plate boundary geometry to moho depths within the 9 03 n and 12 54 n overlapping spreading centers of the east pacific rise abstract multichannel seismic reflection data from the east pacific rise provide a three dimensional view of overlapping spreading centers oscs at 9 03 n and 12 54 n previous studies of the 9 03 n osc based on position of the axial magma chamber top are extended to the base of the crust using the observation that the moho reflection is disrupted in the vicinity of the ridge axis the smaller 12 54 n osc is investigated for comparison like the shallower features the moho disruption is offset within the oscs at the larger 9 03 n osc misalignment between the morphologic ridge axis and the axial magma body extends to the base of the crust where the axial gap in the reflection moho does not consistently underlay the shallower structures spatial relationships within the oscs between axial topography axial magma chamber and axial gap in reflection moho suggest that ridge tips propagate first by brittle fracturing of the upper crust followed by intracrustal magma residence followed finally by establishment or capture of magma pathways from the mantle interpreting all three features as demarcating the axial zone of weakness along which new magma enters the crust the axial plane within the 9 03 n osc appears to dip and to rotate with depth the sense of overlap in the shallow crust is dextral while the sense of offset in the deep crust is sinistral spatial relationships within the 9 03 n osc suggest that the northern propagating ridge tip has already captured a portion of the deep crustal magma conduit connected with the southern ridge

the june july 1993 seismo acoustic event at coaxial segment juan de fuca ridge evidence for a lateral dike injection the coaxial segment juan de fuca ridge was the site of an intense swarm of earthquakes that began at 21 43 gmt on june 26 1993 julian day 177 the swarm started near 46 15 n and migrated northward over the next 40 hours to 46 36 n where the majority of 676 events occurred during the following 3 weeks of activity the earthquakes propagated nne at a velocity of 0 3 0 1 m s 1 from the southern to northern swarm sites the activity went undetected by land based seismic networks along the oregon and washington coasts suggesting that the earthquakes were all m 4 0 the character of this earthquake swarm is very similar to dike injections observed at krafla and kilauea volcanoes the earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the coaxial segment the reservoir acting as the dike s source likely resides beneath or to the south of the initial southern swarm of earthquakes the large magma supply at axial volcano cannot be ruled out as the source for the coaxial dike even though axial volcano exhibited no earthquake activity related to the coaxial swarm the t wave earthquake swarm reported here is the first deep ocean observation of volcanic seismicity associated with what most likely was a mid ocean ridge dike injection event

deepening of the axial magma chamber on the southern east pacific rise toward the garrett fracture zone a wide aperture profile along the ridge axis from 14 29 s to 13 39 s 120 km to 30 km south of the garrett fracture zone is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector five areas along the 90 km line are examined in detail with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area a genetic algorithm code is used to find a best fit model from a comparison of the data and wkbj synthetic seismograms one hundred starting models are generated using a predefined set of velocity nodes with a fixed window of allowable depth variations between nodes an evolutionary process favors the better fitting models in each generation and a satisfactory misfit is usually obtained within 40 generations within individual areas the models were in good agreement with the depth of a given velocity node generally varying by not more than 20 m the depth discretization interval for the models a consistent deepening trend of the axial magma chamber amc is observed across the five areas as the garrett fracture zone is approached the depth varies from 0 99 km at area 1 which is approximately 100 km south of the garrett to 1 23 km at area 5 which is approximately 40 km south of the garrett the depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age for the areas examined here layer 2a is observed to be relatively constant in thickness along the axis although it is about 40 m thicker over area 5 where the axial magma chamber is deepest this variation is within the scatter of previously detailed layer 2a measurements at 13 n on the east pacific rise where an effectively constant thickness is observed this implies that layer 2a thickening is not a significant factor along this profile and that the amc deepening is real rather than apparent theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the garrett fracture zone which marks a major interruption of hundreds of kilometers of continuous ridge axis the deepening may also be interpreted as a downward limb from a central injection point however there is no indication of a similar downward trend in the other southern direction furthermore there is no accompanying systematic variation in axial depth or axial volume both of which are proposed to be indicators of central injection and along axis flow

