segment scale variations in the crustal structure of 150 300 kyr old fast spreading oceanic crust east pacific rise 8 15 n 10 5 n from wide angle seismic refraction profiles summary we have simultaneously inverted seismic refraction and wide angle moho reflection traveltimes for the 2 d crustal thickness and velocity structure of 150 300 kyr old crust along the east pacific rise epr between the siqueiros and clipperton fracture zones fzs our results show a strong correlation between ridge segmentation and upper and mid crustal seismic velocities with higher velocities near segment centres and lower velocities near segment ends low crustal velocities at the clipperton and siqueiros fzs are interpreted as fracturing resulting from brittle deformation of the crust in the transform domain a relict overlap basin left on the pacific plate by the 9 03 n overlapping spreading centre osc as it propagated southward is associated with a large 1 km s 1 negative upper and mid crustal velocity anomaly this anomaly is consistent with the presence of an unusually thick extrusive section within the basin and with tectonic alteration fracturing and shearing arising from rotation of the basin as it was formed the discordant zone left by this osc on the cocos plate is characterized by moderately low crustal velocities probably because of crustal fracturing as the osc propagated into older crust higher crustal velocities near segment centres may reflect a higher ratio of dikes to extrusives in the upper crust and lower intensity tectonic alteration of the crust than near segment ends the mean crustal thickness along the epr between the siqueiros and clipperton fzs is 6 7 6 8 km the thickest crust is found beneath the lamont seamounts 9 km and in a southward pointing v shaped band located just north of the off axis trace of the 9 03 n osc 7 3 7 8 km the thinnest crust 6 km is found proximal to the clipperton and siqueiros fzs the crust associated with the off axis trace of the 9 03 n osc is not anomalously thin suggesting that magma supply beneath the osc is similar to that of the northern and southern segments we see a similar pattern of crustal thickness variation to that determined using multichannel reflection data including a gradual thickening of the crust from north to south along the northern ridge segment and the location of the thickest crust just north of the 9 03 n osc however the magnitude of the along axis crustal thickness variation we observe along the northern ridge segment between 9 50 n and 9 15 n 1 3 1 8 km excluding the lamont seamounts is significantly less than the 2 3 km of variation previously reported weakening the case for the existence of a low density mantle diapir at 9 50 n inferred from gravity data the band of thick crust located just north of the off axis trace of the 9 03 n osc suggests a close genetic link between this feature and the osc thus we attribute the pattern of crustal thickness variations along the northern segment to the kinematics of the southward propagating 9 03 n osc over the past 0 5 myr and not to along axis melt migration away from a mantle diapir as previously proposed

axial high topography and partial melt in the crust and mantle beneath the western galapagos spreading center 1 the hot spot influenced western galapagos spreading center gsc has an axial topographic high that reaches heights of 700 m relative to seafloor depth 25 km from the axis we investigate the cause of the unusual size of the axial high using a model that determines the flexural response to loads resulting from the thermal and magmatic structure of the lithosphere the thermal structure simulated is appropriate for large amounts of cooling by hydrothermal circulation which tends to minimize the amount of partial melt needed to explain the axial topography nonetheless results reveal that the large axial high near 92 w requires that either the crust below the magma lens contains 35 partial melt or that 20 melt is present in the lower crust and at least 3 in the mantle within a narrow column 35 in the crust are considered unreasonable it is likely that much of the axial high region of the gsc is underlain by a narrow region of partially molten mantle of widths approaching those imaged seismically beneath the east pacific rise a narrow zone of mantle upwelling and melting driven largely by melt buoyancy is a plausible explanation

nontransform offsets of the pacific cocos plate boundary and their traces on the rise flank a multi beam bathymetric reconnaissance of the fast spreading crest of the east pacific rise between 7 n and 2 6 n found the axial ridge to be interrupted by four 4 13 km non transform offsets they have probably persisted since initiation by a small change in spreading direction 3 5 ma at each offset the volcanic rift zones veer 15 toward each other and overlap for 5 25 km around deep basins intervening lateral displacements of the rift zones are too short 2 5 km to break the linear axial volcanoes but cause local bends in their plan shapes some examples have a morphology that mimics the larger intervolcano offsets interpretation of a thorough bathymetric and magnetic survey around the 9 km 5 5 n offset indicates that the mainly one sided spreading from the overlapped sections of rift zone occurs by a combination of highly asymmetric dike accretion and episodic small scale jumps of the eruptive zone the northern rift zone is elongating propagating at the expense of the southern one the resulting along axis migration of this and other examples at speeds comparable to spreading rates causes the rise flank scars of the nontransform offsets equivalent to the fracture zones of transform faults to be oblique to the spreading direction these traces are recognizable within the highly lineated fault block terrain of the rise flanks because of the distinctive morphology of the 5 25 km wide bands that accreted at overlapped sections of the rift zones here lineation is less marked and oblique to the spreading direction and volcanic relief is more important than relief caused by faulting the west flank trace of the 5 5 n offset is occupied by a line of volcanoes and shallow volcanic ridges several other volcanic chains previously identified as hot spot traces seem to have grown in similar tectonic settings

evolution of abyssal lavas along propagating segments of the galapagos spreading center abstract the unusual petrological diversity of abyssal lavas erupted along some segments of the galapagos spreading center is a direct consequence of the propagation elongation of these segments into older oceanic crust with increasing distance behind propagating rift tips relatively unfractionated morb erupted close to the tips are joined first by feti basalts bimodal assemblage and then by a wide range of basaltic and siliceous lavas further behind propagating rift tips this broad range diminishes again approaching the narrow compositional range of adjacent normal ridge segments these compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre existing plate we envisage this evolution as proceeding from small isolated ephemeral magma chambers through increasing numbers of larger increasingly interconnected chambers to the steady state buffered system of a normal ridge throughout this evolution magma supply rates gradually increase and cooling rates of crustal magma bodies decrease high degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved at other propagating and non propagating ridge transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas these effects may be evaluated provided a number of tectonic variables including transform length spreading and propagation rates are taken into account

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

sea beam deep tow investigation of an active oceanic propagating rift system galapagos 95 5 w we have tested and corroborated the propagating rift hypothesis with high resolution sea beam and deep tow data collected over the galapagos 95 5 w propagating rift system the propagating rift is continuously breaking through the cocos plate at a velocity of about 50 km m y with an azimuth of about 273 away from the galapagos hotspot this process transfers cocos lithosphere to the nazca plate including the preexisting spreading center which fails as it is gradually preempted by the propagating rift the spreading center azimuth is being changed by about 8 clockwise by this rift propagation the active propagating and failing rift axes overlap by about 15 km and are joined by a broad and anomalously deep zone of nonrigid plate deformation rather than by a transform fault although rift propagation is continuous on a scale of a few kilometers 50 000 years and lithospheric transferral from one plate to the other occurs mostly continuously as the preexisting rift fails it breaks into discrete segments the geometry of seafloor fabric and tectonic elements such as active and failed rifts the v shaped pattern of pseudofaults and the oblique tectonic fabric in the sheared zone of transferred lithosphere are observed as predicted this quantitative verification provides strong support for the propagating rift hypothesis

variations in axial morphology along the galapagos spreading center and the influence of the galapagos hotspot the galapagos spreading center gsc is marked by systematic changes in axial morphology between the inca fracture zone fz at 85 5 w and the 95 5 w propagator we analyze these changes using new swath bathymetry and magnetic data acquired aboard the b o hesperides during the galapagos 96 experiment within 350 km of the galapagos hotspot the ridge axis is associated with an east pacific rise epr like axial high at increasing distance from the hotspot the axial high broadens and deepens forming a distinctive transitional axial morphology tam the axis in this transitional region is typically a broad zone 20 km wide consisting of very rough volcanic and fault generated topography west of 95 w this tam evolves into a 20 40 km wide 400 1500 m deep axial valley typical of the slow spreading mid atlantic ridge mar there is not an abrupt change from axial high to rift valley along the gsc but a distinct tam occurs over a distance of 200 300 km along axis and is accompanied by a gravity estimated crustal thickening of 1 2 km the boundary between an axial high and this tam is quite abrupt and occurs along a segment that is less than 9 km long these changes in axial morphology are primarily caused by variations in magma supply along the gsc due to the entrainment and dispersal of plume mantle from the galapagos hotspot however the changes in morphology are not symmetric about the galapagos fz at 91 w the axial high topography extends farther east of the 91 w fz than to the west and the rift valley which develops west of 94 w is not found at comparable distances along the gsc east of the hotspot axial depth variations are also asymmetric across the 91 w fz this asymmetry in both morphology and axial depth variation is attributed to a full spreading rate increase along the gsc from 46 mm yr at 97 w to 64 mm yr at 85 w off axis depth changes are symmetric about the 91 w fz and suggest that 15 40 of on axis depth variation is dynamically supported

