lithospheric processes and products in the southern canadian cordillera a lithoprobe perspective analyses of lithoprobe and other data from southwestern canada provide new insights on how this portion of the cordillera formed during plate convergence along the western margin of north america crustal rocks are detached from their mantle lithosphere which must have been consumed during subduction detachment occurred at or near the base of the crust beneath the intermontane and or omineca belts probably along the tips of tectonic wedges while the rocks were still outboard of the relatively cool mechanically rigid north american craton during the late cretaceous and early tertiary rotation of detached rocks caught between the north american craton and the oceanic plates accounts for some apparently conflicting results between paleomagnetic data that indicate large northward translation of rocks in the western cordillera and regional geological features that appear to preclude comparable amounts of translation of rocks in the eastern cordillera during the same time interval transpression assoc

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

paleomagnetism and geobarometry of the big creek batholith suggests that the yukon tanana terrane has been a parautochthon since early jurassic abstract the 190 5 ma big creek hornblende syenite and monzogabbro batholith has an outcrop area of 400 km2 within the yukon tanana terrane ytt in the west central part of the yukon territory of canada paleomagnetic analysis of 245 specimens from 21 sites isolates a characteristic remanent magnetization residing principally in magnetite and subordinately in hematite the unit mean direction for the batholith from 16 normal polarity and two reverse polarity sites is d 305 6 i 72 1 95 3 9 al in hornblende geothermobarometry at six locations yields emplacement depths of 16 2 km three arguments suggest that the characteristic remanent magnetization was acquired on exhumation at 180 ma more significantly the pole position indicates no northward translation but counterclockwise rotation implying that the ytt formed close to its present position on the jurassic margin of the north american craton this implies in turn that the allochthonous terranes of the intermontane and coast belts of the western cordillera impacted onto and overrode the ytt upon accretion during late cretaceous to middle eocene time

early mesozoic tectonic history of the boundary area east central alaska thrust faulting in the yukon tanana upland of east central alaska has long been hypothesized but few thrust faults have been definitely identified in the boundary area near the international border between alaska and canada metamorphic rocks of the amphibolite facies have been thrust over rocks of the greenschist facies subordinate thrust faults and imbrication occur within both groups of rocks brecciation slickensides slickenfibers rootless isoclinally folded quartz bands and abrupt lithologic changes from gneiss and amphibolite to quartz white mica chlorite schist characterize the thrust zone ultramafic rocks occur locally on or near the thrust plane thrusting is postulated to have occurred when a terrane composed of the amphibolite facies rocks converged with rocks along the continental margin of north america in late triassic and early jurassic time

collisional orogenesis in the northern canadian cordillera implications for cordilleran crustal structure ophiolite emplacement continental growth and the terrane hypothesis during upper triassic to lower jurassic time arc magmatic rocks of the stikine terrane arc marginal sediments of the whitehorse trough igneous and mantle rocks of the cache creek ophiolite and kutcho assemblage and oceanic sedimentary and volcanic rocks of the cache creek terrane represented a magmatic arc forearc basin forearc basement and subduction complex respectively the cache creek subduction complex was thrust to the southwest over the whitehorse trough forearc basin and the stikine terrane in the middle jurassic during collision with an inboard continental domain interpretation of the various lithotectonic assemblages of the northern intermontane belt in terms of a new plate tectonic model has a number of important implications for the canadian cordillera a the model allows comparisons to be drawn with available seismic reflection interpretations of cordilleran crustal structure b ophiolite emplacement was achieved by ramping of forearc oceanic lithosphere onto thick crustal parts of a subducting plate during collisional orogenesis c island arc collision and accretion were the principal mechanisms for continental growth with relatively minor contributions from sliced off oceanic seamounts and or plateaux and d some terrane bounding faults such as the nahlin fault do not represent major lithospheric scale boundaries and their importance in tectonic reconstructions has been overemphasised

paleomagnetism of the lower jurassic copper mountain intrusions and the geotectonics of terrane i british columbia the 198 ma lower jurassic copper mountain intrusions outcrop in the quesnellia subterrane of terrane i in the canadian cordillera thermal demagnetization at 550 10 c defines a single stable remanence component of 25 9 41 2 95 3 6 in 11 reliable sites the resulting pole position of 11 8 e 57 3 n p 2 7 m 4 4 is discordant for the north american craton and reflects 39 6 of clockwise rotation and 9 5 of northward translation combined with previous paleomagnetic and geologic data a consistent model for the geotectonic evolution of quesnellia and terrane i emerges

post triassic para autochthoneity of the yukon tanana terrane paleomagnetism of the early cretaceous quiet lake batholith

