near axis seamount distribution and its relationship with the segmentation of the east pacific rise and northern pacific antarctic ridge 17 n 56 s abstract we used satellite derived gravity anomaly maps and bathymetry data to analyze the distribution of off axis seamounts on young crust on the flanks of the east pacific rise epr and northern pacific antarctic ridge par from 17 n to 56 s latitude we observed large scale variations in the distribution of the volcanoes which we attribute to variations of the regional magmatic budget of the underlying mantle two distinct settings for off axis volcanism were observed at the scale of second order segments the first one favors volcano growth near first or second order discontinuities the second type occurs preferentially near the centers of second order segments we infer that the distribution of seamounts is controlled both by the availability of melt under the flanks of the ridge and by the vulnerability of the young lithosphere near the northern epr and in some places near the par the off axis volcanism results mostly from fracturing of the crust near axial discontinuities probably due to the thermomechanical stresses that develop in the cooling lithosphere in the southern epr especially in the 14 19 s area the young lithosphere is probably much more vulnerable to off axis volcanism the seamount distribution seems to reflect variations in the off axis melt production at the scale of second order segments 150 km we infer that the difference in volcanic setting close to segment ends or close to segment centers between the northern and southern epr results from the difference in spreading rate and consequently the variation in rheology of the young lithosphere

on the thermal structure of the southern tibetan crust summary we present a quantitative analysis of the heat flow measurements recently made in southern tibet by francheteau et al the data document a sharp heat flow anomaly of 54 12 mw m 2 the sediment ages now available and reported here show that thermal effects due to sedimentation are negligible three physical effects must be taken into account in the interpretation heat refraction across the complex structures of the tibetan crust erosion in transient thermal conditions and unsteady lateral heat transfer we show that the refraction of heat may be responsible for up to 30 per cent of heat flow variations we derive a new formula for the heat flow erosion correction and show that the correction which is usually applied can be a gross overestimate in thermally perturbed regions accounting for the possible magnitude of heat refraction across the geological features of southern tibet the amplitude of the anomaly could be as large as 70 mw m 2 the anomaly can only be due to a shallow and recent heat source we show that in such a case erosion effects are small less than 10 per cent whatever the erosion rate is and therefore do not affect the above values significantly using both the amplitude and the spatial characteristics of the anomaly we show that the source must be younger than 1 myr and shallower than 10 km this is in good agreement with the results of magnetotelluric soundings carried out in the same area the source could well be as young as 50 000 yr which would make it a likely origin for the widespread geothermal activity observed further north on the plateau these results are evidence that melting processes are active today in the tibetan crust near the yalu zangbo suture zone more than 45 myr after the india asia collision and more than 200 km away from the site of underthrusting of the indian lithosphere

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

submarine lava flow direction revealed by neutron diffraction analysis in mineral lattice orientation the ocean crust is formed by the rising of magma from mid ocean ridges and voluminous 1 30 km3 flows of lava away from ridge axes however our understanding of the emplacement kinematics of submarine lava is often limited to plan view geometries of near axis lava drilled cores provide in situ access to the intact internal structure of submarine lavas we used neutron diffraction to study off axis lava flows drilled into the uppermost crust of odp iodp site 1256 cocos plate we provide quantitative insights into submarine lava microstructures and strong evidence for a secondary lava injection into the interior of a solidifying flow of lava along the nw se direction parallel to the paleo ridge axis of the east pacific rise the dynamics of lava inflow are controlled by crystal abundance and the temperature of the lava crystal mixture rather than by local seafloor topography we provide a description of an in situ shear within submarine lavas revealed by composite shape and lattice preferred orientations accounting for a dominant laminar nonuniform type flow

east pacific rise 8 10 30 n evolution of ridge segments and discontinuities from seamarc ii and three dimensional magnetic studies magnetic bathymetric and seamarc ii side scan sonar data from the ridge flanks adjoining the overlapping spreading center osc at 9 03 n on the east pacific rise epr are used to establish the evolution of this ridge axis discontinuity and adjacent ridge segments within the past 2 4 m y a three dimensional inversion for magnetization shows the discontinuity evolved from one of small offset 2 km to an 8 km offset osc within the past approximately 1 0 m y the osc has left a v shaped discordant zone indicating an average southward migration of 42 km m y since the beginning of anomaly 2 time 1 8 m y the west flank discordant zone consists of discrete abandoned overlap basins bounded by highly magnetized ridge tips with anomalous lineations while the east flank discordant zone is a broad swath of high magnetization anomalous lineations and greater depths kinematic modelling indicates that southward migration has not been steady but has been accomplished by a series of episodic propagation events with rates ranging from 500 mm yr ridge tips have been abandoned at intervals of approximately 50 000 100 000 years both by linkage of neighboring ridge segments and by self decapitation magnetic modelling suggests rotations of overlap basins of 25 consistent with predictions of kinematic modelling orientation of magnetic isochrons indicate a 3 6 counterclockwise change in pacifie cocos plate motion within the past 1 m y assuming orthogonal spreading throughout this time although the discontinuity currently located at 9 03 n existed prior to this change in plate motion its growth into a right stepping 8 km offset osc may be causally related to this change the current configuration of intratransform spreading within the siqueiros has developed since onset of the brunhes anomaly 0 73 m y possibly in response to this recent counterclockwise change in plate motion in addition to the osc there is evidence for several small discontinuities offsetting the ridge during anomaly 2 time which have since migrated along the ridge and vanished an axial magnetization high is observed along the crest of the epr change in the magnitude of the axial magnetization high or shifts in its location relative to the bathymetric axis coincide with several devals within the area magnetization maxima are found at ridge transform intersections and the osc these highs may reflect presence of highly fractionated feti basalts erupted from magma chambers with restricted supply located adjacent to both large and short offset ridge axis discontinuities discrete magnetization highs are found off axis adjacent to clipperton and siqueiros fracture zones indicating that production of feti basalts at ridge transform intersections is episodic magnetizations are higher north of clipperton than south of it possibly reflecting a long lived starvation in magma supply north of clipperton and more abundant supply to the south most seamounts within the area have the same magnetization polarity as surrounding seafloor indicating that seamount formation occurs near the ridge axis the exceptions lie near magnetic reversal boundaries and are consistent with seamount formation as close as 10 km from the ridge axis few pronounced magnetization highs are associated with seamounts indicating that the magnetized source layer is no thicker beneath them than surrounding seafloor or that seamount basalts are substantially less magnetic than normal mid ocean ridge basalts

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

accumulations of melt at the base of young oceanic crust as hot solid rock ascends through the mantle the pressure around it decreases while its temperature decreases approximately adiabatically during the ascent the melting temperature decreases faster than the adiabat temperature and if the initial temperature of the rock is high enough the rock will begin to melt at the depth where the adiabat crosses the melting temperature curve models of upwelling beneath mid ocean ridges predict significant melt production over a zone several tens of kilometres wider than the ridge axis region suggesting that some of the ascending melt must arrive away from the ridge axis here i report ocean bottom seismograph observations of an uncommon and previously unreported marine seismic phase which provide strong evidence that melt accumulates in sills at the base of young oceanic crust the phase results from the conversion of compressional p waves to shear s waves on interaction with the crust mantle boundary the moho this phase is rare and very difficult to model but when visible it is strikingly prominent and usually appears at unexpectedly great source to receiver ranges the observations imply the presence of bodies of extremely low rigidity presumably melt sills at the moho close to the receiver melt may be fairly mobile at the base of the crust allowing ridges to accumulate melt through a lateral plumbing system at the locally uplifted boundary between the crust and the underlying thermally expanded mantle escape of this melt to the sea floor may be responsible for the formation of off axis seamounts

discrete plumbing systems and heterogeneous magma sources of a 24 km3 off axis lava field on the western flank of east pacific rise 14 s the largest known mid ocean ridge off axis lava flow field occurs slightly off axis near 14 s along the east pacific rise epr it comprises at least four volcanologically discrete units we collected lava samples from 24 sites within the off axis lava field and 7 sites on the adjacent ridge crest using the shinkai 6500 submersible the lava field comprises at least three distinct morb compositions all of which are different from the lavas collected from the nearby ridge axis the east and west cones and the northern lobe of the lava field consist of normal morb n morb lavas with a low concentration of incompatible elements and low lile hfse and lree hree ratios by contrast the samples from the west plain of the field have a higher concentration of incompatible elements and higher lile hfse and lree hree ratios indicating t morb character the lava samples collected from the summit of the east cone show the highest concentration of lile elements and lrees among the lava field emorb lava the n morb of the off axis lava field are more depleted in incompatible elements than the adjacent epr axis lavas possibly reflecting the re melting of the residual mantle in the off axis region the e morb lava was probably derived from fertile mantle that did not undergo melting beneath the spreading center t morb which occupies the main part of the 14 s lava field is the product of magma mixing between n morb and e morb magmas 2007 elsevier b v all rights reserved

