recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors

geomagnetic intensity and 14c abundance in the atmosphere and ocean during the past 50 kyr we have used a new sedimentary record of geomagnetic field intensity combined with published volcanic data to examine the extent to which geomagnetic changes have affected 14c production in the past the production record was then converted into an atmospheric 14c signal using a 4 box model of past oceanic circulation the results indicate that changes in geomagnetic field intensity account for at least 80 of the 14c shift documented by published u th data which are of the order of 500 per mil with respect to a constant production hypothesis model simulations show that changes in past oceanic circulation account for only 100 per mil of 14c variation which is within the uncertainty of the geomagnetic correction in terms of dating these results show that the radiocarbon ages have to be shifted by 2 to 3 5 kyr towards older ages during the 20 40 ka interval

single crystal 40ar 39ar dating of the olorgesailie formation southern kenya rift single crystal laser fusion sup 40 ar sup 39 ar analyses and several conventional bulk fusion sup 40 k sup 40 ar dates have been used to determine the age of volcaniclastic strata within the olorgesailie formation and of associated volcanic and sedimentary units of the southern kenya rift in the principal exposures along the southern edge of the legemunge plain the formation spans the interval from approximately 500 to 1 000 ka deposition continued to the east along the ol keju nyiro river where a tuff near the top of the formation has been dated at 215 ka in these exposures the formation is unconformably overlain by sediments dated at 49 ka a possible source for the olorgesailie tephra the ol doinyo nyokie volcanic complex contains as ash flow dated at approximately 1 ma extending the known age range of this complex to encompass that of virtually the entire olorgesailie formation in the legemunge plain these geologic examples illustrate the importance of the single crystal sup 40 ar sup 39 ar dating technique whereby contaminant altered or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits the authors have presented a computer modeling procedure based on an inverse isochron analysis thatmore promotes a more objective approach to trimming sup 40 ar sup 39 ar isotope data sets of this type less

isotope and trace element evolution of the naica aquifer chihuahua mexico over the past 60 000 yr revealed by speleothems the espada speleothems of cueva de las espadas naica mine chihuahua mexico comprise a high purity selenite core overlain by successive deposits of calcite gypsum and aragonite gypsum precipitated under water from a hydrothermal solution 58 c when the water table was above the cave level ca 57 ka during the last glaciation and some intervals during deglaciation and the holocene aragonite was deposited at lower temperatures 26 c in a perched lake occupying the cave bottom when the water table dropped below the cave level during brief dry intervals during deglaciation and the early holocene the isotopic composition of gypsum water of crystallization shows that the deglaciation holocene aquifer water was enriched in deuterium by 12 8 8 7 relative to water from the last glaciation this is attributed to an increased relative moisture contribution from the gulf of mexico during deglaciation and the holocene compared to the last glaciation this indicates that drier conditions occurred in the naica area during the holocene than around 57 ka furthermore trace element analyses of gypsum served to deduce the circulation regime of the naica aquifer during the past 60 000 yr and also suggest that higher aquifer recharge occurred during the last glaciation

palaeomagnetism and magnetic fabrics of the late palaeozoic volcanism in the castejon laspaules basin central pyrenees implications for palaeoflow directions and basin configuration the authors thank cristina garcia and teresa roman for her help during field and laboratory work and manuel tricas for the preparation of thin sections this work forms part of the research projects cgl2009 08969 and cgl2006 05817 spanish ministry of education and is included in the objectives of the pre doctoral grant ap 2009 0554 esther izquierdo programa de formacion de profesorado universitario spanish ministry of education funding from the instituto de estudios altoaragoneses is also acknowledged bou acknowledges the jaedoc postdoctoral programme financed with esf the authors acknowledge the careful and constructive revisions by b henry and an anonymous reviewer

soil geomorphology and wet cycles in the holocene record of north central mexico abstract the distinction between the impact of climatic periodicities or land use practices on soil erosion is an important issue for pre hispanic and colonial mexico that question can best be addressed by first documenting the dynamics of changing wet cycles during the holocene in the central mexican region between the northern limits of pre hispanic agriculture and its southern margins in northwestern chihuahua consequently the laguna project targeted a 125 000 km 2 sector of north central mexico 250 km from north to south and 500 km from east to west from saltillo to durango some 40 sedimentary profiles with multiple cumulic soils were studied in the field and laboratory supported by 163 conventional 14 c and ams dates on charcoal and soil humates we distinguish 1 wet floodplains with humic paleosols redox phenomena reflecting high water tables channel ponding sequences and interbedded tufas that imply aquifer recharge sustained base flow and mainly low energy conditions and 2 high energy pulses of discharge that mobilized cobble gravels or forced channel entrenchment gullying and were tied to episodic excessive rains that promoted valley and slope instability in between such wet cycles and recurrent disequilibrium events climate was similar to today probably less humid with limited geomorphologic change or slow soil formation wet cycles were rare at the end of the pleistocene but prominent during the holocene disequilibrium proxies became common and dramatic after 2500 bp the drainages from the eastern and western sierra madres responded in phase but varied in detail around ad 1050 1200 natural erosion led to loss of soil organic carbon as alternating severe droughts and heavy rains destroyed the ground cover and led to ecological aridification well before arrival of spanish miners and settlers the evidence that human activity triggered pre hispanic or colonial erosion in central mexico should therefore be re evaluated global comparisons and interpretations are discussed but with caution since no single theory can explain the whole of the record the soil geomorphology geoarchive of north central mexico primarily is an environmental history of alternating wet cycles rather than of sustained wet or dry climates the critical differences between soaking and excessive rains with their respective impacts may be due to switching between winter and summer storm categories

paleoclimatic significance of late quaternary lacustrine and alluvial stratigraphy animas valley new mexico lacustrine and alluvial stratigraphic sequences in the southern animas valley of new mexico allow reconstruction of late quaternary climates four separate stands of late quaternary lake cloverdale in the southern animas valley are recorded by lacustrine shoreline deposits soils and stratigraphic evidence show that three young lake highstands occurred during the holocene and that a higher lake stand occurred 18 000 to 20 00014c yr b p fluvial systems aggraded the southern animas valley during the middle to late holocene the late quaternary stratigraphy shows that several periods during the late holocene were characterized by higher effective precipitation than at any time since the last glacial maximum

sources of glacial moisture in mesoamerica paleoclimatic records from mesoamerica document the interplay between atlantic and pacific sources of precipitation during the last glacial stage and holocene today and throughout much of the holocene the entire region receives its principal moisture in the summer from an interaction of easterly trade winds with the equatorial calms glacial records from sites east of 95 w in guatemala florida northern venezuela and colombia record dry conditions before 12 ka however west of 95 w glacial conditions were moister than in the holocene for example pollen and diatom data show that lake patzcuaro in the central mexican highlands was cool deep and fresh during this time and fossil pinyon needles in packrat middens in chihuahua sonora arizona and texas indicate cooler glacial climates with increased winter precipitation cold gulf of mexico sea surface temperatures and reduced strength of the equatorial calms can explain arid full and late glacial environments east of 95 w whereas an intensified pattern of winter westerly air flow dominated hydrologic balances as far south as 20 n overall cooler temperatures may have increased effective moisture levels during dry summer months in both areas