geochemical evidence for low magma supply and inactive propagation at the galapagos 93 25 w overlapping spreading center 1 major and trace element concentrations sr nd and pb isotopic ratios and plagioclase and olivine compositions of samples from 92 35 w to 94 10 w on the galapagos spreading center gsc constrain magmatic processes associated with the 93 25 w overlapping spreading center osc significant variations in parental magma compositions and extent of low pressure fractionation over short along axis distances suggest small discontinuous or poorly mixed magma chambers mineralogical petrological and geochemical data indicate this osc is characterized by low overall melt supply regional along axis geochemical gradients along the western gsc e g in 206pb 204pb nb zr and la 8 sm 8 are interrupted by increased variability within the osc consistent with less efficient mixing of melts in this region despite a long term history of westward propagation bathymetric and geochemical data are inconsistent with recent propagation of this offset the eastern limb appears to have recently retreated and left behind a series of abandoned ridges within the overlap zone the eastern limb shows two progressive westward trends of decreasing ratios of highly to moderately incompatible elements one outside and one within the overlap zone the overlapping portion of the western limb also has low ratios of these elements these patterns are consistent with previously depleted mantle being preferentially melted beneath this offset

detailed tectonics near the tip of the galapagos 95 5 w propagator how the lithosphere tears and a spreading axis develops a model for propagation of an active spreading axis into preexisting lithosphere has been developed based on intensive detailed studies using seamarc ii sea beam deep tow and alvin near the tip of the propagator growing westward at 52 km m y along the east west trending galapagos spreading axis near 95 5 w initial lithospheric rifting appears to be accommodated along reactivated roughly east west abyssal hill faults at the tectonic tip extension leads to downdropping of a keystone block which is pervasively cut by normal faults and fissures the initial volcanism initial volcanic tip occurs about 6 5 km behind the tectonic tip as basalt erupts through these fissures covering most of the keystone block the neovolcanic axial tip occurs another 4 5 km back at the western termination of a well defined axial pillow ridge along which volcanism is localized spreading along the propagator axis accelerates to the full rate 29 mm yr half rate at the full rate tip interpreted to be 10 km behind the neovolcanic axial tip or 21 km behind the tectonic tip a complex pattern of strain in the tip area results from the interaction of plate spreading transform shear stress crack propagation dynamic depression of the western end of the axial valley and the transform zone and variations in the strength thickness heterogeneity and anisotropy of the lithosphere southwesterly curvature of the neovolcanic axis near its tip is inferred to result primarily from the response of young thin weak relatively homogeneous lithosphere to counterclockwise rotation of the maximum tensional stress direction relative to the spreading direction due to the influence of shear stress from the transform zone similar stress interactions likely occur at the tectonic tip but are inferred to be accommodated in this older thick strong heterogeneous and anisotropic lithosphere by oblique slip along a set of left stepping reactivated preexisting east west abyssal hill structures although on a time scale of 106 years the overall process of propagation is mostly continuous rifting at the tectonic tip appears discontinuous at a scale of 1500 m or 30 000 years based on the east west spacing of grabens formed as rifting begins the scale of discontinuity of propagation of the neovolcanic axis is 3 5 km or 50 000 100 000 years based on the distribution of northeast trending features interpreted as beheaded western terminations of paleoneovolcanic axes the extension involved in rifting is 10 20 based on summing the horizontal normal separation of all normal faults and fissures integrated and averaged over the zone of rifting these values are approximately doubled if simple shear listric faulting is dominant rather than pure shear normal faulting

high resolution surveys along the hot spot affected galapagos spreading center 2 influence of magma supply on volcanic morphology the galapagos spreading center gsc at 89 95 w exhibits large gradients in magma supply at a relatively constant intermediate spreading rate making this area an ideal natural laboratory to study the effects of magma supply on volcanism at seafloor spreading ridges prior work shows that the gsc develops from axial valley to shallow axial rise and a shallow magma sill much like a typical fast spreading ridge as the contribution of the hot spot increases the volcanic morphology varies with magma supply in a predictable manner that we divide into three terrains based on the characteristic style of volcanic emplacement and edifice construction within each terrain the volcanic cone terrain comprises most of the gsc and is characterized by prominent volcanic cones within a 1 km wide and 100 m deep axial graben approaching the area of maximum mantle plume influence at 91 w the gsc axis lies along an elevated axial rise split by a 1 km wide and 100 m deep axial graben and the style of volcanism shifts to axial volcanic ridge terrain characterized by axis elongate low relief ridges of pillow lava the lava channel terrain comprises only one segment on either side of the maximum magma supply at 91 w where sheet lava flows and lava channels are relatively widespread a general lengthening of seafloor fissures with increasing magma supply suggests a greater tendency toward linear source eruptions in agreement with the volcanic observations these results suggest that magma supply rather than magma chamber depth or rate of tectonic extension is the primary influence on lava morphology hence eruptive processes at seafloor spreading ridges in general in both the axial volcanic ridge and lava channel terrains a single prominent volcanic cone exists within each volcanic segment suggesting a segment centered magma focusing

variations in upper crustal structure due to variable mantle temperature along the southeast indian ridge 1 there is a systematic variation in axial morphology and axial depth along the southeast indian ridge seir with distance away from the australian antarctic discordance an area of cold uppermost mantle since spreading rate 72 76 mm yr and mantle geochemistry appear constant along this portion of the seir the observed variations in axial morphology and axial depth are attributed to a gradient in mantle temperature in this study we report results from a multichannel seismic investigation of on axis crustal structure along this portion of the seir three distinct forms of ridge crest morphology are found within our study area axial highs rifted axial highs and shallow axial valleys axial highs have a shallow 1500 m below seafloor bsf magma lens and a thin 300 m layer 2a along the ridge crest rifted axial highs have a deeper 2100 m bsf magma lens and thicker 450 m layer 2a on axis beneath shallow axial valleys no magma lens is imaged and layer 2a is thick 450 m there are step like transitions in magma lens depth and layer 2a thickness with changes in morphology along the seir the transitions between the different modes of axial morphology and shallow structure are abrupt suggesting a threshold type mechanism variations in crustal structure along the seir appear to be steady state persisting for at least 1 m y portions of segments in which a magma lens is found are characterized by lower relief abyssal hills on the ridge flank shallower ridge flank depths and at the location of along axis mantle bouguer anomaly mba lows the long wavelength variation in ridge morphology along the seir from axial high segments to the west to axial valley segments to the east is linked to the regional gradient in mantle temperature superimposed on the long wavelength trend are segment to segment variations that are related to the absolute motion of the seir to the northeast which influence mantle melt production and delivery to the ridge

petrologic consequences of rift propagation on oceanic spreading ridges abstract the production of anomalously differentiated lava compositions at several mid ocean spreading centers can be attributed to magmatic processes associated with propagating rifts the degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers low supply rates and moderate cooling rates allow advanced degrees of closed system fractionation to occur high supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma propagating rift tips are a special class of ridge transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge this balance is affected by the spreading rate the propagation rate of the rift the length of the bounding transform and proximity to hotspots maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion geochemical anomalies may persist though changing in degree and extent for several million years on ridge segments that stop propagating the concept of magnetic telechemistry is generally supported by our study but in the vicinity of hotspots magnetic anomaly amplitude may be controlled more by bathymetric and or thickened magnetic layer effects than by geochemistry

