extrusive thickness variability at the east pacific rise 9 10 n constraints from seismic techniques we calculate synthetic shot gathers and their corresponding common depth point cdp profiles over plausible east pacific rise epr shallow velocity structures based on the structures obtained from high resolution on bottom seismic refraction experiments we then use these results to analyze the variability in layer 2a thickness at the epr 9 10 n region as measured by cdp wide aperture profile wap on bottom seismic refraction experiments and conventional air gun refraction data the synthetics indicate that the accuracy of correlating the prominent shallow reflector observed in cdp and wide angle data with the layer 2a 2b boundary is strongly dependent on the structure within layer 2a if layer 2a consists of a surficial low velocity layer overlying a steep velocity gradient our gradient model then there is an excellent correspondence between the two way travel times to the shallow reflector and the base of layer 2a however the shallow reflector may originate from a gradient within layer 2a if the upper crust contains more than one high gradient region our step model this implies that independent estimates of layer 2a velocity structure are needed to properly interpret cdp and wide angle data we also determine that the travel time to the layer 2a reflector for identical velocity structure can vary by as much as 50 ms about 125 m for differing experimental geometries this can explain the discrepancy in two way travel time to the layer 2a reflector imaged on zero age cdp and wap lines the depths to a shallow reflector calculated from cdp and wide angle data in the 9 10 n region of the epr generally correlate with estimated layer 2a thicknesses from on bottom refraction profiles and conventional air gun refraction lines which suggests that the upper crustal structure in this area is similar to the gradient model wap and conventional air gun refraction data indicate that there is a 100 200 m decrease in off axis layer 2a thickness at 9 35 n on the epr the present day location of a deviation in axial linearity deval there is no bathymetric expression of the 50 decrease in layer 2a thickness layer 2a can be interpreted to consist of the extrusive section and transition zone with the layer 2a 2b boundary corresponding to the top of the sheeted dikes we suggest that buoyancy forces associated with the axial magma chamber amc are supporting the extrusive layer and sheeted dikes at the neovolcanic zone with distance from the rise axis the amc solidifies the crust cools the buoyancy forces are reduced and the sheeted dike complex subsides concurrently the extrusive layer thickens resulting in significantly less subsidence of the seafloor we speculate that the 50 decrease in dike subsidence and extrusive thickness at the 9 35 n deval is due to a local reduction in magma supply within the axial magma chamber the off axis pattern of layer 2a thickness suggests that the 9 35 n deval has persisted for 175 000 275 000 years

axial summit trough of the east pacific rice 9 10 n geological characteristics and evolution of the axial zone on fast spreading mid ocean ridge the nature and morphological characteristics of axial summit troughs on fast 90 130 mm yr 1 full spreading rate and superfast spreading 130 mm yr 1 mid ocean ridge crests reflect the time integrated effects of long term magmatic cycles short term volcanic episodicity and the tensional stress regime imposed on young ocean crust two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid ocean ridge crests 1 narrow axial troughs 300 2000 m wide and 30 100 m deep on the east pacific rise crest are classified as axial summit graben the dimensions of axial summit graben as well as the morphological and structural character of their walls and floors suggest a primary tectonic origin an axial summit graben may contain a nested axial summit collapse trough implying that processes responsible for these endemic features may be linked near bottom side looking sonar and observational data collected using the towed vehicle argo i and submersible alvin have been used to characterize the axial summit trough of the fast spreading east pacific rise between 9 and 10 n a four stage model is presented for the evolution of this axial summit collapse trough as well as for other well studied portions of the east pacific rise crest from 21 n to 20 s we propose that the transition from a narrow surface collapse dominated axial trough to a broader fault bounded graben is controlled by the relative importance of diking volcanism hydrothermal cooling and tectonism along a ridge segment over time periods 104 years