wet conditions during the last glaciation in the chihuahuan desert alta babicora basin mexico abstract a 467 cm core b94 3 from the alta babicora basin in the chihuahuan desert northern mexico 29 n 108 w documents lake level and climate changes over the past ca 65 000 yr chronological control is provided by four accelerator mass spectrometry 14 c dates and five u series dates on diatom silica the core has been analyzed for magnetic susceptibility loss on ignition carbonate content sediment chemistry and mineralogy and pollen and diatom composition the basin was occupied by a deep freshwater lake throughout the late pleistocene which based on shoreline evidence was at least 19 m deeper than today the lake shallowed after ca 57 000 yr b p high variability typified the period between 54 600 and 38 000 yr b p probably with periodic desiccation and deflation a deep water lake was reestablished after 38 000 yr b p and persisted until ca 29 000 yr b p shallowing occurred through the last glacial maximum although the lake was still deeper than at present the pleistocene holocene transition was marked by a distinctive change in the diatom flora there is no diatom record for much of the holocene but other proxies indicate generally dry stable catchment conditions the babicora record is more akin to those from the southwest united states than it is to central america and northern south america wet conditions in the late pleistocene are attributed to winter rainfall from midlatitudes in contrast to the modern tropical summer rainfall regime

magnetostratigraphy of eocene oligocene boundary sections in italy no evidence for short subchrons within chrons 12r and 13r abstract the marine magnetic polarity record shows many small scale anomalies that could be due to short normal polarity subchrons within chrons 12r and 13r new examination of published magnetostratigraphic results from a section near gubbio shows that suspected short chrons in this section are probably not real the eocene oligocene boundary in the stratotype section at massignano lies just above a tentatively identified normal magnetozone that might correlate with a possible short normal subchron within chron 13r in a new investigation of the weathered marlstones in this section acquisition and thermal demagnetization of irm reveal a magnetic mineralogy dominated by magnetite with minor amounts of maghemite and hematite vector analysis of the natural remanent magnetization discloses a complex structure in some samples at least two and up to four components were isolated the magnetostratigraphy based on a magnetite component correlates well to the main features of the chron 13n 16n polarity sequence no short polarity subchrons are found although the resolution is inadequate to exclude the possibility that they exist in particular we did not find a short normal polarity magnetozone previously located close to the eocene oligocene boundary the results of this study favour the interpretation that low amplitude magnetic anomalies represent geomagnetic intensity fluctuations rather than unresolved short polarity intervals

compaction profiles of ash flow tuffs modeling versus reality abstract density profiles of 4 ash flow deposits in oregon and idaho are simulated using the model of riehle et al 1995 to calculate heat flow degassing and compaction the deposits are all the rattlesnake tuff a more complex ignimbrite sheet was sampled at 3 distal sites near to one another a medial site and 4 proximal sites model results of distal profiles are similar despite more than twofold variation in thickness and comprise 3 early deposits having emplacement temperatures of 724 732 c followed by two hotter deposits 785 790 c and a cooler capping deposit of about 735 c one distal site has an inferred surge deposit at its base density reversals simulated as cooling intervals all range from 3 to 10 days correlated cooling intervals agree to within a factor of two however the duration of a cooling interval cannot be precisely hindcast owing to potential complications by rainfall or accumulation of chilled airfall ash thus the seemingly wide range of model results for the cooling intervals is perhaps not surprising proximal profiles have more density reversals implying more deposits than the distal profiles they also have greater overall compaction and consequential loss of detail in the density profiles some earlier deposits have model temperatures of 736 757 c and may correlate with the lower distal deposits at one site these are overlain by two hotter deposits 790 796 c and a cooler capping deposit 745 c that are almost surely correlatives of the upper distal deposits some sites have additional early hotter deposits and one site has an uppermost deposit that is the coolest of all model deposits thus a picture emerges of thermally stratified near source flows during early eruptive activity and more far travelled flows during later eruptions shards analyzed by streck and grunder 1997 show a compositional range and corresponding magma temperatures of 795 880 c which is permissive of thermally layered deposits and implies cooling during fallback and travel of 50 100 c rare textural evidence for internal flow boundaries as well as our consistent model results among multiple sites despite variation in deposit thickness support the assumption that density reversals represent partial cooling breaks if so then modeling of density profiles can provide important evidence for how large ignimbrites are constructed sheridan and wang 2004 also report stratigraphically consistent results by modeling density reversals in distal bishop tuff as cooling breaks our results contrast with those of wilson and hildreth 2003 who concluded that density reversals in the proximal bishop tuff do not represent cooling breaks but instead are evidence for thermally heterogeneous deposits it may be that the proximal bishop tuff is not amenable to modeling due to its great thickness 100 200 m and deposition on steeply dipping rugged terrain factors which would likely cause turbulent flow obscure subtle stratigraphic details of flow boundaries and complicate resulting compaction zonation

magnetic structural and geochronologic evidence bearing on volcanic sources and oligocene deformation of ash flow tuffs northeast nevada magnetic properties of mid tertiary volcanic rocks west of jiggs in northeast nevada were investigated for the purposes of interpreting igneous structural and tectonic processes in this part of the basin and range province anisotropy of magnetic susceptibility ams patterns reflect flow fabrics and suggest previously unknown sources for these ash flow tuffs paleomagnetic and structural evidence suggest counterclockwise relative rotation of the southern part of the area with respect to the north new stratigraphic chemical and k ar isotopic data support these interpretations dacite to rhyolite ash flow tuffs of the indian well formation were subdivided into two main units the lower and predominant tuff of jiggs 35 8 37 0 ma and the unconformably overlying but areally restricted tuff of hackwood 30 8 ma the jiggs unit has two polarities whereas the hackwood has only a reversed polarity remanence together these units with tilt correction define a pole 92 e 87 n dp 6 dm 8 for approximately 30 37 ma this pole is concordant with coeval north american reference poles indicating that this zone of approximately 30 km ns extent has not undergone significant vertical axis rotation relative to the north american reference andesite lavas of normal magnetic polarity and of 32 5 ma age characterize the diamond hills immediately to the south we interpret this region from both structural evidence and discordant paleomagnetic direction to have rotated approximately 25 counterclockwise relative to the indian well volcanic units to the north the apparent rotation of the diamond hills is possibly the result of drag on the left lateral garcia fault which limits the diamond hills on the southwest analysis of ams data suggests by patterns of the k1 axes two distinct sources for the jiggs unit a northern buried source and a central partially buried source lithologic evidence consistent with proximal vent facies is found near the latter source an unusual pattern in the ams k3 axes suggested as resulting from the tilt correction applied to initial dip is consistent with the interpreted flow directions tight age constraints can be placed on the deformation of the main ash flows of the jiggs unit in the 35 8 to 30 8 ma interval because the diamond hills unit is slightly younger and appears to have been affected by the same deformative episode as the jiggs the regional deformation affecting both units can be more narrowly confined to the 32 5 30 8 ma interval this deformation is inferred to coincide with the major early tertiary episode of crustal extension in this region of the basin and range

