a geodynamic model of mantle density heterogeneity using cenozoic and mesozoic plate motion reconstructions we derive a model of present day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle the thermal buoyancy of these slabs is estimated from the observed thermal subsidence cooling of oceanic lithosphere slab velocities in the upper mantle are computed from the local convergence rate we assume that slabs cross the upper lower mantle interface and continue sinking into the lower mantle with a reduced velocity for a velocity reduction factor between 2 and 5 our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other we have also computed a synthetic geoid from our density model for a viscosity increase of about a factor of 40 from the upper to lower mantle our model predicts the first 8 spherical harmonic degrees of the geoid with statistical confidence larger than 95 and explains 84 of the observed geoid assuming that the model c21 and s21 terms are absent due to a long relaxation time for earth s rotational bulge otherwise 73 of the geoid variance is explained the viscosity increase is consistent with our velocity reduction factor for slabs entering the lower mantle since downwelling velocities are expected to scale roughly as the logarithm of viscosity loge 40 3 7 these results show that the history of plate tectonics can explain the main features of the present day structure of the mantle the dynamic topography induced by this heterogeneity structure consists mainly of about 1 km amplitude lows concentrated along the active continental margins of the pacific basin our model can also be used to predict the time variation of mantle heterogeneity and the gravity field we find that the age of the geoid defined as the time in the past before which the geoid becomes uncorrelated with the present geoid is about 50 m y our model for the history of the degree 2 geoid which is equivalent to the history of the inertia tensor should give us a tool to study the variations in earth s rotation pole indicated in paleomagiietic studies

investigating ultra low velocity zones in the southern hemisphere using an antarctic dataset abstract given limited seismic coverage of the lowermost mantle less than one fifth of the core mantle boundary cmb has been surveyed for the presence of ultra low velocity zones ulvzs investigations that sample the cmb with new geometries are therefore important to further our understanding of ulvz origins and their potential connection to other deep earth processes using core reflected scp waves recorded by the recently deployed transantarctic mountains northern network in antarctica the current study aims to expand ulvz investigations in the southern hemisphere our dataset samples the cmb in the vicinity of new zealand providing coverage between an area to the northeast where ulvz structure has been previously identified and another region to the south where prior evidence for a ulvz was inconclusive this area is of particular interest because the data sample across the boundary of the pacific large low shear velocity province llsvp the weddell sea region near antarctica is also well sampled providing new information on a region that has not been previously studied a correlative scheme between a large database of 1 d synthetic seismograms and the observed scp data demonstrates that ulvzs are required in both study regions modeling uncertainties limit our ability to definitively define ulvz characteristics but also likely indicate more complex 3 d structure given that ulvzs are detected within along the edge of and far from the pacific llsvp our results support the hypothesis that ulvzs are compositionally distinct from the surrounding mantle and are not solely related to partial melt ulvzs may be ubiquitous along the cmb however they may be thinner in many regions than can be resolved by current methods mantle convection currents may sweep the ulvzs into thicker piles in some areas pushing these anomalies toward the boundaries of llsvps

ophiolite melange zone records exhumation in a fossil subduction channel recent models propose that the exhumation of high pressure rocks occurs by means of return flow inside a low viscosity channel of serpentinite situated between the plates to test this hypothesis we investigated a serpentinite melange in the western alps which contains exotic mafic and metasedimentary tectonic blocks recording heterogeneous metamorphic evolutions and variable high pressure ages the peak metamorphic conditions range from eclogite to garnet blueschist facies the structural evidence and the pressure temperature paths of the different blocks suggest coupling between blocks and matrix at least in the blue schist facies 39 ar 40 ar dating indicates eclogite facies peak at ca 43 ma and blueschist facies peak at ca 43 and 40 ma in different blocks respectively these data point to diachronous metamorphic paths resulting from independent tectonic evolutions of the different slices we therefore propose that this melange formed during exhumation of subducted rocks equilibrated at different depths inside a subduction channel this mechanism can be extended to other serpentinite melanges in the alps and other orogens e g the cyclades the coast ranges of california for which a growing heterogeneity in the timing of metamorphic equilibration and of pressure temperature paths can be expected with further investigations

regional tilt of the mount stuart batholith washington determined using aluminum in hornblende barometry implications for northward translation of baja british columbia we have developed a new quantitative method to estimate paleohorizontal in granitic plutons using the aluminum in hornblende ah barometer the method is used to correct previously published paleomagnetic data from the 93 96 ma mount stuart batholith of the cascades mountains washington state for the effects of postemplacement tilting ah barometry was done on 46 samples from the batholith using the compositions of hornblende rims coexisting with the full mineral assemblage required for pressureestimation high contrastback scatteredelectronimagingwas usedtoensurethattheanalyzedhornblendeswerenotsignificantly affected by subsolidus alteration the success of the ah barometry is indicated by two observations first increases in the al content of the hornblendes are governed almost entirely by a pressure sensitive tschermak type substitution second amphibole plagioclase thermometry indicates that assemblage equilibration occurred at or very near magmatic conditions 650 c and that temperature has a negligible effect on our pressure estimates ah barometry results indicate that the depth of crystallization across the batholith decreases systematically from 0 3 gpa in the northwest to 0 15 gpa in the southeast consistentwithindependentbarometryforthecontactaureole ofthebatholithandregionalstructuralandstratigraphicrelations using a best fit planar tilt model and bootstrap analysis of uncertainties we estimate that the paleohorizontal plane has a strike of 43 630 4 and dip of 7 62 0 southeast 695 confidence our estimated paleohorizontal allows us to restore the paleomagnetic data of beck et al 1981 and to estimate the original paleolatitude of the mount stuart batholith beck et al found that the southern part of the batholith yielded a number of sites with a well defined high coercivity remanence the carrier of this remanence was not resolved but the following four lines of evidence stronglysuggestthatthepublisheddirectionswereacquiredshortly after emplacement of the batholith 1 the stable sites all came from the shallowest and most rapidly cooled portions of the batholith as indicated by our ah results and concordant k ar ages for hornblende biotite pairs 2 the high coercivity component was always normal in polarity which is consistent with emplacement of the mount stuart batholith at the end of the cretaceous long normal 3 sites from the batholith and the contact aureole gave similar directions 4 the directions show no indication of tilt related smearing after restoration the paleomagnetic data indicate 42 6 11 clockwise rotation and 3100 6 600 km of northward offset

franciscan complex olistostrome at crescent city northern california an olistostrome and bounding turbidites are exposed within the late mesozoic franciscan complex along the crescent city california coastline facies grade in character from mutti ricci lucchi 1978 mixed facies b c and d to f the olistostrome to mixed a b and e progressing upwards within the franciscan stratigraphic section the facies f unit outcrop is up to 600 m thick and extends 12 km along strike it consists of oblate to tabular blocks up to 200 m in maximum dimension of greenstone tonalite radiolarian chert limestone phyllite and greywacke dispersed in a scaly argillite matrix the olistostromal origin of the unit is indicated by depositional contacts with bounding turbidites by the presence of abundant recycled sedimentary clasts within the unit and by the presence of sedimentary breccias and associated dismembered slump folded turbidites both within the olistostrome and among subjacent turbidites sandstones are chiefly feldspathic litharenites that were very likely derived from the partially dissected late mesozoic sierran klamath magmatic are franciscan rocks record an early pervasive layer parallel flattening strain in such features as extensional faults necking and pinch and swell structures several scales of extensional faulting account for the juxtaposition of turbidites of different facies and or with varying degrees of stratal disruption the formation of sandstone lozenges and the formation of scaly foliation in the olistostrome matrix the latter resulted from the juxtaposition of lenticles with varying concentrations of silt and clay these were ultimately derived from the finer divisions of turbidite beds that were incorporated into the olistostrome the presence of gradational contacts between some sandstone olistoliths and the olistostrome matrix and of sandstone dykes that intrude fractures and associated drag folded turbidite beds indicate that franciscan sediments were not lithified during their early deformation these sediments were deposited in either a trench or trench slope basin and were first deformed either by gravity collapse of the trench slope cover or less likely by vertical loading beneath the toe of the accretionary wedge they later were folded during internal shortening of the growing franciscan accretionary prism

low p wave velocity at the base of the mantle a tool for investigating p wave vp structure at the base of the mantle is presented sks waves at distances around 107 are incident upon the core mantle boundary cmb with a slowness that allows for coupling with diffracted p waves pd at the base of the mantle the p wave diffraction occurs at both the sks entrance and exit locations of the outer core the resulting phase spdks arrives slightly later in time than sks having a wave path through the mantle and core very close to sks the difference time between sks and spdks most strongly depends on vp at the base of the mantle near sks core entrance and exit points digitized long period 5 15s observations from deep focus tonga events recorded by north american wwssn and canadian seismographic network stations and south american events recorded by european and eurasian wwssn stations exhibit anomalously large spdks sks difference times sks and the later arriving spdks are separated by several seconds more than predictions made by the global average prem model models having a pronounced low velocity zone in vp at the base of the mantle best predict the observations these models are perturbations of the prem model whereby the lowermost 50 100 km of the d layer has a negative vp gradient with the mantle side cmb vp reduced from prem by 5 to 13 0 km s at the base of the mantle

