vertical magnetic structure of ocean crust determined from near bottom magnetic field measurements consensus on the source of marine magnetic anomaly stripes has remained one of the most elusive aspects of the vine and matthews seafloor spreading model which has been of such fundamental importance to global plate tectonics the first attempt to measure the vertical magnetic structure of oceanic crust exposed at a scarp face using near bottom magnetic measurements is reported along with the theoretical basis for the reduction and analysis of these measurements magnetometers mounted to deep tow and submersible platforms were used to determine the horizontal and vertical variation of crustal magnetization of the upper 2 km of young oceanic crust exposed by a steep submarine escarpment at the blanco fracture zone in the northeast pacific ocean results show a large amplitude magnetic anomaly is consistently found at the extrusive to intrusive transition indicating that the extrusives are strongly magnetized compared to the weakly magnetized intrusive crust profiles also show the systematic presence of more than one polarity with depth indicating the possibility of dipping polarity boundaries within the extrusives forward models find that a dipping polarity boundary model correctly predicts the amplitude and shape of the overlying sea surface anomaly while a block model with vertical polarity boundaries overestimates the amplitude these in situ results are used to directly assess the contribution made by the extrusive section to the overlying magnetic anomaly signal in essence to define the magnetic source layer it is concluded that the extrusive lavas constitute the main magnetic source for magnetic anomalies in oceanic crust less than 2 m y in age

the cleft revealed geologic magnetic and morphologic evidence for construction of upper oceanic crust along the southern juan de fuca ridge 1 the geology and structure of the cleft segment of the southern juan de fuca ridge jdfr have been examined using high resolution mapping systems observations by remotely operated vehicle rov rov mounted magnetometer and the geochemical analysis of recovered lavas bathymetric mapping using multibeam em300 coupled with in situ observations that focused on near axis and flank regions provides a detailed picture of 0 to 400 ka upper crust created at the southern terminus of the jdfr a total of 53 rock cores and 276 precisely located rock or glass samples were collected during three cruises that included sixteen rov dives our observations of the seafloor during these dives suggest that many of the unfaulted and extensive lava flows that comprise and or cap the prominent ridges that flank the axial valley emanate from ridge parallel faults and fissures that formed in the highly tectonized zone that forms the walls of the axial valley the geochemically evolved and heterogeneous nature of these near axis and flank eruptions is consistent with an origin within the cooler distal edges of a crustal magma chamber or mush zone in contrast the most recent axial eruptions are more primitive higher mgo chemically homogeneous lobate sheet and massive flows that generate a distinct magnetic high over the axial valley we suggest that the syntectonic capping volcanics observed off axis were erupted from near axis and flank fissures and created a thickened extrusive layer as suggested by the magnetic and seismic data this model suggests that many of the lavas that comprise the elevated ridges that bound the axial valley of the cleft segment were erupted during the collapse of a magmatic cycle not during the robust phase that established a new magmatic cycle

temporal and spatial variability in the composition of lavas exposed along the western blanco transform fault the northern scarp of the western blanco transform bt fault zone provides a tectonic window into crust generated at an intermediate rate spreading center exposing a 2000 m vertical section of lavas and dikes the lava unit was sampled by submersible during the blancovin dive program in 1995 recovering a total of 61 samples over vertical distances of 1000 m and a lateral extent of 13 km major element analyses of 40 whole rock samples exhibit typical tholeiitic fractionation trends of increasing feo na2o and tio2 and decreasing al2o3 and cao with decreasing mgo the lava suite shows a considerable range in extent of crystallization including primitive samples mg 64 and evolved feti basalts feo 12 tio2 2 on the basis of rare earth element and trace element data all of the lavas are incompatible element depleted normal mid ocean ridge basalts n morb la smn 1 the geochemical systematics suggest that the lavas were derived from a slightly heterogeneous mantle source and crystallization occurred in a magmatic regime of relatively low magma flux and or high cooling rate consistent with magmatic processes occurring along the present day southern cleft segment the bt scarp reveals the oceanic crust in two dimensional space allowing us to explore temporal and spatial relationships in the horizontal and vertical directions as a whole the data do not appear to form regular spatial trends rather primitive lavas tend to cluster shallower and toward the center of the study area while more evolved lavas are present deeper and toward the west and east considered within a model for construction of the upper crust these findings suggest that the upper lavas along the bt scarp may have been emplaced off axis either by extensive off axis flow or off axis eruption while the lower lavas represent axial flows that have subsided with time a calculation based on an isochron model for construction of the upper crust suggests that the cleft segment requires at least 50 kyr to build the lower extrusive section consistent to first order with independent estimates for the construction of intermediate spreading rate crust

geochemical modeling of nonsteady state magma chambers a case study from an ultrafast spreading ridge east pacific rise 17 19 s we propose a numerical method for modeling the geochemical evolution of nonsteady state changing volume magma chambers using analyses of erupted lavas the method allows estimation of the ratio between concentrations of an incompatible element in the erupted and incoming liquids e1 the relations between e1 the dimensionless magma flux through the reservoir and time are shown graphically we apply this model to the behavior of magma chambers underlying three segments of the east pacific rise between 17 00 s and 18 40 s with different axial morphology the geochemical data vary according to the axial morphology of the segments the lavas sampled along a dome shaped axis display more restricted compositional variations than the lavas from the axial caldera of collapsed segments these geochemical features thought to reflect variable magma chamber states are consistent with a cyclical model of a successively inflating and deflating magma chamber the results of our modeling imply that 1 the inflation could be about twice as long as deflation and 2 the magmatic volume expelled from the reservoir during deflation is at least three times larger than the volume expelled during inflation

an experimental investigation of the influence of water and oxygen fugacity on differentiation of morb at 200 mpa crystallization experiments were performed at 200mpa in the temperature range 1150 950 c at oxygen fugacities corresponding to the quartz fayalite magnetite qfm and mno mn3o4 buffers to assess the role of water and fo2 on phase relations and differentiation trends in mid ocean ridge basalt morb systems starting from a primitive mgo 9 8 wt and an evolved morb mgo 6 49 wt crystallization paths with four different water contents 0 35 4 7 wt h2o have been investigated in primitive morb olivine is the liquidus phase followed by plagioclase clinopyroxene amphibole is present only at water saturated conditions below 1000 c but not all fluid saturated runs contain amphibole magnetite and orthopyroxene are not stable at low fo2 qfm buffer residual liquids obtained at low fo2 show a tholeiitic differentiation trend the crystallization of magnetite at high fo2 mno mn3o4 buffer results in a decrease of melt feo mgo ratio causing a calc alkaline differentiation trend because the magnetite crystallization temperature is nearly independent of the h2o content in contrast to silicate minerals the calc alkaline differentiation trend is more pronounced at high water contents residual melts at 950 c in a primitive morb system have compositions approaching those of oceanic plagiogranites in terms of sio2 and k2o but have ca na ratios and feo contents that are too high compared with the natural rocks implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites

seismic wavefield calculation for laterally heterogeneous whole earth models using the pseudospectral method summary a method for simulating seismic wave propagation in a laterally heterogeneous whole earth model is presented by solving the elastodynamic equations in 2 d cylindrical coordinates using the pseudospectral method psm the psm is an attractive timedomain technique that uses the fast fourier transform for an accurate diierentiation of eld variables in the equations since no dispersion error arises in fourier diierentiation even when using a large grid spacing computer memoryand time are reduced by several orders of magnitude compared to traditional nite diierence methods in order to examine body wave phases with current computing resources a slice through the sphere is approximated with a 2 d cylindrical model an irregular grid spacing is used in the vertical coordinate to improve the treatment of the various structural boundaries appearing in the earth model by matching the heterogeneity in the model synthetic seismograms obtained by the psm calculation are compared with those calculated from an exact simulation method for a spherically homogeneous 1 d earth model and achieve good agreement the psm method is illustrated by constructing the seismic p sv wave eld for strongly heterogeneous earth models including a shield structure near the free surface and velocity perturbations just above the core mantle boundary the visualization of the evolution of seismic p and s waves in time and space is tracked using a sequence of snapshots and synthetic seismograms these displays allow direct insight into the nature of the complex seismic wave behaviour in the earth s interior

a fluid inclusion and stable isotope study of hydrothermal circulation in a transform zone western blanco depression northeast pacific the blanco trough juan de fuca ridge exposes a continuous section of upper oceanic crust basalts and diabases that has been hydrothermally altered in a transform fault environment microthermometric measurements on fluid inclusions and stable oxygen isotope compositions of whole rocks and secondary minerals from pillow lava and diabase units were performed to determine the evolution of hydrothermal fluids a few low 18o values down to 4 8 for diabase and temperatures from 280 to 350 c obtained from fluid inclusions in quartz attest to an early high temperature stage of hydrothermal circulation close to the ridge axis a dense fracture network has maintained moderate temperature hydrothermal circulation about 200 c rooted down to the top of the diabase unit oxygen isotope profiles in two reference sections reveal that the upper part of the oceanic crust exposed in the blanco trough is significantly 18o enriched 6 6 relative to its mantle source 5 7 0 2 we propose that extensive off axis tectonic activity linked to the transform zone favored fluid circulation in crust at maximum temperatures of about 200 c the low temperature water rock interactions t 200 c were more pervasive in the blanco crust despite being 6 times younger than the crust at hole 504b which evolved far from any influence of transform fault activity transform faults which are common in slow spreading ridge environments may therefore significantly affect the hydrothermal activity which contributes to the regulation of the chemical and isotope composition of seawater

a technique for identifying microseismic multiplets and application to the valhall field north sea a fast fully automatic technique to identify microseismic multiplets in borehole seismic data is developed the technique may be applied in real time to either continuous data or detected event data for a number of three component receivers and does not require prior information such as p or s wave time picks peak crosscorrelation coefficients evaluated in the frequency domain are used as the basis for identifying microseismic doublets the peak crosscorrelation coefficient at each receiver is evaluated with a weighted arithmetic average of the normalized correlation coefficients of each component each component is weighted by the maximum amplitude of the signal for that component to reduce the effect of noise on the calculations the weighted average correlations are averaged over all receivers in a time window centered on a fixed lag time the size of the time window is determined from the dominant period in the signal and the lag time is the time that maximizes the average correlation coefficient the technique is applied to a three component passive seismic data set recorded at the valhall field north sea a large number of microseismic doublets are identified that can be grouped into multiplets reducing the total number of absolute event locations by a factor of two seven large multiplets reflect the repeated multiple rerupturing up to 30 times on a single fault and significant stress release two major faults dominate the seismic activity causing at least one fourth of the observed events

bulk rock geochemistry and plagioclase zoning in lavas exposed along the northern flank of the western blanco depression northeast pacific insight into open system magma chamber processes abstract the present study is based on a set of lavas and crosscutting dikes collected by dives along detailed vertical transects on the northern flank of the western part of the blanco transform fault northeast pacific the studied area consists of a small basin the western blanco depression wbd extending from the southern end of the juan de fuca ridge to a pseudofault trace 60 km eastward the northern scarp of the wbd comprises a volcanic unit overlying a sheeted dike complex major and trace element data coupled with sr nd isotope ratios reveal a two component mantle source composed by an isotopically depleted matrix variably veined by more enriched material one chemical group ns2 indistinguishable from the other northern scarp samples on the basis of trace element data has an unusually depleted isotopic composition typical of a nearly pure mantle end member some cogenetic samples of the northern scarp have been used to constrain the differentiation modalities anorthite and mgo content profiles in plagioclase xenocrysts and phenocrysts reveal i the existence of h2o bearing evolved melts in the mushy zones and ii the occurrence of mixing process between these melts and anhydrous mafic liquids the hydration is supported by other petrographic features such as high magmatic fo2 values calculated from fe ti oxide pairs and the presence of pyroxene inclusions in plagioclase phenocrysts mixing consistent with the existence of ni rich ferrobasalts is interpreted to be the consequence of the reservoir refilling by mafic liquids mg 70 these petrological and geochemical evidences are combined with the evolution of mg with depth to suggest a periodic open system magma chamber evolution beneath the southern end of the juan de fuca ridge

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

differential analysis of coda q using similar microearthquakes in seismic gaps part 1 techniques and application to seismograms recorded in the anza seismic gap abstract a large but conflicting literature exists that suggests that the scattering properties of the earth in seismogenic areas may be temporally variable and that this variability could be correlated with the nucleation times of moderate to large earthquakes to search for temporal variations in coda q in the vicinity of an expected moderate earthquake nucleation region along the san jacinto fault zone pairs and clusters of earthquakes recorded by the anza seismic network between 1982 and 1992 are examined to minimize differences arising from path and source variability only the most similar examples of microearthquake wave fields are intercompared to assess coda q differences between similar microearthquake signals we search for systematic temporal changes in moving window spectral amplitude ratios for a single scattering coda model with an envelope decay function of the form t m e ft qc the relationship between the natural log amplitude spectral ratio and q 1 q 1 c 2 q 1 c 1 is linear i e ln r f t ft q 1 f w f where w f is a time independent relative source term to obtain robust uncertainty estimates of q 1 f and w f we utilize multi taper spectral analysis coupled with a nonparametric monte carlo confidence interval estimation procedure for the most similar events in the anza region coda cross correlation values of approximately 0 7 for 16 sec of coda we find that coda q as a function of time is stable at a typical 1 resolution of only between 25 and 50 of the reference coda q value for the area even in the best constrained frequency bands comparison of similar microearthquake coda signals with both short and long temporal separations indicates that all of this variability can be reasonably attributed to random fluctuations in the coda that are driven by source variability and is not indicative of any systematic temporal variability in coda q to obtain estimates of relative coda q that are more precise than this level the spectral ratio analysis technique requires significantly more similar earthquakes than are produced by seismogenic processes in the anza region such earthquakes have been observed to occur in other regions of the san andreas fault system and are analyzed in antolik et al 1996

