fluid inclusion characteristics of intrusion related gold mineralization tombstone tungsten magmatic belt yukon territory canada intrusion hosted low sulfide sheeted vein systems are common within many plutons and stocks of the middle cretaceous tombstone tungsten magmatic belt yukon territory and host significant gold mineralization fluid inclusion characteristics of five such systems namely emerald lake dublin gulch scheelite dome mike lake and mactung constrain the vein forming fluid composition formation temperatures and pressures hydrothermal fluid processes and potential fluid sources the veins contain a wide range of fluid inclusion types ubiquitous type 1a and 1b inclusions are low salinity 1a xnacl 1 kbar at emerald lake and dublin gulch and 2 kbar at mactung and scheelite dome that potentially control fluid composition and evolution a variety of fluid processes may have been responsible for gold precipitation including immiscibility and or release of an evolving magmatic fluid

crustal structure across the northern cordillera yukon territory from seismic wide angle studies omineca belt to intermontane belt a seismic refraction wide angle reflection experiment shot in 1997 in the southern yukon territory crosses the omineca belt which includes the strike slip tintina fault and terminates within the intermontane belt of the northern canadian cordillera lithospheric structure is interpreted from two dimensional forward and inverse modelling of traveltimes combined with forward amplitude modelling and from 2 5 dimensional modelling of gravity data beneath the cassiar terrane and the north america miogeocline average velocities in the upper 20 km of crust are 6 1 km s in the west beneath the accreted cache creek slide mountain and yukon tanana terranes average velocities increase to 6 3 km s in the upper crust the velocity model beneath these terranes thus correlates with more mafic accreted material and not with a subsurface extension of the cassiar terrane the tintina fault is a crustal scale structure across which significant structural differences occur a mid crustal reflector terminates

early diagenesis in differing depositional environments the response of transition metals in pore water the cycling of fe mn ni co cu cr v and mo during early diagenesis was investigated in sediments from five different depositional environments in the california borderland dissolved o2 no3 no2 and so4 were also measured at each site to establish the position of redox boundaries pertinent to this study sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals bottom water o2 concentration sediment redox conditions cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux at 10 m oxygen and above the sequence of terminal electron acceptor utilization was typical of pelagic sediments differing only in scale under these conditions the recycling of mn oxides resulted in an enriched layer near the interface at bottom water 02 concentrations 5 m mn oxides are not recycled under these conditions fe oxides and so42 are important oxidants the depth of the zone of manganese oxidation with respect to the interface affects the efficiency of metal scavenging by manganese oxides trapping and recycling of ni and co with mn oxides results in remobilization from reducing sediments and enrichment in oxic sediments scavenging of these metals is favored by a deep oxic zone in the sediments in contrast cr v and mo appear to be transported to the sediments as reduced species and are released from the sediments by oxidation the source of the reduced species is assumed to be biogenic material the accumulation of these metals is favored by reducing conditions in the sediments cu is enriched in the sediments by transport with detrital biogenic material followed by adsorption onto sediment solids cu is released at the interface and rapidly removed onto the solids at all sites except the one pelagic site at the base of the slope the magnitude of cu released in the slowly accumulating pelagic sediment exceeds the cu binding capacity of the solids resulting in pore water cu concentrations in excess of 100 nm in general metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study

cadmium diagenesis in laurentian trough sediments the depth distributions of cd mn and fe in pore water and sediment were determined on three replicate box cores collected at a 325 m deep station in the laurentian trough the results reveal a surface layer in which the content of solid phase cd first decreases but then increases sharply with depth in contrast mn decreases regularly over the same depth interval in the oxygenated zone of this layer cd probably associated with organic matter is released to the pore water resulting in concentrations that are more than an order of magnitude higher than in the overlying water column some of the dissolved cd is returned to the water column and some migrates downward and is precipitated at depth this part of the cd cycle is virtually complete within the surface layer the base of which corresponds to the base of the zone of mn enrichment in the subsurface layer solid phase cd and mn show little or no concentration change however dissolved cd which reaches often non detectable concentrations 0 05 nm in this layer increases once again deep within the anaerobic zone and attains concentrations even higher than in the surface layer the presence of dissolved cd complexes revealed by 1 5 to 8 fold increases in electrochemically active cd in pore water following uv treatment as well as inconsistent distributions of dissolved and solid phase cd indicate complexities in diagenesis that merit further investigation

intersecting intracontinental tertiary transform fault systems in the north american cordillera in central british columbia north trending dextral strike slip faults that cut late eocene granite also truncate northwest trending dextral strike slip faults the northwest trending strike slip faults bound the wolverine metamorphic complex wolverine complex which has been uplifted primarily by northwest southeast eocene crustal extension and somewhat by late eocene northerly extension the crustal extension is indicated by shallow dipping extensions faults dyke complexes and stretching lineations the wolverine complex and its bounding faults define a crustal pull apart in an en echelon dextral transform the northwest and north trending dextral strike slip faults in central british columbia are the continuations of faults that transect the interior of the north american cordillera and they represent at least two distinct plate boundaries intermittently active during the early to middle eocene and the late eocene to early oligocene each of these systems consists of en echelon strike slip fault

release of dissolved organic carbon doc from sediments of the n w european continental margin goban spur and its significance for benthic carbon cycling abstract pore water samples from n w european continental margin sediments 49 48 n 16 10 w were analyzed for dissolved organic carbon doc using a high temperature combustion method two transects across the margin were investigated a gentle undisturbed slope goban spur 670 4800 m and the centre of a nearby precipitous canyon whittard canyon 180 3680 m concentrations of pore water doc were typically an order of magnitude greater than those from the overlying water therefore the sediments appear to act as a doc source to the bottom water conservative estimates ignoring possible bioirrigation of the doc fluxes from the sediments gave daily fluxes of 0 09 0 15 mmol m 2 d 1 for the goban spur sediments and 0 05 0 16 mmol m 2 d 1 at the canyon transect these relatively low variations in doc concentrations and fluxes under widely differing environmental conditions suggest that production and consumption of labile doc components proceed at similar rates irrespective of what the overall benthic activity is to the total benthic degradation rate of organic carbon the doc fluxes contribute 2 32 median 11 a part that is missing when the carbon degradation rate is solely based on determinations of the oxygen consumption rate or on pore water modeling of the reaction thus doc effluxes are a significant component of benthic carbon budgets in continental margin sediments

the effects of shrimp trawling on sediment oxygen consumption and the fluxes of trace metals and nutrients from estuarine sediments abstract the effects of shrimp trawling on sediment oxygen consumption and the sediment water exchange fluxes of nutrients ammonium phosphate and silicate and trace metals mn ni cu cd and pb were determined for two separate experiments conducted during july 1998 and may 1999 oxygen microelectrode measurements showed that during the 1998 experiment the surface sediments were oxic down to depths of approximately 15 mm suggesting that the study area had not been recently trawled however after passage of the trawl gear oxygen was present to depths of only 5 mm this suggests resuspension of the upper 1 cm of sediment and resulted in ammonium and manganese fluxes increasing by a factor of 2 3 which was attributed to a similar mechanism i e a strengthening of the diffusive gradient the measured fluxes of oxygen phosphate and the trace metals ni cu and pb were not affected by trawling while the flux of cd was affected during the 1999 shrimp trawl experiment pre and post trawl oxygen penetration was limited to the upper 1 mm of sediment suggesting the area had been recently trawled the diffusive fluxes of oxygen were in agreement with directly measured oxygen fluxes both prior to and after trawling further indication that trawling had little to no effect on the exchange of oxygen across the sediment water interface during this time macrofaunal abundance was significantly reduced during 1999 and consistent with sediment oxygen consumption rates which were also lower during this time this resulted in the fluxes of oxygen which were enhanced by biological processes during 1998 becoming diffusion controlled during 1999 this resulted in pre and post trawl fluxes of oxygen ammonium silicate mn ni cu and pb when considering the measurement error which did not differ significantly cd again seemed affected by shrimp trawling activities an increase or decrease in a trace metal s turnover time the time in days required to replenish the water column concentration can have important implications towards the overall health of an estuary in galveston bay trace metal turnover times averaged 0 4 0 5 52 32 27 15 22 23 and 5 3 3 0 d for mn ni cu cd and pb respectively these relatively short turnover times indicate that the sediments can be an important source of trace metals to the overlying water column especially during low flow periods of the trinity river which can sometimes last more than 1 yr this in turn could possibly lead to the non conservative mixing behaviors for both nutrients and trace metals previously reported for the trinity bay region of galveston bay as conservative mixing is observed in the lower estuary our study shows that the effects of shrimp trawling are largely dependent on the prevailing sediment redox conditions due to the shallow penetration depth of both oxygen and the trawl gear therefore it is unlikely that trawling activities adversely affect the overall health of galveston bay however repeated trawling with removal of the upper oxic sediment layers could trend surface sediments towards anoxia and ultimately lead to changes in benthic pelagic coupling

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

giant quartz vein systems in accretionary orogenic belts the evidence for a metamorphic fluid origin from δ 15 n and δ 13 c studies abstract nitrogen isotope compositions of hydrothermal micas in giant structurally hosted quartz vein systems place constraints on the origin of the hydrothermal fluids the vein systems are from neoarchean terranes in the superior province of canada and the norseman terrane western australia the four quartz vein systems studied in different terranes formed at metamorphic grades ranging from lower greenschist to lower amphibolite facies coevally with metamorphism of the host terrane nitrogen resides predominantly as nh 4 in hydrothermal mica for lower to upper greenschist facies quartz veins n contents and 15 n values of micas are between 40 and 200 ppm and 15 and 21 respectively whereas in quartz veins formed at the greenschist to amphibolite transition and lower amphibolite facies micas have n contents of 20 70 ppm and 15 n of 11 24 in contrast micas and k feldspars from granitoids in the neoarchean abitibi and red lake greenstone belts are characterized by systematically lower 15 n of 5 to 5 and generally lower n contents of 20 50 ppm comparable to other granitoids carbon and oxygen isotope compositions of hydrothermal ferroan dolomite and calcite show systematic depletions with increasing metamorphic grade the mean values range from 2 2 to 3 7 for 13 c and 13 8 to 12 3 for 18 o in veins formed at lower to upper greenschist facies but from 5 8 to 7 1 for 13 c and 11 4 to 9 8 for 18 o at higher metamorphic grades nitrogen isotope compositions rule out mantle 15 n mean 5 magmatic 15 n 5 to 15 n mean 4 4 2 0 for the quartz veins accordingly the results are consistent with fluids derived from metamorphic dehydration of oceanic crust and sediments within accretionary orogenic belts

current deformation in the northern canadian cordillera inferred from gps measurements 1 continuous and campaign style global positioning system gps measurements provide new constraints on the first order current deformation pattern of the northern cordillera of nw canada and eastern alaska the yakutat block is currently colliding with north america in the corner of the gulf of alaska our data infer that relative yakutat north america motion is accommodated across the eastern boundary by right lateral motion 40 mm a mainly on the fairweather fault and minor shortening 6 mm a to the north collision is taken up by shortening 31 mm a mainly on the chugach st elias fault system with westward extrusion and possible counterclockwise rotation of the yakutat block and alaskan fore arc facilitated by 23 mm a right lateral motion that is shared by several faults the seismicity pattern indicates that plate boundary deformation is far reaching producing strain throughout eastern alaska yukon and western northwest territories our gps data confirm the transfer of strain from the yakutat collision zone and enable the extent of plate boundary deformation to be mapped thereby defining the western edge of stable north america gps sites in sw yukon show motion of 3 10 mm a to the ne confirming that strain is transferred at least 400 km from the yakutat collision continued transfer north toward the mackenzie delta and ne to the mackenzie mountains indicated by current seismicity is not yet well resolved by the campaign gps data which are hampered by coseismic and postseismic effects of the 2002 mw 7 9 earthquake on the denali fault in eastern alaska

geometric test for late cretaceous paleogene intracontinental transform faulting in the canadian cordillera the tintina trench northern rocky mountain trench tt nrmt fault zone and the fraser river straight creek fr sc fault zone are separate en echelon concentric small circle fault segments of a composite intracontinental transform fault zone more than 2500 km long that cuts diagonally across the canadian cordillera from the outboard part of a tectonic collage of accreted foreign terranes in the south into the north american preaccretionary continental margin in the north most of the 450 km of right hand slip on the tt nrmt fault zone was transformed southward during the late cretaceous and paleocene into oblique convergence in the southern canadian rockies the remainder probably comprising less than 100 km was transformed southwestward during early and middle eocene time via a zone of distributed shear and east west crustal stretching into right hand slip on the en echelon fr sc fault zone these interpretations based on regional systematic mapping of geologic structures are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post mid cretaceous right hand displacement along the general locus of the tt nrmt fault zone involving foreign terranes and the parts of north america to which they had been accreted this paradox must be resolved

asymmetric rift interpretation of the western north american margin geologic evidence shows that the neoproterozoic rift margin of western north america formed as a set of asymmetric detachments leading to alternating upper and lower plate domains distinguishable by the width and gradient of the continental margin thickness of overlying strata uplift subsidence history relative structural complexity and presence or absence of exposed lower crustal rocks the gradient of the cratonal margin is a key feature related to the transition between continental and oceanic crust upper plate margins are marked by steep gradients whereas lower plate margins have gentle gradients in the canadian cordillera the gradient across this transition changes coherently along the margin from shallow to steep to shallow in turn from north to south the sinuosity of the cretaceous tertiary thrust belt front the distribution of basement exposures and patterns of miogeoclinal deposition may therefore be inherited from the original asymmetric neoproterozoic rift geometry

andean flat slab subduction through time the analysis of magmatic distribution basin formation tectonic evolution and structural styles of different segments of the andes shows that most of the andes have experienced a stage of flat subduction evidence is presented here for a wide range of regions throughout the andes including the three present flat slab segments pampean peruvian bucaramanga three incipient flat slab segments carnegie guanacos tehuantepec three older and no longer active cenozoic flat slab segments altiplano puna payenia and an inferred palaeozoic flatslab segment early permian san rafael based on the present characteristics of the pampean flat slab combined with the peruvian and bucaramanga segments a pattern of geological processes can be attributed to slab shallowing and steepening this pattern permits recognition of other older cenozoic subhorizontal subduction zones throughout the andes based on crustal thickness two different settings of slab steepening are proposed slab steepening under thick crust leads to delamination basaltic underplating lower crustal melting extension and widespread rhyolitic volcanism as seen in the caldera formation and huge ignimbritic fields of the altiplano and puna segments on the other hand when steepening affects thin crust extension and extensive within plate basaltic flows reach the surface forming large volcanic provinces such as payenia in the southern andes this last case has very limited crustal melt along the axial part of the andean roots which shows incipient delamination based on these cases a palaeozoic flat slab is proposed with its subsequent steepening and widespread rhyolitic volcanism the geological evolution of the andes indicates that shallowing and steepening of the subduction zone are thus frequent processes which can be recognized throughout the entire system

benthic chamber and profiling landers in oceanography a review of design technical solutions and functioning we review and evaluate the design and operation of twenty seven known autonomous benthic chamber and profiling lander instruments we have made a detailed comparison of the different existing lander designs and discuss the relative strengths and weaknesses of each every aspect of a lander deployment from preparation and launch to recovery and sample treatment is presented and compared it is our intention that this publication will make it easier for future lander builders to choose a design suitable for their needs and to avoid unnecessary mistakes

organic carbon oxidation and benthic nitrogen and silica dynamics in san clemente basin a continental borderland site organic carbon oxidation rates in san clemente basin were determined by benthic chamber experiments using the bottom lander along with studies of pore water chemistry non steady state diagenetic models are developed for interpreting concentration time data from the benthic chamber experiments o2 no3 and so42 are all important oxidants for organic carbon at our study site regenerated fixed nitrogen was consumed by no3 reduction there is a flux of no3 into the sediments and the benthic flux of nh4 is undetectable the total rate at which fixed nitrogen is removed from the oceans at this site is about twice the flux of pon to the sea floor sio2 fluxes calculated from interfacial pore water gradients are in satisfactory agreement with those determined using the lander most silica dissolution must therefore occur within the sediments although interstitial profiles show that little dissolution occurs below 1 cm depth

the variability of benthic fluxes and sedimentary remineralization rates in response to seasonally variable organic carbon rain rates in the deep sea a modeling study

metasomatic diamond growth a multi isotope study 13c 15n 33s 34s of sulphide inclusions and their host diamonds from jwaneng botswana abstract the formation of diamond through metasomatic events from volatiles enriched fluids melts brought into pre existing mantle rocks has been suggested from a series of independent studies however the link between these hypothetical volatile rich fluids and deep seated mineral inclusions mostly silicate and sulphides entrapped in diamonds remains unclear yet the relationship between these two species may provide the key to our understanding of diamond crystallization in order to address the relationship between the origin and formation of diamonds and their mineral inclusions we carried out the first coupled stable isotopic study 13 c 15 n and multiple s isotopes as 34 s 33 s 33 s of diamonds containing sulphide inclusions using samples from the jwaneng kimberlite as our model system sulphides extracted from the present collection of 55 diamonds belong to either eclogitic e type 52 diamonds or peridotitic p type 3 diamonds paragenetic suites as attested by their cr and ni content cr 0 18 wt and ni 14 wt for p types sulphur isotopic compositions have been measured in situ by multicollector secondary ion mass spectrometry and reveal that peridotitic sulphide inclusions in diamonds n 4 lie on the terrestrial fractionation line whereas mass independent sulphur isotope fractionations mif are preserved inside eclogitic sulphides inclusions 0 5 33 s n 33 such large mif values extended here to the negative values recorded in archean sediments are thought to be produced through photochemical reactions in the archean atmosphere which implies that sulphides inclusions contain an archean sedimentary sulphur component that was transferred to the earth s mantle in contrast to this isotopic dichotomy between e type and p type sulphides the geochemical characteristics of their host diamonds 13 c 15 n n content n speciation are largely homogeneous throughout the population and differ from silicate bearing diamonds previously studied in the same locality sulphide bearing diamonds appear to define a distinct population which might reflect a different crystallization process characterized by carbon and nitrogen isotopic compositions falling within the range of unfractionated mantle values the lack of a distinct isotopic signature of recycled sedimentary nitrogen and carbon in sulphide bearing diamonds leads us to infer that sulphide bearing jwaneng diamonds were not derived from the same chemical reservoir as their inclusions one possible formation mechanism that is compatible with our observations is diamond crystallization from a mantle derived carbon bearing fluid associated with pre existing sedimentary sulphide minerals this model has important implications for the interpretation of diamond ages obtained from sulphide inclusions using re os systematics here we suggest that the incorporation of a significant amount of mantle related sulphur during a diamond growth event into the original sulphide could lead to a re equilibration of the re os isotope composition in any inclusion and accordingly account for some scatter on re os isochrons

a paleomagnetic test for northward tectonic transport of the eastern selwyn basin northern canadian cordillera data from the mid cretaceous ragged pluton ragged pluton is a member of the tungsten plutonic suite in the eastern selwyn basin this circular massive 97 ma granitic pluton is 5 km in diameter it intrudes deformed and metamorphosed clastic strata of the neoproterozoic to lower cambrian vampire formation thermal and alternating field step demagnetization data for 216 specimens from 21 plutonic sites isolated a stable characteristic remanent magnetization chrm direction at 20 sites of declination d 325 9 inclination i 80 4 95 2 7 k 142 thermal demagnetization and saturation remanence analysis show that the chrm is carried by both single domain pyrrhotite and magnetite ragged pluton s paleopole at 73 9 n latitude 191 4 e longitude dp 5 0 dm 5 2 is concordant with its co eval 97 ma north american reference paleopole showing at 95 confidence that the pluton has not been significantly rotated or translated relative to north america this paleomagnetic result favours tectonic models in which the eastern selwyn basin is

benthic recycling of biogenic debris in the eastern tropical atlantic ocean abstract distributions of pore water o2 no 2 no 3 nh 4 si oh 4 po3 4 mn2 f and t a were determined at 15 stations in the eastern equatorial atlantic while overall profile characteristics are consistent with previous models of organic matter diagenesis profile shapes suggest that a deep reaction layer rich in organic c is also present at many sites while it is unlikely that the oxidation of organic c in this layer has had a major effect on the ocean c cycle pore water profile shapes are significantly altered despite exposure to seawater so2 4 concentrations for 1000 years decomposition of the organic matter in the layer appears to be restricted to oxic and suboxic processes these results suggest major differences in organic carbon decomposition and preservation under oxic suboxic and anoxic conditions present day benthic fluxes are largest adjacent to the eastern boundary coastal upwelling region and similar in magnitude to values reported for the eastern pacific preliminary estimates suggest that the benthic respiration in the eastern 1 3 of the north atlantic south of 20 n may alone account for 20 of the total deep north atlantic respiration combining these results with estimates of organic c burial and deep water column decomposition suggests that this region is a major location of organic c input into the deep sea

early diagenetic processes fluxes and reaction rates in sediments of the south atlantic abstract porewaters recovered from sediment cores gravity corers box corers and multicorers from various subrogions of the south atlantic amazon river mouth equatorial upwelling congo river mouth benguela coastal upwelling area and angola basin were investigated geochemically objectives included determination of eh ph oxygen nitrate sulfate alkalinity phosphate ammonium fluoride sulfide cal mg sr fe mn and si in order to quantify organic matter diagenesis and related mineral precipitation and dissolution processes porewater profiles from the eastern upwelling areas of the south atlantic suggest that sulfate reduction in the deeper parts of the sediment may be attributed mainly to methane oxidation whereas organic matter degradation by sulfate reduction is restricted to the near surface sediments further a prominent concentration gradient change of sulfate and related mineralization products occurred typically in the upwelling sediments at a depth of 4 to 8 m far below the zone of bioturbation or bioirrigation because other sedimentological reasons seem to fail as explanations an early diagenetic sulfide oxidation to sulfate within the anoxic environment is discussed porewater profiles from the sediments of the amazon fan area are mainly influenced by reactions with fe iii phases the remarkable linearity of the concentration gradients of sulfate supports the idea of distinct reaction layers in these sediments in contrast to the upwelling sediments the sulfate gradient develops from the sediment surface to a sulfate reduction zone at a depth of 5 3 m probably because a reoxidation of sulfide is prevented by the reaction with iron oxides and the formation of iron sulfide minerals a comparison of organic matter degradation rates from the different areas of the south atlantic show the expected relationship to primary productivity oxygen is the dominating oxidant whereas organic matter degradation by nitrate mn iv and probably also fe iii reduction is several orders of magnitude lower sulfate reduction is quantitatively of similar importance as oxygen respiration in the organic rich sediments but may also include methane oxidation reoxidation of ammonium and mn2 by oxygen or nitrate do not alter significantly the estimation of organic matter degradation in the oxic zone but may contribute to the nitrate reduction in suboxic layers

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

contrasting seismic characteristics of three major faults in northwestern canada the lithoprobe slave northern cordillera lithospheric evolution snorcle profiles crossed three major tectonic zones of the northwestern canadian shield and northern canadian cordillera that are diverse in age and in depth of penetration the oldest 2630 2590 ma the yellowknife river fault zone formed as a strike slip fault in a tensional strain regime reflector attenuation or truncations align vertically beneath the fault trace through much of the crust implying a near vertical fault plane the youngest 60 10 ma the tintina fault zone produced cumulative dextral strike slip displacements of 425 km perhaps 800 km tomographic velocity and ray trace models of reflection data indicate that several fault splays form a tectonic zone 30 km wide at the surface but truncations of deeper crustal reflections suggest that the zone thins in the mid crust and widens near the moho this apparent variable width versus depth of the tintina fault is atypical of major strike slip faults worldwide the teslin fault was an active terrane boundary during accretion of terranes onto north america observed reflection geometries indicate that the juxtapositions of highly contrasting metamorphic grades across the teslin fault are confined to the upper crust along snorcle line 3 implying that the fault soles eastward into a mid crustal detachment at the interpreted top of north american crust the limited depth extent of the teslin fault zone is similar to some models of the san andreas fault and may result from their similar histories as convergent margin structures

the 129iodine bomb pulse recorded in mississippi river delta sediments results from isotopes of i pu cs pb and c 129i t1 2 1 56 107 yr has both natural as well as anthropogenic sources anthropogenic sources from nuclear reprocessing discharges and bomb test fallout have completely overwhelmed the natural signal on the surface of the earth in the last 50 years however the transfer functions in and out of environmental compartments are not well known due to temporal variations in the sources of 129i and to a lack of knowledge regarding the forms of iodine from a vertical profile of 129i 127i ratios in sediments located in the mississippi delta region in approximately 60 meters water depth the 129i input function to this region was reconstructed dates in the core were assigned based on the plutonium peak at 20 cm depth assumed to have been deposited in 1963 and the excess 210pb profile in the same depth interval and below that based on the steadily decreasing 240pu 239pu ratios from a ratio of 0 18 at 22 cm to 0 05 at 57 cm depth the 1953 horizon these low 240pu 239pu values are attributed to low yield close in tropospherically transported bomb fallout produced from the nevada test site in the early 1950s which had a value of about 0 035 and strongly suggest a terrestrial source for pu isotopes 129i 127i ratios increased from 2 10 10 at 3 cm to 4 10 10 at 20 cm and from there decreased monotonously to pre anthropogenic values at 53 cm and below 129i concentrations ranged from 8 13 106 atoms g in the top 20 cm and decreased to values of less than 1 106 atoms g below 50 cm atom ratios of 129i 137cs decay corrected to 1962 the year of maximum radionuclide production are about 0 3 very close to the production ratios of about 0 2 during atomic bomb tests this evidence combined with other observations strongly suggests that 129i in mississippi river delta sediments originates from atomic bomb fallout eroded from soils of the mississippi river drainage basin with little alteration of the isotopic ratios during transport from watershed to coastal deposits this is further corroborated by a close correspondence between 129i 127i ratios and other bomb fallout nuclides in this core based on these observations and on laboratory evidence we propose a conceptual model which explains this correspondence and the low 129i 127i ratios found in our sediment core as caused by geochemical i isotope fractionation processes during organic carbon leaching from erodible soils in the mississippi river watershed differences in mobilities of the different chemical forms of 129i and 127i as well as the variances in chemical forms of 129i from nuclear bomb fallout versus nuclear fuel reprocessing are proposed to have created such a correspondence between i isotope ratios and bomb fallout nuclides without revealing recent inputs from nuclear fuel reprocessing releases to the northern hemisphere observed in watersheds of the usa and europe

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

a comment on the nitrogen record of crust mantle interaction and mantle convection from archean to present by b marty and n dauphas earth planet sci lett 206 2003 397 410

c and n isotopic composition and the infrared absorption spectra of coated diamonds evidence for the regional uniformity of co2h2o rich fluids in lithospheric mantle abstract the 13 c and 15 n values nitrogen abundances and nitrogen aggregation states of coated diamonds from botswana angola sierra leone and siberia have been determined a single cubic micro diamond from the northern territory australia was also analysed the cubic coats of the diamonds and the cubic diamond from australia had a restricted range in isotope composition 13 c 7 2 to 4 1 15 n 8 7 to 1 7 in contrast the cores of the coated diamonds were found to be highly variable 13 c 21 1 to 1 9 and 15 n 2 8 to 12 1 with the majority being positive all of the coats gave the type iaa absorption spectra together with others due to micro inclusions dominated by h 2 o and co 2 whereas the cores contained nitrogen that was more highly aggregated the results suggest that coated diamonds were formed following the influx of co 2 h 2 o rich fluids into diamond bearing lithosphere pre existing diamonds acted as seeds for renewed growth and these are now the cores of the diamonds these cores may be very variable in terms of morphology isotopic composition age nitrogen aggregation state and crystallinity depending on the particular history of the source regions and the conditions of diamond growth it is believed that the influx of volatiles and the growth of the coats was linked to kimberlite magmatism the 13 c and 15 n results for the coats indicate that in terms of c and n isotope composition the source of co 2 h 2 o rich fluids is globally quite homogeneous it is probably located beneath continental lithosphere and in addition may have characteristics similar to the source of the ocean island basalts

evidence against large scale strike slip separation of paleozoic strata along the richardson hess fault system northern canadian cordillera a review of geologic relationships across the richardson hess fault system demonstrates that large scale strike slip separation did not occur among rocks younger than proterozoic evidence against major lateral separation is 1 structural units of gently dipping paleozoic strata deformed during columbian laramide orogenesis show only 10 to 20 km total generally right lateral separation across the fault system 2 the only notable discontinuity across the fault system occurs along a prominent lineament at its south end this discontinuity is part of a 200 km long transition of thick lower cambrian quartzite and carbonate strata southwestward into argillite and does not need to be explained by strike slip faulting 3 lower paleozoic paleogeography across the richardson hess system features a complex arrangement of arches basins and troughs all strata in and around these features are at present harmoniously juxtaposed

the impact of shrimp trawling and associated sediment resuspension in mud dominated shallow estuaries abstract to address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries a series of disturbance and monitoring experiments were conducted at a bay bottom mud site 2 5 m depth in galveston bay texas in july 1998 and may 1999 based on pre and post trawl sediment profiles of 7be pore water dissolved oxygen and sulfide concentration and bulk sediment properties it was estimated that the trawl rig including the net trawl doors and tickler chain excavate the seabed to a maximum depth of approximately 1 5 cm with most areas displaying considerably less disturbance water column profile data in the turbid plume left by the trawl in these underconsolidated muds 85 90 porosity

plateau style accumulation of deformation southern altiplano 1 employing surface mapping of syntectonic sediments interpretation of industry reflection seismic profiles gravity data and isotopic age dating we reconstruct the tectonic evolution of the southern altiplano 20 22 s between the cordilleras defining its margins the southern altiplano crust was deformed between the late oligocene and the late miocene with two main shortening stages in the oligocene 33 27 ma and middle late miocene 19 8 ma that succeeded eocene onset of shortening at the protoplateau margins shortening rates in the southern altiplano ranged between 1 and 4 7 mm yr with maximum rates in the late miocene summing rates for the southern altiplano and the eastern cordillera we observe an increase from eocene times to the late oligocene to some 8 mm yr followed by fluctuation around this value during the miocene prior to shutoff of deformation at 7 8 ma and transfer of active shortening to the sub andean fold and thrust belt shortening inverted early tertiary graben and half graben systems and was partitioned in three fault systems in the western central and eastern altiplano respectively the east vergent fault systems of the western and central altiplano were synchronously active with the west vergent altiplano west flank fault system from these data and from section balancing we infer a kinematically linked western altiplano thrust belt that accumulated a minimum of 65 km shortening evolution of this belt contrasts with the eastern cordillera which reached peak shortening rates 8 mm yr in between the above two stages hence local shortening rates fluctuated across the plateau superimposed on a general trend of increasing bulk rate with no trend of lateral propagation this observation is repeated at the shorter length and time scales of individual growth structures that show evidence for periods of enhanced local rates at a timescale of 1 3 myr we interpret this irregular pattern of deformation to reflect a plateau style of shortening related to a self organized state of a weak crust in the central south american back arc with a fault network that fluctuated around the critical state of mechanical failure tuning of this state may have occurred by changes in plate kinematics during the paleogene initially reactivating crustal weak zones and by thermal weakening of the crust with active magmatism mainly in the neogene stage

the 3d geophysical investigation of a middle cretaceous to paleocene regional decollement in the cordillera of northern canada and alaska

geochronology of sediments in the sabine neches estuary texas u s a the accumulation rate of sediments in the sabine neches estuary near beaumont texas was studied in four sediment cores using radioisotopes 210pb and 239 240pu due to very low and variable activities of unsupported 210pb in the sediments the sedimentation rates could not be reliably established using the 210pb dating method however 239 240pu profiles showed distinct peaks corresponding to the maximum fallout in 1963 and using these pu peaks the average sedimentation rates were estimated at 4 5 mm yr 1 in the upper and lower estuary various assumptions that are commonly used in the 210pb dating method were tested it was found that the 226ra activities were in equilibrium with 238u and the possible loss of 222rn was judged to be insignificant the lack of an exponential profile of excess 210pb with depth did not appear to be a direct result of sediment mixing as the post depositional mixing rates estimated using a numerical mixing model were low compared to other coastal areas a significant positive correlation between the amount of 63 m fraction sediments and the total 210pb activity was observed in the surface sediments while the hydraulic residence time was relativelt short in this estuary 10 days the average total residence time of 210pb was 10 days between 50 and 94 of the total 210pb in the water column was found in the 0 5 m fraction resulting in low kd values the measured inventories of excess 210pb and 239 240pu in estuarine sediments were only 10 34 and 19 50 respectively of the total expected inventories it appears that the estuarine retention and sediment inventory of these radionuclides and by analogy other particle reactive metals are a function of the hydraulic residence time grain size and possibly complexation of these nuclides with doc

the cycling and oxidation pathways of organic carbon in a shallow estuary along the texas gulf coast abstract the cycling and oxidation pathways of organic carbon were investigated at a single shallow water estuarine site in trinity bay texas the uppermost lobe of galveston bay during november 2000 radio isotopes were used to estimate sediment mixing and accumulation rates and benthic chamber and pore water measurements were used to determine sediment water exchange fluxes of oxygen nutrients and metals and infer carbon oxidation rates using 7 be and 234 th xs the sediment mixing coefficient d b was 4 3 1 8 cm 2 y 1 a value that lies at the lower limit for marine environments indicating that mixing was not important in these sediments at this time sediment accumulation rates s a estimated using 137 cs and 210 pb xs were 0 16 0 02 g cm 2 y 1 the supply rate of organic carbon to the sediment water interface was 30 3 9 mmol c m 2 d 1 of which 10 or 2 9 0 44 mmol c m 2 d 1 was lost from the system through burial below the 1 cm thick surface mixed layer measured fluxes of o 2 were 26 3 8 mmol m 2 d 1 and equated to a carbon oxidation rate of 20 3 3 mmol c m 2 d 1 which is an upper limit due to the potential for oxidation of additional reduced species using organic carbon gradients in the surface mixed layer carbon oxidation was estimated at 2 6 1 1 mmol c m 2 d 1 independent estimates made using pore water concentration gradients of ammonium and c n stoichiometry equaled 2 8 0 46 mmol c m 2 d 1 the flux of doc out of the sediments doc efflux was 5 6 1 3 mmol c m 2 d 1 in general while mass balance was achieved indicating the sediments were at steady state during this time changes in environmental conditions within the bay and the surrounding area mean this conclusion might not always hold these results show that the majority of carbon oxidation occurred at the sediment water interface via o 2 reduction this likely results from the high frequency of sediment resuspension events combined with the shallow sediment mixing zone leaving anaerobic oxidants responsible for only 10 15 of the carbon oxidized in these sediments

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

structural evolution of the argentine precordillera the rio san juan section abstract the argentine precordillera ap represents a high level fold and thrust belt which was formed during the andean tertiary crustal shortening in a w e transect studied mostly e directed imbrications are combined with folding and involve a pile of cambrian to tertiary sediments the thrust belt is detached above a main decollement within the ordovician to lower devonian strata to the east it is bounded by a back thrust zone directed westwards restored sections allow an estimate of crustal shortening and also provide insight into the structural development during pre tertiary times carbonate platform sequences of cambrian to ordovician age grade into a volcani clastic succession with transitional melanges representing deposits along a western continental slope in the western and easternmost parts of the ap cambrian to silurian deposits are affected by a first folding which to the west is combined with a slight metamorphic overprint during carboniferous to permian times intensified mobility along the eastern rim and in the western parts of the ap leads to crustal extension which partly was triggered by the formation of synsedimentary normal faults in the central ap no angular unconformities at the base of carboniferous permian deposits were found there was no field evidence found to postulate a terrane boundary within the ap area and an onset of thrust tectonics during cretaceous to early tertiary times

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

basin architecture and syndepositional fault activity during deposition of the neoproterozoic mackenzie mountains supergroup northwest territories canada the early neoproterozoic mackenzie mountains supergroup mmsg 4 km thick 779 ma was deposited in a large but poorly understood basin analysis of new and existing stratigraphic data reveals unexpected abrupt changes in thickness and lithofacies patterns in formation scale and smaller units over short distances parallel and perpendicular to depositional strike these isopach and lithofacies patterns depict irregularly subsiding syndepositional sub basins these basins may have developed during basin extension in a lower plate segment of an evolving passive margin as the result of episodic excessive subsidence along growth faults parallel to the basin margin and crustal scale transfer faults at a high angle to the margin the extent of the lower plate area is inferred to correspond to the distribution of the mmsg 500 km strike length upper plate segments are inferred to flank the mmsg embayment to the northwest and southeast a lower plate model for the arcuate mmsg embayment has im

nitrogen and argon signatures in 3 8 to 2 8 ga metasediments clues on the chemical state of the archean ocean and the deep biosphere abstract n and ar elemental and isotopic analyses were conducted on archean metasediments of isukasia west greenland and pilbara craton western australia in order to investigate the n isotopic evolution during the first half of earth s history the selected samples are deep sea sediments and hydrothermal deposits having ages from 3 8 to 2 8 ga and affected by different degrees of metamorphism the release patterns of n and ar obtained by high resolution stepped combustion show the occurrence of at least two trapped components the first is released at 600 c and it is likely contained in fluid inclusions n is released together with primordial 36ar and shows a 15n value of 1 3 1 0 close to that of modern atmospheric n2 15n 0 this component is well preserved in hydrothermal vent silica deposits of north pole pilbara craton and nitrogen may represent ammonium salt dissolved in deep sea hydrothermal fluids the second n component released at temperatures higher than 1000 c is accompanied by radiogenic 40ar and shows a 15n value of 7 4 1 0 in a kerogen rich chert from north pole pilbara craton this n is likely biogenic and negative 15n values may reflect a metabolic isotopic fractionation induced by chemosynthetic bacteria using inorganic nh4 contained in hydrothermal fluids this 15n depleted biogenic component may occur in isukasia banded iron formation 15n 1 7 but further data are needed to confirm such a hypothesis in all other samples metamorphic induced rayleigh distillation has altered the pristine n isotopic signature

multiple growth events during diamond genesis an integrated study of carbon and nitrogen isotopes and nitrogen aggregation state in coated stones the internal structure and growth history of diamonds have been investigated by the study of carbon isotopes and the abundance isotope composition and aggregation state of nitrogen polished plates of zoned diamonds coated stones were laser sectioned for infrared mapping and isotope profiling nine small cubic and three small octahedral diamonds from zaire and two coated stones and five small cubic diamonds specifically from the mbuji mayi area of that country were also investigated the isotope composition and infrared characteristics of the diamonds of cubic habit including coats were found to be indistinguishable with 13c in the range 5 9 to 7 5 and all the nitrogen being present as the paired structure iaa with a 15n of between 3 1 and 8 0 the diamonds of octahedral habit including the cores of the coated stones were more variable 13c varied from 4 9 to 9 8 infrared spectra of the octahedral diamond exhibited either strong iaa characteristics or a combination of iaa and iab platelets were present in most cases 15n values from octahedral diamonds varied from 5 8 to 13 4 the data suggest that the octahedral diamonds represent a variety of growth events separated in time and or space and that the diamonds resided in the lithosphere for a period sufficient for thermal maturation in the case of the coated stones the octahedral diamonds were later involved in an event which added a component from a single relatively homogeneous reservoir cubic habit diamonds were formed at the same time this latter event is believed to be related to the kimberlite eruption comparison of the data with carbon isotope analyses of cubic diamond from other sources suggests that this reservoir could be widespread

ammonium concentration and nitrogen isotope composition in metasedimentary rocks from different tectonometamorphic units of the european variscan belt the content and isotopic composition of ammonium nh4 were measured by using kjeldahl distillation for 63 samples of predominantly metasedimentary rocks from two different geological settings of the european variscides i e the erzgebirge and the zone of erbendorf vohenstrauss in germany the studied erzgebirge schists represent an early paleozoic passive margin sequence that reappears in different deep subducted metamorphic units this allows an efficient examination of nitrogen loss and isotope fractionation during prograde metamorphism ammonium is found to be progressively depleted accompanied by a shift in 15n from the low grade units 2 kbar 300 c with 638 124 ppm and 15n 2 2 0 6 to the garnet phyllite unit 9 kbar 470 c with 621 190 ppm and 15n 3 5 0 9 the mica schist eclogite unit 12 kbar 550 c with 394 113 ppm and 15n 3 9 0 8 and to the gneiss eclogite unit 12 kbar 730 c with 99 32 ppm and 15n 7 7 2 0 using equilibrium models for rayleigh distillation and batch volatilization suggest that the nitrogen depletion took place by ammonia release only for the mica schists this isotope fractionation can be explained by the loss of molecular nitrogen in comparison with the early paleozoic schists the proterozoic ortho and paragneisses of the erzgebirge gneiss unit 6 8 kbar 650 c contain significantly lower amounts of ammonium 70 ppm and reach relatively low 15n values 2 5 to 3 6 the paragneisses from the zone of erbendorf vohenstrauss zev sampled from the ktb pilot hole represents a separate tectonometamorphic unit that has undergone polyphase metamorphic conditions with the major imprint being amphibolite facies 7 kbar 650 700 c the metagraywackes have nh4 contents of 80 ppm and 15n of approximately 6 gneisses in the vicinity of a late cataclastic shear zone at 2000 m depth are characterized by a shift in nh4 250 ppm and 15n up to 12 which suggest late fluid rock interactions with a nitrogen and 15n enriched fluid

a comment on the nitrogen record of crust mantle interaction and mantle convection from archean to present by b marty and n dauphas earth planet sci lett 206 2003 397 410 the earth s upper mantle is depleted in 15n isotope with a n15n value around 35x where n15n n nsample n nair31 1000 relative to atmosphere and crust the 15n depleted character rst identi ed from recent i e 6 380 ma diamonds 1 3 see 4 for review was con rmed by studies focusing on volatiles extracted from mid ocean ridge basalts morb vesicles 5 8 archean diamonds as old as 3 2 ga 9 for review see 10 and references therein con rmed the negative n15n values showing that the 15ndepleted character of the earth s upper mantle is a long term feature 11 12 if the nitrogen present within the external reservoirs of the earth i e atmosphere sediments and crust had been simply outgassed from the mantle then one would expect them to display also negative n15n values however the atmosphere sediments and crust have all n15nv 0x in addition the fractionation of nisotopes occurring during basaltic melt degassing at ridges is opposite to what would be required to achieve positive n15n in vesicles 8 13 see below and 1 14 for discussion accordingly there is a so called isotopic imbalance of nitrogen between the mantle and the external reservoirs of the earth fig 1 which cannot be accounted for by outgassing of the earth s mantle only several models have been proposed to account for the nitrogen isotopic imbalance see fig 1 1 a heterogeneous accretion of the earth s volatiles e g 15 2 core mantle isotopic fractionation 7 13 3 hydrodynamic escape and fractionation of the atmospheric nitrogen 6 and 4 the new model presented by marty and dauphas 16 who consider that the nitrogen now present in the diierent mantle reservoirs is dominantly recycled organic nitrogen according to these authors the nitrogen in the upper mantle as sampled by diamonds and morb has a n15n value around 35x because it contains nitrogen of organic origin recycled in the archean which would have had a n15n around 35x the nitro

