metastable olivine wedge and deep dry cold slab beneath southwest japan abstract oceanic plates subducted at trenches penetrate into the deep mantle and encounter a structural boundary at a depth of 410 km where olivine the dominant element of mantle rocks transforms into a higher density form wadsleyite this transformation may be delayed within the coldest core of subducting plates slabs due to kinetic effects and it has been suggested that metastable olivine may persist deeper than 410 km using high density seismic array data in japan we show the direct image of the structure corresponding to this metastable olivine wedge mow beneath southwest japan numerical simulation of a subducting slab including the kinetic effect of water h 2 o on the olivine wadsleyite transformation indicates that the presence of the imaged mow requires an insignificant amount of water less than 100 wt ppm be present in the slab mantle thus a deep dry cold slab we infer that the transportation of water into the deep mantle occurs along the top surface of the subducting slab but no significant amount within the slab itself we also demonstrate that a numerical simulation including the kinetics of 660 km phase transformation can reconcile the observed deep depression of the 660 km discontinuity with a gentle clapeyron slope

controls on back arc basin formation the relationship between subduction and back arc spreading has been well known since the early days of plate tectonics however the reasons why back arc basins are associated with some subduction systems but not all has remained elusive we examine the kinematic controls on subduction and back arc basins for both the present day and cenozoic to differentiate between the major competing hypotheses for back arc basin formation and to explain their temporal and spatial distribution our new data set of subduction and back arc basin parameters uses a new set of paleo oceanic age grids muller et al 2005 associated with a moving atlantic indian ocean hot spot reference frame o neill et al 2005 the plate model includes detailed reconstructed spreading histories of back arc basins based on marine geophysical and satellite gravity data our combined rotation and oceanic paleo age model provides the age distribution of subducting lithosphere through space and time convergence rates and the absolute motion of the downgoing and overriding plates we find that back arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years additionally we establish an age dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 with back arc spreading our results suggest that back arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge thereby creating accommodation space between the overriding and subducting plates once back arc extension is established it continues regardless of overriding plate motion indicating back arc spreading is not a simple consequence of overriding plate behaviour the landward migration of the overriding plate as a precursor to back arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle however once back arc extension is established rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space our analysis indicates the driving mechanism for back arc extension is a combination of surface kinematics properties of the downgoing slab the effect of lateral mantle flow on the slab and mantle wedge dynamics

quantifying upper ocean turbulence driven by surface waves 1 nearly all operational ocean models use air sea fluxes and the ocean shear and stratification to estimate upper ocean boundary layer mixing rates this approach implicitly parameterizes surface wave effects in terms of these inputs here we test this assumption using parallel experiments in a lake with small waves and in the open ocean with much bigger waves under the same wind stress and adjusting for buoyancy flux we find the mixed layer average turbulent vertical kinetic energy in the open ocean typically twice that in the lake the increase is consistent with models of langmuir turbulence in which the wave stokes drift and not wave breaking is the dominant mechanism by which waves energize turbulence in the mixed layer applying these same theories globally we find enhanced mixing and deeper mixed layers resulting from the inclusion of langmuir turbulence in the boundary layer parameterization especially in the southern ocean

impacts of surface gravity waves on summer ocean dynamics in bohai sea abstract a one way coupled pomgcs swan model is applied to study the impacts of three wave processes i e langmuir turbulence coriolis stokes forcing csf and resolved scale craik leibovich vortex forcing clvf on ocean dynamics in bohai sea in summer both csf and clvf are included in the horizontal momentum equation the level 2 5 mellor yamada turbulence closure scheme is modified to contain the parameterization of langmuir turbulence numerical results show that this improved model reproduces the sea surface temperature sst better than the original one all of these wave effects considerably contribute to temperature variability at different vertical levels with the maximum temperature change of about 0 4 c based on the diagnostic experiments the impacts of the csf and clvf are much weaker than that of langmuir turbulence those wave processes reduce the current velocity and make the current more vertically uniform the horizontal distribution of langmuir number indicates that the langmuir turbulence is prevalent in bohai sea the large vertical kinematic viscosity coefficients containing the langmuir turbulence would lead to a strong vertical mixing

