reply to comment by fernando marques on tectonophysics article deep structure recent deformation and analog modeling of the gulf of cadiz accretionary wedge

mechanics of low stress forearcs nankai and cascadia the nankai and cascadia subduction zones have low stress forearcs where the margin normal horizontal compressive stress is similar to or less than the vertical stress except in the accretionary prisms in the absence of active back arc spreading the forearc margin normal stress is controlled mainly by the total shear force along the subduction fault or the plate coupling force and the gravitational force the plate coupling force creates compression and the gravitational force in the presence of margin topography creates lateral tension we give a simple expression that relates the forearc stress to topography and average fault strength with the latter represented by an effective coefficient of friction we also use a finite element model of two converging plates in frictional contact to model the forearc stresses the analyses indicate that a low stress forearc requires not only the subduction fault to be very weak but also the mechanically coupled fault area to be narrow in the downdip direction and shallow in depth for nankai and cascadia where the coupled fault areas are narrow and shallow because of the young and hot subducting plates is about 0 05 or less and in any case less than 0 09 the low fault strength is probably not a unique property of low stress forearcs since the same low values combined with wider and deeper coupled areas can be shown to create large margin normal compression in addition there does not seem to be any relation between subduction earthquake magnitudes and plate coupling forces great earthquakes occur at subduction zones having low stress forearcs such as nankai and cascadia

numerical modelling of arc continent collision application to taiwan two dimensional finite element numerical modelling is applied to study the deformation and failure of the overriding plate during arc continent collision continental margin subduction this plate has elasto plastic rheology with strain weakening and contains a volcanic arc with thinned and weakened lithosphere the overriding plate deforms due to the normal and tangential stresses applied along the interplate surface these boundary conditions represent the friction and pressure between the plates the latter is due to the flexural rigidity of the subducting lithosphere and the buoyancy force generated by the subducting continental crust the modelling shows that continental margin subduction results in increasing compression and failure of the overriding plate which occurs along the surface dipping under the arc in either of two possible directions the failure mode is largely controlled by the two competitive factors the rigidity of the subducting plate and thickness gradient of the subducted continental crust a high rigidity favors failure along an ocean vergent fault which is followed by a subduction reversal while a high thickness gradient favors failure in the opposite direction which is followed by a fore arc block underthrusting beneath the arc both scenarios seem to have natural analogs we consider one of them the ongoing arc continent collision in taiwan and argue that this process occurs according to the second scenario corresponding to the fore arc underthrusting wavelet statistical analysis of the seismicity distribution to the south of taiwan has clearly displayed a shallow 0 40 km zone of high density coherently distributed seismicity beneath the luzon arc this zone interpreted as a lithospheric scale fault dips from the forearc basin to the east and corresponds to the initiation of the forearc block subduction a self consistent combined mechanical gravity topography model is used to see whether failure of the overriding plate to the south of taiwan can be captured by this model by tuning diverent controlling parameters we did not succeed in obtaining realistic topography and gravity field in a model where failure of the overriding plate was not allowed introduction of this failure and underthrusting of the forearc block under the luzon arc allowed us to fit both topography and gravity data 2000 elsevier science b v all rights reserved

numerical comparison of different convergent plate contacts subduction channel and subduction fault summary at convergent plate boundaries the properties of the actual plate contact are important for the overall dynamics convergent plate boundaries both mechanically decouple and link tectonic plates and accommodate large amounts of strain we investigate two fundamental physical states of the subduction contact one based on a fault and the other based on a subduction channel using a finite element method we determine the specific signatures of both states of the subduction contact we pay particular attention to the overriding plate in a tectonic setting of converging plates where the subducting plate is freely moving the subduction channel reduces compression relative to the fault model in a land locked basin setting where the relative motion between the far field of the plates is zero the subduction channel model produces tensile stress regime in the overriding plate even though the amount of slab roll back is small the fault model shows a stronger development of slab roll back and a compressive stress regime in the upper plate based on a consistent comparison of fault and channel numerical models we find that the nature of the plate contact is one of the controlling factors in developing or not of backarc extension we conclude that the type of plate contact plays a decisive role in controlling the backarc state of stress to obtain backarc extension roll back is required as an underling geodynamic process but it is not always a sufficient condition

weak zone formation for initiating subduction from thermo mechanical feedback of low temperature plasticity abstract we have addressed the problem of subduction initiation with a solid mechanical and fluid dynamical approach using a finite element method the setup has been extended by a rate sensitive coupling at the bottom of a semi brittle lithosphere and a fully coupled thermo mechanical model the central element of our model is a broad asymmetric sedimentary loading function at the passive continental margin which grows with time to 15 km two fundamentally different modes of shear zone formation have been found depending on the rheology of the creep layer mode 1 for cases of low or absent yield stress in the creep layer only the semi brittle top develops a weak zone while the rate sensitive layer acts as a decoupling shear zone mode 2 for cases with a yield strength in the creep layer strain rates above 10 15 s 1 after yielding the entire mechanical lithosphere fails on a major shear zone mode 1 fails to model subduction initiation while mode 2 creates a weak major shear zone that severs through the entire lithosphere

mechanism of rifting considerations based on results of physical modelling and on geological and geophysical data abstract on the basis of results of physical modelling and an analysis of the relevant geological and geophysical data the mechanism and regularity of rifting are studied as a rule rifting develops in extensive and long living hot regions characterized by a reduced thickness of lithosphere rifts develop due to regional extension of the lithosphere in the hot regions and are zones of localization of deformation neckings the style of deformation and faulting in a necking are studied experimentally the plastic layers of low strength in the continental crust play an essential role in rifting those places favourable for the localization of plastic deformation are related to the parameters expressiveness of these layers in various parts of a hot region the development of a necking rifting is accompanied by replacement of the lower crust by high strength mantle ultrabasic material when there is an interruption of the extension this might cause an effective strengthening of the lithosphere in the rift zones and results in jumps when tension is renewed modelling has indicated that rifts can form angular junctions and branching in hot spots and older faulting zones various generations of rifts can converge and intersect in a hot spot in contrast to this cold lithospheric zones of higher strength are detoured by rifts basic magmatism including plateau basalts is often present in such zones

microchemical and sr isotopic investigation of zoned k feldspar megacrysts insights into the petrogenesis of a granitic system and disequilibrium crystal growth k feldspar megacrysts kfm are used to investigate the magmatic evolution of the 7ma monte capanne mc monzogranite elba italy dissolution and regrowth of kfm during magma mixing or mingling events produce indented resorption surfaces associated with high ba contents diffusion calculations demonstrate that kfm chemical zoning is primary core to rim variations in ba rb sr li and p support magma mixing i e high ba and p and low rb sr at rims but more complex variations require other mechanisms in particular we show that disequilibrium growth related to variations in diffusion rates in the melt may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber initial 87 sr 86 sr ratios isr obtained by microdrilling decrease from core to rim inner core analyses define a mixing trend extending towards a high isr rb sr melt component whereas the outer cores and rims display a more restricted range of isr but a larger range of rb sr lower isr at the rim of one megacryst suggests mixing with high k calc alkaline mantle derived volcanics of similar age on capraia trace element and isotopic profiles suggest 1 early megacryst growth in magmas contaminated by crust and refreshed by high isr silicic melts as seen in the inner cores and 2 later recharge with mafic magmas as seen in the outer cores followed by 3 crystal fractionation with possible interaction with hydrothermal fluids as seen in the rim the model is compatible with the field occurrence of mafic enclaves and xenoliths