paleomagnetism of the spences bridge group and northward displacement of the intermontane belt british columbia a second look the spences bridge group is a mid cretaceous 104 ma volcanic succession in the southern intermontane belt of the canadian cordillera 50 5 n 121 w it comprises the pimainus formation mafic to felsic lava volcaniclastic and interbedded epiclastic rocks and the overlying spius formation andesitic lava flows including previous work we have 55 sites distributed among 15 localities representing most of the 3000 m thickness forty seven sites 286 oriented cores 457 specimens mainly andesites yielded acceptable data the beds are gently to moderately tilted partly due to synvolcanic deformation samples taken from a sequence of flows at any one locality have with one exception well grouped magnetization directions polarities are all normal as expected for rocks laid down in the cretaceous normal superchron declinations always are clockwise of that expected of cratonic north america indicating 60 rotation of the spences bridge group as a whole however declinations differ from locality to locality implying relative interlocality rotations about vertical axes hence inclination only analysis has been used to estimate dispersion mean inclination and paleolatitude minimum dispersion for the spius formation 27 sites was achieved after 80 untilting but the changes between 80 and 100 untilting are insignificant indicating that magnetization was acquired predominantly before tilting by contrast minimum dispersion for the pimainus formation 20 sites was achieved after 50 untilting indicating that magnetization was acquired after synvolcanic tilting when buried beneath the overlying spius formation polished thin section studies show that magnetite in the pimainus formation has undergone extensive low temperature hydrothermal alteration whereas magnetite in the spius formation shows little alteration throughout the range of tilt correction from 0 to 100 the mean inclinations of both formations were less than expected from observations obtained from mid cretaceous rocks of cratonic north america the best estimate of paleolatitude from the spius formation is 50 8 5 0 p 0 05 which is 9 5 5 7 less than would be expected had the rocks had been rigidly attached to north america this corresponds to displacement from the south of 1100 600 km displacement between the northern intermontane belt and craton probably was accommodated along major strike slip faults northern rocky mountain trench finlay pinchi etc in the south our results require a major dextral fault the intra quesnellia fault to be situated during late cretaceous or paleocene time within or marginal to the omineca belt along which about 1000 km of dextral motion occurred this could be a southern extension of the pinchi fault whose trace is now obscured by eocene extension and tectonic denudation the results also indicate that the largest tectonic discontinuity in the canadian cordillera occurs not to the east of the intermontane belt as commonly assumed but to the west because the displacements relative to cratonic north america observed from the intermontane belt are only about one third of those observed from the coast belt

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

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

escape tectonics and the extrusion of alaska past present and future the north pacific rim is a tectonically active plate boundary zone parts of which may be characterized as a laterally moving orogenic stream crustal blocks are transported along large magnitude strike slip faults in western canada and central alaska toward the aleutian bering sea subduction zones throughout much of the cenozoic at and west of its alaskan nexus the north pacific rim orogenic stream nprs has undergone tectonic escape during transport relatively rigid blocks acquired paleomagnetic rotations and fault juxtaposed boundaries while flowing differentially through the system from their original point of accretion and entrainment toward the free face defined by the aleutian bering sea subduction zones built upon classical terrane tectonics the nprs model provides a new framework with which to view the mobilistic nature of the western north american plate boundary zone

travels of the cache creek terrane a paleomagnetic geobarometric and 40ar 39ar study of the jurassic fourth of july batholith canadian cordillera abstract the middle jurassic fourth of july batholith and cross cutting mafic dikes have been studied geochronologically geobarometrically and paleomagnetically to estimate subsequent tectonic motion of the cache creek terrane cct in the northern canadian cordillera 40ar 39ar hornblende ages from a granodiorite phase are similar to u pb zircon ages and indicate rapid cooling of the batholith upon intrusion suggesting that the magnetization age is coincident with the 173 ma crystallization age argon ages of biotite from the granodiorite and two mafic dikes have similar ages of 165 ma which dates cooling through 280 c aluminum in hornblende geobarometry indicates differential uplift of the batholith across a north south fault zone along atlin lake with 6 km more uplift on its eastern side also the eastern side has been tilted downward to the south southwest by 9 combined paleomagnetic data from 20 granitoid and 11 mafic dike sites yield an in situ paleopole at 55 w 63 n dp 5 dm 5 and a tilt corrected paleopole at 81 w 55 n dp 5 dm 6 compared to the 173 ma reference pole for the north american craton the tilt corrected pole suggests a significant southward translation of 16 1 3 7 and a significant clockwise rotation of 107 7 the translation estimate is similar to the jurassic teslin crossing pluton in the stikine terrane however the rotation estimate is very different this could indicate that the cache creek terrane was at a similar latitude of the stikine terrane but the two were not yet amalgamated

on the motion of hawaii and other mantle plumes abstract the hawaiian hotspot and its prominent bend at 47 ma have figured prominently in the development of ideas on the nature of mantle plumes and plate motion here i review paleomagnetic analyses associated with ocean drilling program odp leg 197 which together with studies of plate circuits and geodynamic modeling results indicate southward motion of the hawaiian hotspot during formation of the emperor seamounts these analyses confirm the long posited idea that mantle plumes should be influenced by mantle flow moreover these results indicate that late cretaceous early paleogene hotspot motion was the dominant factor in forming the emperor track and thus the famous bend morphology an important corollary of this finding on hotspot motion is that long term polar wander of earth which has been inaccurately assessed by viewing paleomagnetic data in a fixed hotspot reference frame has been far less than previously thought salient points for future research raised by the odp leg 197 paleomagnetic results include the scale of mantle flow recorded by hotspot track segments and the underlying processes that cause track segments to be dominated by mantle flow rather than plate motion for given time intervals