the provenance of middle jurassic sandstones in the scotian basin petrographic evidence of passive margin tectonics the tectonic and geomorphological evolution of the scotian margin and its hinterland is poorly known between late triassic rifting and the early cretaceous progradation of major deltas this study determined sedimentary provenance of middle jurassic mohican formation sandstones from three wells using heavy minerals and mineral chemistry indicator minerals such as xenotime altered ilmenite and varietal types of garnet and tourmaline are similar to those in hauterivian barremian sandstones in the western scotian basin which are almost exclusively derived from the meguma terrane the wells adjacent to the canso ridge have more zircon and less ilmenite indicating a greater contribution of polycyclic reworking but with an ultimate source in the meguma terrane zircon and ilmenite were likely derived in part from carboniferous sandstones in eastern mainland nova scotia and cape breton island any river drainage from the inboard terranes of the appalachians either was diverted through the fundy basin or entered the easternmost scotian basin where the mohican formation is 5 5 km thick along the linear continuation of the southwest grand banks transform such sediment did not reach the canso ridge suggesting that the cobequid chedabucto fault zone in orpheus graben was not a significant physiographic feature this tectonically controlled paleogeography in the middle jurassic is quite different from that during active rifting in the late triassic early jurassic middle jurassic quiescence was followed in the tithonian early cretaceous by renewed tectonic uplift associated with rifting of grand banks from iberia and labrador from greenland

the waveguide effect of metastable olivine in slabs we use a finite difference algorithm to compute p sv synthetic seismograms for deep earthquakes recorded at regional seismic stations located above a subducting slab we calculate synthetics for 2 d slab models in which the 410 km discontinuity is uplifted owing to the exothermic nature of the transition the equilibrium model and models in which the 410 km discontinuity is depressed owing to inhibited kinetics of the olivine transition the metastable model in the latter case the existence of the low velocity wedge of metastable olivine has a profound influence on the waveforms of deep focus earthquakes the wedge acts as a geometrical waveguide with the energy focusing effect apparent even after the wavefront has traveled the several hundred kilometers from the wedge boundary to the earth s surface the guided energy is most pronounced for receivers located near the surface projection of the deep slab but is observable over a range of 300 km and so may provide a valuable diagnostic for discriminating between equilibrium and metastable models of subduction a recent search for such guided energy using regionally recorded p and s waves from deep events in tonga yielded negative results

the role of premagmatic rifting in shaping a volcanic continental margin an example from the eastern north american margin both magmatic and tectonic processes contribute to the formation of volcanic continental margins such margins are thought to undergo extension across a narrow zone of lithospheric thinning 100 k

the influence of crosswind tidal currents on langmuir circulation in a shallow ocean 1 langmuir circulation lc is a turbulent process driven by wind and surface waves that plays a key role in transferring momentum heat and mass in the oceanic surface layer on the coastal shelves the largest scale lc span the whole water column and thus couple the surface and bottom boundary layers and enhance turbulent mixing observations and large eddy simulations les of a shallow coastal ocean demonstrate that these relatively large scale langmuir cells are strongly influenced by crosswind tidal currents two mechanisms by which crosswind tidal shear may distort and disrupt langmuir cells are proposed the first mechanism involves cell shearing due to differential advection across the whole cell for the second mechanism middepth vertical lc currents advect sheared mean crosswind current leading to the attraction of upwelling and downwelling regions so that lc cells are unsustainable when both regions overlap scaling arguments indicate that lc cells are more susceptible to crosswind shear distortion for smaller lc surface velocity convergence and greater cell aspect ratio vertical to horizontal lc scale which is consistent with the results obtained from the observations and les these results imply that scaling of lc characteristics in a coastal ocean differs from that in the open ocean which has important practical implications for parameterizing enhanced mixing due to lc

the cambrian to devonian odyssey of the brabant massif within avalonia a review with new zircon ages geochemistry sm nd isotopes stratigraphy and palaeogeography abstract this study provides an up to date and comprehensive review of the early palaeozoic evolution of the brabant massif belonging to the anglo brabant deformation belt situated at the southeastern side of avalonia microplate it is the only well known part of the northern passive margin of the rheic ocean the cambrian silurian sedimentary pile is 13 km thick with 9 km for the cambrian only the unraveling of this continuous registration reflects the successive rifting and drifting of avalonia from the gondwana mainland followed by soft collisional processes with baltica and finally the formation of laurussia based on recently established detailed stratigraphy sedimentology and basin development on u pb la icp ms analyses of igneous and detrital zircon grains along with geochemical data including sm nd isotopes a new geodynamic and palaeogeographic evolution is proposed brabant megasequence 1 lower cambrian to lowermost ordovician 9 km thick represents an embayment of the peri gondwanan rift from which the rheic ocean has evolved detrital zircon ages demonstrate that the brabant is a typical peri gondwanan terrane with a major pan african neoproterozoic age and a mixed west african and amazonian source palaeoproterozoic archaean and some mesoproterozoic age the transition towards the avalonia drifting is marked by an unconformity and a short volcanic episode the northward drift of avalonia towards baltica is recorded by the megasequence 2 middle to upper ordovician 1 3 km thick the source for mesoproterozoic zircons vanished as the result of the rheic ocean opening and the isolation from amazonian sources the transition to megasequence 3 is marked by a drastic change in palaeobathymetry and an important sub volcanic episode during a tectonic instability period 460 430 ma reflecting the avalonia baltica soft docking as also shown by the reappearance of mesoproterozoic detrital zircons typical of baltica unradiogenic nd isotope signature end 4 5 and tdm model ages 1 3 1 7 ga for brabant magmatic rocks indicate an old recycled component megasequence 3 uppermost ordovician to lowermost devonian 3 5 km thick includes the onset of a silurian foreland basin that reflects the tectonic inversion of the core of the massif brabantian orogeny in response to the baltica avalonia laurentia collision finally the comparison with the strikingly similar cambrian successions of the harlech dome wales avalonia and the meguma terrane nova scotia peri gondwana allows the construction of a new early cambrian palaeogeographic model for the whole avalonia microplate in which the meguma terrane is included

effects of fluid flow cooling and deformation as recorded by 40ar 39ar rb sr and zircon fission track ages in very low to low grade metamorphic rocks in avalonian se cape breton island nova scotia canada 40ar 39ar in situ uv laser ablation of white mica rb sr mineral isochrons and zircon fission track dating were applied to determine ages of very low to low grade metamorphic processes at 3 5 0 4 kbar 280 30 c in the avalonian mira terrane of se cape breton island nova scotia the mira terrane comprises neoproterozoic volcanic arc rocks overlain by cambrian sedimentary rocks crystallization of metamorphic white mica was dated in six metavolcanic samples by 40ar 39ar spot age peaks between 396 3 and 363 14 ma rb sr systematics of minerals and mineral aggregates yielded two isochrons at 389 7 ma and 365 8 ma corroborating equilibrium conditions during very low to low grade metamorphism the dated white mica is oriented parallel to foliations produced by sinistral strike slip faulting and or folding related to the middle late devonian transpressive assembly of avalonian terranes during convergence and emplacement of the neighbouring meguma terrane exhumation occurred earlier in the nw mira terrane than in the se transpression was related to the closure of the rheic ocean between gondwana and laurussia by nw directed convergence the 40ar 39ar spot age spectra also display relict age peaks at 477 465 ma 439 ma and 420 428 ma attributed to deformation and fluid access possibly related to the collision of avalonia with composite laurentia or to earlier ordovician silurian rifting fission track ages of zircon from mira terrane samples range between 242 18 and 225 21 ma and reflect late palaeozoic reburial and reheating close to previous peak metamorphic temperatures under fluid absent conditions during rifting prior to opening of the central atlantic ocean

meguma terrane orocline u pb age and paleomagnetism of the silurian mavillette gabbro nova scotia canada

significance of langmuir circulation in upper ocean mixing comparison of observations and simulations received 4 february 2009 revised 27 march 2009 accepted 20 april 2009 published 28 may 2009 1 representing upper ocean turbulence accurately in models remains a great challenge for improving weather and climate projections langmuir circulation lc is a turbulent process driven by wind and surface waves that plays a key role in transferring momentum heat and mass in the oceanic surface layer we present a direct comparison between observations and large eddy simulations based on the wave averaged navier stokes equation of an lc growth event the evolution of cross wind velocity variance and spatial scales as well as mixed layer deepening are only consistent with simulations if lc effects are included in the model our results offer a validation of the large eddy simulation approach to understanding lc dynamics and demonstrate the importance of lc in ocean surface layer mixing citation kukulka t a j plueddemann j h trowbridge and p p sullivan 2009 significance of langmuir circulation in upper ocean mixing comparison of observations and simulations geophys res lett 36 l10603 doi 10 1029 2009gl037620

a petrological and geochronological study of a 360 ma metallogenic event in maritime canada with implications for lithophile metal mineralization in the canadian appalachians the occurrence of ca 360 ma greisen related molybdenite mineralization in the clayton hill biotite muscovite leucomonzongranite 1 km2 is unusual for the southwestern meguma terrane of the maritime appalachians where 380 ma granite related sn mineralization dominates the presence of extensive hornfelsing and occurrence of mineralized sn w cu zn skarns in the area suggests a larger intrusion occurs at depth the intrusion and mineralization are constrained to 361 ma based on concordant re os molybdenite and 40ar 39ar muscovite ages which equates to other ca 360 ma granites in the area hence magmatism of this age is more widespread than previously considered the intrusion is highly evolved 74 wt sio2 metaluminous a cnk 1 1 05 a cnk molar ratio al2o3 cao na2o k2o and depleted in cafemic and transition elements rare earth elements ree 106 23 ppm are strongly fractionated with light rare earth elements lree enriched la lun 10 and negative eu anomalies biotite is

impact of langmuir turbulence on upper ocean response to hurricane edouard model and observations tropical cyclone intensity is strongly affected by the air sea heat flux beneath the storm when strong storm winds enhance upper ocean turbulent mixing and entrainment of colder water from below the thermocline the resulting sea surface temperature cooling may reduce the heat flux to the storm and weaken the storm recent studies suggest that this upper ocean turbulence is strongly affected by different sea states langmuir turbulence which are highly complex and variable in tropical cyclone conditions in this study the upper ocean response under hurricane edouard 2014 is investigated using a coupled ocean wave model with and without an explicit sea state dependent langmuir turbulence parameterization the results are compared with in situ observations of sea surface temperature and mixed layer depth from axbts as well as satellite sea surface temperature observations overall the model results of mixed layer deepening and sea surface temperature cooling under and behind the storm are consistent with observations the model results show that the effects of sea state dependent langmuir turbulence can be significant particularly on the mixed layer depth evolution although available observations are not sufficient to confirm such effects some observed trends suggest that the sea state dependent parameterization might be more accurate than the traditional sea state independent parameterization

derivation of the early carboniferous wedgeport pluton by crystal fractionation of a mafic parental magma a rare case of an a type granite within the meguma terrane nova scotia canada the cambrian ordovician metasedimentary rocks of the meguma terrane canadian appalachians were extensively intruded by silicic plutons during middle devonian to early carboniferous times syn plutonic but volumetrically minor mafic ultramafic intrusions were also emplaced in most localities the silicic plutons and mafic ultramafic intrusions do not appear to be petrogenetically related and are likely derived from different sources the attwoods brook gabbronorite of sw nova scotia yielded an in situ zircon weighted mean 206pb 238u age of 357 9 3 3 ma that is within the uncertainty of the age of the neighbouring wedgeport pluton 357 1 ma the wedgeport pluton is a rare example of a mantle derived peraluminous a type granite within the meguma terrane the similar ages and nd isotopes of the attwoods brook gabbronorite end t 1 1 to 4 0 and wedgeport pluton end t 2 1 to 3 3 suggest the two intrusions are petrogenetically related fractional crystallization modelling demonstrates that a parental magma similar to the attwoods brook gabbronorite can produce residual silicic liquids that resemble the granites of the wedgeport pluton indicating that they could be members of the same intrusive complex the emplacement of the gabbronorite and wedgeport pluton occurred during a period of tensional plate stress that was contemporaneous with rifting of the maritimes basin that produced the fountain lake continental flood basalts and a type granites of the cobequid highlands within the avalon terrane it is possible that the early carboniferous rocks of sw nova scotia are related to the rifted related magmatism within the maritimes basin

the influence of olivine metastability on deep subduction of oceanic lithosphere abstract the dynamics of subduction of oceanic lithosphere is mainly controlled by thermal buoyancy forces and mineralogy in self consistent models the influence of metastable olivine mo on the dynamics and evolution of subducting oceanic lithosphere is investigated we consider both phase boundaries olivine ol to spinel sp 410 km and sp to perovskite pv magnesiowustite mw 660 km treating the kinetics of the phase transition in a simplified way while models do not allow the formation of mo penetrate straight into the lower mantle slabs with a sufficient amount of mo flatten out at the 660 km boundary before penetrating at a later stage furthermore our models show that the effect of mo to slow down subduction velocities within the transition zone is rather moderate

the provenance of jurassic and lower cretaceous clastic sediments offshore southwestern nova scotia jurassic and cretaceous sandstones in the shelburne subbasin and fundy basin offshore nova scotia are poorly known but are of current interest for petroleum exploration the goal of this study is to determine the provenance of sandstones and shales which will contribute to a better understanding of regional tectonics and paleogeography in the study area mineral and lithic clast chemistry was determined from samples from conventional cores and cuttings from exploration wells using scanning electron microscopy and an electron microprobe whole rock geochemical composition of shales was used to test the hypotheses regarding provenance of mesozoic clastic sedimentary rocks in the sw scotian basin lower jurassic clastic sedimentary rocks in the fundy basin contain magnetite biotite and chlorite suggesting local supply from the north mountain basalt and meguma terrane whereas pyrope and anthophyllite suggest small supply from distant sources in the sw scotian basin detrital minerals lithic clasts and shale geochemistry from middle jurassic to early cretaceous indicate a predominant meguma terrane source and transport by local rivers rare spinel and garnet grains of meta ultramafic rocks only in the middle jurassic at the mohawk b 93 well suggest minor supply from the rising labrador rift via the same river that transported distant sediments to the fundy basin lower cretaceous sandstones from the mohican i 100 well contain minor garnet spinel and tourmaline from meta ultramafic rocks characteristic of sediment supplied to the central scotian basin at that time the dominant meguma terrane provenance precludes thick deep water sandstones in the eastern part of the shelburne subbasin but the evidence of middle jurassic distant river supply through the fundy basin is encouraging for deep water reservoir quality in the western part

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

metastable olivine wedge beneath the japan sea imaged by seismic interferometry the metastable olivine wedge mow within subducted slabs has long been hypothesized to host deep focus earthquakes 300 km its presence would also rule out hydrous slabs being subducted into the mantle transition zone however the existence and dimensions of mow remain debatable here we apply inter source interferometry which converts deep earthquakes into virtual seismometers to detect the seismic signature of mow without influence from shallow heterogeneities with data from the hi net we confirm the existence of mow beneath the japan sea and constrain its geometry to be 30 km thick at 410 km depth and gradually thinning to a depth of 580 km at least our result supports transformational faulting of metastable olivine as the initiation mechanism of deep earthquakes although large events m7 0 probably rupture beyond the wedge furthermore the slab core must be dehydrated at shallower depth and only transports negligible amount of water into the transition zone

bottom friction and wind drag for wave models abstract waves propagating in shallow water dissipate energy in a thin turbulent boundary layer near the bottom this friction can be estimated with a simple quadratic friction law scaled with an empirical coefficient two values of this coefficient have been recommended by previous studies for sandy bottoms a high value for waves in a storm and a low value for swell we show here that in contrast to current practise the lower value should be used for both applications the reason is that the high value dating from the early 1980s was inferred from observations in a severe storm using a relatively high wind drag our review of a large number of more recent observations gives a new wind drag parameterization with lower values with this new parameterization we infer from the same storm the lower value of the bottom friction coefficient using this lower value also improves the estimates of wave growth in shallow water and of low frequency wave decay in a tidal inlet independent of the wind drag

interaction of langmuir turbulence and inertial currents in the ocean surface boundary layer under tropical cyclones abstractbased on a large eddy simulation approach this study investigates the response of the ocean surface boundary layer osbl and langmuir turbulence lt to extreme wind and complex wave forc

detrital muscovite geochronology and the cretaceous tectonics of the inner scotian shelf southeastern canada1 2 geochronology of detrital muscovite from the lower cretaceous of the central scotian basin has shown predominant supply of meguma terrane detritus including muscovite with alleghanian mid carboniferous to permian ages from the inner scotian shelf in this study 87 detrital muscovite grains from five wells in the eastern scotian basin representing depositional ages from tithonian to early albian were dated by single grain 40ar 39ar techniques previous work shows that the provenance of this sediment was different from wells in the central scotian basin compared with the central scotian basin the muscovite age populations from the eastern scotian basin have more 20 versus 7 pre devonian ages derived from inboard terranes of the appalachians in both the central and eastern scotian basin most old grains are found in the tithonian and the aptian albian with fewer in the valanginian and almost none in the kimmeridgian and the hauterivian barremian these data suggest rapid unroofing of the megum

hydrogen diffusivity in wadsleyite and water distribution in the mantle transition zone abstract the kinetics of the hydrogen diffusivity in synthesized polycrystalline wadsleyite was measured by ir spectroscopy in order to determine the diffusion coefficient of hydrogen in wadsleyite the major constituent mineral in the mantle transition zone with the composition of mg0 89fe0 11 2sio4 the hydration experiments were conducted at 15 16 gpa and temperature range from 900 to 1200 c with mg oh 2 as the water source by kawai type multi anvil apparatus the diffusion rate obtained here is considered to be an effective diffusion coefficient with the grain size of 9 m involving contributions both from the lattice diffusion and the grain boundary diffusion the temperature dependence of diffusion of hydrogen in polycrystalline wadsleyite was determined to be dh 9 6 10 6 exp 123 32 kj mol 1 rt at 15 gpa hydrogen diffusion rate in wadsleyite is roughly consistent with the average diffusivity of hydrogen in olivine in recent years several authors have suggested a possibility of the hydrous transition zone and the distribution and transport properties of water are still debated on the basis of water dependence of the electrical conductivity the water content in the mantle transition zone was estimated from the observed conductivity by using the nernst einstein relation the results obtained here show that the distribution of water should be quite heterogeneous throughout the mantle transition zone

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

seismological evidence for metastable olivine inside a subducting slab the nature and location of the olivine spinel phase transition inside subducting slabs differ greatly from the situation in the surrounding mantle due to the different temperature distribution inside the slabs two models have been proposed for this phase transition one in which the location of the phase boundary between olivine and modified phase spinel is determined by equilibrium thermodynamics1 and the other including a metastable olivine phase which persists to a depth of 550 km ref 2 the location of the olivine spinel transition in the slab may be relevant to the generation of deep earthquakes3 5 and to the buoyancy forces driving subduction6 here we use travel time residuals from deep earthquakes recorded by the dense seismograph network in japan to investigate the configuration of the olivine spinel phase boundary inside the subducting pacific plate theoretical travel time residuals for the equilibrium model do not fit the observed residuals whereas those for the metastable model do implying the presence of metastable olivine inside the subducting slab

petrology age and tectonic setting of the seal island pluton offshore southwestern nova scotia the seal island pluton outcrops only on small islands located on the continental shelf 45 km south of nova scotia although geophysical data indicate that the pluton is part of large granitoid units that cover thousands of square kilometres farther offshore based on the island outcrops the seal island pluton consists of biotite monzogranite and muscovite biotite monzogranite of uncertain relative age metasedimentary xenoliths combined with characteristic magnetic patterns indicate that the pluton intruded the cambrian ordovician meguma group compared with the biotite monzogranite the muscovite biotite monzogranite is higher in sio2 more peraluminous and more depleted in heavy rare earth elements and also has lower end 1 39 versus 0 82 possibly the result of more contamination by meguma group sedimentary rocks the biotite monzogranite yielded a late devonian u pb zircon age of 362 8 0 7 ma although the relatively minor petrological differences between the two units do not preclude a co m

a global perspective on langmuir turbulence in the ocean surface boundary layer the turbulent mixing in thin ocean surface boundary layers osbl which occupy the upper 100 m or so of the ocean control the exchange of heat and trace gases between the atmosphere and ocean here we show that current parameterizations of this turbulent mixing lead to systematic and substantial errors in the depth of the osbl in global climate models which then leads to biases in sea surface temperature one reason we argue is that current parameterizations are missing key surface wave processes that force langmuir turbulence that deepens the osbl more rapidly than steady wind forcing scaling arguments are presented to identify two dimensionless parameters that measure the importance of wave forcing against wind forcing and against buoyancy forcing a global perspective on the occurrence of wave forced turbulence is developed using re analysis data to compute these parameters globally the diagnostic study developed here suggests that turbulent energy available for mixing the osbl is under estimated without forcing by surface waves wave forcing and hence langmuir turbulence could be important over wide areas of the ocean and in all seasons in the southern ocean we conclude that surface wave forced langmuir turbulence is an important process in the osbl that requires parameterization citation belcher s e et al 2012 a global perspective on langmuir turbulence in the ocean surface boundary layer geophys res lett 39 l18605 doi 10 1029 2012gl052932

mixed siliciclastic carbonate systems and their impact for the development of deep water turbidites in continental margins a case study from the late jurassic to early cretaceous shelburne subbasin in offshore nova scotia progradation of deltaic systems that reach the shelf edge is considered a primary mechanism to deliver sand to continental margins sediment bypass processes can dominate the outer shelf to upper slope transition causing poor preservation of reservoir quality sandstones turbidites can carry the bulk of the coarse fraction downdip where a thicker sand pile can be deposited once the channel to lobe transition is reached predicting this lateral and vertical variability along the slope is challenging this configuration can be further complicated when mixed siliciclastic carbonate systems are present the roseway missisauga case study from nova scotia is used here to explore potential implications associated with the development of deep water turbidites that are time equivalent to outer shelf mixed siliciclastic carbonate units two scenarios are possible 1 the carbonate factory is dominant and the development of carbonate reefs and pinnacles on the outer shelf prevents the passage of siliciclastic systems beyond the shelf break in this case the siliciclastic component is sequestered within the inner outer shelf 2 favorable conditions for carbonate production gradually deteriorate by the activation of fluviodeltaic systems that prograde outboard reaching the slope region in this last scenario low relief and lateral discontinuous carbonate shoals are ubiquitous in the outer shelf representing the last outboard remnants of the carbonate factory shelf edge deltas circumvent or breach these carbonate shoals establishing sedimentary pathways on the shelf break region that connect with deep water turbidites observations suggest that this last scenario is the most likely in this part of the scotian margin during the late jurassic to early cretaceous

langmuir turbulence in shallow water part 1 observations during extended deployment at an ocean observatory off the coast of new jersey a bottom mounted five beam acoustic doppler current profiler measured large scale velocity structures that we interpret as langmuir circulations filling the entire water column these circulations are the large eddy structures of wind wave driven turbulent flows that occur episodically when a shallow water column experiences prolonged strong wind forcing many observational characteristics agree with former descriptions of langmuir circulations in deep water the three dimensional velocity field reveals quasi organized structures consisting of pairs of surface intensified counter rotating vortices aligned approximately downwind maximum downward velocities are stronger than upward velocities and the downwelling region of each cell defined as a pair of vortices is narrower than the upwelling region maximum downward vertical velocity occurs at or above mid depth and scales approximately with wind speed the estimated crosswind scale of cells is roughly 3 6 times their vertical scale set under these conditions by water depth the long axis of the cells appears to lie at an angle 10 20 to the right of the wind a major difference from deep water observations is strong near bottom intensification of the downwind jets found typically centred over downwelling regions accessible observational features such as cell morphology and profiles of mean velocities turbulent velocity variances and shear stress components are compared with the results of associated large eddy simulations reported in part 2 of shallow water flows driven by surface stress and the craik leibovich vortex forcing generally used to represent generation of langmuir cells a particularly sensitive diagnostic for identification of langmuir circulations as the energy containing eddies of the turbulent flow is the depth trajectory of invariants of the turbulent stress tensor plotted in the lumley triangle corresponding to realizable turbulent flows when langmuir structures are present in the observations the lumley map is distinctly different from that of surface stress driven couette flow again in agreement with the large eddy simulations les unlike the les observed velocity fields contain two distinct and significant scales of variability documented by wavelet analysis of observational records of vertical velocity variability with periods of many minutes is that expected from langmuir cells drifting past the instrument at the slowly time varying crosswind velocity shorter period variability of the order of 1 2 min has roughly the observed periodicity of surface wave groups suggesting a connection with the wave groups themselves and or the wave breaking associated with them in high wind conditions

recognising different sediment provenances within a passive margin setting towards characterising a sediment source to the west of the british late carboniferous sedimentary basins to investigate whether sediments with subtly different provenances could be recognised within a passive margin setting samples were taken from westphalian sedimentary rocks of the pennine basin south wales coalfield culm basin of britain and from northern ireland although previous work has shown that 3 major source areas were responsible for the infill of the pennine basin this study is restricted to sediments believed to have been derived from the west their nominally similar western provenance was confirmed using sm nd isotope techniques and their deposition in a passive margin setting was established using elemental discrimination diagrams while the overall sediment provenance may have been from the west the trace element geochemistry indicates that at least two major sediment pathways can be distinguished plots of eu eu gdn ybn along with ternary plots of cr v ti zr gdn ybn or sc cr suggest different source areas for the culm basin and the south wales coalfield in the culm basin the cr v ratio generally exceeds 1 25 with a small eu anomaly and a low gdn ybn ratio the cr v ratio is less than 1 25 in the south wales with a larger eu ratio and a higher gdn ybn than the culm basin significant separation of the culm basin and the south wales coalfield during the early westphalian by the bristol channel bray fault may have been a factor contributing to the provenance differences late carboniferous dextral displacement along this fault brought these basins into close juxtaposition and it is unlikely that the bristol channel landmass formed a common source area for these two basins

role of langmuir circulation in the deepening of the ocean surface mixed layer helical motions known as langmuir circulation are a key physical process in the upper ocean but have not yet been incorporated into ocean models here surface mixed layer deepening by langmuir circulation was added to that due to convection or velocity shear langmuir circulation is more important than shear if the velocity difference across the mixed layer base is less than about 1 percent of the wind speed in an upper ocean data set evidence was found for the deepening of the mixed layer by both mechanisms thus langmuir circulation influences upper ocean diurnal and seasonal changes in stratification

a revised biostratigraphic and well log sequence stratigraphic framework for the scotian margin offshore eastern canada12 1this article is one of a series of papers published in this cjes special issue on the theme of mesozoic cenozoic geology of the scotian basin 2earth sciences sector contribution 20120137 in this study we build on a foundation of previous biostratigraphic studies for the scotian basin offshore eastern canada by conducting new quantitative multidisciplinary biostratigraphic studies of the mesozoic sections in eight wells bonnet p 23 chebucto k 90 cohasset l 97 glenelg j 48 glooscap c 63 mohican i 100 south desbarres o 76 and south griffin j 13 these wells were chosen to provide good spatial coverage stratigraphic penetration and correlation with the seismic grid we have also evaluated pre existing biostratigraphic data and undertaken the well log sequence stratigraphic interpretation of upper triassic to cenozoic sediments in 16 additional wells using a consistent multidisciplinary event scheme derived from the new wells key to the dating of some horizons has been integration of the palynology and micropaleontology disciplines mostly used previously on the scotian margin with new nannofossil data simultaneously we have closely integrated the biostratigraphic results with