zircon u pb age and sr nd hf isotope geochemistry of the ganluogou dioritic complex in the northern triassic yidun arc belt eastern tibetan plateau implications for the closure of the garze litang ocean abstract the triassic yidun arc belt yab lies between the jinshajiang suture zone to the west and the garze litang suture zone to the east eastern tibetan plateau to study the yab can not only help us to better understand the evolutionary history of the garze litang ocean but can also provide some important information to constrain the evolution of the eastern paleo tethys in this paper the geochronological and geochemical data of the ganluogou dioritic complex were systematically investigated in order to decipher the geodynamic setting of the complex and to further determine the final closure time of the garze litang ocean the ganluogou dioritic complex is located in the northern part of the yab it consists of ferrodiorite diorite and a mixing zone between them and is the largest intermediate mafic pluton in the yab the ferrodiorites were emplaced at 213 2 ma have low sio 2 and high fe 2 o 3 contents whereas the diorites formed at 209 2 ma and have relatively higher sio 2 na 2 o k 2 o th u zr and hf contents but lower al 2 o 3 mgo cao co and sr contents than the ferrodiorites relative to the primitive mantle both the ferrodiorites and diorites are depleted in nb and ta however the ferrodiorites exhibit strong depletion in zr and hf whereas the diorites contain relatively higher th and u contents without negative zr and hf anomalies both rock types have similar chondrite normalized rare earth element patterns with la yb n ratios 4 4 to 18 2 and show weak eu anomalies with eu eu of 0 47 to 1 they both show narrow ranges in sr nd hf isotopic compositions however the ferrodiorites contain lower initial 87 sr 86 sr ratios 0 7052 0 7057 and relatively higher e nd t values 3 8 to 2 4 than the diorites which record values of 0 7062 0 7066 and 5 5 to 5 7 respectively for the zircon hf isotopic composition the ferrodiorites also exhibit higher 176 hf 177 hf ratios 0 282738 0 282804 and more depleted e hf t values 3 4 5 6 than those of the diorites 176 hf 177 hf ratios of 0 282690 0 282728 and e hf t values of 1 7 2 8 these geochemical and isotopic features suggest that the ferrodiorites and diorites were derived from the different magma sources we propose that the ferrodiorites were produced by differentiation of mantle derived tholeiitic arc magma whereas the diorites were derived either from dehydration melting of meta basaltic rocks from the kangding complex the basement of the west yangtze craton or young arc volcanic rocks but mixed with a small amount of sediments both ferrodiorites and diorites were emplaced at relatively low pressure 3 3 3 9 kbar but with quite different magma temperature ferrodiorites 821 960 c diorites 692 788 c the ganluogou dioritic complex was therefore formed under an extension setting that was associated with the closure of the garze litang ocean previous studies on late triassic magmatism in the area have shown that there was slab break off event at 216 ma combined with regional geological background we conclude that the final closure of the garze litang segment of the paleo tethys ocean occurred prior to 216 ma

ophiolite obduction pulses as a proxy indicator of superplume events abstract a major new synthesis of ophiolite geochronology and map of global phanerozoic distribution indicates that previously noted pulses of ophiolite obduction can be linked to superplume related tectonism a marked cyclicity is evident in obduction ages obtained from minerals in ophiolite metamorphic soles and obduction related minor intrusions this episodicity is in phase with periods of predominantly uniform polarity of the geomagnetic field formation of massive carbon rich deposits sea level highstands and formation of large flood basalt provinces all generally considered to be superplume proxy indicators a key to interpreting ophiolite obduction as a further proxy is the mid cretaceous superplume associated ocean margin compressional deformation during this event thermal rejuvenation and increased buoyancy of ocean lithosphere caused arc terrane collision marginal basin shoaling back arc basin closure and ophiolite obduction convergent margins were placed in compression with increased coupling between subducting and overriding plates resulting in major ocean margin deformation being directly datable and tectonically and petrologically distinctive makes ophiolite assemblages of considerable use for identifying superplume events

variability in flow and temperatures within mantle subduction zones 1 a series of laboratory experiments is used to model three dimensional aspects of flow in subduction zones and the consequent temperature variations in the slab and overlying mantle wedge the effects of longitudinal rollback and slab steepening components of motions are considered along with different thicknesses of the over riding lithosphere the results show that the style of plate sinking influences the evolution of subduction zones both in terms of the speed orientation and temperature of flow in the overlying wedge and in terms of the temperatures at the surface of the descending slab in the simplest case of longitudinal sinking without rollback motion velocities in the mantle wedge are 30 40 of the downdip plate speed return flow paths in the shallow mantle wedge are nearly horizontal for slow slab speeds and steepen with higher slab speeds and there is no mass flux around the edges of a slab segment of finite width rollback subduction leads to flow both around and beneath the sinking slab with larger velocities in the wedge up to 150 of the slab speed and flow focused toward the center of the plate segment rollback subduction in which either the trench migrates or the plate steepens with time induces shallow and steep wedge return flow trajectories respectively the thermal evolution of the plate is strongly influenced by sinking style and rate highest temperatures are along the edges of the slab for longitudinal sinking but along the centerline of the slab segment for rollback motion slab surface temperatures when scaled to the mantle are higher than those given by previous two dimensional numerical models and are consistent with recent observational and experimental data on melt compositions

numerical modelling of lithosphere asthenosphere interaction in a subduction zone we developed a new 2 d numerical approach to study solid fluid coupling applied to subduction zones the lithosphere is characterised by an elastic or elastoplastic behaviour and the asthenosphere by a homogeneous isoviscous fluid the temperature effects are ignored and viscosity and density are constant in time the solid and the fluid problem are discretised by the finite elements method fem the same solid code used in hassani et al hassani r jongmans d chery j study of plate deformation and stress in subduction processes using two dimensional numerical models j geophys res 1997 102 17951 17965 has been used to compute the solution of the solid problem the stokes problem is solved by a direct solver with a stabilisation procedure we used a very simple staggered coupling method where the fluid domain is regularly re meshing we observed numerical instabilities when the time step is not sufficiently small especially when strong coupling between the solid and the fluid occurs we have tested different configurations where the lithosphere is elastic or elastoplastic and show how the slab geometry the topography and the stress regime in the plates are affected by the viscous resistance of the mantle we observed that the asthenosphere viscosity is a fundamental parameter in the subduction process for subduction with an extensional regime in the upper plate we observe a linear decrease of the extensional stress as a function of the asthenospheric viscosity

overriding plate deformation and variability of fore arc deformation during subduction insight from geodynamic models and application to the calabria subduction zone in nature subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates such large plates can impose far field boundary conditions that influence the style of subduction and overriding plate deformation here we present dynamic laboratory models of progressive subduction in three dimensional space in which the far field boundary conditions at the trailing edges of the subducting plate sp and overriding plate op are varied four configurations are presented free both plates free sp fixed op fixed and sp op fixed we investigate their impact on the kinematics and dynamics of subduction particularly focusing on overriding plate deformation the results indicate that the variation in far field boundary conditions has an influence on the slab geometry subduction partitioning and trench migration partitioning our models also indicate that in natural narrow subduction zones assuming a homogeneous overriding plate the formation of back arc basins e g tyrrhenian sea aegean sea and scotia sea is generally expected to occur at a comparable location 250 700 km from the trench irrespective of the boundary condition in addition our models indicate that the style of fore arc deformation shortening or extension is influenced by the mobility of the overriding plate through controlling the force normal to the subduction zone interface trench suction our geodynamic model that uses the sp op fixed setup is comparable to the calabria subduction zone with respect to subduction kinematics slab geometry trench curvature and accretionary configuration furthermore the model can explain back arc and fore arc extension at the calabria subduction zone since the latest middle miocene as a consequence of subduction of the narrow calabrian slab and the immobility of the subducting african plate and overriding eurasian plate this setting induced strong trench suction driving fore arc extension and forced subduction to be accommodated almost entirely by slab rollback not trenchward subducting plate motion while trench retreat was accommodated almost entirely by back arc and fore arc extension not trenchward overriding plate motion comparable to our sp op fixed model

physical modeling of slow seafloor spreading a properly scaled thermo mechanical experimental model of slow seafloor spreading is developed to investigate the mechanism of lithosphere accretion the melt of a specially fabricated hydrocarbon compositional system is cooled from above in a tank the crystallizing upper layer the lithosphere possesses semiplastic semibrittle properties horizontal tension of the layer is produced by a paddle drawn at a constant rate spreading in the model is very unstable asymmetric and involves regular jumps of the spreading center scaling up the experimental results to match seafloor spreading gives the length of the jumps of the order of 10 km and the period of the order of 105 106 years the jumps normally occur in the wall of a rift valley resulting in quasiplastic stretching of the lithosphere at this place and then in the formation of two intersecting major normal fault zones crossing the plate from some shallow depth to the base the wedge separated by these zones rises up symmetrically prior to the cessation of movement along one zone with motion only continuing along the other from this moment the deformation becomes asymmetrical the material crystallized at the base of the lithosphere is dragged along the active zone to the upper horizons of the plate and to the surface this process leads to both the nonisostatic uplifting of the central part of the pulled block and subsidence near the inlet on the surface of the fault detachment zone which forms the center of this asymmetric lithospheric accretion at a specific stage under the action of both horizontal tension and vertical nonisostatic forces the strength of the pulling block is exceeded within the rift valley the result is the next jump of the spreading center to the point of breakage and a repetition of the whole cycle the jump can also occur at other places weakened during previous cycles normally the asymmetry of the spreading alternates at a new center the dimensions of the valley as well as the overall seafloor relief are strongly dependent on the spreading rate v the lower the v value the rougher the relief the jumps of the spreading centers are not synchronous along the plate boundary moreover they can be of different types with varying distances and in opposite directions on adjacent segments of the spreading axis leading to the occurrence of transfer or accommodation zones

