new u pb and ar ar isotopic age constraints on the timing of eocene magmatism fort fraser and nechako river map areas central british columbia twenty three new precise eocene u pb and 40 ar 39 ar age determinations for calc alkaline volcanic rocks of the ootsa lake group and associated intrusive rocks widespread in the nechako plateau in central british columbia constrain the timing of the eocene magmatism to 53 2 47 6 ma with a local duration of as little as 2 3 million years the new dates show that magmatism in the study area is partly coeval with that responsible for the babine igneous suite newman volcanic suite 53 50 ma to the north and for the endako group 51 45 ma which overlies the ootsa lake group however locally the three magmatic suites are distinct in age and or the strata record a magmatic hiatus of as much as 7 5 million years the ages generally young from north to south 52 47 ma along the western portion of the study area the babine igneous suite newman volcanic suite represents the oldest member of this series however in the east the ootsa lake group volcanic rocks are generally older 53 51 ma the anomalously older ages may be related to the interaction of magmatism and formation of a nearby and coeval core complex which ongoing studies show was uplifted at about the same time during the eocene felsic plutonism associated with the ootsa lake group occurred between 50 5 and 47 3 ma these plutons were emplaced in an extensional setting along north northeast trending faults the new dates stratigraphic relationships and suggested correlations of eocene strata in the study area with that to the west and north require a revision of the stratigraphic nomenclature for the ootsa lake and endako groups 696

age of emplacement and basement character of the cache creek terrane as constrained by new isotopic and geochemical data new u pb and rb sr isotopic and major and trace element geochemical data are reported for late triassic to eocene granite bodies that intrude the cache creek and stikine terranes in the atlin bennett area of northwestern british columbia the u pb isotopic age data help constrain closure of the cache creek ocean and obduction of the cache creek terrane to before 172 ma middle jurassic low 87sr 86sr initial ratios 0 7037 0 7046 and lack of evidence for inheritance in zircons suggest that rocks underlying the cache creek terrane are largely primitive in nature with derivation of intrusive rocks predominantly from unevolved sources a genetic link between late triassic granitic plutons and stuhini arc volcanics of stikinia is supported by trace and rare earth element data which indicate generation of magma in a volcanic arc setting geochemical patterns for postaccretion middle jurassic and late cretaceous to early tertiary age plutons are similar to those of the late triassic indicating that middle ju

evaluation of low latitude pi2 pulsations as indicators of substorm onset using polar ultraviolet imagery impulsive pi2 pulsations have long been recognized as one of the key signatures of magnetic activity during substorm periods due to their wide observable range both in latitudes and longitudes it is well documented that there is usually more than one pi2 wave burst associated with a substorm and only one of them corresponds to the onset of the substorm this observational fact poses obstacles to determining substorm onsets with pi2 signals although the pi2 have become one of the most popular indicators for substorm onsets the reliability of using the pi2 in this fashion has not been seriously investigated in this paper we address this question with a statistical approach by using 650 auroral substorm onsets identified with polar ultraviolet images for a time interval from april 1996 to may 1997 a comparison of the low latitude pi2 pulsation onsets from kakioka l 1 07 with the auroral breakups indicates that identifying substorm onset with the pi2 alone is often ambiguous of a total of 119 isolated defined as 10 min of quiet time preceding the onset pi2 bursts seen within 10 min from a magnetic positive bay there were 65 events 55 taking place within 3 min from breakups and 34 events 29 indicating no sign of an auroral breakup within 10 min of the pi2 burst this result suggests that pi2 may not be as a good indicator of the substorm onset as it was thought to be interestingly it is always possible to associate pi2 pulsations with some forms of auroral intensification when compared to auroral breakups pi2 onsets are subject to a small delay of 1 3 min with a peak around l min delays of pi2 onsets are revealed to be a function of location relative to auroral breakup this dependence is found to be consistent with the time of flight for a fast mode wave in a plasmapause cavity mode model propagating in the magnetosphere

