listric faulting and continental rifting in western mexico a paleomagnetic and structural study abstract the western trans mexican volcanic belt is characterized by several large scale tectonic depressions which recently have been interpreted in terms of active rifting and independent motion of the jalisco block in this paper we report results for the northwestern part of the jalisco block within the ameca tectonic depression the trend of the central part of the depression changes where it is bounded by the ameca river fault there the fault which is characterized by a steep high scarp and a pronounced surficial curvature bounds the pleistocene amatlan de canas volcanic field which consists of small eroded volcanic structures and mafic lava flows intercalated with conglomeratic talus debris the sinuous pattern of outcrop of the fault indicates that the fault plane dips toward the volcanic field the conglomeratic units are abruptly truncated at the fault contact and generally dip 12 to 20 toward the fault plane paleomagnetic results for the young volcanic units give a mean direction which is interpreted to be the result of 12 to 20 of tilting toward the fault regional listric faulting block sliding of the entire block occupied by the volcanic field is postulated to explain the structural stratigraphic and paleomagnetic patterns observed listric faulting occurs within a transtensional tectonic environment related to the southwesterly motion of the jalisco block as it rifts away from mainland mexico

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

palaeomagnetism of the western and central sectors of the trans mexican volcanic belt implications for tectonic rotations and palaeosecular variation in the past 11 ma summary new palaeomagnetic data from the central and western sectors of the trans mexican volcanic belt tmvb are presented and analysed for palaeosecular variation psv and tectonic rotations fifty one sites with geochronological control were collected from selected volcanic lava flows which cover the temporal and spatial activity in the magmatic arc for the past 11 ma rock magnetic experiments reveal that generally ti poor titanomagnetites sometimes maghemitized are the magnetic carriers of the characteristic remanent magnetizations after analysis of detailed progressive demagnetization data 47 mean site directions are determined in addition data from previous regional palaeomagnetic study in the eastern tmvb sector 53 sites are re analysed palaeomagnetic data are grouped according to geographic distribution three structural sectors and according to age late miocene pliocene or quaternary to avoid discarding tectonic effects the less stringent criterion i e a fixed cut off angle to the mean equal to 45 was applied to identify transitional data affecting four sites palaeomagnetic results backed by positive reversal tests indicate no palaeomagnetically detectable vertical axis block rotations in the study areas the nearly geocentric axial dipole gad inclinations suggest no significant tilting effects and negligible quadrupolar contributions since the mean palaeomagnetic directions for the study areas do not differ from reference directions from the north american polar wander path two data sets for 11 5 and 5 0 ma are tested for geomagnetic purposes virtual geomagnetic poles vgps were selected using both fixed 45 and optimum variable cut off angles this resulted in vgp dispersions that increase back on time and are consistent with those obtained from globally distributed observations at the site latitude for their respective age ranges and cut off criteria palaeomagnetic data from late miocene and pliocene tmvb rocks can be considered in the databases for time averaged field taf and palaeosecular variation from lavas psvl analyses and geomagnetic field geometry characterization during the past 11 ma

active faults and state of stress in the central part of the trans mexican volcanic belt mexico 1 the venta de bravo fault the central part of the trans mexican volcanic belt is being deformed by seismically active west east striking normal faults that cause pronounced scarps structural and seismic along strike discontinuities allow the following segmentation 1 the western segment the area between morelia and the los azufres volcanic center is characterized by 25 km long fault scarps that define the cuitzeo graben and the santa ana maya horst which has a relief 850 m with respect to the cuitzeo graben the faults cut several quaternary scoria cones as well as alluvium 2 the central neotectonic segment extends between maravatio in the west and altamirano volcano in the east the most prominent structure of this segment is the 45 km long venta de bravo fault with a relief 300 m the february 22 1979 mb 5 3 maravatio earthquake occurred on this fault the late quaternary vertical slip rate along the easternmost part of the venta de bravo fault is estimated at 2 mm yr the lateral extension of the segment is defined by the limits of the venta de bravo fault and the eastern termination of the cuitzeo graben 3 the eastern segment is made up of the acambay graben the limit between the central and eastern segments is defined by an extensional jog between the venta de bravo and pastores faults which indicates a left lateral strike slip component the most recent rupture of this segment occurred in the november 19 1912 ms 6 9 acambay earthquake that caused vertical displacements 50 cm along the faults flanking the graben stress determinations based on quaternary scoria cone alignments the focal mechanism of the 1979 earthquake and the inversion of striation measurements indicate a stress field with sv sene snnw the fault striations as well as the focal mechanism indicate a minor but consistent left lateral strike slip component the observed stress and deformation can be explained by the superposition of two sources of stress isostatically compensated surface loads related to the high elevation of the trans mexican volcanic belt probably cause horizontal deviatoric tension and the observed crestal normal faults which strike parallel to the axis of the volcanic belt the observed left lateral strike slip component on the other hand can be explained by compressional far field stresses caused by loads at the boundary between the cocos and north american plates the orientation of the faults with respect to the direction of plate convergence is a prefered orientation for left lateral slip along the observed faults alternatively the observed left lateral strike slip component may accommodate trench parallel motion of crustal blocks in southwestern mexico

high pressure metamorphism during the llano orogeny inferred from proterozoic eclogite remnants mineral chemistries and textures in mafic garnet pyroxene rock within the lost creek gneiss in the purdy hill quadrangle mason county texas record three distinct p t events two of them at pressures higher than any previously documented for these grenville age rocks the earliest metamorphic episode produced an eclogitic assemblage of pyrope rich almandine sodian augite orthopyroxene pargasitic amphibole some orthopyroxene cores record pressures greater than 15 kbar even for the most conservative estimated temperature of 500 c garnet and orthopyroxene rims and matrix clinopyroxene and amphibole record a second event at 750 c and 8 kbar symplectites of clinopyroxene plagioclase and coronas around garnet of plagioclase amphibole magnetite formed in response to reheating and hydration associated with postkinematic intrusion at the amphibolite facies conditions 600 650 c 3 5 5 kbar commonly recorded in the region recognition of early high pressure metamorphic episodes markedly strengthens the correlation between these rocks and other grenville age exposures along the continental margin and provides petrologic support for models invoking proterozoic subduction accompanying the llano orogeny

39ar recoil artifacts in chloritized biotite biotite separates from the metamorphosed granitic rocks of eastern taiwan yield disturbed 40ar39ar age spectra their disturbed profiles become more pronounced with increasing degree of chloritization and are attributed to mixed release of argon from sub micron scale biotite and chlorite intercalations 39ar internal recoil best explains the discordance of the age spectra chlorite is randomly intercalated with biotite at scales ranging from 10 a to several tens of microns both 40ar and k were evidently lost during the chloritization reaction during the neutron irradiation 39ar atoms recoil into the chlorite from the adjacent biotite intercalations the recoil and chloritization combine to produce a variable 40ar39ar spatial distribution in the intercalated biotite chlorite during 40ar39ar step heating analyses argon in biotite is released in two pulses centered at 650 c and 950 1050 c which represent two stages of dehydroxylation of the biotite chlorite out gasses in two pulses centered at 550 c and 700 900 c during dehydroxylation and then decomposes this differential thermal release of argon from each mineral phase with variable 40ar39ar ratios yields discordant incremental heating age spectra

land use and geologic controls on the major elemental and isotopic δ15n and 87sr 86sr geochemistry of the connecticut river watershed usa abstract we studied the relationship between land use geology and water chemistry in the connecticut river watershed of northeastern usa by combining geographic information systems gis with nitrogen and strontium isotopic analyses and major cation and anion geochemistry river water was sampled at 35 sites in the summer and fall of 1998 and the spring of 1999 in an attempt to distinguish spatial and temporal changes in geochemical signatures related to land use and bedrock geology we coupled 87sr 86sr values and major element concentrations from river water to quantify the relationship between bedrock geology and riverine geochemistry the results in our study show that strontium isotopes of tributary waters reflect their catchment bedrock lithology our results also suggest that major cations in the connecticut river water are dominated by carbonate weathering despite the limited presence of calcareous metasedimentary rocks 10 of watershed bedrock in addition the relative contribution of carbonate to silicate weathering increases downriver as a result of input from tributaries draining paleozoic carbonate bearing rocks in contrast nitrate concentrations and nitrogen isotopic ratios of nitrate in water were highly variable nitrate concentrations were highest for tributaries draining developed and agricultural sites suggesting anthropogenic inputs however it was difficult to determine the source of nitrate because of the wide scatter in the nitrogen isotopic data which for the most part did not correlate with land use the only systematic trends in nitrogen isotopic values are found in waters collected in the summer however these data show that 15n values of nitrate from agricultural sites were lower than 15n values of nitrate from forested sites a result that is inconsistent with previous studies our study suggests that strontium isotopes provide a better biomarker for tracking atlantic salmon salmo salar than nitrogen isotopes because unlike 15n 14n ratios 87sr 86sr ratios are both predictable and seasonally constant in water

mylonites from the towaliga fault zone central georgia products of heterogeneous non coaxial deformation abstract retrograde mylonitic rocks occur in the segment of the towaliga fault southeast of jackson georgia mylonite protoliths are heterogeneous quartz feldspathic gneisses upper amphibolite facies sillimanite zone from the pine mountain and piedmont terranes protoliths contain no distinctive lithological markers that can be used to determine fault displacement shear sense can only be determined using microstructural criteria and type porphyroclast systems shear bands asymmetric microfolded quartz ribbons subgrain fabrics in quartz ribbons the most common porphyroclast system developed in mylonites and ultramylonites is type type systems do not have readily identifiable tails but do produce distinctive asymmetrical pile ups in the matrix such that the angle of intersection between the perturbed zone and the mylonitic foliation points in the shear direction differences in morphology between and type systems can be related to differences in the recrystallization rate deformation rate ratio r reflecting changes in ductility contrast between matrix and porphyroclast that controls the amount of recrystallized material in a porphyroclast mantle that is potentially available to form tails in early stages of mylonitization with low ductility contrasts and large r ratios large mantles occur around porphyroclasts and tailed systems develop in advanced stages of mylonitization with large ductility contrasts and small r ratios either very small or partially or totally missing mantles occur and type systems develop microstructural criteria ubiquitously record a right lateral strike slip motion component to the deformation textural evidence e g unidirectional shear bands consistent vorticity recorded by porphyroclasts and asymmetrical microfolds suggest that the mylonites are the products of non coaxial deformation the local presence of fabrics with coaxial symmetry result from heterogeneity in the deformation the right lateral offset on the towaliga fault most likely occurred during the late paleozoic alleghanian orogeny

cr isotope systematics in the connecticut river estuary abstract stable chromium cr isotopes expressed in 53cr as the deviation of 53cr 52cr in samples from the international standard nist srm 979 are an emerging paleoredox proxy that has been used to track earth s redox evolution over a range of timescales there have been several major advances in our understanding of the cr isotope system but we are still developing a basic knowledge of the global cr isotope mass balance and there are key cr fluxes with poorly constrained isotopic compositions the cr isotope behavior in estuaries is one such poorly constrained aspect of the modern cr cycle therefore we collected water and suspended particulate samples from the estuary of the connecticut river and present the first cr isotope dataset from a salinity gradient as ph and salinity increases from brackish to salt water dissolved cr increases from 118 1 25 7 ng kg 1 n 6 to 225 1 26 0 ng kg 1 n 8 while particulate cr decreases from 341 3 319 8 ng kg 1 n 6 to 208 4 364 5 ng kg 1 n 8 dissolved 53cr decreases from 1 30 0 27 1 n 6 to 0 64 0 16 1 n 8 whereas particulate 53cr remained stable at 0 11 0 07 1 n 9 these data suggest that cr is lost from particles to solution when transitioning from brackish to salt water in the connecticut river estuary if these observations can be confirmed in other estuaries estuary systems can potentially enhance riverine dissolved cr fluxes and thus lower the cr oceanic residence time which has important implications for the cr isotope system as a paleoceanographic redox proxy there is a clear inverse correlation between dissolved cr and 53cr in the connecticut river estuary which yields a cr isotope fractionation factor of 0 89 that is indistinguishable from previous estimates based on open seawater samples large 53cr variation in oxygenated open and estuary seawater suggests that marine 53cr variations need not necessarily be exclusively tied to cr vi reduction in low oxygen zones therefore interpreting 53cr in the sedimentary record requires taking multiple processes into consideration

