the 1978 brooks peninsula vancouver island earthquakes a pair of mw 5 5 earthquakes occurred beneath the brooks peninsula on the northwest coast of vancouver island on 2 june and 25 july 1978 these are the largest and best recorded earthquakes to date in the vicinity of northern vancouver island and the adjacent coast margin a detailed study of these earthquakes was undertaken to examine the contemporary tectonics of this region specifically the poorly understood interaction between the explorer plate and the north american plate at the northern end of the cascadia subduction zone both earthquakes were followed by well defined aftershock sequences a four station temporary seismograph array deployed on the brooks peninsula following the 2 june mainshock allowed for accurate aftershock locations this earthquake was located at 50 15 n 127 84 w based on the center of a 9 km diameter circular region of aftershocks the 25 july earthquake was located 4 to 7 km to the northeast of the june epicenter based on waveform comparison of the two events both earthquakes occurred at 15 to 16 km depth the focal mechanisms as determined from body wave modeling are nearly identical and show left lateral strike slip motion along a shallow north dipping east west striking fault the focal mechanism and depth of these two earthquakes indicates that they were not megathrust events on the explorer north america plate boundary but rather that they occurred within the north american plate 5 to 10 km above the megathrust the northeast directed pressure axes for these earthquakes suggests coupling across the explorer north america segment of the cascadia subduction zone consistent with contemporary convergence of the explorer plate with the north american plate in a northeast southwest direction

plate tectonics at an awkward junction rules for the evolution of sovanco ridge area ne pacific summary the elegant geometrical rules of plate tectonics do not allow for a gradual shift in plate motion directions or the gradual as opposed to sudden cessation of subduction at the scale of the small plates in the ne pacific imperfections in boundary processes have a large effect on the net torque on the plates and heavily influence the evolution of the geometry in this area the rotation of the spreading directions and the diminution of true subduction along the southern canadian coast has not occurred by the sudden switching of plate motions from one stable condition to another instead it appears as if the dominant factor for the evolution is the resistance of the ocean floor to formation of new smoothly slipping transform faults compressive deformation of even young lithosphere is not only mechanically unlikely but is not helpful to the particular configurations found in this area instead a migrating shear zone and an episode of highly en echelon spreading along a new axis nearly perpendicular to the present juan de fuca ridge have resulted the present sovanco ridge was never a transform fault neither is the nootka fault a shear zone but the locus of stretching between plates whose motions are congruent at the juan de fuca ridge but diverge toward the continental margin

pacific north america plate boundary reorganization in response to a change in relative plate motion offshore canada 1 the transition from subduction in cascadia to the transform queen charlotte fault along western canada is often drawn as a subduction zone yet recent studies of gps and earthquake data from northern vancouver island are not consistent with that model in this paper we synthesize seismic reflection and gravity interpretations with microseismicity data in order to test models of 1 microplate subduction and 2 reorganization of the preexisting strike slip plate boundary we focus on the critical region of outer queen charlotte sound and the adjacent offshore on much of the continental shelf several million years of subsidence above thin crust are a counterindicator for subduction an undated episode of compression uplifted the southernmost shelf but subsidence patterns offshore show that recent subduction is unlikely to be responsible previously unremarked near vertical faults and a mix of extensional and compressional faults offshore indicate that strike slip faulting has been a significant mode of deformation seismicity in the last 18 years is dominantly strike slip and shows large amounts of moment release on the revere dellwood fault and its overlap with the queen charlotte fault the relative plate motion between the pacific and north american plates rotated clockwise 6 ma and appears to have triggered formation of an evolving array of structures we suggest that the paleo queen charlotte fault which had defined this continental margin retreated northward as offshore distributed shear and the newly formed revere dellwood fault propagated to the northwest

