jurassic sedimentary melange and associated facies baja california mexico the middle jurassic coloradito formation of cedros island baja california is a fine grained flysch sequence that includes some coarse debris flow filled channels sedimentary melange olistostrome is widespread and is composed of sheared black argillite with megablocks of triassic cherts shallow water origin paleozoic limestone and continental sandstone derived from the north this unit represents the earliest record of arc continent interaction in the baja area the eugenia formation is locally present in erosional contact above the coloradito formation the unit is a northerly derived channel fill deposit in which the lower one third is dominated by metasedimentary detritus and first cycle calc alkaline volcanic detritus dominates above this the long axes of clasts in sheared argillite of the coloradito have orientations parallel to clast imbrication in associated conglomerate channels suggesting shearing due to downslope sliding contrasting deformation styles sheared versus open folded of fine grained coloradito units may be due to abundance of smectite in the latter allowing higher water retention and a more plastic style of deformation jurassic sedimentary sequences on isla cedros record a complex history of explosive island arc volcanism gran canon formation arc continent collision coloradito formation and renewal of magmatic arc volcanism eugenia formation

is there a oaxaca california megashear conflict between paleomagnetic data and other elements of geology after two decades paleomagnetic study of cretaceous plutonic volcanic and sedimentary rocks consistently shows that both the continental borderland and the batholithic terrane of peninsular california have moved north 14 degree plus minus 5 degree and rotated 25 degree to 40 degree clockwise between the end of the cretaceous and the end of the paleocene a variety of geologic data across the borderland batholith boundary with the peninsula across the gulf of california and within the state of sonora mexico appear to preclude the existence of a megashear along which peninsular california could have traveled northward lacking an acceptable scenario for moving the rocks from their proposed position of magnetization to their present position the data of paleomagnetism and other elements of geology are in direct conflict the resolution of this enigma will require an important revision of current assumptions

lithology and biostratigraphy of franciscan like chert and associated rocks in west central baja california mexico we present the result of biostratigraphic and lithologic studies of ribbon chert and associated rocks in a mesozoic subduction complex in west central baja california mexico the subduction complex terrane is divided into three subterranes each of which consists of an originally coherent sequence of interbedded oceanic rocks including ocean floor basalt terrigenous siliciclastic turbidites radiolarian ribbon chert and limestone parts of the original stratigraphy are preserved in the least deformed least metamorphosed subterrane ocean floor pillow basalt on cedros island is overlain by about 40 m of radiolarian ribbon chert ranging in age from early jurassic or older to early cretaceous the chert is overlain by thin bedded turbidites on san benito west island radiolarian ribbon chert is interbedded with volcanogenic rocks including a megabreccia that contains blocks of metabasites folded late jurassic early cretaceous chert and marble in a mid cretaceous radiolarite matrix chert samples from tectonically thinned sequences elsewhere on cedros and san benito islands contain late triassic to early cretaceous radiolarian assemblages the 40 m thick chert section on cedros island probably accumulated atop oceanic crust in a large ocean basin for at least 65 m y and was later capped by terrigenous turbidites as it approached the subduction zone along the western margin of north america faulted chert sections on cedros and san benito central and west islands are subsets of the main section on cedros island the chert megabreccia section on san benito west island is interpreted to have accumulated at the foot of a volcanic edifice on the ocean floor for example seamount fracture zone or during extension and normal faulting of the downgoing oceanic plate in a subduction zone sedimentary structures and stratigraphic relations indicate that all ribbon chert in this subduction complex was deposited by sediment gravity flows that is they are turbidites

plume modified orogeny an example from the western united states plate tectonic theory implies that orogeny at convergent margins results from several processes including subduction of oceanic crust subduction of aseismic ridges accretion of terranes and continental collision each of these processes involves the consumption of an oceanic tract and each has its own characteristic style of tectonothermal activity an additional potentially important orogenic process has been largely overlooked oceanic regions contain chains of volcanic islands formed at hotspots which are generally considered to reflect the sites of rising plumes in a hotspot reference frame where active continental margins advance and override a plume the plume9s buoyant swell may profoundly change the tectonothermal expression of subduction and hence orogenic processes at the continental margin the late cretaceous tertiary evolution of the western united states may be an example of this process which has been overlooked in the development of orogenic paradigms

