crustal contamination in palaeogene east greenland flood basalts plumbing system evolution during continental rifting abstract palaeogene central east greenland flood basalts have sr nd and pb isotope characteristics consistent with crustal contamination of icelandic type basalts in the lower basalts of central east greenland amphibolite facies gneiss is the dominant contaminant at low stratigraphic levels whereas contamination with granulite facies gneiss prevails in the upper part of the lava succession the contamination probably occurred by partial melting of the crust and can be modelled by assimilation fractionation crystallisation processes similar contaminants have been detected in lavas from odp leg 152 off the se greenland coast although in a reverse sequence contamination in lower basalts and odp leg 152 lavas which erupted through an archaean proterozoic substrate differs from lavas in ne greenland which overly the caledonian fold belt and large mesozoic sediment deposits in the ne greenland lavas the dominant contaminant appears to be a dacitic component formed by partial melting of mesozoic sediments changes in magma supply rates from the mantle may be the determining factor for the sequence of contaminants and may also control the depth of ponding in crustal chambers the compositional variation of the least contaminated east greenland basalts suggests that melts were produced from mantle sources similar to present day iceland and north atlantic morb at the time of continental breakup but distinction between mantle sources is very complicated ree systematics indicate a maximum in initial depth of mantle melting at kangerlussuaq during formation of the east greenland flood basalts consistent with a plume being centered in that region

intercalibration of astronomical and radioisotopic time the 40ar 39ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in earth s history but the accuracy of this method is limited by the accuracy with which the ages of neutron fluence monitors dating standards are known calibrating the ages of standards by conventional means has proved difficult and contentious the emerging astronomically calibrated geomagnetic polarity time scale apts offers a means to calibrate the ages of 40ar 39ar dating standards that is independent of absolute isotopic abundance measurements seven published 40ar 39ar dates for polarity transitions nominally ranging from 0 78 to 3 40 ma are based on the fish canyon sanidine standard and can be compared with apts predictions solving the 40ar 39ar age equation for the age of the fish canyon sanidine that produces coincidence with the apts age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27 78 to 28 09 ma with an inverse variance weighted mean of 27 95 0 18 ma normalized residuals are minimized at an age of 27 92 ma indicating the robustness of the solution

formation of low δ 18 o magmas of the kangerlussuaq intrusion by addition of water derived from dehydration of foundered basaltic roof rocks the kangerlussuaq intrusion in east greenland is concentrically zoned from quartz nordmarkite quartz syenite at the margin through pulaskite to foyaite nepheline syenite in the centre with no apparent intrusive contacts the 18o values of coexisting minerals are consistent with oxygen isotope equilibrium at magmatic temperatures most of the intrusion formed from low 18o magma magma 18o values generally increased upwards from about 3 3 in the quartz nordmarkites to 5 6 in the foyaites the lowest magma 18o value of about 1 0 is from the upper part of the nordmarkites where there is a high concentration of foundered basaltic xenoliths stoped from the roof of the intrusion the amphiboles in the syenites have d values that range from those typical of hydrous mantle derived minerals to much lower values 86 to 157 as do whole rock samples of xenolith and country rock 125 to 148 the low magma 18o and d values are consistent with continuous incorporation exchange and upward escape of low 18o and d fluids released from stoped basaltic roof material mass balance suggests that the integrated amount of water involved was 7 wt of the volume of the magma but locally reached 30 wt water the requirement for large amounts of water with low 18o value is satisfied only if the foundered basalt contained most of its water in cavities as opposed to hydrous minerals even with this requirement the volume of stoped basalt would have been equal to the volume of the magma repeated recharge of the residual magma with progressively less contaminated silica undersaturated melt resulted in a gradual shift across the low pressure thermal divide crystallisation was suppressed by the depression of the liquidus due to water saturation of the residual magma ph2o 1 kbar

40ar 39ar ages of intrusions in east greenland rift to drift transition over the iceland hotspot abstract sixteen 40 ar 39 ar ages are presented for alkaline intrusions to appraise prolonged post breakup magmatism of the central east greenland rifted margin the chronology of rift to drift transition and the asymmetry of magmatic activity in the northeast atlantic igneous province the alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and occur up to 100 km inland the area south of the kangerlussuaq fjord includes at least four tectonic lineaments and the intrusions are confined to three time windows at 56 54 ma 50 47 ma and 37 35 ma in the kangerlussuaq fjord which coincides with a major tectonic lineament possibly the failed arm of a triple junction the alkaline plutons span from 56 to 40 ma to the north and within the continental flood basalt succession alkaline intrusions of the north south trending wiedemann fjord kronborg gletscher lineament range from 52 to 36 ma we show that post breakup magmatism of the east greenland rifted margin can be linked to reconfiguration of spreading ridges in the northeast atlantic northwards propagation of the proto kolbeinsey ridge rifted the jan mayen micro continent away from central east greenland and resulted in protracted rift to drift transition the intrusions of the wiedemann fjord kronborg gletscher lineament are interpreted as a failed continental rift system and the intrusions of the kangerlussuaq fjord as off axis magmatism the post breakup intrusions south of kangerlussuaq fjord occur landward of the greenland iceland rise and are explained by mantle melting caused first by the crossing of the central east greenland rifted margin over the axis of the iceland mantle plume 50 47 ma and later by uplift associated with regional plate tectonic reorganization 37 35 ma the iceland mantle plume was instrumental in causing protracted rift to drift transition and post breakup tholeiitic and alkaline magmatism on the east greenland rifted margin and asymmetry in the magmatic history of the conjugate margins of the central northeast atlantic

trans atlantic correlation of the palaeogene volcanic successions in the faeroe islands and east greenland before continental break up in the ne atlantic the faeroe islands and central east greenland were within a distance of 100 120 km chemical and lithological data for complete sections through the 5 km thick piles of contemporaneous palaeogene flood basalts in the faeroe islands and in the nansen fjord area in east greenland show very similar basalt compositions and evolution patterns with time the faeroes lower basalt formation and the equivalent nansen fjord formation in east greenland form a pre break up succession overlain by a sediment horizon a syn break up succession consists of the faeroes middle and upper basalt formations and the equivalent milne land formation in east greenland in which five intervals can be correlated with a compositional evolution from ti rich magnesian basalts and picrites at the base to a dominance of morb like low ti basalts at the top the successions were generated in the same mantle melting column beneath a thinning continent with a rift zone that eventually ruptured the continent the evolution pattern is very similar to that seen on the se greenland margin but spreading according to the palmason model of 1973 was not yet established the pre and syn break up successions formed volcanic megasystems stretching across the rift zone with areal extents of 70 000 and 220 000 km 2 and volumes of 120 000 and 250 000 km 3 rocks from the pre and syn break up successions can be discriminated based on a simple major element plot the overlying succession was 3 3 5 km thick in e greenland but was thin or absent in the faeroes the energy source for the melting appears to have been concentrated on the greenland side

pyroclastic deposits within the east greenland tertiary flood basalts stratigraphic geochemical and mineralogical characterization of pyroclastic deposits on the gronau west nunatak of east greenland indicates that both alkaline and basaltic tephras occurred during the eruption of flood basalts associated with the opening of the north atlantic ocean in the early tertiary within the exposed section of gronau west nunatak c 1 km thick representing c 1 6 ma 17 of the horizons are phreatomagmatic basaltic tephras near the top of the section an alkaline tuff was dated at 53 8 0 3 ma by 40 ar 39 ar method this tuff contains a distinctive mineralogy sanidine mg katophorite aegirine and geochemistry melt inclusion compositions and trace and rare earth element abundances that indicate it was erupted from the gardiner melanephelinite carbonatite volcanic complex located 175 km to the se which was active from c 55 to 52 ma the alkaline tuff can be correlated to age equivalent compositionally similar alkaline pyroclastic horizons reported in north atlantic sediment cores and in outcrops in northern europe making it an important regional time stratigraphic marker this study indicates that north atlantic explosive volcanic events were prevalent regionally widespread and originated in east greenland during continental rifting suggesting the need for reappraisal of the impact of north atlantic volcanism on climate change during the early tertiary

flow resistance in ice covered alluvial channels this paper describes the influences of floating level ice covers on flow and bed load transport in alluvial channels and proposes a semi empirical procedure for calculating flow resistance in such channels the procedure can be applied in conjunction with several existing methods for estimating flow resistance in alluvial channels under open water conditions previously cover influence was poorly understood with the consequence that flow resistance calculations for ice covered alluvial channels entailed an inordinate amount of guesswork the cover influences described herein were determined from the results of experiments conducted with a laboratory flume the main findings of the flume experiments are that ice covers increase flow depth decrease bulk flow velocity increase dune length and significantly decrease bed load transport rate additionally it was found that sediment transport relationships developed for open water flow are applicable to ice covered flow provided they are used in conjunctio

formation and emplacement of the josephine ophiolite and the nevadan orogeny in the klamath mountains california oregon u pb zircon and 40ar 39ar geochronology cordilleran ophiolites typically occur as basement for accreted terranes in the klamath mountains ophiolitic terranes were progressively accreted by underthrusting beneath north america the josephine ophiolite is the youngest of the klamath ophiolites and forms the basement for a thick late jurassic flysch sequence galice formation this ophiolite flysch terrane forms an east dipping thrust sheet sandwiched between older rocks of the klamath mountains above and a coeval plutonic volcanic arc complex below the outcrop pattern of the roof orleans thrust indicates a minimum displacement of 40 km and geophysical studies suggest 110 km of displacement the basal madstone cabin thrust is associated with an amphibolitic sole and has a minimum displacement of 12 km a rapid sequence of events from ophiolite generation to thrust emplacement has been determined using 40 ar 39 ar and pb u geochronology ophiolite generation occurred at 162 164 ma a thin hemipelagic sequence was deposited from 162 to 157 ma and flysch deposition took place between 157 and 150 ma tight age constraints on thrusting and low grade metamorphism associated with ophiolite emplacement nevadan orogeny are provided by abundant calc alkaline dikes and plutons ranging in age from 151 to 139 ma deformation and metamorphism related to the nevadan orogeny appears to have extended from 155 to 135 ma most of the crustal shortening took place by thrusting constrained to have occurred from 155 to 150 ma on both the roof and basal thrusts minimum rates of displacement are 2 4 and 3 6 mm year for the basal and roof thrusts respectively but correlations with coeval thrusts yield rates of 8 4 and 22 mm year within the range of plate velocities the high displacement rates and synchronous movement along the basal and roof thrusts suggest that the ophiolite may have behaved as a microplate situated between western north america and an active arc from 155 to 150 ma a steep thermal gradient was present in the josephine galice thrust sheet from 155 to 150 ma with amphibolite facies conditions developed along the basal thrust after accretion of the ophiolite by underthrusting the ophiolite and overlying flysch underwent low grade dynamothermal regional metamorphism from 150 to 135 ma the upper age limit is tightly constrained by a 135 ma k feldspar cooling age syntectonic plutons as young as 139 ma and a lower cretaceous angular unconformity very rapid exhumation is indicated by the late valanginian to hauterivian age 130 ma of the unconformably overlying strata suggesting unroofing by extensional tectonics

temporal evolution of a long lived syenitic centre the kangerlussuaq alkaline complex east greenland abstract we present new mineral and whole rock compositions and sr nd hf pb o h isotope data on samples from the kangerlussuaq alkaline complex 1000 km2 in central east greenland part of the north atlantic igneous province this complex mainly consists of the kangerlussuaq intrusion but includes at least 13 separate satellite intrusions emplaced in the uppermost crust close to the unconformity between archaean gneisses and overlying palaeogene flood basalts the complex is divided into i older satellite intrusions 55 53 ma composed of multiple syenites and granites and minor gabbros and peridotites ii the voluminous kangerlussuaq intrusion 50 ma which displays a gradual transition from quartz syenites nordmarkites at the margin to nepheline syenites foyaites in the centre and iii younger satellite intrusions 47 45 ma of minor syenites granites and diorites concentrated southeast of the kangerlussuaq intrusion the complex displays a temporal evolution in which sio2 decreases 74 56 wt and total alkalis 6 16 wt amphibole na k content 206pb 204pbmeas endi and ehfi 3 to 11 increase from the older intrusions through the nepheline syenites this is followed by a reversal to higher silica 62 73 wt and lower total alkalis 9 12 wt amphibole na k content 206pb 204pbmeas endi and ehfi 13 to 2 in the younger satellite intrusions temporal changes in the location of magma plumbing systems and in magma production rates played a profound role in controlling silica content alkalinity and degree of crustal contamination during development of the complex phonolitic magma was only generated after prolonged magmatism had shielded the conduits from interaction with country rock the parental magmas were probably basanitic to alkali olivine basaltic in composition the older satellite intrusions and the kangerlussuaq intrusion have low 18o magma values 1 to 6 compared to the younger more crustally contaminated satellite intrusions 4 5 to 7 it appears that the magmas only had sufficient over pressure to intrude the basalt cover when larger volumes of less contaminated trachyte magma were produced resulting in the generation of low 18o magmas due to dehydration of hydrothermally altered basalt xenoliths

emplacement and deformation of granitoids during volcanic arc construction in the foothills terrane central sierra nevada california abstract emplacement of a series of late jurassic to early cretaceous tonalites and diorites at depths of 4 to 9 km accompanied volcanic arc construction in the foothills terrane central sierra nevada california emplacement of these plutons occurred after initiation of faulting and rigid rotation of bedding and immediately before widespread cleavage development pluton emplacement occurred by permissive emplacement in the hinges of large anticlines or domes or as structurally controlled sill like bodies in a ductile shear zone and was accompanied by widespread mingling of magmas subsequent deformation rotated but did not internally deform one pluton strongly deformed several others and shortened the overlying volcanic and sedimentary sequences by 30 to 50 we suggest that deformation was active throughout volcanic arc construction but evolved from early more brittle deformation to ductile flow immediately after pluton emplacement and back to brittle deformation as the arc was uplifted and cooled at this location arc magmatism lasted at least 30 ma whereas deformation lasted at least 35 ma and possibly much longer the different emplacement mechanisms changing style of deformation and differences in viscosity contrast between plutons and wall rocks resulted in an impressive variety of structural characteristics and apparent timing relationships for the subvolcanic plutons

evidence from the rare earth element record of mantle melting for cooling of the tertiary iceland plume widespread flood basalt volcanism and continental rifting in the northeast atlantic in the early tertiary period 55 myr ago have been linked to the mantle plume now residing beneath iceland1 2 3 4 5 although much is known about the present day iceland plume6 7 8 9 its thermal structure composition and position in the early tertiary period remain unresolved estimates of its temperature for example range from 1 600 c in some plume models3 to 1 500 c based on the volume and composition of basaltic crust10 11 12 several recent studies4 have emphasized similarities in the thermal and chemical structure of the tertiary and present day plumes to argue for stability of the mantle anomaly whereas others12 13 relate variations in basalt volumes and compositions to changes in plume flux moreover some authors1 2 13 have assumed that the plume was rift centred for its entire history whereas others argue that it became ridge centred only after plate separation14 15 here we report compositional data for 6 000 metres of flood basalts erupted in east greenland close to the inferred plume axis that we use to constrain the tertiary plume structure rare earth element systematics place limits on the pressures and extents of mantle melting and show that the mantle was initially moderately hot 1 500 c but that its temperature declined during flood volcanism these observations are difficult to reconcile with current plume head models and call for important lithospheric control5 10 16 17 18 on actively upwelling mantle along the rifted margin

geochemical recognition of a captured back arc basin metabasaltic complex southwestern oregon an extensive fault bounded amphibolite terrane of late jurassic 145 pm 2 ma metamorphic age occurring in the northeastern klamath mountains of southern oregon has been recognized as the remnants of an ancient back arc basin in spite of thorough metamorphic recrystallization under amphibolite facies conditions the amphibolite locally displays relict igneous textures which suggest that the protoliths included basaltic dikes or sills shallow diabase intrusions and gabbros the major and minor element chemistry of the amphibolite indicates that some alteration of original compositions has occurred nevertheless certain elements e g zr ti y hf p th ta and the ree were probably immobile and retain their original igneous abundances chondrite normalized ree patterns are similar to those of mid ocean ridge basalt morb concave downward and flat or slightly lree depleted with abundances about six to 30 times chondritic values patterns are slightly lree enriched and hree depleted relative t

cyclical variation in composition in continental tholeiites of east greenland the prinsen of wales bjerge pwb part of the tertiary volcanic province of east greenland consists of tholeiitic basalts overlain by alkalic basalts that were erupted 100 150 km west of the original axis of continental rifting and active ocean floor development during the creation of the north atlantic ocean they have many features of continental flood basalts but are somewhat enriched in fe and in ti relative to fe and have slightly lower al2o3 they have slight enrichments in the light rare earth elements la yb 3 4 a nunatak within the pwb displays four cycles of tholeiitic basalt each about 50 m thick which are defined by trace element variations ni cr sr zr and zr y in three of the four cycles the lowermost flows are the most highly differentiated and successive flows are increasingly primitive these changes are thought to be the result of frequent injection of primitive mantle derived tholeiitic magma into small crustal magma chambers that contain evolved tholeiitic magma the re

overthrust model for the sierra nevada the structure of the northern sierra nevada california is a series of east directed thrust faults modified by upright and west directed folds and faults the highest tectonic unit is a nappe composed of the smartville ophiolitic complex and related rocks these structures collectively represent the middle jurassic nevadan orogeny this overthrust backfold model resembles middle jurassic events in british columbia and the cretaceous tertiary tectonics of the western european alpine belt and it is consistent with the hypothesis that the nevadan orogeny reflects the collision of an oceanic volcanic arc with north america

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

baddeleyite and zircon u pb ages from the kaerven area kangerlussuaq implications for the timing of paleogene continental breakup in the north atlantic abstract we report u pb zircon and baddeleyite age results for two mafic units in the kangerlussuaq area which are part of the paleogene east greenland igneous province the kangerlussuaq macrodyke which represents a feeder to some flood basalts and the nearby kaerven gabbro baddeleyite from the macrodyke yielded a concordant age of 54 7 0 4 2 ma and this is among the most precise age determinations from the province this age is interpreted to reflect the time of emplacement the macrodyke belongs to a swarm of dykes emplaced perhaps in a failed arm during continental rifting and the age predates the completion of continental breakup at the end of emplacement of the plateau basalts of the east greenland eruption sequence geochemically the macrodyke is correlated with the milne land formation lavas the magma experienced slight crustal contamination based on the age and geochemistry of the macrodyke the age of the skaergaard intrusion and skraenterne formation lavas an age span of 1 2 ma is confirmed for the extrusion of the entire plateau basalts u pb analysis of zircon from the kaerven gabbro yields an age of 53 0 0 3 2 ma the kaerven gabbro was intruded after extension had ceased in the area and we suggest that the post plateau basalt magmatism did not take place at a distinct later time but instead that igneous activity continued for some time at a lower rate slightly fractionated hrees indicate that the kaerven gabbro magma was extracted at relatively shallow levels in the mantle

timescales of orogeny jurassic construction of the klamath mountains an electronic supplement of this material may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 94ycj2454 timescales of orogeny jurassic construction of the klamath mountains b r hacker m m donato c g barnes m o mcwilliams and w g ernst diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts forty five new 40ar 39ar ages from the klamath mountains illuminate the deformation metamorphism magmatism and sedimentation involved in the jurassic construction of that orogen leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes the new geochronologic relationships show that many jurassic units of the klamath mountains had 200 ma or older volcanoplutonic basement subsequent formation of a large 170 ma arc was followed by contractional collapse of the arc collision with a spreading ridge may have led to large scale nw se extension in the central and northern klamaths from 167 to 155 ma coincident with the crystallization of voluminous plutonic and volcanic suites marked cooling of a large region of the central klamath mountains to below 350 c at 150 ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels these data demonstrate that the klamath mountains and perhaps other similar orogens were constructed during areally and temporally variant episodes of contraction extension and magmatism that do not fit classical definitions of orogeny

age of tertiary volcanic rocks on the west greenland continental margin volcanic evolution and event correlation to other parts of the north atlantic igneous province radiometric ages for undated parts of the volcanic succession and intrusions in west greenland were obtained by the 40ar 39ar incremental heating method acceptable crystallization ages were obtained for 27 samples combined with published results the new data provide a volcanic stratigraphy correlatable throughout the nuussuaq basin the thick onshore volcanic pile consists of four widespread formations 62 5 61 ma picrites vaigat formation 61 60 ma depleted basalts malig t formation and the hellefisk 1 well 60 58 ma less depleted basalts svartenhuk formation and 56 54 ma enriched basalts naqerloq formation two local successions comprise 53 5 ma alkali basalts erqua formation and 38 7 ma transitional basalts talerua member a central volcano developed on ubekendt ejland leading to the sarq ta qaq gabbro granophyre intrusion at 57 55 ma pre break up volcanism took place further south as early as 64 63 ma the offshore volcanic succession most probably comprises the known onshore succession plus some younger lavas the change in spreading direction near the paleocene eocene boundary took place west of the nuussuaq basin c 56 2 ma some tectonomagmatic events are correlatable across the entire north atlantic igneous province a quiescent 58 56 ma period correlates with similar periods in east greenland and the faroes and the naqerloq formation is coeval with the eocene basalts in east greenland the paleocene and eocene tholeiitic basalts are distinguishable chemically in early eocene time mantle typical of the iceland plume seems to have extended beneath the whole west greenland margin as well as the central east greenland margin

and mg fe si al2 compositional control on argon behaviour in high pressure white micas a continuous laser probe study from the dora maira nappe of the internal western alps italy abstract high pressure white micas from a restricted area in the dora maira nappe of the western alps italy were analyzed by the conventional 40 ar 39 ar bulk sample step heating method and by the step heating and the spot fusion 40 ar 39 ar continuous laser probe techniques on single grains high si ferich phengites from fine grained gneisses display homogeneous 35 40 ma plateau and spot fusion ages consistent with the upper eocene 40 ar 39 ar phengite ages documented over the whole internal western alps in contrast phengite separates and single grains of high si and mg rich to intermediate composition selected from metapelitic and mg rich mica schist samples yield older discordant apparent ages even when sampled in the same outcrop as the fine grained gneisses these local age variations are interpreted to reflect primarily an increasing ar retentivity with increasing mg content in phengites in metapelite derived white micas the isotopic discordance is marked by 1 1 a characteristic bulk sample upward convex spectrum shape 2 2 single grain disturbed age spectra 3 3 a systematic radially outward decrease in spot ages across individual mica cleavage planes and 4 4 strong grain to grain spot age variations in rock sections 1 1 in mg rich mica schists the isotopic discordance is observed as a reproducible upward convex bulk sample age spectrum with high 140 ma intermediate apparent ages 2 2 upward convex single grain age spectra and 3 3 irregular intra and intergrain age gradients on geochemical grounds it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the mg fe si al2 celadonitic re equilibration of the high pressure phengites during greenschist overprinting in mg rich phengites this chemical re equilibration may induce ar loss below their presumed higher blocking temperature a model of ar behaviour in high pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the mg fe compositional control on ar retentivity and the mg fe si al2 modifications occurring in phengites during progressive overprinting

an expanded view of jurassic orogenesis in the western united states cordillera middle jurassic pre nevadan regional metamorphism and thrust faulting within an active arc environment klamath mountains california basaltic to basaltic andesitic volcaniclastic rocks and their contemporaneous mafic ultramafic intrusive complexes delineate a middle jurassic arc terrane within the klamath mountain province of northern california exposures of the supracrustal volcaniclastic rocks are restricted to a single fault bounded terrane but the deeper level intrusive complexes were emplaced into most if not all the pre late jurassic terranes of the klamath mountain region the pre late jurassic terranes thus constitute the basement of the middle jurassic arc u pb zircon analyses of 39 zircon fractions from 12 intrusive complexes plus k ar dating of the volcaniclastic strata demonstrate magmatic activity over the interval of 177 159 ma the active arc and its basement were imbricated by a compressive deformational event the signature of which included thrust faulting isoclinal folding and regional metamorphism several diverse lines of evidence including k ar dating of metamorphic rocks crosscutting relations of dated intrusive complexes to thrust faults and u pb dating of synmetamorphic intrusive complexes establish a distinctly pre nevadan middle jurassic age ongoing at 169 ma and over by at least 161 ma for this compressive deformational episode outboard and structurally beneath the middle jurassic arc and its basement are several terranes that collectively comprise the western jurassic belt these terranes were deformed and regionally metamorphosed during the late jurassic nevadan orogeny which occurred within the time interval of 157 150 ma as upper jurassic plutons with 150 to 142 m y old zircon ages have contact aureoles that overprint the nevadan fabric and the 157 m y old rogue formation was deformed in the nevadan event the middle and late jurassic compressive deformational events were thus distinct and separated by as much as 15 20 m y the relation between middle and late jurassic magmatism and deformation suggests that the klamath mountain province records the evolution of a considerably long lived arc system that evolved above an east dipping subduction zone in addition we suggest that this are system may represent an oceanic continuation of the long recognized early mesozoic arc terrane of the western u s cordillera

significance of xenocrystic precambrian zircon contained within the southern continuation of the josephine ophiolite devils elbow ophiolite remnant klamath mountains northern california the josephine ophiolite of the western jurassic belt klamath mountain province of california and oregon is the expression of a well documented late jurassic suprasubduction zone rift basin that formed between an active late jurassic arc to the west and a remnant middle jurassic arc to the east the devils elbow ophiolite remnant deo exposed along the south fork of the trinity river represents the southernmost continuation of the josephine ophiolite and provides important new constraints on the early history of this rift basin the deo consists of pillow lavas and breccias and a well developed sheeted dike complex that are depositionally overlain by a clast supported breccia derived almost exclusively from ophiolitic detritus dikes and irregular pods of plagiogranite are conspicuous elements of the deo and exhibit mutually crosscutting relations with mafic dikes thus they are interpreted as genetically related elements of the dike complex zircon separates from two widely separated plagiogranite dike localities yielded two distinct zircon populations a clear euhedral magmatic population and a reddish rounded xenocrystic population the isotopic systematics of four zircon fractions from these two dike localities indicate a crystallization age for the deo of 164 1 ma and an age of 1 7 ga for the xenocrystic zircon component the occurrence of a xenocrystic precambrian zircon component within the plagiogranites of the deo provides unequivocal evidence that rifting occurred within preexisting zircon bearing crustal rocks of the klamath mountains as there is no precambrian crust within the klamath mountains the xenocrystic zircon population must have been derived from supracrustal sedimentary sequences the most likely source for the older zircon component is a terrigenous metasedimentary sequence contained within the rattlesnake creek terrane which formed part of the structural basement of the rifted middle jurassic arc the incorporation of xenocrystic precambrian zircon from the rifted basement of the middle jurassic arc could only have occurred during the earliest stages of rifting thus the deo must represent a fragment of an initial rift that ultimately formed the late jurassic basin now floored by the josephine ophiolite in support of the zircon data dikes and lavas of the deo have geochemical characteristics transitional between island arc and mid ocean ridge magma series as would be expected within an initial rift setting of a suprasubduction zone ophiolite

cryptic tectonic domains of the klamath mountains california and oregon abstract numerous fragments of oceanic crust and island arcs make up the klamath mountains province these fragments were joined together amalgamated in an oceanic setting during paleozoic and mesozoic collisional events and were accreted to north america as a composite unit during latest jurassic or earliest cretaceous time the roughly arcuate and concentric distribution of the terranes of the klamath mountains does not now seem to be a result of simple oroclinal bending as earlier believed although commonly described as a west facing arcuate structure the province is cut diagonally by a vaguely defined nw trending zone of discontinuity or hinge line that divides the province into ne and sw tectonic domains the zone of discontinuity is marked by a number of lithic and structural anomalies and particularly by the distribution of a remarkable series of belts of plutonic rocks the terranes regional structures and plutonic belts of the ne domain trend ne and are generally wider and more coherent than the narrow nw trending terranes and plutonic belts of the sw domain most plutonic belts of the ne domain do not have equivalents in the sw domain paleomagnetic evidence suggests that all the plutonic belts except possibly the youngest the earliest cretaceous shasta bally belt were emplaced before the klamath mountains terranes finally accreted to north america

structure and stratigraphy of the early tertiary basalts of the blosseville kyst east greenland the central part of the 65 000 km 2 lava plateau in east greenland between kangerlussuaq and scoresby sund has been investigated by multi model photogrammetry and profile sampling and a three dimensional framework for the lava plateau has been established post volcanic movements have divided the region into several structural blocks with dips varying from 1 2 n ne in the inland areas to 12 se at the coast a major n s trending fracture zone may reflect an underlying caledonian structure early volcanic and sedimentary rocks are overlain by widespread plateau basalts locally with an angular unconformity of 8 the basalt stratigraphy established in the scoresby sund area has been extended over most of the region the plateau basalts were erupted over a zone more than 150 km wide from the present coast and inland the succession attains a vertical thickness of more than 5 5 km in the central coastal areas and thins inland to 2 3 km a differential sagging of the coastal areas by at least 2 3 km took place during the emplacement of the lavas accelerating with time the features suggest increasing focusing of the magmatic production into a developing rift zone close to the present coast

estimation of mean flow velocity in ice covered channels point measurement methods for estimating the mean velocity of ver tical distributions of streamwise velocity in ice covered channels are evaluated in this paper the evaluation uses profiles generated numerically based on a two power law description of vertical distribution of streamwise velocity the two power law simplifies into the well known power law expression for open water velocity profiles its validity was verified using measured velocity profiles obtained from rivers and a laboratory flume the profiles are representative of flows subject to various combinations of bed and ice cover conditions values of estimation bias the percent error in estimating average velocity om a few point measurements instead of integrating over the entire vertical velocity profile were determined for se eral point measurement methods including those currently used by the united states geological survey usgs and the water survey of canada and proposed new methods the systematic variation of bias values with ice cover and bed resistance conditions was also determined the method with the least overall bias is the so called usual two point method in which velocity measurements taken at 0 2 and 0 8 of flow depth are averaged to obtain an estimate of mean velocity this method produces a remarkably small bias its maximum value is close to 2 estimation accuracy is enhanced if a coefficient of 0 98 is applied to the two point method

reassembling the paleogene eocene north atlantic igneous province new paleomagnetic constraints from the isle of mull scotland abstract the paleomagnetic data sets from the british tertiary igneous province btip have recently been criticized as being unreliable and discordant with data from elsewhere in the north atlantic igneous province naip riisager et al earth planet sci lett 201 2002 261 276 riisager et al earth planet sci lett 214 2003 409 425 we offer new paleomagnetic data for the extensive lava flow sequence on the isle of mull scotland and can confirm the paleomagnetic pole positions emanating from important earlier studies our new north paleomagnetic pole position for eurasia at 59 0 2 ma has latitude 73 3 n longitude 166 2 e dp dm 5 2 7 0 a re evaluation and an inter comparison of the paleomagnetic database emanating from the naip were carried out to test for sub province consistency we find a general agreement between the eurasian part of naip btip and faeroes and east greenland data however a compilation of west greenland data displays a large and unexplained dispersion we speculate on if this is related to different sense of block rotation of the tertiary west greenland constituents combining all data from the naip constituents give a pole position at 75 0 n 169 9 e n 25 k 84 3 a95 3 2 in eurasian reference frame

igneous petrogenesis of magnesian metavolcanic rocks from the central klamath mountains northern california mafic meta igneous supracrustal rocks in the east central part of the western triassic and paleozoic belt are interlayered with and appear to predominantly overlie fine grained clastic and cherty metasedimentary strata this complex constitutes a lithostratigraphic terrane exposed in the vicinity of sawyers bar california basaltic flows dikes and sills are informally referred to as the yellow dog greenstones dark colored amygdaloidal flow breccias rich in titanium iron phosphorus and light rare earth elements lree9s constitute the lowest mildly alkalic members of the mafic flow series intimately interlayered with fine grained terrigenous detritus element proportions suggest that the dark ti fe p rich basaltic lavas may have been extruded in an oceanic intraplate setting interstratification with distal turbiditic strata indicates concurrent submarine volcanism and deep sea sedimentary deposition the main mass of the stratigraphically higher extrusive sequence is a light colored massive tholeiite series poorer in ti fe p lree9s hypabyssal rocks belong exclusively to this second igneous suite compositions of the pale colored overlying basaltic diabasic mass of the yellow dog greenstone section are consistent with eruption in an immature magmatic arc dikes presumably comagmatic with terminal stage yellow dog volcanism cut the overlying more eastern stuart fork terrane the yellow dog greenstones contain relict magmatic clinopyroxene and or pargasitic hornblende both melt series include mg rich units ave 17 mgo 14 4 1 9 wt cr 650 200 ppm ni 270 100 ppm sea floor alteration apparently affected these mafic rocks in the marine environment following eruption as reflected by 18 o mgo and cao al 2 o 3 values and by minor alkali and silica metasomatism yellow dog igneous activity was extinguished during mid jurassic suturing of the oceanward sawyers bar terrane against and beneath the pre existing landward stuart fork blueschist complex of late triassic metamorphic age subsequently the terrane amalgam was invaded by middle late jurassic granitoids minor chemical alteration including 18 o enrichment also probably accompanied later regional and especially contact metamorphism nevertheless many major minor and rare earth element variations in the yellow dog greenstones are compatible with igneous fractional crystallization trends but exclude the assimilation of significant amounts of sialic crust

precise 40ar 39ar age for the initiation of palaeogene volcanism in the inner hebrides and its regional significance sanidine crystals from two tuff layers at the base of the palaeocene eigg lava formation on the isle of muck inner hebrides yield step wise ar release plateau ages of 62 8 0 6 2 and 62 4 0 6 ma using the laser 40 ar 39 ar dating technique these determinations which are in accord with but significantly more precise than earlier results provide a definitive age for the inception of igneous activity in the small isles from stratigraphical inferences the muck ages provide precise maximum constraints on the possible age of skye magmatism and possibly most of the british tertiary volcanic province the new ages significantly predate the oldest ages determined so far for tertiary magmatism in ne greenland but are similar to ages obtained from w greenland and offshore se greenland and support the concept that the initiation of regional volcanism was primarily controlled by lithospheric thin spots the muck sanidine age appears to precisely date the first manifestation of the impingement of the iceland mantle plume on the lithosphere beneath nw scotland

episodic volcanism during break up of the north atlantic evidence from the east greenland plateau basalts abstract the lower tertiary upper paleocene lower eocene plateau basalts in the scoresby sund region of east greenland formed during three distinct magmatic episodes the activity of the first episode was centred in the present inland areas to the northwest and produced a voluminous regional basalt sequence lower sequence the activity of the second episode was centred to the southeast offshore the present atlantic coast and this also produced a voluminous regional basalt sequence upper sequence the third episode was centred on the atlantic coast and gave rise to a dense coastal dyke swarm and local lavas which are now only locally preserved by downfaulting the lavas are all tholeiitic basalts after an initial phase the two regional lava sequences of the first and second episodes show a similar vertical compositional zonation pattern a thin lower formation with variable composition is followed by a thick uniform formation which shows systematic compositional change with height from tio2 rich feti basalts to tio2 poor basalts and ending with a reversal to tio2 rich basalts each of these cyclic patterns apparently resulted from a temporary rifting episode and much of the compositional variation found can be explained by magma development in open magma chambers the dykes and lavas of the third episode of activity have chemical affinities to ocean floor basalts and their tio2 p2o5 ratios indicate formation at a spreading ridge centre the pattern of two failed rifting episodes and one event that produced oceanic crust can be correlated with stages in the complicated initiation of seafloor spreading in the region and the volcanism itself is closely related to this process

u pb sm nd and rb sr geochronological and isotopic study of northern sierra nevada ophiolitic assemblages california distinct ophiolitic assemblages occur as oceanic basement within three of the four regional tectonic belts of the northern sierra nevada new u pb zircon sm nd and rb sr data are presented for each assemblage providing critical geochronological and isotopic constraints on the petrogenesis and tectonic evolution of the ophiolitic and associated ensimatic assemblages ophiolitic assemblages include from west to east the smartville complex central belt and feather river belt the smartville complex represents an island arc volcanic plutonic sequence with a major late stage sheeted dike swarm the sm nd systems from a wide compositional spectrum of rocks record a 178 21 ma petrogenetic age and an nd t 9 2 0 6 zircon u pb systems on an uppermost dacite yield a 164 2 ma age and on a number of plagiogranite screens and dikes from the sheeted complex 162 1 ma ages the central and feather river belts are structurally complex polygenetic assemblages the u pb zircon and sm nd systems record major 205 ma and 315 ma petrogenetic events respectively both involving depleted mantle derived magmas such magmatism probably occurred in marginal basin transform systems developed within an older oceanic depleted mantle basement regime both sm nd and u pb zircon systems show local components of proterozoic sialic material the sialic contaminants were probably introduced into the system as craton derived detritus it is doubtful that any of the ophiolitic assemblages studied represent genetically related crust upper mantle sequences generated during the development of new oceanic lithosphere integration of the geochronological data with geological relations reveals a pattern of petrogenesis and tectonics whereby progressively younger ensimatic terranes were added to the continental margin through time by plate convergence and were ultimately welded into north american sial by a crosscutting batholithic belt this accretionary pattern is reflected in both the protolith ages and deformation metamorphic ages of each of the regional belts which progressively young westward crustal components of the accreted ensimatic terranes grew by mainly basaltic igneous activity within island arc marginal basin and leaky transform systems adjacent to the continent edge prior to final tectonic accretion such complexities are suggested to be typical of cordilleran type ophiolites and representative of the circum pacific erogenic style

evolution of the northern sierra nevada metamorphic belt petrological structural and ar ar constraints the sierra nevada metamorphic belt constitutes an important record of the growth of continental crust from essentially oceanic materials in the northern sierra the central part of the belt is made up of volcanoplutonic arcs and sediment dominated units inferred to be accretionary wedges or closed ocean basins the latter are broken formation and melange composed of radiolarian chert lava and volcanogenic and continental turbidites sedimentary detritus in the largest of these units can be plausibly linked to sources farther east in the sierra suggesting that deposition occurred near the eastern sierran arc isoclinal folds steeply dipping foliations and steeply plunging down dip lineations are characteristics structures the westernmost unit is only feebly recrystallized and deformation was accomplished principally by stress solution and local redeposition in veins more easterly inboard units are compositionally similar but they recrystallized at pumpellyite actinolite and blueschist facies conditions and deformed via solution transfer and dislocation creep phengite silica contents the degree of quartz veining and the locations of pseudo isograds support an eastward increase in metamorphic pressure and temperature metamorphic conditions during the growth of pumpellyite and actinolite ranged from approximately 150 350 degrees c and 200 400 mpa compatible with recrystallization and deformation in subduction zones or the deeper levels of magmatic arcs ar armore ages of volcanisclastic rocks and crosscutting plutons constrain the age of deformation and metamorphism in the western part of the region to 174 165 ma deformation and recrystallization in more easterly units may have been coeval or begun as early as triassic time 58 refs 14 figs 4 tabs less

mean flow and turbulence structure over fixed two dimensional dunes implications for sediment transport and bedform stability detailed measurements of flow velocity and its turbulent fluctuation were obtained over fixed two dimensional dunes in a laboratory channel laser doppler anemometry was used to measure the downstream and vertical components of velocity at more than 1800 points over one dune wavelength the density of the sampling grid allowed construction of a unique set of contour maps for all mean flow and turbulence parameters which are assessed using higher moment measures and quadrant analysis these flow field maps illustrate that 1 the time averaged downstream and vertical velocities agree well with previous studies of quasi equilibrium flow over fixed and mobile bedforms and show a remarkable symmetry from crest to crest 2 the maximum root mean square rms of the downstream velocity values occur at and just downstream of flow reattachment and within the flow separation cell 3 the maximum vertical rms values occur within and above the zone of flow separation along the shear layer and this zone advects and diffuses downstream extending almost to the next crest 4 positive downstream skewness values occur within the separation cell whereas positive vertical skewness values are restricted to the shear layer 5 the highest reynolds stresses are located within the zone of flow separation and along the shear layer 6 high magnitude high frequency quadrant 2 events ejections are concentrated along the shear layer kelvin helmholtz instabilities and dominate the contribution to the local reynolds stress and 7 high magnitude high frequency quadrant 4 events occur bounding the separation zone near reattachment and close to the dune crest and are significant contributors to the local reynolds stress at each location these data demonstrate that the turbulence structure associated with dunes is controlled intrinsically by the formation magnitude and downstream extent of the flow separation zone and resultant shear layer furthermore the origin of dune related macroturbulence lies in the dynamics of the shear layer rather than classical turbulent boundary layer bursting the fluid dynamic distinction between dunes and ripples is reasoned to be linked to the velocity differential across the shear layer and hence the magnitude of the kelvin helmholtz instabilities which are both greater for dunes than ripples these instabilities control the local flow and turbulence structure and dictate the modes of sediment entrainment and their transport rates

mean and turbulent flow fields in a simulated ice covered channel with a gravel bed some laboratory observations northern rivers experience freeze up over the winter creating asymmetric under ice flows field and laboratory measurements of under ice flows typically exhibit flow asymmetry and its characteristics depend on the presence of roughness elements on the ice cover underside in this study flume experiments of flows under a simulated ice cover are presented open water conditions and simulated rough ice covered flows are discussed mean flow and turbulent flow statistics were obtained from an acoustic doppler velocimeter adv above a gravel bed surface a central region of faster flow develops in the middle portion of the flow with the addition of a rough cover the turbulent flow characteristics are unambiguously different when simulated ice covered conditions are used two distinct boundary layers near the bed and in the vicinity of the ice cover near the water surface are clearly identified each being characterized by high turbulent intensity levels detailed profile measurements of reynolds stresses and turbulent kinetic energy indicate that the turbulence structure is strongly influenced by the presence of an ice cover and its roughness characteristics in general for y d 0 4 where y is height above bed and d is local flow depth the addition of cover and its roughening tends to generate higher turbulent kinetic energy values in comparison to open water flows and reynolds stresses become increasingly negative due to increased turbulence levels in the vicinity of the rough ice cover the high negative reynolds stresses not only indicate high turbulence levels created by the rough ice cover but also coherent flow structures where quadrants one and three dominate copyright 2012 john wiley sons ltd

changes in velocity profiles at roughness transitions in coarse grained channels heterogeneous coarse grained channels are often characterized by local transitions in bed surface roughness distinct spatial zones in terms of grain size have been reported for example sand ribbons and bedload sheets the transition from areas of finer to coarser grained surface sediment is often abrupt however the effects of these transitions on the shape of the velocity profile and associated shear velocity and roughness length estimates have not been investigated in detail in coarse grained channels this paper therefore examines the combined effects of a sudden change in surface roughness and of superimposed scales of resistance on the structure of the turbulent boundary layer measurements along roughness transitions from smooth to rough beds were conducted in a flume using artificial roughness features and in a natural gravel bed river immediately at the transition from a zone of close packed roughness to a rougher section dominated by obstacles superimposed on the more or less uniform roughness surface boundary shear stress and roughness length increase considerably downstream from this transition velocity profiles become concave upwards downstream and upstream sections show significant differences in terms of near bed velocities deceleration downstream of the transition velocity gradient and turbulence intensity of the streamwise velocity component comparing the mean velocity profiles corresponding to these two different roughness surfaces gives some indication of the proportion of total shear velocity or shear stress associated with the pressure drag produced by large and isolated obstacles

permo triassic accretionary subduction complex southwestern klamath mountains northern california the western paleozoic and triassic subprovince of the southwestern klamath mountains has previously been subdivided into three relatively steeply dipping fault bounded litho tectonic terranes in descending structural order from east to west these terranes are 1 the north fork terrane 2 the hayfork terrane and 3 the rattlesnake creek terrane in a general fashion each of these terranes exhibits its own particular set of lithologic structural and temporal relationships that sets it apart from adjacent terranes this threefold subdivision of the southwestern klamath mountains needs modification only as far as the hayfork terrane is concerned the hayfork terrane is actually a composite terrane consisting of a structurally lower assemblage of predominantly basaltic metavolcaniclastic rocks and a structurally higher assemblage of chaotically disrupted metasedimentary rocks juxtaposed along a previously unrecognized major regional thrust these two lithologically and structurally distinct assemblages have been elevated to the status of a terrane in this report and are herein referred to as the western hayfork terrane structurally lower metavolcaniclastic assemblage and the eastern hayfork terrane structurally overlying metasedimentary assemblage and the fault which juxtaposes these two terranes is herein referred to as the wilson point thrust this paper will focus on the eastern hayfork and north fork terranes and their relationship to other roughly coeval tectonic elements of the klamath mountain province the eastern hayfork terrane is best described as a tectono stratigraphic unit consisting of a complex chert argillite and quartzose sandstone melange and broken formation that contains radiolarian cherts as young as late triassic in age scattered throughout this terrane are various types of blocks some of which are clearly exotic metachert is by far the most common type of metamorphic block and these blocks appear to have been derived via sedimentary processes from the paleozoic central metamorphic belt and or the mesozoic stuart fork formation shallow water permian limestone blocks are also plentiful and are interpreted as reefal accumulations which flanked seamounts that became incorporated into the melange and broken formation as proto pacific ocean floor was subducted beneath the western margin of north america during at least the permo triassic to early jurassic the north fork terrane has at its base basaltic rocks tentatively interpreted as an oceanic crustal remnant generated within an open ocean ridge setting that was removed from any appreciable source of continental or volcanic arc derived detritus overlying the basaltic rocks is a sequence of pelagic hemipelagic terrigenous and tuffaceous metasedimentary rocks that have yielded a late triassic and early or middle jurassic radiolarian fauna the metasedimentary rocks of the north fork terrane are lithologically very similar to those of the eastern hayfork terrane both terranes consist predominantly of chert and argillite contain blocks of metamorphic rock and permian limestone and are at least in part of overlapping age however the metasedimentary rocks of the north fork terrane are structurally coherent unlike the chaotic melange and broken formation of the eastern hayfork terrane taken together the north fork and eastern hayfork terranes bear remarkable similarities to both modern and ancient rock assemblages that occupy sites within zones of lithospheric plate convergence and are interpreted as tectonic elements of an underthrust plate margin developed oceanward of a volcanic arc represented by the permo triassic to middle jurassic volcanic strata of the eastern klamath belt

exsolution in hornblende and its consequences for 40ar39ar age spectra and closure temperature abstract step heating studies of hornblende and biotite which coexist in an amphibolite near north walpole vermont reveal dramatically different 40 ar 39 ar age spectra the biotite mg 59 is characterized by a plateau over 98 percent 39ar release at 343 5 ma in strong contrast to the amphibole which exhibits both a complex age spectrum and a k ar age significantly younger than the biotite the amphibole release pattern contains three complete age cycles which range between extremes of 199 ma and 408 ma interestingly the k ca ratio calculated from reactor produced isotopes also varies with ar release but follows a trend which is antipathetic to the 40 ar 39 ar age pattern detailed microstructural examination including transmission electron microscopy tem reveals a complex exsolution structure that is consistent with both the complexity of the release pattern as well as the low degree of radiogenic 40ar 40ar retentivity in the amphibole this result underscores the variability of closure temperature tc for 40ar in metamorphic amphiboles and should warn against accepting a single universal value of tc

the influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers a reply clifford et al 1992 argue that differences in hydraulic conditions at the moment of sediment entrainment in turkey brook flood by flood are attributable to variations in the form drag that arises from cluster microforms while not denying the possibility of variable cluster concentration we show that the time dependent bed consolidation and cementation that arises as a function of interstitial matrix development can also account for variation in the traction threshold clifford et al further postulate that the growth of pebble clusters just prior to bedload entrainment is a cause of the observed differences in traction thresholds at the start and finish of bedload during a flood event this merits further examination however we point out that clusters are not only observed at low flow and therefore already present and able to contribute significant form drag as the next bedload generating flood waxes but that they are known both to survive more or less intact and to entrap mobile particles during bed disturbing events the revelation by clifford et al that flow structure in the river quarme reflects differences in bed rugosity between riffles and pools and that it relates in scale to bed roughness elements perhaps clusters is not surprising there is however some difficulty in distinguishing the precedence of chicken and egg when using this data set to elucidate the cause of cluster spacing nevertheless the inference that flow and form are dynamically linked is in sympathy with our own conclusions that cluster spacing is an equilibrium condition governed by the interplay of flow resistance and sediment transport

subsurface configuration of the orleans fault implications for deformation in the western klamath mountains california geologic and geophysical evidence from western klamath mountains california provides quantitative information on the shape and regional subsurface extent of the orleans fault this major jurassic thrust fault lies beneath all exposures of the western paleozoic and triassic belt within the study area and mainly dips gently or is subhorizontal the regional extent of the juxtaposition of different units across the fault implies a minimum horizontal displacement of 110 km due to westward overthrusting the fault cuts the wooley creek batholith which indicates that thrusting took place after intrusion at 163 3 ma the shape of the fault near condrey mountain is consistent with post thrusting neogene uplift of the condrey mountain dome and suggests a minimum of 6 km of differential uplift

stable isotope systematics in mesozoic granites of central and northern california and southwestern oregon 18o d and h2o contents were measured for whole rock specimens of granitoid rocks from 131 localitics in california and southwestern oregon with 41 new determinations in the klamath mountains and sierra nevada initial strontium isotope ratios are known for 104 of these samples large variations in 18o 5 5 to 12 4 d 130 to 31 water contents 0 14 to 2 23 weight percent and initial strontium isotope ratios 0 7028 to 0 7095 suggest a variety of source materials and identify rocks modified by secondary processes regular patterns of variation in each isotopic ratio exist over large geographical regions but correlations between the ratios are generally absent except in restricted areas for example the regular decrease in d values from west to east in the sierra nevada batholith is not correlative with a quite complex pattern of 18o values implying that different processes were responsible for the isotopic variations in these two elements in marked contrast to a good correlation between 87sr 86sr o and 18o observed in the peninsular ranges batholith to the south such correlations are lacking except in a few areas d values on the other hand correlate well with rock types chemistry and 87sr 86sr o except in the coast ranges where few of the isotopic signatures are primary the uniformly low d values of samples from the mojave desert indicate that meteoric water contributed much of the hydrogen to the rocks in that area even so the 18o values and 18o fractionations between quartz and feldspar are normal in these same rocks

40ar39ar geochronology of tertiary mafic intrusions along the east greenland rifted margin relation to flood basalts and the iceland hotspot track abstract the east greeland tertiary igneous province includes the largest exposed continental flood basalt sequence within the north atlantic borderlands more than ten layered gabbro complexes including the 55 ma skaergaard intrusion and a large dolerite sill complex are the plutonic equivalents of flood basalts both lavas and intrusions have been regarded as synchronous with continental breakup at 57 54 ma we report ten new ages of the mafic intrusions determined by40ar 39ar incremental heating experiments demonstrating that the mafic intrusions formed in two distinct time windows only intrusion ii of the imilik gabbro complex the skaergaard intrusion and the sorgenfri glestcher sill complex formed at 57 55 ma coeval with the eruption of regional flood basalts and continental breakup other layered gabbro intrusions at imilik intrusion iii kruuse fjord igtutarajik nordre aputite q kap edvard holm and lilloise are distinctly younger and formed between 50 and 47 ma plate kinematic models indicate the axis of the ancestral iceland mantle plume was located under central greenland at 60 ma and subsequently crossed the east greenland rifted continental margin we propose that tholeiitic magmatism along the east greenland rifted margin largely occurred in three distinct pulses at 62 59 ma lavas and dykes 57 54 ma lavas dykes sills and some gabbros and 50 47 ma gabbros dykes and rare lavas related to discrete mantle melting episodes triggered by plume impact continental breakup and passage of the plume axis respectively this model implies northwestward continental drift of greenland relative to the plume axis by 3 9 5 0 cm yr between 60 and 49 ma consistent with estimates from seismic studies of submerged flood basalts

the relationship between k ar mineral ages mica grainsizes and movement on the moine thrust zone nw highlands scotland muscovite and biotite separated from moine schists close to the moine thrust zone yield a range of k ar ages incompatible with the simple cooling regime previously proposed for the caledonides of northern scotland differences of only 1 ma between muscovite and biotite of the same grainsize contrast with differences of 12 13 ma between the 125 200 m and 250 500 m grainsize fractions this is contrary to the normally accepted dominance of mica type over grainsize effects dynamic recrystallization of the micas during early deformation associated with the moine thrust zone is thought to play a role as important as temperature in determining the k ar ages the conclusions drawn have important implications for the use of uplift and cooling rates derived from the absolute k ar closure temperatures of micas close to thrust zones although the k ar ages from micas with diameters greater than 200 m are not reproducible the data can be used to show that deformation associated with thrusting commenced between c 443 ma and c 435 ma and that the thrust zone was active until at least 422 ma ago

polygenetic ophiolite belt of the california sierra nevada geochronological and tectonostratigraphic development the assumption that ophiolite sequences are generated at essentially one point in geologic time by the process of sea floor spreading is critical for modern concepts in the tectonics of ophiolites and for topics dealing with their structure and petrology however this assumption has only been verified in a few locations by an integrated geochronological and structural stratigraphic approach many ophiolite sections are reconstructed from structurally disrupted sequences with the idealized ocean floor model in mind such reconstructions are prone to error without adequate age control on each of the reconstructed fragments this is a significant problem in structurally complex regions where more than one generation of ophiolite may be present in this paper new pb u zircon ages are presented for key locations along a 375 km segment of the western sierra nevada ophiolite belt these age data are combined with structural stratigraphic observations and published ages and significant tectonic implications for the ophiolite belt emerge three different ophiolitic assemblages are recognized with igneous ages of about 300 200 and 160 m y b p rocks of the 300 m y assemblage are in a completely disrupted array of metamorphic tectonite slabs and serpentinite matrix melange fragments of upper paleozoic seamounts occur in association with the ophiolitic melange and together these assemblages constitute the basement framework for the western sierra pb u and k ar isotopic systematics are complex within this framework and indicate a polymetamorphic history systematics in the 200 and 160 m y assemblages are less complex and give tighter igneous age constraints rocks of the 200 m y assemblage are in a semi intact state with only local tectonite and melange zones rocks of the 160 m y assemblage are intact but nevertheless deformed both the 200 and 160 m y assemblages have equivalent age basinal volcanic sedimentary sequences that lie unconformably above the ophiolitic melange basement in each case the basinal sequences locally extend conformably into the upper stratigraphic levels of the age equivalent ophiolite sections these relations along with vestiges of intrusive contacts between the edges of both younger ophiolites and the melange basement indicate that the younger ophiolites underwent igneous formation in proximity to the melange basement the sierran ophiolite belt is considered to have formed by a multistage process initiated by the early mesozoic tectonic accretion of upper paleozoic sea floor in general proximity to the ancient continental margin regional metamorphism and ophiolitic melange resulted this accretionary nucleus became the basement of jurassic age primitive volcanic arc terranes which underwent rifting episodes during the production of the 200 and 160 m y ophiolites the rifting episodes resulted in the formation of sedimentary basins which were the depositional sites of volcanic sedimentary sequences non volcanic sources for the basinal sedimentary rocks include the melange basement and continental margin terranes contact zones between pre existing basement and the juvenile ophiolitic sequences created during the rifting episodes consist of dynamothermal metamorphic aureoles protoclastic deformation zones and cross cutting dikes such edge zone assemblages are in most localities obscurred or destroyed by superimposed deformations resulting from convergent and perhaps transform motions along basin edges both the 200 and 160 m y basins were destroyed by compressional orogenic episodes shortly after their formational episodes destruction of young ophiolite floored basins may be a common course of events when small oceanic type plates are generated along continental margin environments such tectonic settings are ideal for the emplacement of young ophiolite sheets

finite strain patterns of nevadan deformation western klamath mountains california finite strain analyses of galice formation metagraywackes from the footwall block of the orleans fault western klamath mountains california show a dramatic increase in strain ratios structurally upward approaching the orleans fault r xz strain values range from less than 3 at a distance of 2 km from the fault to 6 5 adjacent to the fault strain ellipse orientations are relatively consistent x directions are commonly northwest southeast and subhorizontal and trend subparallel to the trace of the orleans fault finite strain ellipse orientations and the clockwise rotational strain recorded by microscopic kinematic indicators are both compatible with the interpretation of dextral oblique convergence on the orleans fault during the nevadan orogeny

native terranes of the central klamath mountains california the klamath mountains of northern california and southern oregon contain several good examples of terranes which have developed in situ the term native is proposed for five of these terranes and all terranes whose development can be tied to an adjacent cratonal area the klamath terranes discussed herein include disrupted stratigraphic and metamorphic types three disrupted terranes the rattlesnake creek eastern hayfork and north fork contain fossiliferous blocks derived from both north american and exotic sources the unique mixed faunal assemblage stratigraphic ties to north america in the source terranes of the blocks and paleomagnetic evidence indicate that the tectonic and sedimentary processes responsible for mixing these blocks occurred in proximity to north america not distant from the terranes present positions coeval blueschist metamorphism in a fourth inboard terrane the stuart fork suggests that all four terranes developed during a late triassic to early jurassic subduction event a fifth stratigraphic terrane the western hayfork was constructed upon the assembled disrupted and metamorphic terranes in the middle jurassic disrupted terranes with similar mixtures of north american and exotic faunas occur throughout the cordillera from central california possibly as far north as british columbia late triassic deformation has been documented in several of these terranes suggesting that 1 subduction operated along at least this portion of north america during the late triassic early jurassic and 2 many additional cordilleran terranes should also be considered native

1 min in the life of a river selecting the optimal record length for the measurement of turbulence in fluvial boundary layers in the analysis of instantaneous velocity measurements it is important to consider carefully the properties of the instruments as well as the sampling frequency and the record length although several studies have examined the influence of instrument settings on velocity measurements in rivers little information is available on the effects of the record length on these measurements and on the turbulence parameters that can be extracted from the time series decisions on the record length are often made ad hoc here we quantify the effects of the sampling period on the precision of turbulence statistics e g mean standard deviation skewness reynolds stress quadrant analysis obtained from velocity measurements in fluvial turbulent boundary layers a bootstrapping method is used to determine the standard errors s e of turbulence statistics from 19 time series the time series were sampled in three gravel bed rivers using both electromagnetic current meters ecm and acoustic doppler velocimeters adv the record lengths ranged between 20 and 60 min the sampling frequencies were 20 and 25 hz for the ecms and advs respectively we show that for most turbulence statistics the optimal record length minimum sampling effort to achieve low standard errors ranges between 60 and 90 s we discuss the implication of these record lengths for standard errors estimates and variance losses

timing and duration of volcanism in the north atlantic igneous province implications for geodynamics and links to the iceland hotspot abstract we combine new and published 40 ar 39 ar age determinations from incremental heating experiments on whole rocks and mineral separates to assess the timing duration and distribution of volcanic activity during construction of the north atlantic igneous province we use these ages together with volume estimates of erupted magmas and their cumulates to calculate melt production rates for the early tertiary flood basalts of east greenland and the faeroes islands the lavas lie at opposite ends of the greenland iceland faeroes ridge the postulated iceland hotspot track and record volcanic activity leading up to during and following continental breakup between greenland and europe dominantly basaltic but also alkalic and picritic magmas were erupted from beneath thick continental lithosphere simultaneously at 61 ma from baffin island to the western and eastern margins of greenland to the faeroe islands and the western british isles a roughly circular area 2000 km in diameter volcanic activity was increasingly intermittent by 57 56 ma but at 56 1 0 5 ma the average melt production rate increased by more than an order of magnitude over previous levels from 3000 km 3 km of rift my coinciding with continental rifting and shallow decompressional melting of the mantle that fed subaerial volcanic activity along most of the eastern margin of greenland and its complement on the northwestern european margin offshore abnormally thick oceanic crust was produced along the line of plate separation the upper part of this crust comprises seismically imaged seaward dipping subaerially erupted lavas by 50 ma eruption rates had diminished drastically and volcanic activity had narrowed to a much restricted portion of the east greenland margin at the western end of the greenland iceland ridge that subsequently connected in an age progressive trend to present hotspot activity in southeastern iceland the startup mantle plume head and tail model with moderate excess temperature t 100 c and active upwelling best explains the time space and compositional aspects of volcanism in this province

tectonic implications of isotopic variation among jurassic and early cretaceous plutons klamath mountains jurassic and early cretaceous plutons in the klamath mountains province of california and oregon were intruded both before and after thrusting during the nevadan orogeny ca 155 148 ma plutons intruded before nevadan thrusting range from calc alkalic to calcic and display a range of compositions that changes consistently with time among this group rb k 2 o and initial 87 sr 86 sr sr i range 0 7034 to 0 7047 decrease and nd range 2 0 to 4 8 increases in younger plutons within individual plutons sr i varies considerably whereas nd is nearly constant the sr and nd isotopic data typically plot to the right of the mantle array 18 o values in these plutons are greater than 8 0 tonalitic and trondhjemitic plutons intruded after nevadan thrusting are typically calcic contain low rb and k 2 o contents and have 18 o less than 8 0 these plutons have a narrow range of sr i 0 7033 0 7036 and nd 4 7 to 7 4 values that plot in the mantle array the isotopic compositions of the pre nevadan plutons requires assimilation of metasedimentary rocks or mixing with crustal melts at deep crustal levels the deep crustal component is not exposed in the province and was presumably truncated during nevadan thrusting post nevadan plutons show little evidence of assimilation of metasedimentary crust but display trace element evidence low k 2 o rb and rare earth element concentrations consistent with an origin by partial melting of young ophiolitic rocks underthrust during the nevadan orogeny

the transition from sedimentation to flood volcanism in the kangerlussuaq basin east greenland basaltic pyroclastic volcanism during initial palaeogene continental break up primary basaltic pyroclastic deposits compose volumetrically c 35 50 of the lower palaeogene vandfaldsdalen mikis and haengefjeldet formations within the lower volcanic series of the east greenland flood basalt province the eruption and emplacement mechanisms of these deposits can be used to evaluate the volcanotectonic evolution of continental break up and initiation of flood basalt volcanism we present a revised stratigraphy for the pre volcanic sedimentary rocks and synvolcanic pyroclastic and epiclastic deposits and lavas from the kangerlussuaq basin east greenland primary basaltic pyroclastic deposits range from airfall tuffs to bomb beds surge deposits and vent sites with cinder cones early volcanism was apparently restricted to localized basins and formed shield volcano structures 30 40 km in diameter that are characterized by a heterogeneous sequence of intercalated hyaloclastites sediments pyroclastic deposits and lavas the lateral variation of deposit morphology reflects distance from source and indicates a migrating locus of volcanism along the axis of rifting towards the ne paired with continued down dropping of inactive volcanic centres the complex stratigraphic architecture of the initial volcanic deposits reflects local tectonic and sedimentary processes interacting with regional flood basalt volcanism and continental rifting

geochemical transitions in the ancestral iceland plume evidence from the isle of mull tertiary volcano scotland the mull tertiary volcano lies on the eastern edge of the north atlantic igneous province and was active soon after the initiation of the iceland plume detailed sampling of the mull lava pile has allowed temporal changes in the mantle source to be determined a significant change at 700 m above sea level from n morb like to icelandic marks the first arrival beneath the region of mantle from the core of the ancestral plume the earlier basalts represent an outer plume sheath of heated upper mantle from the initiation of magmatism in mull it took 1 9 0 4 ma for this transition to occur

volcanic features of the north rockall trough application of visualisation techniques on 3d seismic reflection data understanding the volcanic processes operating during continental break up is hampered by the subsequent burial of the majority of the volcanic pile beneath thick sedimentary sequences currently located in bathymetrically deep offshore regions although portions of these volcanic systems are currently exposed on land a full understanding of the volcanic structure the eruptive styles and their evolution is not possible as these localities have been partially eroded furthermore as the onshore exposure represents a volumetrically minor part of the entire system the documented eruptive styles may not be representative the increasing availability of 3d seismic reflection data has the potential to significantly enhance our understanding of break up related volcanism as it allows direct access to detailed information from the buried volcanic succession however conventional seismic interpretation methodologies cannot determine lava flow morphologies and as a result eruptive styles and their evolution are still largely based on extrapolation from the accessible onshore outcrop data new 3d seismic volume visualisation techniques allow the buried basalt morphologies to be examined for the first time in a manner similar to outcrop aerial photographic or satellite based observations applying this new approach for 3d seismic data to the north rockall trough u k atlantic margin demonstrates that a range of volcanic features indicative of eruptive style can be determined the data reveal a complex terrain containing lava flows originating from discrete volcanic centres contemporaneous normal faults linear fissures a few kilometres long radial fissures and inflation ridges lava flow morphologies that are indicative of tube fed inflated sheetflows intracanyon flows and elongate subaerial flows that enter water downslope to produce a large hyaloclastite delta are observed

constraints on timing of deformation in the benton range southeastern california and implications to nevadan orogenesis lower paleozoic precambrian miogeoclinal strata of the benton range are deformed into structures whose form and orientation are often associated with the nevadan orogeny a swarm of undeformed dikes that postdate folding was intruded between 209 and 168 ma thus major compressional deformation in the benton range predates type nevadan orogenesis by at least 10 m y the classic nevadan and subsequent orogenies did not produce pervasive deformation throughout the eastern parts of the sierra nevada and klamath orogens a nevadan age cannot be confidently assigned to structures in the western north american cordillera solely on the basis of their form and or orientation

timing and structural expression of the nevadan orogeny sierra nevada california the nevadan orogeny was a very short lived event in the late jurassic that involved the deformation of a great variety of rock types and paleozoic and mesozoic terranes throughout the extent of the sierra nevada the nevadan structures show great variation in style but relatively constant orientations these relations can be explained by considering the prior histories of the various terranes slaty cleavages and tight folds are the characteristic main phase structures in the western belt of jurassic island arc volcanic rocks and flysch type sedimentary rocks a strip of phyllites and greenschists along the eastern edge of the belt apparently represents similar jurassic rocks that were deformed and metamorphosed at greater depths probably during underthrusting of the western belt beneath the central belt the central belt of paleozoic metasedimentary and metavolcanic rocks shows the most extreme variation in style of main phase structures from weak spaced to crenulation cleavages in the south where polyphase deformed rocks formed a structural basement to slaty and phyllitic cleavages and asymmetric to isoclinal folds in the north where most of the paleozoic basement rocks lack penetrative pre nevadan fabrics eastward directed thrust faulting apparently was important only in the northern part of the range where main phase deformation was most intense the eastern belt of jurassic and triassic magmatic arc volcanic and sedimentary rocks defines the core of a major synclinorium and the rocks contain penetrative slaty cleavages and asymmetric tight to isoclinal folds a late phase of nevadan structures consisting of northeast trending cleavages and minor folds also shows a marked variation in style from relatively intensely developed in the north to very weakly developed in the south the regional extent and geometry of the nevadan structures indicate that the nevadan orogeny involved underthrusting of island arc rocks on the west and significant crustal shortening in the central and eastern belts these features suggest that the nevadan orogeny resulted from the collision of the island arc western belt with an andean type arc eastern belt situated at the western edge of north america

paleoenvironmental and biostratigraphic significance of siliceous microfossils of the permo triassic redding section eastern klamath mountains california abstract permo triassic rocks of the redding section eastern klamath mountains represent a vertical succession from a shallow water carbonate environment of the mccloud limestone to a deeper water environment of the pit formation this resulted from an episode of subsidence which started in the early permian and continued through the middle triassic subsidence is demonstrated by the existence of a set of progressively deepening lithofacies in dekkas and basal pit cherts dekkas cherts are hematitic with abundant sponge spicules and radiolarians indicative of highly oxygenated waters in full communication with oceanic circulation basal pit cherts are pyritic spicule poor and fine grained indicative of poorer circulation and greater depths extensive pyritic black shales of the pit and modin formations overlie the basal pit cherts and correspond to a restriction of circulation resulting in the stagnation of bottom water and the development of anoxic conditions normal marine circulation returned briefly in the late triassic as evidenced by the hosselkus limestone a fossiliferous micritic limestone containing abundant marine fauna including radiolarians radiolarians recovered from dekkas cherts are sufficiently well preserved to yield an age of early guadalupian based on albaillellid fauna this fauna was calibrated with a fusulinid fauna collected 7 km south in the lower section of the same member one new genus and three new species are described

north american apparent polar wander plate motion and left oblique convergence late jurassic early cretaceous orogenic consequences the north american apparent polar wander apw path indicates an episode of unusually rapid absolute northward motion of western north america between 150 and 135 ma during this time the northward component of absolute motion of points along the washington oregon california coast was in excess of 150 km m y and perhaps as high as 230 km m y we believe that such high absolute northward velocity for north america probably ensures that relative motions of oceanic plates and terranes influenced by them were to the south at this time the inception of rapid northward motion and left oblique convergence was abrupt and should be recorded in the geology of the western cordillera it is tempting to correlate this period of unusual pacific basin north american interaction with the nevadan orogeny in the klamath mountains as well as with left lateral strike slip structures such as the pine nut fault and bear mountains fault zone significant differences exist between north american plate motion recorded by the late jurassic cretaceous apw path and that predicted by a fixed hotspot model we believe that this discrepancy reflects uncertainty associated with pre late cretaceous hotspot tracks and poorly constrained relative plate motions during the cretaceous normal polarity superchron

pressure conditions for the solidification of the skaergaard intrusion eruption of east greenland flood basalts in less than 300 000 years abstract primary granophyres are differentiated from olivine tholeiitic magma and occur interstitially throughout the cumulus stratigraphy of the skaergaard intrusion east greenland samples from the lower zones a c lza c the middle zone mz and the sandwich horizon sh are included in the present study together with granophyric accumulations in gabbroic pegmatite from lza c fluid inclusions in quartz and the mineral assemblage in the granophyres record the pressure under which the skaergaard intrusion crystallised pegmatitic granophyre from lza c consists mainly of quartz plagioclase an 4 7 and alkali feldspar or 40 80 enclosing an earlier formed assemblage of ferrohastingsitic and ferroedenitic hornblende fayalite fo 4 5 titanite biotite and fluor apatite granophyric quartz albite and alkali feldspar crystallised from water saturated granitic melts near eutectic minimum conditions between 680 and 660 c the pressure of granophyre crystallisation was modelled by the intercept between fluid inclusion isochores and the minimum melt solidus for granitic compositions pressures recalculated to the roof pendant of the intrusion are 0 7 0 5 for lza 2 0 0 2 for lzb c 2 3 0 8 for mz the triple group level and 3 3 1 3 kb for sh amphibole geobarometry independently confirm the pressure estimates for pegmatitic granophyres in lza c the granophyres formed as the intrusion cooled through the minimum melt solidus in lza lzb lzc mz and sh respectively the pressure increase from lza to sh granophyres is explained by progressive burial during cooling of the intrusion and contemporaneous outpouring of 5 3 6 3 2 7 km of flood basalts during the initial opening of the northeast atlantic ocean accordingly the skaergaard intrusion evolved from a subvolcanic magma chamber at emplacement to a more deep seated igneous system during terminal crystallization when the majority of the intercumulus phases formed the present cooling history suggests the intrusion cooled through the temperature of granophyre formation in less than 300 000 years the increase in the pressure of granophyre formation therefore implies that 5 3 6 3 2 7 km of flood basalts were emplaced within this time window

jurassic cordilleran dike swarm shear zones implications for the nevadan orogeny and north american plate motion a cogenetic and coeval tonalitic and mafic dike swarm has been identified within a southern fragment the owens mountain area of the western foothills terrane california the dikes were mylonitized and transposed rotated into subparallel orientation during emplacement from 155 to 148 ma u pb zircon data which coincides in time with the nevadan orogeny steeply southeast plunging fold axes and s fold geometries indicate a sinistral sense of shear possibly with some dip slip motion as well this shear zone may be the southern and possibly deeper extension of the bear mountains fault zone this and other late jurassic cordilleran dike swarms record a complex pattern of sinistral sense transtension transpression that developed at the apparent polar wander j2 cusp 150 ma and during subsequent rapid northwestward acceleration of north america the late jurassic nevadan orogeny is a manifestation of these dramatic changes in magnitude and direction of north american motion

evolution of an ophiolitic tectonic melange marble mountains northern california klamath mountains the marble mountain terrane of northern california constitutes a metamorphosed ophiolitic tectonic melange occupying the lowest structural levels of the western paleozoic and triassic belt of the klamath mountains province on the basis of lithology and structural position it probably represents the metamorphosed equivalent of the rattlesnake creek terrane the marble mountain terrane consists of sedimentary oceanic mafic volcanic and ultramafic rocks and is structurally and probably stratigraphically overlain by arc derived meta volcanic and volcaniclastic rocks of the western hayfork terrane the contact between the two terranes is interpreted to be a zone of tectonic and stratigraphic interlayering but superimposed metamorphism has largely obscured its original character metamorphism of the assembled melange and overlying western hayfork terrane took place under low pressure amphibolite facies conditions a conclusion based primarily on the presence of prograde andalusite in pelitic rocks metamorphism was accompanied by penetrative deformation which produced two generations of northeast trending subhorizontal isoclinal recumbent folds and associated axial planar foliation mineral and stretching lineations also formed at this time these events preceded the intrusion of the 162 2 ma wooley creek batholith a later episode of deformation and recrystallization locally produced north northeast trending upright folds and steeply dipping crenulation cleavage greenschist to lower amphibolite facies metamarphic assemblages that were developed at this time overprint the earlier structures the latter deformational event may reflect the westward thrusting of the western paleozoic and triassic belt over the subjacent western jurassic belt during late jurassic time the marble mountain rattlesnake creek terranes probably represent oceanic crust which lay to the west of an active subduction zone along the western margin of north america during late triassic and early jurassic time and which was trapped in the forearc region when subduction stepped westward in earliest middle jurassic time newly initiated subduction led to the inception of the western hayfork arc which was built upon the marble mountain rattlesnake creek oceanic basement the melange may have been formed by imbrication of the oceanic lithosphere trapped between the new trench and the magmatic axis of the incipient arc continued convergence led to juxtaposition of oceanic and arc derived rocks to regional metamorphism and to deformation of the melange and overlying volcaniclastic rocks the low pressure character of the metamorphism supports an arc setting rather than a subduction zone regime for this tectonism

petrogenesis of syenites at a rifted continental margin origin contamination and interaction of alkaline mafic and felsic magmas in the astrophyllite bay complex east greenland the astrophyllite bay complex in east greenland part of the palaeogene north atlantic igneous province consists of an alkaline diorite plug with detached trachyandesitic pillows surrounded by co magmatic syenite that was emplaced into archaean basement the diorite intrusion has yielded a 47 11 0 68 ma rb sr isochron age saw cut profiles through pillow syenite gneiss sections have been taken to resolve close spatial elemental and isotopic sr nd hf pb o variations the diorite and syenite formed from alkaline basaltic mantle derived melts with complex histories of prolonged assimilation and fractional crystallisation each evolved to different extents in separate magma chambers during the establishment of new plumbing systems in the kangerlussuaq area the diorite is dominated by lower crustal granulite facies contamination whereas the syenite shows evidence for greater degrees of upper crustal amphibolite facies contamination indicating stalling and fractionation of magmas at different levels within the crust the syenite and diorite magmas were subsequently emplaced as separate pulses into the basement gneisses at astrophyllite bay giving rise to superimposed local contamination trends between pillow syenite and syenite gneiss respectively

implications of new jurassic stratigraphic geochronometric and paleolatitudinal data from the western klamath terrane smith river and rogue valley subterranes combined biostratigraphic chronostratigraphic and geochronometric studies of the rogue and galice formations rogue valley subterrane southwestern oregon indicate that the oxfordian kimmeridgian boundary should be placed at 154 1 5 ma rather than at 156 6 ma as advocated in the decade of north american geology 1983 geologic time scale in the smith river subterrane northwestern california southwestern oregon well preserved radiolaria were recovered not only from strata overlying the josephine ophiolite but also from within the volcanic member of the ophiolite u pb geochronometric data constrain the age of the mirifusus first occurrence event to after 162 1 ma and to being 157 1 5 ma or slightly older radiolarian faunal data indicate that the josephine ophiolite originated at central tethyan paleolatitudes during the latest callovian 162 ma and was carried northward to northern tethyan and southern boreal paleolatitudes during the oxfordian