quaternary redox transitions in deep crystalline rock fractures at the western margin of the greenland ice sheet when planning for long term deep geological repositories for spent nuclear fuel knowledge of processes that will influence and change the sub surface environment is crucial for repositories in nor

detecting the near surface redox front in crystalline bedrock using fracture mineral distribution geochemistry and u series disequilibrium oxidizing conditions normally prevail in surface waters and near surface groundwaters but there is usually a change to reducing conditions in groundwater at greater depth dissolved o2 originally present is consumed through biogenic and inorganic reactions along the flow paths fracture minerals participate in these reactions and the fracture mineralogy and geochemistry can be used to trace the redox front an important task in the safety assessment of a potential repository for the disposal of nuclear waste in crystalline bedrock at an approximate depth of 500 m in sweden is to demonstrate that reducing conditions can be maintained for a long period of time oxygen may damage the cu canisters that host nuclear waste additionally in the event of a canister failure oxidizing conditions may increase the mobility of some radionuclides the present study of the near surface redox front is based on mineralogical redox sensitive minerals geochemical redox sensitive elements and u series disequilibrium investigations of mineral coatings along open fractures the fractures have been sampled along drill cores from closely spaced 100 m deep boreholes which were drilled during the site investigation work in the laxemar area south eastern sweden carried out by the swedish nuclear fuel and waste management co skb the distribution of the redox sensitive minerals pyrite and goethite in open fractures shows that the redox front switch from mainly goethite to mainly pyrite in the fractures generally occurs at about 15 20 m depth calcite leaching by recharging water is indicated in the upper 20 30 m and positive ce anomalies suggest oxidation of ce down to 20 m depth the u series radionuclides show disequilibrium in most of the samples indicating mobility of u during the last 1 ma in the upper 20 m u is mainly removed due to oxidation or has experienced complex removal and or deposition at depths of 35 55 m both deposition and removal of u are indicated below 55 m recent deposition of u is generally indicated which suggests removal of u near surface oxidation and deposition of u below the redox front scattered goethite occurrences below the general redox front down to ca 80 m and signs of u removal at 35 55 m mostly correlate with sections of high transmissivity and or high fracture frequencies this shows that highly transmissive fractures are generally required to allow oxygenated groundwaters at depth greater than ca 30 m removal of u oxidation below 55 m within the last 300 ka is not observed although penetration of glacial waters to great depths has been confirmed in the study area their potential o2 load seems to have been reduced near the surface

early miocene change in direction of least principal stress southwestern united states conflicting inferences from dikes and metamorphic core detachment fault terranes previous investigations have suggested that the direction of least principal stress in the western united states has shifted from northerly to easterly between 50 to 30 ma an effort is made to more tightly constrain the time of this shift using igneous dike host rock systems in which the isotopic age of both host and dike is known and differs by less than 10 ma in such systems the possibility that magma intruded along old fractures unrelated to the state of stress at the time of intrusion is minimized and the axis of least principal stress is considered to be normal to the strike of the subvertical dike during the time interval between the ages of the dike and host rock these systems indicate that a northerly least principal stress orientation persisted more recently than previously believed until probably after 30 ma possibly about 26 ma and conceivably as recently as 18 ma in some areas the data are insufficient and too loosely bracketed in time to decide how fast the change in state of stress occurred in any one place and if by systematic rotation or abrupt shift however the change could be diachronous earlier in the southeast and later in the northwest major easterly trending magmatic and tectonic elements of tertiary age in the great basin of utah and nevada are compatible with the dike data however numerous metamorphic core complexes and detachment fault terranes seem to indicate more or less easterly extension throughout the time period represented by the dikes this inconsistency is not clearly resolvable but may reflect differences between the regional plate stress field and gravitational stresses in a crust overthickened by mesozoic compression

does subduction polarity control metallogeny the mediterranean case the distribution of ore deposit types in different sectors of the circum mediterranean realm that have been affected by subduction processes since the cretaceous varies in space and time sectors involved in w directed subduction sardinia apennines maghrebides internal betics tyrrhenian western eastern carpathians are dominated by relatively low sulphidation epithermal vms deposits orogens formed by ne directed subduction dinarides hellenides pontides anatolides taurides dhpat were initially dominated by pluton related porphyry skarn high sulphidation epithermal associations these distinct metallogenic styles can be related to the systematic tectono magmatic asymmetry of e ne and w directed subduction systems and are analogous to the relationship observed in circum pacific belts exceptions to this simple pattern occurred in the dhpat in the cenozoic when deposit associations typical of both e directed and w directed systems were formed such exceptions are interpreted to reflect superimposition of contrasting subduction trends and inheritance from earlier metallogenic stages apuseni or the interference of subduction processes with subduction unrelated extension hellenides west anatolia

dyke intrusion into a pre existing joint network the aiguablava lamprophyre dyke swarm catalan coastal ranges abstract a structural analysis has been performed in the upper permian lamprophyric dyke swarm of aiguablava costa brava ne iberia emplacement of sub vertical dykes is related to the presence of a widespread joint network mostly developed during the cooling and decompression of the late variscan leucogranitic host rocks the joint pattern consists on multiple sub vertical joint sets among which two orthogonal sets at n23 and n113 are predominant a sub horizontal set is also prominent the sub vertical dykes have a mean n100 n125 trend which corresponds to the trend of one of the main joint sets however dyke segmentation is noticeable at the dm to cm scale and this is inferred to be related to dyke propagation and emplacement along the variably oriented pre existing joints a mean robust sub horizontal nne ssw net dilation direction was measured from matching dyke jogs markers in the host rock and analysis of maximum dyke thicknesses and this is in line with the minimum principal stress axis 3 derived from a three dimensional paleostress analysis from dyke orientations the inferred maximum principal stress axis 1 is sub vertical indicating that dykes intruded under conditions of tectonic extension furthermore a mohr construction allowed calculation of the stress ratio 0 29 close to a prolate stress ellipsoid and a driving pressure ratio r 0 27 which corresponds to a magmatic pressure almost equal to the intermediate principal stress axis 2 it is inferred that many of the pre existing joint sets were exploited by the magma the wnw ese joint set normal to 3 being the most favorable for dyke emplacement the present study highlights the structural control of a pre existing fracture network on emplacement of the aiguablava lamprophyres in the upper crust during late permian nne ssw brittle extension

contractional tectonics and magma paths in volcanoes abstract this study aims to contribute a possible explanation for magma migration within volcanoes located in contractional tectonic settings based on field data and physically scaled experiments the data demonstrate the occurrence of large stratovolcanoes in areas of coeval reverse faulting in spite of the widely accepted idea that volcanism can develop only in extensional transcurrent tectonic settings the experiments simulate the propagation of deformation from substrate reverse faults with different attitudes and locations into volcanoes the substrate fault splits into two main shear zones within the volcano a shallow dipping one with reverse motion propagates towards the lower volcano flank and a steeper dipping one with normal motion propagates upwards in plan view the reverse fault zone is arcuate and convex outwards whereas the normal fault zone is rectilinear structural field surveys at volcanoes located in contractional settings show similar features the plio quaternary trohunco and los cardos centinela volcanic complexes argentina lie above plio quaternary reverse faults the late pleistocene holocene el reventador volcano ecuador is also located in a coeval contractional tectonic belt these volcanoes show curvilinear reverse faults along one flank and rectilinear extensional fracture zones across the crater area consistent with the experiments these data consistently suggest that magma migrates along the substrate reverse fault and is channelled along the normal fault zone across the volcano

development of the continental margins of the labrador sea a review abstract the labrador sea is a small oceanic basin that developed when the north american and greenland plates separated an initial period of stretching in early cretaceous time formed sedimentary basins now preserved under the continental shelves and around the margins of the oceanic crust the basins subsided thermally during late cretaceous time and a second episode of tectonism took place during latest cretaceous and early paleocene time before the onset of sea floor spreading in mid paleocene time around the northern labrador sea davis strait and in southern baffin bay voluminous picrites and basalts were erupted at and shortly after the commencement of sea floor spreading volcanism occurred again in early eocene time at the same time as sea floor spreading commenced in the northern north atlantic farther southeast along the labrador and southern west greenland margins oceanic crust is separated from continental crust by highly stretched but non magmatic transition zones which developed before sea floor spreading a complex transform zone which developed during sea floor spreading in late paleocene and early eocene time separates continental and oceanic crust along the baffin island margin the greenland and labrador ocean continent transitions are asymmetric across the only available conjugate cross sections however a cross section through the labrador margin farther north resembles the greenland cross section in the conjugate pair more than it does the labrador cross section of this pair consideration of the geological history of the area suggests that the non magmatic transition zones may have formed by slow extension of a few millimetres per year through a period of 53 ma during cretaceous and early paleocene time

u pb zircon age of the southwest lobe of the havre saint pierre anorthosite complex grenville province canada u pb zircon dating of the southwest lobe of the havre saint pierre anorthosite intrusion indicates that it is 1062 4 ma old parallelism of magmatic and solid state foliations with the adjacent abbe huard lineament suggest that anorthosite parental magmas rose up this shear zone which was active at that time the age of igneous crystallization is much younger than that of a spatially associated mangerite intrusion but accords with age data from other granitoid intrusions elsewhere in the grenville province evidence points to a widespread 1 09 1 05 ga magmatic event that included massif anorthosite intrusions this magmatic event coincided with late stage convergent tectonics in the southwestern grenville province