cerro toledo rhyolite jemez volcanic field new mexico 40ar 39ar geochronology of eruptions between two caldera forming events the cerro toledo rhyolite comprises a group of domes and tephra which were erupted during the interval between two caldera forming ignimbrites the tshirege member and otowi member of the bandelier tuff in the jemez volcanic field new mexico to provide a chronologic framework for geochemical and isotopic studies on these rhyolites which record the evolution of the bandelier magma system during this interval a 40 ar 39 ar geochronology study was undertaken pumice from major pyroclastic fall deposits within the rhyolite tephra and samples from the rhyolite domes were dated as well as the stratigraphically bracketing bandelier tuff analyzed crystal populations range from being fairly homogeneous juvenile material to very heterogeneous mixed juvenile and xenocrystic assemblages in most cases dominant groups of juvenile sanidine crystals define 40 ar 39 ar ages which agree with stratigraphic constraints plagioclase analyses are distinctly more scattered and do not typically define reasonable ages the 40 ar 39 ar ages for the two members of the bandelier tuff yield an interval of 380 20 k y between these caldera forming eruptions during this interval nine major pyroclastic pumice units were deposited in the sections studied for which six yield isochron ages one a weighted mean age one a maximum age and one no reliable age due to lack of sanidine 40 ar 39 ar dates on pumice fall units within the cerro toledo rhyolite tephra indicate that eruptive activity occurred at 1 59 1 54 1 48 1 37 and 1 22 ma 40 ar 39 ar dating of cerro toledo rhyolite domes indicates these were erupted within the caldera at 1 54 1 45 1 38 1 34 and 1 27 ma the dates obtained indicate that eruptive activity occurred throughout the 380 k y interval between the two members of the bandelier tuff but suggest that eruptions producing both tephra and domes occurred during discrete intervals at ca 1 54 1 48 and 1 38 1 34 ma the interval from 1 34 to 1 38 ma was particularly active 7 of 18 units dated are these ages

geochemistry and tectonic affinities of a proterozoic bimodal igneous suite west texas diabasic sills from the 1 27 b y old carrizo mountain group of west texas have immobile element abundances similar to those of modern mid ocean ridge basalts morb the presence of thick terrigenous sedimentary rocks and peraluminous rhyolitic ash flow tuffs within the carrizo mountain group preclude the diabases forming in a typical morb environment field petrographic and geochemical evidence suggests that the carrizo mountain group was deposited in an incipient continental margin back arc basin that may have extended northeastward into the texas panhandle region the overlying allamoore formation consists of sedimentary rocks and basalt accumulated within a mature marginal basin behind the tholeiite dominated island arc postulated for the llano area

40ar39ar thermochronological constraints on the structural evolution of the mesoproterozoic natal metamorphic province se africa abstract ten 40 ar 39 ar age spectra are presented for hornblende grains separated from mylonitized and weakly deformed amphibolites from the mesoproterozoic 1 2 1 0 ga natal metamorphic province nmp since the proposal of the sweat hypothesis the nmp has become a crucial area in which to study grenville aged locally termed namaquan accretion in this part of rodinia gondwana unlike many segments of the world s 1 1 ga belts the nmp is particularly suited to such studies since it is devoid of a high temperature pan african 500 ma overprint in this study we have dated hornblendes from within the earliest d1 ne directed thrust dominated structural domains of the belt the d1 domains are characterized by ne directed recumbent fold and thrust nappes and a pervasive generally s to sw dipping metamorphic foliation with down dip stretching lineations the new data constrain the minimum age of this early event which includes obduction of the northernmost tugela terrane onto the kaapvaal craton to 1135 9 ma in the d1 domains of the mzumbe terrane to the south cooling to below 550 c had only been attained by 1005 ma we also dated amphibolites from the later d2 structural domains which are characterized by sub vertical sinistral mylonite belts with sub horizontal to oblique stretching lineations our data show that d2 oblique shearing commenced at 1050 1035 ma in the mzumbe terrane and only terminated at 980 ma the youngest movements also included reactivation of major d1 structures such as the melville thrust mzumbe margate terrane boundary at 990 ma finally indications of minor pan african resetting was detected during the first 5 of the ar ar spectra from mylonites of the lilani matigulu shear zone tugela mzumbe terrane boundary indicating the polyphase nature of this major structure it has been suggested that the d1 event records the early ne sw directed arc continent collision history of the belt whilst the d2 event represents a continuation of essentially the same convergence vectors after extensive crustal thickening the new data place important time constraints on the major tectonic events in the nmp which appear comparable in many ways to other parts of the global grenvillian orogen

structural analysis of sheath folds in the sylacauga marble group talladega slate belt southern appalachians remnant blocks of marble from the moretti harrah dimension stone quarry provide excellent exposure of meter scale sheath folds tubular structures with elliptical cross sections 4 ryz 5 are the most common expression of the folds the tubes are elongate subparallel to stretching lineation and are defined by centimeter scale layers of schist eccentrically nested elliptical patterns and opposing asymmetry of folds s and z are consistent with the sheath fold interpretation sheath folds are locally numerous in the moretti harrah quarry but are not widely distributed in the sylacauga marble group reconnaissance in neighboring quarries provided no additional observations the presence of sheath folds in part of the talladega slate belt indicates a local history of plastic non coaxial deformation such a history of deformation is substantiated by petrographic study of an extracted hinge from the moretti harrah quarry the sheath folds are modeled as due to passive amplification of initial structures during simple shear using both analytic geometry and graphic simulation as indicated by these models relatively large shear strains y 9 and longitudinal initial structures are required the shear strain presumably relates to nw directed displacement of overlying crystalline rocks during late paleozoic orogeny

a shale hosted cr isotope record of low atmospheric oxygen during the proterozoic the emergence and expansion of animal life on earth represents a dramatic shift in the structure and complexity of the biosphere a lack of firm constraints on surface oxygen levels during the mid proterozoic has resulted in heated debate as to whether the rise and earliest diversification of animals was directly linked to a change in environmental oxygen levels or instead simply reflects the timing of innovations in gene expression and developmental regulation and was independent of a direct environmental trigger here we present chromium cr isotope data from marine black shales that provide evidence for minimal cr oxidation throughout the mid proterozoic leading up to the diversification of eukaryotes and the rise of animals during the late neoproterozoic this observation requires very low background oxygen levels 1 of present atmospheric levels accepting previously proposed estimates of p o2 levels needed to induce cr isotope fractionation our data provide support for the persistence of an earth system in which baseline atmospheric p o2 would have been low enough to inhibit the diversification of animals until ca 800 ma more generally evidence for a delayed rise of atmospheric oxygen strongly suggests that environmental factors have played a fundamental role in controlling the emergence and expansion of complex life on earth

neotectonics in central mexico regional neotectonic activity in central mexico was mapped through analysis of eleven landsat thematic mapper images the study area stretches from mexico city on the east to the gulf of california on the west between 17 and 21 n latitude fault scarps of plio quaternary age define several large scale tectonic features within the mexican volcanic belt dominated by rifting transtension and shear with probable sinistral components three large crustal blocks south of the mexican volcanic belt are separated from the north american plate and from each other by zones of neotectonic activity that express the relative motions between the adjacent blocks a simple vector diagram for plates and crustal blocks in central mexico has been constructed using information from the literature combined with the orientations of fault zones as mapped in this study the relative motion between the blocks varies from about 2 5 to 8 mm year 1 which is comparable to motions of crustal blocks in areas of active rifting in other parts of the world in the mexican volcanic belt there is a clear relationship between volcanism and neotectonic activity fault zones that traverse nearly the entire region have the same location and orientation as the volcanic belt this may indicate that deformation within the crust of the overriding slab plays a significant role in determining the unusual orientation of the mexican volcanic belt relative to the trench the ultimate factor controlling the location of plio quaternary volcanism and tectonism in central mexico may be large zones of crustal weakness formed during major mesozoic and cenozoic tectonic events

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

late middle proterozoic hazel formation near van horn trans pecos texas evidence for transpressive deformation in grenvillian basement the latest middle proterozoic hazel formation near van horn in the trans pecos region of west texas is an orogenic elastic wedge that provides insight into paleotectonic evolution of the southern margin of the laurentian craton the hazel formation is 2 5 km thick adjacent to the contemporaneous streeruwitz thrust fault and consists exclusively of terrestrial sedimentary rocks boulder sized clasts in conglomerate were deposited within alluvial fans north of the streeruwitz thrust sheet 0along the southern margin of a basin of unknown geometry and extent the alluvial fan conglomerate interfingers with eolian sedimentary rocks in the northern part of the basin alluvial fan conglomerate megasequences indicate that at least two episodes of faulting accompanied hazel sedimentation alluvial megasequences are overall aggradational within the hazel succession as revealed by clast size variations within the conglomerate systematic variation of pebble composition up through the hazel conglomerate provides an erosional history within the source terrain carbonate and subordinate mafic volcanic rocks derived from the pre orogenic allamoore and tumbledown formations were the principal source off the hazel formation metavolcanic and metasedimentary rocks from the carrizo mountain group in the present tectonic hinterland are notably absent in the hazel conglomerate moreover a rhyolite terrain presently not exposed in the van horn area was a significant component of the hazel source juxtaposition between source terrain and sedimentary basin coupled with aggradation of alluvial fan megasequences along the southern fault bounded basin margin supports a transpressive regime for this orogenic belt rather than a purely convergent margin deformation associated with the hazel orogenic sequence post dated a 1 126 to 1 070 m y old regional felsic igneous event grenville deformation in the llano uplift of central texas predated the same igneous event this implies that the hazel tectonic episode was a second later grenvillian deformation the terms llano orogeny and hazel orogeny are introduced here for the grenville province in texas and may be comparable with the elzevirian and ottawan orogenies within the grenville province of canada

tectonics along the trans mexican volcanic belt according to palaeomagnetic data abstract subduction of the cocos plate beneath the southern mexico continental margin takes place at the middle america trench volcanism of calc alkaline character occurs along the e w trending trans mexican volcanic belt tmvb which includes several active stratovolcanoes monogenetic cinder cone fields and silicic centres however a direct association between the volcanism and plate subduction has been questioned for a number of reasons including the oblique orientation of the tmvb with respect to the trench most palaeomagnetic directions for the cretaceous to quaternary within or immediately to the south of the tmvb appear discordant the declinations are rotated anticlockwise relative to expected declinations this angular divergence may be explained by rotations of crustal blocks by different degrees in response to large scale left lateral shear across central mexico the fault system may have acted as an efficient structural control for the rise of magmas which may explain some of the peculiarities of the tmvb

superimposed folding and thrusting by two phases of mutually orthogonal or oblique shortening in analogue models abstract orogens may suffer more than one phase shortening resulting in superposition of structures of different generations superimposition of orthogonal or oblique shortening is studied using sandbox and centrifuge modelling results of sand models show that in orthogonal superimposition the two resulting structural trends are approximately orthogonal to each other in oblique superimposition structures trend obliquely to each other in the relatively thin areas of the model foreland and mutually orthogonal in areas where the model is thickened during the first phase of shortening i e the hinterland thrusts formed during the first shortening phase may be reactivated during the later shortening phase spacing of the later phase structures is not as wide as expected considering they across the pre existing thickened wedge superposition of structures results in formation of type 1 fold interference pattern bedding is curved outwards both in the dome and basin structures folded layers are dipping and plunging outwards in a dome while they are dipping and plunging inwards in a basin in the areas between two adjacent domes or basins i e where an anticline is superimposed by a syncline or a syncline is superimposed by an anticline bedding is curved inwards and the anticlines plunge inwards and the synclines outwards the latter feature could be helpful to determine the age relationship for type 2 fold interference pattern in tectonic regions where multiple phases of shortening have occurred the orogenic scale dome and basin and arrowhead shaped interference patterns are commonly formed as in the models however in some areas the fold interference pattern might be modified by a later phase of thrusting similar to models results superimposition of two and or even more deformation phases may not be recorded by structures all over the tectonic area

