean plateau apulia peninsula south adriatic sea ionian basin and sirte plain can be minimized by a single rigid rotation around a pole located in the sirte plain compatible with the opening the pelagian rifts 2 2 5 mm yr and seismotectonics in libya we interpret the trenchward ultraslow motion of the calabrian arc 2 2 5 mm yr as pure collapse the calabrian subduction being now inactive in the absolute plate motion reference frame our modeled velocity field depicts two toroidal crustal patterns located at both ends of the hellenic subduction zone clockwise in nw greece and counter clockwise in western anatolia we suggest the nw greece toroidal pattern is the surface expression of a slab tear and consequent toroidal asthenospheric flow

mechanics of fore arc slivers insights from simple analog models 1 convergent margins with oblique subduction commonly include a fore arc sliver a portion of the overlying plate bounded by the trench and a major strike slip fault system it has long been noted that relative plate motion near the deformation front as indicated by earthquake focal mechanisms is generally closer to margin normal than would be expected from overall relative plate motion this results from sliver motion as well as from shear within the fore arc we carry out some simple calculations to determine how the rate of sliver motion should vary with plate convergence obliquity we find that the results are related to rheology but that the measurement of sliver rates at natural fore arcs is unlikely to yield real insight into physical properties even for very simple models the system is too complicated and its relation to rheology is not unique the proportion of oblique convergence taken up by sliver motion in simple tabletop experiments depends on the rate of slip and the smoothing of asperities similarities in taper and style of strain between frontal wedges forming with and without slivers suggest that structural observations of exhumed accretionary wedges are unlikely to allow geologists to draw definitive conclusions about the degree of obliquity of relative plate motion at the time when the wedges were formed and in some cases not even whether or not a sliver plate was present at the time of deformation

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

evolution of the late miocene mediterranean atlantic gateways and their impact on regional and global environmental change marine gateways play a critical role in the exchange of water heat salt and nutrients between oceans and seas as a result changes in gateway geometry can significantly alter both the pattern of global ocean circulation and associated heat transport and climate as well as having a profound impact on local environmental conditions mediterranean atlantic marine corridors that pre date the modern gibraltar strait closed during the late miocene and are now exposed on land in northern morocco and southern spain the restriction and closure of these miocene connections resulted in extreme salinity fluctuations in the mediterranean leading to the precipitation of thick evaporites this event is known as the messinian salinity crisis msc the evolution and closure of the mediterranean atlantic gateways are a critical control on the msc but at present the location geometry and age of these gateways are still highly controversial as is the impact of changing mediterranean outflow on northern hemisphere circulation here we present a comprehensive overview of the evolution of the late miocene gateways and the nature of mediterranean atlantic exchange as deduced from published studies focussed both on the sediments preserved within the fossil corridors and inferences that can be derived from data in the adjacent basins we also consider the possible impact of evolving exchange on both the mediterranean and global climate and highlight the main enduring challenges for reconstructing past mediterranean atlantic exchange

a mechanism for offshore transport across the gulf stream this paper explores a possible mechanism to explain an atypical phenomenon observed in the gulf stream between the florida straits and cape hatteras namely upgradient momentum transport and associated eddyto mean energy conversion on the cyclonic shear side the proposed mechanism is based on the results of an experimental study of free surface turbulent jets in a rotating system and on theoretical descriptions of rotating shear flows unlike field situations where coriolis effects are always present the laboratory experiment provides a convenient means of comparing turbulence characteristics in rotating and nonrotating systems rotation is shown to alter turbulence eddy orientation thus affecting the momentum transport and energy conversion processes

komatiite flooding of a rifted archean rhyolitic arc complex geochemical signature and tectonic significance of the stoughton roquemaure group abitibi greenstone belt canada the 0 2 2 km thick archean stoughton roquemaure group srg in the northern volcanic zone of the abitibi greenstone belt quebec canada is composed of tholeiitic basalt komatiitic basalt and komatiite the mafic and ultramafic rocks are pillowed brecciated and massive columnar jointed flows the srg conformably overlies the 2730 ma hunter mine group a volcanic complex dominated by calc alkaline felsic rocks the tholeiitic basalts of the srg resemble morb the komatiitic basalts and komatiites have positive nd values overlapping those of the tholeiitic basalts komatiitic basalts with low al2o3 tio2 ratios 10 and fractionated heavy ree patterns are similar to al depleted komatiites in contrast the komatiites have high al2o3 tio2 20 unfractionated heavy ree patterns and resemble al undepleted munro type komatiites the al depleted komatiitic basalts occur at the base of the srg whereas the al undepleted komatiites are prevalent higher up in the stratigraphy the association of calcalka

geochemistry of the archean kam group yellowknife greenstone belt slave province canada abstract the geochemistry and isotope systematics of archean greenstone belts provide important constraints on the origin of the volcanic rocks and tectonic models for the evolution of archean cratons the kam group is a 10 km thick pile of submarine tholeiitic mafic and subordinate felsic volcanic rocks erupted between 2712 and 2701 ma that forms the bulk of the yellowknife greenstone belt in the dominantly granite metasedimentary slave province mafic rocks range from normal mid ocean range basalt like basalts to slightly light rare earth element enriched lree enriched but nb depleted basaltic andesites and andesites whereas dacitic to rhyodacitic felsic rocks are strongly lree enriched and highly depleted in nb the documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcomma

geochronological and geochemical constraints on the petrogenesis of the 2 6 2 5 ga amphibolites low and high al ttgs in the wangwushan area southern north china craton implications for the neoarchean crustal evolution abstract the crustal evolutionary processes of the north china craton ncc during the late neoarchean ca 2 6 2 5 ga are still under large debate the systematic investigations of widespread ca 2 6 2 5 ga meta volcanic basalts and ttg assemblages can provide a better understanding of tectonic evolution at this period the wangwushan neoarchean amphibolites high and low al ttgs in southern ncc were formed at 2 57 2 52 ga the amphibolites are tholeiitic in composition they have whole rock end t values of 2 8 4 0 tdm1 3 7 2 6 ga and zircon ehf t values of 1 2 7 2 tdm1 2 81 2 57 ga indicative of a depleted mantle source with involvement of older crustal materials their compositional spectrum accords with those in island arc field or straddles the boundary between morb and island arc fields suggesting significant contributions from subduction zone derived fluids during magma formation the high al ttgs contain high mg cr and ni contents and la yb n and sr y ratios together with positive whole rock end t values 0 6 5 2 tdm2 2 95 2 48 ga indicating that they derived from partial melting of a subducted oceanic slab with older crustal materials their geochemical features are corresponding to slab melt identification criterion in an arc setting and illustrate a deep source 45 km with garnet amphibolite in the residue without rutile the low al ttgs show low mg and la yb n ratios negative eu anomalies relatively flat hree and y patterns their zircon ehf t 2 2 8 8 and 1 3 6 9 and whole rock end t values ca 2 23 are positive all the evidences support that the low al ttgs derived from a low degree partial melting of basaltic crust with the residue lack of garnet consistent with a shallower depth about