magmatic processes at superfast spreading mid ocean ridges glass compositional variations along the east pacific rise 13 23 s major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading 150 mm yr east pacific rise epr 13 23 s and near ridge seamounts comprise 212 chemical groups we interpret these groups to represent the average composition of individual lava flows or groups of closely related flows groups slightly enriched in k2o t morb are distributed variably along the axis in contrast to the galapagos spreading center where t morb are extremely rare this result is consistent with the interpretation that t morb magmas arise from low melting temperature k rich heterogeneities in the subaxial epr mantle the galapagos spreading center which is migrating to the west in an absolute reference frame is underlain by mantle previously processed and depleted in the t morb component during melting events giving rise to earlier epr magmas excluding t morb there are nearly monotonic twofold increases in k ti and k p of axial lavas from 23 s to 13 s from 22 s to 17 s these gradients correlate with isotopic ratios but north of 17 s there is a reversal of isotopic gradients indicating recent decoupling of the isotopic and minor element ratios in the subaxial mantle a strong southward increase in degree of differentiation for approximately 200 km north of the large offset at 20 7 s correlates with a gradient in bathymetry consistent with previous interpretations that this offset is propagating to the south samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive evolved fractionation signatures of rift propagation suggesting that propagating rift tips have been abandoned preferentially to failing rift tips glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset and possibly also south of 22 s the latter region may be affected by deformation accompanying northward growth of the easter microplate near ridge seamounts on the pacific plate between 18 19 s comprise two distinct populations those aligned approximately parallel to the spreading direction are extremely variable in major element composition but consistently enriched in sr relative to nearby axial lavas smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and sr relative to axial lavas the degree of differentiation of axial lavas between 18 19 s can be related to the structural development of the rift axis and or vigor of hydrothermal activity of individual segments glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area primary magmatic segmentation mainly reflects mantle source variations the boundaries of which correlate with the largest physical offsets in the rise axis between the easter microplate and garrett transform zone a secondary magmatic segmentation defined by the along axis continuity of similar parental magma compositions or liquid lines of descent occurs with a length scale varying from 11 to 185 km with an average of 69 57 1 km the boundaries of these segments mainly occur at overlapping spreading centers all first second and third order physical offsets correspond to secondary magmatic segment boundaries but some secondary magmatic segment boundaries also occur at small fourth order ridge axis discontinuities the secondary magmatic segments define the length scale of mantle melting variations mainly variations in extent of melting but not the scale of melt extraction processes that feed the axis this scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth order physical discontinuities along axis there is a good positive correlation of average secondary magmatic segment length with spreading rate for four well sampled areas varying from 20 to 150 mm yr secondary magmatic segments also become more variable in axial length with increasing spreading rate the average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature defined by extent of magmatic differentiation which bears little systematic relation to physical or other kinds of magmatic segmentation at 13 23 s the length scale of this variation is 217 60 1 km approximately corresponding to the wavelength of rolls in the gravity field observed off axis taken together the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere magma supply to the axis and crustal magmatic temperature reflect independent regionally decoupled processes

history of rift propagation and magnetization intensity for the cocos nazca sspreading center analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the cocos nazca spreading center forward models of the magnetic anomalies accounting for spreading rate variations ridge jumps asymmetric spreading magnetization intensity variations and bathymetry show excellent agreement with observed anomalies spreading rates can be constrained to a common finite rotation history through anomaly 4a with three changes in rates rate changes at about 1 5 ma and 4 1 ma correspond to changes in rate gradients and occur during the well calibrated part of the reversal timescale so they can unquestionably be identified as true changes in plate motion a 15 rate decrease at about 5 2 ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale the change at 4 1 ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0 6 ma rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis propagation has been predominantly away from the hotspot with jumps predominantly south ward propagation rates have ranged from 30 to 120 mm yr commonly near 70 mm yr origin of most propagation sequences is difficult to interpret but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot magnetic anomaly amplitude appears to be a reliable tracer of fe content of lavas several generalizations can be drawn about along axis variations in magnetization intensities since 8 ma high magnetizations are only observed at the far ends relative to the galapagos hotspot of segments at least 150 km long offset at the end of a high magnetization segment is at least 15 km and there are no offsets larger than 30 45 km between high magnetization segments and the reconstructed position of the hotspot we interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along axis flow of evolved magma the gradients in supply result from subaxial flow of hotspot derived asthenosphere in a narrow conduit this flow is only partly obstructed by an offset of 20 30 km but entirely blocked by an offset of 50 km

the axial summit graben and cross sectional shape of the east pacific rise as indicators of axial magma chambers and recent volcanic eruptions abstract the axis of the east pacific rise epr undulates up and down hundreds of meters over distances of 30 200 km along strike the deep areas occurring at transform faults and other ridge axis discontinuities such as overlapping spreading centers oscs we have suggested that systematic variations in depth and cross sectional shape of the rise are indicators of the changes in the local axial magmatic budget along a given ridge segment 1 a comparison of recently collected multichannel seismic mcs data 2 with our sea beam and seamarc ii data have allowed us to test and advance this hypothesis along the epr from 9 to 13 n there is an excellent correlation between three parameters that are all directly related to the phase of a magmatic cycle along a given ridge segment the cross sectional shape of the rise the presence or absence of an axial summit graben and the presence or absence of a shallow axial magma chamber as interpreted from mcs data where the axial magma chamber is present the cross sectional shape of the ridge is broad and an axial summit graben is recognized along the axis in contrast where the cross sectional shape of the rise is narrow and triangular an axial magma chamber is not detected and an axial summit graben is absent these ridge axis characteristics tend to occur along deeper portions of a given ridge segment often near ridge axis discontinuities we suggest that these systematic variations in ridge axis morphology cross sectional shape and structure presence or absence of an axial graben reflect spatial and temporal variations in the magmatic budget of the ridge axis where the magmatic budget is waxing shallow level magma reservoirs in the crust and underlying upper mantle swell creating a broad axial bulge with a summit graben where the magmatic budget is diminished the crustal magma chamber is small this proposed correlation of shape structure and magmatic parameters fails along only two short portions of the ridge in these areas there is evidence for an axial magma chamber and the rise has a broad cross sectional shape but there is no summit graben bottom photographs and submersible results however show that in these areas the rise crest is covered with very fresh lavas undisrupted by faulting suggesting that the summit graben has been recently filled in by lava flows and the development of a summit graben or a linear caldera by volcano tectonic collapse has not yet occurred

extensional transform zones and oblique spreading centers extensional transform zones etzs are plate boundary segments of order 100 km long that strike at angles between 15 and 45 to the extension direction they are characterized by neovolcanic tectonic zones comprising overlapping en echelon volcanic systems and or faults that trend 30 75 to the extension direction sometimes accompanied by a riedel shear below these surficial en echelon structures the deformation is aseismic and ductile and the plate boundary is probably continuous etzs occur in fast and slow spreading and rifting environments and may persist in a stable configuration for several million years etzs link oblique spreading segments to transform faults in the manus and probably the lau backarc basins the reykjanes peninsula and tjornes fracture zone in iceland and the mak arrasou in afar are etzs that link subaerial to submarine spreading or rifting segments the brawley and cerro prieto seismic zones appear to be etzs in the imperial and mexicali valleys that link the san andreas imperial and cerro prieto transform faults experimental and analytical models of transtensional deformation in brittle ductile systems match many of the observed characteristics of etzs and oblique spreading centers including variably sigmoidal to straight en echelon faults that are not parallel to the extension direction the contrasting fault patterns reflect the rheology of the models and lithosphere they are more sigmoidal when the strain in the lower ductile layer is more focused causing the axial faults to curve as they propagate toward parallelism with the less ductile rift margins the angle o between the faults and the extension direction decreases with the angle between the strike of the zone and the extension direction etzs occur in the range 15 45 whereas oblique spreading centers have 45 90 and transform relay zones have 0 15 oblique fast spreading segments exhibit ridge parallel faults and volcanic systems o presumably reflecting locally rotated stress fields whereas at oblique slow spreading centers o is closer to orthogonal o 90