inter and intracrystalline isotopic disequilibria techniques and applications in the last 20 years we have seen a significant expansion of techniques related to geochemical and isotopic microsampling of materials in the earth sciences from constraining pre eruptive histories of flood basalt magmas to identifying the natal rivers of origin of anadromous fishes these techniques have had significant impacts in a wide variety of scientific fields nowhere has the impact been greater than in identifying the sources processes and timing of processes involved in igneous magmatic systems both technique refinements and the development of new technologies have aided in advancing microsampling applications thus allowing for a better understanding of the sources and mechanisms responsible for changing geochemical and isotopic signatures in natural systems in this chapter we focus on the techniques and technologies associated with radiogenic isotope microsampling and review applications of these techniques as utilized in scientific investigations isotope microsamping is a logical extension of earlier studies that evaluated individual components of magmas and magma systems including melts and minerals from the use of petrographic microscopes and the later introduction of the electron microprobe the focus on internal chemical variations in melts and minerals is critical to assessing the petrogenetic histories of igneous rocks even today these technologies are used to ensure that further trace element and isotopic analyses are undertaken in a textural and major element context for trace elements and isotopes early studies e g cortini and van calsteren 1985 confirmed variations in the melt and mineral components of many igneous rocks but focused on mineral or glass separates potential information associated with isotopic variations retained by individual crystals or internal variations within individual crystals was lost later studies such as geist et al 1988 focused on isotope variations within single megacrysts to constrain mixing scenarios between basaltic andesitic and rhyolitic magmas and davidson et al 1990 used

micrometer silicon isotope diagnostics of soils by uv femtosecond laser ablation this study presents the first si isotope data of the principle si pools in soils determined by a uv femtosecond laser ablation system coupled to a multicollector inductively coupled plasma mass spectrometer mc icp ms this method reveals accurate and precise si isotope data on bulk materials and at high spatial resolution on the mineral scale the following si pools have been investigated a the si source to soils on all major silicate minerals on thin sections from bedrock fragments in the soil profiles b bulk soils particle size 2 mm after fusion to glass beads with an iridium strip heater or pressed into powder pellets c separated clay fractions as pressed powder pellets and e separated phytoliths as pressed powder pellets multiple analyses of three rock standards bhvo 2 agv 1 and rgm 1 as fused glass beads and as pressed powder pellets reveal 30si values within the expected range of igneous rocks the mpi ding reference glass kl2 g exhibits the same si isotope composition after remelting by an iridium strip heater showing that this technique does not alter the isotope composition of the glass we used this approach to investigated two immature cambisols developed on sandstone and paragneiss in the black forest germany respectively bulk soils show a largely uniform si isotope signature for different horizons and locations which is close to those of primary quartz and feldspar with 30si values around 0 4 soil clay formation is associated with limited si mobility which preserves initial si isotope signatures of parental minerals an exception is the organic horizon of the paragneiss catchment where intense weathering leads to a high mobility of si and significant negative isotope signatures as low as to 1 00 in bulk soils biogenic opal in the form of phytoliths exhibits negative si isotope signatures of about 0 4 these results demonstrate that uv femtosecond laser ablation mc icp ms provides a tool to characterize the si isotope signature of the principle si pools left behind after weathering and si transport have altered soils these results can now serve as a fingerprint of the residual solids that can be used to explain the isotope composition of dissolved si in soil solutions and river water which is mostly enriched in the heavy isotopes

matrix independent fe isotope ratio determination in silicates using uv femtosecond laser ablation abstract uv femtosecond laser ablation coupled to mc icp ms provides a promising in situ tool to investigate elemental and isotope ratios by non matrix matched calibration in this study we investigate fe isotope composition in siliceous matrices including biotite hornblende garnet fayalite and forsterite san carlos olivine and an oceanic fe mn crust using the iron reference material irmm 014 for calibration to test the accuracy of the laser ablation data fe isotope compositions were obtained independently by solution icp ms after chromatographic separation of fe sample materials with low cr content i e biotite hornblende fayalite and the fe mn crust reveal 56 54 fe and 57 54 fe values that agree with those from solution icp ms data within the measured precision for high cr concentration 54 cr 54 fe 0 0001 i e in the garnet and forsterite sample 56 54 fe and 57 54 fe values were derived from 57 fe 56 fe ratios as correction of the isobaric interference of 54 cr on 54 fe is unsatisfactory this approach provides accurate results for both minerals moreover the garnet crystal exhibits isotopic zonation with differences of 0 3 in 56 54 fe showing that substantial heterogeneities exist in high temperature metamorphic minerals multiple analyses of homogeneous sample materials reveal a repeatability of 0 1 2 sd for 56 54 fe and 0 2 2 sd for 57 54 fe respectively this study adds to the observations of horn et al 2006 who have shown that the determination of fe isotope ratios in various matrices including iron alloys iron oxides and hydroxides iron sulfide and iron carbonates can be performed with high accuracy and precision at high spatial resolution using uv femtosecond laser ablation icp ms these results demonstrate that femtosecond laser ablation icp ms is a largely matrix independent method which provides a substantial advantage over commonly employed nanosecond laser ablation systems

effect of shallow level contamination on the helium isotope systematics of ocean island lavas the bimodal distribution of helium isotope ratios in ocean island lavas has provided powerful constraints on the composition and evolution of the earth s mantle1 3 high 3he hotspot ratios 3he 4he greater than in mid ocean ridge basalt morb found on hawaii4 and iceland5 are thought to trace pristine plumes from the deep er mantle whereas low 3he hotspot values 3he 3he morb value at tristan da cunha6 and st helena7 are considered to characterize plumes composed in part of recycled oceanic or continental crust here we report the observation of both high 3he and 6low 3he characteristics8 in lavas from a single ocean island heard island in the indian ocean whereas the high 3he lavas provide unambiguous evidence for the involvement of a deep seated plume in their genesis we argue that the low 3he 4he ratios in other lavas result from shallow level contamination by radiogenic helium before eruption these observations call into question the presumed association between low 3he ratios at heard island and elsewhere and ancient crustal material recycled back into the mantle

horizontal lithosphere compression and subduction constraints provided by physical modeling physical modeling of the subduction is performed with a two layered mantle model elastico plastic lithosphere and low viscosity asthenosphere and is governed by the criteria of similarity compression of the lithosphere in the area of a passive continental margin has been shown to produce a buckling instability in the oceanic plate with wavelengths of 200 km on the average later a localization of deformation occurs in sagging at some distance from the margin where a strongly dislocated linear ridge is formed due to the thrusting the plate then experiences a failure along the inclined zone and subduction starts the inner trench slope which forms therefore has a scraped structure and include a block of crushed and dislocated oceanic crust and sediments in the lower section if there is an old inclined fault striking across the compression of an oceanic lithosphere it is on that fault that a subduction zone is initiated the inner trench slope has then a different structure and forms due to normal faulting in the frontal part of the overriding plate the formation of a subduction zone requires a compression that is smaller than that in the preceding case by a factor of 2 or 3 the subducting plate experiences an elastico plastic bending and under specific conditions thrusting along the zone dipping from under the overriding plate oceanward and crossing the entire lithosphere the best agreement between generalized relief in subduction zone in the model and nature is achieved when a shear yield limit s 1 3 108 pa modulus of elasticity e about a few times 1011 pa and a thickness h 60 km are adopted for the real lithosphere

the okinawa trough genesis of a back arc basin developing along a continental margin abstract the okinawa trough is the only example of a marginal back arc basin type at a young stage of evolution that developed along a continental margin the first phase of crustal stretching and subsidence occurred during late miocene early pliocene times a second major phase occurred after the latest pliocene or early pleistocene with en echelon rift axes along the axes of the okinawa trough oblique to the general trend of the arc and trench crustal separation and active spreading has occurred in the central and southern okinawa trough but the north okinawa trough is only in a rifting stage the development of the basin has several correlations with the general tectonism of the arc system 1 1 there was a synchronism between opening and subsidence of the okinawa trough and tilting and subsidence of the fore arc terrace 2 2 back arc rifting was initiated in the volcanic arc and propagated along it during the neogene remnants of a volcanic arc are found on the continental side of the basin the beginning of the back arc spreading observed in the central and southern okinawa trough correspond to a relative minimum in the activity of the volcanic arc 3 3 the timing and kinematic evolution of the taiwan collision and the south okinawa trough opening suggest a connection between these two events such a collision lateral back arc opening model could explain the initiation of the opening of back arc basins but additional phenomena such as thermal convection retreating trench model or anchored slab model are indispensable to maintain extension in the back arc basin

