along axis gravity gradients at mid ocean ridges implications for mantle flow and axial morphology along axis gradients in mantle bouguer anomalies mbas were calculated for detailed gravity surveys at the southwest central and southeast indian ridges with half spreading rates of 0 9 2 3 and 3 1 cm yr respectively mba gradients at the very slow spreading southwest indian ridge are 0 4 0 7 mgal km similar to the gradients found at the slow spreading mid atlantic ridge whereas gradients at the central indian and southeast indian ridges are 0 2 0 35 mgal km the results of this study when combined with other published data show that axial mba gradients at ridge segments with an axial high are about 0 1 mgal km independent of spreading rate gradients at segments with a median valley are higher and generally decrease with increasing spreading rate the spreading rate independent mba gradient at ridge segments with an axial high represents the mantle gravity signal due to the focusing of upwelling within segments the larger gravity gradients at segments with an axial valley result from the superposition of intrasegment variations in crustal thickness on this mantle signal the consistent relation between along axis mba gradient and across axis axial morphology suggests that the axial morphology is related to the efficiency of along axis transport of melt

hydrothermal activity on near arc sections of back arc ridges results from the mariana trough and lau basin 1 the spatial density of hydrothermal venting is strongly correlated with spreading rate on mid ocean ridges with the interesting exception of hot spot affected ridges evidently because spreading rate is a reliable proxy for the magma budget this correlation remains untested on spreading ridges in back arc basins where the magma budget may be complicated by subduction induced variations of the melt supply to address this uncertainty we conducted hydrothermal plume surveys along slow spreading 40 60 mm yr and arc proximal 10 60 km distant sections of the southern mariana trough and the valu fa ridge lau basin on both sections we found multiple plumes overlying 15 20 of the total length of each section a coverage comparable to mid ocean ridges spreading at similar rates these conditions contrast with earlier reported results from the two nearest arc segments of a faster spreading 60 70 mm yr back arc ridge the east scotia ridge which approaches no closer than 100 km to its arc there hydrothermal venting is relatively scarce 5 plume coverage and the ridge characteristics are distinctly slow spreading small central volcanic highs bookended by deep median valleys and axial melt lenses restricted to the volcanic highs two factors may contribute to an unexpectedly low hydrothermal budget on these east scotia ridge segments they may lie too far from the adjacent arc to benefit from near arc sources of melt supply and subduction aided migration of mantle from the bouvet hot spot may reduce hydrothermal circulation by local crustal warming and thickening analogous to the reykjanes ridge thus the pattern among these three ridge sections appears to mirror the larger global pattern defined by mid ocean ridges a well defined trend of spreading rate versus hydrothermal activity on most ridge sections plus a subset of ridge sections where unusual melt delivery conditions diminish the expected hydrothermal activity

bathymetry of mariana trench arc system and formation of the challenger deep as a consequence of weak plate coupling 1 the challenger deep in the southernmost mariana trench western pacific ocean is the deepest point on the earth s surface 10 920 m below sea level its location within a subduction trench where one plate bends and descends below another is not surprising however why is it located in the southernmost mariana trench and not at its central part where the rate of subduction is higher where the lithosphere is the oldest and densest on the earth and where the subducted lithosphere pulling down is the longest in the earth 1000 km or more according to seismic tomography we suggest that although subduction rate and slab age generally control trench depth the width of the plate coupling zone is more important beneath the central marianas the subducted slab is attached to the upper plate along a 150 km wide surface that holds the shallow portion of the subducted plate nearly horizontal in spite of its great load and thus counters trench deepening in contrast along the south mariana trench the subducted length of the lithosphere is much shorter but its attachment to the upper plate is only along a relatively narrow 50 km wide surface in addition a tear in the slab beneath this region helps it to sink rapidly through the mantle and this combination of circumstances allows the slab to steepen and form the deepest trench on the earth in a wider perspective the interrelations shown here between trench deepening ridge shallowing slab steepening and forearc narrowing may shed light on other subduction zones located near edges of rapidly steepening slabs