imbricate architecture of the upper paleozoic to jurassic oceanic cache creek terrane central british columbia upper paleozoic to lower jurassic oceanic rocks of the cache creek terrane near fort st james in central british columbia form a stack of thrust sheets cut by steeply dipping strike slip faults paleontologically dated upper paleozoic strata include bioclastic shallow water limestone and ribbon chert isotopically dated permian rocks consist of tonalite sills and stocks and rhyolite flows intercalated with basalt flows paleontologically dated lower mesozoic rocks include greywacke sandstone siltstone argillite ribbon chert conglomerate limestone and basalt tuff trembleur ultramafite unit of the cache creek complex in places part of an ophiolite suite forms thrust sheets and klippen that overlie lower mesozoic sedimentary rocks sedimentological lithochemical paleontological petrological and textural comparisons with other areas and established models demonstrate that cache creek terrane is an accretionary complex a structurally stacked assemblage of rocks that originated in diverse and

extrusion of high pressure cache creek rocks into the triassic stikinia quesnellia arc of the canadian cordillera implications for terrane analysis of ancient orogens and palaeogeography

a comprehensive sulfur and oxygen isotope study of sulfur cycling in a shallow hyper euxinic meromictic lake abstract mahoney lake is a permanently anoxic and sulfidic euxinic lake that has a dense plate of purple sulfur bacteria positioned at mid water depth 7 m where free sulfide intercepts the photic zone we analyzed the isotopic composition of sulfate 34 s so4 and 18 o so4 sulfide 34 s h2s and the water 18 o h2o to track the potentially coupled processes of dissimilatory sulfate reduction and phototrophic sulfide oxidation within an aquatic environment with extremely high sulfide concentrations 30 mm large isotopic offsets observed between sulfate and sulfide within the monimolimnion 34 s so4 h2s 51 and within pore waters along the oxic margin 34 s so4 h2s 50 are consistent with sulfate reduction in both the sediments and the anoxic water column given the high sulfide concentrations of the lake sulfur disproportionation is likely inoperable or limited to a very narrow zone in the chemocline and therefore the large instantaneous fractionations are best explained by the microbial process of sulfate reduction pyrite extracted from the sediments reflects the isotopic composition of water column sulfide suggesting that pyrite buried in the euxinic depocenter of the lake formed in the water column the offset between sulfate and dissolved sulfide decreases at the chemocline 34 s so4 h2s 37 a trend possibly explained by elevated sulfate reduction rates and inconsistent with appreciable disproportionation within this interval water column sulfate exhibits a linear response in 18 o so4 34 s so4 and the slope of this relationship suggests relatively high sulfate reduction rates that appear to respond to seasonal changes in the productivity of purple sulfur bacteria although photosynthetic activity within the microbial plate influences the 18 o so4 34 s so4 relationship the biosignature for photosynthetic sulfur bacteria is restricted to the oxic anoxic transition zone and is apparently minor relative to the more prevalent process of sulfate reduction operative throughout the light deprived deeper anoxic water column and sediment pore waters

major dextral transcurrent displacements along the northern rocky mountain trench and related lineaments in north central british columbia the northern rocky mountain trench and a number of other prominent lineaments along and east of the eastern margin of the intermontane belt mark faults along which dextral transcurrent movements have been dominant offsets of shelf to off shelf facies boundaries in lower paleozoic rocks indicate a cumulative displacement of at least 750 km and probably 900 km within the system of faults related to those in the northern rocky mountain and the tintina trenches farther west another system of faults appears to offset plutons and stratigraphic assemblages along the eastern margin of the intermontane belt by as much as 300 km these faults including the kutcho and the pinchi connect in part with the teslin suture zone in yukon territory and probably with the fraser river straight creek fault zone in southern british columbia although dextral transcurrent faulting may have taken place between the middle jurassic and early cenozoic the most convincing evidence points to middle cretaceous and particularly to early cenozoic eocene displacements the eocene movements were temporally related to plutonism volcanism lamprophyre dike emplacement high heat flow sedimentation in grabens and rapid uplift of northwesterly trending elongate ranges climactic episodes of granite emplacement particularly in and near the northern omineca crystalline belt at 100 m y 70 m y and 50 m y ago may have been facilitated by changes from dominantly compressional to dominantly transcurrent and related tensional strain

