segment scale variations in the crustal structure of 150 300 kyr old fast spreading oceanic crust east pacific rise 8 15 n 10 5 n from wide angle seismic refraction profiles summary we have simultaneously inverted seismic refraction and wide angle moho reflection traveltimes for the 2 d crustal thickness and velocity structure of 150 300 kyr old crust along the east pacific rise epr between the siqueiros and clipperton fracture zones fzs our results show a strong correlation between ridge segmentation and upper and mid crustal seismic velocities with higher velocities near segment centres and lower velocities near segment ends low crustal velocities at the clipperton and siqueiros fzs are interpreted as fracturing resulting from brittle deformation of the crust in the transform domain a relict overlap basin left on the pacific plate by the 9 03 n overlapping spreading centre osc as it propagated southward is associated with a large 1 km s 1 negative upper and mid crustal velocity anomaly this anomaly is consistent with the presence of an unusually thick extrusive section within the basin and with tectonic alteration fracturing and shearing arising from rotation of the basin as it was formed the discordant zone left by this osc on the cocos plate is characterized by moderately low crustal velocities probably because of crustal fracturing as the osc propagated into older crust higher crustal velocities near segment centres may reflect a higher ratio of dikes to extrusives in the upper crust and lower intensity tectonic alteration of the crust than near segment ends the mean crustal thickness along the epr between the siqueiros and clipperton fzs is 6 7 6 8 km the thickest crust is found beneath the lamont seamounts 9 km and in a southward pointing v shaped band located just north of the off axis trace of the 9 03 n osc 7 3 7 8 km the thinnest crust 6 km is found proximal to the clipperton and siqueiros fzs the crust associated with the off axis trace of the 9 03 n osc is not anomalously thin suggesting that magma supply beneath the osc is similar to that of the northern and southern segments we see a similar pattern of crustal thickness variation to that determined using multichannel reflection data including a gradual thickening of the crust from north to south along the northern ridge segment and the location of the thickest crust just north of the 9 03 n osc however the magnitude of the along axis crustal thickness variation we observe along the northern ridge segment between 9 50 n and 9 15 n 1 3 1 8 km excluding the lamont seamounts is significantly less than the 2 3 km of variation previously reported weakening the case for the existence of a low density mantle diapir at 9 50 n inferred from gravity data the band of thick crust located just north of the off axis trace of the 9 03 n osc suggests a close genetic link between this feature and the osc thus we attribute the pattern of crustal thickness variations along the northern segment to the kinematics of the southward propagating 9 03 n osc over the past 0 5 myr and not to along axis melt migration away from a mantle diapir as previously proposed

applications of texture attribute analysis to 3d seismic data in this study texture attribute analysis application to 3d surface seismic data is presented this is done by choosing a cubic texel texture element from the seismic data to generate a gray level occurrence matrix which in turn is used to compute second order statistical measures of textural characteristics the cubic texel is then successively made to glide through the 3d seismic volume to transform it to a plurality of texture attributes application of texture attributes to two case studies from alberta confirm that these attributes enhance understanding of the reservoir by providing a clearer picture of the distribution volume and connectivity of the hydrocarbon bearing facies in the reservoir

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

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

fine scale segmentation of the crustal magma reservoir beneath the east pacific rise mid ocean ridges are composed of segmented faults and magma reservoirs seismic images from the east pacific rise show that the magma reservoirs are segmented on the same fine scale as the surface faults and that distinct lava eruptions are sourced from largely isolated magma lenses

olivine enigma why alteration controls the seismic properties of oceanic gabbros 1 the objective of this study was to determine whether the effects of low mg content and or cracks that typically populate olivine grains can explain why p and s wave velocities in samples of oceanic gabbro do not increase with increasing olivine content the mg content of olivine in these rocks is known to be appreciably less than fo90 and optical sem and electron microprobe analyses indicate that the olivine is actually an aggregate of olivine and 5 15 crack fill consisting of talc and or serpentine plus magnetite voigt average models show that there is a strong positive correlation between p wave velocities and fo90 content but no correlation with fo73 because of its mg content olivine that is typical of oceanic gabbros does not influence seismic velocities adding 11 alteration of olivine to serpentine plus magnetite in the form of crack fill to the fo73 model yields a nearly exact match between the model and measured p and s wave velocities the fact that the crack fill consists largely of talc and or serpentine indicates that these minerals formed by hydrothermal alteration in young hot 150 c lower crust hence our results apply to gabbros in situ as well as to the laboratory samples seismic velocities in lower oceanic crust produced at high and intermediate spreading rates are controlled principally by the degree of hydrothermal alteration of the gabbros that compose the lower crust

the origin of the upper palaeozoic chanaral melange of n chile the fragmented and mixed strata of the chanaral melange extend for some 220 km from n to s in the coastal ranges of n chile 26 20 to 28 20 s the melange was produced in an accretionary wedge resulting from the ne subduction of the ancestral pacific oceanic plate beneath the gondwana land margin during carboniferous times the melange consists essentially of blocks of sandstone in a pelitic matrix these sediments were deposited as deep sea basin plain turbidites prior to their fragmentation other blocks in the melange include mafic volcanic rocks with the geochemical characteristics of oceanic island basalts and rare shallow marine limestones structures in the melange suggest an origin by two distinct deformation processes involving unlithified sediments initial boudinage and break up of the strata was apparently accomplished by intrastratal movement resulting from imbricate thrusting within the accretionary wedge the second process produced cross cutting zones of breccia which are interpreted as fluid escape conduits resulting from the high pore pressures produced during underthrusting

on the cause of the asymmetric distribution of seamounts about the juan de fuca ridge ridge crest migration over a heterogeneous asthenosphere abstract the distribution of non hotspot seamounts in the northeast pacific is highly asymmetric small seamount chains and isolated edifices are numerous on the pacific plate but nearly absent on the juan de fuca plate we propose a hypothesis for the asymmetric generation of seamounts near a ridge axis in which upwelling and early melting of upper mantle heterogeneities occurin advance of a spreading centre which migrates with respect to the asthenospheric frame of reference plate motion solutions indicate that the juan de fuca ridge is migrating to the west at a ridge perpendicular rate of 20 mm a 1 which is large compared to the half spreading rate of 30 mm a 1 migration of the ridge axis will result in the initiation of upwelling in the upper mantle in advance of the spreading centre if slightly enriched lower melting temperature heterogeneities are present in the upper mantle they will intersect their solidus at a greater depth and will begin to melt earlier than the host peridotite seamount volcanism will occur preferentially on the pacific plate because of two factors firstly more asthenosphere and hence more early melt heterogeneities must ascend to supply the pacific plate which is translating more rapidly than the relatively stationary juan de fuca plate secondly the asthenosphere that is required to ascend to supply the thickening of the juan de fuca plate will have been flushed of its significant shallow level heterogeneities by the previous advance and passing of the ridge few should remain to cause volcanism on the juan de fuca plate testable physical and petrologic predictions of this model can be identified for example within a small seamount chain the age difference between individual seamounts and the crust on which they lie will decrease toward the ridge axis variation in lava chemistry along a small seamount chain which is interpreted to represent magmatism from a single large heterogeneity will provide the major discriminant of this hypothesis systematic variations should be observed with earlier formed edifices being constructed of more enriched lavas and later younger ones of more depleted lavas isotope ratios should be unaffected by varying degrees of partial melting but could display increased dilution towards the ridge axis as melting of the host mantle and magma mixing become significant