japan sea opening history and mechanism a synthesis the respective tectonic effects of back arc spreading and continental collision in asia are considered either as two independent processes or as closely interrelated extrusion tectonics assumes that the opening of the south china sea and the left lateral motion along the red river fault are geometrically linked in a pull apart manner this model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated in the case of the japan sea we can show without ambiguity that back arc opening was controlled by large intracontinental strike slip faults which can be easily understood as effects of the india asia collision far from the indenter the japan sea opened in the early miocene in a broad pull apart zone between two major dextral strike slip shear zones the first one extends from north sakhalin to central japan along 2000 km it has accommodated about 400 km of finite displacement deformation along it varies from dextral transpression in the north to dextral transtension in the south the second is between korea and sw japan and has accommodated a smaller displacement of about 200 km the extensional domain in between lies in the back arc region of japan distributed stretching of the arc crust resulted in the formation of most of the japan sea while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the japan basin the first oceanic crust was formed in a small triangle based on the eastern shear zone and spreading propagated westward inside the pull apart region timing of oceanic crust formation of formation of the dextral shear zones and of block rotation in between as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history this evolution is discussed by comparison to other manifestations of the arc and back arc activity such as the history of sedimentation and volcanism the paper then suggests that the collision of india can have tectonic consequences as far north as japan and sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion

discussion of temporal evolution of the kerguelen plume geochemical evidence from 38 to 82 ma lavas forming the ninetyeast ridge by f a frey and d weis

micro scale tracing of fe and si isotope signatures in banded iron formation using femtosecond laser ablation we have detected micrometre scale differences in fe and si stable isotope ratios between coexisting minerals and between layers of banded iron formation bif using an uv femtosecond laser ablation system connected to a mc icp ms in the magnetite carbonate chert bif from the archean old wanderer formation in the shurugwi greenstone belt zimbabwe magnetite shows neither intra nor inter layer trends giving overall uniform d 56 fe values of 0 9 but exhibits intra crystal zonation bulk iron carbonates are also relatively uniform at near zero values however their individual d 56 fe value is highly composition dependent both siderite and ankerite and mixtures between both are present and d 56 fe end member values are 0 4 for siderite and 0 7 for ankerite the data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by fe iii reducing bacteria or more likely an abiotic reaction of organic carbon and fe iii during low grade metamorphism si isotope composition of the old wanderer bif also shows significant variations with d 30 si values that range between 1 0 and 2 6 for bulk layers these isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal rich waters interestingly fe and si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures moreover the changes of fe and si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal rich water which govern the rates of fe and si precipitation and therefore also the development of layering during periods of low hydrothermal activity precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate chert layers during periods of intensive hydrothermal activity precipitation rates of ferric oxyhydroxide were high and subsequent diagenesis triggered only partial reduction forming magnetite carbonate chert layers we are confident that our micro analytical technique is able to detect both the solute flux history into the sedimentary bif precursor and the bif s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors

a proposal for the kinematic modelling of w dipping subductions possible applications to the tyrrhenian apennines system in w dipping subduction zones there is a general eastward progression of the back arc basin accretionary wedge foredeep complex with the forward progression early stages of the complex are revealed by slices of upper crust and sedimentary cover abandoned to the west left floating above a new section of mantle a major shear zone should form at the new moho separating upper crust slices of earlier accretionary stages and the eastward flowing mantle the mantle wedging at the top of the subduction plane could be responsible for the uplift of the central parts of the belt the retreating of the subduction hinge is interpreted as due to the push generated by the eastward mantle flow detected in the hot spot reference frame the foredeep depth is mainly a function of the radius of curvature of the subduction hinge the frontal wedge is constructed from the stacking of the upper layers of the subducting plate and the syntectonic clastics that fill the foredeep which are progressively involved in thrusting and later by extension in order to preserve volume balance the lithosphere of the eastern plate before subduction has to be the same size as that which has been subducted due to the longer length of the arc with respect to the original length of the linear margin between the two converging plates laterally stretched subducted lithosphere is predicted at depth w dipping subductions usually have a short life probably due to their inherent capability to produce new lateral heterogeneities of the lithosphere the thin back arc which are a key factor in controlling and generating new subductions both e and w dipping this model is applied to the apennines tyrrhenian sea system

temporal evolution of the kerguelen plume geochemical evidence from 38 to 82 ma lavas forming the ninetyeast ridge abstract basaltic basement has been recovered by deep sea drilling at seven sites on the linear ninetyeast ridge in the eastern indian ocean studies of the recovered lavas show that this ridge formed from 82 to 38 ma as a series of subaerial volcanoes that were created by the northward migration of the indian plate over a fixed magma source in the mantle the sr nd and pb isotopic ratios of lavas from the ninetyeast ridge range widely but they largely overlap with those of lavas from the kerguelen archipelago thereby confirming previous inferences that the kerguelen plume was an important magma source for the ninetyeast ridge particularly important are the 81 ma ninetyeast ridge lavas from dsdp site 216 which has an anomalous subsidence history coffin 1992 these lavas are feti rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic upper miocene lavas in the kerguelen archipelago the isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the kerguelen plume weis et al 1993a b despite the overlap in isotopic ratios there are important compositional differences between lavas erupted on the ninetyeast ridge and in the kerguelen archipelago the ninetyeast ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios such as la yb and zr nb which are intermediate between those of indian ocean morb mid ocean ridge basalt and the transitional to alkalic basalts erupted in the kerguelen archipelago these compositional differences reflect a much larger extent of melting for the ninetyeast ridge lavas and the proximity of the plume to a spreading ridge axis this tectonic setting contrasts with that of the recent alkalic lavas in the kerguelen archipelago which formed beneath the thick lithosphere of the kerguelen plateau from 82 to 38 ma there was no simple systematic temporal variation of sr nd and pb isotopic ratios in ninetyeast ridge lavas therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume although class et al 1993 propose that some of the isotopic variations reflect such aging we infer that most of the isotopic heterogeneity in lavas from the ninetyeast ridge and kerguelen archipelago can be explained by mixing of the kerguelen plume with a depleted morb like mantle component however with this interpretation some of the youngest 42 44 ma lavas from the southern ninetyeast ridge which have206pb 204pb ratios exceeding those in indian ocean morb and kerguelen archipelago lavas require a component with higher206pb 204pb such as that expressed in lavas from st paul island

evolution of the downgoing lithosphere and the mechanisms of deep focus earthquakes summary the thermal evolution of the lithospheric slab at subduction zones and its geophysical effects are numerically calculated an alternating direction implicit finite difference scheme is used to compute the thermal models taking into account all heating sources and phase boundaries these models with the appropriate spreading rates and dip angles are compared with different island arc systems temperatures inside the slab are strongly controlled by the conductivity and by the time elapsed since the initiation of descent the depth to which temperature anomalies persist is generally about 700 km or less the thermal results are used to construct the seismic velocities and ray paths density anomalies and the resulting stress distribution comparing the theoretical stress distribution and the focal mechanism studies of intermediate and deep focus earthquakes indicates the importance of both the mantle s rheology and the temperature dependence of the slab s elastic properties the intermediate and deep focus earthquakes are located along the coolest region of the slab the theoretical results explain the source mechanisms and the orientation of principal stresses under major island arcs

the tectonic development of the hellenic arc and the sea of crete a synthesis abstract we use field analyses together with interpretation of aerial photographs orbital imagery and sea beam data to define more accurately the geometry of fault systems in the hellenic arc the tectonics of the area are dominated by extensional processes related to normal faulting we then use sea beam data previous oceanographic studies and extrapolation of field analyses on surrounding islands to describe the fault pattern of the cretan sea basin which is found consistent with the land fault pattern the mechanisms of the late neogene and quaternary deformation of the central and southern aegean region are analysed in terms of stress trajectories as revealed by field studies of fault populations we then examine the subsidence of the sea of crete in terms of a model of uniform stretching of the lithosphere the opening of the sea of crete was cylindrical and did not result in bending of crete this leads us to modify the southern portion of an earlier kinematic reconstruction of aegea in agreement with recent paleomagnetic results the implications of the opening of the sea of crete on the tectonic evolution of the straits to the west and east of crete are discussed finally the uplift of the outer arc is explained by underplating due to subduction

the development of slabs in the upper mantle insights from numerical and laboratory experiments we have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab a two dimensional finite element code is applied to model newtonian creep in the numerical experiments scaled analog media are used in the laboratory a sand mixture models the brittle crust silicone putty simulates creep in the lower crust and mantle lithosphere and glucose syrup is the asthenosphere analog both model approaches show similar results and reproduce first order observations of the subduction process in nature based on density and viscosity heterogeneities in a stokes flow model subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold we find that the subduction velocity and angle are time dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670 km discontinuity the style of subduction is controlled by the prescribed velocity of convergence the density contrast between the plates and the viscosity contrast between the oceanic plate and the mantle these factors can be combined in the buoyancy number f which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate variations in f control the stress in the plates the speed and the dip of subduction and the rate of trench retreat reproducing the contrasting styles of subduction observed in nature the subduction rate is strongly influenced by the work of bending the lithosphere as it subducts