stratigraphy sedimentology and physical volcanology of the henik group central ennadai rankin greenstone belt northwest territories canada late archean paleogeography of the hearne province and tectonic implications abstract extensive areas of the hearne and rae provinces are underlain by archean metavolcanic and metasedimentary rocks in the rae province supracrustal sequences with prominent continental to shallow water quartz arenites have been interpreted by others as rift and passive margin deposits in the hearne province to the southeast the ennadai rankin greenstone belt is exposed in an area ca 700 200 km forming the second largest greenstone belt in canada it comprises mainly greenschist facies grade mafic and felsic volcanic siliciclastic chemogenic and volcaniclastic rocks stratigraphic sedimentologic and physical volcanologic data based on 1 50 000 scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau slope basin system that was well removed from a continental influence water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron formation units absence of features suggestive of subaerial exposure oscillatory flow or tidal currents and lack of interfingering rocks that could represent laterally adjacent shelf coastal or continental environments reconnaissance level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the hearne province consisting of a large mafic volcanic plain isolated felsic volcanic centres with lenses and belts of deep water sedimentary rocks at least two speculative yet testable tectonic models are consistent with the inferred paleogeography for both the rae province is considered to be the western continental hinterland to the ensimatic hearne province the rae and hearne provinces may record late archean continental extension sea floor spreading oceanic volcanic arc growth diachronous back arc spreading and collapse and arc continent collision with thrusting of oceanic rocks on to attenuated rae crust alternatively the hearne province may represent northwestward accretion of multiple volcanic arc trench pairs

messinian salinity crisis regulated by competing tectonics and erosion at the gibraltar arc the persistent shallow connection between the atlantic ocean and the mediterranean sea that caused the messinian salinity crisis may have been sustained by a balance between tectonic uplift and erosion due to inflow about six million years ago a series of geological factors drastically reduced the exchange of water between the atlantic ocean to the mediterranean sea through what is now the strait of gibraltar the resulting widespread salt precipitation in the mediterranean linked to a decrease in sea level of about 1 5 kilometres due to evaporation is known as the messinian salinity crisis the exact mechanism that caused the crisis remains uncertain here daniel garcia castellanos and antonio villasenor propose a competition between the obstruction of the gibraltar arc seaway by tectonic uplift and its erosional deepening by water inflow controlled by the resulting changes in sea level difference across the gibraltar arc as a possible mechanism for the crisis the messinian salinity crisis1 2 5 96 to 5 33 million years ago was caused by reduced water inflow from the atlantic ocean to the mediterranean sea resulting in widespread salt precipitation and a decrease in mediterranean sea level of about 1 5 kilometres due to evaporation3 the reduced connectivity between the atlantic and the mediterranean at the time of the salinity crisis is thought to have resulted from tectonic uplift of the gibraltar arc seaway and global sea level changes both of which control the inflow of water required to compensate for the hydrological deficit of the mediterranean1 4 however the different timescales on which tectonic uplift and changes in sea level occur are difficult to reconcile with the long duration of the shallow connection between the mediterranean and the atlantic5 needed to explain the large amount of salt precipitated here we use numerical modelling to show that seaway erosion caused by the atlantic inflow could sustain such a shallow connection between the atlantic and the mediterranean by counteracting tectonic uplift the erosion and uplift rates required are consistent with previous mountain erosion studies with the present altitude of marine sediments in the gibraltar arc6 7 and with geodynamic models suggesting a lithospheric slab tear underneath the region8 9 10 the moderate mediterranean sea level drawdown during the early stages of the messinian salinity crisis3 5 can be explained by an uplift of a few millimetres per year counteracted by similar rates of erosion due to atlantic inflow our findings suggest that the competition between uplift and erosion can result in harmonic coupling between erosion and the mediterranean sea level providing an alternative mechanism for the cyclicity observed in early salt precipitation deposits and calling into question previous ideas regarding the timing of the events that occurred during the messinian salinity crisis1

late neoarchean thick skinned thrusting and paleoproterozoic reworking in the macquoid supracrustal belt and cross bay plutonic complex western churchill province nunavut canada abstract in the western churchill province canadian shield neoarchean supracrustal and plutonic rocks intruded by paleoproterozoic mafic dykes and granitic rocks comprise the macquoid supracrustal belt and the structurally overlying cross bay plutonic complex they form part of the northwestern hearne subdomain that occupies an intermediate position between the continental rae domain to the north and west and the oceanic central hearne subdomain to the south and east new geological mapping and supporting geoscience are compatible with the presence of 2550 2500 ma southeast directed mid crustal thick skinned thrusting that juxtaposed the plutonic complex over the supracrustal belt the structural contact between the macquoid supracrustal belt and the cross bay plutonic complex potentially represents a fundamental boundary between isotopically distinct crustal blocks the 2190 ma macquoid mafic dyke swarm cuts across neoarchean deformation fabrics but records 1 9 ga deep crustal regional metamorphism that affected both the supracrustal belt and the plutonic complex other paleoproterozoic deformation events that occurred at 1850 1810 ma are of local extent and appear to be relatively minor manifestations of more important events elsewhere related to the trans hudson orogen

plate motions slab dynamics and back arc deformation back arc extension or compression is often related to a particular behavior of the slab dip change and or forward backward migration with respect to the upper plate slabs act either passively when anchored in the deep mantle whereas back arc deformation accommodates the backward or forward motion of the upper plate with respect to the trench or actively when slab pull increasing with slab age forces the trench to rollback in addition to these two mechanisms i e slab anchor and slab rollback numerous observations support the existence of dynamic mantle flow that can exert an overpressure on one side of the slab causing its forward or rearward migration with respect to the arc based on a compilation of upper plate absolute motion trench absolute motion back arc deformation rate upper plate strain regime and slab age for all oceanic subduction zones excluding any kind of collision with continents arcs or plateaus we have examined how the combined effects of these parameters can account for the observed back arc deformations our main results are 1 a global correlation exists between upper plate absolute motion and back arc deformation i e back arc extension when upper plate retreats and vice versa 2 there are as many advancing trenches as retreating ones with trench motion globally limited to 50 mm y 1 furthermore there is no positive correlation between trench retreat and slab age and 3 upper plate absolute motion often fails to explain the back arc deformation rates and the trench motions observed at several subduction zones as tonga new hebrides sandwich or ryukyu we propose that the trench migration of these subduction zones are on the influence of mantle flows we conclude that spontaneous trench rollback related to slab pull is negligible with respect to upper plate motion back arc deformation regime is mostly controlled by the upper plate absolute motion relative to a partly anchored slab trench motion is limited to 50 mm y 1 even if locally mantle flows force slab to move