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

stress relaxation at the lower dislocation edge of great shallow earthquakes abstract the problem of post seismic stress evolution at the lower dislocation edge of great shallow earthquakes is investigated by a model reproducing a transcurrent boundary zone subject to a constant strain rate dislocation models with frictional stress threshold for slip arrest must be considered in order to study post seismic stress evolution in the proximity of dislocation edges it is found that the boundary zone is vertically divided in two regions an upper region where stress increases with time and a lower region where stress decreases dislocations which are nucleated in the seismogenic region can propagate downward into the aseismic region in this case earthquakes are followed by relaxation of the stress concentration at the lower dislocation edge this may have effects on the space time distribution of aftershocks

fragment of the coast range ophiolite and the great valley sequence in the san juan islands washington the decatur terrane is a small allochthonous fragment of upper jurassic oceanic crust with an overlying blanket of upper jurassic lower cretaceous sedimentary rocks in the san juan islands the stratigraphy and petrotectonic elements are dissimilar to those of coeval sequences in the pacific northwest oregon washington and southern british columbia but are virtually identical to those of the coast range ophiolite and overlying great valley sequence south of san francisco california this stratigraphic correlation together with regional considerations suggests that the decatur terrane was detached and transported during cretaceous time the resultant displacement is in excess of 1000 km

stratigraphy age and paleogeography of the eocene chuckanut formation northwest washington the eocene chuckanut formation of washington s north cascades comprises as much as 6000 m of alluvial strata and is one of the thickest nonmarine sequences in north america it is exposed in several disconnected outcrop belts that are remnants of what was probably an extensive fluvial system in western washington in this study seven stratigraphic members are defined in the main outcrop belt of the chuckanut formation near the town of bellingham their ages based on fission track zircon dates palynologic studies and correlation by mapping extend from the early to the late eocene three types of fluvial systems contributed to the chuckanut formation and are distinguished on the basis of their lithology sedimentology and petrology the first comprises fine load meandering river deposits of the lower eocene bellingham bay member and the middle eocene slide member sandstones are arkosic and had their main source in rapidly uplifted high grade metamorphic terranes in eastern washington the second type

a viscoelastic shear zone model of compressional and extensional plate boundaries a model is proposed describing the mechanical evolution of a shear zone along compressional and extensional plate boundaries subject to constant strain rate the shear zones are assumed as viscoelastic with maxwell rheology and with elastic and rheological parameters depending on temperature and petrology stress and strain are computed as functions of time and depth for both kinds of boundaries the model reproduces the existence of a shallow seismogenic zone characterized by a stress concentration the thickness of the seismogenic layer is evaluated considering the variations of shear stress and frictional strength on faults embedded in the shear zone assuming that a fault dislocation takes place the brittle ductile transition is assumed to occur at the depth at which the time derivative of total shear stress changes from positive to negative values the effects of different strain rates and geothermal gradients on the depth of the brittle ductile transition are studied the model predictions are consistent with values inferred from seismicity data of different boundary zones

geographic correlation between hot spots and deep mantle lateral shear wave velocity gradients abstract hot spot volcanism may originate from the deep mantle in regions exhibiting the earth s most pronounced lateral s wave velocity gradients these strong gradient regions display an improved geographic correlation over s wave velocities to surface hot spot locations for the lowest velocities or strongest gradients occupying 10 of the surface area of the core mantle boundary cmb hot spots are nearly twice as likely to overlie the anomalous gradients if plumes arise in an isochemical lower mantle plume initiation should occur in the hottest thus lowest velocity regions or in the regions of strongest temperature gradients however if plume initiation occurs in the lowest velocity regions of the cmb lateral deflection of plumes or plume roots are required the average lateral deflections of hot spot root locations from the vertical of the presumed current hot spot location ranges from 300 to 900 km at the cmb for the 10 30 of the cmb covered by the most anomalous low s wave velocities the deep mantle may however contain strong temperature gradients or be compositionally heterogeneous with plume initiation in regions of strong lateral s wave velocity gradients as well as low s wave velocity regions if mantle plumes arise from strong gradient regions only half of the lateral deflection from plume root to hot spot surface location is required for the 10 30 of the cmb covered by the most anomalous strong lateral s wave velocities we find that strong gradient regions typically surround the large lower velocity regions in the base of the mantle which may indicate a possible chemical in addition to thermal component to these regions

seismic evidence of partial melt within a possibly ubiquitous low velocity layer at the base of the mantle three source regions show evidence of a low velocity layer that is less than 15 kilometers thick on top of the core mantle boundary and require about a 3 1 ratio of shear to compressional velocity reduction which is consistent with partial melt layer thickness is correlated with travel time residuals of the seismic phases that are most sensitive to the lowermost mantle velocity these observations suggest that the layer is thinned beneath downwellings but is present everywhere along the core mantle boundary low viscosity accompanying partial melt can localize the upwelling of warmed mantle making the low velocity layer a plausible source of mantle plumes

seismic reflection images beneath puget sound western washington state the puget lowland thrust sheet hypothesis seismic reflection data show that the densely populated puget lowland of western washington state is underlain by subhorizontal paleogene and neogene sedimentary rocks deformed by west and northwest trending faults and folds from south to north beneath the lowland features seen on the seismic data include the horizontally stratified 3 5 km thick tacoma sedimentary basin the seattle uplift with south dipping 20 strata on its south flank and steeply 50 to 90 north dipping strata and the west trending seattle fault on its north flank the 7 5 km thick northward thinning seattle sedimentary basin the antiformal kingston arch and the northwest trending transpressional southern whidbey island fault zone swif interpreting the uplifts as fault bend and fault propagation folds leads to the hypothesis that the puget lowland lies on a north directed thrust sheet the base of the thrust sheet may lie at 14 to 20 km depth within or at the base of a thick block of basaltic crescent formation its edges may be right lateral strike slip faults along the base of the cascade range on the east and the olympic mountains on the west our model suggests that the seattle fault has a long term slip rate of about 0 25 mm year and is large enough to generate a m7 6 to 7 7 earthquake

paleomagnetism and tectonic significance of albian and cenomanian turbidites ochoco basin mitchell inlier central oregon 1 understanding continental growth and convergent margin dynamics associated with terrane accretion and modification of the cordilleran margin of north america is prevented by conflicts in paleogeographic models for major terranes oceanic plates and the north american margin we present new paleomagnetic data suggesting that the blue mountains superterrane located at an inboard portion of the cordilleran margin has undergone substantial northward translation and clockwise rotation relative to north america since mid cretaceous time positive baked contact conglomerate and fold tests provide evidence that the magnetization of these rocks dates from their deposition these results yield a mean direction of d 10 8 i 58 5 k 127 95 4 1 and n 11 comparison of magnetic inclinations and magnetic fabrics shows no evidence for inclination error a paleolatitude of 39 2 4 8 4 3 n is obtained indicating a paleolatitude anomaly of 15 9 4 1 northward translation of 1760 460 km and cw rotation of 37 7 3 with respect to north america since circa 93 ma application of an ad hoc 5 inclination error yields translation of 1200 460 km through comparison with other units in the western cordillera we infer that the mitchell inlier was located adjacent to the sierra nevada arc during mid cretaceous time and that the blue mountains province may correlate with the intermontane superterrane in british columbia other interpretations such as large inclination errors or an independent displacement history for the mitchell inlier are possible but unlikely more work is needed to fully test these hypotheses and resolve major questions of cordilleran tectonics