u pb geochronology of detrital zircons from a continental margin assemblage in the northern coast mountains southeastern alaska metamorphic rocks within and west of the northern coast mountains in southeastern alaska consist of an upper proterozoic to upper paleozoic continental margin assemblage that we interpret to belong to the yukon tanana terrane u pb geochronologic analyses of single detrital zircon grains from four samples of quartzite suggest that the zircons were shed from source regions containing rocks of 495 ma 750 ma 1 05 1 40 ga 1 75 2 00 ga 2 3 ga 2 5 2 7 ga and 3 0 ga multigrain fractions from two samples yield upper intercepts between 2 0 and 2 3 ga but the scarcity of single grains of similar age suggests that these fractions comprise a mixture of 2 3 ga grains zircons in these rocks generally overlap in age with i detrital zircons in metasedimentary rocks of the yukon tanana terrane in eastern alaska and yukon ii detrital zircons in strata of the cordilleran miogeocline and iii plutonic and gneissic rocks that intrude or are overlain by miogeoclinal strata in addition the p

the yukon tanana terrane part of north america at 215 ma from paleomagnetism of the taylor mountain batholith alaska abstract the massive unmetamorphosed 212 ma taylor mountain batholith underlies 648 km2 of the yukon tanana terrane ytt in east central alaska and intrudes klippen thrust sheets of the fortymile river assemblage and the chicken metamorphic complex structural evidence indicates that the batholith is neither tilted nor rotated about a vertical axis paleomagnetic analysis of 295 specimens from 24 sites in the batholith and cross cutting mafic dikes isolates a stable primary characteristic remanent magnetization in magnetite with unblocking temperatures of 500 to 585 c that places the batholith s paleopole at 61 4 n 102 2 e n 17 95 5 2 on the north american apparent polar wander path apwp at 215 5 ma 1 the taylor mountain paleopole alone and all ytt paleopoles together show that the ytt has been a para autochthonous part of the north american margin since the late triassic further the intermontane belt imb paleopoles are distinct from the north american and ytt paleopoles prior to the mid eocene and provide two apwp segments one imb segment includes the quesnel cache creek eastern coast belt and northern stikine terranes and the second more speculative imb segment includes the central and southwestern stikine and methow terranes this analysis supports tectonic models for the northern cordillera in which the imb terranes were obducted onto north america during the late cretaceous to mid eocene co eval with the laramide orogeny

crustal structure and lithology of the northern canadian cordillera alternative interpretations of snorcle seismic reflection lines 2a and 2b seismic reflectors at shallow crustal levels recorded in snorcle slave northern cordillera lithospheric evolution lines 2a and 2b in the northern canadian cordillera can be reconciled with many stratigraphic and structural elements known from geological mapping clearly evident examples are the crustal penetrating northern rocky mountain trench nrmt fault the several kilometres thin slices of slide mountain sylvester allochthon and cache creek terranes and the northward dipping hotailuh fault interpretations of lithological successions and structures at deeper crustal levels are more speculative of fundamental significance is a southwesterly tapering wedge of continental crust overlain in its distal part by the allochthonous terrane stikinia and northeast of thibert fault by a thick probably structurally thickened sequence of sedimentary rocks and the extensive granitic cassiar batholith the top of the wedge is interpreted to be a tectonic accretion surface resulting from collision of sti

jurassic location of the yukon tanana terrane from palaeomagnetism of the folded mississippian tatlmain batholith and ragged stock

the big flush paleomagnetic signature of a 70 ma regional hydrothermal event in displaced rocks of the northern canadian cordillera the 70 ma carmacks group a subaerial volcanic succession which once covered much of central southwest yukon has a paleomagnetic remanent direction which passes the fold test and the reversal test a new collection of 13 sites combined with 13 sites from a previous study renders a pole 088 6 e 78 4 n a95 7 8 which is far sided with respect to the pole for cratonic north america and implies a displacement from the south of 1900 700 km late triassic mandanna member red beds and early jurassic nordenskild formation tuffs deformed in the late jurassic fail the fold test and conglomerate test but pass a contact test with eocene dykes the postdeformational remanent direction is identical to that isolated from the carmacks group the magnetic signature contained in these older formations is probably an overprint produced by an extensive hydrothermal system active during carmacks extrusion geological work indicates that the carmacks group is plume related given its paleomagnetic latitude and geological nature we hypothesize that the carmacks group is a displaced segment of the yellowstone hot spot track and the hydrothermal system which remagnetized the older rocks was established by mantle upwelling below the region resume le groupe de carmacks go de 70 ma est une succession volcanique subaorienne qui couvrait jadis la majeure partie du centre sud ouest du yukon il possde une direction paloomagnotique romanente certifioe par le test du pli et le test des inversions une nouveau groupe de 13 sites combino 13 sites diune otude antorieure fournit un pule 088 6 e 78 4 n a95 7 8 qui est considorablement oloigno par rapport au pule du craton de liamorique du nord et qui implique un doplacement partir du sud de 1900 700 km les couches rouges du membre de mandanna du trias tardif et les tufs du formation de nordenskild du jurassique prococe qui subirent une doformation durant le jurassique tardif ont ochouo le test du pli et le test de conglomorat mais ils ont passo avec succs le test de contact avec les dykes de li ocne la direction romanente post tectonique est identique celle isoloe du groupe de carmacks la signature magnotique qui caractorise ces formations plus anciennes est probablement une roaimantation produite par un systme hydrothermal otendu qui fut actif durant liextrusion de carmacks liotude goologique rovle que le groupe de carmacks est apparento un panache tant donno sa latitude paloomagnotique et sa nature goologique nous avanaons lihypothse que le groupe de carmacks reprosente un segment doplaco du traco du point chaud de yellowstone et que le systme hydrothermal qui a roaimanto les roches les plus anciennes est ds liascension du manteau sous la rogion otudioe