ground subsidence quaternary faults and vulnerability of utilities and transportation networks in thessaly greece ground fissures and subsidence have caused extensive damage in structures and the utility networks roads railroad line larisa nato airport telecommunication network in the plain of thessaly central greece in the last 20 years the fact that these fissures are parallel to quaternary faults controlling the structural evolution of the thessaly basin marked by intense recent seismic activity led some scientists to regard them as manifestation of tectonic creep even premonitory phenomena of major future seismic events a careful study of the geometric characteristics and of the kinematics of these ground fissures however revealed that they only reflect shear failure of the ground due to compaction of loose sediments following significant decline of the groundwater level due to over pumping i e a geotechnical and not a tectonic process furthermore theoretical considerations and modeling predict surface fissuring along zones of anomalous basement normal fault controlled topography this explains the correlation between ground fissures and active quaternary normal faults

glacier fluctuations of jostedalsbreen western norway during the past 20 years the sensitive response of maritime mountain glaciers the steep outlet glaciers of jostedalsbreen western norway are good examples of sensitively reacting maritime mountain glaciers their changes in length frontal position and lower tongue s morphology during the past 20 years have been well documented at first they experienced a strong frontal advance after ad 2000 glacier behaviour was dominated by a strong frontal retreat in some cases causing a separation of the lowermost glacier tongue in this paper the glacier length changes are presented both visually and numerically supplemented by mass balance and meteorological data the glacier behaviour is interpreted and its causes are discussed whereas the factors controlling the advance during the 1990s seem clear the interpretation of the most recent retreat still leaves some uncertainties the actual glacier front behaviour cannot fully be related to the mass balance data terminus response times and relations between individual mass balance and meteorological parameters have changed some hypoth

constraining crustal emplacement processes from the variation in seismic layer 2a thickness at the east pacific rise abstract a stochastic model for the emplacement of dikes and lava flows at a fast spreading ridge can generate an upper oceanic crustal structure similar to that observed in seismic data from the east pacific rise epr in ocean drill holes and in ophiolites in this model the location of successive dike intrusion events relative to the ridge axis is determined by a gaussian probability function and the cumulative flow lengths of the erupted lavas are chosen to build a gaussian shaped lava pile we interpret wide angle seismic reflections from the steep velocity gradient at the base of seismic layer 2a to be the extrusive sheeted dike contact seismic data from the northern and southern epr place constraints on the on axis extrusive layer thickness 230 50 m the distance over which the thickening of the extrusive layer occurs width of the accretion zone 1 3 km and its off axis thickness 300 800 m ophiolites and ocean drill holes dsdp hole 504b provide additional estimates of the thickness of the extrusive layer and constrain the thickness of the transition region from extrusives to sheeted dikes 100 200 m a simple stochastic emplacement model where the lavas are described by one mean flow length fits the thickening of the extrusive layer off axis inferred from the deepening of seismic layer 2a but the predicted transition from sheeted dikes to extrusives is too thick in order to match the dimensions and flat topped shape of the seismic layer 2a boundary as well as the thickness of the extrusive sheeted dike transition a bimodal distribution of lava flows is used short flows confined within the axial summit caldera asc build up approximately half the extrusive volume occasional voluminous flows spill out of the asc or erupt outside of the asc and pond at a considerable distance off axis to build up the remainder of the extrusive section the upper part of the final extrusive section will be dominated by the off axis flows while the lower portions will be primarily composed of short flows erupted within the asc magnetic transition widths predicted from the overlap of lavas 2 km in this model are similar to those measured in deep tow studies assuming a smoothing function which acts over one seismic wavelength the upper crustal velocity structure predicted by the bimodal lava emplacement model is consistent with the shallow seismic velocity structure measured on the epr the ages of seafloor lavas in this model are younger than the tectonic spreading model ages by 30 70 kyr in agreement with anomalously young lava ages obtained from radioisotope dating of seafloor basalts near the epr