island arc tectonics abstract tectonic stress states in arc backarc systems have been examined on simplified model structures corresponding to the chilean mariana and intermediate arc types two sources of tectonic stress are assumed the stress due to lateral density variations and the stress transmitted from the subducting oceanic plate to the forearc the average vertical stress zz of the former type has been calculated for each isostatically compensated model lithospheric plate using the difference from a reference structure from the sign of xx zz one can estimate whether the tectonic situation is extentional or compressional if we assume that xx is caused by mechanical coupling between the subducting oceanic slab and is laterally constant in one arc backarc system we can estimate at least the upper or lower boundary of the mechanical coupling for the chilean type arcs where the truly great interplate earthquakes occur and no backarc spreading takes place the transmitted average stress must be larger than 660 bar for the mariana type arcs where no large interplate earthquakes are recorded and spreading takes place in the backarc region xx should be smaller than 100 bar in any arc backarc system the most extentional feature is expected at the highest topography of the volcanic arc once spreading has started the viscosity under the backarc region drops dramatically and the spreading will continue in the backarc even if its topography is subdued in comparison with the now separated main and remnant arcs

geochemical and isotopic study of metasomatic apatite implications for gold mineralization in xindigou northern china abstract the 0 7 moz xindigou gold deposit lies on the north margin of north china craton the orebodies are controlled by nww trending shear zones and hosted by an archean metavolcanosedimentary sequence these host schist amphibolite and marble rocks were intruded by late paleozoic granites and mafic dikes two independent magmatic hydrothermal assemblages were identified based on petrographic evidence a pre ore magmatic hydrothermal magnetite apatite pyrite zircon assemblage and an ore related potassium feldspar muscovite sulfide minor monazite assemblage apatite that has undergone later alteration shows significant intracrystalline spatial variability with respect to the concentrations of rare earth elements rees si and sr scanning electron microscope backscattered images and statistical interrogation of the compositional data acquired by la icp ms indicate that apatite grains in the pre ore magnetite apatite assemblage are homogeneous and enriched in rees and si ap1 where sulfide related alteration of mineralization stage was overlapped these apatite grains display a distinctive texture of homogeneous cores and turbid rims this dissolution reprecipitation process preferentially transported elements from the original apatite rees and si gradually decrease from the core ap1 to the altered rim ap2 ap3 whereas sr increases localized leaching of rees especially lrees from apatite was followed by precipitation of secondary monazite grains in the metasomatic apatite rims and the muscovite sulfide quartz phosphate sequence in situ sr isotope analyses of the apatite show the 87sr 86sr ratios varying from the lowest value 0 707874 in unaltered ap1 to the highest 0 714160 in altered ap3 zircon grains from migmatite display a peak metamorphic age at 2512 9 ma whereas zircons from the early magnetite apatite association formed near 1776 12 ma and late hydrothermal monazite yields a th pb age of 301 0 1 7 ma these geochronology results show that the mineralization occurred during the late paleozoic orogenesis long after neoarchean regional metamorphism and late paleoproterozoic rift related magmatism the typical hydrothermal features of orogenic gold deposit could be inferred from the fluorapatite the elemental patterns of fluorapatite grains indicate that a carbon and sr enriched acid slightly oxidized and low salinity hydrothermal system was responsible for apatite metasomatism and gold mineralization strontium isotopes further reveal that the fluid related to gold deposition was derived from mafic lower crust with extensive radiogenic sr accumulation during fluid rock interaction this study demonstrates the potential application of apatite in exploration for orogenic gold deposits and its use in tracking hydrothermal ree re mobilization during metasomatism

in situ iron isotope ratio determination using uv femtosecond laser ablation with application to hydrothermal ore formation processes abstract the feasibility of in situ stable fe isotope ratio measurements using uv femtosecond laser ablation connected to a multiple collector inductively coupled plasma mass spectrometer mc icp ms has been investigated different types of matrices independently determined by solution mc icp ms after chromatographic separation of fe have been analysed by laser ablation using the isotopically certified iron reference material irmm 014 as the bracketing standard the samples have been pure iron metal jm puratronic fe meteorites north chile glenormiston and toluca the meteorite phases kamacite and taenite in toluca and fe sulphides furthermore fe isotope ratios from hydrothermal hematite siderite and goethite from an old mining area in the schwarzwald germany and of magnetite from the metamorphic biwabik iron formation have been determined the results show that a precision of better than 0 1 2 sigma can be achieved with laser ablation and that all the results obtained agree with those determined by solution icp to better than 0 1 this precision and accuracy is achievable in both raster and spot ablation mode a matrix matched bracketing standard is not required and all these materials can be measured accurately against a metal standard the hydrothermal minerals show significant fe isotope zonations in some samples the range of 56fe in a single aggregate encompasses the entire spectrum of ratios found by bulk solution analyses in multiple samples distributed over the whole mining district for example isotopic zonations found in secondary fibrous hematites show a continuous change in the 56fe values from 0 5 in the core to 1 8 in the rim primary hydrothermal siderite shows the reverse pattern with lighter values in the core than in the rim while the siderite is thought to record primary fluid histories the hematite pattern is interpreted as a reworked isotopic signature generated by oxic dissolution of primary zoned siderite and immediate close range re precipitation of the oxidized fe abrupt changes are documented for secondary goethite showing a distinct overgrowth that is 0 4 lighter than the core of the grain if indeed fe isotopes in secondary minerals from hydrothermal ore deposits record the initial isotopic signatures of their precursor minerals and these in turn record hydrothermal fluid histories then the tools are in place for a detailed reconstruction of the deposit s genesis we expect similar observations from other fe rich deposits formed at intermediate and low temperatures e g banded iron formations laser ablation now provides us with the spatial resolution that adds a further dimension to our interpretation of stable fe isotope fractionation

quantifying the energy dissipation of overriding plate deformation in three dimensional subduction models in a subduction system the force and the energy required to deform the overriding plate are generally thought to come from the negative buoyancy of the subducted slab and its potential energy respectively such deformation might involve extension and back arc basin formation or shortening and mountain building how much of the slab s potential energy is consumed during overriding plate deformation remains unknown in this work we present dynamic three dimensional laboratory experiments of progressive subduction with an overriding plate to quantify the force fopd that drives overriding plate deformation and the associated energy dissipation rate opd and we compare them with the negative buoyancy fbu of the subducted slab and its total potential energy release rate bu respectively we varied the viscosity ratio between the plates and the sublithospheric upper mantle with sp um 157 560 and the thickness of the overriding plate with top 0 5 2 5 cm scaling to 25 125 km in nature the results show that fopd fbu has average values of 0 5 2 0 with a maximum of 5 3 and opd bu has average values of 0 05 0 30 with a maximum of 0 41 the results indicate that only a small portion of the negative buoyancy of the slab and its potential energy are used to deform the overriding plate our models also suggest that the force required to deform the overriding plate is of comparable magnitude as the ridge push force furthermore we show that in subduction models with an overriding plate bending dissipation at the subduction zone hinge remains low 3 15 during steady state subduction

oscillatory sr isotopic signature in plagioclase megacrysts from the damiao anorthosite complex north china implication for petrogenesis of massif type anorthosite abstract formation of proterozoic massif type anorthosites is known to be related to polybaric process involving early high pressure crystallization of plagioclase and high al orthopyroxene megacrysts at mantle crust boundary followed by emplacement of plagioclase dominated mushes to shallow level crust therefore the plagioclase megacrysts record important information about the magmatic sources and crystallization process of anorthosites the 1 74 ga damiao complex north china comprises 90 vol anorthosites and leuconorites containing abundant plagioclase megacrysts associated with minor high al orthopyroxene megacrysts haoms the plagioclase megacrysts are generally euhedral to subhedral mostly 1 to 30 cm in diameter and some of them contain very fine lamellae of orthoclase and undefined fe ti rich minerals 1 5 m the haoms occur as subhedral grains or as angular grains with an intercumulus relationship to the plagioclase megacrysts they contain abundant thin regular lamellae of exsolved plagioclase 15 20 vol indicative of originally high al features originally 6 0 7 3 wt al2o3 based on the al in orthopyroxene geobarometry the haoms and associated plagioclase megacrysts are constrained to be crystallized together at pressures of 9 4 11 2 kbar 33 36 km indicative of their crystallization at lower crust depths or crust mantle boundary in contrast orthopyroxene grains from late differentiation phases such as oxide apatite gabbronorites do not contain plagioclase lamellae and have much lower al2o3 contents 1 5 to 1 7 wt indicating final crystallization at