chromium fluxes and speciation in ultramafic catchments and global rivers abstract the role of ultramafic rock weathering in global chromium cr budgets and how these may be linked to changes in earth s climate over geological time was investigated by compiling dissolved cr speciation and fluxes in i 73 of the worlds large rivers representing 35 of global river discharge to oceans and in ii an ultramafic catchment putah creek watershed in the california coast range mountains usa weathering of ultramafic rocks creates ideal conditions for cr release and redox cycling alkaline river water draining ultramafic catchments is naturally enriched in cr up to 582 nmol l chromium concentrations increase with the extent of rock water interaction as indicated by correlations with mg and hco3 concentrations and ph whereas cr cycling depends on the availability of electron donors and acceptors e g fe ii organics mn iii iv oxides thus dissolved cr is exported from ultramafic watersheds as both the soluble hexavalent cr vi species mgcro4 cro42 and also as trivalent cr iii species croh co3 22 cr oh 3 the latter cr iii species were previously thought to have low solubility ultramafic catchments have higher area normalized cr and major ion fluxes for a given runoff when compared to global rivers and may thus have a disproportional impact on global cr budgets long term carbon cycling and moderation of the earth s climate over million year timescales riverine export fluxes of cr are linearly correlated with fluxes of ca mg and alkalinity in both ultramafic catchments and global rivers this suggests that silicate weathering is a key control on cr fluxes and that major element weathering fluxes may be a useful proxy for estimating cr and other trace element fluxes globally present day riverine dissolved cr fluxes to oceans are spatially variable and estimated to be 1 7 109 mol yr three times higher than previously reported however when geochemical reactions in estuaries are considered riverine cr fluxes may be lower throughout earth history cr weathering fluxes also may have varied with changes in the global distribution of ultramafic rocks cr concentrations in continental rocks or climactic conditions e g atmospheric co2 levels or runoff such variation should be considered when interpreting both modern and past cr seawater isotopic compositions and residence times

stress pattern evolution in the central sector of the mexican volcanic belt abstract stratigraphic studies and detailed field analyses of brittle and plastic deformations allow the reconstruction of the stress pattern and the tectonic evolution of the central sector of the mexican volcanic belt several compressional phases affected the metamorphic basement while the last one also involved the volcanic cover of early miocene age in middle miocene early pliocene times and ene wsw extensional phase developed representing the southern protraction of the basin and range province of the western united states in pliocene quaternary times there then followed the development of a complicated deformational cycle showing the superficial evidence of a sinistral lateral shear zone at the beginning the area was affected by a ne sw compression including reverse dextral and sinistral strike slip faulting this was followed by early pleistocene sinistral transtension characterized by a nw se direction of least principal stress and sinistral normal slip faulting during late pleistocene holocene times the transtension was concentrated along e w faults tectonic control in the distribution of volcanism has also been recognized the basin and range phase controlled the deposition of the miocene volcanic and clastic units particularly the effusion of basaltic lavas connected with the latest extensional movements of the same phase the mexican volcanic belt sequence is strictly linked with the development of a large e w sinistral lateral shear zone active from the pliocene to the present which shows the extensional component progressively prevailing over the shear one

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

present day kinematics of the rivera plate and implications for tectonics in southwestern mexico we model the present day motion of the rivera plate relative to the pacific north america and cocos plates and combine the results with the nuvel i model for cocos north america motion to examine present day deformation in southwestern mexico the pacific rivera data which include 25 three m y average spreading rates determined from original surface ship magnetic data and 22 azimuthal data along the rivera transform are systematically misfit by the nuvel i pacific north america and pacifie cocos euler vectors indicating that the rivera plate is kinematically distinct from north america and cocos an f test shows that the improvement in fit to the pacific rivera data from a model with a distinct rivera plate exceeds that expected solely from adding three model parameters the rivera north america euler vector derived from closure of the rivera pacific north america plate circuit predicts slower and more trenchnormal convergence along the acapulco trench than prior models in addition the model predicts convergence normal to the eastern tamayo fracture zone at a rate 60 slower than the right lateral strike slip predicted by prior models an observed systematic misfit of the pacific rivera euler vector to azimuths from the western rivera transform fault may have several causes there may be a time averaging problem between the 3 0 m y average rates and the shorter time average transform fault azimuths and earthquake slip vectors alternatively rivera transform trends may be biased by deformation within lithosphere adjacent to the transform valley or future detailed mapping of the transform may reveal the misfit to be an artifact of the presently available bathymetry along the entire pacific rivera rise spreading rates averaged over the past 0 7 m y are systematically faster than 3 0 m y average rates with the difference increasing southward along the rise the post 3 ma change in the rate gradient has caused a southward migration of the pacific rivera euler pole since 3 ma possibly in response to the 3 ma completion of a spreading reorganization that doubled the length of the rivera transform fault and halved the area of the rivera plate we hypothesize that deformation in southwestern mexico including opening along the colima rift is related to oblique subduction of the cocos plate along the northern middle america trench three unes of evidence support this hypothesis first the sense of oblique subduction predicted by nuvel i along the middle america trench is consistent with a model in which part of western mexico located southeast of the colima rift moves to the southeast relative to north america second field geologic data and landsat imagery suggest that in the past few million years the chapala oaxaco fault zone has accommodated several kilometers of sinistral motion and sinistral transtension has occurred along the trans mexican volcanic belt either or both of these fault zones could accommodate any southeastward coastal block motion that might result from oblique subduction third the observation that earthquake slip vectors from the northern middle america trench trend systematically counterclockwise from the predicted cocos north america convergence direction is also consistent with southeastward motion the discrepancy between the observed and predicted slip directions along the middle america trench suggests sinistral slip is 0 10 mm yr 1 if oblique subduction is driving southeastward transport of a coastal sliver the colima rift is a passive pull apart zone at the northwestern end of this sliver rather than an incipient spreading ridge that will replace the pacific rivera rise

grenville age a type and related magmatism in southern laurentia texas and new mexico u s a abstract large grenville age plutons are known to be present in the 1340 1370 ma basement in texas and eastern new mexico but their distribution and compositional ranges are poorly known new u pb zircon dates shrimp rg stanford university on subsurface samples have identified widespread presence of compositionally diverse grenville age 1070 1110 ma plutonic rocks in the texas panhandle core from a 175 m thick sill like gabbroic intrusion yielded a date of 1081 8 3 ma in situ differentiation of this tholeiitic magma led to a 7 fold increase in incompatible element concentrations as ree patterns remained essentially flat trace element data suggest an e morb like source to the south alkali feldspar granite with a type affinities was recovered from drill core from the abilene gravity minimum near albany texas it yielded a date of 1078 23 ma similar to undeformed post orogenic granites in the llano uplift further west an anorthosite xenolith from the eocene three sisters intrusion in el paso texas yielded a date of 1068 30 ma which is within error of the 1110 19 ma age determined for the main stage of the nearby a type red bluff granitic suite xenoliths from potrillo maar volcano in the center of the southern rio grande rift include monzonitic xenoliths with granulitic texture which yielded dates of 1072 ma in contrast potrillo maar xenoliths with igneous textures yielded u pb dates of 27 ma development of the granulitic texture is interpreted to reflect granulite facies metamorphism during tertiary time 26 27 ma on the basis of age zoning and u concentrations in the zircon crystals our results show that grenville age magmatism in the texas and new mexico subsurface was widespread and was coeval with syn and post deformation granites in the llano uplift of central texas the compositions of dated samples suggest that mafic magmas were broadly tholeiitic in character and that granitic magmas were a type these magmatic affinities are thought to be associated with crustal extension rather than with subduction moreover the presence of anorthosite xenoliths in the el paso kilbourne hole region the broadly tholeiitic character of mafic rocks and the a type nature of the granitic rocks are broadly similar to amcg anorthosite mangerite charnockite granite suites present in the grenville province of eastern laurentia

fluid evolution and transport during metamorphism evidence from the llano uplift texas a field petrologic and stable isotopic investigation of the marbles and calc silicates of the 1 15 b y valley spring gneiss documents the dilution of internally evolved co2 rich fluids by externally derived aqueous fluids introduced along channelways reaction textures within calcsilicates record the evolution through time of initially co2 rich fluids toward increasingly more aqueous compositions assemblage zonations within calc silicates require equilibration within local gradients of the mole fraction of co2 in the fluid and suggest that the infiltration of aqueous fluids was largely channelized along more permeable lithologies localized depletions in 13c and 18o corroborate petrologic evidence for channelized infiltration isotopic compositions reflect both devolatilization and the introduction of low 18o fluids estimated minimum oxygen equivalent fluid to rock ratios are near unity both mineralogical and stable isotopic systematics document the essential role of infiltration in driving decarbonation reactions during calc silicate formation the calc silicate assemblages which equilibrated with fluids of the lowest mole fraction of co2 record isotopic exchange equilibrium with fluids of 18o typical of those derived from normal granites as do the granitic aplites and pegmatites which transect most calcsilicate occurrences thus the infiltrating fluids are believed to be genetically linked to the intrusion of a suite of granitic plutons emplaced after the peak of regional metamorphism

variation of cenozoic extension and volcanism across the southern sierra madre occidental volcanic province mexico the middle to late cenozoic tectonic magmatic evolution of the sierra madre occidental volcanic province south of the tropic of cancer is summarized and analyzed for the first time based on new geologic and structural work and published information in the eastern part of the study region mesa central physiographic province silicic volcanism occurred in a short lived episode culminating at ca 30 ma and was followed by crustal scale extension between 30 and 27 ma in the western part of the study area sierra madre occidental physiographic province a voluminous episode of ignimbrite volcanism at 24 21 ma was succeeded by east west extension that produced regularly spaced grabens affecting only the upper crust in the westernmost part of the study region an andesitic to rhyolitic arc formed between 17 and 12 ma was affected by crustal scale north northwest trending extensional faulting leading to the formation of the gulf of california in the mesa central the maximum extension was oriented approximately east west and amounted to 20 in the eastern sierra madre occidental physiographic province extension was only 8 and oriented approximately east west we observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late oligocene early miocene and late miocene time comparison with the offshore tectonics indicates that the first two tectonic magmatic pulses coincide with periods of fast spreading at the pacific farallon boundary south of the shirley fracture zone we propose that increases in the spreading rate are related to periods of high subduction rate which in turn correspond to episodes of retreating subduction a retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate boundary conditions i e plate tectonics ultimately determined timing magnitude and orientation of extension whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism

uranium and molybdenum isotope systematics in modern euxinic basins case studies from the central baltic sea and the kyllaren fjord norway abstract recent investigations have revealed significant fractionation of 238u 235u between organic rich sediments of anoxic marginal seas and seawater indicating redox dependent u isotope fractionation this study explores the conditions controlling u isotope fractionation in selected modern anoxic basins baltic sea landsort and gotland deeps and the kyllaren fjord in norway and compares u with mo isotope fractionation therefore the concentrations and isotope compositions of dissolved u and mo from the water column and organic rich sediments from three currently anoxic basins have been analysed the water column samples from the kyllaren fjord display a moderate depletion of u and a strong depletion of mo with increasing depth these variations are associated with a decrease in 238u and an increase in 98mo with depth from 0 35 to 0 70 and from 2 4 to 2 6 respectively from the u isotope composition of the deep euxinic water column of the kyllaren fjord a minimum value of 238ured 0 7 for the u isotope fractionation during reduction is inferred due to the high sedimentation rate in the kyllaren fjord surface sediment samples are only moderately enriched in u and mo 4 g g and 6 g g to 37 g g respectively and display 238u and 98mo averages of 0 26 0 10 and 2 18 0 21 respectively water column samples from the baltic sea display a minor decrease in the u and mo concentrations and only negligible u and mo isotope fractionation compared to open seawater the sediment layers from the baltic sea landsort deep which were deposited under mostly anoxic conditions are moderately enriched in u up to 8 g g and highly enriched in mo up to 222 g g however u and mo isotopic compositions are significantly lighter with 238u of 0 36 0 08 and 98mo of 0 04 0 11 than those of typical organic rich sediments from anoxic basins these findings reveal that compared to mo the u isotopic composition of sediments in restricted and strongly euxinic basins like the kyllaren fjord is significantly more dependent on 1 the extent of u removal from the water column which is lower than that of mo and on 2 the sedimentation rate i e the fraction of authigenic u relative to detrital u in the sediment which is also lower compared to that of mo in more open and only temporarily euxinic basins such as the investigated basins of the baltic sea strong mo is coupled with weak u isotope fractionation between water and sediment these signatures were likely the result of isotope fractionation under weakly sulfidic conditions and the generation of isotopically light mo during frequently occurring flushing events with oxygen rich water from the open sea this implies that mo and u isotope signatures of sediments only record paleo water column redox conditions of restricted basins if the water column was permanently stratified

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

proterozoic ocean redox and biogeochemical stasis the partial pressure of oxygen in earth s atmosphere has increased dramatically through time and this increase is thought to have occurred in two rapid steps at both ends of the proterozoic eon 2 5 0 543 ga however the trajectory and mechanisms of earth s oxygenation are still poorly constrained and little is known regarding attendant changes in ocean ventilation and seafloor redox we have a particularly poor understanding of ocean chemistry during the mid proterozoic 1 8 0 8 ga given the coupling between redox sensitive trace element cycles and planktonic productivity various models for mid proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients with potential ecological constraints on emerging eukaryotic life here we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates we find that the metal enrichment record implies a proterozoic deep ocean characterized by pervasive anoxia relative to the phanerozoic at least 30 40 of modern seafloor area but a relatively small extent of euxinic anoxic and sulfidic seafloor less than 1 10 of modern seafloor area our model suggests that the oceanic mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of mo and chromium enrichments through time is consistent with the possibility of a mo n colimited marine biosphere during many periods of earth s history

active rifting in southwestern mexico manifestations of an incipient eastward spreading ridge jump a major ongoing episode of continental rifting has formed a system of three intersecting grabens in the western mexican volcanic belt these structures may mark the initial stage of an eastward spreading ridge jump beneath continental crust eastward ridge segment jumps have been propagating northward along this part of the east pacific rise for the past 12 m y but this is the first plate boundary reorganization event to involve continental crust since the separation of baja california in the miocene since pliocene time small volumes of unusual alkaline magmas have erupted in the graben areas in close association with the more abundant subduction related calc alkaline magmas erupted from andesitic stratovolcanoes the alkaline magmas are varieties found in zones of active rifting elsewhere in the world

chromium isotope signature during continental crust subduction recorded in metamorphic rocks the chromium isotope compositions of 27 metamorphic mafic rocks with varying metamorphic degrees from eastern china were systematically measured to investigate the cr isotope behavior during continental crust subduction the cr isotope compositions of all samples studied were bulk silicate earth bse like with 53crnist979 of greenschists amphibolites and eclogites ranging from 0 06 to 0 17 0 05 to 0 27 and 0 01 to 0 24 respectively the lack of resolvable isotopic variability among the metamorphic rocks from different metamorphic zones indicated that no systematic cr isotope fractionation was associated with the degree of metamorphism however the cr isotopic variability among homologous samples may have reflected effects induced by metamorphic dehydration with a change of redox state rather than protolith heterogeneity i e magma differentiation in addition the differences in 53cr 53crcpx gt between coexisting clinopyroxene cpx and garnet gt from two garnet pyroxenites were 0 06 and 0 34 respectively indicating that significant inter mineral cr isotope disequilibria could occur during metamorphism to provide a basis for comparison with metamorphic rocks and to provide further constraints on the potential cr isotope heterogeneity in the mantle and in the protolith of some metamorphic rocks we analyzed mantle derived chromites and the associated peridotites from luobusa and we obtained the following general order chromite free peridotites 0 21 to 0 11 chromite bearing peridotite 0 07 chromite 0 06 these findings imply potential mantle heterogeneity as a result of partial melting or fractional crystallization associated with chromite

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

an integrated paleomagnetic study of rio grande de santiago volcanic succession trans mexican volcanic belt revisited abstract we carried out an integrated paleomagnetic rock magnetic and paleointensity study of miocene volcanic succession from the trans mexican volcanic belt tmvb north of guadalajara a total of 37 consecutive basaltic lava flows 326 oriented standard paleomagnetic cores were collected at lazo locality continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases reasonably reversible curves with curie points close to that of pseudo single domain magnetite two geomagnetic reversals were observed in the 300 m thick composite section paleosecular variation was lower than the one observed in general during miocene it appears that the volcanic units have been emplaced during a relatively short time span of about 1 ma the mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle miocene the mean paleomagnetic direction calculated from all data is i 31 1 d 354 6 k 124 and 95 2 1 n 37 seventy two samples with apparently preserved primary magnetic mineralogy and without secondary magnetization mostly belonging to reverse polarity chron were pre selected for thellier paleointensity determination the flow mean paleointensity values are ranging from 22 4 3 4 to 53 8 6 0 t and the corresponding virtual dipole moments vdms are ranging from 5 4 0 8 to 12 0 1 4 1022 a m2 this corresponds to mean value of 7 7 2 2 1022 a m2 which is close to present day geomagnetic field strength altogether our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations

palaeomagnetism of the miocene jantetelco granodiorites and tepexco volcanic group and inferences for crustal block rotations in central mexico abstract palaeomagnetic directions isolated from seven sites in three bodies of the miocene jantetelco granodiorites and from two sites in two lava flows of the miocene tepexco volcanic group of central mexico point about 50 west of the expected miocene and middle tertiary geomagnetic field directions for the area both alternating field and thermal demagnetization experiments indicate that these remanent magnetizations are stable and free from any significant secondary overprinting the granodiorites present both normal and reverse polarity and at one site there is evidence of the coexistence of both polarities suggesting that the cooling of the intrusive spanned at least one polarity transition of the geomagnetic field the lavas present reverse polarity the angular deflection is attributed to the tectonic rotation of a crustal block about a pivot located somewhere nearby the tectonic rotation is likely related to lateral shear acting along the trace of the mexican volcanic belt the mechanism may have involved rotations of rigid crustal fragments while the area was translated in response to lateral shear along large regional bounding faults alternatively the rotations may have been associated with the formation of pullapart basins again in conjuction with the regional lateral shear

the fate of chromium during tropical weathering a laterite profile from central madagascar abstract we performed a mineral geochemical and cr sr pb isotope study on a laterite profile developed on ca 540 ma old tonalitic bedrock in madagascar with special emphasis on the behavior of chromium during tropical weathering the observed strong depletions of ca si and p and enrichment of fe and al in the soil relative to bedrock and underlying saprolite are the characteristic features pertinent to laterites the enrichment of fe in topsoil horizon can be correlated with enrichment of p and the redox sensitive elements mn and cr and indicates redistribution of these elements related to oxidation reduction processes the slight scatter of 87sr 86sr and 206 207 208pb 204pb values in the profile is one consequence of such redistribution processes our results are compatible with a two stage process during alteration 1 an incipient alteration stage characterized mainly by a pervasive effective and complete loss of sulfides accompanied by patchy alteration constrained along fissures and cracks and 2 a main stage of soil formation actual laterization with loss of na k ca and si and accumulation of fe and mn relative to the unaltered bedrock this two stage evolution is also depicted by cr concentrations and 53cr values along the studied profile where the first incipient alteration stage caused a pronounced depletion of cr particularly hosted by sulfides and where the second alteration laterization led to redistribution and a small increase in cr concentration in the uppermost portions of the soil profile relative to stage one altered saprolite this gain in cr is accompanied by decreasing 53cr values and can be explained by partial immobilization possibly by adsorption coprecipitation on with fe oxy hydroxides of mobile cr iii during upward transport in the weathering profile the negatively fractionated 53cr values measured in the weathering profile relative to the unaltered tonalitic bedrock characterized by a high temperature magmatic inventory cr isotope signature are consistent with loss of a positively fractionated cr vi pool formed during weathering the predicted existence of a former positively fractionated and mobile chromium pool has been experimentally constrained in circumneutral and basic leachates of powdered tonalite bedrock where 53cr of 0 21 to 0 48 was measured our results show that mobilization of chromium is effective under highly oxidative conditions which in well drained sulfide bearing parent bedrocks potentially lead to both acid dissolution of sulfide hosted cr and redox promoted mobilization of cr vi from silicates during later stages of weathering under basic ph conditions

sedimentary chromium isotopic compositions across the cretaceous oae2 at demerara rise site 1258 abstract in order to advance our understanding of the emerging chromium cr isotope system as a paleoredox proxy we measured the 53cr 52cr of black shales deposited before during and after cretaceous oceanic anoxic event 2 oae2 we observed a 1 coherent negative 53cr 52cr excursion and significant drawdown in cr enrichments during oae2 coincident with a large positive carbon isotope excursion our observed negative 53cr 52cr excursion during oae2 is most easily linked to an increase in the ratio of euxinic to reducing conditions additional work on other oae sections is needed to determine the spatial significance of this cr isotope trend

seismicity and tectonics of the rivera plate and implications for the 1932 jalisco mexico earthquake seismicity on and near the boundaries of the rivera plate is examined in order to determine the subduction regime of the rivera plate and its influence on the earthquake behavior of the northernmost section of the mid america trench in particular we address the question of a rivera cocos boundary there have been several large historic earthquakes in the coastal areas of the mexican states colima and jalisco but the last large event was in june 1932 the 1932 jalisco earthquake ms 8 1 the present quiescence has lasted longer than the average recurrence interval for the mexican subduction zone and the jalisco area is now categorized as a seismic gap however if subduction in this area is dominated by the motion of the rivera plate which is subducting at half the rate of the cocos plate the deficiency of earthquake activity may not be unusual in an attempt to better delineate the rivera cocos boundary intermediate sized earthquakes along and near the rivera fracture zone and the mid america trench were relocated with the joint epicenter determination method we found that in general the catalog locations are fairly accurate with the average change in epicenter upon relocation being 12 km seismicity along the rivera fracture zone follows the physiographic trend of the fracture zone but east of its intersection with the east pacific rise seismicity is more diffuse two intermediate sized earthquakes in this diffuse area have focal mechanisms suggesting that they represent rivera cocos boundary motion reexamination of arrival time data for the 1932 jalisco earthquake yields an epicenter at 19 57 n 104 42 w close to the boundary zone inferred from these two earthquakes we conclude that the possibility that the 1932 jalisco earthquake broke the northernmost section of the cocos north american plate interface as opposed to the rivera north american plate interface cannot be ruled out

late thermal evolution of proterozoic rocks in the northeastern llano uplift central texas abstract the llano uplift of central texas usa contains large exposures of polydeformed grenvillian metaigneous and metasedimentary rocks that were intruded by late to post orogenic granites new u pb and 40 ar 39 ar single mineral ages and rb sr mineral isochrons document the retrograde cooling history that followed grenville age metamorphism in the north central and northeastern llano uplift and explain discrepancies between previous u pb crystallization ages and younger determinations based on rb sr and k ar isotopic systems dating has been carried out in the valley spring gneiss vsg and the post tectonic lone grove pluton lgp in three areas of the llano uplift from west to east vsg rocks in the babyhead area north of llano texas yielding hornblende muscovite and biotite 40 ar 39 ar ages and rb sr mineral isochron ages from 1098 to 1014 ma a sample from the lgp at petrick quarry yielding u pb titanite and hornblende and biotite 40 ar 39 ar ages from 1096 to 1076 ma and vsg samples from the inks lake area in the eastern llano uplift yielding u pb titanite hornblende and biotite 40 ar 39 ar and rb sr mineral isochron ages from 1087 to 1029 ma titanite from the oatman granite to the southwest of these areas yields a u pb age of 1119 6 ma these ages combined with closure temperature estimates document cooling in the llano uplift from 1100 1000 ma at rates between 5 and 14 c ma 1 although rapid uplift following early high pressure metamorphism has important regional thermal effects the cooling rates documented in this study are interpreted to represent the relaxation of local geotherms following local magmatic activity the low temperature history documented in this study is probably unrelated to uplift rates this supports the suggestion that magmatic heating was an important factor in the late high temperature low pressure metamorphism of the llano region it further suggests that the late pressure and temperature conditions recorded in the rocks were likely not synchronous across the region

late miocene volcanism and intra arc tectonics during the early development of the trans mexican volcanic belt abstract the early stage of the trans mexican volcanic belt hereafter tmvb is marked by widespread mafic to intermediate volcanism emplaced between 11 and 7 ma from the pacific coast to the longitude of mexico city to the north of the modern volcanic arc petrological and geochronological data support the hypothesis that this volcanism made up a unique late miocenic central mexican comagmatic province mafic lavas at the mouth of the gulf of california and along the northwestern sector of the tmvb made up the nayarit district which includes calc alkaline to transitional varieties the central sector of the tmvb is characterized by two basaltic districts the jalisco guanajuato and the queretaro hidalgo which are distinguished from the westernmost ones by their lower nb la and generally lower hfse lile values as well as by spider diagrams characterized by larger negative spikes at th ta nb and ti the surface occurrence of the late miocene basalts appears to be controlled by pre existing zones of crustal weakness that channeled the mafic magmas field observations suggest that these structures have been reactivated in a transtensional fashion induced by differential tectonic motion of crustal blocks to the south and to the north of the tmvb starting from 12 ma the tmvb separates a northern tectonic domain subject to the developing divergent pacific north america plate boundary from a southern tectonic domain characterized by oblique subduction of the rivera and cocos plates apparently far field stresses related to these complex plate boundaries reactivated older suture zones allowing rapid uprise of mantle derived magmas the subduction related signature shown by miocene mafic lavas of the jalisco guanajuato district argues against the existence of mantle plumes beneath this sector of the north america plate on the other hand the occurrence in the western tmvb and in the guadalajara region of a large volume of mafic magmas which sometimes show characteristics transitional to ocean island basalts could be due to passive upwelling of the subslab asthenosphere that might have interacted with subduction related magmas and continental lithosphere to produce the observed basaltic varieties subslab magmas may have flowed through slab free areas along the northern and eastern edges of the subducting rivera plate which in the late miocene was already detached from the farallon plate remnants and diverging from the cocos plate

an 40 ar 39 ar investigation of the contact effects of a dyke intrusion kapuskasing structural zone ontario an 40ar 39ar study on biotite and hornblende from baked tonalite gneiss adjacent to a 14 m wide kapuskasing dyke in the chapleau block of the kapuskasing structural zone employed resistance furnace step heating of bulk mineral separates single grain laser step heating and laser spot dating of individual grains in thick section all three methods yielded concordant results for the biotite indicating that it has been uniformly overprinted at ca 2 05 ga this date is probably a close estimate of the age of the dyke intrusion the argon data from the hornblende are more difficult to interpret all three methods suggest that the hornblende was partially overprinted and then affected by a later influx of excess argon from the external environment laser spot dates record an apparent age gradient extending radially outward from 2 25 ga in the grain cores to 2 4 2 7 ga in the grain rims corresponding 37ar 39ar values show no systematic variation across the hornblende although 38ar 39ar ratios display a similar pattern to the age distribution with lower values in the cores than in the rims the bulk separate and single grain step heating spectra mimic a volume diffusion ar loss profile that would appear to be inconsistent with the laser spot dates three possible interpretations to explain this apparent discrepancy may involve either experimental difficulties an effective radius of diffusion which is much smaller than the actual grain radius or an effective radius of diffusion that is of the order of the grain radius it is clear that for the 40ar 39ar dating of samples that have experienced complex geologic histories it is important to utilize both resistance furnace and laser techniques

characterization and tectonic evolution of a mesoproterozoic island arc in the southern grenville orogen llano uplift central texas abstract integrated field and u pb geochronologic data document the presence and tectonic evolution of a distinct lithotectonic terrane in the southeastern llano uplift here termed the coal creek domain ccd the ccd consists of tonalitic and amphibolitic gneisses foliated gabbroic tonalitic and granodioritic intrusions and the structurally emplaced coal creek serpentinite the oldest rocks in the ccd consist of tonalitic gneisses with protolith crystallization ages of 1326 ma and 1301 ma and pre 1292 ma amphibolitic gneisses all ages are u pb zircon they were intruded by dominantly mafic and intermediate with subordinate amounts of felsic low k plutonic rocks between 1286 ma and 1275 ma high temperature metamorphism and local anatexis that resulted in the formation of gneisses and migmatites occurred between 1301 ma and 1286 ma and is suggested to have occurred at 1292 ma metamorphism at 1256 ma resulted in the growth of zircons in mafic rocks the regional significance of this metamorphic event is unclear but it may be related to dynamothermal metamorphism of the ccd the contemporaneous emplacement of low k gabbro tonalite and granodiorite and their spatial association with serpentinite combined with published geochemical data strongly suggest that the ccd represents an ensimatic arc the coal creek serpentinite is interpreted as a fragment of oceanic crust that formed either basement to the arc or in a related back arc or intra arc setting combined field u pb geochronologic isotopic and geochemical data indicate that the magmatic metamorphic and structural history of the ccd differs from that of coeval rocks elsewhere in the llano uplift the boundary between the ccd and deformed and metamorphosed granitic and supracrustal rocks to the north is a zone of intense mylonitization and polyphase deformation this zone is interpreted as the structural boundary between the arc terrane and units to the north

the chromium isotopic composition of seawater and marine carbonates article history chromium isotopes are fractionated during redox reactions and have the potential to provide a record of changes in the oxygenation levels of the oceans in the geological past however cr is a trace metal in seawater and its low concentrations make isotopic measurements challenging here we report the first determinations of 53 cr for seawater from open ocean argentine basin and coastal southampton water settings using a double spike technique the total chromium concentration in seawater from southampton water is 1 85 nm whereas the cr content of argentine basin samples is 5 8 6 6 nm the 53cr value of seawater from the argentine basin is 0 491 0 556 in intermediate and deep waters and varies between 0 412 and 0 664 in surface waters 150 m the 53

structure of the carrizo mountain group southeastern carrizo mountains west texas a transpressional zone of the grenville orogen 1 the west texas segment of the grenville orogen includes an oblique dextral transpressive zone the orogen exposed near van horn texas displays a transition from the mid amphibolite facies metamorphic core to a foreland fold and thrust belt in this paper detailed structural mapping and analyses presented for key exposures of the metamorphic rocks the mesoproterozoic carrizo mountain group show a southeastward increase in number of deformational phases and intensity of deformation the central portion is dominated by oblique northwest convergence which produced two phases of northwest verging folds and synchronous oblique reverse and dextral shearing the thick parts of pre tectonic mafic plutons which had been emplaced as locally discordant sills control the spatial distribution of structures two dominant phases of folds form type 0 interference patterns essentially coaxial and coplanar i e fold tightening was dominant this central belt also contains en echelon reverse to oblique shear zones and comprises a transfer zone within the orogen between structurally higher and lower major shear zones overall the mapped part of the central carrizo mountains represents localized dextral transpression within a larger region dominated by reverse motion possibly indicating a change through time from orthogonal to oblique contraction further south in the bass canyon region the earlier fold generations are refolded into type 2 mushroom interference patterns these structures show a westward vergence and are kinematically linked to those further north across a zone of disharmonic folding and heterogeneous shear the structure and kinematics of the polyphase deformation and ductile shearing in the metamorphic portion of the orogen is compatible with that seen in the foreland fold thrust belt suggesting that in both foreland and hinterland transpression was a characteristic feature of the west texas grenville orogeny the timing and kinematics of deformation in the van horn exposures contrasts with that seen in the llano uplift but compares favorably with those of the natal belt south africa a relationship with implications for recently proposed reconstructions of rodinia

the isotopic composition of authigenic chromium in anoxic marine sediments a case study from the cariaco basin chromium cr isotopes are an emerging proxy for tracking redox processes at the earth s surface however there has been limited exploration of the cr isotope record of modern and recent marine sediments the basic inorganic chemistry of cr suggests that anoxic marine basins should factor prominently in the global cr cycle and that sediments deposited within anoxic basins may offer a valuable cr isotope archive throughout earth s history here we present 53 cr data from sediments of the cariaco basin venezuela a type environment for large perennially anoxic basins with a relatively strong hydrological connection to the global oceans we document a marked positive shift in bulk 53 cr values following the termination of the last glacial maximum followed by relative stasis based on a suite of independent redox proxies this transition marks a switch from oxic to persistently anoxic and sulfidic euxinic depositional conditions within the basin we find good agreement between two independent approaches toward estimating the 53 cr composition of authigenic cr in euxinic cariaco basin sediments and that these estimates are very similar to the 53 cr composition of modern open atlantic ocean seawater these data together with considerations of reaction kinetics and mass balance within the cariaco basin are consistent with the hypothesis that anoxic marine settings can serve as a chemical archive of first order trends in seawater 53 cr composition additionally the cariaco basin data suggest that there has been secular stability in the average 53 cr value of atlantic seawater over the last 15 kyr

oxidative release of chromium from archean ultramafic rocks its transport and environmental impact a cr isotope perspective on the sukinda valley ore district orissa india abstract this study investigates cr isotope fractionation during soil formation from archean 3 1 3 3 ga ultramafic rocks in a chromite mining area in the southern singhbhum craton orissa india the cr isotope signatures of two studied weathering profiles range from non fractionated mantle values to negatively fractionated values as low as 53 cr 1 29 0 04 local surface waters are isotopically heavy relative to the soils this supports the hypothesis that during oxidative weathering isotopically heavy cr vi is leached from the soils to runoff the impact of mining pollution is observed downstream from the mine where surface water cr concentrations are significantly increased accompanied by a shift to less positive 53 cr values relative to upstream unpolluted surface water a microbial mat sample indicates that microbes have the potential to reduce and immobilize cr vi which could be a factor in controlling the hazardous impact of cr vi on health and environment the positive cr isotope signatures of the brahmani estuary and coastal seawater collected from the bay of bengal further indicate that the positively fractionated cr isotope signal from the catchment area is preserved during its transport to the sea isotopically lighter cr vi downstream from the mine is probably back reduced to cr iii during riverine transport leading to similar cr isotope values in the estuary as observed upstream from the mine

grenville age polyphase deformation of mid proterozoic basement nw van horn mountains trans pecos texas psammitic pelitic and mafic schists in a basement horst in the nw van horn mountains of west texas preserve evidence of a multiphase synmetamorphic grenville age ductile deformation three phases of progressive isoclinal folding followed by two later less intense nearly coaxial folding phases have been identified the second phase of deformation produced the dominant foliation and foliatipn intersection lineation observed throughout the area this deformation is inferred to have resulted from a regional scale folding possibly associated with nappe formation at or near peak metamorphic conditions mineral assemblages indicate amphibolite facies and fe mg exchange thermometry provide maximum temperatures of 640 pm 50 circ c corresponding to mid amphibolite facies conditions three subsequent fold generations f 3 f 5 formed under progressively lower metamorphic conditions f 3 produced a weakly developed foliation s 3 whereas neither of the two later folding phases fo

rb and sr diffusion in alkali feldspars with implications for cooling histories of rocks volume diffusion of sr and rb have been measured in orthoclase feldspar as well as sr diffusion in albite using isotope exchange hydrothermal solutions and analysis by ion microprobe for water pressures of 1000 bars the kinetics of diffusion parallel to the c axis are given by the arrhenius relations strontium transport in the or is isotropic to within a factor of four the measurements use a new procedure in which only the elements in question are present in the starting fluid cooling rates of igneous rocks can now be determined from closure temperatures material balance and the rb sr dating method based on the departures of the mineral data from the whole rock isochron for cooling rates less than approximately 100 c my

the cr isotope signature of surface seawater a global perspective abstract this study presents chromium isotope data expressed as 53 cr and chromium concentrations cr of surface seawater 0 2 m depth from a variety of locations worldwide in addition to samples from the open ocean arctic southern pacific and atlantic oceans samples were analysed from areas with more restricted water exchange rates the mediterranean sea oresund and baltic sea the data indicate a heterogeneous distribution of 53 cr in seawater with a total range from 0 13 to 1 24 the data are in agreement with a previous study which focused on depth profiles suggesting that seawater heterogeneity in 53 cr and cr concentration can be explained using a single fractionation factor for cr reduction e 0 79 0 06 2sd in basins with limited water exchange with the open ocean however local factors seem to control the cr isotope composition

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

response of the cr isotope proxy to cretaceous ocean anoxic event 2 in a pelagic carbonate succession from the western interior seaway abstract chromium offers a redox sensitive isotopic proxy with potential for tracing past oxygen levels in the oceans we explore this potential in a pelagic succession of marine carbonate sediment deposited during cretaceous ocean anoxic event 2 oae 2 in the western interior seaway colorado wis using the usgs portland 1 core reduction of cr vi causes light isotopes of cr to preferentially partition into cr iii because cr vi is the thermodynamically favoured species in oxygenated seawater and cr iii is relatively insoluble under the same conditions increased removal of cr iii into anoxic marine sediment during ocean anoxic events should cause positive shifts in seawater 53cr values this assumes that isotopic fractionation associated with all cr removal fluxes from the oceans was constant during oae 2 and that there was no change in cr input fluxes to the oceans here we report findings that counter this prediction namely evidence for a negative shift in seawater 53cr during oae 2 of 1 1 in the wis the magnitude of the excursion depends on the speciation of cr removed in this setting over the duration of oae 2 as well as the fractionation factor accompanying the removal flux of chromate into carbonate sediment both of which are uncertain the 53cr excursion reaches values as low 0 09 during oae 2 which is indistinguishable from the high temperature igneous rock baseline value of 0 124 0 101 2 moreover the minima in the 53cr profile coincides with a peak in cr concentrations in the study core as well as anomalous enrichments of other trace metals of basaltic affinity building on previous studies linking oae 2 to a massive episode of submarine volcanic activity we attribute the decrease in carbonate 53cr values during oae 2 to the expansion of a hydrothermal superplume created during eruptions of the caribbean large igneous province lip this metal laden plume enriched in cr iii and probably anoxic moved from deep waters of the eruption site in the eastern pacific into deep waters of the proto north atlantic through an oceanic gateway in the central americas once inside anoxic metal rich waters upwelled against the surrounding continental margins where isotopically unfractionated cr iii whose solubility was improved by organic complexation and or oxidation to cr vi mixed with the isotopically heavy pool of oceanic chromate in surface waters above the redoxcline application of a simple mixing equation shows that hydrothermal cr contributed 90 of the authigenic cr in the trace metal enriched interval of the study core the increased flux of hydrothermal cr was large enough to mask the expected isotopic response of the ocean cr cycle to increasing anoxia we consider this interpretation to be robust in that it is not substantially affected by uncertainty in the speciation of hydrothermal cr nor uncertainty in the fractionation factor sw caco 3 associated with chromate removal into marine carbonate sediment which we measured in the modern caribbean sea to be 0 46 0 14 2

statistical examination of the existence and relative motion of the jalisco and southern mexico blocks the continental lithosphere of mexico south of the trans mexican volcanic belt may consist of two lithospheric blocks the jalisco and southern mexico blocks the existence of these two blocks can be examined employing the statistical f test formulated to test for the presence of significant misclosure around a plate circuit to plate motion data derived from marine magnetic anomaly lineations earthquake slip vectors and transform fault azimuths the result of the f test applied to the pacific cocos north american plate circuit indicates that there is no significant at the 99 confidence level misclosure around this plate circuit therefore if the southern mexico block exists its motion relative to the surrounding lithospheric plates is too small to be resolved from these data in contrast the result of the f test applied to the rivera pacific north american plate circuit indicates the presence of a significant misclosure about the rivera pacific north american plate circuit the cause of which is uncertain one interpretation of the misclosure around the rivera pacific north american plate circuit is that it is due to the presence of an independent jalisco block however it is more likely that this misclosure is instead primarily due to the effects of recent changes in the relative motion between the pacific and rivera plates assuming the second explanation is correct a new rivera north american euler pole location 21 8 n 110 4 w is determinable solely from earthquake slip vectors located along the rivera north american boundary

grenville orogenic affinities in the red mountain area llano uplift texas the red mountain area in the southeast corner of the llano uplift contains evidence of a grenville age polyphase non coaxial deformational history four synmetamorphic deformational phases affected the precambrian click formation of the packsaddle group and a single shear related deformation is observed in the big branch and red mountain gneisses the first phase of deformation produced tight folds with an axial planar metamorphic foliation the second and most intense pervasive deformational phase produced the regional metamorphic layering isoclinal folds transposition of bedding and mylonitic textures in the gneisses the third deformation is characterized by tight to isoclinal north verging folds with an axial planar differentiation crenulation cleavage the fourth phase of deformation resulted in a north northwest trending differentiation crenulation cleavage that is axial planar to small east northeast verging folds the growth of cordierite over all foliations indicates that low pressure me

physical models of regional fold superposition the role of competence contrast abstract physical models of superposed folds demonstrate that rheologic contrast strongly controls the style of fold interference rheologic contrast affects strain partitioning between layer parallel shortening and buckling and affects the development of structural anisotropies parallel to first generation folds laminates with insignificant competence contrast are characterized by circular to elliptical domes and basins type 1 interference in addition to folding the layers accommodate shortening by layer parallel thickening preferentially in the fold culminations laminates with significant competence contrast are characterized by buckled fold hingelines and axial surfaces type 2 interference the less competent layers thicken in the hinges the more competent layers maintain their initial thickness in plan view the f 1 hingelines are refolded in a lobate cuspate to box style whose axial traces form conjugate pairs the models are dynamically scaled to represent folds with km scale wavelengths the evolution of surface structures map patterns at different depths and equal area projections synthesized from structure contour maps were used for analysis comparable to regional analysis common to field studies gravitational body forces effectively damp the vertical amplitudes of both the first and second generation folds thereby enhancing the formation of type 2 interference

authigenic chromium enrichments in proterozoic ironstones abstract chromium isotopes have recently emerged as a proxy for atmospheric oxygen levels in the precambrian chromium cr enrichments along with a suite of other redox sensitive metals have also been used as marine redox proxies for decades despite significant interest and growing use of this system which has been used to argue for low oxygen levels through the mid proterozoic leading up to the rise of animals there is a poor understanding of the host phases of cr in the sedimentary record without this context interpretation of measured isotopic signatures and concentrations of cr in various sedimentary archives can be ambiguous in order to shed light on this problem we have used electron microprobe elemental mapping to identify authigenic and detrital cr phases we have examined a range of ironstone samples that have been previously analyzed for cr isotopes and find that cr is predominantly associated with iron rich allochems instead of detrital cr mineral phases we did not find evidence for detrital chromite or other cr rich detrital minerals additionally we find evidence for authigenic ti enrichments and decoupling of al and ti elements traditionally used as detrital tracers similar to what has been reported in other slowly accumulating metal rich sediments e g iron manganese nodules lastly we have explored a range of possible sedimentary cr isotope signals that could result from changes in the percentage of detrital versus authigenic cr incorporated into a sample providing clear evidence for the ambiguity in interpretations of this isotope system without petrographic context

comparison of calcite dolomite thermometry and carbonate silicate equilibria constraints on the conditions of metamorphism of the llano uplift central texas u s a temperatures based on the composition of calcite coexisting with dolomite calcite dolomite thermometry range from 475 to 600 c for 63 marbles from the llano uplift of central texas the highest temperatures 600 c were obtained by carefullyreintegrating calcite containing exsolved lamellae of dolomite in some cases these high temperatures were determined for marbles that contain an isobarically invariant assemblage consisting of calcite dolomite tremolite diopside forsterite at a pressure of 3 kbar these five minerals are stable at 630 ocand xco 0 62 in contrast relatively low calcite dolomite temperatures of 475 480 cwere obtained for marbles containing the assemblage calcite dolomite tremolite talc this talc bearing assemblage is stable at 475 oc depending on fluid composition at a pressure of 3 kbar additional isobarically univariant equilibria are stable at intermediate temperatures generallybetween 535 and 630 0q and these are also generallyconsistent with results obtained from calcite dolomite thermometry agreement between the calcite dolomite temperatures and those inferred from silicate carbonate equilibria in the marbles indicates that the temperatures generally reflect peak conditions of metamorphism although some resetting has occurred the marbles having the highest calcite dolomite temperatures as well as those containing the hightemperature isobarically invariant assemblage are generally found close to post tectonic pluton contacts indicating that some of the amphibolite facies assemblagesare related to the emplacement ofthe granitic intrusions relatively low temperatures are recorded within approximately 2 km of pluton contacts suggestinga possible thermal aureole however relatively high temperatures of 550 600 c are recorded in marbles that are not spatially related to post tectonic granites 2 km from pluton contacts these temperatures may be relicts of an earlier metamorphic event although they could be related to granites that were emplaced above or below the current erosional level

last gasp of the grenville orogeny thermochronology of the grenville front tectonic zone near killarney ontario we present u pb titanite zircon and 40 ar 39 ar hornblende mica k feldspar data from a transect across the western part of the grenville front tectonic zone gftz near killarney ontario high grade metamorphic assemblages 1450 ma in this part of the gftz pre date the grenvillian orogeny and were primarily exhumed with little or no metamorphic overprinting by grenvillian deformation the titanite and zircon data form a discordant array with an upper intercept of 1454 pm 8 ma and a lower intercept of 978 pm 13 ma these data are interpreted in terms of partial lead loss during a short lived thermal event that increased in intensity from west to east across the transect 40 ar 39 ar data from hornblende indicate cooling through 450 c at 993 979 ma multiple diffusion domain models for the interpretation of discordant k feldspar spectra indicate cooling through 365 340 c at 990 960 ma and muscovite data indicate cooling through 320 c at 930 ma biotite data are not easily

the isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans despite important biological and biogeochemical consequences of extensive ocean anoxic events their identification is controversial the marine isotope geochemistry of molybdenum can help quantify the past oxygenation state of the ocean if the riverine input of mo isotopes is known analysis of a set of rivers that account for 28 of global river runoff suggests more variable mo isotopic ratios in rivers that are also isotopically enriched in the heavy isotopes suggesting near total anoxia in the proterozoic ocean and during mesozoic ocean anoxic events despite its important biological and biogeochemical consequences1 2 3 the identification of extensive oceanic anoxia in the geological record is controversial in particular global anoxia is difficult to distinguish from spatially restricted anoxia in the deep ocean or in tectonically isolated basins such as the modern black sea the marine isotope geochemistry of molybdenum mo can help quantify the past oxygenation state of the ocean4 5 6 7 8 9 10 11 12 13 14 because to first approximation under oxic conditions lighter isotopes of mo are preferentially removed to sediments whereas in euxinic conditions quantitative removal leads to no fractionation however the isotopic composition of the mo input from rivers the main contributor of mo to the oceans is poorly constrained and had been assumed to be isotopically comparable to the narrow range found in a small dataset of continental rocks4 5 6 7 8 9 10 11 12 13 14 here we present an isotopic analysis of mo in a set of rivers that together account for 22 of the global riverine water discharge we find a broad range of variability in the mo isotopic composition of these rivers with almost all samples enriched in the heavy isotopes compared with continental rocks our data remove key uncertainties associated with the marine mo isotope budget14 and strongly suggest near total anoxia in the mid proterozoic ocean8 as well as during mesozoic ocean anoxic events13

thermochronology of the grenville orogeny in west texas abstract the carrizo mountain group cmg comprises the westernmost exposures of the metamorphic core of the grenville orogen in texas unlike the correlative llano uplift 500 km to the east the cmg underwent one phase of medium p t metamorphism peak t was attained during d2 in all cmg exposures the peak metamorphic paleotemperatures decrease to the northwest in the overall direction of tectonic transport from staurolite to biotite grade garnet in the southeast carrizo mountains grew isothermally and probably during decompression i e part of a clockwise p t loop at ca 520 c in between biotite grade d1 and d3 events hornblende in this region cooled below closure t during d2 at 1057 6 ma within error of muscovite closure in the northwest carrizo mountains at 1059 2 ma hornblende in higher grade exposures to the southeast cooled through closure t as late as 1002 7 ma muscovite in the garnet grade and higher exposures closed to ar diffusion at 992 4 ma to 981 3 ma biotite and partially recrystallized hornblende from the northwest carrizo mountains cooled through closure in this same time period we interpret these data as signifying 1 ar loss due to peak metamorphism in hornblende southeast carrizo mountains and muscovite northwest carrizo mountains at ca 1059 ma 2 post tmax cooling through 500 c in the higher grade outcrops until ca 1000 ma and 3 cooling of all cmg exposures through 300 c due to uplift along the streeruwitz thrust and erosion from 1000 to 980 ma these ages significantly post date known tectonism in the llano uplift and extend the duration of grenville orogenesis in texas by ca 115 million years the 1000 980 ma time period is a time of thrusting in frontal regions of the grenville province of canada and the northeastern us and shearing in the natal province south africa suggesting ultimate closure of the grenville orogen and supporting recent rodinia reconstructions

petrological significance of prograde homogenization of growth zoning in garnet an example from the llano uplift in the llano uplift of central texas usa prograde homogenization of garnet growth zoning took place during moderate to high pressure dynamothermal metamorphism over a narrow temperature range near the transition from the amphibolite to the granulite facies this subtle record of early dynamothermal metamorphism survived subsequent static metamorphism at low pressures in the middle amphibolite facies despite the destruction of most high pressure mineral assemblages that originated in the early metamorphic episode geographically systematic variations in the degree of homogenization indicate that the uplift as a whole underwent high pressure metamorphism in accord with emerging tectonic models for the mid proterozoic evolution of the southern margin of the north american continent

kinematic constraints on rodinia reconstructions from the core of the texas grenville orogen abstract mesoproterozoic metamorphic rocks exposed in the llano uplift central texas were involved in a 1 15 1 12 ga grenville age collisional orogenic event along the southern margin of laurentia this event is characterized by polyphase ductile deformation synchronous with transitional amphibolite granulite to eclogite facies metamorphism results of detailed structural mapping in the southeastern part of the llano uplift show that these rocks record a deformational history involving northeast directed ductile thrusting and regional scale polyphase folding all related to northward collision of a southern continent and an arc with the southern margin of laurentia we use the structural metamorphic and kinematic history of the southeastern llano uplift to critically evaluate collisional models for the grenville orogen in laurentia and discuss the implications for reconstructions of rodinia

weathering on land and transport of chromium to the ocean in a subtropical region misiones nw argentina a chromium stable isotope perspective abstract we have investigated the pathway of chromium from its mobilization on land and along its riverine transport in a subtropical region of south america misiones province argentina in an attempt to link cr stable isotope compositions recently measured in seawater with signals prevailing in rivers and ultimately with cr isotope effects observed during oxidative surface weathering in subtropical red soils cr concentrations and stable cr isotopic compositions expressed as 53cr values in two typical and representative surface profiles of weathered basalt show significant depletion of cr in the soils of up to 50 together with pronounced negatively fractionated 53cr values which are indicative of oxidative mobilization of heavy cr vi into the run off the behavior of cr in the studied weathering profiles is not correlated with that of other redox sensitive elements such as ce and u this is essentially due to the affinity of ree and u but not cr with secondary phosphates which form during weathering processes smaller tributaries in nw argentina to the parana river second largest river in south america carry dissolved cr in the order of 0 7 1 4 ppb 13 27 nm with 53cr values of 0 2 to 0 4 balancing the negatively fractionated weathering products the isotope composition and concentration of dissolved cr in the ca 1200 km long parana river from misiones to its estuary and discharge area into the south atlantic ocean remains relatively constant with an average cr concentration around 2 4 ppb 46 nm and an average 53cr value of 0 32 the cr concentration in the estuary itself drops by ca 50 but with only minor change in its cr isotope composition results from the parana estuary are identical with recently analyzed surface seawater from the argentine basin with cr contents of 0 3 ppb 6 nm and 53cr values 0 4 bonnand et al 2013 and indicate that there is only a minimal cr isotopic variability during riverine transport even during long transport distances as shown in our example of the parana river simple cr input flux calculations reveal that the parana river accounts for 5 of the total yearly cr flux to the world s oceans today and that its isotopic signature seems to be at least locally imparted to the surface seawater of the argentine basin whether or not this cr isotope signature is generally exhibited by the world s oceans needs further investigations particularly the characterization of seawater around the globe

grenville age orogeny in the llano uplift of central texas deformation and metamorphism of the rough ridge formation field and petrographic study of the precambrian rough ridge formation of the packsaddle group documents a more complex tectonic history for the llano uplift than has previously been reported the rough ridge formation was affected by at least two metamorphic events and five phases of deformation that may be part of a single progressive event the predominant mineral assemblages in pelitic and mafic rocks are characteristic of the amphibolite facies at relatively low pressure ubiquitous granoblastic polygonal textures overprinting multiple foliations indicate that post tectonic recrystallization followed dynamothermal metamorphism a probable consequence of regional reheating during later granite intrusion the most intense deformation produced isoclinal folds and pervasive axial planar foliation s2 resulting in complete transposition of original bedding this foliation cuts an earlier local foliation s1 and is folded by small f3 folds and a crenulation cleavage s3 both s2 and s3 are cut by a differentiated crenulation cleavage s4 the most prominent folds in the area and the associated pervasive axial planar cleavage s5 overprint all earlier structures in one pelitic unit oriented inclusions show that garnet formed after s3 and cordierite after s5 growth and recrystallization of micas quartz and feldspars span all phases of deformation the timing of tectonic and metamorphic events is bracketed by the pretectonic intrusion of the 1 167 15 m y old red mountain gneiss and the post tectonic intrusion of a 1 080 30 m y old melarhyolite dike structural and petrologic characteristics of the rough ridge formation indicate that the llano uplift underwent a complex tectonic and metamorphic development similar to that described for the grenville orogeny elsewhere in north america

tubular folds and sheath folds definitions and conceptual models for their development with examples from the grapesvare area northern sweden abstract two parameters are used in the classification of highly non cylindrical folds the hinge line angle and the ratio of the length of the cone axis to that of the long axis of the cross section of the structure at the position where the hinge angle was measured if in a non cylindrical fold a cross section can be selected so that the hinge angle w measured where the cross section cuts the cone is less than 90 and the length of the cone axis x is more than a quarter of the length of the long axis y of the cross section the fold is a sheath fold a tight sheath fold with w x y 1 can be termed a tubular fold a tubular fold is thus an extreme type of sheath fold while sheath folds with w 20 but x y 0 25 but tubular folds from the grapesvare area comply to the definition given above and they occur in a structural setting that is consistent with the models described

the influence of initial fold geometry on type 1 and type 2 interference patterns an experimental approach abstract experiments on superposed folding were performed in a plane strain pure shear rig using paraffin wax as an analogue for rocks a series of pre formed cylindrical folds were refolded with the compression direction acting parallel to the initial fold hinge direction the intermediate axis perpendicular to the first fold axial plane and the extension direction parallel to the first fold axial plane and perpendicular to its axis type 1 interference geometry roundness tightness and amplitude were varied to investigate the influence of first fold geometry on the interference patterns experimental folding of an initially planar layer oriented parallel to the first fold envelope provides a reference geometry for comparison the results suggest that hinge roundness has less influence on the interference pattern than other factors close initial folds refold into type 2 interference patterns which become even more pronounced as the tightness increases open folds produce type 1 interference patterns folds with the same interlimb angle and the same roundness produce different interference patterns depending on relative amplitude folds with relatively large amplitude are refolded into type 2 patterns whereas folds with small amplitude give clear dome and basin structures in a second set of experiments the compression direction also acted parallel to the first fold hinge but the orientations of the intermediate and extension axes were interchanged the interference patterns obtained are very similar to those in type 1 interference geometry suggesting that the major factor in determining whether type 1 or type 2 interference patterns develop is the initial fold geometry and not the kinematics of the second deformation

chromium isotope systematics in the connecticut river abstract limited constraints on cr isotope fractionation during weathering and river transport is currently a gap in our understanding of the chromium cr isotope system which is an emerging proxy in environmental and paleoenvironmental studies here we investigate cr mobility and isotope fractionation from the temperate connecticut river usa including cr concentrations and isotopic compositions of river water suspended particles riverbed sediments and weathering profiles the 53cr values of the connecticut river water range from 0 17 to 0 92 which are similar to or higher than the weathered rocks in the catchment 0 08 to 0 29 we find seasonal variations in dissolved 53cr values in some but not all tributaries suggesting that dissolved 53cr is not a simple function of seasonality but may also be influenced by sub catchment heterogeneity in lithology in contrast to dissolved cr we found consistent seasonal difference in suspended cr concentration and 53cr suspended 53cr is 0 1 higher than the unfractionated bse in the fall 0 01 0 13 but indistinguishable from the bse in the spring 0 11 to 0 00 the suspended cr concentration is also lower in the spring and with higher al mn fe concentrations the lower suspended cr concentration and 53cr in spring may be linked to increased silicate and oxide load with depleted cr due to stronger hydrological flux building from our dataset there is not a consistent correlation with climate zones in a compilation of 53cr and cr concentration data from river water and weathering profiles suggesting that climate is not a dominating factor controlling cr isotopic behavior during weathering suggesting that other factors e g local catchment conditions and dissolved organic matter may also be responsible for the observed river water 53cr variability

chromium isotope variations δ53 52cr in mantle derived sources and their weathering products implications for environmental studies and the evolution of δ53 52cr in the earth s mantle over geologic time here we report chromium isotope compositions expressed as delta cr 53 52 in per mil relative to nist 979 measured in selected cr rich minerals and rocks formed by the primary magmatic as well as the secondary metamorphic and weathering processes the main objectives of this study were i to further constrain the isotope composition of the earth s mantle cr inventory and its possible variation during geological history based on the analysis of globally distributed and stratigraphically constrained mantle derived chromites and ii to investigate the magnitude and systematics of cr isotope fractionation during oxidative weathering and secondary alteration i e hydration serpentinization of the magmatic cr sources specifically we analyzed delta cr 53 52 in a set of globally distributed mantle derived chromites femgcr2o4 n 30 collected from various locations in europe asia africa and south america and our results confirm that a chromite hosted earth s mantle cr inventory is uniform at 0 079 0 129 2sd which we named here as a canonical mantle d 53 52 cr signature furthermore our dataset of stratigraphically constrained chromites whose crystallization ages cover most of the earth s geological history indicate that the bulk cr isotope composition of the chromite hosted mantle inventory has remained uniform within about 0 100 since at least the early archean times similar to 3500 million years ago ma to investigate the systematics of cr isotope fractionation associated with alteration processes we analyzed a number of secondary cr rich minerals and variably altered ultramafic rocks i e serpentinized harzburgites lherzolites that revealed large positive delta cr 53 52 anomalies that are systematically shifted to higher values with an increasing degree of alteration and serpentinization the degree of aqueous alteration and serpentinization was quantified by the abundances of fluid mobile rb k elements and by the loss on ignition loi parameter which determines the amount of structurally bound water oh h2o present in secondary hydrated minerals like serpentine overall we observed that altered ultramafic rocks that yielded the highest loi values and the lowest amounts of fluid mobile elements also yielded the heaviest delta cr 53 52 signatures therefore we conclude that secondary alteration i e hydration serpentinization of ultramafic rocks in near surface oxidative environments tend to shift the bulk cr isotope composition of the weathered products to isotopically heavier values pointing to a dynamic redox cycling of cr in the earth s crustal and near surface environments hence if validated by future

paleosecular variation and time averaged field recorded in late pliocene holocene lava flows from mexico 1 this paper presents results from 13 paleomagnetic sites from an area west of mexico city and 7 sites from an area of dispersed monogenetic volcanism in the state of san luis potosi accompanied by seven 40ar 39ar radiometric dates an analysis of secular variation and time averaged paleomagnetic field in the trans mexican volcanic belt tmvb using compiled data both newly obtained and from the literature is presented interpretation can best be constrained after excluding from the data set sites that appear to be tectonically affected the selected data include 187 sites of late pliocene holocene age the mean direction among these sites is dec 358 8 inc 31 6 95 2 0 k 29 this direction does not overlap the expected geocentric axial dipole gad but is consistent with a gad plus a 5 quadrupole the virtual geomagnetic pole scatter of this group of sites 12 7 with lower and upper 95 confidence limits of 11 9 and 14 1 is consistent with the value expected from model g 13 6

late mesoproterozoic deformation of sw amazonia rondonia brazil geochronological and structural evidence for collision with southern laurentia abstract proposed assembly of the rodinia supercontinent in the late mesoproterozoic involved the collision of the amazon craton with some portion of the southern or eastern margin of cratonic north america previously reported paleomagnetic data from the sw amazon craton suggest a paleogeographic link between grenvillian deformation of the sw amazon craton and late mesoproterozoic tectonometamorphism in southern laurentia a structural geochronological and petrological investigation of the western amazon basement rocks rondonia brazil was carried out in order to document evidence of a grenvillian collision connecting the amazon to laurentia integration of 40ar 39ar data and feldspar thermometry data from regionally extensive strike slip mylonitic shear zones ji parana shear zone network indicates that deformation took place at 450 550 c between 1 18 and 1 15 ga an older ca 1 35 ga event found exclusively in less deformed basement rocks is interpreted as recording cooling from an earlier met

the chromium isotope composition of reducing and oxic marine sediments the chromium cr isotope composition of marine sediments has the potential to provide new insights into the evolution of earth surface redox conditions there are significant but poorly constrained isotope fractionations associated with oxidative subaerial weathering and riverine transport the major source of seawater cr and with partial cr reduction during burial in marine sediments the major sink for seawater cr a more comprehensive understanding of these processes is needed to establish global cr isotope mass balance and to gauge the utility of cr isotopes as a paleoredox proxy for these purposes we investigated the cr isotope composition of reducing sediments from the upwelling zone of the peru margin and the deep cariaco basin chromium is present in marine sediments in both detrital and authigenic phases and to estimate the isotopic composition of the authigenic fraction we measured 53cr on a weakly acid leached fraction in addition to the bulk sediment in an effort to examine potential variability in the cr isotope composition of the detrital fraction we also measured 53cr on a variety of oxic marine sediments that contain minimal authigenic cr the average 53cr value of the oxic sediments examined here is 0 05 0 10 2 n 25 which is within the range of 53cr values characteristic of the bulk silicate earth this implies that uncertainty in estimates of authigenic 53cr values based on bulk sediment analyses is mainly linked to estimation of the ratio of cr in detrital versus authigenic phases rather than to the cr isotopic composition of the detrital pool leaches of cariaco basin sediments have an average 53cr value of 0 38 0 10 2 n 7 which shows no dependency on sample location within the basin and is close to that of atlantic deepwater cr 0 5 this suggests that authigenic cr in anoxic sediments may reliably reflect the first order cr isotope composition of deepwaters for peru margin samples the average 53cr values of bulk sediments 0 59 0 06 2 and leach fractions 0 61 0 06 2 are also comparable with those of cariaco basin samples and modern deepwater values finally we found that the 53cr of peru margin samples correlates with 15n on glacial interglacial timescales which we attribute to secular variation in basinal or global ocean redox conditions thus the 53cr stratigraphic record of peru margin sediments indicates that cr isotopes may be suited to tracking geologically short term changes in ocean oxygenation

p t x conditions of calc silicate formation evidence from fluid inclusions and phase equilibria llano uplift central texas usa the llano uplift in central texas is a grenville aged c 1 1 ga metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages the formation of these assemblages has been attributed to the emplacement of relatively late granite plutons throughout the area two types of granitic intrusion have previously been recognized 1 town mountain granites which occut as relatively large circular shaped bodies of coarse grained granite and 2 younger granites which are present as smaller and more irregular bodies of finer grained granite in the central part of the uplift wollastonite bearing calc silicate rocks occur within the valley spring gneiss the development of these calc silicate rocks has been linked to infiltrating fluids presumably derived from spatially associated younger granites the stability of coexisting quartz calcite wollastonite grossular and anorthite and coexisting quartz calcite wollastonite andradite and hedenbergite shows that the calc silicate rocks equilibrated under h20 rich conditions with xcs 0 10 fluid inclusions present within the calc silicate minerals are h o rich with salinities of 17wt equivalent naci the absence of any detectable co in the fluid inclusions may indicate entrapment of the inclusions at lower pressures and more h20 rich conditions compared to the stability of the peak metamorphic mineral assemblage homogenization temperatures measured for texturally primary inclusions range from 360 to 368 c corresponding to a density range from 0 53 to 0 82g cm3 isochores for these fluid inclusions when combined with the stability of the solid solid equilibria grs qtz wo an yield formation conditions of xhl 550 c at 1 2 kbar this indicates that the granitic intrusions involved in the formation of the blount mountain calc silicates were emplaced at a pressure of at least 1 2 kbar

the stable cr isotope inventory of solid earth reservoirs determined by double spike mc icp ms abstract we present the first comprehensive set of stable cr isotope data for the major igneous silicate earth reservoirs cr iii rich ores and minerals and hydrothermal chromates these were determined by mc icp ms using a double spike technique mantle xenoliths ultramafic rocks cumulates as well as oceanic and continental basalts share a common cr isotope composition with an average 53 52 cr value of 0 124 0 101 2 sd relative to the isotopically certified chromium standard nist srm 979 an isotopic difference between mantle xenoliths and basalts as was reported for iron was not observed for chromium thus the change in oxidation state that is observed when solid mantle rocks containing only trivalent chromium partially melt to form basaltic melts which predominantly contain bivalent chromium does not cause any measurable cr isotope fractionation chromite separates from major chromitite seams of the bushveld and great dyke layered igneous complexes are invariable in their cr isotope compositions and reproduce within uncertainties the average 53 52 cr srm 979 value of igneous silicate earth reservoirs this is important for environmental stable cr isotope studies because it labels the approximate isotope composition of industrial cr vi pollutants the cr isotope compositions of hydrothermal lead chromates crocoites pbcr vi o 4 from various localities yield 53 52 cr srm 979 values between 0 640 and 1 037 these cr isotope compositions are substantially heavier than those of igneous silicate rocks from which the chromium was leached precipitation experiments revealed an isotope fractionation of 53 52 cr crocoite cr vi aq of only ca 0 10 0 05 at a temperature of 20 c thus the heavy cr isotope signature of crocoites is most likely the result of repeated redox cycling of chromium in hydrothermal processes

global variability of chromium isotopes in seawater demonstrated by pacific atlantic and arctic ocean samples abstract seawater chromium cr isotope and concentration data are presented from multiple sites in the arctic ocean and three locations in the atlantic and pacific oceans a 2400 m profile illustrates the heterogeneity of 53cr in the arctic ocean with depth and water mass source pacific vs atlantic the highest 53cr values occur in pacific sourced waters which also have the lowest cr concentration chromium concentration and 53cr data from these locations in conjunction with published data for the south atlantic ocean yield a simple logarithmic function as predicted by rayleigh fractionation in a closed system the observed cr isotope signature is hypothesized to arise from fractionation during the reduction of cr vi in surface waters and oxygen minimum zones scavenging of isotopically light cr iii to deeper water and sediment and subsequent release of this seawater derived cr iii back into seawater either as organic complexes with cr iii or after oxidation to cr vi the isotopic fractionation factor e associated with cr cycling in seawater is estimated to be 0 80 0 03 2 samples from the sea ice affected surface mixed layer of the arctic ocean 10 m depth deviate from the general trend and samples proximal to rivers illustrate geographic variation in 53cr values for continental runoff but prompt loss of this signature away from the source

precise u pb ages for grenvillian and pre grenvillian thrusting of proterozoic and archean metamorphic assemblages in the grenville front tectonic zone canada while the position of the grenville front is placed at a structural feature associated with the onset of high grade rocks when approached from the north the time of first metamorphism in these rocks is in general pre grenvillian and highly variable at killarney amphibolite to granulite grade rocks were metamorphosed lineated and foliated to their present front parallel configuration mainly by 1453 6 ma north of north bay amphibolite grade migmatites have been subjected to a pre 2650 ma metamorphism granulite grade gneisses south of the grenville front near val d or record a 2640 ma metamorphic event south of chibougamau mafic enclaves in tonalite south of the front were converted to migmatitic amphibolite at about 2680 to 2690 ma in central labrador a 1650 ma foliated granite is present south of the grenville front and at least some of the banding is this old at the atlantic coast granulite facies metamorphism in front parallel gneisses predates a 1790 1662 ma deformation in contrast each of these areas shares a near common age of second metamorphism that is variable in degree and accompanies nw directed thrusting or crustal loading near killarney on georgian bay titanite records an episode of 977 2 ma partial resetting the degree of which increases with distance into the grenville province here grenvillian deformation where present is manifest as feldspar porphyroclastic fabric with strong se plunging lineations near north bay archean pegmatites in the grenville front zone were similarly deformed at 988 10 ma and strongly rodded ductile amphibolite underwent late melting to produce postlineation tonalite in which new zircon tips grew at 987 2 ma a 991 4 ma cooling age for titanite in this melt is analytically identical to the zircon growth age hence rapid cooling occurred south of chibougamau new zircons grew in high pressure coronite in response to tectonic loading at 995 6 ma and in central labrador melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 995 2 ma titanite in the granitic host rock has an analytically identical age of 992 4 ma again indicating rapid cooling this second event is recorded by 28 titanite and zircon analyses for samples up to 225 km apart that record a 1651 5 ma to 988 5 ma two age mixing line grenville resetting at about 1037 19 ma in the grenville front zone on the labrador coast is indicated but major thrusting is pre 1662 ma u pb isotopic results indicate that the grenville front zone is characterized along its length by partially reset u pb systems high pressure assemblages local melting and rapid cooling crustal loading and shearing with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling are required

paleomagnetic study of the eastern sector of chapala lake and implications for the tectonics of west central mexico abstract western mexico is characterized by several large scale tectonic depressions which have been interpreted in terms of active continental rifting tectonic transpression and coastal sinistral lateral transport of terranes in this paper we report results of a paleomagnetic study of 148 samples from 22 sites in the neogene volcanics from the eastern sector of the chapala graben western end of the e w chapala tula fault zone characteristic remanent magnetization directions have been isolated after detailed thermal demagnetization for sixteen sites six sites present reverse polarities and ten sites present normal polarities two sites show a low latitude vgp and are considered as transitional the normal and reverse polarity directions are almost antipodal the overall characteristic pole position for the normal and reverse polarity sites n 16 p lat 74 n p long 160 e k 25 and a 95 7 6 lies to the left of the neogene segment of the north american or northern mexico apparent polar wander path which suggest the occurrence of a counterclockwise rotation of 15 5 7 4 to 16 5 6 9 results are interpreted in terms of counterclockwise vertical axis rotation associated with regional transtension and left lateral shear as a result of oblique subduction of the cocos plate along the middle american trench we suggest that the chapala graben developed early in the miocene in a left lateral strike slip environment within the regional e w chapala tula fault zone results support recent studies that propose relative motion for southern mexico along the volcanic arc and trench parallel strike slip faulting

theoretical estimates of equilibrium chromium isotope fractionations equilibrium chlorine isotope 37 cl 35 cl fractionations have been determined by using published vibrational spectra and force field modeling to calculate reduced partition function ratios for cl isotope exchange ab initio force fields calculated at the hf 6 31g d level are used to estimate unknown vibrational frequencies of 37 cl bearing molecules whereas crystalline phases are modeled by published lattice dynamics models calculated fractionations are principally controlled by the oxidation state of cl and its bond partners molecular mass or the absence of c h bonds also appears to play a role in determining relative fractionations among simple cl bearing organic species molecules and complexes with oxidized cl i e cl 0 cl etc will concentrate 37 cl relative to chlorides substances with cl at 298 k clo 2 containing cl 4 and clo 4 containing cl 7 will concentrate 37 cl relative to chlorides by as much as 27 and 73 respectively in rough agreement with earlier calculations among chlorides 37 cl will be concentrated in substances where cl is bonded to 2 cations i e fecl 2 mncl 2 micas and amphiboles relative to substances where cl is bonded to 1 cations such as nacl by 2 to 3 at 298 k organic molecules with c cl bonds will be even richer in 37 cl 5 to 9 at 298 k precipitation experiments in combination with our results provide an estimate for cl isotope partitioning in brines and suggest that silicates to the extent that their cl atoms are associated with nearest neighbor 2 cations analogous with fecl 2 and mncl 2 will have higher 37 cl 35 cl ratios than coexisting brine by 2 to 3 at room temperature calculated fractionations between hcl and cl 2 and between brines and such alteration minerals are in qualitative agreement with both experimental results and systematics observed in natural samples our results suggest that cl bearing organic molecules will have markedly higher 37 cl 35 cl ratios by 5 8 to 8 5 at 295 k than coexisting aqueous solutions at equilibrium predicted fractionations are consistent with the presence of an isotopically heavy reservoir of hcl that is in exchange equilibrium with cl aq in large marine aerosols

behaviour of chromium and chromium isotopes during estuarine mixing in the beaulieu estuary uk abstract rivers are the principal source of chromium cr to seawater and the cr isotopic signatures of ancient marine sediments are widely considered to provide a record of the presence or absence of oxidative weathering processes on land this assumes however that the 53 cr value of river water is faithfully transferred to the oceans and is not modified in the estuarine mixing zone to test this assumption we have determined the concentration and 53 cr values of inorganic cr cr iii cr vi and also cr speciation for water samples collected within the estuarine mixing zone of the beaulieu river uk the 53 cr values of dissolved inorganic cr ranged from 0 59 to 1 68 cr vi concentrations from 0 39 to 1 83 nmol kg 1 and cr iii concentrations from 0 11 to 3 21 nmol kg 1 both cr vi concentrations and 53 cr values increased linearly as a function of salinity while cr iii concentrations decreased linearly with salinity thus 53 cr cr iii and cr vi all showed conservative behaviour in the estuarine mixing zone and the 53 cr signature of beaulieu river water was modified only by mixing between the river and seawater endmembers the calculated average 53 cr value of the river water endmember 0 39 0 08 was however lower than the range that has been observed in other rivers which we attribute to input of organically bound cr iii released by anoxic weathering processes this is supported by the fact that cr recovered by uv irradiation was found to have low 53 cr values 0 11 to 0 75 while input of cr from anoxic weathering processes is unlikely to be an important source of cr to the oceans today this suggests that processes other than oxidative weathering may have an influence on the 53 cr values of estuarine and coastal waters on the local scale the 53 cr value of the coastal seawater endmember 1 6 0 4 was also higher than the range observed in the deep open ocean due to in situ biogeochemical cycling of cr these factors need to be considered in the interpretation of marine sedimentary 53 cr records