coulomb stress changes following the 2012 mw 7 8 haida gwaii canada earthquake implications for seismic hazard this article examines spatial changes to the local stress field resulting from the 28 october 2012 m w 7 8 haida gwaii earthquake off the west coast of moresby island british columbia this event occurred on a northeast dipping potentially blind thrust fault rather than on the subvertical queen charlotte fault qcf that represents the pacific north american plate boundary this was the largest earthquake along the canadian portion of this plate boundary since the 1949 m s 8 1 queen charlotte earthquake the u s geological survey coulomb software is used to quantitatively estimate the effect of the mainshock on the background stress field the known aftershock nodal planes and the nearby qcf we use two different mainshock finite fault models both of which are seismologically derived by lay et al 2013 and hayes 2013 separately and subsequently adapted by k wang to account for the motion detected at four nearby global positioning system stations see nykolaishen et al 2015 for more information we also use the best located set of aftershocks with information provided by a temporary array of ocean bottom seismometers results indicate an apparent clustering of aftershocks slightly seaward of the main thrust which is consistent with the modeled zone of promoted normal failure likely related to extension in the footwall using existing models we found a high number of aftershocks to be consistent with triggering by the mainshock suggesting that static stress is a dominant control in the months following a large earthquake in this area further we find loading greater than the triggering threshold on the qcf in an area interpreted as a seismic gap this work improves understanding of the evolving seismic hazard along the queen charlotte margin and tests the usefulness of coulomb modeling in this complex tectonic environment online material figures of focal mechanisms and maximum coulomb stress change and table of aftershock moment tensor parameters

model for the origin of the yakutat block an accreting terrane in the northern gulf of alaska a composite oceanic and continental terrane the yakutat block is currently colliding with and accreting to north america in the northern gulf of alaska marine geophysical data indicate that the block has moved with the pacific plate during pliocene and quaternary time but are indeterminant about its previous movement history a model that explains some observed geologic and structural features of the block is that it originated as a result of subduction of the kula farallon spreading center beneath north america in this model the spreading center that formed the basaltic basement of the yakutat block was offset from a spreading center to the south by a left lateral transform fault 400 500 km long subduction of the northern spreading center about 45 m y ago sliced off part of the north america continental margin attached it to the kula plate and initiated northward movement of the composite terrane as the yakutat block during eocene and oligocene time migration of the southern spreading center toward north america truncated the yakutat block on the south and left a fossil fracture zone that is now seen as the transition fault the southern spreading center subducted about 25 m y ago and the yakutat block has since traveled with the pacific plate to southern alaska

correlation of plate motions with continental tectonics laramide to basin range some of the major tectonic and magmatic events of the last 150 ma in the western cordillera can be correlated with a new model for the displacement histories between western north america and adjacent oceanic plates sierra nevada plutonism ended and laramide compression began during increasingly rapid convergence of the farallon plate with north america and during a moderate increase in the westward motion of north america in the hotspot reference frame the end of the laramide and beginning of widespread arc magmatism and extension correlates with slowing of both of these motions the spectacular slowing of faralion north america convergence is attributed to the decreasing age of the farallon plate entering the trench and thus to a change from negative to positive buoyancy the transition from widespread arc related magmatism and rapid extension to basaltic volcanism and moderate extension in the basin and range province finds no ready explanation in the plate motions of the pacific basin a change to oblique spreading in the basin and range province accompanied growth in length of the san andreas fault as the mendocino triple junction progressed northward

triassic to neogene geologic evolution of the queen charlotte region a wealth of new geological and geophysical data from recent studies of the queen charlotte region are integrated into a coherent model in this paper we summarize these new studies and discuss possible correlations with other areas four tectonostratigraphic divisions are distinguished by stratigraphic structural and magmatic character and each is separated by a major unconformity the oldest division comprises widely distributed upper paleozoic through middle jurassic strata of wrangellia that accumulated in volcanic arc and stable shelf and basinal settings no significant deformation occurred in the queen charlotte islands region during the accumulation of these rocks a middle and upper jurassic assemblage comprises two plutonic suites and volcanic and epiclastic rocks the unconformity below the middle and upper jurassic assemblage marks a regional southwest vergent contractional deformation that is the most significant mesozoic or cenozoic deformation in the region jurassic plutons in the que

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

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

current deformation in the northern canadian cordillera inferred from gps measurements 1 continuous and campaign style global positioning system gps measurements provide new constraints on the first order current deformation pattern of the northern cordillera of nw canada and eastern alaska the yakutat block is currently colliding with north america in the corner of the gulf of alaska our data infer that relative yakutat north america motion is accommodated across the eastern boundary by right lateral motion 40 mm a mainly on the fairweather fault and minor shortening 6 mm a to the north collision is taken up by shortening 31 mm a mainly on the chugach st elias fault system with westward extrusion and possible counterclockwise rotation of the yakutat block and alaskan fore arc facilitated by 23 mm a right lateral motion that is shared by several faults the seismicity pattern indicates that plate boundary deformation is far reaching producing strain throughout eastern alaska yukon and western northwest territories our gps data confirm the transfer of strain from the yakutat collision zone and enable the extent of plate boundary deformation to be mapped thereby defining the western edge of stable north america gps sites in sw yukon show motion of 3 10 mm a to the ne confirming that strain is transferred at least 400 km from the yakutat collision continued transfer north toward the mackenzie delta and ne to the mackenzie mountains indicated by current seismicity is not yet well resolved by the campaign gps data which are hampered by coseismic and postseismic effects of the 2002 mw 7 9 earthquake on the denali fault in eastern alaska