why large earthquakes do not nucleate at shallow depths although by definition tectonic earthquakes may nucleate anywhere within the seismogenic layer in almost all cases those earthquakes that become large nucleate near the base of the seismogenic layer because frictional strength and stress drop should increase with depth and in situ stress measurements suggest that the ambient stress increases with depth we have examined spontaneous rupture models with gradients of both average stress drop and average strength relative to ambient stress to determine whether this explains the observed phenomena we report here that ruptures that are nucleated with in the low stress drop shallow regions of the model are inhibited from propagating but those that are nucleated within the high stress drop regions can propagate over the entire fault plane

magma chamber profiles from the bay of islands ophiolite complex models of the oceanic lithosphere derived from constraints imposed by the geology of ophiolite complexes require a magma chamber to produce the cumulate and other plutonic portions of the oceanic crust and upper mantle evidence from the bay of islands ophiolite complex indicates that layered plutonic rocks have not formed solely due to differential crystal settling to the horizontal floor of a magma chamber but have formed largely by heterogeneous nucleation and in situ crystallization along gently inclined as well as steeply inclined bounding surfaces of a large magma reservoir coherent well defined and systematically oriented large scale arcuate patterns described by mesoscopic igneous layering have been identified in the plutonic sections of two ophiolite massifs in the bay of islands complex these layering patterns seem to outline the shape of a large continuously evolving steady state magma chamber that extended along a ridge axis and ended at a transform fault intersection

shock induced melting of mgsio3 perovskite and implications for melts in earth s lowermost mantle 1 new shock wave equation of state eos data for enstatite and mgsio3 glass constrain the density change upon melting of mg silicate perovskite up to 200 gpa the melt becomes denser than perovskite near the base of earth s lower mantle this inference is confirmed by shock temperature data suggesting a negative pressure temperature slope along the melting curve at high pressure although melting of earth s mantle involves multiple phases and chemical components this implies that the partial melts invoked to explain anomalous seismic velocities in the lowermost mantle may be dynamically stable index terms 3919 mineral physics equations of state 3944 mineral physics shock wave experiments 8124 tectonophysics earth s interior composition and state 1212 citation akins j a s n luo p d asimow and t j ahrens 2004 shock induced melting of mgsio3 perovskite and implications for melts in earth s lowermost mantle geophys res lett 31 l14612 doi 10 1029 2004gl020237

kinematic evidence for extensional unroofing of the franciscan complex along the coast range fault northern diablo range california franciscan metagraywacke immediately below the del puerto ophiolite an outlier of the coast range ophiolite in the northern diablo range was sheared during top to the east displacement on the coast range fault this represents normal faulting and extensional offset it was accompanied by attenuation of the coast range ophiolite and great valley sequence in the hanging wall along layer parallel normal faults that sole into the coast range fault extension occurred as the franciscan complex moved relatively west out from under north american lithosphere and across the subducting ocean plate below this effected a lengthening and thinning in the wedge of material above the down going plate presumably in response to instability brought about by subduction shallowing krueger and jones 1989 and accretion of the franciscan central belt in the latest cretaceous to early paleocene as a result blueschist facies terranes of the uppermost part of the franciscan complex are now juxtaposed directly against hanging wall units that bear only low grade metamorphism

a plate tectonic model for late jurassic ophiolite genesis nevadan orogeny and forearc initiation northern california recently published age and structural data allow the reconciliation of previously conflicting models for late jurassic genesis of the josephine smartville and coast range ophiolites and the nevadan orogeny in the klamath mountains and sierra nevada the resulting model is consistent with the mode of initiation location and geometry of the great valley forearc basin and with the lack of a significant forearc basin west of the klamath mountains the coast range ophiolite formed by backarc spreading west of an east facing intraoceanic arc soon thereafter a remnant arc was calved off the west side of this arc and the smartville ophiolite formed by backarc interarc spreading during this time the sierran phase of the nevadan orogeny began as the intraoceanic arc encountered the west facing continental margin arc of north america an east west trending calcalkaline dike swarm in the sierra nevada foothills may mark the trajectory of the colliding arcs at the initiation of the collision simultaneously a new subduction zone was initiated west of the collision suture zone and this new trench propagated southward thus trapping the coast range ophiolite in the new forearc area south of the klamath area intense deformation in the sierran region resulted from this collision and both magmatic arcs became inactive as the last remnant of intervening oceanic crust was subducted continued westward relative movement of the north american arc was permitted north of the sierra nevada owing to the lack of a colliding intraoceanic arc the result was the westward rifting of the continental margin arc by intraarc spreading which formed the josephine ophiolite in the klamath area the klamath phase of the nevadan orogeny resulted from contraction of the west facing intraoceanic arc and josephine backarc basin beneath the continental margin basal sediments of the great valley forearc basin were derived primarily from the sutured arc ophiolite terranes and were deposited on top of the coast range ophiolite the southern edge of the klamaths and the western side of the sierra nevada a new late mesozoic magmatic arc was superposed across the previously accreted terranes and formed the primary sediment source for the cretaceous forearc basin

deep mantle seismic modeling and imaging detailed seismic modeling and imaging of earth s deep interior is providing key information about lower mantle structures and processes including heat flow across the core mantle boundary the configuration of mantle upwellings and downwellings phase equilibria and transport properties of deep mantle materials and mechanisms of core mantle coupling multichannel seismic wave analysis methods that provide the highest resolution deep mantle structural information include network waveform modeling and stacking array processing and 3d migrations of p and s wave seismograms these methods detect and identify weak signals from structures that cannot be resolved by global seismic tomography some methods are adapted from oil exploration seismology but all are constrained by the source and receiver distributions long travel paths and strong attenuation experienced by seismic waves that penetrate to the deep mantle large and small scale structures with velocity variations ranging from a fraction of a percent to tens of percent have been detected and are guiding geophysicists to new perspectives of thermochemical mantle convection and evolution

teleseismic slowness azimuth station corrections for the international monitoring system seismic network abstract the prototype international data center pidc in arlington virginia has been developing and testing software and procedures for use in the verification of the comprehensive test ban treaty after three years of operation with a global network of array and three component stations it has been possible to characterize various systematic biases of those stations that are designated in the treaty as part of the international monitoring system ims these biases include deviations of azimuth and slowness measurements from predicted values caused largely by lateral heterogeneity for events recorded by few stations azimuth and slowness are used in addition to arrival time data for location by the pidc corrections to teleseismic azimuth and slowness observations have been empirically determined for most ims stations providing data to the pidc application of these corrections is shown to improve signal association and event location at some stations an overall systematic bias can be ascribed to local crustal structure or to unreported instrumental problems the corrections have been applied in routine operation of the pidc since february 1998

the structure of the core mantle boundary from diffracted waves diffracted p and s waves pd sd traveling around the core mantle boundary cmb of the earth give us information about the velocity structure and therefore the thermochemistry of d the base of the earth s mantle by examining pd and sdarrivals we determined the apparent ray parameter for different regions at the base of the mantle by comparing the data slownesses to those found from reflectivity synthetic seismograms we were able to quantify d average velocities using these averaged velocities with a thermochemical modeling of lower mantle minerals using a birch mumaghan equation of state we have been able to make chemical and physical inferences as to the causes of lateral variations at the cmb examinations found significant lateral heterogeneity at the base of the mantle amounting to 4 for both p and s velocities these velocities did not always vary in parallel and the poisson ratio varied regionally by almost 6 the most unusual region of the cmb found was under indonesia where velocities 3 slower than the preliminary reference earth models were found adjacent to a region of faster than average velocities these regions currently correspond to areas of core up welling and down welling respectively found by voorhies 1986 which if mostly held in place by core mantle coupling might cause a flux of heat and iron into the mantle making the anomaly both thermally and chemically derived at the cmb under the northern pacific rim the fastest shear velocities were found but the same region yielded slower than average p velocities while the presence of fast shear velocities here would support the idea that we are seeing the cold dregs of mantle convection perhaps continuing down from the north pacific subduction zones the presence of slow p velocities suggests additional complications our thermochemical modeling suggests that the d poisson ratio is very sensitive to variations in the silicate oxide ratio and that a decrease in the amount of perovskite relative to magnesiowustite may play an important role in this region