subduction influence on magma supply at the east scotia ridge despite a spreading rate of 65 70 km ma 1 the east scotia ridge has along most of its length a form typically associated with slower rates of sea floor spreading this may be a consequence of cooler than normal mantle upwelling which could be a feature of back arc spreading at the northern end of the ridge recently acquired sonar data show a complex rapidly evolving pattern of extension within 100 km of the south sandwich trench new ridge segments appear to be nucleating at or near the boundary between the south american and scotia sea plates and propagating southwards supplanting older segments the most prominent of these north of 56 30 s has been propagating at a rate of approximately 60 km ma 1 for at least 1 ma and displays a morphology unique on this plate boundary a 40 km long axial high exists at the centre of this segment forming one of the shallowest sections of the east scotia ridge beneath it seismic reflection profiles reveal an axial magma chamber or amc reflector similar to those observed beneath the east pacific rise and valu fa ridge simple calculations indicate the existence here of a narrow 1 km wide body of melt at a depth of approximately 3 km beneath the sea floor from the topographic and seismic data we deduce that a localised mantle melting anomaly lies beneath this segment rates of spreading in the east scotia sea show little variation along axis hence the changes in melt supply are related to the unique tectonic setting in which the south american plate is tearing to the east perhaps allowing mantle flow around the end of the subducting slab volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melting observed a southward mantle flow may have existed beneath the axis of the east scotia ridge throughout its history

morphology of the valu fa spreading ridge in the southern lau basin multibeam echosounder surveys and seismic reflection profiling were conducted in the southern lau back arc basin sw pacific the bathymetric survey covered a 130 km long section of the active spreading ridge 3 cm yr half spreading rate the n s trending valu fa ridge vfr while seismic profiles concentrated on a 35 km long ridge section analysis of these data shows a morphologically segmented ridge which is underlain by a continuous magma chamber the ridge shows similarities but also differences to the segmentation model developed for mid ocean ridges analogous to this model the section of the vfr studied displays third and fourth order segmentation the amplitudes of the along axis undulations however are a factor of 5 to 10 greater than at mid ocean ridges additionally the vfr has numerous small volcanic cones along the axis which have previously only been observed at much slower spreading ridges in cross section the vfr has a very steep narrow triangular shape which is thought to result from the high viscosity of its andesitic lavas the axial magma chamber appears to vary in width 1 4 km and location relative to the axis beneath different morphological segments perhaps the most significant deduction from our combined data is that the magma chamber is widest beneath the most prominent axial discontinuity a small overlapping spreading center osc this is counter to observations from mid ocean ridges off axis traces of ridge segments allow us to reconstruct the evolution of this osc for the past 220 kyr we conclude that this back arc spreading axis shows a segmentation pattern which is similar to mid ocean ridges but it is influenced and modified by the geochemistry of its lavas and its proximity to the island arc

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

seismic attenuation due to wave induced flow analytical expressions for three p wave attenuation mechanisms in sedimentary rocks are given a unified theoretical framework two of the models concern wave induced flow due to heterogeneity in the elastic moduli at mesoscopic scales scales greater than grain sizes but smaller than wavelengths in the first model the heterogeneity is due to lithological variations e g mixtures of sands and clays with a single fluid saturating all the pores in the second model a single uniform lithology is saturated in mesoscopic patches by two immiscible fluids e g air and water in the third model the heterogeneity is at microscopic grain scales broken grain contacts and or micro cracks in the grains and the associated fluid response corresponds to squirt flow the model of squirt flow derived here reduces to proper limits as any of the fluid bulk modulus crack porosity and or frequency is reduced to zero it is shown that squirt flow is incapable of explaining the measured level of loss 10 sup 2 q sup 1 10 sup 1 within the seismic band of frequencies 1 to 10 sup 4 hz however either of the two mesoscopic scale models easily produce enough attenuation to explain the field data

evolution of backarc rifting mariana trough 20 24 n the mariana trough is an actively opening backarc basin in the western pacific the trough formed by extension which longitudinally split an earlier arc massif creating a crescent shaped basin between the remnant west mariana ridge and eastern active volcanoes of the mariana arc opening increases southward from the volcano islands near 24 n where the two arcs join at 18 n in the central mariana trough the basin is widest and may be opening primarily by seafloor spreading we present a synthesis of closely spaced shipboard gravity magnetic and bathymetry measurements from the northern basin 20 24 n an area that undergoes a significant progressive southward increase in extension we have identified three stages in the evolution of rifting in this area 1 asymmetric rifting from 24 n to 22 15 n where faulting and magmatism have migrated laterally to remain near the active volcanic arc side of the basin 2 a localization of rifting from 22 15 n to 21 n where the primary zone of rifting separates from the active volcanic arc and 3 a further concentration of rifting from 21 n to 20 n leading to the formation of deep tectonic grabens near the center of the basin this last stage may be a precursor to or incipient seafloor spreading we describe a new type of magnetic lineation resolved in a three dimensional seafloor magnetization inversion of the trough between 20 and 24 n the detailed character of these lineations their association with tectonic structures and other geophysical observations indicate that they are not seafloor spreading lineations but rather result from magnetic intrusions and volcanism emplaced within preexisting less magnetic rifted arc crust the development of these well defined magnetization bands indicates that the zone of magmatism remains relatively narrow at any one time throughout the opening of the trough although tectonic deformation appears to be more widely distributed in addition the geometry of the magnetization bands with respect to the remnant arc border faults indicates that rifting propagated rapidly northward

