a sea floor spreading event captured by seismometers two thirds of earth s surface is formed at mid ocean ridges yet sea floor spreading events are poorly understood because they occur far beneath the ocean surface at 9 50 n on the east pacific rise ocean bottom seismometers recently recorded the microearthquake character of a mid ocean ridge eruption including precursory activity a gradual ramp up in activity rates since seismic monitoring began at this site in october 2003 suggests that eruptions may be forecast in the fast spreading environment the pattern culminates in an intense but brief 6 hour inferred diking event on 22 january 2006 followed by rapid tapering to markedly decreased levels of seismicity

aftershocks of the 2010 mw 7 2 el mayor cucapah earthquake reveal complex faulting in the yuha desert california 1 we detect and precisely locate over 9500 aftershocks that occurred in the yuha desert region during a 2 month period following the 4 april 2010 mw 7 2 el mayor cucapah emc earthquake events are relocated using a series of absolute and relative relocation procedures that include hypoinverse velest and hypodd location errors are reduced to 40 m horizontally and 120 m vertically aftershock locations reveal a complex pattern of faulting with en echelon fault segments trending toward the northwest approximately parallel to the north american pacific plate boundary and en echelon conjugate features trending to the northeast the relocated seismicity is highly correlated with published surface mapping of faults that experienced triggered surface slip in response to the emc main shock aftershocks occurred between 2 km and 11 km depths consistent with previous studies of seismogenic thickness in the region three dimensional analysis reveals individual and intersecting fault planes that are limited in their along strike length these fault planes remain distinct structures at depth indicative of conjugate faulting and do not appear to coalesce onto a throughgoing fault segment we observe a complex spatiotemporal migration of aftershocks with seismicity that jumps between individual fault segments that are active for only a few days to weeks aftershock rates are roughly consistent with the expected earthquake production rates of dieterich 1994 the conjugate pattern of faulting and nonuniform aftershock migration patterns suggest that strain in the yuha desert is being accommodated in a complex manner

the role of magma injection in localizing black smoker activity the mechanisms for localization of black smoker systems at mid ocean ridges remain to be fully understood seismic data for a segment of the juan de fuca ridge with long lived black smoker vents reveal ongoing magma recharge into the crustal magma chamber thereby providing an explanation for the localization

temperate carbonate debrites and short lived earliest miocene yo yo tectonics eastern taranaki basin margin new zealand abstract this study examines two unconformity bound earliest miocene temperate shallow marine limestones at remote gibson beach western north island on the eastern margin of taranaki basin new zealand s only producing hydrocarbon province the local geology comprises an upper limestone of up to 6 m of spectacular conglomeratic limestone rudstone papakura limestone whose pebble to boulder sized clasts were derived from cannibalisation of the lower skeletal limestone bryomol grainstone otorohanga limestone multiple lines of evidence require the otorohanga limestone to have been well indurated before erosion and clast generation cementation occurred from pressure dissolution of calcitic skeletons at burial depths of probably 400 m or more subsequent uplift led to local subaerial exposure and karstification of a fault block cliffed coastline where physical erosion sourced talus limestone debris onto adjacent high energy pocket beaches storm and or seismic events periodically triggered mass flows of the limestone clasts offshore onto the contemporary mixed siliciclastic carbonate shelf as channelised carbonate debrites lithification of these papakura limestone debrites occurred via pressure dissolution and required a second burial episode karst pinnacles preserved on the eroded upper surface of the papakura limestone demand uplift again and erosion in a subaerial setting before subsequent deep burial by early miocene siliciclastic shelf sand and turbidite deposits strontium isotope dating of brachiopod fossils suggests the two postulated burial uplift cycles involving movements up to 400 100 m occurred very rapidly within several 100 kyr speculative drivers of the yo yo tectonic events are earliest miocene movements on the nearby major taranaki fault thrust in association with regional changes in subduction tectonics at the australian pacific plate boundary along with pulsed emplacement of obducted gravity slide deposits northland allochthon onto northern north island the two limestones record a previously undocumented degree of eastern taranaki basin margin tectonic mobility relevant to a fuller appreciation of petroleum system development in that basin

high resolution bathymetry of the east pacific rise axial summit trough 9 49 51 n a compilation of alvin scanning sonar and altimetry data from 1991 1995 bathymetric data acquired during six cruises to the east pacific rise crest 9 49 51 n are compiled into a single bathymetric database and used to create high resolution bathymetry maps of two geologically and biologically active areas scanning sonar and alvin altimetry were collected during 23 alvin dives to the p vent bio9 and biomarker 141 areas during 1991 95 bathymetry data were filtered edited and processed as individual dives then combined into a single database for each cruise and statistically evaluated cruise data sets were numerically and visually compared and cross correlated to known bottom features and fixed seafloor reference markers then horizontally shifted to establish a consistent database with 3 m vertical and 5 m horizontal resolutions the compiled and corrected bathymetry was used to produce bathymetric maps of two areas within the axial summit collapse trough a 2 104 m area around p and bio9 vents and a 1 1 10 m area around biomarker 141 the resulting detailed bathymetry maps reveal topography dominated by low relief volcanic constructional features and post eruptive collapse of lobate and sheet lava surfaces

frequency magnitude distribution of microearthquakes beneath the 9 50 n region of the east pacific rise october 2003 through april 2004 1 relocated hypocentral data from a 7 month deployment october 2003 to april 2004 of ocean bottom seismometers provide an opportunity to map microearthquake frequency magnitude distributions fmds along the 9 49 52 n region on the east pacific rise these analyses which incorporate more than 9000 earthquakes represent the first investigation of the 3 d spatial and temporal patterns of fmds along any mid ocean ridge spreading center the data are described well by a gutenberg richter model indicating a power law or fractal relationship between earthquake size and frequency the scaling exponent or b value shows significant spatial variability exceeding a value of 2 0 at the shallowest depths on axis and dropping below 1 0 away from the axial trough this spatial pattern is consistent with an inverse relationship between b value and ambient stress conditions with the lowest stress levels at shallow depths and relatively high stress levels or low pore pressures observed away from the axial zone intermediate b values are observed on axis above the ridge system s melt lens however within this region there also exists significant spatial variability this indicates that stress conditions and or structural heterogeneity may vary at subkilometer scales within the hydrothermal circulation cell although the observational period is characterized by increasing seismicity rates building toward an eruptive episode in january 2006 the first order spatial pattern of b values is sustained with no overall temporal trend as a byproduct of this b value analysis the detection capabilities of the array are assessed empirically

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

marine cements in mid tertiary cool water shelf limestones of new zealand and southern australia large areas of southern australia and new zealand are covered by mid tertiary limestones formed in cool water shelf environments the generally destructive character of sea floor diagenesis in such settings precludes ubiquitous inorganic precipitation of carbonates yet these limestones include occasional units with marine cements 1 within rare in situ biomounds 2 within some stacked cross bedded sand bodies 3 at the top of metre scale subtidal carbonate cycles and 4 most commonly associated with certain unconformities the marine cements are dominated by isopachous rinds of fibrous to bladed spar interstitial homogeneous micrite and interstitial micropeloidal micrite often precipitated sequentially in that order internal sedimentation of microbioclastic micrite may occur at any stage the paradox of marine cemented limestone units in an overall destructive cool water diagenetic regime may be explained by the precipitation of cement as intermediate mg calcite from marine waters undersaturated with respect to aragonite in some of the marine cemented limestones aragonite biomoulds may include marine cement sediment internally suggesting that dissolution of aragonite can at times be wholly marine and not always involve meteoric influences we suggest that marine cementation occurred preferentially but not exclusively during periods of relatively lowered sea level probably glacio eustatically driven in the mid tertiary at times of reduced sea level there was a relative increase in both the temperature and the carbonate saturation state of the shelf waters and the locus of carbonate sedimentation shifted towards formerly deeper shelf sites which now experienced increased swell wave and or tidal energy levels fostering sediment abrasion and reworking reduced sedimentation rates and freer exchange of sediment pore waters energy levels were probably also enhanced by increased upwelling of cold deep waters onto the southern ocean margins of the australasian carbonate platforms where water mass mixing warming and loss of co2 locally maintained critical levels of carbonate saturation for sea floor cement precipitation and promoted the phosphate glauconite mineralization associated with some of the marine cemented limestone units

platform drowning leading to cool water carbonate deposition evolution of a late ordovician turinian chatfieldian mixed sediment platform within the taconic orogen long point group newfoundland appalachians late ordovician turinian chatfieldian drowning of a mixed carbonate siliciclastic platform within the taconic orogen newfoundland appalachians is recorded by net deepening of an initial warm shallow water platform succession lourdes formation culminating in a metre scale thick condensed interval that characterizes a drowning succession punctuated by storm deposits composition of transported material suggests that seaward drowning was coupled with back stepping of a high energy carbonate factory related to hinterland uplift and erosion that would eventually lead to drowning of the outer platform beneath marine transported siliciclastic sediments winterhouse formation in the new offshore shelf setting a sparse reciprocal stratigraphy of fine to very coarse grained phosphatic carbonate and mixed sediment is interpreted to document gravity flow deposition downgradient from either a sustained or episodically developed high energy cool water carbonate source along the inner shelf transported carbo