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

new paleomagnetic data for the triassic guichon batholith of south central british columbia and their bearing on terrane i tectonics the 198 ma guichon batholith outcrops in the southern end of the intermontane belt or composite terrane i of the western north american cordillera thermal demagnetization of specimens from 19 sites at 200 450 and 560 c isolates a stable primary component at 560 c in 13 sites giving a pole position of 347 w 52 n p 5 m 9 these data support earlier results obtained by the author that indicate the batholith has undergone a clockwise rotation about a vertical axis of 43 7 they also show that the batholith has undergone northward motion of 13 6 which supports recent arguments that the terrane underwent 14 2 of northward translation between late cretaceous and pre miocene time

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

discordant paleomagnetic poles from the canadian coast plutonic complex regional tilt rather than large scale displacement stratigraphic petrologic and isotopic data indicate that parts of the coast plutonic complex and the north cascade range have been tilted northeast side up by angles of approximately 30 degree about north northwest trending axes these tilts can account for discordant paleomagnetic directions observed in mid cretaceous plutons from these regions without large scale displacement relative to north america

evolution of the yukon tanana terrane evidence from southeastern yukon territory the yukon tanana terrane ytt in southeastern yukon consists predominantly of a mid paleozoic volcanic plutonic arc assemblage built on continental crust the ytt had experienced strong deformation and metamorphism by late triassic time by mid cretaceous the metamorphic complex and late triassic clastic rocks derived from it were imbricated with middle and upper paleozoic ophiolitic sheets the ytt differs markedly from adjacent parts of the north american continental margin in both its depositional and tectonic histories the composition and tectonic history of the ytt is comparable to that of the kootenay and barkerville terranes of southern and central british columbia from which the ytt may have been displaced

deciphering shallow paleomagnetic inclinations 1 implications from correlation of albian volcanic rocks along the insular intermontane superterrane boundary in the southern canadian cordillera 1 geologic and paleomagnetic data lead to two contradictory hypotheses regarding the paleoposition of the insular and intermontane superterranes that presently constitute the western canadian cordillera paleomagnetic data from the insular and intermontane superterranes suggest a southerly origin coinciding with the latitude of mexico and the northwest united states respectively during the mid cretaceous geologic evidence points to a northerly origin for these same tectonic entities during this period both models cannot be correct geologic and paleomagnetic data from the empire valley churn creek area in south central british columbia 51 5 n 122 5 w are critical to resolving these contradictory hypotheses late cretaceous rocks correlated to the insular superterrane with large paleomagnetic displacements unconformably overlie mid cretaceous rocks correlative to the spences bridge group of the intermontane superterrane we provide paleomagnetic evidence of this correlation based on similar magnetic properties opaque mineral assemblages demagnetization behavior fold test results mean inclinations clockwise vertical axes rotations and statistically indistinguishable paleomagnetic poles and displacement estimates this correlation and the observed geologic relationships in the empire valley churn creek area indicate that the insular and intermontane superterranes were linked by the mid cretaceous sites from the two previous spences bridge group studies are combined with their correlatives in the empire valley churn creek area to give 81 sites that yield a paleomagnetic pole of 60 5 n 304 5 e dp 3 7 dm 5 5 which corresponds to 1050 450 km of displacement from the south this new displacement estimate suggests that the spences bridge arc formed at the latitude of southern oregon during the mid cretaceous

yellowstone in yukon the late cretaceous carmacks group the late cretaceous carmacks group a thick subaerial volcanic succession that once covered much of southwest yukon was deposited on an uplifted terrane and is divisible into a lower fragmental unit and an upper flood basalt unit coeval hydrothermal activity resulted in widespread alteration and gold mineralization the lavas are shoshonites enriched in large ion lithophile and light rare earth elements but depleted in high field strength elements ankaramitic absarokite flows in the upper carmacks group range up to 15 wt mgo requiring a high liquidus temperature 1400 c at 1 bar dry high k 2 o contents 3 of these magnesian lavas indicate that the potassic character of the volcanic suite was established in the mantle although previously interpreted as subduction related the carmacks group was erupted during a cordilleran wide magmatic lull and lacks coeval calc alkalic batholiths the lavas are petrologically similar to plume related eocene to pliocene potassic lavas of the western united states new paleomagnetic collections combined with previous work place the carmacks group 17 2 6 5 1900 700 km south of its present position relative to the craton during deposition near the paleolocation of the yellowstone hotspot the spatial coincidence similarity of tectonic setting and lithologic similarity of the carmacks group and yellowstone volcanic successions suggest that the carmacks group is the 70 ma effusion of the yellowstone hotspot subsequent northward displacement of the carmacks group is attributed to coupling with the kula plate correlation of the carmacks group and the yellowstone hotspot fixes the paleolatitude and the paleolongitude of the terranes of the northern intermontane belt at 70 ma