influence of enhanced melt supply on upper crustal structure at a mid ocean ridge discontinuity a three dimensional seismic tomographic study of 9 n east pacific rise we present a three dimensional upper crustal model of the 9degrees03 n overlapping spreading center osc on the east pacific rise that assists in understanding the relationship between melt sills and upper crustal structure at a ridge discontinuity with enhanced melt supply at crustal levels our p wave velocity model obtained from tomographic inversion of similar to 70 000 crustal first arrival travel times suggests that the geometry of extrusive emplacement are significantly different beneath the overlapping spreading limbs extrusive volcanic rocks above the western melt sill are inferred to be thin similar to 250 m more extensive accumulation of extrusives is inferred to the west than to the east of the western melt sill the extrusive layer inferred above the eastern melt sill thickens from similar to 350 at the neovolcanic axis to 550 m to the west of the melt sill volcanic construction is likely to be significant in the formation of ridge crest morphology at the osc particularly at the tip of the eastern limb on the basis of our interpretation of the velocity model we propose that enhanced magma supply at crustal levels at the osc may provide an effective mechanism for the migration of ridge discontinuities this dynamic magma supply model may explain the commonly observed nonsteady migration pattern of ridge discontinuities by attributing this to the temporal fluctuations in melt availability to the overlapping spreading limbs

a multichannel seismic investigation of upper crustal structure at 9 n on the east pacific rise implications for crustal accretion reprocessed multichannel seismic profiles from the 9 n segment of the east pacific rise reveal prominent shallow subbasement events these events are identified as wide angle reflections from the base of seismic layer 2a based upon modeling of expanding spread profile data and velocity functions the layer 2a reflections typically increase from 0 15 s after the seafloor reflection at the rise axis to 0 3 0 45 s within 1 2 km of the axis corresponding to an increase in layer thickness of 200 600 m no further systematic increase in layer thickness is observed although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis however the intermittent character of the imaged layer 2a reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer the base of layer 2a is interpreted as corresponding to the transition between the extrusive section pillow basalts and sheet flows and a sheeted dike complex the rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone lateral variability of layer 2a can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2a are approximately half that of layer 2b for an along axis profile apparent along axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2a caused by wandering of the profile relative to axis within the resolution of the data the time delay of the magma chamber reflection relative to the base of layer 2a is constant

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

three dimensional crustal structure and magma chamber geometry at the intermediate spreading back arc valu fa ridge lau basin results of a wide angle seismic tomographic inversion in this paper the results of a tomographic analysis of a 3 d wide angle seismic refraction data set acquired at the valu fa ridge vfr in 1995 are presented the vfr is an intermediate spreading ridge located in the southern lau back arc basin in the southwest pacific the ridge comprises three morphological segments the southern central and northern valu fa ridges separated by overlapping spreading centres oscs previous seismic experiments have identified a robust axial magmatic system beneath the central segment cvfr and the osc with the northern segment nvfr the experiment described in this paper aimed to resolve details of the structure of this magma chamber and the adjacent post rift crust a regularized inversion scheme that minimizes model roughness was applied to the first arrival traveltime picks made from the wide angle data a quantitative approach for determining data uncertainties is described based on the signal to noise ratio of the arrivals several initial model assumptions were tested including one with a thin melt lens representing a seismic reflector identified in previous studies explicitly included in the initial model the inversion results suggest that crustal layer 2 exhibits northward thickening which mirrors a similar northward thickening of the whole crust in addition local thinning of layer 2 is identified in the vicinity of the boundary between pre and post rift crust which is thought to represent thinning of the crust prior to the onset of rifting axial low velocity anomalies are identified in layer 2b c and layer 3 the models are consistent with a continuous 6 km wide negative velocity anomaly in layer 3 with an amplitude of 0 7 0 9 km s 1 relative to off axis post rift crust this anomaly is consistent with the presence of an axial mush zone comprising a small percentage 1 per cent of partial melt the negative velocity anomaly in layer 2b c is modelled with its largest amplitude 0 5 km s 1 beneath the northern osc possible origins for this anomaly include locally thicker crust or locally higher porosity near the osc or a high temperature anomaly associated with the axial magmatic system

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

variation in cross sectional area of the axial ridge along the east pacific rise evidence for the magmatic budget of a fast spreading center along the fast and ultrafast spreading east pacific rise the cross sectional area of the axial ridge varies significantly over length scales similar to its morphologic segmentation using an automated method we measure the ridge area volume per kilometer along axis where there is complete bathymetric coverage along 3500 km of the northern and southern east pacific rise our two study areas encompass numerous transform faults large overlapping spreading centers small overlapping spreading centers and smaller discontinuities first second third and fourth order discontinuities the cross sectional area variation mimics the undulation of the ridge crest depth local area maxima occur toward the middle of segments and the axial area decreases by 40 or more at first and second order discontinuities third order discontinuities are generally marked by smaller disruptions in the ridge area and fourth order offsets do not systematically correspond with features of the cross sectional area profile the correlation between shallower ridges and larger ridge areas breaks down at some locations because axial cross sectional area represents a longer term average of the ridge s magmatic state than axial depth a correlation between large ridge areas and negative residual gravity anomalies indicates that inflated ridges are underlain by low density crust and mantle also a correlation between larger area and higher mgo content of axial basalts suggests that inflated areas generally erupt hotter magmas which are presumably supplied more rapidly to the neovolcanic zone the cross sectional area of the axial ridge appears to correlate with the width of the axis centered low velocity zone in the crust these observations as well as the absence of large relict axial ridges off axis indicate that the axial ridge originates from buoyancy due to thermal expansion and the presence of melt in the crust and mantle within about 10 km of the rise axis portions of the northern east pacific rise underlain by a magma chamber reflector generally occur where the ridge cross sectional area is greatest this supports the connection between processes which inflate the axial ridge and those which heat the crust and upper mantle and produce melt thus while the axial high on fast spreading centers resembles a constructional volcano in cross section it is more like a long narrow balloon whose cross sectional area is a sensitive indicator of magma supply using this relationship we predict with 75 confidence that at least 45 of the unsurveyed northern east pacific rise 18 n to 13 n and 9 n to 5 n and at least 60 of the southern east pacific rise 4 s to 23 s is underlain by a magma chamber reflector

contribution of volcanism and tectonism to axial and flank morphology of the southern east pacific rise 17 10 17 40 s from a study of layer 2a geometry multichannel seismic images from the east pacific rise 17 10 17 40 s are used to study the geometry of seismic layer 2a in the near axis region wave equation datuming is applied to stacked common midpoint data to remove the distorting effects of seafloor topography on the layer 2a event processed stacks are compared with seafloor morphology and tectonic fabric imaged in side scan sonar data assuming layer 2a corresponds with the extrusive crust these data are used to study the relationship between volcanism and tectonism in the accumulation of the extrusive layer and to assess the contribution of extrusives to ridge crest and ridge flank morphology we find variations in axial thickening of layer 2a which imply twofold variation in the width of the extrusive layer accumulation zone as well as systematic changes in the pattern of accumulation of this layer on the ridge flanks the layer 2a horizon mimics abyssal hill relief consistent with a horst and graben origin for this topography short wavelength variations in the thickness of layer 2a are superimposed on this relief which we attribute to volcanic modification of tectonic topography at the edge of the neovolcanic zone the integrated off axis thickness of layer 2a is correlated with ridge cross sectional area and indicates persistent 100 000 years spatial gradients in extrusive layer thickness systematic changes in the cross axis shape of the ridge observed over distances of 5 10 km cannot be attributed to the extrusive layer and it appears that axial structure beneath the volcanics governs the cross axis morphology of the ridge

distribution of magma beneath the east pacific rise between the clipperton transform and the 9 17 n deval from forward modeling of common depth point data we have reprocessed seven cross axis common depth point cdp profiles between the clipperton transform and the 9 17 n deval deviation in axial linearity on the east pacific rise epr to understand the relationship between axial magma chamber amc width and seafloor morphology forward modeling of cross axis cdp profiles suggests a segmented amc in which significant variations in width occur across minor rise axis discontinuities e g devals the modeled rise segment widths bounded by the 9 53 n 9 35 n devals the 9 35 n 9 17 n devals and south of the 9 17 n deval were 0 7 km 1 0 1 2 km and 4 15 km respectively transition in amc width across these discontinuities is unclear due to the sparseness of cross axis line spacing however a simple association of devals with decreased magma supply is doubtful the minimum 250 m and maximum 4150 m amc widths are found near the 9 35 n and 9 17 n devals respectively the reprocessing of cdp profiles included repicking stacking velocities to ensure a proper stack of the amc reflector and its associated diffractions imaging postcritical reflections from the base of layer 2a finite difference time migration ray theoretical depth migration and travel time modeling of amc diffraction patterns constraints on amc width were derived from forward modeling techniques based on analytic raytracing velocity models were constructed from seabeam bathymetry and modified expanding spread profile esp velocity depth functions esp velocity models were altered to compensate for off axis thickening of layer 2a as imaged in the cdp reflection data continuous two dimensional velocity models constructed from modified esp velocity depth functions and seabeam bathymetry should account for ray bending at the seafloor basalt interface and any lateral velocity gradients induced by a thickening layer 2a stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration reflector positions were input into our forward modeling scheme to produce a zero offset travel time response of our migrated solution the travel time response was then overlain on the stacked section to ensure an adequate match especially to diffractions generated at the amc edge forward modeling of amc diffraction patterns reveals that original amc width estimates were too large the under migration of the amc reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual cmp gathers thus resulting in improperly collapsed diffraction hyperbolae ship wandering relative to the amc edge can account for variations in amc reflector amplitude and dropout on the along axis cdp line the continuity of the amc appears unbroken across several ridge axis discontinuities between the clipperton transform and the 9 17 n deval which suggests an amc whose along axis dimension exceeds 75 km reflectivity modeling of cmp gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush such a thin layer of melt might inhibit along axis mixing of magmas and thereby account for variations in magma composition along the rise crest this melt lens model for the amc would also produce strong diffraction patterns as imaged in the cdp data

asymmetric melt sills and upper crustal construction beneath overlapping ridge segments implications for the development of melt sills and ridge crests a new three dimensional tomographic velocity model and depth converted reflection images of the melt sills beneath the 9 03 n overlapping spreading center on the east pacific rise show that the upper crustal construction at this ridge discontinuity is highly asymmetric with reference to the bathymetric ridge crests of the overlapping limbs despite the similarly curved ridge crests the asymmetries are markedly different under the two limbs and appear to be related to the contrasting evolutionary history of the limbs the overlap basin is closely related to the propagating eastern limb in terms of its seismic structure by contrast the western limb forms a distinct morphologic region that displays little structural relationship to the adjacent overlap basin and other relict basins as the overlapping spreading center is migrating southward the differential development of melt sills and ridge crests may be inferred from the results of this study ridge propagation appears to involve two major processes the advancement of the melt sill at the ridge tip and the development of ridge crest morphology and the neovolcanic axis to the north of the overlap basin region near the existing propagating limb the latter process may result in the abandonment of the current neovolcanic axis leading to the self decapitation of the propagating limb by contrast the self decapitation of the western limb is related to the receding melt sill which lags behind the anticlockwise rotational motion of the ridge crest

crustal structure accretionary processes and rift propagation a gravity study of the intermediate spreading valu fa ridge lau basin summary the valu fa ridge is an intermediate spreading full rate of 60 mm yr 1 ridge located in the lau basin in 1995 this ridge was surveyed using a multidisciplinary geophysical approach to image crust and upper mantle structure with the aim of investigating the processes of oceanic crustal accretion in a back arc tectonic environment as part of this experiment a network of gravity profiles was acquired together with seismic magnetic swath bathymetry and controlled source electromagnetic data presented in this paper are the results of forward modelling of a subset of the acquired gravity profiles two oriented ridge perpendicular and one ridge parallel using the preferred seismic models of turner et al 1999 as a basis of initial model construction in addition the gravity data set in its entirety has been used to calculate the mantle and residual mantle bouguer anomalies with the aim of investigating variability in crustal structure both density and layer thickness and the nature of the underlying upper mantle of particular interest are the overlapping spreading centre between the central and northern valu fa ridges where seismic modelling implies a generally thickened crust and a magma chamber located beneath the overlap basin rather than separate chambers supplying each ridge and the propagating rift tip and associated basin bounding pseudo fault modelling results suggest that the pre and post rift crusts have different compositional origins with lower densities required 12 km off axis to fit the observed free air gravity anomaly the locations of the transitions into regions of lower density correspond with those of turner et al 1999 derived from seismic modelling which in turn correspond in location to the rift related pseudo fault identified by wiedicke collier 1993 calculation and interpretation of the mantle and residual mantle bouguer anomalies also confirms the lower off axis densities and indicates a general increase in crustal thickness northwards towards the overlapping spreading centre between the central and northern valu fa ridge segments the presence of thicker crust corresponding to a region of negative mantle and residual mantle bouguer anomaly implies that the region surrounding the overlapping spreading centre has been in the recent geological past or is presently the site of an increased magma supply the residual mantle bouguer anomaly also reveals features related directly to off axis lateral variation in density and layer thickness associated with the southward propagation of the valu fa ridge into the havre trough modelling of the north south variation in crustal thickness along axis shows that the anomaly trend can be explained simply by thicker crust beneath each of the valu fa ridge segments and the overlapping spreading centre between the northern and central valu fa ridges the observation of segment centred crustal thickening is consistent with models of ridge segmentation that suggest that melt influx is located towards segment centres and that segment length is controlled by the maximum lateral extent to which this melt can flow macdonald et al 1988 tolstoy et al 1993 however the overlapping spreading centre remains anomalous from these models in that it also appears to be associated with thicker crust and an increased melt supply

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

distribution of magma beneath the east pacific rise near the 9 03 n overlapping spreading center from forward modeling of common depth point data we have reprocessed six cross axis and three along axis common depth point cdp profiles near the 9 03 n overlapping spreading center osc to understand the relationship between axial magma chamber amc width and seafloor morphology travel time modeling of the amc reflector reveals an asymmetric distribution of melt across the 9 03 n osc the variation of modeled amc width beneath either osc limb is minimal but the width increases nearly fourfold across the offset attaining an estimated maximum width of 4 15 km near the 9 17 n ridge axis discontinuity additionally melt distribution underlying the eastern rise limb is not symmetric with respect to the rise axis neovolcanic zone but is displaced toward the western rise flank depth migration based on a continuum velocity model consistent with postcritical reflections from the base of layer 2a places the skewed amc reflector beneath a nearly constant thickness sheeted dike section which dips 10 away from the rise axis to confirm amc continuity beneath the western rise flank we use the maslov synthetic seismogram method to show that amplitude enhancement of the amc reflector is consistent with a continuous melt body underlying a thickening extrusive layer analysis of along strike cdp profiles indicates an amc which is neither overlapping nor discontinuous when projected onto the along strike plane identifying intracrustal events on along axis cdp lines however requires extreme caution we have modeled out of plane scattering using a kirchhoff formulation and we show that a coherent event identified beneath the overlap basin results from diffraction off the amc which lies nearly 3 km to the west of the profile we attribute the asymmetric pattern of melt to a decoupling of melt supply from preexisting weaknesses in the brittle upper crust in this model melt ascends upward buoyancy forces until deflected by the impermeable sheeted dike complex melt then migrates updip beneath the base of the sheeted dikes toward the neovplcanic zone where fissuring produces a temporary conduit for emplacement discrete jumps in modeled amc width toward the overlap basin represent a further displacement defocusing of melt supply western amc edge relative to the neovolcanic zone eastern amc edge the asymmetric pattern of melt therefore represents a gradual en echelon accommodation of melt supply across the 9 km of ridge axis offset at 9 03 n thus for asymmetric configurations amc width may not correlate solely with magmatic robustness but may signify the amount of decoupling which exists between melt supply and extrusive emplacement within the neovolcanic zone here we present a new model for osc development which invokes a significant component of cross axis melt migration moreover abrupt changes in amc width near ridge axis discontinuities e g 9 17 n deviation in axial linearity suggest that any along axis melt migration is confined to subsegments of the ridge and seem to preclude the segment length migration of melt proposed in some current models the transition of melt supply beneath the overlap basin might favor a continuous low velocity zone underlying this feature if true basin development may be related to the subsidence of a mechanically weak crustal lid the proposed model for osc development therefore views ridge axis discontinuities as the surficial response of misalignment and or defocusing of melt supply in the uppermost mantle