crustal thickness along the western galapagos spreading center and the compensation of the galapagos hotspot swell wide angle refraction and multichannel reflection seismic data show that oceanic crust along the galapagos spreading center gsc between 97 w and 91 25pw thickens by 2 3 km as the galapagos plume is approached from the west this crustal thickening can account for v52 of the 700 m amplitude of the galapagos swell after correcting for changes in crustal thickness the residual mantle bouguer gravity anomaly associated with the galapagos swell shows a minimum of 325 mgal near 92 15pw the area where the gsc is intersected by the wolf darwin volcanic lineament wdl the remaining depth and gravity anomalies indicate an eastward reduction of mantle density estimated to be most prominent above a compensation depth of 50 100 km melting calculations assuming adiabatic passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of v30 c the associated thermal expansion and increase in melt depletion reduce mantle densities but to a degree that is insufficient to explain the geophysical observations the largest density anomalies appear at the intersection of the gsc and the wdl our results therefore require the existence of compositionally buoyant mantle beneath the gsc near the galapagos plume possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the galapagos islands that is later transported to the gsc our estimate for the buoyancy flux of the galapagos plume 700 kg s 31 is lower than previous estimates while the total crustal production rate of the galapagos plume 5 5 m 3 s 31 is comparable to that of the icelandic and hawaiian plumes

segmentation and eruptive activity along the east pacific rise at 16 n in relation with the nearby mathematician hotspot the 16 n segment of the east pacific rise is the most overinflated and shallowest of this fast spreading ridge in relation with an important magma flux due to the proximity of the mathematician hot spot here we analyze the detailed morphology of the axial dome and of the axial summit trough ast the lava morphology and the geometry of fissures and faults in regard to the attributes of the magma chamber beneath and of the nearby hotspot the data used are 1 m resolution bathymetry combined with seafloor photos and videos at the dome summit the ast is highly segmented by 10 third order and fourth order discontinuities over a distance of 30 km often two contiguous and synchronous asts coexist such a configuration implies a wide 1100 m minimum zone of diking the existence of contiguous asts their mobility their general en echelon arrangement accommodating the bow shape of the axial dome toward the hotspot plus the existence of a second magma lens under the western half of the summit pla teau clearly reflect the influence of the hotspot on the organization of the spreading system the different asts exhibit contrasted widths and depths we suggest that narrow asts reflect an intense volcanic activity that produces eruptions covering the tectonic features and partially filling the asts ast widening and deepening would indicate a decrease in volcanic activity but with continued dike intrusions at the origin of abundant sets of fissures and faults that are not masked by volcanic deposits

correlated geophysical geochemical and volcanological manifestations of plume ridge interaction along the galapagos spreading center 1 as the galapagos hot spot is approached from the west along the galapagos spreading center there are systematic increases in crustal thickness and in the k ti nb zr 3he 4he h2o and na2o content of lavas recovered from the spreading axis these increases correlate with progressive transitions from rift valley to axial high morphology along with decreases in average swell depth residual mantle bouguer gravity anomaly magma chamber depth average lava mg ca al ratio and the frequency of point fed versus fissure fed volcanism magma chamber depth and axial morphology display a threshold effect in which small changes in magma supply result in large changes in these variables these correlated variations in geophysical geochemical and volcanological manifestations of plume ridge interaction along the western galapagos spreading center reflect the combined effects of changes in mantle temperature and source composition on melt generation processes and the consequences of these variations on magma supply axial thermal structure basalt chemistry and styles of volcanism

high resolution surveys along the hot spot affected galapagos spreading center 3 black smoker discoveries and the implications for geological controls on hydrothermal activity 1 to explore effects of hot spots on mid ocean ridge hydrothermal systems we conducted nested sonar hydrothermal plume and near bottom photographic surveys along the portion of the galapagos spreading center gsc influenced by the galapagos hot spot from longitude 95 89 5 w we report the first active high temperature black smokers to be found on the gsc at longitudes 94 4 5 w and 91 56 2 54 3 w describe two areas of recently inactive smokers at longitudes 91 23 4 23 7 w and 91 13 8 w and document an older inactive site at longitude 90 33 4 w all imaged vents issue either from dike induced fissures along linear axial volcanic ridges and collapses or from a caldera magmatic control of hydrothermal systems also is revealed by spatial clustering of plumes within the topographically elevated middles of volcanic ridge segments with inferred centralized melt supply in searched areas smokers are more typical than diffuse flow vents but total gsc plume incidence is half of that expected from the spreading rate why dike fed fissures provide permeable pathways for efficient hydrothermal extraction of magmatic heat but cones without calderas do not among many point source cones surveyed only the two with calderas had detectable plumes possibly dominance of point source over linear source melt delivery on the gsc decreases plume incidence also similar maturities of observed vents and their host lava flows indicate that hydrothermally active volcanic segments along the western gsc are contemporaneously in a waning phase of volcanic hydrothermal activity perhaps ridge hot spot interaction produces melt pulses that drive near synchronous volcanic hydrothermal activity on the volcanic segments spanning the hot spot during active periods hydrothermally active dike fed fissures and calderas may be more abundant than we currently observe

reconstructing lava flow emplacement processes at the hot spot affected galapagos spreading center 95 w and 92 w 1 volcanic eruptions at mid ocean ridges mors control the permeability internal structure and architecture of oceanic crust thus establishing the foundation for the evolution of the ocean basins to better understand the emplacement of submarine lava flows at mors we have integrated submersible based geologic mapping with remote sensing techniques to characterize the lava flow morphology within previously mapped lava flow fields produced during single eruptive episodes at the galapagos spreading center gsc detailed attributes describing the surface geometry and texture of the lava flows have been extracted from high resolution sonar data and combined with georeferenced visual observations from submersible dives and camera tows based on signatures contained in these data a fuzzy logic based classification algorithm categorized lava flow morphology as pillows lobates or sheets the resulting digital thematic maps offer an unprecedented view of gsc lava morphology collectively covering 77 km2 of ridge axis terrain at a resolution of 2 m 2 m error assessments with independent visual reference data indicate approximately 90 agreement comparable to subaerial classification studies the digital lava morphology maps enable quantitative spatially comprehensive measurements of the abundance and distribution of lava morphologies over large areas of seafloor and within individual eruptive units a comparison of lava flow fields mapped at lower and higher magma supply settings 95 and 92 w respectively indicates that effusion rates increase along with magma supply and independent of spreading rate at the gsc although a complete range of eruptive behavior exists at each setting

a 1 5 ma record of plume ridge interaction at the western galapagos spreading center 91 40 92 00 w abstract shallow elevated portions of mid ocean ridges with enriched geochemical compositions near hotspots document the interaction of hot geochemically enriched plume mantle with shallow depleted upper mantle whereas the spatial variations in geochemical composition of ocean crust along the ridge axis in areas where plume ridge interaction is taking place have been studied globally only restricted information exists concerning temporal variations in geochemistry of ocean crust formed through plume ridge interaction here we present a detailed geochemical study of 0 1 5 ma ocean crust sampled from the western galapagos spreading center wgsc axis to 50 km north of the axis an area that is presently experiencing a high influx of mantle material from the galapagos hotspot the tholeiitic to basaltic andesitic fresh glass and few bulk rock samples have incompatible element abundances and sr nd pb isotopic compositions intermediate between depleted normal mid ocean ridge basalt n morb from 95 5 w along the wgsc and enriched lavas from the galapagos archipelago displaying enriched e morb type compositions only limited and no systematic geochemical variations are observed with distance from the ridge axis for