using natural distributions of short lived radium isotopes to quantify groundwater discharge and recharge radium activity in pore water of wetland sediments often differs from the amount expected from local production decay and exchange with solid phases this disequilibrium results from vertical transport of radium with groundwater that flows between the underlying aquifer and surface water in situations where groundwater recharge or discharge is significant the rate of vertical water flow through wetland sediment can be determined from the radium disequilibrium by a combined model of transport production decay and exchange with solid phases we have developed and tested this technique at three sites in the freshwater portion of the everglades by quantifying vertical advective velocities in areas with persistent groundwater recharge or discharge and estimating a coefficient of dispersion at a site that is subject to reversals between recharge and discharge groundwater velocities v were determined to be between 0 and 20 5 cm d 21 for a recharge site and 1 5 6 0 4 cm d 21 for a discharge site near levee 39 in the everglades strong gradients in 223 ra and 224 ra usually occurred at the base of the peat layer which avoided the problems of other tracers e g chloride for which greatest sensitivity occurs near the peat surface a zone readily disturbed by processes unrelated to groundwater flow this technique should be easily applicable to any wetland system with different production rates of these isotopes in distinct sedimentary layers or surface water the approach is most straightforward in systems where constant pore water ionic strength can be assumed simplifying the modeling of radium exchange

spatiotemporal distribution of events during the first week of the 2012 haida gwaii aftershock sequence abstract we systematically relocated the 2012 haida gwaii aftershock sequence by re examining continuous seismic waveforms of the canadian national seismographic network cnsn because of the sparse station density in the source area and the offshore location of the majority of events precise determination of hypocenters is extremely challenging constraints on the back azimuth and incident angle of ray paths were derived from the cross correlation of three component waveforms and incorporated into the locating process to improve the results we located 1228 aftershocks for the first week after the mainshock effectively double the size of the cnsn routine earthquake catalog the distribution of aftershocks tends to form two linear trends roughly parallel to the strike of the plate margin in the northwest southeast direction the trend located updip from the mainshock within the subducting pacific plate appears to have three clusters spanning a lateral distance of 80 km the other trend coincides with the surface trace of the queen charlotte fault qcf spanning the northern two thirds of the main rupture zone the overall area of aftershock distribution is 120 km 40 km aftershocks overlap the southern part of the estimated rupture zone of the 1949 earthquake with no activity in the seismic gap farther to the south therefore it is likely that the 2012 earthquake only partially released the accumulated stress on the strike slip transform fault system in this scenario the possibility of a major strike slip earthquake along the southernmost part of the qcf should not be ignored online material table of source parameters of relocated mainshock and aftershocks

the relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls abstract we derive an analytical expression that relates the shape of a fault in cross section to the shape of the bedding horizons in its hanging wall block the expression assumes that the hanging wall deforms by simple shear and that the footwall remains undeformed throughout although this paper concentrates on normal faults the expression is equally valid and applicable to thrust faults the direction of simple shear in the hanging wall block is arbitrary and has a dramatic effect on the predicted fault or bedding geometry there is no reason to believe that the simple shear occurs on vertical planes as is commonly assumed in graphical approaches to this problem and ignoring the presence of inclined simple shear is likely to lead to considerable underestimates of the amount of extension across normal faults and in the amount of shortening across thrusts similar though more complicated expressions can be obtained when compaction within the hanging wall block is taken into account for a planar normal fault such compaction may result in the development of a hanging wall syncline

geological and geophysical characteristics of the tuzo wilson seamounts implications for plate geometry in the vicinity of the pacific north america explorer triple junction seamarc ii imagery seabeam bathymetry seismic reflection profiles and gravity and magnetic data are used to establish the tectonic significance of the tuzo wilson seamounts two submarine volcan

an abrupt transition in the mechanical response of the upper crust to transpression along the queen charlotte fault the queen charlotte fault qcf is a major strike slip fault that forms the boundary between the pacific and north american plates from 51 to 58 n near 53 2 n the angle of oblique convergence predicted by the mid ocean ridge velocity morvel interplate pole of rotation decreases from 15 in the south to 15 in the north south of 53 2 n the convergent component of plate motion results in the formation of a 40 km wide terrace on the pacific plate west of qcf and earthquakes with thrust mechanisms including the 2012 haida gwaii earthquake sequence are observed north of 53 2 n in the primary rupture zone of the m 8 1 strike slip earthquake of 1949 the linear terrace disappears and topography of the continental slope west of the qcf is characterized by a complex pattern of ridges and basins that trend obliquely to the primary trace of the qcf deformation within the pacific plate appears to occur primarily through strike slip faulting with a minor thrust component on secondary synthetic faults the orientations of these secondary faults as determined from seismic reflection and bathymetric data are consistent with the reactivation of faults originally formed as ridge parallel normal faults and as thrust faults formed parallel to the qcf south of the bend at 53 2 n and subsequently translated to the north we suggest that an oblique convergence angle of 15 represents a critical threshold separating distinct crustal responses to transpression this result is consistent with theoretical and analog strain models of transpressive plate boundaries the sharpness of this transition along the qcf in contrast to purely continental transform boundaries may be facilitated by the relatively simple structure of oceanic crust and the presence of pre existing optimally oriented faults in the young pacific plate

eocene basalts from the yakutat terrane evidence for the origin of an accreting terrane in southern alaska basalts from the yakutat terrane a composite oceanic and continental tectonostratigraphic terrane are chemically diverse large ion lithophile element lile depleted and lile enriched tholeiites interpreted as normal mid ocean ridge and oceanic island basalt that originated on seamounts near the kula farallon spreading center during the early to middle eocene coeval and geochemically similar basalts occurring sporadically in a linear belt from southern vancouver island to the southern oregon coast range are correlative with the yakutat terrane basalts both basaltic sequences were accreted to the continental margin during subduction of the kula farallon ridge and kula plate about 48 ma the yakutat basalts were emplaced along the coast of washington or british columbia the yakutat terrane probably originated during the neogene as a composite terrane when it was sliced off the continental margin and started to move northward along the queen charlotte fairweather transform fault toward its present location in southern alaska