comparison of two depth averaged numerical models for debris flow runout estimation this paper analyses an important aspect of the continuum numerical modelling of rapid landslides as debris flows by using the same rheological parameter values are the results obtained with code

magmatic and tectonic extension at mid ocean ridges 1 controls on fault characteristics we use 2 d numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction m of the total spreading rate i e m 1 for fully magmatic spreading heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust we examine a range of intrusion rates and axial thermal structures by varying m spreading rate and the efficiency of crustal cooling by conduction and hydrothermal circulation fault development proceeds in a sequential manner with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis fault spacing and heave are primarily sensitive to m and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis contrary to what is often cited in the literature but consistent with prior results of mechanical modeling we find that thicker axial lithosphere tends to reduce fault spacing and heave in addition fault spacing and heave are predicted to increase with decreasing rates of off axis lithospheric thickening the combination of low m particularly when m approaches 0 5 as well as a reduced rate of off axis lithospheric thickening produces long lived large offset faults similar to oceanic core complexes such long lived faults produce a highly asymmetric axial thermal structure with thinner lithosphere on the side with the active fault this across axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes

source rock seismic velocity models gassmann versus backus source rocks are described by a porous transversely isotropic medium composed of illite and organic matter kerogen oil and gas the bulk modulus of the oil gas mixture is calculated by using a model of patchy saturation then the moduli of the kerogen fluid mixture are obtained with the kuster and toksoz model assuming that oil is the inclusion in a kerogen matrix to obtain the seismic velocities of the shale we used backus averaging and gassmann equations generalized to the anisotropic case with a solid pore infill in the latter case the dry rock elastic constants are calculated with a generalization of krief equations to the anisotropic case we considered 11 samples of the bakken shale data set with a kerogen pore infill the backus model provides lower and upper bounds of the velocities whereas the krief gassmann model provides a good match to the data alternatively we obtain the dry rock elastic moduli by using the inverse gassmann equation instead of using krief equations four cases

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

p wave seismic attenuation by slow wave diffusion effects of inhomogeneous rock properties recent research has established that the dominant p wave attenuation mechanism in reservoir rocks at seismic frequencies is because of wave induced fluid flow mesoscopic loss the p wave induces a fluid pressure difference at mesoscopic scale inhomogeneities larger than the pore size but smaller than the wavelength typically tens of centimeters and generates fluid flow and slow diffusion biot waves continuity of pore pressure is achieved by energy conversion to slow p waves which diffuse away from the interfaces in this context we consider a periodically stratified medium and investigate the amount of attenuation and velocity dispersion caused by different types of heterogeneities in the rock properties namely porosity grain and frame moduli permeability and fluid properties the most effective loss mechanisms result from porosity variations and partial saturation where one of the fluids is very stiff and the other is very compliant such as a highly permeable sandstone at shallow depths saturated with small amounts of gas around 10 saturation and water grain and frame moduli variations are the next cause of attenuation the relaxation peak moves towards low frequencies as the background permeability decreases and the viscosity and thickness of the layers increase the analysis indicates in which cases the seismic band is in the relaxed regime and therefore when the gassmann equation can yield a good approximation to the wave velocity