subduction of the lithosphere and back arc dynamics insights from physical modeling physical modeling of the subduction of an elastico plastic lithosphere underlained by a low viscosity asthenosphere and driven by both horizontal compressional force and gravitational sinking of the subducting plate has shown the subduction regime to be strongly affected by 1 a where a and 1 are the asthenosphere and lithosphere densities at 0 a steady state process develops which requires a horizontal lithosphere compression of h 0 3 0 4 s 108 pa where s is the lithosphere yield limit the central part of the leading wedge of the overriding plate is nonisostatically elevated in this case especially in the region of the frontal arc which corresponds to a free air gravity anomaly maximum of 200 400 mgal a decrease in 0 compression of the overriding plate is considerably less from the onset of subduction and then decreases during subduction at some point compression changes into tension tensional nonhydrostatic stresses grow during the process to a value near h 0 1 0 2 s 0 4 108 pa both subsidence of the overriding wedge below the isostatic equilibrium level and an increase in the benioif dip angle accompany this process the gravity anomaly maximum is displaced into the volcanic arc in this subduction regime where it is normally less than 150 mgal if some weaknesses are present in the overriding plate tensional stresses result in a failure of the lithosphere at these locations and the broken lithosphere segment the arc plate starts to move counter to the subduction plate the arc plate slides along the underthrusting plate s surface without loss of contact and causes back arc opening hydrostatic suction prevents the plates from being separated this effect is caused by the peculiarity of a mechanical system whereby a low viscosity asthenosphere and a plastic lithosphere which effective yield limit is considerably lower than the hydrostatic stresses in the plate if the overriding lithosphere is not weakened enough to fail under the existing tension then the hanging slab breaks down under its own weight when it reaches a specific length both interacting plates undergo isostatic recovery in this case and the whole cycle is repeated again the hydrostatic suction effect also provides back arc tension under other conditions such as when asthenospheric flow forces the subducted plate to move away from the island arc

in situ sr isotopic analysis by laser ablation we demonstrate that the sr isotopic compositions of geological materials can be measured in situ through laser ablation using an icp multiple collector double focusing magnetic sector mass spectrometer mc icpms this provides rapid texturally sensitive high precision 0 004 sr isotopic analysis without the need for chemical preparation of samples the system used employs a q switched nd yag infrared laser the laser ablated material is carried by ar flow to an icp torch the resulting ions are extracted from the plasma and focused with d c quadrupole lenses and an electrostatic analyzer prior to entering a magnetic sector analyzer and collection in an array of faraday collectors a feldspar megacryst with independently known 87sr86sr of 0 703117 13 and a modern gastropod shell with independently known 87sr86sr of 0 709170 10 were used evaluate the method both have 2000 ppm sr laser power was adjusted to give an average 88sr of 2 10 11 amps during analysis to replicate the beam size during typical thermal ionization mass spectrometry tims within run precisions in 87sr86sr of 4 10 5 compare favorably with the precision from tims thirteen analyses of the cameroon feldspar megacryst yielded a weighted average 87sr86sr of 0 703106 22 in perfect agreement with the tims measurement similarly 22 analyses of the modern gastropod shell gave a weighted average of 0 709182 22 identical to the known value no memory effect was observed when switching from the cameroon feldspar sample run for several hours to the modern shell material the power of this technique is illustrated with a preliminary study of systematic differences in 87sr86sr of 0 0004 between two feldspar crystals of a long valley basalt one feldspar has identical 87sr86sr to the host basalt confirming the lack of any matrix effect with this method

petrogenesis of the flood basalts forming the northern kerguelen archipelago implications for the kerguelen plume the thick 20 km crust of the kerguelen archipelago formed as the tectonic setting of the kerguelen plume changed from an oceanic ridge centered location at 43 ma to its present location beneath the antarctic plate the uppermost crust is dominantly flood basalt with a thickness of up to 10 km inverse isochron 40ar 39ar ages for upper and lower lavas in a 630 m section of basalt flows from mont bureau are 30 4 and 29 0 ma re os isotopic systematics are consistent with this age most of the lavas in two stratigraphic sections mont bureau and mont rabouillere from the northern part of the archipelago have sr nd and pb isotopic characteristics similar to the youngest upper miocene to pleistocene lavas erupted in the southeast part of the archipelago i e initial 87sr 86sr 0 7050 143nd 144nd 10 mgo alkalic lavas in the southeast province of the archipelago in these northern sections it is confined to transitional lavas with

models of lithospheric scale deformation during plate collision effects of indentor shape and lithospheric thickness abstract analogue models designed to test the effects of lithosphere thickness and indentor shape during continental collision indicate that strength and density differences between lithosphere types control structural vergence during collision models presented here include layers of putty and sand to form four three and two layer lithosphere types craton arc and oceanic equivalent respectively which collide during unidirectional shortening in most cases the stronger thicker four layer lithosphere does not deform but simply acts to bulldoze weaker material aside deformation concentrated in the thinnest layers consistently verges away from the indenting continent irrespective of indentor shape lateral strike slip component deformation occurs in the early stages of most models even with simple unidirectional shortening and is enhanced by highly oblique indentor shapes the effects of extrusion during collision tested by repeating models with and without a free edge which allows extrusion consistently indicate that with the ability to extrude structures are smaller and more numerous without extrusion fewer larger structures tend to develop two geological examples from the precambrian part of the tasman orogenic belt and the modern day collision at the leading edge of australia illustrate aspects of the models

rapid determination of initial 87sr 86sr and estimation of the rb sr age of plutonic rocks by la icpms of variably altered feldspars an example from the 1 14 ga great abitibi dyke ontario canada abstract sr isotope analysis of plutonic rocks is an important method for deriving the provenance and age of ancient magmatic episodes and the petrogenetic processes operating during intrusive activity we determined the 87 sr 86 sr isotopic compositions of fresh and variably altered feldspars from the 1 14 ga gabbroic great abitibi dyke ontario canada through the use of laser ablation inductively coupled plasma mass spectrometry la icpms the data show an alteration trend between fresh low rb plagioclase with 87 sr 86 sr ratios of 0 7045 and high rb alkali feldspar with 87 sr 86 sr ratios of 0 7059 which may be used to constrain the 87 sr 86 sr ratio of the mantle source this trend is incompatible with simple mixing and instead has age significance as post intrusive alteration through hydrothermal circulation starts immediately following emplacement la icpms analysis allows rapid estimation of initial 87 sr 86 sr ratios and intrusive age suitable for sample screening and selection for more precise dating for the great abitibi dyke the rb sr age estimate of emplacement of 1 10 0 23 ga 2 is concordant with the precise u pb emplacement age of the dyke 1140 6 2 0 ma the most altered high rb alkali feldspars return younger ages suggesting that post intrusive alteration lasted several hundred million years this study highlights the need for microanalysis to differentiate between alteration and magmatic processes on basis of sr isotopic signatures and outlines a rapid method to determine initial 87 sr 86 sr ratios and to estimate rb sr ages of plutonic rocks in terranes with long histories of multiple intrusive magmatic episodes

recycling of k feldspar antecrysts in the baishiya porphyritic granodiorite east kunlun orogenic belt northern tibet plateau implications for magma differentiation in a crystal mush reservoir abstract it is popular in geochemical models to assume that chemical differentiation occurs by crystal fractionation from high melt fraction magma reservoirs however a crystal mush reservoir characterized by low melt fraction has been gradually accepted in the past decade magma differentiation in this scenario remains poorly understand located in the east kunlun orogenic belt northern tibet plateau the baishiya granodiorite is an ideal example to investigate magma reservoir processes although major elements of the baishiya granodiorite are relatively homogeneous there is a distinct variation of magmatic texture trace elements concentrations and isotopic ratios from top to bottom the textural and geochemical signatures above cannot be explained by assimilation fractionation crystallization magma mixing or convective self mixing alone within the concept model of crystal mush we propose a multistage formation process of the studied granodiorite first a rhyolitic mushy reservoir once existed it was replenished by episodic basaltic melts from below resulting in the textural coarsening of k feldspar when the heat and or melts injection was high enough the crystal mush could be gradually rejuvenated and subsequently stirred some k feldspar antecrysts would be partly recycled to form mantled plagioclase through convective self mixing thus the evolved and buoyancy melts exhibited an equilibrated geochemical signature with plagioclase and ultimately formed the porphyry phase on the top moreover recycling of k feldspar antecrysts contributed to a depletion of heavy rare earth elements and y contents as well as an elevation of cr concentrations and eu eu ratios in the residual crystal mush and finally formed the porphyritic granodiorite due to a continuous injection and convective self mixing the overall magma reservoir became more mafic gradually until it reached a granodioritic composition within a waning intensity of magma flux the magma reservoir would be locked again the injected melts would go through a rapid crystallization and subsequently form microlites such as the dioritic enclaves and the ground mass of the studied granodiorite consequently traditional mechanisms that are based on low crystallinity magmas may be a specific part of the protracted history rather than the whole story of the magma reservoir geochemical index of granitoid rocks should be used with caution due to this inherited affinity it should be prerequisite to investigate crystal genesis carefully before utilizing geochemical diagrams

