escape hypothesis for the stikine block comparison of stratigraphic faunal and paleomagnetic characteristics of the stikine terrane of british columbia with other terranes in the cordilleran collage reveals broad similarities with a group of terranes that formed a volcanic belt marginal to north america in paleozoic and early mesozoic time unlike stikine these terranes lie inboard of another belt of terranes that represents an early mesozoic subduction complex or melange belt thus in british columbia the marginal volcanic belt is apparently doubled in the columbia embayment region the marginal volcanic and melange belts are missing and rocks of the outermost major component of the collage the wrangellia superterrane are juxtaposed directly against cratonic rocks we propose that stikine is the missing fragment from the columbia embayment its northward tectonic escape was driven by the early stages of collision of the wrangefia superterrane with north america in middle to late jurassic time the escaped fragment was then trapped between melange and more northerly later arriving parts of the wrangellia superterrane in early to mid cretaceous time

seismicity of the forearc marginal wedge accretionary prism three different types of seismic data have been examined for seismic events occurring within the zone called the accreted wedge or forearc marginal wedge that underlies the inner trench wall of some arcs these types of data are 1 teleseismically recorded earthquakes that have been reported in the literature as occurring in major arc trench regions these events fail to demonstrate that earthquakes occur within the accreted wedge because the uncertainty of focal depth usually exceeds the depth dimension of the accreted wedge these data include many tsunamigenic earthquakes 2 local earthquakes located by combined ocean bottom seismograph and land networks in the arc trench region in the new hebrides and the central and eastern aleutian trench none of the more reliable of these hypocenters lies within the accreted wedge 3 s p intervals measured at stations on islands located on the outer ridge or at ocean bottom seismograph stations on the forearc marginal wedge these data do not show the existence of events occurring within the accreted wedge e g from 18 ocean bottom seismograph stations with a cumulative operation time of about 1 year the smallest s p time is about 2 5 s for events in the new hebrides and about 4 s for events in the adak and kodiak regions we found no s p time smaller than 2 s from 6 years of seismograms recorded at middleton island alaska and no s p time smaller than 4 s from 25 years of seismograms recorded on barbados all of the events could have occurred outside the forearc marginal wedge although we have located no events with hypocenters which are situated unambiguously within the accreted wedge we have located many events which occurred in the vicinity of the accreted wedge some occur in the upper portion of the benioff zone shallower than about 40 km and others occur in the leading edge of the overriding plate these locations reported in this paper which are within or close to the seismic networks comprise some of the most reliably located shallow events ever reported in convergent margins of island arcs the absence of seismic activity within the accreted wedge in any island arc that we studied suggests that the deformation evident in this region occurs aseismically

triassic jurassic silica undersaturated and silica saturated alkalic intrusions in the cordillera of british columbia implications for arc magmatism alkalic igneous rocks of early mesozoic age are found in both the quesnel and stikine terranes in the canadian cordillera and include both silica undersaturated and silica saturated types the saturated complexes are most abundant in quesnellia and are multiphase complexes dominated by monzonite to diorite intrusions undersaturated complexes are distributed through both terranes are dominated by syenite with lesser monzonite and pyroxenite and when present as a single intrusion are characterized by concentric zoning igneous layering and planar mineral fabrics both types of complex are associated with cu au mineralization accompanied by potassic and distinctive sodic and calc potassic alteration assemblages although undersaturated and saturated alkalic intrusions are petrographically distinct a petrogenetic association is suggested by their spatial coincidence in some districts and similarities in their tectonic environment and associated alteration the undersaturated complexes represent a distinctive suite of alkalic intrusion with magmatic arc affinities and their emplacement into both stikinia and quesnellia between 210 and 200 ma suggests that these terranes were either linked at that time or have shared unusual but similar magma generating tectonic events at identical times

structural aspects of suspect terranes and accretionary tectonics in western north america abstract the regional structural relationships between the cordilleran suspect terranes and cratonic north america and within the suspect terranes themselves are reviewed for eastern alaska western canada and united states and northern mexico a distinctive characteristic of the relationships between the suspect terranes and the north american craton is that although much modified by post accretionary strike slip faulting thrust faulting and extensional detachment faults the generally more distal and or oceanic terranes are frequently found as rootless nappes or thin thrust sheets sitting upon more inboard terranes or upon the craton itself the amount of tectonic transport implied is large strike slip faulting has been a very significant aspect of the structural evolution of the north american cordillera with total distributed displacements of over 2000 km documented on geologic relationships alone and much more implied by paleomagnetic evidence at least since later mesozoic time to the present movements were such that the terranes moved northward post accretionary intraplate telescoping along thrust faults transpressional strike slip faulting and delamination at mid to upper crustal levels were important processes in cordilleran tectonic evolution