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

geochemistry of axial seamount lavas magmatic relationship between the cobb hotspot and the juan de fuca ridge axial seamount located along the central portion of the juan de fuca ridge axis and at the eastern end of the cobb eickelberg seamount chain is the current center of the cobb hotspot the axial seamount lavas are transitional between n type and e type mid ocean ridge basalt morb characteristics that they share with lavas along the rest of the juan de fuca ridge there are however subtle but distinct differences between the seamount lavas and those of the adjoining ridge segments these include higher na2o cao and sr at a given mgo content and lower silica saturation in the seamount lavas as compared with the ridge lavas lava chemistry and bathymetry indicate that axial seamount is a discrete volcanic unit with a more productive shallow magmatic plumbing system separate from the adjacent ridge segments these high magma supply rates have sustained a continuously replenished steady state magma reservoir that has erupted remarkably homogeneous lavas over a long time period despite this classic association of spreading center and hotspot volcanic activity there is no evidence in the lavas for geochemical or isotopic enrichment typical of hotspot or mantle plume activity the differences in composition between the axial seamount lavas and the juan de fuca ridge lavas are attributed to melting processes rather than to any fundamental differences in their mantle source compositions the higher magma production rates higher sr and lower silica saturation in the seamount lavas relative to the ridge lavas are thought to be a consequence of melt initiation at greater depths the melting column producing the seamount lavas is thought to be initiated in the stability field of spinel peridotite whereas the ridge lavas are produced from a melting column initiated at shallower levels possibly within or close to the stability field of plagioclase peridotite implicit in this interpretation is the conclusion that the juan de fuca ridge lavas and by analogy most morb are generated at shallow mantle levels mostly within the stability field of plagioclase peridotite this interpretation also requires that for the upwelling mantle to intersect the solidus at different depths the mantle supplying axial seamount must be hotter than the rest of the juan de fuca ridge axial seamount therefore reflects a thermal anomaly in the mantle rather than a geochemically enriched ocean island basalt type mantle plume

magma evolution recorded in plagioclase zoning in 1991 pinatubo eruption products plagioclase the most abundant phenocryst at mount pinatubo displays varying textures and compositions within the 1991 eruption products in june 7 14 dome forming andesite plagioclase phenocrysts show prominent rims with higher mgo 0 04 0 06 wt jo fe20j 0 6 0 8 wt o and k20 at given an than the interiors the compositions of the rims are identical to those of microlites which are abundant in the groundmass glass white dacitic pumice the most voluminous product of the june 15 eruption contains partly corroded plagioclase phenocrysts but no prominent rims and no microlites the interiors of phenocrysts from the dacitic pumice and the dome forming andesite are remarkably similar in terms of textures and compositions they show oscillatory zoning mostly an35 6o low mgo 0 02 wtojo and fe20j 0 10 0 30 wt jo and similar k20 at given an this similarity indicates that the two types of plagioclase phenocrysts formed in the same rhyolitic melt the oscillatory zoning likely formed by temperature fluctuations in the convecting magma and incorporation and degassing of external fluids a portion of the felsic magma 800 c mixed with a mafic melt 1000 1100 c to become an andesitic magma that extruded to form the june 7 14 dome all plagioclase phenocrysts in the andesite were derived from the felsic magma the mixing caused destabilization of phenocrysts forming sieve textures dusty zones and partial resorption extrusion of the mixed magma resulted in overgrowths on once resorbed phenocrysts and the nucleation of plagioclase microlites in the groundmass glass in the unmixed remaining portion of the felsic magma some plagioclase underwent partial resorption but did not develop overgrowths this lack of overgrowths and the absence of micro lites in the groundmass glass of the june 15 dacitic pumice indicate rapid magma ascent during the eruption and a short time span between the injection of a mafic melt and the cataclysmic eruption supporting the linkage between the two

internal structure of uppermost oceanic crust along the western blanco transform scarp implications for subaxial accretion and deformation at the juan de fuca ridge 1 the uppermost 2 km of oceanic crust created near the southern end of the juan de fuca ridge 60 mm yr full spreading rate is exposed in a tectonic window created along the north wall of the blanco transform fault a total of 27 submersible dives transect this escarpment over a distance of 30 km corresponding to 1 m y of spreading history in this exposure weakly deformed basaltic lavas grade downward into much more intensely fractured lavas cut by basaltic dikes lava flow surfaces dip persistently to the northwest toward the spreading center and the dikes dip southeast away from the spreading center the underlying sheeted dike complex is composed of subparallel dikes that dip moderately to the southeast discrete faults and more distributed cataclastic deformation zones separate the panels of sheeted dikes from one another disruption of the lava and dike units are interpreted as a result of dramatic subaxial vertical mass transport that created the accommodation space for the accumulation of a thickness of 1000 1500 m of basaltic lava the observed patterns of tilting of lavas and dikes imply subsidence beneath the region of active volcanism 3 km wide that is most rapid near the axis and diminishes quickly off axis deformation that accommodates this subsidence has generated extensive fracture porosity in the subaxial region that could strongly influence upper crustal hydrothermal pathways alteration history seismic properties and the distribution of subsurface biological communities

microseismicity and permeability enhancement of hydrogeologic structures during massive fluid injections into granite at 3 km depth at the soultz hdr site summary a high rate injection of 20 000 m3 of water into granite between 2 8 and 3 4 km depth at the soultz hot dry rock hdr test site in france in 1993 september led to a 200 fold increase in borehole transmissivity and produced a subvertical cloud of microseismicity of dimensions 0 5 km wide 1 2 km long 1 5 km high and oriented 25 nw the resulting data set is unusually complete and well suited to studying permeability creation enhancement processes in crystalline rock and the utility of microseismic data for revealing them although the microseismic cloud defined using joint hypocentre determination jhd locations was diffuse and showed little structure application of the collapsing method showed it to be composed largely of discrete tubes and planes that propagated coherently one prominent structure that extended 350 m downwards from the vicinity of a flow inlet early in the injection and that appears to contain a major flow path was subjected to detailed investigation to establish its hydrogeologic nature and the mechanisms underpinning its inferred permeability enhancement high resolution microseismic mapping techniques i e multiplets and clustering showed it to be a subvertical nnw sse striking fracture zone of width 10 20 m the strike and scale of the structure identifies it as a member of a family of hydrothermally altered cataclastic shear structures that constitute the primary permeable paths for fluid migration within the rock mass both under ambient and forced fluid flow conditions the microseismicity occurred on subvertical small scale fractures within the cataclastic shear zone whose azimuths scatter within 22 of parallel to the parent structure although the structure is likely to have been naturally permeable to some degree its permeability appears to have been significantly enhanced as a consequence of the injection the most likely mechanism of permeability enhancement which is in accord with the strong preference for the microseismicity to grow downwards involves strike slip shearing which produced the opening of vertical tubes at along strike jogs in the fault the so called hill mesh seismic moment release averaged over the structure suggests shear displacements of at least 0 3 mm occurred which are sufficient to generate aperture changes that are hydraulically significant the preponderance of discrete structures within the microseismic cloud after collapsing suggests that significant flow and permeability enhancement i e stimulation within the rock mass is largely confined to the interiors of shear zones that appear to have a spacing of approximately 100 m