geodynamic controls on tertiary arc magmatism in ecuador constraints from u pb zircon geochronology of oligocene miocene intrusions and regional age distribution trends abstract we obtained u pb zircon ages of late tertiary intrusions in the northern andes to provide robust time calibration points for the intrusive geochronologic framework of ecuador which is mostly based on k ar data magmatism associated with intrusion emplacement ranges from 31 to 7 ma with ages mainly pooling in the late oligocene early miocene where both k ar and u pb data exist for a given intrusive system ages obtained by the different methods are usually concordant within 1 4 m y suggesting that k ar ages are accurate indicators for the timing of tertiary arc magmatism on a regional multi m y scale except for a single sample the investigated intrusions completely lack externally inherited zircons in agreement with dominantly zircon poor oceanic crustal basement domains spatio temporal distribution trends of tertiary arc magmatism inferred from screened u pb k ar and zircon fission track geochronologic data allow tracking of the progressive along arc broadening of a flat slab segment below southernmost ecuador in the mid to late miocene and of moderate slab shallowing in northern central ecuador in the late miocene these regional arc migration patterns correlate in time with the subduction of the buoyant inca plateau and the carnegie ridge seamount chain the temporal distribution of tertiary ecuadorian arc magmatism indicates a late oligocene early miocene arc magmatic flare up event comprising widespread ignimbrite eruption and batholith construction initiation of the flare up event coincides in time with accelerating less oblique farallon nazca south america plate convergence suggesting a positive feedback between convergence rates asthenospheric melt production mantle crust melt flux and upper crustal arc magmatic productivity in ecuador

the determination of true shear senses from the deflection of passive markers in shear zones the sense of movement in shear zones can often be determined from the reorientation of pre existing planar or linear features within the zone this cannot always be done simply by examining the change in strike or trend of these features sometimes these will show the opposite sense of rotation to the true sense of shear in the zone this is illustrated for a generalized oblique slip zone using the deflection method9 described here the true movement sense can be deduced from the apparent rotation the emphasis being on a three dimensional approach consideration of the movement on a large scale shear zone the laxford front underlines the importance of being aware of this aspect of shear zone geometry the method may be applied to brittle fault offsets as well as to deflections in ductile shear zones

evidence for a two phase palmer land event from crosscutting structural relationships and emplacement timing of the lassiter coast intrusive suite antarctic peninsula implications for mid cretaceous southern ocean plate configuration 1 new analysis of the relationships between geological structural data and radiometric ages for the lassiter coast intrusive suite indicate that the collisional mid cretaceous palmer land event orogeny in the antarctic peninsula has had two kinematic phases forming an intersection orocline one of which can be related to cretaceous southern ocean plate motions both are compressional phases along the eastern palmer land shear zone phase 1 occurred at 107 ma with a principal paleostrain axis of 341 and is best expressed in southern palmer land although evident elsewhere on the antarctic peninsula phase 2 occurred at 103 ma with a principal paleostrain axis of 259 5 but is confined to between 68 s and 74 s a peak in lassiter coast intrusive suite magma emplacement rate was coeval with phase 1 whereas phase 2 may have coincided with a lull during phase 1 the allochthonous central and western domain terranes may have been transported to the gondwana margin represented by the para autochthonous eastern domain on board the phoenix plate or on board the south american plate the variable provenance indicators from the central and western terranes can be cited to support either or a combination of these scenarios the convergence direction evident from phase 2 structures cannot easily be reproduced in regional plate kinematic models this and the localized evidence for superposition of phase 2 structures on phase 1 structures suggests that phase 2 is unlikely to have occurred solely in response to changes in the kinematics of the large plates considered

syntectonic emplacement of the middle jurassic concon mafic dike swarm coastal range central chile 33 s abstract the concon mafic dike swarm cmds consists of basaltic to andesitic dikes emplaced into deformed late paleozoic granitoids during the development of the jurassic arc of central chile the dikes are divided into an early group of thick dikes 5 12 m and a late group of thin dikes 0 5 3 m two new amphibole 40ar 39ar dates obtained from undeformed and deformed dikes constrain the age of emplacement and deformation of the cmds between 163 and 157 ma based on radiometric ages field observations ams studies and petrographic data we conclude that the emplacement of the cmds was syntectonic with the jurassic arc extension and associated with sinistral displacements along the nw trending structures that host the cmds the common occurrence of already deformed and rotated xenoliths in the dikes indicates that deformation in the granitoids started previously the early thick dikes and country rocks appear to have been remagnetized during the exhumation of deep seated coastal rocks in the early cretaceous around 100 ma the remanent magnetization in late thin dikes is mainly retained by small amounts of low ti magnetite at high temperature and pyrrhotite at low temperature the magnetization in these dikes appears to be primary in origin paleomagnetic results from the thin dikes also indicate that the whole area was tilted 23 to the nnw during cooling of the cmds the nnw sse extension vectors deduced from the paleomagnetic data and internal fabric of dikes are different with respect to extension direction deduced for the middle late jurassic of northern chile pointing to major heterogeneities along the margin of the overriding plate during the mesozoic or differences in the mechanisms driving extension during such period

235u fission tracks and238u series disequilibria as a means to study recent mobilization of uranium in archaean pyritic conglomerates uranium transport in archaean sediments was investigated by means of two carefully selected suites of variably weathered sediment samples from different localities of the pongola supergroup kaapvaal craton south africa u remobilization on a micron to meter scale was analyzed using u fission track micromapping and spectrometry bulk measurements these studies were supported by ore microscopy neutron activation analysis and spectrometry by combining particle track technique and spectrometry detailed information is obtained on the u transport history of weathered surface samples a simple model of u redistribution processes in geologic systems based on238u series disequilibria is developed and applied to the samples investigated some implications for radiometric field methods are discussed

isotopic variation in modern lake waters from western greenland the stable isotope composition of lake waters along sindre strimfjord in western greenland have been measured both spatially coast to ice sheet and temporally up to five times between 1998 2000 and the data compared to local climatic information there is a climatic gradient along sindre strimfjord with the head of the fjord characterised by a low arctic continental climate while the coast has a more maritime climate increased precipitation and reduced annual temperature range the change in climate from continental to maritime is reflected in the isotopic composition of the lakes the inland lakes have 18o and dd values which lie on evaporation trends away from meteoric water composition the coastal lakes have 18o and dd compositions close to meteoric water short term seasonal trends are superimposed on longer term variation for example the relatively dry years precipitation lower than the long term annual mean of 1998 and 1999 are seen in the smaller lakes as isotopic enrichment but the

strain and strain rate in a synkinematic trondhjemitic dike evidence for melt induced strain softening during shearing bohemian massif czech republic abstract emplacement related deformation of a cambrian trondhjemitic dike located at the western border of the tepla barrandian unit central european variscides is studied in detail the trondhjemitic melt was emplaced at t 750 c into an active low temperature ca 350 c east northeast trending dextral transcurrent shear zone thermal modelling indicates that the rheologically critical melt fraction and the solidus of the dike were achieved after 1 and 3 days of cooling respectively the major part of the shearing occurred within 8 days after the melt emplaced the zonation of the dike in strain magnitude mineralogy and geochemistry can be explained by the laterally varying rheology from margin to centre during cooling and shearing the deformation was volume constant and the strain data plot in the apparent constrictional field the unusually high value of the calculated longitudinal strain rate 2 8 10 6 s 1 equivalent to a displacement rate of 2 cm h 1 is probably related to the intruding melt that lubricated the shear zone and thus enhanced its displacement velocity consequently melts that intrude as dikes into active transcurrent shear zones may significantly weaken the strength of the middle and upper continental crust

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

paleogeographic and tectonic implications of the geology of the tiefort mountains northern mojave desert california geologic mapping petrologic and structural analyses and u pb dating in the tiefort mountains northern mojave desert california provide data on paleozoic paleogeography and mesozoic intra arc tectonics metasedimentary rocks consist primarily of quartzose schists and gneisses that are provisionally correlated with late precambrian cordilleran passive margin facies recognition of precambrian augen gneiss and metasedimentary rocks with craton margin protoliths suggests that the craton miogeocline boundary is in or near the tiefort mountains the presence of eugeoclinal rocks in ranges to the west implies truncation or telescoping of the miogeocline in this area the metasedimentary units are intruded by middle jurassic granite and diorite orthogneisses dated by u pb as ca 164 and ca 160 ma deformed rocks record amphibolite facies metamorphism and mylonitization during middle to late jurassic southeast vergent ductile shear pervasively annealed microstructures indicate that recrystallization outlasted deformation at south tiefort mountain the mylonitic fabric is deformed by northwest trending folds and crosscut by a swarm of mafic and felsic dikes dated as 148 14 ma at tiefort mountain synkinematic biotite granite gneiss is dated as ca 105 ma and mylonitic fabrics are folded about north trending axes the presence of the cretaceous orthogneiss suggests that two deformational events jurassic and cretaceous in age may have affected part of the study area dikes dated as ca 82 ma that crosscut all fabrics are interpreted to be part of a regional intrusive event the timing and vergence of shear zones at south tiefort mountain are similar to contractional shear zones in adjacent areas forming a belt of contractional deformation from the central mojave desert to the east sierran thrust system this deformation may be related to a period of late middle to late jurassic sinistral oblique subduction

roof and walls of the red mountain creek pluton eastern sierra nevada california usa implications for process zones during pluton emplacement abstract the red mountain creek pluton has a sub circular outline in map section generally flat roof and steep sides the pluton roof sharply truncates host rock markers has a highly irregular geometry with rectangular steps and passes continuously into a steep wall the roof wall transition is not offset by faults and shows no evidence of extensive diking or synemplacement ductile strain which requires that the pluton roof and its wall remained attached to each other and were not faulted or ductilely deformed during emplacement of the pluton we propose that the exposed section through the red mountain creek pluton may represent the crestal portion of a vertically extensive piston shaped plutonic system the upper part of which was largely emplaced by magmatic stoping although other material transfer processes may have previously operated during emplacement characteristics of the pluton roof presented in this paper as well as in other papers on pluton roofs directly rule out some emplacement models commonly applied to plutons in the sierra nevada place limitations on other models and fit well the expected characteristics of visco elastic diapirs around which variable material transfer processes largely displace host rock downwards in the upper crustal plutons the most widespread preserved process of downward transport of host rock was magmatic stoping