remagnetization during cretaceous normal superchron in eastern san juan islands wa implications for tectonic history abstract the eastern san juan islands expose an island arc ophiolite sequence and one or more ocean floor terranes cover rocks of the former including red radiolarian tuff and arc derived sandstone have no penetrative fabric and lack high pressure minerals in contrast pillow basalts and ribbon cherts of the latter are strongly deformed and the overlying metagraywacke is folded with a penetrative axial plane cleavage all ocean floor rocks have aragonite in veins indicative of higher pressure conditions than experienced by the island arc rocks a downward magnetization is common to the ocean floor rocks consistent with widespread remagnetization during the cretaceous normal superchron this magnetization is carried by magnetite and in red rocks by hematite fluid flow during or after deformation and high p metamorphism likely facilitated remagnetization at low temperature the magnetic directions are somewhat scattered but most are moderately to steeply down to the southeast they are far from directions expected with or without northward transport of baja bc rotation of observed directions into agreement with those expected for either present or more southerly locations also rotate structural elements in the host rocks three alternative rotation histories explored here all produce structural reconstructions that differ considerably from the present structure a tectonic history of the eastern san juan islands based on any of these reconstructions would be superior to one based on present attitudes of structures this is because at least a constraint that the restored paleomagnetic directions match a paleomagnetic field direction is satisfied choices among alternative reconstructions would become clearer with better understanding of relative ages of specific structures and their magnetizations improved geologic understanding of the regional setting where the rocks could have been deformed and remagnetized would also help

criteria helpful in recognizing shear zone and diapiric melanges examples from the hoh accretionary complex olympic peninsula washington a study of the middle miocene hoh melanges exposed on the western olympic peninsula washington provides criteria for differentiating between shear zone and diapiric melanges the shear zone melange is characterized by a consistent foliation orientation throughout the outcrop intense pervasive scaly foliation uniform fold vergence directions exotic clasts predominantly phacoidal clasts and consistent clast long axis lineations the diapiric melange is characterized by radial foliation in map view well developed scaly foliation at the contacts and poorly developed scaly foliation in the center of the melange opposite fold vergence directions across the melange and rare exotic clasts additionally clasts vary in shape from phacoidal at the contacts to angular in the center in the diapiric melange large clasts are found only in the center of the melange and clast long axes vary from tightly clustered at the contacts to near random orientations in the center the systematic variation in clast size in the diapir may be due to increased shear at the margins and or clast migration to the center of the diapir caused by a velocity viscosity contrast interpretation of field data indicates that some clasts rotate continuously in the matrix yet clast long axes have a definite preferred orientation this contradiction may be resolved by theoretical models indicating that clasts rotate fastest at high angles to the shear plane thus although clasts above a critical aspect ratio asymptotically approach stability parallel to the shear plane the long axes of rotating clasts spend a proportionally greater amount of time close to the shear plane parasitic folds in the matrix indicate that clast long axes align parallel to transport direction small scale folds in the matrix clast long axis lineations and the mean foliation indicate that the hogsback section of the hoh melange was emplaced by a high angle oblique thrust directed to n10 e

evaluation of stresses in two geodynamically different areas stable foreland and extensional backarc areas which are geodynamically different have different behaviors both in their thermal regime and seismic activity a stable area has a geotherm which can be considered as standard extensional and compressional areas have respectively high and low temperature gradients the italian region includes different geodynamical areas and all such situations are present we consider the apulian platform as an example of a stable area and the tuscany latium as an example of an extensional area for both of them the present geotherms are calculated taking into account for the tuscany latium its thermal history assuming that each region is subject to a constant strain rate the stresses are calculated as functions of depth and time the rheological behavior is assumed to be linear viscoelastic with viscosity dependent on temperature and elastic parameters dependent on lithology the geothermal profile and the rheological structure of the lithosphere remarkably affect the processes of stress accumulation which control the distribution of seismic activity the abrupt decrease of the temperature gradient at the moho produces considerably higher stress values with respect to the case of uniform gradient thus favoring subcrustal seismicity in the case of a standard temperature gradient subcrustal seismicity is predicted and a gap in seismicity indicating a soft intracrustal layer exists if there is a discontinuity in rheology by contrast in the case of a high temperature gradient subcrustal seismicity is not to be expected even in the presence of a discontinuity in rheology since subcrustal temperatures are already too high to permit a sufficient stress accumluation

a model of interseismic stress evolution in a transcurrent shear zone abstract it has been observed that most shallow earthquakes occur in a seismogenic layer which extends to a depth of a few tens of kilometres while at greater depths the relative plate motion must take place aseismically such behaviour is reproduced by a model where a wide deformed fault region is encompassed by two transcurrent plates and subjected to a constant strain rate the shear zone is treated as a viscoelastic body for which a power law constitutive relation is employed temperature and therefore rheology depend on depth z also rigidity depends on depth the model determines a maximum depth h for earthquake nucleation on the faults in the shear zone if a frictional resistance linearly increasing with depth is assumed the interseismic shear stress evolution on a vertical fault is obtained analytically for n 1 2 3 and 4 where n is the power law exponent it is found that the rate of stress increase does not change appreciably as a function of n for z h while the effect of non linearity becomes more sensible at larger z moreover for z h the state of stress as obtained from this model is very different from estimates obtained from purely viscous models even for much longer times than are considered in seismology

intraplate lithosphere deformation and the strength of the lithosphere summary the intraplate deformation of continental lithosphere in response to applied stress has been investigated using a mathematical model which incorporates the elastic ductile and brittle response of lithosphere material ductile deformation is assumed to be controlled in the crust by dislocation creep in quartz and in the mantle by dislocation creep and plasticity in olivine brittle failure is predicted using modified griffith theory a fundamental feature of the model is the redistribution of stress within the lithosphere following stress release by both ductile and brittle deformation this redistribution produces high levels of stress in the middle or lower crust immediately above the elastic ductile or brittle ductile transition lithosphere deformation is shown to be critically dependent on the temperature structure of which surface heat flow is a convenient indicator for higher geothermal gradients the release of stress in the lower lithosphere by ductile deformation is more rapid and complete and results in large stress levels in the upper lithosphere for sufficiently large applied stresses or steep geothermal gradients the stress levels in the upper and middle crust will cause complete fracture of the upper lithosphere whole lithosphere failure wlf then results by continued brittle and ductile deformation causing geologically significant strains the critical value of applied stress required to give wlf has been calculated for both tensional and compressional deformation as a function of surface heat flow the predicted lithosphere bulk strength is then compared with expected levels of intraplate stress arising from plate boundary forces and isostatically compensated loads which are thought to give net stress levels in the continental lithosphere in the range 0 25 to 0 25 kb using these expected maximum stress levels the model predicts significant extensional deformation in regions of moderate heat flow with q c 60mwm e g central europe as well as for areas of high heat flow like the basinand range province significant compressional deformation is predicted for areas of high heat flow with q c 75 mwm 2 but only for restricted conditions of stress combination these results are in good agreement with

implications of stress concentration on a strike slip fault in an elastic plate subject to basal shear stress summary we consider a long strike slip fault in a lithosphere modelled as an elastic slab to the base of the slab a shear stress distribution is applied which simulates the viscous drag exerted by the asthenosphere the resulant stress on the fault plane may directly fracture the lithosphere in its brittle upper portion alternatively it may give rise at first to a stable aseismic sliding in the lower portion in the latter case stress concentration due to the deep aseismic slip is the relevant feature of the pre seismic stress acting on the upper section of the lithosphere the two cases are examined by use of dislocation theory and their observable effects compared different depths of the aseismic slip zone and the presence or absence of a uniform friction on the seismic fault are allowed for if the model is applied to the san andreas fault region where a steady sliding condition actually seems to be present at shallow depth it turns out that the slip amplitudes commonly associated with large earthquakes are consistent with average basal stress values which can be substantially lower than a few bars a value often quoted as the steady state basal stress due to a velocity gradient in the upper asthenosphere