paleoclimatic changes during the late pleistocene holocene in laguna babicora near the chihuahuan desert mexico the paleoclimatic variability of northern mexico since the late wisconsinan is determined from sedimentological geochemical and stratigraphic studies of the lacustrine sequence of laguna babicora 29 4 n 107 7 w elevation 2100 m asl chronological control is based on 13 radiocarbon dates 6 from this study and 7 previously reported from four stratigraphic profiles that cover a range from 16 342 to 2800 bp two major periods of increased humidity are recognized by lake level variation during the late wisconsinan and early holocene the late wisconsinan humid period may be related to jet stream migration to southerly latitudes and the influence of the mobile polar high this period has been identified in the sedimentary record of el diablo profile with a radiocarbon date of 16 342 201 bp the early holocene humid period is recorded in el diablo 10 976 115 bp and cano profiles 9614 130 bp it is interpreted as a result of the younger dryas climatic anomaly characterized by a temperature decrease and cold paleoclimatic conditions similar to those for the late wisconsinan the early holocene was followed by an overall gradual drying that reached a maximum around 6000 bp during the middle holocene for the late holocene evidence of widespread erosion sedimentation changes flooded surfaces and paleosoils indicates that laguna babicora was subjected to short term climatic changes

hydrologic segmentation of high temperature shear zones structural geochemical and isotopic evidence from auriferous mylonites of the renco mine zimbabwe abstract combined structural mineralogical and geochemical observations in auriferous mylonites of the renco mine hosted by late archaean high grade metamorphic granitoids in southern zimbabwe are used to describe the spatially heterogeneous fluid flow and metasomatism that occurred synchronous with deformation at mid to upper amphibolite facies metamorphic conditions significantly the narrow on average 1m wide mylonitic shear zones are internally zoned reflecting a pronounced hydrologic segmentation during deformation shear zones typically consist of two distinct domains i anastomosing quartz feldspar biotite hornblende mylonites and or quartz mylonites and ii tabular shaped pods referred to as lithons that are enveloped by mylonites and that exhibit evidence of transient episodes of brittle fracturing and ductile creep whole rock geochemistry and mass balance calculations indicate dramatic element volume up to 100 and associated mass gains for the brittle ductile lithons that are mineralogically reflected in a volumetrically abundant sulphide mineralisation and the formation of a pervasively developed silicate alteration paragenesis in contrast mylonites have experienced only minor element and volumetric changes and minor alteration 18 o values for whole rocks and quartz are enriched in lithons compared to wall rocks and enveloping mylonites which implies the influence of externally derived fluids these results indicate a strongly domainal fluid flow and mass transfer fluid advection was dominated by microscopic and macroscopic fracture permeabilities related to periods of coseismic dilatancy in lithons the patchy but partly interconnected distribution of lithons enveloped by mylonites indicates that deformation in the high temperature shear zones was characterised by transient periods of seismic slip rather than continuous aseismic creep based on the well preserved internal mineralogical and textural development of the renco shear zones a more general model applicable for fluid advection coupled with the rheological behaviour of mid crustal shear zones is presented

on archeomagnetic secular variation curves and archeomagnetic dating abstract secular variation sv of the earth s magnetic field can be used for dating purposes by comparing archeomagnetic directions of unknown ages with a well dated reference curve in this study we propose a dating technique based on the statistics of mcfadden and mcelhinny geophys j int 103 1990 725 for testing the hypothesis that two fisherian distributions of individual directions share a common mean direction the statistics are adapted to test the degree of compatibility between one individual fisherian mean direction and a reference curve constructed using the bivariate extension of the fisher distribution furthermore as the density of the data which define the archeomagnetic reference curve varies in time we suggest that one computes the mean directions with moving windows of varying duration where both the window widths and the time shifts between successive mean directions are fixed when a minimum threshold density of data is reached within each time interval in our paper we apply this new procedure to the french archeomagnetic data set

orbital and internal forcing of climate on the yucatan peninsula for the past ca 36 ka abstract a 19 6 m sedimentary sequence from lake quexil guatemala is presented as a proxy record of climatic change on the low lying yucatan peninsula over the last ca 36 ka long term climatic fluctuations are attributed to orbital forcing of insolation while abrupt changes during the late glacial period are the result of internal forcing mechanisms that remain imperfectly modelled gcms simulating meltwater cooling of the gulf of mexico may predict some but not all climatic responses in the circum caribbean lake quexil did not experience greater aridity than predicted by orbital forcing during late glacial zones q lg 1 and 2 which were drier only in comparison with holocene conditions q lg 1 appears to have been colder than expected based on insolation alone and is contemporary with the younger dryas event between meltwater pulses 1a and 1b thus other feedbacks also may have cooled surface temperatures in the gulf of mexico such as unique late glacial circulation patterns increased albedo for the florida and yucatan peninsulas or surface temperatures on the north american continent additional terrestrial sequences are needed to improve the chronology and confirm the distribution of sub regional climatic variability in the circum caribbean

late pleistocene and holocene paleoenvironments of chalco lake central mexico abstract in order to establish paleoenvironmental conditions during the late quaternary four cores from the basin of mexico central mexico were drilled in chalco lake located in the southeastern part of the basin the upper 8 m of two parallel cores were studied using paleomagnetic loss on ignition pollen and diatom analyses based on 11 14c ages the analyzed record spans the last 19 000 14c yr b p volcanic activity has affected microfossil abundances both directly and indirectly resulting in absence or reduction of pollen and diatom assemblages important volcanic activity took place between 19 000 and 15 000 yr b p when the lake was a shallow alkaline marsh and an increase of grassland pollen suggests a dry cold climate during this interval abrupt environmental changes with increasing moisture occurred from 15 000 until 12 500 yr b p the lake level increased and the pollen indicates wetter conditions the highest lake level is registered from 12 500 to ca 9000 yr b p the end of the pleistocene is characterized by an increase in humidity from 9000 until ca 3000 yr b p chalco lake was a saline marsh and the pollen record indicates warmer conditions after 3000 yr b p the lake level increased and human disturbance dominates the lacustrine record