near n s paleo extension in the western deccan region india does it link strike slip tectonics with india seychelles rifting this is the first detailed report and analyses of deformation from the w part of the deccan large igneous province dlip maharashtra india this deformation related to the india seychelles rifting during late cretaceous early paleocene was studied and the paleostress tensors were deduced near n s trending shear zones lineaments and faults were already reported without significant detail an e w extension was envisaged by the previous workers to explain the india seychelles rift at 64 ma the direction of extension however does not match with their n s brittle shear zones and also those faults sub vertical ne sw nw se and few n s we report and emphasize in this work slickenside bearing fault planes brittle shear zones and extension fractures in meso scale enabled us to estimate the paleostress tensors directions and relative magnitudes the field study was complemented by remote sensing lineament analyses to map dykes and shear zones dykes emplaced along pre existing n s to ne sw nw se shears fractures this information was used to derive regional paleostress trends a nw se ne sw minimum compressive stress in the oldest kalsubai subgroup and a n s direction for the younger lonavala wai and salsette subgroups were deciphered thus a nw ne to n s extension is put forward that refutes the popular view of e w india seychelles extension paleostress analyses indicate that this is an oblique rifted margin field criteria suggest only ne sw and nw se with some n s strike slip faults brittle shear zones we refer this deformation zone as the western deccan strike slip zone wdsz the observed deformation was matched with offshore tectonics deciphered mainly from faults interpreted on seismic profiles and from magnetic seafloor spreading anomalies these geophysical findings too indicate oblique rifting in this part of the w indian passive margin we argue that the seychelles microcontinent separated from india only after much of the dlip erupted further studies of magma rich passive margins with respect to timing and architecture of deformation and emplacement of volcanics are required

the link between volcanism and tectonics in the southern volcanic zone of the chilean andes a review abstract the nature of the interplay between tectonics and volcanism is a major question in continental margin tectonics the southern andes volcanic zone svz located at the obliquely convergent nazca south america plate margin between 33 s and 46 s offers a unique opportunity to address this question because of along strike changes in crustal thickness tectonic style and well constrained long term and short term kinematic history the complex interaction between tectonic and magmatic processes is evidenced by both the architecture and geochemical signature of volcanic systems main first order factors accounting for the along strike variations in the nature and composition of volcanism are crustal thickness and the existence of a major intra arc fault system the liquine ofqui fault zone lofz second order factors include the local nature of the volcanic arc basement two main categories of volcano tectonic associations have been identified according to the spatial distribution and internal organization of individual volcanoes and clusters of volcanoes with respect to both the overall strike of the volcanic arc and the first and second order active inactive basement faults kinematically coupled associations those that are directly related to the current dextral transpressional tectonic regime include ne trending volcanic alignments of stratovolcanoes and or monogenetic cones kinematically uncoupled associations in which coupling with the current regime is not required include stratovolcanoes built on top of ancient reverse and strike slip faults of the volcanic arc basement and monogenetic cones lying along master faults of the lofz whereas mostly primitive magmas are found in the first category more evolved terms can occur genetically associated with the second category we propose a simplified three dimensional model for the magma plumbing system along and across the svz that integrates kinematically coupled and kinematically uncoupled volcano tectonic associations within the framework of first and second order geological and structural constraints

paleo redox boundaries in fractured granite at the earth s surface fe ii often oxidises and forms insoluble fe iii oxyhydr oxides whose particle size and structure depend on solution composition and temperature during formation and afterwards bacterial processes and exposure to reducing environments reduces them again releasing dissolved iron to the groundwater during such cycling the fe isotopes fractionate to an extent that is expected to depend on temperature in this study we report on the use of fe oxides as paleo redox indicators using their structure morphology and fe composition as a clue for formation conditions in samples taken from 120 m drill cores in granite from se sweden x ray amorphous superparamagnetic nanometre sized fe oxides are confined to fractures of the upper 50 m whereas well crystalline fe oxides with particle sizes typical for soils occur down to 110 m we also identified hematite with a particle size of 100 nm similar to hematite of hydrothermal origin the fe isotope composition of the fine grained fe oxides 1 56fe 1 irmm 14 referenced scatter significantly compared to the distribution previously observed for hydrothermal material 0 26 56fe 0 12 and they are dominantly heavier than fe bearing silicates from fractures 0 56 56fe 0 35 this is consistent with formation by low temperature weathering where fe silicates dissolve fe ii oxidises and fe iii oxides precipitate the x ray amorphous nanometre sized nature of near surface fe oxides suggests recent formation the deeper situated well crystalline fe oxides are more mature and we interpret that they record earlier oxidising events they exist in fractures that are not significantly altered indicating formation during periods of oxidation our results show that oxygenated water may reach depths of 110 m in fractured granite the absence of natural low temperature fe oxides from deeper drill cores suggests that oxygenated waters do not readily penetrate beyond about 100 m and suggests that radioactive waste repositories located at a depth of 500 m should be well protected from oxygenated waters

improved smoothing of an altimetric ocean tide model with global proudman functions summary a set of global proudman functions computed on a 2 finite difference grid is used to smooth raw tidal estimates previously derived by cartwright and ray from a two year analysis of geosat altimeter data the least squares problem provides an interesting application of ridge regression and hansen s l curve analysis the final smoothed maps are found to agree more closely to a set of 80 ground truth constants than did the original tidal models

direct observation of tetravalent cerium in ferromanganese nodules and crusts by x ray absorption near edge structure xanes abstract x ray absorption near edge structure xanes was successfully applied for the direct determination of ce valence in hydrogenetic ferromanganese nodules two samples and crust one sample rare earth element ree patterns of the hydrogenetic ferromanganese nodules and crust recovered from the pacific ocean show large positive ce anomalies especially when normalized to seawater despite some disadvantages in the spectroscopic analysis of ce in natural samples e g low concentration and interference from other elements intense synchrotron radiation and measurement of fluorescent xanes with semiconductor detectors having high energy resolution enabled us to detect xanes at ce l iii edge in the ferromanganese nodule or crust in the xanes spectra of the nodule or crust large double peaks were observed showing that the oxidation state of ce in the nodule or crust is predominantly tetravalent with little contribution from ce iii this is the first in situ quantitative analysis of ce iv among total ce in natural samples as a trace element as far as we know the results obtained by xanes seem to be consistent with the percentage of ce iv estimated from seawater normalized ree patterns suggesting that seawater is a precursor of ree in the marine deposits the present results also confirm that ce enrichment relative to other rees in hydrogenetic ferromanganese deposits is due to oxidative removal of ce from seawater to the ferromanganese nodules and crust in the marine environment comparison of xanes spectra of ce sorbed on synthetic manganese oxide and ceo 2 vs those of the ferromanganese nodules and crust suggests that the initial reaction of ce with the ferromanganese nodules and crust is the sorption of dissolved ce iii to the manganese oxide surface

cerium sequestration and accumulation in fractured crystalline bedrock the role of mn fe hydr oxides and clay minerals abstract this study focuses on the mechanisms of ce sequestration and accumulation in the fracture network of the upper kilometer of the granitoid bedrock of the baltic shield in southeast sweden laxemar area sweden the material includes 81 specimens of bulk secondary mineral precipitates fracture coatings collected on fracture walls identified in 17 drill cores and 66 groundwater samples collected from 21 deep boreholes with equipment designed for retrieval of representative groundwater at controlled depths the concentrations of ce in the fracture coatings although varying considerably 10 90th percentiles 67 438 mg kg 1 were frequently higher than those of the wall rock 10 90th percentiles 70 118 mg kg 1 linear combination fitting analysis of ce l iii edge x ray absorption near edge structure xanes spectra obtained for 19 fracture coatings with relatively high ce concentrations 145 mg kg 1 and a wide range of ce anomaly values revealed that ce iv occurs frequently in the upper 10 m of the fracture network ce iv ce total 0 06 1 00 in 8 out of 11 specimens and is mainly associated with mn oxides modeled as ce oxidatively scavenged by birnessite these features are in line with the strong oxidative and sorptive capacities of mn oxide as demonstrated by previous studies and abundant todorokite and birnessite like mn oxides identified in 3 out of 4 specimens analyzed by mn k edge x ray absorption spectroscopy xas in the upper parts of the fracture network down to 5 m for a specimen with very high ce concentration 1430 mg kg 1 and nasc normalized ce anomaly 3 63 the analysis of ce xanes and mn xas data revealed i a predominance of ce oxide in addition to ce scavenged by mn oxide and ii a large fraction of poorly crystalline hexagonal birnessite and aqueous mn 2 suggesting a recent or on going oxidation of mn 2 in this fracture in addition the ce oxide precipitates on this fracture observed by in situ sem eds contained considerable amounts of mn these spectroscopic and microscopic features led us to suggest that the remarkable accumulation of ce iv in this fracture is a result of repeated formation and dissolution of mn oxides that is formation of mn oxide followed by oxidative scavenging of ce as ce oxide nanoparticles which largely remained during the subsequent reductive dissolution of the mn oxides in addition the xanes data indicate that goethite has the capability to oxidize ce at near neutral ph under our experimental conditions goethite reacted with 0 001 m ce for 48 h in a glove box with o 2

sheeted intrusion of the synkinematic mcdoogle pluton sierra nevada california field microstructural and geochronologic evidence indicates that the late cretaceous mcdoogle pluton located near the eastern margin of the sierra nevada batholith was emplaced as a subvertically sheeted complex into a steep reverse sense shear zone evidence of internal subvertical sheeting includes abundant concordant wall rock inclusions and screens that separate the pluton from adjacent jurassic plutons preservation of a ghost tectonostratigraphy in the distribution of the inclusions and rare interphase contacts the solid state tectonic fabric in the wall rocks and the magmatic submagmatic and weak solid state fabrics in the mcdoogle pluton all are concordant and record northeast southwest horizontal shortening subvertical extension and a significant component of northeast side up simple shear intrusive contacts generally are concordant with the fabrics but where discordances occur the wall rock fabric invariably is truncated by the contact however late synkinematic emplacement of the mcdoogle pluton is indicated by synintrusive boudinage of apophyses of the pluton and by the overall concordance of pluton fabrics including magmatic lineation zircon u pb isotope dates were obtained from the quartz monzodiorite central phase of the mcdoogle pluton 94 4 ma the mafic granodiorite border phase 94 8 0 6 ma and an older hornblende granodiorite included in the central phase 97 6 0 4 ma granodiorite orthogneiss in the wall rock yielded a u pb zircon date of 164 2 1 6 8 ma which we interpret to be the crystallization age and which is indistinguishable from a new age of 164 5 2 3 ma for the twin lakes pluton a u pb titanite date of 94 4 0 9 ma from the 164 ma orthogneiss sample may reflect thermal effects of the intruding mcdoogle pluton synkinematic growth of titanite in the shear zone or perhaps both the age of the sawmill lake shear zone that hosts the mcdoogle pluton is bracketed between 148 ma the age of the independence dike swarm and 92 ma the age of the lamarck pluton that crosscuts both the mcdoogle pluton and the shear zone fabrics regional evidence suggests a shear zone age younger than 110 ma but older deformation is possible the predominance of horizontal shortening over transcurrent motion and an age older than 90 ma exclude a direct relationship to the dextral sierra crest shear system which has been proposed to pass near the study area the location age constraints and kinematics of the shear zone are consistent with both the later stages of movement in the east sierran thrust system and isostatic sinking of the arc magmatic complex into its substrate

implications for deglaciation chronology from new ams age determinations in central west greenland abstract new evidence has been obtained for the age of the umivit keglen and orkendalen moraine systems close to the present ice sheet margin in central west greenland the umivit keglen moraine system is dated at 7500 to 6500 14 c yr b p which is older than the previously assumed date of 7300 to 6000 14 c yr b p the orkendalen system is now dated at 6200 to 5600 14 c yr b p against earlier estimates of 300 to 700 14 c yr b p the new age is based on ams radiocarbon dated organic material within depressions between morainic ridges belonging to the orkendalen system a major implication of the new age is that ice margin positions prevailed for about 6000 years behind the present ice sheet margin the retreat behind the present margin could be substantial and in the light of deglaciation rates prior to the orkendalen phase may be ca 10 s of kilometers rather than kilometers circumstantial evidence is found for the retreat of the ice sheet margin behind its present position during the holocene climatic optimum the results placed into a time frame of deglaciation since the last glacial maximum enable comparison with greenland ice sheet models and ice core records

the liquine ofqui fault zone a long lived intra arc fault system in southern chile abstract plate reconstructions for the cenozoic document relatively steady right oblique subduction of the farallon nazca plate beneath the chilean continental margin the kinematic signature recorded along the intra arc liquine ofqui fault zone lofz a major feature of the southern andes may help to understand the way in which the nazca south america slip vector has been partitioned into strain and displacement along and across the continental margin the lofz consists of two nne trending right stepping straight lineaments a strike slip duplex at the right step and curved features which splay off the straight lineaments toward the northwest the lofz runs mostly through heterogeneously deformed cenozoic plutonic rock of the north patagonian batholith and patchy metamorphic wall rock early cenozoic volcano sedimentary rocks and dyke swarms found in close spatial association with the strike slip duplex are believed to have developed in strike slip related basins quaternary volcanoes are aligned parallel to the lofz both ductile and brittle kinematic indicators within centimeter to meter wide high strain zones document late cenozoic dextral shear deformation contrasting left lateral deformation recorded on older and wider mylonitic zones suggests that the lofz may be a long lived shear zone that accommodated continental scale deformation arising from the farallon nazca south america plate convergence a block rotation pattern as indicated by paleomagnetic data is consistent with the geometry and cenozoic kinematics of the lofz

uranium lead isotopic ages from the sierra nevada batholith california this study provides new information on the timing and distribution of mesozoic magmatic events in the sierra nevada batholithic complex chiefly between 36 and 37 n latitude u pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks granitoid rocks in this area range in age from 217 to 80 m y triassic intrusions are restricted to the east side of the batholith jurassic plutons occur south of the triassic plutons east of the sierra nevada as isolated masses within the cretaceous batholith and in the western foothills of the range cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the sierra nevada no granitic intrusions were emplaced for 37 m y east of the sierra nevada following the end of jurassic plutonism however following emplacement of the eastern jurassic granitoids regional extension produced a fracture system at least 350 km long into which the dominantly mafic calc alkalic independence dike swarm was intruded 148 m y ago the dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the nevadan orogeny intrusion of cretaceous granitic plutons began in large volume about 120 m y ago in the western sierra nevada and migrated steadily eastward for 40 m y at a rate of 2 7 mm y this slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone such steady conditions could be necessary for the production of large batholithic complexes such as the sierra nevada the abrupt termination of plutonism 80 m y ago may have resulted from an increased rate of convergence of the american and eastern pacific plates and dramatic flattening of the subduction zone u pb ages of the giant forest alaskite sequence in sequoia national park are all in the range 99 3 m y indicating a relatively short period of emplacement and cooling for this nested group of plutons u pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma comparison of isotopic ages determined by different methods such as zircon u pb sphene u pb hornblende k ar and biotite k ar suggests that zircon u pb ages generally approximate the emplacement age of a pluton however some plutons probably contain inherited or entrained old zircons and the zircons of some samples are disturbed by younger thermal and metamorphic events the ages reported here are consistent with u pb age determinations previously made on granitic rocks to the north stern et al 1981 the age distribution of granitic belts determined here is in general agreement with those established by k ar dating evernden and kistler 1970 but does not differentiate the five epochs of plutonism determined in their study

limited hydrologic response to pleistocene climate change in deep vadose zones yucca mountain nevada abstract understanding the movement of water through thick vadose zones especially on time scales encompassing long term climate change is increasingly important as societies utilize semi arid environments for both water resources and sites viewed as favorable for long term disposal or storage of hazardous waste hydrologic responses to pleistocene climate change within a deep vadose zone in the eastern mojave desert at yucca mountain nevada were evaluated by uranium series dating of finely layered hyalitic opal using secondary ion mass spectrometry opal is present within cm thick secondary hydrogenic mineral crusts coating floors of lithophysal cavities in fractured volcanic rocks at depths of 200 to 300 m below land surface uranium concentrations in opal fluctuate systematically between 5 and 550 g g age calibrated profiles of uranium concentration correlate with regional climate records over the last 300 000 years and produce time series spectral peaks that have distinct periodicities of 100 and 41 ka consistent with planetary orbital parameters these results indicate that the chemical compositions of percolating solutions varied in response to near surface climate driven processes however slow micrometers per thousand years relatively uniform growth rates of secondary opal and calcite deposition spanning several glacial interglacial climate cycles imply that water fluxes in the deep vadose zone remained low and generally buffered from the large fluctuations in available surface moisture during different climates

palaeohydrogeology a methodology based on fracture mineral studies abstract two sites on the east coast of sweden forsmark and laxemar simpevarp are currently being investigated as potential geologic hosts for a deep repository isolating high level nuclear waste in this paper a methodology for fracture mineral studies is suggested with focus on the variation in depth of the fresh saline water interface and location of the redox front in the bedrock the most commonly precipitated fracture minerals in crystalline rocks are chlorite calcite quartz k feldspar ca al silicates like epidote prehnite and laumontite sulphides and fe oxides of these calcite is the mineral best suited for palaeohydrological studies since it precipitates during a wide range of conditions including low temperature conditions during the pleistocene and holocene epochs sulphides and fe oxides hydroxides provide information on the position of the redox front in order to carry out palaeohydrological studies a number of prerequisites are required such as high quality drill core material geological knowledge of the sequence of fracture mineralizations the post glacial holocene evolution in the area high quality groundwater chemistry including stable isotopes and a conceptual model of the hydrogeochemistry that is to be tested the choice of methods used here is based on the fact that both the forsmark and laxemar simpevarp sites are situated in palaeoproterozoic crystalline rocks with reactivation of fractures over at least 1 5 ga and they have been exposed to glaciations deglaciations and transgressions regressions of the baltic sea during the quaternary this has resulted in a palaeohydrology with a range of groundwaters of quite different chemistry and stable isotopic composition the suggested scheme for solving the variation in depth of the fresh saline water interface focuses on fracture calcite it includes a step by step procedure with 1 initial 18 o and 13 c analyses and complementary petrographic studies of thin sections and crystal morphology followed by 2 87 sr 86 sr and trace element analyses and finally 3 fluid inclusion analyses and cathodoluminescence cl studies of zoning for redox front investigations the following methods are suggested 1 study of the distribution of redox sensitive minerals 2 u series measurements 3 trace element analyses of fracture fillings 4 fe iii fe ii analyses fe isotope analyses it is concluded that fracture mineral studies can reveal a palaeohydrological record in crystalline rock that is essential to understand the stability or evolution of the groundwater system over a time scale that is relevant to performance assessment for a spent nuclear fuel repository it is also concluded that the suggested methodology for palaeohydrogeological studies is site specific

a plate tectonic model for late jurassic ophiolite genesis nevadan orogeny and forearc initiation northern california recently published age and structural data allow the reconciliation of previously conflicting models for late jurassic genesis of the josephine smartville and coast range ophiolites and the nevadan orogeny in the klamath mountains and sierra nevada the resulting model is consistent with the mode of initiation location and geometry of the great valley forearc basin and with the lack of a significant forearc basin west of the klamath mountains the coast range ophiolite formed by backarc spreading west of an east facing intraoceanic arc soon thereafter a remnant arc was calved off the west side of this arc and the smartville ophiolite formed by backarc interarc spreading during this time the sierran phase of the nevadan orogeny began as the intraoceanic arc encountered the west facing continental margin arc of north america an east west trending calcalkaline dike swarm in the sierra nevada foothills may mark the trajectory of the colliding arcs at the initiation of the collision simultaneously a new subduction zone was initiated west of the collision suture zone and this new trench propagated southward thus trapping the coast range ophiolite in the new forearc area south of the klamath area intense deformation in the sierran region resulted from this collision and both magmatic arcs became inactive as the last remnant of intervening oceanic crust was subducted continued westward relative movement of the north american arc was permitted north of the sierra nevada owing to the lack of a colliding intraoceanic arc the result was the westward rifting of the continental margin arc by intraarc spreading which formed the josephine ophiolite in the klamath area the klamath phase of the nevadan orogeny resulted from contraction of the west facing intraoceanic arc and josephine backarc basin beneath the continental margin basal sediments of the great valley forearc basin were derived primarily from the sutured arc ophiolite terranes and were deposited on top of the coast range ophiolite the southern edge of the klamaths and the western side of the sierra nevada a new late mesozoic magmatic arc was superposed across the previously accreted terranes and formed the primary sediment source for the cretaceous forearc basin

