accretion of southern alaska abstract paleomagnetic data from southern alaska indicate that the wrangellia and peninsular terranes collided with central alaska probably by 65 ma ago and certainly no later than 55 ma ago the accretion of these terranes to the mainland was followed by the arrival of the ghost rocks volcanic assemblage at the southern margin of kodiak island poleward movement of these terranes can be explained by rapid motion of the kula oceanic plate mainly from 85 to 43 ma ago according to recent reconstructions derived from the hot spot reference frame after accretion much of southwestern alaska underwent a counterclockwise rotation of about 50 as indicated by paleomagnetic poles from volcanic rocks of late cretaceous and early tertiary age compression between north america and asia during opening of the north atlantic 68 44 ma ago may account for the rotation

paleomagnetism of the late triassic hound island volcanics revisited the collision and accretion of the alexander terrane profoundly influenced the geologic history of alaska and western canada however the terrane s displacement history is only poorly constrained by sparse paleomagnetic studies we studied the paleomagnetism of the hound island volcanics in order to evaluate the location of the alexander terrane in late triassic time we collected 618 samples at 102 sites in and near the keku strait alaska from the late triassic hound island volcanics the permian pybus formation and 23 ma gabbroic intrusions we found three components of magnetization in the hound island volcanics the high temperature component component a resides in hematite and magnetite and was found only in highly oxidized lava flows in a geographically restricted area we think it is primary or acquired soon after eruption of the lavas principally because the directions pass a fold test the paleolatitude indicated by this component 19 2 10 3 is similar to those determined for various portions of wrangellia consistent with the geologic interpretation that the alexander terrane was with the wrangellia terrane in late triassic time we found two overprint directions in the hound island volcanics component b was acquired 23 m y ago due to intrusion of gabbroic dikes and sills this interpretation is indicated by the similarity of upper hemisphere directions in the hound island volcanics to those in the gabbro component c found in both the hound island volcanics and the permian pybus formation is oriented northeast and down fails a regional fold test and was acquired after regional deformation around 90 to 100 ma this overprint direction yields a paleolatitude similar to but slightly higher than slightly older rocks from the coast plutonic complex suggesting that the alexander terrane was displaced 17 in early late cretaceous time the occurrence of these two separate overprinting events provides a satisfying explanation of the earlier puzzling results from the hound island volcanics hillhouse and gromme 1980 finally great circle analysis of the paleomagnetic data from the pybus formation suggests the alexander terrane may have been in the northern hemisphere in permian time

paleomagnetic study of the eastern klamath terrane california and implications for the tectonic history of the klamath mountains province paleomagnetic study of permian through jurassic volcanic and sedimentary strata of the eastern klamath terrane has shown the remanent magnetization of many of these rocks to be prefolding and most likely primary similarities in magnetic declinations recorded by coeval strata over a broad area are consistent with the hypothesis that the terrane in general has behaved as a single rigid block paleomagnetic data indicate that the volcanic island arc represented by this terrane the nucleus of the province was facing toward the present southwest during late paleozoic time although its orientation during earlier periods is unknown whether the arc was separated from the north american craton by a small marginal basin or originated far offshore cannot be determined from paleomagnetic data the declination anomalies for both permian and triassic strata are similar average 106 12 so we infer that clockwise rotation of the late paleozoic arc did not begin until latest triassic or earliest jurassic time the arc may have completed its initial rotation with respect to stable north america by middle jurassic time after some retrograde motion the arc was again facing west by the late jurassic by which time some of the more westerly terranes of the province had become attached to the eastern klamath terrane the composite klamath mountains terranes continued to rotate until the final 60 of clockwise rotation was nearly complete by the early cretaceous coincidence of the waning stages of rotation at about 136 ma with the beginning of deposition of the basal great valley sequence onto the klamath basement probably represents the completion of accretion of the klamath mountains terranes to the north american continent nearly all the rotation occurred while the klamath mountains terranes were part of a converging oceanic plate with only about 20 of rotation in mid tertiary time during basin and range extension no data currently available show evidence for any significant latitudinal displacement of any klamath mountains terranes relative to cratonic north america