equatorial waves in the lower stratosphere i a novel detection method wavenumber frequency spectral analysis and linear wave theory are combined in a novel method to quantitatively estimate equatorial wave activity in the tropical lower stratosphere the method requires temperature and velocity observations that are regularly spaced in latitude longitude and time it is therefore applied to the ecmwf 15 year re analysis dataset era 15 signals consistent with idealized kelvin and rossby gravity waves are found at wavenumbers and frequencies in agreement with previous studies when averaged over 1981 93 the kelvin wave explains approximately 1 k2 of temperature variance on the equator at 100 hpa while the rossby gravity wave explains approximately 1 m2s 2 of meridional wind variance some inertio gravity wave and equatorial rossby wave signals are also found however the resolution of era 15 is not sufficient for the method to provide an accurate climatology of waves with high meridional structure copyright 2006 royal meteorological society

waxing and waning volcanism along the east pacific rise on a millennium time scale microbathymetric maps of the southern east pacific rise reveal subtle field relations between volcanic features and provide new insight on seafloor spreading processes along one of the shallowest and broadest sections of ridge at 17 28 s lavas have erupted from a fissure system and flooded the axis through a network of lava tubes and lava channels along the neighboring ridge segment at 18 15 s the axial area has subsided and formed a broad tectonized trough a swath of newly accreted crust has since widened that trough late stage volcanism consists of small circular pillow mounds we propose that these contrasting eruptive styles reflect the waxing and waning phases of a common magmatic evolution spanning a few millennia

monitoring velocity variations in the crust using earthquake doublets an application to the calaveras fault california we present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates we compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal a moving window analysis of the entire seismograms including the coda gives t the difference in arrival times versus running time along the seismogram the time resolution of the method is an order of magnitude better than the digitization interval the t technique is illustrated with a pair of microearthquakes m 1 7 and 2 0 that occurred before and after the coyote lake california earthquake m 5 9 of august 6 1979 and on the same segment of the calaveras fault that ruptured during the earthquake the coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet so that its seismogram appears as a stretched version of the earlier event we interpret this systematic variation in t along the coda as a change in the average s velocity in the upper crust in the time interval between the two doublets s wave velocities appear to have decreased by 0 2 in an oblong region 5 10 km in radius at the south end of the aftershock zone

diagenetic comparisons between non tropical cenozoic limestones of new zealand and tropical mississippian limestones from indiana usa is the non tropical model better than the tropical model abstract mississippian limestones exposed in indiana u s a were deposited in a shallow tropical ocean however many properties of these limestones are more like those of modern and cenozoic non tropical limestones such as those found in new zealand the dominant skeletal grains in the indiana limestones are calcitic echinoderms bryozoans and brachiopods the dominant skeletal grains in most cenozoic limestones of new zealand are calcitic bryozoans echinoderms bivalve molluscs and foraminifera in contrast modern and cenozoic tropical limestones contain an abundance of aragonitic green algae corals and molluscs early in diagenesis the metastable aragonite dissolves and reprecipitates as calcite causing early cementation of the sediments originally aragonitic fossils that have dissolved can be identified as molds that are commonly filled with secondary calcite because they contained little aragonite most of the indiana and new zealand limestones did not have an abundant source of early cement except for local cases in which grains were cemented in contact with carbonate supersaturated seawater grainstones were relatively deeply buried with little cement between the grains this resulted in mechanical and chemical compaction of skeletal grains producing a fitted fabric with greatly reduced pore space either open or filled with cement between the grains cement in these aragonite poor grainstones comes largely from pressure dissolution between grains and along stylolitic seams in the rock features that are common only after burial beyond a few hundred meters the final product of deeply buried up to 2000 m cenozoic new zealand grainstones is similar to the mississippian grainstones of indiana in the indiana limestones we have only the final product of this extensive burial diagenesis however the new zealand sediments and rocks reveal all steps of formation of the final deeply buried limestone the reason for the scarcity of originally aragonitic fossil grains in paleozoic rocks worldwide is unknown organisms with aragonitic skeletons such as some molluscan groups and calcareous green algae were present but seldom in much abundance the aragonitic scleractinian corals had not yet evolved previous researchers have noted that non skeletal precipitates such as ooids and cements have at times during the paleozoic been predominantly aragonite and at other times calcite they have attributed this difference to secular variation in seawater chemistry icehouse vs greenhouse seas abundance of aragonitic and calcitic skeletal grains does not follow this pattern

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

burial dominated cementation in non tropical carbonates of the oligocene te kuiti group new zealand abstract cementation of bryozoan echinoid benthic foraminiferal temperate shelf carbonates of the oligocene te kuiti group north island new zealand occurred mainly during subsurface burial the calcite cements in the limestones are dominated by equant and syntaxial rim spar which typically becomes ferroan given an iron supply and compared to the skeletal material with normal marine 18o values from 2 to 1 more depleted in 18o with depth of burial the 18o composition of bulk cement samples ranging from 1 to 7 these trends reflect the establishment in pore waters during sediment burial of reducing conditions and gradually increasing temperatures 20 50 c respectively the 13c values 0 to 3 of the cements remain the same as the host marine shells suggesting the source of carbon in the cements was simply redistributed marine carbonate derived from shell dissolution two gradational burial diagenetic environments influenced by marine derived porewaters are arbitrarily distinguished shallow burial phase and moderate burial phase during the shallow burial phase down to 500 600 m sub bottom depth the carbonates lost at least 25 of their original porosity by mechanical compaction and were selectively cemented by non ferroan or usually ferroan variably luminescent slightly 18o depleted sparry calcite cement 18o 2 to 4 mainly as syntaxial rims about echinoid grains these shallow burial cements form less than about 10 of total cement in the majority of the limestones and their source was probably mainly mild intergranular dissolution of calcitic skeletal fragments accompanying the onset of chemical compaction during the moderate burial phase between about 600 and 1100 m sub bottom depth porosity loss continued typically to about 70 of its original value as a result of pressure solution of calcitic bioclasts associated with more advanced stages of chemical compaction this involved development of a wide variety of non sutured and microstylolitic solution seams including both single and composite wispy or continuous bedding parallel types and non parallel reticulate forms the released carbonate was precipitated as ferroan or non ferroan where iron supply was negligible dull luminescent strongly 18o depleted 18o 4 to 7 mainly equant calcite spar cement occluding available pore space in the limestones

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

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

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

toomey et al reply replying to s s singh k c macdonald 10 1038 nature07887 2009 we think that the singh and macdonald 1 description of the experimental uncertainties in our data2 is a misrepresentation of what we have done and that they are incorrect in stating that our results2 on the structure of the uppermost mantle in the subaxial region beneath the east pacific rise are unconstrained by data

a new model for submarine volcanic collapse formation 1 collapse pits and an associated suite of collapse related features that form in submarine lava flows are ubiquitous on the global mid ocean ridge crest collapse pits the lava tube systems they expose and lenses of talus created by the collapse process combine to produce a permeable region in the shallow ocean crust and are thought to contribute significantly to the 100 300 m thick low velocity zone observed at intermediate to fast spreading mid ocean ridges this horizon of low density high porosity material is likely to be an important aquifer for the transfer of hydrothermal fluids in the upper ocean crust in a recent survey of the east pacific rise at 9 37 n we used photographs video and observations from the submersible alvin and dsl 120a side scan data to determine that 13 of the 720 000 m2 of seafloor imaged had foundered to form collapse pits in 98 of the images collapse pits occurred in lobate flows and the rest in sheet flows on the basis of our observations and analyses of collapse features and incorporating data from previous models for collapse formation plus laboratory and theoretical models of basalt lava behavior in the deep ocean we develop a detailed multistage physical model for collapse formation in the deep ocean in our model lava extruded on the seafloor traps pockets of seawater beneath the flow that are instantly vaporized to a briny steam the seawater is transformed to vapor at temperatures above 480 c with a 20 times expansion in volume bubbles of vapor rise through the lava and concentrate below the chilled upper crust of the lava flow creating gas filled cavities at magmatic temperatures fluid lava from the cavity roofs drips into the vapor pockets to create delicate drip and septa structures a process that may be enhanced by water vapor diffusing into the magma and reducing its melting point as the vapor pocket cools the pressure within it drops causing a pressure gradient to develop across the upper crust the pressure gradient often causes the roof crust to collapse during cooling though vapor pocket geometry may be such that the roof remains intact during subsidence of the underlying lava alternatively drainaway of the molten lava may cause collapse in locations where inflated lava roof crusts are not supported from below by bounding walls or lava pillars post eruption seismicity lava movement or hydrovolcanic explosions may cause continued collapse of the lava carapace after the eruption