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 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

an examination of along axis variation of magma chamber width and crustal structure on the east pacific rise between 13 30 n and 12 20 n we investigate the along axis variations of magma chamber width and crustal structure along the east pacific rise epr from 13 30 n to 12 20 n through reprocessed common depth point cdp reflection profiles the magma lens is predominantly a continuous feature in the study area with an average width of 500 m as determined from migrated cross axis cdp profiles this value is similar to widths estimated elsewhere along the epr suggesting that the axial magma chamber amc width is not spreading rate dependent once the threshold for a steady state magma chamber is reached the axial morphology of the 13 n area is generally not a good predictor of magma lens width or continuity a fairly continuous melt lens is imaged where the triangular axial topography might suggest waning magma supply in fact between 13 05 n and 13 01 n a shallow melt lens has been imaged which may be indicative of recent or impending eruptive activity this shoaling is similar to that observed near the 17 26 s region of the epr where the rise axis summit is domed and highly inflated generally the thickness of seismic layer 2a beneath the ridge crest is uniform and comparable to that estimated for 9 n 14 s and 17 s on the epr suggesting that the axial extrusive layer is invariant along fast spreading ridges uniformity of layer 2a thickness along axis implies that variations in magma chamber depth are directly attributed to changes in thickness of the sheeted dike complex seismic layer 2b contrary to expectations of decreasing melt sill depth with increasing spreading rate the average thickness of seismic layer 2b is slightly less 165 m at 13 n than at the faster spreading more robust 9 n area finally geochemical petrologic boundaries which may delineate separate melt supply regions occurring at the 13 20 n and 12 46 n devals deviation in axial linearity are observed to coincide with subtle changes in amc and layer 2a reflection characteristics

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

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

single plume model for asynchronous formation of the lamont seamounts and adjacent eeast pacific rise terrains sonar reflectance is used to quantify the relative ages of young volcanic terrains on a seamount chain and a nearby spreading center in the north equatorial pacific the reflectance is a measure both of the burial of lava by pelagic sedimentation and of the microrelief of the lava the extent of sediment cover and the lava type distribution are calibrated with thousands of bottom photographs the relative ages indicate that the locus of volcanic eruptions has migrated toward the east pacific rise axis zero age activity lies just to the west of the east pacific rise south of the clipperton transform and is probably the cause of an anomalously shallow neovolcanic zone the evidence fits a scenario in which eruptive centers appear evolve and become extinct in succession with little or no overlap in time the eruptive centers are fed in sequence through conduits from buoyant tightly focused magma plumes the near identical heights of many of the centers at the time of their latest eruptions suggest that the seamounts tap the same mantle depth range broad summit plateaus with calderas are formed if the plume volume is such that the hydraulic head is reached and magma is trapped within the volcano in ephemeral magma chambers or conduits if the plume volume is small and the hydraulic head is not reached the seamount remains small with only a small crater the plumes ascend asynchronously from a common region of nucleation in the asthenosphere plumes that follow in close succession build a composite seamount with a string of summit calderas plumes that rise less often result in a line of discrete circular volcanoes

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

seismic attenuation structure of the east pacific rise near 9 30 n we describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9 32 n on the east pacific rise over 3500 estimates of t are obtained from 0 4 to 0 7 s long windows aligned with crustal p phases including diffractions above and below a midcrustal magma chamber and moho reflections which cross the rise axis at a range of lower crustal depths we apply a smoothest model inversion algorithm to the t measurements to derive images of apparent crustal q 1 both 20 km off axis and within a 16 16 km area centered on the rise crest the models resolve regions of high attenuation in the uppermost crust and in a low q zone which extends from midcrustal to lower crustal depths beneath the rise axis off axis q values in the upper 1 km average 35 50 while at depths greater than 2 3 km q is at least 500 1000 the high levels of attenuation in the uppermost crust probably result from the combined effect of factional fluid flow and scattering mechanisms within 1 3 km of the rise axis q increases markedly in the uppermost 1 km to about 65 if the increase in attenuation off axis is entirely due to the 300 m increase in the thickness of layer 2a extrusives required by seismic velocity measurements then q in layer 2a must be about 10 20 no measurements of q are obtained in the immediate vicinity of the 1 6 km deep axial magma lens because no wave paths cross the rise axis through this region the diffractions beneath the magma chamber and the moho reflections require a low q region with minimum q values of 20 50 which extends from no more than 2 5 km depth to the base of the crust these values are similar to laboratory measurements of q obtained at solidus temperatures and constrain the low q region to contain no more than a few percent melt the axial magma chamber which comprises a melt lens and an underlying crystal mush zone must be confined to a narrow 1 km thick region through which no rise crossing paths pass inversions for along axis structure in the low q anomaly show a 20 25 increase in attenuation at 2 3 km depth north of 9 34 n but resolve no such trend at 4 6 km depth the along axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along axis variations in magma supply