tertiary extension and tilting in the queen charlotte islands evidence from dyke swarms and their paleomagnetism the queen charlotte islands form the western margin of the tertiary queen charlotte basin which is situated on the western margin of the north american plate they contain seven major dyke swarms of late eocene to miocene age a period when the relative motions of the pacific and the north american plates in this region were dominantly dextral strike slip transform margin with intervals of highly oblique divergence and convergence within each swarm dykes have a systematic trend however trends vary from swarm to swarm indicating that the stress field varied a total of 678 cores 1352 specimens were collected from 129 dykes in six swarms over a distance of about 200 km magnetic stability is variable one hundred and one dykes yielded records of the paleofield data are also reported from an oligocene pluton 5 sites 27 cores 52 specimens and miocene lavas 8 sites 52 cores 101 specimens both normal and reversed magnetizations occur but irrespective of sign the mean directions of remanen

the composition and emplacement of basaltic magmas produced during the development of continental margin basins the gulf of california mexico the gulf of california is a region of active ensialic basin development and transform faulting legs 64 and 65 of the deep sea drilling project have demonstrated the diversity of basement stratigraphy drilling at sites 474 482 483 and 485 at the mouth of the gulf recovered massive basaltic flows and occasional pillow lavas and sills whereas drilling in the central part of the gulf in the guaymas basin recovered a sequence of sediments and massive doleritic sills very high sedimentation rates over 1000 m ma are characteristic of the guaymas basin and prevent extrusion of the basaltic magma onto the seafloor thus resulting in at least partial intercalation of oceanic layers 1 and 2 such a process of ocean crust formation could be locally important during the early stages of ocean and marginal basin development when sedimentation rates may be very high over 200 samples of basalts and dolerites from the gulf of california have been analysed by x ray fluorescence and neutron activation techniques basalts from the mouth of the gulf are tholeiitic and contain very low abundances of the more hygromagmatophile more hyg elements k rb ba th ta nb and light ree relative to the hyg elements y zr ti and heavy ree thus resembling depleted n type morb from elsewhere along the e pacific rise epr dolerites from the guaymas basin also have a tholeiitic chemistry and some epr basalt characteristics e g la n sm n

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

shear wave velocity structure in the vicinity of the 2012 mw 7 8 haida gwaii earthquake from receiver function inversion abstract the thrust mechanism of the 2012 m w 7 8 haida gwaii earthquake suggests convergence across the transpressive pacific north america plate boundary in the region is accommodated by underthrusting with important consequences for seismic and tsunami hazard analysis this article investigates the crustal structure and extent of subduction beneath haida gwaii by nonlinear inversion of receiver function data processed from teleseismic recordings at five land based seismograph stations three of these stations were deployed since the 2012 earthquake to extend coverage to the southeast and have not been analyzed previously the inversions provide estimates of the shear wave velocity structure beneath much of moresby island results indicate a positive velocity contrast at approximately 18 26 km depth interpreted as a shallow continental moho a 12 17 km thick low shear wave velocity zone is also identified which increases in depth from 25 to 42 km along the direction of plate convergence which is interpreted as subducting oceanic material these results provide the first evidence that the subducting oceanic plate extends beneath the entirety of moresby island

tectonic history of the queen charlotte islands and adjacent areas a model the region including queen charlotte islands hecate strait and queen charlotte sound is underlain by two allochthonous terranes wrangellia and the alexander terrane the suture between them occurs in central graham island and central hecate strait and is coincident with the traces of the sandspit and rennell sound fault zones each of which developed in response to crustal rifting in queen charlotte sound during mid tertiary time the stratigraphic succession comprises four tectonic assemblages 1 the allochthonous assemblages comprise paleozoic rocks of the alexander terrane and upper triassic and jurassic rocks of wrangellia which on the basis of paleomagnetic and biogeographical data are clearly exotic the distribution of these terranes beneath queen charlotte sound and hecate strait is supported by geophysical information and subsurface data obtained from offshore wells 2 the suture assemblage is represented by extremely coarse conglomerates massive graywackes and turbidites of early cretac

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

reconciling late neogene pacific absolute and relative plate motion changes new models of pacific absolute plate motion relative to hot spots and models of relative plate motion involving the pacific plate all agree there was a significant change in the late neogene chron 3a 5 89 ma reflecting a more northerly absolute motion than previously determined as pacific absolute plate motion became slightly more northerly left stepping transform segments came under compression some left stepping segments became microplates with clockwise rotation others show clear evidence of compressional deformation conversely right stepping transforms came under tension and many developed intratransform spreading centers or show similar evidence for transform magmatism several large left stepping transform offsets represent portions of the nazca plate protruding into the pacific and as such act as obstacles to the more northerly pacific absolute plate motion we suggest these obstructions act to enhance the generally tensile equatorial pacific stress regime caused by distant slab pull as a consequence the greater french polynesia region has experienced diffuse volcanism that increased considerably following the chron 3a plate motion change we propose that since the hawaii emperor bend time the distant pacific slab pull and the friction between the large buoyant ontong java plateau and the northern margin of the australia plate have produced episodes of increased tensile stresses in the equatorial pacific we believe these stress excursions are responsible for much of the intraplate volcanism observed in a wide triangular region from samoa to the east pacific and pacific antarctic rises

p wave velocity structure of the earth s crust beneath vancouver island refraction data were recorded from three shot points out to a maximum distance of 330 km as part of the 1980 vancouver island seismic project visp80 these vertical component data are partially reversed and so can be interpreted in terms of two dimensional structures by iterative modeling of p wave travel times and amplitudes the structure of the upper crust is the best constrained part of the model it consists generally of a gradually increasing velocity from 5 3 km s at the surface to 6 4 km s at 2 km depth to 6 75 km s at 15 5 km depth where the velocity increases sharply to 7 km s below 20 km depth the model becomes speculative because the data provide only indirect constraints on velocities at these depths an interpretation that fits the observed times and amplitudes has a low velocity zone in the lower crust and a moho at 37 km depth the only significant departure from this general structure is beneath the central part of vancouver island where the 15 5 km boundary in the model atta

amount of extension on small faults an example from the viking graben extension estimates based on palinspastic restoration of only the largest faults in a deformed terrane may be significantly deficient due to neglect of small faults i e unsampled faults most of which have smaller displacements than do sampled faults small faults individually account for little extension but there may be large numbers of them the cumulative extension on small faults can be calculated from the fractal size distributions of fault displacements independent extension estimates support the inference from fault population statistics that small faults account for significant amounts of extension for example restorations of mesozoic faults interpreted from seismic reflection profiles across the viking graben in the north sea yield extension estimates that are only 40 75 of independent estimates the fractal size distribution of fault displacements in the viking graben is consistent with the hypothesis that the remaining 25 60 of extension resulted from displacements on small faults

age origin and significance of brittle faulting and pseudotachylyte along the coast shear zone prince rupert british columbia northwest striking brittle faults containing cataclasite fault gouge and in one location pseudotachylyte are common in the vicinity of the coast shear zone near prince rupert british columbia the pseudotachylyte locality found in a dextral strike slip fault zone along the coast shear zone contains spherulites alkali feldspar microlites and amygdules suggesting that the pseudotachylyte crystallized rapidly from a melt phase within 5 km of the surface 4 0 ar 3 9 ar incremental heating of the pseudotachylyte matrix yielded a weighted mean plateau of 29 8 0 6 ma and an inverse isochron of 29 8 1 5 ma with an 4 0 ar 3 6 ar intercept of 296 0 15 2 these results show that pseudotachylyte associated with brittle faulting can be dated precisely and imply that some dextral coast parallel displacement occurred across the coast shear zone in the oligocene and that the majority of exhumation in the coast mountains at the latitude of prince rupert 54 n was accomplished by 30 ma

episodic north america and pacific plate motions the north america plate motion for the period 20 0 m y is modeled by two absolute angular velocity vectors with a change occurring at about 9 m y the model incorporates new geologic constraints based on 1 the recognition of a discontinuity in north america plate motion at 9 m y 2 a reexamination of the iceland and yellowstone hotspot traces 3 improved delineation of the present north america pacific relative motion and 4 a revised history of pacific absolute motion the two stage model is consistent with several features of the late cenozoic geologic history of north america including geological observations relating to tectonism along a large portion of both the eastern and western boundaries of north america this plate model combined with a previously established pacific plate model makes specific predictions concerning the 15 0 m y history of the north america pacific plate boundary which compare favorably with the known history these models together resolve most of the discrepancies previously encountered between absolute and relative motion models of the two plates the implications of the north america plate model are the greatest for those events related to the 9 m y change in its motion among the geologic consequences of this change are rifting in the western united states intense compressional deformation along the northern caribbean plate boundary a hiatus in aleutian arc volcanic activity and a change in the orientation of the lesser antilles arc this picture reinforces the notion that the major plates behave rigidly and transmit stresses over great distances the predictive power of the model may help in interpreting the history of the north america plate boundary in those areas where clear observations are lacking an extra force needs to be applied to a plate to cause a change in plate motion by treating the plate as a rigid spherical shell overlying a uniformly viscous fluid we derive a simple class of forces sufficient to cause a given change in plate motion such force balance considerations indicate that an additional force applied in the northwest pacific region toward the northeast would result in the observed change in north america plate motion this force is surprisingly similar to that which apparently caused a large change in pacific plate motion about 5 m y later we propose that a common mantle process is responsible for both changes and is related to the ability of the northwest pacific subducted lithosphere to enter into the lower mantle

accretion and recent deformation of sediments along the northern cascadia subduction zone new multichannel seismic reflection profiles acoustic imagery and swath bathymetric data from the vancouver island continental margin reveal the surface morphology cross sectional geometry and deformation style of this part of the cascadia accretionary prism the incoming 3 to 5 km thick sediment section fails most commonly along landward dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust the faults are listric with approximately circular cross sections and hanging wall sediments are back tilted in a planar manner high near lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement and from the long distances over which the decollement has propagated seaward the faults dip more steeply at 40 45 near the surface where pleistocene turbidites are displaced similar folds have developed where the turbidites are particularly thick 4 km and near hydrostatic pore pressures are inferred to occur in the turbidite part of the section an increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites the top of the downgoing juan de fuca plate which defines the base of the accreted sedimentary wedge beneath the margin is well imaged in the new seismic reflection profiles and is further defined by land reflection data seismic refraction and earthquake hypocenters the landward dipping continental backstop to the prism is also well located if these constraints are used for the geometry of the accretionary prism a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42 m y accretion history this result along with the observation that deformation front faults penetrate most or all of the sediment section suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the eocene application of the critically tapered wedge model of davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism particularly the seawardmost part beneath the lower slope beneath the continental shelf the dip of the subducting slab increases to greater than 11 a dip that according to the critically tapered wedge model is too steep to allow upward growth of an accretionary prism this provides a simple explanation for ongoing subsidence of the tofino sedimentary basin over the older part of the accretionary complex to the southeast the juan de fuca plate dips much more gently beneath the olympic peninsula of northern washington allowing the accretionary prism to grow upward well above sea level to form the olympic mountains

paleogene history of the kula plate offshore evidence and onshore implications paleocene to middle eocene magnetic anomalies were mapped over oceanic crust that accreted at the kula pacific spreading center and is now obliquely entering the western aleutian trench between 179 e and 168 e the strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56 55 ma when north south spreading veered to northwest southeast 310 130 kula pacific spreading ceased in 43 ma a 75 km long section of the fossil kula rift axis has avoided subduction although it now intersects the trench axis almost orthogonally near 171 5 e a narrow remnant of the former kula plate northwest of this fossil spreading center is bounded by a fossil kula pacific transform with a high transverse ridge alongside a sediment filled transform valley anomalies on this remnant show that eocene kula pacific spreading was highly asymmetric 2 1 the 56 55 ma change in kula plate rotation inferred from the change in spreading direction coincided with birth of the aleutian subduction zone and was probably a consequence of the resulting change in slab pull stresses on the oceanic lithosphere the change in direction of kula north american motion is a plausible explanation for the detachment of continental terranes from the pacific northwest and their migration around the gulf of alaska and for the early eocene demise of alaska range are volcanism the cessation of kula pacific spreading coincides with a major change in pacific plate rotation and the subsequent direction of convergence of the pacific plate with the aleutian arc was similar to the 55 43 ma direction of kula plate convergence

late neogene motion of the pacific plate oceanic crust between 38 s and 65 s along the pacific antarctic ridge records a complete history of neogene pacific antarctic motion we have modeled magnetic stages corresponding to portions of isochrons 1 2 2a 3 3a 4 and 5 from 17 high quality marine magnetic traverses of the ridge to determine the neogene finite rotation poles for this plate pair we chose to model velocities over time intervals of about 0 5 m y because this greatly increased the number of estimates of pacific antarctic plate velocity our analysis shows that a change in pacific antarctic relative motion occurred between chron 2a 3 40 ma and the beginning of chron 3 time 3 86 ma we used 203 estimates of pacific antarctic spreading velocity to calculate a 0 3 4 ma and 3 86 10 3 ma euler pole combined with tracks of seamounts that record the passage of the pacific plate over mantle plumes our data show that this change in relative motion corresponds to a change in the absolute motion of the pacific plate with respect to the hotspot frame of reference using these euler poles to calculate pacific antarctic africa north america circuit euler poles show that this recent change in pacific plate absolute motion corresponds to a change from strike slip to transpressive motion along the coastal california pacific north america plate boundary interestingly this age of increased tectonism agrees in time with deformation in new zealand and northern japan

variation in fold geometry in the yuso basin northern spain implications for the deformation regime first generation structures in greywackes of the yuso group from the cantabrian mountains of northern spain show a distinct variation in geometry with depth in a regional synclinal structure curavacas and lechada synclines they are easily distinguished from other deformation events in the structurally uppermost level we find flap folds flap folds are recumbent structures with the inverted limb preserved below this level cascade folds are found these structures have a vergence opposite to that of parasitic folds the nomenclature adopted is from harrison and falcon characteristically these structures have shallowly dipping axial surfaces in agreement with the shallow dip of the axial plane regional cleavage in the lowermost structural level upright parasitic folds with a steep cleavage are present the variation in fold geometry is accompanied by a general steepening of the regional cleavage with increasing depth in the absence of overprinting relationships the f1 fold geometries are included in a single deformation event the steepening of the cleavage with depth reflects the change in orientation of the maximum shortening direction from sub vertical in the upper part of the syncline to sub horizontal in the lower part with increasing depth the deformation regime during f changed from bending to buckling the deformation regime on the regional scale however is associated with basement subsidence and passive formation of the regional synclinal structure furthermore the absence of a distinct microfabric for the different f1 folds indicates that on a small scale a similar deformation regime was present we conclude therefore that the scale at which we study a structure only reflects the deformation regime at that particular scale consequently the overall deformation regime cannot be determined from single outcrops or microstructural analysis alone

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

hanging canyons of haida gwaii british columbia canada fault control on submarine canyon geomorphology along active continental margins abstract faulting commonly influences the geomorphology of submarine canyons that occur on active continental margins here we examine the geomorphology of canyons located on the continental margin off haida gwaii british columbia that are truncated on the mid slope 1200 1400 m water depth by the queen charlotte fault zone qcfz the qcfz is an oblique strike slip fault zone that has rates of lateral motion of around 50 60 mm yr and a small convergent component equal to about 3 mm yr slow subduction along the cascadia subduction zone has accreted a prism of marine sediment against the lower slope 1500 3500 m water depth forming the queen charlotte terrace which blocks the mouths of submarine canyons formed on the upper slope 200 1400 m water depth consequently canyons along this margin are short 4 8 km in length closely spaced around 800 m and terminate uniformly along the 1400 m isobath coinciding with the primary fault trend of the qcfz vertical displacement along the fault has resulted in hanging canyons occurring locally the haida gwaii canyons are compared and contrasted with the sur canyon system located to the south of monterey bay california on a transform margin which is not blocked by any accretionary prism and where canyons thus extend to 4000 m depth across the full breadth of the slope

fault displacement distance relationships as indicators of contractional fault related folding style we present a method of using fault displacement distance profiles to distinguish fault bend shear fault bend and fault propagation folds and use these insights to guide balanced and retrodeformable interpretations of these structures we first describe the displacement profiles associated with different end member fault related folding models then provide examples of structures that are consistent with these model based predictions natural examples are imaged in high resolution two and three dimensional seismic reflection data sets from the niger delta sichuan basin sierras pampeanas and cascadia to record variations in displacement with distance updip along faults termed displacement distance profiles fault bend folds exhibit constant displacement along fault segments and changes in displacement associated with bends in faults shear fault bend folds demonstrate an increase in displacement through the shearing interval and fault propagation folds exhibit decreasing displacement toward the fault tip more complex structures are then investigated using this method demonstrating that displacement distance profiles can be used to provide insight into structures that involve multiple fault related folding processes or have changed kinematic behavior over time these interpretations are supported by comparison with the kinematics inferred from the geometry of growth strata overlying these structures collectively these analyses illustrate that the displacement distance approach can provide valuable insights into the styles of fault related folding

queen charlotte fault zone microearthquakes from a temporary array of land stations and ocean bottom seismographs a temporary array of land and ocean bottom seismograph stations was used to accurately locate microearthquakes on the queen charlotte fault zone which occurs along the continental margin of wester

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

the northern cascadia subduction zone at vancouver island seismic structure and tectonic history the structure and tertiary tectonic history of the northern cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the

geomorphic regime modulates hydrologic control of chemical weathering in the andes amazon abstract the interplay between the hydrologic processes that supply store and route water in catchment systems and the chemical weathering reactions that add and remove solutes acts as an important control on chemical weathering fluxes in this study we use paired measurements of solute chemistry and runoff in four nested catchments that span the transition from the andes mountains to the amazonian foreland floodplain in peru in order to investigate the links between hydrology and weathering processes and to determine how these links change across a geomorphic gradient all of the sites show variation in elemental concentrations and ratios with runoff consistent with hydrologically driven changes in lithologic sources the extent of secondary mineral precipitation and potentially fluid flow paths in the andean sites solute concentrations are relatively constant despite large changes in runoff this is in direct contrast to the foreland floodplain site where solute concentrations are diluted as runoff increases in the andes amazon the concentration runoff behavior is correlated with the mean catchment slope angle which suggests that erosional processes by modulating the timescales over which weathering reactions occur within the critical zone can be an underlying control on solute production and therefore on chemical weathering due to the co variation between the geomorphic and hydrologic controls on chemical weathering weathering fluxes in andean sites are more sensitive to seasonal changes in runoff than in the foreland floodplain site

distribution nature and origin of neogene quaternary magmatism in the northern cordilleran volcanic province canada the northern cordilleran volcanic province encompasses a broad area of neogene to quaternary volcanism in northwestern british columbia the yukon territory and adjacent eastern alaska volcanic rocks of the northern cordilleran volcanic province range in age from 20 ma to ca 200 yr b p and are dominantly alkali olivine basalt and hawaiite a variety of more strongly alkaline rock types not commonly found in the north american cordillera are locally abundant in the northern cordilleran volcanic province these include nephelinite basanite and peralkaline phonolite trachyte and comendite the most mgo rich nephelinites basanites and alkaline basalts from throughout the northern cordilleran volcanic province show trace element abundances and isotopic compositions that are consistent with an asthenospheric source region similar to that for average oceanic island basalt and for post 5 ma alkaline basalts from the basin and range our petrologic observations help constrain the origin of northern cordilleran volcanic province magmatism as well as lithosphere changes between the four major basement terranes that underlie the province results from phase equilibria calculations and the spatial distributions of volcanic rock types and magmatic inclusions are more consistent with the existence of thicker lithosphere beneath stikinia which underlies the southern part of the northern cordilleran volcanic province than beneath the cache creek and yukon tanana terranes which underlie the northern part of the northern cordilleran volcanic province our results support a model for initiation of northern cordilleran volcanic province magmatism due to incipient rifting of the northern cordillera driven by changes in relative plate motion between the pacific and north american plates ca 15 10 ma

plate boundary adjustments of the southernmost queen charlotte fault abstract a revision of the plate boundary configuration offshore southern haida gwaii which is the locus of the 2012 m w 7 8 thrust earthquake is based on a re evaluation of regional geophysical data and leads to recognition of fault segments critical for seismic hazard analysis off western canada the strike slip queen charlotte fault qcf and revere dellwood fault rdf constitute the transpressive plate boundary between the pacific and north american plates the southernmost 80 km of the qcf strikes at the highest angle 20 to the relative plate motion yet geomorphology adjacent to the fault is the most subdued synthesizing geomorphology seismicity sidescan sonar gravity and seismic reflection profiles indicates that the plate boundary is evolving to a less transpressive configuration than previously interpreted the rdf is 100 km longer than previously mapped it has propagated northward and overlaps the qcf by 120 km propagation is defined by strike slip earthquakes 3 9 m w m w 7 8 and the 1949 m 8 1 earthquakes

relationships between plate motions and late cretaceous to paleogene magmatism in southwestern alaska four major late cretaceous to paleogene magmatic belts have been recognized in southwestern alaska and tentative boundaries have been drawn for each the belts indicate minimum ages by which the tectonostratigraphic terranes they overprint must have been linked together though there has been significant postmagmatic tectonism the kuskokwim mountains belt 73 60 m y b p the alaska range belt 74 55 m y b p and the gulf of alaska belt 62 57 m y b p all may have been related to a period of rapid north northeastward motion of the kuia plate from about 74 to 56 m y b p the angle of convergence was low with respect to the present orientation of the belts suggesting the possibility of postmagmatic rotation of the belts a higher angle of convergence may have existed with respect to the alaska range belt a probable arc if postmagmatic rotation resulted from 300 400 km of right lateral displacement on the denali fault the gulf of alaska belt is in an anomalous forearc position with respect to the alaska range belt but probably formed by a convergence related mechanism the kuskokwim mountains belt is in an apparent back arc position but the mechanism of its origin is uncertain it may have been part of an unusually broad alaska range arc or less likely it may have been a separate arc magmatic quiescence occurred throughout south western alaska during a period of plate reorganization from about 56 to 43 m y b p a period which began with extremely rapid northward motion of the kuia plate magmatism resumed when the present period of rather slow northwestward pacific plate motion began at about 43 m y b p the aleutian arc probably formed at this time and has been episodically active in about the same location ever since

evidence for underthrusting beneath the queen charlotte margin british columbia from teleseismic receiver function analysis summary the queen charlotte fault zone is the transpressive boundary between the north america and pacific plates along the northwestern margin of british columbia two models have been suggested for the accommodation of the 20 mm yr 1 of convergence along the fault boundary 1 underthrusting 2 internal crustal deformation strong evidence supporting an underthrusting model is provided by a detailed teleseismic receiver function analysis that defines the underthrusting slab forward and inverse modelling techniques were applied to receiver function data calculated at two permanent and four temporary seismic stations within the queen charlotte islands the modelling reveals a 10 km thick low velocity zone dipping eastward at 28 interpreted to be underthrusting oceanic crust the oceanic crust is located beneath a thin 28 km eastward thickening 10 continental crust

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

a high resolution aftershock catalog of the magnitude 7 5 craig alaska earthquake on 5 january 2013 abstract the m w 7 5 craig alaska earthquake on 5 january 2013 was the second largest earthquake on the queen charlotte fault qcf in 40 yrs representing the transform plate boundary between the pacific and north american plates earthquake hazard along the qcf was known from historical earthquakes but the absence of major earthquakes in the digitally instrumented era has limited studies of earthquakes here we present an analysis of aftershocks of the craig earthquake we first use waveform cross correlation and double difference relative relocation to relocate the aftershock catalog of the alaska earthquake center aec our relocated catalog reveals a complex fault structure activated in response to the mainshock and provides insight into the faulting mechanics of this graduated transpressive plate boundary we then use network matched filtering nmf to detect additional aftershocks using 351 aftershocks located by the aec in 2013 as template events we identify 1785 aftershocks through may 2013 this is a greater than fivefold increase in aftershock detection rate compared with the standard single event network processing utilizing regional seismic stations and waveform modeling we are able to characterize the faulting parameters for 12 aftershocks six aftershocks show focal mechanisms consistent with the mainshock faulting parameters and six aftershocks show either thrust or strike slip mechanisms with nodal planes rotated 45 from the mainshock mechanism the six aftershocks that are inconsistent with the qcf have p axes that are perpendicular to the plate boundary suggesting that transpression remains at this location we show that five of these large aftershock families were activated within a day of the mainshock whereas seven families were delayed by multiple days before activating we show evidence for migrating patterns of seismic quiescence and elevated seismicity relative to the predicted omori utsu decay which is related to the largest aftershock in the sequence online material aftershock catalog of the 5 january 2013 craig alaska earthquake

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

deep structure and structural inversion along the central california continental margin from edge seismic profile ru 3 deep penetration seismic reflection profile ru 3 crosses the central california continental margin revealing a subducted oceanic plate a modified accretionary prism and complex structures of the overlying sedimentary basins this structural framework was established by subduction processes during paleogene and earlier time and subsequently was modified by neogene transform motion combined with apparent components of extension and compression subducted rocks are indicated by deep gently dipping reflectors that extend beneath the continental margin for at least 38 km at a depth of about 15 km we interpret the subducted crust as either a part of the pacific plate or more likely a subducted fragment derived from the farallon plate a set of more steeply dipping deep events may indicate faulting within the subducted plate or its boundary with a no slab zone the overlying largely nonreflective layer of accreted material rapidly reaches 10 km in thickness landward of the paleotrench and increases to 15 km in thickness near the coast the santa lucia basin landward of the steep continental slope originated as a slope basin during paleogene subduction the lower strata of this basin were deposited onto and partially incorporated into the accretionary complex the offshore santa maria basin exhibits a variety of compressional structures that formed in the last 3 5 m y and whose locations correspond to an earlier framework of extensional faults structural inversion has occurred in miocene depocenters adjacent to the santa lucia bank fault and at the queenie structure miocene and lower pliocene strata also thicken toward the hosgri fault zone where subsequent compression is characterized by low angle thrusts and folding

basalt geochemistry of the explorer ridge area northeast pacific ocean forty two fragments of young fresh basalts dredged from the explorer ridge paul revere ridge fracture zone and dellwood knolls have been analysed for 34 major minor and trace elements and 87sr 86sr ratios have been determined in seven of the fragments the explorer ridge basalts have major element compositions similar to most mid ocean ridge basalts morb particularly to the iron rich basalts of the southern juan de fuca ridge the basalts exhibit variability in trace element content largely attributable to crystal fractionation at low pressure with respect to morb the incompatible minor and trace elements are weakly to strongly enriched in the explorer samples and are most strongly enriched in basalts from explorer deep adjacent ridge segments erupt basalts with variable incompatible element concentrations and ratios this could be an indicator of a chemically heterogeneous mantle source or may be the result of intermittent injection of enriched magmas from a hotspot beneath explorer deep

evidence of paleogene sedimentation on graham island queen charlotte islands west coast canada the palynomorph taxa ctenosporites eskerensis pesavis tagluensis and tilia spp demonstrate that an unnamed sedimentary unit in the union port louis well and float from mud bay creek skidegate i

an overview of the 28 october 2012 m w 7 7 earthquake in haida gwaii canada a tsunamigenic thrust event along a predominantly strike slip margin the boundary between the pacific and north america plates along canada s west coast is one of the most seismically active regions of canada and is where canada s two largest instrumentally recorded earthquakes have occurred although this is a predominantly strike slip transform fault boundary there is a component of oblique convergence between the pacific and north america plates off haida gwaii the 2012 mw 7 7 haida gwaii earthquake was a thrust event that generated a tsunami with significant run up of over 7 m in several inlets on the west coast of moresby island several over 6 m with a maximum of 13 m damage from this earthquake and tsunami was minor due to the lack of population and vulnerable structures on this coast

a revised estimate of pacific north america motion and implications for western north america plate boundary zone tectonics for the past 20 years much effort has been directed to determining the present day relative motion of the pacific and north american plates using two independent approaches one uses geologic observations and geodetic measurements along the san andreas fault and other faults in the plate boundary zone the other is based on plate motion models that incorporate spreading rates from marine magnetic anomalies transform azimuths and earthquake slip vectors geologic and geodetic studies find two principal elements of deformation slip along the san andreas of 34 mm yr directed n41 w and extension across the basin and range province of about 10 mm yr directed n56 w in contrast plate motion studies find 56 60 mm yr directed n35 w the discrepancy between these estimates a vector of about 15 mm yr oriented nearly due north is often attributed to a combination of slip along faults parallel to the san andreas and shortening normal to it here we revise the estimate of pacific north america motion by analyzing marine magnetic profiles from the gulf of california since 3 ma spreading has averaged 48 mm yr 10 mm yr slower than estimated before and consistent with the 49 mm yr spreading predicted by a new global plate motion model derived without any data along the pacific north america boundary the discrepancy with geodetic and geologic estimates is thus reduced to only 5 mm yr parallel to the san andreas 60 less than estimated before and 7 mm yr of shortening across the san andreas similar to prior estimates these results suggest that strike slip motion on faults west of the san andreas is less than thought before a conclusion consistent with geodetic seismological and other geologic observations

the mt edgecumbe volcanic field alaska an example of tholeiitic and calc alkaline volcanism major element trace element and isotopic composition data for lava samples have been collected from a holocene stratovolcano in southeastern alaska results indicate that a wide variation in chemical compositions exist 48 71 sio 2 10 14 16 mgo and the suite may be divided into two series on the basis of chemical mineralogical and isotopic characteristics the tholeiitic series lavas consist of generally phenocryst poor olivine basalts whose chemical and isotopic compositions are transitional between normal morb and lree enriched lavas of atypical areas such as the azores platform cao is high mgo feo 1 fe 2 o 3 k 2 o rb and sr are low while mgo and ni are high data show that the basalts are lree depleted and sr 87 sr 86 70304 the basalts compose 65 of the volcanic field and crystal fractionation appears to have produced the compositions of the more silicic basalts the calc alkaline series lavas are composed of basaltic andesites andesites and dacites of

pliocene change in pacific plate motion the absolute motion of the pacific plate has been nearly constant during the past 15 myr but epp1 recognized that some change took place between 15 myr and the present and tentatively chose 1 myr for the time of the change cox and engebretson2 found a change in pacific absolute motion based on hotspot tracks and a change in pacific antarctic relative motion the pliocene deformation in california and the juan de fuca rotations led them to postulate a 5 myr age for the change the manner and timing of the change are explored more thoroughly here and we put the time for the change in motion of the pacific plate at between 5 and 3 2 myr basing the younger limit on the correlation of computed hotspot tracks with pacific and nazca hotspot traces the change may explain several observations on the neighbouring continental and oceanic plate boundaries including direct adjustments at the pacific spreading ridges a likely mechanism for the change is a large clockwise torque caused by acceleration of the sinking slab beneath the kurile trench where a fossil spreading ridge is inferred to have been subducted

change in motion of pacific plate at 5 myr bp alignments of volcanoes such as the hawaiian seamount chain record the passage of oceanic plates over mantle plumes or hot spots and changes in the orientation of a seamount chain may indicate changes in the direction of motion of the plate within the hotspot frame of reference1 this hypothesis can be tested for the pacific plate by comparing data from the hawaiian volcanoes with tectonic movements inferred from the geology of adjacent plates we have now found that the motion of the pacific plate changed to a more northerly direction 5 myr ago probably as a result of the detachment of a piece of pacific sea floor which had been subducted beneath the fiji plateau during the tertiary

the queen charlotte islands refraction project part i the queen charlotte fault zone the active margin between the continental north american plate and oceanic pacific plate west of the queen charlotte islands was the site of an extensive onshore offshore seismic refraction project

neogene strontium isotope stratigraphy foraminifer biostratigraphy and lithostratigraphy from offshore wells queen charlotte basin british columbia canada strontium isotope ages of foraminifers from early miocene to late pliocene neogene sequences 21 2 3 4 ma are reported for the first time from the queen charlotte basin qcb in queen charlotte

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

queen charlotte fault zone heat flow measurements the heat flow pattern from the deep sea across the queen charlotte terrace to the west coast of the queen charlotte islands has been determined through 20 marine heat flow measurements and by 11 borehole measurements at one land site there is a continuous transition from very high deep sea heat flow through intermediate values on the terrace to low continental heat flow the deep sea values are scattered probably reflecting hydrothermal circulation in the crust but their mean is in agreement with the theoretical value for the 7 ma deep sea oceanic crust the mean heat flow at the land site of 47 mw m 2 is similar to the average for the coast insular belt to the south and east the measurements on the 30 km wide 2 km deep terrace are less scattered and much lower than those on the deep sea floor the main thermal contrast is at the seaward edge of the terrace rather than near the coast where the earthquakes of the main plate boundary are located numerical models suggest that no reasonable steady stat