microearthquakes on the endeavour segment of the juan de fuca ridge 1 we report the results of a 55 day microearthquake experiment on the endeavour segment of juan de fuca ridge the network covered a 5 km section of the ridge axis centered on the main vent field and extended 15 km off axis on the west flank the ridge axis and flanks were seismically active and 1750 earthquakes were located with a minimum of five travel time picks including at least one s wave over half the earthquakes occurred in swarms and a waveform cross correlation technique was used to obtain relative locations on the western flank the hypocenters for four swarms at midcrustal depths are compatible with steeply dipping fault planes that strike at 035 050 n and oblique to the abyssal hills focal mechanisms determined from p wave first motions and p s amplitude ratios are predominantly strike slip with north south compression and appear to be affected by the reorganization of the explorer plate earthquakes beneath the ridge axis are concentrated in a band of intense seismicity at 1 5 3 5 km depth to the north of high rise vent field the seismicity defines a plane striking parallel to the ridge axis and dipping east at 70 and the earthquakes appear to extend beneath an axial reflector previously imaged at 2 3 km depth farther south the hypocenters are not compatible with a single fault plane focal mechanisms are characterized by subhorizontal tension axes oriented in all directions except parallel to the ridge and suggest a stress field that is about equally influenced by ridge spreading and hydrothermal cooling

quasi biennial oscillation and tropical waves in total ozone westward and eastward propagating tropical waves in total ozone are investigated in 13 years 1979 1991 of version 6 total column ozone data from the nimbus 7 total ozone mapping spectrometer toms satellite instrument a clear synchronization between the stratospheric quasi biennial oscillation qbo zonal winds and the fast periods 15 days propagating waves in tropical toms data is detailed largest total ozone wave amplitudes 3 6 dobson units occur when their phase propagation direction is primarily opposite the singapore qbo lower stratospheric winds this effect is most apparent in meridionally symmetric components examination of specific episodes including cross spectral calculations with singapore rawinsonde wind data 10 70 hpa reveals signatures of tropically confined eastward propagating kelvin waves of zonal wavenumbers 1 2 during the descending eastward qbo phase consistent with acceleration of that qbo phase by kelvin waves the toms results are also consistent with possible forcing of the westward qbo wind phase by episodes of both meridionally symmetric and asymmetric westward waves w events in the text however in contrast to the case of eastward kelvin waves the strongest westward events w events in the text appear to be filtered by rather than forcing the westward phase of the stratospheric qbo wind these dominant westward episodes are interpreted as meridionally symmetric westward global normal modes and tropically confined equatorial rossby waves 2 6 the events exhibit phase and group speeds characteristic of wave dynamics rather than simple wind advection these results underscore the utility of the long time series and excellent horizontal coverage of toms data for dynamical investigations in the relatively observation poor tropical stratosphere

seismogenic structure and processes associated with magma inflation and hydrothermal circulation beneath the east pacific rise at 9 50 n a dense ocean bottom seismometer array recorded more than 7 300 microearthquakes between october 2003 and april 2004 on the fast spreading east pacific rise at 9 50 n a previous initial analysis of these data revealed the general structure of an along axis oriented hydrothermal circulation cell with an inferred down flow zone near a 4th order axial discontinuity a horizontal band of seismicity overlying the axial magma chamber amc and an upflow zone that correlates with activity patterns of hydrothermal vents here we present cross correlation based double difference hypocenter locations that reveal further insight into the detailed structure and kinematics that control seismic failure we find that the majority of events within the inferred down flow pipe occur near the surface where the entrenched seawater presumably lowers the frictional strength of faults generated by local tectonic stresses beneath the eastern side of the ridge axis just above the amc at 1 4 km depth we observe repeated shear failure along well defined steeply east dipping faults composite focal mechanisms indicate reverse motion suggesting that slip on these faults is caused by amc inflation and possibly injection of magma into a narrow sill we resolve a small counterclockwise skew in the strike of the reverse faults relative to the direction of the ridge axis consistent with nuvel 1a spreading direction suggesting that the regional tectonic stress field may be controlling the seismogenic structures at depth earthquakes on the reverse faults occur preferentially during peak extensional tidal stresses indicating that these faults are critically stressed

210po 210pb dating of recent volcanic eruptions on the sea floor the globe encircling mid ocean ridge system accounts for most of the earth s volcanism and influences its heat budget morphology and the composition of the oceans it is therefore important to constrain the size frequency and compositional variability of ridge eruptions but this is made difficult by the remoteness of much of the sea floor and the paucity of applicable radioactive chronometers remote monitoring has provided indirect evidence for active ridge volcanism1 3 which in some cases has been strengthened by submersible observations4 5 but it has not been possible to date these eruptions directly here we present a new chronometer based on 210po 210pb radioactive disequilibrium which allows us to date glassy eruption products within a few years of their eruption our first results on lavas from 9 50 n on the east pacific rise confirm that the ridge erupted only months before sample collection and indicate that the eruptions extended over at least one year the chronometer begins with po volatilization during eruption all glasses analysed were 75 100 degassed a crystalline pillow interior was 20 degassed however implying that the flow surface may control po degassing during deep submarine eruptions although the 210po 210pb chronometer is useful for only a short time frame it should prove valuable for dating recent but unobserved submarine eruptions and determining details of eruptive sequences

ordovician carboniferous tectono sedimentary evolution of the north nuratau region uzbekistan westernmost tien shan abstract the tien shan is a c 2500 km long orogenic belt of which the nuratau region of eastern uzbekistan forms the western part petrographical and field analysis of the ordovician carboniferous succession in the north nuratau region provided the basis for a reconstruction of the depositional settings along part of the northern margin of the alai continent and their evolution during the period of closure of the turkestan ocean which separated the alai and the kazakh kyrgyz continents initial sedimentation ordovician was broadly carbonate dominated although by mid late ordovician times siliciclastic input predominated in some areas these variations between clastic and carbonate dominated regions may have been related to tectonic activity within the alai continent carbonate sedimentation was reestablished in the wenlock with broad shelf systems forming along the continental margin volcanic activity in the early devonian records a period of tectonic instability and this was followed by the reestablishment of the carbonate mosaic albeit with a greater degree of instability as indicated by stratigraphic gaps than in the silurian this pattern extended up into the carboniferous culminating in backarc related magmatic activity final closure of the turkestan ocean involved significant folding and thrusting as well as a major change from compressional to strike slip movement

basal waitemata group lithofacies rapid subsidence in an early miocene interarc basin new zealand the abrupt transition from coastal and shallow shelf sediments to bathyal sediments provides a record of rapid subsidence and deepening of the early miocene waitemata basin basal shallow marine strata kawau subgroup accumulated upon a highly dissected surface that overlies deformed mesozoic metagreywacke the early miocene coast was characterized by an embayed and cliffed shoreline with numerous sea stacks and islands kawau subgroup lithofacies which include pocket beach shallow shelf and base of cliff talus deposits reflect rapidly changing coastline configuration and water depths as the rugged bedrock surface was buried the response to continued rapid subsidence and transgression in waitemata basin was a decrease in the supply of coarse clastic sediment beach gravels were locally displaced to greater water depths by avalanching down steep bedrock slopes the first bathyal turbidite facies which abruptly overlie the shallow water kawau subgroup include locally derived sediment gravity flows commonly ponded by remnant bedrock submarine highs when this local supply of sediment had been exhausted coarse sediment starvation ensued and bathyal muds accumulated with the resumption of sediment supply and gradual burial of submarine bedrock relief submarine fans coalesced and increased in lateral extent subsidence of the waitemata basin to bathyal depths is thought to have occurred in less than a million years from the above hypothesis a general model of sedimentation is proposed

tectonic reconstruction of the clipperton and siqueiros fracture zones evidence and consequences of plate motion change for the last 3 myr bathymetry side looking sonar and magnetics data from the northern east pacific rise have been analyzed to determine the tectonic history of the clipperton fracture zone cfz and the siqueiros fracture zone sfz over the last 2 3 m y results of tectonic reconstructions indicate a series of counterclockwise changes in spreading direction at 2 5 ma 1 2 1 5 ma 1 2 and 0 5 ma 4 5 along the right stepping clipperton transform the most recent change in spreading direction has resulted in fracture zone normal compression which we propose has created a median ridge and transform parallel troughs along the active transform fault volcanic intersection highs located near the ridge transform intersections rti of the cfz are interpreted to be the result of fracture zone normal extension that has created pathways for magma emplacement into onto the older lithosphere across from the rti along the left stepping sfz we propose the changes in spreading direction have generated extension across the transform and have resulted in the formation of intratransform spreading centers and flexural transverse ridges tectonic reconstructions indicate a single euler pole is unable to describe the motion of the pacific cocos plate boundary between the cfz and sfz during periods of spreading direction change since 3 ma transtensional transforms will adjust to a new spreading direction almost immediately while transpressional transforms will experience compression for a period of time comparable to half the age offset of the compressional transform

pulse of the seafloor tidal triggering of microearthquakes at 9 50 n east pacific rise 1 unequivocal evidence of tidal triggering is observed for microearthquakes 0 4 to 2 0 ml recorded between october 2003 to april 2004 near 9 50 n on the east pacific rise epr although semidiurnal tidal stress changes are small 99 9 nonrandom temporal distribution with events occurring preferentially near times of peak extension due to the proximity of this site to an ocean tidal node where changes in sea surface height are minimal periodic stress changes are dominated by the solid earth tide in contrast previous studies on the juan de fuca ridge have shown microearthquake triggering to be a response to seafloor unloading during times of low ocean tide the modulation of 9 50 n microearthquakes by small amplitude periodic stresses is consistent with earthquake nucleation within a high stressing rate environment that is maintained near a critical state of failure by on axis magmatic and hydrothermal processes

the mid cenozoic challenger rift system of western new zealand and its implications for the age of alpine fault inception analysis of the structure and sedimentary geology of western new zealand has identified a middle eocene to early miocene continental rift system 1 200 km long and 100 200 km wide named here the challenger rift system four phases of rift development occurred 1 infra rift subsidence 2 active axial trough subsidence 3 expanded rift subsidence involving collapse of the rift shoulders and 4 incipient sea floor spreading the spatial and temporal distribution of these phases identifies a north island and a south island rift segment and shows that rifting propagated toward the center of the rift from both ends the northern segment shows a simple pattern of rifting that is comparable with vink9s model of rift propagation the southern segment with locked zones and rift nucleation segments is comparable to courtillot9s model of rift propagation the sea floor spreading history of the southwest pacific shows that the northern rift segment probably linked with a sea floor spreading center in the norfolk basin and the southern segment linked with the southeast indian ridge this is corroborated by the good correlation between the ages of sea floor magnetic anomaly lineations that are aligned with the rift and the biostratigraphic ages of rifting the probable continuity of the rift system in its early development precludes pre miocene transcurrent displacement on the alpine fault an early miocene 23 m y b p age of alpine fault inception is indicated by the age and pattern of rift disruption attributed to compression that originated at the australia pacific plate boundary the modern australia and pacific plates were not discrete entities therefore until the early miocene

reef talus a popular misconception reef fronts have traditionally been regarded as comprising debris derived by contemporaneous erosion of the reef however evidence from wave transport indicates that on present day reefs the bulk of the debris generated in this way accumulates in the back reef area with only finer grained sediment carried off reef by retreating flows or by overwash nevertheless in contrast to this observation fore reef debris slopes are commonly considered characteristic of phanerozoic reefs this apparent error reflects the conflation of processes defining contemporary growth and accretion of the reef and the corresponding long term accretion of the carbonate platform on which it rests present day reefs are commonly although not exclusively additions to long lived carbonate platforms growth of the latter is intermittent and has been moderated by changes in sea level that for recent reefs have been on time scales of less than 100 ka during low sea level stands growth ceases or is translated downslope and earlier deposits are subject to lithification and subaerial erosion similar changes are applied on a larger scale to the aggrading growth of carbonate platforms but the bulk accretion of these includes quite different processes and reflects far longer timescales during low sea level stands the margins of platforms commonly become unstable with instability reflected in slope failure and in the shedding of blocks ranging from metres to kilometres in diameter associated with the generation of debris flows and turbidites it is argued that these are the materials that are commonly described as reef talus in ancient structures although their formation is largely independent of any contemporary reef growth difficulties arise where the reef and the platform are treated as a single functional entity it is important to recognize the conceptual distinction between them reef talus is a misleading description of the debris predominantly generated by platform erosion and slope failure

the role of tectonism in sequence development and facies distribution of upper oligocene cool water carbonates coromandel peninsula new zealand the torehina formation is part of a cool water carbonate succession of oligocene age in new zealand that crops out on the coromandel peninsula north island it contains two major transgressive sequences that record successive onlap of a once emergent landmass the first sequence records marine flooding of non marine to marginal marine fan delta estuarine facies followed by deepening upward and formation of a low energy deep 100 m muddy carbonate ramp the capping sequence boundary is characterized by differential uplift and varies considerably in its character over a small 9 km2 area varying from a burrowed glauconitic firmground to an erosional hardground to an undulatory marine contact to a palaeokarst with planktic echinoderm and bivalve limestones in another this variation disappears upsection which suggests that initial bathymetric differences were eliminated with renewed rise in sea level yielding deeper water inner shelf sediment facies followed by the accumulation of still deeper but higher energy outer shelf bivalve and bryozoan facies the sequence architecture of the torehina formation is controlled by tectonism both long term subsidence and short term differential uplift this arose as a result of increasing tectonic activity throughout proto new zealand during the late oligocene in such a system local and regional variation in tectonism among adjacent basins can impose subtle to marked differences in the timing of sequence boundaries and the character of basin fill patterns

tectonic volcanic segmentation and controls on hydrothermal venting along earth s fastest seafloor spreading system epr 27 32 s 1 we have collected 12 khz seabeam bathymetry and 120 khz dsl 120 side scan sonar and bathymetry data to determine the tectonic and volcanic segmentation along the fastest spreading 150 km myr part of the global mid ocean ridge system the southern east pacific rise between the easter and juan fernandez microplates this area is presently reorganizing by large scale dueling rift propagation and possible protomicroplate tectonics fracture patterns observed in the side scan data define structural segmentation scales along these ridge segments these sometimes but not always correlate with linear volcanic systems defining segmentation in the seabeam data some of the subsegments behave cohesively with in phase tectonic activity while fundamental discontinuities occur between other subsegments we also collected hydrothermal plume data using sensors mounted on the dsl 120 instrument package as well as ctdo tow yos to determine detailed structural and volcanic controls on the hydrothermal vent pattern observed along 600 km of the pacific nazca axis here we report the first rigorous correlation between coregistered hydrothermal plume and high resolution marine geophysical data on similar scales and over multisegment distances major plume concentrations were usually found where axial inflation was relatively high and fracture density was relatively low these correlations suggest that hydrothermal venting is most active where the apparent magmatic budget is greatest resulting in recent eruptions that have paved over the neovolcanic zone areas of voluminous acoustically dark young lava flows produced from recent fissure eruptions correlate with many of the major hydrothermal vent areas increased crustal permeability as gauged by increased fracture density does not enhance hydrothermal venting in this area axial summit troughs and graben are rare probably because of frequent volcanic resurfacing in this superfast spreading environment and are not good predictors of hydrothermal activity here many of the hydrothermal areas are found in inflated areas near the ends of segments suggesting that abundant magma is being supplied to these areas

systematic along axis tidal triggering of microearthquakes observed at 9 50 n east pacific rise 1 hydrothermal fluid circulation at mid ocean ridges facilitates the exchange of heat and chemicals between the oceans and the solid earth and supports chemosynthetic microbial and macro faunal communities the structure and evolution of newly formed oceanic crust plays a dominant role in controlling the character and longevity of hydrothermal systems however direct measurements of subsurface processes remain technologically challenging to obtain previous studies have shown that tidally induced stresses within the subseafloor modulate both fluid flow and microearthquake origin times in this study we observe systematic along axis variations between peak microearthquake activity and maximum predicted tidal extension beneath the hydrothermal vent site at 9 50 n east pacific rise we interpret this systematic triggering to result from pore pressure perturbations propagating laterally through the hydrothermal system based on our observations and a one dimensional pore pressure perturbation model we estimate bulk permeability at 10 13 to 10 12 m2 within layer 2b over a calculated diffusive lengthscale of 2 0 km

rapid emplacement of a mid ocean ridge lava flow on the east pacific rise at 9 46 51 n in april 1991 during a submersible diving expedition to the east pacific rise epr crest at 9 46 51 n a new volcanic eruption on the sea floor was discovered here we report results from numerical modeling of that eruption which indicate that 4 106 6 106 m3 of lava was emplaced in 1 2 h with an average eruption rate of 103 106 m3 s 1 comparable to rates observed in hawaii at kilauea s east rift zone if the rapid emplacement of the 1991 epr lava and its short eruption duration are typical of volcanic events at fast spreading mid ocean ridge crests these characteristics have broad implications for our ability to detect monitor and understand the evolution of magmatic and volcanic processes within the axial zone

hydrothermal fluid flow system around the iheya north knoll in the mid okinawa trough based on seismic reflection data seismic reflection data around the iheya north knoll hydrothermal field provide insights into geological structures that control subseafloor hydrothermal fluid flow in the sediment covered continental backarc basin of the mid okinawa trough we identified the seismic expression of widespread porous volcaniclastic pumiceous deposits and intrusions as a result of silicic arc volcanism the porous and permeable volcanic deposits are distributed in an area extending updip from the thick succession of the deep trough to the seafloor at the hydrothermal field their regional structure focuses the flow of hydrothermal fluids derived from the surrounding trough fill sediments and directs them to the vents of the hydrothermal field the high concentrations of ch4 and nh4 in the fluids of the hydrothermal field are likely derived from the interaction of migrating fluids with trough fill sediments

upper tropospheric equatorial waves in ecmwf analyses tropical wind fields from the european centre for medium range weather forecasts operational analyses are analysed to search for equatorially trapped waves in the upper troposphere data are studied for 1980 87 zonal wind spectra show a predominance of power at low zonal wave numbers and low frequencies these data do not show distinct spectral peaks corresponding to eastward propagating kelvin waves or westward propagating equatorially trapped rossby waves the zonal wind spectra do show episodic evidence for westward propagating 5 day zonal wave 1 features with maxima over the equator these are likely related to rossby normal mode oscillations previously reported meridional wind spectra show a predominance of westward propagating power in the 6 to 10 day period range primarily concentrated in zonal waves 4 7 enhanced meridional wind activity at the equator occurs episodically in time with events persisting for one to several months duration wave packets are longitudinally localized with preferred occurrence over the eastern pacific ocean most cases show evidence of meridional wind maxima in the tropics suggesting a degree of equatorial trapping the horizontal structure of these oscillations is that of mixed rossby gravity waves although they are not necessarily centred directly over the equator these waves have largest amplitudes in the upper troposphere showing weak coherence with the surface they appear to be fundamentally distinct from fast 4 to 5 day rossby gravity waves observed in the western pacific a detailed study of a group of intense rossby gravity waves observed over the eastern pacific during august september 1985 is presented the reality of these features is confirmed in direct rawinsonde observations correlations are found between bursts of equatorial wave activity and enhanced meridional momentum fluxes from the subtropics suggesting extratropical forcing as at least one mechanism for their generation the waves are also strongly coupled to the upper tropospheric zonal wind field which likely accounts for their longitudinal and temporal confinement

an introductory perspective on non tropical shelf carbonates abstract this brief account introduces the topic of non tropical shelf carbonate deposits both modern and ancient specific aspects of which are described and discussed in the 22 accompanying papers in this special issue of sedimentary geology non tropical shelves are loosely defined as lying poleward of the present limit of development of hermatypic coral reefs or beyond the mean annual surface water isotherm of about 20 c typically near 30 latitude most geoscientists have traditionally regarded shallow water carbonate formation as a low latitude phenomenon however wherever the rate and extent of terrigenous sediment dilution is small the potential exists for extensive deposits of skeletal carbonate to accumulate on shelves beyond the sub tropics the facies attributes of the modern non tropical carbonates are distinctive and contrast significantly with the structural lithological faunal floral mineralogical and diagenetic character of their tropical counterparts in the rock record non tropical limestones probably occur throughout the phanerozoic but generally remain to be identified and or interpreted as such they are presently described mainly from the ordovician permian and tertiary quaternary periods of major continental glaciations and accentuated latitudinal thermal gradients a uniformitarian approach to the recognition and interpretation of ancient shallow marine limestones requires acceptance of both tropical and non tropical carbonate facies models

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

post eruption inflation of the east pacific rise at 9 50 n in june 2008 we installed a geodetic network at 9 50 n on the east pacific rise to track the long term movement of magma following the 2005 6 eruption this network consists of 10 concrete benchmarks stretching from the ridge to 9 km off axis during three campaign style surveys measurements of vertical seafloor motions were made at each of these benchmarks by precisely recording ambient seawater pressure as a proxy for seafloor depth with a mobile pressure recorder mpr the mpr was deployed using the manned submersible alvin in 2008 and 2009 and the remotely operated vehicle jason in 2011 the mpr observations are supplemented with data from a multiyear deployment of continuously recording bottom pressure recorders bprs extending along this segment of the ridge that can record rapid changes in seafloor depth from seafloor eruptions and or dike intrusions these measurements show no diking events and up to 12 cm of volcanic inflation that occurred from december 2009 to october 2011 in the area of the 2005 6 eruption these observations are fit with an inflating point source at a depth of 2 7 km and volume change of 2 3 106 m3 yr located on the ridge axis at approximately 9 51 166 n 407 m from our northernmost benchmark suggesting that the magma chamber underlying this segment of the ridge is being recharged from a deeper source at a rate that is about half the long term inflation rate observed at axial seamount on the juan de fuca ridge these data represent the second location that active volcanic uplift has been measured on a mid ocean ridge segment and the first on a nonhotspot influenced segment

mechanical decoupling and thermal structure at the east pacific rise axis 9 n constraints from axial magma chamber geometry and seafloor structures abstract we study the relationships between the seafloor structures and the axial magma chamber geometry in the 9 n overlapping spreading center osc area on the fast spreading east pacific rise epr our observations are based on a new high resolution bathymetric map of the 9 n osc area derived from picks of the seafloor arrival on 3d seismic data and on previously published data that constrain the presence and distribution of melt below the 9 n osc differences in the orientation of structures between the seafloor and the magma chamber indicate a sharp change in principal stress directions with depth suggesting that the brittle crust above the melt sill is decoupled from the melt sill itself and the ductile crust underlying it the stress field within the brittle upper crust results from a local interaction of the two overlapping spreading centers whereas the stress field in the crust below the melt sill corresponds to the regional stress field imposed by plate separation given this mechanical structure of the crust the melt sill shape and location appear to be controlled by the following factors the location of the deep melt source below the melt sill the ambient stress field at the depth of the melt sill and the stress field in the brittle upper crust above the melt sill which thermally shapes the roof of the melt sill through repeated eruptions

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

kinematics and geometry of active detachment faulting beneath the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge newly acquired seismic refraction and microearthquake data from the trans atlantic geotraverse tag segment of the mid atlantic ridge at 26 n reveal for the first time the geometry and seismic character of an active oceanic detachment fault hypocenters from 19 232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an 15 km long dome shaped fault surface that penetrates to depths 7 km below the seafloor on a steeply dipping 70 interface a tomographic model of compressional wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall which appears to roll over to a shallow dip of 20 5 and become aseismic at a depth of 3 km outboard of the detachment the exhumed lithosphere is deformed by ridge parallel antithetical normal faulting our results suggest that hydrothermal fluids at the tag field exploit the detachment fault to extract heat from a region near the crust mantle interface over long periods of time

modification of fracture porosity by multiphase vein mineralization in an oligocene nontropical carbonate reservoir taranaki basin new zealand the nontropical oligocene carbonate rich tikorangi formation is an important oil producer in the taranaki basin new zealand hydrocarbons are hosted and produced from mineralized natural fracture systems petrographic trace element stable isotope 18o and 13c and fluid inclusion data have enabled a complex sequence of eight paragenetic events to be determined the tikorangi formation host rock was cemented by low mg calcite event 1 during burial diagenesis from temperatures of 27c corresponding to 0 5 km burial and continued until 37c 1 km burial depth producing tight pressure dissolved fabrics with essentially no porosity and permeability the host rock was partially dolomitized 5 50 event 2 by ca and fe rich dolomite rhombohedra at burial depths and temperatures of 1 0 1 5 km and 35 50c without secondary porosity development subsequent brittle fracturing formed by neogene compression event 3 is constrained to a period following lithification and dolomitization but before precipitation of first generation vein calcite event 4 this initial ferroan low mg vein calcite formed after a period of burial from fe rich meteorically modified fluids at temperatures of about 50 60c and 1 4 1 9 km burial depth baroque dolomite formed event 5 following a period of mg enriched basinal fluid input precursory to hydrocarbon emplacement per se the dolomite formed mainly as a primary cement but also as a calcite replacement at temperatures following further burial to 2 2 5 km and temperatures of 65 80c formation of celestite and quartzine phases event 6 coincided with or marginally postdated dolomite at similar depths and temperatures to event 6 and formed as both replacements and cements second generation ferroan vein calcite formed event 7 at cooler temperatures 53 65c perhaps resulting from the introduction of cooler meteoric fluids from upsection the presence of petroleum fluid inclusions in the second generation calcite suggests precursory hydrocarbon bearing fluids have migrated along with aqueous fluids from about 10 ma with hydrocarbon emplacement event 8 occurring in the last 6 m y following a period of rapid late miocene burial an improved understanding of the paragenesis of the tikorangi formation may assist in hydrocarbon production from its reservoirs

evidence for tidal triggering of earthquakes as revealed from statistical analysis of global data 1 we observe tidal triggering of earthquakes by measuring the correlation between the earth tide and earthquake occurrence we used the times locations and focal mechanisms of the 9350 globally distributed earthquakes with magnitude 5 5 or larger from the harvard centroid moment tensor catalog the tidal stress was theoretically computed by using the preliminary reference earth model and a recent ocean tide model nao 99b we considered the shear stress on the fault plane and the trace of stress tensor j1 defining the tidal phase angle at the occurrence time for each earthquake we statistically tested the phase selectivity using the schuster s method for all the earthquakes no significant correlation is found between the earth tide and earthquake occurrence both for the shear stress and for j1 by classifying the data set according to fault types however we find a high correlation with the shear stress for reverse fault type the correlation is particularly clear for shallow and smaller earthquakes of this type significant correlation with j1 also appears for larger earthquakes of reverse fault type and for shallow and larger ones of normal fault type we find no correlation for strike slip type for all the cases of high correlation earthquakes tend to occur when the tidal stress accelerates the fault slip indicating that high correlation is not coincidental but is physically justified this result strongly suggests that a small stress change due to the earth tide encourages earthquake occurrence when the stress in the future focal area is near a critical condition

influence of fortnightly tides on earthquake triggering at the east pacific rise at 9 50 n the study of tidal forcing provides an opportunity to view the response of the solid earth to known changes in stress earthquakes triggered by diurnal and semidiurnal tidal forces have been well documented for the deep ocean and some terrestrial environments however few studies examine the influence of fortnightly tides on earthquake triggering and only in terrestrial environments we assess triggering of microearthquakes related to true fortnightly tides and fortnightly tidal modulations along the east pacific rise at 9 50 n where strong diurnal and semidiurnal triggering of earthquakes has been observed an ocean bottom seismograph experiment collected microearthquake data between october 2003 and january 2007 a time period that included a well documented submarine eruption culminating in january 2006 we examine how triggering changed through the build up to and aftermath of this seafloor spreading event results show that earthquakes occur preferentially during times of increasing peak volumetric stress leading up to the eruption with strong statistical significance schuster test p values up to 2 22e 31 however the correlation loses its strength immediately prior to and during the eruptive period p value as high as 0 2837 when competing tectonic and magmatic stresses may occur post eruption triggering reappears p value 1 45e 26 but with opposite phase statistical significance is also explored using monte carlo method highlighting limitations of swarm dominated time series for the schuster test nevertheless the observations are consistent with a shift in the dominating stress triggering the earthquakes from extensional tectonics to magmatic processes followed by a post eruption period of cooling relaxation and possibly magma chamber refilling

seismological evidence for three dimensional melt migration beneath the east pacific rise the extent to which crustal processes along mid ocean ridges are controlled by either the pattern of mantle upwelling or the mode of magma injection into the crust is not known models of mantle upwelling vary from two dimensional passive flow1 to three dimensional diapiric flow2 3 4 similarly beneath a ridge segment bounded by tectonic offsets crustal magma chambers may be replenished continuously along the ridge5 6 7 or at a central injection zone2 3 4 from which magma migrates towards the segment s ends here we present tomographic images that reveal the seismic structure and anisotropy of the uppermost mantle beneath the east pacific rise the anisotropy is consistent with two dimensional mantle flow diverging from the rise whereas the anomalous isotropic structure requires a three dimensional but continuous distribution of melt near the crust mantle interface our results indicate that crustal magma chambers are replenished at closely spaced intervals along axis and that crustal systems inherit characteristics of scale from melt transport processes originating in the mantle

a skeletal assemblage classification system for non tropical carbonate deposits based on new zealand cenozoic limestones abstract cenozoic limestones in new zealand are mainly skeletal grainstones and packstones formed under non tropical climatic conditions in open marine shelf or ramp environments following petrographic analysis of the nature and abundance of the skeletal components in nearly 500 samples of these limestones a complete linkage cluster analysis identified seven major skeletal assemblages that may be regarded as subdivisions of the single foramol skeletal association defined by lees and buller 1972 for temperate region carbonate deposits the seven assemblages are given contracted names as follows a barnamol barnacle bivalve dominated b bimol bivalve dominated c bryomol bryozoan bivalve dominated d echinofor echinoderm benthic foraminiferal dominated e nannofor nannofossil planktonic foraminiferal dominated f rhodalgal calcareous red algal dominated and g rhodechfor calcareous red algal echinoderm benthic foraminiferal dominated a composite triangular classification diagram has been devised for naming the skeletal assemblage of an unknown sample on the basis of its three main skeletal components the diagram successfully characterises more than 85 of the new zealand cenozoic limestone samples and also appears to be applicable for the skeletal assemblage designation of many overseas examples of non tropical carbonate deposits limitations relate mainly to locally common skeletal types e g serpulids brachiopods that are presently not incorporated into the new zealand based scheme the general ecological preferences of the main skeletal contributors in each of the seven skeletal assemblages form a basis for relating the assemblages to broad shelf habitats consequently as well as the benefits of providing a more consistent skeletal assemblage terminology for comparative studies between different workers the scheme can assist with the paleoenvironmental interpretation of non tropical skeletal carbonate facies

temporal and spatial patterns of biological community development at nascent deep sea hydrothermal vents 9 50 n east pacific rise the april 1991 discovery of newly formed hydrothermal vents in areas of recent volcanic eruption between 9 45 n and 9 52 n on the east pacific rise provided a unique opportunity to follow temporal changes in biological community structure from the birth of numerous deep sea hydrothermal vents in march l992 dsv alvin was used to deploy an on bottom observatory the biologic geologic transect to monitor faunal succession along a 1 37 km segment of the axial summit caldera between 9 49 61 n and 9 50 36 n depth 2520 m photo and videographic documentation of megafaunal colonization and chemical analyses of diffuse hydrothermal fluids associated with many of these developing communities within the transect were performed in march 1992 december 1993 october 1994 and november 1995 photographic and chemical time series analyses revealed the following sequence of events in low temperature venting areas 1 immediately following the 1991 eruption hydrogen sulfide and iron concentrations in diffuse fluids were extremely high 1 mmol kg 1 and microbially derived material blanketed active areas of venting in the form of thick microbial mats 2 mobile vent fauna e g amphipods copepods octopods and galatheid and brachyuran crabs and non vent fauna e g nematocarcinid shrimp proliferated in response to this increased biological production 3 within 1 yr of the eruption areal coverage of microbial mat was reduced by 60 and individuals of the vestimentiferan tube worm tevnia jerichonana settled gregariously in areas where diffuse flow was most intense 4 two years after the eruption maximum levels of h2s decreased by almost half from 1 90 to 0 97 mmol kg 1 and dense thickets of the vestimentiferan riftia pachyptila dominated vent openings previously inhabited by tevnia jerichonana 5 three years after the eruption maximum hydrogen sulfide levels declined further to 0 88 mmol kg 1 and mussels bathymodiolus thermophilus were observed on basaltic substrates 6 four years after the eruption galatheid crabs and serpulid polychaetes increased in abundance and were observed close to active vent openings as maximum hydrogen levels decreased to 0 72 mmol kg 1 also by this time mussels had colonized on to tubes of riftia pachyptila 7 between 3 and 5 yr after the eruption there was a 2 to 3 fold increase in the number of species in the faunal assemblages in the absence of additional volcanic tectonic disturbance we predict that mytilid and vesicomyid bivalves will gradually replace vestimentiferans as the dominant megafauna 5 10 yr following the eruption we also anticipate that the abundance of suspension feeders will decline during this period while the abundance of carnivores will increase we hypothesize that the above series of events 1 7 represents a general sequence of biological successional changes that will occur at newly formed low temperature deep sea hydrothermal vents along the northern east pacific rise and contiguous ridge axes megafaunal colonization at deep sea hydrothermal vents is considered to be the consequence of an intimate interaction of the life history strategies of individual species physical oceanographic processes and the dynamic hydrothermal environment our observations indicate that the successful sequential colonization of dominant megafaunal vent species from tevnia jerichonana to riftia pachyptila to bathymodiolus thermophilus also may be strongly influenced by temporal changes in geochemical conditions additional evidence demonstrating the close link between diffuse vent flux fluid geochemistry and faunal succession included the rapid death of several newly formed biological assemblages coincident with abrupt changes in the geochemical composition of the venting fluid and the local refocusing or cessation of vent flow these correlations suggest that future models of faunal succession at hydrothermal vents along intermediate to fast spreading mid ocean ridges should consider not only the interplay of species specific life history strategies community productivity and physical oceanographic processes but also the influence of changing geochemical conditions on the sequential colonization of megafaunal species

one to two month oscillations in the stratosphere during southern winter abstract stratospheric disturbances on the 35 60 day time scale are investigated with particular emphasis on the southern hemisphere the data used are stratospheric brightness temperatures from 90 to 1 5 hpa covering seven 6 month southern winter periods from april 1980 to march 1987 global time lag correlation plots are constructed from which tropical extratropical connections three dimensional wave structure and propagation characteristics are studied horizontal correlation patterns at 90 hpa reveal a strong connection between the indonesian tropics and the winter extratropics vertical correlation patterns in the southern winter extratropics reveal westward tilt with height and vertical propagation of 35 60 day zonal wavenumber 1 perturbations from tropospheric regions up to great heights at least as high as the stratopause region disturbances are found to propagate from 90 hpa to 1 5 hpa in generally 6 to 9 days in contrast the vertical correlation plots in the tropics indicate little or n

volcanic eruption of the mid ocean ridge along the east pacific rise crest at 9 45 52 n direct submersible observations of seafloor phenomena associated with an eruption event in april 1991 in april 1991 we witnessed from the submersible alvin a suite of previously undocumented seafloor phenomena accompanying an in progress eruption of the mid ocean ridge on the east pacific rise crest at 9 45 n 52 n the volume of the eruption could not be precisely determined although comparison of pre and post eruption seabeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were 10 m high effects of the eruption included 1 increased abundance and redistribution of hydrothermal vents disappearance of numerous vent communities and changes in characteristics of vent fauna and mineral deposits within the eruption area since december 1989 2 murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates 3 destruction of a vent community by lava flows mass wasting and possible hydrovolcanic explosion at a site known as tubeworm barbecue in the axial summit caldera asc at 9 50 6 n 4 near critical temperatures of hydrothermal vent fluids ranging up to 403 c 5 temporal variations over a 2 week interval in both temperatures and chemical isotopic compositions of hydrothermal fluids 6 unusual compositions of end member vent fluids with ph values ranging to a record low of 2 5 salinities ranging as low as 0 3 wt nacl one twelfth that of seawater and dissolved gases reaching high concentrations 65 mmol l for both co2 and h2s 7 venting at temperatures above 380 c of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices 8 disorganized venting of both high temperature fluids black and gray smoke and large volumes of cooler diffuse hydrothermal fluids directly from the basaltic seafloor rather than from hydrothermal mineral constructions 9 rapid and extensive growth of flocculent white bacterial mats species unknown on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid and 10 subseafloor downslope migration of magma normal to the ridge axis in a network of small scale 1 5 m diameter lava tubes and channels to distances at least 100 200 m outside the asc we suggest that in april 1991 intrusion of dikes in the eruption area to 200 m beneath the asc floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor rich hydrothermal fluids through the overlying rubbly cavernous lavas low salinities and gas rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid vapor phase separation boiling and phase segregation hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and or hydrovolcanic explosions these fissures and pits were lined with white mats of a unique fast growing bacteria that was the only life associated with the brand new vents the prolific bacteria which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids may thrive on high levels of gases in the vapor rich hydrothermal fluids initially escaping the hydrothermal system white bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic blizzard up to 50 m above the bottom the bacterial proliferation of april 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling and or by grazing as other organisms are re established in the biologically devastated area

diagenesis of early permian high latitude limestones lower parmeener supergroup tasmania the darlington sakmarian and berriedale artinskian limestones are neritic deposits that accumulated in high latitude environments along the south eastern margin of pangea in what is now tasmania these rocks underwent a series of diagenetic processes that began in the marine palaeoenvironment continued during rapid burial and were profoundly modified by alteration associated with the intrusion of mesozoic igneous rocks marine diagenesis was important but contradictory although dissolution took place there was also coeval precipitation of fibrous calcite cement phosphate and glauconite as well as calcitization of aragonite shells these processes are interpreted as having been promoted by mixing of shelf and upwelling deep ocean waters and enabled by microbial degradation of organic matter in contrast to warm water carbonates where meteoric diagenesis is important the darlington and berriedale limestones were largely unaffected by meteoric diagenesis only minor dissolution and local cementation took place in this diagenetic environment although mechanical compaction was ubiquitous correlation with burial history curves indicates that chemical compaction became important as burial depths exceeded 150 m promoting precipitation of extensive ferroan calcite this effect resulted from burial by rapidly deposited overlying thick late permian and triassic terrestrial sediments this diagenetic pathway was however complicated by the subsequent intrusion of massive mesozoic diabases and associated silicifying diagenetic fluids finally fractures most probably connected with cretaceous uplift were filled with late stage non ferroan calcite cement this study suggests that both carbonate dissolution and precipitation occur in high latitude marine palaeoenvironments and therefore the cold water diagenetic realm is not always destructive in terms of diagenesis furthermore it appears that for the early permian of southern pangea at least there was no real difference in the diagenetic pathways taken by cool water and cold water carbonates

resolving apparent conflicts between oceanographic and antarctic climate records and evidence for a decrease in pco2 during the oligocene through early miocene 34 16 ma abstract an apparent mismatch between published oxygen isotopic data and other paleoclimate proxies for the span from 26 16 ma is resolved by calibration against global sea level estimates obtained from backstripping continental margin stratigraphy ice volume estimates from calibrated oxygen isotope data compare favorably with stratigraphic and palynological data from antarctica and with estimates of atmospheric pco2 throughout the oligocene to early miocene 34 16 ma isotopic evidence for an east antarctic ice sheet eais as much as 30 larger than its present day volume at glacial maxima during that span is consistent with seismic reflection and stratigraphic evidence for an ice sheet covering much of the antarctic continental shelf at the same glacial maxima palynological data suggest long term cooling during the oligocene with cold near tundra environments developing along the coast at glacial minima no later than the late oligocene a possible mechanism for this long term cooling is a decrease in atmospheric pco2 from the middle eocene to oligocene reaching near pre industrial levels by the latest oligocene and remaining at those depressed levels throughout the miocene

seismicity and tremor in a submarine hydrothermal field the northern juan de fuca ridge we have made the first multi year deployments of a small number of ocean bottom seismometers on a mid ocean ridge we located our instruments in and around a vigorous hydrothermal vent field on the endeavour segment of the juan de fuca ridge the deployments have revealed a consistent pattern of activity comprising several types of events most abundant are swarms of extremely small microearthquakes that we suggest are associated with cracking of the oceanic crust we also observe local tectonic earthquakes that occur about four times per day preliminary hypocentral locations place these tectonic events away from the ridge axis and outside the hydrothermal field the seismicity on the endeavour segment is remarkably different from the southern juan de fuca ridge where little if any tectonic activity is occurring we speculate that the endeavour and southern juan de fuca ridge are in two different phases of their long term evolution the latter has only rare activity that is only beginning to focus hydrothermal circulation along faults in contrast the endeavour segment is in an active tectonic phase and the faults have had time to develop robust hydrothermal circulation resulting in large hydrothermal vent deposits

foreshock sequence of the 1992 landers california earthquake and its implications for earthquake nucleation the june 28 1992 landers california earthquake mw 7 3 was preceded for about 7 hours by a foreshock sequence consisting of at least 28 events in this study we examine the geometry and temporal development of the foreshocks using high precision locations based on cross correlation of waveforms recorded at nearby stations by aligning waveforms rather than trying to obtain travel time picks for each event independently we are able to improve the timing accuracy greatly and to make very accurate travel time picks even for emergent arrivals we perform a joint relocation using the improved travel times and reduce the relative location errors to less than 100 m horizontally and less than 200 m vertically with the improved locations the geometry of the foreshock sequence becomes clear the landers foreshocks occurred at a right step of about 500 m in the mainshock fault plane the nucleation zone as defined by the foreshock sequence is southeast trending to the south and nearly north trending to the north of the right step this geometry is confirmed by the focal mechanisms of the foreshock sequence which are right lateral and follow the trend as determined by the foreshock locations on the two straight segments of the fault and are rotated clockwise for foreshocks that occur within the step the extent of the foreshock sequence is approximately 1 km both vertically and horizontally modeling of the coulomb stress changes due to all previous foreshocks indicates that the foreshocks probably did not trigger each other this result is particularly clear for the mw 4 4 immediate foreshock since stress transfer in the sequence appears not to have played a significant role in its development we infer an underlying aseismic nucleation process probably aseismic creep other studies have shown that earthquake nucleation may be controlled by fault zone irregularities this appears to be true in the case of the landers earthquake although the size of the irregularity is so small that it is not detectable by standard location techniques

submeter bathymetric mapping of volcanic and hydrothermal features on the east pacific rise crest at 9 50 n 1 recent advances in underwater vehicle navigation and sonar technology now permit detailed mapping of complex seafloor bathymetry found at mid ocean ridge crests imagenex 881 675 khz scanning sonar data collected during low altitude 5 m surveys conducted with dsv alvin were used to produce submeter resolution bathymetric maps of five hydrothermal vent areas at the east pacific rise epr ridge2000 integrated study site 9 50 n bull s eye data were collected during 29 dives in 2004 and 2005 and were merged through a grid rectification technique to create high resolution 0 5 m grid composite maps these are the first submeter bathymetric maps generated with a scanning sonar mounted on alvin the composite maps can be used to quantify the dimensions of meter scale volcanic and hydrothermal features within the epr axial summit trough ast including hydrothermal vent structures lava pillars collapse areas the trough walls and primary volcanic fissures existing autonomous benthic explorer abe bathymetry data 675 khz scanning sonar collected at this site provide the broader geologic context necessary to interpret the meter scale features resolved in the composite maps the grid rectification technique we employed can be used to optimize vehicle time by permitting the creation of high resolution bathymetry maps from data collected during multiple coordinated short duration surveys after primary dive objectives are met this method can also be used to colocate future near bottom sonar data sets within the high resolution composite maps enabling quantification of bathymetric changes associated with active volcanic hydrothermal and tectonic processes

the relative roles of compaction and early cementation in the destruction of permeability in carbonate grainstones a case study from the paleogene of west central florida u s a abstract how compaction can yield a progressive decrease in the permeability and pore structure of carbonate grainstones is not well documented in this study the destruction of permeability by compaction relative to early cementation is examined in forty five shallow buried 30 440 m grainstones with a range in permeability of 12 to 5180 md compaction and cementation indices calculated from thin section point count data indicate a group of grainstones dominated by cementation and another by compaction log linear cross plots of permeability versus these indices suggest that permeability loss in these two groups occurs by different processes the nature of those processes is elucidated by capillary pressure data and the distribution of cement and compaction fabrics as a function of permeability as cement abundance increases in the cement dominated samples a continuous distribution of cement develops along grain surfaces which is followed by the development of partial and then complete pore filling mosaics permeability is progressively reduced with this succession of cement fabrics and an increasingly greater percentage of pore space is positioned behind progressively smaller throats permeability loss results from porosity reduction progressive constriction of pore throats and increased tortuosity of flow paths once permeability is reduced to tens of millidarcies effective pore radii are all less than 1 m and many are less than 0 1 m but there is no evidence for complete blocking of all pore throats in this suite of samples in absolute terms the pore lining cements reduce permeability by thousands of millidarcies whereas the pore filling cements reduce permeability only by hundreds of millidarcies mechanical compaction is the dominant cause of permeability reduction between 5000 and about 500 md in the compaction dominated samples linear grain contacts and embayed grains become more abundant and close to overly close packing textures become more homogeneously distributed as permeability decreases however there is no concurrent change in the amount of pore space accessed by large or small throats indicating that mechanical compaction does not uniformly constrict or seal pore throats instead the data suggest that mechanical compaction reduces permeability by reducing porosity lengthening pore throats and increasing tortuosity at about 500 md the compaction dominated trend steepens on the log linear plot of permeability versus porosity a change that corresponds to the development of grain to grain pressure solution unlike mechanical compaction grain to grain pressure solution constricts and eliminates pore throats and isolates pore space resulting in a loss of large throats and an increase in the amount of pore volume accessed by small throats in contrast to cement dominated samples the most effective pore radii are still greater than 1 m at tens of millidarcies of permeability such low amounts of permeability can be reached even when pressure solution is not pervasively distributed throughout a sample

testing for astronomical forcing of cycles and gamma ray signals in outer shelf upper slope mixed siliciclastic carbonates upper oligocene new zealand abstract we evaluate whether outer shelf upper slope settings preserve a record of astronomical forcing using gamma ray logs from the upper oligocene upper chattian tikorangi formation onshore taranaki basin north island new zealand the formation is a siliciclastic carbonate succession deposited in outer neritic to upper bathyal depths at paleolatitudes of c 45os gamma ray and sonic logs calibrated to core indicate four lithofacies based on percent carbonate these are calcareous siliciclastic mudrock highest gamma ray values highly calcareous siltstones high gamma ray values very fine to fine siliciclastic muddy packstones intermediate gamma and winnowed very fine to medium grain dominated packstones and grainstones low gamma detrended gamma ray logs were tuned using minimal tuning methods 405 kyr long eccentricity cycle or where this was poorly developed the 100 kyr short eccentricity cycle with sedimentation rates tested statistically against astronomical forcing the tuned data show well developed long and short term eccentricity obliquity and a weak precessional forcing obliquity modulation is observed at the 1 2 myr scale one well and 175 kyr scale two wells the floating time scale was tied to the absolute time scale via published biostratigraphy the la2004 eccentricity solution the atlantic odp site 1264 18o record and the mi 1 isotope event at c 23 ma sea level fluctuations typically were moderate 20 to 40 m estimated from the 18o record the high gamma siliciclastic prone units formed during warmer and wetter phases and possibly slightly higher sea levels whereas the cleaner carbonates with abundant inner shelf derived fine carbonate developed during cooler drier phases and possibly slightly lower sea levels gamma ray logs from outer shelf upper slope settings provide valuable data sets for documenting the effects of astronomical forcing on the sedimentary record the study supports climate models for new zealand which predict generally increased precipitation with warming over the next 100 years the study appears to be the first to record astronomical forcing in the oligocene marine record of new zealand

the seismogenic zone of the continental crust in northwest iberia and its relation to crustal structure the distribution of seismicity at the western end of the cantabrian mountain range nw iberia reflecting current crustal dynamics is investigated integrating seismically active structures long term structures and the topographic features at the surface the thickness of the seismogenic zone within the continental crust is established in 20 km two crustal domains can be distinguished in the study area in terms of the seismicity pattern the style of alpine structures and the relief the presence of crustal fluids arises as a very likely contributing factor to the excessive thickness of the seismogenic zone in the study area more importantly a switch in tectonic style at the transition between crustal domains in coincidence with the lateral termination of orogenic frontal thrusts is envisaged as involving sufficient stress heterogeneity and amplification to explain the current concentration and characteristic pattern of historical seismicity in the region overall the distribution of seismicity in the crust is sensitive to the type and style of crustal structures

complex rupture of the 2014 m s 6 6 jinggu earthquake sequence in yunnan province inferred from double difference relocation to study the seismogenic structure of the 7 october 2014 ms 6 6 jinggu earthquake we use the cut and paste cap method to obtain the focal mechanisms and depths of the mainshock and the two largest aftershocks with ms 5 then we take advantage of waveform cross correlations to obtain precise time difference data and apply the double difference method to relocate the aftershocks between october 7 and december 31 we relocate 2076 earthquakes approximately 73 6 of the total with an average horizontal accuracy of approximately 150 m and a depth uncertainty of approximately 700 m the ms 6 6 mainshock epicenter originated at 23 378 n 100 475 e at a depth of 15 38 km the jinggu earthquake sequence exhibited a linearly concentrated distribution in the northwest southeast direction with obvious segmentation features and thus it can be divided into four branches spatially and two stages temporally the aftershocks first appeared around the ms 6 6 mainshock and then extended on either side of the mainshock along the nw se direction the nw branch the earthquake rupture direction experienced a slight clockwise deflection at the nw end and rotated clockwise by approximately 90 toward the sw the ne branch at the se end then the rupture propagated a short distance along the sw direction and rotated counterclockwise by approximately 90 into an sse direction the nnw branch there is a small angle between the nw and nnw branches the depths of the whole sequence with vertical dip angles were shallower toward the nw and deeper with two seismogenic layers toward the se according to the distributions of the earthquakes and nearby fractures it is highly possible that the jinggu earthquake sequence occurred along a hidden fracture section of the wuliangshan fault in the nw direction moreover we can confirm that the three large earthquakes all occurred along right lateral strike slip faults

rossby gravity waves in tropical total ozone randel 1992 has recently reported evidence for rossby gravity waves in tropical data fields produced by the european centre for medium range weather forecasts ecmwf the purpose of this paper is to show that similar features are observable in fields of total column ozone from the total ozone mapping spectrometer toms satellite instrument the observed features are episodic have zonal east west wavelengths of approximately 6 000 10 000 km and oscillate with periods of 5 10 days the modes exhibit westward phase progression and eastward group velocity a simple linear model is used to estimate the magnitude of total ozone perturbations induced by rossby gravity waves the model is able to account for the magnitude of observed toms features as well as their asymmetry about the equator the significance of finding rossby gravity waves in total ozone fields is that 1 the report of similar features in ecmwf tropical fields is corroborated with an independent data set and 2 the toms data set is demonstrated to possess surprising versatility and sensitivity to relatively smaller scale tropical phenomena

observation of 3 6 day meridional wind oscillations over the tropical pacific 1973 1992 vertical structure and interannual variability abstract rawinsonde data from tropical pacific stations were examined for westward propagating 3 6 day meridional wind oscillations in the troposphere and lower stratosphere 1973 1992 four types were identified from cross spectrum and principal component analysis 1 the dominant oscillation near 250 mb had a period slightly greater than 5 days zonal wavenumber 4 6 and modified rossby gravity structure near the date line 2 in the western pacific lower troposphere there was broadband activity with short zonal scale coupled to upper tropospheric waves in nh summer 3 in the central pacific during nh autumn there was a well defined 4 day oscillation with maximum amplitude in the lower troposphere and baroclinic phase tilt above the vertical structure suggested coupling to deep tropical convection this interpretation was supported by correlation of meridional wind with antisymmetric outgoing longwave radiation 4 in the stratosphere rossby gravity waves had periods 4 days and zonal wavenu

non double couple earthquake mechanisms at the hengill grensdalur volcanic complex southwest iceland the hengill grensdalur area in iceland generates frequent small non double couple earthquakes with explosive volumetric components we collected high quality three component digital recordings of 4 000 earthquakes on a purpose designed 32 station network in 1991 and determined focal mechanisms for 100 of the best recorded earthquakes by inverting amplitude ratios many of the mechanisms are consistent within the errors with simultaneous shear and tensile faulting with tensile faults parallel to the local spreading ridge and shear faulting similar to that in the south iceland transform fault zone some events cannot be explained by this model however and require other processes such as crack closing and partial compensation of tensile cracks by fluid flow

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

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