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

cache creek terrane entrapment oroclinal paradox within the canadian cordillera exotic and far traveled oceanic crustal rocks of the cache creek terrane cc are bordered by less exotic quesnel qn and stikine st arc terranes to the east north and west all of these terranes are enveloped by an arcuate belt of displaced continental margin rocks the kootenay ko nisling ns and parts of the yukon tanana ytt terranes that have indirect ties to ancestral north america na initial 87sr 86sr isopleths conform to this arcuate pattern such a pattern of concentric belts presents a geological conundrum how did the qn st and cc come to be virtually enveloped by terranes with ties to na past and current models that explain assembly of the canadian cordillera are deficient in their treatment of this problem we propose that early mesozoic qn and st were joined through their northern ends as two adjacent arc festoons that faced south toward the cache creek ocean panthalassa oceanic plateau remnants within the cc today were transported from the tethyan realm and collided with these arcs during subduction of the cache creek ocean counterclockwise oroclinal rotation of st and ns terranes in the late triassic to early jurassic caused enclosure of the cc rotation continued until these terranes collided with qn in the middle jurassic paleomagnetic declination data provide support for this model in the form of large average anticlockwise rotations for permian to early jurassic sites in st but moderate clockwise rotations for sites in qn specific modern analogues for the cordilleran orocline include the yap trench where the caroline rise is colliding end on with the mariana arc and the banda arc located on the southeastern tail of the asian plate which is being deformed into a tight loop by interactions with the australian and pacific plates

evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the canadian cordillera sm nd and rb sr isotopic studies of several phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked pre existing continental crust in contrast isotopic data collected from two of the largest terranes in the canadian cordilleran orogenic belt the alexander and stikine terranes show that these tectonic fragments are composed of mantle derived continental crust in many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some proterozoic crustal regions

detrital zircon geochronology and regional correlation of metasedimentary rocks in the coast mountains southeastern alaska u pb ages have been determined for 55 detrital zircon grains from a metasedimentary sequence along the west flank of the coast mountains in southeastern alaska these rocks belong to the port houghton assemblage which consists of upper paleozoic pelitic and psammitic schist metaconglomerate metabasalt and marble the port houghton assemblage rests unconformably on metamorphosed and deformed mid paleozoic arc type volcanics endicott arm assemblage which gradationally overlie upper proterozoic lower paleozoic continental margin strata tracy arm assemblage three main clusters of ages are present 330 365 ma 19 grains 1710 2000 ma 27 grains and 2450 2680 ma 6 grains additional grains are approximately 2334 2364 and 3324 ma comparison of these ages with detrital zircon ages in other cordilleran assemblages supports previous interpretations that metasedimentary rocks in the coast mountains i form a southwestern continuation of the yukon tanana terrane of eastern alaska and yukon

the nicola group late triassic and early jurassic subduction related volcanism in british columbia mafic lavas of the nicola group are divided into three distinct petrographic and geochemical types type 1 lavas are strongly augite porphyritic picrites basalts and andesites that belong to a high potassium to shoshonitic rock series type 2 lavas are augite and plagioclase porphyritic basalts and andesites that belong to a low potassium calc alkaline series and type 3 lavas are petrographically variable tholeiitic to transitional basalts and andesites low concentrations of ti zr y and nb and moderate to high concentrations of k rb ba and sr in type 1 and 2 lavas clearly indicate a subduction related tectonic setting of eruption type 3 lavas show chemical affinities intermediate between modern day island arc and intraplate volcanics type 1 shoshonitic lavas generally lie east of and are younger than type 2 calc alkaline lavas a relationship that implies an east dipping early mesozoic subduction zone beneath the nicola arc these interpretations resolve previous uncertainties regarding th

the northern termination of the cache creek terrane in yukon middle triassic arc activity and jurassic cretaceous structural imbrication the northernmost part of the cache creek terrane lies in south central yukon and comprises metavolcanic rocks hemipelagic chert and shale newly identified volcaniclastic and clastic rocks michie

earthquake activity at the kodiak continental shelf alaska determined by land and ocean bottom seismograph networks the spatial pattern of earthquakes determined by a combined land and ocean bottom seismometer obs network in the kodiak island shelf region differs systematically from the pattern determined by a land network and from locations determined by the international seismological centre isc as a part of a larger study of seismic risk on the continental shelf near kodiak island we augmented the university of alaska land network by deploying 11 recoverable obs units south of kodiak island for 2 months in the summer of 1979 despite a relatively short operation time and various instrument malfunctions the combined network detected 19 locatable earthquakes in the shelf region because of the structural heterogeneity of this area the earthquakes were located with a scheme which allowed different velocity models to be used for travel time calculations of phases traveling to different stations in the network the locations of earthquakes determined using data from both land and obs networks were displaced about 12 km from the hypocenters of the same earthquakes determined using only land network data for these events on the continental shelf azimuthal control of the joint land obs network is excellent and thus the joint land obs network locations are considerably more reliable than locations determined with the land network data alone when the locations of the combined land obs network are compared to 15 yr of teleseismic locations reported by the isc the center of teleseismic activity appears to be about 20 to 30 km north of the center of activity determined in our study this difference between locally determined and teleseismically determined location is similar to that observed in other studies of earthquakes and nuclear explosions in the aleutian arc