precambrian crustal evolution seismic constraints from the canadian shield whether or not plate tectonic processes operated on a younger hotter earth remains ambiguous seismic data from new networks in the hudson bay region of the canadian shield where the precambrian geological record spans more than 2 billion years offer fresh scope to address this problem using receiver function analyses we show that the crust of the rae domain which exhibits ages of paleo to neoarchean 3 9 2 7 ga is likely felsic to intermediate in composition average vp vs 1 73 and seismically transparent with a sharp moho there is little evidence for modern style plate tectonics and based on the simplicity and spatial extent of the felsic crust models favouring vertical tectonic processes such as crustal delamination or plume activity appear better suited to the results data from the hearne domain which exhibits widespread 2 7 ga granite and greenstone geology show a more complex crust with higher vp vs ratios consistent with a greater mafic component the trans hudson orogen tho proposed to be a himalayan scale mountain belt during the paleoproterozoic is thought to have formed during the 1 8 ga collision of the superior and churchill plates results from the quebec baffin island segment of the tho appear to map out the first order shape of the underthrusting superior plate with elevated vp vs ratios likely representing the rifted margin of the superior craton consistently thicker crust is observed beneath central and southern baffin island 43 km coincident with widespread high grade metamorphic surface geology these features can be explained by crustal thickening due to stacking of accreted terranes during continent continent collision analogous to the present day tibetan plateau followed by erosion when reviewed in light of age and compositional constraints from the geological record our seismic observations point towards secular crustal evolution from non plate tectonic during the paleo to mesoarchean evolving towards fully developed modern style plate tectonics during the paleoproterozoic

petrogenesis of neoarchaean volcanic rocks of the macquoid supracrustal belt a back arc setting for the northwestern hearne subdomain western churchill province canada abstract rocks exposed in the macquoid gibson lakes region northwest hearne subdomain western churchill province canada comprise three major lithotectonic assemblages the principal volcanic belt the metasedimentary macquoid homocline and the cross bay plutonic complex neoarchaean supracrustal rocks of the belt range in age from 2700 ma crust likely did not occur ndt 2680 ma values for three specimens one from each of the arc like suites and one babb like basalt are slightly lower than the remainder suggesting very minor incorporation of slightly older crust these features imply that the processes that generated the macquoid supracrustal belt required simultaneous tapping of geochemically distinct mantle reservoirs with concomitant anatexis of sialic crust garnet stability field and fractionation of felsic magmas in upper crustal magma chambers shallow water deposition of abundant volcaniclastic rocks and semipelite along with minor conglomerate and quartzite was broadly contemporaneous with this magmatism we envisage a geodynamic setting characterized by tectonomagmatic processes similar to those of modern supra subduction zone back arc marginal basins such as the sea of japan therein an extensional back arc setting likely proximal to continental crust provides an explanation for a broad swath of diverse mantle derived rocks intercalated with less common felsic rocks as well as an abundance of immature clastic metasedimentary rocks

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

the diffuse plate boundary of nubia and iberia in the western mediterranean crustal deformation evidence for viscous coupling and fragmented lithosphere a spatially dense gnss based crustal velocity field for the iberian peninsula and northern africa allows us to provide new insights into two main tectonic processes currently occurring in this area in particular we provide for the first time clear evidence for a large scale clockwise rotation of the iberian peninsula with respect to stable eurasia euler pole component n42 612 w1 833 clockwise rotation rate of 0 07 deg myr we favor the interpretation that this pattern reflects the quasi continuous straining of the ductile lithosphere in some sectors of south and western iberia in response to viscous coupling of the nw nubia and iberian plate boundary in the gulf of cadiz we furnish evidence for a fragmentation of the western mediterranean basin into independent crustal tectonic blocks which are delimited by inherited lithospheric shear structures among these blocks an oceanic like western algerian one is currently transferring a significant fraction of the nubia eurasia convergence rate into the eastern betics se iberia and likely causing the eastward motion of the baleares promontory these processes can be mainly explained by spatially variable lithospheric plate forces imposed along the nubia eurasia convergence boundary

qingyuan high grade granite greenstone terrain in the eastern north china craton root of a neoarchaean arc abstract the qingyuan high grade granite greenstone terrain is one of the first recognized granite greenstone terrains in the eastern north china craton similar to other archaean terrains its tectonic environment is debated was it an intra continental rift or a modern analogy of arc occurrence and petrogenesis of major igneous series the ultramafic mafic and felsic volcanic rocks 2510 ma and the plutons of the quartz diorite 2570 2510 ma ttg 2570 2510 ma and quartz monzodiorite 2510 2490 ma series in xinbin area are investigated the mafic intrusives and volcanic rocks have high mgo content 5 4 7 5 wt and mg number 48 61 they show slightly depleted to flat trace element patterns the ultramafic rocks serpentinite could be genetically related to the mafic rocks the meta dacite rhyolite is adakitic with enriched light and middle rees and liles but significantly depleted hfses the quartz diorite has high mg number 60 64 moderately enriched light and middle rees and liles the ttg shows distinct light but moderate middle ree enrichment prominent nb ta depletion but zr hf enrichment zr sm 100 the quartz monzodiorite has moderate light and middle ree enrichment significant nb ta depletion and negative eu sr anomalies the ttg has more depleted sr nd isotopes endt 2 6 87sr 86srt 0 700 than all the others endt 0 2 87sr 86srt 0 701 703 their petrogenesis can be explained by an archaean style subduction defined as a mantle wedge absent flat hot subduction with significant vertical tectonism in the overriding slab the ultramafic mafic rocks were originated from primitive mantle the meta dacite rhyolite was originated from the eclogite facies overriding crust the quartz diorite was a mixture of melts from mantle and the overriding crust the ttg was from the subducting slab under amphibolite to amphibole bearing eclogite facies and the quartz monzodiorite was from the subducting slab after the derivation of the ttg the qingyuan terrain represents remnants of the root of a neoarchaean arc its anticlockwise p t path recorded the initiation of the arc to its cession and exhumation cratonization at 2480 ma

geochemistry of lau basin volcanic rocks influence of ridge segmentation and arc proximity side scan sonar imaging of the central lau basin sw pacific has revealed a central lau spreading centre clsc propagating southwards at the expense of an eastern lau spreading centre elsc with a small intermediate lau spreading centre ilsc forming a relay between the two volcanic rocks sampled along these spreading centres and from two adjacent seamounts are glassy to fine grained pillow lavas and sheet flows of basalts ferrobasalts and andesites the evolved rocks are mostly confined to the propagating tip of the clsc and can be explained by a high rate of cooling relative to magma supply as invoked for magma genesis at propagating ridges elsewhere compared with equivalent rocks from the eastern pacific the most evolved members of the clsc suite require similarly high degrees 90 of fractional crystallization from their basaltic parents fractional crystallisation cannot however account for the compositional differences between clsc and elsc lavas whereas the composition of the clsc lavas lies just within the compositional spectrum of typical n morb the elsc lavas are distinctly enriched in alkali and alkaline earth elements reach oxide and apatite saturation at lower fe ti and p concentrations and generally show greater vesicularity despite slightly greater depths of eruption all indicative of a water rich subduction component they also have lower contents of ni sc na and fe and higher contents of si at a given mgo concentration that indicate a more depleted and more hydrous mantle beneath the elsc compared with the clsc these results provide further evidence that beneath the central lau basin the source composition changes progressively from morb type to island arc tholeiite type as the subduction zone is approached both eastwards from the clsc to elsc and southwards along the elsc to the valu fa ridge they also indicate that the composition of the subduction component may vary systematically away from the arc with th lree ba rb and h as h2o all present close to the arc only ba rb and h2o present at intermediate distances and just h2o perceptible at the furthest distances