magnetic properties and emplacement of the bishop tuff california anisotropy of magnetic susceptibility ams and characteristic remanence were measured for 45 sites in the 0 76 ma bishop tuff eastern california thirty three sites were sampled in three stratigraphic sections two in owens gorge south of long valley caldera and the third in the adobe lobe north of long valley the remaining 12 sites are widely distributed but of limited stratigraphic extent weakly indurated highly porous to dense welded ash flow tuffs were sampled saturation magnetization vs temperature experiments indicate two principal iron oxide phases low ti magnetites with 525 570 c curie temperatures and maghemite with 610 640 c curie temperatures af demagnetization spectra of isothermal remanent magnetizations are indicative of magnetite maghemite predominantly in the multidomain to pseudo single domain size ranges remeasurement of ams after application of saturating direct fields indicates that randomly oriented single domain grains are also present the degree of anisotropy is only a few percent typical of tuffs the ams ellipsoids are oblate with kmin axes normal to subhorizontal foliation and kmax axes regionally aligned with published source vents for 12 of 16 locality means kmax axes plunge sourceward confirming previous observations regarding flow sense topographic control on flow emplacement is indicated by the distribution of tuff deposits and by flow directions inferred from kmax axes deposition east of the benton range occurred by flow around the south end of the range and through two gaps benton notch and chidago gap flow down mammoth pass of the sierra nevada is also evident at least some of the adobe lobe in the northeast flowed around the west end of glass mountain eastward flow directions in the upper owens gorge and southeast directions in the lower owens gorge are parallel to the present canyon suggesting that the present drainage has been established along the pre bishop paleodrainage characteristic remanence directions from 45 sites 267 samples yield an overall mean of d 348 i 53 for the bishop tuff a correlation is found in two of the three profiles between density and remanence inclination a mean remanence direction based on 13 localities together with data from uncompacted xenoliths and data from the ash fall tuff at lake tecopa is d 353 i 54 k 172 95 2 9 n 15

eocene oligocene paleoceanographic changes in the stratotype section massignano italy clues from rock magnetism and stable isotopes 1 we have conducted high resolution paleomagnetic and rock magnetic studies in addition to stable isotope analyses of the massignano sedimentary section which is the global stratotype section and point gssp for the eocene oligocene boundary our research builds upon the many past studies of the massignano section in seeking to understand the timing and nature of the paleoenvironmental variations that occurred during the transition for the earth s climate system from greenhouse to icehouse the new paleomagnetic results provide a refined magnetostratigraphy of the section and new age for the eocene oligocene boundary at 33 7 ma abrupt and large alternations in magnetic concentration composition and grain sizes that occur in the high resolution rock magnetic record are interpreted to be the result of rapid bimodal shifts in deep sea circulation that affect sediment sources or transport we speculate that currents flowing through the gateway between the atlantic and indo pacific oceans may have turned on and off as the gateway was progressively closing resulting in the deposition of two different assemblages of magnetic minerals at massignano finally stable isotope 18o and 13c data collected on ostracod valves also suggest significant changes in sea bottom circulation in the neo tethys ocean at and about 2 m y before the eocene oligocene boundary

relationships between tectonics and magmatism in a transtensive transform setting an example from faial island azores portugal the relationships between tectonics and hotspot related magmatism in transform transtensive settings are poorly known the azores archipelago lying where the transform plate boundary between the nubian and eurasian plates meets the mid atlantic ridge is a rare site to investigate these relationships the distinct tectono magmatic features of faial make it the ideal island to focus the study here we analyze the relationships between tectonics and magmatism using remote sensing field and paleomagnetic analyses dominant wnw ese trending lineaments correspond to major faults with transtensive dextral motion and ne sw opening probably related to the nearby transtensive terceira rift most 60 dikes vent elongations and alignments are parallel to this system the lavas forming the basement of the distinctive wnw ese trending pedro miguel graben strike parallel to the graben axis dipping outward with a tilt consistent with that of paleomagnetic data these lavas are dissected by the wnw ese trending transtensive faults therefore the graben consists of outward tilted fault bounded blocks forming two opposite verging dominoes its estimated stretching factor 1 35 and mean extension rate between 3 4 and 8 2 mm yr are similar or slightly larger than those of terceira rift to the east part of the graben extension may be magma induced via diking we suggest that 1 faial along with the nearby pico island is a major locus of extension within the azores directly above the imaged hotspot and 2 the faial pico magmatic segment constitutes the offset westward magmatic continuation of the terceira rift the segment opening highlights the importance of extensional strain along magmatic transtensive plate boundaries

cooling and welding history of the bishop tuff in adobe valley and chidago canyon california abstract a numerical model for cooling and welding of ash flow tuffs was developed to obtain a quantitative understanding of the welding compaction that occurs during the cooling of ash flow sheets and to evaluate the relative effects of emplacement temperature thickness and emplacement histories on compaction zonation density profiles of large ash flow sheets this model includes a set of two phase compressible navier stokes equations for the transport of mass momentum and energy for a variable density volatile outfluxing through a porous media ash flow tuff with this model we calculated theoretical compaction profiles density vs elevation to match actual data measured in sections of bishop tuff compaction profiles from the adobe basin indicate that there were 3 periods of non deposition 5 10 and 40 days separating 4 distinct rapidly emplaced packets rep of pyroclastic flow deposition in contrast the 3 periods of quiescence 400 20 and 800 days between the 4 reps in chidago canyon were considerably longer these data constrain models of eruption for the bishop tuff by implying an eruption cycle that lasted longer than 3 years the average calculated deposition temperatures for all 4 reps in the adobe basin are fairly uniform at 667 33 c in chidago canyon two of the reps had a higher deposition temperature 660 c than the other two 534 c

limnologic history of lago de patzcuaro michoacan mexico for the past 48 000 years impacts of climate and man abstract lago de patzcuaro lies at an elevation of 2035 m 19 36 n 101 39 w on the west part of the volcanic meseta central of mexico about 50 km east southeast of morelia pollen diatom and geochemical records from a 15 m core taken in 6 m of water continuously track vegetation and limnological changes in the patzcuaro basin for the past 48 ky calendar years pine oak and alder pollen dominate both the wisconsin and holocene vegetation record and indicate that this region did not witness strongly arid climates during that time before the holocene juniperus type and artemisia pollen document cooler climates of more effective moisture at least seasonally isoetes characterizes the full and late glacial interval 25 13 ka and implies along with the diatoms aulacoseira and stephanodiscus cool deep freshwater conditions at lago de patzcuaro the freshest and deepest lacustrine phase in the lake occurred 38 to 25 ka during this time lago de patzcuaro may have drained into the lerma river system between 30 and 10 ka diatom species that bloom in the winter and early spring imply moisture during those seasons evaluation of contemporaneous marine and terrestrial records west and east of lago de patzcuaro suggests that this moisture probably originated from westerly storm tracks shifted south by the laurentide ice sheet after 10 ka lago de patzcuaro became shallower and more eutrophic rooted aquatic plants prospered under a climate regime of negative hydrologic budgets increased seasonality and moisture arriving during the summer as it does today the lake became alkaline enough by the mid holocene to deposit ostracode rich marls at the core site for the first time in the past 48 ky indigenous agriculture in the patzcuaro drainage and around the lake appears about 4 ka with the appearance of zea and chenopodiineae pollen iron and potassium track increased soil erosion and planktic diatoms cyclostephanos record nutrients flushed into the lake from the drainage basin

lake levels since about 40 000 years ago at lake chalco near mexico city abstract diatoms magnetic susceptibility organic content and14c ages of sediments from a 26 m core suggest that lake chalco in the southern part of the basin of mexico went through a series of major fluctuations during the late pleistocene and the holocene before ca 39 00014c yr b p the lake was very deep about 8 10 m alkaline and saline it then became shallow