structures in the canyon mountain ophiolite indicate an island arc intrusion abstract lithosphere created in an interarc basin is expected to be characterized by features distinguishing it from normal oceanic lithosphere apart from island arc geochemical affinities and from the occurrence of hydrous high t parageneses in the mantle and deep crustal sequences it is expected that due to a low rate of spreading vertical transport prevails over lateral drifting the canyon mountain complex located in an island arc environment of permo triassic age offers a remarkable illustration of these expected geological characteristics in particular mantle diapirism is deduced from the structural study smaller diapirs are formed in crustal formations the intrusions took place at variable temperatures 1300 800 c and were accompanied by multistage melting in hydrous conditions

structural chronology oroclinal deformation and tectonic evolution of the southeastern klamath mountains california devonian to jurassic volcanic sedimentary and minor intrusive rocks of the redding section rs accumulated in an intermittently active island arc setting general stratigraphic concordancy and the downsection continuity of metamorphic facies series indicate that these rocks shared a common postdepositional geologic history burial metamorphism produced secondary mineral assemblages of the zeolite prehnite pumpellyite pumpellyite actinolite and greenschist facies close correspondence between pressures calculated from existing stratigraphic overburden and those predicted from the observed metamorphic facies series indicates that less than 3 km of strata have been eroded from the presently exposed top of the early middle jurassic potem formation shortly after deposition of the potem formation compressional deformation f1 produced open to isoclinal folds foliations and small thrust faults k ar dates from post tectonic intrusions indicate that f1 deformation was completed before 168 ma this minimum age of f1 deformation is corroborated by a postfolding paleomagnetic overprint acquired at about 171 ma a second suite of intrusions dated at 120 130 ma provided a thermal pulse that variably reset the k ar system in f1 fabric defining celadonite and other relatively ar unretentive materials the nevadan orogeny late jurassic did not significantly deform rocks of the rs if at all nor is evidence for permo triassic sonoman deformation apparent the geometry of f1 structures varies systematically with location in the western bollibokka quadrangle bq f1 folds are upright with subvertical axial planes and associated foliations in the eastern bq f1 folds are overturned to the east with axial planes and associated foliations dipping as little as 40 to the west regional relationships suggest that f1 may have been a response to coeval tectonism in the western klamaths wherein crustal thickening during regional westward thrusting induced gravitational detachment of the rs from its partly serpentinized ophiolitic basement f1 structures subsequently were broadly warped about vertical axes producing variable paleomagnetic declination anomalies that range from 0 to greater than 100 clockwise the timing and cause of this f2 deformation event are as yet relatively unconstrained but were almost certainly post jurassic and may have been associated with development of the columbia embayment paleomagnetic data indicate north american paleolatitudes for the rs since at least the permian and apparently weigh against an allochthonous or exotic origin for this terrane paleontologic data indicate strong faunal similarities with north america and to a lesser extent with a tethyan province the significance of this apparent dual provinciality remains uncertain but extreme endemism of tethyan fusulinids corals brachiopods and pelecypods is questionable

paleomagnetism and tectonic significance of albian and cenomanian turbidites ochoco basin mitchell inlier central oregon 1 understanding continental growth and convergent margin dynamics associated with terrane accretion and modification of the cordilleran margin of north america is prevented by conflicts in paleogeographic models for major terranes oceanic plates and the north american margin we present new paleomagnetic data suggesting that the blue mountains superterrane located at an inboard portion of the cordilleran margin has undergone substantial northward translation and clockwise rotation relative to north america since mid cretaceous time positive baked contact conglomerate and fold tests provide evidence that the magnetization of these rocks dates from their deposition these results yield a mean direction of d 10 8 i 58 5 k 127 95 4 1 and n 11 comparison of magnetic inclinations and magnetic fabrics shows no evidence for inclination error a paleolatitude of 39 2 4 8 4 3 n is obtained indicating a paleolatitude anomaly of 15 9 4 1 northward translation of 1760 460 km and cw rotation of 37 7 3 with respect to north america since circa 93 ma application of an ad hoc 5 inclination error yields translation of 1200 460 km through comparison with other units in the western cordillera we infer that the mitchell inlier was located adjacent to the sierra nevada arc during mid cretaceous time and that the blue mountains province may correlate with the intermontane superterrane in british columbia other interpretations such as large inclination errors or an independent displacement history for the mitchell inlier are possible but unlikely more work is needed to fully test these hypotheses and resolve major questions of cordilleran tectonics