deciphering shallow paleomagnetic inclinations 2 implications from late cretaceous strata overlapping the insular intermontane superterrane boundary in the southern canadian cordillera 1 significant discrepancies exist between paleomagnetic and geologic estimates of the timing and magnitude of terrane displacement in the southern canadian cordillera lithostratigraphic palynological geochronologic geochemical and structural data to which we add paleomagnetic data demonstrate that upper cretaceous strata in churn creek are laterally equivalent facies of the silverquick powell creek spc succession these rocks therefore comprise a single overlap sequence linking the insular and intermontane superterranes by late albian cenomanian time 95 ma contradicting previous assertions based solely on paleomagnetic interpretation that major 2000 km latitudinal displacement separated the two regions the churn creek succession retains primary magnetic remanence as demonstrated by positive fold conglomerate and contact tests the sediments do not hold the anisotropy signature of compaction shallowed inclinations the mean inclination of all spc sites from both mount tatlow and churn creek is 55 6 2 3 n 36 sites corresponding to a paleolatitude of 36 1 2 4 this reflects northward translation of the composite insular intermontane superterrane of 3050 450 km between 85 and 50 ma the silverquick powell creek succession at churn creek unconformably overlies the northern end of the spences bridge group a late early cretaceous volcanic arc assemblage on the intermontane superterrane paleomagnetic data in the companion paper place it 1050 450 km south of its current position between 100 and 105 ma integration of these new data sets require that the insular and intermontane superterranes formed a single enormous crustal block that underwent complex episodic south north translation between circa 105 and 50 ma the timing rate and vectors of translation are problematic in terms of geologic constraints along the continental margin

further paleomagnetic results from the jurassic topley intrusives in the stikinia subterrane of british columbia the 142 ma old jurassic topley intrusions are exposed in the middle of the intermontane belt or stikinia subterrane of composite terrane i of the canadian cordillera in central british columbia thermal demagnetization at 560 c leads to the isolation of a single stable magnetic component directed at 324 2 54 1 95 8 6 in 17 of the 22 sites the pole position of 238 2 w 58 4 n p 8 3 m 11 8 for the component is discordant relative to the north american craton this position supports previous work in suggesting that stikinia has been translated northward by 14 2 or about 1400 km between mid cretaceous and pre miocene time it also suggests that the central portion of stikinia has not been rotated about a vertical axis by a statistically significant amount whereas minor marginal blocks of the subterrane have been subjected to large and variable amounts of both clockwise and counterclockwise rotation

paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia this study contributes the first paleomagnetic data from dated cretaceous units of the coast belt 51 3 n 123 8 w in which paleohorizontal bedding surfaces can be identified with certainty we sampled 38 sites through a 1 5 km stratigraphic interval of cenomanian silverquick sedimentary strata and cenomanian to campanian powell creek volcanics in the tatlow syncline in the tyaughton methow basin magnetizations at 21 sites have high unblocking temperatures tub and pass the tilt test precision k 5 before untilting maximum k 48 after 100 untilting magnetic minerals of volcanics show original textures mean declinations in volcanic and sedimentary rocks are statistically identical and polarity is consistently normal the mean inclination 55 4 3 4 probability p 0 05 of this pretilting magnetization is 20 1 4 0 shallower than that expected if the rocks were deposited in their present position relative to cratonic north america field and laboratory observations rule out other causes for these anomalous shallow inclinations these rocks therefore have been displaced northward by 3000 500 km relative to cratonic north america since mid cretaceous time this displacement is almost 3 times greater than that estimated from mid cretaceous volcanics in the now adjacent intermontane belt evidently the two belts were not amalgamated until latest cretaceous or early tertiary time these results are inconsistent with all previous geologically based tectonic reconstructions of the cordillera magnetizations at eight sites in silverquick and powell creek beds have low tub and directional characteristics similar to those in three younger diorite intrusions reversed and normal polarities occur the rocks are commonly highly altered and maximum precision occurs after 40 untilting the magnetizations therefore are interpreted as overprints of latest cretaceous age acquired during folding

u pb geochronology of late cretaceous and early tertiary plutons in the northern coast mountains batholith u pb geochronologic studies demonstrate that steeply dipping sheetlike tonalitic plutons along the western margin of the northern coast mountains batholith were emplaced between 83 and 57 perhaps 55 ma less elongate tonalitic granodioritic bodies in central portions of the batholith yield ages of 59 58 ma coeval with younger phases of the tonalitic sheets large granite granodiorite bodies in central and eastern portions of the batholith were emplaced at 51 48 ma trends in ages suggest that the tonalitic bodies generally become younger southeastward and that at the latitude of juneau plutonism migrated northeastward across the batholith at 0 9 km ma variations in the age shape location and degree of fabric development among the various plutons indicate that late cretaceous paleocene tonalitic bodies were emplaced into a steeply dipping dip slip shear zone that was active along the western margin of the batholith postkinematic eocene plutons were emplaced at shallow crustal levels inhe