primitive permo triassic volcanism in the canadian cordillera tectonic and metallogenic implications volcanogenic massive sulfide deposits of various ages types and geochemical affinities occur within allochthonous and para autochthonous terranes of the cordillera of western canada the terrane affiliation age and chemical association of volcanic rocks can be used as a guide in the exploration for volcanogenic massive sulfide mineralization the kutcho massive sulfide deposit occurs within volcanic rocks of the kutcho assemblage in the fault bounded king salmon allochthon in northern british columbia the kutcho creek deposit is characterized by a permo triassic age of mineralization and primitive chemical and radiogenic isotope compositions of the host rocks which are distinct from those of other volcanogenic massive sulfide deposits in the canadian cordillera the kutcho deposit is interpreted to have formed within a primitive intraoceanic are environment distinct from the setting ascribed to more typical felsic volcanic hosted kuroko massive sulfide deposits the age mineralogy and chemical and isotopic compositions of rocks from the kutcho assemblage are comparable to those of two other fault bounded silvers in the cordillera the sitlika assemblage and the venables valley red hill area these similarities suggest that rocks from the three areas may have formed within the same period and tectonic environment as such the latter two areas are prospective for volcanogenic massive sulfide exploration in the canadian cordillera

new u pb zircon ages integrated with ammonite biochronology from the jurassic of the canadian cordillera the accuracy of published jurassic time scales is compromised by a paucity of sufficiently accurate and stratigraphically well constrained isotopic dates u pb dating of volcaniclastic rocks within fossiliferous marine successions is a method suitable of providing additional useful calibration points stikinia and wrangellia two of the major accreted terranes of the canadian cordillera comprise volcanosedimentary assemblages of magmatic arc origin within them we selected sections from which we report 14 new u pb zircon ages that are integrated with ammonite biochronology at the zonal level poor zircon recovery and complex u pb systematics made it difficult to obtain precise ages for many of the samples the following ages were determined with 2 errors 194 0 9 1 1 8 ma from the plesechioceras harbledownense assemblage upper sinemurian 191 5 0 8 ma from the plesechioceras harbledownense assemblage upper sinemurian to the whiteavesi zone lower pliensbachian 185 6 6 1 0 6 ma from the frebo

strain partitioning in accretionary orogens and its effects on orogenic collapse insights from western north america changes in relative plate motions during the construction of accretionary orogens generally result in varying structural styles along the length of the orogen these disparate structural styles can be interpreted as having been formed by different tectonic regimes along the orogenic axis that formed at the same time if the orogen is considered at the large scale the differences in the way in which the crust responds during accretion can be explained by large scale strain partitioning within the same overall tectonic environment the westernmost canadian cordillera records the transition from late cretaceous dextral strike slip faulting to near orthogonal compression along the orogenic axis we postulate that the transition between strike slip dominated to compression dominated tectonics represents a late cretaceous partitioning of strain that resulted in a significant difference in crustal rheology along strike of the orogeny this had a dramatic effect on subsequent tertiary orogen scale extension we propose that plate readjustments in the tertiary led to orogen perpendicular collapse in portions of the orogen facilitated by decoupling between the middle and lower crusts along thermally weakened layers in contrast localized orogen parallel extension occurred in other portions of the orogen along kinematically linked large dextral strike slip faults where the upper crust remained coupled to the middle and lower crust new data indicate that partitioning of strain occurs across very large regions within any orogenic system and that the way in which strain is partitioned can lead to dramatic differences in future orogenic processes it becomes apparent from these data that orogens must be examined as a whole and that differing structural styles of similar ages are likely responses to the same overall tectonic regime