quartz osl dating of late quaternary chinese and serbian loess a cross eurasian comparison of dust mass accumulation rates abstract reconstructing dust mass accumulation rate mar from loess deposits is critical to understanding past atmospheric mineral dust activity and requires accurate independent age models from loess deposits across europe and asia previous correlations of loess in europe and china have tended to focus on multi millennial timescales with no detailed examination of dust mar at the two ends of the eurasian loess belt on shorter sub orbital scales here we present a detailed quartz optically stimulated luminescence osl chronology from the serbian titel loess plateau veliki surduk loess core and the chinese loess plateau lingtai section the luminescence ages pass internal tests and show consistent increases in age with depth with no obvious major hiatuses however as reported before it seems the quartz osl technique is only capable of accurate age determination up to accrued doses of ca 150 gy ca 30 40 ka due to approaching field saturation of the quartz osl signal two age depth models were used to reconstruct dust mars where one utilises osl data solely and the other additionally makes assumptions about sedimentation rates although short term fluctuations in mar are model dependent general mar patterns between the two sites are very similar with peak mar occurring rather late in the last glacial ca 13 25 ka this suggests that at least broad scale trends in dust activity within the last glacial period may be similar at a continental scale

origin and age of the border ranges fault of southern alaska and its bearing on the late mesozoic tectonic evolution of alaska local geologic relationships in the western chugach mountains together with regional considerations suggest that in the early cretaceous a subduction zone was formed along the southern edge of the wrangellia peninsular alexander composite terrane talkeetna superterrane of csejtey et al 1982 of southern alaska the effects of this event include 1 a shattering of older pre cretaceous crystalline rocks along a complex fault system 2 emplacement of a tectonic melange beneath the shattered crystalline rocks and 3 a subgreenschist facies prograde metamorphism of the melange and retrograde metamorphism of the older crystalline rocks this event created a regionally mappable structural contact between broken crystalline rocks and the melange the border ranges fault of mackevett and plafker 1974 regional stratigraphy as well as radiometric ages of rocks predating and postdating the border ranges fault appear to bracket the age of the border ranges fault and its associated deformational effects to the interval between about 135 and 120 ma further regional tectonostratigraphic associations suggest that the deformation along the border ranges fault represents the nucleation of a north and or east dipping subduction zone beneath the talkeetna superterrane and that the magmatic arc associated with this juvenile subduction zone is the gravina nutzotin belt of southeast alaska mismatches in the distribution of different elements of this early cretaceous arc trench system are probably a result of late cretaceous or early tertiary strike slip motion but early cretaceous ridge trench interaction suggested by the occurrence of early cretaceous near trench plutons may have played a role as well the talkeetna superterrane is generally thought to be an exotic block that collided with the cordillera in the middle cretaceous coney et al 1980 the age data commonly cited as evidence for a middle cretaceous age for the collision are however misleading a model is proposed here in which the initiation of convergence along the trailing edge of the talkeetna superterrane records the time of initial impingement of two irregular margins whereas intense middle cretaceous deformation recognized along the leading edge of the talkeetna superterrane csejtey et al 1982 records destruction of a syncollisional flysch basin during the final phase of the collision