geochemical characteristics of the youngest volcano mount ross in the kerguelen archipelago inferences for magma flux lithosphere assimilation and composition of the kerguelen plume mount ross is the youngest 1 0 1 ma volcanic edifice recognized in the kerguelen archipelago which is located on the northern part of the submarine kerguelen plateau lava types range from basaltic trachyandesite to trachyte and extensive glaciation has exposed an intrusive core ranging from gabbro to syenite mount ross rocks are not as silica undersaturated as the basanite to phonolite upper miocene lavas erupted in the southeast province of the archipelago however both lava suites are characterized by relatively high 87sr 86sr 0 7051 0 7054 and 0 7054 0 7058 respectively and low 206pb 204pb 18 02 18 14 and 18 06 18 27 respectively the abundant trachytes and phonolites in the age range from lower miocene to pleistocene on the kerguelen archipelago indicate that the supply of basaltic magma has been low for the last 20 m y despite the prolonged time period required for extensive mineral fractionation there is no evidence for assimilation of geochemically distinctive lithosphere i e most of the trachytes and syenites have initial isotopic ratios within the narrow range of the more mafic rocks volcanic and plutonic rocks of diverse composition and age 0 1 30 ma in the kerguelen archipelago have similar initial isotopic ratios consequently we infer that the kerguelen plume is characterized by 87sr 86sr 0 7051 0 7058 143nd 144nd 0 51263 0 51249 and 206pb 204pb 18 02 18 27

trench motion slab geometry and viscous stresses in subduction systems summary a semi analytic 3 d model for subduction within a newtonian viscous upper mantle provides a dynamically consistent means of computing viscous stress trench motion and slab geometry in subduction systems although negative slab buoyancy provides the basic driving force for subduction slabs that extend from the surface to the base of the upper mantle are oversupported by viscous stresses in the shallow 100 150 km mantle and undersupported by viscous stresses at greater depth in the upper mantle these deeper parts of the subduction system act as an engine for subduction while shallower parts act as a brake on trench motion trench migration rates and slab geometry reflect a competition between these two effects during steady state subduction trench migration rates vary approximately linearly with slab buoyancy and model rates of trench motion are in good agreement with the range of observed rates for a two layer upper mantle viscosity of 2 10 20 pa s above 300 km and 5 10 20 pa s below steady state slab dip increases as slab density decreases especially for very low density slabs which dip significantly more steeply than high density slabs the horizontal velocity at the top of the lower mantle measured relative to the foreland has a very large effect on trench migration rates rivalling or even exceeding that of slab buoyancy slab width parallel to the trench also has a significant effect on trench migration rates due to the viscous pressure of toroidal flow around the slab the stiffness of the subducting lithosphere does not exert a significant effect on trench migration rates or slab geometries for rigidities compatible with oceanic lithosphere very stiff slabs with elastic plate thicknesses more that 40 km or viscosities in the range of 10 25 10 26 pa s subduct significantly more slowly than weak slabs with trench migration rates in the range of half to a third that of weak slabs large unexpected effects on trench migration rates and slab geometry are exerted by the structure and density of the frontal prism and overriding plate indicating that local geology can exert important constraints on slab dynamics during non steady state subduction rates of trench migration respond rapidly as variably buoyant lithosphere penetrates into the asthenosphere in the absence of other driving forces for convergence trench migration rates can change by a factor of two or more in as little as 2 3 myr for example from 35 to 70 mm yr 1 when an oceanic piece of slab follows a continental one into the subduction system subduction of variable buoyancy lithosphere is accompanied by changes in slab dip with depth and through time

fluid sources and hydrothermal architecture of the sudbury structure constraints from femtosecond la mc icp ms sr isotopic analysis of hydrothermal epidote and calcite abstract this study presents new sr isotopic data on hydrothermal mineral assemblages from different stratigraphic levels within the sudbury impact structure determined using femtosecond la mc icp ms these data provide new insights into the origin of the hydrothermal fluids that have circulated through and around the sudbury igneous complex sic and further define the overall hydrothermal architecture of the structure early blocky miarolitic epidote from the sic has a narrow range of initial 87 sr 86 sr 0 70710 to 0 70742 similar to the sic 0 70648 to 0 70736 that is consistent with the precipitation from orthomagmatic fluids derived from the sic the 87 sr 86 sr values of later replacement miarolitic and vein epidote from different stratigraphic levels of the sic and of replacement and amygdule epidote from the overlying onaping formation are distinctly higher up to 0 71753 than those of the blocky miarolitic epidote these data suggest that the fluids responsible for this late epidote were not derived from the sic and more likely represent basement derived fluids that passed through the sic and onaping formation the sr isotopic composition of syngenetic laminated calcite in the post impact vermillion formation suggests precipitation from proterozoic seawater 87 sr 86 sr 0 70295 whereas some vermilion formation related syngenetic laminated calcite and epigenetic discordant and replacement mineralized calcite with more radiogenic values suggest precipitation from seawater modified through interaction with the vitric and lithic fragments present in the onaping formation the formation of some replacement and amygdule carbonate in the onaping formation also involved mixing between seawater and basement derived fluids of similar chemical and isotopic composition to those responsible for the replacement epidote hosted by the sic and onaping formation the sr isotopic composition of hydrothermal epidote from throughout the sudbury structure indicates a fluid connection between the footwall and hanging wall of the sudbury igneous complex

strontium isotope ratios in fossil teeth from south africa assessing laser ablation mc icp ms analysis and the extent of diagenesis abstract this study tests the precision and accuracy of laser ablation multi collector inductively coupled plasma mass spectrometry la mc icp ms for measuring strontium isotope ratios 87 sr 86 sr in fossil teeth from swartkrans and sterkfontein in south africa and explores the extent of diagenesis in these specimens compared to solution mc icp ms or thermal ionization mass spectrometry tims methods la mc icp ms can be faster and less expensive but has lower precision and accuracy we found that 87 sr 86 sr ratios in fossil rodent teeth measured by la mc icp ms were offset from solution values by a mean of 0 0005 0 0010 2 because of the very wide spread of biologically available 87 sr 86 sr ratios 0 710 to 0 790 in the region la mc icp ms is more than sufficient for assessing the geographic origins of the fossil rodents the concentration of strontium in modern and fossil enamel from the area is statistically indistinguishable as is the range of 87 sr 86 sr ratios in modern rodents and fossil rodents from swartkrans thus there is little evidence of diagenetic sr in the fossil enamel in contrast the fossil dentine may contain up to 50 diagenetic strontium although many dentine 87 sr 86 sr ratios are still far removed from the background cave value of 0 729 we propose that la mc icp ms will also be of use for investigating the geographic origins of other non rodent fossil teeth from the sterkfontein and swartkrans collections and may prove of similar utility at other suitable sites

flow models for back arc spreading abstract a secondary flow model for back arc spreading is developed in this paper that shows some of the characteristics of observed back arc spreading back arc spreading has formed marginal seas around the west and southwest rim of the pacific the episodic spreading and different directions of opening are not completely understood however there does appear to be a limited lifetime

possible effects of collisions on plate motions abstract a large scale collision at a plate boundary is expected to play an important role not only in the deformation at the boundary but also in the motion of the plate carrying the buoyant material to be accreted possible changes in rates and directions of such motions may be calculated provided that certain assumptions are made about the nature of the driving forces in this model we shall assume basically that 1 1 an oceanic plate is driven by slab pull and ridge push being resisted by basal asthenospheric drag and slab resistance and 2 2 because of detachment slab pull is lost upon collision if however the calculated motion following collision has a convergent component at the boundary a new subduction zone with an increasing slab pull force forms seaward of the accreting buoyant material calculations were first made on an idealized planar octagonal plate results indicated that so long as the scale of collision is limited changes in motion take place but the motion returns to the initial state however for large scale collisions the plate motion suffers a large directional change which also changes the nature of some boundaries and hence the motion approaches a new steady state motion irreversibly calculations were then conducted on an idealized philippine sea plate on a spherical earth for a sizable collision along the taiwan philippine mindanao boundary this boundary and perhaps the bonin mariana boundary may become a transform fault provided that slab pull ridge push is the driving force these results provide some support to uyeda mccabe model 1983 of episodic spreading of the philippine sea basins as well as the predominance of strike slip motion along collision boundaries finally in agreement with prior studies subduction of a ridge system assumed to have been located along the southwestern rim of the paleo pacific plate was shown to be consistent with the change in direction of motion of that plate at 43 m y b p calculations conducted in this study show that predicted changes are model dependent so that comparisons with observed or inferred motions may be useful as a means of estimating the relative importance of the various potential driving forces