precambrian crust beneath the mesozoic northern canadian cordillera discovered by lithoprobe seismic reflection profiling 1 the cordillera in northern canada is underlain by westward tapering layers that can be followed from outcrops of proterozoic strata in the foreland belt to the lowermost crust of the orogenic interior a distance of as much as 500 km across strike they are interpreted as stratified proterozoic rocks including 1 8 0 7 ga supracrustal rocks and their basement the layering was discovered on two new deep seismic reflection profiles in the yukon line 3 650 km and northern british columbia line 2 1245 km in two segments that were acquired as part of the lithoprobe slave northern cordillera lithospheric evolution snorcle transect in the mackenzie mountains of the eastern yukon the layering in line 3 is visible between 5 0 and 12 0 s 15 to 36 km depth it is followed southwestward for nearly 650 km 500 km across strike and thins to less than 1 0 s 3 0 3 5 km thickness near the moho at the yukon alaska international boundary in the northern rocky mountains of british columbia the upper part of the layering on line 2 correlates with outcrops of proterozoic 1 76 1 0 ga strata in the muskwa anticlinorium at this location the layering is at least 15 km thick and is followed westward then southward into the middle and lower crust for 700 km 300 km across strike it disappears as a thin taper at the base of the crust 150 km east of the coast of the alaskan panhandle the only significant disruption in the layering occurs at the tintina fault zone a late to postorogenic strike slip fault with up to 800 km of displacement which appears as a vertical zone of little reflectivity that disrupts the continuity of the deep layering on both profiles 300 km apart the base of the layered reflection zone coincides with the moho which exhibits variable character and undulates in a series of broad arches with widths of 150 km in general the mantle appears to have few reflections however at the southwest end of line 3 near the alaska british columbia border a reflection dips eastward from 14 0 s to 21 0 s 45 to 73 km depth beneath exposed eocene magmatic rocks it is interpreted as a relict subduction surface of the kula plate our interpretation of proterozoic layered rocks beneath most of the northern cordillera suggests a much different crustal structure than previously considered 1 ancient north american crust comprising up to 25 km of metamorphosed proterozoic to paleozoic sediments plus 5 10 km of pre 1 8 ga crystalline basement projects westward beneath most of the northern canadian cordillera 2 the lateral 500 km by at least 1000 km and vertical up to 25 km extent of the proterozoic layers and their internal deformation are consistent with a long lived margin for northwestern north america with alternating episodes of extension and contraction 3 the detachments that carry deformed rocks of the mackenzie mountains and northern rocky mountains are largely confined to the upper crustal region above the layering 4 accreted terranes include thin klippen that were thrust over north american pericratonic strata e g yukon tanana and terranes such as nisling and stikinia that thicken westward as the underlying proterozoic layers taper and disappear 5 the ages of exposed rocks are not necessarily indicative of the ages of underlying crust a frequent observation in lithoprobe interpretations so that estimates of crustal growth based on surface geology may not be representative

a duplex model for the eastern cape fold belt evidence from the palaeozoic witteberg and bokkeveld groups cape supergroup near steytlerville south africa abstract the eastern part of the cape fold belt near steytlerville south africa reveals a typical pattern of numerous north verging thrust faults and associated folds interpreted as part of a large duplex structure that formed along the southern margin of gondwana during the late palaeozoic steeply dipping fore and backthrusts occur in the bokkeveld group middle cape supergroup where strata are composed of predominantly argillaceous rocks whereas in the more arenaceous witteberg group upper cape supergroup there are fewer recognizable and less closely spaced thrusts open style folds characterize areas in which the bokkeveld group crops out but in areas of witteberg outcrop folds especially those adjacent to thrusts are often overturned in spite of a general absence of marker horizons a displacement of at least 500 metres can be inferred for one prominent thrust the jackalsbos thrust this fault the northernmost in the area investigated is probably the sole thrust in the duplex structure linked through southward dipping imbricates to a projected roof thrust the baviaanskloof thrust cropping out immediately south of the study area displacements on imbricates within the duplex are difficult if not impossible to measure but the net effect is certainly accumulative and incremental truncation by a roof thrust and subsequent erosional processes may explain why so few of the many thrusts so far identified in the eastern part of the fold belt can be successfully mapped and their displacements measured normal and strike slip faults less common than thrust faults formed during extensional tectonism related to the breakup of gondwana during the mesozoic

paleomagnetic and geobarometric study of the mid cretaceous whitehorse pluton yukon territory this is the first of several lithoprobe paleomagnetic studies underway to examine geotectonic motions in the northern canadian cordillera except for one controversial study estimates for terranes underlying the intermontane belt in the yukon have been extrapolated from studies in alaska southern british columbia and the northwestern united states the whitehorse pluton is a large unmetamorphosed and undeformed tonalitic body of mid cretaceous age 112 ma that was intruded into sedimentary units of the whitehorse trough in the stikinia terrane geothermobarometric estimates for eight sites around the pluton indicate that postmagnetization tilting has been negligible since cooling through the hornblende crystallization temperature and that the pluton is a high level intrusion paleomagnetic measurements for 22 of 24 sites in the pluton yield a well defined characteristic remanent magnetization chrm direction that is steeply down and northwards the chrm direction gives a paleopole of 285 5 e 81 7 n