paleogene continental margin truncation in southwestern mexico geochronological evidence the reasons for and mechanisms of continental margin truncation in sw mexico where mesozoic cenozoic plutons are situated directly on the pacific coast are not yet well understood large scale dextral and or sinistral displacements of the continental margin terranes now forming parts of baja california or the chortis block have been proposed the well defined along coast nw se decreasing granitoid intrusion age trend 1 2 cm yr in the 100 ma 40 ma time interval between puerto vallarta and zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the chortis block from those latitudes toward the se changes in the dip and velocity of the nnw sse trending cretaceous tertiary subduction zone resulted in a landward migration of the magmatic arc taking into account certain stratigraphic affinities of chortis and the oaxaca and mixteca terranes together with the known displacement rates along the north america caribbean plate boundary the northwesternmost paleoposition of the chortis block with respect to sw mexico was near zihuatanejo in contrast between zihuatanejo and the isthmus of tehuantepec the cessation of the tertiary magmatism decreased more rapidly 7 7 cm yr although the trend is not so obvious starting in the late eocene chortis moved about 1100 km to the se along a transform boundary associated with the opening of the cayman trough based on our geochronological data and structural relationships between mylonite zones and plutons in the acapulco tehuantepec area we propose an approximately 650 km se movement of chortis from about 40 25 ma with a velocity of 6 5 4 3 cm yr since this is considerably slower than the decreasing age trend obtained by us using the geochronological data we consider batholith formation in this segment to predate and postdate the offshore passage of the north america farallon caribbean triple junction geological observations and paleomagnetic data do not give strong support for large scale right lateral displacements of crustal blocks like the baja california given the isotopic data presented the continental margin truncation in sw mexico seems to be the consequence of an interaction of mechanisms of these we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment on the other hand the lateral removal of material associated with the displacement of chortis is more important in the southeastern segment

paleomagnetism magnetic anisotropy and mid cretaceous paleolatitude of the duke island alaska ultramafic complex we report paleomagnetic results from layered igneous rocks that imply substantial post mid cretaceous poleward motion of the insular superterrane western canadian cordillera and southeast alaska relative to north america the samples studied are from the stratiform zoned ultramafic body at duke island which intruded rocks of the alexander terrane at the south end of the southeastern alaska archipelago at about 110 ma thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the curie temperature of magnetite this remanence is likely carried by low ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts the duke island intrusion exhibits a well developed gravitational layering that was deformed during initial cooling but below 540 c into folds that plunge moderately to the west southwest the characteristic remanence clearly predates this early folding and is therefore primary the fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site mean directions are corrected by restoring the layering to estimated paleohorizontal all samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion and we show that it has no significant or systematic effect on the site mean directions of remanence at least some of the anisotropy derives from secondary magnetite formed during partial serpentinization the mean paleomagnetic inclination 56 10 corroborates paleomagnetic results from five coeval silicic plutons of the canadian coast plutonic complex to the south and southeast and implies 3000 km 1300 km of poleward transport relative to the north american craton between mid cretaceous and middle eocene time the insular superterrane and coast plutonic complex shared a common paleolatitude history with more poleward transport than coeval inboard terranes