u th ra disequilibria and the extent of off axis volcanism across the east pacific rise at 9 30 n 10 30 n and 11 20 n there is widespread interest in the distance that mid ocean ridge magmatism extends beyond the neovolcanic zone off axis magmas also provide a means to map out variations across the melting zone we present 238u 230th 226ra data for 35 well characterized samples that extend up to 50 km away from the ridge axis across the east pacific rise at 9 30 n 10 30 n and 11 20 n the 230th 238u ratios range from 1 00 to 1 19 and the 226ra 230th ratios range from 1 to 2 78 the samples have a bimodal 230th 238u distribution with approximately half overlying published axial data on the u th diagram and the remainder lying close to the equiline the u series disequilibria in the majority of the samples can be explained by aging subsequent to eruption in a zone 8 km wide about the neovolcanic zone consistent with visual evidence for sample age nevertheless seven of the samples lie above calculated 230th 238u axial decay curves and or have 226ra excesses implying eruption tens of kilometers off axis these are consistent with evidence from seamounts and seismic interpretations that magmatism can extend up to 20 km off axis the implication is that magma is not as efficiently focused beneath the ridge axis as has generally been believed there is a decrease in initial 230th 238u in both these and published samples inferred to have formed off axis but there is no compelling evidence that this reflects source heterogeneity simple modeling suggests that this could be explained by a decrease in fertility and melt column length as the overlying lithosphere thickens with age and the solidus shallows

magmatic cycles and formation of the upper oceanic crust at spreading centers geochemical study of a continuous extrusive section in the oman ophiolite 1 the data and analysis presented in this paper provide an assessment of lava morphologies and the geochemistry of lavas from the oman ophiolite in order to provide detailed constraints on the construction of the upper oceanic crust a continuous volcanic transect 300 m thick was sampled at high frequency in the semail ophiolite along wadi shaffan the wadi shaffan section is composed mainly of pillow lavas interbedded with massive flows and occasional hyaloclastites the sampling performed along wadi shaffan implies temporal variations in the activity of the ridge the section is characterized by chemical compositions consistent with those of v1 geotimes volcanism the wadi shaffan transect was built through two main petrological and geochemical sequences of volcanic activity trace element ratios e g zr nb and la yb allow us to distinguish two main sequences with two different parental magmas differences in the degree of partial melting are required to explain these trace element ratio variations beyond these differences in parent melt composition variations in trace element abundances tio2 zr ree involve differentiation processes prior to emplacement in the lower sequence less differentiated lavas are in the upper part of the cycle magma mixing is proposed to explain this reversed geochemical evolution through time in the upper sequence geochemical analysis suggests a different magma chamber process this sequence consists of multiple events of magma emplacement variations in trace element abundance suggest four magmatic cycles each magmatic cycle is characterized by primitive lavas evolving to more differentiated lavas with time the upper sequence lavas appear to be in equilibrium with clinopyroxene and lower sills from the mtz mantle crust transition zone and with lower gabbros we propose a model in which the upper sequence lavas were directly derived from the mtz and lower gabbro sills and then transported to the surface without interaction with higher crustal levels