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

investigation of a vine matthews magnetic lineation from a submersible the source and character of marine magnetic anomalies we report here the first attempt to map directly the boundary of a vine matthews magnetic stripe on the seafloor our objectives are to study the processes of oceanic crustal accretion as recorded in the reversal transition zone and to investigate the formation of the magnetic source of vine matthews magnetic anomalies our deep tow and alvin based magnetic studies focus on the matuyama brunhes reversal transition on the flanks of the east pacific rise near 21 n while the sea level magnetic anomalies are less than average in clarity for the east pacific rise a three dimensional inversion of deep tow data reveals a sharp strike linear polarity transition less than 1 8 km wide macdonald et al 1980a these measurements have been augmented by mounting a vertical magnetic gradiometer on alvin and making 280 reliable polarity determinations along and across the polarity transition zone even on long traverses across both sides of the boundary we find that nearly every magnetic target has the correct polarity i e the same polarity as the regional magnetic lineation this homogeneity in polarity of the magnetic lineations is surprising the magnetic polarity transition in the outcropping volcanic section is sharp and linear along strike delineated in some cases by a clear geologic contact of opposing flow fronts of different ages several weakly magnetized outcrops mapped within the transition zone may have erupted during the time in which the geomagnetic field was reversing the reversal boundary mapped on the seafloor from alvin is displaced 250 m to 500 m nw away from the spreading axis relative to the position of the average boundary as derived from inversion of the deep tow and sea level magnetic data this offset provides a means for estimating the spillover of lava flows away from the spreading axis during the time the crust was formed the combination of deep tow and alvin measurements suggests that circa 0 7 m y ago the crustal accretion zone magnetized volcanic intrusive and plutonic rocks was 2000 2800 m wide while the zone of recent volcanism alone was only 1000 2000 m wide the determination for the most recent reversal agrees well with submersible observations at the present spreading center where the zone of recent volcanism neovolcanic zone varies between 600 and 2000 m in width this very orderly picture for the formation of magnetic lineations and crustal accretion processes appears to conflict with complex deep sea drilling project dsdp magnetic results from the atlantic we suggest that the crustal generating processes and resulting magnetic structure vary significantly with spreading rate on the slow spreading mid atlantic ridge major episodes of volcanism are likely to be infrequent 104 years the magma chamber may be non steady state and the neovolcanic zone shifts or varies in width considerably this sporadic start and stop spreading process will contribute to a highly heterogeneous and complex crustal and magnetic structure as seen in dsdp holes in addition significant faulting and tilting may disrupt slow spreading crust for intermediate to fast spreading centers more frequent volcanism 50 600 years a nearly steady state magma chamber and a narrow stable neovolcanic zone will create a less complex magnetic and crustal structure both as seen from alvin and as inferred from clear sea level magnetic anomalies in the pacific

a new view of ridge segmentation and near axis volcanism at the east pacific rise 8 12 n from em300 multibeam bathymetry 1 new high resolution bathymetry for the east pacific rise between 8 n and 12 n was collected over a 6 km wide swath centered on the ridge axis using the 30 khz simrad em300 multibeam system the coverage area corresponds latitudinally to the designated ridge2000 integrated studies site iss for fast spreading ridges the em300 data gridded at 30 m latitude by 50 m longitude represent a greater than 4x improvement in horizontal resolution over previously available multibeam data and a 2x improvement in depth resolution the new bathymetry was used to update the locations and hierarchy of ridge offsets for this area among the many applications for this data it enables us to tabulate volcanoes half the size that could be previously detected the distribution of near axis volcanic cones 25 m high suggests that this population of small near axis cones results from low effusion rate eruptions of the ridge axis

two forms of volcanism implications for mantle flow and off axis crustal production on the west flank of the southern east pacific rise seamarcii side scan sonar and bathymetric data on the west flank of the east pacific rise 16 18 s reveal a large number of seamounts organized primarily in chains the easternmost ends of the chains near the ridge axis appear to be active as indicated by fresh lava flows in addition areas of unusually high reflectivity representing recent lava flows were found as far as 60 to 80 km from the spreading axis the flows are sometimes dammed by abyssal hill scarps formed near the ridge axis that have maximum relief of the order of 100 m in other cases the scarps appear to be buried by flows the disappearance of scarps can be used to estimate the extent of old lava flows or volcanic debris that are not detected acoustically in the sonar image under this criterion lava flows and volcanic debris surrounding the seamounts cover 40 50 of the survey area of about 18 000 km2 with an estimated average thickness of about 100 m the volume of lava flows and volcanic debris is about 0 8 0 3 of that of the total crust the volume of the seamounts is estimated by isolating the seamounts from the background topography with an anisotropic median filtering technique the volume of seamount edifices is 1 05 0 05 of the volume of the crust thus the total volume of volcanic material extruded off axis is about 1 5 to 2 2 of the volume of the crust the decreasing volcanic activity away from the ridge axis suggests that near axis seamounts may be associated with the upwelling system beneath the ridge perhaps by preferential melting of embedded passive heterogeneities however the decrease in volcanism with increasing distance from the axis also could be caused by a decrease in lithospheric vulnerability to penetration by magma since no apparent east west faults and no simultaneous volcanic activity along the length of a seamount chain are observed north south extension and ridge perpendicular convection are unlikely to be major causes of seamount formation in the study area the existence of closely spaced linear chains and the tendency for fresh flows to be found at near ridge ends of chains suggest that there are discrete sources in the upper mantle active for extended periods over 1 8 m y passive heterogeneities embedded in the upwelling mantle would have to be highly elongated along streamlines to produce persistent nearly stationary melting anomalies a variety of mechanisms may be responsible for the off axis volcanism but the single hypothesis most consistent with all observations is a mini hotspot origin although fresh lavas 80 km off axis are consistent with broad mantle upwelling beneath the ridge the possibility of narrow upwelling cannot be ruled out because the seamounts could be caused by independent buoyant upwelling of mini plumes that can penetrate thin lithosphere more easily near the axis

seamount statistics in the pacific ocean we apply the wide beam sampling technique of jordan et al 1983 to approximately 157 000 km of wide beam profiles to obtain seamount population statistics for eight regions in the eastern and southern pacific ocean population statistics derived from wide beam echograms are compared with seamount counts from sea beam swaths and with counts from bathymetric maps we find that the average number of seamounts with summit heights h h is well approximated by the exponential frequency size distribution h oe h the exponential model for seamount sizes characterized by the single scale parameter 1 is found to be superior to a power law self similar model which has no intrinsic scale in describing the average distribution of pacific seamounts and it appears to be valid over a size spectrum spanning 5 orders of magnitude in abundance large scale regional variations in seamount populations are documented we observe significant differences in seamount densities across the murray fracture zone in the north pacific and the eltanin fracture zone system in the south pacific the eltanin discontinuity is equally evident on both sides of the pacific antarctic ridge in the south pacific regions symmetrically disposed about the ridge axis have very similar seamount densities despite the large difference between pacific plate and antarctic plate absolute velocities evidently any differences in the shear flows at the base of the pacific and antarctic plates do not affect seamount emplacement systematic variations in o and are observed as a function of lithospheric age with the number of large seamounts increasing more rapidly than small seamounts these observations have been used to develop a simple model for seamount production under the assumptions that 1 an exponential size frequency distribution is maintained 2 production is steady state and 3 most small seamounts are formed on or near the ridge axis the limited data available from this study appear to be consistent with the model but they are insufficient to provide a rigorous test of the assumptions or determine accurately the model parameters however the data from the south pacific indicate that the off axis production of large seamounts probably accounts for the majority of seamounts with summit heights greater than 1000 m

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

controls on permo carboniferous precipitation over tropical pangaea a gcm sensitivity study abstract a series of late paleozoic climate model simulations was developed using the genesis atmospheric general circulation model to investigate the independent roles of gondwanan deglaciation atmospheric co 2 rise and regional tectonism on the long term 10 6 yr evolution of continental precipitation over tropical pangaea the model results indicate that either deglaciation or a co 2 increase from 1 to 8 present atmospheric levels could have caused substantial drying and warming at low latitudes a result that is consistent with late paleozoic proxy records yet the atmospheric processes that led to drying differ between these cases the deglaciation of gondwana reduces low latitude temperature gradients in the southern hemisphere decreasing by as much as 60 the meridional overturning in the southern hemisphere winter cell of the hadley circulation the decline in overturning reduces convective precipitation over equatorial pangaea in contrast high levels of co 2 have little effect on the large scale meridional overturning circulation but increase continental temperatures leading to high evaporation rates and reduced available soil moisture the source of convective precipitation on land in comparison to deglaciation and p co 2 regional uplift erosion of the central pangaean mountains cpms has a secondary effect on tropical precipitation in genesis and cannot explain the long term aridification and warming of western pangaea we suggest that the deglaciation of gondwana and a concomitant rise in atmospheric p co 2 could explain much of the late paleozoic climate change over low latitude pangaea