dynamics of magma mixing and degassing recorded in plagioclase at stromboli aeolian archipelago italy crystal rich materials scoriae and lava flows emitted during the 1985 2000 activity of stromboli were taken into consideration for systematic study of bulk rock matrix glass chemistry and in particular for the study of chemical and textural zoning of plagioclase the most abundant mineral phase over the considered time period bulk rock composition remained fairly constant in both major sio2 49 2 50 9 wt and k2o 1 96 2 18 wt and trace elements the quite constant chemistry of matrix glasses also indicates that the degree of crystallization of magma was maintained at around 50 vol plagioclase ranges in composition between an62 and an88 and is characterized by alternating 10 100 m thick bytownitic and labradoritic concentric layers although the dominant and representative plagioclase of scoriae is an68 the labradoritic layers an62 70 show small scale 1 5 m oscillatory zoning are free of inclusions and appear to record episodes of slow crystal growth in equilibrium with a degassed liquid having the composition of the matrix glass in contrast the bytownitic layers an70 an88 are patchy zoned show sieve structure with abundant micrometric glass inclusions and voids and are attributed to rapid crystal growth

origins of amplitude variations in seismic doublets source or attenuation process summary accurate estimates of spectral ratios of two doublets i e similar seismic events occurring along the san andreas fault in the hollister region have been computed using a cross spectral method both events of each doublet occurred within a 24 hr interval although both waveforms of a doublet are remarkably similar computed spectral ratios exhibit strong but steady variations with frequency in the first arrivals the attenuation process can explain neither the sign nor the order of magnitude of these variations on the contrary they are closely related to the source mechanism we show that the slope computed from the linear and coherent part of the spectral ratio is an estimation of the variation in the apparent rupture duration between the two events one doublet exhibits an azimuthal distribution of the spectral ratio variations that is interpreted as the result of a change in the position of the hypocentres relative to the rupture area for this doublet location of hypocentres varies only from 10 to 30 per cent of the source length this points out that small changes in rupture kinematics can lead to significant variations in the spectral ratio of earthquake doublets i e in the amplitude of each event for the other doublet the variations are related to a change in source length we also have computed spectral ratios all along the seismograms using a moving window technique when an azimuthal distribution of the spectral ratio is observed in the first arrivals we notice a decrease of the spectral ratio all along the seismograms the doublet with strong but non azimuthal variations of the spectral ratio in the first arrivals does not indicate such a decrease of the spectral ratio along the seismogram this implies that the variations of the coda decay are related not only to temporal or spatial changes of attenuation but also to source parameters these variations have the same order of magnitude as those provoked by a temporal change in coda q of 10 per cent they can occult time changes of spectral ratio for instance in case the attenuation in the crust would vary before a strong earthquake

earthquake cluster what can we learn from waveform similarity abstract the seismograms of earthquakes which have closely spaced hypocenters tend to be similar due to the similarity of the green s functions characterizing the source receiver paths based on the 4 criterion it is frequently argued that similar earthquakes may represent repeated slip of the same patch of a fault because of the phenomenological nature of waveform similarity such interpretations are strongly dependent on seismic signal characteristics and on the way the waveform similarity is defined in this article we use two dimensional synthetic wave field simulations in lateral heterogeneous media to investigate how the waveform similarity of closely spaced hypocenters changes with interevent separation we analyze the influence of correlation window length signal frequency bandwidth and source receiver geometry on the waveform similarity and discuss under which conditions the 4 criterion can be applied to the synthetic data set with the correlation window length defined as 2 8 times the travel time difference between the s and p phase onsets we find a correlation threshold value of 0 95 independent of the signal frequency bandwidth we use the same threshold value for two field data examples that are similar to the synthetic data in frequency content and waveform complexity and we discuss the implications of the 4 criterion for three microearthquakes occurring during a fluid injection experiment at the german deep drilling site kontinentale tiefbohrung ktb the interevent separation constrained by the 4 criterion is sufficient to identify these events as a sequence of repeating earthquakes in the sense that at least a fraction of the source area experienced repeated slip for a second data example of four natural micro earthquakes occurring near the island of crete the 4 criterion does not sufficiently constrain the hypocenter location to identify these events as repeating earthquakes due to the lack of high frequency information

the mafic silicic layered intrusions of saint jean du doigt france and north guernsey channel islands armorican massif gabbro diorite layering and mafic cumulate pegmatoid association abstract the saint jean du doigt france and north guernsey channel islands intrusive complexes hereafter referred to as sjic and ngic respectively are examples of mafic silicic layered intrusions in the armorican massif both are characterized by the occurrence of 1 a basal peripheral gabbroic unit interlayered with sheets generally dioritic in composition occasionally gabbroic and crossed by leucocratic diapirs and pipes from monzodioritic to q monzonitic in composition 2 peripheral pegmatoids associated with mafic cumulates and 3 coeval granitoids beside these main similarities some contrasted features lead us to propose two distinct models of formation the variscan sjic includes tholeiitic mafic rocks monzogabbro that locally mingle and mix with leucocratic components monzonite or q monzonite the cadomian ngic is calc alkaline the sjic sheet bearing gabbro is homogeneous from a petrologic point of view whereas the ngic exhibits gabbroic macrorhythmic sequences with mineral layering the sr nd isotopic compositions of the sjic gabbros are significantly different from those of the associated dioritic layers this is not the case in the ngic where the magmas could be cogenetic we argue that the sjic gabbro was a liquid that crystallized in situ without significant crystal settling by contrast the rhythmic sequences of the ngic are consistent with crystal accumulation subsequently both can be seen as mafic reservoirs which were repeatedly invaded by magmas of intermediate composition we interpret the sheets in the sjic as the result of horizontal spreading of dioritic metastable magmas into a gabbroic reservoir crystallizing from below at levels of neutral buoyancy injections and convection in the central part of the reservoir possibly resulted in spectacular mixing mingling structures in the ngic the emplacement of the dioritic sheets was rather controlled by pre existing rhythmic cumulative structures in both intrusions late differentiated diapirs were extracted from the dioritic sheets associated peripheral pegmatoids are thought to result from the crystallization of liquids issued from a mafic intercumulus melt in the presence of a fluid phase this extraction might have been enhanced by the disruption of the peripheral cumulate stack perhaps following pressure drops