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

distinct regional differences in crustal thickness along the axis of the mariana trough inferred from gravity anomalies we have compiled extensive gravity and bathymetry data for the whole mariana trough which were collected during several japanese scientific cruises over the last few years this study aims to clarify the lateral distribution of the local differences in geochemical signatures which have been observed locally in the mariana trough shipboard free air gravity anomaly data from eight japan agency for marine earth science and technology jamstec cruises were compiled with those crossover errors of 2 85 mgal mantle bouguer anomalies mba were calculated by subtracting the predictable gravity signal due to the seawater crust and crust mantle density boundaries the crustal thickness variation along the spreading axis was estimated from the mba different features in crustal thickness its variation and segment length for each segment allow us to identify four distinct regional differences in magmatic activity along the spreading axis of the mariana trough segment in region a to the north of 20 35 n shows the largest sectional dimensions of crust along the axis and it is probably affected by an additional supply from island arc magma sources a variety of crustal thickness values and of along axis crustal thickness variations in region b between 15 38 n and 20 35 n suggests two types of segments one is similar to a slow spreading ridge segment that has a plume like mantle upwelling under the spreading axis and the other is a magma starved segment region c between 14 22 n and 15 38 n is a less magmatic region individual crustal thickness averages of 3 4 4 1 km region d to the south of 14 22 n has higher individual crustal thickness averages of 5 9 6 9 km suggesting higher magmatic activity with a sheet like mantle upwelling under the spreading axis different features in the mba for off axis areas suggest that these four regions have existed since the mariana trough started spreading moreover comparison between our results of crustal thickness and previous geochemical results indicates that less magmatic spreading segments with thin crust which are locally distributed in both regions b and c probably result from mantle source depleted of water and incompatible elements this suggests that lateral compositional variation of water and incompatible elements exists on a segment scale in the mantle source beneath the spreading axis of the mariana trough

a three plate kinematic model for lau basin opening 1 we present revised compilations of bathymetry magnetization acoustic imagery and seismicity data for the lau basin and surrounds we interpret these data to more precisely locate the plate boundaries in the region and to derive a three plate kinematic model for the opening of the lau basin during the brunhes chron our tectonic model includes the niuafo ou microplate which is separated from the australian plate along the peggy ridge lau extensional transform zone central lau spreading center and from the tongan plate along the newly discovered fonualei rift and spreading center our model shows that australia tonga plate motion between 15 5 s and 19 s is partitioned across the microplate and it resolves the former apparent conflict between geodetic versus seafloor spreading velocities regionally this implies that the angular rate of opening of the lau basin has been faster than that of the havre trough to the south and that the rapid current rates of pacific subduction at the tonga trench 240 mm yr at 16 s have been sustained for at least 0 78 m y

why is the challenger deep so deep abstract recent sidescan surveys of the deepest segment of the southern mariana trench in the western pacific ocean provide the first detailed images of this plate boundary which includes the world s greatest ocean depth the challenger deep the surveys reveal details of the southern mariana plate margin identify another deep rivaling the challenger and document widespread deformation of the overriding plate our data show a subduction generated deep ocean trench not the transform fault boundary suggested by other work d e karig geol soc am bull 82 1971 323 344 d e karig et al j geophys res 83 1978 1213 1226 d e karig b ranken in the tectonic and geologic evolution of southeast asian seas and islands part 2 american geophysical union washington dc 1983 pp 266 280 k fujioka et al geophys res lett 19 2002 1 4 we present the geological characteristics of the region including seismic evidence for a tear in the subducting plate that has influenced the deformation of the overriding plate the rollback caused by this tear creates greater depths along the southern part of the trench than elsewhere along its length

malaguana gadao ridge identification and implications of a magma chamber reflector in the southern mariana trough 1 six channel seismic reflection data reveal a magma chamber reflector beneath the malaguana gadao ridge the southernmost segment of the spreading center in the mariana trough for most of its length the spreading center in this active back arc basin is morphologically similar to slow spreading mid ocean ridges having a deep central graben flanked by a zone of abyssal hill fabric this southernmost segment however has a broad smooth cross section lacks a deep central graben and is thus similar in morphology to fast spreading ridges e g the east pacific rise we identify a magma chamber at 1 5 s two way travel time below the crest of the ridge observations from remotely operated vehicles along the ridge reveal not only fresh pillows lobate and sheet lava flows but also an abundance of volcaniclastic debris and intense hydrothermal activity these observations together with the fast spreading morphology of the ridge suggest that this segment has a considerably higher magma supply than is typical in the mariana trough we suggest that the volcanic arc or enhanced melting of a hydrated mantle is supplying volatile rich magma as evidenced by a highly negative coefficient of reflection 0 42 for this mcr and the presence of evolved highly vesicular lava and volcaniclastic materials the southeastern mariana back arc basin spreading ridge does not compare readily with mechanical models for global mid ocean ridge data sets because of marked asymmetry in both volcanism and deformation that may be the consequence of slab related geometry in this part of the convergent margin system