neoarchean convergent margin ni cu mineralization axis lake and nickel king ni cu deposits in the south rae craton of the canadian shield abstract the southern portion of the rae craton in the canadian shield is host to multiple ca 2 63 ga ni cu rich metamorphosed mafic intrusions with noritic noritic anorthositic and pyroxenitic compositions including those hosting the undeveloped axis lake and nickel king deposits found in northern saskatchewan and southeast northwest territories in order to understand the setting of these intrusions and their concomitant mineralization we investigated their geochemical and isotopic characteristics mineralized intrusions at axis lake and nickel king are metamorphosed up to granulite facies with local partial melting the intrusions preserve whole rock major element geochemistry consistent with a sub alkaline basaltic composition e g sio2 46 54 wt tio2 the nd isotope values of the axis lake intrusion ndt2630 0 3 to 2 1 are more juvenile and closer to the depleted mantle value relative to those of the nickel king deposit ndt2630 2 4 to 0 4 this agrees with their also distinctive th yb and nb yb distributions suggesting oceanic and continental arc like settings respectively the axis lake and nickel king nd isotope compositions are indistinguishable from other arc like 2 61 to 2 58 ga mafic to felsic intrusions and volcanic rocks recognized in the rae craton and the neighboring chesterfield block the age spatial distribution and geochemistry of the studied intrusions coincide with the initial stages of a recently defined 2 63 2 60 ga continental arc the roots of which are found in the south rae craton our data indicate that neoarchean intrusions with an arc like geochemistry may be a significant exploration target in the rae craton this broadens possible host suites beyond the established paleoproterozoic rift related rocks most commonly considered as the prime target for ni cu pge mineralization

active faulting and tectonics of burma and surrounding regions this study of the active tectonics of burma and surrounding regions is based mostly on an interpretation of landsat imagery and on fault plane solutions for shallow and intermediate earthquakes fold axes in the indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the indoburman ranges fault plane solutions of shallow earthquakes however do not show underthrusting at the present time instead p axes are oriented roughly north south parallel not perpendicular to the fold axes in the ranges p axes for fault plane solutions of intermediate shocks also trend north south parallel to the strike of the subducted slab and may indicate that the hanging slab is being dragged north by india through the surrounding asthenosphere the sagaing fault probably accommodates most of the right lateral slip of india past indochina but india s northward movement also causes internal deformation within burma thailand and yunnan differences between the tectonic fabric seen on the landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion in the orientations of strain and in the style of deformation in the late cenozoic presumably these changes reflect the transition from subduction to collision and strike slip motion as india penetrated into eurasia past burma large parts of burma and thailand may have rotated several tens of degrees clockwise in this process

subduction driven recycling of continental margin lithosphere seismic images of the subducted atlantic slab under northeastern south america and the alboran slab beneath the gibraltar arc region show that subducting oceanic plates viscously entrain and remove the bottom of the continental thermal boundary layer from adjacent continental margins driving surface tectonics and pre conditioning the margins for further deformation alan levander and co authors present seismic images that show larger than expected volumes of high seismic velocity mantle associated with the subducted atlantic slab under northeastern south america and the alboran slab beneath the gibraltar arc region the anomalous regions are under and aligned with the continental margins at depths greater than 200 km with the lithospheric mantle under the continental margins significantly thinner than expected the authors conclude that subducting oceanic plates viscously entrain and remove the bottom of the continental thermal boundary layer from adjacent continental margins driving surface tectonics and preconditioning the margins for further deformation by creating topography along the lithosphere asthenosphere boundary whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics the recycling of continental lithosphere appears to be far more complicated and less well understood1 delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts2 3 4 5 here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings we have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted atlantic slab under northeastern south america and the alboran slab beneath the gibraltar arc region6 7 the anomalies are under and are aligned with the continental margins at depths greater than 200 kilometres rayleigh wave analysis8 9 finds that the lithospheric mantle under the continental margins is significantly thinner than expected and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores taking these data together here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins this drives surface tectonics and pre conditions the margins for further deformation by creating topography along the lithosphere asthenosphere boundary this can lead to development of secondary downwellings under the continental interior probably under both south america and the gibraltar arc8 10 and to delamination of the entire lithospheric mantle as around the gibraltar arc11 this process reconciles numerous sometimes mutually exclusive geodynamic models proposed to explain the complex oceanic continental tectonics of these subduction zones12 13 14 15 16 17

crustal deformation in eastern betics from cuateneo gps network abstract the eastern betic cordillera spain is the most seismically active area within the iberian peninsula we present a global positioning system gps derived horizontal crustal deformation obtained from five occupations of the cuateneo gps network 1997 2002 2006 2009 and 2011 that clearly shows continuing tectonic activity in the se betics the most prominent feature of the gps velocity field is the nw oriented motion of the majority of the stations at rates ranging from 2 mm yr near the coast to 0 5 mm yr inland this type of deformation indicates that the main driving force responsible for the observed velocities is related to the on going convergence between nubia and eurasia plates the calculated deformation field shows evidence for localized deformation related to active faults within the area most of the deformation is concentrated on the alhama de murcia fault the source of the 2011 lorca earthquake m w 5 2 we estimate a reverse sinistral geodetic slip rate of 1 5 0 3 mm yr for this fault our crustal deformation field and analyses are important contributions to estimating seismic hazard for the eastern betics since it is the first time crustal deformation rates at this scale are presented