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

induced seismicity during the stimulation of a geothermal hfr reservoir in the cooper basin australia a long term fluid injection experiment was performed in the cooper basin australia in 2003 to stimulate a geothermal reservoir more than 20 000 m 3 of water were injected into the granitic crust at 4250 m depth during reservoir stimulation about 27 000 induced seismic events were detected by a local eight station seismic monitoring system deployed in nearby boreholes hypocenter locations for 11 068 events were determined by using an averaged velocity model that was calibrated by associating early events with the injection point the spatial hypocenter distribution forms a nearly subhorizontal structure with a lateral extension of 2 km 1 5 km and an apparent thickness of approximately 150 200 m which is in the order of the hypocenter location confidence limits the hypocenter distribution exhibits a high degree of spatiotemporal ordering with the seismic activity systematically migrating away from the injection well with increasing time previously activated regions become seismically quiet indicating relaxation processes high resolution relative hypocenter locations determined for clusters of similar events locally reduce the apparent thickness of the structure to the level of a few tens of meters indicating that the reservoir is dominated by a single fracture zone only consistent with these findings a subsequently drilled well intersects a dominating high permeable fracture within 15 m of the predicted intersection depth based on drilling and logging information the fracture zone is interpreted as a preexisting possibly tectonically formed feature that partly sheared during stimulation triggering of the induced seismicity is found to be predominantly controlled by the increase of fluid pressure implicating a local reduction of the effective normal stress resolved on the fracture plane additionally perturbations of the stress field caused by the largest magnitude events may trigger seismicity aftershocks on a local short ranging scale

pb sr and nd isotopes in basalts and sulfides from the juan de fuca ridge pb sr and nd isotopes of seven basalt glasses collected by the submersible alvin from the southern juan de fuca ridge sjfr are almost identical 206pb 204pb 18 45 207pb 204pb 15 47 208pb 204pb 37 81 87sr 86sr 0 70249 143nd 144nd 0 51315 whereas all basalts appear cogenetic four of the samples have uniform abundances of u th rb nd sm pb and sr indicating that they are also comagmatic two basalt glasses dredged previously at the sjfr have similiar isotopic compositions but higher concentrations of u th and pb the 206pb 204pb ratios are intermediate between generally less radiogenic ridge basalts from south of the juan de fuca ridge jfr and often more radiogenic basalts from the northern jfr and ne pacific seamounts sr and nd isotopic compositions closely resemble data of other ridge basalts from the northernmost east pacific rise and are intermediate between isotopically more diverse seamount basalts produced nearby eight sulfides collected from three hydrothermal vent sites at sjfr have very similar pb isotopic compositions to that of the associated basalts sr and nd isotopic ratios of a single sulfide sample lie on a mixing curve between seawater and basalt data indicating that the sulfide precipitated after mixing of hydrothermal solutions and ambient seawater

fine scale magnetic anomaly field over the southern juan de fuca ridge axial magnetization low and implications for crustal structure sea surface deep tow and submersible magnetic surveys show that the spreading axis of the cleft segment of the southern juan de fuca ridge sjdf is overlain by a distinctive magnetic anomaly low in contrast to the more typical magnetic anomaly high found over the adjacent vance segment and most mid ocean ridge spreading centers forward iterative thickness modeling based on the near bottom magnetic data shows that this axial magnetic low can be produced by a thin magnetic source layer beneath the cleft axial valley thick crust is required to produce the magnetic anomaly high over the vance axial valley this magnetic modeling demonstrates that variations in the base of the magnetic source layer which is assumed to be equivalent to the base of the extrusive lavas can produce a significant proportion of the observed short wavelength magnetic anomaly signal these magnetic layer thickness models also correlate quite well with nearby seismic results which show that seismic layer 2a thins beneath the axial valley of the cleft and is thicker beneath the vance axial valley recent seismic results over other fast and medium spreading rifts also show rapid changes in the thickness of the uppermost crust near the axis of spreading which could produce significant magnetic contrasts the results of this study and the correlation with independent seismic measurements suggest that while magnetization may vary due to geomagnetic behavior or because of petrology or alteration first order change in upper crustal thickness is one of the most important variables to consider in the production of fine scale magnetic anomalies in young ocean crust

gabbros from iodp site 1256 equatorial pacific insight into axial magma chamber processes at fast spreading ocean ridges the odp iodp multileg campaign at odp site 1256 cocos plate eastern equatorial pacific provides the first continuous in situ sampling of fast spreading ocean crust from the extrusive lavas through the sheeted dikes and down into the uppermost gabbros this paper focuses on a detailed petrographic and microanalytical investigation of the gabbro section drilled during iodp expedition 312 the marked patchy and spotty features that can be observed in many hole 1256d gabbros is mostly due to a close association of two different lithological domains in variable amounts 1 subophitic domains and 2 a granular matrix major and trace element mineral compositions geothermometry and petrological modeling suggest that subophitic and granular domains follow one single magma evolution trend formed by in situ fractionation the subophitic domains correspond to the relative primitive high temperature end member compositionally similar to the basalts and dikes from the extrusive unit upsection while the granular domains fit with a magma evolution by crystal fractionation to lower temperatures up to a degree of crystallization of 80 our results support the following scenario for the fossilization of the axial melt lens at odp site 1256 relatively primitive morb melts under near liquidus conditions fill the melt lens and feed the upper extrusive crust near the melt lens sheeted dike boundary at lower temperatures crystallization starts with first plagioclase before clinopyroxene in a mushy zone forming the subophitic domains at decreasing temperatures the subophitic domains continue to crystallize finally forming a well connected framework evolved residual melt is finally trapped within the subophitic network crystallizing at near solidus conditions to the granular matrix another important textural feature in hole 1256d gabbros is the presence of microgranular domains which are interpreted as relics of stoped assimilated sheeted dikes transformed to granoblastic dikes by contact metamorphism all these different domains can be observed in close association often at the thin section scale demonstrating the extremely complex petrological record of combined crystallization assimilation processes ongoing in the axial melt lens very similar gabbros with a marked spotty patchy appearance and bearing the same close association of lithological domains as observed at site 1256 are known in the so called varitextured gabbro unit from the oman ophiolite located at the same structural level between cumulate gabbros and granoblastic dikes the close petrological similarity of the gabbro dike transition between both iodp hole 1256d and the oman ophiolite suggests that in situ fractionation and dike assimilation contamination are major magmatic processes controlling the dynamics and fossilization of the axial melt lens at fast spreading oceanic ridges