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

structural pattern of the galapagos microplate and evolution of the galapagos triple junctions a 13 000 km2 microplate intervenes to prevent the junction of the nazca cocos and pacific plates in the eastern equatorial pacific ocean west of the galapagos islands the microplate is rotating clockwise at 6 m y about an absolute pole 400 km southeast of its center so that although its northern margin has a similar motion to adjacent parts of the cocos plate its southern margin is 50 slower the perimeter of the galapagos microplate is best defined by bathymetry and has been surveyed with sea beam and examined by deep tow two of the three triple junctions on its margins are ridge ridge ridge rrr junctions 160 km apart on the crest of the east pacific rise epr the most like a classic rrr junction is the pacific nazca galapagos intersection at 1 10 n where the epr axis is joined by a narrow volcanic ridge 70 km long and 1 km high the cocos nazca galapagos junction at the west end of the cocos nazca spreading center is inferred to be of the rff type but the transform fault branches are represented by large nontransform offsets lack of stable transform faults is related to rapid westward migration of the junction caused by lengthening of the cocos nazca axis at 60 70 mm yr this rate which was once required to maintain a stable nazca cocos pacific rrr junction has persisted even after birth of the microplate about 1 ma continuing propagation of the cocos nazca axis has opened up the hess deep rift basin in the eastern epr flank in advance of the eruptive propagator tip this rift valley has a relief of more than 3 km from its uplifted rims to the floor where severe attenuation exposes basal layers of the epr crust the similar dietz deep rift basin at the southeastern margin of the microplate 70 km from hess deep was formed when a lengthening galapagos nazca spreading axis rifted through another part of the epr flank

volcanic growth faults and the origin of pacific abyssal hills the topographic features known as abyssal hills characterize 30 of the ocean floor and yet their origin has been the subject of vigorous debate for over 40 years submersible based investigations show that pacific abyssal hills are created on the flanks of the east pacific rise as horsts and grabens which lengthen with time hills are bounded on one side by ridge facing scarps produced by normal faulting and on the other by more gentle slopes produced by volcanic growth faulting

mantle fluids in the karakoram fault helium isotope evidence abstract the karakoram fault kkf is the 1000 km long strike slip fault separating the western himalaya from the tibetan plateau from geologic and geodetic data the kkf is argued either to be a lithospheric scale fault with hundreds of km of offset at several cm a or to be almost inactive with cumulative offset of only a few tens of kilometers and to be just the upper crustal localization of distributed deformation at depth here we show 3he 4he ratios in geothermal springs along a 500 km segment of the kkf are 3 100 times the normal ratio in continental crust providing unequivocal evidence that a component of these hydrologic systems is derived from tectonically active mantle mantle enrichment is absent along the indus yarlung suture zone isz just 35 km southwest of the kkf suggesting that the mantle fluids flow only within the kkf within the last few ma the kkf must have accessed tectonically active tibetan mantle northeast of the mantle suture which we therefore locate vertically beneath the kkf very close to the surface trace of the isz hence in southwestern tibet indian crust may not now be underthrusting substantially north of the isz even though miocene underthrusting may have placed indian crust north of the isz in the lower half of the tibetan plateau crust this is in significant contrast to central and eastern tibet where underthrust indian material not only forms the lower half of the tibetan crust but is also currently underthrusting for 200 km north of the isz our new constraint on kkf penetration to the mantle allows an improved description of the continuously evolving karakoram fault as a tectonically significant yet perhaps geologically ephemeral lithospheric structure

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

hundreds of small volcanoes on the median valley floor of the mid atlantic ridge at 24 30 n we have identified 481 seamounts from 6 000 km2 of sea beam swaths of the median valley floor of the mid atlantic ridge mar between 24 and 30 n we find that seamount abundances are an order of magnitude larger on average 80 per 1 000 km2 with relief above 50 m than estimated in previous studies along mid ocean ridges in the pacific1 and south atlantic2 suggesting that at this section of the mar seamount volcanism plays an important part in the formation of new oceanic lithosphere the neovolcanic ridge often observed on the inner valley floor3 appears to be constructed totally of individual coalesced seamounts so that our counts represent a minimum estimate the frequency distribution of summit heights of the mar seamounts is consistent with the exponential model describing the heights of off axis seamounts in the eastern pacific4 6 but with different parameters such a distribution of heights may be a fundamental aspect of seamount volcanism related to lithospheric properties and or magmatic plumbing seamount dominated axial volcanism must create a crustal structure very different from classic fissure fed models

an exhumed late paleozoic canyon in the rocky mountains abstract landscapes are thought to be youthful particularly those of active orogenic belts unaweep canyon in the colorado rocky mountains a large gorge drained by two opposite flowing creeks is an exception its origin has long been enigmatic but new data indicate that it is an exhumed late paleozoic landform its survival within a region of profound late paleozoic orogenesis demands a reassessment of tectonic models for the ancestral rocky mountains and its form and genesis have significant implications for understanding late paleozoic equatorial climate this discovery highlights the utility of paleogeomorphology as a tectonic and climatic indicator

reconciling lithospheric deformation and lower crustal flow beneath central tibet a viscous region with deformable boundaries is used to model simultaneous crustal flow and lithospheric coupling in northern tibet this model suggests that 1 the deformation of northern tibet is different from that of southern tibet because of structural differences between the regions 2 the viscosity contrast between the crust and the mantle lithosphere is relatively small beneath northern tibet and 3 crustal flow is compatible with crust mantle coupling under these conditions

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

abundances distribution and sizes of volcanoes in the pacific ocean and implications for the origin of non hotspot volcanoes abstract in this paper i present data on the abundances sizes and crustal age for all volcanoes volcanic islands and seamounts which appear on published bathymetric charts of the pacific ocean these new data shed light on the origin of non hotspot volcanoes and are important in combination with data on the chemical compositions of seamounts and volcanic islands for estimates of the bulk composition of ocean crust these data also provide firm constraints on off ridge oceanic volcanism models results of this study show that the size frequency distribution of pacific volcanoes is poisson like and that the smallest volcanoes are much more abundant than large ones this study shows clearly that the most abundant volcanoes on the earth are the submerged oceanic volcanoes which comprise 5 25 of the oceanic volcanic layer on pacific crust of eocene age and younger the abundance of volcanoes number of volcanoes per unit area increases monotonically with increasing age assuming steady state the production rate of new off ridge volcanoes number of volcanoes per unit area per unit time is inversely proportional to the square root of the lithosphere age 1 on crust older than eocene the number of volcanoes per unit area of crust decreases monotonically with increasing age however the total volume of lava represented by these edifices increases with increasing age size frequency distributions of volcanoes on swaths of successively older crust indicate that these abundance patterns are partly due to the effect of sediment burial of small edifices on old pacific crust as well as the effect of increased lithosphere thickness on seamount size these general patterns are not appreciably changed by omitting from consideration known hotspot volcanoes 2 and volcanoes built at fossil constructional plate margins 3

seismic constraints on shallow crustal emplacement processes at the fast spreading east pacific rise we present the results of nine on bottom seismic refraction experiments carried out over young east pacific rise crust the experiments are unusual in that both the source and receiver are located within a few meters of the seafloor allowing high resolution determinations of shallow crustal structure three experiments were located within the axial summit caldera asc over zero age crust the seismic structure at these three locations is fundamentally the same with a thin 60 m surficial low velocity 2 5 km s layer a 100 to 150 m thick transition zone with velocities increasing by 2 5 km s and a layer with velocities of 5 km s at a depth beneath the seafloor of 130 190 m the surficial low velocity layer and transition zone are defined as seismic layer 2a and the 5 km s layer is defined as the top of layer 2b both the surficial low velocity layer and the transition zone double in thickness within 1 km of the rise axis we model layer 2a as the extrusive sequence and transition zone and the 2a 2b boundary as the top of the sheeted dikes the primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from 150 m to 300 m within 1 km of the asc the thickening of the extrusive layer is interpreted to be due to lava that either overflows the asc walls is emplaced through eruptions outside of the asc or travels laterally from the asc through conduits the most probable cause for the thickening of the transition zone is sill emplacement outside of the asc either from magma that does not reach the surface in an off axis eruption or magma that is transported laterally during the drainage process creating the asc we suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2a

identification and implications of off axis lava flows around the east pacific rise 1 off axis eruptions at ocean ridges provide critical information with respect to underlying crustal plumbing and mantle melting systems a detailed study of basaltic glass samples around the east pacific rise from 12 00 to 12 30 n provides geological evidence for the existence of off axis eruptions with a distinctive chemical composition this composition has not been found along the axis of the epr from 8 to 14 n except at a ridge transform intersection but has been recovered in numerous locations that were farther than 1 km off axis these off axis normal mid ocean ridge basalts or oa nmorb are distinguished by low na2o sr and al2o3 and are unusually depleted in incompatible elements moderately enriched off axis transitional mid ocean ridge basalts oa tmorb with the same compositional tendencies can also be identified comparison of epr axis lavas and the oa type suggests that they come from the same range of unmelted mantle source compositions but that the source of the oa magmas was depleted in incompatible trace elements by removal of a small degree partial melt this would be consistent with the oa type as an epr pooled melt that is missing the low degree melt fraction from deep in the melting regime which provides a reasonable physical model for their formation in this case off axis magmas do not represent the same range of chemical variation as magmas delivered to the axial magma system the oa nmorb are similar to depleted lavas from near epr seamounts other seamount lavas with depleted trace elements have tmorb like major elements and may be classified as oa tmorb the similarity between seamount lavas and the lavas erupted off axis close to the epr suggests that the two are manifestations of the same phenomenon we suggest that seamount type volcanism effectively starts within 1 2 km of the axis this is within the range where lavas derived from the axial plumbing system may also erupt therefore there is a narrow zone where young lava flows from the axial plumbing system and from the off axis systems may overlap lavas erupted off axis may ultimately cover 20 of the seafloor around the epr which is substantially more than previous estimates that were based primarily on morphological studies