paleomagnetism of the tertiary clarno formation of central oregon and its significance for the tectonic history of the pacific northwest the clarno formation a mostly eocene and partly early oligocene sequence of andesitic lavas and volcaniclastic rocks is the oldest tertiary formation exposed in north central oregon remanent magnetization directions at 46 sites in the lavas provide a paleomagnetic pole at 84 n 278 e with a 95 confidence cone of 7 comparison of this pole with the north american reference pole for eocene time indicates that the clarno formation has rotated 16 clockwise with an uncertainty of 10 but has undergone no significant latitudinal displacement further comparison with paleomagnetic data from the miocene columbia river basalt group and steens basalt shows that large scale block rotations in the eastern columbia embayment ceased sometime between 38 m y and 15 m y a paleogeographic reconstruction for 38 m y is offered which is consistent with the observed rotation of the clarno and with the other paleomagnetic data from the pacific northwest which are briefly reviewed this reconstruction shows that it is possible to account for virtually all of the paleomagnetically indicated rotations in pre miocene tertiary rocks of the pacific northwest by an extensional tectonic model as a consequence only part of the rotations in the pre tertiary rocks of the eastern columbia embayment need to be the result of accretionary tectonics the initiation of magmatism in the eastern columbia embayment that is represented by the clarno formation itself may have been contemporaneous with the beginning of crustal extension in the pacific northwest

stratigraphic record of triassic jurassic collisional tectonics in the blue mountains province northeastern oregon sedimentary and volcanic rocks in the blue mountains province bmp of northeastern oregon preserve a well studied record of triassic jurassic magmatism basin evolution and terrane accretion terranes of the bmp represent two magmatic arcs wallowa and olds ferry terranes an intervening oceanic subduction and accretionary complex baker terrane and a complex thick succession of sedimentary rocks commonly known as the izee terrane we divide volcanic and sedimentary rocks into two regionally correlative unconformity bounded megasequences 1 ms 1 late triassic to early jurassic deposits that change up section from 1a older volcanic and volcaniclastic deposits of the wallowa and olds ferry arcs to 1b marine turbidites shale and argillite with chert clast conglomerate and olistostromes derived from the emergent baker terrane and 2 ms 2 early to late jurassic marine deposits that overlap older rocks and structures and record 20 to 40 m y of deep crustal subsidence in a large marine basin many of the known stratigraphic relationships in the blue mountains cannot be explained using the existing model for a middle triassic to late jurassic west facing non collisional volcanic arc and forearc basin we propose a new tectonic model for the bmp based on prior studies and comparison to modern analogues which includes 1 middle triassic magmatism in the wallowa and olds ferry arcs during subduction and progressive closure of an ocean basin 2 late triassic collision between facing accretionary wedges of the wallowa and olds ferry arcs and growth of marine basins on both sides of the emergent baker terrane thrust belt 3 early to middle jurassic terrane continent collision which resulted in closure of a wide back arc basin crustal thickening and loading in nevada and growth of a large marine collisional basin in the bmp and 4 late jurassic thrusting regional shortening and final accretion of the basin and underlying terranes to western north america this analysis suggests that collisional tectonics may have played a significant role in plate interactions that drove triassic jurassic crustal thickening mountain building and basin development in the western north american cordillera

northward displacement and accretion of wrangellia new paleomagnetic evidence from alaska wrangellia is a vast terrane that accreted along the pacific margin of north america from oregon to central alaska previous paleomagnetic studies of the triassic volcanic rocks of wrangellia in south central alaska and british columbia have given paleolatitudes of 10 17 which are anomalously low in comparison to results from similar aged rocks of nuclear north america the discrepancy in paleolatitude implies substantial northward drift of the terrane relative to the craton we have extended paleomagnetic sampling to the northern boundary of the terrane in the mount hayes and healy quadrangles of the central alaska range this collection of 46 triassic lava flows yields a mean paleolatitude of 13 9 95 3 8 the polarity of the data and hence the hemisphere of origin are not resolved with certainty in contrast triassic paleomagnetic poles from the stable part of north america predict a paleolatitude of 42 1 n 95 4 6 for the region the northernmost fragment of wrangellia probably reached mainland alaska in the middle cretaceous when upper jurassic and lower cretaceous flysch within the intervening basin underwent severe deformation tertiary transcurrent faulting has modified the positions of wrangellian fragments in alaska but such displacements are probably minor as indicated by paleomagnetic data from early tertiary volcanic rocks that overlap the terrane

nd and sr isotopic characterization of the wrangellia terrane and implications for crustal growth of the canadian cordillera nd and sr isotopic data are reported from samples of most of the major lithologic units of the wrangellia terrane exposed on vancouver island and the queen charlotte islands initial epsilon nd values range from 1 0 to 7 3 and initial 87 sr 86 sr ratios range from 0 70323 to 0 70481 these ratios are similar to those of modern island arcs and demonstrate the unevolved juvenile nature of this terrane these data indicate that wrangellia probably resided in an intra oceanic environment until its accretion to north america nd crustal residence ages for clastic sedimentary rocks of wrangellia mean t cr 0 77 pm 0 33 ga t dm 0 57 pm 0 35 ga are much closer to their stratigraphic ages than is the case for the ages of many other phanerozoic sedimentary rocks thus providing strong evidence for phanerozoic crustal addition to north america early proterozoic crustal components cannot comprise more than about 6 of wrangellia therefore the terrane consists of 94 new mantle d

paleomagnetic evidence for motions of parts of the canadian cordillera abstract paleomagnetic evidence from major terranes of the western cordillera in canada referred to collectively as baja british columbia indicates that they have moved as a whole about 2300 km from the south and rotated 60 clockwise relative to north america in the late cretaceous and earliest tertiary 85 50 ma sometime between the early jurassic and the mid cretaceous 190 100 ma these same terranes moved south over 1000 km and underwent irregular but where well constrained counter clockwise rotations the terranes amongst themselves seem to have shared much the same paleolatitude during and since the late triassic although they may have undergone relative rotations in the later jurassic and early cretaceous these motions are now known to be broadly consistent with the motions of oceanic plates to the west as calculated by independent procedures and indicate that the terranes of the western cordillera were for part of their existence coupled to oceanic plates first to the farallon plate during the late jurassic and early cretaceous and then to the kula plate during the late cretaceous and early tertiary it was this change of allegiance from farallon to kula that caused first the southerly and then the northerly motion the paleomagnetic evidence despite claims to the contrary is consistent with geologically inferred displacements along major faults in the canadian cordillera when due account is taken of the age constraints on those motions

petrology geochemistry and genesis of high al tonalite and trondhjemites of the cornucopia stock blue mountains northeastern oregon tonalitic and trondhjemitic compositions span a narrow range of trondhjemite sio2 content 65 74 wt and exhibit characteristics of a highal tonalite trondhjemite granitoid ttg suite including light rare earth element lree enrichment low y nb and rb sr and high al2o3 and sr these compositions are consistent with an

geochemical reconnaissance of cretaceous inliers in north central oregon cretaceous inliers in north central oregon represent isolated exposures of potential source rocks in the region interpreted neritic to upper bathyal mudstones of the hudspeth formation represent late deposition within the mesozoic fore arc terrane accretionary terranes that are represented by the blue mountain province are in part inundated by voluminous tertiary volcanic sequences most notably the columbia river group both pyrolysis and solvent extraction source rock data from the hudspeth formation present in local cretaceous inliers support a mature oil prone mixed marine source rock environment a trend reflected in these data indicates that the western part of the study area is more mature if these same strata and conditions prevail beneath the volcanic cover the potential for the presence of paraffinic oil accumulations exists near surface soil gas investigations have shown the occurrence of light hydrocarbon microseep gases which cluster in the vicinity of the mitchell inlier the soil gas signal received in the near surface far removed from the inliers is identical to the soil gas signature measured on the cretaceous sediments it is suggested that the application of reconnaissance surface geochemical methods may offer additional insight in defining potentially favorable source rocks peripheral to the columbia river plateau as well as projecting their presence into the interior of the plateau