cache creek ocean closure or enclosure exotic tethyan faunas within the cache creek terrane contrast markedly with faunas and lithologic associations in the adjacent quesnel and stikine terranes in northern british columbia and southeast yukon all three terranes are enveloped in the north by pericontinental rocks of the yukon tanana terrane a geometry that imposes severe constraints on terrane assembly models for the northern canadian cordillera our solution to the problem invokes a northern join between the stikinia and quesnellia arcs through the yukon tanana terrane forming an orocline that encloses the cache creek terrane this model involves 1 collision of a linear oceanic plateau at the cusp between quesnellia and stikinia 2 anticlockwise rotation of stikinia about an axis in the yukon tanana terrane 3 simultaneous enclosure of the cache creek ocean and 4 emplacement of quesnellia onto the margin of ancestral north america and the cache creek terrane onto stikinia during final closure of the orocline early mesozoic paleomagnetic declinations in stikinia are permissive of the large anticlockwise rotations predicted by the model similar large scale rotations and ocean basin enclosure are common features in the southwest pacific this model accounts for paleozoic and younger linkages between yukon tanana and both northern stikinia and quesnellia the striking similarity between triassic jurassic arcs east and west of the cache creek terrane and the profound early mesozoic deformational event in the yukon tanana terrane

sm nd isotopic geochemistry of precambrian to paleozoic granitoid suites and the deep crustal structure of the southeast margin of the newfoundland appalachians in the eastern central mobile belt of the newfoundland appalachians late precambrian basement inliers have end from 3 to 2 but cambro ordovician metasedimentary rocks have initial end below 7

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

late triassic arc related potassic igneous rocks in the north american cordillera igneous rocks of late triassic age are widespread in the cordillera of western north america and except in wrangellia consist of subduction related plutonic and volcanic suites many of these including those in the stikinia quesnellia rattlesnake creek and jackson terrenes and in southern california are clinopyroxene rich and belong to high potassium and shoshonitic rock series features that are generally absent from older and younger igneous rocks in the same terranes the late triassic subduction related rocks are exposed in two discontinuous belts that lie east and west of the cache creek terrane in canada and correlative melange terranes farther south stratigraphic and structural data suggest that these belts were spatially separate magmatic arcs in late triassic time tectonic implications of this analysis include an explanation of middle jurassic cordilleran deformation as the result of collision of the western with the eastern belt absence of late triassic links between stikinia and quesnellia disassociation of stikinia with terranes in northwestern nevada and tentative correlation of the wallowa seven devils terrane with stikinia rather than wrangellia

nina creek group and lay range assemblage north central british columbia remnants of late paleozoic oceanic and arc terranes in north central british columbia a belt of upper paleozoic volcanic and sedimentary rocks lies between mesozoic arc rocks of quesnellia and ancestral north america these rocks belong to two distinct terranes the nina creek group of the slide mountain terrane and the lay range assemblage of the quesnel terrane the nina creek group is composed of mississippian to late permian argillite chert and mid ocean ridge tholeiitic basalt formed in an ocean floor setting the sedimentary and volcanic rocks the mount howell and pillow ridge successions respectively form discrete generally coeval sequences interpreted as facies equivalents that have been interleaved by thrusting the entire assemblage has been faulted against the cassiar terrane of the north american miogeocline west of the nina creek group is the lay range assemblage correlated with the harper ranch subterrane of quesnellia it includes a lower division of mississippian to early pennsylvanian sedimentary and volcanic rocks some with con

the p velocity within the tonga benioff zone determined from traced rays and observations p waves with travel time residuals between 0 s and 12 s are observed at regional stations in samoa afi and raoul island rao for 39 earthquakes in tonga with focal depths between 70 km and 300 km these anomalously large residuals apparently are produced because seismic phases travel along the strike of the tonga benioff zone within the high velocity subducted lithosphere for up to 1200 km before arriving at afi and rao to eliminate erroneous residuals caused by poor event locations we selected 11 stations and reread the available p times at these stations for the 39 events these arrivals and pp p intervals were used to relocate the events using a variant of the joint hypocenter determination method then the pattern of residuals at afi and rao not used in the relocation was compared to the pattern of residuals expected for various models of the subducted lithosphere as determined by ray tracing the observed pattern of residuals at afi is consistent with the ray traced models if some of the first arrivals are produced by rays traveling directly along the strike of the subducting lithosphere and if others are produced by rays which reflect once off the upper surface of the subducting lithosphere before arriving at afi the observed residuals can be explained by a model where the p velocity in the subducted lithosphere is 8 higher than the velocity in the herrin model the residuals are fit even better by a layered slab model in which the seismic velocity is about 6 higher than the herrin velocity at the upper surface of the subducted lithosphere and about 9 higher at the bottom of the slab these velocity contrasts could be produced if the temperature in the slab was 700 c cooler than the surrounding mantle and if there were no partially melted material within the slab the existence of these anomalously large residuals suggests that the high velocity region in the upper 300 km of the mantle beneath tonga must be fairly continuous over distances of 1000 km and more

geochemistry of the neoproterozoic tilemsi belt of iforas mali sahara a crustal section of an oceanic island arc abstract the pre pan african neoproterozoic tilemsi belt of the western iforas mali sahara comprises a predominantly bimodal mafic and felsic volcanic assemblage associated with numerous intrusions mainly of mafic composition the volcanics include island arc tholeiites and the calc alkali rocks rhyolites dacites and andesites the volcanic domain shows similarities to recent oceanic island arc systems as well as to archaean greenstone belts the tilemsi magmatic sequence represents an example of a section through the middle and upper levels of a neoproterozoic intra oceanic island arc crust that is composed nearly exclusively of igneous material the middle crustal section is mainly of mafic composition while the upper crustal level contains more silica rich plutonic volcanic and volcaniclastic assemblages the mafic intrusions include a large proportion of cumulates indicating that fractional crystallization played an important role during the evolution of the oceanic arc suites compositionally the arc changed through time from an immature to a relatively developed system the rocks became more silica and incompatible element rich with higher sr isotopic ratios this trend reflects the increasing growth and maturity of the arc

pebbly versus bouldery rock glaciers morphology structure and processes abstract differences in rockwall geology cause two types of rock glaciers a bouldery rock glacier having the active layer composed of matrix free boulders derived from crystalline rocks and massive limestone and a pebbly rock glacier consisting of matrix supported debris derived from less resistant shale and platy limestone such material composition controls transport processes responsible for the shape size and internal structure of the two types of rock glaciers this paper compares the major processes controlling the morphology and structure of the two types in the swiss alps based on mapping description of geological and morphological characteristics direct observation of stratigraphy and geophysical soundings of internal structure in the swiss alps pebbly rock glaciers are distinguished from bouldery rock glaciers by the clast size of 15 20 cm in the mean b axis diameter the former are fed by small scale rockfalls debris flows and solifluction whereas the latter mainly originate from large rockfalls pebbly rock glaciers are generally smaller 100 k m in bouldery rock glaciers appears to originate from snow banks buried with deposits of large rockfalls from the large source rockwall

petrogenesis of the natkusiak continental basalts victoria island northwest territories canada the natkusiak flows 1100 m thick sills and dykes of victoria island northwest territories are part of an extensive late hadrynian magmatic event in the northern canadian shield the flows are subaerially erupted plateau basalts and together with associated doleritic sills and dykes display many geochemical features typical of continental tholeiites in addition to fractional crystallization the rocks were affected by crustal contamination but to a smaller degree than usually encountered in continental tholeiites elsewhere the basalts were derived from a depleted upper mantle source very similar to that of n type mid ocean ridge basalts although it is suggested that the melting residue contained small amounts of garnet 2

upper triassic takla group volcanic rocks stikine terrane north central british columbia geochemistry petrogenesis and tectonic implications the upper triassic takla group volcano sedimentary assemblage is part of the stikine terrane of the intermontane belt in the canadian cordillera and covers an area of more than 30 000 km2 in a belt up to 50 km wide and more than 800 km long in the mcconnell creek area of north central british columbia the assemblage consists of plagioclase clinopyroxene phyric dominantly basaltic to andesitic flows and pyroclastic rocks interlayered with volcanogenic sedimentary rocks compositionally the volcanic rocks are intermediate between tholeiitic and calc alkaline their mantle normalized trace element patterns are characterized by a moderate large ion lithophile element enrichment and nb and ti depletion suggesting that magmatism occurred in a volcanic arc setting flat heavy rare earth element chondrite normalized patterns with la yb n ratios from 2 to 4 5 suggest that the parent magma was produced by mantle melting in the spinel stability field the low sr isotopic ratios 87sr 86sri approximately equa