boundary layer crystallization in a basaltic magma chamber evidence from rishiri volcano northern japan the evolution of basaltic magma by mushy boundary layer processes introduction in a magma chamber is documented for an alkali basalt kutsugata the evolution of magma chambers is of major petrologic lava from rishiri volcano northern japan on the basis of zoning interest and has been studied extensively experimental patterns of plagioclase phenocrysts and their spatial distribution in and theoretical studies that have investigated the cooling the lava plagioclase phenocrysts with complex zoning patterns can of magma chambers have demonstrated that a magma be divided into four types type 1 is characterized by an extremely chamber solidifies through formation of a mushy boundan rich core an71 90 which is commonly corroded and filled with ary layer composed of crystals and interstitial melt along sodic plagioclase type 2 has an ab rich core surrounded by a calcic the chamber walls e g brandeis jaupart 1986 turner mantle characteristic of partial dissolution type 3 is characterized by et al 1986 marsh 1988 tait jaupart 1992 layered igneous complexes e g morse 1969 shirley 1987 an ab rich core that is commonly surrounded by a reversely zoned mcbirney 1996 provide useful information on how slightly calcic mantle type 4 has an ab rich core without a calcic mafic magma chambers evolve and consolidate but they mantle the an rich cores of the type 1 plagioclase are too calcic do not offer much insight into the instantaneous state of to have crystallized from a liquid represented by any whole rock a chamber because those intrusions integrate processes composition at the estimated temperatures of the kutsugata magma that occurred over long solidification times mcbirney system the an rich cores are inferred to have been formed in a noyes 1979 tait 1988 volcanic products such as mushy boundary layer along the wall of a magma chamber where lava flows and tephra deposits can sample the inthe magma was relatively cool and rich in water because of significant stantaneous state of a magma chamber there are a few olivine fractionation and possibly because of addition of water studies along this line of approach tait 1988 de silva expelled from the chilled margin and or groundwater from the 1989 nakada et al 1994 these studies focused on surrounding crust the ab rich cores of types 2 4 plagioclase crystal aggregates in volcanic products and concluded phenocrysts are interpreted to have crystallized in the main magma that the crystal formation in a mushy boundary layer body of the reservoir the types 2 and 3 ab rich cores were brought plays a dominant role in magmatic evolution in a magma to the mushy layer by convection the temporal and spatial variations reservoir however these studies were mostly on silicic of zoning patterns of plagioclase phenocrysts in basaltic lava flows magmas and there are significant differences in physical give useful information on the evolution of mafic magma chambers and chemical features between silicic and mafic magma chambers in this paper the magmatic evolution of an alkali basalt kutsugata lava from rishiri volcano northern japan is studied the kutsugata lava is divided into three lava flows on the basis of whole rock chemical

spatio temporal microseismicity clustering in the cretan region abstract spatio temporal clustering of microseismicity in the central forearc of the hellenic subduction zone in the area of crete is investigated data for this study were gathered by temporary short period networks which were installed on the islands of crete and gavdos between 1996 and 2004 the similarity of waveforms is quantified systematically to identify clusters of microseismicity waveform similarities are calculated using an adaptive time window containing both the p and s wave onsets the cluster detection is performed by applying a single linkage approach clusters are found in the interplate seismicity as well as in the intraplate seismicity of the continental crust in the region of the transtensional ptolemy structure the majority of the clusters are off the southern coast of crete in a region of elevated intraplate microseismic activity within the aegean plate clusters in the gavdos region are located at depths compatible with the plate interface while cluster activity in the region of the ptolemy trench is distributed along a nearly vertical structure throughout the crust extending down to the plate interface most clusters show swarm like behaviour with seismic activity confined to only a few hours or days without a dominant earthquake and with a power law distribution of the interevent times for the largest cluster precise relocations of the events using travel time differences of p and s waves derived from waveform cross correlations reveal migration of the hypocenters this cluster is located in the region of the ptolemy trench and migration occurs along the strike of the trench at 500 m day relocated hypocenters as well as subtle differences in the waveforms suggest an offset between the hypocenters and thus the activation of distinct patches on the rupture surface the observed microseismicity patterns may be related to fluids being transported along the plate interface and escaping towards the surface in zones of crustal weakness ptolemy structure triggering swarm like cluster activity along its way

microearthquake cluster detection based on waveform similarities with an application to the western swiss alps summary highly similar waveforms of different earthquakes are due to similar focal mechanisms and common propagation paths the relative hypocentral locations of events in clusters of similar earthquakes can provide useful insights into geometry and style of faulting at depth within the crust the detection of such earthquake clusters within large data sets can only be accomplished efficiently by means of an automatic procedure therefore we developed an algorithm based on correlation analysis that detects and associates events with similar waveforms the algorithm has been applied to a data set recorded in the western swiss alps 619 out of a total of 1497 events exhibit similarities with other events based on a more detailed investigation of two selected clusters with known focal mechanisms it could be shown that the active fault planes correspond to neotectonic structures mapped in the study area due to their oblique orientation relative to the larger scale epicentral alignment these faults have been interpreted as riedel shears

low pressure phase equilibria of anhydrous anorthite bearing mafic magmas 1 one of the most persistent questions regarding the phase equilibria of mid ocean ridge basalts morb pertains to the petrogenesis of the anorthitic plagioclase phenocrysts an90 that are characteristic of the more primitive members of such suites anorthitic phenocrysts are present in many if not most morb suites in spite of the fact that no naturally occurring morb glasses have ever been discovered to be in equilibrium with plagioclase more calcic than an85 we have addressed this paradox by attempting to saturate natural basalts with anorthite in a series of 1 atm experiments using three different natural basaltic starting compositions an n morb an e morb and a continental high alumina basalt to ensure duplication of the olivine and anorthite saturation observed in natural anorthite bearing basalt the experiments were run in an93 6 capsules with fo92 olivine added to the starting glass the compositions of experimental liquids are generally colinear with the trends observed in the lava suites used as the source material for the starting glasses significantly aluminous spinel al2o3 contents of 61 68 wt was produced at 1290 c in all compositions and chromites al2o3 contents of 33 42 wt at lower temperatures in n morb derived liquids despite no spinel having been added to the starting mixture in addition the experiments produced basaltic liquid in equilibrium with both fo89 olivine and an85 feldspar at temperatures of 1230 and 1210 these liquids have compositions with mg at mg mg fet 100 that range from 63 to 85 the tio2 mgo correlation indicates large 16 23 amounts of crystallization for each percent decrease in mgo these results suggest the possibility that dry anorthite bearing basaltic magmas are the product of the interaction between primary melt and al spinel bearing upper mantle in addition the results indicate that morb magmas can undergo a large amount 50 of crystallization prior to reaching 8 mgo further although anorthite bearing magmas have characteristics consistent with their being a significant volumetric component of morb parent magmas the reaction mechanism suggested for their petrogenesis indicates that they are not necessarily primary magmas

seismological studies at parkfield vi moment release rates and estimates of source parameters for small repeating earthquakes abstract waveform data from a borehole network of broadband seismographic stations have been used to study microearthquakes along the parkfield segment of the san andreas fault saf analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences quasiperiodic sequences of earthquakes that are essentially identical in terms of waveform size and location scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the stone canyon section of the fault and the main parkfield sequence these estimates show that seismic moment is being released as a function of time in a very regular manner measurements of the moment release rate combined with an assumed tectonic loading rate lead to estimates of the seismic parameters source area slip and recurrence interval such parameters exhibit a systematic dependence upon source size over a range of 10 10 in seismic moment that can be described by three simple scaling relationships several implications of these scaling relationships are explored including the repeat time of earthquakes average stress drop strength of the fault and heat generated by earthquakes what emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1 of the fault area this means that the fault is highly heterogeneous with respect to stress strength and heat generation such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes such as why faults appear weak why significant heat flow is not observed how significant high frequencies can be generated by large earthquakes and how various geologic features such as pseudotachylytes might form