tectonic history of the eastern edge of the alexander terrane southeast alaska rocks exposed west of the coast plutonic complex in southern southeast alaska form an imbricate thrust belt that overprints the tectonic boundary between two of the largest allochthonous crustal fragments in the north american cordillera the insular and intermontane composite terranes in the alexander terrane insular composite terrane lower paleozoic metavolcanic and metasedimentary rocks descon formation and dioritic plutons are unconformably overlain by lower devonian clastic strata karheen formation these rocks are overlain locally by upper triassic basalt rhyolite and marine clastic strata hyd group upper jurassic and lower cretaceous metavolcanic and metasedimentary strata of the gravina sequence unconformably overlie the alexander terrane the gravina sequence forms a structural package over 15 km thick and records intermittent arc volcanism along the eastern edge of the alexander terrane the gravina sequence is structurally overlain by upper paleozoic and lower mesozoic metamorphosed basaltic strata marble and argillite alava sequence and locally by lower paleozoic supracrustal rocks and orthogneiss kah shakes sequence together these constitute the taku terrane which we correlate with the intermontane composite terrane local unconformity of gravina sequence strata over the alava sequence demonstrates that the gravina sequence overlapped an earlier structural boundary between the intermontane and insular composite terranes the rocks were deformed in the mid cretaceous by west vergent thrusting that was was broadly coeval with arc magmatism deformation involved emplacement of west directed thrust nappes over the structurally intact and relatively unmetamorphosed alexander terrane basement mid cretaceous tonalite granodiorite and quartz diorite intrude rocks of the thrust belt and are locally affected by the deformation mid cretaceous deformation occurred during two episodes that were contemporaneous with the emplacement of large sill like plutons older structures record ductile southwest vergent folding and faulting regional metamorphism and development of axial planar foliation the second generation structures developed during the later stages of southwest directed thrust faulting which juxtaposed rocks of contrasting metamorphic pressures and temperatures structural stratigraphic and geochronologic data indicate that the two phases of regional thrusting in southeast alaska occurred between 113 ma and 89 ma rocks in the western part of the thrust belt were uplifted regionally by 70 ma deformation involved the collapse of a marginal basin s and a magmatic arc and overprinted the older tectonic boundary between the insular composite terrane and the late mesozoic western margin of north america at that time the intermontane composite terrane contractional deformation along the length of the thrust belt was broadly coeval with arc magmatism and thus records intra arc tectonism late paleocene to early eocene igneous activity and extensional deformation subsequently affected the thrust belt

origin of the columbia plateau and snake river plain deflection of the yellowstone plume previous models relating the snake river plain and columbia river basalt northwest united states to the yellowstone plume have not taken into account the effects of the subducted farallon and juan de fuca plates not only did subduction shield the surface from mantle plume activity but it deflected the plume in the direction of plate motion a variety of geophysical and geologic evidence suggests that the plume penetrated the subducted slab at 17 5 ma causing eruption of the columbia river basalt the plate boundary then migrated over the plume and the yellowstone plume has recovered to a more nearly vertical conduit forming the snake river plain and yellowstone plateau

guerrero terrane of mexico its role in the southern cordillera from new geochemical data the guerrero terrane makes up most of the western part of mexico is one of the largest terranes of the north american cordillera and is characterized by an upper jurassic lower cretaceous volcanic sedimentary sequence of are affinity metamorphic rocks that crop out in the western area of the terrane arteaga complex may represent its basement they are mostly composed of terrigenous sediments varales formation with minor basaltic pillow lavas chert tuff and limestone initial nd values 13 and rare earth element ree values for pillow lavas of the arteaga complex are characteristic of mid ocean ridge basalts morb in contrast the varales formation sedimentary rocks from the arteaga complex have negative initial nd 6 2 and 7 2 and are enriched in light rees these data indicate that the sediments of the varales formation were supplied from an evolved continental crust the overlying jurassic cretaceous arc related rocks have initial nd 7 9 to 3 9 and ree patterns similar to those of evolved intraoceanic island arcs these data show that the evolution of the guerrero terrane had an early pre cretaceous stage which consisted of an oceanic crust receiving sediments from a continental source and a cretaceous stage which was the development of an island arc the oceanic continental isotopic signature of the arteaga complex is different from other western north american cordilleran terranes e g alexander wrangelfia that are more completely oceanic in affinity nevertheless the extensive jurassic cretaceous arc represents additions of juvenile material to the western north american cordillera

measurements of the cretaceous paleolatitude of vancouver island consistent with the baja british columbia hypothesis a previously unsampled outcrop of gently dipping or flat lying upper cretaceous sedimentary strata in the vancouver island region which contains unaltered aragonitic mollusk fossils yielded a stable remanent magnetization that is biostratigraphically consistent with cretaceous magnetochrons 33r 33n and 32r these results characterized by shallow inclinations indicate an upper cretaceous paleolatitude of about 25 3 degrees north which is equivalent to that of modern day baja california these findings are consistent with the baja british columbia hypothesis which puts the insular superterrane well south of the oregon california border in the late cretaceous

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

evaluation of the baja controversy using paleomagnetic and faunal data plume magmatism and piercing points abstract the controversy over late cretaceous locations of baja alaska baja bc stikinia and western quesnellia centres around whether a they were within 1000 km of their present positions or b located 1000 5000 km south along the western coast of laurentia to be transported northwards by the kula plate during the late cretaceous and cenozoic the latter is supported by paleomagnetic data including data that pass a fold contact or reversal test whereas the former is based upon correlation of strata across faults although various critical tests have been proposed and tried interpretations are equivocal e g alternative interpretations for the source of detrital zircons in baja bc we propose that the presence of plume magmatism provides unique vertical piercing points thus correlation of the 70 ma plume magmatism in northern stikinia with the yellowstone hotspot locates stikinia 2000 km to the south and 1000 km off the western coast of laurentia on the other hand the absence of 90 70 ma yellowstone plume magmatism in the presently adjacent baja bc but the presence of 60 47 ma yellowstone plume spreading ridge magmatism in southern baja bc southern vancouver island and coast ranges of washington and oregon implies that it moved at least 1200 km northwards between 70 and 60 ma i e a rate of 12 cm year the spreading ridge may be correlated with the kula farallon ridge furthermore the presence of middle late triassic plume magmatism in wrangellia and the peninsula and alexander terranes suggests 2000 km of relative dextral movement between baja alaska and baja bc applying these conclusions to pre 90 ma paths of baja alaska bc across the pacific ocean suggests that it had three components 1 northeasterly between 230 and 180 ma 2 eastsoutheasterly between 180 and 140 ma and 3 northeasterly between 140 and 90 ma whereas the latter component is consistent with published terrane tracks the 180 140 ma component is at odds suggesting that this latter segment be reassessed the weight of evidence in favour of correlating geological piercing points across the gulf of california appears to outweigh paleomagnetic evidence for 1000 2500 km of apparent northward displacement of baja california in the cretaceous and early tertiary which may be explained by 15 20 tilting of fault blocks recorded by 40 ar 39 ar cooling ages thus we conclude that whereas baja alaska baja bc stikina and western quesnellia were transported between 1000 and 5000 km northwards along the western margin of laurentia during the late cretaceous and cenozoic baja california has not moved more than that required to open the gulf of california

coastal and baja california paleomagnetism reconsidered systematic reappraisal of paleomagnetic data from upper mesozoic and lower cenozoic sedimentary rocks and igneous rocks of late mesozoic age in coastal and baja california indicates that pluton tilt and shallowing of paleomagnetic directions by sedimentary compaction resolve observed discordances with the north american apparent polar wander apw path when the best available reference paleopoles are used for comparison the hypothesis of major tectonic transport in excess of distances inherent in current models for evolution of the san andreas transform system is unnecessary and in conflict with geologic observations fresh insights derived from recent data sets and improved analysis include the following 1 the limited paleomagnetic data recording primary remanence for upper jurassic and lower cretaceous volcanic rocks in coastal california are concordant with the north american apw path 2 the tilt of the peninsular ranges batholith inferred from paleobarometric data eliminates its declination discordance and reduces its inclination discordance to a level where ovals of confidence for observed and expected paleomagnetic directions overlap 3 key cretaceous and paleogene red bed successions on the salinian block of coastal california and in baja california have yielded concordant paleolatitudes 4 the discordances of different apparent paleolatitudes inferred from cretaceous and paleogene marine sedimentary strata lie within a range that can be attributed to compaction shallowing on the basis of both experimental information and observations from elsewhere 5 turbidites are more susceptible to compaction shallowing than shelf and deltaic strata 6 the uniform rotation of paleomagnetic directions inferred previously for peninsular california is not supported by the full array of data now available from multiple localities 7 basement ties of the salinian block in california and of the baja california peninsula to the continental interior preclude any hypothesis for tectonic transport much in excess of san andreas transform slip

samarium neodymium chronology and rubidium strontium systematics of an allende calcium aluminum rich inclusion with implications for 146sm half life abstract calcium aluminum rich inclusions cais are primitive objects that formed within the protoplanetary disk surrounding the young sun recent pb pb chronologic studies have demonstrated that cais are the oldest solar system solids crystallizing 4567 ma ago amelin et al 2002 connelly et al 2012 the isotope systematics of cais therefore provide critical insight into the earliest history of the solar system although sm nd and rb sr geochronometers are highly effective tools for investigating cosmochemical evolution in the early solar system previous studies of cais have revealed evidence for isotopically disturbed systems here we report new age data for allende cai al3s4 derived from both the long lived 147sm 143nd and short lived 146sm 142nd isotopic systems the 147sm 143nd chronometer yields an age of 4560 34 ma that is concordant with 207pb 206pb ages for cais and indicates that the sm nd system was not significantly disturbed by secondary alteration or nucleosynthetic processes the slope of the 146sm 142nd isochron defines the solar system initial 146sm 144sm of 0 00828 0 00044 this value is significantly different from the value of 0 0094 determined by kinoshita et al 2012 ages recalculated from all published 146sm 142nd isochron data using the traditional 103 ma half life and the initial 146sm 144sm value determined here closely match pb pb and 147sm 143nd ages determined on the same samples in contrast ages recalculated using the 68 ma half life determined by kinoshita et al 2012 and either of the initial 146sm 144sm values are often anomalously old this is particularly true for the youngest samples with 146sm 142nd isochron ages that are most sensitive to the choice of 146sm half life used in the age calculation in contrast to the sm nd isotope system the rb sr system is affected by alteration but yields an apparent isochron with a slope corresponding to a much younger age of 4247 110 ma although the rb sr system in cais appears to be disturbed the initial 87sr 86sr value determined from the isochron is 0 698942 0 000008 and closely approximates estimates of the initial solar system value although this isochron may be a mixing line it might also record alteration on the allende parent body in which rb was added to the al3s4 cai that was initially largely devoid of rb

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

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

paleogene history of the kula plate offshore evidence and onshore implications paleocene to middle eocene magnetic anomalies were mapped over oceanic crust that accreted at the kula pacific spreading center and is now obliquely entering the western aleutian trench between 179 e and 168 e the strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56 55 ma when north south spreading veered to northwest southeast 310 130 kula pacific spreading ceased in 43 ma a 75 km long section of the fossil kula rift axis has avoided subduction although it now intersects the trench axis almost orthogonally near 171 5 e a narrow remnant of the former kula plate northwest of this fossil spreading center is bounded by a fossil kula pacific transform with a high transverse ridge alongside a sediment filled transform valley anomalies on this remnant show that eocene kula pacific spreading was highly asymmetric 2 1 the 56 55 ma change in kula plate rotation inferred from the change in spreading direction coincided with birth of the aleutian subduction zone and was probably a consequence of the resulting change in slab pull stresses on the oceanic lithosphere the change in direction of kula north american motion is a plausible explanation for the detachment of continental terranes from the pacific northwest and their migration around the gulf of alaska and for the early eocene demise of alaska range are volcanism the cessation of kula pacific spreading coincides with a major change in pacific plate rotation and the subsequent direction of convergence of the pacific plate with the aleutian arc was similar to the 55 43 ma direction of kula plate convergence

microbial habitat dynamics and ablation control on the ward hunt ice shelf the ward hunt ice shelf 83 02 n 74 00 w is an 40 m thick ice feature that occupies a large embayment along canada s northernmost coast sediments cover 10 of its surface and provide a habitat for diverse microbial communities these assemblages form an organo sedimentary matrix microbial mat composed of cold tolerant cyanobacteria and several other types of organisms we investigated the environmental properties temperature irradiance conductivity and nutrient concentration of the microbial mat habitat and the effect of the microbial mats on the surface topography of the ice shelf the low albedo of microbial mats relative to the surrounding snow and ice encouraged meltwater production thereby extending the growth season to 61 days despite only 52 days with mean temperatures above 0 c we found large excursions in salinity near the microbial mat during freeze up and melt and 54 of all ponds sampled had conductivity profiles indicating stratification nutrient concentrations within the microbial mats were up to two orders of magnitude higher than those found in the water column which underscores the differences between the microbial mat microenvironment and the overall bulk properties of the cryo ecosystem the average ice surface ablation in the microbial mat rich study site was 1 22 m year 1 two times higher than values measured in areas of the ice shelf where mats were less prevalent we demonstrate with topographic surveys that the microbial mats promote differential ablation and conclude that the cohesive microbial aggregates trap and stabilize sediment reduce albedo and thereby influence the surface morphology of the ice shelf copyright 2006 john wiley sons ltd

evolution of a middle jurassic back arc basin cedros island baja california evidence from a marine volcaniclastic apron a rifted arc ophiolite assemblage and over lying volcaniclastic rocks all of late middle jurassic age represent a fragment of the frontal arc margin of a back arc basin now exposed on cedros island in baja california mexico ophiolite generation was immediately followed by progradation of a deep marine volcaniclastic apron gran cannon formation contemporaneous with the growth of island arc volcanoes this back arc apron shows relatively simple uniform sedimentation patterns that may be recognizable in other back arc basins isolated from terrigenous sediment influx the progradational back arc apron sequence includes from base to top 1 the tuff lithofacies thin bedded well sorted laterally continuous tuffs that thicken into basement paleo lows these were deposited from dilute sediment gravity flows generated by eruptions from a deeply submerged nascent arc 2 the lapilli tuff tuff breccia lithofacies medium to very thick bedded lapilli tuffs and tuff breccias these were deposited from debris flows on proximal parts of the apron and high density turbidites were deposited on more distal parts this lithofacies records increasing generation and resedimentation of scoriaceous and hyaloclastic debris as the summits of the arc volcanoes grew nearer to sea level 3 the primary volcanic lithofacies monolithologic dacite pyroclastic flows were produced during the eruption of differentiated magmas which climaxed the growth of the island arc basalt lava flows were fed from fissures that extended down the apron within the back arc basin pyroclastic rocks of the back arc apron were blanketed with silty basinal turbidites epiclastic facies that record abrupt cessation of volcanism and erosion of the arc within about 10 m y of ophiolite generation reflecting the temporal episodicity of back arc basins it is predicted from this study that the frontal arc side of back arc spreading centers is preferentially preserved in the geologic record relative to the center of the basin it is suggested that the hot frontal arc side of the basin is prone to intra plate volcanism but is unaffected by faulting subsequent to basin initiation several factors lead to slumps occurring more frequently on the arc flank of back arc basins than in any other tectonic setting except perhaps intra arc marine basins

mantle plume influence on the neogene uplift and extension of the u s western cordillera espite its highly extended and thinned crust much of the western cordillera in the united states is elevated more than 1 km above sea level therefore this region cannot be thought of as thick crust floating isostatically in a uniform mantle rather the lithospheric mantle and or the upper asthenosphere must vary in thickness or density across the region utilizing crustal thickness and density constraints we modeled the residual mass deficit that must occur in the mantle lithosphere and asthenosphere beneath the western cordillera a major hot spot broke out during a complex series of cenozoic tectonic events that included lithospheric thickening backarc extension and transition from a subduction to a transform plate boundary we suggest that many of the characteristics that make the western cordillera unique among extensional provinces can be ttributed to the mantle plume that created the yellowstone hot spot

paleomagnetism of 122 ma plutons in new england and the mid cretaceous paleomagnetic field in north america true polar wander or large scale differential mantle motion a paleomagnetic study of cretaceous white mountains plutonic complexes in new hampshire and vermont yields high unblocking temperature dual polarity magnetizations in different types of igneous rocks the resulting pole position for three plutons 71 9 n 187 4 e a95 6 9 age 122 5 ma agrees with previously published mid cretaceous poles for north america which together give a mid cretaceous standstill reference pole slightly revised from globerman and irving 1988 at 71 2 n 194 1 e a95 3 7 n 5 studies we argue on the basis of the wide geographic distribution of these studies the variety in tectonic settings and rock types positive reversal tests and an overall reversal pattern consistent with geomagnetic polarity time scales that this mean pole represents the north american mid cretaceous reference field for nominally 36 m y 124 to 88 ma the standstill pole limits to within 4 the motion of the north american plate relative to the earth s spin axis during the same mid cretaceous interval the new england hotspot track 124 ma monteregian hills 122 5 ma cretaceous white mountains and 103 to 84 ma new england seamounts requires 11 4 of north poleward motion of north america in direct conflict with the paleomagnetic standstill a similar 13 discrepancy is independently demonstrated between the spin axis and the tristan da cunha hotspot track on the african plate during the mid cretaceous interval the hotspot spin axis discrepancies ended by 90 ma when it is shown that both atlantic hotspots agree with north american and african dipole paleolatitudes and present day locations nondipole fields are an unlikely explanation of the uniform motion of these two widely separated hotspots with respect to the spin axis leaving as possible interpretations true polar wander and large scale but differential mantle motion the southerly motion of the mid cretaceous louisville hotspot relative to the spin axis is ostensively at odds with what would be predicted under the true polar wander interpretation and points to differential mantle kinematics the motions of the three widely separated mid cretaceous hotspots with respect to the spin axis may be related to the recently proposed increase in global oceanic lithosphere production rates which gave rise to the mid cretaceous superplume

paleomagnetic evidence for a post cretaceous accretion of wrangellia in order to test the various estimates of the accretion age of some of the southern alaska terranes a paleomagnetic investigation of the maccoll ridge formation latest cretaceous age in the wrangell mountains wrangellia was undertaken the data pass the fold test and reversal test thus affirming the primary nature of the magnetic remanence the mean paleomagnetic direction corresponds to a paleolatitude of 32 9 which is far south of the predicted paleolatitude assuming a north american paleogeography therefore these results apparently rule out a pre cenozoic accretion age of wrangellia and its associated terranes composing the southern alaska superterrane the data also imply about 130 counterclockwise rotation which is the opposite sense of rotation usually observed in western north america

coupled variations in helium isotopes and fluid chemistry shoshone geyser basin yellowstone national park abstract early studies of 3he 4he variations in geothermal systems have generally attributed these fluctuations to either differences in the source of the magmatic 3he rich helium or to local differences in the deep flux of magmatic 3he rich helium kennedy et al 1987 however show that near surface processes such as boiling and dilution may also drastically affect 3 he 4 he ratios of geothermal vapors helium isotope ratios were determined for several hot springs at shoshone geyser basin of yellowstone national park for this study along with other noble gas data stable isotope data and water and gas chemistry data for each spring were also compiled the water chemistry indicates that there is one deep hot thermal water in the area which is mixing with dilute meteoric water that has entered the system at depth spring hco3 concentrations correlate with 3 he 4 he values as in nearby lower geyser basin this correlation is attributed to variable amounts of deep dilution of thermal waters with a relatively cool water that inhibits boiling at depth thus preventing the loss of co2 and therefore hco3 and magmatic he in the most diluted samples oxygen and hydrogen isotope data also support a boiling and dilution model but to produce the observed fractionations the boiling event would have to be extensive with steam loss at the surface whereas the boiling that affected the helium isotope ratios was probably a small scale event with steam loss at depth it is possible that deep boiling occurred in the basin and that small amounts of steam escaped along fractures at about 500 m below the surface while all subsequently produced steam was lost near or at the surface