canadian cordilleran displacement palaeomagnetic results from the early jurassic hazelton group terrane i british columbia canada summary the early jurassic telkwa formation comprising the base of the hazelton group was sampled at three areas in the central part of the stikine terrane british columbia detailed alternating field and thermal step demagnetization on samples from nine sites from the red canyon area and 10 sites from the zymoetz river area both of which are in the bulkley ranges and eight sites from the telkwa range area isolate stable remanence magnetization components the components are interpreted to be primary magnetizations because of the presence of two polarities and the much improved agreement of their inclinations after full tilt correction of the sites from the bulkley ranges within locality declinations agree well between locality declinations are discordant the expected cratonic declination is 340 whereas declinations from this study are 265 227 and 328 for the red canyon zymoetz river and telkwa range areas respectively these results are similar to those of monger irving 1980 and we concur that the discordant declinations are the result of differential rotation about vertical axes between sample localities our observed mean inclination of 52 is in good agreement with monger irving s 1980 result of 54 they interpreted this to indicate 1300 km of northward displacement since the early jurassic and their interpretation was substantiated by several later studies however with revisions to the geologic time scale and the early jurassic reference pole for north america the early jurassic canadian cordillera palaeomagnetic data base appears latitudinally concordant this indicates that terrane i and southern wrangellia were in much the same latitudinal position relative to the north american craton as they are now tectonic displacement models developed in view of discordant cretaceous palaeomagnetic data from the canadian cordillera must now consider latitudinally concordant permian to early jurassic data and middle eocene to recent data this tectonic senario appears more reconcilable with the regional tilt hypothesis however this hypothesis cannot account for discordant cretaceous data from bedded volcanic rocks in terrane i it is therefore probable that cretaceous cordilleran tectonics were characterized by both regional tilting and moderate latitudinal displacement

cretaceous coast belt paleomagnetic data from the spetch creek pluton british columbia evidence for the tilt and moderate displacement model well grouped stable magnetizations have been isolated at 14 of 20 sites sampled from the spetch creek pluton the single polarity primary magnetization directed at d 026 2 i 73 5 95 4 4 paleolatitude 59 7 n paleopole 73 n 078 w a95 7 was acquired around 88 ma during the cretaceous normal polarity superchron 118 84 ma this direction is discordant from the expected mid cretaceous direction d 332 5 i 75 1 for north america the difference could be caused by one of two end member models the all tilt model requires a 15 east side up tilt about a horizontal axis striking 353 and the displacement rotation model requires 330 770 km of northward displacement combined with 54 14 of clockwise rotation regardless this result provides a negative test of the baja british columbia model which requires 2400 km of northward displacement a review of previously observed mid cretaceous magnetizations from the coast belt which are also discordant indicates that they exhibit co

review of radiometric data from the yukon crystalline terrane alaska and yukon territory the results of more than 20 years of geochronological studies in the yukon crystalline terrane in east central alaska and the western yukon territory suggest at least six igneous and thermal metamorphic events plutonism during mississippian early jurassic mid cretaceous late cretaceous and early tertiary times is indicated evidence also indicates that mississippian early jurassic late early cretaceous and late cretaceous thermal metamorphic events have affected parts of the terrane the western part of the terrane was affected by a significant regional metamorphic event in late early cretaceous time followed by a terrane wide mid cretaceous plutonic event the pattern of k ar ages allows division of the terrane into domains bounded by northeast trending lineaments

impact of amazonian evapotranspiration on moisture transport and convection along the eastern flanks of the tropical andes the weather research and forecasting wrf model was used to investigate the impact of amazonian evapotranspiration et on moisture transport and convection along the eastern flanks of the andes eads to isolate the role of surface et quasi idealized simulations down to 1 2 km grid spacing were conducted where over the amazon lowlands amzl and at every time step the surface sensible heat effects are identical to the realistic reference runs while surface latent heat fluxes are prevented from entering the atmosphere the results show that without surface et daily precipitation within the amzl decreases by as much as 75 but nearly doubles over the surrounding mountainous regions this dramatic influence is attributed to a dipole structure of convergence divergence anomalies over the amzl primarily due to the considerable cooling of the troposphere associated with suppressed convection further examination of moist static energy evolution indicates that the net decrease in convective available potential energy over the amzl is due to the removal of surface et that is only partially compensated by related regional circulation changes because of the concave shape of the andean mountain range enhanced low level divergence promotes air mass accumulation to the east of the central eads this perturbation becomes sufficiently strong around nightfall and produces significant eastward low level pressure gradient force rendering stronger winds away from the andes moisture convergence and convection over the eads vary accordingly strengthened in the day but attenuated at night nocturnal convective motion is however more widespread analytical solutions of simplified diagnostic equations of convective fraction suggest that reduction of lower troposphere evaporation is the driving mechanism additional exploratory experiments with varied surface et magnitude demonstrate that the connection between the amzl et and eads precipitation is robust