the winona basin structure and tectonics a compilation of published and new geophysical data from the winona basin off northern vancouver island has allowed a detailed interpretation of the sedimentary and tectonic history of the region to be made the basin is forming as a result of the asymmetric subsidence of a recently isolated lithospheric block that is slowly converging with the continental margin the crust beneath the basin is young 1 5 ma increasing in age from southeast to northwest and of normal oceanic thickness it is virtually non magnetic however probably because of its having been rapidly buried by turbidite sedimentation subsidence of the basin and uplift of the paul revere ridge began in the early pleistocene ca 1 8 ma and since that time up to 8 km of turbidite sediments has accumulated in the basin the nature of the fanning of the deposits suggests that the basin has been kept full throughout its history the minimum average supply rate necessary to accomplish this is about 70 106 mg year 1 this pleistocene ave

thermal conductivity measurement of rock fragments using a pulsed needle probe a heat pulse generated inside a needle probe can be used to measure the thermal conductivity of surrounding rock fragments or drill cuttings as the pulse of heat is conducted outwards into the surrounding aggregate of rock fragments and water the decrease in temperature inside the probe is recorded as a function of time an asymptotic relation between probe temperature conductivity and inverse time since the heat pulse is shown to be accurate for the range of times used the relatively slow thermal response of the probe in samples with higher conductivities is accommodated by a delay in the origin time of the pulse the combined correction for finite pulse length and slow probe response is shown to be small and predictable the thermal conductivity of rock fragments is calculated from a model that is dependent on the water content of the sample as in other methods results using this method and a divided bar apparatus are equivalent given the expected accuracy of a divided bar the measured thermal

crustal thickness beneath the queen charlotte basin canada results of a seismically constrained gravity inversion a gravity inversion routine was used to calculate crustal thickness variations in the queen charlotte basin western canada first a crustal bouguer anomaly was derived using a digital well constrained depth to basement map and densities determined from well logs and rock samples this anomaly was subsequently inverted for crustal thickness assuming a single density contrast at depth and an average crustal thickness provided by seismic refraction data the results restricted to that portion of the basin for which there are good quality data show crustal thickness variations of the order of 12 km regions of thin crust are located off northern vancouver island 20 km along the axis of hecate strait and in northern queen charlotte sound 24 km thicker crust is associated with central queen charlotte ridge and northeastern hecate strait tests show these trends in crustal thickness prevail for all reasonable ranges of input parameters calculated residual gravity anomalies contain information useful for mapping the distribution of volcanics interbedded with sediments in subbasins and for refining estimates of depth to basement derived from seismic data alone our results are for the most part in excellent agreement with seismically derived estimates of moho depth they show minimum crustal thickness beneath regions of maximum sediment accumulation supporting an extensional origin for the basin they indicate that this extension was quite heterogeneous and yield minimum estimates which vary from 13 in some parts of the basin to as much as 65 in others the results can be used to discriminate between models on the tertiary evolution of the basin the observed trends are not predicted by the lithospheric flexure or simple margin perpendicular extension models but are consistent with a model of crustal thinning by distributed shear in a transtensional regime

continental signature of a ridge trench triple junction northern vancouver island south of the juan de fuca pacific north america triple junction located off northern vancouver island there is subduction and to the north there is oblique convergence along the queen charlotte transform fault in the mid tertiary periods of extension including crustal thinning and volcanism formed queen charlotte basin heat flow on northern vancouver island 67 mw m2 is the same as in queen charlotte basin to the north but it decreases abruptly to the south over a distance of 70 km to an average value of 46 mw m2 compared to the rest of the island the northern island has more subdued topography a lower mean elevation slightly higher bouguer gravity values and lower seismicity late tertiary volcanics which occur only on the northern part of the island geochemically resemble the tertiary masset formation which formed during periods of extension in queen charlotte basin these data are consistent with a tertiary history including periods of extension for northern vancouver island similar to queen charlotte basin and distinguish it from the rest of vancouver island the southern boundary of this northern block extends northeastward from brooks peninsula approximately above the inferred northern edge of the subducted juan de fuca plate and parallel to a series of linear magnetic anomalies associated with a northeasterly trending fault zone the abrupt transition in observed heat flow is most likely associated with the edge of the subducted plate which produces large temperature differences at depths of 30 40 km the time constant for these thermal effects to reach the surface conductively indicates that this boundary has been stable over tens of millions of years and therefore the location of the subducting plate s northern edge must have been stationary or moving very slowly over this period consequently it is likely that the triple junction has been located on average for the last 40 myr off brooks peninsula