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

propagation and linkage of oceanic ridge segments we have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates the ridge propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other the ridge propagation force increases as ridge ends approach but then declines sharply as the ends pass so ridge segments may overlap somewhat the sign of the path factor changes as ridge ends approach and pass so the overlapping ridge ends may diverge and then converge following a hook shaped path the magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting for different loading conditions simulating ridge push plate pull and ridge suction we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated for all conditions the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap thus crust in this zone may fracture and weaken in preparation for the formation of a through going transform fault the calculated shear planes rotate toward an orientation about 90 from the strike of ridges as the ends pass thus favoring the orthogonal arrangement of ridges and transforms the magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated the mean stress is tensile in the interaction zone so basins may form there except in the case of ridge push loading the planes across which the maximum tension acts are oblique to ridges thus favoring obliquely oriented normal faults bounding the transform valley

crustal structure beneath the galapagos archipelago from ambient noise tomography and its implications for plume lithosphere interactions 1 to constrain the seismic velocity structure of the crust beneath the galapagos archipelago we conducted a tomographic study using high frequency rayleigh waves obtained from cross correlations of ambient noise we analyzed waves with periods between 5 and 8 5 s sensitive to shear wave velocity vs structure between about 3 and 10 km depth after accounting for the effect of water depth crustal velocities are up to 25 lower than those of very young crust at the east pacific rise and are comparable to those of hawaii we attribute the lower than normal velocities to the combined effect of heating and the presence of melt in the crust above the galapagos plume as well as the construction of a highly porous volcanic platform emplaced atop preexisting oceanic crust on average vs between 3 and 10 km depth beneath the western archipelago is up to 15 lower than beneath the eastern archipelago we attribute the west to east velocity increase to a decrease in porosity of the volcanic platform and to cooling of the crust after its passage above the galapagos plume the results of this study in combination with previous work indicate that many of the unusual aspects of the galapagos archipelago are the result of variations in the thickness and internal structure of the chemical and thermal lithospheres our findings indicate that observed variations in the flexural response to loading observed in the galapagos cannot be explained by the current thermal state of the lithosphere instead the flexural response likely represents varying elastic strength at the time of loading we also propose that the northwest and northeast trending alignments of volcanic centers found throughout the archipelago the darwinian lineations may be associated with preexisting zones of weakness in the lithosphere formed during earlier episodes of ridge jumping and ridge propagation that were later reactivated by stresses generated by plume lithosphere interactions

influence of ridge migration and proximity to hot spots on the morphology of slow and intermediate spreading centers recent observations of ridge bathymetry suggest magmatic segmentation at fast and intermediate spreading centers is linked to the migration of the spreading axis over the mantle at fast and intermediate spreading centers carbotte et al 2004 observed that leading segments those that are offset in the direction of ridge migration are typically shallower interpreted to be magmatically robust relative to trailing segments across first and second order discontinuities the model set forth for this correlation invokes asymmetrical mantle upwelling in response to the absolute motion of the ridge axis and the entrainment of melt from across discontinuities in this investigation differences in ridge axis depth across first and second order discontinuities are examined within the context of absolute plate motions for portions of the slow spreading mid atlantic ridge mar 22 36 n and 25 35 s latitude intermediate spreading galapagos spreading center gsc 83 98 w and intermediate spreading south east indian ridge seir 77 114 e portions of each of these areas display chemical and or physical anomalies resulting from hot spot ridge interaction along non hot spot influenced sections of intermediate spreading ridges leading segments coincide with shallower segments across 72 of the first and second order discontinuities 86 of transform faults and 55 of second order discontinuities depth asymmetries vary with ridge offset length with maximum asymmetries for ridge offset lengths of 50 100 km a weaker correlation is observed between ridge migration direction and ridge morphology at the slow spreading mar where leading segments are shallower across 60 of first and second order discontinuities for hot spot influenced spreading centers hot spot proximity dominates ridge morphology at intermediate spreading centers but it is not a consistent predictor of axial depth asymmetries at slow spreading centers this spreading rate dependent influence of ridge migration and hot spot proximity on axial morphology may reflect a more limited entrainment of melt from across slow spreading discontinuities due to the predominance of three dimensional upwelling and melt focusing to segment centers

tectonic evolution of ridge axis discontinuities by the meeting linking or self decapitation of neighboring ridge segments recent deep tow sea beam and seamarc ii studies indicate that ridge axis discontinuities along the east pacific rise evolve in several different ways where magmatic pulses along the spreading center happen to meet head on a low point or saddle point occurs along the axial depth profile of the spreading center where the magmatic pulses misalign overlapping spreading centers develop and two distinct evolutionary paths are possible one spreading center tip may cut through to and link with the adjacent en echelon ridge chopping off the opposing ridge tip as proposed by macdonald and fox 1983 in addition either spreading center may cut inside or outside of itself repeatedly decapitating its own ridge tip crack propagation studies show that the crack propagation force g drops significantly when the ratio of crack overlap to crack offset l w exceeds 3 applying this relation to spreading centers we suggest that propagation of the individual spreading center tips may stall when l w is greater than 3 if linkage has not yet occurred then the next magmatic pulse that propagates along the ridge may be deflected away from the path of the existing ridge tip decapitating the ridge tip in a process we call self decapitation subsequent magmatic pulses may be deflected or derailed from the existing path of the ridge because freezing of the axial magma chamber near ridge axis discontinuities creates a core of coherent unfaulted gabbroic rock along the spreading axis which is strong relative to the intensely faulted lithosphere on either side of the frozen magma chamber in addition the local stress field rotates near the discontinuity so that subsequent magma pulses and associated cracking fronts will tend to deflect away from the preexisting path of the ridge for propagating rifts which represent first order changes in plate boundary geometry l w remains 1 5 so that g is maintained near its maximum value and episodes of continued propagation in the same direction are enhanced

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

galapagos hot spot spreading center system 1 spatial petrological and geochemical variations 83 w 101 w we report on the petrology and geochemistry of basalts dredged at 40 50 km intervals along the galapagos spreading center between 83 sup 0 w and 101 sup 0 w 40 stations emphasis is on spatial variations of whole rock major elements rare earths trace metals of the first transition series and the nature of phenocryst assemblages and their abundances these results provide new constraints on the nature and scale of mantle source heterogeneities melting conditions thermal field and dynamics of crustal formation of the region we suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements which appears of worldwide extent we reject vogt and deboer s 1976 model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line because this model would not explain the gradients in ree observed about the galapagos platform our preferred model combines the mantle plume binary mixing model of schilling 1973 with the concept of recurring rift propagation proposed by hey 1977a we further propose that pulsating mantle plume flux perhaps inmore the form of a chain of blobs may initiate the development of new rifts and their propagation the present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow a two million year period is suggested for the last 4 m y from wilson and hey s 1979 information rigorous testing of our preferred model is possible less

galapagos hot spot spreading center system 2 87sr 86 sr and large ion lithophile element variations 85 w 101 w thirty eight samples of basalts dredged from the galapagos spreading center gsc between 85 w and 101 5 w longitudes have been analyzed for k rb cs ba and sr contents as well as for 87sr 86sr ratios basalts between 85 w and 89 w segment of the gsc range from 0 7025 to 0 7028 in 87sr 86sr and those between 95 5 w and 101 5 w range from 0 7026 to 0 7027 these basalts are characterized by lree depleted patterns basalts from 89 w to 95 5 w segment having slightly lree depleted to lree enriched patterns la sm e f between 0 8 and 2 4 range from 0 7027 to 0 7031 in 87sr 86sr the longitudinal 87sr 86sr variation shows a maximum at about 92 w where the gsc is nearest to darwin island the alkali and alkaline earth elements as well as several element ratios also show maxima in their longitudinal profiles but these occur at about 91 w the apparent shift of 87sr 86sr maximum with respect to that of the trace elements may be related to differences in the degree of partial melting the highest 87sr 86sr from the gsc is only slightly lower than the highest 87sr 86sr for tholeiitic basalts from the galapagos archipelago several possible hypotheses have been examined to account for these geochemical and sr isotopic variations in the light of the presently available data the most promising explanation seems to be a binary mixing of hot spot or plume type material with the source of normal mid ocean ridge basalts and resulting rift propagation both end members seem to be heterogeneous and therefore it is at present difficult to put severe constraints on this binary mixing nevertheless the flow pattern of mantle material from the hot spot does not appear to be radial but is preferentially channeled along the 91 w and 92 5 w fracture zones toward the rift

a three dimensional study of a crustal low velocity region beneath the 9 03 n overlapping spreading center overlapping spreading centers oscs play a key role in models of magma distribution at fast spreading ridges to investigate the relationship between ridge axis discontinuities and magma supply we conducted a three dimensional seismic reflection and tomography experiment at the 9 03 n osc along the east pacific rise tomographic analysis imaged a broad mid crustal low velocity zone lvz beneath parts of the overlapper and the associated overlap basin demonstrating that it is magmatically robust the complementary datasets reveal a complex storage and tapping of melt the lvz and melt sill at either end of the overlap basin are not simply centered beneath the rise crest but are skewed inwards the subsequent focussing of the lvz and sill beneath the axis of the eastern limb appears to be due to melt migration toward the tip the osc western limb is less magmatically robust and may be in the process of dying