deep tow studies of the overlapping spreading centers at 9 03 n on the east pacific rise the deeply towed instrument package of the scripps institution of oceanography was used to study for the first time the fine scale structure of an overlapping spreading center osc system the 9 03 n osc on the east pacific rise epr the eastern and western limbs of this osc system which are 8 km apart and overlap by 27 km show marked differences away from the tips the spreading centers are highly tectonized and differ greatly from the ridge tips the western ridge tip exhibits a few fissures but no sign of recent volcanic activity whereas the eastern ridge tip is highly fissured and has experienced more recent volcanic episodes which are confined to a narrow zone within this highly fissured area in both cases the ridge tip fissures are 10 15 oblique to the strike of the faults that occur on the flanks of the adjacent portion of the opposite spreading axis indicating a recent change in the direction of deviatoric tension and suggesting that the eastern and western spreading centers have been propagating southward and northward respectively the eastern ridge tip fissures are transecting the flank of the western spreading axis suggesting that the eastern spreading center is about to become the through going trace of the east pacific rise in contrast the western spreading center disappears into abyssal hill terrain that was presumably created at the eastern ridge axis some of the very few tectonic features observed in the overlap basin may reflect the existence of a short lived shear couple due to the interaction of the two overlapping spreading centers sediment cover over the whole area and especially in the overlap basin is relatively thick supporting the idea of a time averaged deficit in the magmatic budget at this osc we suggest that the 9 03 n osc developed where two magmatic pulses independent in space and time and propagating along the strike of the east pacific rise away from the loci of melt emplacement failed to meet misalignment of the magmatic conduits resulted in the propagation of the two spreading centers past each other and the development of the ensuing osc geometry independent lines of evidence derived from the three dimensional inversion of the magnetic field at the 9 03 n osc sempere et al 1984 from numerical modelling of the growth of two en echelon elastic cracks in a tensile stress field sempere and macdonald 1986 and from geochemical data langmuir et al 1986 support our interpretation of the deep tow data we suggest that deviations from axial linearity of the spreading centers devals langmuir et al 1986 and small nonoverlapping offsets snoos batiza and margolis 1986 are simply local lows or saddle points along the axial depth profile that arise when two magmatic pulses propagating toward one another meet head on oscs and some saddle points i e devals snoos are all small rapidly evolving ridge axis discontinuities which may represent the surficial expression of the distal ends of small scale longitudinal convection cells beneath the east pacific rise

overlapping rift zones at the 5 5 s offset of the east pacific rise a seabeam and magnetometer survey of the pacific nazca plate boundary around 5 5 s mapped a nontransform offset whose geology and kinematics seem typical of a whole class of structures formed where fast spreading rises are laterally offset for distances less than the width of a subaxial magma chamber though the spreading center is abruptly displaced by 15 km right laterally there is no trace of strike slip faulting instead the ends of two 100 km long axial volcanic ridges veer 15 toward each other and overlap for more than 20 km in this overlapping region between 5 22 s and 5 32 s accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one sided spreading a continuous but dog legged magma chamber is inferred to underlie both rift zones and an intervening 9 km wide volcano studded basin a strong magnetic signature from the crust which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank in the past 0 5 106 years the southern axial ridge has propagated north at 40 mm yr 55 of the spreading half rate

volcanic mound fields on the east pacific rise 16 19 s low effusion rate eruptions at overlapping spreading centers for the past 1 myr 1 volcanic mound fields identified on seamarc ii and hmr1 12 khz side scan data from the southern east pacific rise sepr occur near overlapping spreading centers oscs and migration traces of oscs the volcanic mound fields appear as a distinctive hummocky seafloor fabric due to side scan backscatter reflections from clusters of mound shaped reflectors the lack of growth of the mound fields away from the ridge axis and their occurrence in association with osc traces suggests that mound fields form along the ridge crest near oscs volcanic mound fields are found where 120 khz side scan and visual observations find fields of pillow mounds since pillow mounds are constructed by low effusion rate eruptions the volcanic mound fields found near the oscs and in their migration traces indicate that volcanic effusion rates tend to be lower near ridge discontinuities than midsegment regions this tendency for low effusion rate eruptions at oscs is documented for the past 1 myr three independent measurements of ridge segmentation 1 volcanic segment boundaries marked by the low effusion rate eruptions 2 tectonic segments defined by oscs and 3 magmatic segment boundaries based on continuity of parental magma composition all coincide in the study area high backscatter off axis lava fields not associated with seamounts are found on seafloor younger than 0 2 ma the 0 2 ma corridor corroborates previous results from the distribution of small isolated volcanoes that indicates randomly distributed off axis eruptions mainly occur on crust younger than 0 2 ma

practical geological comparison of some seafloor survey instruments seafloor survey instruments are integral to the study of marine geology because understanding their resolution and limitations is critical we compare how different survey systems represent the seafloor coincident data collected at the galapagos propagator gloria seamarc ii sea beam deep tow camera sled and alvin allow comparisons of how well seafloor features e g faults and volcanoes observed and characterized in high resolution data are represented in lower resolution coarser scale data sets our reported values for the minimum sizes of detected and well represented features show that practical geological resolutions are generally 2 10 times lower than theoretical resolutions care must be taken in evaluating which system to use to address a particular problem

comparison of seafloor tectonic fabric at intermediate fast and super fast spreading ridges influence of spreading rate plate motions and ridge segmentation on fault patterns we have conducted a comparative study of the tectonic morphology of young seafloor using seamarc ii side scan sonar surveys of the intermediate spreading ecuador rift the fast spreading east pacific rise epr 8 30 10 n and the super fast spreading epr 18 19 s we find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments i e with proximity to large and short offset discontinuities we also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer for example regions of super fast spreading are characterized by the largest numbers of short faults the smallest average fault spacing and throw and the highest fault density in addition clusters of short closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area presumably the result of a thinner weaker brittle layer faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets these differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge fault lengths and spacings in all areas approximate exponential distributions the extensional strain represented by fault populations is calculated from the displacement and length distributions of faults and strain estimates of 4 are obtained for each area assuming that fault spacing reflects fracture depth extent where faults initiate we infer a brittle layer thickness of 1 km when faulting begins fault populations are examined for ridge segment scale variations in amagmatic extension we see evidence for greater amagmatic extension associated with long term reduced magma supply along the eastern third of the ecuador rift evidence for local increased brittle extension is also found within 15 km of transform faults discordant zones left by overlapping spreading centers oscs are characterized by low fault abundances at oscs discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting fault azimuths do appear to be useful indicators of plate motion within the epr 8 30 10 n area fault trends record a recent change in pacific cocos plate motion 3 6 at 1 m y consistent with magnetic anomaly and fault lineation data from elsewhere along the northern epr within the ecuador rift fault azimuths scatter within 3 of predicted trends and are consistent with constant spreading about one pole for the past 1 5 m y

trade off in production between adjacent seamount chains near the east pacific rise bathymetric data and side scan sonar images collected near the southern east pacific rise between 15 and 19 s1 2 have revealed many seamounts on the west flank of the rise most of which are organized into chains running perpendicular to the rise axis the number of seamounts of 5 km diameter is at least twice that expected for average east pacific sea floor3 the formation of numerous small 5 myr linear seamount chains and the tendency for fresh lava flows to be found primarily at the near ridge ends of the chains suggest that there are plume like sources in the upper mantle which are relatively stationary active for extended periods and affected by adjacent sources

the relationship between the height of a volcano and the depth to its magma source zone a critical reexamination it is commonly assumed that hydrostatic pressure balance arguments can be used to establish a relationship between the maximum height to which a volcanic edifice is able to grow and the depth at which the partial melts providing its magma supply are formed such a relationship has been used to infer various aspects of the thermal and stress state of the lithosphere beneath volcanic constructs on earth mars io and venus we examine the assumptions behind this relationship which are that 1 a continuous pressure connection exists between source and summit 2 the pressure around the magma source is the local hydrostatic pressure dictated by the depth below the geoid and 3 the melt erupting at the summit has a net positive buoyancy and show that many of them require geologically unreasonable conditions we then critically assess the evidence cited in the literature for the relationship and find that there are other factors that may explain the observations we conclude that volcano heights on the terrestrial planets cannot be related in any simple way to lithospheric thickness or depth to the magma source zone and we review the range of other factors controlling volcano height

seismic imaging of oceanic layer 2a between 9 30 n and 10 n on the east pacific rise from two ship wide aperture profiles we analyze two ship multichannel seismic wide aperture profiles wap acquired on the east pacific rise epr between 9 30 c and 10 n in may and june 1985 for offsets between 2 and 5 km most of the wap common depth point cdp gathers exhibit a strong retrograde reflection like seismic arrival that corresponds to rays turned within a very strong velocity gradient zone underlying the uppermost crustal layer 2a stacking of this postcritical arrival for hundreds of cdp gathers along the wap lines yields remarkably clear and virtually continuous images of the bottom of layer 2a for several tens of kilometers along and across the epr an innovative combination of this image with others of the seafloor sf and of the axial magma chamber amc provides an unusually clear composite picture of young upper crust we combine these images with detailed seismic velocity information also obtained from the cdp gathers to achieve an accurate determination of the layer 2a thickness in the area along the epr axis between 9 30 n and 10 n layer 2a shows a relatively constant thickness between 100 and 150 m 80 120 ms two way travel time across the axis however a line near 9 30 n indicates that layer 2a thickness rapidly increases by approximately a factor of 2 within 2 4 km of the axis and remains nearly constant afterwards thus the final thickness of layer 2a is attained before the development of large scale tectonic faulting strongly suggesting that the thickening of layer 2a away from the axis is due to successive episodes of volcanic activity increasing the thickness of the uppermost crustal section from the top rather than tectonic fracturing lowering the uppermost crustal seismic velocities and pushing the bottom of layer 2a downwards as the crust moves away from the axis