evolution of the hazelton arc near terrace british columbia stratigraphic geochronological and geochemical constraints on a late triassic early jurassic arc and cu au porphyry belt understanding the development of island arcs that accreted to the north american craton is critical to deciphering the complex geological history of the canadian cordillera in the case of the hazelton arc part of the stikine terrane or stikinia in northwestern british columbia understanding arc evolution also bears on the formation of spatially associated porphyry cu au epithermal and volcanogenic massive sulfide deposits the hazelton group is a regionally extensive long lived and exceptionally thick upper triassic to middle jurassic volcano sedimentary succession considered to record a successor arc that was built upon the paleozoic and triassic stikine and stuhini arcs in central stikinia near terrace british columbia the lower hazelton group telkwa formation comprises three volcanic intrusive complexes mt henderson mt o brien and kitselas that at their thickest constitute almost 16 km of volcanic stratigraphy basal telkwa formation conglomerates and volcanic rocks were deposite

origin of the eskay creek precious metal rich volcanogenic massive sulfide deposit fluid inclusion and stable isotope evidence the eskay creek deposit is an unusual polymetallic au ag rich volcanogenic sulfide sulfosalt deposit located in the iskut river area of northwestern british columbia canada economic concentrations of precious and base metals are contained in the 21 zone which is divided into a number of subzones as of december 31 1998 total production and proven probable reserves are 1 9 million tons mt at 60 2 g t au 2 652 g t ag 3 2 percent pb 5 2 percent zn and 0 7 percent cu the 21b zone which contains the bulk of the reserves began production in 1995 the mineralization occurs mainly as well preserved stratiform clastic beds of sulfide sulfosalt debris and also as discordant footwall quartz sulfide veins the hydrothermal system that formed the eskay deposit was low temperature

magnetic dipolarization with substorm expansion onset 1 we compare the timing and spatial extent of dipolarization with auroral breakup by using magnetic field data 1 min time resolution from goes 8 and goes 9 and global auroral images 37 s frame rate from the polar ultraviolet imager we survey the entire year of 1997 data and obtain 32 unambiguous dipolarization events that had ionospheric footprints within 2 hours in magnetic local time mlt from their associated auroral breakups it is found that the auroral breakup preceded the arrival of dipolarization at goes by an average of 1 7 2 7 min 1 0 2 1 min for events 1 hour from the auroral breakup the delay of geosynchronous dipolarization represents a propagation time as the substorms rarely initiated at the goes location propagation velocities derived from the propagation time are 60 km s 1 eastward 75 km s 1 westward and 270 km s 1 earthward at geosynchronous orbit we also show for the first time that auroral bulge and dipolarization region closely map the bulge is the region of currents out of the ionosphere the eastern edge of the dipolarization region where field aligned currents are flowing into the ionosphere is based on the t89 magnetic field model located at 0 5 hour in mlt eastward of the eastern edge of the auroral bulge within the initiation site meridian the dipolarization onset was observed 1 min prior to the auroral breakup at the goes altitude this time is associated with alfven transit time and therefore depends on the radial location of the onset the azimuthal scale of the initiation site is found to be 0 5 hour in mlt in the ionosphere or less than 1 earth s radius at 6 6 re these results support current diversion into the ionosphere as a critical part of substorms finally a simple timing analysis of specific substorm onset time features suggests that near earth reconnection cannot occur before current disruption and therefore cannot be the cause of substorm

a coral reef in the telkwa range british columbia the earliest jurassic example an end triassic mass extinction profoundly affected reef ecosystems that flourished in the late triassic tethys seaway the collapse of late triassic coral sponge reefs was followed by an early jurassic hettangian sinemurian perturbation interval with a near global absence of reefs and sharp reductions in diversity a jurassic sinemurian reef in the hazelton group of central british columbia appears to fill the gap its paleoecology and composition show it to be the first large framework example in north america and perhaps the world it demonstrates that the reef building triassic coral phacelostylophyllum survived the extinction event and was constructing reefs in early jurassic time during a global reef eclipse the reef is a 48 m thick bioherm that grew within the island arc complex of stikinia following a decrease in volcanism reef building began with bivalves growing upon water lain tuffs the reef was dominated by large dendroid phaceloid corals primarily phacelostylophyllum rugosum a spe

geochemistry and nd isotopes of the francois lake plutonic suite endako batholith host and progenitor to the endako molybdenum camp central british columbia the endako low f granodiorite type porphyry mo deposit is hosted by the triassic to eocene endako batholith which comprises five temporally distinct plutonic suites only one of which is mineralized pre mineralization suites range in composition from diorite to granodiorite the synmineralization jurassic cretaceous francois lake suite includes two granodiorite to monzogranite bearing subsuites postmineralization phases include the eocene sam ross creek monzogranite the batholith spans a silica range of 44 80 wt and consists of metaluminous to slightly peraluminous low to high k i type granitoids the sam ross creek phase is an a type granite positive e nd t values 1 1 to 7 2 indicate derivation predominately from juvenile source materials but with variable input from an older crustal component evidence suggests generation of older plutonic suites in a juvenile arc type setting and younger k rich felsic suites via recycling of juvenile arc crust without significant mantle derived contrib

geochronology of mid cretaceous to eocene magmatism babine porphyry copper district central british columbia new u pb and 40 ar 39 ar isotopic dating in the babine porphyry copper district of central british columbia documents three distinct magmatic events at 107 104 85 78 and 54 50 ma the earliest event involved emplacement of rhyolite domes into submarine volcanic rocks of the rocky ridge formation the rhyolite domes and related dacitic to basaltic volcanic rocks gave a u pb age of 107 9 0 2 ma and an 40 ar 39 ar age of 104 8 1 2 ma the rhyolites which were previously mapped as eocene are reinterpreted to be part of a previously unrecognized mid cretaceous cauldron subsidence complex the regionally extensive late cretaceous magmatic event is also recognized in the babine district and is represented by 40 ar 39 ar ages of 85 2 2 8 and 78 3 0 8 ma on two bulkley intrusions one of which has associated porphyry copper mineralization the final magmatic event is the most widespread and involved emplacement of the babine intrusions and formation of numerous porphyry copper deposits including the bell and granisle past producers twenty one new 40 ar 39 ar isotopic ages for these intrusions and coeval andesites of the newman formation have a narrow range from 53 6 0 9 to 49 9 0 6 ma whereas previous k ar isotopic dating had a possible range of 15 ma the mid cretaceous late cretaceous and eocene magmatic suites in the babine dis trict are interpreted to be part of a long lived volcano plutonic complex that was the site of periodic magmatism and porphyry copper mineralization over a 60 ma time period this complex may have evolved within a zone of extension pull apart basins situated between dextral strike slip faults that were active during periods of rapid oblique plate con vergence

late jurassic magmatism metamorphism and deformation in the blue mountains province northeast oregon an early to mid mesozoic record of sedimentation magmatism and metamorphism is well developed in the blue mountains province of northeast oregon detailed studies both north and south of the blue mountains province e g terranes of the intermontane belt klamath mountains and western sierra nevada have documented a complex middle to late jurassic orogenic evolution however the timing of magmatic metamorphic and deformational events in the blue mountains and the significance of these events in relationship to other terranes in the western north american cordillera remain poorly understood in this study we investigate the structural magmatic and metamorphic histories of brittle to semibrittle deformation zones that indicate widespread late jurassic orogenesis in the blue mountains province folding and faulting associated with contractional deformation are primarily localized along terrane boundaries e g baker wallowa and baker izee olds ferry boundaries and within the composite baker oceanic melange terrane e g bourne greenhorn subterrane boundary these brittle to semibrittle deformation zones are broadly characterized by the development of e w oriented slaty to spaced cleavage in fine grained metasedimentary rocks of the baker terrane e g elkhorn ridge argillite approximately n s bivergent folding and n and s dipping reverse and thrust faulting on opposite flanks of the baker terrane similarly oriented contractional features are also present in late middle triassic to early late jurassic i e oxfordian stage ca 159 ma sedimentary rocks of the john day and huntington areas of northeast oregon radiometric age constraints from youngest detrital zircons in deformed sedimentary rocks and crystallization ages of postkinematic plutons which intrude the deformation zones limit deformation to between ca 159 and ca 154 ma we suggest that the widespread approximately n s directed contractional features in the blue mountains province record a short lived intense early late jurassic deformational event and preserve an example of upper crustal strain localization associated with terminal arc arc collision between the olds ferry and wallowa island arc terranes the age interval of deformation in the blue mountains province is younger than middle jurassic deformation in the canadian cordillera and klamath mountains siskiyou orogeny and predates classic nevadan orogenesis

a geomagnetic polarity timescale for the permian calibrated to stage boundaries the reverse polarity kiaman superchron has strong evidence for at least three or prob ably four normal magnetochrons during the early permian normal magnetochrons are during the early asselian base ci1r 1n at 297 94 0 33 ma late artinskian ci2n at 281 24 2 3 ma mid kungurian ci3n at 275 86 2 0 ma and roadian ci3r an at 269 54 1 6 ma the mixed polarity illawarra superchron begins in the early wordian at 266 66 0 76 ma the wordian capitanian interval is biased to normal polarity but the basal wuchiapingian begins the beginning of a significant reverse polarity magnetochron lp0r with an overlying mixed polarity interval through the later lopingian no significant magnetostratigraphic data gaps exist in the permian geomagnetic polarity record the early cisuralian magnetochrons are calibrated to a succession of fusulinid zones the later cisuralian and guadalupian to a conodont and fusulinid biostratigraphy and lopingian magnetochrons to conodont zonations age calibration of the magnetochrons is obtained through a bayesian approach using 35 radiometric dates and 95 confidence intervals on the ages and chron durations are obtained the dating control points are most numerous in the gzhelian asselian wordian and changhsingian intervals this significant advance should provide a framework for better correlation and dating of the marine and non marine permian

polar ultraviolet imager observation of auroral breakup 1 the nasa s polar satellite completed its mission in march 2008 during its 12 year operation the ultraviolet imager uvi on board polar acquired a large number of auroral images in fuv bands with unprecedented spatial and temporal resolutions the purpose of this brief report is to provide a list of auroral substorm onset identified with uvi to the community for full use a total of 2003 auroral substorm onsets are identified in the northern hemisphere between 1996 and 2000 and 536 onsets in southern hemisphere in 2007 distributions of the onset locations are near gaussian with a population mean of 65 9 standard deviation 2 2 in magnetic latitude mlat and 22 6 1 1 in magnetic local time mlt for the northern hemispheric events and 68 0 2 3 in mlat and 22 6 1 1 in mlt for the southern hemispheric events a comparison with previously published image onset results suggests that substorms occur more frequently and intensely in the descending than in the ascending phase of solar cycle the list of onset events includes the onset times and locations and is provided as a supplement

mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group the age relations geochemistry and sedimentology of the rocky ridge formation of the skeena group are used to test competing tectonic reconstructions for the mid cretaceous canadian cordillera as well as the timing and location of the accretion of the insular superterrane pollen and macrofossil assemblages indicate that these intrabasinal basalts were erupted along the southern margin of the bowser basin in the early albian to early cenomanian single crystal fusion and step heating 40ar 39ar dating of hornblendes in one basalt flow from the uppermost part of the formation yielded middle cenomanian ages of 94 3 0 4 95 6 1 6 and 95 0 1 6 ma vesicular basalt flows interbedded with crystal rich tuff breccias contain evidence for hot emplacement as pyroclastic flows individual eruptive centers are identified by their proximal facies paleoflow indicators within the lava flows paleoflow indicators within interbedded volcaniclastic fluvial deposits geochemical differences and geographic isolation of volcanic deposits major and trace element geochemistry from 20 sampled lava flows indicates an alkali basalt composition for the volcanics the basalts of the northern rocky ridge volcanic center show enrichment of light rare earth and large ion lithophile elements with strong negative nb ta anomalies whereas the basalts of the southern tahtsa lake volcanic center show depletion to slight enrichment of light rare earth elements slight enrichment of large ion lithophile elements with minimal negative nb ta anomalies the geochemistry combined with paleogeographic and regional tectonic reconstruction suggests a continental arc setting with intraarc extension the presence of deeper marine facies to the west and the lack of a western sediment source in the skeena group indicate that the technically active insular superterrane was not west of the study area during mid cretaceous time thus we reconsider the omineca belt as the main axis of a mid cretaceous continental arc placing the intermontane superterrane in the intraarc to forearc position with the rocky ridge volcanics erupted along the forearc side of the omineca arc coeval regional strike slip faulting and reconstructed oblique plate convergence suggest a transtensional setting for rocky ridge intraarc extension an electronic supplement of tables a1 a2 may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 95tc03496 mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group kari n bassett and karen l kleinspehn diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

relationship between substorm onset locations and nightside convection pattern features 1 this paper presents a study using the super dual auroral radar network superdarn convection database in conjunction with a list of substorm onsets determined from the image satellite to examine the relationship between substorm onset locations and features of the convection pattern particular attention is devoted to the latitude of onset locations compared to the latitude of the near midnight convection reversal boundary crb and the latitude at which the highest flow velocity the flow peak was observed equatorward of the crb because of this focus the set of observations examined is limited to cases where superdarn provided sufficient observations in the midnight sector to determine the latitude of the crb distributions of the latitude difference between the onset location and the crb and flow peak were examined for different times leading up to substorm onset and for a few different conditions it was found that the onsets occurred equatorward of the crb in more than 95 of the cases and that the distribution was approximately centered on the flow velocity peak

stratigraphic evolution of the paleozoic stikine assemblage in the stikine and iskut rivers area northwestern british columbia the stikine assemblage the basement of stikinia extends 500 km along the western flank of the intermontane belt east of younger coast belt plutons four different stratigraphic successions are characteristic of lower to middle devonian carboniferous and permian rocks in the stikine and iskut rivers area west of forrest kerr creek are penetratively deformed lower to middle devonian island arc volcaniclastic rocks coralline limestone and felsic tuff fringing carbonate buildups in an arc setting are best illustrated in the sequence at round lake where lower carboniferous mafic dominated bimodal submarine volcanic rocks grade upward into two distinctive coarse echinoderm limestone units and medial siliceous siltstone and limestone conglomerate conodont colour alteration indices for lower carboniferous rocks near newmont lake indicate an anomalously low temperature thermal history upper carboniferous permian polymictic volcanic conglomerate and lower permian limestone overlie these strata there t

timing and tectonic setting of stikine terrane magmatism babine takla lakes area central british columbia new bedrock mapping completed as part of the nechako natmap project indicates that the area between babine and takla lakes in central british columbia is underlain by rocks of the early permian asitka late triassic takla and early to middle jurassic hazelton volcanic arc assemblages of the stikine terrane these are cut by large composite stocks of quartz diorite granodiorite and quartz monzonite previously mapped as the late triassic to early jurassic topley intrusions new u pb n 6 and laser 40ar 39ar n 10 isotopic age dates reported in this paper suggest there are two distinct ages of plutons the topley intrusive suite with isotopic ages between 218 and 193 ma and east of babine lake the new spike peak intrusive suite with isotopic ages ranging from 179 to 166 ma west of the main plutonic belt is a thick volcanic succession of subaerial porphyritic andesite flows volcanic breccias and rhyolitic ash flow tuffs that have isotopic ages between 185 and 174 ma these rocks are assigned t