coupled sr o isotope variations as an indicator of source heterogeneity for the northern peninsular ranges batholith primary 18o values for tonalitic rocks from the san jacinto intrusive complex range from 9 0 to 10 6 initial87sr 86sr for the same samples varies from 0 7058 to 0 7076 rocks with low 18o tend to have higher initial87sr 86sr the majority of samples have a limited range of 18o values 10 0 10 4 that is anticorrelated with colour index this variation is compatible with crystallization of rocks with varying mineral abundances from an homogeneous for oxygen reservoir more mafic rocks show a large range of 18o values reflecting original variations in source material compositions the combined sr o isotope data demonstrate that three or more distinct components were involved in the genesis of these rocks these three components are also seen in batholithic rocks from the mojave block and sierra nevada to the north these data are interpreted as indicating a 0 35 oxygen atom basis contribution to the san jacinto rocks from an old continental lithosphere source

marine triassic faunas of north america their significance for assessing plate and terrane movements the distribution of marine triassic invertebrate faunas in north america is reviewed to recognize forms that may have paleolatitudinal significance

paleomagnetic evidence for motion of the hawaiian hotspot during formation of the emperor seamounts the bend in the hawaiian emperor chain is the best example of a change in plate motion recorded in a fixed hotspot frame of reference alternatively the bend might record primarily differences in motion of the hawaiian hotspot relative to the pacific lithosphere new paleomagnetic data from the emperor chain support the latter view althouth the rate of motion is difficult to constrain because of uncertainties posed by true polar wander and limited sampling of the chain the best available paleomagnetic data suggest pacific hotspots may have moved at rates comparable to those of lithospheric plates 30 mm yr 1 in late cretaceous to early tertiary times 81 43 ma if correct this requires a major change in how we view mantle dynamics and the history of plate motions in the early to mid cretaceous 128 95 ma hotspots in the atlantic moved at similar rates these episodes during which groups of hotspots appear to move rapidly are separated by times of much slower motion such as the past 5 m y

case for northward transport of baja and coastal southern california paleomagnetic data analysis and alternatives this paper which accompanies the preceding article by gastil reviews cretaceous paleomagnetic evidence for baja and coastal southern california like cretaceous paleomagnetic data for the cordillera as a whole results from baja and southern california require northward transport of 1000km or more relative to interior north america agreement between independent results for different rock types and experimental techniques is overwhelming dextral shear driven by right oblique subduction readily accounts for the paleomagnetic results alternative explanations such as experimental errors rock magnetic effects anomalous geomagnetic behavior errors in dating or local structure are inadequate improbable or both some geologists have difficulty reconciling the findings of paleomagnetism with their own results hence as emphasized by gastil a conflict and enigma exist to resolve them will require a willingness on both sides to honestly scrutinize basic assumptions

metchosin igneous complex southern vancouver island ophiolite stratigraphy developed in an emergent island setting the metchosin igneous complex of southern vancouver island has three components gabbro stocks formerly sooke gabbro a newly recognized sheeted dike complex and a basaltic volcanic sequence formerly the metchosin volcanics the sheeted dikes occupy a position transitional between the gabbro stocks and submarine volcanics which they tie together in an ophiolitic pseudostratigraphy the submarine volcanics shoal upward and pass into subaerial sheet flows the complex formed in an area of regional extension as an emergent island