aqueous and isotope geochemistry of mineral springs along the southern margin of the tibetan plateau implications for fluid sources and regional degassing of co2 springs issuing from different faults and shear zones along the crest of the himalayas tap three different levels of crust beneath the tibetan plateau from structurally highest to lowest these are the tingri graben the south tibetan detachment system stds and the ama drime massif adm the aqueous chemistry reflects water rock interactions along faults and is consistent with mapped rock types major ion chemistry and calculated temperatures indicate that spring waters have circulated to greater depths along the n s trending faults that bound the tingri graben and ama drime detachment add compared to the stds suggesting that these structures penetrate to greater depths springs have excess co2 n2 he and ch4 compared to meteoric water values implying addition from crustal sources the 3he 4he ratios range from 0 018 to 0 063 ra and are consistent with a crustal source for he the 13c values of dissolved inorganic carbon dic and co2 gas range from 5 5 to 3 8 and 13 1 to 0 3 versus peedee belemnite respectively sources of carbon are evaluated by calculating isotopic trajectories associated with near surface effervescence of co2 positive 13c values of the tingri graben and stds springs are consistent with decarbonation of marine carbonates as the source of co2 negative values for the add springs overlap with mantle values but are best explained by metamorphic devolatilization of reduced sedimentary carbon the 15n values of n2 range from 2 2 to 2 1 versus air and are explained by mixtures of air derived nitrogen metamorphic devolatilization of sedimentary nitrogen and nitrogen from near surface biogenic processes co2 flux is estimated by scaling from individual springs 105 mol a 1 per spring to extensional structures across the southern limit of the tibetan plateau and likely contributes between 108 and 1011 mol a 1 up to 10 to the global carbon budget

a geological and geophysical investigation of baby bare locus of a ridge flank hydrothermal system in the cascadia basin baby bare is one of three small basement outcrops on the eastern sedimentburied juan de fuca ridge flank that have localized heat loss and fluid movement within 3 5 ma oceanic crust low temperature 25 c hydrothermal vents near the summit of baby bare represent the highest temperature occurrence of off axis hydrothermal activity found in oceanic crust older than 1 million years this site has been investigated with seismic reflection profiling towed camera surveys and an alvin dive series that included heat flow measurements to document the detailed geological setting of these off axis vents a new geologic map based on visual observations of baby bare shows that the distribution of rock sediment and biota appears to be controlled by seafloor slope and elevation while specific vent locations are controlled by faulting and occur only in areas of thin or no sediment cover alvin heat flow data indicate that conductive heat loss from the edifice is 4 5 times greater than that from the sediment blanketed area around the outcrops although the outcrop is generally conical seismic reflection profiles reveal that the sediment buried portions of the edifice have an asymmetric morphology strongly suggesting that baby bare is a volcano built upon a preexisting fault generated abyssal hill this evidence combined with previously published petrologic data and results of ocean drilling program leg 168 drilling is consistent with the hypothesis that baby bare formed by off axis volcanism rather than at the adjacent ridge axis sediment thickness and fossil assemblages indicate that it could be as young as 2 7 ma off axis volcanoes such as baby bare increase the overall roughness of basement topography and thus delay complete sediment burial during normal lithospheric aging particularly in areas where near axis sediment accumulation is rapid partially buried seamounts play an important role in focusing hydrothermal exchange between the oceans and young oceanic crust and if baby bare is representative may contribute as much as 85 of the heat flux from a sedimented ridge flank

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

small scale spatial and temporal variations in mid ocean ridge crest magmatic processes data from a suite of closely spaced lava flows recovered within the axial summit caldera and on the crestal plateau of the east pacific rise around lat 9 31 n indicate that eruptions on this fast spreading part of the mid ocean ridge occur throughout the crestal region and are not restricted to the axis these eruptions contribute to a complex distribution of basalts of various ages and a significant thickening of seismic layer 2a away from the axis in our study area small scale 600 m diversity and nonsystematic distribution of lava types may reflect rapid changes in magma chemistry that occur during crystallization and replenishment in small magma lenses coupled with the effects of frequent low volume eruptions both within and outside of the axial summit caldera

the regional tectonic fabric of the east pacific rise from 12 50 n to 15 10 n seamarc ii backscatter data for the east pacific rise are used to create structural maps of ridge parallel fabric on the crest and flanks of the rise from 12 50 n to 15 10 n the structural data are statistically analyzed to describe the spacing density azimuth facing direction and length of faults results of the statistical studies are compared with predictions for the width of the zone of active fault formation models for the generation of abyssal hills plate kinematic predictions models of along strike segmentation and observations of the asymmetric subsidence of oceanic crust on the cocos and pacific plates comparisons of the number of faults with distance from the rise crest and examination of the stratigraphic relationship between seamounts and the ridge parallel tectonic fabric illustrate that the zone where new faults are created is located within a few kilometers of the ridge crest fault density data reveal that fault distributions do not resemble periodic processes examination of fault density and spacing data along strike indicates that there is a high probability that the mean number of faults per crustal block differs significantly from north to south changes in along strike statistics correlate well with the occurrence of three overlapping spreading centers located within the survey area additionally fault spacing s determined from seamarc i and seamarc ii backscatter imagery demonstrate that quantitative analyses of seafloor fabric are dependent upon the instrument used to collect the data analysis of inward and outward facing faults indicates that especially in the northern portion of the survey area half graben models are better predictors of abyssal hill morphology than full graben models within the survey area inward facing faults are more abundant and affect more of the oceanic crust than outward facing faults although the cocos and pacific plates subside at different rates this asymmetry is not reflected in the tectonic component of morphology fault azimuth is observed to vary as a function of crustal age although the overall trend of the change in azimuthal values agrees with the trend predicted by relative poles of opening for the pacific and cocos plates the variability in azimuthal data suggests that other processes contribute to the orientation of lineations formed near the axis of the east pacific rise finally the statistical analyses demonstrate that all fault parameters are better correlated about the ridge axis than along strike of the axis the differences between the northern and southern portions of the survey area are reflected in the seamarc ii bathymetric data which depict a continuous narrow ridge crest in the southern region and an irregularly shaped ridge crest in the northern region that shoals and deepens every 30 to 40 km this along strike variability observed over distances of less than 100 km suggests that large scale plate stresses are not the only processes responsible for generating the tectonic fabric observed on the flanks of the east pacific rise

near axis seamounts on the flanks of the east pacific rise 8 n to 17 n with complete seabeam and seamarcii bathymetry and nearly complete seamarcii side scan coverage of the northern east pacific rise and its flanks from 8 to 17 n we characterize the entire population of seamounts on young seafloor 0 to 2 ma formed along this fast spreading center there are 179 seamounts in this area taller than 200 m most of which belong to one of 21 seamount chains these chains are oriented between the relative and absolute plate motion directions of the pacific and cocos plates and most of the larger chains are more voluminous near their centers than near their ends several of the seamount chains contain bends in orientation which are not coincident with each other the two largest seamount chains at 9 55 n lamont seamounts and 15 40 n are aligned with portions of the east pacific rise crest which are among the shallowest and broadest of this area overall the maximum abundance and size of seamounts increase with shallower and broader ridge axes no near axis seamounts form on the summit of the axial ridge nonetheless within 15 km 0 3 ma of the axis the abundance of seamounts achieves its overall average observations of the numbers and cumulative volume of seamount edifices versus distance from the axis are consistent with a model where the majority of near axis seamounts are formed in a narrow zone on crust aged 0 1 0 3 ma 5 15 km from the axis with significant growth in a zone several times wider side scan sonar reflectivity and seamount magnetization analyses are largely consistent with the predictions of this model two exceptions to this model however are seamount chains at 15 40 n and 8 25 n which show evidence for activity over a broader region this may be due to the presence of fracture zones within about 30 km of these chains a compilation of seamount counts from a wide range of spreading centers indicates that seamounts taller than 400 m are more abundant at greater spreading rates and with transitions from a rift valley to an axial ridge the variations with spreading rate of melt availability and of lithosphere penetrability probably combine to produce the observed results although these influences cannot be distinguished in this analysis

flow of mantle fluids through the ductile lower crust helium isotope trends heat and mass are injected into the shallow crust when mantle fluids are able to flow through the ductile lower crust minimum 3he 4he ratios in surface fluids from the northern basin and range province western north america increase systematically from low crustal values in the east to high mantle values in the west a regional trend that correlates with the rates of active crustal deformation the highest ratios occur where the extension and shear strain rates are greatest the correspondence of helium isotope ratios and active transtensional deformation indicates a deformation enhanced permeability and that mantle fluids can penetrate the ductile lithosphere even in regions where there is no substantial magmatism superimposed on the regional trend are local high 3he 4he anomalies indicating hidden magmatic activity and or deep fluid production with locally enhanced permeability identifying zones with high resource potential particularly for geothermal energy development