multiple mantle sources during island arc magmatism u pb and hf isotopic evidence from the kohistan arc complex pakistan this paper presents high precision u pb ages and initial hf isotopic compositions of zircon from mafic to felsic rocks of the kohistan arc complex pakistan three magmatic pulses tapping geochemically different reservoirs are distinguished partial melting of mantle with morb type isotopic characteristics generated 99 92 ma old magmas plutonism around 85 ma tapped a more fertile mantle source most likely consisting of a 600 ma old metasomatically enriched mantle or of mantle contaminated by an old sedimentary component 82 ma old felsic peraluminous dykes have morb type isotopic compositions considered to be inherited from remelting earlier magmas in the deep base of the arc the isotopic results demonstrate several and rather rapid changes in melt source region during arc development they also show that there was subordinate continental influence and negligible importance of slab components for the hf budget during the generation of the kohistan arc complex

effect of margin curvature on plate deformation in a 3 d numerical model of subduction zones summary the large amount of data acquired along most subduction zones underlines the importance of three dimensions in the subduction mechanism in this work we use a fully 3 d mechanical numerical model to analyse the impact of the plate boundary geometry on the deformation of the upper plate the model consists of two initially horizontal independent plates overlying an inviscid fluid and continuously pushed towards each other the initial geometry of the contact zone between the two plates controls the stress regime and the strain pattern in the upper plate the case with a linear plate boundary and orthogonal convergence is first considered and used as a reference model with this simple model the effects of some important parameters i e the interplate friction and the lithosphere asthenosphere density contrast are investigated finally cases with curved plate boundaries are considered and the results are used to shed light on the importance of the dip direction of the interplate plane for the upper plate strain pattern unnecessary oceanward convexity causes an accumulation of the subducted material beneath the upper plate and induces an important uplift of the convex area on the contrary the material escapes from an oceanward concavity and provokes subsidence in the forearc zone such a behaviour induces preferential zones of weakeness in the overriding plate and may allow for explaining some local stress regime variations along convergent margins

continental subduction and a mechanism for exhumation of high pressure metamorphic rocks new modelling and field data from oman abstract the physical model presented reveals two principal regimes of continental subduction a highly compressional hc regime and a low compressional lc regime characterised by high and low pressure between the overriding and subducting plates respectively the pressure is inversely proportional to the pull force which depends on the difference between average density of the subducting lithosphere and density of the mantle the subducting continental crust reaches a maximum depth which is proportional to the strength of the crust inversely proportional to the interplate pressure and is ca 200 km on average the crust then fails forming a major crustal thrust the location of failure depends on the regime of subduction on the interplate pressure in the hc regime failure occurs near the front of the subduction collision zone whereas the crust fails at greater depth under the base of the overriding plate in the lc regime the failure is followed by buoyancy driven uplift of the subducted crustal slice while the lithospheric mantle keeps subducting the uplift causes a normal sense displacement formation of a normal fault along the upper surface of the crustal slice for the lc regime failure of the crust near the base of the overriding plate a rapid spontaneous crustal uplift intrusion into the interplate zone brings the deeply subducted crust to shallow depths under the pressure of this crust the frontal part of the overriding plate undergoes local extension and then fails forming a tectonic window the rising material the high pressure rocks is exhumed within this window in the hc regime uplift of the subducted crust after its failure in front of the overriding plate is possible only with erosion of the relief which provides an unloading effect allowing the subducted crustal slice to rise up the exhumation depth is generally smaller in this regime but the volume of the exhumed material is larger the hc subduction regime has been shown earlier to match the himalayan situation the lc regime fits the situation in the oman mountains considered in this paper

improved in situ sr isotopic analysis by a 257 nm femtosecond laser in combination with the addition of nitrogen for geological minerals abstract in situ sr isotope analysis of geological materials by laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icp ms is a powerful tracer technique for tracking magmatic source components and geological processes however the accuracy and precision of the 87sr 86sr ratio are limited in the analysis of natural minerals because of the low sr concentration the isobaric interference or small grains with complex textural contexts especially for transparent minerals such as feldspars in this study analytical results demonstrated that ablation rates in fs laser ablation were consistent for various samples 0 08 0 11 m per pulse but those in ns laser ablation were obviously material properties dependent such as the rates of 0 026 m per pulse and 0 144 m per pulse for feldspar and pyrite respectively in addition at similar energy fluences the sensitivities of sr in feldspars analyzed by the fs laser were 3 4 times higher than those analyzed by the ns laser due to the higher ablation efficiency of the fs pulse these advantages of the fs laser not only offer the benefit of eliminating or weakening the matrix effect during the laser ablation processes but also help to improve the analytical precision for transparent minerals we also demonstrated that the isobaric interferences of calcium dimers and argides caar caca and kr were dramatically reduced by factors of 6 5 11 7 and 5 12 5 in the presence of 6 12 ml min 1 n2 respectively furthermore with the addition of n2 12 ml min 1 the sensitivity of rb was inhibited resulting in a decrease of 1 47 times in rb sr signal ratios due to the effect of suppressing interferences by adding n2 both the stability and accuracy of the 87sr 86sr and 84sr 86sr ratios show improvement especially for the rb rich feldspars combining the advantages of the fs laser system with the addition of nitrogen an improved in situ sr isotope analytical method is then developed the satisfactory accuracy and precision of the 87sr 86sr ratio from natural plagioclases a k feldspar with high rb sr ratios 0 46 and a low sr clinopyroxene were obtained demonstrating the reliability of the proposed method four feldspars which have different contents of the major elements sr and rb showed homogeneous sr isotope compositions and were recommended as potential suitable reference materials for in situ sr isotope analysis as an application two plagioclases in mafic microgranular enclaves mmes with small grain sizes 200 300 m and wide ranges of rb sr ratios were analyzed and showed obvious variations of the 87sr 86sr ratios from core to rim which indicated that the proposed method in this study can provide high spatial resolution geochemical information for a single mineral

contrasting geophysical and geochemical signatures of a volcano at the axis of the wharton fossil ridge n e indian ocean the styx volcano is a prominent seamount 20 30 km diameter 2500 m high located at the axis of the wharton fossil spreading center n e indian ocean where seafloor spreading stopped 40 ma ago gravity modeling shows that it was emplaced on a weak lithosphere in agreement with an on axis origin when seafloor spreading was active the rocks dredged at the summit of the volcano are extreme typical alkaline basalts indicating a deep enriched mantle source we show here that most of this seamount was emplaced during the final stage of the seafloor spreading in the wharton basin we then discuss how alkaline magmatism can be emplaced at the summit of the styx

back arc extension in the southern andes a review and critical reappraisal the interpretation that the mafic rocas verdes green rocks complex of the southern andes represents part of the uplifted floor of a late jurassic to early cretaceous back arc basin has proved particularly useful in understanding the geological evolution of the southern andes the north scotia ridge and the antarctic peninsula clear field evidence of the back arc setting of the 9rocas verdes gabbro sheeted dyke pillow lava ophiolitic assemblages has encouraged fruitful petrological and geochemical comparison with mid ocean ridge and marginal basin basalts other onshore ophiolite complexes and archaean greenstone belts uncertainty still surrounds estimates of the original width and depth of the basin as well as the proportion of new mafic crust compared with relict sialic crust in the basin floor these questions are unresolved owing mainly to the considerable lower cretaceous turbiditic basin infill and the effects of mid cretaceous compressional deformation while the field relations clearly indicate that the rocas verdes basin is not an older piece of ocean floor trapped behind a volcanic arc it is not yet clear whether the basin is directly subduction related or falls in the category of back arc leaky transforms like the proto gulf of california or apparent rip off features like the andaman sea

the subduction of young lithosphere studies using a variety of techniques of the subduction beneath western south america and the southwest honshu kyushu region of japan indicate that volcano free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere the subduction is initiated at normal dip angles but the plate deforms at some depth 100 km under central peru to travel horizontally immediately beneath continental lithosphere the most plausible reason for this geometry is that the subducted plate is buoyant a model is developed constrained by age of the plate bathymetry and heat flow estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts if the high density tranformation of crustal basalt to eclogite is retarded by low temperatures the plate can remain buoyant to considerable depth for long periods of time heat flow data from western south america are consistent with this model

magma recharge contamination and residence times revealed by in situ laser ablation isotopic analysis of feldspar in volcanic rocks we illustrate a rapid and effective method for determining variations in 87sr 86sr across single plagioclase feldspar crystals using laser ablation interfaced to a multicollector inductively coupled plasma source mass spectrometer the speed of analysis proves to be a particular advantage permitting representative populations of crystals to be characterized or enabling reconnaissance analysis of rocks to test for mineral scale isotopic heterogeneity our results from magmatic crystals in lavas from el chichon volcano confirm trends previously obtained by the more time consuming microdrilling method for the first time with confidence the laser data allow us to associate abrupt changes in 87sr 86sr with distinct petrographic discontinuities such as dissolution surfaces in the crystal because the isotopic variations within crystals would be expected to equilibrate diffusively with time the width over which abrupt changes in 87sr 86sr occur can also be used to constrain timescales at which the crystal has been held at magmatic temperatures

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins