three dimensional tomographic velocity structure of upper crust coaxial segment juan de fuca ridge implications for on axis evolution and hydrothermal circulation three dimensional models of compressional velocity and azimuthal anisotropy from tomographic inversions using 23 564 ocean bottom seismometer p wave arrivals define systematic lateral variations in seismic structure of the coaxial segment of the juan de fuca ridge jdfr over much of the segment the across axis structure is roughly axisymmetric characterized by a progressive increase in dike velocities moving away from the ridge axis this trend is most apparent in the basal dikes where on axis velocities are about 800 m s slower than those measured elsewhere within the rift valley the on axis sheeted dikes also exhibit ridge oriented azimuthal anisotropy with a peak to peak amplitude of about 600 m s outboard of the rift valley beneath ridge flanks with fault scarps velocities in the upper 1500 m of crust are reduced the maximum amplitude of this anomaly is about 700 m s located near the top of the sheeted dikes variations in the three dimensional velocity model are believed to reflect changes in crustal porosity from which we infer an axisymmetric porosity model for seismic layer 2 of the coaxial segment as the crust ages the evolution of layer 2 porosity could occur in the following way 1 the porosity of zero age on axis dikes is set at formation by the contraction of molten material 2 hydrothermal alteration fills pore spaces as the dikes move away from the center of the axial valley and 3 normal faulting on the ridge flank scarps opens fractures and increases porosity of the upper dikes as they move off axis at the north end of the segment dike velocities are several hundred meters per second slower on average and the across axis structure is lost the transition from a coherent aligned seismic structure to a less distinct pattern with reduced velocities may represent a shift from magmatic to amagmatic extension moving away from the cobb hotspot on the ridge axis the porosity structure we have derived for the coaxial segment suggests an alternative to the usual hydrothermal circulation model of cross axis convection cells a circulation model with along axis convection cells located entirely within the axial valley appears to be more compatible with our data

upper crustal seismic velocity structure and microearthquake depths at the endeavour segment juan de fuca ridge 1 we present the results of a study to invert microearthquake and explosive shot data from the endeavour segment of the intermediate spreading juan de fuca ridge the average isotropic p wave velocity structure derived from the shot data in the uppermost 1 5 km of the oceanic crust is characterized by an increase with age of 8 from the axis to at least 0 5 ma that is attributed to the sealing of layer 2a porosity by hydrothermal processes superimposed on this variation are axis parallel 2 km wide alternating bands of high and low velocity with a peak to peak variation of 5 12 high and low velocities away from the axis correspond to bathymetric trenches and ridges respectively and are likely due to variations in layer 2a thickness p wave azimuthal anisotropy is present in the data that is best fit with a model of 9 anisotropy at 750 m depth decreasing to 1 at 3 km depth and is likely due to the preferential alignment of vertical cracks and fissures in the along axis direction anisotropy and velocity heterogeneity are coupled anisotropy alone may explain the form but not the magnitude of the axis parallel bands there are strong trade offs between the hypocentral depths of microearthquakes and the p and s wave velocity structures changing the mean hypocentral depth by up to 0 5 km leads to only modest increases in the travel time rms but the resulting velocity models appear more feasible when the earthquakes are forced deeper than when they are forced shallower

glacial lake camelsfoot a late wisconsinan advance stage proglacial lake in the fraser river valley gang ranch area british columbia in the gang ranch area british columbia interaction between the regional physiography and ice flow during the late wisconsinan fraser glaciation resulted in the formation of a proglacial lake confined to valleys of the fraser river and its tributaries lithostratigraphic and geomorphic evidence suggests that ponding was initiated in the big bar creek area where the fraser river is confined to a deep canyon in the camelsfoot range during ice advance a proglacial lake system developed that progressively deepened and reached a minimum upper elevation of approximately 710 m asl prior to being overridden by ice we propose that this system be formally named glacial lake camelsfoot a composite stratotype comprising lithostratigraphic units associated with glacial lake camelsfoot is described from eight reference sections along fraser river churn and lone cabin creeks in the gang ranch area additional geomorphic evidence indicates that at the fraser glaciation maximum the cordilleran ice sheet in th

finite element study of uplift and strain across vancouver island geological evidence for sudden coastal subsidence along the west coast of southern vancouver island points to the occurrence of great prehistorical subduction earthquakes contemporary uplift and crustal shortening patterns in southern vancouver island appear to indicate that the subduction megathrust fault is currently locked to understand better the dynamics of the observed surface deformation we develop a finite element model of earthquake cycles for the northern cascadia subduction zone across southern vancouver island using a linear viscoelastic rheology the model consists of the continental and oceanic lithospheres the asthenospheric mantle with a viscosity of 5 1019 pa s and a low viscosity 1018 pa s mantle wedge between the subducted oceanic plate and the overlying continental plate the shallow geometry of the subducted juan de fuca plate is well defined by the results of various geophysical surveys and the deep geometry is constrained by the results of seismic tomography the model me

seismic velocities in the uppermost oceanic crust age dependence and the fate of layer 2a it has been widely accepted that upper crustal seismic velocities gradually increase from low values in the youngest crust to typical crustal velocities 5 2 km s 1 over a period of 30 40 million years with a corresponding decrease in the observed heat flow deficiency caused by hydrothermal circulation some recent seismic studies however suggest that the seismic velocities actually rise rapidly nearly doubling in 5 km s 1 and velocities 3 km s 1 are not observed in crust older than 5 ma there is no detectable systematic change in the seismic properties of the uppermost oceanic crust after 10 million years median and average velocities increase from 2 3 to 4 4 km s 1 by 7 5 ma and remain unchanged thereafter if layer 2a is an interval capping the oceanic crust in which velocities are significantly lower than velocities that are typical of the underlying rocks layer 2b 5 2 km s 1 it follows that layer 2a does not disappear but is a persistent feature of the oceanic crust these results are consistent with geochemical evidence that most hydrothermal alteration occurs within 3 million years after the formation of the oceanic crust and suggest that there is little correlation between seismic velocities and ongoing hydrothermal circulation

variable crustal structure along the juan de fuca ridge influence of on axis hot spots and absolute plate motions multichannel seismic and bathymetric data from the juan de fuca ridge jdfr provide constraints on axial and ridge flank structure for the past 4 8 ma within three spreading corridors crossing cleft northern symmetric and endeavour segments along axis data reveal south to north gradients in seafloor relief and presence and depth of the crustal magma lens which indicate a warmer axial regime to the south both on a regional scale and within individual segments for young crust cross axis lines reveal differences between segments in moho two way traveltimes of 200 300 ms which indicate 0 5 1 km thicker crust at endeavour and cleft compared to northern symmetric moho traveltime anomalies extend beyond the 5 15 km wide axial high and coincide with distinct plateaus 32 and 40 km wide and 200 400 m high found at both segments on older crust moho traveltimes are similar for all three segments 2100 100 ms indicating little difference in average crustal production prior to 0 6 and 0 7 ma the presence of broad axis centered bathymetric plateau with thickened crust at cleft and endeavour segments is attributed to recent initiation of ridge axis centered melt anomalies associated with the cobb hot spot and the heckle melt anomaly increased melt supply at cleft segment upon initiation of axial volcano and southward propagation of endeavour segment during the brunhes point to rapid southward directed along axis channeling of melt anomalies linked to these hot spots preferential southward flow of the cobb and heckle melt anomalies and the regional scale south to north gradients in ridge structure along the jdfr may reflect influence of the northwesterly absolute motion of the ridge axis on subaxial melt distribution

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

microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

detailed geomorphology and neotectonics of the endeavour segment juan de fuca ridge new results from seabeam swath mapping a 36 km wide corridor of seabeam bathymetry was collected on the juan de fuca ridge north of 48 00 n in may 1983 these data merged with previous seabeam bathymetric compilations for the cobb offset as well as with sea marc i and sea marc ii side scan sonar seismic reflection magnetometer and deep tow photography data have been used to identify morphological variations along the spreading center of the endeavour segment of the juan de fuca ridge between the cobb offset and sovanco fracture zone the endeavour segment consists of two ridge sections separated by the 13 km wide endeavour offset which formed when spreading jumped from middle valley to west valley 200 000 yr ago the locus of spreading occurs along the entire endeavour segment as a narrow 1 2 km wide shallow 10 30 m deep inner rift which is superimposed upon larger structures that range from broad 5 10 km wide deep as much as 3 000 m median valleys to narrow 5 km wide shallow 2 100 m volcanic ridges an abrupt discontinuity in the tectonic fabric occurs at about 48 05 n south of this latitude spreading occurs within an axial high endeavour ridge which gradually deepens to the south south endeavour valley as it approaches the zone of overlap with the propagating rift at the cobb offset north of 48 05 n the axis is characterized by broad deep median valleys north endeavour valley and west valley these large scale morphological variations reflect a complex interplay of the spreading center with seamount volcanism waning magma supply at the distal ends of ridge axes thermal contrasts across ridge offsets propagating rifts and an unstable triple junction the pronounced axial deep of west valley is interpreted as reflecting the youth of the structure 200 000 yr and may be primarily due to collapse with little extension and magmatism the rift axis jump to west valley fortuitously isolated endeavour seamount the youngest edifice in the heck seamount chain from the pacific plate and trapped it between a pair of overlapping spreading centers at the newly formed endeavour offset development of a remarkably simple overlapping conjugate rift pair at endeavour offset has resulted in spite of the complex and highly variable crustal thickness magma supply and pre existing structural grain rifting in south west valley has caused pre existing topography generated at the endeavour ridge to be destroyed by subsidence and burial thereby creating the apparent discontinuity in morphology north of 48 05 n

geology of a vigorous hydrothermal system on the endeavour segment juan de fuca ridge a high precision high resolution geologic map explicitly documents relationships between tectonic features and large steep sided sulfide sulfate silica deposits in the vigorously venting endeavour hydrothermal field near the northern end of the juan de fuca ridge water depth in the vent field varies from 2220 to 2200 m location of the most massive sulfide structures appears to be controlled by intersections of ridge parallel normal faults and other fracture fissure sets that trend oblique to and perpendicular to the overall structural fabric of the axial valley the fractured basaltic substrate is primarily composed of well weathered pillow and lobate flows as presently mapped the field is about 200 by 400 m on a side and contains at least 15 large 1000 m3 sulfide edifices and many tens of smaller commonly inactive sulfide structures the larger sulfide structures are also the most vigorously venting features in the field they are commonly more than 30 m in diameter and up to 20 m in height actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller maximum venting temperatures of 375 c are associated with the smaller structures in the southeastern portion of the field highest temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20 30 c lower hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture controlled black smoker activity near the top of the structures a different type of high temperature venting takes place from the upper sides of the structures in the form of overflow from fully exposed quiescent pools of buoyant 350 c vent water trapped beneath overhanging sulfide sulfate silica ledges or flanges these flanges are attached to the upper outer walls of the large sulfide edifices two types of diffuse venting in the endeavour field include a lower temperature 8 15 c output through colonies of large tubeworms and 25 50 c vent fluid that seems to percolate through the tops of overhanging flanges the large size and steep walled nature of the these structures evidently results from sustained venting in a mature hydrothermal system coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development

shallow crustal structure beneath the juan de fuca ridge from 2 d seismic refraction tomography summary the formation of oceanic lithosphere along ocean ridges and the role that crustal magma chambers play in the accretionary process continue to be fundamental issues in plate tectonics to address these issues a multireceiver airgun oceanbottom seismograph refraction line was shot across the endeavour segment of the juan de fuca ridge near 48 n 129 w 2 d traveltime tomography applied to the data results in a three layer model for the upper crust layer 1 is 250 650m thick with u1 2 5 km s and vzvl 0 5 s layer 2 is 800 m thick u2 4 8 km s and vzuz 1 0 layer 1 and layer 2 probably represent the sequence of extrusives and the transition to layer 3 u3 5 8 km s l vzv3 0 5 s is associated with the extrusives dike complex transition an abrupt velocity transition between layer 1 and layer 2 may be a metamorphic front within the basalt pillows or it may be the depth to which surface fissures penetrate a low velocity anomaly velocities decreased by 0 45 km s exists beneath the ridge in layer 2 and upper layer 3 it is interpreted as a zone of increased fracture porosity and or permeability associated with axial hydrothermal circulation and correlates reasonably well with a sub axial reflector no evidence is found for the existence of a crustal magma chamber in the depth range of 1 5 3 5 km sub bottom however the velocity anomalies observed in layer 3 suggest that crustal temperatures in this layer are elevated by 150 250 c beneath and to the east of the ridge relative to temperatures west of the ridge

microearthquake characteristics and crustal vp vs structure at the mid atlantic ridge 35 n we report on the results of a microearthquake experiment conducted at the along axis bathymetric high of the slow spreading ridge segment near 35 n on the mid atlantic ridge mar of a total of 255 microearthquakes recorded during the 43 day experiment 31 were located near the oceanographer transform at the northern end of the segment 79 occurred at the nontransform offset at the southern end of the segment and 145 were at the segment center at the segment center earthquake epicenters lay within the median valley inner floor and formed a 12 km long trend paralleling a steep scarp bounding the western wall of the inner valley focal depths were 3 4 km below the seafloor most 80 of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend from a joint inversion for hypocenters and p and s wave velocity structures we determined a horizontally averaged v p v s ratio that decreases from 2 9 in the shallowmost 300 m to 1 7 at 2 km depth and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth the maximum depth of seismicity 4 km is anomalously shallow compared with other mar segments at which microearthquake experiments have been carried out cross axis relief is also anomalously low for this segment s center and on the basis of this and other mar microearthquake experiments there appears to be a correlation between cross axis relief and maximum depth of seismicity from the correlation of cross axis relief and inferred crustal thickness we suggest a relationship between thick crust high crustal temperatures and low cross axis relief in qualitative agreement with thermomechanical models for the depth of the axial valley

seismic attenuation structure beneath the juan de fuca ridge from tomographic inversion of amplitudes tomographic inversion of seismic amplitudes for attenuation structure has been applied to first arrival amplitudes from a seismic refraction profile across the juan de fuca ridge to further constrain a velocity structure previously obtained by travel time velocity tomography and to obtain a model for attenuation variations within the shallow crust amplitude analysis has identified an additional boundary defined by a decrease in vertical velocity gradient at 2 0 km subbottom which was not required by the travel time data this transition zone likely represents the oceanic layer 2 layer 3 boundary the analysis also identified zones of relatively high attenuation qe 20 100 at subbottom depths of 0 3 2 0 km beneath the ridge axis which correlate well with low velocity zones velocities reduced by 0 4 km s in the velocity model the subaxial zone is interpreted as a region of increased fracture porosity and or permeability associated with axial hydrothermal circulation within the extrusives and sheeted dike complex

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

microearthquakes at intermediate spreading rate ridges the cleft segment megaplume site on the juan de fuca ridge we discuss models for earthquake generation at intermediate spreading rate ridges and present results from an ocean bottom seismograph obs deployment on the cleft segment of the juan de fuca ridge jdfr although the cleft segment is spreading at approximately 6 cm yr an intermediate spreading rate it has no seismic activity detectable by land networks which provide a detection threshold of ml 4 or by northeast pacific underwater hydrophone arrays with a detection threshold of ml 2 to monitor seismicity four obss were deployed at the cleft segment for 11 days during august 1990 these obss detected 34 regional earthquakes associated with adjacent transform faults in addition 10 local microearthquakes ml 0 to 1 were observed over a 10 km segment of the ridge crest four local events were detected sufficiently well by multiple instruments to allow their hypocenters to be determined all these events occurred within the rift valley of the cleft segment spreading axis three events lie directly beneath a linear surface zone of hydrothermal venting this zone has been identified as the site of a seafloor volcanic eruption dike injection and large scale hydrothermal venting or megaplume event that occurred during 1986 and 1987 these earthquakes were located at depths coincident with the dike injection zone a possible mechanism for earthquake production might be tectonic release of stress acquired during the dike injection either along the dike zone or along the bounding faults of the rift valley an alternative mechanism may be stress release associated with hydrothermal activity and cooling of the dike a fourth event was located southeast of the linear hydrothermal venting and dike injection zone at a depth coincident with the spreading center crustal melt zone these observations suggest that the cleft segment may primarily experience seismic events correlated with periods of volcanic intrusion

regional stratigraphy and age of chilcotin group basalts south central british columbia mio pliocene chilcotin group basalt flows form a 50 000 km2 lava plateau in south central british columbia two periods of eruptive activity at 2 3 and 6 10 ma are indicated by a compilation of available age data including 10 new k ar age determinations and basalts from these two periods are chemically indistinguishable the chilcotin group consists of thin crudely columnar jointed pahoehoe flows some thick tiered flows pillow lava and pillow breccia and rare silicic tephra layers the presence of many vesicle sheets and cylinders and collapsed pahoehoe toes suggests that the basalts were volatile rich known vents gabbro and basalt plugs for the basalt flows form a northwest trend along the axis of the lava plateau the plateau appears to have formed from the overlap of many low profile shield volcanoes and is similar in morphology to other plains basalts such as the snake river plain and parts of iceland glaciation has stripped off an unknown volume of the flat lying basalt flows

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

imaging and regional distribution of basalt flows in the faeroe shetland basin we demonstrate that the use of long offset seismic data allows wide angle reflections and diving waves to be recorded and that these can be used in conjunction with prestack depth migrations to constrain and to image the base of the basalt flows and the underlying structure in the faeroe shetland basin crustal velocity models are built first by inverting the traveltimes of the recorded reflections and diving waves using ray tracing methods finer details of the velocity structure can then be refined by analysis of the amplitudes and waveforms of the arrivals we show that prestack depth migration of selected wide angle arrivals of known origin such as the base basalt reflection using the crustal velocity model allows us to build a composite image of the structure down to the pre rift basement this has the advantage that the wide angle first arriving energy must be primary and not from one of the many multiples or mode converted phases that plague near offset seismic data this allows us to tag these primary arrivals with confidence and then to identify the same arrivals on higher resolution prestack migrations that include data from all offsets examples are drawn from the faeroe shetland basin with a series of regional maps of the entire area showing the basalt depths and the thickness of the basalt flows and underlying sediment down to the top of the pre rift basement the maps show how the basalts thin to the southeast away from their presumed source west of the present faeroe islands and also show the extent to which the structure of the pre rift basement controls the considerable variations in sediment thickness between the basement and the cap formed by the overlying basalt flows

compressional and shear wave anisotropy in the oceanic lithosphere the ngendei seismic refraction experiment summary synthetic seismogram modelling of seismic refraction data from the 1983 ngendei expedition to the south pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle we calculate average p and s wave velocity versus depth functions for two orthogonal azimuths n3ooe approximately aligned with the fast mantle direction and n120 e close to the fast crustal direction probable lateral heterogeneities at the ngendei site prevent perfect matching of data and synthetic waveforms crustal anisotropy is indicated by a 28 pattern of both p and s wave travel times as a function of azimuth and is consistent with an approximate 0 2 km s difference in p wave velocities and 0 1 km s difference in s wave velocities upper mantle anisotropy is characterized by pn velocity differences of 7 95 8 4 km s l and a nearly uniform s velocity of 4 65 km s we use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle the crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge resulting in a fast direction perpendicular to the fossil spreading direction the upper mantle anisotropy is consistent with aligned olivine crystal models in which the fast direction is parallel to the fossil spreading direction for both of these models we use bounds from the ngendei data to place constraints on the physical properties of the lithosphere

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

crustal structure of endeavour ridge segment juan de fuca ridge from a detailed seismic refraction survey a detailed seismic refraction survey was carried out over the endeavour segment of the juan de fuca ridge a medium rate spreading center which lies off western north america to investigate the creation and evolution of oceanic crust a bathymetric high and the presence of hydrothermal vents suggested that the study area was the most recent locus of spreading travel time and amplitude data from 15 in line air gun ocean bottom seismometer profiles were forward modeled using asymptotic ray theory to obtain two dimensional velocity models consisting of four primary layers which correlate well with classic models of oceanic crust significant lateral variations in thicknesses and velocities on the scale of a few to 10 km are superimposed on this basic velocity structure but they appear to be random rather than distributed symmetrically about the ridge we attribute them to fracturing which causes porosity changes hydrothermal circulation which fills voids and fractures with alteration products and variations in magmatic and or deformational processes at the spreading center layer 2a is found to have low 2 6 2 8 km s velocities to average 0 4 km in thickness with variations up to 0 2 km and to be bounded at its base by a sharp velocity increase to 4 8 km s along the axial ridge velocities 0 4 0 6 km s higher than average are interpreted for layers 2b and 2c but these values are confined to a 2 km wide zone centered below the ridge velocities along ridge parallel lines offset 10 km are normal indicating that maturation to off ridge structure has occurred within at most 0 3 ma layer 3 velocities decrease by 0 1 0 2 km s for arrivals traveling along and under the axial ridge perhaps caused by higher temperatures however we find no anomalously low velocities beneath the ridge indicating that no large crustal magma chamber exists on the basis of this study we conclude that magmatic accretion is a fully three dimensional process within ridge segments such as endeavour ridge

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

cenozoic thermal evolution and tectonics of the coast mountains of british columbia 1 fission track dating apparent uplift rates and patterns of uplift the high relief of the coast mountains of british columbia has been produced in part by rapid late neogene uplift the relationship of this uplift to the presently diverse and complex plate tectonic regime off the west coast of british columbia however is not clear cut to study this problem the areal patterns rates and total amounts of cenozoic uplift from 50 to 55 n have been determined by using fission track dating of zircon and apatite separated from rocks collected at high and low altitudes along several traverses through the mountains accepting closure temperatures of 105 c 10 c and 175 c 20 c for apatite and zircon respectively and a likely closure temperature difference between zircon and apatite of 70 15 c paleogeothermal gradients have been measured at kemano c km 35 ma ago and near ocean falls king island c km 20 ma ago subcrustal heat flow is suggested to have significantly increased in the late neogene near king island uplift rates in the axial part of the northern 52 55 n coast mountains were 0 1 0 2 km ma from 25 to 15 ma ago this uplift was temporally and tectonically related to part of the subsidence in the adjacent queen charlotte basin uplift rates increased somewhat in the late miocene to about 0 4 km ma probably due to passage of the anahim hot spot beneath the area late miocene recent erosion in the north 52 55 n has reduced summit altitudes and been more extensive than farther south despite this relicts of miocene river valleys and topography are still locally preserved partly beneath 7 10 ma basalts the area of the present southern 50 to 52 n coast mountains was near sea level and experienced very low uplift rates 0 5 km ma of the southern coast mountains the profile of this uplift was broadly plateaulike and this uplift led to the inversion of the miocene coastal topography middle cenozoic uplift from 52 to 55 n was probably the result of denudation and diminishing uplift after the terminal paleocene eocene orogenic episode which grossly thickened the crust this middle cenozoic erosion resulted in the restoration of a normal crustal thickness late neogene uplift in the coast mountains was the result of thermal expansion in the mantle caused by a mantle hot spot near 53 n and changes in the geometry of the subducted slab farther south

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

fine scale seismic structure of young upper crust at 17 20 s on the fast spreading east pacific rise 1 the detailed upper crustal structure of the east pacific rise epr at 17 20 s is examined by applying a genetic algorithm based waveform inversion to five multichannel seismic lines one on axis and two to either side along 42 and 85 kyr old crust on axis a double stepped velocity pattern is recorded beneath 70 100 m of low velocity extrusives 2 1 2 4 km s 1 we define the upper velocity contrast as the base of seismic layer 2a due to its severity and continuity along and across axis the more subdued and intermittent lower velocity step is not observed off axis material between the two high gradient intervals is proposed to represent the pillow dike transition bounded above by a sharp increase in dike fraction with depth and below by an abrupt change in rheology and or deformation extrusive velocities increase quite rapidly in this area with velocities 3 km s 1 common in crust 85 kyr old this plus a rapid 300 400 m thickening of layer 2a observed within 1 4 km of the rise axis indicates that this segment is undergoing focused melt delivery 500 m wide dike intrusion zone and elevated hydrothermal activity these findings demonstrate the ability of single ship multichannel data to record detailed information on the reflectivity and velocity of the upper crust and the ability of the genetic algorithm to efficiently construct accurate seismic models based on this information

new geophysical models for subsurface velocity structure in the nechako chilcotin plateau from 2 5 d waveform tomography1 seismic inversion is applied to generate physical property models p wave velocity and numerical attenuation for four profiles in the nechako chilcotin plateau region of south central british colu

tracking stress and hydrothermal activity along the eastern lau spreading center using seismic anisotropy abstract using p wave refraction data from the l scan controlled source seismic experiment the three dimensional anisotropy structure of the upper crust is determined along the eastern lau spreading center elsc the tomographic image of anisotropy magnitude and orientation is constructed from 197 000 travel time measurements of p wave arrivals recorded on 83 ocean bottom seismometers providing the most detailed and extensive view of upper crustal spreading center anisotropy to date p wave speeds vary with the azimuth of the seismic ray path a type of anisotropy produced by stress aligned lithospheric cracks and microcracks across the study area the fast axes of anisotropy are generally oriented parallel to the trend of the spreading center as expected for ridge parallel cracks that form in association with seafloor spreading the seismic study encompassed four individual spreading segments of the elsc separated by three overlapping spreading centers osc the two larger oscs exhibit high anisotropy that penetrates deep into the upper crust surrounding these oscs and in the wake of the largest are regions where the fast axes of anisotropy are misaligned relative to the general trend in these areas the observations agree with numerical models and seafloor morphology that suggest tensile stress concentrations and a high degree of brittle crack formation the tomographic images indicate that the location and dynamics of persistent ridge offsets can be tracked back through time via their anomalous anisotropy along the spreading centers and away from ridge tips the anisotropy is greater where the melt supply is inferred to be greater and smaller where the melt supply is inferred to be smaller this is opposite to the trend expected if simple tectonic stress models govern anisotropy alternatively hydrothermal activity is expected to increase with magma supply and can explain higher anisotropy via hydrofracturing this study provides the first evidence that seismic anisotropy tracks variations in hydrologic activity along the crests of oceanic spreading centers

evolution of the subglacial hydrologic system beneath the rapidly decaying cordilleran ice sheet caused by ice dammed lake drainage implications for meltwater induced ice acceleration abstract a positive correlation between ice dammed lake drainage and ice acceleration at antarctic ice sheets ais and land terminating sections of the greenland ice sheet gris has been implicated in enhanced ice sheet decay however the paucity of direct measurements at the ice sheet bed restricts our understanding of subglacial drainage system evolution in response to transient water inputs we present evidence that two meltwater corridors on the former bed of the thin 600 m at last glacial maximum over the interior plateaus of british columbia and rapidly decaying cordilleran ice sheet cis were generated subglacially in response to the drainage of an ice dammed lake and operated as canals tunnel channels geomorphological ground penetrating radar gpr and electrical resistivity tomography ert data reveal a simple event sequence that includes initial propagation of a broad at least 2 5 km wide floodwave inefficient drainage from an ice dammed lake over relatively short 3 24 km zones at the corridor heads that collapsed into efficient canals large up to 0 25 2 5 km wide channels incised down into the sediment bed and up into the ice downglacier canal formation on the southern fraser plateau involved synchronous along the full canal length system development including elements of headward erosion and plunge pool formation our data suggest that ice dammed lake drainage beneath a rapidly decaying thin ice mass that has an efficient antecedent drainage network is not conducive to large scale ice acceleration these data may aid better assessment of the role of ice dammed lake drainage on the dynamics of former as well as contemporary ice sheets

the thickness of neogene and quaternary cover across the central interior plateau british columbia analysis of water well drill records and implications for mineral exploration potential abstract analysis of over 10 000 water well records has been used to produce new depth to bedrock maps for areas around five cities on the central interior plateau of central british columbia 100 mile house prince george quesnel vanderhoof and williams lake hitherto exploration for mineral and hydrocarbon resources has been hampered by a lack of basic knowledge of the thickness of neogene and quaternary lithologies interpretation of these new maps provides first order constraints on the localization of thick drift in pre late wisconsinan bedrock paleovalleys some of which are now buried basalt lavas of the chilcotin group are restricted to erosional remnants of previously extensive sheets emplaced onto an older peneplain our results confirm that the neogene and quaternary cover is primarily controlled by paleotopography and is generally thin and patchy across much of the region increased understanding of the three dimensional distribution of cover produces a corresponding increase in the utility of geological geochemical and geophysical exploration techniques and a reduction in the risk for future mineral exploration activities especially when combined with more sophisticated data sets

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

seismic velocities of the uppermost igneous crust versus age summary seismic refraction velocity data from the acoustic basement called layer 2a have been compiled for different mid ocean ridges the data from post 1970 studies show a strong correlation between velocity and basement age importantly velocities double in less than 10 myr for all ridges but for older crust up to 160 myr in age velocities do not increase significantly

crack induced seismic anisotropy in the oceanic crust across the east pacific rise 9 30 n the seismic anisotropy of the shallow oceanic crust across the east pacific rise 9 30pn is studied with p wave refraction data collected during a controlled source three dimensional tomography experiment the anisotropy is indicated by a cos 2a pattern of travel time residuals where a is the receiver to shot azimuth the travel time data are best fit by a model containing 4 anisotropy from 500 m to 1 km depth below the sea floor 2 from 1 to 2 km and 0 below 2 km depth the upper 500 m of crust is not independently constrained by the data since ray paths at these depths are oriented near vertically the fast direction of p wave propagation is aligned along a trend oriented 6 10 counterclockwise from the rise axis the results are consistent with the presence of vertically aligned cracks that form on or very near the rise axis are oriented nearly parallel to the rise trend penetrate to less than 2 km depth and provide 6 10 of the total pore space in the shallow crust in comparison with similar studies across slow and intermediatespreading ridges there is no apparent spreading rate dependence of the magnitude of anisotropy fl 2001 published by elsevier science b v

distribution of paleogene and cretaceous rocks around the nazko river belt of central british columbia from 3 d long offset first arrival seismic tomography across the nechako chilcotin plateau of british columbia the distribution of cretaceous sedimentary rocks which are considered prospective for hydrocarbon exploration is poorly known due to the surface cover of glacial deposits and tertiary volcanic rocks to constrain the subsurface distribution of these cretaceous rocks in 2008 geoscience bc acquired seven long up to 14 4 km offset vibroseis seismic reflection lines across a north northwest trending belt of exhumed sedimentary rocks inferred to be part of the taylor creek group p wave velocity models which are consistent with sonic logs from nearby wells have been estimated using three dimensional first arrival tomography to depths ranging from 1 to 4 km igneous basement can be identified on most lines using the 5 5 km s isovelocity contour which locates the top of the basement to an accuracy of 400 m where its depth is known in exploration wells there is no general distinction on the basis of seismic velocity between cretaceous sedimentary

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

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

genesis of an esker like ridge over the southern fraser plateau british columbia implications for paleo ice sheet reconstruction based on geomorphic inversion abstract robust interpretations of meltwater systems operating during ice sheet decay are integral to reconstructing deglacial patterns and style yet over reliance on meltwater landform morphology with limited attention to morpho sedimentary relationships and basin scale geomorphic and stratigraphic context can lead to unreliable geomorphic inversion based paleo ice sheet reconstructions this problem is illustrated by the evolution of young lake esker like ridge yler formed in the young lake basin ylb on bc s southern fraser plateau during decay of the last cordilleran ice sheet cis we integrate data from digital elevation models aerial photographs sedimentary outcrops water wells and shallow geophysics ground penetrating radar electrical resistivity tomography previous interpretations of yler as both an esker and an ice contact poorly sorted stratified deposit emplaced by westerly flowing meltwater imply an eastward retreating ice margin geophysical data from a flat topped component of yler reveal slipface and planar bedded sand and gravel overlying lacustrine sediments characteristic of a hjulstrom delta eastward dipping foresets in a gilbert delta exist at the eastern terminus contextually our observations suggest despite esker like morphology yler was not deposited within a subglacial ice tunnel instead it formed through deposition of subaerial outwash between and or on dead ice in front of a regionally backwasting ice margin the complex deglacial evolution of ylb including a drainage reversal and formation of two glacial lakes supports northwestward backwasting of the cis and dead ice within ylb we conclude that accurate geomorphic inversion of meltwater landforms for deglacial paleo ice sheet reconstruction requires knowledge of landform scale morpho sedimentary relationships and basin scale geomorphic and stratigraphic context

eruptive hummocks building blocks of the upper ocean crust the spreading axis at many slow spreading mid ocean ridges is marked by an axial volcanic ridge in this study we use a combination of high resolution remote sensing methods to elucidate the detailed nature of volcanoes in such a ridge we find that the hummocks described in previous sidescan sonar studies are dome or cone shaped edifices 5 150 m high with diameters of 30 330 m we estimate they form quickly in single eruptions each of which may produce several hummocks hummock collapse is common and hummocks of all heights are prone to failure collapses generally occur down the regional seafloor slope suggesting control by local topography approximately 33 of hummocks lose 40 of their volume by collapse so 12 of all material erupted on the axial volcanic ridge is rapidly converted to talus the higher porosity of these deposits may increase average upper crustal porosity by several percent contributing 0 5 km s 1 to seismic velocity decrease in the upper oceanic crust and may be one of the dominant mechanisms for increasing porosity in upper slow spreading oceanic crust

lithostratigraphic and tectonic framework of jurassic and cretaceous intermontane sedimentary basins of south central british columbia1this article is one of a series of papers published in this special issue on the theme of new insights in cordilleran intermontane geoscience reducing exploration risk in the mountain pine beetle affected area british columbia the south central intermontane belt of british columbia has a complex architecture comprising late paleozoic to mesozoic volcanic and plutonic arc magmatic suites marine and nonmarine clastic basins high grade metamorphic complexes and accretionary rocks jurassic and cretaceous clastic basins within this framework contain stratigraphy with hydrocarbon potential the geology is complicated by cretaceous to eocene deformation dismemberment and dislocation the eocene to neogene history of the southern intermontane belt is dominated by non arc volcanism followed by pleistocene to recent glaciation the volcanic and glacial cover makes this a difficult region to explore for resources much recent work has involved re evaluating the challenges that the overlying volcanic cover has historically presented to geophysical imaging of the sedimentary rocks in this region in light of technological advances in geophysical data collection and analysis this paper summarizes the lithological and stratigraphic fra

cracked media poisson s ratio and the structure of the upper oceanic crust summary theoretical models of water filled cracks within a solid are in general agreement and indicate that the poisson s ratio of the composite material will typically increase for thin cracks with aspect ratios less than a but will decrease for thick cracks with aspect ratios between and i near spherical pores cause little change in poisson s ratio this is true both for randomly oriented isotropic crack models and at certain directions for aligned crack models which predict seismic anisotropy observed anomalous values of poisson s ratio in oceanic layer 2 can often be explained using these models as an effect of cracks and do not necessarily require the presence of rocks of unusual composition in particular the low poisson s ratios reported by spudich orcutt 1980 and au clowes 1984 can approximately but not always exactly be fitted with a model consisting of thick cracks within higher poisson s ratio rock more typical of the upper oceanic crust however this interpretation is near the limit of existing theories of cracked material so other explanations for the anomalous observations must also be considered

mapping quaternary paleovalleys and drift thickness using petrophysical logs northeast british columbia fontas map sheet nts 94ithis article is one of a selection of papers published in this special issue on the theme geology of northeastern british columbia and northwestern alberta diamonds shallow gas gravel and glaciers the relatively subdued topography of british columbia s northern interior plains does not reflect the irregular buried bedrock surface many areas have been deeply incised by preglacial rivers tha

large massive sulfide deposits in a newly discovered active hydrothermal system the high rise field endeavour segment juan de fuca ridge submersible observations and side scan sonar imagery delineate ten large venting sulfide sulfate silica structures located on an uplifted block of the axial valley floor on the central portion of the endeavour segment juan de fuca ridge this newly discovered vent field lies 1 8 km north of the main endeavour vent field the axial valley a horst and faults controlling sulfide deposition trend parallel to the ridge axis the horst rises 10 to 15 m above the valley floor and offsets a lava lake which extended 500 m in width across the valley the largest sulfide structures are perched along the major horst bounding faults one structure is the tallest single edifice 45 m reported in the pacific ocean smaller less active and inactive sulfide edifices are associated with faulted remnants of the lava lake surface fissured pressure ridges and at the edge of collapse pits based on the relationship between faults and sulfide structures the morphology of sulfide edifices and their venting characteristics we infer that sulfide structures grow vertically by chimney accumulation and laterally by flange development late stage deposition of amorphous silica toughens the large deposits and may be responsible for the unusual size and aspect ratio of endeavour edifices

deep seismic reflection constraints on paleogene crustal extension in the south central intermontane belt british columbia following growth of the canadian cordillera during the mesozoic the southern cordillera was subject to extension during the paleocene and eocene that correlated with widespread volcanic activity in south central british columbia including across much of the nechako chilcotin plateau in 2008 geoscience bc acquired 330 km of deep vibroseis reflection profiles on the plateau mostly over the stikinia arc terrane but also over its eastern contact with the oceanic cache creek terrane all seven seismic reflection lines reveal a strongly reflective lower crust that extends from 7 to 9 s down to the moho which is defined by the downward termination of reflectivity at 11 12 s in the uppermost crust extension occurred by block faulting with faults soling into subhorizontal to shallowly dipping detachments above 10 km depth extension in the deeper upper and middle crust which was partly controlled by antiforms likely related to earlier shortening was accommodated on a network of anastomosing shear zones

growth of large sulfide structures on the endeavour segment of the juan de fuca ridge abstract mapping and sampling with dsrv alvin has established that sulfide blocks 0 5 m across dredged from the axial valley of the endeavour segment at 47 57 n are samples of unusually large sulfide structures the steep sided structures up to 30 m in length 20 m in height and 10 15 m across are localized by venting along normal faults at the base of the western axial valley wall and are distributed for about 200 m along strike paralleling the 020 trend of the ridge crest high temperature fluids 350 to more than 400 c pass through the massive sulfide structures and enter seawater through small concentric nozzle like features projecting from the top or the sides of the larger vent structures diffuse low temperature flow is pervasive in the vicinity of the active sulfide structures exiting from basalt and sulfide surfaces alike evidence of recent volcanic activity is sparse the two largest samples taken with the dredge would not have been recoverable using the submersible these samples represent massive complex portions of the sulfide structures which were not closely associated with rapid high temperature fluid flow at the time of sampling they contain textural evidence of sealed hydrothermal fluid exit channels mineralogy is dominated by fe sulfides nnd amorphous silica pyrite marcasite wurtzite chalcopyrite and iss are the most common sulfide phases pyrrhotite galena and sphalerite are present in trace amounts barite amorphous silica and chalcedony are the only non sulfide phases anhydrite is not observed in any of the dredge samples although it is common in the chimney like samples recovered by alvin specific mineralogical textural zones within the dredge samples are anaoogous to individual layers in east pacific rise at 21 n and southern juan de fuca ridge samples with two exceptions a coarse grained highly porous fe sulfide rich interior containing sulfidized tubeworm casts and a 2 5 cm thick zone near the outer margin of the samples dominated by late stage amorphous silica the porous interior may have formed by dendritic crystal growth from a slowly circulating fluid within a large enclosed chamber the amorphous silica deposited from a seawater hydrothermal fluid mixture percolating slowly through the walls of the enclosed chamber conductive cooling of the fluid as it traversed the walls allowed amorphous silica to precipitate these silica rich zones are the densest most durable portions of the structures and may be responsible for the lasting stability of the large sulfide features observations in these samples are consistent with two distinct phases of development phase 1 is analogous to chimney growth and construction at 21 n and ends when flow channels become sealed to rapid flow of through going fluid the flow is evidently redirected within the structure phase 2 includes dissolution of anhydrite and precipitation of amorphous silica during conductive cooling of sluggishly circulating hydrothermal fluid or seawater hydrothermal fluid mixtures evolution of vent structures through phase 2 allows lateral and vertical growth of unusually large structures

an investigation of seismic anisotropy due to cracks in the upper oceanic crust at 45 n mid atlantic ridge summary a detailed seismic refraction survey using three 3 component ocean bottom seismographs over young 1 1 3 4 ma old mid atlantic ridge at 45 n recorded compressional wave velocity anisotropy in the upper crust of about 5 per cent and associated particle motion deviations of up to 8 the magnitudes and azimuths of these anisotropic variations suggest that they are generated by sets of subparallel near vertical water filled cracks in layer 2 the directions of the cracks cannot be determined uniquely but they lie between the seafloor spreading and the ridge axis directions in the range 30 60 with respect to the ridge axis we explain this unexpected crack orientation by the occurrence of cracking and fracturing induced in the upper crust as it moves away from the spreading axis the principal horizontal stresses responsible for this fracturing are parallel and perpendicular to the ridge axis the apparently negligible seismic influence exerted by ridge parallel sets of shallow cracks observed at the ridge axis is explained by their infilling by sediment and hydrothermal precipitates and by their closure under ridge normal compression we find a normal oceanic crustal velocity structure that can be explained by decreasing numbers of open cracks with increasing depth through the upper 2 km of the crust lateral variability across the 25 km square survey is limited in magnitude typically 0 05 s variations in delay times and the observed travel times are consistent with a model of the crust in which the isovelocity surfaces at depth are a subdued copy of the basement topography

mid ocean ridge sulfide deposits evidence for heat extraction from magma chambers or cracking fronts numerous seafloor observations show that the sizes of high temperature hydrothermal sulfide edifices vary dramatically with spreading rate on fast spreading ridges venting occurs from spindly chimneys which reach heights of about 10 m while on slow spreading ridges vents are frequently located near the tops of large sulfide mounds whose volumes may reach 105 106 m3 we argue that such variations are the result of a profound difference in the nature of hydrothermal heat extraction between fast spreading ridges where spreading occurs predominantly by magmatism and slow spreading ridges where there is a significant component of tectonic extension along fast spreading ridges a steady state axial magma chamber can be insulated by a relatively thick conductive boundary layer because heat extraction is limited by low permeability in such systems episodes of vigorous venting are linked to diking events not only because these introduce a significant heat source into the upper crust but also because the intrusion of a dike is the primary source of increased permeability near the ridge axis over a time scale of the order of a decade mineral deposition tends to clog the reaction and upflow zones and there is a high probability that young sulfide structures will be buried by subsequent eruptions on slow spreading ridges tectonic extension maintains the fluid pathways necessary to support vigorous convection in the waning stages of magmatism hydrothermal circulation driven by a downward migrating cracking front can cool the entire crust leading to the formation of very large sulfide deposits as the depth of circulation increases overburden pressures reduce the permeability and the hydrothermal heat fluxes decrease progressively once hydrothermal fluids penetrate the moho serpentinization clogs the fluid pathways and high temperature venting ceases until it is reactivated by a fresh magmatic intrusion

spatial and temporal evolution of magmatic systems beneath the endeavour segment juan de fuca ridge tectonic and petrologic constraints major and trace element data for a suite of lavas from fifty six dredges and alven dives on the ridge axis and adjacent abyssal hills have been used to investigate the geometry and evolution of magmatic systems beneath the endeavour segment juan de fuca ridge the morphology of the endeavour segment between the northward propagating cobb offset and the recently formed 0 2 m y endeavour offset is dominated by a shallow rifted elongate crestal volcano endeavour ridge that deepens along strike into a broad deep basin at each offset a set of ridges interpreted to be previous crestal volcanoes rifted apart by spreading flank the endeavour ridge and chronicle the dueling propagator history of the cobb offset the tectonic evidence strongly suggests that a large portion of the endeavour segment may be a failing rift segment at this time lavas from the current axis of the endeavour segment are moderately fractionated mgo 6 8 5 wt and have generally higher sio2 al2o3 na2o and k2o and lower feo man lavas from south of the cobb offset soco lavas incompatible trace element abundances and ratios indicate the endeavour lavas are primarily enriched e morbs and t morbs e g zr nb 7 24 zr y 2 5 5 9 and ba tio2 6 64 in contrast with the soco lavas which are more depleted in character thus the 30 km wide cobb offset appears to mark a major geochemical boundary beneath the juan de fuca ridge in contrast with the endeavour segment axial lavas samples from adjacent abyssal hills are more similar to the soco lavas in their major and trace element characteristics these observations suggest that the parental magmas of the endeavour segment exhibit temporal variability with more enriched material arriving only recently beneath the ridge axis pronounced compositional variability is observed at small spatial scales within the endeavour segment axial lavas which does not correlate with axial morphology this variability is interpreted to reflect ubiquitous small scale mantle heterogeneity and poor mixing of multiple parental magmas during migration from the melt region or within axial magma chambers highly enriched samples zr nb 10 are localized near the summit region of endeavour ridge whereas slightly less enriched samples occur along the length of the axis and on the older flanking ridges recent enrichment may result from diminishing extents of partial melting of a heterogeneous source in response to tectonic reconfiguration causing more fusible enriched domains to dominate the chemical signature of melts produced small scale heterogeneity along strike seems incompatible with models of centralized upwelling of melts beneath the summit region of the ridge axis with shallow lateral injection of melts to distal ends of the segment unless these spatial variations actually reflect temporal variations in the source composition and collapse of the shallow magmatic systems toward the summit region as rift failure has progressed

pleistocene reversal of the fraser river british columbia the fraser river in british columbia canada is the longest non dammed river on the west coast of north america and supplies 20 10 6 t yr of sediment to the pacific ocean abundant geomorphological evidence indicates that the fraser river reversed its course to southward flow in the recent geological past investigation of two volcanic dams at dog creek demonstrates northward flow of the fraser until at least 1 06 ma before reversal and erosion of the 270 km long fraser canyon we propose that the submarine nitinat fan off the coast of british columbia records the reversal and sudden input of coarse continental derived sediment ca 0 76 ma this study confirms reversal of the fraser river and places a firm constraint on the maximum age of that reversal reversal likely followed stream capture in response to enhanced glaciofluvial erosion and uplift of the coast mountains

towards a morphogenetic classification of eskers implications for modelling ice sheet hydrology abstract validations of paleo ice sheet hydrological models have used esker spacing as a proxy for ice tunnel density changes in crest type cross sectional shape along esker ridges have typically been attributed to the effect of changing subglacial topography on hydro and ice dynamics and hence subglacial ice tunnel shape these claims assume that all eskers formed in subglacial ice tunnels and that all major subglacial ice tunnels produced a remnant esker we identify differences in geomorphic context sinuosity cross sectional shape and sedimentary architecture by analysing eskers formed at or near the margins of the last cordilleran ice sheet on british columbia s southern fraser plateau and propose a morphogenetic esker classification three morphogenetic types and 2 subtypes of eskers are classified based on differences in geomorphic context ridge length sinuosity cross sectional shape and sedimentary architecture using geophysical techniques and sedimentary exposures they largely record seasonal meltwater flows and glacial lake outburst floods glofs through sub en and supraglacial meltwater channels and ice walled canyons general principles extracted from these interpretations are 1 esker ridge crest type and sinuosity strongly reflect meltwater channel type eskers formed in subglacial conduits are likely to be round crested with low sinuosity except where controlled by ice structure or modified by surging and contain faults associated with flank collapse eskers formed near or at the ice surface are more likely to be sharp crested highly sinuous and contain numerous faults both under ridge crest lines and in areas of flank collapse 2 esker ridges containing numerous flat crested reaches formed directly on the land surface in ice walled canyons unroofed ice tunnels or in ice tunnels at atmospheric pressure and therefore likely record thin or dead ice 3 eskers containing macroforms exhibiting headward and downflow growth likely record flood scale flows possibly glofs where a lake can be inferred these conclusions suggest that esker crest type sinuosity and geomorphic context when understood along with sedimentary architecture largely reflect formational position with respect to the ice surface reconstructions of ice sheet hydrology need to account for variation in esker morphology because basing hydrodynamic inferences on the presence or absence of an esker alone ignores encoded differences in water source supply flow magnitude and frequency and conduit position

the late wisconsinan deglacial history of the east central taseko lakes area british columbia late wisconsinan fraser glaciation retreat phase deposits and landforms in the east central taseko lakes area british columbia are used to demonstrate a four phase model of deglaciation during phase i at the onset of ice retreat the cordilleran ice sheet occupied much of the study area blocking southward drainage of fraser river phase ii was marked by the deglaciation of uplands and plateaux meltwater drainage patterns were controlled by stagnating glaciers confined to valleys phase iii commenced as remnant ice in the fraser valley downwasted to between 850 and 760 m elevation at this time interlobate glacial lakes formed in hanging valleys east of fraser river drainage of glacial lakes occurred subglacially and was accompanied by disintegration of remnant ice and an increase in mass movements in valleys these events were followed by decreased sedimentation rates reflecting lower meltwater volumes and exhaustion of unstable glacial debris during phase iv postglaciation valley fill was subj

microearthquakes in the black smoker hydrothermal field east pacific rise at 21 n in july and august 1980 an array of five ocean bottom seismographs was deployed within 3 km of the 350 c hydrothermal vents at the rivera submersible experiment rise site at 21 n on the east pacific rise two of these instruments were placed within 600 m of the vents using a transponder navigation network the array detected four basic types of events the first type consisted of local very small microearthquakes locations obtained for 11 of these events place three within 1 km of the vents with the others elsewhere along the rise crest they appear to originate either from movement on the faults in the area or from the hydrothermal system beneath this area a study of the s p times of this type indicates a maximum hypocentral depth of 2 3 km implying a similar limit to the depth of hydrothermal circulation and brittle fracturing in the vicinity of the vents the second type of event found consisted of emergent earthquakes that have many of the characteristics of volcanic harmonic tremor the frequency of these events falls in the 1 5 hz range and are similar in appearance to those seen at mount st helens prior to and during its may 1980 eruption they may be either hydrothermal or volcanic in origin the third type of event produced a very monochromatic high frequency seismogram with the energy concentrated at 20 hz these events also appear to have a local origin the particle motion of these earthquakes shows almost pure stonely wave propagation and they appear to originate from a regularly oscillating dilatational source the nature of the source of event types two and three is clearly not the usual sort of motion on a fault and is still not understood the fourth type of earthquake recordings were regional events apparently from the nearby rivera and tamayo fracture zones the low frequency acoustic noise in the water was 16 64 times higher within 300 m of the vents than it was 2 km away this implies that the black smoker vents may be generating a large acoustical disturbance as the 350 c water pours into the ocean the observations are among the first of seismicity on moderate to fast spreading ridges and many of the events appear to be related to the hydrothermal activity found there

plate motion at the ridge transform boundary of the south cleft segment of the juan de fuca ridge from gps acoustic data 1 we measure the present day plate velocity of the juan de fuca ridge 25 km off axis to be 63 6 3 6 mm a at s67 2 e 7 9 degrees 1 relative to the pacific plate pa this velocity was derived from gps acoustic gpsa measurements in 2000 2001 2002 and 2003 that observed the position of a seafloor array 44 43 n 130 03 w 2900 m depth with a repeatability of 4 6 mm three transient events at the juan de fuca ridge and blanco transform account for 10 of this motion in viscoelastic modeling suggesting that the observed gpsa pa velocity is due primarily to steady state plate dynamics subtracting the modeled transient motion gives a velocity of 57 3 3 9 mm a at s72 9 e 12 1 degrees 1 which is consistent at the 95 confidence level with the velocity calculated from the wilson 1993 0 0 78 ma euler pole therefore this site is interpreted to be in a region of continuous full rate plate motion a robust result of this study which holds with and without correcting for transient motions these results provide direct geodetic evidence that spreading occurs predominantly within 25 km of the axis at this intermediate spreading rate ridge previously reported geodetic monitoring across the 1 km wide axial valley from 1994 1999 and 2000 2003 shows no significant extension chadwell et al 1999 hildebrand et al 1999 chadwick and stapp 2002 w w chadwick personal communication 2006 and seismic monitoring shows no activity this suggests the crust between 0 5 and 25 km off axis accommodates 26 mm of aseismic deformation each year through some combination of near axis fault motion and elastic strain accumulation

eocene and neogene volcanic rocks in the southeastern nechako basin british columbia interpretation of the canadian hunter seismic reflection surveys using first arrival tomography geological investigation of the near surface in the southeastern nechako basin is difficult shallow seismic reflection imaging is poor due in part to an extensive cover of eocene and neogene volcanic rocks outcrops of these volcanic rocks and the primarily cretaceous bedrock are commonly obscured by quaternary deposits and vegetation estimates of near surface p wave velocity are derived from the tomographic inversion of seismic first arrivals an effective tool when seismic imaging is poor tomographic model velocities are in agreement with sonic logs and laboratory samples except for those from the neogene chilcotin group cretaceous sedimentary rocks have velocities of 2800 4200 ms 1 the eocene endako and ootsa lake groups which have velocities of 3000 4200 ms 1 are not distinguishable based on velocity the velocity the character density focus and penetration depth of rays and ties with well and surface geology constrain the subsurface extent of the endako group adjacent to well b 82 c

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

seismic structure and crustal magmatism at the mid atlantic ridge 35 n we have determined the three dimensional p wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the mid atlantic ridge mar near 35 n the best fitting one dimensional model is 0 8 km s lower in velocity at a given depth than for fast spreading ridges while the isotropic three dimensional velocity structure shows a strong lateral heterogeneity up to 1 4 km s peak to peak variation at 1 km depth in both the along and cross axis directions an anomalous region with velocities as much as 0 8 km s lower than average and 3 5 km in lateral extent is located beneath a near axis seamount we interpret this feature as a region of near solidus temperatures with possibly a small degree of partial melt from the variation of travel time residuals with azimuth we determine a best fitting p wave anisotropy model that has 4 anisotropy from 500 m to 1 km depth 2 anisotropy in the depth range 1 1 5 km and no anisotropy at greater depth this finding is consistent with vertical cracks aligned parallel to the axial valley we find no evidence for the shallow high velocity ridge parallel feature commonly observed at the axis of fast spreading ridges and attributed to high velocity dikes beneath a thin extrusive layer the absence of this anomaly at the mar implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25 000 years an unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes which may account for the generally lower velocities beneath the mar compared with the east pacific rise and a lateral variation in upper crustal lithology

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

neogene chilcotin basalts in south central british columbia geology ages and geomorphic history an extensive but generally thin mantle of basalt flows the chilcotin group covers much of the interior plateau of south central british columbia it provides material for dating and for reconstructing the original form of the paleosurface on which it was deposited k ar whole rock dates demonstrate that several ages of basalt are represented from early miocene or even late oligocene to early pleistocene with particularly abundant eruptions about 14 16 9 6 and 1 3 ma ago basalts of middle miocene and later ages if not the early miocene relics as well clearly rest on land surfaces of low local relief in places the low relief surfaces had been incised to depths of 100 200 m and the valleys backfilled with mid or late cenozoic sediments prior to burial by the basalts the low relief surfaces throughout the area are believed to have been developed close to a common base level and regional differences in their present elevation are thus largely a product of post basalt deformation this is recorded

impact of crustal evolution on changes of the seismic properties of the uppermost ocean crust one of the most important observations of oceanic crustal evolution is that the uppermost part of the igneous crust layer 2a appears to thin and eventually disappear as its seismic velocity doubles from approximately 2 2 to 4 5 km s 1 in the first 40 m y the most likely cause is decreasing porosity in extruded basalts that compose the uppermost igneous crust by filling cracks with hydrothermally generated minerals a by product of off axis hydrothermal circulation and alteration of basalts new insights from two recent papers on crustal hydrothermal circulation patterns and modeling of the effects of crack geometry distributions upon seismic velocities provide guidance to relate filling of cracks within the basement to seismic velocity increases a qualitative model of layer 2a crustal evolution is outlined that satisfies many geophysical and geochemical observations some seemingly unrelated somewhat surprisingly the crustal age at which layer 2a seismic velocities reach their maximum value is primarily controlled by the integrated permeability of the overlying sediment which in turn controls the hydrologic regime necessary for secondary mineral precipitation speculative predictions can be used to verify refine or invalidate this dynamic model of crustal evolution major scientific and technical obstacles must be overcome before a definitive model can be quantified with such a rigorous model of crustal upper crustal evolution in hand remote geophysical sensing tools can begin to fulfill the promise of interpreting the physical state composition and past history of oceanic crust

the gang ranch big bar area south central british columbia stratigraphy geochronology and palynology of the tertiary beds and their relationship to the fraser fault tertiary rocks in the gang ranch big bar area south central british columbia consist of 1 early or middle eocene 50 ma lavas breccias and tuffs capped by a mappable unit of conglomerate a

correcting airborne gravity data for overburden thickness a case study from the nechako interior plateau british columbia large areas of bedrock in canada such as in the interior plateau of british columbia are covered by a thick glacial overburden lateral variations in overburden thickness can create spurious anom

anisotropy in the oceanic lithosphere theory and observations from the ngendei seismic refraction experiment in the south west pacific summary p wave travel time data from a seismic refraction experiment on the 1983 ngendei expedition to the south pacific indicate anisotropy at two levels in the oceanic lithosphere in the upper mantle p wave velocities vary between 8 0 and 8 5 km s 1 with the fast direction at n30 e crustal anisotropy within layer 2 is characterized by azimuthal p wave velocity differences of about 0 2 0 4 km s 1 with the fast direction at n120 e orthogonal to the upper mantle anisotropy the observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust we examine a model of p wave propagation from a point source in an isotropic layer above an anisotropic half space and show that under these conditions anisotropy can cause seismic ray paths to deviate from the vertical plane connecting source and receiver this deviation has a negligible effect on seismic travel times but must be taken into account in modelling p wave polarization anomalies p wave polarization anomalies within the anisotropic half space are the sum of the particle motion deviation from the ray path and ray path deviation from the source receiver azimuth and are typically range dependent p wave polarization anomalies will also occur in the overlying isotropic layer purely as a result of the ray path deviation with greater deviations at long ranges a simple model of upper mantle anisotropy 6kms 1 crust above 8 0 8 6 km s 1 anisotropic mantle produces surface polarization anomalies of 1 5 3 5 in practice p wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels we examine the implications of anisotropic ray path deviations on travel times and p wave polarizations for the ngendei data as well as some previous marine seismic refraction experiments

asymmetric deep crustal structure across the juan de fuca ridge a multichannel line was shot across the endeavour segment of the juan de fuca ridge careful processing of 25 km of the line centered about the rise axis shows that young crustal structure is asymmetric an intermittent reflector from the top 1 km of crust follows topography on both the pacific and the juan de fuca plates it could be either the pillow basalt diabase dike contact or a metamorphic front within the pillow basalts a reflection exists 2 3 km below the rift valley floor diffractions from its edges indicate that the rift valley9s velocity structure is markedly different from the structure of the flanks the deep crustal structure is highly asymmetric moho exists within 4 km of the rift valley on the pacific plate in contrast to the juan de fuca plate which shows a reflector at intermediate depths dipping away from the rise to reach moho depth 12 km from the rift valley this reflector9s origin is enigmatic we speculate that the thermal history and structure of the two plates differ because of local differences in mantle structure caused by either ridge migration or a melting anomaly

poisson s ratio structure of young oceanic crust we have applied full waveform inversion to wide aperture seismic reflection data from the southern east pacific rise near 14 s the data contain clear compressional wave and doubly converted shear wave arrivals that provide good constraints on the p and s wave velocities vp vs and hence poisson s ratio and seismic attenuation qp qs structure of seismic layer 2 layer 2a is highly attenuating qp 18 30 and qs 8 15 and layer 2b is moderately attenuating qp 30 50 and qs 20 25 our results show high at the seafloor and in layer 2a 0 48 across the top of the 2a b transition the rapid increase in vp is accompanied by a sharp drop in to 0 25 within just 200 m of the seafloor we perform simple calculations to gain an insight into the porosity and crack distribution with depth these calculations suggest that porosity is in excess of 30 in layer 2a but reduces to 6 7 at the top of the 2a b transition and to about 5 at a depth of 600 m below seafloor within layer 2b our results suggest that there is an increase in the average aspect ratio with depth across the 2a b transition the most likely explanation is that numerous thin cracks either mechanically close or are infilled at depth our results show there to be an abrupt change in the pore structure across the 2a b transition which is consistent with a lithologic transition from extrusives to dykes but is equally consistent with a transition mechanical or hydrothermal within the extrusive pile

seismic anisotropy of the shallow crust at the juan de fuca ridge microearthquake data recorded on four ocean bottom seismometers are used to study shear wave splitting on the endeavour segment of the juan de fuca ridge the covariance matrix decomposition method is used to determine the sensor orientation from explosive shot data and to estimate the anisotropy parameters for 238 earthquake records at three of the four sites the results show a remarkably consistent fast direction parallel to the ridge axis the time delays between the fast and the slow waves range from 40 to 200 ms with an average of 90 ms they are not clearly related to earthquake range focal depth or source receiver azimuth the splitting of the shear waves is interpreted as an effect of structural anisotropy due to the presence of ridge parallel cracks in the shallow crust if we assume that anisotropy is concentrated in the upper 1 2 km the splitting times require a high crack density of 0 1

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

seismic anisotropy in the upper oceanic crust seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at deep sea drilling project dsdp site 504b on dsdp leg 92 particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis there are four diagnostic patterns 1 along symmetry axes where vertically polarized shear waves sv are generated but horizontally polarized shear waves sh are not generated the particle motions are purely vertical 2 for azimuths at which both sv and sh are generated and the sh velocity is significantly faster than sv a cruciform pattern with horizontal first motion is observed 3 for azimuths at which both are generated and the sv velocity is significantly faster than sh a cruciform pattern with vertical first motion is observed and 4 for azimuths at which both are generated and sv and sh velocities are similar elliptical particle motions are observed the shear wave particle motions and compressional wave travel times from a 2 km radius circle are consistent with an anisotropic model with hexagonal symmetry the compressional wave velocity has a two theta azimuthal variation between 4 0 and 5 0 km s the symmetry axis is horizontal with an azimuth of n20 w 10 the spreading direction at the site 6 m y age is north south the observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting

shear wave splitting and crustal anisotropy at the mid atlantic ridge 35 n 1 shear wave splitting observed in microearthquake data at the axis of the mid atlantic ridge near 35 n has a fast polarization direction that is parallel to the trend of the axial valley the time delays between fast and slow s wave arrivals range from 35 to 180 ms with an average of 90 ms and show no relationship with ray path length source to receiver azimuth or receiver location the anisotropy is attributed to a shallow distribution of vertical fluid filled cracks aligned parallel to the trend of the axial valley joint modeling of the shear wave anisotropy and coincident p wave anisotropy results using recent theoretical models for the elasticity of a porous medium with aligned cracks suggests that the crack distribution that causes the observed p wave anisotropy can account for at most 10 ms of the shear wave delay most of the shear wave delay thus likely accrues within the shallowmost 500 m seismic layer 2a and the percent s wave anisotropy within this highly fissured layer is 8 30 isolated fluid filled cracks at 500 m to 3 km depth that are too thin or too shallow to be detected by the p wave experiment may also contribute to the shear wave delays the joint analysis of p and s wave anisotropy is an important approach for constraining the crack distributions in the upper oceanic crust and is especially suited for seismically active hydrothermal systems at slow and intermediate spreading mid ocean ridges

hydrogen and oxygen isotopic compositions of water vapor over the bay of bengal during monsoon 1 dv and 18ov of 70 water vapor samples collected at 6 and 25 m above sea level over the bay of bengal bob during july august 2012 are reported this helps characterize the isotopic signature of monsoon vapor no significant vertical variation is observed in dv 18ov or deuterium excess defined as d 8 18o dv and 18ov are significantly correlated r 0 92 at each height the deuterium excess values do not because the variation of dv and 18ov relative to their uncertainties is much larger than that of the latter the temporal variations of dv and 18ov correlate well with air temperature rather than sea surface temperature the control of normalized humidity on deuterium excess is less prominent while the distribution of water vapor isotopologues over the bob is primarily determined by the ocean surface conditions they are significantly altered by laterally advected vapor from rain en route during the monsoon

increase of seismic velocities in upper oceanic crust the superfast spreading east pacific rise at 14 14 s one of the most important observations of oceanic crustal evolution is that seismic compressional wave velocities in the upper igneous crust layer 2a increase as the crust ages the most viable explanation for this phenomenon is decreasing porosity due to filling of open void spaces with hydrothermally generated minerals but the idea that seismic velocity could be a function of plate age however depends primarily on airgun sonobuoy profiles collected more than 20 years ago houtz and ewing 1976 therefore in late 1995 new seismic refraction measurements were obtained on the eastern flank of the superfast spreading east pacific rise south of the garrett transform we found that velocities rapidly increase by about 45 50 within only 0 5 1 m y double within 5 m y or less and remain constant thereafter

ice streams of the late wisconsin cordilleran ice sheet in western north america abstract the late wisconsin cordilleran ice sheet cis of western north america is thought to have reached its maximum extent 2 5 106 km2 as late at c 14 5 ka most 80 of the ice sheet s bed consists of high mountains but its core zone sited on plateaux of the intermontane belt of british columbia and coterminous parts of the usa shows broad swaths of subglacially streamlined rock and sediment broad scale mapping from new digital imagery data identifies three subglacial bed types 1 hard beds of variably streamlined bedrock 2 drumlinized soft beds of deformation till reworked from antecedent sediment and 3 mixed beds of variably streamlined bedrock protruding through drumlinized sediment drumlins on soft beds appear to be erosional features cut into till and antecedent sediments and identify the catchment areas of paleo ice streams expressed downglacier as flow sets of megascale glacial lineations msgls grooved and cloned drumlins appear to record the transition from drumlins to msgls the location of paleo ice streams reflects topographic funneling of ice from plateau surfaces through outlet valleys and a soft bed that sustained fast flow rock cut msgls are also present locally on the floors of outlet valleys cis disintegrated in

upper crustal evolution across the juan de fuca ridge flanks 1 recent p wave velocity compilations of the oceanic crust indicate that the velocity of the uppermost layer 2a doubles or reaches 4 3 km s found in mature crust in 10 ma after crustal formation this velocity change is commonly attributed to precipitation of low temperature alteration minerals within the extrusive rocks associated with ridge flank hydrothermal circulation sediment blanketing acting as a thermal insulator has been proposed to further accelerate layer 2a evolution by enhancing mineral precipitation we carried out 1 d traveltime modeling on common midpoint supergathers from our 2002 juan de fuca ridge multichannel seismic data to determine upper crustal structure at 3 km intervals along 300 km long transects crossing the endeavor northern symmetric and cleft ridge segments our results show a regional correlation between upper crustal velocity and crustal age the measured velocity increase with crustal age is not uniform across the investigated ridge flanks for the ridge flanks blanketed with a sealing sedimentary cover the velocity increase is double that observed on the sparsely and discontinuously sedimented flanks 60 increase versus 28 over the same crustal age range of 5 9 ma extrapolation of velocity age gradients indicates that layer 2a velocity reaches 4 3 km s by 8 ma on the sediment blanketed flanks compared to 16 ma on the flanks with thin and discontinuous sediment cover the computed thickness gradients show that layer 2a does not thin and disappear in the juan de fuca region with increasing crustal age or sediment blanketing but persists as a relatively low seismic velocity layer capping the deeper oceanic crust however layer 2a on the fully sedimented ridge flank sections is on average thinner than on the sparsely and discontinuously sedimented flanks 330 80 versus 430 80 m the change in thickness occurs over a 10 20 km distance coincident with the onset of sediment burial our results also suggest that propagator wakes can have atypical layer 2a thickness and velocity impact of propagator wakes is evident in the chemical signature of the fluids sampled by odp drill holes along the east endeavor transect providing further indication that these crustal discontinuities may be sites of localized fluid flow and alteration

age variations of oceanic crust poisson s ratio inversion and a porosity evolution model porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities porosity is particularly important in the uppermost young crust where rapid variations in velocities with depth and crustal age are observed knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of poisson s ratio are available from observations of compressional and shear seismic velocities p and s in this paper i present a joint seismic waveform inversion for p and s velocities are found while maximizing or minimizing poisson s ratio using a hypothesis testing mechanism i apply this method to ocean bottom hydrophone data in 140 ma atlantic crust the resulting solution corridor agrees with laboratory measurements without the low poisson s ratio anomalies at depths of 0 8 1 5 km found by spudich and orcutt 1980 and au and clowes 1984 on younger 15 ma pacific crust compiling other published p and s solutions an age dependent pattern emerges none of the solutions for crust older than 60 ma display the poisson s ratio anomaly i propose a simple crustal evolution model using thin and thick cracks to explain these observations thin cracks preferentially close at shallow depths in the crust producing the localized poisson s ratio anomaly sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements this model is consistent with similar models of crack populations and their evolution from shallow measurements

geology and venting characteristics of the mothra hydrothermal field endeavour segment juan de fuca ridge the 500 m long mothra hydrothermal field is the largest venting site in areal extent on the endeavour segment of the juan de fuca ridge within this field there are at least five actively venting sulfide complexes spaced 40 to 200 m apart along a trend of 020 the clusters are composed of multiple steep sided pinnacles that rise up to 20 m above the seafloor most of the sulfide structures are awash in diffusely venting fluids 30 200 c that support rich and diverse macrofaunal and microbial communities isolated black smoker chimneys vent 302 c fluids the linear arrays of sulfide complexes delineate a zone of active faulting near the western wall of the axial valley this fault network serves as the conduit for the rising fluids

the quaternary stratigraphic record of british columbia evidence for episodic sedimentation and erosion controlled by glaciation the terrestrial quaternary stratigraphic record of british columbia is largely a product of brief depositional events separated by long periods of nondeposition and erosion thick stratified quaternary sediments are present mainly in valleys and coastal lowlands and accumulated during periods of growth and decay of the cordilleran ice sheet at glacial maxima till was deposited over large areas of low and moderate relief however at the same time much of the landscape was eroded by glaciers sedimentation has been more restricted and has occurred at lower rates during nonglacial periods than during glaciations on land the only important sediment accumulation sites during nonglacials have been lakes floodplains and fans however large amounts of sediment have accumulated offshore especially in fjords and basins such as the strait of georgia because of the restricted aspect of sedimentation during nonglacials the stratigraphic record of these periods is meagre in most places true nonglacial uni