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

spectral roughness and glacial erosion of a land terminating section of the greenland ice sheet spectral roughness offers a significant potential for understanding the evolution of glaciated landscapes here we present the first roughness study combining a high resolution 250 to 500 m dem

evaluation of pleistocene groundwater flow through fractured tuffs using a u series disequilibrium approach pahute mesa nevada usa abstract groundwater flow through fractured felsic tuffs and lavas at the nevada national security site represents the most likely mechanism for transport of radionuclides away from underground nuclear tests at pahute mesa to help evaluate fracture flow and matrix water exchange we have determined u series isotopic compositions on more than 40 drill core samples from 5 boreholes that represent discrete fracture surfaces breccia zones and interiors of unfractured core the u series approach relies on the disruption of radioactive secular equilibrium between isotopes in the uranium series decay chain due to preferential mobilization of 234 u relative to 238 u and u relative to th samples from discrete fractures were obtained by milling fracture surfaces containing thin secondary mineral coatings of clays silica fe mn oxyhydroxides and zeolite intact core interiors and breccia fragments were sampled in bulk in addition profiles of rock matrix extending 15 to 44 mm away from several fractures that show evidence of recent flow were analyzed to investigate the extent of fracture matrix water exchange samples of rock matrix have 234 u 238 u and 230 th 238 u activity ratios ar closest to radioactive secular equilibrium indicating only small amounts of groundwater penetrated unfractured matrix greater u mobility was observed in welded tuff matrix with elevated porosity and in zeolitized bedded tuff samples of brecciated core were also in secular equilibrium implying a lack of long range hydraulic connectivity in these cases samples of discrete fracture surfaces typically but not always were in radioactive disequilibrium many fractures had isotopic compositions plotting near the 230 th 234 u 1 1 line indicating a steady state balance between u input and removal along with radioactive decay numerical simulations of u series isotope evolution indicate that 0 5 to 1 million years are required to reach steady state compositions once attained disequilibrium 234 u 238 u and 230 th 238 u ar values can be maintained indefinitely as long as hydrological and geochemical processes remain stable therefore many pahute mesa fractures represent stable hydrologic pathways over million year timescales a smaller number of samples have non steady state compositions indicating transient conditions in the last several hundred thousand years in these cases u mobility is dominated by overall gains rather than losses of u

cenozoic noncoaxial transtension along the western shoulder of the ross sea antarctica and the emplacement of mcmurdo dyke arrays field data on cenozoic faults and the mcmurdo dyke arrays in the reeves glacier mawson glacier area victoria land antarctica allow us to support noncoaxial transtensional tectonics along the n s trending western shoulder of the ross sea dyke injection within a crustal scale right lateral strike slip shear zone is testified by magma filled tension gash like arrangements within some master fault zones and by the left stepping arrangements of dykes in the intrafault zones the noncoaxiality of deformation is shown by the re activation of many dyke walls as right lateral strike slip faults this suggests an increase in the strike slip component over time along the western shoulder of the ross sea our data support the relevance of transtensional to strike slip tectonics for triggering melting and controlling the geometry and modes of magma emplacement

magnitude and significance of miocene crustal extension in the central mojave desert california the newly recognized waterman hills detachment fault whdf of the central mojave desert california is significant because it provides the first unambiguous evidence for large scale core complex style crustal extension in the central mojave desert and because it has significantly rearranged the pre miocene paleogeography of the mojave desert the whdf places steeply dipping to overturned miocene volcanic and sedimentary rocks upon mylonitic pre tertiary basement the mylonites which apparently formed during extension are predominantly l tectonites which manifest top to northeast shear the whdf dips to the northeast beneath dominofaulted ranges of the central mojave desert and detachment faults of the colorado river trough forming an imbricated early miocene system of detachment faults extension continued in the colorado river trough after extension had ceased in the central mojave desert tentative correlations of mesozoic intrusions suggest about 40 km of slip across the whdf which carries eugeoclinal paleozoic rocks in its hanging wall and cratonal miogeoclinal paleozoic rocks in its footwall restoration of 40 km of slip 1 removes a prominent kink in the boundary between eugeoclinal and cratonal miogeoclinal facies 2 aligns cratonal miogeoclinal strata near victorville more closely with the late paleozoic continental margin farther north 3 places cratonal miogeoclinal rocks structurally beneath eugeoclinal rocks implying that the facies were stacked by thrusting and 4 straightens the western margin of the late jurassic independence dike swarm

tracing the palaeoredox conditions at forsmark sweden using uranium mineral geochronology abstract u pb isotope systems have been used to constrain the timing of formation alteration and oxidation of u minerals from the meta granitic bedrock at forsmark eastern sweden secondary ion mass spectrometry sims has been used to collect u pb data from uraninite discordant data suggest a 1 8 ga emplacement of uraninite bearing pegmatites and an event of uraninite alteration at 1 6 ga the latter age is contemporaneous with the gothian orogeny in scandinavia which was associated with hydrothermal fluid circulation in the fennoscandian shield ca uranyl silicates haiweeite and uranophane predominately formed 1 3 1 2 ga contemporaneous with the emplacement of the satakunta complex of the central scandinavian dolerite group a palaeozoic group of ca u vi silicates is also present which indicates that the geochemical composition of geologic fluids was heterogeneous throughout the fracture network during this time low pb concentrations in the u vi silicates of several samples are compatible with a recent

new evidence for cretaceous strike slip faulting in the united states cordillera and implications for terrane displacement deformation patterns and plutonism early to mid cretaceous dextral strike slip faulting involving up to 400 km displacement has been documented in the southern sierra nevada of california in western idaho the salmon river suture juxtaposes unrelated oceanic and continen tal rocks along a major near vertical shear zone that was active in the early to mid cretaceous these two structural zones have been treated as separate entities however new data indicate that they likely connect via a structural discontinuity in northwest nevada and southeast oregon herein called the western nevada shear zone wns current structures within the wns record dip slip contractional deformation however regional relations structural and stratigraphic indicate a mismatch in the early mesozoic geology of rocks across the wns that is best explained by dextral strike slip displacement of several 100 km these relations argue that an early strike slip structure along the wns was reactivated and obscured during later regional shortening deformation most likely related to the cretaceous sevier orogeny the wns is along strike and broadly coeval with the salmon river suture zone and strike slip boundaries in the sierra nevada we hypothesize that all these features are related elements of a major structural boundary in the western united states cordil lera this boundary is interpreted to have accommodated significant dextral strike slip displacement of outboard arc terranes of the cordillera in the early cretaceous followed by reactivation as a localized contractional zone in the mid cretaceous during the sevier orogeny cretaceous batholiths of the united states cordillera closely parallel the boundary which we suggest formed a major crustal flaw that localized and facilitated emplacement of arc magmas

active stress field and fault kinematics of the greater caucasus abstract this work contributes to depict the current seismicity fault kinematics and state of stress in the greater caucasus territories of georgia azerbaijan and russia we merged and homogenized data from different earthquake catalogues relocated 1000 seismic events created a database of 366 selected focal mechanism solutions 239 of which are new and performed a formal stress inversion preferential alignments of crustal earthquake foci indicate that most seismic areas are located along the southern margin of the belt and in the north eastern sector this is consistent with the presence of dominant active wnw ese faults parallel to the mountain range in the entire greater caucasus a dominant nne ssw oriented greatest principal stress 1 controls the over all occurrence of earthquakes of minor and major magnitude main earthquakes are characterized by a vertical least principal stress 3 corresponding to reverse kinematics reverse slip is more common along the southwestern and north eastern foothills of the greater caucasus although in these areas there are also scattered strike slip events this suggests the presence of local stress fields with horizontal 1 and 3 in the central southern part of the mountain belt in correspondence of the local collision between the lesser and the greater caucasus 1 rotates to nnw sse the strike slip events instead dominate along the southern flank of the central eastern mountain range this is interpreted as the effect of the collision that promotes eastward escape of the tectonic blocks located to the east

plutonism oblique subduction and continental growth an example from the mesozoic of california major episodes of mesozoic plutonism in california correlate with periods of oblique subduction and trench parallel transport of western california along intrabatholithic faults major episodes of plutonism occurred in the late jurassic during left oblique convergence and in the mid cretaceous during right oblique convergence in contrast a conspicuous lull in plutonism but continuation of volcanism in the earliest cretaceous coincides with a time when the north america farallon convergence vector although large in magnitude was oriented perpendicular to the trench this correlation suggests that plutonism is facilitated by strike slip faulting within the batholithic belt one explanation which helps to solve the plutonic room problem is that plutons are passively emplaced at releasing bends in the strike slip faults and volume is conserved by thrusting at the trench if this correlation is generally applicable then it implies that mid crustal plutonism is limited beneath areas in which the convergence vector is subperpendicular to the trench continental growth in such areas may occur dominantly by volcanism

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

interplay of plutonism and regional deformation in an obliquely convergent arc southern coast belt british columbia the coast plutonic complex is an extensive zone of continental growth that formed along the mesozoic convergent margin of northwestern north america the orogeny creating this belt involved terrane accretion and assembly massive upward transfer and emplacement of sial in the form of batholiths constituting a magmatic arc and transformation of broad tracts of sedimentary and volcanic rocks into crystalline continental crust all operating in more or less the same space and time the mechanisms and interplay of these orogenic processes are well displayed in the harrison lake area of the southern coast belt british columbia great structural relief across the area exhibits a present day architecture defined by thin thrust stacked terrane sheets and early concordant pluton sheets folded on a macroscopic scale all truncated by oblique dextral reverse faults and overlain by later floored plutons construction of this complex began with terrane assembly on orogen normal thrusts during a lull in plutonism in the early cretaceous low grade metamorphism during this event indicates only modest crustal thickening subsequent plutons intruded into the assembled terranes appear to be composites of sheets early pluton sheets are now steeply dipping due to folding but were likely intruded as horizontal bodies large ovoid post folding plutons are mostly subhorizontal floored bodies at least in part sheeted these plutons are underlain by barrovian mineralogic aureoles that indicate downward vertical displacement of 10 km or more during plutonism suggesting pluton emplacement by vertical inflation magmatic fabrics in these bodies and the discordant relation of plutons to regional structures preclude emplacement in active fault zones penetrative strain aureoles flanking plutons are mostly limited to zones a few hundred meters wide and regional tectonic structures are widely preserved tectonic deformation of the arc is characterized by contraction and strike slip not orogen normal extension plutons played a greater role than terrane accretion in causing crustal thickening and high grade metamorphism

early permian transform margin development of the southern new england orogen eastern australia eastern gondwana the southern new england orogen part of the eastern margin of gondwana evolved from a zone of high angle plate convergence during the carboniferous into either a transform margin or a highly oblique convergent margin by the early permian fault bounded narrow elongate rapidly filled sedimentary basins that developed along the peel manning fault system in the early permian provide support for this hypothesis they are similar to sedimentary basins developed along strike slip fault zones worldwide strike slip dislocations provide the best explanation for juxtaposition of terranes with distinctly different geological histories and discontinuities in regional geological trends across these basins a close spatial relationship also exists between occurrences of serpentinite matrix ophiolitic melange the peel manning fault system and related secondary faults early permian transtension may have facilitated the diapiric rise of serpentinites which originated from an older underlying ultramafic precursor up these fault zones rise and early permian emplacement of the bundarra plutonic suite an s type batholith across which regional geological trends are offset is also best accommodated in a model that involves a transtensional regime copyright 1992 by the american geophysical union

southern extension of the independence dike swarm of eastern california new geochronologic data show that jurassic dikes in the western mojave desert and eastern transverse ranges are part of the independence dike swarm of eastern california these occurrences extend the length of the dike swarm to more than 500 km u pb zircon concordia intercept ages for dikes near stoddard well in the western mojave desert and big wash in the eagle mountains are identical within analytical uncertainties to 148 ma zircon ages for independence dikes north of the garlock fault recognition of this widespread magmatic and tectonic event with a restricted age has important ramifications 1 crosscutting relations between the dikes and pre upper jurassic and cretaceous rocks help establish relative ages 2 persistence of dikes across several inferred terrane boundaries constrains terrane accretion ages 3 the dike rocks represent magmatic processes in a wide range of wall rock settings 4 the dikes are structural markers that record translations and rotationsof crustal flakes 5 regional persistence and dilational emplacement of the independence dike swarm suggest that it is the last extension in a series of alternating extensional and compressional events during the jurassic the tectonic regime responsible for the independence dikes may be related to arc normal extension changes in plate motions or oblique subduction with left lateral shear

structural control on volcanoes and magma paths from local to orogen scale the central andes case abstract assessing the parameters that control the location and geometry of magma paths is of paramount importance for the comprehension of volcanic plumbing systems and geo hazards we analyse the distribution of 1518 monogenic and polygenic volcanoes of miocene quaternary age of the central volcanic zone of the andes chile bolivia argentina and reconstruct the magma paths at 315 edifices by analysing the morphostructural characteristics of craters and cones then we compare these data with outcropping dykes tectonic structures and state of stress most magma paths trend n s nw se and ne sw in decreasing order of frequency the n s and nw se paths coexist in the northern and southern part of the study area whereas n s paths dominate east of the salar de atacama outcropping dykes show the same trends the regional holocene stress state is given by an e w greatest horizontal principal stress n s and nne ssw reverse faults and folds affect deposits of 4 8 3 2 and 1 3 ma bp especially in the central and southern study areas a few nw se left lateral strike slip faults are present in the interior of the volcanic arc part of which belong to the calama olacapato el toro fault the volcanic chain is also affected by several n s and nw se striking normal faults that offset pliocene and quaternary deposits the results indicate different scenarios of magma tectonic interaction given by n s normal and reverse faults and n s fold hinges that guide volcano emplacement and magma paths magma paths are also guided by strike slip and normal nw se faults especially in the northern part of the study area zones with verticalized strata with bedding striking ne sw also acted as preferential magma paths these data suggest that at convergence zones with continental crust shallow magma paths can be more sensitive to the presence and geometry of upper crustal weakness zones than to the regional state of stress

a new method for the determination of ceiii ceiv ratios in geological materials application for weathering sedimentary and diagenetic processes abstract relative abundances of rare earth elements ree in geological materials are used widely to investigate diverse geochemical issues such as the origins of igneous rocks or the degree of stratification of water columns in oceans one of the ree cerium ce can exist in either trivalent or tetravalent forms depending on the redox condition thus knowledge of the oxidation state of ce in rocks and minerals could potentially be used to constrain the redox states of past and present geological environments however the use of ce for this purpose has been hampered by an inability to measure its oxidation state directly here we present a new method employing the x ray absorption near edge structure for making such determinations in samples with ce concentrations as low as 15 ppm by analyzing a range of diverse geological materials granites manganese nodules and cherts we showed that the method could identify significant differences in ce speciation between materials formed under widely differing redox conditions used together with the degrees of ce anomalies measured in ree patterns this information has promise for refining our knowledge of various geochemical processes such as weathering sedimentary and diagenetic processes that control the ree behavior such a refinement should in turn improve geological interpretations based upon ree data

estimation of main tidal constituents from topex altimetry using a proudman function expansion tidal models for the main diurnal and semidiurnal constituents have been computed from topex altimeter data and a set of proudman functions computed numerically in the space defined by the ocean basins the accurate modeling of the ocean tides is necessary in order to interpret the height measurements of the ocean surface obtained from satellite altimeters it is also an interesting dynamical problem in its own right the surface height field due to any tidal constituent can be expanded in terms of the eigenfunctions of the velocity potential proudman functions with coefficients estimated in a least squares sense from a field of discrete data points obtained from altimetry tide gauges bottom pressure sensors etc the proudman functions constitute a mass conserving orthogonal basis their computation does not require any assumption concerning friction or energy dissipation only a numerical grid expressing the shape of coastline and the bathymetry of the ocean basins they have the space structure of standing waves and can be identified as the zero rotation gravitational normal modes they have to be evaluated numerically only once for each particular grid resolution in this investigation the proudman functions were computed by means of finite differences in spherical coordinates over a 2ox2 o grid covering most of the world s oceans for a total of 8608 degrees of freedom the data field used in this study consists of approximately 15 months of topex altimetry in the form of collinear differences results for the major semidiurnal and diurnal constituents m2 s2 n2 k2 k1 o1 p 1 and q1 have been obtained in terms of corrections to a priori values obtained by fitting schwiderski s 1980 models the new models goddard space flight center gsfc94a are tested at a set of ground truth data points these tests indicate substantial improvement for most of the constituents as compared with schwiderski s solutions use of gsfc94a results in a 7 8 cm reduction in the rms overlap difference of 15 5 cm the gsfc94a model yielded a mean rms sea surface variability of 7 9 cm compared with the 9 4 cm obtained when using schwiderski s model

incremental emplacement and syn tectonic deformation of late triassic granites in the qinling orogen structural and geochronological constraints abstract granitoids are important components of major orogenic belts and provide important information about the regional geodynamic evolution the emplacement mechanism of granite plutons and its relationship with regional tectonics has long been discussed although it still remains debated the qinling orogen within the central china orogen was marked by the emplacement of numerous late triassic granitic plutons although the petrology geochemistry and geochronology of these intrusions have been addressed in various studies their tectonic setting remains controversial particularly since the structural aspects not been evaluated in detail in this study we attempt to reconstruct the emplacement process of the late triassic dongjiangkou pluton in the south qinling belt field observations show extensive syn plutonic deformations both in the pluton and its contact zones microstructural observations demonstrate that fabrics in the pluton were mainly acquired during submagmatic flow to high t solid state deformation zircon u pb ages reveal that the pluton is a composite intrusion which is composed of two juxtaposed small plutons with distinct ages 210 ma and 200 ma al in hornblende thermobarometer indicates that the pluton was formed at depths ranging from 4 7 km to 8 8 km with an increasing depth trend from the inner unit to the outer unit distribution of the internal fabrics shows two concentric patterns which are concordant with pluton margins at the pluton scale and were probably induced by the regional sinistral transpression integrating these analyses an incremental emplacement model is proposed for the syn tectonic pluton this model not only solves the room problem but also accounts for the zoned petrological features of the pluton combined with previous studies we suggest that the late triassic granite plutons in the qinling orogen were emplaced under a syn collisional convergence setting and that the granite magmatism was probably controlled by regional tectonics additionally the incremental emplacement model may be a common mechanism for the late triassic granite plutons

granite generation and rapid unroofing related to strike slip faulting aysen chile a stock of biotite muscovite garnet leucogranite crops out in the lower course of rio cisnes as an unusual minor lithology within the predominantly dioritic to tonalitic north patagonian batholith foliated and unfoliated varieties are present the former are nearer to the main lineament of the liquin e ofqui fault zone lofz two feldspar thermometry indicates equilibration temperatures above 600 c for pressures probably not over 3 kbar as suggested by the mn rich garnet composition a rb sr whole rock isochron age of 9 6 0 4 ma 1 error probably indicates the time of magma crystallization 40ar 39ar ages of 6 6 0 3 ma on muscovite and 5 5 0 4 ma on biotite are cooling ages from which a moderate average uplift denudation rate 1 mm yr may be calculated paucity of occurrence distribution close to the lofz and a near minimum melt composition all suggest that the leucogranite magma was derived by partial melting of the lower crust perhaps by decompression melting at a time when uplift denudation rates were high 4 mm yr or more are required regional evidence for rapid holocene uplift in the immediate vicinity of the lofz substantiates the feasibility of the proposed petrogenetic model which may be valid in other strike slip orogenic environments

evaluation of multiple emplacement mechanisms the huichizi granite pluton qinling orogenic belt central china three end member granite emplacement mechanisms host rock ductile shortening hrds host rock rigid displacement hrrd and magmatic absorption ma are identified in this study of the huichizi pluton the internal fabrics are mainly flattening typesona 0 4 0 9u and strain is highoesa 1 0 0 6u near the boundaries and lowoesa 0 7 0 30u toward the central parts of the pluton in the 1 5 km wide contact aureole extension e is measured in syn emplacement deformed veins and the calculated average shortening normal to the sides and tips of the pluton are 1 14 km and 0 74 km respectively from this we conclude that the hrds produced only about 20 space for the present volume of the pluton the regional hrrd derived from the uplift by about 8 km of the qinling group in a flower structure contributed 19 032 space ms including stoping and material transfer processes yielded another 37 space of this stoping contributed a minimum of 1 space estimated from the surface area of 2 8 km 2 covered by xenoliths transfer processes provided about 36 of the needed space deduced from the average 64 36 ratio of hybridization of mantle derived to crustal qinling group derived magma the three mechanisms may act nearly simultaneously as a multiple emplacement mechanism this situation may apply to many other plutons as well 2000 elsevier science ltd all rights reserved

vertical axis rotations in the mojave evidence from the independence dike swarm the 500 km long late jurassic independence dike swarm in california extends from the central sierra nevada through the mojave desert to the eastern transverse range the dikes are mostly vertical and their azimuth in the sierra nevada is 315 however in the mojave region their azimuth varies significantly 300 320 in the argus range the spangler hills and the stoddard well and soda mountain areas and 340 350 in the granite mountain alvord mountain and eagle mountain areas apparently the dikes in the granite alvord and eagle mountains areas have rotated 50 clockwise about vertical axis relative to the sierra nevada and the stoddard well area these rotations are remarkably consistent with known post early miocene paleomagnetically derived rotations the clockwise rotations are associated with the east west trending left lateral strike slip faults in the northeastern mojave and the eastern transverse range regions in contrast the data show that little or no rotation occurred in the central mojave domain which is characterized by northwest trending faults

oblique opening and noncoaxial emplacement of the jurassic independence dike swarm california abstract several features of the independence dike swarm ids indicate that dikes opened obliquely to their walls in a sinistral sense the ids is largely latest jurassic in age 148 ma and has been traced for more than 600 km along its nw se strike from east central to southern california field and petrographic observations from the ids in the sierra nevada indicate that i some and perhaps all of the dikes initially opened perpendicular to their margins and then were sinistrally sheared and ii sinistral displacement occurred during and shortly after dike injection dike emplacement overlapped in time with the formation of wall rock mylonite zones which accommodated shear varying in sense from sinistral to west side down reverse we suggest that the ids intruded dilatant fractures within a regional sinistral shear system the swarm is located within or along the eastern flank of the cordilleran subduction related magmatic arc the ids and associated wall rock deformation may record partitioning into the magmatic arc of the sinistral component of strongly oblique late jurassic subduction

kinematics of plate convergence deduced from mesozoic structures in the western cordillera mesozoic structures in the western cordillera of the united states yield important constraints for kinematic reconstruction of plate motions relative to north america structures in the mesozoic arc s of the northern sierra nevada and of northeastern oregon are remarkably uniform in timing style and with late cretaceous and or cenozoic rotations removed orientation rocks of both regions experienced three major episodes of contraction occurring in the triassic early jurassic the late jurassic and the early cretaceous cross folds were developed after the first two deformations but were not associated with significant shortening the three major deformations are nearly homoaxial and have shortening axes oriented subperpendicular ne sw to the regional trend of the arc system in the back arc region of the northwestern great basin deposition was continuous during the triassic and early jurassic and the region was shielded from tectonism in the arc to the west development of a regional fold thrust belt associated with major nw se shortening was initiated in the back arc region during the middle or late jurassic and continued through the early cretaceous corresponding deformation did not occur in the arc and the arc and back arc were decoupled by a major left lateral fault system that subparallelled the arc axis in the mid cretaceous approximately 100 m y b p regional compression in the back arc abruptly changed to ne sw and both the arc and back arc experienced major shortening development of homoaxial structures in the sierran arc is probably analogous to shortening in present day arc systems undergoing oblique subduction in which the shortening axis is parallel to the normal component of convergence thus the dominant structures in the sierran arc simply may be a reflection of the orientation of the trench and may not constrain the sense of obliquity of convergence strike slip faults and related compressional structures such as the back arc left lateral fault and associated fold thrust belt are probably the response to the strike slip component of convergence and indicate a period of left oblique subduction relative plate motions constrained by paleomagnetic data are in agreement with regional structures and indicate strong right lateral components of motion for the last 100 m y before 100 m y b p relative plate motions are inadequately constrained by paleomagnetic data and regional structures indicate a period of left oblique convergence from the middle or late jurassic to approximately 100 m y b p during the triassic and early jurassic the relative sense of convergence is equivocal and the possibility exists that continuous left oblique convergence occurred throughout the late paleozoic and early mesozoic until 100 m y b p based on the available data we cannot discount the possibility that a complex convergence history existed however with changes from right to left convergence in the middle or late jurassic and from left to right convergence in the mid cretaceous

the uranium ore from mina fe salamanca spain as a natural analogue of processes in a spent fuel repository abstract in the frame of the enresa natural analogue programme the uranium ore from the mina fe salamanca spain has been studied as a natural analogue of radioactive spent fuel behaviour this uranium mine is hosted in highly fractured schistose rocks a geological setting that has not been envisaged in the spanish options for radioactive waste burial however some analogies with the processes that might be involved in the evolution of these geological repositories suggested this investigation the pitchblende pyrite carbonate paragenesis has been studied in situ as natural analogue of the nuclear spent fuel behaviour under extremely oxidative dissolution conditions similarly secondary fe oxyhydroxides and clay minerals have also been considered as relevant analogue materials for the retention of uranium and other analogous trace metals a multidisciplinary characterisation of the site has been performed in order to study these processes though the intense mining activities in the site hindered precise determination of the original hydrogeological and hydrochemical features of the investigated zone boa fault zone the mineralogy and geochemistry of fracture fillings mineralisation and associated clayey materials have allowed the geochemical evolution of the system to be established three geochemical zones have been clearly differentiated i the oxidised zone from the surface to approximately 20 m depth ii the redox transition zone from 20 to 50 m depth and iii the reduced zone located below the transition zone the oxidised zone is characterised by the presence of the typical mineral association resulting from the strong acid conditions caused by the total oxidation of pyrite and other sulphides the total oxidation dissolution and leaching of u iv as uranyl sulphate aqueous complexes prevailed in this oxidised zone the redox transition zone is characterised by the coexistence of the primary uranium paragenesis oxidised minerals as well as numerous secondary solid phases as a result of the physico chemical changes in the environment the optimal physico chemical conditions for the coffinitisation of pitchblende and the co precipitation of fe iii u vi took place in this zone in the reduced zone where the primary uranium paragenesis is present we currently find the necessary physico chemical conditions to stabilise pitchblende pyrite and carbonates the physico chemical conditions of the oxidised zone are not relevant to disposal conditions in the transition zone two main geochemical processes take place i the coffinitisation of pitchblende which may be an important process for the stability of spent fuel in reducing conditions and ii the co precipitation of the fe iii and u vi as oxyhydroxides another relevant mechanism for the retention of uranium the physico chemical conditions that prevail below 50 m depth should be sufficient to stabilise a spent nuclear fuel repository in the same way as they have been able to preserve the 34 ma old uranium deposit of the mina fe

3d gravity modelling of the aguablanca stock tectonic control and emplacement of a variscan gabbronorite bearing a ni cu pge ore sw iberia the aguablanca stock is a variscan mafic pluton located in the ossa morena zone southern iberian massif hosting an unusual ni cu pge mineralization associated with magmatic breccia pipes which intruded its northern part the emplacement of the aguablanca stock and the mineralized breccia pipes are related to the activity of the cherneca ductile shear zone a variscan sinistral shear zone that favoured magma ascent through the upper crust a detailed gravity study has been carried out in order to investigate the 3d geometry of the aguablanca intrusion and to get insights about the emplacement mechanism and tectonic controls of the mineralization the three dimensional gravity modelling shows that the stock has an inverted drop geometry with a feeder zone in contact with the cherneca ductile shear zone the inferred orientation of the feeder zone suggests that the emplacement probably took place along an open tensional crack formed within the strain field of the adjacent cherneca ductile shear zone the modelling of the breccia pipes hosting the ni cu pge ore shows that they are included inside the feeder zone thus their emplacement is probably controlled by successive opening events of this tensional crack

jurassic magmatism in the central mojave desert implications for arc paleogeography and preservation of continental volcanic sequences exposures of jurassic magmatic rocks in the west central mojave desert provide insight into the changing character of volcanism throughout jurassic time and the paleogeography of the continental arc middle jurassic explosive volcanism lower sidewinder volcanic series resulted in collapse of multiple calderas this was followed by north south extension broadly coeval with batholith emplacement late jurassic effusive volcanism upper sidewinder volcanic series appears to reflect transtension over a larger region during intrusion of the independence dike swarm the lower sidewinder volcanic series consists of a subaerial nested caldera complex with an aggregate thickness of 4 km the first caldera formed during the eruption of crystal poor rhyolite ignimbrite outflow and intracaldera facies are intercalated with quartzose sandstone of probable cratonal provenance the second caldera formed during the eruption of crystal rich rhyolite to dacite ignimbrite this ignimbrite exhibits complex mineralogical zoning and contains two units of probable collapse related mesobreccia within the 1 400 m thick intracaldera sequence a third caldera nested within the second is filled with crystal rich biotite dacite ignimbrite and tuff breccia a fourth caldera largely coincident with the second formed during eruption of lithic and pumice lapilli dacite ignimbrite eruption and collapse appear to have been multistage as several units of reworked tuff and fallout tuff occur within the 1 750 m thick intracaldera sequence and caldera collapse breccias occur at the base and near the middle of the sequence the top of this sequence contains reworked tuffs and epiclastic rocks suggesting that the fourth caldera provided a posteruptive depocenter for accumulation of sediment normal faulting tilting and erosion followed explosive volcanism and was broadly contemporaneous with intrusion of middle jurassic porphyritic quartz monzonite plutons plutons and tilted lower sidewinder volcanic series are intruded and unconformably over lain by latest jurassic volcanic rocks upper sidewinder volcanic series the upper sidewinder volcanic series is characterized by alkalic basalt to basaltic andesite and rhyolite lavas hypabyssal intrusions and dikes including the 148 ma independence dike swarm regional northeast southwest extension is suggested by the presence of the northwest striking dike swarm and by bimodal volcanism the absence of debris flow deposits and epiclastic rocks suggests an intra arc region of low relief although local caldera related subsidence was the principal control on accumulation of lower sidewinder volcanic rocks preservation was likely enhanced by middle jurassic extension late jurassic extension was of broader regional extent but lesser magnitude because it did not create basins or cause significant tilting of strata the geometry timing and regional setting of middle and late jurassic extension and magmatism suggest a sinistral oblique subduction regime for the jurassic arc of the southern u s cordillera

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

uranium series disequilibrium in tuffs from yucca mountain nevada as evidence of pore fluid flow over the last million years abstract samples of tuff from boreholes drilled into fault zones in the exploratory studies facility esf and relatively unfractured rock of the cross drift tunnels at yucca mountain nevada have been analysed by u series methods this work is part of a project to verify the finding of fast flow paths through the tuff to esf level indicated by the presence of bomb 36cl in pore fluids secular radioactive equilibrium in the u decay series i e when the radioactivity ratios 234u 238u 230th 234u and 226ra 230th all equal 1 00 might be expected if the tuff samples have not experienced radionuclide loss due to rock water interaction occurring within the last million years however most fractured and unfractured samples were found to have a small deficiency of 234u weighted mean 234u 238u 0 95 0 01 and a small excess of 230th weighted mean 230th 234u 1 10 0 02 the 226ra 230th ratios are close to secular equilibrium weighted mean 0 94 0 07 these data indicate that 234u has been removed from the rock samples in the last 350 ka probably by pore fluids within the precision of the measurement it would appear that 226ra has not been mobilized and removed from the tuff although there may be some localised 226ra redistribution as suggested by a few ratio values that are significantly different from 1 0 because both fractured and unfractured tuffs show approximately the same deficiency of 234u this indicates that pore fluids are moving equally through fractured and unfractured rock more importantly fractured rock appears not to be a dominant pathway for groundwater flow otherwise the ratio would be more strongly affected and the th and ra isotopic ratios would likely also show disequilibrium application of a simple mass balance model suggests that surface infiltration rate is over an order of magnitude greater than the rate indicated by other infiltration models and that residence time of pore fluids at esf level is about 400 a processes of u sorption precipitation and re solution are believed to be occurring and would account for these anomalous results but have not been included in the model despite the difficulties the u series data suggest that fractured rock specifically the sundance and drill hole wash faults are not preferred flow paths for groundwater flowing through the topopah spring tuff and by implication rapid flow within 50 a from the surface to the level of the esf is improbable

oceanic tides from geosat altimetry direct tidal analysis of the altimetry from the geosat exact repeat mission s first year is used to derive estimates of the diurnal and semidiurnal oceanic tides the geoid is removed by collinear differences at 34 1 day separation and the orbit error is reduced by subtracting a slowly modulated 1 cycle revolution term a sequence of independent analyses at grid areas of 1 latitude 1 5 longitude using orthotide functions ensures complete definition of the diurnal and semidiurnal species global admittance maps for m2 and s2 within the latitudes 58 n and 59 s compare well with ground truth at 66 open ocean sites maps of differences between geosat and the naval surface weapons center model show important areas of the order of 10 15 cm the integral for the global power input from the moon gives 2 51 tw m2 0 44 tw s2 lower than many previous estimates but in agreement with figures deduced from a recent analysis of satellite orbits

magmatic flow direction indicators in a giant diabase feeder dike connecticut recent studies have suggested that many large diabase dikes have been emplaced laterally rather than vertically these studies report only unidirectional flow which is usually based on measurements of anisotropy of magnetic susceptibility however in the 250 km long higganum dike of southern new england united states seven independent flow direction indicators record a complex history of forward and backward moving magma we would expect other similar size dikes to have had equally complex histories measurements within one en echelon segment of the higganum dike show that as the magma rose it spread laterally toward the ends of the segment and that backflow was in the reverse direction of the forward flow the higganum dike is interpreted to have formed from a linear array of intrusive fingers each finger spreading laterally to form an en echelon dike segment as it rose through the lithosphere

geochemical evolution of the minto block a 2 7 ga continental magmatic arc built on the superior proto craton abstract a 400 km long by 100 km wide geological transect across the minto block of northeastern superior province reveals five plutonic suites and minor amphibolite to granulite grade supracrustal remnants the oldest regionally significant plutonic rocks include 3 0 0 1 ga foliated to gneissic tonalite characterized by high na k ratios high al contents light ree enrichment and initial gend 1 8 the leaf river plutonic suite is the most widespread with crystallization ages of about 2725 5 ma across the transect the suite comprises massive to foliated pyroxene and hornblende bearing granodiorite tonalite granite diorite and minor gabbro pyroxenite and syn plutonic mafic dykes the rocks are i type calc alkaline have variable na k ratios moderate to high ba and sr contents and ree patterns with variable light ree enrichment depletion of heavy rees with increasing silica and neutral negative or positive eu anomalies initial nd values for the suite range from 0 5 to 1 3 mean 0 4 and initial 87 sr 86 sr ratios isr are 0 7020 the diatexite plutonic suite 2713 ma occurs within a 150 km wide zone and consists of orthopyroxene garnet granodiorite containing 25 50 inclusions of granulite grade mafic supracrustal and tonalitic inclusions the variable major and trace element chemistry of the suite with generally high ni and cr contents reflects heterogeneous source materials and the presence of non liquid components initial nd values are from 0 1 to 1 7 monzogranite and orthopyroxene granite dykes plugs and plutonic masses both 2690 ma comprise the last important plutonic suites both biotite monzogranite and orthopyroxene granite are light ree enriched but the former have negative eu anomalies and the latter positive initial nd values range from 0 7 to 3 2 for monzogranite isr 0 7016 to 0 7067 and 0 7 to 0 2 for orthopyroxene granite isr 0 701 to 0 702 the initial nd and isr values of the 2 7 ga plutonic suites are lower and higher respectively than estimatesf of the contemporaneous depleted mantle suggesting significant and wide scale reworking of older tight ree enriched high rb sr lithosphere early tonalite orthogneiss from goudalie domain may be derived by partial melting of mafic crust at high pressures the lithological and geochemical diversity of the 2725 ma leaf river plutonic suite indicate a range of parental magma compositions and petrogenetic processes but fractional crystallization of mantle derived basaltic melts which had assimilated small amounts of significantly older lower crust may have been the dominant mechanism crustal inclusion charged granodioritic diatexite 2713 ma was probably derived by partial melting of heterogeneous lower crustal materials under relatively dry conditions late biotite monzogranite was generated by partial melting of older sialic crust and orthopyroxene granite by deeper melting or fractional crystallization of more juvenile source materials with garnet as a residual mineral we infer from the presence of coeval mafic dykes in all plutonic suites that heat for melting was ultimately derived from intrusion of basaltic magmas at depth the 2 7 ga magmatism in the minto block occurred during a period of active production and accretion of juvenile oceanic terranes in the southern superior province the northwest directed subduction regime inferred from the southern superior province arc terranes may also have been responsible for the magmatism that acted on 3 ga lithosphere in the northern superior province melting within the mantle wedge beneath the proto cratonic lithosphere and in the lower crust occurred within a tectonic setting similar to that of modem continental magmatic arcs

tectonomagmatism in continental arcs evidence from the sark arc complex abstract the island of sark channel islands uk exposes syntectonic plutons and country rock gneisses within a precambrian cadomian continental arc this sark arc complex records sequential pulses of magmatism over a period of 7 ma ca 616 609 ma the earliest intrusion ca 616 ma was a composite sill that shows an ultramafic base overlain by a magma mingled net vein complex subsequently deformed at near solidus temperatures into the amphibolitic and tonalitic tintageu banded gneisses the deformation was synchronous with d 2 deformation of the paragneissic envelope with both intrusion and country rock showing flat top to the south ls fabrics later plutonism injected three homogeneous quartz diorite granodiorite sheets the creux moulin pluton 150 250 m ca 614 ma the little sark pluton 700 m 611 ma and the northern pluton 500 m 609 ma similar but thinner sheets in the south derrible hogsback dixcart and west port es saies brecqhou are interpreted as offshoots from the creux moulin pluton and little sark pluton respectively all these plutons show the same ls fabric seen in the older gneisses with rare magmatic fabrics and common solid state fabrics recording syntectonic crystallisation and cooling the cooling rate increased rapidly with decreasing crystallisation age 9 ma for the oldest intrusion to cool to lower amphibolite conditions 7 8 ma for the creux moulin pluton 5 6 ma for the little sark pluton and 2 the initiation of the d 2 event is suggested to have been a response to the intrusion of the tintageu magma which promoted a rapid increase in strain rate 10 14 s 1 that focussed extensional deformation into the sark area the increased rates of extension allowed ingress of the subsequent quartz diorite granodiorite sheets although strain rate slowly declined as the whole complex cooled during exhumation the regional architecture of syntectonic cadomian arc complexes includes flat lying sark type and steep guernsey type domains produced synchronously in shear zone networks induced by oblique subduction a pattern seen in other continental arcs such as that running from alaska to california

interconnected sills and inclined sheet intrusions control shallow magma transport in the ferrar large igneous province antarctica field observations and structural data from intrusive complexes at allan hills and terra cotta mountain south victoria land antarctica demonstrate that interconnected sills and inclined sheets transported magma through the shallow subsurface these sills and sheets represent the upper crustal top 4 km plumbing system of the 183 ma ferrar large igneous province the sheets are short in length