asymmetric accretion along the slow spreading mariana ridge 1 we report the results of several surface geophysical surveys of the central part of the mariana basin western pacific analysis of bathymetric and magnetic data along three distinct spreading segments indicates highly asymmetric spreading processes the half spreading rate being two to three times more important on the western side of the basin than on its eastern side such an asymmetry is as strong at the centers of segments as at their ends and does not systematically correlate with an asymmetric shape of the rift valley a half graben shape of the rift valley is observed only at segment ends at segments centers a symmetric graben shape characterizes the valley the asymmetric shape of the valley at segment ends probably reflects the localization of the tectonic strain due to changes in lithospheric rheology and thickness asymmetric spreading is likely accomplished through a different amount of deformation and magma emplacement on both sides of the spreading axis it may result from the combined effects of 1 asymmetric rheologic melting and stress conditions imposed by the influence of the pre existing volcanic arc and its melt sources on the eastern side of the basin 2 the northwestward movement of the philippine sea plate while the almost vertical pacific slab in the mariana subduction zone resists to this motion 3 a roll back component at the mariana subduction zone

asymmetric deep crustal structure across the juan de fuca ridge a multichannel line was shot across the endeavour segment of the juan de fuca ridge careful processing of 25 km of the line centered about the rise axis shows that young crustal structure is asymmetric an intermittent reflector from the top 1 km of crust follows topography on both the pacific and the juan de fuca plates it could be either the pillow basalt diabase dike contact or a metamorphic front within the pillow basalts a reflection exists 2 3 km below the rift valley floor diffractions from its edges indicate that the rift valley9s velocity structure is markedly different from the structure of the flanks the deep crustal structure is highly asymmetric moho exists within 4 km of the rift valley on the pacific plate in contrast to the juan de fuca plate which shows a reflector at intermediate depths dipping away from the rise to reach moho depth 12 km from the rift valley this reflector9s origin is enigmatic we speculate that the thermal history and structure of the two plates differ because of local differences in mantle structure caused by either ridge migration or a melting anomaly

enhanced magma supply at the southern east scotia ridge evidence for mantle flow around the subducting slab abstract bathymetric and seismic data show that the southernmost segment of the east scotia ridge segment e9 is anomalous in its curved plan form and in the presence of a large axial volcanic ridge avr spreading commenced only within the past 1 million years on this segment which appears to have propagated both northward and southward the presence of a caldera at the summit of the avr indicates that a shallow magma chamber of limited extent existed recently beneath the ridge eruption from this magma chamber probably within the past 0 1 million years may have led to the formation of the avr itself careful examination of seismic reflection profiles suggests that small pockets of magma may still exist beneath the avr magmatism on segment e9 may be enhanced as a result of the flow of shallow mantle around the southern end of the subducting slab beneath the south sandwich arc in a manner similar to that proposed for the northern end this mantle probably carries a slightly enriched hotspot signature and is affected by volatiles released from the edge of the slab both tending to increase the supply of magma to the back arc spreading centre

seismic mapping of a magma chamber beneath the valu fa ridge lau basin we analyze a dense grid of seismic normal incidence and wide angle data collected over an active back arc spreading center in the lau basin of the southwest pacific our survey area covers the whole of a 35 km long morphological segment and a small overlapping spreading center a bright reflector which is coincident with a velocity inversion at a depth of 3 2 0 2 km below the seafloor is observed on every one of the 40 across axis profiles we interpret this reflector as being the roof of a crustal magma chamber the widest magma chamber reflector occurs beneath the overlapping spreading center where it extends up to 4 km being imaged beneath both ridges and in places beneath the overlap basin elsewhere the width of the reflector varies between 0 6 and 2 3 0 4 km the narrowest reflectors are observed beneath deviations devais of the ridge axis in addition to being observed on each of the kilometer spaced across axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis the reflection coefficient of the event is between 0 34 and 0 65 this requires the melt at the top of the chamber to have a p wave velocity less than 2 7 km s if the interface is planar or less than 3 8 km s if the interface is layered