variations in thickness of layer 3 dominate oceanic crustal structure abstract seismic refraction studies have determined that while much of the crust underlying the world s oceans is about 7 km thick it also occurs in lesser and considerably greater thicknesses here we show that vastly different thicknesses of oceanic crust formed at ridges by seafloor spreading are generally characterized by the two gradient velocity structure recognized as typical of 6 7 km thick oceanic crust adopting a standard we study ninety published seismic velocity structures of oceanic crust ranging in thickness from 2 to 37 km several structures formed in midplate settings ocean islands and seamounts are also considered for comparison regardless of origin ridge or midplate we find that the percentage of the whole crust formed by layer 2 systematically decreases with increasing total crustal thickness while the average velocity of layer 2 varies widely for all crust the average velocity of layer 3 increases systematically with increases in the average whole crustal thickness and velocity crust formed in midplate settings differs in that the thickness of layer 2 velocity crust is generally two to three times that of crust formed at ridges residual depths were calculated for oceanic crust by removing the effects of sediment loading and subsidence due to lithospheric cooling most of the data fall close to a model whereby different crustal thicknesses result from different extents of partial melting of oceanic upper mantle the crustal sections were restored to zero age assuming the average depth of emplacement for 7 km thick oceanic crust is 2 5 km below sea level from this restoration it appears that the depth to the top of layer 3 at the time it was formed is 4 25 1 25 km below sea level regardless of whole crustal thickness or spreading rate these results suggest that the mechanism of construction of oceanic crust is in many ways remarkably uniform despite very large changes in the total thickness of the crust produced very basic questions remain it is for instance not at all clear why layer 2 and layer 3 exist in the proportions they do while neutral buoyancy or melt trapping at the base of a brittle lid may account for local properties of the layer 2 3 boundary they do not seem adequate to account for the global phenomenon

crustal thickness and moho character of the fast spreading east pacific rise from 9 42 n to 9 57 n from poststack migrated 3 d mcs data we computed crustal thickness 5740 270 m and mapped moho reflection character using 3 d seismic data covering 658 km2 of the fast spreading east pacific rise epr from 9 42 n to 9 57 n moho reflections are imaged within 87 of the study area average crustal thickness varies little between large sections of the study area suggesting regionally uniform crustal production in the last 180 ka however individual crustal thickness measurements differ by as much as 1 75 km indicating that the mantle melt delivery has not been uniform third order but not fourth order ridge discontinuities are associated with changes in the moho reflection character and or near axis crustal thickness this suggests that the third order segmentation is governed by melt distribution processes within the uppermost mantle while the fourth order ridge segmentation arises from midcrustal to upper crustal processes in this light we assign fourth order ridge discontinuity status to the debated ridge segment boundary at 9 45 n and third order status at 9 51 5 n to the ridge segment boundary previously interpreted as a fourth order discontinuity our seismic results also suggest that the mechanism of lower crustal accretion varies along the investigated section of the epr but that the volume of melt delivered to the crust is mostly uniform more efficient mantle melt extraction is inferred within the southern half of our survey area with greater proportion of the lower crust accreted from the axial magma lens than that for the northern half this south to north variation in the crustal accretion style may be caused by interaction between the melt sources for the ridge and the lamont seamounts

faulting and extension rate over the last 20 000 years in the offshore whakatane graben new zealand continental shelf oblique rifting in the offshore taupo volcanic zone new zealand is expressed in widely distributed active normal faulting in the 20 km wide whakatane graben active faults are identified along seafloor scarps and displacements of the post last glacial transgressive ravinement surface 400 faults we derive an extension rate at seismogenic depths 6 10 km across the graben of 13 6 mm yr by summing surface measurements assuming an average crustal fault dip of 45 15o and correcting for the discrepancies between surface and deep crustal extension estimates structural and kinematic data implies an extension direction 20o oblique to the rift axis resulting in up to 4 6 2 1 mm yr of dextral motion parallel to the rift axis the strike slip motion is accommodated by dip slip displacements on oblique faults in the centre of the graben and oblique slip faulting along the rift margins pure dip slip in the graben centre represents 50 of the total slip with the rangitaiki fault accommodating 25 of the total extension in the graben

overlapping spreading centres 3 d inversion of the magnetic field at 9 03 n on the east pacific rise summary overlapping spreading centres oscs represent a new type of plate boundary interaction in which en echelon rise segments overlap significantly and are not joined by a transform fault a three dimensional fourier inversion of the magnetic field was performed on an overlapping spreading centre to remove the effects of topography and ridge orientation a magnetic high exists at the tip of one of the two ridge segments forward modelling suggests that the anomalous magnetic field cannot be attributed to the effects of topography alone the inversion reveals the existence of a magnetization high at the tip of the eastern spreading centre maximum magnetization values are consistent with ones obtained in other high amplitude zones in the pacific as well as with the measured magnetization of samples dredged in the same areas we suggest that the magnetization high over the eastern ridge tip of the 9 03 n osc is associated with highly evolved basalts enriched in iron and titanium such enrichment may be caused by enhanced crystal fractionation within an axial magma chamber which is intermittent and occasionally freezes as the eastern spreading axis propagates into older lithosphere

analogue modelling of orthogonal and oblique rifting abstract the three dimensional progressive development of orthogonal and oblique rift systems has been simulated using scaled analogue sandbox models the orientation of the zone of rifting where angle between the rift axis and the extension direction was varied systematically from 90 orthogonal rifting to 75 60 and 45 oblique rifting completed models were impregnated and sectioned both vertically and horizontally orthogonal rift models are characterized by long straight rift border faults and intra rift faults perpendicular to the extension direction oblique rift models are characterized by shorter segmented rift border faults and intrarift faults in the oblique models the rift border faults formed en echelon arrays parallel to the inderlying zone of basement stretching whereas the intra rift faults formed at a high angle to the extension vector increased extension in the moderately oblique and oblique rift systems 60 and 45 caused the intra rift faults to rotate towards parallelism with the rift border faults no strike slip or oblique slip transfer faults were found in any experiments rift border faults and intra rift faults in the models are segmented along strike with relay ramp structures formed between overlapping fault tips along the strike of the rift variations in half graben polarities and offsets of depocentres are accomplished by accommodation zones formed by interlocking conjugate fault arrays the experimental fault patterns compare well with the fault architectures found in those modern or relatively young rift systems where well defined basement controlled strike slip or oblique slip transfer faults are absent an accommodation zone model consisting of interlocking conjugate extension faults is proposed for the along strike switching of half graben asymmetries and offsetting of depocentres in rift systems the fault geometries in the analogue models can be used as templates for recognition of fault styles in both orthogonal and in oblique rift environments

segmentation of mid ocean ridges attributed to oblique mantle divergence mantle flow beneath mid ocean ridges was thought to respond passively to plate motions analysis of seismic data from ridges reveals a skew between the directions of plate motion and mantle flow implying mantle flow may stress the plates

upper crustal vp vs ratios at the endeavour segment juan de fuca ridge from joint inversion of p and s traveltimes implications for hydrothermal circulation

seismic structure of the endeavour segment juan de fuca ridge correlations with seismicity and hydrothermal activity 1 multichannel seismic reflection data collected in july 2002 at the endeavour segment juan de fuca ridge show a midcrustal reflector underlying all of the known high temperature hydrothermal vent fields in this area on the basis of the character and geometry of this reflection its similarity to events at other spreading centers and its polarity we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface the endeavour magma chamber reflector is found under the central topographically shallow section of the segment at two way traveltime twtt values of 0 9 1 4 s 2 1 3 3 km below the seafloor it extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends on cross axis lines the axial magma chamber amc reflector is only 0 4 1 2 km wide and appears to dip 8 36 to the east while a magma chamber underlies all known endeavour high temperature hydrothermal vent fields amc depth is not a dominant factor in determining vent fluid properties the stacked and migrated seismic lines also show a strong layer 2a event at twtt values of 0 30 0 09 s 380 120 m below the seafloor on the along axis line and 0 38 0 09 s 500 110 m on the cross axis lines a weak moho reflection is observed in a few locations at twtt values of 1 9 2 4 s below the seafloor by projecting hypocenters of well located microseismicity in this region onto the seismic sections we find that most axial earthquakes are concentrated just above the magma chamber and distributed diffusely within this zone indicating thermal related cracking the presence of a partially molten crustal magma chamber argues against prior hypotheses that hydrothermal heat extraction at this intermediate spreading ridge is primarily driven by propagation of a cracking front down into a frozen magma chamber and indicates that magmatic heat plays a significant role in the hydrothermal system morphological and hydrothermal differences between the intermediate spreading endeavour and fast spreading ridges are attributable to the greater depth of the endeavour amc and the corresponding possibility of axial faulting

the late palaeozoic to early mesozoic evolution of southern south america a plate tectonic interpretation the limits of 3 distinct upper palaeozoic to lower mesozoic tectono stratigraphic zones of the ancestral pacific margin of gondwana in southern south america are documented these are a fore arc region consisting in part of oceanic components a magmatic arc that is coincident with carboniferous to triassic continental sedimentation and a back arc region comprising the upper palaeozoic epicratonic sequences of the samfrau geosyncline these zones traceable 2300 km from 29 56 s cape horn document the semi continuous subduction of the ancestral pacific floor from the middle devonian to the triassic between 29 s and 37 s the palaeo subduction zone was roughly coincident with the present pacific coast line between 37 s and cape horn the subduction zone migrated progressively westward as an alaskan style accretionary prism grew to a width of 250 km while in the northern sector jurassic are intrusions are coincident with the upper palaeozoic are intrusions in the s they intruded the upper palaeozoic to lower mesozoic accretionary prism 200 km w of their palaeozoic equivalents no post middle palaeozoic sutures have been identified in southern south america hence there is no evidence of late palaeozoic mesozoic or cenozoic accretion of discrete microplates

prediction of 3 d fluid permeability and mudstone distributions from ground penetrating radar gpr attributes example from the cretaceous ferron sandstone member east central utah a 3 d fluid permeability distribution is estimated inside a channel sandstone reservoir analog in the cretaceous ferron sandstone fluvio deltaic complex in east central utah from ground penetrating radar gpr attributes fluid permeability measurements at 5 cm spacing along four boreholes and one pseudohole section at the adjacent cliff face are used together with instantaneous amplitude and frequency attributes of gpr data to predict fluid permeabilities away from the measured vertical transects and to delineate the distribution and geometry of mudstone layers inside the reservoir analog statistically significant relationships are determined between the well data fluid permeability and mudstone content and the gpr attributes these calibrations are applied to the entire gpr volume to estimate the 3 d fluid permeability variation and the lateral development of mudstone units measured and predicted fluid permeabilities range from 0 1 to 290 md one of the five units considered contained no mudstone lay

a 100 m y long window onto mass flow processes in the patagonian mesozoic subduction zone diego de almagro island chile diego de almagro island was formed by the subduction and accretion of several seafloor derived tectonic slices with very heterogeneous ages and pressure temperature time p t t paths the highest element of the pile the lazaro unit evidences subduction in the high p granulite field 1 3 gpa 750 c at ca 163 ma below it a thin tectonic sliver the garnet amphibolite unit preserves eclogite facies remnants 570 c and 1 7 gpa formed at ca 131 ma in situ u pb zircon rim ages peak assemblages were nearly fully amphibolitized during decompression down to 1 2 gpa and 600 c at 125 120 ma rb sr multimineral dating the underlying blueschist unit has 50 m y younger metamorphic ages and exhibits slightly cooler peak burial conditions 520 c 1 7 gpa ca 80 ma in situ white mica ar ar ages and multimineral rb sr dating and is devoid of amphibolitization the mylonites from the sinistral strike slip seno arcabuz shear zone bounding diego de almagro island to the east also exhibit amphibolite facies 620 c and 0 9 gpa deformation at ca 117 ma multimineral rb sr ages in situ white mica ar ar dating and multimineral rb sr dating of low t mylonites 450 c along the base of the lazaro unit reveal partial resetting of high t assemblages during tectonic displacement between 115 and 72 ma and exhumation of the slice stack detrital zircon u th pb ages indicate that the material accreted on diego de almagro island has been mostly recycled from a permian triassic accretionary wedge madre de dios accretionary complex exposed along the subduction buttress geological and geochronological constraints suggest that the rocks of the seno arcabuz shear zone and the lazaro unit were tectonically eroded from the buttress while the underlying garnet amphibolite and blueschist units instead derive from the subducted oceanic basin with increasingly younger maximum depositional ages the very long residence time of the rocks 90 m y for the lazaro unit along the hanging wall of the subduction interface recorded long term cooling along the patagonian subduction zone during the mesozoic diego de almagro island therefore represents a unique window onto long term tectonic processes such as subduction interface down stepping tectonic erosion and episodic underplating near the base of an accretionary wedge 40 50 km

mantle upwelling magmatic differentiation and the meaning of axial depth at fast spreading ridges since the fi rst systematic mapping of the east pacifi c rise its axial depth profi le has been used to infer variability in magma supply or mantle temperature here however we show that segment scale rise parallel undulations of both on and off axis seafl oor depth result pri marily from variations in the bulk density of oceanic crust using seismic images of crustal and upper mantle structure combined with gravity data and lava chemistry we demonstrate that segment scale variations in crustal density are caused by magmatic differentiation rise parallel changes in magmatic differentiation are attributed to a skew between the axes of mantle upwelling and plate spreading we conclude that segmentation of axial depth along fast spreading ridges is controlled primarily by the geometry of mantle upwelling

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

a seismic swarm and regional hydrothermal and hydrologic perturbations the northern endeavour segment february 2005 the february 2005 swarm at the overlapping spreading center osc on the northern end of the endeavour segment is the first swarm on the juan de fuca ridge recorded on a local seafloor seismic network the swarm included several larger earthquakes and caused triggered seismicity and a hydrothermal response in the endeavour vent fields as well as regional scale hydrologic pressure perturbations the spatial and temporal pattern of over 6000 earthquakes recorded during this seismic sequence is complex small magnitude events dominate and seismicity rates wax and wane indicating a magmatic process the main swarm initiates at the northern end of the endeavour ridge however most of the moment release including six strike slip events occurs in the southwest endeavour valley where the swarm epicenters generally migrate south the main swarm is contemporaneous with a hydrologic pressure response at four sealed seafloor boreholes 25 105 km away we infer that the seismic sequence is driven by a largely aseismic magma intrusion at the northern endeavour axis resulting stress changes trigger slip on tectonic faults and possibly dike propagation at the opposing limb of the endeavour osc in the southwest endeavour valley consistent with the eventual decapitation of the endeavour by the west valley segment furthermore 2 5 days after the start of the main swarm seismicity is triggered beneath the endeavour vent fields and temperature increases at a diffuse vent in the mothra field we infer that this delayed response is due to a hydrologic pressure pulse that diffuses away from the main magma intrusion

surface rupture earthquakes over the last 1000 years in the wellington region new zealand and implications for ground shaking hazard the wellington region is cut by five active right lateral strike slip faults wairarapa wellington ohariu shepherds gully pukerua and wairau faults that have average recurrence intervals of meter scale surface rupture that range from 500 years to 5000 years and lateral slip rates that range from 1 to 10 mm yr only the wairarapa fault has ruptured since european settlement since circa a d 1840 paleoseismological studies on these faults have allowed the compilation of a complete record of surface rupture events over the past 1000 years in the wellington region within this time period there does not appear to be any temporal clustering of surface rupture events on adjacent faults the m 8 a d 1855 wairarapa earthquake did not trigger rupture on any other fault in the region the most recent surface faulting event on the wellington fault 290 440 cal years b p cal years are calendar years before a d 1950 does not coincide with rupture of any other onland fault and over 300 years separate the timing of the second most recent rupture on the wellington fault 660 720 cal years b p and the most recent rupture of the ohariu fault 1060 1140 cal years b p the most recent rupture of the shepherds gully pukerua fault is probably older than that of the ohariu fault the apparent nonclustering of surface rupture earthquakes in the wellington region has been documented only for the on land strike slip faults there are other possible seismogenic sources in the region and thus important issues remain to be addressed regarding the history of large earthquakes in the wellington region 1 the seismogenic potential and earthquake recurrence interval of the subduction thrust beneath wellington is not known 2 the timing of rupture events on the offshore portion of the wairau fault is not known and 3 paleoseismic data are not available for the section of the wellington fault north of the wellington hutt valley segment estimates of earthquake hazard in the wellington region for all return times greater than 50 years that incorporate paleoseismicity data are between one and two modified mercalli mm intensity units higher than the hazard based solely on the historical seismicity catalog and the hazard is spatially more variable using a deterministic attenuation model the level of shaking hazard approaches near maximum values within a return time of 500 years largely reflecting the recurrence interval 500 770 years of surface rupture earthquakes on the wellington fault inclusion of a plausible model for magnitude 8 subduction zone earthquakes does not affect the level of mm intensity in wellington region at return times greater than 500 years but does make a small contribution to the hazard at return times between 50 and 500 years

interpretation of reflection attributes in a 3 d gpr survey at vallee d ossau western pyrenees france to study the attributes of reflected radar waves a 3 d ground penetrating radar gpr data set was recorded processed with promax 3 d seismic processing software and analyzed with stratimagic 3 d seismic interpretation software the zone studied was an old glacial valley vallee d ossau western pyrenees where fluvio glacial deposits 8 10 m thick overlay a karstic limestone bedrock reflections were picked to 250 ns 9 m depth with velocity 0 07 m ns analysis of the isochron map of the bedrock surface reflector showed north south undulations corresponding to the displacement of the old glacier together with sharp discontinuities interpreted as local faults or karstic zones amplitude variations along the reflector were interpreted as being from water saturation and the nearby north south alignment of springs at the surface in the fluvio glacial deposits paths of old channels were also imaged using amplitude analysis the radar wave amplitude attenuation was also characterized within the d

influence of pressure and mineralogy on seismic velocities in oceanic gabbros implications for the composition and state of the lower oceanic crust 1 we have analyzed the relationship between p wave velocities measured at pressures of 40 100 and 200 mpa and modal mineralogy in oceanic gabbro samples from ocean drilling program holes 735b 894g and 923a at all pressures increasing velocities correspond to increasing pyroxene contents and decreasing alteration phyllosilicate and amphibole content but indicate little or no variation of olivine content in part because the olivine compositions are in the range fo65 to fo73 a voigt reuss hill inverse model reveals that the effective bulk density and elastic moduli of olivine in the gabbros are low even relative to fo73 possibly because the olivine grains contain ubiquitous networks of cracks even if the cracks are not present in situ on average gabbros with velocities typical of seismic layer 3 6 7 7 0 km s 1 contain 5 15 alteration products including 5 15 amphibole and 0 5 phyllosilicates these results suggest that the lower crust is on average slightly to moderately altered

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

visualization of active faults using geometric attributes of 3d gpr data an example from the alpine fault zone new zealand three dimensional ground penetrating radar gpr data are routinely acquired for diverse geologic hydrogeologic archeological and civil engineering purposes interpretations of these dataareinvariablybasedonsubjectiveanalysesofreflectionpatterns such analyses are heavily dependent on interpreter expertiseandexperience usingdataacquiredacrossgravelunitsoverlyingthealpinefaultzoneinnewzealand wedemonstratethe utilityofvariousgeometricattributesinreducingthesubjectivity of3dgprdataanalysis weuseacoherence basedtechniqueto compute the coherency azimuth and dip attributes and a graylevel co occurrence matrixglcmmethod to compute the texture basedenergy entropy homogeneity andcontrastattributes a selection of the gpr attribute volumes allows us to highlight key aspects of the fault zone and observe important features not apparent in the standard images this selection also provides information that improves our understanding of gravel deposition andtectonicstructuresatthestudysite anewdepositional structuralmodellargelybasedontheresultsofouranalysisofgprattributes includes four distinct gravel units deposited in three phases and a well defined fault trace this fault trace coincides with a zone of stratal disruption and shearing bound on one side by upward tilted to synclinally folded stratified gravels and on the other side by moderately dipping stratified alluvial fan gravelsthatcouldhavebeenaffectedbylateralfaultdrag whenused in tandem the coherence and texture based attribute volumes can significantly improve the efficiency and quality of 3d gpr interpretation especially for complex data collected across activefaultzones

termination of a 6 year ridge spreading event observed using a seafloor seismic network on the endeavour segment juan de fuca ridge 1 we present automatically determined epicenters and magnitudes for 36 523 earthquakes recorded along the endeavour segment between august 2003 and october 2006 using a local ocean bottom seismometer obs network the catalog is dominated by two swarm sequences in january and february 2005 in the vicinity of the endeavour overlapping spreading center which included earthquakes in west valley the northern portion of the endeavour segment southwest endeavour valley and the endeavour vent fields these swarms are attributed to volcanism including a dike intrusion on the northern endeavour in february 2005 and smaller diking events on the propagating tip of the west valley segment in both swarms the dike on the northern endeavour propagated to the south which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment following the swarms seismic activity on the endeavour segment decreased on average to 15 of pre swarm values and almost ceased at the segment ends we infer that a 6 year non eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate spreading stresses the inferred coupling between the 1999 and 2005 events the observation of extensive precursory activity prior to the 2005 swarms and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation hydraulic diffusion and magma withdrawal and replenishment

a detailed study of the cobb offset of the juan de fuca ridge evolution of a propagating rift the cobb offset on the northern portion of the juan de fuca ridge has been identified as the tip of a northward propagating rift hey and wilson 1982 map compilations of magnetic and seismic data from four new cruises define the present locus of spreading and volcanism on the two ridge segments abutting the offset and permit detailed modeling of the recent evolution within this transform zone the axis of recent spreading on the southern ridge segment bends from the normal ridge trend n20 e to a n s trend north of 47 15 n the spreading axis on the northern ridge segment generally defines a n20 e trend except at the southern terminus where the spreading center is offset slightly to the east the two spreading centers overlap by about 33 km in the offset vicinity and there is evidence of recent volcanism on both segments present ridge axis morphology exhibits a transitional sequence from a symmetrical axial high along the more normal portions of each ridge segment to a grabenlike depression as the tip is approached the magnetic anomaly patterns observed in the cobb offset vicinity are not consistent with the patterns predicted by models of continuous northward propagation the magnetic anomaly patterns of the brunhes epoch require an event of rapid northward propagation about 0 7 m y b p followed by a more gradual southward propagation in the middle brunhes epoch most recently the spreading center on the southern ridge has extended northward to its present configuration prior to the brunhes epoch modeling of the magnetic anomaly patterns does not indicate a unique solution however net propagation has been northward we present alternative models for the period beginning 1 7 m y b p in the first model the cobb offset has evolved by a series of northward and southward events of propagation with net advance to the north in the second model stable asymmetric spreading from overlapping ridge segments has evolved into a transform fault offset by gradual decay of the northern ridge both models indicate that propagation has been discontinuous and relatively rapid compared to spreading rate and that realignment of ridge segments near the propagator tip has been an important component

near axis crustal structure and thickness of the endeavour segment juan de fuca ridge a model of crustal thickness and lower crustal velocities is obtained for crustal ages of 0 1 1 2 ma on the endeavour segment of the juan de fuca ridge by inverting travel times of crustal paths and non ridge crossing wide angle moho reflections obtained from a three dimensional tomographic experiment the crust is thicker by 0 5 1 km beneath a 200 m high plateau that extends across the segment center this feature is consistent with the influence of the proposed heckle melt anomaly on the spreading center the history of ridge propagation on the cobb overlapping spreading center may also have influenced the formation of the plateau the sharp boundaries of the plateau and crustal thickness anomaly suggest that melt transport is predominantly upward in the crust lower crustal velocities are lower at the ends of the segment likely due to increased hydrothermal alteration in regions influenced by overlapping spreading centers and possibly increased magmatic differentiation

stratigraphy and geochemistry of accreted fragments of the ancestral pacific floor in southern south america abstract the outer pacific archipelago of southern chile has allochthonous fore arc rocks that were accreted prior to the late jurassic along the ancestral pacific margin of the southern south american sector of gondwana the fore arc rocks in the archipielago madre de dios 50 50 47 s are divisible into three assemblages the tarlton limestone a sequence of massively bedded upper carboniferous to lower permian limestone the duque de york complex a sequence of flysch like sedimentary rocks and the denaro complex an association of pillow basalt bedded chert detrital calcareous rocks and shale the denaro complex is interpreted as a sequence of the late paleozoic ancestral pacific floor the tarlton limestone may represent a shallow water carbonate platform also built on a basaltic substrate located within the ancestral pacific realm the basalt units of the denaro complex have tholeiitic major element patterns similar to those of modern day mor and within plate basalts chemical data for the denaro complex allow for the partial discrimination of biogenic hydrothermal hydrogenous and detrital components within the sedimentary units and facilitate comparison with modern day sediments as well as with other ophiolite sequences chemical data together with stratigraphic and structural field relations suggest that the evolution of the denaro complex and tarlton limestone involved seafloor spreading hydrothermal activity seamount volcanism carbonate reef development and convergence of the oceanic realm with the margin of gondwana

three dimensional seismic structure of the mid atlantic ridge 35 n evidence for focused melt supply and lower crustal dike injection 1 we gathered seismic refraction and wide angle reflection data from several active source experiments that occurred along the mid atlantic ridge near 35 n and constructed three dimensional anisotropic tomographic images of the crust and upper mantle velocity structure and crustal thickness the tomographic images reveal anomalously thick crust 8 9 km and a low velocity bull s eye from 4 to 10 km depth beneath the center of the ridge segment the velocity anomaly is indicative of high temperatures and a small amount of melt up to 5 and likely represents the current magma plumbing system for melts ascending from the mantle in addition at the segment center seismic anisotropy in the lower crust indicates that the crust is composed of partially molten dikes that are surrounded by regions of hot rock with little or no melt fraction our results indicate that mantle melts are focused at mantle depths to the segment center and that melt is delivered to the crust via dikes in the lower crust our results also indicate that the segment ends are colder receive a reduced magma supply and undergo significantly greater tectonic stretching than the segment center

gravity and seismic study of crustal structure along the juan de fuca ridge axis and across pseudofaults on the ridge flanks variations in topography and seismic structure are observed along the juan de fuca jdf ridge axis and in the vicinity of pseudofaults on the ridge flanks left by former episodes of ridge propagation here we analyze gravity data coregistered with multichannel seismic data from the jdf ridge and flanks in order to better understand the origin of crustal structure variations in this area the data were collected along the ridge axis and along three ridge perpendicular transects at the endeavor northern symmetric and cleft segments negative mantle bouguer anomalies of 21 to 28 mgal are observed at the axis of the three segments thicker crust at the endeavor and cleft segments is inferred from seismic data and can account for the small differences in axial gravity anomalies 3 7 mgal additional low densities elevated temperatures within and or below the axial crust are required to explain the remaining axial mba low at all segments gravity models indicate that the region of low densities is wider beneath the cleft segment gravity models for pseudofaults crossed along the three transects support the presence of thinner and denser crust within the pseudofault zones that we attribute to iron enriched crust on the young crust side of the pseudofaults a 10 20 km wide zone of thicker crust is found reflection events interpreted as subcrustal sills underlie the zones of thicker crust and are the presumed source for the iron enrichment

the spreading rate dependence of three dimensional mid ocean ridge gravity structure we analyze over 1300 km of high resolution along axis gravity profiles at ridges with half spreading rates ranging from 1 2 to 5 5 cm yr the results show consistently higher along axis gradients of mantle bouguer anomaly at the slow spreading mid atlantic ridge mar 0 3 1 2 mgal km than at the intermediate to fast spreading cocos nazca ridge and east pacific rise epr 0 1 0 2 mgal km the regional peak to trough amplitude of mantle bouguer anomaly is also greater along the mar 30 60 mgal than the cocos nazca ridge and the epr 10 20 mgal with increasing spreading rate the regional peak to trough amplitude of axial seafloor depth decreases from 1000 1700 m to 200 700 m 3 d numerical experiments suggest that mantle contributions to the gravity can be significant only near large offset transforms at the more commonly observed non transform offsets gravity anomalies will reflect crustal thickness variations the along axis gravity data thus indicate that the amplitude of along axis crustal thickness variation decreases with increasing spreading rate we propose that this spreading rate dependent crustal accretion style may originate in the mantle finite amplitude mantle upwelling is intrinsically plume like 3 d beneath a slow spreading ridge but more sheet like 2 d beneath a fast spreading ridge such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate small amplitude 3 d upwellings may occur at a fast spreading ridge but their effects on crustal thickness variations will be significantly reduced by along axis melt flows along a persistent low viscosity crustal magma chamber in contrast the large crustal thickness variations due to 3 d mantle upwellings will be maintained at a slow spreading ridge because less along axis melt flows can occur in the colder and more rigid crust there

constraints on rift propagation history at the cobb offset juan de fuca ridge from numerical modeling of tectonic fabric abstract rift propagation has been proposed as the mechanism for the reorganization of the juan de fuca ridge during the cenozoic in particular magnetic anomalies in the cobb offset region of this ridge have been interpreted in terms of a complex dueling propagator history in which rift propagation has occurred alternately in opposing directions on adjacent ridge segments although the overall propagation history has been inferred from the magnetic data these data are limited in their resolution for the detailed history by the length of time during which magnetic polarity remains constant we thus examine whether more detailed resolution within polarity chrons can be obtained using seafloor tectonic fabric from acoustic imagery by modeling lineations as isochrons synthetic fabric can be generated for a given rift propagation history and the fit of the model to the data can be compared numerically for the cobb we used a propagation history model with a given timing and direction of propagation and generated synthetic fabric for different propagation rates the fabric is best fit for propagation rates between 400 and 900 mm yr slower 900 mm yr propagation rates yield slightly poorer fits the fabric results are thus in accord with the interpretation based on magnetics alone that the rates of rift propagation significantly exceeded the spreading rates an interesting feature of the model is that the complex tectonic fabric is reasonably well fit on this scale 1 km by simple rift propagation models which do not invoke shear zone behavior and thus deviate from rigid plate tectonics

3 d seismic discontinuity for faults and stratigraphic features the coherence cube seismic data are usually acquired and processed for imaging reflections this paper describes a method of processing seismic data for imaging discontinuities e g faults and stratigraphic features one application of this nontraditional process is a 3 d volume or cube of coherence coefficients within which faults are revealed as numerically separated surfaces figure 1 compares a traditional 3 d reflection amplitude time slice with the results of the new method to our knowledge this is the first published method of revealing fault surfaces within a 3 d volume for which no fault reflections have been recorded

rift topography linked to magmatism at the intermediate spreading juan de fuca ridge new seismic observations of crustal structure along the juan de fuca ridge indicate that the axial rift topography reflects magma induced deformation rather than alternating phases of magmatism and tectonic extension as previously proposed contrary to predictions of the episodic models crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2 1 2 6 km the shallow rift valley or axial graben associated with each juan de fuca segment is 50 200 m deep and 1 8 km wide and is well correlated with a magma body in the subsurface analysis of graben dimensions height and width shows that the axial graben narrows and graben height diminishes where the magma body disappears rather than deepening and broadening as expected for rift topography due to tectonic extension we propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor in this model an evolving axial topography results from feedbacks between the rheology of the crust above a magma sill and dike intrusion rather than episodic magma delivery from the mantle

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

the gondwana glaciation in chile description of alleged glacial deposits and paleogeographic conditions bearing on the extension of the ice cover in southern south america abstract during the late paleozoic three main elevated areas can be recognized in southern south america the protoprecordillera the asuncion height and the present coastal area of uruguay and of the buenos aires province the protoprecordillera located along the western margin of gondwana and extending at least from present southern bolivia to the tip of the continent represented a major source for clastic deposits accumulated in continental intramontane and adjacent marine basins the septosyringothyris sea to the west the chaco mesopotamia basin to the east and the salta bolivia basin to the northeast the asuncion height located in present northeastern argentina and paraguay had a more reduced extension and separated the chaco mesopotamia and the salta bolivia basins to the west from the parana basin to the east the elevated area along the coastal area of uruguay and of the buenos aires province bounded the parana basin to the east breccia deposits along the north central chilean coast originally considered to be of glacial origin have no typical glacial features and no similarities with the gondwanan glacial deposits in argentina bolivia and brazil they may be of triassic age and may correspond to detritus covering an erosion surface cliff deposits as well as fore arc basin deposits the alleged glacial deposits of the el toco and la caleta formations have ages that preclude a glacial origin related to the gondwana glaciation further studies are needed to define the age and origin of the diamictites of puchuncavi the available paleomagnetic data for south america indicate that 1 during the late paleozoic central chile and western argentina were located at distances greater that 30 latitude from the south pole 2 during the early and middle carboniferous the present chilean coast had an approximately e w orientation and during the late carboniferous and early permian a ne sw orientation the proximity of these areas to the late paleonzoic south pole do not demonstrate the existence of a glaciation the fossil faunal and floral evidence in the late paleozoic deposits suggests general moderate to cold but not polar humid climates with seasonal variations on the basis of this paleogeographic setting mountain glaciations can be postulated for the protoprecordillera and the asuncion height the east margin of the parana basin and part of it was covered by the margin of a polar ice calotte centered in centered south africa these glaciations are responsible for the existence of abundant diamictite deposits known in the late paleozoic of southern south america the protoprecordillera formed a barrier for the humid oceanic westerly winds and separated two main climatic domains a milder rainy coastal domain and a more dry continental type eastern domain there is no evidence supporting the existence of land emerging from the septosyringothyris sea along the present chilean coastal area the existence of late paleozoic glacial covers in southern south america can be attributed to two main factors the relative movement of gondwana and the south pole and the existence of elevated areas the existence of a regional polar glaciation is thus rejected for the western part of southern south america

structure and metamorphism of the gondwanan basement in the bariloche region north patagonian argentine andes in the bariloche region the gondwanan basement consists of metamorphic and deformed igneous rocks intruded by mesozoic and tertiary granitoids metasediments and amphibolites have been affected by three main deformation events d1 d2 and d3 d1 s1 foliation is well shown preserved in the microlithons of the s2 regional foliation and in albite and garnet porphyroblasts both foliations have been folded by upright open folds with associated crenulation s3 locally developed in the d3 folds hinge zones an early stage syn d1 subduction episode can be invoked based on evidence of high pressure hp metamorphism in schists and amphibolites as well as on the presence of pre collisional i type granitoids d2 d3 developed under regional intermediate pressure low pressure ip lp metamorphic conditions high temperature ht conditions were reached as deduced from regional migmatization and the intrusion of syn to late orogenic s type granitoids the transition from hp to ip metamorphic conditions could be associated with the emplacement of ne directed crustal scale thrusts during d2

waveform inversion of seismic refraction data and applications to young pacific crust summary we present a method for constraining the velocity depth structure in the earth using seismic refraction waveform data we test the method with synthetic data from known models and apply it to a set of data collected in 1982 june from the east pacific rise at 13 n from the magma expedition in this iterative process wkbj seismograms are computed for a starting model the difference between these and the observed seismograms is used to update the model subject to physical constraints an important first step in the inverse scheme is the linearization of the wkbj seismogram equation allowing us to compute differential seismograms partial derivatives of the synthetic seismogram with respect to specific model parameters this linearization provides the means for estimating required model perturbations based on the misfit in the seismograms the choice of a suitable numerical strategy for computing an updated model is a crucial second step in formulating a working algorithm because the data contain noise synthetic seismograms can only fit the data to this noise level in this case infinitely many models fit the data to this tolerance and some of these estimates are non physical involving negative layer thicknesses a successful strategy must choose from among these possibilities a well defined physically reasonable new model in a commonly used approach to solving non linear problems the perturbation to the starting model is minimized while improving the fit to the data after several iterations the final model which possesses no special properties still tends to resemble the starting model when used with the magma data this technique essentially does not perturb the model at all a method we find much more satisfactory involves solving for the new model directly while applying physically important constraints as constraints we require the velocity gradient remain below a fixed value and penalize the roughness of the new model we thus solve for the smoothest model fitting the data to the specified misfit

explorer deformation zone evidence of a large shear zone and reorganization of the pacific juan de fuca north american triple junction recently collected multibeam bathymetry and hydroacoustic earthquake data are used to investigate the recent tectonics of the explorer juan de fuca ridge systems the boundaries between the pacific and northern juan de fuca plates the bathymetric and seismic evidence presented is consistent with a zone of shear extending well south of the sovanco fracture zone to include the heck and heckle seamounts and potentially as far south as the springfield seamounts and cobb offset along the juan de fuca ridge this indicates that the triple junction between the pacific juan de fuca north american plates may be reorganizing southward to establish at the cobb offset

shallow geometry and displacements on the san andreas fault near point arena based on trenching and 3 d georadar surveying 1 to characterize buried portions of the san andreas fault saf near its northern extremity in california three trenches were excavated perpendicular to it and a 3 d georadar data set was acquired similar sequences of unconsolidated fluvial sediments franciscan serpentinite and active fault zones were observed in the three trenches fault traces in the trenches were initially assumed to be part of a single saf strand however the georadar data demonstrated the existence of two saf strands one of which was exposed in the central and southern trenches and the other in the northern trench offset of a linear trending georadar feature possibly a fluvial paleochannel suggests that 4 5 5 5 m of horizontal displacement occurred across the eastern saf strand either during the 1906 san francisco earthquake or earlier georadar reflections from flat lying and trough shaped sedimentary boundaries are vertically offset by 0 2 0 3 m along both fault strands

upper crustal seismic structure of the endeavour segment juan de fuca ridge from traveltime tomography implications for oceanic crustal accretion the isotropic and anisotropic p wave velocity structure of the upper oceanic crust on the endeavour segment of the juan de fuca ridge is studied using refracted traveltime data collected by an active source three dimensional tomography experiment the isotropic velocity structure is characterized by low crustal velocities in the overlapping spreading centers oscs at the segment ends these low velocities are indicative of pervasive tectonic fracturing and persist off axis recording the history of ridge propagation near the segment center velocities within the upper 1 km show ridge parallel bands with low velocities on the outer flanks of topographic highs these features are consistent with localized thickening of the volcanic extrusive layer from eruptions extending outside of the axial valley that flow down the fault tilted blocks that form the abyssal hill topography on axis velocities are generally relatively high beneath the hydrothermal vent fields likely due to the infilling of porosity by mineral precipitation lower velocities are observed beneath the most vigorous vent fields in a seismically active region above the axial magma chamber and may reflect increased fracturing and higher temperatures seismic anisotropy is high on axis but decreases substantially off axis over 5 10 km 0 2 0 4 ma this decrease coincides with an increase in seismic velocities resolved at depths 1 km and is attributed to the infilling of cracks by mineral precipitation associated with near axis hydrothermal circulation the orientation of the fast axis of anisotropy is ridge parallel near the segment center but curves near the segment ends reflecting the tectonic fabric within the oscs

gravity anomalies and crustal thickness variations along the mid atlantic ridge between 33 n and 40 n multibeam bathymetry and gravity data have been obtained along an 800 km long section of the mid atlantic ridge from just south of the hayes fracture zone at 33 n to the northern edge of the azores platform near 40 n a three dimensional analysis of these gravity and topography data combined with results from earlier seismic refraction studies in this area reveal two different scales of crustal heterogeneity a systematic along axis segment scale 8 9 km near the middle of spreading segments and the thinnest crust 3 4 km near segment offsets the magnitude of this along axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset there also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs inferred to be thinner crust beneath the inside corner highs adjacent to these offsets this segment scale crustal heterogeneity is similar to that reported from the kane to atlantis section of the mid atlantic ridge and from parts of the intermediate spreading southern mid atlantic ridge it is superimposed on a second longer wavelength variation in gravity and crustal thickness associated with the azores hot spot the most pronounced effect of the azores hot spot on the mid atlantic ridge occurs between 38 n and 40 n where the ridge axis rapidly shoals by more than 1000 m the crust thickens by over 2 km and the rift valley largely disappears the absence of a deep axial rift valley on the azores platform and near the midpoints of some ridge segments along the mid atlantic ridge to the south agrees well with the predictions of recent ridge crest thermal and rheological models that suggest a dependence of axial morphology on both crustal thickness and spreading rate the transition from a rift valley to an axial high morphology at these spreading rates 11 mm yr half rate occurs at a crustal thickness of about 9 1km

the portland hills fault uncovering a hidden fault in portland oregon using high resolution geophysical methods abstract the portland metropolitan area historically is the most seismically active region in oregon at least three potentially active faults are located in the immediate vicinity of downtown portland with the portland hills fault phf extending directly beneath downtown portland the faults are poorly understood and the surface geologic record does not provide the information required to assess the seismic hazards associated with them the limited geologic information stems from a surface topography that has not maintained a cumulative geologic record of faulting in part due to rapid erosion and deposition from late pleistocene catastrophic flood events and a possible strike slip component of the faults we integrated multiple high resolution geophysical techniques including seismic reflection ground penetrating radar gpr and magnetic methods with regional geological and geophysical surveys to determine that the portland hills fault is presently active with a zone of deformation that extends at least 400 m the style of deformation is consistent with at least two major earthquakes in the last 12 15 ka as confirmed by a sidehill excavation trench high resolution geophysical methods provide detailed images of the upper 100 m across the active fault zone the geophysical images are critical to characterizing the structural style within the zone of deformation and when integrated with a paleoseismic trench can accurately record the seismic history of a region with little surface geologic exposure

evidence of displaced terranes from permian to jurassic faunas around the pacific margins for the permian to jurassic time interval tethyan marine invertebrate faunas which are generally accepted as having lived at low latitude can be distinguished from less diverse higher latitude faunas displacement of these low latitude faunas to positions of high latitude around the pacific margins therefore provides key evidence for the movement of so called suspect or displaced terranes the fullest story so far has been worked out for the western margin of n america as far north as southern alaska but there is also convincing evidence for the substantial northward movement of continental segments along the ne asian margin as regards the southern hemisphere the torlesse terrane of new zealand appears to have moved a considerable distance southwards but there is no fauna1 evidence of comparable far travelled suspect terranes along the andean margin of south america not only tethyan and boreal but e and w pacific elements are detectable in the faunas and the distribution of these argues for a panthalassia studded with islands of various sizes fringing the adjacent continents rather than for the disintegration of a single continent pacifica

surface breakthrough of a basement fault by repeated seismic slip episodes the ostler fault south island new zealand 1 the ostler fault is one of the major active reverse faults in the piedmont of the southern alps se of the alpine fault we present a new geological and morphotectonic map of the southern ostler fault integrated with two seismic reflection profiles across the active central segments of the fault segmented subparallel scarps define a n s belt 40 km long and 2 3 km wide of pure reverse faults which upthrow and back tilt a panel of plio pleistocene terrestrial units 2 4 1 0 ma plus the overlying glacial outwash 200 ka uplift gradients the chronology of newly faulted markers and tectonically controlled diversion of paleodrainages all indicate progressive s to n breakthrough of the surface trace of the ostler fault in the last 2 4 ma the new seismic data define a main fault segment dipping 50 60 w to depths of 1 5 km with a vertical throw of 800 m and a shortening of 30 the fault geometry and kinematics and the subsurface data favor the interpretation that the ostler fault propagated updip across the plio quaternary terrestrial sequence as the emerging high angle splay of an inherited late cretaceous paleocene normal fault that underwent repeated cycles of compressional reactivation in the last 2 4 ma

seismological study of lau back arc crust mantle water magmatic differentiation and a compositionally zoned basin abstract on the basis of seismic tomography analyses a structural model of crust formed along the eastern lau spreading center elsc is presented as evidence for a transition from a hydrous type of oceanic crust to a more typical oceanic crust the seismic data indicate that as the spreading center moved away from the active arc the crust thinned from 8 9 km to 7 km the lower crust changed from high p wave velocity values 7 2 7 4 km s to typical values for oceanic crust 7 0 7 2 km s and the upper crustal volcanic layer changed from a thick low velocity layer to a thinner layer with more typical wave speeds the seismic results in combination with other geophysical and geochemical data suggest that crustal formation along the elsc is strongly controlled by the influence of slab water when a spreading center is near the active arc water from the downgoing slab is entrained in the melting zone beneath the ridges where it enhances melting thereafter the water enhances crustal differentiation within sub ridge magma chambers this creates an anomalous hydrous form of oceanic crust with a thick felsic volcanic layer and a mafic ultramafic lower crust features that are not typically observed in crust formed at mid ocean ridges the lau basin has a zoned structure with an abrupt transition from this type of oceanic crust to more typical oceanic crust which resulted from a rapid change in the influence of slab water as the ridge moved away from the arc the abundance and high rate of production of the hydrous crust suggests that such crust may make up a significant proportion of the arc like crust that forms continents

paleomagnetic evidence for a very large counterclockwise rotation of the madre de dios archipelago southern chile a paleomagnetic study of late paleozoic exotic units exposed in the madre de dios archipelago and diego de almagro island southern chile is reported these units correspond to the tarlton limestone and the metabasaltic pillow lavas of the denaro complex nineteen sites were drilled 14 in limestone and five in lavas standard demagnetization procedures show that the primary remanence is not present in the investigated rocks preventing the determination of the original latitudinal position of these rocks however a post tectonic secondary magnetic component was isolated in eight sites at the madre de dios archipelago in both limestone and basaltic pillow lavas rock magnetic experiments and microscopic observations indicated that the carrier of this remanence is secondary titanomagnetite in the limestone and titanomagnetite titanomaghemite in the lavas the most likely event to produce this secondary magnetization is the nearby intrusion of the south patagonian batholith in the early cretaceous mean direction from eight sites shows a very large 117 7 29 9 counterclockwise rotation of the study area with negligible paleolatitude anomaly this rotation is only constrained as post early cretaceous restoration of the structures to their pre rotation position indicates that accretion of the exotic block took place from the northwest and not from the south as previously proposed this is more consistent with a low latitude origin for the limestone and the general paleogeographic framework different tectonic mechanisms and ages for this large rotation are possible

interaction between the chile ridge and chile trench geophysical and geothermal evidence geophysical and geothermal data are examined from the three southernmost sections of the chile ridge starting at 44 s and continuing south to the triple junction of the nazca antarctic and south america plates at 47 s these sections represent three progressively younger stages in a ridge trench collision event corresponding to 3 m y before the collision 1 m y before the collision and culminating in an ongoing collision at the triple junction magnetic and bathymetric data across the chile rise indicate that there is little change in the configuration of the spreading center as the ridge approaches and then collides with the trench in the collision zone a normal looking rift valley can be traced for 40 km before it disappears beneath the toe of the landward trench slope there is no evidence for the complex pattern of ridge jumps and spreading center rotations that occurred when the pacific farallon spreading center collided with north america in contrast the overriding plate is greatly affected by the ridge collision the landward trench slope steepens and narrows as the collision zone is approached at the southern end of the collision zone a ridge associated with the taitao fracture zone is colliding with the trench and may be in the process of being obducted onto the landward trench slope geothermal measurements were made along three transects of the margin corresponding to the time 3 m y before the collision during the collision and 6 m y after the collision the heat flow measurements in the collision zone document a large pulse of heat associated with the subduction of the ridge values as high as 345 mw m2 were recorded on the lower trench slope a prominent bottom simulating reflector bsr observed over a wide area of the landward trench slope north of the triple junction and more locally south of the triple junction is used to expand the grid of heat flow observations excellent agreement is found between measured heat flow and estimates of heat flow based on the depth to the bsr the heat flow measurements compare favorably with a theoretical model assuming conductive heat flow we estimate that the accretionary prism has been substantially removed in the collision zone and conclude that the landward trench slope is undergoing an episode of rapid tectonic erosion periodic ridge collisions in the past may account for the apparent truncation of the andean forearc region

characterization of the shallow structures of active fault zones using 3 d ground penetrating radar data 1 where they can be correlated with geological exposures and trenches 3 d ground penetrating radar gpr data can contribute critical subsurface information to paleoseismic investigations because active faults are typically characterized by complicated near surface structures that vary with the styles of faulting and the types of rock that are ruptured gpr data can be difficult to interpret we have acquired 3 d gpr data sets across three active fault zones within new zealand that have different deformation styles the strike slip wellington fault zone reverse faults of the ostler fault zone and normal faults of the maleme fault zone to improve our interpretation of the processed gpr volumes we employed two suites of geometric attributes the first suite was computed using a coherence based algorithm it provided estimates of the coherency azimuth and dip of reflections the second suite quantified the volumetric textures of reflections which allowed different reflection facies to be defined objectively we have demonstrated how some attributes were more successful at visualizing certain structural or depositional characteristics than others for example the coherency attribute was an excellent tool for highlighting normal faults within volcanic deposits of the maleme fault zone whereas the texture based attributes were most useful for discriminating between the gravel and metasediment units juxtaposed by the wellington fault zone our gpr data sets and associated attribute volumes showed details of near surface fault geometry that were not obvious from surface mapping and they revealed evidence of off fault deformation gravitational collapse and topple structures

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data