a compositional origin to ultralow velocity zones we analyzed vertical component short period scp waveforms for 26 earthquakes occurring in the tonga fiji trench recorded at the alice springs array in central australia these waveforms show strong precursory and postcursory seismic arrivals consistent with ultralow velocity zone ulvz layering beneath the coral sea we used the viterbi sparse spike detection method to measure differential travel times and amplitudes of the postcursor arrival scsp and the precursor arrival spcp relative to scp we compare our measurements to a database of 340 000 synthetic seismograms finding that these data are best fit by a ulvz model with an s wave velocity reduction of 24 a p wave velocity reduction of 23 a thickness of 8 5 km and a density increase of 6 this 1 1 vs vp velocity decrease is commensurate with a ulvz compositional origin and is most consistent with highly iron enriched ferropericlase

role of extensional tectonics in exhumation of eclogites and blueschists in an oblique subduction setting northeastern venezuela eclogites and blueschists in northeastern venezuela belong to a mid to late cretaceous subduction complex fore arc terrane related to right oblique subduction of the proto caribbean plate beneath the farallon plate along the leeward antilles volcanic island arc these rocks were metamorphosed at high pressures and subsequently underwent retrogradation at decreasing pressures but at constant or slightly increasing temperatures they underwent five phases of deformation three synmetamorphic and two postmetamorphic the major principal extensions x axes of each of the three synmetamorphic deformations are subparallel to the volcanic arc the extension parallel to the arc and the associated exhumation and uplift of the subduction complex fore arc terrane may have resulted primarily from an increase of the arc parallel displacement rate of this terrane caused by an increase of obliquity of plate convergence

localized boundary layer below the mid pacific velocity anomaly identified from a pcp precursor dense record sections from deep earthquakes in fiji and argentina recorded on hundreds of short period stations in california at distances of 81 to 85 are used to investigate the detailed p wave velocity structure above the core mantle boundary cmb in the fiji data a secondary phase arriving 2 to 4 s after the direct p is identified as a precursor to pcp this phase provides good evidence for a reflection off the top of a thin low velocity layer above the cmb comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5 10 compared to the overlying mantle a record section from an argentina event does not show the pcp precursor indicating that the low velocity layer is not a global feature this thin low velocity layer is in the same place as a much larger s wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the cmb under the mid pacific

phase transition and density of subducted morb crust in the lower mantle abstract phase relations mineral chemistry and density of a natural mid oceanic ridge basalt morb composition were investigated up to 134 gpa and 2300 k by a combination of in situ x ray diffraction measurements and chemical analyses using transmission electron microscope tem results demonstrate that the morb composition consists of mgsio 3 rich perovskite stishovite casio 3 perovskite and cafe 2 o 4 type al phase in the upper part of the lower mantle the most abundant mineral of mgsio 3 rich perovskite undergoes phase transition to a cairo 3 type post perovskite phase above 110 gpa and 2500 k post perovskite phase is similar in composition to perovskite except considerably high na 2 o content stishovite transforms to cacl 2 type sio 2 phase above 62 gpa and 2000 k and further to pbo 2 type phase above 110 gpa pbo 2 type sio 2 phase includes large amount of al 2 o 3 which significantly expands its stability relative to cacl 2 type phase phase transition of casio 3 perovskite from tetragonal to cubic was also observed with increasing temperature cafe 2 o 4 type al phase is stable to the bottom of the mantle the density of morb crust was calculated using volume data combining with measured chemical compositions and calculated mineral proportions the former morb crust is denser than the average lower mantle at all depths greater than 720 km contrary to earlier predictions the subducted basaltic crust may have accumulated at the base of the mantle

shallow earthquakes in a viscoelastic shear zone with depth dependent friction and rheology summary most crustal earthquakes of the world are observed to occur within a seismogenic layer which extends from the earth s surface to a depth of a few tens of kilometres at most a model is proposed in which the shear zone along a transcurrent plate margin is represented as a viscoelastic medium with depth dependent power law rheology a frictional resistance linearly increasing with depth is assumed on a vertical transcurrent fault within the shear zone such a model is able to reproduce a continuous transition from the brittle behaviour of the upper crust to the ductile behaviour at depth assuming that the shear zone is subjected to a constant strain rate from the opposite motions of the two adjacent plates it is found that there exists a maximum depth h below which tectonic stress can never reach the frictional threshold this may be identified as the maximum depth of earthquake nucleation the value of h is consistent with observations for plausible values of the model parameters the stress evolution in the shear zone is calculated in the linear approximation of the constitutive equation a change in rigidity with depth which is also introduced in the model may reproduce the high vertical gradient of shear stress which has been measured across the san andreas fault and the fact that most earthquakes are nucleated at some depth in the seismogenic layer a crack which drops the ambient stress to the dynamic frictional level is then introduced in the model to this aim a crack solution is employed without a stress singularity at its edges which is compatible with a frictional stress threshold criterion for fracture a constraint on the vertical friction gradient is obtained if such cracks are assumed to be entirely confined within the seismogenic layer

unmetamorphosed sedimentary melange with high pressure metamorphic blocks in a nascent forearc basin setting abstract melanges crop out within unmetamorphosed basal great valley group gvg forearc basin strata and between gvg and the underlying coast range ophiolite cro in the san francisco bay region of coastal california these melanges include high pressure hp metamorphic blocks of the franciscan subduction complex that structurally underlies the unmetamorphosed cro as well as blocks of gvg and cro the matrix consists of foliated shale and serpentinite locally interleaved at centimeter scale the melanges strike and dip parallel to bounding gvg sandstones and conglomerates the matrix locally consists of sedimentary breccia and conglomerate made up of clasts of serpentinite and shale gvg sandstones within and bounding the melanges have detrital serpentinite clasts the field relationships indicate a sedimentary origin of the melanges as olistostromal deposits within latest jurassic to earliest cretaceous basal gvg the melanges correlate to units along the eastern margin of the northern coast ranges about 250 km to the north with restoration of post subduction dextral faulting and differ from the latter in the higher proportion of shale stronger matrix foliation and common occurrence of hp blocks similar units may have mistakenly been assigned to the franciscan owing to the foliated nature of the matrix and occurrence of hp blocks this and the broad distribution of localities indicate that these deposits are more widespread than previously believed exhumation rates of coarse grained hp melange blocks may have been 2 to 10 mm yr or higher based on the ages of similar blocks in the coast ranges burial depth of the blocks and depositional age of the enclosing strata exhumation and deposition of franciscan blocks in these gvg melanges predates preserved accretion of similar materials in the adjacent franciscan by at least 30 m y suggesting subduction erosion of previously accreted material or exhumation of the blocks in forearc serpentinite shale diapirs

polyphase brittle and ductile deformation of late jurassic ophiolitic basalt argillite matrix melange and stratigraphic overlap sequence northwestern washington state abstract a belt of jurassic to cretaceous ophiolitic rocks borders the western margin of the u s cordillera and stretches from central california to northwestern washington state the northern end of this belt lies between the san juan islands and the northwest cascades within this region ophiolitic rocks consist of a succession of oceanic and arc affinity igneous and sedimentary rocks which form a sedimentary melange and sedimentary overlap sequence which is imbricated during the mid cretaceous the melange contains blocks and olistoliths of peridotite plagiogranite chert basalt and volcanoclastic conglomerate which range in size from a meter to over 1 km and are contained within a matrix of argillite and volcanoclastic breccia and conglomerate peridotites were exposed to the sub aqueous surface along serpentinized shear zones prior to their incorporation into the melange and the sedimentary matrix of the melange underwent brittle deformation during the earliest stages of its structural history melange rocks are overlain in angular unconformity by a jura cretaceous arc sourced sedimentary succession which is at least 500 meters thick and passes upward from a basal breccia containing clasts of plagiogranite gabbro tonalite chert and basalt into argillite containing late jurassic radiolarians the argillite is overlain by poorly sorted greywacke and conglomerate with clast populations similar to those of the basal breccia the conglomerate fines upward into a massive to bedded feldspathic lithic arenite and greywacke that yields mid cretaceous detrital zircons the overlap succession and the melange are deformed by two generations of highly penetrative structures d1a and d1b which produced north to east vergent tight to isoclinal folds and axial planar pressure solution cleavages all units are further deformed by three generations of penetrative structures the successively younger nne to nw ne and e w to wnw trending folds have foliations that cross cut the earlier structural fabrics and faults formation of the melange required differential elevations during the time of deposition and the presence of rocks which are sourced from both arc and oceanic crust extension within the forearc provides a mechanism to exhume peridotites and generate differential topography for arc and oceanic affinity rocks to erode and be incorporated into the melange as part of olistostromal deposits