the wave driven ocean circulation oceanic surface gravity waves have a mean lagrangian motion the stokes drift the dynamics of winddriven basin scale oceanic currents in the presence of stokes drift are modified by the addition of so called vortex forces and wave induced material advection as well by wave averaged effects in the surface boundary conditions for the dynamic pressure sea level and vertical velocity some theoretical analyses previously have been made for the gravity wave influences on boundary layer motions including the ekman currents the present paper extends this theory to the basin scale depth integrated circulation in a bounded domain it is shown that the sverdrup circulation relation with the meridional transport proportional to the curl of the surface wind stress applies to lagrangian transport while the associated eulerian transport is shown to have a component opposite to the stokes drift transport a wave induced correction to the relation between sea level and surface dynamic pressure is also derived preliminary assessments are made of the relative importance of these influences using a global wind climatology and an empirical relationship between the wind and wave fields recommendations are made for further development and testing of this theory and for its inclusion in general circulation models

the german bank pluton offshore sw nova scotia age petrology and regional significance for alleghanian plutonism permian plutonism is widespread in the u s appalachians but not in the canadian sector of the appalachians the german bank pluton sampled in two boreholes offshore southwestern nova scotia gave a u pb age of 300 1 ma determined by isotope dilution thermal ionization mass spectrometry id tims on thermally annealed and chemically etched zircon grains electron microprobe dating of monazite yielded an age of 308 13 ma late paleozoic plutonism in the canadian appalachians was dominated by northwestward subduction of the rheic ocean whereas the rheic ocean in the u s appalachians closed by southeasterly subduction a slab tear therefore developed at the southwestern margin of the rheic ocean slab beneath the canadian appalachians the high sr and ba metaluminous magnetite granite of the german bank pluton resulted from melting of underplated lower crust and or lithospheric mantle by a rising asthenospheric diapir related to this slab tear geochemically the german bank pluton is similar to most 300 265 ma plutons of the southern appalachians except in its strongly negative nd isotopic composition that results from the underthrusting of a ca 2 ga lower crustal block of saharan craton

modeling the tonga slab can travel time data resolve a metastable olivine wedge we present the results of detailed modeling of the tonga slab with the goals of determining whether high resolution travel time data 1 can be fit by simple thermal and petrological slab models and 2 can resolve a metastable olivine wedge at depths greater than the equilibrium olivine spinel phase boundary we model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers obs and island broadband seismic stations extending from the lau backarc basin across the tonga trench and onto the pacific plate the data consist of 388 local p wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3 month obs deployment in late 1994 we locate the events using both local and teleseismic arrival times and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference earth model the modeling consists of grid searches using a three dimensional finite difference algorithm to compute local first arriving travel times for equilibrium and metastable p wave velocity models constructed from thermal mineralogical and morphological constraints the travel time anomalies are well fit by standard slab thermal models and p velocity temperature derivatives of 0 4 to 0 3 ms 1 c 1 forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low velocity region wedge velocity models provide a slightly better fit to the data than equilibrium models but f tests indicate the improvement is not significant at the 95 level our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary

pre mesozoic basement rocks offshore nova scotia canada new constraints on the accretion history of the meguma terrane basement samples from offshore wells and geophysical data show that the meguma terrane forms the basement of mesozoic cenozoic sedimentary basins as far as 200 km offshore nova scotia the data provide constraints on avalon meguma terrane interaction during middle to late paleozoic events in the appalachian orogen the basement consists of metasedimentary rocks intruded by late paleozoic granitoid plutons principally magnetite bearing granodiorites which contrast with more peraluminous ilmenite bearing devonian granodiorite and granite onshore offshore tonalites have a subduction related trace element signature that is similar to the signature in minor devonian gabbro and tonalite intrusions on land nd sr and pb isotopes suggest three sources for meguma terrane plutons 1 2 ga basement possibly saharan shield and sediments derived therefrom this basement may be present beneath the southwestern scotian shelf 2 late proterozoic underthrust avalonian basement and basement autochthonous to the meguma terrane and 3 melts from mantle last remobilized in the late proterozoic pan african orogeny seismic reflection profiles show meguma terrane thrust northward over a wedge of avalon basement extending 30 km south of the surface suture there is no evidence that the devonian tonalites are directly related to subduction the lack of either ophiolites or paleozoic pelagic sediment suggests that convergence of the avalon and meguma terranes was achieved principally by strike slip motion resulting from low angle continent continent convergence with only a minor thrust component the widespread devonian carboniferous igneous activity of the amalgamated meguma and avalon terranes was a consequence of regional extension which resulted in decompression melting of upper mantle and production of mafic magmas variably mixed with crustal material

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes