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

extensional forced folding and decollement of the pre rift series along the rhine graben and their influence on the geometry of the syn rift sequences

magnesium isotopic systematics of mafic rocks during continental subduction abstract magnesium isotopic compositions of a set of prograde metamorphosed rocks greenschists amphibolites and eclogites from east china were measured in order to understand the behavior of mg isotopes during continental subduction the 26mg values of the greenschists vary from 0 269 0 057 to 0 133 0 042 2sd and correlate negatively with the mgo contents and nb u ratios possibly due to the assimilation of isotopicallly heavy felsic schists 26mg up to 0 099 during the genesis of morb like protoliths the two subgroups of amphibolites have slightly different mg isotopic compositions the group i amphibolites with cumulate origin have 26mg values ranging from 0 243 0 061 to 0 192 0 050 which reflects clinopyroxene accumulation nature of the protolith the group ii amphibolites have relatively light 26mg values varying from 0 358 0 061 to 0 224 0 056 which may result from either source contamination or crustal assimilation of isotopically light components e g carbonates eclogitic garnet and clinopyroxene display large inter mineral mg isotope fractionations 26mgcpx grt 1 097 0 056 to 1 645 0 081 equilibrated at peak metamorphic temperatures the 26mg values of the eclogites vary from 0 348 0 041 to 0 137 0 063 overlapping the ranges of greenschists and amphibolites such mg isotopic variations are not related to unrepresentative sampling nor host eclogite chemical interactions instead they reflect the protolith heterogeneities collectively the similarity of mg isotopic compositions among the greenschists 0 196 0 044 amphibolites 0 271 0 042 and eclogites 0 226 0 044 suggests that mg isotope fractionation during continental subduction if any is limited

the cenozoic evolution of the roer valley rift system integrated at a european scale the roer valley rift system rvrs is located between the west european rift and the north sea rift system during the cenozoic the rvrs was characterized by several periods of subsidence and inversion which are linked to the evolution of the adjacent rift systems combination of subsidence analysis and results from the analysis of thickness distributions and fault systems allows the determination of the cenozoic evolution and quantification of the subsidence during the early paleocene the rvrs was inverted laramide phase the backstripping method shows that the rvrs was subsequently mainly affected by two periods of subsidence during the late paleocene and the oligocene quaternary time intervals separated by an inversion phase during the late eocene during the oligocene and miocene periods the thickness of the sediments and the distribution of the active faults reveal a radical rotation of the direction of extension by about 70 80j counter clockwise integration of these results at a european scale indicates that the late paleocene subsidence was related to the evolution of the north sea basins whereas the oligocene quaternary subsidence is connected to the west european rift evolution the distribution of the inverted provinces also shows that the early paleocene inversion laramide phase has affected the whole european crust whereas the late eocene inversion was restricted to the southern north sea basins and the channel area finally comparison of these deformations in the european crust with the evolution of the alpine chain suggests that the formation of the alps has controlled the evolution of the european crust since the beginning of the cenozoic

the late oligocene to early miocene early evolution of rifting in the southwestern part of the roer valley graben the roer valley graben is a mesozoic continental rift basin that was reactivated during the late oligocene the study area is located in the graben area of the southwestern part of the roer valley graben rifting initiated in the study area with the development of a large number of faults in the prerift strata some of these faults were rooted in preexisting zones of weakness in the mesozoic strata early in the late oligocene several faults died out in the study area as strain became focused upon others some of which were able to link into several kilometer long systems within the late oligocene to early miocene northwestward prograding shallow marine syn rift deposits the number of active faults further decreased with time a relatively strong decrease was observed around the oligocene miocene boundary and represents a further focus of strain onto the long fault systems miocene extensional strain was not accommodated by further growth but predominantly by displacements along the long fault systems since the oligocene miocene boundary coincides with a radical change in the european intraplate stress field the latter might have contributed significantly to the simultaneous change of fault kinematics in the study area

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

kinematic and geometric analysis of fault related folds in a rift setting the dannemarie basin upper rhine graben france abstract the upper rhine graben urg developed in the alpine foreland from mid eocene to pliocene as part of the west european rift system the dannemarie basin occupies the sw termination of the urg and is bordered by the vosges fault zone west and the illfurth fault zone east we constructed a 3d model of faults and principal pre and syn rift stratigraphic surfaces using all available subsurface and surface geological and geophysical data the model shows 1 syn rift strata thinning across kilometre scale fault parallel monoclinal flexures associated with basin margin faults and 2 major basin margin faults terminating laterally into major flexures sequential cross section restorations show that these flexures accommodated deposition of over 1300 m of syn rift strata salt series which is 42 63 of the basin fill these folds are interpreted as syn rift fault propagation folds that developed above normal faults their geometry is replicated in a trishear numerical model using reasonable parameter values fold development was controlled principally by 1 reactivation of favourably oriented basement faults 2 the presence of a thick viscous layer principally triassic evaporites allowing decoupling between crystalline basement and competent jurassic cover and 3 a relatively slow deformation rate

massive magnesium depletion and isotope fractionation in weathered basalts abstract magnesium isotopes have been shown to fractionate significantly during continental weathering however the degree and direction of fractionation varies from one study to the next and the main factors that control mg isotope fractionation during weathering have yet to be delineated here we report mg contents and isotopic compositions for two 10 m deep drill cores through bauxites developed on columbia river basalts crbs samples from these two drill cores have very low mgo contents 0 12 0 25 wt and variable but high 26mg values from 0 1 to up to 1 8 the heaviest isotopic composition ever reported for mg relative to the average values of fresh crbs 26mg 0 23 0 07 and mgo 5 9 wt these features reflect the near complete loss of mg in the isotopically heavy regolith the most highly weathered bauxites at the tops of the profiles have the lowest 26mg values reflecting the addition of isotopically light eolian dust excluding these samples 26mg in bauxites displays a weak positive correlation with gibbsite abundance suggesting that gibbsite preferentially retains 26mg in the bauxites the integrated mg isotopic fractionation factors between bauxite and fluid 26mgbauxite fluid inferred from rayleigh fractionation calculations vary from 0 05 to 0 4 which is similar to the range inferred from previous studies of igneous rock weathering profiles though none of these previous studies found the extremely heavy 26mg and massive mg depletion seen here collectively studies of weathering profiles suggest loss of isotopically light mg from the continents which must contribute to the very light 26mg seen in river water and seawater

calcium and magnesium isotope systematics in rivers draining the himalaya tibetan plateau region lithological or fractionation control abstract in rivers draining the himalaya tibetan plateau region the 26mg 24mg ratio has a range of 2 and the 44ca 42ca ratio has a range of 0 6 the average 26 mg values of tributaries from each of the main lithotectonic units tethyan sedimentary series tss high himalayan crystalline series hhcs and lesser himalayan series lhs are within 2 standard deviation analytical uncertainty 0 14 the consistency of average riverine 26 mg values is in contrast to the main rock types limestone dolostone and silicate which range in their average 26 mg values by more than 2 tributaries draining the dolostones of the lhs differ in their 44 42 ca values compared to tributaries from the tss and hhcs the chemistry of these river waters is strongly influenced by dolostone solute mg ca close to unity and both 26 mg 1 31 and 44 42 ca 0 64 values are within analytical uncertainty of the lhs dolostone these are the most elevated 44 42 ca values in rivers and rock reported so far demonstrating that both riverine and bedrock 44 42 ca values may show greater variability than previously thought although rivers draining tss limestone have the lowest 26 mg and 44 42 ca values at 1 41 and 0 42 respectively both are offset to higher values compared to bedrock tss limestone the average 26 mg value of rivers draining mainly silicate rock of the hhcs is 1 25 lower by 0 63 than the average silicate rock these differences are consistent with a fractionation of 26 mg values during silicate weathering given that the proportion of mg exported from the himalaya as solute mg is small the difference in 26mg 24mg ratios between silicate rock and solute mg reflects the 26mg 24mg isotopic fractionation factor silicate dissolved mg between silicate and dissolved mg during incongruent silicate weathering the value of silicate dissolved mg of 0 99937 implies that in the tss solute mg is primarily derived from silicate weathering whereas the source of ca is overwhelmingly derived from carbonate weathering the average 44 42 ca value in hhcs rivers is within uncertainty of silicate rock at 0 39 the widespread hot springs of the high himalaya have an average 26 mg value of 0 46 and an average 44 42 ca value of 0 5 distinct from riverine values for 26 mg but similar to riverine 44 42 ca values although rivers draining each major rock type have 44 42 ca and 26 mg values in part inherited from bedrock there is no correlation with proxies for carbonate or silicate lithology such as na ca ratios suggesting that ca and mg are in part recycled however in spite of the vast contrast in vegetation density between the arid tibetan plateau and the tropical lesser himalaya the isotopic fractionation factor for ca and mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of ca and mg in these catchments the discrepancy between solute and solid ca and mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of ca and mg bound as minerals in rock and released as cations in rivers the fractionation of ca and mg isotope ratios may prove useful for tracing mechanisms of chemical alteration ca isotope ratios of solute riverine ca show a greater variability than previously acknowledged the variability of ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global ca cycling if past variations in oceanic ca isotope ratios are to be of use in constraining the past carbon cycle

positive correlation between li and mg isotope ratios in the river waters of the mackenzie basin challenges the interpretation of apparent isotopic fractionation during weathering during chemical weathering magnesium mg is released by the dissolution of both carbonate and silicate sources the degree to which solute concentrations and isotopic compositions of rivers reflect the relative proportions of these two inputs or cycling by a series of processes associated with weathering is poorly constrained in the river waters of the mackenzie basin canada the mg content is high and mg isotope ratios 26mg 24mg expressed as mg 26 show in excess of one per mil variability part of this variability is attributed to the 3 range in the carbonate and silicate rocks drained despite this inherent lithological control on river water mg 26 values there is also evidence for a fractionation control a linear positive covariation between lithium 7li 6li expressed as li 7 and mg isotope ratios in the river waters of the mackenzie basin is reported this covariation is not expected because previously reported fractionation related to physicochemical processes associated with clays or oxides should induce a negative covariation with mg isotope ratios

boron and magnesium isotopic composition of seawater the isotopic systems of boron and magnesium are increasingly being used as proxies for a number of environmental variables and processes the isotopic composition of seawater for both systems plays a central role in these studies and is an important interlaboratory standard given the long residence times of both elements 10 7 years it is commonly assumed that seawater is isotopically homogenous for these systems yet no systematic studies currently exist here we present the b and mg isotopic composition of 26 28 seawater samples from a number of ocean basins that encompass a significant range in salinity 32 to 38 psu temperature 0 3 to 25 9c and water depth 0 to 1240 m we find no significant or systematic variation for either system in accordance with their long residence times we recommend that the mean values we report 11 b 39 61 0 04 2 s e n 28 25 mg 0 43 0 01 2 s e n 26 26 mg 0 82 0 01 2 s e n 26 be used in future studies involving mg and b isotopes copyright 2010 by the american geophysical union

southern continuation of the coast shear zone and paleocene strain partitioning in british columbia southeast alaska this paper documents the newly recognized southern continuation of the early tertiary coast shear zone extending its known length by 350 km to more than 1200 km three sites along the shear zone in british columbia douglas channel bella coola and machmell river have similar histories during the period ca 65 55 ma the shear zone is 2 11 km thick and is defined by well developed mylonite zones that strike northwest and dip steeply northeast motion on the shear zone was predominantly reverse with the northeast side up synkinematic plutons are common in the shear zone lower plate rocks high grade gneiss derived from an ancient continental margin assemblage show little evidence of the extensive deformation and plutonism in the shear zone north of bella coola high grade gneiss forms the upper plate but to the south weakly metamorphosed rocks of stikinia compose the upper plate geochronologic data show that the shear zone was active between ca 60 and 55 ma at douglas channel ca 62 and 56 ma at bella coola and after 66 and before 56 ma near the machmell river these features match those of the coast shear zone in southeast alaska and adjacent british columbia together the shear zones formed a continental scale reverse ductile fault in paleocene time plate reconstructions show an oblique component to the dominantly dextral transcurrent paleocene margin partitioning of this motion across the continental margin produced a regional system of strike slip faults and contraction on the coast shear zone

deformation of stratiform znpb barite deposits in the northern canadian cordillera abstract the major stratiform sediment hosted zn pb barite deposits of the northern canadian cordillera occur in the palaeozoic selwyn basin kechika trough they formed in euxinic fine grained clastic dominated extensional basins that developed within the carbonate platform of the passive margin of ancestral north america during the late jurassic to mid cretaceous these basins and the stratiform ore deposits were subjected to low greenschist facies regional metamorphism and fold and thrust deformation as they were incorporated into the western part of the rocky mountains mackenzie mountains foreland fold and thrust belt this paper summarises the results of detailed field and microscopic studies on four of these deposits the tom deposit the howards pass deposit the cirque deposit and the driftpile creek deposit the dominant mineralogy of these deposits is sphalerite galena pyrite and in some cases significant barite is present low greenschist grade metamorphism and deformation is characterised by cataclastic flow and brecciation in coarse grained massive pyritic sulphides and by folding recrystallisation and cleavage development in fine grained sulphides and barite relict primary depositional diagenetic structures are found within the orebodies deformation is characterised by strong pressure solution fabrics in barite in fine grained laminated pyrite layers and in mixed sulphide carbonate beds marked local transposition is found with significant remobilisation of galena barite and pyrite the deformation mechanics of these sulphide barite deposits are illustrated using schematic deformation mechanism maps

paleoseismological evidence from lake sediments for recent movement on the denali and other faults yukon territory canada abstract most of the pacific north america plate motion in northwestern canada and southeastern alaska occurs along coastal faults that are the inland extension of the queen charlotte dextral transform fault more ancient faults farther east such as the denali and duke river faults exhibit moderate earthquake activity and may be accommodating part of the pacific plate motion paleoseismological results based on seismic resuspension of lake sediments covering a period of up to 3000 years show that the seismicity on the denali duke river fault system is remarkably recent spanning just the last three to five centuries there is a northerly trending zone of seismicity from the active fairweather contact fault system on the coast across to the part of the denali fault that is currently as well as prehistorically active this partial transfer eastward of pacific plate motion may be analogous to a similar proposed transfer of motion northward to the denali fault from the same fairweather contact fault system farther west in alaska

characterization and quantification of active faulting in the roer valley rift system based on high precision digital elevation models the roer valley rift system rvrs is located in the northern prolongation of the upper rhine graben during the cenozoic the evolution of the rvrs was influenced by two different rift systems situated in the north and in the south the north sea rift system and the west european rift system respectively during the last decades moderate seismicity revealed the continuous activity of the graben border faults the peel boundary fault zone pbfz and the feldbiss fault zone ffz we use a high precision digital elevation model dem to characterize and quantify the present day deformation along these faults the fault pattern shows similarity to a strike slip structure however analysis of the dem reveals that the recent to quaternary formations and landforms affected by the fault activity are only vertically offset this suggests a pure normal faulting mode for the main border faults and a direction of extension perpendicular to the graben i e ne sw quantification of the offset dated markers allows the determination of the displacement rates along the fault segments in the southeastern part of the rvrs the vertical displacement rates inferred for the ffz and the southeastern pbfz range between 55 and 65 mm ky and around 65 mm ky respectively in contrast the displacement rates determined for the northwestern segment of the pbfz are around 200 mm ky we explain these differences between the northwestern and southeastern parts of the rvrs by the large scale geometry of the graben the rvrs being symmetric in the south east and asymmetric half graben in the north west the deformation is accommodated by two border faults ffz and the south eastern part of the pbfz in the south east and by only one fault in the north west the north western segment of the pbfz in addition the thickness of the neogene main depocentre in the northwestern half of the rvrs indicates a larger amount of extension in this part of the graben than in the south east the combination of the graben geometry and the amount of extension can explain the differences in the displacement rate

an overview of the nature distribution and inferred tectonic setting of granitoids in the late archaean norseman wiluna belt abstract the highly mineralized late archaean norseman wiluna belt yilgarn block western australia has lithostratigraphic associations and a structural style and history that has recently been interpreted in terms of a subduction related tectonic setting it is important to investigate whether the geochemistry and petrology of the granitoids in the belt are compatible with such a model the granitoid suite within the belt can be subdivided into association 1 large composite batholiths of calc alkaline affinity spatially and temporally associated with calc alkaline volcanic centres association 2 plutons ranging from quartz diorite and tonalite to monzogranite and syenogranite spatially related to dike swarms of felsic to intermediate porphyries and calc alkaline lamprophyres and association 3 peralkaline to alkaline batholiths and stocks which are spatially and temporally associated with rhyolitic volcanic centres and major lineaments the calc alkaline associations 1 and 2 have characteristics comparable with suites of variable fractionated i type granitoids in phanerozoic continental margin are settings including 1 a wide range in sio 2 2 meta to peraluminous compositions 3 a progression from granodiorite to granite with plutons becoming more k 2 o rich and hornblende poor with time 4 lile enrichment and relative depletion of ta nb and ti in contrast to most phanerozoic granitoids the norseman wiluna belt granitoids have highly fractionated ree patterns and low contents of yb and y with respect to average post archaean equivalents ree data and trace element characteristics suggest derivation from a mafic source leaving hornblende and or garnet as residual phases peralkaline granitoids of association 3 exhibit enrichments on sio 2 na 2 o k 2 o zr y nb and ta with respect to the calc alkaline granitoids they post date associations 1 and 2 and are consistent with the concept that they represent a continuation of magmatism in a post orogenic environment thus the nature and distribution of the granitoids of the norseman wiluna belt are consistent with interpretations that the belt formed in a subduction related convergent margin setting

investigation of magnesium isotope fractionation during basalt differentiation implications for a chondritic composition of the terrestrial mantle to investigate whether magnesium isotopes are fractionated during basalt differentiation we have performed high precision mg isotopic analyses by multi collector inductively coupled plasma mass spectrometry mc icp ms on a set of well characterized samples from kilauea iki lava lake hawaii usa samples from the kilauea iki lava lake produced by closed system crystal melt fractionation range from olivine rich cumulates to highly differentiated basalts with mgo content ranging from 2 37 to 26 87 wt our results demonstrate that although these basalts have diverse chemical compositions mineralogies crystallization temperatures and degrees of differentiation their mg isotopic compositions display no measurable variation within the limits of our external precision average 26 mg 0 36 0 10 and 25 mg 0 20 0 07 uncertainties are 2sd this indicates that mg isotopic fractionation during crystal melt fractionation at temperatures of 1055 c is undetectable at the level of precision of the current investigation calculations based on our data suggest that at near magmatic temperatures the maximum fractionation in the 26 mg 24 mg ratio between olivine and melt is 0 07 two additional oceanic basalts two continental basalts bcr 1 and bcr 2 and two primitive carbonaceous chondrites allende and murchison analyzed in this study have mg isotopic compositions similar to the kilauea iki lava lake samples in contrast to a recent report u wiechert a n halliday non chondritic magnesium and the origins of the inner terrestrial planets earth and planetary science letters 256 2007 360 371 the results presented here suggest that the bulk silicate earth has a chondritic mg isotopic composition 2007 published by elsevier b v

origins of two types of archean potassic granite constrained by mg isotopes and statistical geochemistry implications for continental crustal evolution abstract the chemical composition of the upper continental crust ucc dramatically changed from predominantly sodic to more potassic during the late archean 3000 2500 ma although how this change occurred remains poorly constrained the origins of neoarchean potassic granites may provide clues for this crucial change as their formation dominated the compositional transition of the ucc in this study we conducted high precision mg isotopic analyses of neoarchean potassic granites 2558 2520 ma and tonalite trondhjemite granodiorites ttgs 2595 2574 ma from the north china craton and statistical geochemistry in k2o na2o ratios of global archean granitoids results show that these ttgs with k2o na2o ratios of 0 61 and 26mg values of 0 39 to 0 22 were generated from heterogeneous basaltic sources in contrast potassic granites yield two distinctive groups of 26mg values and can be subdivided into i and s types the i type granite has mantle like 26mg values of 0 28 to 0 22 and weak enrichment in k2o k2o na2o 1 0 1 5 and was likely derived by partial melting of igneous sources the s type is characterized by much higher 26mg values of 0 60 to 0 91 and strong k2o enrichment k2o na2o 1 57 23 26 with geochemical characteristics suggesting derivation by partial melting of sediments based on the geochemical characteristics of the s type potassic granites in this study we sort the s type granites out of 4066 archean felsic igneous rocks worldwide their temporal distribution suggests that s type potassic granites have appeared in the middle archean 3500 3000 ma and became widespread in the late archean 3000 2500 ma but were scarce in the early archean 4000 3500 ma the onset of s type granites indicates reworking of supracrustal rocks may have started since 3500 ma and widespread s type potassic granites during 3000 2500 ma play an important role for the high k2o na2o ratio of ucc at the end of archean a mass balance calculation shows that 25 88 wt i type 4 12 wt s type potassic granite and 70 wt ttgs could increase the k2o na2o ratio of the ucc to 0 83 which is the global average value for felsic rocks at the end archean to increase k2o na2o of the archean ucc contributions made by i and s type potassic granites could probably reach 3 2 respectively consequently the evolved crustal composition from predominantly sodic to potassic results from the synergy of re working of igneous unaltered and sedimentary weathered rocks by which the former contributed more quantitatively while the latter contributed more efficiently

relationship between basement linked and gravity driven fault systems in the ukcs salt basins abstract extension of basement below significant detachment layers such as salt results in the formation of structures in the cover strata above the detachment layer in two ways thick skinned cover structures evolve to balance basement stretching these are linked to the basement faults via detachment in the salt layer or are directly linked to the basement structure depending on the amount of basement fault displacement relative to salt layer thickness basement extension also results in rotation of major basement fault blocks tilting the detachment layer and precipitating gravity driven thin skinned slip of the cover involving detached extensional faulting and folding detachment layers consisting specifically of salt play two roles in facilitating the development of thin skinned systems salt constitutes the detachment layer and also lends buoyancy to the cores of anticlinal folds reducing resistance to shortening the detachment layer may become tilted away from the basement fault scarp resulting in synthetic slip systems with the same polarity as the underlying basement fault or the detachment may be tilted toward the basement scarp giving antithetic slip systems of the opposite polarity in both cases the slides may terminate downdip in detachment folds and or reverse faults both types of thin skinned system will interact with those cover structures directly balancing basement extension and all of these components must be considered during section restoration tilt associated with 3d basement fault geometry e g relay zones may result in decoupled cover faults locally trending at high angles to the trend of underlying basement faults contrasting strain rates of basement faulting and related cover slip can lead to continuation of cover structural evolution after cessation of basement extension giving for example cover faults and folds of lower cretaceous age in the central north sea though basement extension had occurred in the upper jurassic synthetic systems may include footwall derived olistostromes emplaced above basement hangingwalls these commonly occur above basement relay ramps on segmented basin margins examples from the central graben and channel basin are discussed examples of antithetic systems from the irish sea central graben and a field example from the bristol channel basin are discussed

mg isotope fractionation during partial melting of garnet bearing sources an adakite perspective abstract garnet that has a distinctly light mg isotopic composition compared to other mafic minerals is commonly present in deep crustal and mantle sources whether partial melting of these garnet bearing sources can lead to significant mg isotope fractionation or not remains poorly understood in order to investigate mg isotope fractionation during partial melting of garnet bearing sources we report mg isotopic data of 17 well characterized low mg adakitic rocks lma from the dabie orogen central china these lma are known for derivation from mafic sources leaving eclogitic residua with variable abundance of garnet our data together with previously published results show that fresh dabie lma have homogeneous mg isotopic compositions with an average 26mg of 0 22 0 08 2sd n 18 except that two samples display higher 26mg 0 05 to 0 01 due to alteration during sericitization and chloritization of biotite the lack of correlation between 26mg and dy yb n of the dabie lma indicates an unresolvable residual garnet effect given that mg isotopic compositions of the dabie lma are identical with local mafic basement within the current precision 26mgmelt cpx is estimated to be around 0 02 limited isotope fractionation of the dabie lma could be attributed to the dilution effect of residual clinopyroxene co existing with garnet the estimated 26mgmelt cpx is then applied to predict the possible isotope fractionation that may occur during hydrous and dehydration melting of subducted oceanic crust with a lower melting temperature and more abundant garnet in the residua slab derived melts may have 26mg higher than their sources by up to 0 36 which however can be easily erased by later melt mantle interaction in this regard mg isotopes can serve as a powerful tracer of recycled supracrustal materials

using mg isotope ratios to trace cenozoic weathering changes a case study from the chinese loess plateau abstract the magnesium isotope system is a useful tracer of weathering processes due to its sensitivity to secondary mineral formation we present elemental and isotopic data in bulk loess and paleosol sediments from the luochuan profile in the chinese loess plateau clp due to a relatively constant pre weathering mineralogy changes in the sediment chemistry with depth can be interpreted to reflect changes in weathering processes that are ultimately controlled by climate we find that the luochuan sediments are all enriched in light mg relative to the upper continental crust ucc 0 22 the isotopically light signature is a result of three way mixing between mg sourced from silicate calcite and dolomite changes in 26 mg between loess and paleosol reflect the higher weathering intensities during pedogenesis and corresponding loss of easily leachable calcite enriched in light mg isotopes from the loess layers this is confirmed by the results of a simple leaching procedure in which the leachate phase contains high ca concentrations and isotopically light 26 mg values of up to 2 any isotopic fractionation of mg imparted by isotopically heavy clay mineral formation appears to be insignificant and is obscured by the presence of detrital dolomite that is resistant to acid leaching and is isotopically light the isotopic composition of mg at luochuan broadly follows the magnetic susceptibility record in that loess layers are relatively isotopically light and have lower magnetic susceptibilities than their adjacent paleosol as such the changes in 26 mg values with depth do correspond to changes in chemical weathering however the driving factor behind these changing 26 mg values is carbonate preservation which in itself is not a highly sensitive paleoclimate proxy from a global perspective the 26 mg value of luochuan loess is similar to other loess sediments which are often isotopically light relative to the ucc we have recalculated the average 26 mg value of global loess sediments to be 0 58 1 18 this reflects the fact that 26 mg values in loess are highly heterogeneous and are generally enriched in 24 mg compared to typical ucc rocks due to preservation of high amounts of isotopically light carbonate minerals for this reason loess sediments cannot be used to approximate the isotopic composition of mg in the earth s crust

magnesium isotopic systematics of the makran arc magmas iran implications for crust mantle mg isotopic balance abstract continental crust loses mg by lower crustal foundering and chemical weathering to become its intermediate silicic composition both processes should preferentially retain heavy mg isotopes in the crust yet the estimated mg isotopic ratio for the bulk continental crust is indistinguishable from that for unmodified mantle this can be elucidated by two notions that are not mutually exclusive i the basaltic parent to the crust has mg isotopes lighter than the mantle and has since become mantle like as a result of mg loss by igneous and weathering fractionation and ii heavy mg isotopes in the continental crust are constantly balanced by a hidden input of light mg isotopes arc magmas are arguably building blocks of the continental crust and their mg isotopic signature could be used to place limits on the above views here we conducted a mg isotopic study of the makran arc a rare continental arc in iran within the alpine himalayan orogenic belt the measured mg isotopic ratios for most mafic and intermediate samples are mantle like with 26mg values ranging from 0 32 to 0 20 after excluding dubious samples affected by alteration the silicic samples span a broad range of isotopic ratios with 26mg values ranging from 0 32 to 0 32 beyond that of unmodified mantle 26mg 0 25 0 04 the isotopic data form a trend from mantle like 26mg at relatively high mgo towards elevated 26mg at low mgo requiring the isotopic variability be related to differentiation in an open system involving assimilation of high 26mg crustal rocks our results are generally consistent with mg isotopic data compiled for global arcs where magmas have mg isotopic ratios ranging from mantle like to 0 7 per mil above the mantle range and compositions substantially below that range are rare thus heavy mg isotopes in the continents predicted by igneous and weathering fractionation might have been compensated by a hidden input of light mg isotopes which is presumably in the form of carbonates leading to a bulk crustal mg isotopic composition that is mantle like

forced folds and fractures this volume is concerned with defining the major similarities and difference between forced folds and buckle folds in order that these differences can be used to recognize the type of folding and therefore the expected fracture pattern present in regions of poor exposure or where the geologist has to rely on seismic images an understanding of the differences between the two fold types their 3d geometry spatial organization fracture patterns etc provides an invaluable tool for earth scientists concerned with assessing the possible role of folds and their associated fracture patterns in controlling fluid migration and concentration within the crust the papers presented here are grouped into 4 sections contributions in the first section describe the use of numerical analyses to investigate the formation of fractures in forced folds including compaction folds and the section contains a description of large scale compaction folding in which the associated fracturing has given rise to the development of large sandstone dykes the papers in the second section deal with the formation of forced folds as a result of normal faulting in various geological environments including along a classical graben margin and around a resurgent caldera the third section contains papers relating to forced folding in compressional and strike slip regimes the final section considers the temporal and spatial relationships between forced folds and buckle folds the formation of crustal scale folds and a method of determining the distribution of strain on any folded surface a key parameter controlling the distribution of fractures

lithospheric structure in northwestern canada from lithoprobe seismic refraction and related studies a synthesis the snorcle refraction wide angle reflection r war experiment snore 97 included four individual lines along the three transect corridors a combination of snore 97 results with those from earlier studies permits generation of a 2000 km long lithospheric velocity model that extends from the archean slave craton to the present pacific basin using this model and coincident near vertical incidence nvi reflection data and geological information an interpreted cross section that exemplifies 4 ga of lithospheric development is generated the velocity structural models correlate well with the reflection sections and provide additional structural compositional and thermal constraints geological structures and some faults are defined in the upper crust at a larger scale the seismic data identify a variety of orogenic styles ranging from thin to thick skinned accretion in the cordillera and crustal scale tectonic wedging associated with both paleoproterozoic and mesozoic collisions models of poisson

syn accretionary multistage assembly of an early jurassic alaskan type intrusion in the canadian cordillera u pb and 40ar 39ar geochronology of the turnagain ultramafic mafic intrusive complex yukon tanana terrane the magmatic evolution of the early jurassic turnagain alaskan type intrusion is inextricably linked to terrane accretionary events at the north american continental margin we have calibrated the

major dextral transcurrent displacements along the northern rocky mountain trench and related lineaments in north central british columbia the northern rocky mountain trench and a number of other prominent lineaments along and east of the eastern margin of the intermontane belt mark faults along which dextral transcurrent movements have been dominant offsets of shelf to off shelf facies boundaries in lower paleozoic rocks indicate a cumulative displacement of at least 750 km and probably 900 km within the system of faults related to those in the northern rocky mountain and the tintina trenches farther west another system of faults appears to offset plutons and stratigraphic assemblages along the eastern margin of the intermontane belt by as much as 300 km these faults including the kutcho and the pinchi connect in part with the teslin suture zone in yukon territory and probably with the fraser river straight creek fault zone in southern british columbia although dextral transcurrent faulting may have taken place between the middle jurassic and early cenozoic the most convincing evidence points to middle cretaceous and particularly to early cenozoic eocene displacements the eocene movements were temporally related to plutonism volcanism lamprophyre dike emplacement high heat flow sedimentation in grabens and rapid uplift of northwesterly trending elongate ranges climactic episodes of granite emplacement particularly in and near the northern omineca crystalline belt at 100 m y 70 m y and 50 m y ago may have been facilitated by changes from dominantly compressional to dominantly transcurrent and related tensional strain

structure and tectonic development of the southern rocky mountain trench the rocky mountain trench is one of the youngest most prominent and most enigmatic structures of the canadian cordillera approximately 650 km of seismic reflection data providing regional three dimensional coverage over an area of 10 000 km include six crossings of the rocky mountain trench between 49 n and 50 15 n prominent reflections from mid proterozoic moyie sills outline thrust and fold structures of a late jurassic to early cretaceous fault system that was cut by the rocky mountain trench fault in the tertiary the near basement reflections outline a 10 km high west facing basement ramp the hinge line of which spatially coincides with the rocky mountain trench in this area this ramp is part of a mid proterozoic margin upon which the belt purcell supergroup was deposited and is preserved beneath the trench during mesozoic contraction the basal detachment of the foreland belt closely followed the craton cover contact across the basement ramp thrusting ceased and extension was initiated when a culmination of thick basinal strata was juxtaposed with the basement ramp in the eocene miocene the basement ramp and the culmination above it focused stress reactivating the basal detachment and causing extensional faulting in the southern rocky mountain trench the rocky mountain trench fault may be linked via the basal detachment to the flathead fault on the east and the eocene extensional faults that flank the omineca belt on the west

magnesium isotopic variations in loess origins and implications abstract loess deposits serve as important continental archives for studying quaternary climatic variations and for estimating the average chemical composition of the upper continental crust here we report high precision mg isotopic data for 19 loess samples from china argentina and europe which were previously used to estimate the composition of the upper continental crust the results show that global loess samples have heterogeneous mg isotopic compositions with 26mg per mil deviation of the 26mg 24mg ratio from the dsm3 standard values ranging from 1 64 to 0 25 and a weighted average of 0 89 which is lighter than both crust and mantle silicates mgo content and 26mg of loess positively correlate with cao al2o3 ratio suggesting a two component mixing between carbonates and secondary silicate minerals the large variation in mg isotopic composition of loess results from a combination of factors including source heterogeneity eolian sorting during transport of the loess and chemical weathering during formation of the loess deposit our results suggest that mg isotopic composition of loess has potential to be a proxy indicator to characterize the paleoclimatic change but may not represent the average mg isotopic composition of the upper continental crust due to mixing sorting of isotopically distinct components and isotope fractionation during loess deposit formation

intensified chemical weathering during early triassic revealed by magnesium isotopes abstract marine ecosystem recovery after the latest permian mass extinction lpme was a protracted process during the early triassic 252 247 ma owing to repeated climatic and environmental perturbations chemical weathering can supply nutrients to the ocean and may have played an important role in the early triassic carbon cycle and biological recovery however only limited geochemical records of chemical weathering during the early triassic have been presented to date and the relationship between changes in weathering intensity and the slow recovery of marine ecosystems is effectively unknown here we report magnesium mg isotopic compositions of the siliciclastic components from shallow marine carbonates in two well studied upper permian lower triassic sections in iran and south china to track changes in chemical weathering intensity after the lpme both sections display a wide range of 26mg values 2 09 to 1 10 at zal 2 30 to 0 33 at zuodeng we identified two distinct stages i and ii in each section based on 26mg values and major elemental ratios variations of 26mg values in stage i changhsingian to mid dienerian are mainly controlled by mineralogical composition that can obscure weathering signals by contrast 26mg variations in stage ii upper dienerian to upper spathian are independent of lithology and inferred to reflect control by chemical weathering intensity the trends in chemical weathering intensity within stage ii correspond to first order variations in climate and carbon cycling marine ecosystem recovery during the early triassic may have been linked to recurrent episodes of intense chemical weathering caused by co2 degassing and climate warming this study demonstrates the potential utility of the mg isotopic compositions of the silicate fraction in marine carbonates as a proxy for chemical weathering intensity laying the groundwork for general applications of this method to deep time earth systems

co variation of mg and c isotopes in late precambrian carbonates of the siberian platform a new tool for tracing the change in weathering regime abstract this study reports significant mg isotopic variations in 32 samples of late precambrian dolomites from various parts of the siberian platform exhibiting a range of 13 c from 10 to 6 v pdb in the ediacaran dolomites from the n and ne part of the patom paleobasin a systematic increase of 26 mg from 2 3 0 2 to 0 8 0 3 dsm3 along with a 13 c decrease from 6 to 10 26 mg 0 072 13 c 1 83 with a correlation coefficient of r 2 0 73 n 20 are observed within the stratigraphic sequence of ediacaran and ediacaran lower cambrian transition successions all positive c isotope excursions co vary with negative mg isotope shifts and vice versa other samples from the siberian platform lay on this regression with r 2 0 66 n 28 except for 4 samples probably diagenetic dolostones that exhibit a positive 26 mg 13 c correlation the enrichment of sedimentary dolomite in heavy carbon and light mg isotopes may be interpreted as a result of a complex interplay of contrasting sedimentation conditions sea regression versus sea transgression regime yielding different weathering regimes and relative contributions of carbonate versus silicate rocks the enrichment of both oceanic water and carbonate sediments by 26 mg could occur during transgression and would coincide with sedimentation of low 13 c carbonates whereas sedimentation of the depleted in 26 mg and enriched in 13 c carbonates could happen under regression regime when exposed carbonate platforms and coastal planes are subjected to intense weathering

growth folds above propagating normal faults abstract growth folds above the upper tips of normal faults are ubiquitous in extensional settings especially during the early phases of extension and in salt rich basins as slip accumulates on the underlying normal fault the geometry and size of the fold changes these changes reflect the dip throw displacement and propagation rate of the underlying normal fault as well as the thickness and rheology of the overlying cover these changes also have a marked impact on the architecture and distribution of synkinematic sediments as well as the styles of secondary deformation accommodating strain within the growing fold here we analyse a large dataset of natural and physically and numerically modelled growth folds to i characterise their diagnostic features ii investigate the controls on their geometry size and differences and iii describe how they grow with increasing extensional strain we demonstrate that larger fault throws and a thicker and weaker cover are associated with larger growth folds in contrast small fault throws as well as thin and strong brittle cover are associated with smaller growth folds we show that the geometry and size of growth folds vary through time the width and thus the wavelength of the fold is established relatively early during fold growth whereas fold amplitude increases gradually with increasing fault throw fold width and amplitude become increasingly similar during fold evolution until the fold is breached by the underlying normal fault we also derive a number of preliminary empirical relationships between readily observable structural and stratigraphic parameters in our dataset that may help estimate the geometry and size of poorly exposed i e in the field or imaged i e in the subsurface growth folds in addition we discuss how fault growth models i e constant length vs propagating may impact the three dimensional evolution of growth folds finally our work shows that growth folds are likely more common than previously thought for example although they are well documented in areas characterised by weak ductile cover strata and low strain rates our dataset illustrates that growth folds may also occur in brittle relatively strong rocks and in regions with high strain rates however the underlying controls on fold occurrence remain elusive

crustal evolution of the eastern canadian cordillera the cordillera is not like the himalayas the himalayas are supported by the indian continent that was thrust under the asian continent the eastern cordillera is supported not by imbricated continents but by imbricated continental margin of the ancient western north america that ancient margin was compressed and imbricated as it was confined between the north american craton and the minicontinent of stikinia in the eastern canadian cordillera tectonic thickening is in the collision zone and not on either side of it as for example in the tibetan plateau where the underthrust indian continent has duplicated the thickness of the crust in the eastern canadian cordillera jurassic and early cretaceous thrusts and faults are directed away from the collision zone toward the converging blocks the oppositely directed thrust faults outline wedge shaped sheets of rock sedimentary and volcanic rocks cover the crystalline rocks on which they were deposited basement and the underlying upper mantle may have been detached from each other during the compression the basement therefore would have imbricated internally between the cover and the mantle as a duplex structure confinement of the ancient margin s basement between the colliding blocks requires that the volume of basement under the margin be the same as that presently between the colliding blocks the possibility of such an equality has been tested and confirmed in cross section the cross section of the ancient western margin from that test has many features comparable to cross sections recently drawn through the eastern continental margin of canada

paleomagnetism and geochronology of 23 ma gabbroic intrusions in the keku strait alaska and implications for the alexander terrane samples of tertiary gabbro from 24 sites in the keku strait alaska help constrain the displacement history of the alexander terrane step heating experiments on a plagioclase separate from these previously undated intrusions indicate a discordant 40ar 39ar age of 23 1 1 7 ma the characteristic magnetization resides in magnetite is easily isolated by thermal and alternating field demagnetization and has both normal and reversed polarities the mean paleomagnetic pole with no structural correction is latitude 87 1 n longitude 141 6 e a95 10 1 with n 20 sites this pole indicates insignificant tectonic displacement 0 5 8 2 southward and rotation 0 6 15 2 counterclockwise we therefore conclude that any northward displacement or vertical axis rotation of the alexander terrane and or tilting in the vicinity of the keku strait must have occurred before 23 ma

mantle heterogeneities and their significance results from lithoprobe seismic reflection and refraction wide angle reflection studiesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent lithoprobe contribution 1486 within lithoprobe s 10 transects data from more than 20 000 km of multichannel seismic mcs reflection profiling and 12 refraction wide angle reflection r war surveys were acquired while the

structural divergence and transpression in the teslin tectonic zone southern yukon territory the structural and tectonic evolution of the teslin tectonic zone a complex belt of ductilely deformed rocks forming the southern extension of the yukon tanana terrane in the northern canadian cordillera is elucidated with new field data and by the reinterpretation of existing data the zone includes greenschist to amphibolite facies metasedimentary and metaplutonic rocks of the nisutlin and anvil assemblages characterized by s and l s tectonite fabrics primary contact relationships and ages show that most rocks in the zone were contiguous by mississippian time mapping identified a number of strain domains which preserve s fabrics dipping northeasterly or southwesterly variable mineral lineation orientations and a variety of shear directions including easterly and westerly vergent thrust shear down to the east and down to the west normal shear and dextral strike slip shear regional constraints and microstructures suggest that latest ductile deformation was late triassic to early jurassic in age structural characteristics are explained most effectively by convergent dominated transpression in combination with tectonic wedging and associated back thrusting above a low angle west dipping detachment oblique eastward and upward movement of the rocks over the detachment which may have coincided with the top of thinned north american continental crust produced easterly vergent shear and localized or widespread tectonic wedging and back thrusting produced westerly vergent shear the transpressive deformation postdates formation of the nisutlin assemblage by mississippian time and subduction of the slide mountain ocean beneath the nisutlin assemblage in permian to early triassic time the western margin of the teslin tectonic zone was truncated by cretaceous strike slip faults and translated northward rocks of the peraluminous orthogneiss assemblage of the yukon tanana terrane in central yukon acted as the tectonic wedge and were overthrust by rocks of the nisutlin and anvil assemblages in early jurassic time before unroofing by local extension in cretaceous time

magnesium isotope fractionation during granite weathering abstract the mg isotope compositions of weathering profiles developed on granite and granodiorite bedrock have been analyzed to investigate mg isotope behavior during the weathering of felsic rocks in a continental environment 26mg generally exhibits a small increase from the bedrock values which range from 0 17 0 06 to 0 07 0 06 until the upper two meters in which a decrease is observed producing near surface values between 0 14 0 06 and 0 09 0 06 the small observed increase in isotopic composition below 2 m is likely due to mg loss to the hydrosphere during biotite weathering and in situ illite production while the decrease at depths

response of continental magmatic arcs to regional tectonic changes recorded by synorogenic plutons in the middle crust an example from the coast mountains of british columbia abstract the coast mountains of british columbia record an increase in magmatic activity acceleration in exhumation rate and a change from transpression to extension between 60 and 52 ma structural analysis of fabrics in three mid crustal plutons and country rocks leads to conclusions about pluton emplacement mechanisms and strain partitioning during changing tectonic conditions the quottoon and kitlope plutons 60 ma have steep foliations and lineations consistent with partitioned transpressional deformation the chief matthew s pluton 57 55 ma intruded during the formation of a sub horizontal transposition foliation and has radially distributed lineations consistent with sub vertical flattening during extension the change in orientation of the foliation represents an almost orthogonal rotation of the shortening direction from sub horizontal to sub vertical the chief mathew s pluton is interpreted to intrude initially into gently dipping fractures perpendicular to the steeply dipping foliation these melt filled fractures acted as conduits for melt triggering horizontal flow and eventually formed kilometer scale sills the steeply dipping fabrics of the coast shear zone provided pathways for melt to ascended from lower to middle crustal depths partitioning deformation into three dimensional domains of flattening simple shear and constrictional strain created the space for plutons this pattern of deformation reflects the interaction of regional deformation with magma emplacement

decoupled extensional faulting and forced folding in the southern part of the roer valley graben belgium abstract during late oligocene incipient rifting the southern part of the roer valley graben was characterized by normal faulting and forced folding of its paleogene pre rift strata the 2d seismic data used in this study shows that these faults and forced folds were geometrically decoupled from faults or fault zones in the underlying triassic and older strata geometric decoupling consistently took place in an interval that comprises latest triassic to early jurassic soft claystones on top of triassic alternations of evaporites and claystones layers this mechanically weak interval probably inhibited the upward propagation of re activated underlying faults resulting in the formation of the observed forced folds monoclines in the overlying paleogene pre rift strata strain from the sub detachment faults was distributed along the mechanically weak interval towards detachment edges leading to the consistent presence of faults in the footwall domain of the supra detachment monoclines the mechanically weak interval was thereby able to maintain the kinematic coherency between geometrically decoupled under and overlying deformation throughout late oligocene rifting

investigating the behaviour of mg isotopes during the formation of clay minerals abstract we present elemental and isotopic data detailing how the mg isotope system behaves in natural and experimentally synthesized clay minerals we show that the bulk mg isotopic composition 26 mg of a set of natural illite montmorillonite and kaolinite spans a 2 range and that their isotopic composition depends strongly on a balance between the relative proportions of structural and exchangeable mg after acid leaching these natural clays become relatively enriched in isotopically heavy mg by between 0 2 and 1 6 results of exchange experiments indicate that the mg that has adsorbed to interlayer spaces and surface charged sites is relatively enriched in isotopically light mg compared to the residual clay the isotopic composition of this exchangeable mg 1 49 to 2 03 is characteristic of the isotopic composition of mg found in many natural waters further experiments with an isotopically characterized mgcl 2 solution shows that the clay minerals adsorb this exchangeable mg with little or no isotopic fractionation although we cannot discount the possibility that the uptake of exchangeable mg does so with a slight preference for 24 mg to characterize the behaviour of mg isotopes during clay mineral formation we synthesized brucite mg oh 2 which we consider to be a good analogue for the incorporation of mg into the octahedral sheet of mg rich clay minerals or into the brucitic layer of clays such as chlorite in our experiment the brucite mineral becomes enriched in the heavy isotopes of mg while the corresponding solution is always relatively enriched in isotopically light mg the system reaches a steady state after 10 days with a final fractionation factor solid solution of 1 0005 at near neutral ph this result is consistent with the general consensus that secondary clay minerals preferentially take up isotopically heavy mg during their formation however our results also show that exchangeable mg is an important component within bulk clay minerals and can have an important influence over the bulk clay 26 mg value modeling shows that in certain soils or sediments where the percentage of exchangeable mg is 30 and the isotopic composition of the exchangeable mg is around 2 the generation of bulk 26 mg values of 26 mg values that are closer in composition to the ucc despite this results of modeling together with experimental observation suggests that the uptake of exchangeable mg into these clay rich sediments could cause a decrease in the bulk 26 mg value by up to 0 3 0 4 this should be accounted for when assessing the 26 mg value of sediments on a crustal scale

the magnesium isotope budget of the modern ocean constraints from riverine magnesium isotope ratios we have measured the magnesium mg isotope ratios in 45 rivers including 16 of the largest rivers in the world covering a range of geologic tectonic and climatic drainage basin environments the range in riverine 26mg is 2 5 half the variation in terrestrial rock although the mg isotopic composition of the source rock is important for small rivers at a global scale lithology is of limited significance for mg isotope ratios in rivers part of the variability at a global scale may be attributed to fractionation in the weathering environment the rivers analysed constitute 30 of the global mg riverine flux to the oceans and a flux weighted mg isotope composition of global runoff has been estimated at 1 09 even taking into account uncertainty this is distinct from seawater at 0 82 this difference arises either from the fractionation of mg isotope ratios in the ocean and or a mg budget which is not at steady state the difference is consistent with fractionation by carbonate precipitation in the simplest steady state scenario where the oceanic mass balance is maintained by riverine input and hydrothermal and dolomite output mg isotope ratios imply a minimum dolomite mg flux of 9 of the total output mg flux the implications of the distinct isotopic composition of the riverine flux from modern seawater is that the mg isotope ratio of seawater has probably varied outside of current analytical uncertainty mg isotope ratios may therefore provide a valuable new tracer in palaeo oceanography

transpressive inversion of a mesozoic extensional forced fold system with an intermediate decollement level in the basque cantabrian basin spain 1 in the basque cantabrian basin spain normal faulting and associated folding occurred during late jurassic to early cretaceous rifting cenozoic pyrenean thick skinned transpressive inversion in the western parts of the basin preserved the first order extensional architecture integration of geological maps and seismic profiles has permitted to fully constrain the style of extensional deformation and subsequent inversion in the western portion of the basque cantabrian basin extensional faults offset the paleozoic basement up to lower triassic rocks the presence of an efficient decollement level represented by triassic evaporites produced the decoupling between basement rocks and the upper triassic to middle jurassic prerift cover sequence extensional forced folding occurred in the cover driven by basement faulting and the migration of evaporites toward the hanging wall of the extensional faults with salt welds developing away from them upper jurassic to lower cretaceous syn rift sediments deposited synchronously with forced folding which led to the development of extensional growth geometries associated with both master faults and nearly transverse faults syn rift growth sequences are characterized by downlap and onlap relationships with the underlying prerift units interpreted as the result of along strike variations of master fault extensional displacement rate cenozoic pyrenean contraction generated the right lateral transpressive inversion of basement master faults and the almost dip slip reactivation of transverse extensional faults

magnesium isotope systematics of the lithologically varied moselle river basin france abstract magnesium and strontium isotope signatures were determined during different seasons for the main rivers of the moselle basin northeastern france this small basin is remarkable for its well constrained and varied lithology on a small distance scale and this is reflected in river water sr isotope compositions upstream where the moselle river drains silicate rocks of the vosges mountains waters are characterized by relatively high 87sr 86sr ratios 0 7128 0 7174 in contrast downstream of the city of epinal where the moselle river flows through carbonates and evaporites of the lorraine plateau 87sr 86sr ratios are lower down to 0 70824 magnesium in river waters draining silicates is systematically depleted in heavy isotopes 26mg values range from 1 2 to 0 7 relative to the value presently estimated for the continental crust and a local diorite 0 5 in comparison 26mg values measured in soil samples are higher 0 0 this suggests that mg isotope fractionation occurs during mineral leaching and or formation of secondary clay minerals on the lorraine plateau tributaries draining marls carbonates and evaporites are characterized by low ca mg 1 5 3 2 and low ca sr 80 400 when compared to local carbonate rocks ca mg 29 59 ca sr 370 2200 similar to other rivers draining carbonates the most likely cause of the mg and sr excesses in these rivers is early thermodynamic saturation of groundwater with calcite relative to magnesite and strontianite as groundwater chemistry progressively evolves in the aquifer 26mg of the dissolved phases of tributaries draining mainly carbonates and evaporites are relatively low and constant throughout the year from 1 4 to 1 6 and from 1 2 to 1 4 respectively within the range defined for the underlying rocks downstream of epinal the compositions of the moselle river samples in a 26mg vs 87sr 86sr diagram can be explained by mixing curves between silicate carbonate and evaporite waters with a significant contribution from the vosgian silicate lithologies 70 temporal co variation between 26mg and 87sr 86sr for the moselle river throughout year is also observed and is consistent with a higher contribution from the vosges mountains in winter in terms of runoff and dissolved element flux overall this study shows that mg isotopes measured in waters rocks and soils coupled with other tracers such as sr isotopes could be used to better constrain riverine mg sources particularly if analytical uncertainties in mg isotope measurements can be improved in order to perform more precise quantifications

magnesium isotope fractionation during low mg calcite precipitation in a limestone cave field study and experiments abstract the chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings despite intensive research including modelling and novel approaches speleothem data remain difficult to interpret a possible way foreword is to apply a multi proxy approach including non conventional isotope systems for the first time we here present a complete analytical dataset of magnesium isotopes 26 mg from a monitored cave in nw germany bunker cave the data set includes 26 mg values of loess derived soil above the cave 1 0 0 5 soil water 1 2 0 5 the carbonate hostrock 3 8 0 5 dripwater in the cave 1 8 0 2 speleothem low mg calcite stalactites stalagmites 4 3 0 6 cave loam 0 6 0 1 and runoff water 1 8 0 1 in the cave respectively magnesium isotope fractionation processes during weathering and interaction between soil cover hostrock and solute bearing soil water are non trivial and depend on a number of variables including solution residence times dissolution rates adsorption effects and potential neo formation of solids in the regolith and the carbonate aquifer apparent mg isotope fractionation between dripwater and speleothem low mg calcite is about 1000ln mg cc mg aq 2 4 a similar mg isotope fractionation 1000ln mg cc mg aq 2 1 is obtained by abiogenic precipitation experiments carried out at aqueous mg ca ratios and temperatures close to cave conditions accordingly 26 mg discrimination during low mg calcite formation in caves is highly related to inorganic fractionation effects which may comprise dehydration of mg 2 prior to incorporation into calcite surface entrapment of light isotopes and reaction kinetics relevance of kinetics is supported by a significant negative correlation of mg isotope fractionation with the precipitation rate for inorganic precipitation experiments

integrated study on the topographic and shallow subsurface expression of the grote brogel fault at the boundary of the roer valley graben belgium abstract the grote brogel fault gbf is a major wnw ese striking normal fault in belgium that diverges westward from the nw se striking western border fault system of the roer valley graben the gbf delimits the topographically higher campine block from the subsiding roer valley graben and is expressed in the digital terrain model dtm by relief gradients or scarps by integrating dtm electrical resistivity tomography ert cone penetration test cpt and borehole data we studied the quaternary activity of the gbf and its effects on local hydrogeology in the shallow subsurface

large scale coast parallel displacements in the cordillera a granitic resolution to a paleomagnetic dilemma abstract resolution of the paleomagnetic dilemma the discrepancy between large paleomagnetically determined dextral displacement of outboard portions of the northern cordillera and much smaller offsets implied by mapping and stratigraphic correlations is fundamental to understanding the tectonic evolution of the cordillera this paper presents structural orientation data from the middle cretaceous dawson range batholith of west central yukon and its wallrocks and suggests that some of the missing displacement may be found in intrusions the elongate northwest trending batholith has a margin parallel foliation a sub horizontal stretching lineation and records syn intrusive dextral shearing in country rocks adjacent to the batholith north trending lineations are deflected clockwise into near parallelism with the batholith s margins lineations from wallrock screens within the batholith are all aligned parallel with the batholith s long axis the big creek strike slip fault forms the north margin of the batholith and accommodated a minimum of 20 km of dextral slip these observations imply that the batholith invaded an active dextral shear zone accommodated shearing while crystallizing and focused post crystallization fault development the batholith is conservatively estimated to have accommodated 45 km of syn intrusive shearing collectively middle cretaceous intrusions of the northern cordillera may account for 400 km of previously unrecognized dextral displacement

simultaneous normal faulting and extensional flexuring during rifting an example from the southernmost upper rhine graben the southern end of the upper rhine graben urg is formed by a major continental transfer zone which was localised by the reactivation of ene oriented basement faults of late palaeozoic origin a combination of subcrop data derived from exploration wells and reflection seismic lines and palaeostress analysis provided new constraints on the timing and kinematics of interacting basement faults rifting in the southern urg began in the upper priabonian under regional wnw ese directed extension oriented roughly perpendicular to the graben axis in the study area this led to the formation of nne trending half grabens simultaneously ene trending basement faults situated in the area of the future rhine bresse transfer zone rbtz were reactivated in a sinistrally transtensive mode in the sedimentary cover the strike slip component was accommodated by the development of en echelon aligned extensional flexures flexuring and interference between the differently oriented basement faults imposed additional but locally confined extension in the sedimentary cover which deviated by as much as 90 from the regional wnw ese extension the interference of regional and local stresses led to a regime approaching radial extension at the intersection between the urg and rbtz

adsorption and desorption controlled magnesium isotope fractionation during extreme weathering of basalt in hainan island china magnesium isotopic compositions of a set of clay rich saprolites developed on the neogene tholeiitic basalt from hainan island in southern china have been measured in order to document the behavior of mg isotopes during continental weathering compared with unaltered basalts 26mg 0 36 the overlying saprolites are strongly depleted in mg i e th mg 99 1 to 92 9 and display highly variable 26mg ranging from 0 49 to 0 40 magnesium concentration and 26mg value of the saprolites display a general increasing trend upwards in the lower part of the profile but a decreasing trend towards the surface in the upper part the variations of mg concentration and isotopic composition in this weathering profile can be explained through adsorption and desorption processes 1 adsorption of mg to kaolin minerals kaolinite and halloysite with preferential uptake of heavy mg isotopes onto kaolin minerals and 2 desorption of mg through cation exchange of mg with the relatively lower hydration energy cations in the upper profile evidence for adsorption is supported by the positive correlation between 26mg and the modal abundance of kaolin minerals in saprolite of the lower profile while negative correlations between 26mg and concentrations of lower hydration energy cations e g sr and cs in the upper profile support the desorption process our results highlight that adsorption and desorption of mg on clay minerals play an important role in behavior of mg isotopes during extreme weathering which may help to explain the large variation in mg isotopic composition of river waters

constraining magnesium cycling in marine sediments using magnesium isotopes abstract magnesium concentrations in deep sea sediment pore fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust to better characterize and differentiate these processes we have measured magnesium isotopes in pore fluids and sediment samples from ocean drilling program sites 1082 1086 1012 984 1219 and 925 that span a range of oceanographic settings at all sites magnesium concentrations decrease with depth at sites where diagenetic reactions are dominated by the respiration of organic carbon pore fluid 26mg values increase with depth by as much as 2 because carbonates preferentially incorporate 24mg low 26mg the increase in pore fluid 26mg values at these sites is consistent with the removal of magnesium in mg carbonate dolomite in contrast at sites where the respiration of organic carbon is not important and or weatherable minerals are abundant pore fluid 26mg values decrease with depth by up to 2 the decline in pore fluid 26mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore fluid the identity of this enriched magnesium sink is likely clay minerals using a simple 1d diffusion advection reaction model of pore fluid magnesium we estimate rates of net magnesium uptake removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites independent estimates of magnesium isotope fractionation during dolomite precipitation from measured 26mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites suggesting that local exchange of magnesium between sediment and pore fluid at these sites can be neglected our results indicate that the magnesium incorporated in dolomite is 2 0 2 7 depleted in 26mg relative to the precipitating fluid assuming local exchange of magnesium is minor at the rest of the studied sites our results suggest that magnesium incorporated into clay minerals is enriched in 26mg by 0 to 1 25 relative to the precipitating fluid this work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources sinks in low temperature aqueous systems

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

the major ion δ44 40ca δ44 42ca and δ26 24mg geochemistry of granite weathering at ph 1 and t 25 c power law processes and the relative reactivity of minerals abstract we dissolved boulder creek granodiorite in a plug flow reactor for 5794 h at ph 1 and t 25 c the primary purpose of the experiment was to identify controls on dissolved 44 40ca 44 42ca and 26 24mg values during granite weathering herein we also examine the origin of ca and mg isotopic variability among minerals composing the boulder creek granodiorite and we constrain fundamental characteristics of granite weathering important for quantifying the elemental and isotopic geochemistry of the reactor output nine ca bearing minerals display an 8 80 range of 44 40ca values and a 0 51 range of 44 42ca values three mg bearing minerals display a 1 53 range of 26 24mg values these ranges expressed at the mineralogical scale are higher than the ranges thus far reported for bulk igneous rocks most of the 44 40ca variability reflects 40ca enrichment in k feldspar and to a lesser extent biotite due to the radioactive decay of 40k over the 1 7 ga age of the rock whereas the entire range of 44 42ca values reflects mass dependent isotope fractionation during igneous differentiation and crystallization the range of 26 24mg values may represent either fractionation during the chloritization of biotite or interaction of the boulder creek granodiorite with mg rich metamorphic fluids having low 26 24mg values the elemental and isotopic composition of the reactor output varied substantially during the experiment we synthesize the mineralogical and fluid data using coupled mass conservation equations solved at non steady state model calculations reveal an intricate balance between increasing specific surface area and decreasing mineral concentrations while surface area normalized dissolution rate constants were time invariant specific surface area increased as a power law function of time through positive feedbacks between mechanical disaggregation chemical dissolution and mineral depletion variations in dissolved 44 40ca 44 42ca and 26 24mg values reflect conservative mixing rather than fractionation apatite and calcite initially control 44 40ca and 44 42ca values followed by biotite titanite epidote hornblende and plagioclase the release of radiogenic 40ca clearly defines the period where biotite dissolution dominates the brucite layer of chlorite initially controls 26 24mg values followed by biotite the tot layer of chlorite and hornblende through direct isotopic tracking these results demonstrate that trace minerals such as apatite and calcite in the case of ca and brucite in the case of mg dominate elemental release during the incipient stages of granite weathering the results further show that biotite dissolution dominates the middle stages of granite weathering and that plagioclase dissolution only becomes important during relatively late stages the ca and mg isotope variations associated with these stages are distinct and potentially resolvable in soil mineral weathering studies

lithium magnesium and silicon isotope behaviour accompanying weathering in a basaltic soil and pore water profile in iceland this study presents lithium magnesium and silicon isotope ratios from pore waters and soils from a well characterised histic andosol in south west iceland the soil 7li composition ranges between values slightly lighter than basalt to those that are much heavier 1 1 to 26 8 and are possibly influenced by sea salt in contrast precipitation corrected dissolved pore water 7li values 1 8 10 0 appear to reflect preferential adsorption of 6li onto secondary minerals where allophane supersaturation results in high 7li values conversely low 7li together with high li are probably due to destabilisation of allophane at low ph and thus desorption of li when compared to icelandic river values it would appear that soil pore waters reflect an intermediate isotope composition between basalts and river waters precipitation corrected pore water mg isotope ratios 26mg range between 0 46 and 0 12 and correlate with the amount of heavy mg adsorbed onto the soil exchange complex silicon isotopes in the soils are isotopically lighter 30si 0 91 to 0 53 than basalt 0 29 whereas pore waters are heavier 0 13 to 1 03 soil 30si values show a clear evolution between unweathered basalt and a hypothetical isotopically light endmember representing secondary minerals dissolved si isotopes also respond to chemical weathering processes and show that isotopically heavy 30si corresponds to high cation fluxes and high secondary mineral formation however comparison of all these proposed isotopic weathering tracers suggests that they respond differently to the same chemical weathering conditions this indicates a differing behaviour during secondary mineral neoformation or adsorption depending on whether the incorporated element is a major or trace constituent in turn this behaviour can potentially yield important information on secondary mineral behaviour and destabilisation and thus on the chemical weathering processes

influence of basement faults on the development of salt structures in the danish basin scaled centrifuged models based on interpreted seismic profiles were used to simulate the salt structures in the danish basin the models consisted of a viscous layer simulating rock salt overlain by layers of cohesive sand that mimic brittle failure in natural overburdens model results support the hypothesis that basement faults trigger the growth of many salt diapirs in the danish basin in plan view model diapirs are localized as buoyant walls along faults in the overburden which collapsed due to basement fault movement with further burial single diapirs rose from these buoyant walls some of the diapirs surfaced through the faulted overburden a few were trapped beneath the competent cretaceous units and others were starved due to lack of supply from below in section model diapirs were asymmetric and were rooted on the tip of the faults or located on the rotated footwall blocks model diapirs not located over faults were triggered by differential loading and collapsing of overburden layers due to basement block rotation models show that the presence of fewer salt structures in the northern than the southern part of the basin is due to differential subsidence and flow of buoyant material from north to south where they rise diapirically in the model the pressure due to loading by overburden layers changed from 1 3 102 pa in the south to 2 3 102 pa in the northern part of the basin in general the model results illustrate that basement faults influence diapirs by accumulating buoyant material in half grabens and introducing a slope and steps at the base of the buoyant layer weakening its overburden and causing differential loading and compaction the spreading overhangs of salt diapirs beneath the cretaceous chalk in the danish basin would seal any potential pre cretaceous reservoir rocks

pressure temperature and tectonic evolution of triassic lawsonite aragonite blueschists from pinchi lake british columbia a blueschist and eclogite terrane is associated with one of the largest faults in the canadian cordilleran orogen the pinchi fault blueschists in situ and retrogressed eclogite blocks occur along the pinchi fault zone near 54 30 n and 124 w critical blueschist facies mineral assemblages include lawsonite glaucophane jadeite lawsonite glaucophane quartz and aragonite white mica 40ar 39ar spectra on blueschist and eclogite yield ages in the range 221 8 1 9 to 223 5 1 7 ma establishing a direct link between the blueschists and eclogites preservation of aragonite sets rigid constraints on the pressure temperature fluid time conditions of unroofing k ar dates indicate that this is some of the oldest documented metamorphic aragonite comparison with computed petrogenetic grids suggests that metamorphic temperatures were in the range 200 300 c with pressures greater than 8 10 kbar 1 kbar 100 mpa unroofing likely occurred during collision of the cache creek terrane with quesnellia in the lat

lower crustal deformation during terrane dispersion along strike slip faults abstract after accretion terranes caught between obliquely converging plates are often displaced several hundreds of kilometers how this motion is accommodated at depth is not well known spurred by our trans alaskan crustal transect tact seismic studies of the yukon tanana terrane alaska i propose that the lower crust responds ductilely to accommodate differential movement between upper crust and upper mantle during margin parallel translation the locations of these ductile regions are dependent on temperature composition and strain rate for geotherms based on moderate surface heat flow 60 90 mwm 2 and reasonable geological strain rates 100 in 3 ma typical lower crustal rocks can deform ductilely the resultant subhorizontal layered fabric should be detectable with seismic techniques two terranes that have experienced hundreds of kilometers of margin parallel translation the yukon tanana terrane sampled by our seismic refraction wide angle reflection profiles and the salinian block california sampled by both seismic refraction and reflection have reflective lower crust both of these terranes have moderate to high surface heat flows have relatively thin crust and are interpreted seismically to be underlain by a quartzo feldspathic lower crust conditions favorable for ductile deformation in the lower crust interpretations of lower crustal layering for these two terranes is based on an increase in lower crustal reflectivity this layering is attributed to tectonic banding consistent with ductile deformation

reply to comment on paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia by p j wynne et al and on paleomagnetism of the spences bridge group and northward displacement of the intermontane belt british columbia a second look by e irving et al

magnesium isotope fractionation during shale weathering in the shale hills critical zone observatory accumulation of light mg isotopes in soils by clay mineral transformation abstract magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments but few have focused on the mechanisms that control mg isotope fractionation during shale weathering in this study we focus on understanding mg isotope fractionation in the shale hills catchment in central pennsylvania mg isotope ratios were measured systematically in weathering products along geochemical pathways of mg during shale weathering from bedrock to soils and soil pore water on a planar hillslope and to sediments stream water and groundwater on a valley floor significant variations of mg isotopic values were observed 26mg values 0 6 to 0 1 of stream and soil pore waters are about 0 5 to 1 lighter than the shale bedrock 26mg values of 0 4 consistent with previous observations that lighter mg isotopes are preferentially released to water during silicate weathering dissolution of the carbonate mineral ankerite depleted in the shallow soils but present in bedrock at greater depths produced higher mg2 concentrations but lower 26mg values 1 1 in groundwater 1 5 lighter than the bedrock 26mg values 0 2 to 0 4 of soil samples on the planar hillslope are either similar or up to 0 2 lighter than the bedrock hence a heavy mg isotope reservoir complementary to the lighter mg isotopes in soil pore water and stream water is missing from the residual soils on the hillslope in addition soil samples show a slight but systematic decreasing trend in 26mg values with increasing weathering duration towards the surface we suggest that the accumulation of light mg isotopes in surface soils at shale hills is due to a combined effect of i sequestration of isotopically light mg from soil water during clay dissolution precipitation reactions and ii loss of isotopically heavy particulate mg in micron sized particles from the hillslope as suspended sediments this latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation stream sediments 26mg values of 0 3 to 0 5 accumulated on the valley floor are 0 2 heavier than the bedrock and are thus consistent with that mobile particulates are the heavy mg isotope reservoir our study provides the first field evidence that changes in clay mineralogy lead to accumulation of lighter mg isotopes in residual bulk soils this example also demonstrates that transport of isotopically distinct fine particles from clay rich systems could be a new and important mechanism to drive the mg isotope compositions of silicate weathering residuals this mechanism drives fractionation in an opposite direction as might be expected from previous studies i e residual soils are driven to lighter mg values and sediments become isotopically heavier

magnesium retention on the soil exchange complex controlling mg isotope variations in soils soil solutions and vegetation in volcanic soils iceland understanding the biogeochemical cycle of magnesium mg is not only crucial for terrestrial ecology as this element is a key nutrient for plants but also for quantifying chemical weathering fluxes of mg and associated atmospheric co2 consumption requiring distinction of biotic from abiotic contributions to mg fluxes exported to the hydrosphere here mg isotope compositions are reported for parent basalt bulk soils clay fractions exchangeable mg seasonal soil solutions and vegetation for five types of volcanic soils in iceland in order to improve the understanding of sources and processes controlling mg supply to vegetation and export to the hydrosphere bulk soils 26mg 0 40 0 11 are isotopically similar to the parent basalt 26mg 0 31 whereas clay fractions 26mg 0 62 0 12 exchangeable mg 26mg 0 75 0 14 and soil solutions 26mg 0 89 0 16 are all isotopically lighter than the basalt these compositions can be explained by a combination of mixing and isotope fractionation processes on the soil exchange complex successive adsorption desorption of heavy mg isotopes leads to the preferential loss of heavy mg from the soil profile leaving soils with light mg isotope compositions relative to the parent basalt additionally external contributions from sea spray and organic matter decomposition result in a mixture of mg sources on the soil exchange complex vegetation preferentially takes up heavy mg from the soil exchange complex 26mgplant exch 0 50 0 09 and changes in 26mg in vegetation reflect changes in bioavailable mg sources in soils this study highlights the major role of mg retention on the soil exchange complex amongst the factors controlling mg isotope variations in soils and soil solutions and demonstrates that mg isotopes provide a valuable tool for monitoring biotic and abiotic contributions of mg that is bioavailable for plants and is exported to the hydrosphere

investigation of magnesium isotope fractionation during granite differentiation implication for mg isotopic composition of the continental crust abstract high precision mg isotopic analysis was performed on a suite of well characterized i type granitoids and associated hornblende and biotite minerals from the dabie orogen in central china to address the behavior of mg isotopes during granite differentiation although these granitoids formed through different degrees of partial melting and fractional crystallization with large variations in elemental and mineral compositions their 26 mg values vary from 0 26 to 0 14 and are indistinguishable within our analytical precision 0 07 2 sd coexisting hornblendes and biotites in these granitoids display similar mg isotopic composition with 26 mg ranging from 0 31 to 0 14 in hornblendes and 0 23 to 0 12 in biotites the inter mineral fractionation factors 26 mg hbl bt 26 mg hbl 26 mg bt vary from 0 10 to 0 02 with an average 0 06 0 08 2 sd the limited inter mineral fractionation agrees with the theoretic prediction that mg cations in both hornblende and biotite are octahedrally coordinated with oxygen which restricts the magnitude of equilibrium isotope fractionation overall data from both bulk granitoids and associated mineral separates suggest that mg isotope fractionation during i type granite differentiation is limited collectively granitoids studied here have mg isotopic composition similar to that of terrestrial basalts and peridotites 26 mg 0 21 0 07 vs 0 25 0 07 2 sd confirming that magmatic processes do not significantly fractionate mg isotopes the continental crust in the dabie orogen as sampled by these i type granitoids has a mantle like mg isotopic composition given that significant mg isotope fractionation occurs during chemical weathering processes mg isotopes may potentially be used for tracing granite genesis in particular if sedimentary materials are involved in granite sources

cenozoic right lateral wrench tectonics in the western pyrenees spain the ubierna fault system abstract a study of macro and mesostructural deformation patterns of the southern margin of the cantabrian area western pyrenees spain has revealed a complex cenozoic tectonic framework right lateral tectonics reactivated inherited wnw ese striking faults which developed during late paleozoic and early triassic events and late jurassic to early cretaceous main rifting stage the ubierna fault represents the southern boundary of the mesozoic basin during the oligocene even eocene to present day deformation this fault and the ventaniella fault located to the south in the study area acted as right lateral slightly transpressive elements forming a 120 km long and 15 km wide overstep area here named ubierna fault system where the cumulative right lateral displacement exceeds 15 km the cenozoic tectonic framework of the ubierna fault system includes reactivation along the wnw ese faults development of negative and mostly positive flower structures branch faults strike slip duplexes and releasing and restraining bends ne sw to ene wsw striking reverse faults and contractional horsetail terminations and nnw sse striking normal faults and joints are produced by the wnw ese right lateral strike slip motion the extensional elements are well developed and deformation progression implied their incorporation in the strike slip system as right lateral faults forming part of strike slip duplexes the abundance of flower structures striking wnw ese and paralleling the main strike slip faults together with the overall uplift of the overstep area testifies for a slight compressional component at a regional scale the ubierna fault system represents the most prominent element of a cenozoic transpressional belt which incorporates the western portion of the basque cantabrian basin and the asturian massif area lateral transition between this transpressive belt and the dip slip belt located to the east occurs across an area experiencing along strike shortening which developed to accommodate the eastward extrusion of the transpressional belt

displacement rates of normal faults previous estimates of displacement rates on individual faults have been limited to neotectonic faults and averaged over time intervals of about 200 kyr or less1 2 3 4 5 these estimates have been highly variable which has led to a belief that longer term displacement rates on individual faults are likely to be variable as well here we report estimates of long term normal fault displacement rates averaged over time intervals ranging from 1 to 40 myr and based on observed decreases in displacement of progressively younger horizons intersected by syn sedimentary faults we find that displacement rates are remarkably stable over these longer time periods and within a given fault system the rates are strongly dependent on the relative size of the fault as measured by cumulative vertical displacement taken together these results indicate that faults become large relative to nearby faults by having higher displacement rates even when small rather than as a consequence of having been active for longer our analyses also show that high regional strain rates tend to be accommodated by high fault displacement rates rather than high fault densities

active normal faulting beneath a salt layer an experimental study of deformation patterns in the cover sequence scaled experimental models show that the presence of a viscous layer such as salt facilitates the development of extensional forced folds above active normal faults the geometries of the extensional forced folds and their associated secondary fault patterns depend on the thickness and viscosity of the viscous layer the thickness of the cover sequence the strength and ductility of the cover sequence and the magnitude and rate of displacement on the underlying master normal fault increasing the thickness of the viscous layer and the cohesive strength and ductility of the overburden enhances the decoupling between the deep and shallow deformation alternatively increasing the viscosity of the viscous layer the thickness of the overburden and the magnitude and rate of displacement on the master normal fault reduces the decoupling between the deep and shallow deformation enhanced decoupling facilitates the formation of broad extensional forced folds and the development of detached secondary faults both near and far from the master normal faults the model predicted deformation patterns closely resemble those observed in the gulf of suez the haltenbanken area of offshore norway and the jeanne d arc basin of the grand banks offshore southeastern canada

paleomagnetic data from alaska reliability interpretation and terrane trajectories virtually the entire body of paleomagnetic data collected from southern alaska depicts a clear decrease in paleolatitude with increasing age strongly suggesting that southern alaska represents a displaced terrane in this paper paleomagnetic studies from southern alaska have been classified with respect to a quality index that is based on four criteria these criteria are the presence of both polarities of magnetic remanence stepwise thermal or alternating field demagnetization of specimens principal component analysis of demagnetization data and a successful fold test of the 51 studies compiled only four from southern alaska and one from western canada are demonstrated to satisfy all criteria and fall therefore in the category of group 3 highly reliable two studies from southern alaska satisfy three of the four criteria lacking both polarities of characteristic remanence and are judged to be of group 2 reliable two of these paleomagnetic studies constrain the accretion time of the southern alaska terrane to the relatively stationary region of central alaska north of the denali fault four paleomagnetic studies from the southern alaska terrane show a distinct paleolatitude anomaly when compared with their expected paleolatitudes from the north american apparent polar wander path using the model of engebretson et al 1984 a series of models are presented to best fit these highly reliable and reliable paleomagnetic studies the model preferred in this article assumes an accretion time with north america of 50 ma and documents pre 50 ma displacement of the southern alaska terrane on the kula plate if the ghost rocks paleomagnetic magnetizations plumley et al 1983 are assumed to be of earliest tertiary age this model fits all of the low paleolatitudes observed in southern alaska models incorporating coastwise translation of the southern alaska terrane along the western boundary of the north america plate and a 50 ma suturing age of this terrane to north america do not satisfy the paleomagnetic constraints four of the reliable studies can be modeled assuming strike slip motion of the southern alaska terrane along the western boundary of the north america plate if a 40 ma suturing age is assumed in this model however the paleomagnetic data must be presumed to represent the highest permissible paleolatitude and the ghost rocks formation paleolatitudes are problematically lower than is possible to model with coastwise translation

high temperature inter mineral magnesium isotope fractionation in eclogite from the dabie orogen china abstract to investigate the magnitude and mechanism of inter mineral mg isotope fractionation at high temperatures we report high precision analyses of mg isotopes for 10 whole rocks and 13 mineral separates for a set of eclogites from bixiling in the dabie orogen china magnesium isotopic compositions of whole rocks 26mg of 0 44 to 0 26 are similar to the estimated 26mg values of the mantle suggesting mg isotopic inheritance from a gabbroic protolith with limited mg isotope fractionation during eclogite facies metamorphism by contrast mineral separates are highly heterogeneous with 26mg values ranging from 0 30 to 0 60 in phengite from 0 16 to 0 40 in omphacite and from 0 95 to 0 74 in garnet phengite and omphacite are 1 heavier in 26mg than coexisting garnet indicating large high temperature inter mineral mg isotope fractionations the constant 26mgomphacite garnet 26mgomphacite 26mggarnet value 1 14 0 04 together with homogeneous mineral chemistry and equilibrium oxygen isotopic partitioning between omphacite and garnet suggests an equilibrium mg isotope fractionation controlled by the difference in coordination number of mg between omphacite six and garnet eight the 1 14 fractionation is the largest high temperature equilibrium inter mineral mg isotope fractionation observed so far and makes the omphacite garnet mg isotope fractionation a potential geothermometer by contrast mg isotope fractionations between phengite and garnet and between phengite and omphacite vary from 1 25 to 1 47 and from 0 14 to 0 32 respectively this implies mg isotopic disequilibria between phengite and garnet omphacite which might result from the mg isotopic variation in phengites due to mg isotope exchange between phengites and retrograde fluids in bixiling eclogites

mechanisms of magnesium isotope fractionation in volcanic soil weathering sequences guadeloupe magnesium mg stable isotopes are increasingly used as a weathering proxy in soils and rivers but the impact of the mineralogy of secondary phases on isotopefractionation remains obscure a better understanding of the behaviour of mg isotopes during weathering processes is a mandatory step toward deployment of this new tracer for understanding chemical fluxes exported from the critical zone here we investigate isotopic variations in 26mg in bulk soils and clay fractions relative to their parent andesite in three soilweatheringsequences from guadeloupe formed under contrasting climatic conditions soils formed in drier conditions low precipitation contain smectite whereas soils formed under wet conditions high rainfall are characterized by halloysite and fe oxides or kaolinite all clay fractions have mg isotopic compositions 26mg 0 41 to 0 10 similar to or heavier than their parent andesite 26mg 0 47 supporting the preferential incorporation of heavy mg isotopes in secondary mg bearing clay minerals with the first direct measurements on clay fractions soils with lighter mg isotope compositions have greater quantities of exchangeable mg the data support a contribution from sea spray to the exchangeable mg pool correlated to the soilweathering degree this study highlights for the first time that the soil 26mg not only depend on 26mg of the parent rock and on any fractionation that might occur but also on the mg retention on the exchange complex which could in turn be controlled by external inputs such as sea spray