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

geochemical and structural studies of the lamont seamounts seamounts as indicators of mantle processes abstract a chain of seamounts located just west of the east pacific rise epr axis at 09 55 n herein called the lamont seamounts have been investigated using sea marc i and sea beam sonar surveys alvin submersible dives deep sea camera tows and dredging the most recently active volcanic vents on the summits of these volcanoes are located at the periphery of each caldera or crater rim or at the margins of caldera floors the distribution of recent volcanic vents and the morphology of the summit plateaus suggest the possible existence of ring fractures on these seamounts ring fractures could be important in focusing post caldera collapse eruptions around summit craters and building summit plateaus structural and morphological data and the petrology of recovered lavas suggest that calderas in the lamont seamounts probably formed by collapse associated with magma withdrawal into large primary vertical magma conduit systems rather than by repeated inflation deflation cycles associated with steady state shallow level magma chambers calderas in the lamont seamounts have evolved through coalescence of adjoining craters and the plan view shape and morphology of each caldera suggest that primary magma conduits have migrated beneath the summit of each seamount through time compared to lavas from the adjacent epr the majority of lamont seamount and cone lavas are more primitive depleted in incompatible elements and compositionally heterogeneous variations in light rare earth elements relative to major elements e g tio 2 na 2 o na 2 o cao and radiogenic isotopes cannot be ascribed to differences in the amount of partial melting of a single n type morb source rather it appears that lamont seamount lavas were derived from heterogeneous mantle sources variably depleted in highly incompatible elements comparison with lavas from the adjacent ridge crest suggests that magma plumbing and ascent mechanisms beneath the lamont seamounts are separate from those along the epr in particular seamount magmas appear to reflect near primary melting events in the mantle whereas epr lavas are the products of shallow level crystallization and mixing in crustal magma chambers absence of long lived magma chambers within the lamont seamounts may account for the low level of hydrothermal activity observed throughout the chain furthermore because of the tight control on sample location and the recovery of lavas having distinct parents from individual volcanoes as well as the lamont chain as a whole we propose that the scale of mantle chemical heterogeneities is less than 5 km

small off axis volcanoes on the east pacific rise abstract a study of sea beam bathymetry and seamarc ii side scan sonar data allows a quantitative measure of the contribution of off axis volcanism to the creation of abyssal hill topography on the east pacific rise epr 9 15 n 9 50 n in order to assess the role of off axis volcanism the distribution of volcanic edifices within 35 km of the ridge was determined we measure the size and location of 55 edifices defined as local highs greater than or equal to 40 m with aspect ratios less than 2 the distribution of small volcanic edifices is notably different from that of larger volcanic constructions seamounts in that the former do not appear to be organized into discrete chains the volcanic edifices form 5 10 km off axis and are typically 40 70 m high in contrast the seamounts 200 m high measured by scheirer and macdonald 1 first form between 5 and 15 km off axis and then continue to grow in volume and size over a distance 2 4 times larger the smaller edifices make a minute contribution to crustal volume 0 02 0 03 but can cover 7 11 of the mature seafloor seamounts contribute 0 3 1 to the volume of oceanic crust and cover only 6 of the seafloor 1 2 we propose that within 10 km of the ridge seamounts and smaller volcanic edifices compete for a limited magma supply the mechanism of melt delivery to seamounts at these distances is not yet fully mature and some magma may leak in a random fashion onto the seafloor to form volcanic edifices typically less than 70 m high beyond 10 km the mechanism of melt delivery to seamounts is mature and no further formation or growth of small volcanic edifices occurs

anomalous cold in the pangaean tropics comment comment soreghan et al 2008 1 present the hypothesis that glaciation existed to near sea level in the tropical region of pangaea in late paleozoic time their most important evidence for this comes from unaweep canyon a wind gap that crosses the uncompahgre plateau colorado united states the

anomalous cold in the pangaean tropics reply we thank w hood et al 2009 1 for their comment on our geology paper soreghan et al 2008 2 we recognize that our hypothesis of episodic cold in the pangaean tropics which relies in part on our proposal of a late paleozoic age and glacial origin for unaweep canyon is radical hence

anomalous cold in the pangaean tropics the late paleozoic archives the greatest glaciation of the phanerozoic whereas high latitude gondwanan strata preserve widespread evidence for continental ice the permo carboniferous tropics have long been considered analogous to today9s warm and shielded from the high latitude cold here we report on glacial and periglacial indicators that record episodes of freezing continental temperatures in western equatorial pangaea an exhumed glacial valley and associated deposits record direct evidence for glaciation that extended to low paleoelevations in the ancestral rocky mountains furthermore the permo carboniferous archives the only known occurrence of widespread tropical loess in earth9s history the volume chemistry and provenance of this loess ite is most consistent with glacial derivation together with emerging indicators for cold elsewhere in low latitude pangaea these results suggest that tropical climate was not buffered from the high latitudes and may record glacial interglacial climate shifts of very large magnitude coupled climate ice sheet model simulations demonstrate that low atmospheric co 2 and solar luminosity alone cannot account for such cold and that other factors must be considered in attempting to explain this best known analogue to our present earth

the evolution of craters and calderas on young seamounts insights from sea marc i and sea beam sonar surveys of a small seamount group near the axis of the east pacific rise at 10 n a northwest trending group of seamounts located just west of the east pacific rise epr axis has been mapped using high resolution sea marc i side scan sonar and sea beam multibeam bathymetry these seamounts have 9 13 km basal diameters and the shapes of the volcanos change from conical domes near the epr axis to truncated cones and larger elliptical truncated cones farther away from the rise axis the detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions craters to large complex calderas and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse the creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera the concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau the steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera the steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain as a seamount evolves the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits the final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau

oxygen and hydrogen isotope compositions of paleosol phyllosilicates differential burial histories and determination of middle late pennsylvanian low latitude terrestrial paleotemperatures abstract the clay mineralogy chemistry and stable hydrogen and oxygen isotope compositions were measured from 20 phyllosilicate samples representing 11 pennsylvanian age paleosol profiles taken from three cores in the illinois basin in order to assess their utility as proxies of low latitude terrestrial paleotemperatures the majority of the samples are mineralogical mixtures of illite smectite i s kaolinite and rarely discrete illite samples from a shallowly buried locality in the northern part of the basin are dominantly composed of smectite rich i s with variable amounts of kaolinite and no discrete illite samples from deeply buried interior parts of the basin are composed of illite rich i s variable amounts of kaolinite and discrete illite these phyllosilicate mixtures have 18 o v smow and d v smow values that range from 17 2 to 23 0 and 56 to 27 respectively assuming that the phyllosilicates preserve a record of isotopic equilibrium with pennsylvanian meteoric waters these oxygen and hydrogen isotope values correspond to crystallization temperatures ranging from 22 3 c to 55 3 c the clay mineralogy phyllosilicate 18 o and d values and calculated crystallization temperatures of 44 c to 55 c from deeply buried localities in the interior of the basin are not consistent with a pedogenic origin instead these trends are considered to be the result of diagenetic recrystallization of pedogenic minerals in response to greater depths of burial by 1 5 to 3 km in the southerly basin center localities as well as an interval of middle permian elevated heat flow associated with magmatic intrusions in the southern part of the basin phyllosilicate mineralogy 18 o and d values and calculated phyllosilicate crystallization temperatures from a shallowly buried northern locality in the illinois basin are consistent with a pedogenic origin and reveal a long term warming trend from an average temperature of 23 3 c in the lower desmoinesian to an average temperature of 32 3 c in the missourian this temperature change is coincident with a significant change in the composition of wetland vegetation in euramerica which has been attributed to a shift in low latitude pennsylvanian climate towards warmer and drier conditions in the late pennsylvanian this study reveals the presence of a dynamic late paleozoic paleoequatorial icehouse climate characterized by significant low latitude temperature variability unprecedented on modern earth

a model based evaluation of tropical climate in pangaea during the late palaeozoic icehouse abstract the late palaeozoic ice age lpia is the earth s penultimate icehouse climate geological proxies for tropical pangaean climate during the lpia are significantly modulated on 100 400 kyr and sub 100 kyr scales in addition some geological proxies suggest that equatorial continental areas may have been colder and more arid during lpia glacial intervals than during late cenozoic glacial intervals furthermore the relationship between polar and tropical climate variability remains controversial the climate dynamics underlying these phenomena are only partly understood nevertheless past modeling of lpia climate has suggested that precipitation near the equator was modulated by monsoonal circulation that itself was modulated by orbital variability here the earth s climate during asselian sakmarian time 299 284 ma was simulated with the community climate system model version 3 ccsm3 these simulations address model climate sensitivity to direct and indirect effects of glaciation as well as variability in the earth s orbit the results of these simulations suggest that sea level and orbitally forced monsoon variability were the primary controls on tropical precipitation when ice sheets were confined to polar latitudes any impact of orbital forcing on glaciation therefore could have led to aliasing between glacial and monsoonal effects in particular regions perhaps explaining contrasting geologically based interpretations of how tropical climate responded to glaciation glaciation of the central pangaean mountains would have led to widespread cold and aridity in equatorial pangaea but forming these glaciers would have required an unknown additional climate forcing with an unknown impact on tropical precipitation

midcrustal reflector on indepth wide angle profiles an ophiolitic slab beneath the india asia suture in southern tibet the wide angle seismic experiment of project indepth international deep profiling of tibet and the himalayas focused on the structure of the india asia continental collision suture yarlung zangbo suture in southern tibet three component portable seismographs recorded the explosive sources of the indepth seismic reflection profile across the suture zone we image a prominent subhorizontal reflector the yarlung zangbo reflector yzr dipping 4 n at 20 km depth beneath the outcrop position of the yarlung zangbo suture with a total area of at least 90 80 km possible geological interpretations for the yzr include fault shear zones fluids magmatic metamorphic or hydrothermal or lithotectonic contacts high amplitude reflections off the yzr out to postcritical offsets and refracted phases with a velocity of 7 km s 1 provide evidence that it is a solid solid interface and the top of a mafic to ultramafic lithological unit we suggest that the yzr is a tethyan ophiolitic slab involving several kilometers of mafic ultramafic rocks we interpret the origin of the slab and its position in the midcrust in the context of the emplacement history of oceanic lithosphere and accreted material onto the northwestern and northern indian passive margin and the tectonic history of the yarlung zangbo suture zone the origin of the yzr body and its position in the midcrust are explained by the emplacement of an ophiolite nappe accretionary wedge complex onto india s passive margin prior to continental collision the subduction of this nappe complex beneath the forearc basin and the asian gangdese magmatic arc and its further burial by the gangdese arc from the north and the indian passive margin sedimentary sequence from the south our model of the yzr constituting an ophiolitic sheet can explain several features of southern tibetan tectonics namely the pre collisional deformation within the northern indian passive margin the lack of strong deformation during collision and the bivergent thrust belt along the suture zone and doming within central southern tibet e g kangmar dome our interpretation implies that the yarlung zangbo suture is subhorizontal in the middle crust in southern tibet