structural styles in mesozoic and cenozoic melanges in the western cordillera of north america the term melange is currently used to describe several different kinds of mudstone rich rocks that are broadly characterized by an obscure stratigraphy stratal disruption or a chaotic block in matrix fabric four types of melange which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition are particularly widespread and distinctive type i includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer parallel extension type ii consists of similarly deformed thin layers of green tuff radiolarian ribbon chert and minor sandstone originally interbedded with black mudstone disruption in both types i and ii which probably occurred while the sediments were incompletely consolidated has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation type iii comprises inclusions of diverse shapes sizes and compositions enveloped in a locally scaly pelitic matrix the ultimate source of fragments is obscure because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks although some type iii melanges may have originated deep within accretionary prisms final emplacement as olistostromes muddy debris flow deposits or mud diapirs seems likely type iii melanges are mechanically analogous to scaly sheared serpentinites many probably have been tectonically remobilized or even intruded into shallow level fault zones type iv consists of lenticular inclusions bounded by an anastomosing network of subparallel faults their fabric records progressive slicing in brittle fault zones each of the four types of melange described here could in theory have formed in a variety of settings on or within an accretionary wedge at an active convergent margin none can yet be singled out as a uniquely diagnostic subduction melange

thermochemical structures within a spherical mantle superplumes or piles 1 heterogeneous compositional mantle models are frequently invoked to explain some observations obtained from geochemistry and seismology in particular two regions in the earth s lower mantle under the pacific and africa are often interpreted as being either piles or superplumes of dense material we perform numerical modeling of thermochemical convection in a three dimensional spherical geometry to investigate whether the geometry can focus a dense chemical component into a small number of isolated rounded piles or superplumes of material we study the effect of temperature and compositionally dependent viscosity and we find two different modes of dense layer deformation temperature dependent rheology leads to a low viscosity dense layer which is passively swept aside by downwellings into linear piles that are spread throughout the entire lower mantle the addition of compositionally dependent rheology in which the dense layer is more viscous results in active deformation of the dense material forming large isolated superplumes our results indicate that piles and superplumes are separate features which in general do not occur together and in order for isolated rounded superplumes to form an intrinsic compositional viscosity increase is required for the dense material

a very slow basal layer underlying large scale low velocity anomalies in the lower mantle beneath the pacific evidence from core phases a multi phase analysis using long period world wide standardized seismograph network and canadian network data has been conducted using core phases for deep focus events from the southwest pacific these include sks s2ks svd iff and sp dks the last phase emerges from sks near 106 and is associated with a p wave diffracting along the bottom of the mantle patterns in s2ks sks differential travel times t s2ks sks correlate with those in sp dks sks t spdks sks t s2ks sks values strongly depend on variations in v s structure in the lower third of the mantle whereas t spdks sks values mainly depend on v p structure and variations in a thin zone 100 km or less at the very base of the mantle anomalously large t s2ks sks and t spdks sks values relative to the preliminary reference earth model prem are present for fiji tonga and kermadec events recorded in north and south america along with anomalously large sv diff amplitudes well into the core s shadow more northerly paths beneath the pacific to north america for indonesian and solomon events display both prem like and anomalous times a model compatible with the observations is presented and contains a thin very low velocity layer at the base of the mantle that underlies the large volumetric lower mantle low velocity regions in the southwest pacific a low velocity layer of 20 100 km thickness with reductions of up to 5 10 relative to prem can reproduce t spdks sks as well as sv diff amplitudes large scale more than 1000 km lower mantle v s heterogeneity 2 4 can explain long wavelength trends in t s2ks sks the exact thickness and velocity reduction in the basal layer is uncertain owing to difficulties in resolving whether anomalous structure occurs on the source and or receiver side of wavepaths at the cmb

active displacement partitioning and arc parallel extension of the aleutian volcanic arc based on global positioning system geodesy and kinematic analysis global positioning system geodesy and structural analysis of parts of the aleutian island chain support the interpretation of forearc migration westward along right lateral transcurrent faults measured displacements are essentially arc parallel in the eastern unalaska island and western attu island parts of the arc and vary from 3 1 mm yr to 31 4 mm yr respectively at the center of the arc adak subduction of the amlia fracture zone disrupts the pattern of arc parallel displacement and records an upper plate velocity of 9 6 mm yr parallel to plate motion where active displacement partitioning accommodates 30 50 of the strike slip component of plate convergence in the area of impeded subduction partitioning ceases suggesting that the process is sensitive to the degree of coupling across the plate boundary megathrust the differential arc parallel displacements observed at the ends of the arc together with structural relations indicate substantial arc parallel extension which may play an important role in exhumation of high pressure low temperature metamorphic rocks in ancient arc systems

spdks analysis of ultralow velocity zones beneath the western pacific 1 we collected a new data set of 1354 broadband spdks waveforms sampling the western pacific ocean region these data indicate that multiple ultralow velocity zones ulvzs exist in this region we compared these data to 2 5 d synthetic seismograms computed with the psvaxi method for a suite of 517 ulvz models the region beneath the north and south philippine sea shows no evidence for ulvz presence the region beneath the coral sea shows a large ulvz which is approximately 700 700 km in lateral dimensions and up to 20 km thick multiple small scale ulvzs approximately 180 180 km and up to 10 km thick are inferred beneath the south china sea our findings are consistent with previous efforts utilizing the scp seismic phase and provide additional constraints on ulvz position and size

perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon abstract the formation of oceanic plagiogranite has been attributed primarily to either 1 extreme fractional crystallization of a mantle melt or 2 partial melting of hydrated mafic crust with support for the latter from field evidence and recent melting experiments remelting of hydrothermally altered ocean crust could yield rocks and minerals with diverse primary magmatic 18 o values forming proximal to the magmatic center where crustal growth is occurring to constrain the magmatic 18 o of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis we characterized the 18 o of zircons in 22 plagiogranite samples tonalite and trondhjemite from 8 different ophiolites and one dacite sampled along the east pacific rise using secondary ion mass spectrometry sims the 18 o values of 202 magmatic zircons from ophiolites range from 3 9 to 5 6 n 244 spots average 4 9 0 6 2sd extending 1 below typical zircon in equilibrium with mantle and from gabbroic massifs along slow spreading mid ocean ridges 4 7 5 9 east pacific rise dacite zircons range from 4 6 to 5 0 n 12 spots plagiogranite from the dike gabbro transition zone of the northern oman ophiolite yield the lowest 18 o zrn with rock average values of 4 3 5 0 the low 18 o values are best explained by remelting of crust altered by hydrothermal fluids with seawater like isotopic compositions at high temperatures possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high temperature hydrothermal system in the overlying crust if the partial melt was assimilated into a fractionating melt lens with morb like 18 o as envisioned for km scale plagiogranite bodies in oman up to 20 contamination by a protolith with 18 o 2 would be required previous oxygen isotope constraints from quartz in oman plagiogranite suggested melting of both high and low 18 o crust had occurred comparison of quartz zircon pairs indicates that quartz has been modified in most samples and we find no evidence for the involvement of high 18 o rocks during plagiogranite formation