evidence from western north america for rapid shifts in climate during the last glacial maximum the estancia basin in southwestern united states contains evidence for strong and rapid pulsations in the supply of moisture brought into the region during the last ice age the pulses were recorded during episodes of stream discharge that spread plumes of fresh water laden with quartz sand over the saline lake the largest pulses in stream discharge lasted only a few decades were organized into cycles that were spaced approximately 200 to 250 and 2000 years apart and were of sufficient magnitude to freshen and maintain the lake at its maximum recorded elevation

late quaternary lacustrine history and paleoclimatic significance of pluvial lake cochise southeastern arizona abstract during the latest quaternary freshwater pluvial lakes intermittently formed in the topographically closed willcox basin arizona a lacustrine sequence of six separate high stands of lake cochise is documented by stratigraphic studies 19 radiocarbon ages and supplementary evidence two stands of pluvial lake cochise older than 14 000 yr b p reached elevations above 1290 m the prominent 1274 m shoreline of lake cochise which circumscribes the basin was largely created during a high stand between 13 750 and 13 400 yr b p during the holocene water filled the willcox basin three times to an elevation slightly below the crest of the 1274 m shoreline this occurred once during the early holocene around or before 8900 yr b p and twice during the later part of the middle holocene since the middle holocene only shallow ephemeral lakes have occupied the deflated central portion of ancient lake cochise a depression known as the willcox playa the lacustrine sequence of lake cochise provides an independent evaluation of late quaternary paleoclimatic reconstructions for southern arizona and the american southwest

anisotropy of magnetic susceptibility in relation to source vents of ignimbrites empirical observations the anisotropy of magnetic susceptibility ams of young undeformed ashflow deposits e g bishop tuff california bandlier tuff new mexico show alignment of the maximum axis of magnetic susceptibility kmax with inferred flow lines the sense of flow is deduced from the predominant plunge bias of kmax axes these principles are discussed in relation to four deformed settings of ashflow deposits in the great basin of the western united states the magnetic susceptibility data provide valuable but not always unambiguous insight into probable source areas various complications arise in interpreting the ams data in terms of flow directions but these can be reduced by integration with geologic observations

insights into deposition and deformation of intra caldera ignimbrites central nevada abstract magnetic studies together with structural and 40 ar 39 ar age analyses were used to investigate the development and deformation of ignimbrites of the caetano tuff formation in central north nevada magnetic susceptibility susceptibility anisotropy ams and magnetic remanence approaches were used to decipher events accompanying emplacement and subsequent deformation of that tuff for both intracaldera tuffs and outflow deposits 40 ar 39 ar ages both those presented here and previously published ages indicate about 33 8 ma for the age of caetano ignimbrites approximately at the eocene oligocene boundary although extensive faulting associated with regional extension of the great basin region has dissected these deposits the thickness of the intra caldera ignimbrites has been estimated at 3 5 km or more thermal modeling for cooling of a caldera filling of 3 5 km thickness indicates acquisition of remanent magnetization can persist up to 25 ka sufficient to average secular variation this contrasts with rapid cooling and remanence acquisition in outflow ignimbrites typically only tens of meters thick a 750 m thick reference section was selected for more detailed study about 10 km to the west of the type area this reference section fault bounded at both base and top represents the upper part of the formation its lower member includes the uppermost 250 m of the lower massive resistant unit of ignimbrites a middle member of 375 m of well stratified pyroclastic deposits vitrophyres and sediments and an upper member of about 125 m of recessive ignimbrite and thin bedded clastic and pyroclastic deposits structural and magnetic remanence measurements suggest 13 eastward tilting of the caldera floor occurred early during deposition of the middle member flow directions inferred from ams k max axes in the middle member diverge about 45 relative to those in the underlying member and suggest a change in vent source accompanying the deposition of the middle member correcting for e w extension the outflow ignimbrites reached approximately 20 km to the west and south of the caldera north of the caldera ignimbrites formerly assigned to the caetano tuff but now assigned to the tuff of cove mine exhibit variable flow directions from an indeterminate source significantly however the intracaldera ams magnetic lineations conventionally used as proxies for flow lines trend mainly wnw parallel to a proposed major paleodrainage which stretches from the southern cortez range through 40 km to the tobin range caetano tuff magnetic remanence is entirely reversed and corresponds to magnetization acquired in the younger part of magnetochron 13r the magnetic remanence directions of both the intra caldera and outflow ignimbrites are similar d 151 i 60 significantly different from those predicted by a coeval reference pole for north america raising the possibility of a regional counterclockwise rotation of the sampling area of 15 6 presumably this began during faulting and major regional extension about 16 ma ago during the middle miocene and possibly continued later

polyphase deformation of mylonites from the renco gold mine zimbabwe identified by crystallographic preferred orientation of quartz abstract gold mineralization at renco mine in the northern marginal zone of the limpopo belt zimbabwe is hosted by a set of intersecting steep and shallow dipping shear zones locally termed reefs both steep and shallow reefs are closely associated with quartz mylonites the crystallographic preferred orientation of quartz from samples of these quartz mylonites indicate that deformation in steep reefs occurred under amphibolite facies conditions pole figures and microstructures of samples from shallow reefs in contrast show evidence for an origin of the auriferous mylonites at amphibolite facies conditions that were subsequently overprinted by deformation under lower greenschist facies conditions since mineralization is hosted by both reef geometries gold mineralization is inferred to have occurred during deformation and associated fluid infiltration along mylonites at amphibolite facies grades coeval with late archaean thrusting of rocks of the northern marginal zone onto rocks of the zimbabwe craton the crystallographic preferred orientation patterns of quartz point to a normal sense of movement during reactivation of the shallow reefs probably related to the meso proterozoic tectonometamorphic event that has affected the central parts of the limpopo belt at ca 2 0 ga normal faulting along the boundary between the northern marginal zone and zimbabwe craton has to date not yet been documented and may provide some important constraints for the understanding of the kinematics of the meso proterozoic tectonics in the limpopo belt

atlantic ocean modulated hydroclimate of the subtropical northeastern mexico since the last glacial maximum and comparison with the southern us abstract proxy records generated from chemical composition and radiocarbon dating of a sediment sequence collected from the el potosi basin provide information about variable hydroclimate of the subtropical northeastern mexico since the last glacial maximum lgm minimal amounts of runoff and more carbonate deposition during 19 ka represent the lgm the deglaciation was wetter compared to the lgm and enhanced runoff during 19 12 ka transported more ti bearing clastics from a relatively humid watershed over the deglaciation the basin received more runoff during the bolling allerod compared to the heinrich stadial 1 and younger dryas reduction in the amount of runoff during 10 1 ka and deposition of unweathered si bearing clastics during 4 1 ka suggest that the holocene was drier compared to the deglaciation hydroclimates of the northeastern mexico and southern us were synchronous during the lgm as well as deglaciation change from the drier lgm to wetter bolling allerod occurred as the gulf of mexico became warmer and the region received more summer precipitation however the dissimilarities observed in regional hydroclimates of the holocene need further evaluation