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

off axis volcanism at the east pacific rise detected by uranium series dating of basalts recent detailed surveys of the east pacific rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast spreading ocean ridge crests1 7 in parallel with geological and geochemical investigations it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium series disequilibria methods8 11 here we use 238u 230th and235u 231pa ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9 31 n on the east pacific rise most of the ages are younger than would be expected based on off axis distance and spreading rate we infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0 5 2 km outside the axial summit caldera with some volcanism occurring as far as 4 km off axis melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas

a 220 km2 recently erupted lava field on the east pacific rise near lat 8 s seamarc ii records show a 220 km2 area of unusually high reflectivity on the east pacific rise epr near lat 8 s which we interpret to be a recently erupted lava field this is the only segment of the epr exhibiting high reflectivity over such a large area in our continuous seamarc ii coverage from lat 8 n to 18 n and 3 s to 23 s the lavas appear to issue from the axial neovolcanic zone between lat 8 15 and 8 17 s the lava field extends up to 18km off axis and inundates scarps up to 100 m high on the basis of measurements of scarp heights just outside areas where they are buried we estimate the average thickness of the field to be 70 20 m thus the volume of the lava field is 15 4 km3 in comparison the average annual volcanic budget for the earth is approximately 4 5 km3 and the largest historic basaltic eruption from the icelandic laki fissure in 1783 was approximately 12 3 km3 when did the eruption occur comparative studies of acoustic reflectivity and photographed bottom characteristics elsewhere along the epr suggest that the lava field has little or no sediment cover and is very young this is also one of the only areas on the epr where earthquakes occur that do not appear to be caused by transform faulting if the earthquakes are related to the lava field then major eruptions may have occurred in 1964 1965 and 1969 a large helium plume near lat 13 s may be caused in part by eruptions of the lat 8 s lava field this is consistent with estimates of currents at 2 3 km depth which are such that a plume generated at lat 8 s would eventually appear off axis at lat 13 s our observations suggest that this spreading segment which is bounded by small ridge axis discontinuities at lat 8 05 s and 8 47 s may have been the most volcanically active segment along the epr during the past 25 yr

architecture of on and off axis magma bodies at epr 9 37 40 n and implications for oceanic crustal accretion abstract crustal accretion at fast spreading mid ocean ridges is believed to be concentrated in a narrow zone up to a few kilometers wide centered beneath the ridge axis however there is increasing evidence for off axis magmatism occurring beyond this narrow zone here we present 3d multichannel seismic mcs images from the east pacific rise 9 37 40 n extending to 11 km on the ridge flanks in the axial region two offset axial magma bodies underlie a small ridge axis discontinuity at 9 37 n displaying an overlapping geometry similar to that of the seafloor structures above on the ridge flanks a series of off axis magma lenses oaml are imaged they are located 2 10 km from the ridge axis at 700 to 1520 ms two way travel time below seafloor bsf 1 6 to 4 5 km bsf with variable areas ranging from 0 5 km2 to 5 2 km2 the largest body is centered 4 km east of the ridge axis and is composed of a large continuous flat topped lens and a series of small discontinuous westward dipping bodies along its western edge the flat crest of the oaml lies at approximately the same depth beneath layer 2a as the axial magma lens and we infer that this oaml has formed by aggregation of ascending melts that accumulate at the base of the sheeted dike section a cluster of reflections underlying the oaml at 1260 1510 ms bsf are observed that may be deeper lenses feeding melts to the upper lens this largest oaml is associated with moho travel time anomalies of 120 260 ms within a zone that extends up to 2 km from the edge of the oaml suggesting a lower crust that is partially molten with lower crustal velocities reduced by 8 18 and or thicker than normal by up to 1 km local volcanic edifices are found above two of the three oamls imaged in our study area and are inferred to be the eruptive products of the oamls from the volume of these edifices and the moho travel time anomalies we estimate the potential contribution of off axis magmatism to the total volume of the crust to be 0 01 3 the oamls imaged in our study area are present over roughly the same distance range as the zone of formation of near axis seamounts we speculate that oamls and the volcanic edifices found above them are small scale manifestations of the off axis magmatism that gives rise to near axis seamounts

morphology and distribution of seamounts surrounding easter island we investigate the morphology and distribution of a seamount population on a section of seafloor influenced by both superfast seafloor spreading and hotspot volcanism the population under investigation is part of a broad chain of seamounts extending eastward from the east pacific rise near easter island in order to define the morphological variability of the seamounts basal shape cross sectional area volume flatness and flank slope are plotted against height for 383 seamounts with heights greater than 200 m based on bathymetry data collected by glori b and seabeam 2000 during three cruises onboard the r v melville in the spring of 1993 nearly complete swath mapping coverage of the seamounts is available for the analysis of size and shape distribution we quantitatively describe the seamount population of this active region in which seamounts cover 27 of the seafloor and account for 4 2 of the total crustal volume over 50 of the total volume 61 000 km3 of seamounts used in this study is made up by the 14 largest seamounts and the remaining volume is made up by the 369 smaller seamounts 200 m in height our analysis indicates there are at least two seamount populations in the easter island salas y gomez island 25 29 s 113 104 w study area one population of seamounts is composed of short seamounts 1200 m shield like pointy cones flatness 1200 m originate exclusively from a hotspot source but only a portion of the smaller volcanoes 1200 m are formed from a hotspot source the remainder would be presumably formed by a normal mantle or mixed source

distribution of isolated volcanoes on the flanks of the east pacific rise 15 3 s 20 s volcanic constructions not associated with seamount or volcano chains are abundant on the flanks of the east pacific rise epr but are rare along the axial high the distribution of isolated volcanoes based on multibeam bathymetric maps is approximately symmetric about the epr axis this symmetry contrasts with the asymmetries in the distribution of volcano chains more abundant on the west flank the seafloor subsidence rates slower on the west flank and the distribution of plate motion parallel gravity lineaments more prominent on the west flank most of the isolated volcanoes complete their growth within 14 km of the axis on crust younger than 0 2 ma while seamount chain volcanoes continue to be active on older crust volcanic edifices within 6 km of the ridge axis are primarily found adjacent to axial discontinuities suggesting a more sporadic magma supply and stronger lithosphere able to support volcanic constructions near axial discontinuities the volume of isolated near axis volcanoes correlates with ridge axis cross sectional area suggesting a link between the magma budget of the ridge and the eruption of near axis volcanoes within the study area off axis volcanic edifices cover at least 6 of the seafloor and contribute more than 0 2 to the volume of the crust the inferred width of the zone where isolated volcanoes initially form increases with spreading rate for the mid atlantic ridge 4 km northern epr 20 km and southern epr 28 km so that isolated volcanoes form primarily on lithosphere younger than 0 2 ma 4 6 km brittle thickness independent of spreading rate this suggests some form of lithospheric control on the eruption of isolated off axis volcanoes due to brittle thickness increased normal stresses across cracks impeding dike injection or thermal stresses within the newly forming lithosphere

regional and temporal variations in co2 3he 3he 4he and δ13c along the north anatolian fault zone turkey abstract new he and c relative abundance isotope and concentration results from nine geothermal locations situated along an 800 km transect of the north anatolian fault zone nafz turkey that were monitored during the period november 2001 november 2004 are reported the geothermal waters were collected every 3 6 months to study possible links between temporal geochemical variations and seismic activity along the nafz at the nine sample locations the he isotope ratios range from 0 24 to 2 3 r a 13 c values range from 4 5 to 5 8 and co 2 3 he ratios range from 5 10 9 to 5 10 14 the following geochemical observations are noted 1 the highest 3 he 4 he ratios are found near the galatean volcanic region in the central section of the nafz 2 at each of the nine sample locations the 3 he 4 he ratios are generally constant however co 2 3 he ratios and he contents both show one order of magnitude variability and 13 c values show up to 4 variability and 3 at all locations except resadiye 13 c values show positive correlations with co 2 contents the results indicate that at least three processes are necessary to account for the geochemical variations 1 binary mixing between crustal and mantle derived volatiles can explain the general characteristics of 3 he 4 he ratios 13 c values and co 2 3 he ratios at the nine sample locations 2 preferential degassing of he from the geothermal waters is responsible for variations in co 2 3 he values and he contents at each sample location and 3 co 2 dissolution followed by calcite precipitation is responsible for variations in co 2 contents and 13 c values at most locations for each of the geochemical parameters anomalies are defined in the temporal record by values that fall outside two standard deviations of average values at each specific location geochemical anomalies that may be related to seismic activity are recorded on june 28 2004 at yalova where a m 4 2 earthquake occurred 43 days earlier at 15 km distance from the sample location and on april 7 2003 at efteni where a m 4 0 earthquake occurred 44 days later at a distance of 12 km at both locations the sampling periods containing geochemical anomalies were preceded by an increase in m 3 earthquakes occurring within 60 days and less than 40 km distance