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

geochemical constraints on the origin of the annieopsquotch ophiolite belt newfoundland appalachians the early ordovician annieopsquotch ophiolite belt occurs immediately west of the main iapetus suture zone and imposes important constraints on the tectonic processes associated with closure of the peri laurentian portion of iapetus the annieopsquotch ophiolite the most prominent ophiolite within the annieopsquotch ophiolite belt exposes a 5 5 km thick section of gabbros sheeted dikes and pillow basalts in which three magmatic episodes have been recognized based on field and geochemical data the first phase is composed of layered troctolites which are preserved as enclaves within the gabbro zone trace element modeling suggests the troctolites crystallized from boninitic melts the troctolite substrate was intruded by the dominant second tholeiitic magmatic phase which formed a gabbro sheeted dike basalt sequence all tholeiites have suprasubduction zone chemical characteristics but the suprasubduction zone signature decreases toward the top of the basalt sequence the third magmatic episode is composed of primitive dikes which are interpreted as off axis intrusions other ophiolites within the annieopsquotch ophiolite belt have comparable geochemical signatures suggesting they may have constituted a single piece of oceanic lithosphere based on geochemical and regional tectonic constraints the annieopsquotch ophiolite belt is interpreted to have formed during initiation of west directed subduction fast rollback of the subducting slab would have induced volatile fluxed decompression melting of previously depleted mantle yielding boninitic melts the suprasubduction zone tholeiite sequence would have formed from ascending fertile mantle fluxed with sub duction related fluids as rollback continued this suggests that the annieopsquotch ophiolite belt does not represent the remnants of normal oceanic crust or backarc basin crust as previously thought our model constrains the initiation and early evolution of a west dipping peri laurentian subduction zone that was responsible for formation of several arc backarc complexes currently preserved in the annieopsquotch accretionary tract

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

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

central anomaly magnetization high documentation of crustal accretion along the east pacific rise 9 55 9 25 n near bottom magnetic data collected along the crest of the east pacific rise between 9 55 and 9 25 n identify the central anomaly magnetization high camh a geomagnetic anomaly modulated by crustal accretionary processes over timescales of 104 years a significant decrease in camh amplitude is observed along axis from north to south with the steepest gradient between 9 42 and 9 36 n the source of this variation is neither a systematic change in geochemistry nor varying paleointensity at the time of lava eruption instead magnetic moment models show that it can be accounted for by an observed 50 decrease in seismic layer 2a thickness along axis layer 2a is assumed to be the extrusive volcanic layer and we propose that this composes most of the magnetic source layer along the ridge axis the 9 37 n overlapping spreading center osc is located at the southern end of the steep camh gradient and the 9 42 9 36 n ridge segment is interpreted to be a transition zone in crustal accretion processes with robust magmatism north of 9 42 n and relatively low magmatism at present south of 9 36 n the 9 37 n osc is also the only bathymetric discontinuity associated with a shift in the camh peak which deviates 0 7 km to the west of the axial summit trough indicating southward migration of the osc camh boundaries defined from the maximum gradients lie within or overlie the neovolcanic zone nvz boundaries throughout our survey area implying a systematic relationship between recent volcanic activity and camh source maximum flow distances and minimum lava dip angles are inferred on the basis of the lateral distance between the nvz and camh boundaries lava dip angles average 14 toward the ridge axis which agrees well with previous observations and offers a new method for estimating lava dip angles along fast spreading ridges where volcanic sequences are not exposed