formation of large summit troughs along the east pacific rise as collapse calderas an evolutionary model summit troughs wider than 500 m and 30 110 m deep are present along 15 20 of the fast spreading east pacific rise epr they occur only along ridge segments with large cross sectional areas indicative of a time averaged robust magma supply where available seismic data confirm that these troughs are underlain by an axial magma chamber amc 1 2 1 6 km below the seafloor furthermore detailed investigation of the large summit troughs which notch the epr between 17 56 and 18 35 s indicates that the vertical relief of the troughs tends to be maximum where the amc is shallowest both of these observations are inconsistent with the predictions from a model in which large summit troughs form by rifting of the brittle upper crust during phases of amagmatic extension rather we propose that they represent elongated collapsed calderas that form when the melt supply to formerly inflated amcs wanes or nearly ceases because seismic studies constrain the melt lens along the epr to be only 30 80 m thick the proposed existence of collapsed calderas 40 110 m deep implies that the entire magma reservoir comprising the melt lens and the underlying crystal mush zone deforms and compacts during periods of waning magma supply in particular we suggest that a voluminous crystal mush zone will stretch in response to steady state seafloor spreading when the magma supply temporarily decreases the resulting caldera will further widen with each subsequent dike intrusion when an abundant melt supply finally resumes the associated tumescence of the neovolcanic zone and profuse lava flows will combine to smooth out the caldera

estimation of arrival times in intense seismic sequences using a master events methodology based on waveform similarity summary intense seismic sequences involve a large number of earthquakes densely clustered in space and time their detailed analysis is important for the geometry of the activated faults which contributes to studies of the seismotectonic characteristics of an area the sheer number of small events as well as their low energy content renders their processing problematic in this work we have developed and applied a methodology to automatically pick the arrival times of p and s waves using a correlation detector event detection is performed using the waveform recordings of a reference station located close to the epicentral area of an intense seismic sequence such as aftershocks or swarms cross correlation matrices are constructed followed by nearest neighbour clustering and the formation of multiplets a master event is chosen from each cluster and its arrival times are picked manually the automatic algorithm uses the p o rs wave of each master event as a correlation detector searches the waveforms of the other events of the same multiplet and imposes the corresponding arrival time when the bestfit is achieved the picks are characterized by observation weights which derive from the quality of thefit the type of the available waveform components and the consistency between multiple measurements the proposed methodology was applied to an important seismic sequence that occurred between 2010 january 18 and 26 near the city of efpalio greece this procedure has the potential to increase 10 fold the amount of information and provide sufficient detail for a subsequent analysis of the spatiotemporal distribution of a seismic series

petrology and geochemistry of basalts from the southern juan de fuca ridge controls on the spatial and temporal evolution of mid ocean ridge basalt three morphologically distinct regions within the neovolcanic zone of the cleft segment of the southern juan de fuca ridge were investigated and sampled in detail using the dsrv alvin additional along strike and off axis samples were recovered by dredge the southernmost region the southern cleft site is characterized by a 3 km wide axial valley floored by ponded sheet flows and bisected by a 30 to 50 m wide cleft farther north at the young sheet flow site the ridge axis is characterized by a distinct 500 m wide inner graben that is largely covered by distinctly younger looking sheet flows the northernmost of the three regions is defined by a linear series of discontinuous constructional pillow mounds that extend the trend of the cleft segment well into the zone of overlap with the neighboring vance segment the pillowed lavas at the young pillow mound site represent the most recent episode of volcanism along the cleft segment strong correlations exist between degrees of fractionation relative ages of lavas and latitude lavas are progressively younger looking and more mafic to the north the compositional range of mid ocean ridge basalts from the neovolcanic zone can generally be accounted for by 35 40 low pressure fractional crystallization of relatively primitive but not primary depleted n type melts scatter of the geochemical data about calculated liquid lines of descent is probably the result of mixing of magmas with slightly different parental compositions generated from small scale mantle heterogeneities furthermore the chemical variability may be the result of mixing of very depleted and more enriched sources or melts that are present beneath the southern juan de fuca ridge the more primitive nature of the young pillow mound basalts and their slightly different chemical characteristics indicates they cannot be simply related to the older southernmost lavas by along axis flow in a continuous axial magma chamber or conduit rather the data suggest lavas were derived from discrete magma chambers or lenses each in a different stage of evolution the youngest events may be associated with a new influx of magma into the northern part of the segment and subsequent northward diking and propagation to form the new pillow mounds the oldest stage 40 additional crystallization of the most mafic composition is associated with focused hydrothermal activity and tectonic extension whereas the youngest events are characterized by ridge inflation diking and dispersed hydrothermal activity geochemical and tectonomagmatic features observed along the cleft segment are similar to those recently documented along the east pacific rise from 9 30 n 10 n suggesting the scales processes and stages of magmagenesis are similar along medium to fast spreading ridge segments

the petrogenesis of plagioclase phyric basalts at mid ocean ridges 1 plagioclase ultraphyric basalts pubs have been sampled along most mid ocean ridges with ultraslow to intermediate spreading rates over the past 40 years the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation however when the global population of pub lavas is examined this hypothesis becomes less compelling pub host lavas demonstrate a large range of compositions and densities similar to aphyric glasses from the same ridge segments most importantly the majority of pub host liquids are less dense than their phenocryst cargo meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment furthermore pub lavas have never been sampled on axis at fast spreading centers or from locations with noted contemporaneous axial magma chambers where pubs should be abundant if plagioclase is buoyant in mid ocean ridge basalt morb instead we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust possibly followed by loss of olivine during magma ascent pubs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts which precludes long crustal residence times for these magmas in addition to being a proxy for lower spreading rates our findings also suggest that pub eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber

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

the effects of sliding velocity on the frictional and physical properties of heated fault gouge the frictional properties of a crushed granite gouge and of gouges rich in montmorillonite illite and serpentine minerals have been investigated at temperatures as high as 600 c confining pressures as high as 2 5 kbar a pore pressure of 30 bar and sliding velocities of 4 8 and 4 8 10 2 m sec the gouges showed nearly identical strength behaviors at the two sliding velocities all four gouges however showed a greater tendency to stick slip movement and somewhat higher stress drops in the experiments at 4 8 10 2 m sec varying the sliding velocity also had an effect on the mineral assemblages and deformation textures developed in the heated gouges the principal mineralogical difference was that at 400 c and 1 kbar confining pressure a serpentine breakdown reaction occurred in the experiments at 4 8 10 2 m sec but not in those at 4 8 m sec the textures developed in the gouge layers were in part functions of the gouge type and the temperature but changes in the sliding velocity affected among other features the degree of mineral deformation and the orientation of some fractures