a 2 5 ga fore arc subduction accretion complex in the dengfeng granite greenstone belt southern north china craton abstract the dengfeng granite greenstone belt dggb located in the southern segment of the central orogenic belt cob of the north china craton ncc consists of a volcano sedimentary assemblage intruded by tonalite trondhjemite granodiorite ttg suite diorite granites and late mafic dikes the volcano sedimentary assemblage in the dggb mainly consists of tectonically imbricated basaltic amphibolites meta gabbroic rocks with minor ultramafic rocks and metagreywacke marble and quartzite consistent with characterisitics of typical phanerozoic subduction accretion complexes the basaltic amphibolites yield a metamorphic zircon 207pb 206pb age of 2507 26 ma interpreted to represent the peak age of amphibolite facies metamorphism that took place during subduction accretion of the basaltic protolith the basaltic amphibolites are characterized by a tholeiitic affinity and flat lree patterns with minor negative nb and zr anomalies based on mixed morb and arc affinities the basaltic amphibolites in the dggb are interpreted to have formed in a fore arc tectonic setting one late potassic granite dike cutting across the fabrics of the volcano sedimentary assemblage yields an intrusion age of 2492 35 ma constraining the minimum deformation age for tectonic assembly of the package the ttg gneisses and diorites intruded the western margin and the center of the subduction accretion complex respectively one ttg sample and one diorite sample yield igneous zircon 207pb 206pb ages of 2514 26 ma and 2518 36 ma respectively constraining their intrusion ages the ttg gneisses display high ratios of la yb cn and sr y and depletion in hfse with negative nb ta and ti anomalies consistent with those of typical archean ttgs the ttg gneisses are therefore considered to be generated from partial melting of a shallowly subducting oceanic slab and or accreted arc amphibolites the diorites have high concentrations of mgo 2 89 6 05 wt ni 148 178 ppm and cr 85 7 120 6 ppm and highly fractionated ree patterns and are depleted in hfses with negative nb ta zr hf and ti anomalies we suggest that the high mg diorites in the dggb may have been derived from a hydrated mantle wedge which was previously metasomatized by subduction derived melts and or fluids collectively a neoarchean subduction accretion collision event is therefore proposed to have generated the dggb the volcano sedimentary assemblage in the dggb represents a fore arc subduction accretion complex which is interpreted to be related to the suture zone of ca 2 5 ga arc continent collision between a ttg dominated arc terrane in the cob and the eastern block of the ncc we further propose a long n s striking neoarchean suture zone occurring in the eastern margin of the cob mainly consists of the ca 2 5 ga subduction accretion complex in the dggb to the south the ca 2 5 ga melange belt in the zanhuang complex in the central and the ca 2 5 ga ophiolitic melange belt in the zunhua dongwanzi structural belt to the north which separate an arc terrane in the cob from the eastern block of the ncc

eddy induced cross slope exchange maintaining summer high productivity of the bering sea shelf break 1 eddy related cross slope exchange along the bering sea shelf break was investigated using a hydrographic observations data set and a numerical model results of observations in summer of 2001 showed a shelf break front that formed at a shelf break near an anticyclonic eddy high nitrate nitrite concentrations in the subsurface layer and high chlorophyll a chl a concentrations 6 mg m 3 in the surface layer a hydrographic observation in summer of 2002 exhibited relatively high chl a concentrations at the surface around the anticyclonic eddy tracer experiments revealed two types of cross slope exchange under isopycnals nutrient rich water in the basin is transported to the shelf and there is about a 64 53 increase in integrated nitrate nitrite on shelf flux 50 m depth bottom when mesoscale eddies are formed and propagated along the shelf break at the surface high chl a waters in the shelf are advected to the deep basin area by eddy transport and propagation these indicate that 1 mesoscale eddies supply nutrients and sustain primary productivity at the shelf break and 2 eddies expand the high chl a area to the basin then to the highly productive area so that the green belt is maintained

new gps constraints on active deformation along the africa iberia plate boundary abstract we use velocities from 65 continuous stations and 31 survey mode gps sites as well as kinematic modeling to investigate present day deformation along the africa iberia plate boundary zone in the western mediterranean region the gps velocity field shows southwestward motion of the central part of the rif mountains in northern morocco with respect to africa varying between 3 5 and 4 0 mm yr consistent with prior published results stations in the southwestern part of the betic mountains of southern spain move west southwest with respect to eurasia 2 3 mm yr the western component of betics motion is consistent with partial transfer of nubia eurasia plate motion into the southern betics the southward component of betics motion with respect to iberia is kinematically consistent with south to southwest motion of the rif mountains with respect to africa we use block modeling constrained by mapped surface faults and seismicity to estimate the geometry and rates of strain accumulation on plate boundary structures our preferred plate boundary geometry includes one block between iberia and africa including the sw betics alboran sea and central rif this geometry provides a good fit to the observed motions suggesting a wide transpressive boundary in the westernmost mediterranean with deformation mainly accommodated by the gloria azores fault system to the west and the rif tell lineament to the east block boundaries encompass aspects of earlier interpretations suggesting three main deformation styles i extension along the ne sw trending trans alboran shear zone ii dextral strike slip in the betics corresponding to a well defined e w seismic lineament and iii right lateral strike slip motion extending west to the azores and right lateral motion with compression extending east along the algerian tell we interpret differential motion in the rif alboran betic system to be driven both by surface processes related the africa eurasia oblique convergence and sub crustal dynamic processes associated with the long history of subduction of the neotethys ocean lithosphere the dextral slip identified in the betic mountains in southern spain may be related to the offshore fault that produced the great 1755 lisbon earthquake and as such may represent a significant seismic hazard for the west mediterranean region

age and geological setting of the rankin inlet greenstone belt and its relationship to the gold endowment of the meliadine gold district nunavut canada abstract archean greenstone belts host a significant proportion of the world s gold typically in deposits that formed late during greenstone belt formation and cratonization however this is not always the case and in the multiply reworked western churchill province wcp orogenic lode gold deposition post dates greenstone belt formation by nearly one billion years the spatial link between proterozoic gold and archean greenstone belts in the wcp is thus particularly striking although its significance is still not fully understood the meliadine gold district 2 8 moz contained au in reserves plus an indicated and inferred resources of 5 8 moz au represents an important example of this deposit style and is hosted within the rankin inlet greenstone belt rigb which occupies a critical but controversial position along the largely inferred boundary between the hearne craton and the chesterfield block rigb felsic volcanic rocks ca 2 66 ga are structurally intercalated and broadly coeval with mafic volcanic and volcaniclastic rocks 2 66 2 64 ga turbidite 2 66 2 64 ga argillite auriferous banded iron formation successions and syn volcanic granodioritic to tonalitic intrusions 2 67 2 64 ga neoarchean basaltic to andesitic volcanic rocks possess calc alkaline to primitive arc like tholeiitic magmatic affinities along with lesser morb like basaltic compositions geochemically evolved lavas yield depleted 144nd 143nd ratios nd2 66 ga 1 1 to 1 6 that reflect variable interaction with an evolved and hitherto undocumented meso to neoarchean basement underlying the rigb whereas transitional arc like primitive tholeiitic and morb like basaltic samples overlap with the nd isotopic composition of depleted mantle at ca 2 66 ga nd2 66 ga 1 6 to 2 7 these neoarchean volcano sedimentary panels represent the main auriferous rock package within the meliadine gold district and are intercalated with deformed paleoproterozoic conglomerate 2 50 and 2 155 ga the latter are in turn unconformably overlain by a geochemically distinct pillowed basalt sequence and a unique carbonate siliciclastic package that presumably represent the remnants of paleoproterozoic basins and are not known to host gold the geological setting of gold deposits thus likely reflects this favourable neoarchean lithostratigraphy in addition to metamorphism and fluid focusing along the reactivated faults during the collision of the hearne and combined chesterfield block rae craton at 1 90 1 85 ga

stratigraphy and geochemistry of the igneous rocks in the elu link between hope bay and elu greenstone belts northeast slave craton tectonic setting and implications for gold mineralization geological investigation of the rocks in the elu link has provided new information on the geodynamic origin of the neoarchean ca 2716 2663 ma hope bay and elu granite greenstone belts stratigra

inversion of relative motion data for estimates of the velocity gradient field and fault slip abstract a new method is presented for a purely kinematic analysis of relative motion data of crustal deformation we start by developing a general integral formulation that links the relative motion between two positions to the velocity gradient field next fault slip is explicitly incorporated leading to the final observation equation this equation provides a new complete and in practice exact description of the time dependent relation between surface motion and the underlying kinematics the combination of equations belonging to pairs of observation sites e g of a geodetic network leads in a natural way to a linear inverse problem from which the velocity gradient field and slip on active faults can be estimated the method is successfully tested in a synthetic experiment

geochemistry of ultramafic rocks and hornblendite veins in the fiskenaesset layered anorthosite complex sw greenland evidence for hydrous upper mantle in the archean abstract the fiskenaesset complex sw greenland contains the world s best preserved archean 2970 ma layered anorthosite leucogabbro gabbro and ultramafic association the complex was emplaced into archean oceanic crust distal from continental lithosphere and later intruded by tonalites trondhjemites and granodiorites ttg constituting archean continental crust the complex and bordering ttg intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions this study presents new whole rock major and trace element petrographic and sem scanning electron microscope bse backscatter electron image data for a 45 60 m thick ultramafic sill and cross cutting hornblendite veins the ultramafic sill is composed of 75 olivine pyroxene hornblendite 15 pyroxene hornblendite and 10 hornblende pyroxenite despite granulite facies metamorphism and multiple phases of deformation the fiskenaesset ultramafic sill shares many petrographic characteristics of unmetamorphosed mafic to ultramafic layered intrusions and alaskan type ultramafic complexes hornblende appears to have originated as an igneous mineral but underwent extensive recrystallization during granulite facies metamorphism formation of orthopyroxene magnetite symplectitic vermicular intergrowths mainly at the expense of olivine is attributed to chemical reactions between late stage residual hydrous melts and olivine field geochemical and petrographic observations suggest that the ultramafic sill did not undergo significant fractional crystallization following its intrusion high mgo 16 31 wt contents suggest that the sill was emplaced as a crystal mush rather than as melt only petrographic observations and bse images indicate that olivine orthopyroxene and clinopyroxene were the major crystal phases whereas hornblende represents the main interstitial liquid phase in the crystal mush the sill exhibits three different ree patterns corresponding to three cryptic layers although the origin of these ree patterns is not fully understood they may reflect the intrusion of three batches of magma containing different hornblende forming melt crystal ratios geochemical data indicate that the cross cutting hornblendite veins likely originated from evolved late stage melts but were not related to the melts of the ultramafic sill large negative nb anomalies nb nb 0 04 0 66 suggest that the fiskenaesset hornblendites hornblende pyroxenites and hornblendite veins were derived from a hydrous sub arc mantle peridotite phanerozoic hornblendites are typically associated with supra subduction zone ophiolites and magmatic arcs accordingly it is suggested that water was recycled to the source of the fiskenaesset hornblendites through subduction of altered oceanic crust recycling of water to the upper mantle via subduction not only resulted in the generation of hornblendites but also played an important role in the formation of ttg dominated archean continental crust therefore the field petrographic and geochemical data presented in this study suggest that the origin of the mesoarchean to neoarchean terrane in the fiskenaesset region sw greenland is consistent with subduction zone geodynamic processes rather than non uniformitarian density driven vertical crustal overturn and diapiric processes

an overview of the geochemistry of eoarchean to mesoarchean ultramafic to mafic volcanic rocks sw greenland implications for mantle depletion and petrogenetic processes at subduction zones in the early earth abstract this study reviews the geochemical characteristics of eoarchean to mesoarchean ultramafic to mafic volcanic rocks now amphibolites in sw greenland and compares them with those of cenozoic oceanic island arc basalts to evaluate archean subduction zone petrogenetic processes emphasis is placed on the th ree hfse zr ti and nb systematics of the ca 3800 and ca 3700 ma arc suites in the isua greenstone belt the ca 3075 ma ivisaartoq ujarassuit greenstone belt and amphibolites associated with the ca 2970 ma fiskenaesset layered anorthosite complex on n morb normalized diagrams the isua ivisaartoq ujarassuit and fiskenaesset volcanic rocks are all characterized by depletion of nb relative to th and lree consistent with a supra subduction forearc arc backarc geodynamic setting similarly on the th yb nb yb projection these suites plot within the field of cenozoic oceanic island arc basalts on log transformed immobile trace element ratio nb th la th sm th and yb th diagrams they display a trend projecting from morb mid ocean ridge basalt to iab island arc basalt on the iab crb continental rift basalt oib ocean island basalt morb diagram as for cenozoic oceanic island arc basalts accordingly these trace element compositions are interpreted as reflecting the enrichment of archean depleted upper mantle morb source by subduction derived melts and fluids following the initiation of intra oceanic subduction and arc migration concentrations of mgo and ni in sw greenland archean basalts overlap with but extend to 2 to 4 times higher than those in cenozoic oceanic island arc counterparts in contrast the majority of archean basalts have ree and hfse concentrations 2 to 4 times lower than cenozoic oceanic island arc basalts consistent with more depleted sub arc mantle wedge peridotites in the archean than in cenozoic counterparts such depletion reflects the extraction of large volumes of mafic to ultramafic melts from hotter archean mantle we infer that less refractory compositions of the present day depleted upper mantle the source of morb and conservative hfse in arc basalts have resulted from re mixing with the less depleted to enriched deeper mantle material transferred to shallower depths by mantle convection and plumes in post archean times archean basalts in sw greenland share the negative nb and ti anomalies of cenozoic oceanic island arc basalts on average archean basalts however have lower th nb la nb sm tio2 and gd tio2 ratios than cenozoic oceanic island arc counterparts given that rutile controls the nb and ti budgets in arc magmas the lower th nb la nb sm tio2 and gd tio2 ratios in sw greenland basalts are attributed to the mobility of nb and ti in slab derived melts involving rutile fusion in archean subduction zones these elements are less mobile in fluids originating from cenozoic subducting oceanic crust where rutile appears to be generally stable accordingly it is suggested that higher geothermal gradients in the archean may have provided optimized conditions for slab melting and metasomatism of the sub arc mantle wedge by slab derived melts

evidence for active subduction beneath gibraltar we report on a marine seismic survey that images an active accretionary wedge west of gibraltar ramp thrusts offset the seafloor and sole out to an east dipping decollement indicating ongoing westward vergent tectonic shortening new traveltime tomographic re sults image a slab of oceanic lithosphere descending from the atlantic domain of the gulf of cadiz passing through intermediate depth 60 120 km seismicity beneath the west ernmost alboran sea and merging with a region of deep focus earthquakes 600 660 km below granada spain together these new data provide compelling evidence for an active east dipping subduction zone