subducted sedimentary serpentinite melanges record of multiple burial exhumation cycles and subduction erosion abstract serpentinite matrix melanges give insight into large scale convergent plate margin processes particularly because of the derivation of the serpentinite from oceanic mantle similar to shale matrix melanges a field geologist may easily recognize the sedimentary origins of little deformed serpentinite matrix melanges but melanges within accretionary prisms have undergone significant deformation and recrystallization of matrix serpentinite melanges of the franciscan subduction complex of california have a seemingly intact and foliated matrix such exposures contrast sharply with the granular undeformed sedimentary serpentinite melanges of the coeval great valley group gvg forearc basin deposits that depositionally overlie coast range ophiolite that structurally overlies the franciscan nonetheless franciscan serpentinite melanges display evidence of sedimentary origins including sedimentary breccia composed of exotic block material tolay ridge sedimentary serpentinite breccia panoche pass road basal serpentinite conglomerate with exotic clasts sunol regional wilderness and serpentinite sandstones and conglomerates including a basal conglomerate overlying coherent metagraywacke tiburon peninsula these examples record two burial exhumation cycles to blueschist facies depths in addition a melange breccia in the panoche pass area may have components that record three burial exhumation cycles to blueschist or greater depth exhumation rates for various cycles ranged from about 1 2 to 10 mm year the tiburon peninsula serpentinite melange occupies the structurally highest horizon in the franciscan of the san francisco bay area and regional field relationships indicate deposition at ca 100 ma apparently about 65 ma of subduction erosion non accretion followed initiation of franciscan subduction in this region the oldest franciscan serpentinite melanges are at least 35 ma younger than sedimentary serpentinites of the gvg subduction erosion may have facilitated reworking of forearc sedimentary serpentinite deposits into the trench evidence of multiple burial exhumation cycles indicates reworking of subduction complex material consistent with existence of clastic units that have fossils indicative of significantly older ages than detrital zircon ages

depth of the brittle ductile transition in a transcurrent boundary zone a model is proposed which describes the boundary zone between two transcurrent plates as a viscoelastic body with rheological properties changing with depth in this model the brittle ductile transition is defined as the depth at which the time derivative of shear stress changes from positive to negative values variations of this depth are studied as functions of geothermal gradient rheological parameters and strain rate using a power law rheology with exponent ranging from 1 to 4 stress relaxation in the ductile zone is controlled by a local characteristic time which depends on petrology temperature and in the case of non newtonian rheology on strain rate the composition and the hydration degree of crustal rocks may also sensibly influence the depth of the brittle ductile transition the model predictions are compared with observations regarding the san andreas imperial valley and north anatolian faults it is found that values ofn from 1 to 3 are more appropriate to reproduce the transition depth inferred by the seismicity distribution

coupling of early tertiary extension in the great valley forearc basin with blueschist exhumation in the underlying franciscan accretionary wedge at mount diablo california in the california coast ranges east of san francisco post 3 5 ma folding and erosion of the mount diablo anticline have created unusual three dimensional exposures of late mesozoic cenozoic stratigraphic and structural relations among rocks of the franciscan subduction complex coast range ophiolite and great valley forearc basin these relations offer new insights into the kinematics of late cretaceous early tertiary synsubduction extension attenuation and blueschist exhumation within the ancestral california forearc region map relations and subsurface data reveal that marine strata of the great valley forearc basin northeast of mount diablo accumulated in an 100 km long north south trending extensional graben system normal faults in the graben system cut steeply downsection through late cretaceous and early tertiary growth strata and terminate against the clayton marsh creek fault which is at a low angle to bedding and is interpreted to be the basal detachment for part of the graben system the clayton marsh creek fault in turn is linked to a system of faults that juxtapose blueschist facies rocks of the underlying ancestral franciscan accretionary prism against attenuated remnants of forearc crust that were never deeply buried including the coast range ophiolite and basal great valley strata these faults are the local expression of the coast range fault in this area franciscan rocks exposed beneath the faults were metamorphosed at 20 km depths after 108 ma based on jadeitic pyroxene and 108 ma detrital zircons u pb in metagraywackes apatite fission track data indicate that these blueschist facies rocks were subsequently uplifted and exhumed from 9 km to 3 km depths in latest cretaceous early tertiary time coeval with the subsidence and extension in the structurally overlying forearc graben system much of their earlier rise from 20 km to 9 km depth presumably also occurred coeval with graben development although our data cannot determine this directly these relations demonstrate that the california forearc around mount diablo was in an extensional regime undergoing active sedimentation during major franciscan exhumation and thus support models for exhumation of franciscan blueschist facies rocks via synsubduction extensional processes rather than via erosional processes

processes in continental collision zones preface abstract formation and exhumation of high pressure hp to ultrahigh pressure uhp metamorphic rocks in continental subduction zones are the two fundamental geodynamic aspects of collisional orogensis this volume is based on the session 08c titled geochemical processes in continental collision zones at goldschmidt 2010 in knoxville usa it focuses on micro to macro scale processes that are temporally and spatially linked to different depths of crustal subduction exhumation and associated mineralogical changes they are a key to understanding a wide spectrum of phenomena involving hp uhp metamorphism and syn post collisional magmatism papers in this volume report progresses in petrological geochronological and geochemical studies of uhp metamorphic rocks and their derivatives in china with tectonic settings varying from arc continent collision to continent continent collision microbeam in situ analyses of metamorphic and magmatic minerals are successfully utilized to solve various problems in the study of continental deep subduction and uhp metamorphism in addition to their geochronological applications to dating of hp to uhp metamorphic events during continental collision microbeam techniques have also served as an efficient means to recognize different generations of mineral growth during continental subduction zone metamorphism furthermore metamorphic dehydration and partial melting of uhp metamorphic rocks during subduction and exhumation are highlighted with respect to their effects on fluid action and element mobilization these have provided new insights into chemical geodynamics in continental subduction zones

the geodynamic evolution of the eastern eurasian margin in mesozoic times abstract the cenozoic features of eurasia such as marginal seas and intracontinental deformation are unraveled using available paleomagnetic and field data the late cretaceous shape of eurasia is then reconstructed from the examples of sw japan and south russian far east the major features of the mesozoic eurasian margin suture zones marked by ophiolites or high pressure metamorphic rocks olistostromes characterized by the lithology of the olistoliths calc alkaline magmatism and strike slip faults are outlined correlations of the ophiolitic belts and the olistostromes along the eastern eurasian margin from borneo to siberia allow the definition of four microcontinents the west philippines south japan and the anuy and okhotsk sea microblocks from south to north the mesozoic structure of the eurasian margin is due to three processes 1 1 oceanic subduction and the accretion of subduction complexes 2 2 the collision of these microcontinents with eurasia between the late jurassic and the late cretaceous and 3 3 the reactivation of the ophiolitic sutures by left lateral wrench faulting

tectonic significance of post accretion rifting of a mesozoic island arc terrane in the northern sierra nevada california this paper presents a new interpretation for the tectonic evolution of the smartville complex that forms the western belt of the foothills metamorphic belt in the northern sierra nevada california new geological petrological and geochronological information suggest that the smartville complex may form part of an early jurassic island arc terrane that was probably built on a pre existing oceanic basement inferred oceanic basement is represented by multiply deformed and metamorphosed ophiolitic rocks that crop out in the central belt north and east of the smartville complex that are intruded by numerous lower jurassic dikes of an incipient island arc character lower jurassic volcanic and plutonic rocks of the slate creek and possibly the lake combie complexes in the central belt are interpreted as fragments of the early jurassic arc terrane accretion of the ensimatic arc terrane into the continental margin of north america may have occurred along east vergent thrusting around middle jurassic time and

detection of a ulvz at the base of the mantle beneath the northwest pacific 1 we used the yellowknife seismic array yka to measure the slowness of 1 371 p and pdiff waves from earthquakes occurring in the circum pacific region the corresponding anomalies in p velocity show a sharp reduction of up to 6 across a patch of the lowermost mantle beneath the northwest pacific with lateral dimensions of several hundred kilometers the location of this ultra low velocity zone ulvz correlates with a long wavelength compositional boundary revealed by probabilistic mantle tomography we interpret the ulvz as partial melt created by paleo slab material that is being swept laterally from northwestern pacific subduction zones towards the large chemically distinct province beneath the south central pacific

small scale ultralow velocity zone structure imaged by scp 1 short period teleseismic scp phases from events in the tonga fiji region recorded at the short period small aperture warramunga array in australia are used to study structure at the core mantle boundary cmb we examine the data for evidence of precursors and postcursors to scp such additional arrivals are generated by the interaction of scp with structure at the reflection and conversion point at or near the cmb we show precursors and postcursors to scp indicating an ultralow velocity zone ulvz at the cmb in the region between tonga fiji and australia one dimensional forward modeling of the short period data leads to models of the ulvz with thickness varying between 5 and 20 km velocity reductions of 0 10 and 10 30 for p velocity and s velocity respectively and 0 50 density increase many events however show simple waveforms consistent with a locally simple cmb models for proximal events can vary greatly indicating very complex three dimensional structure with scale lengths of a few tens of kilometers these results point to complex chemical and physical processes in the lowermost mantle