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

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

geology of the alarcon rise southern gulf of california author s clague da caress dw dreyer bm lundsten l paduan jb portner ra spelz madero r bowles ja castillo pr guardado france r le saout m martin jf santa rosa del rio ma zierenberg ra abstract 2018 the authors meter scale auv bathymetric mapping and rov sampling of the entire 47 km long alarcon rise between the pescadero and tamayo transforms show that the shallowest inflated portion of the segment hosts all four active hydrothermal vent fields and the youngest hottest and highest effusion rate lava flows this shallowest inflated part is located 1 3 of the way between the tamayo and pescadero transforms and is paved by a 16 km2 channelized flow that erupted from 9 km of en echelon fissures and is larger than historic flows on the east pacific rise or on the gorda and juan de fuca ridges starting 5 km south of the pescadero transform 6 5 km of the alarcon rise is characterized by faulted ridges and domes of fractionated lavas ranging from basaltic andesite to rhyolite with up to 77 3 wt sio2 these are the first known rhyolites from the submarine global mid ocean ridge system silicic lavas range from g11 7 ka to as young as 1 1 ka a basalt to basaltic andesite sequence and an andesite to dacite to rhyolite sequence are consistent with crystal fractionation but some intermediate basaltic andesite and andesite formed by mixing basalt with dacite or rhyolite magmatism occurred along the bounding tamayo and pescadero transforms as extensive channelized flows the flows erupted from ring faults surrounding uplifted sediment hills inferred to overlie sills the transforms are transtensional to accommodate magma migration from the adjacent alarcon rise

seismic structure and indicators of magma budget along the southern east pacific rise in this paper we re examine the relationship between seismically constrained variations in crustal structure along the southern east pacific rise sepr and the segment scale variations in axial depth morphology basalt geochemistry and hydrothermal activity that have often been attributed to along axis differences in the supply of magma to the mid ocean ridge along 800 km of the fast spreading sepr good correlations exist between axial depth ridge cross sectional area mantle bouguer anomaly and the mgo weight percent of basalts recovered from the rise axis these correlations indicate along axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of 100 000 years in contrast we show that the depth and width of the midcrustal magma sill the thickness of seismic layer 2a and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross sectional area we suggest that the emplacement geometry width of the intrusion zone and flow lengths not magma supply controls extrusive layer seismic layer 2a thickness we hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events dike intrusion eruptions faulting which occur on much shorter timescales 10 100 years than the longer term variations in magma supply reflected in along axis changes in the shape and depth of the ridge axis

uniform accretion of oceanic crust south of the garrett transform at 14 15 s on the east pacific rise using migrated common depth point reflection profiles we find the structural differences along the ultrafast spreading 150 mm yr east pacific rise south of the garrett fracture zone are second order suggesting a remarkably uniform process of crustal accretion the rise axis south of the garrett transform is underlain by a narrow 1 0 km melt lens which shows great along strike continuity the depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading east pacific rise at 9 30 n this observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent melt sill width however shows little variation along the east pacific rise suggesting no dependence of magma chamber size on spreading rate the melt reservoir decreases in width toward across the 14 27 s ridge axis discontinuity by a modest 250 300 m and appears to be continuous across this feature given the small aspect ratio 1 0 km by 50 m by tens of kilometers of the axial melt lens the previously recorded jump in mgo content across the 14 27 s offset is likely the result of a mixing boundary which is sustained through an along strike impedance in convection wide angle reflections originating at the base of seismic layer 2a assumed to coincide with the extrusive layer reveal a twofold to threefold increase 200 250 to 500 600 m in thickness within 1 2 km of the rise axis the pattern of extrusive thickening imaged south of the garrett transform is similar to that observed along the slower spreading 110 120 mm yr east pacific rise at 9 n outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile this implies a degree of temporal stability of the along strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone the inferred uniformity of off axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14 27 s discontinuity are caused by long term reductions in magma supply second order differences in the style of extrusive thickening may be related to structural differences within the low velocity zone underlying the rise axis and or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis images of mono on cross axis profiles may be traced to within 1 0 km of the melt sill edge this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber

magmatic processes and segmentation at a fast spreading mid ocean ridge detailed investigation of an axial discontinuity on the east pacific rise crest at 9 37 n 1 geophysical and petrological boundaries on mid ocean ridges provide ideal locations to study the relationships between magmatic tectonic and hydrothermal processes alvin based observational data and geochemical data for basalts and hydrothermal fluids are used to investigate these relationships at an axial discontinuity on the east pacific rise epr crest between 9 36 n and 9 38 n this ridge crest discontinuity is morphologically expressed by the overlap of an eastern and western axial summit collapse trough asct that delimits the primary volcanic and hydrothermal loci along the ridge crest in this area the ascts overlap by 3 km and are offset in a right lateral sense by 0 45 km near bottom imaging of this area in 1989 and 1991 shows changes in volcanic morphology and increases in hydrothermal and biological activity consistent with the occurrence of a magmatic event during that time interval when combined with the inferred age and structure of the seafloor basalt geochemistry and hydrothermal fluid chemistry these temporal changes suggest active southward propagation of the eastern asct and show that the western asct was unaffected by the recent magmatic event numerous extinct hydrothermal vents and older looking lava flow surfaces suggest waning of magmatic activity in the western asct young looking lava flows within or proximal to the eastern asct have anomalously high mg numbers relative to the regional trend and are chemically similar to lava erupted in 1991 along the 9 46 52 n epr region we propose that the young looking lava flows in the eastern asct are related to the 1991 eruption data show that the 9 37 n axial discontinuity marks a magmatic and hydrothermal boundary along the epr ridge crest and we argue that it be classified as a third order discontinuity this result is consistent with geophysical evidence suggesting fundamental differences in the crust and upper mantle north and south of the 9 37 n discontinuity

channelized lava flows at the east pacific rise crest 9 10 n the importance of off axis lava transport in developing the architecture of young oceanic crust 1 submarine lava flows are the building blocks of young oceanic crust lava erupted at the ridge axis is transported across the ridge crest in a manner dictated by the rheology of the lava the characteristics of the eruption and the topography it encounters the resulting lava flows can vary dramatically in form and consequently in their impact on the physical characteristics of the seafloor and the architecture of the upper 50 500 m of the oceanic crust we have mapped and measured numerous submarine channelized lava flows at the east pacific rise epr crest 9 10 n that reflect the high effusion rate and high flow velocity end member of lava eruption and transport at mid ocean ridges channel systems composed of identifiable segments 50 1000 m in length extend up to 3 km from the axial summit trough ast and have widths of 10 50 m and depths of 2 3 m samples collected within the channels are n morb with mg indicating eruption from the ast we produce detailed maps of lava surface morphology across the channel surface from mosaics of digital images that show lineated or flat sheets at the channel center bounded by brecciated lava at the channel margins modeled velocity profiles across the channel surface allow us to determine flux through the channels from 0 4 to 4 7 103 m3 s and modeled shear rates help explain the surface morphology variation we suggest that channelized lava flows are a primary mechanism by which lava accumulates in the off axis region 1 3 km and produces the layer 2a thickening that is observed at fast and superfast spreading ridges in addition the rapid high volume flux eruptions necessary to produce channelized flows may act as an indicator of the local magma budget along the epr we find that high concentrations of channelized lava flows correlate with local across axis ridge morphology indicative of an elevated magma budget additionally in locations where channelized flows are located dominantly to the east or west of the ast the ridge crest is asymmetric and layer 2a appears to thicken over a greater distance from the ast toward the side of the ridge crest where the channels are located