paleomagnetic evidence for counterclockwise block rotation in the north nevada rift region paleomagnetic data from mid miocene dikes and flows at three localities within the north nevada rift indicate that some crustal blocks have rotated approximately 19 degree plus minus 7 degree counterclockwise relative to stable north america as one possible consequence a revised mid miocene extension direction for the area is about n89 degree e s89 degree w indicating an approximately 25 degree difference between mid miocene and modern least principal stress directions the rotation may be accommodated by a right lateral component of slip on northwest trending oblique slip faults these as well as data from other parts of the basin and range province indicate that strike slip faulting and associated block rotation contributed to cenozoic extension

evidence for a very long term trend in geomagnetic secular variation reconstructions of palaeosecular variation suggest that the earth s magnetic field reversed less frequently 2 82 to 2 45 billion years ago relative to the cenozoic era this suggests a long term trend of decreasing geodynamo stability since the archaean eon the earth s inner core is believed to inhibit rapid fluctuations in the geomagnetic field from developing into full polarity reversals1 2 consequently during the precambrian the smaller size of the inner core might suggest that polarity reversals could occur more readily it is therefore surprising that there are indications that reversals were rare during this period3 4 here we use new and existing palaeomagnetic data from three continents to examine the stability of the earth s magnetic field from 2 82 to 2 45 billion years ago we show that on average geomagnetic secular variation the field variations produced by normal geodynamo action during the late archaean and early proterozoic was different from that of the past 200 million years specifically the apparent variability of the geomagnetic pole as viewed at low and mid latitudes was reduced relative to the past 200 million years according to both dynamo simulations4 and more recent palaeomagnetic field observations5 the observed pattern of secular variation suggests a lower frequency of polarity reversals 2 5 billion years ago this may imply that the geodynamo is becoming progressively less stable over long timescales consistent with some numerical simulations6 7 possibly as a result of changing outer core geometry that has accompanied inner core growth

stratigraphy paleomagnetism and magnetic fabric of the toba tuffs constraints on the sources and eruptive styles the toba depression in north central sumatra is a complex of several overlapping calderas resulting from three major ignimbrite forming eruptions within the depression the upland masses of samosir and northern uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment hitherto interpreted to be two parts of a single resurgent dome this study has demonstrated that the welded tuffs of samosir and uluan have different magnetic polarities and therefore at least two different ignimbrites are present the samosir uluan massif may consist of parts of two resurgent domes the first ignimbrite eruption occurred at 0 84 ma and produced a very thick 400 m densely welded unit having a reversed polarity anisotropy of magnetic susceptibility ams flow direction and lithic size data indicate that the source lies in the southern part of the toba depression and the thick deposit of uluan is thought to have ponded in a 40 km wide caldera the second ignimbrite is normally magnetized ams flow direction data indicate two separate source vents one to the north in the haranggaol caldera and another to the south the thick deposit at samosir is thought to have ponded in the southern caldera coarse sediments then accumulated over samosir and northern uluan and were capped by lacustrine deposits a renewed episode of resurgence then uplifted samosir island and possibly the northern part of uluan at approximately 0 075 ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the toba depression this ignimbrite is mostly nonwelded and normally magnetized part of the uluan dome was destroyed by collapse of the sibandung caldera and latung graben and concomitant with renewed subsidence of the haranggaol and porsea calderas

apparent polar wander paths for the major continents 200 ma to the present day a palaeomagnetic reference frame for global plate tectonic reconstructions summary synthetic apparent polar wander apw paths for north america south america eurasia india central africa australia and antarctica for the last 200 myr are proposed computation of these apw paths is based upon the latest version 4 5a of the global paleomagnetic database gpmdb a revised global plate tectonic model since the early jurassic and a new technique for generating smoothed apw paths the smoothing technique includes the following steps 1 pre selection of palaeopoles including pre filtering parameters number of sites number of samples per site 95 per cent confidence circle about mean direction cleaning procedure and time uncertainty 2 generation of palaeolatitude and declination plots for a reference site on each continent that combines palaeopoles via a global plate tectonic circuit 3 independent spline regression analyses of the palaeolatitude and declination plots 4 removal of palaeolatitude or declination data that deviate by more than 10 from the regression curves post filtering process 5 generation of synthetic apw paths from the resulting palaeolatitude and declination plots these synthetic apw paths are then rotated into african coordinates to determine the best fit apw path and a global palaeomagnetic reference frame four representative plate tectonic reconstructions and global plate velocity fields are presented for the three time intervals that correspond to globally synchronous changes in plate motion

a holocene record from a former coastal lagoon in bahia kino gulf of california mexico a 10 m long core was recovered from the coastal flood plain on the eastern margin of the gulf of california bahia kino sonora sediments were analysed for loss on ignition magnetic susceptibility pollen diatoms ostracodes and foraminifera three levels were dated by 1 4 c the combined data suggest that the site was flooded during the middle holocene as sea level increased at about 6600 yr bp the area was a freshwater to brackish marsh dominated by typha domingensis and cyperaceae freshwater to brackish conditions suggest the presence of a local source of freshwater during this time and therefore a climate wetter than present continued rise in sea level was associated with an increase in depth and salinity by c 6350 6250 yr bp the coring site was an inner lagoon by c 6125 yr bp a central lagoon and by c 5900 yr bp the site was close to the mouth of the lagoon lower lagoon lower lagoon conditions persisted until the top of the record showing sea level stabilization which was followed by the formation of the dune ridge that presently separates the coring site from the sea

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

prehistoric and colonial land use in the chihuahuan desert as inferred from pollen and sedimentary data from coahuila mexico abstract pollen along with context sedimentary data of two desert bottom sites in south coahuila mexico reconstruct holocene environment in the chihuahuan desert in relation to possible human influence on vegetation during the holocene examined pollen sites here derive from the margins and bottoms of the playa lake of the laguna mayran lm first at the reported site of cuota 4 lm4 archaeologically dated deposits of the initial spanish colonial period here are correlated with a xeric environment followed by an early modern mesic period of increased rainfall with wetland plants second at the reported site of coahuila highway 30 ch30 unconformable discontinuous deposits with intermittent representation of early middle and late holocene sedimentation are analyzed showing periodic changes in hydrology at ch30 possible cultivation based on cucurbitaceae pollen occurs in a defined wet phase dating to circa 1500 bp finally origins of pollen taphonomy at ch30 are defined by analog samples from the chihuahuan desert zone in the menchacas basin near cuatro cienegas are analyzed the pilot pollen analog study is based on inferred pollen recruitment mechanics in different sediments as well as moss pollsters in the menchacas basin in the chihuahuan desert in middle coahuila

paleomagnetic and structural evidence for middle tertiary counterclockwise block rotation in the dixie valley region west central nevada paleomagnetic data from late oligocene to early miocene ash flow tuffs at four localities in the northern dixie valley region west central nevada indicate that parts of the crust have rotated counterclockwise by at least 25 sup 0 and perhaps significantly more in late cenozoic time field relations in white rock canyon stillwater range suggest that rotation 1 was accommodated by right lateral slip on northwest trending faults 2 spanned ash flow tuff emplacement and 3 probably ceased before eruption of overlying middle miocene basalts accurate estimates of cenozoic extension as well as evaluation of earlier mesozoic structures must include the strain partitioned into rotation in the area