investigation of the moho discontinuity beneath the chinese mainland using deep seismic sounding profiles abstract we herein describe the depth distribution of the moho beneath the chinese mainland determined via compilation and resampling of the interpreted results of crustal p wave velocity structures obtained from deep seismic soundings dsss performed since the pioneering dss work carried out in the qaidam basin in 1958 for the present study 114 wide angle seismic profiles acquired over the last 50 years were collated we included results for crustal structures from several profiles in japan and south korea to improve the reliability of the interpolation of the moho depth distribution our final moho map shows that the depth of the moho ranges from 10 to 85 km the deepest moho discontinuity at approximately 70 85 km beneath the tibetan plateau was formed by ongoing continent continent collision the moho beneath the eastern north china craton at a relatively constant 30 35 km has endured mantle lithosphere destruction the moho depths determined from active seismology are consistent within 3 5 km with results obtained from gravity inversion and surface wave tomography the spatial variation of the moho depth crustal formation and composition of different tectonic blocks contribute to controls on the spatial distribution of the seismicity and rheology in the crust beneath mainland china

the axial topographic high at intermediate and fast spreading ridges abstract an axial topographic high is commonly observed at both fast spreading ridges and some segments of intermediate spreading ridges at fast rates the axial high is primarily created by the buoyancy of hot rock and magma beneath the rise as newly formed crust is transported off axis little vestige of an axial high is observed on the ridge flanks in contrast at intermediate rates a significant component of the positive topography may be a volcanic construction preserved on the ridge flanks as abyssal hills which are split axial volcanoes we suggest this difference in the nature of the axial high reflects a lithosphere strong enough to support construction of a volcanic crestal ridge at intermediate spreading rates but only rarely at fast rates relict overlap ridges found within the discordant zones left by overlapping spreading centers is one class of ridge flank topography which appears to have a significant volcanic constructional component even at fast spreading ridges unlike topography away from these discontinuities the relief and shape of overlapping spreading centers is preserved as relict ridge tips are rafted onto the ridge flanks reduced magma supply at these discontinuities may give rise to an axial lithosphere strong enough to support volcanic construction of overlap ridges low axial lithospheric strength may also account for the lack of normal faults within the innermost 1 2 km of fast and some intermediate spreading ridges with a thin weak brittle layer at the ridge crest tensile failure will predominate and few normal faults will form depths to the axial magma chamber reflector observed in multi channel seismic data limit the thickness of the brittle layer on axis to less than 1 2 km for much of the epr this depth is comparable to depths over which tensile failure within the oceanic crust will predominate estimated from the griffith criteria for fracture initiation 0 5 1 5 km as the brittle layer thickens strengthens away from the ridge shear failure begins and the large scale normal faults associated with abyssal hill relief develop

ephemeral magma chambers in the trinity peridotite northern california abstract the trinity massif comprises the major lithologies of an ophiolite as defined at the 1972 penrose conference previous studies have shown however that it differs from the semail oman or table mountain newfoundland ophiolitic massifs particularly because its crustal section is thin and because its mantle section has vertical plastic flow planes these features have led to an interpretation of the trinity massif as a fragment of slow spreading oceanic lithosphere le sueur et al 1984 boudier and nicolas 1985 in this paper we show that the trinity gabbros occur in discontinuous kilometre sized pockets intrusive into the mantle peridotites the internal stratigraphy and the petrological characteristics of these gabbros suggest that they formed in short lived magma chambers these ephemeral magma chambers developed after the end of the plastic deformation in the surrounding mantle when it had cooled down to lithospheric temperatures we discuss the possibility that these small and ephemeral magma chambers formed at a slow spreading oceanic ridge