regional heat flow in western anatolia using silica temperature estimates from thermal springs regional trends of variation of heat flow in western anatolia have been outlined using the silica temperature estimator on thermal springs silica heat flow values from 187 springs have been calculated the data are corrected for local long term mean annual surface temperature a mean value of heat flow for the western part of anatolia of 107 45 mw m 2 has been obtained which is about 60 above the world average the silica heat flow data are compared with available conventional heat flow values for this region a general agreement is observed and new patterns are recognized a close association exists between areas of high silica heat flow values above 100 mw m 2 and areas of tertiary and younger volcanism high heat flow is also observed in palaeozoic units of the menderes massif which is under tensional stress the highest heat flow estimate of 247 mw m 2 is obtained near gediz 38 57 n 29 13 2 e this area is part of a seismically active region and is located near a middle eocene subduction zone which is an area of later collision tectonics the new heat flow data from western anatolia improve our knowledge of the thermal conditions within the crust this information is important for the analysis of various geophysical and geological phenomena including seismicity

an exhumed late paleozoic canyon in the rocky mountains a reply we thank william hood for his discussion of soreghan et al 2007 we recognize that our interpretation of the events leading to the formation of unaweep canyon particularly our proposed paleozoic age of the ancestral canyon represents a significant departure from established models validation of our hypotheses regarding its age and origin would force revision of several longaccepted models ranging from the cenozoic tectonic and geomorphic evolution of this region to the climatic and perhaps tectonic framework of the permo pennsylvanian tropics represented by this system accordingly our work deserves close scrutiny and hood s discussion provides such an opportunity hood 2009 begins his discussion by stating that soreghan et al 2007 presented the hypothesis that unaweep canyon is a permian glacial valley that was filled by paleozoic sediment and subsequently exhumed by cenozoic rivers to clarify the focus of soreghan et al 2007 is the hypothesized paleozoic age of the canyon although we posed the question of a possible glacial origin in the final sentences of the article a more complete analysis of the evidence for a glacial origin however appears in soreghan et al 2008 although only abstracts of this aspect e g soreghan et al 2004 were published at the time that hood submitted his discussion nevertheless here we address all of the points raised by hood we treat each of his points using the subheadings he provides reexamination of the field evidence and paleomagnetic data

measuring the seismic properties of tibetan bright spots evidence for free aqueous fluids in the tibetan middle crust seismic bright spots are commonly interpreted to mark fluid concentrations but their nature melt or aqueous is usually inferred only from circumstantial evidence of the geologic setting a band of bright spot reflections has been imaged by project indepth international deep profiling of tibet and the himalayas at about 15 km depth along 150 km of the northern yadong gulu rift southern tibet we use indepth three component wide angle seismic data to measure seismic velocities at the bright spot reflector and theoretical rock physics bounds to constrain the nature of the fluids merging of data from multiple bright spots allows us to use a one dimensional approximation travel time modeling yields average p and s velocities for the upper crust above the bright spots of 5 3 0 2 and 3 2 0 2 km s 1 respectively reflection amplitude variation with offset avo modeling constrains the p and s velocities of the bright spots to 3 0 0 8 and 1 6 0 8 km s 1 respectively multiple modeling procedures suggest these velocities are not model dependent our results imply that of the order of 10 volume of free aqueous fluids in the tibetan middle crust produces the observed bright spot reflections the presence of relatively large quantities of free aqueous fluids presumably mostly saline supercritical h2o does not preclude the presence of melt but does constrain the maximum temperature at the bright spots to the wet granite solidus about 650 c and thus the maximum surface heat flow to 110 mw m 2 the observed bright spots can alternatively be explained as a result of transient flow of aqueous fluids through a lower temperature and lower heat flow southern tibetan crust

on origin of near axis volcanism and faulting at fast spreading mid ocean ridges at fast and superfast spreading mid ocean ridges such as the east pacific rise a plate boundary is defined by a narrow tens to hundreds of meters wide neovolcanic zone within which the bulk of the upper oceanic crust is created however detailed near bottom observations indicate that the volcanic construction may occasionally persist several kilometers off of the ridge axis it has been proposed that off axis volcanism manifests tapping of a wide melting region that supplies magma to the ridge axis or spatial migration of magmatic sources in the crust and upper mantle we demonstrate that off axis eruptions may be a natural consequence of variations in magma supply rate even if the ridge axis is stationary in space and the magma delivery is perfectly focussed at the ridge axis theoretical modeling and field observations indicate that off axis volcanism may result from magma emplacement in sills that propagate toward the surface after their characteristic horizontal size exceeds their emplacement depth volcanic construction and faulting due to sill intrusions may contribute to the formation of abyssal hills arguably the most abundant relief form on earth fl 2001 elsevier science b v all rights reserved

adjacent releases of mantle helium and soil co2 from active faults observations from the marmara region of the north anatolian fault zone turkey volatile releases in terms of co2 efflux and noble gas of mantle origin are investigated in the marmara region of the seismically active north anatolian fault naf zone a total of 180 co2 efflux measurements 1 to 309 g m 2d 1 were carried out along four transects across the naf spatial variability of surface co2 effluxes was higher at faulted sites relative to comparable background areas the 13c values of soil co2 are 24 0 to 15 6 without correlation with co2 efflux values which suggest that co2 efflux anomaly is caused by fault related biogenic gas flow furthermore 36 gas and water samples from spring sites were investigated for 3he 4he ratios which resulted in a range from 0 26 r ra to 4 22 r ra large mantle helium contributions were observed from several sources along the naf the high co2 efflux sector on the gazikoy saros fault along the naf coincides with the area of high mantle helium flux this shows that fault zones play an important role both in transferring mantle helium to the surface and in causing fault related biogenic gas flow

helium isotope variations in turkey relationship to tectonics volcanism and recent seismic activities the distribution of helium isotope ratios in the various tectonic provinces of turkey is examined through a synthesis of previously published data and the results of a recent survey along the north anatolian fault zone nafz following the catastrophic 1999 earthquakes the r ra values where r sample 3he 4he and ra air 3he 4he cover a wide range from 0 05 to 7 87 and the mantle derived helium is clearly identified in most locations the mantle derived component is high 50 of total he in a regions of central and eastern anatolia both of which are associated with historically active volcanoes and b the seismically active west to central segment of nafz while the mantle contribution reaches a maximum at nemrut volcano in eastern anatolia a region of dominantly compressional tectonics and with moderate enthalpy geothermal fields it is relatively low 50 in the western anatolian extensional province even though this region has the highest geothermal potential the average 3he enthalpy ratios estimated for the different provinces suggest lithospheric stretching and rise of the geotherm as the major mechanism of high heat flow yet limited intrusive activity in western anatolia is suggested as the reason for the comparatively low he additions and consequently low 1 7 106 atoms j 3he enthalpy ratios a more extensive magmatic activity appears to be responsible for the greater input of both heat and helium in eastern and central anatolia with intra province 3he enthalpy variations from 3 1 106 atoms j in eastern to 0 25 106 atoms j in central anatolia reflecting the ageing of hydro magmatic systems the relatively high 3he concentrations in low enthalpy waters of northern anatolia 2 8 106 atoms j are particularly significant since there is no evidence of volcanic activity associated with the strike slip motion along the seismically active segment of nafz continuous monitoring of he isotope compositions along nafz should lead to a better understanding of this apparent anomaly as well as the relationship if any between he isotope variations and seismic activities in this region

satellite geodetic imaging reveals internal deformation of western tibet 1 it is often assumed that the majority of the interseismic strain accumulates around the major mapped geological block boundaries however numerous recent earthquakes in the continents have occurred on faults that were previously unidentified existing deformation data from tibet are insufficiently dense to map the distribution of interseismic strain here we combine 265 interferograms formed from 166 radar images with gps data to constrain a high resolution velocity field covering 200 000 km2 of western tibet we confirm that the slip rate of the karakoram fault is low 6 mm yr but show that areas of focused strain do not coincide with the major fault structures some of this strain is due to postseismic relaxation after a mw 6 8 earthquake that occurred in 1996 on a structure that is difficult to identify in satellite imagery models of seismic hazard that rely on imperfect knowledge of the boundaries of crustal blocks can therefore underestimate hazard from unknown faults

a low velocity belt beneath northern and eastern tibetan plateau from pn tomography 1 new pn tomography on tibetan plateau tp and neighboring region is conducted with data assembled from various sources the improved pn tomographic image delineates several distinct geophysical regimes that are approximately separated by geological boundaries at surface dense distribution of the stations in the eastern half of the plateau allows us to infer a 3d view of crustal thickness variation of the region the most striking feature is the bended low velocity belt lvb which extends from west to east beneath qiangtang block and songpan ganzi fold belt sgfb in northern tp and then turns consistently southward with sgfb beneath eastern tp and further south to indo china the consistency between geological boundaries on surface and sharp velocity contrast boundaries in uppermost mantle may suggest strong deformation coupling between crust and at least uppermost mantle if not whole lithosphere a north south striking lvb is also observed cutting through the southern tp coinciding with the yadong gulu rift at surface our tomographic results are generally consistent with dynamic models that involve with underthrusting in the southern tp shortening in the northern tp and extrusion to the east and the southeast furthermore our results suggest significant east west variations in the structure and dynamics