geodetic evidence of back arc spreading in the mariana trough 1 repeated gps surveys in the mariana islands show that the mariana block is moving apart from the philippine sea plate the velocities of the islands relative to the philippine sea plate range from about 15 mm yr in the northern islands to about 45 mm yr near guam the data also suggest convergence rates for the mariana forearc with respect to the pacific plate of 35 45 mm yr at 19 n increasing to 55 70 mm yr at 13 5 n in addition the velocity vectors show a slight north south expansion of the arc the estimated location of the euler pole of the mariana forearc with respect to the philippine sea plate is well south of the geographical point where the back arc basin narrows to zero width

rifting history of the northern mariana trough seamarc ii and seismic reflection surveys seamarc ii sidescan sonar data and seismic reflection profiles of the northernmost portion of the mariana trough suggest a rifting rather than a seafloor spreading origin for lithosphere within this active backarc basin of the western pacific variations in morphology along the strike of the backarc basin suggest both a spatial and a temporal variation in the style of extension of the basin the northern mariana trough widens from 70 km near 23 n to 135 km near 21 30 n major active fault zones bound both sides of the backarc basin active rifting and volcanism are predominant in a roughly 20 km wide active rift zone along the eastern arc side of the basin from 24 n to 22 n the locus of active rifting has remained near the volcanic front of the arc during extension and is probably controlled by lithospheric weakness which is greatest at the volcanic front the zone of rifting intersects the active volcanic arc at nikko seamount the distinction between the mechanism for formation of the mariana trough proposed here and that of slow mid ocean ridge spreading is that no true plate boundary has developed in the northernmost mariana trough rather the backarc basin is composed of arc lithosphere intruded by arc and backarc basin magmas volcanism both in the rift zone and throughout the basin is fault controlled south of 22 n the zone of active rifting diverges westward from the arc as the transition to true seafloor spreading is approached the transition to true mid ocean ridge spreading occurs to the south of the survey area described here

back arc basin basalt systematics the mariana east scotia lau and manus back arc basins babs have spreading rates that vary from slow 100 mm yr and extension axes located from 10 to 400 km behind their island arcs axial lava compositions from these babs indicate melting of mid ocean ridge basalt morb like sources in proportion to the amount added of previously depleted water rich arc like components the arc like end members are characterized by low na ti and fe and by high h2o and ba la the morb like end members have the opposite traits comparisons between basins show that the least hydrous compositions follow global morb systematics and an inverse correlation between na8 and fe8 this is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures lau manus hotter than mariana scotia this interpretation accords with numerical model predictions that faster subduction induced advection will maintain a hotter mantle wedge the primary compositional trends within each bab a positive correlation between fe8 na8 and ti8 and their inverse correlation with h2o 8 and ba la are controlled by variations in water content melt extraction and enrichments imposed by slab and mantle wedge processes systematic axial depth as a proxy for crustal production variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate hydrous fluxing enhances decompression melting allowing depleted mantle sources just behind the island arc to melt extensively producing shallow spreading axes flow of enriched mantle components around the ends of slabs may augment this process in transform bounded back arcs such as the east scotia basin the re circulation by mantle wedge corner flow to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the east lau spreading center manus southern rifts and mariana trough and the very depleted lavas of east scotia segments e8 e9 the crust becomes mid ocean ridge mor like where the spreading axes further away from the island arc and subducted slab entrain dominantly fertile mantle

seismic velocity and q anisotropy in fractured poroelastic media a fluid saturated poroelastic isotropic medium with aligned fractures behaves as a transversely isotropic and viscoelastic tiv medium when the predominant wavelength is much larger than the average distance between fractures a planar fracture embedded in a fluid saturated poroelastic background medium can be modeled as a extremely thin and compliant porous layer p waves traveling in this type of medium induce fluid pressure gradients at fractures and mesoscopic scale heterogeneities generating fluid flow and slow diffusion biot waves causing attenuation and dispersion of the fast modes mesoscopic loss a poroelastic medium with embedded aligned fractures exhibits significant attenuation and dispersion effects due to this mechanism which can properly be represented at the macroscale with an equivalent tiv medium in this work we apply a set of compressibility and shear harmonic finite element fe experiments on fractured highly heterogeneous poroelastic samples to determine the five complex and frequency dependent stiffnesses characterizing the equivalent medium the experiments consider brine or patchy brine co 2 saturated samples and a brine saturated sample with a heterogeneous fractal skeleton with fractures we show that fractures induce strong seismic velocity and q anisotropy both for qp and qsv waves enhanced either by patchy saturation or frame heterogeneity