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

seismic structure and ultra low velocity zones at the base of the earth s mantle beneath southeast asia abstract we constrain seismic structure and ultra low velocity zones near the earth s core mantle boundary cmb beneath southeast asia we first determine the average shear velocity structure near the cmb in the region based on travel time analysis of s scs p and scp phases we then map seismic scattering in the lowermost mantle using the pkp precursors observed at the usarray the inferred average shear velocity perturbations in the lowermost 200 km of the mantle range from about 6 to 6 and exhibit a complex geographic distribution of alternate low and high velocity patches adjacent to each other surrounded by a high velocity anomaly in the south the inferred strong seismic scatterers exhibit a crescent shape distributed from the south china sea to the maluku islands and coincide with the westernmost low velocity patch suggesting that the strong scatterers represent ultra low velocity zones ulvzs we suggest that the seismic structure in the region likely results from a complex interaction between a downwelling and a low velocity region near the cmb the downwelling the high velocity patches displaces the low velocity region into many low velocity patches and pushes the ulvzs to the edge of the low velocity region

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

the interaction between the post perovskite phase change and a thermo chemical boundary layer near the core mantle boundary numerical simulations of thermo chemical mantle convection with the newly discovered post perovskite ppv phase change are used to characterize its dynamical consequences for the dq region which has frequently been interpreted as being thermo chemical in origin a strong lateral variation in the appearance of the ppv region is observed with high cmb temperatures leading to bdouble crossingq of the phase boundary in average or cold regions or no crossing in hot regions lower cmb temperatures result in a single crossing everywhere but with highly variable depth the strongly exothermic nature of the post perovskite phase change induces an anti correlation between regions containing a thick ppv layer and piles of chemically dense material formed by the segregation of the crustal component from subducted slabs the ppv phase change destabilizes chemically dense material allowing subducted slabs to easily penetrate into a dense layer forming isolated patches of the post perovskite phase whereas the lateral spectrum of thermal anomalies in the deep mantle is dominated by long wavelengths the lateral spectrum of composition is relatively flat and is the dominant contributor to seismic velocity variations at all wavelengths except the very longest ones d 2005 elsevier b v all rights reserved

the josephine ophiolite northwestern california the 157 m y old josephine ophiolite is a complete ophiolite consisting of the following units harzburgite tectonite cumulate ultramafics cumulate gabbro interlayered with cumulate ultramafics cumulate gabbro high level gabbro and diorite a 1 5 km thick sheeted dike complex and 0 4 km of pillow lava and breccia the ophiolite is conformably overlain by the galice formation consisting of a thin siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental margin sources the ophiolite and overlying galice formation were intruded by numerous 150 to 151 m y old calc alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite pumpellyite to lower greenschist facies the josephine peridotite makes up the base of the ophiolite and covers 800 km2 it is an alpine type peridotite that represents upper mantle material and the residue of partial melting ultramafic rocks overlying the josephine peridotite are olivine spinel clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene olivine plagioclase cumulates and finally to clinopyroxene plagioclase cumulates high level gabbros and diorites are characterized by highly variable textures complex intrusive relationships strongly zoned plagioclase and abundant mafic dikes plagiogranites occur at this level as dikes small intrusions and screens between mafic dikes but they are rare the sheeted dike complex consists of mafic dikes averaging 0 6 m in thickness chilled margins are locally well preserved and many of the dikes have been split and intruded by younger dikes resulting in half and marginless dikes the dike complex is composed of 100 sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts the dikes are inclined at 40 to bedding after correcting for folding and are inferred to have been rotated shortly after formation the extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas some of the breccias formed as talus deposits whereas others are graded and matrix supported indicating transport and redeposition possibly in an area of rugged submarine topography the dikes and lavas have geochemical affinities to island arc tholeiites but are in some aspects transitional to mid ocean ridge basalts the dikes and lavas are cogenetic with the underlying plutonic sequence as indicated by similarity in crystallization sequence contradictory intrusive relationships between sheeted dikes and high level plutonic rocks and lack of diabase dikes in the lowermost cumulates and harzburgite the upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center resulting in downward changes in mineralogy texture and 18o in addition metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs a geothermal gradient of 250 350 c km is inferred from mineral assemblages on the basis primarily of its regional setting the ophiolite is inferred to have formed in a back arc basin in particular it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex remnant arc this interpretation is consistent with abundant volcanic arc detritus in graywackes overlying the ophiolite the island arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back arc spreading

fault slip beyond a barrier on a transform plate boundary summary a strike slip plate boundary is considered with non uniform strength depth dependent slip on the fault surface is assumed to occur aseismically in the lower fault section in response to a basal shear stress and seismically in the brittle upper section the upper fault section acts as a barrier preventing aseismic slip from reaching shallower depths if however friction decreases in the proximity of the earth s surface brittle fracture may occur beyond the barrier it is found that when friction in the uppermost fault section exceeds the applied shear stress fault slip occurs there with a smaller amplitude than at larger depth this may explain the discrepancies which have been sometimes reported between fault offsets observed at the earth s surface and fault offsets inferred from seismograms by using simpler dislocation models

a new probe of ulvz s wave velocity structure array stacking of scs waveforms 1 s wave velocity reductions in thin layers above the core mantle boundary cmb are resolved by modeling precursors to the tangential components of core reflected scs phases unlike most phases used to study ultra low velocity zone ulvz structure scs provides direct sensitivity to shear velocity structure vs recordings of deep earthquakes in the tonga fiji and south american subduction zones at broadband seismic networks in western north america are used to map small scale ulvz variability a two layer low vs structure exists under the central pacific an abrupt 0 8 2 0 decrease 60 86 km above the cmb is underlain by a 24 30 km thick ulvz layer having reductions of 3 3 7 4 with respect to prem no corresponding velocity reductions are found under the cocos plate the structure above the ulvz may be associated with reverse transformation of post perovskite to perovskite in a steep thermal gradient in a boundary layer above the cmb

evolution of a conjugate passive margin pair in mesozoic southern turkey in the western tauride belt of southern turkey a north pointing cusp isparta angle occurs along the mesozoic carbonate axis and encompasses passive continental margin sequences rift assemblages mantle peridotites an ophiolite complex and platform fragments the contacts between these tectonostratigraphic units are commonly tectonic with the carbonate platforms forming the autochthonous basement to the west and east directed thrust sheets both in the west and the east rift assemblages consist of chert pelagic limestone mudstone sandstone and alkaline lavas ranging in age from upper triassic to upper cretaceous and locally include serpentinite lenses mantle peridotites are commonly harzburgitic cut by numerous doleritic dikes and underlain by a syntectonic melange the ophiolite complex occurs in the south adjacent to and overlying the western platform bey daglari and consits mainly of peridotites and cumulate gabbros with a poorly developed dike complex and rare extrusive rocks platform fragments are represented by north south trending slivers of limestones that have affinity with the western platform they are technically overlain by the rift assemblages and or mantle rocks within the isparta angle rock types and associated structures within the isparta angle indicate a progressive evolution from continental rifting and passive margin development to ocean basin formation in the tauride belt of the neotethyan realm during triassic to late cretaceous time the pronounced asymmetry in the crustal architecture of the conjugate margins suggests that initial stages of rifting and subsequent extension were probably facilitated by crustal failure along an east dipping in present coordinates low angle fault continued rifting and associated crustal thinning resulted in upwelling and emplacement of the mantle beneath the lower plate and in partial melting of this elevated mantle generating alkaline lavas further extension in the south established a spreading system along which cretaceous oceanic crust now represented by the tekirova ophiolite was created contractional deformation in late cretaceous to late tertiary time caused telescoping of rock units within the isparta angle and emplacement of thrust sheets onto the platform carbonates both in the east and the west the opening and closure stages of the ocean basin within the isparta angle were diachronous while the bounding carbonate platforms were probably revolved around a pivoting pole located at or near the apex of this sharp bend in the tauride belt