recent dueling propagation history at the fastest spreading center the east pacific rise 26 32 s the east pacific rise epr 26 32 s located between the easter microplate and the juan fernandez microplate has the world s fastest spreading rate and tectonics characterized by dueling ridge propagation a glori b and sea beam 2000 side scan bathymetry survey together with other geophysical data collection was conducted aboard the r v melville covering an approximately 5 by 5 area centered on this part of epr the side scan and bathymetric data reveal the complete geometry of the propagator system the western and eastern ridges both curve inwardly and are overlapped 120 km with an offset to overlap ratio of nearly one several abandoned ridges and rifts are clearly mapped on the east flank and a few failed rifts are also mapped near the western ridge tip a fracture zone discovered south of easter island indicates a stage of stable ridge transform intersection in the past an inversion with magnetic anomaly vectors was carried out for the area a resultant magnetic boundary strike map was interpreted with the downward component profiles to construct a magnetic isochron map the spreading and propagation rates were estimated from the magnetic isochrons the net southward propagation rate of the western ridge is 120 km m y for the last 1 9 m y the spreading rate of the eastern ridge has been 150 km m y and its current propagation rate is estimated as 500 km m y toward the north intense asymmetric spreading as high as 30 is observed at both ridges together with the side scan and bathymetric data the magnetic lineations provide significant constraints on the evolution of the propagator system and a propagation model with cyclic rift failure was applied to model the tectonic evolution during the last 2 m y a normal ridge transform intersection evolved into a nontransform overlapping offset 1 95 ma and the eastern and the western ridges have propagated alternately the dueling propagation history is characterized by the dominant southward propagation of the western ridge and the transitory propagation events of the eastern ridge with a variety of overlap lengths widths and ridge curvatures in the transition zone

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

continuous mantle melt supply beneath an overlapping spreading center on the east pacific rise tomographic images of upper mantle velocity structure beneath an overlapping spreading center osc on the east pacific rise indicate that this ridge axis discontinuity is underlain by a continuous region of low p wave velocities the anomalous structure can be explained by an approximately 16 kilometer wide region of high temperatures and melt fractions of a few percent by volume our results show that oscs are not necessarily associated with a discontinuity in melt supply and that both osc limbs are supplied with melt from a mantle source located beneath the osc we conclude that tectonic segmentation of the ridge by oscs is not the direct result of magmatic segmentation at mantle depths

variability in oceanic crustal thickness and structure multichannel seismic reflection results from the northern east pacific rise multichannel seismic reflection data acquired between 8 50 and 9 50 n and between 12 30 and 13 30 n along the east pacific rise provide a three dimensional view of the young oceanic crust seafloor to moho reflection travel times vary by up to 0 9 s within our study areas the total range of crustal travel times in the 9 n area is 1 55 to 2 45 s the total range in the 13 n area is 1 60 to 2 05 s the variation is systematic indicating thinner crust locally associated with overlapping spreading centers oscs and in the 9 n area segment scale variation along crustal isochrons crustal travel time is found to be a valid proxy for oceanic crustal thickness outside of the axial low velocity volume thickness can be calculated from time to 500 m even in the axial region thickness can be calculated to 1 km if low velocity zone position is known crustal thicknesses calculated from travel times vary by 2 6 km in the 9 n area and by 1 5 km in the 13 n area the majority of this variation is attributed to seismic layer 3 the lower crust segment scale variation of 1 8 km 5 5 to 7 3 km thickness is observed in the 9 n area with thinnest crust formed between 9 40 and 9 50 n and thickest formed between 9 15 and 9 20 n results imply a three dimensional pattern of magma supply to the 9 n segment the osc at 9 03 n is associated with major disruptions of the segment scale pattern in the form of local thin areas within the discordant zone the smaller osc at 12 54 n is not associated with dramatic changes in thickness of the surrounding crust in the absence of oscs the process of crustal formation displays more temporal uniformity along flow lines than spatial uniformity along isochrons within a segment thicker crust does not always correlate with shallower ridge bathymetry broader axial cross section or more negative mantle bouguer or subcrustal gravity anomaly variable thickness of the crust mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result we hypothesize that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick moho transition zone and a subcrustal gravity low such a diapiric upwelling center appears to be now located beneath the east pacific rise near 9 40 to 9 50 n