mafic to felsic magmatism and crustal recycling in the obonga lake greenstone belt western superior province evidence from geochemistry nd isotopes and u pb geochronology abstract the central wabigoon subprovince of the western superior province contains mesoarchean granitoid and supracrustal rocks 3 01 2 83 ga and 2 78 2 69 ga granitoid plutons and supracrustal sequences it is a key area for understanding the relationship between greenstone belts and surrounding granitoid rocks that may have acted as basement the obonga lake greenstone belt contains two distinct assemblages 1 a ta nb th they have end values of 0 7 to 2 4 a dacitic unit in the northern assemblage has high la yb high sr y low nb y and heavy ree and is interpreted to represent a mantle modified slab melt similar to adakites in cenozoic arcs the associated enriched basalts may represent melts from the mantle wedge modified by slab melt adakitic metasomatism or they may represent an enriched oib like asthenospheric source rocks of the southern assemblage mostly show th and light ree enrichment and negative nb and ta anomalies low end values down to 0 9 can be modelled through contamination of a mafic liquid by 3 3 3 2 ga sialic crust although rocks of that age have not been observed suggested mesoarchean basement to the southern assemblage may therefore have had a complex history spanning several hundred million years the southern assemblage volcanism may have occurred in a continental arc to continental back arc system where crustal recycling played an important role

lithofacies associations and structural evolution of the archean rio das velhas greenstone belt quadrilatero ferrifero brazil a review of the setting of gold deposits abstract the rio das velhas greenstone belt is located in the quadrilatero ferrifero region in the southern extremity of the sao francisco craton central southern part of the state of minas gerais se brazil the metavolcano sedimentary rocks of the rio das velhas supergroup in this region are subdivided into the nova lima and maquine groups the former occurs at the base of the sequence and contains the major au deposits of the region new geochronological data along with a review of geochemical data for volcanic and sedimentary rocks suggest at least two generations of greenstone belts dated at 2900 and 2780 ma seven lithofacies associations are identified from bottom to top encompassing 1 mafic ultramafic volcanic 2 volcano chemical sedimentary 3 clastic chemical sedimentary 4 volcaniclastic association with four lithofacies monomictic and polymictic breccias conglomerate graywacke graywacke sandstone graywacke argillite 5 resedimented association including three sequences of graywacke argillite in the north and eastern at greenschist facies and in the south at amphibolite metamorphic facies 6 coastal association with four lithofacies sandstone with medium to large scale cross bedding sandstone with ripple marks sandstone with herringbone cross bedding sandstone siltstone 7 non marine association with the lithofacies conglomerate sandstone coarse grained sandstone fine to medium grained sandstone four generations of structures are recognized the first and second are archean and compressional driven from nne to ssw the third is extensional and attributed to the paleoproterozoic transamazonian orogenic cycle and the fourth is compressional driven from e to w is related to the neoproterozoic brasiliano orogenic cycle gold deposits in the rio das velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along archean thrust shear zones of the second generation of structures sedimentation occurred during four episodes cycle 1 is interpreted to have occurred between 2800 and 2780 ma based on the ages of the mafic and felsic volcanism and comprises predominantly chemical sedimentary rocks intercalated with mafic ultramafic volcanic flows it includes the volcano chemical sedimentary lithofacies association and part of the mafic ultramafic volcanic association the cycle is related to the initial extensional stage of the greenstone belt formation with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains cycle 2 encompasses the clastic chemical sedimentary association and distal turbidites of the resedimented association in the eastern sector of the quadrilatero ferrifero it was deposited in the initial stages of the felsic volcanism cycle 2 includes the coastal and resedimented associations in the southern sector in advanced stages of subduction in this southern sedimentary cycle it is also possible to recognize a stable shelf environment following the felsic volcanism cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations largely in the northern sector of the area the characteristics of both associations indicate a submarine fan environment transitional to non marine successions related to felsic volcanic edifices and related to the formation of island arcs cycle 4 is made up of clastic sedimentary rocks belonging to the non marine lithofacies association they are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles

u pb geochronological constraints on neoarchean tectonism multiple compressional events in the northwestern hearne domain western churchill province canada in the yathkyed lake area the tyrrell shear zone tsz marks the boundary between the northwestern and central hearne subdomains of the western churchill province the tsz is dominated by proterozoic dextral strain but in areas of low dextral strain older shear fabrics are consistent with an earlier thrusting event four syntectonic granite sheets from within low dextral strain zones in the tsz and contiguous shear zones at the base of the yathkyed belt have ages of 2636 2629 2652 8 2665 2639 and 2644 3 ma the yathkyed belt occurs in the hanging wall of these shear zones and comprises a greenschist to amphibolite grade overturned panel that is overlain by an upper amphibolite grade polydeformed panel the geometry is consistent with a thrust imbricated stack two episodes of deformation in the hanging wall are bracket between ca 2660 and 2616 6 4 ma and between 2616 6 4 ma and ca 2 60 ga the yathkyed belt is interpreted to have initially been emplaced as a thick skinned thrust nappe alon

gulf stream dynamics along the southeastern u s seaboard abstractthe gulf stream strongly interacts with the topography along the southeastern u s seaboard between the straits of florida and cape hatteras the dynamics of the gulf stream in this region is investigated with a set of realistic very high resolution simulations using the regional ocean modeling system roms the mean path is strongly influenced by the topography and in particular the charleston bump there are significant local pressure anomalies and topographic form stresses exerted by the bump that retard the mean flow and steer the mean current pathway seaward the topography provides through bottom pressure torque the positive input of barotropic vorticity necessary to balance the meridional transport of fluid and close the gyre scale vorticity balance the effect of the topography on the development of meanders and eddies is studied by computing energy budgets of the eddies and the mean flow the baroclinic instability is stabilized by the slope everywhere except past the bump the flow