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

anatomy and global context of the andes main geologic features and the andean orogenic cycle the andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin subduction began soon after the breakup of rodinia in late proterozoic times and since that time it has been intermittently active up to the present the evolution of the pacifi c margin of south america during the paleozoic occurred in the following stages 1 initial proterozoic rifting followed by subduction and fi nal re amalgamation of the margin in early cambrian times as depicted by the puncoviscana and tucavaca basins and related granitoids in southern bolivia and northern argentina 2 a later phase of rifting in the middle cambrian and subsequent collisions in middle ordovician times of parautochthonous terranes derived from gondwana such as paracas arequipa and antofalla and exotic terranes originating in laurentia such as cuyania chilenia and chibcha 3 fi nal permian collision between south america and north america to form pangea during the alleghanides orogeny leaving behind rifted pieces of laurentia as the tahami and tahuin terranes in the northern andes and other poorly known orthogneisses in the cordillera real of ecuador in the late permian early triassic and 4 amalgamation of the mejillonia and patagonia terranes in early permian times representing the last convergence episodes recorded in the margin during the gondwanides orogeny these rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of gondwana related to global plate reorganizations during proterozoic to paleozoic times generalized rifting during pangea breakup in the triassic concentrated extension in the hanging wall of the sutures that amalgamated the paleozoic terranes the opening of the indian ocean in early jurassic times was associated with a new phase of subduction along the continental margin the northeastward absolute motion of western gondwana produced a negative trench roll back velocity that controlled subduction under an extensional regime until late early cretaceous times the northern andes of venezuela colombia and ecuador record a series of collisions of island arcs and oceanic plateaus from the early cretaceous to the middle miocene as a result of interaction with the caribbean plate the remaining central and southern andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the oligocene based on the generalized occurrence of fl at slab subduction episodes through time as recorded in most ramos v a 2009 anatomy and global context of the andes main geologic features and the andean orogenic cycle in kay s m ramos v a and dickinson w r eds backbone of the americas shallow subduction plateau uplift and ridge and terrane collision geological society of america memoir 204 p 31 65 doi 10 1130 2009 1204 02 for permission to copy contact editing geosociety org 2009 the geological society of america all rights reserved

a reinterpretation of the crustal n isotope record evidence for a n 15 enriched archean atmosphere a new compilation of n isotope and abundance data for metasedimentary rocks and hyrdothermal micas that proxy for bulk crust show systematic patterns 1 15n values of kerogen in precambrian cherts are more negative relative to siliciclastic counterparts probably due to a mantle hydrothermal component 2 there is a secular trend from average 15n 15 3 1 8 in archean shales through intermediate values in the proterozoic to phanerozoic counterparts where 15n averages 3 5 3 hydrothermal micas in metamorphic hydrothermal systems of palaeozoic and mesozoic age that proxy for crust have 15n within the range of contemporaneous sedimentary rocks 4 hydrothermal micas track the secular trend of 15n for kerogen from 2 7 ga to the phanerozoic 5 within precambrian datasets 15n does not increase with decreasing n content accordingly high 15n values cannot stem either from metamorphism or form rayleigh fractionation 6 previous studies show isotopic shifts during metamorphism are only 1 to 3 up to amphibolite facies values of 10 24 are attributed to a high 15n archean atmosphere a residual signature of ci carbonaceous chondrites where 15n is 30 to 42

superimposed quesnel late paleozoic jurassic and yukon tanana devonian mississippian arc assemblages cassiar mountains northern british columbia field u pb and igneous petrochemical evidence allochthons in the cassiar mountains of northern british columbia contain assemblages belonging to two distinct canadian cordilleran terranes yukon tanana ytt and quesnellia these assemblages of pre late devonian devonian mississippian pennsylvanian permian and early jurassic age occur in intrusive and depositional as well as structural contact with each other the allochthons are gently dipping thrust panels interrupted by the mid cretaceous cassiar batholith a key element for correlation across the batholith is the mississippian and older pericratonic dorsey complex new devonian mississippian u pb ages for deformed plutons within it document an igneous suite like those in type yukon tanana exposures farther north other characteristics of the dorsey complex that ally it with ytt are orthoquartzites and grits and amphibolite bodies with transitional mid ocean ridge basalt morb to ocean island basalt oib petrochemical signatures unconformities deformed clasts in the late paleozoic sequences and a shared mid permian intrusive suite show that later arcs onlapped the mid paleozoic and older ytt assemblage the early jurassic intrusive suite cuts all major contacts and fabrics except the terrane bounding fault between the slide mountain and combined ytt quesnel terranes it represents a northern continuation of a plutonic belt that extends the length of the mesozoic quesnel magmatic arc these relationships carry important implications for cordilleran terrane history and the tectonic evolution of the north american margin at least some of the major terranes were not unrelated entities prior to their accretion to the continent but a system of superimposed and interconnected arcs that developed over a protracted time interval with complex and evolving paleogeographic configurations much like the modern western pacific province

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

40ar 39ar age constraints on the thermal history of the archean abitibi greenstone belt and the pontiac subprovince implications for terrane collision differential uplift and overprinting of gold deposits 40ar 39ar mineral age spectra of granitic and metamorphic rocks in conjunction with existing conventional zircon geochronology indicate that at least two major late archean thermal events affected tectonic blocks of the abitibi southern volcanic zone svz and the juxtaposed pontiac subprovince the earlier thermal activity 2690 2670 ma was accompanied by the intrusion of voluminous syntectonic plutons and caused low pressure greenschist facies metamorphism in the svz and intermediate pressure metamorphism in the pontiac subprovince the second thermal event 2660 2630 ma was coeval with the emplacement of syncollisional s type garnet muscovite granites in the pontiac subprovince and the higher grade lacorne block of the abitibi svz and reset the k ar systems in preexisting rocks magmatic amphibole from the syntectonic round lake batholith 2695 ma u pb zircon age of the abitibi svz has a slightly disturbed ar release spectrum with an upper plateau age of 2669 6 ma signifying that the low gra

nitrogen isotope compositions of metasedimentary rocks in the catalina schist california implications for metamorphic devolatilization history abstract in the catalina schist subduction zone metamorphic complex california metasedimentary rocks show a decrease in n concentration and an increase in 15nair with increasing metamorphic grade lowest grade lawsonite albite rocks contain 632 185 ppm n with 15 n 1 9 0 6 whereas highgrade amphibolite equivalents contain 138 76 ppm n with 15 n 4 3 0 8 loss of n accompanied devolatilization reactions that evolved h2o rich c o h s n fluids through consumption of chlorite and phengitic white mica and production of mineral assemblages containing muscovite biotite garnet and kyanite whole rock n concentrations of up to 200 ppm in veins and pegmatites produced during high p t metamorphism reflect the redistribution of n during devolatilization and partial melting of the metasedimentary rocks bulk fluid rock n isotope fractionations 15n 15nfluid 15nrock of 1 5 1 were calculated with the rayleigh distillation equation taking into account variability in rock composition by comparison of samples with similar k2o concentrations these fractionations are similar to but slightly lower than published calculated fractionations for n2 nh4 exchange at the temperature range of 350 600 c over which most of the devolatilization occurred in the catalina schist approximately 3 4 to 2 25 the n systematics appear to be explained by n2 nh4 exchange and a devolatilization process intermediate in behavior to batch volatilization and rayleigh distillation the observed shifts in n concentration and 15n cannot be explained by nh3 nh4 exchange at these temperatures using the equilibrium models the distillation devolatilization process implicated in this study may govern the behavior of other trace elements partitioned into hydrous fluids during devolatilization e g b cs u similarity of the calculated fluid 15n 1 5 to 5 5 with compositions of natural gases inferred to be derived from metasedimentary sources indicates the possibility of using n as a tracer of large scale volatile transport

transition from basement involved to thin skinned thrusting in the cordillera oriental of southern bolivia the eastern flank of the central andes in southern bolivia and northern argentina is underlain by a fold and thrust belt of neogene age it comprises an external thin skinned segment with basal detachment in silurian shales subandean ranges and an internal segment where units as deep as precambrian metasedimentary and igneous rocks are involved in thrusting eastern cordillera oriental between the cordillera oriental and the subandean ranges an intermediate segment interandean zone intervenes north of the argentinian bolivian border 22 s the structure of the interandean zone is characterized by folded and thrust faulted cambrian to triassic strata balanced cross sections across the western subandean ranges the interandean zone and the eastern cordillera oriental are based on previously available data and recent field work they suggest that the geological structure can be interpreted in terms of two major basement involved thrusts here referred to as interandean and subandean thrusts the more internal interandean thrust caused the development of broad basement anticlines in the eastern cordillera oriental however at shallower levels its displacement was accommodated by thin skinned folding and thrusting in the interandean zone in sequence activation of the subandean thrust uplifted the interandean zone on top of an imbricate basement sheet and caused the development of the thinskinned subandean ranges farther east published radiometric data and subsidence history of the subandean foreland basin suggest that the interandean thrust was active between about 10 ma and 5 ma the subandean thrust is still active southward displacement transfer from the subandean thrust onto the interandean thrust and a southward dipping lateral ramp of the interandean thrust result in the gradual transition from the interandean zone north of 22 s to the cordillera oriental of northern argentina a similar but larger scale transfer may be involved in the southern termination of the thin skinned subandean ranges of argentina at 23 s shortening in the western part of the thrust belt is of the order of 80 90 km total shortening in the eastern andean thrust belt from the undeformed foreland to the eastern margin of the cordillera oriental is 140 150 km

constraints on the composition of the crust and uppermost mantle in northwestern canada vp vs variations along lithoprobe s snorcle transect shear wave seismic data recorded along four profiles during the snore 97 1997 slave northern cordillera refraction experiment refraction wide angle reflection experiment in northwestern canada are analyzed to provide s wave velocity vs models these are combined with previous p wave velocity vp models to produce cross sections of the ratio vp vs for the crust and upper mantle the vp vs values are related to rock types through comparisons with published laboratory data the slave craton has low vp vs values of 1 68 1 72 indicating a predominantly silicic crustal composition higher values 1 78 for the great bear and eastern hottah domains of the wopmay orogen imply a more mafic than average crustal composition in the western hottah and fort simpson arc values of vp vs drop to 1 69 these low values continue westward for 700 km into the foreland and omineca belts of the cordillera providing support for the interpretation from coincident seismic reflection studies that much of the crust fro

an 1800 km cross section of the lithosphere through the northwestern north american plate lessons from 4 0 billion years of earth s history the lithoprobe slave northern cordillera lithospheric evolution snorcle study across northwestern north america in combination with related crustal studies has been synthesized into an 1800 km long cross section of the lithosphere that is constrained by high resolution geophysical data seismic reflection refraction electromagnetic potential fields and detailed bedrock geology the cross section offers one of the longest continuous profiles of the continental lithosphere anywhere in the world that is constrained by combined geophysical measurements and electromagnetic properties and exposed bedrock geological relationships the primary conclusion of the study is that during all major orogenic episodes recorded from archean to present in that part of earth s lithosphere the crust and perhaps much of the mantle was reorganized and redistributed rather than being differentiated from the mantle at the time of orogenesis the observed subsurface geometries of relict subduction zones accretion

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

the anvil plutonic suite faro yukon territory the anvil plutonic suite consists of three phases a peraluminous muscovite biotite granite mount mye phase and two metaluminous to peraluminous hornblende biotite granodiorite and minor granite intrusions orchay and marjorie phases the suite is massive or foliated equigranular or seriate and contains alkali feldspar megacrysts the marjorie phase is characteristically porphyritic geochemical trends are irregular for the suite and for individual phases high k2o low cao and low mgo compositions typify the silicic calc alkaline suite hornblende bearing phases contain less sio2 k2o and rb and more cafemic oxides tio2 sr ba and y than the mount mye phase and are compositionally similar to coeval south fork volcanics isochrons from some of the orchay phase whole rock samples t 99 2 5 ma 87sr 86sri 0 7161 0 0001 and from whole rocks and minerals of the mount mye phase t 100 2 ma 87sr 86sri 0 7405 0 0001 indicate they are coeval but not comagmatic accounting for the litho

surface area control of organic carbon accumulation in continental shelf sediments the relationship between organic carbon oc and grain size found in most continental shelf sediments is here reinterpreted in terms of the surface area of the sediments cores from many north american shelf environments show downcore decreases in oc to similar refractory background concentrations if expressed relative to the surface area of the sediments this consistent concentration is 0 86 mg oc m 2 which is equivalent in concentration to a monolayer of organic matter coating all mineral surfaces a more global collection of sediment water interface samples show that this relationship is even more extensive with exceptions occurring in areas of very high riverine sediment input organic pollution or low oxygen water columns density separations indicate that organic matter is largely adsorbed to mineral grains the microtopography of surfaces was examined with n2 sorption and most surface area was found to be inside pores of 10 nm width these data lead to a hypothesis that organic matter is protected by its location inside pores too small to allow functioning of the hydrolytic enzymes necessary for organic matter decay such protection would likely work in concert with other protection mechanisms such as humification this consistent surface area correlation with oc concentration may explain control of spatial and temporal variations in oc burial rates by sedimentation rates the pore protection hypothesis provides a causal mechanism for this observed control