new 42 ma cratonic north american paleomagnetic pole from the yukon underscores another cordilleran paleomagnetism geology conundrum eocene posttectonic plutons of the beaver river alkalic complex in southeastern yukon intruded devonian mississippian and triassic sandstones in the foothills of the canadian cordillera a paleomagnetic collection of 27 sites from three separate plutons produced 326 specimens that were analyzed using alternating field and thermal step demagnetization methods the a component characteristic remanent magnetization chrm resides in magnetite with normal polarity in the 42 6 0 8 ma beaver river pluton reversed polarity in the 42 1 0 7 ma larson creek east pluton and both polarities in the 41 3 0 4 ma larson creek west pluton corresponding with magnetic polarity chrons 20n 19r and the boundary between chron 19r and 18n respectively the chrms of the plutons are indistinguishable 2 with a mean for the 42 0 0 5 ma complex of d 158 8 i 73 1 n 21 sites 95 3 0 k 116 8 a positive paleomagnetic contact test shows the a component to be primary and the poorly isolated b component

rethinking yellowstone in yukon and baja british columbia paleomagnetism of the late cretaceous swede dome stock northern canadian cordillera 1 paleomagnetic results obtained from the 69 8 ma swede dome stock emplaced into the pericratonic yukon tanana terrane ytt west of dawson yukon indicate a minimal northward translation of 360 575 km and a clockwise rotation of 20 23 with respect to north america in accord with the displacement of 425 km along the nearby tintina fault coeval carmacks group volcanics in contrast have previously indicated 1900 km northward translation and minimal rotation and have been proposed to be a displaced manifestation of the fixed yellowstone hot spot thus providing an independent estimate of the late cretaceous paleoposition of the baja british columbia terranes of western north america a compilation of recent mid and late cretaceous paleomagnetic results from the ytt and intermontane belt imb terranes reveals that the carmacks group paleomagnetic result is anomalous the ytt and imb terranes which form the bulk of baja british columbia appear instead to have experienced 1000 km northward translation with respect to north america since 70 ma consistent with geological estimates of motion along plausible accommodating faults in addition past mobility of the hawaii emperor hot spot implies that the pacific hot spots may have experienced 1200 km of southward motion from 81 to 47 ma if so then the paleoposition of the yellowstone hot spot if it existed at 70 ma would likely have produced the carmacks magmatism 1000 km north of yellowstone s current latitude consistent with a moderate 1000 km post 70 ma northward translation of the ytt and imb terranes to their present position in north america

testing self organized criticality in the crust using entropy a regionalized study of the cmt global earthquake catalogue 1 we test the notion of self organized criticality soc and the proximity to the critical point in the brittle crust if the system were strictly critical we would expect an infinite correlation length with minimal temporal or spatial predictability an alternative view is that the earth is in a self organized subcritical sosc state with a finite and systematically fluctuating correlation length this would imply a system that is sufficiently near critical to maintain power law scaling relations over a finite scale range but can intermittently reach criticality in the form of a single large earthquake when the correlation length becomes effectively infinite over the scale of the observed region here we address the question of proximity to criticality from the viewpoint of statistical physics by describing a regionalized study equivalent to the ensemble approach in thermodynamics flinn engdahl regionalization of global seismicity is used to calculate the expectation of the logarithm of energy lne and entropy s from centroid moment tensor cmt data for different seismic regions we compare a phase diagram for s and lne from the data with an analytical statistical mechanical solution and find that they are in good agreement the analysis shows systematic spatial heterogeneity in entropy that is associated with the tectonic deformation style oceanic ridges are seen to be low entropy relatively ordered and subduction zones have higher entropy less ordered with collision zones scattered at the two extremes statistically resolvable phase variations in the system as a whole point toward it being better described as subcritical in the spatial ensemble

paleomagnetic and geobarometric study of the late cretaceous mount lorne stock southern yukon territory the 75 ma mount lorne monzodiorite stock has been studied paleomagnetically to estimate the tectonic motion of the northern canadian cordillera since the late cretaceous the stock is one of sever

morphology and geometry of duplexes formed during flexural slip folding abstract bedding parallel slip on the limbs of chevron style folds during flexural slip folding results occasionally in the formation of small scale duplexes the duplexes described here from upper carboniferous turbidites in sw england generally range in thickness from 0 3 to 30 cm and are up to 2 4 m long they have smooth nearly flat roofs and both the internal thrusts and the slickenfibres on them are commonly oblique to the mean transport direction as indicated by slickenfibres on the floor and roof thrusts the morphology of the duplexes suggests that they developed between active floor and roof thrusts and they show a characteristic lack of hangingwall anticlines on the link thrusts their shear sense always corresponds to that required by flexural slip and the total bedding parallel displacement across each is commensurate with that predicted by the flexural slip model all thrust surfaces in the duplexes are marked by quartz fibre veins a feature which distinguishes them from soft sediment and other pre folding duplexes and carry slickenfibres whose mean orientation and complex variations in slip direction on different fibre sheets on the same slip surface are identical to those on flexural slip movement horizons from the same fold limb the duplexes form either as a result of resistance to thrust propagation by local facies or bed thickness changes or develop as transfer structures between the tips of movement horizons propagating along adjacent slip surfaces late stage duplexes develop from imbricated fibre veins and also form on slip surfaces oblique to the axial planes of major folds