interaction of a mantle plume and a segmented mid ocean ridge results from numerical modeling abstract previous investigations have proposed that changes in lithospheric thickness across a transform fault due to the juxtaposition of seafloor of different ages can impede lateral dispersion of an on ridge mantle plume the application of this transform damming mechanism has been considered for several plume ridge systems including the reunion hotspot and the central indian ridge the amsterdam st paul hotspot and the southeast indian ridge the cobb hotspot and the juan de fuca ridge the iceland hotspot and the kolbeinsey ridge the afar plume and the ridges of the gulf of aden and the marion crozet hotspot and the southwest indian ridge this study explores the geodynamics of the transform damming mechanism using a three dimensional finite element numerical model the model solves the coupled steady state equations for conservation of mass momentum and energy including thermal buoyancy and viscosity that is dependent on pressure and temperature the plume is introduced as a circular thermal anomaly on the bottom boundary of the numerical domain the center of the plume conduit is located directly beneath a spreading segment at a distance of 200 km measured in the along axis direction from a transform offset with length 100 km half spreading rate is 0 5 cm yr in a series of numerical experiments the buoyancy flux of the modeled plume is progressively increased to investigate the effects on the temperature and velocity structure of the upper mantle in the vicinity of the transform unlike earlier studies which suggest that a transform always acts to decrease the along axis extent of plume signature these models imply that the effect of a transform on plume dispersion may be complex under certain ranges of plume flux modeled in this study the region of the upper mantle undergoing along axis flow directed away from the plume could be enhanced by the three dimensional velocity and temperature structure associated with ridge transform ridge geometry it is suggested that for a setting where a plume ridge system has one or more transforms a location specific model with appropriate plate boundary geometry be used to assess the importance of ridge offsets on upper mantle geodynamics

comment on sensitivity of seafloor bathymetry to climate driven fluctuations in mid ocean ridge magma supply olive et al reports 16 october 2015 p 310 argue that 10 fluctuations in melt supply do not produce appreciable changes in ocean ridge bathymetry on time scales less than 100 000 years and thus cannot reflect sea level forcing spectral analysis of bathymetry in a region they highlight as being fault controlled however shows strong evidence for a signal from sea level variation

magmatic and tectonic extension at mid ocean ridges 2 origin of axial morphology 1 we investigate the origin of mid ocean ridge morphology with numerical models that successfully predict axial topographic highs axial valleys and the transition between the two the models are time dependent simulating alternating tectonic and magmatic periods where far field extension is accommodated by faulting and by magmatism respectively during tectonic phases models predict faults to grow on either side of the ridge axis and axial height to decrease during magmatic phases models simulate magmatic extension by allowing the axial lithosphere to open freely in response to extension results show that fault size and spacing decreases with increasing time fraction spent in the magmatic phase fm magmatic phases also simulate the growth of topography in response to local buoyancy forces the fundamental variable that controls the transition between axial highs and valleys is the rise sink ratio fm ft t m where fm ft is the ratio of the time spent in the magmatic and tectonic periods and t m is the ratio of the characteristic rates for growing topography during magmatic phases 1 m and for reducing topography during tectonic phases 1 t models predict the tallest axial highs when fm ft t m 1 faulted topography without a high or valley when fm ft t m 1 and the deepest median valleys when fm ft m t 1 new scaling laws explain a global negative correlation between axial topography and lithosphere thickness as measured by the depths of axial magma lenses and microearthquakes exceptions to this trend reveal the importance of other behaviors such as a predicted inverse relation between axial topography and spreading rate as evident along the lau spreading center still other factors related to the frequency and spatial pervasiveness of magmatic intrusions and eruptions as evident at the mid atlantic and juan de fuca ridges influence the rise sink ratio fm ft t m and thus axial morphology

tectonics of the failing spreading system associated with the 95 5 w galapagos propagator the failing spreading system associated with the galapagos 95 5 w propagator system comprises a complex set of volcanic and structural provinces data from detailed surveys using gloria seamarc ii sea beam deep tow the submersible alvin magnetics and bottom cameras constrain the plate boundary geometry and evolution of this area although the failing spreading system may involve diffuse spreading the patterns of sedimentation volcanism and faulting suggest localization the failed spreading axis composed of a set of right stepping en echelon basins extends and ages southeastward from offset and overlapping doomed failing axes the failing axis is offset 13 km left laterally from the doomed axis although there may have been a preexisting offset in the doomed axis lack of structural evidence for this and lack of an offset of this magnitude in the brunhes matuyama magnetic reversal argue against this possibility a model whereby the offset is transient does not explain the pattern of failed spreading basins which leads directly away from the currently failing axis detailed structural and volcanic patterns suggest that this offset may be caused by a westward progression of northward jumps in the location of the failing axis like a secondary propagator probably in association with the migrating strain stress field of the migrating transform zone of the propagator system this is modelled as a migrating extensional relay zone merz similar to the relay zones observed at some fast slipping transform faults except that it propagates westward with the overall system bathymetric and magnetic evidence for tectonic elements associated with the merz supports this model although the expected extinct trace of the doomed axis is not obvious in the present incomplete data set unlike other models for the plate boundary geometry the merz model does not require serendipity for temporal and spatial juxtaposition of the primary propagator and the offset within the failing spreading system observation of a similar geometry in the lau basin supports this preferred model present data from the galapagos failing spreading system are inconclusive however and the merz model remains speculative if valid this model implies that in at least some cases paired propagators act together the merz is too short and deep to drive its own propagation according to standard cracking models

magmatic and tectonic segmentation of the intermediate spreading costa rica rift a fine balance between magma supply rate faulting and hydrothermal circulation 3 d tomographic modelling of wide angle seismic data recorded at the intermediate spreading costa rica rift has revealed a p wave seismic velocity anomaly low located beneath a small overlapping spreading centre that forms a non transform discontinuity at the ridge axis this low velocity zone displays a maximum velocity anomaly relative to the background ridge axis crustal structure of 0 5 km s 1 has lateral dimensions of 10 5 km and extends to depths 2 5 km below the seabed placing it within layer 2 of the oceanic crust we interpret these observations as representing increased fracturing under enhanced tectonic stress associated with the opening of the overlapping spreading centre that results in higher upper crustal bulk porosity and permeability evidence for ongoing magmatic accretion at the costa rica rift ridge axis takes the form of an axial magma lens beneath the western ridge segment and observations of hydrothermal plume activity and microearthquakes support the presence of an active fluid circulation system we propose that fracture pathways associated with the low velocity zone may provide the system through which hydrothermal fluids circulate these fluids cause rapid cooling of the adjacent ridge axis and any magma accumulations which may be present the costa rica rift exists at a tipping point between episodic phases of magmatic and tectonically enhanced spreading the characteristics inherited from each spreading mode have been preserved in the crustal morphology off axis for the past 7 myr using potential field data we contextualize our seismic observations of the axial ridge structure at the whole segment scale and find that the proposed balance between magmatic and tectonically dominated spreading processes observed off axis may also be apparent along axis and that the current larger scale magma supply system at the costa rica rift may be relatively weak based on all available geophysical observations we suggest a model for the inter relationships between magmatism faulting and fluid circulation at the costa rica rift across a range of scales which may also be influenced by large lithosphere scale structural and or thermal heterogeneity

constructing the crust along the galapagos spreading center 91 3 95 5 w correlation of seismic layer 2a with axial magma lens and topographic characteristics the seafloor and narrow 0 5 1 5 km cross axis width we also image a thin seismic layer 2a 0 24 0 42 km that thickens away from the ridge axis by as much as 150 west of 92 7 w the magma lens is deeper 2 5 4 5 km and wider 0 7 2 4 km and layer 2a is thicker 0 36 0 66 km and thickens off axis by 40 the positive correlation between layer 2a thickness and magma lens depth supports the interpretation of layer 2a as the extrusive volcanic layer with thickness controlled by the pressure on the magma lens and its ability to push magma to the surface our findings also suggest that narrower magma lenses focus diking close the ridge axis such that lava flowing away from the ridge axis will blanket older flows and thicken the extrusive crust off axis flow of lava away from the ridge axis is probably promoted by the slope of the axial bathymetric high which is largest east of 92 5 w west of 94 wt he transitional axial morphology lacks a prominent bathymetric high and layer 2a no longer thickens off axis we detect no magma lens west of 94 7 w where a small axial valley appears the above changes can be linked to the westward decrease in the magma and heat flux associated with the fading influence of the galapagos hot spot on the galapagos spreading center index terms 3035 marine geology and geophysics midocean ridge processes 7220 seismology oceanic crust 0935 exploration geophysics seismic methods 3025