evidence for two zones of debris entrainment beneath the greenland ice sheet investigation of the ice debris and stable isotope characteristics of basal ice exposed at the margin of the greenland ice sheet provides a means of testing and refining models of ice sheet erosion and deposition the basal ice exposed at the ice sheet margin gives evidence of the processes operating at the bed at different locations which can be used to constrain theoretical models predicting varying basal conditions1 2 improved understanding of the effects of isotope fractionation in hydrogen and oxygen caused by freezing processes at the bed which modify the isotope characteristics of the basal ice layers enables us to report evidence of two separate zones of debris entrainment beneath the ice sheet

seismic structure across the active subduction zone of western canada the vancouver island seismic project was conducted in 1980 to study the structure of the subducting oceanic juan de fuca plate and the overriding continental america plate the principal seismic refraction line line i was a 350 km onshore offshore profile perpendicular to the continental margin an array of 32 receivers was located on the america plate on the mainland and across vancouver island and extended offshore with three ocean bottom seismometers obs s two shots were fired at the eastern end of the line and 17 shots were located along the westernmost 100 km of the profile control for the interpretation of the onshore offshore profile was provided by a reversed refraction profile along the length of vancouver island and by a marine refraction profile recorded on the obs s modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two dimensional media the major features of the refraction structural model are that 1 the oceanic lithosphere dips at 3 or less beneath the continental slope so the bend in the subducting slab occurs landward of the foot of the slope 2 the oceanic lithosphere dips at 14 16 beneath the continental shelf until it passes beneath the continental moho at 37 km depth below western vancouver island 3 an upper mantle reflector may correspond to the base of the subducting lithosphere and 4 a segment of high velocity material above the downgoing crust with velocity 7 7 km s and depth range 20 25 km may represent a remnant of subducted lithosphere perhaps detached when the subduction zone jumped westward to its present position

queen charlotte area cenozoic tectonics and volcanism and their association with relative plate motions along the northeastern pacific margin cenozoic relative plate interactions along the queen charlotte margin based on published models for the north america and northeast pacific plates are related to the tectonic and igneous history of the region plate motion models indicate convergence and subduction prior to the eocene since that time there has been transcurrent motion with varying small amounts of oblique extension or compression assuming that the pacific america farallon triple junction was to the south of the region a late miocene or early pliocene about 4 ma to present period of oblique convergence is well resolved the present queen charlotte fault zone along the west coast of the islands may have initiated more recently within the cenozoic transcurrent regime a period of oblique extension in the mid tertiary 36 20 ma in one model is supported by a variety of volcanic and tectonic evidence a small plate motion change at about 20 ma may have resulted in the transfer of the yakutat terrane from the north america plate to the pacific plate and the resulting motion of the terrane northwestward to its present position along the margin of alaska the onset of tertiary masset volcanism that is extensive on the queen charlotte islands corresponds within a few million years to the time of major plate reorganization at 43 ma that has tectonic expression around the entire pacific basin the period of most extensive masset volcanism and plutonism appears to correlate with the model oblique extension in the mid tertiary for one model the geochemistry and physical volcanology of the masset volcanic s are indicative of an extension regime the main syntectonic deposition in the queen charlotte basin skonun formation as inferred from seismic reflection and well data also appears temporally correlated with this time interval of oblique extension a variety of data including plate models dikes normal faults basin subsidence crustal thickness from seismic refraction and present and paleo heat flow indicate mid tertiary crustal extension of at least 20 in the queen charlotte region with up to 150 up to 2 5 in the main queen charlotte basin posttectonic basin subsidence and deposition may correlate with the model time interval of general transcurrent or oblique convergence motion from 20 to 4 ma more recent shortening deformation observed in outcrop and seismic sections of the northern parts of the basin may correlate with plate model oblique convergence from 4 ma to the present the latter convergence is associated with underthrusting that formed a trough or trench and an accretionary sedimentary prism off the west coast of the queen charlotte islands and with inferred uplift and erosion of the western part of the islands

current tectonics of northern cascadia from a decade of gps measurements 1 global positioning system gps measurements to study regional deformation were initiated in northern cascadia in the late 1980s and early 1990s on the basis of a decade of gps data we derive a crustal velocity field for nw washington sw british columbia the permanent and campaign gps velocities are defined with respect to north america in the itrf2000 reference frame velocity uncertainties are estimated using a model of time series noise spectra this new velocity field is the basis for interpretation of the tectonics of the northern cascadia subduction system gps velocities are interpreted in terms of interseismic loading of the megathrust using different coupling models our data confirm that the upper part of the megathrust is nearly fully locked an exponential model for the downdip transition zone gives slightly better agreement with the data compared to the common linear transition the landward decrease of forearc strain loading is smaller than predicted by any of the current subduction interseismic models this could be a consequence of a small 0 3 mm yr long term motion of the southern vancouver island forearc with respect to north america or of a concentration of interseismic strain across the elastically weaker cascadia volcanic arc in northern vancouver island our velocity field supports the existence of an independent explorer microplate currently underthrusting underneath north america at least up to brooks peninsula further north gps velocities indicate transient and or permanent deformation of northernmost vancouver island related to the interaction with the explorer microplate and possibly with the queen charlotte transform margin

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

a microseismicity study of the queen charlotte islands region nineteen land and three ocean bottom seismographs were operated in the queen charlotte islands region for periods of up to 9 weeks and 5 days respectively during the summer of 1983 three hundred and seventeen seismic events were detected one hundred and nine earthquakes ranging in size from magnitude 0 5 to 5 1 were well recorded at three or more stations and could be accurately located of these 84 lie on or close to the queen charlotte fault most within the rupture zone of the great 1949 earthquake ms 8 1 the seismic gap between the rupture zones of the 1949 event and the 1970 earthquake ms 7 4 that occurred just south of the queen charlotte islands exhibited little activity eighteen earthquakes the largest with ml 3 8 were located east of the fault on northern graham island or in adjacent hecate strait focal depths were generally less than 20 km and none could be associated with known faults composite focal mechanism solutions were obtained for four suites of earthquakes along

seismotectonics of the explorer region 1 the explorer region offshore western canada is a tectonically complex area surrounded by the pacific north america and juan de fuca plates existing tectonic models for the region differ fundamentally proposed plate configurations range from multiple independent plate fragments to an explorer plate now fused to north america along the continental margin and cut by pacific north america transform faults in the west we present new seismological data constraining the region s current tectonics we use three component regional waveforms to determine the source parameters of 84 earthquakes with magnitude greater than 4 combined with 34 harvard centroid moment tensor solutions they represent the region s largest earthquake source parameter data set obtained by robust waveform modeling techniques in addition we perform joint epicenter determination to relocate larger earthquakes recorded since 1918 the source parameters and improved locations provide a consistent tectonic picture earthquake slip vector azimuths along the pacific plate boundary change smoothly and are significantly less northerly oriented than the pacific north america plate motion direction requiring an independent explorer plate the present day pacific explorer boundary is formed by transform faults subparallel to the revere dellwood wilson fault plate motion vectors indicate that the winona block is part of the explorer plate current explorer motion is more northerly than indicated by magnetic anomalies prior to 2 ma implying a recent change possibly coinciding with a northwestward ridge jump near explorer plate s northern end transferring the winona block from the pacific to the explorer plate in response to these plate motion changes the region north of the western sovanco fracture zone was assimilated into the pacific plate the region around the eastern sovanco fracture zone characterized by broadly distributed seismicity is composed of well defined sets of conjugate faults bounding rotating crustal blocks earthquake fault strikes agree with the dominant northwest southeast fault sets however the conjugate sets must be also active to fully accommodate present day explorer plate motion the sw portion of the strike slip nootka fault zone the explorer juan de fuca plate boundary is well defined by focused seismicity however its full extent under nootka island remains unresolved the explorer north america boundary shows sporadic low magnitude seismicity our explorer north america rotation pole predicts convergence varying from negligible at the boundary s northwest end to 2 cm yr at the se end this convergence can be accommodated either by subduction or by crustal thickening extending to the north american continent we favor subduction based on low deformation rates observed by onshore gps sites the present explorer plate system configuration is a result of stepwise reorientation of the explorer ridge system each step successively reducing the subduction rate relative to north america

subsidence and thermal history of queen charlotte basin a tectonic model for the formation subsidence and thermal history of queen charlotte basin is developed based upon regional geological and geophysical data subsidence data from offshore wells in hecate strait and queen charlotte sound and thermal criteria derived from present heat flow and vitrinite reflectance information queen charlotte basin is seen to have resulted from two distinct mechanisms 1 during a period of broad regional uplift rifting and crustal extension occurred in queen charlotte sound up to about 17 ma ago and the queen charlotte islands were displaced northwards toward their present position by transcurrent motion along the louscoone inlet sandspit fault system the rifting generated a significant thermal anomaly and a restricted deep basin as a consequence of crustal thinning and subsequent thermal cooling 2 beginning about 6 ma ago oblique underthrusting commenced along the margin resulting in flexural uplift of the western part of the queen charlotte islands and comp

integrated geophysical modelling of terranes and other structural features along the western canadian margin an integrated geophysical data set has been used to develop structural models across the continental margin west of vancouver island canada a modern accretionary complex underlies the continental slope and shelf and rests against and below the allochthonous crescent and pacific rim terranes these terranes in turn abut against the pre tertiary wrangellia terrane that constitutes most of the island gravity and magnetic anomaly data constrained by seismic reflection seismic refraction and other data were interpreted to determine the offshore positions of these terranes and related features iterative 2 5 dimensional forward models of anomaly profiles were stepped laterally along the margin to extend areal coverage over a 70 km wide swath oriented normal to the tectonic features an average model was then developed to represent this part of the margin the pacific rim terrane appears to be continuous and close to the coastline along the length of vancouver island consistent with emplacement by strike

structure beneath queen charlotte sound from seismic refraction and gravity interpretations a combined multichannel seismic reflection and refraction survey was carried out in july 1988 to study the tertiary sedimentary basin architecture and formation and to define the crustal structure and associated plate interactions in the queen charlotte islands region simultaneously with the collection of the multichannel reflection data refractions and wide angle reflections from the airgun array shots were recorded on single channel seismographs distributed on land around hecate strait and queen charlotte sound for this paper a subset of the resulting data set was chosen to study the crustal structure in queen charlotte sound and the nearby subduction zone two dimensional ray tracing and synthetic seismogram modelling produced a velocity structure model in queen charlotte sound on a margin parallel line moho depth was modelled at 27 km off southern moresby island but only 23 km north of vancouver island excluding the approximately 5 km of the tertiary sediments the crust in the latter area is onl

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

migration characteristics of seismic tremors in the northern cascadia margin 1 seismic waveforms from a dense regional array were analyzed to study episodic tremor and slip ets events in northern cascadia the april 2005 episode occurred mostly in northern vancouver island vi where the explorer plate subducts beneath the north america plate while the may 2005 episode was mainly in middle vi all tremor occurrences are positively correlated with the presence of subducted slabs besides the steady along strike migration and halting behavior found in previous ets events a new pattern jumping is found this time visual inspection of previous seismic records suggests that the jumping behavior is probably common to ets tremors in all regions of vi the majority of tremors occurred between 25 and 45 km depth in places where local seismicity is sparse or absent our observations are compatible with the interpretation that fluids released from dehydration of subducted materials are involved in tremor generation

petrologic and geologic variations along the southern explorer ridge northeast pacific ocean mid ocean ridge basalts morb from the well defined southern explorer ridge segment ser in the northeast pacific ocean are moderately to strongly enriched in incompatible elements enriched morb were erupted at the highest and widest part of the segment culmination where the magma supply may be the greatest and also at the northern and southern ends of the ser variations in basalts incompatible element enrichment occur over short distances and suggest that the underlying mantle is heterogeneous on a small scale the variations also preclude the existence of a long lived well mixed magma chamber beneath the robust culmination instead magma chambers are probably temporally and spatially isolated as evidenced by the presence of highly differentiated lavas less enriched transitional morb were erupted along the central part of the ser 11 27 km south of the culmination and were supplied to the ridge separately from a less enriched pan of the mantle a more continuous magma chamber cannot be ruled out for this section on geochemical grounds all of the morb from the ser have undergone significant amounts of fractional crystallization unlike certain segments of the east pacific rise the most evolved morb were erupted near the culmination of the ridge segment aphyric lavas have been recovered only from the vicinity of the culmination while phyric lavas have erupted along the entire length of the ser there is no correlation between composition and total crystal abundances despite the requirement of multiple parental magmas most of the lavas from the culmination and the distal ends of the ridge fall on a common liquid line of descent indicating that they formed at similar depths and extents of partial melting the transitional morb from the central part of the ridge have probably formed by lesser extents of partial melting and separated from their mantle source at greater depths based on abundances of na and moderately incompatible elements such as sm and zr enriched high melt fraction lavas at the magma starved southern end of the ser closely resemble lavas from the culmination either the southernmost lavas have flowed laterally within the crust from near the culmination or there is another melting anomaly that supplies the southern end of the ser if the latter case is true correspondence between ridge morphology and mantle thermal state is poor

the neogene alert bay volcanic belt of northern vancouver island canada descending plate edge volcanism in the arc trench gap abstract the alert bay volcanic belt trends northeasterly across northern vancouver island coincident with the trace of the subducted juan de fuca explorer plate edge volcanism began in the west at brooks peninsula about 8 ma ago but occurred in most centers 3 5 1 ma ago there is a suggestion of eastward migration of activity and shift from basalt to dacite or rhyolite with time most of the volcanism was coincident with a time of rapid changes in the geometry of subduction as inferred from offshore magnetic patterns and with a hiatus in mainland cascade volcanic arc activity geometry and chronometry suggest this is a descending plate edge volcanic belt where disruption of steady state plate consumption patterns triggered magma genesis chemically the rocks are quite variable with divergent fractionation trends one trend resembles that of mull hebrides with a plagiophyric basalt of transitional alkaline subalkaline mildly tholeiitic and aluminous character which differentiated to clinopyroxene andesite and eventually to tholeiitic rhyolite and mildly tholeiitic calc alkaline dacite both of k poor magma type the other trend is like the cascades with aluminous aphyric calc alkaline basalt hornblende and or hypersthene andesite and k poor dacite this divergent character is also evident in ba rb nb and zr fractionation trends major and trace element discriminant diagrams generally identify the basalts as within plate types the 87 sr 86 sr isotope ratio is relatively low averaging 0 70325 and shows no trend with rock type or differentiation series oxygen in the entire suite is relatively heavy 18 o averaging 7 1 even the basalts are 18 o enriched oxygen shows no trend with degree of hydration rock type or series these isotopic and chemical data are compatible with minor crustal contamination of mafic primary magmas followed by fractional crystallization under different oxidation and hydration conditions

high velocities and seismic anisotropy in pleistocene turbidites off western canada summary the winona basin is a young deep water basin along the base of the continental slope off central british columbia which is filled primarily with turbidite sediments seismic reflection profiles in the basin indicate that in places the sediment thickness is greater than 4 0 s two way travel time and simple interpretations of seismic refraction data indicate that velocities up to 6 0 kni s exist within the sediment section because the turbidite sediments are acoustically layered with large velocity contrasts between layers the sediments are probably highly anisotropic a simple model to account for this has been used to estimate the errors involved in using refraction data to determine layer thicknesses and average velocities if the effects of anisotropy are ignored as they usually are values for thickness and velocity may be overestimated by as much as 10 20 per cent even the corrected velocities are still high however and the inferred high densities and high degrees of lithification are remarkable when the youth of the sediments is considered nearly the entire depositional history is contained within the pleistocene and the age of the sediments whose velocity exceeds 4 km s l is estimated to be only 1 o myr old corrected velocities in the deepest part of the basin where the sediment section is over 6 kin thick exceed 5 km s the nature of samples dredged from the basin mudstones to coarse grained sandstones and conglomerates suggests a mechanism for initial diagenesis the mudstones are moderately porous 40 60 per cent and possess little carbonate whereas the coarser grained rocks are all well cemented with calcium carbonate with carbonate contents ranging up to 37 per cent measured porosities are as low as 8 per cent but porosities calculated on a carbonate free basis range from 35 to 55 per cent it appears that the fine grained sediments are first consolidated primarily by gravitational and or tectonic compaction and the coarsegrained sediments are cemented by carbonate which is precipitated from pore fluids escaping through the highly permeable coarse grained layers as lithification deeper in the basin continues pore fluids must continue to escape tectonic fracturing may facilitate this

ephemeral plate tectonics at the queen charlotte triple junction three plate boundaries the queen charlotte transform the cascadia subduction zone and the juan de fuca ridge meet in a complex triple junction offshore vancouver island some interpretations of the plate tectonics of the region have included the explorer ridge the dellwood knolls and the tuzo wilson volcanic field as part of the spreading system requiring up to three triple junctions in the region new seabeam bathymetry interpreted with existing regional data sets indicate that the dellwood knolls and tuzo wilson volcanic field are not independent plate boundaries but the result of leaky transform tectonics between two overlapping transform faults seismicity shows that the explorer plate is being deformed by pacific north american relative motion as a new transform plate boundary forms and cuts off the explorer ridge the system is evolving to a single triple junction at the northern terminus of the juan de fuca ridge where it meets the nootka deformation zone thus the explorer microplate which was spawned 5 ma is an ephemeral adjustment to mechanical difficulties at the triple junction this new model implies that the explorer subduction zone is no longer active ephemeral oceanic microplates have also existed at pacific north american triple junctions off southern california and baja california

dipping seismic reflectors electrically conductive zones and trapped water in the crust over a subducting plate the lithoprobe program across the northern cascadia subduction zone at vancouver island has obtained geophysical data that suggest a dipping zone of trapped free pore water in the overlying continental crust land and marine multichannel seismic reflection profiles show several bands of reflectors the primary one at a depth of about 30 km is about 5 km thick dips inland and appears to coincide with an electrically conductive layer defined by magnetotelluric data the downgoing oceanic crust is imaged as a thin reflector dipping at about 15 and 10 km deeper than the primary reflector a location that is consistent with seismicity and seismic refraction data both the reflectors and the conductor have been modeled by near horizontal lamellae with 1 4 saline water porosity detailed geothermal measurements defining heat flow that decreases inland indicate that the dipping bands are nearly isothermal at a temperature of 400 500 c it is suggested that free water generated by dehydration of the downgoing oceanic plate migrates upward until reduced temperature results in hydration reactions and mineral precipitation that forms an impermeable barrier low vertical permeability is required to keep the water from separating to the top of the layer this can be reconciled with the low electrical resistivity if pore interconnection is through thin grain boundary tubes as predicted by theoretical equilibrium pore geometry although very small effective grain size is necessary an abrupt transition from high permeability to very low permeability is predicted at a several percent porosity in agreement with the modeled porosity such pore geometry with low permeability may be restricted to temperatures above about 400 c i e in greenschist to amphibolite facies conditions where grains will deform to equilibrium geometry and cracks are annealed the region above the reflective and conductive band is estimated to be within blueschist conditions consistent with the inferred high seismic velocity from refraction data and high density from gravity data

u pb zircon geochronology of basal granite by la icp ms in yitong basin northeast china implications for origin of limestone abstract massive deposits of limestone were encountered for the first time by wells drilled into the bedrocks of yitong basin northeast china u pb zircon dating analyses with la icp ms were then employed to determine the geological ages of the deposits and analyze the impact of a later magmatic intrusion upon the hydrocarbon generation and preservation environment in the deposits analyses of 6 granite samples taking from the limestone shows that the basement of the yitong basin contains numerous zircons of magmatic origin and these zircons are characterized in cl images as light color clear oscillatory zoning and intact crystalline form in terms of ree rare earth elements content and its distributive pattern the zircons are shown with mild loss of eu high th u ratio and left slanting distributive curves concordia plots based on u pb dating of 61 zircons of magmatic origin showed concentrated data points along the concordia line indicating no obvious pb loss the crystallization ages of 6 granite samples were dated as 220 ma 192 ma 261 ma 185 ma 270 ma and 273 ma respectively based on the understanding we inferred that the limestone of sedimentary genesis imbedded with the permian formations of the basin was formed during deposition in the permian era the magma activity was the most active during the jurassic magmatic intrusion was thought to be originated from deeper parent rocks below the permian layers it cut through the permian limestone from the bottom up and released great amount of heat to bake the limestone this accelerated hydrocarbon generation in the limestone and increased its ktg kerogen to gas rate to 0 9 or more during 190 ma and 185 ma early hydrocarbon generation and poor preservation are the two hydrocarbon related features of the limestone in terms of reservoir lithology the limestone went through thermal contact metamorphism with the heat and volatile components from magmatic activities it is also observed that contact metamorphism of the limestone was the most active near the eruption points of magma

crustal structure of the cascadia subduction zone southwestern british columbia from potential field and seismic studies the northern cascadia subduction zone is a region of convergence between the oceanic explorer and northern juan de fuca plates and the continental north american plate potential field and new seismic reflection data coupled with previous seismic results and geology enable derivation of a series of density magnetic susceptibility cross sections and a block density model from the ocean basin to the volcanic arc from 2 5 and 3 dimensional interpretations the lateral extent and thickness of the accreted wedge vary significantly along the zone the narrow metasedimentary pacific rim terrane lies immediately west of wrangellia and extends the length of vancouver island consistent with its emplacement by strike slip faulting following the accretion of wrangellia the ophiolitic crescent terrane is a narrow slice lying seaward of the pacific rim terrane but not extending northward beyond the juan de fuca plate in this region the crescent terrane was emplaced in a strike slip or obliquely convergent style

differences in recent ground surface warming in eastern and western canada evidence from borehole temperatures because of the perturbation caused by a recent climatic warming most borehole temperature depth profiles in canada show reduced or negative gradients at shallow depths however the onset time and magnitude of the warming are different in eastern and western british columbia and southern yukon canada we determined the average subsurface temperature perturbations associated with the recent warming from borehole data for 34 and 51 respectively well distributed sites in these two regions of canada if the ground surface temperature is assumed to have increased linearly since and onset time the results indicate that the ground surface has warmed by 1 5 k since the mid 19th century in eastern canada but only by 0 8 k since the late 19th century in british columbia and southern yukon