u series dating of impure carbonates an isochron technique using total sample dissolution abstract u series dating is a well established technique for age determination of late quaternary carbonates materials of sufficient purity for nominal dating however are not as common as materials with mechanically inseparable aluminosilicate detritus detritus contaminates the sample with extraneous th we propose that correction for contamination is best accomplished with the isochron technique using total sample dissolution tsd experiments were conducted on artificial mixtures of natural detritus and carbonate and on an impure carbonate of known age results show that significant and unpredictable transfer of radionuclides occur from the detritus to the leachate in commonly used selective leaching procedures the effects of correcting via leachate residue pairs and isochron plots were assessed isochrons using tsd gave best results followed by isochron plots of leachates only

paleosol organic matter and pedogenic carbonates and paleontological δ13c records applied to the paleoecology of late pleistocene holocene in mexico abstract stable isotopes of carbon 13 c in terrestrial geological materials paleosol humus and carbonates as well as fossil bones collagen and tooth enamel are established as highly specific records on paleoenvironmental characteristics it was assumed that comparing the results of different types of materials on regional or local scales provides more accurate and reliable reconstructions and even opens new aspects of interpretation this was tested by the comparison of the data obtained from the late pleistocene paleosols and or remains of pleistocene megafauna in various locations in northwestern mexico sonora paleosols formed in mis3 mis2 and mis1 central northern mexico san luis potosi teeth corresponding to mis2 and mis1 and central southern mexico puebla paleosols corresponding to mis3 and mis2 and teeth associated with these paleosols and state of mexico teeth corresponding to mis2 the results of isotopic studies are complemented with other paleoenvironmental proxies the 13 c values of paleosols of sonora is 19 8 to 16 95 in organic matter and shows little difference between paleosols and holocene soils 13 c values of pedogenic carbonates are 6 4 to 4 52 in san luis potosi the 13 c values of paleosol are 12 16 in organic matter in puebla they vary from 24 6 in paleosols to 15 16 in the modern surface soil pointing to contrasting vegetation change the 13 c values in fossils range from 10 7 to 1 2 in slp of 2 73 and 2 93 in puebla and 6 5 to 1 3 in state of mexico the 13 c values of paleosols and analysis of megafauna in northwest and central northern mexico matched each other indicating mixed c3 c4 vegetation while for central southern mexico paleosols indicated predominance of c3 plants and megafauna indicated mixed c3 c4 composition in general the results indicate that in northern mexico environmental conditions were slightly wetter than at present while for the central area there was a large change in the environmental conditions of high humidity in mis3 and mis2 with a trend to drier climates in mis1

magnetic fabric flow directions and source area of the lower miocene peach springs tuff in arizona california and nevada we have used anisotropy of magnetic susceptibility ams to define the flow fabric and possible source area of the peach springs tuff a widespread rhyolitic ash flow tuff in the mojave desert and great basin of california arizona and nevada the tuff is an important stratigraphic marker from the colorado plateau to barstow california a distance of 350 km however the location of its source caldera is unknown dated at 18 5 ma by 40ar 39 ar the tuff erupted during the early stages of miocene extension along the lower colorado river the thicker accumulations 100 m occur at kingman arizona and in the piute mountains california on opposite sides of the colorado river extensional corridor our ams studies produced well defined magnetic lineations in 30 of 42 sites distributed throughout the tuff typical ratios of the principal ams axes are 1 01 for the magnetic lineation kmax kint and 1 02 for the foliation kint kmin the bulk magnetic susceptibility of the peach springs tuff averages 2 0 10 3 in the si unit system the subhorizontal lineations which presumably parallel the flow directions form a pattern radiating outward from the approximate center of the outcrop area magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff the lineation and imbrication indicate a source region near the southern tip of nevada defining the best intersection of the ams lineations required restoration of major extension strike slip faulting and associated tectonic rotation in the disrupted tuff the optimum intersection of magnetic lineations lies in the southern black mountains of arizona on the eastern side of the colorado river extensional corridor no caldera structures are known from that area but the area contains thick sections of the peach springs tuff above a silicic volcanic center the caldera may be buried under younger deposits in the mohave valley of arizona tertiary granite in the newberry mountains may represent a deeper level of the peach springs tuff vent that has been exhumed by detachment faulting

geochemical changes recorded in lynch s crater northeastern australia over the past 50 ka there are many geochemical reconstructions of environmental change in the mid and high latitudes but relatively few in the tropical latitudes despite their considerable potential for reconstructing environmental processes that cannot be identified using more traditional proxies here we present one reconstruction of environmental change for the tropics this reconstruction covers the past 50 ka using a suite of geochemical data from the high resolution sequence of lynch s crater in northeast queensland australia a region highly sensitive to el nino southern oscillation enso activity the 23 major oxides and trace elements measured could be summarised by extracting three axes using principal components analysis accounting for 72 of the variability the data indicate that the greatest variability in the geochemical data accounted for erosional activity within the catchment that was associated with past changes in the frequency of enso activity though this was less sensitive during wetter periods probably as a result of buffering by high vegetation cover the remaining variability was largely explained by elements that form complexes with organic compounds e g humic acids and those that are important nutrients for specific vegetation types and therefore a measure of vegetation distribution for more detailed reconstructions further work is required to disentangle the complex controls of clements within sedimentary sequences c 2005 elsevier b v all rights reserved

flow directions in ash flow tuffs a comparison of geological and magnetic susceptibility measurements tshirege member upper bandelier tuff valles caldera new mexico usa this study concludes that the elongation axis k 1 of the ellipsoid of anisotropic magnetic susceptibility ams is a suitable proxy for flow axis in ashflow tuffs 153 oriented samples 176 specimens were studied from 18 sites in the 1 1 ma tshirege member of the bandelier tuff these sites are distributed around the valles caldera at distances of 5 25 km outside of the rim k 1 axes correlate well with postulated radial flow axes at 13 sites k 1 also agrees with measured geological flow indicators mainly imbricated larger clasts at 7 sites at 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredk 1 axes thek 1 axes correspond well with geological flow indicators indicating that the divergence of flow from the predicted radial flow pattern is real two major topographic buttresses are suggested as the cause of flow divergence for the tshirege ash flows the san pedro buttress northwest of the caldera and the san miguel buttress in the southeast in situk 1 axes plunge about 7 toward the source at two thirds of the sites therefore the plunge ofk 1 is a plausible in situ indicator for thedirection of flow multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash flow emplacement these observations raisre the question of how best to represent mean flow directions in ash flow sheets by eigenvector methods by vector sum methods or by modes a method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined imbrication determined in this way at one third of the sites dips toward the source i e up flow the minimum k 3 axis of the ams ellipsoid correlates with the flow foliation rather than with the larger clast imbrication the flow axes of ash flows correspond with thek 1 axes not with the declination ofk 3 axes as suggested by some authors initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction which is reversed with a mean ofd 173 5 i 38 4 95 3 4 n 18 this mean is not different at the 95 confidence level from that of earlier workers the mean pole at 098 0 e 74 8 n a 95 3 3 n 18 is about 15 far sided relative to the expected time averaged geomagnetic pole suggesting a history of emplacement too short to adequately average secular variation