morphology and segmentation of the western galapagos spreading center 90 5 98 w plume ridge interaction at an intermediate spreading ridge author posting american geophysical union 2003 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geochemistry geophysics geosystems 4 2003 8515 doi 10 1029 2003gc000609

neodymium isotopic evidence for galapagos hotspot spreading centre system evolution nd and sr isotope variations along the galapagos spreading centre support morgan s preferential subcrustal flow channel connecting the galapagos mantle plume with the northeastward migrating ridge axis decoupling of la sm and nd isotope variations near 91 w further suggests that basalts are derived from smaller degrees of melting and are deeper beneath this elevated part of the ridge

geology and petrogenesis of lavas from san cristobal island galapagos archipelago volcanism became emergent on san cristobal island 2 3 m y b p in the eruption of floods of alkaline basalt volcanism then coalesced in a central shield volcano which erupted continually until 0 6 m y b p at 0 9 m y b p activity began to jump to the northeast and the style of volcanism changed to fissure fed eruptions that have continued almost to historical time three distinct magma series occur on san cristobal an alkaline series a series of morb like tholeiites and a series of incompatible element enriched tholeiites that are typical of oceanic islands each series is represented by very primitive basalts that have been little modified since segregation from their sources each series also has more evolved lavas which range from low mg basalts to hawaiites and icelandites these evolved lavas are the result of simple fractional crystallization of magmas which produces major and compatible element compositions similar to those of observed primitive lavas the isotope and trace element data suggest that the different series are generated by different degrees of partial melting and mixing of both a depleted and an enriched source the first source is thought to be the depleted asthenosphere which generates the morb9s of the galapagos spreading center and the enriched source is the mantle plume associated with the galapagos hotspot

overlapping spreading centers implications from crack growth simulation by the displacement discontinuity method overlapping spreading centers osc s are a fundamental aspect of accretionary processes at intermediate and fast spreading centers and typically occur at deep points along the axial depth profile they have a characteristic geometry consisting of two en echelon overlapping curving ridges separated by an elongated depression the length to width ratio of this overlap basin is typically 3 1 we have been successful in reproducing the overlapping spreading center geometry by modelling the growth of two initially parallel elastic cracks of given length and offset in a tensile stress field at infinity a boundary element displacement discontinuity method was used to solve this problem our calculated results are compared with seafloor observations in terms of the size and shape of the overlap region for small osc s there is a very good agreement between calculations and observations but for large ones the overlap basin tends to be longer than our predicted results indicate this suggests that the assumptions made in the model i e perfectly elastic isotropic and homogeneous medium are perhaps valid for the brittle lid above the magma chamber that underlies osc s with small offsets 2 km but oversimplified for osc s with large offsets our modelling shows that the initial interaction of closely spaced surface ruptures along spreading centers is to deflect away from one another as they approach the deflection will be the greatest for small misalignments of the fracture systems thus even minor misalignments of the spreading centers may result in the development of osc s where the misalignment is less than the width of the cracking front the fracture systems may meet head on creating a saddle point along the axial depth profile our results support the hypothesis suggested by macdonald et al 1984 in which overlapping spreading centers develop where two magmatic pulses migrate toward each other along the strike of the spreading center following fracture systems and magmatic conduits which are imperfectly aligned

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

estimation of earthquake source parameters of the may 4 1972 event of the hellenic arc by the inversion of waveform data abstract average source parameters of the may 4 1972 m s 6 5 earthquake southwest of crete are obtained by the inversion of body waves a source depth of 40 km is determined which places the source in the mantle the source parameters of the major double couple are strike 309 dip 18 rake 89 and seismic moment m 0 2 62 10 25 dyn cm the source time function is of 5 s duration with a long rise time the mechanism obtained corresponds to a pure thrust with the p axis almost perpendicular to the hellenic arc although a point source is assumed the resulting synthetics match satisfactorily the observed waveforms

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

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

upper crustal structure and axial topography at intermediate spreading ridges seismic constraints from the southern juan de fuca ridge 1 we use multichannel seismic reflection data to image the upper crustal structure of 0 620 ka crust along the southern juan de fuca ridge the study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow cleft and wide vance axial summit grabens asg along most of the axis of both segments we image the top of an axial magma chamber amc the amc along cleft deepens from south to north from 2 0 km beneath the ridge cleft observatory and hydrothermal vents near the southern end of the segment to 2 3 km at the northern end near the site of the 1980s eruptive event along the vance segment the amc also deepens from south to north from 2 4 to 2 7 km seismic layer 2a interpreted as the basaltic extrusive layer is 250 300 m thick at the ridge axis along the cleft segment and 300 350 m thick along the axis of the vance segment however off axis layer 2a is similar in both segments 500 600 m indicating 90 and 60 off axis thickening at the cleft and vance segments respectively half of the thickening occurs sharply at the walls of the asg with the remaining thickening occurring within 3 4 km of the asg along the full length of both segments layer 2a is thinner within the asg compared to the ridge flanks previous studies argued that the asg is a cyclic feature formed by alternating periods of magmatism and tectonic extension our observations agree with the evolving nature of the asg however we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply thus the asg rather than being formed by excess volcanism is a rifted flexural axial high the changes in axial morphology affect the distribution of lava flows along the ridge flanks as indicated by the pattern of layer 2a thickness the fluctuations in magma supply may occur at all spreading rates but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges

high resolution surveys along the hot spot affected galapagos spreading center 1 distribution of hydrothermal activity the spatial density of hydrothermal activity along most mid ocean ridges is a robust linear function of spreading rate or magmatic budget but extreme crustal properties may alter this relationship in 2005 2006 we tested the effect of thickened crust on hydrothermal activity using high resolution mapping of plumes overlying the hot spot affected galapagos spreading center from 95 to 89 42 w 560 km of ridge crest plume mapping discovered only two active high temperature vent fields subsequently confirmed by camera tows though strong plume evidence indicated minor venting from at least six other locations total plume incidence ph the fraction of ridge crest overlain by significant plumes was 0 11 0 014 about half that expected for a non hot spot mid ocean ridge with a similar magmatic budget plume distributions on the galapagos spreading center were uncorrelated with abrupt variations in the depth of the along axis melt lens so these variations are apparently not controlled by hydrothermal cooling differences we also found no statistical difference for a significance level of 0 05 in plume incidence between where the seismically imaged melt lens is shallow 2 0 56 km ph 0 108 0 045 and where it is deep 3 4 0 7 km ph 0 121 0 015 the galapagos spreading center thus joins mid ocean ridges near the iceland reykjanes ridge st paul amsterdam south east indian ridge and ascension mid atlantic ridge hot spots as locations of anomalously scarce high temperature venting this scarcity implies that convective cooling along hot spot affected ridge sections occurs primarily by undetected diffuse flow or is permanently or episodically reduced compared to normal mid ocean ridges

geochronology of galapagos seamounts lavas from seamounts of the central galapagos platform and the carnegie ridge increase in age with distance from the western edge of the platform consistent with a hotspot model for the generation of these features the areal distribution of seamount ages suggests that the pattern of dispersed volcanism seen on the present islands also prevailed between 5 and 6 ma age distance relationships are consistent with a decrease in the velocity of the nazca plate relative to the galapagos hotspot during the past 9 m y most likely at 5 ma when there was a change in pacific hotspot motion alternatively the plate velocity remained constant and volcanism at a single seamount endured up to 7 m y lavas from seamounts along the wolf darwin lineament to the northwest of the galapagos platform are young 1 ma with the youngest measured ages closest to the platform

plume ridge interaction lithospheric stresses and the origin of near ridge volcanic lineaments 1 in many hot spot ridge systems formation of off axis volcanic chains or lineaments occurs between the hot spot centers and nearby mid ocean ridges in some cases such as galapagos kerguelen and possibly the mid pac mountains and tristan these lineaments appear to meet in a focus zone near the hot spot and fan outward in the direction of the ridge axis the origins of these lineaments are not well known and do not easily fit into typical conceptual models of ridge or hot spot volcanism it has been proposed for the galapagos region that such lineaments are caused by channeling of hot asthenosphere from off axis mantle plumes toward mid ocean ridges where enhanced volcanism initiates island formation morgan 1978 alternatively other workers suggest that these lineaments are controlled by patterns of stress in the lithosphere we examine this latter hypothesis by considering the effects of buoyant uplift and asthenospheric shear on the base of the lithosphere induced by an expanding mantle plume using thin plate theory we calculate the two dimensional plan view pattern of depth integrated stresses in a plate of varying thickness both a straight continuous ridge and a ridge transform ridge system are simulated by a displacement discontinuity boundary element model ridge and transform segments have imposed normal and zero shear tractions and we test different idealized tectonic or far field stress conditions assuming that volcanism is promoted by lithospheric tension and aligns along lineaments parallel to trajectories of least tensile stress calculations reproduce the rough fan shaped pattern of lineaments between the galapagos archipelago and the galapagos spreading center the focus of the fan shape is over the plume center when it interacts with a straight ridge but the fan focus appears closer to a segmented ridge offset by a transform fault if the ridge segments are more tensile than the far field stress this condition provides a plausible explanation for the apparent focus at the galapagos north of the hot spot center the width of the fan pattern along the ridge axis is predicted to increase as the plume ridge separation increases and as the far field stress becomes more tensile parallel to the ridge the observed width along the galapagos spreading center is consistent with a nearly isotropic remote stress models predict the lithospheric tension caused by plumes to promote lineament formation on only young lithosphere in support of this prediction a compilation of data from 23 hot spots shows that lineament formation is common near hot spots with plume ridge separations of 1250 km or on lithosphere of 25 ma but not on older lithosphere

rift valley no rift valley transition at mid ocean ridges a simple mechanical model for the axial topographic and gravity variations with spreading rate is developed the model emphasizes the rapid transition from one mechanism to another with changing spreading rate our model has a brittle layer over a ductile layer which are being pulled by forces away from the ridge axis there exists a potential failure zone of width lf where the stresses within the brittle plate exceeds its yield strength if the yield strength of the brittle plate is nearly constant this width is largely determined by the strain rate and the viscosity of the strong mantle the high ductility of oceanic crust at high temperatures relative to that of mantle at the same temperatures leads to the concept of a decoupling region a critical feature in this model the size of the decoupling region ld is determined by the thickness of the oceanic crust and temperatures within the crust if the decoupling region is small ld lf as shown to be the case for fast spreading ridges then each of the two layers responds separately to the horizontal stretching the brittle plate will be broken only at the axis and any buoyant force of the region would lift it up to form the rise crest high as a result of local isostasy the changing size of the decoupling region gives rise to the transition from atlantic type axial topography to pacific type as well as the transition from a high amplitude highly variable gravity signature to a low amplitude very uniform gravity signature parameters can be chosen so that such a transition occurs at a half rate of 35 mm yr which is the critical half rate suggested by both topographic and gravity data

a simple thermal mechanical model for mid ocean ridge topographic variation summary a simple dynamic model based on the geometry of mantle divergence and thermal parameters controlling equilibrium size of the axial magma chamber explains the variation in topography along mid ocean ridges among morphological characters accounted for are 1 the change from axial valley to axial high type ridge crests with increasing spreading rate 2 the localized occurrence of deeps at ridge transform intersections and 3 the correlation of average transform spacing with spreading rate the model also yields an explanation for anomalous ridge topography associated with oceanic hot spots incorporation of smaller scale bathymetric and ophiolite data into this scheme permits construction of a comprehensive model of ocean crust accretion

effect of the galapagos hotspot on seafloor volcanism along the galapagos spreading center 90 9 97 6 w abstract studies along the mid atlantic ridge mar and east pacific rise epr show that seamount abundance is a strong function of spreading rate at the mar small axial seamounts are a dominant morphologic feature of the inner valley floor while at the epr seamounts are rarely observed within the neovolcanic zone the galapagos spreading center gsc provides an excellent location to test the influence of hotspot related magma supply variations on seamount formation at a relatively constant spreading rate in this study we use multibeam bathymetry data to examine the distribution of axial seamounts with distance from the hotspot along the gsc a numerical algorithm is developed to identify isolated volcanic edifices by searching bathymetry for closed concentric contours protruding above the surrounding seafloor seamount populations are fit with a maximum likelihood model to estimate the total number of seamounts per unit area and the characteristic seamount height the number of seamounts in the axial zone decreases significantly as the galapagos hotspot is approached varying from mar like abundances west of 95 5 w to epr like abundances east of 92 7 w the along axis variation in seamount density corresponds to a change in lava flow morphology from dominantly pillow flows in the west to a combination of pillows and sheet lobate flows in the east these changes in volcanic style suggest a transition from point source to fissure fed eruptions as magma supply increases we find no evidence for along axis variations in lava viscosity and therefore interpret the differences in seamount abundance and flow morphology to indicate an increase in effusion rates approaching the hotspot comparing seamount abundance with axial morphology crustal thickness and the presence and depth of an axial magma chamber amc we find that the transition from point source to fissure fed eruptions is most sensitive to the presence of a steady state amc in summary the correlation between seamount abundance lava morphology and magma supply at the gsc suggests that effusion rate is directly related to the presence of a steady state crustal magma chamber

lithospheric structure and compensation mechanisms of the galapagos archipelago volcanic islands of the galapagos archipelago are the most recent subaerial expression of the galapagos hotspot these islands and numerous seamounts are constructed mainly upon a broad volcanic platform that overlies very young 10 m y oceanic lithosphere just south of the active galapagos spreading center the 91 w fracture zone crosses the platform and creates an estimated 5 m y age discontinuity in the lithosphere major tectonic features of the galapagos include an unusually broad distribution of volcanic centers pronounced structural trends such as the nw se wolf darwin lineament wdl and a steep escarpment along the western and southern margins of the archipelago we use shipboard gravity and bathymetry data along with geosat geoid data to explain the tectonic and structural evolution of the galapagos region we model the gravity anomalies using a variety of compensation models including airy isostasy continuous elastic flexure of the lithosphere and an elastic plate with embedded weaknesses and we infer significant lithospheric strength variations across the archipelago the outboard parts of the southern and western escarpment are flexurally supported with an effective elastic thickness of 12 km this area includes the large shield volcanoes of fenandina and isabela islands where the lithosphere regionally supports these volcanic loads the central platform is weaker with an elastic thickness of 6 km or less and close to airy isostasy the greatest depths to the moho are located beneath eastern isabela island and the central platform thinner lithosphere in this region may account for the broad distribution of volcanoes the extended period of eruption of the central volcanoes and their reduced size the transition from strong to weak lithosphere along the southern escarpment appears to be abrupt within the resolution of our models and can be best represented by a free end or faultlike discontinuity also modeling the wdl as a lithospheric fault increases the match to the observed gravity anomalies the primary feature is the weak central platform whose existence is a logical convergence of several factors the 91 w transform age offset the possible increase in age of the lithosphere along the southern part of the platform due to ridge jumping and the formation of the central platform at a spreading ridge

crustal thickness variations along the southeast indian ridge 100 116 e from 2 d body wave tomography 1 axial morphology along the southeast indian ridge seir systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100 116 e west of the australian antarctic discordance aad basalt geochemistry has a consistent indian mid ocean ridge basalt morb type isotopic signature so changes in axial topography are attributed to variations in both mantle temperature and melt supply wide angle seismic refraction lines were shot to four ocean bottom hydrophones within seir segments p1 p2 s1 and t where each segment is characterized by a different morphology we constructed 2 d crustal velocity models by jointly inverting hand picked p wave refraction pg and moho reflection pmp traveltime data using a top down minimum structure methodology the results show a 1 5 km eastward decrease in crustal thickness across the study area with segment averages ranging from 6 1 km at p1 to 4 6 km at t melt generation models require a 30 c decrease in mantle temperature toward the aad to account for the crustal thickness trend significant changes in axial morphology accompany small scale variations in crustal thickness consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature melt supply and cooling from hydrothermal circulation layer 3 thins by 3 0 km as layer 2 thickens by 1 4 km between segments p1 and t reflecting the eastward decrease in melt supply and increase in melt lens depth the trade off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material