geochemistry of volcanic rocks associated with cu zn and ni cu deposits in the abitibi subprovince volcanogenic massive sulfide vhms deposits in the abitibi subprovince are preferentially associated with volcanic successions containing 150 m thicknesses of felsic volcanic rocks approximately 50 by area of volcanic terranes and are found within volcanic sequences of at least three distinct affinities group i which is host to greater than half of the volcanogenic massive sulfide deposits by tonnage and which comprises only approximately 10 percent by area of volcanic terranes is composed of bimodal tholeiitic basalt basaltic andesite and high silica rhyolite the basaltic andesites and high silica rhyolites are characterized by high high field strength element and heavy rare earth element ree contents low light to heavy ree ratios most with la n yb n 0 8 3 and strong negative eu anomalies the kamiskotia matagami and chibougamau lower cycle volcanogenic massive sulfide areas all of which are also underlain by large synvolcanic gabbroic complexes are associated with group i volcanic sequences the kidd creek potter normetal and horne deposits are also included in this category group ii which is host to one third of the volcanogenic massive sulfide deposits by tonnage and which is also approximately 10 percent by area of volcanic terranes is composed of bimodal transitional tholeiitic to calc alkalic andesitc and rhyolite characterized by intermediate high field strength element contents and slightly higher ree ratios la n yb n 1 4 noranda camp excluding the horne deposit and val d or camp group iii is host to only one known deposit the selbaie mine which is unusual in that much of its mineralization cuts stratigraphy the selbaie mine sequence contains calc alkalic andesite rhyolite with relatively low high field strength element and ree contents and higher ree ratios la n yb n 3 9 the vast majority of volcanogenic massive sulfide bearing mafic and felsic volcanic rocks in the abitibi subprovince have la n yb n 5 barren volcanic sequences are group iv calc alkalic basaltic andesite to rhyodacite with low high field strength element and relatively high ree ratios la n yb n 8 20 represented by the upper skead group the quebec hunter mine group and the upper cycle chibougamau rocks and group v mafic to felsic alkalic volcanic rocks with high ree ratios la n yb n 12 62 represented by the timiskaming opemisca and ridout series rocks group i is most similar to thickened oceanic rift suites e g galapagos spreading center iceland east rift group ii is similar to suites in rifled island arcs e g hokuroku district japan and group iii is comparable to continental arc suites e g southern volcanic zone central chile groups iv and v are comparable to arc related suites derived from metasomatized mantle with variable amounts of crustal contamination e g setouchi area japan roman province italy known ni cu deposits in volcanic sequences are hosted exclusively in komatiitic flows and hypabyssal sills represented by chill compositions with high mgo contents 20 35 wt anhydrous very low incompatible element contents and depleted signatures la n sm n 0 5 0 8 zr y 2 5 alexo marbridge and shaw dome deposits in comparison to most barren komatiites in the abitibi subprovince their depleted trace element signatures indicate minimal crustal assimilation which would appear contrary to the widely held view that crustal contamination e g addition of silica sulfur oxygen of a mantle derived melt is necessary for sulfur saturation and magmatic sulfide segregation this problem may be reconciled by considering the dynamics of komatiite flows lesher and arndt 1990 where contaminated present address 23 euclid avenue ottawa ontario canada kis 2w2 komatiite liquid segregated and deposited sulfide and was subsequently swept downstream and replenished by uncontaminated magma over the deposit as the flow ebbed and cooled when considered together with geologic features such as evidence for thermal erosion in the footwall langmuir deposits and the presence of sulfidic footwall rocks shaw dome deposits the distinctive depleted geochemical signature of mineralized komatiites is useful in exploration for ni cu deposits

paleoproterozoic reworking of an archean thrust fault in the hearne domain western churchill province u pb geochronological constraints the nature timing and control of archean crustal architecture on the distribution of proterozoic reworking in western churchill province are investigated in the yathkyed lake area central nunavut the tyrrell shear zone tsz marks the boundary between the northwestern and central hearne subdomains respectively in the hanging wall and footwall metamorphism and deformation in the hanging wall are ca 2 66 2 50 ga two episodes of metamorphism are recorded in a foliated granodiorite in the footwall at 1818 5 2 and 1827 5 6 ma the latter age is interpreted as the time of amphibolite facies metamorphism and deformation in the footwall a crosscutting dyke provides a minimum age of 1818 2 ma for this metamorphism and deformation the tsz is dominated by dextral oblique slip shear fabrics a suite of leucogranite dykes provide a maximum age of 1816 2 ma for dextral shearing dextral deformation is bracketed between 1815 2 and 1811 1 ma the ages of two other granitoid dykes in an area of low d

toward a generalized plate motion reference frame an absolute plate motion apm model is required to address issues such as the thermochemical evolution of earth s mantle all apm models have to rely on indirect inferences including those based on hot spots and seismic anisotropy each with their own set of uncertainties here we explore a seafloor spreading aligned reference frame we show that this reference frame fits azimuthal seismic anisotropy in the uppermost mantle very well the corresponding euler pole is close to those of hot spot reference frames ridge motion minimizing models and geodynamic estimates of net rotation and predicts clear trench motion patterns we conclude that a net rotation pole guided by the spreading aligned model at 64 e 61 s with moderate rotation of 0 2 0 3 myr could indeed represent a standard comprehensive reference frame for present day plate motions with respect to the deep mantle

age and evolution of the lower crust beneath the western churchill province u pb zircon geochronology of kimberlite hosted granulite xenoliths nunavut canada abstract zircon from granulite facies xenoliths can preserve unique geodynamic records of deep and mostly inaccessible levels of the continental crust within archean cratons we present the first such records from beneath two archean crustal blocks that were affected by multiple episodes of regional scale proterozoic metamorphism and are located within the western churchill province nunavut canada zircon u pb geochronology and trace element analysis were carried out on samples that originated in the deep crust at the time of kimberlite eruption as confirmed with raman spectroscopy zircon was recovered from four metabasites grt cpx plg from the rankin inlet area of the chesterfield block and five metabasites grt cpx plg hbl opx and three metatonalite anorthosites plg cpx plg hbl opx from the repulse bay area of the rae craton sem cl imaging reveals several types of growth zoning diagnostic of igneous and metamorphic zircon crystallization the latter are related to local high temperature recrystallization and micro fracturing and are spatially associated with zones of apparent pb loss and or intracrystalline pb variability evidence for the formation of paleoarchean mesoarchean to neoarchean crust is preserved in oscillatory zoned igneous cores having 207pb 206pb ages of ca 3 5 ga and ca 3 0 2 6 ga two periods of paleoproterozoic metamorphism and zircon growth followed with the oldest episode at ca 2 0 1 9 ga in the chesterfield block metamorphic zircon rims from all twelve xenoliths have an age range of ca 1 75 to 1 70 ga the trace element compositions of the metamorphic rims differ from those of older igneous and metamorphic cores with the rims characterized by lower hree contents and low lu gd n suggesting that garnet crystallization occurred syn to pre rim growth at ca 1 75 ga the archean and paleoproterozoic ages correspond broadly to the upper crustal histories of the western churchill province however the prevalence of ca 1 75 1 70 ga metamorphic rims indicates regional scale metamorphic reworking of the lower crust across the region during a period of intracratonic extension and associated magmatism regional magmatic underplating and or remobilization of the lithosphere at ca 1 75 1 70 ga to an extent not widely appreciated represents a terminal stage in western churchill cratonization including stabilization of reworked archean crust relative to underlying diamondiferous mantle

horizontal shear flow in the mantle beneath the tonga arc in the tonga subduction zone a systematic shear deformation in which deep material moves relatively to the south is superimposed on the mode of down dip shortening which is characteristic of the region we interpret this observation together with a tectonic reconstruction in terms of the interaction between the subducting lithospheric slab and a horizontal shear flow in the mantle