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

structural evolution of the east pacific rise axis from 13 10 n to 10 35 n interpretations from seamarc i data in 1983 the seamarc i and the sea beam systems imaged the east pacific rise epr and the clipperton transform fault from 13 10 n to 9 50 n revealing both magmatic and tectonic segmentation along the rise axis tectonic segments are defined by an undulating axial zone of extension which widens and narrows at an average wavelength of 45 km in addition axial fissure fault density and length curves reveal wavelengths of 30 and 56 km the narrowest axial zones 0 5 km in width correspond to regions of hydrothermal activity and are located atop regional morphologic highs quasi harmonically spaced at 155 km these highs define the major magma centers on the rise axis about which seamounts cluster superimposed on the intermediate wavelength highs are smaller highs from 10 to 50 km in length which are truncated by overlapping spreading centers osc s at their distal edges the rise axis widens near osc s while it narrows and disappears near the clipperton transform fault suggesting that transforms and osc s have different origins imagery on and adjacent to a few osc s reveals that in some cases osc s migrate along the axis forming relict off axial structural traces at oblique angles 285 330 to the axis off axial fault density and length curves have wavelengths similar to the axial curves however fault densities to the west of the axis are somewhat antisymmetric to fissure fault densities on the axis in contrast faults to the east of the axis nearly mimic fissure fault density and length curves on the axis suggesting that the fracture pattern is controlled by the state of deviatoric stress in the adjacent cocos plate to the east the data suggest that large deep seated magma diapirs may dominate the magmatic segmentation of the epr generating dynamically supported 155 km wavelength highs smaller shallower magma cupulas may rise as fingers to the seafloor above the large diapirs where the magma erupts may be a factor of the thin skin tectonic segmentation of the rise axis formed in response to the state of deviatoric stress in the upper crust small ocs s may form at crustal weak points and may migrate in response to magma body movement along the axis the instability of both the magmatic and tectonic segmentation is apparent from the oblique alignment of both seamount chains and relict osc s with their present day counterparts

a baseline for upper crustal velocity variations along the east pacific rise at 13 n a wide aperture profile of the east pacific rise at 13 n provides data necessary to make a high resolution seismic velocity profile of the uppermost crust along a 52 km segment of ridge crest automated and objective processing steps including p analysis and waveform inversion allow the construction of models in a consistent way so that comparisons are meaningful a continuous profile is synthesized from 70 independent one dimensional models spaced at 750 m intervals along the ridge the resulting seismic velocity structure of the top 500 m of crust is remarkable in its lack of variability the main features are a thin low velocity layer 2 a at the top with a steep gradient to layer 2b the seafloor velocity is nearly constant at 2 45 km s 3 along the entire ridge the velocity at the top of layer 2b is 5 0 km s 10 the depth to the 4 km s isovelocity contour within layer 2a is 130 20 m from 13 to 13 20 n north of which it increases to 180 m the increase in thickness is coincident with a deviation from axial linearity deval noted by both a slight change in axis depth and orientation and in geochemistry the waveform inversion providing more details plus velocity gradient information shows a layer 2a with about 80 m of constant velocity material underlain by 150 m of high velocity gradient material putting the base of layer 2a at approximately 230 m depth south of 13 20 n and about 50 m thicker north of the deval the overall lack of variability combined with other recent measurements of layer 2a thickness along and near the axis indicates that the thickness of volcanic extrusives is controlled not by levels of volcanic productivity but the dynamics of emplacement the homogeneity along axis also provides a baseline of inherent variability in crustal structure of about 10 against which other observed variations in similar regimes can be compared

paleointensity applications to timing and extent of eruptive activity 9 10 n east pacific rise placing accurate age constraints on near axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges geomagnetic paleointensity of submarine basaltic glass sbg holds promise for placing quantitative age constraints on near axis flows in one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful sbg paleointensity estimates from 189 near axis 4 km sites at the east pacific rise 9 10 n paleointensities range from 6 to 53 t and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field samples within and adjacent to the axial summit trough ast have values approximately equal to or slightly higher than the present day samples out to 1 3 km from the ast have values higher than the present day and samples farther off axis have values lower than the present day the on axis samples 500 m from the ast provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique results from samples collected near a well documented eruption in 1991 1992 suggest there may be a small negative bias in the paleointensity estimates limiting resolution of the dating technique possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain anomalously high magnetic unblocking temperatures leading to a small cooling rate bias and or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories paleointensity remains useful in approximating age differences in young flows and a clear along axis paleointensity contrast near 9 50 n is suggestive of a 150 200 year age difference paleointensity values of off axis samples are generally consistent with rough age interpretations based on side scan data furthermore spatial patterns in the paleointensity suggest extensive off axis flow emplacement may occur infrequently with recurrence intervals of 10 20 kyr results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows with flow size linked to time between eruptions