paleomagnetism and mesozoic tectonics of the seven devils volcanic arc in northeastern oregon upper triassic volcanogenic rocks of the seven devils group and huntington formation of brooks 1979b in northeastern oregon record two episodes of magnetization the older magnetization which predates late jurassic deformation of the volcanic arc terrane comprising the seven devils group was probably acquired in late triassic time the inclinations of both normal and reversed polarity magnetizations indicate that the triassic rocks originated approximately 18 from the paleoequator the results are consistent with paleomagnetic data from similar triassic rock assemblages on vancouver island and in south central alaska we interpret these assemblages as displaced fragments of wrangellia a once coherent late paleozoic volcanic arc and early mesozoic volcanic plateau that was torn apart and carried north by plate motion along the continental margin we interpret the younger magnetization which fails the fold test as a widespread thermochemical overprint caused by late jurassic and early cretaceous plutonism this magnetization yields a paleomagnetic pole at 60 n 295 e 95 7 and is similar to the pole obtained from plutons that intrude the seven devils group the angular difference between the pole from the plutons and the late jurassic early cretaceous reference pole for stable north america suggests that the seven devils volcanic arc has undergone a large clockwise rotation 66 21 the rotation may reflect the last stages of the arc s accretion to the continental margin or later large scale block movements in the pacific northwest

soil c and n sequestration in organic and mineral layers of two coeval forest stands implanted on pyroclastic material mount vesuvius south italy tree species influence organic matter sequestration in forest soil we explored the pattern of soil organic carbon soc and soil organic nitrogen son sequestration in two 40 year old adjacent forest stands one with black pine pinus nigra arn and one with black locust robinia pseudoacacia l we compared the patterns of annual carbon c and nitrogen n sequestration in the organic and mineral soil layers of the two forests with literature data for 100 years old deciduous and coniferous forests from different climatic zones carbon and n inputs with aboveground litter fall were significantly higher in the black pine than in the black locust stand in the soil profile of the black pine stand the amount of soc was significantly higher and that of son significantly lower than in the black locust stand the average annual sequestration rates were 41 3 and 75 0 g c m 2 and 2 9 and 2 6 g n m 2 respectively in the black locust and the black pine stands in both forests most of soc and son were stored in the organic layers ol and of oh in the mineral soil layers the amount of soc was similar for the two stands whereas the amount of son was significantly higher in the black locust stand 23 7 vs 14 6 g n m 2 in all soil layers the concentrations of water soluble organic matter wsom were higher in the black locust stand compared to that with black pine manganese accumulated in the of oh layer of the black pine stand the results suggest different sequestration patterns with higher soc sequestration rates in the black pine stand and higher son sequestration rates in the black locust stand comparison with literature data suggests this to be a generality in c and n sequestration patterns for coniferous and deciduous forests

paleomagnetism and the motion of large and small plates the focus of paleomagnetic research during past decades has varied greatly the first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements an enormous effort followed in the form of individual paleomagnetic studies of cratonic rocks culminating with the construction of apw apparent polar wander paths for the continents which could be used to test paleogeographic reconstructions coupled with parallel research in marine geophysics and geology the refined apwp which result from this effort have helped to develop a reasonable first order history of cenozoic mesozoic paleogeography for the major plates

paleogeographic evolution of the united states pacific northwest during paleogene time paleogeographic reconstructions for oregon and washington during paleogene time illustrate a major transition from a dominantly compressional prior to middle eocene time to an extensional tectonic regime this transition resulted in the development of three phases of paleogene basin evolution in the united states pacific northwest during the initial phase basins formed along the continental margin during collision of oceanic islands sediments in these basins were derived from nearby orogenic highlands the second phase of basin development began in middle eocene time and consisted of rapid subsidence of individual basins that formed within a broad forearc region nonmarine basins that formed during this phase were caused by extension possibly associated with transcurrent faulting rapid sedimentation in both marine and nonmarine basins during this time consisted dominantly of sandstone derived from cretaceous plutonic sources far to the east the final stage of basin development was the modification

magnetic fabric of late jurassic arc plutons and kinematics of terrane accretion in the blue mountains northeastern oregon abstract the blue mountains province of northeastern oregon western idaho and southeastern washington usa consists of the amalgamated wallowa distal island arc baker melange bearing accretionary wedge forearc and olds ferry fringing island arc terranes anisotropy of magnetic susceptibility ams and particularly the orientation of magnetic lineations in middle to late jurassic diorite tonalite granodiorite plutons intruding the baker terrane indicate a change in tectonic regime from flattening with principal stretching at a high angle to the local orientation of the terrane boundaries at around 160 ma to constriction with boundary subparallel subhorizontal stretching at ca 146 ma the former magnetic fabric is compatible with magnetic lineations recording strain related to top to the southwest back thrusting onto the olds ferry arc in contrast the 146 ma boundary parallel stretching may record strain partitioning whereby a part of the baker terrane was extruded laterally parallel to terrane boundaries after the terrane collision after restoration for the clockwise post mid cretaceous terrane rotation the 160 ma restored principal stretching directions are compatible with the overall left oblique subduction and convergence of the wallowa distal and olds ferry fringing arc terranes magnetic fabric data from the plutons thus support the interpretation that the early displacement of the blue mountains superterrane and perhaps also its possible correlative intermontane superterrane of the canadian cordillera was southward along the truncated north american continental margin during the late middle jurassic

contrasting structural histories of the salmon river belt and wallowa terrane implications for terrane accretion in northeastern oregon and west central idaho the salmon river belt is composed of metamorphic tectonites with poorly dated mesozoic and paleozoic protoliths that lie between volcanic arc assemblages of northeastern oregon and the western margin of the north american continent in west central idaho to the west the salmon river belt overlies late paleozoic and mesozoic volcanic arc rocks of the wallowa terrane on a shallowly east dipping fault to the east the belt is separated from north american rocks by the western idaho shear zone the salmon river belt is divided into western and eastern assemblages composed of greenschist and amphibolit facies metamorphic tectonites respectively and is deformed in three and locally four generations of penetrative structures first generation structures are synmetamorphic west vergent tight to isoclinal folds with a penetrative axial planar cleavage and downdip stretching lineation second and third phase structures are postmetamorphic and consist of open to close upright to slightly overturned folds with north south and east west trending penetrative axial planar cleavages respectively low angle faults truncate metamorphic structures within the salmon river belt and juxtapose rocks with different metamorphic grades the faults formed together with second generation structures and are deformed by third generation folds the age of metamorphic and postmetamorphic structures is constrained by deformed carbonate rocks of late triassic age and clastic rocks of late early jurassic age and by the ca 145 ma emplacement of an undeformed tonalitic pluton along the eastern margin of the salmon river belt structures are overprinted and transposed by a younger steeply east dipping mylonitic shear zone the mid cretaceous western idaho shear zone synmetamorphic structures within the salmon river belt formed before amalgamation of the wallowa terrane and the north american fringing arc complex and developed as part of a regional belt of intra arc metamorphic tectonites the pr cretaceous docking of the wallowa terrane is recorded within the salmon river belt by development of postmetamorphic structures and predates formation of the western idaho shear zone the western idaho shear zone formed after the amalgamation of the arc terranes and records the intraplate deformation associated with the collapse of the fringing arc and back arc basin system

crustal genesis on the oregon plateau the areal and temporal distribution and the chemical and isotopic characteristics of cenozoic volcanic rocks from the oregon plateau reflect the changing tectonic conditions affecting this area over the last 20 m y following subduction zone related calc alkaline volcanism flood basalt eruptions occurred in the time interval 18 14 ma this primarily basaltic volcanism is interpreted to be the result of a major shift in spreading orientation of the juan de fuca ridge that resulted in changes in the geometry of subduction against the pacific northwest the compositions of the mid miocene flood basalts show signs of extensive crystal fractionation in some cases accompanied by crustal contamination roughly 11 m y ago basalt eruptions showed an increasing tendency toward smaller volumes and less evolved compositions this implies that as extension in the oregon plateau proceeded the crust became less of an impediment to the passage of relatively unfractionated mantle melts initial sr nd and pb isotopic compositions of all the basalts show more correlation with the geographic position of eruption than with the composition of the lava primary control over the isotope systematics appears to be due to changes in the age and chemical characteristics of the crust and mantle throughout this area these observations suggest that an area of some 130 000 km2 of crust and basalt depleted subcontinental mantle has been added to northwestern north america over the last 20 m y the intensity of basalt volcanism as manifest in the columbia river group and in the oregon plateau does not appear to reflect the presence of a hot spot beneath the area both the areal and temporal distribution of the lavas and their geochemical characteristics are more easily reconciled with the suggestion that this crustal section grew by back arc spreading in a manner analogous to the formation of the marginal basins of the western pacific this spreading initiated along the western border of the archean wyoming craton with the location of mantle ascent melt production and extraction controlled by the changing geometry of subduction the existence of a zone of lithospheric weakness and the presence of a thick low density subcontinental mantle keel existing beneath the archean craton in this area

u pb geochronology and geochemistry of intrusive rocks from the cougar creek complex wallowa arc terrane blue mountains province oregon idaho the cougar creek complex is a compositionally diverse intrusive suite constructed of variably deformed and nondeformed dikes and plutons interpreted as the roots of the wallowa arc terrane in the blue mountains province oregon idaho new high precision u pb zircon ages geochemical data and structural analysis define two compositionally and temporally distinct cycles of magmatism and two episodes of contractional deformation in the cougar creek complex from middle permian to early triassic time 265 4 0 2 ma to 248 8 0 1 ma the wallowa arc was dominated by silicic arc magmatism during the early to middle triassic 248 8 0 1 ma to 229 4 0 1 ma an apparent hiatus in intrusive activity suggests a cessation or lull in arc magmatism or a shift of the arc axis this lull in volcanic activity coincides with d1 deformation uplift and erosion of the wallowa arc in the late triassic 229 4 0 1 ma to 229 1 0 5 ma voluminous mafic to intermediate magmatism exhibiting trace element characteristics of a subduction modified strongly depleted mantle source was initiated and this represents a change in normal arc magmatism we interpret the sequence of d1 deformation an apparent gap in igneous activity exhumation of the wallowa arc and a switch from normal silicic arc magmatism to depleted mid ocean ridge basalt like mafic volcanism as a response to spreading ridge subduction and subsequent slab window magmatism beneath the overriding wallowa arc late triassic d2 synmagmatic left lateral mylonitic shear zones in the cougar creek complex illustrate strain partitioning in an intrusive transpressional arc axis environment undergoing sinistral oblique subduction synchronous u pb crystallization ages 40ar 39ar cooling ages the abundance of plutonic clasts within conglomerate units and unimodal late triassic detrital zircon ages for volcanic sandstone units suggest uplift and erosion of the wallowa arc during the late triassic this is inferred to be related to the upwelling of buoyant asthenosphere through an opening slab window beneath the wallowa arc we propose that late triassic mafic igneous rocks from the wallowa terrane and wrangellia resulted from the subduction of a spreading ridge and slab window magmatism beneath a late paleozoic arc thus linking the late triassic geologic evolution of two fundamental components of the cordilleran orogen

major late triassic strike slip displacement in the seven devils terrane oregon and idaho a result of left oblique plate convergence abstract the shortening direction in rocks deformed in collision or subduction zones is not directly related to the plate convergence vector rather it is perpendicular to the collision zone or subduction zone even in cases where plate convergence is oblique the component of convergence parallel to the subduction collision zone is expressed by strike slip displacement in the arc region behind the subduction zone such strike slip shear zones have been recognized in the seven devils terrane of northeastern oregon and adjacent idaho one of these the oxbow shear zone consisting of cataclasite mylonite and ultra mylonite trends northeasterly from oxbow oregon to cuprum idaho the original rock types of the shear zone were plagiogranite gabbro diabase bassalt and keratophyre the age of the mylonitization is constrained by 40ar 39ar dates as late triassic meso and microscopic structures textures and quartz c axes fabrics indicate that the shear zone was formed by left lateral strike slip motion a minimum left lateral displacement of 65 km has been estimated but the true displacement may have been much larger the oxbow shear zone is interpreted as an intra arc strike slip zone of the seven devils terrane related to left oblique plate convergence during the triassic