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

magma emplacement at anomalous spreading ridge constraints due to plagioclase crystals from basalts of marsili seamount southern tyrrhenian back arc abstract plagioclase crystals from four basalt samples dredged from different portions of the marsili seamount southern tyrrhenian italy provide a detailed record of the changes occurring in the surrounding magma during its ascent in the crust the textural features and chemical zoning i e an content and fe mg sr and ba concentrations of the plagioclases from each sample show that aggregation of various generations of crystals occurred in the host melt prior to eruption plagioclases resulting from crystallisation in small sill intrusions where local flotation of plagioclase may have occurred are ubiquitous instead large up to 1 mm homogeneous an rich plagioclases are rare and have only been found in two of the studied basalts recovered from the northern and southern portions of the volcano respectively these crystals record crystallisation events occurring long before eruption and probably derive from deep seated crystalline material at the highest an values the sr and ba concentrations of the studied plagioclases also indicate the existence of two distinct mafic magmas within the lower crustal plumbing system of the marsili volcano one with lower sr and ba contents than the other three of the four studied basalts came from magma with low sr and ba whereas the fourth which erupted from the southern portion of the volcano sampled magma richer in sr and ba the ascent of magma caused resorption of the previously formed plagioclase crystals and regrowth of normally or inversely zoned portions depending on the water saturated or water undersaturated conditions of the surrounding magma respectively small shallow magmatic reservoirs may be located under the southern part of the marsili volcano as testified by the resorbed sodic cores an52 57 found in the two basalt samples from this portion

quasi static slip rate shielding by locked and creeping zones as an explanation for small repeating earthquakes at parkfield in a recent study of microearthquakes along the parkfield segment of the san andreas fault nadeau et al 1995 have found that much of the seismicity in the region is characterized by quasi periodic repeating sequences of small earthquakes that are essentially identical in waveform size and location nadeau and johnson 1998 interpreted these as repeated slip on a given asperity driven by a steady slip rate of 2 3 cm yr and concluded that the stress drops needed to be extremely high of the order of 20 kilobars we propose another explanation for these small repeating events namely that an inner asperity is surrounded by a larger creeping zone which in turn is surrounded by a still larger locked zone this geometry produces a local slip velocity much less than the overall creep velocity observed on a still larger scale slip velocity shielding we have constructed a foam rubber model to illustrate the phenomenon the time sequences of small events at the asperity punctuated by large events which rupture the whole block look very similar to the cumulative moment plots of nadeau and johnson the actual dynamic stress drops are of the same order as for the large events thus the results of the model correspond to the observations of nadeau and johnson and suggest that the model may be appropriate to explain their observations without requiring super strong asperities

minor and trace element zoning in plagioclase implications for magma chamber processes at parinacota volcano northern chile textural and compositional zoning in plagioclase phenocrysts in a sample from parinacota volcano chile was investigated using backscattered electron images and electron microprobe analysis of major and trace elements large 2 mm oscillatory zoned crystals type i with resorption surfaces of moderate an discontinuities 10 an and decreasing trace element contents sr mg ti towards the rim reflect melt differentiation and turbulent convection in the main magma body early recharge with a low sr mafic magma is seen in the core small scale sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years smaller crystals type ii with low trace element an ratio reflect the influence of an h2o rich melt probably from a differentiated boundary layer closed system in situ crystallisation mafic magma recharge and the role of a water rich differentiated boundary layer can be distinguished from the an trace element relationships crystals apparently move relatively freely between different parts and regimes in the magma chamber evidence for convective crystal dispersion high sr type ii crystals indicate an earlier input of sr rich mafic magma recharge of two distinct mafic magma types is thus identified high sr and low sr which must have been present at increasing recharge rates with time in the plumbing system throughout the volcano s history

diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals we present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects in particular we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements e g mg the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension we show that contrary to intuition diffusive fluxes of trace elements may be coupled to major element concentration gradients and ignoring such coupling can lead to incorrect estimates of timescales we illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a holocene dacitic lava flow of volcan san pedro chilean andes 36 s the inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 30 to 148 yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods

geochemistry of basalt from escanaba trough evidence for sediment contamination mid ocean ridge basalts morbs erupted in escanaba trough introduction the southernmost segment of the gorda ridge have greater isotopic crustal contamination of basaltic magmas is generally and geochemical variability than those from the remainder of the recognized for continental volcanoes e g taylor 1980 ridge samples from the sediment free northern sites in the trough leeman hawkesworth 1986 glazner et al 1991 the are depleted normal morb similar to those from the northern composition thickness and low melting temperatures of gorda ridge however samples from the sediment covered portion continental crustal rocks facilitate assimilation that can of the trough are significantly enriched in incompatible elements be easily identified however the extent to which basaltic and sr nd and pb isotopic ratios are shifted toward the composition magmas are contaminated by oceanic crust is a topic of of local sediments pb isotopic ratios in particular lie on a mixing intense debate e g bohrson reid 1995 clague et al line with local sediments assimilation fractional crystallization 1995 eiler et al 1996 bohrson et al 1996 and references afc calculations suggest that 25 40 fractional crystallization therein thirlwall et al 1997 the debate has focused coupled with assimilation of 1 2 of local sediments can produce on distinguishing the chemical characteristics magmas the enrichment observed for some escanaba basalts variations in acquire during ascent crustal storage and emplacement the amount of enrichment of elements with similar incompatibility from those that have been used to identify heterogeneous for comparably differentiated basalts and uncorrelated isotopic mantle sources for most ocean island volcanoes the ratios suggest variable amounts of assimilation of compositionally crustal components available to serve as contaminants heterogeneous assimilants by parental magmas assimilation of are poorly characterized but include the underlying sediment by morb magmas probably resulted when laccolithic altered oceanic crust pelagic to hemipelagic sediment intrusions ponded at the sediment basement interface beneath the and the hydrothermally altered volcanic edifice thick sediment cover in the southern part of escanaba trough mid ocean ridge basalts morbs are relatively unigeochemical and isotopic evidence for contamination can be detected form geochemically with depleted incompatible element in these lavas because of the strong compositional contrast between and radiogenic isotope compositions compared with lavas the parental magma and assimilant the depleted morb magma from ocean island or continental volcanoes however and highly enriched sediment assimilant are end member comexcess chlorine content and cl k ratios in fresh morb positions that provide a sensitive indicator for the smallest amount glasses support the idea that morb magmas may also of contamination of oceanic magma by crustal processes be affected by assimilation of hydrothermally altered oceanic crust e g michael schilling 1988 michael cornell 1996 chlorine like other incompatible elements shows the greatest enrichment in strongly frac

direct measurement of magnetic reversal polarity boundaries in a cross section of oceanic crust magnetic field measurements made by submersible define the cross sectional geometry of a magnetic polarity rever sal boundary and the vertical variation of crustal magnetization in upper oceanic crust measured polarity boundaries show a systematic pattern of shallow dip towards the spreading axis within the upper extrusive lavas and steeper dip in the lower extrusive lavas this geometry is a consequence of the em placement of extrusive lava at a midocean ridge reversal boundary geometry and magnetization estimates are used to calculate the magnetic contribution of the extrusive lava se quence to the overlying marine magnetic anomaly signal from the forward modeling the highly magnetized extrusive lavas contribute the majority 50 75 of the observed sea surface magnetic anomaly consistent with the extrusive crust forming the primary source layer for young marine magnetic anomalies