instantaneous record of the geomagnetic field direction of various facies from pyroclastic flow deposits tests for consistency in paleomagnetic directions abstract a comparative study of the paleomagnetism of various lithofacies of pyroclastic flow deposits has been undertaken to examine the accuracy of their remanent magnetization direction at the time of deposition the aso 2 pyroclastic flow deposit which erupted at 141 ka from aso caldera japan was chosen as the site of our investigation because the deposit formed over a short time period and the paleomagnetic directions of the deposit are expected to record substantial contemporaneous volcanism paleomagnetic samples of densely welded lava like rheomorphic and non welded pyroclastic deposits were collected at nine sites from the aso 2 pyroclastic flow deposit remanence directions from densely welded pyroclastic deposits display good within and between site consistency and are considered to accurately record the ambient geomagnetic field direction at the time of emplacement rheomorphic pyroclastic deposits had directions that show good within site consistency and it is considered that this material obtains its remanence direction parallel to the ambient field corresponding to the time of its emplacement remanence directions from non welded pyroclastic deposits show large confidence limits or deviations from their expected directions such deposits would likely be prone to modification of remanence direction introduced from random rotations of remanence carrying material during syn or post depositional stages in conclusion we suggest that among various lithofacies of pyroclastic flow deposits remanence directions observed from non welded pyroclastic deposits may need to be interpreted cautiously

a diatom record of climate and hydrology for the past 200 ka from owens lake california with comparison to other great basin records abstract diatoms from lake sediments beneath owens lake playa inyo county california document a nearly continuous paleolimnological record of climate and hydrologic change since the penultimate glacial interglacial cycle based on a chronology established by radiocarbon tephrochronology and paleomagnetic control freshwater planktic diatoms especially species of stephanodiscus plagioclase feldspar rich sediments with high magnetic susceptibility and juniperus type pollen characterized the penultimate glaciation at owens lake saline diatoms dominated in the following interglacial period and there are several episodes during which freshwater planktic diatoms became abundant between 100 and 50 ka that may represent interstadial climatic conditions saline diatoms fell to low values after 50 ka but warm season aulacoseira species indicate episodes of significant summer precipitation in the hydrologic balance of owens lake prior to the last glacial maximum by 25 ka glacial environments were again characterized by abundant juniperus plagioclase feldspar and stephanodiscus species generally arid holocene climates were recorded in owens lake by short term fluctuations of saline and freshwater diatoms desiccation and oolitic sediments barren of diatoms comparison to paleoclimate records both north and south of owens lake suggest a southerly displacement of storm tracks originating from the aleutian low during glacial episodes

u series chronology of lacustrine deposits in death valley california uranium series dating on a 186 m core dv93 1 drilled from badwater basin in death valley california and on calcareous tufas from nearby strandlines shows that lake manly the lake that periodically flooded death valley during the late pleistocene experienced large fluctuations in depth and chemistry over the last 200 000 yr death valley has been occupied by a long standing deep lake perennial shallow saline lakes and a desiccated salt pan similar to the modern valley floor the average sedimentation rate of about 1 mm yr for core dv93 1 was punctuated by episodes of more rapid accumulation of halite arid conditions similar to the modern conditions prevailed during the entire holocene and between 120 000 and 60 000 yr b p from 35 000 yr b p to the beginning of the holocene a perennial saline lake existed over 70 m at its deepest a much deeper and longer lasting perennial lake manly existed from about 185 000 to 128 000 yr b p with water depths reaching about 175 m if not 330 m this lake had two significant dry excursions of 102 103yr duration about 166 000 and 146 000 yr b p and it began to shrink to the point of halite precipitation between 128 000 and 120 000 yr b p the two perennial lake periods correspond to marine oxygen isotopic stages ois 2 and 6 based on the shoreline tufa ages we do not rule out the possible existence 200 000 yr ago of yet a third perennial lake comparable in size to the ois 6 lake the234u 238u data suggest that u in tufa owes its origin mainly to ca rich springs fed by groundwater that emanated along lake shorelines in southern death valley and that an increase of this spring water input relative to the river water input apparently occurred during ois 6

response of the north american monsoon to regional changes in ocean surface temperature 1 the north american monsoon nam an onshore wind shift occurring between july and september has evolved in character during the holocene largely due to changes in northern hemisphere insolation published paleoproxy and modeling studies suggest that prior to 8000 cal years bp the nam affected a broader region than today extending westward into the mojave desert of california holocene proxy sst records from the gulf of california goc and the adjacent pacific provide constraints for this changing nam climatology prior to 8000 cal years bp lower goc ssts would not have fueled northward surges of tropical moisture up the goc which presently contribute most of the monsoon precipitation to the western nam region during the early holocene the north pacific high was further north and ssts in the california current off baja california were warmer allowing monsoonal moisture flow from the subtropical pacific to take a more direct northwesterly trajectory into an expanded area of the southwestern u s west of 114 w a new upwelling record off southwest baja california reveals that enhanced upwelling in the california current beginning at 7500 cal year bp may have triggered a change in nam climatology focusing the geographic expression of nam in the southwest usa into its modern core region east of 114 w in arizona and new mexico holocene proxy precipitation records from the southwestern u s and northwestern mexico including lakes vegetation pollen and caves are reviewed and found to be largely supportive of this hypothesis of changing holocene nam climatology

secular variation in mexico during the last 40 000 years paleomagnetic data are available for mexico from various archives covering irregularly the last 40 000 years data obtained from lake sediments are affected by ambiguous conversion of age to depth and the paleomagnetic data themselves are not of high quality as samples were only demagnetized at one field intensity these lake sediment data are therefore considered as less reliable for construction of a secular variation curve although they may give some qualitative information for the last 25 000 years archeological samples provide data for the period 100 1200 a d their 14 c ages are some times not well constrained with unrecoverable analytical and statistical procedures and often ages were only inferred from the stratigraphic position and the paleomagnetic record the paleomagnetic record of these materials is of high quality with very low internal dispersion although no magnetic cleaning techniques have been applied variations of inclination and declination with time are hampered by large age uncertainties and thus of limited value for the construction of secular variation curves paleomagnetic data from an u th dated stalagmite provide reasonable directions for most of the last 1200 years being of lower quality in those parts with very low remanence intensity the u th age determinations were interpreted in terms of a constant growth rate which might be only valid as a first order approximation recent volcanic rocks from the trans mexican volcanic belt provide 19 paleomagnetic directions of reasonable to high quality and often also absolute paleointensities ages have been determined from 14 c analysis of charcoal or paleosols found below the lavas and extend over the last 40 000 years therefore volcanic rocks provide the most adequate archive for the construction of a secular variation curve but much more volcanic structures need to be dated to obtain a semi continuous secular variation curve comparison of the data from different archives is possible mainly for the last 2500 years and despite the aforementioned problems similar variations of declination and inclination are observed this holds as well when comparing them to data from holocene volcanic rocks from western north america 2002 elsevier science b v all rights reserved

brittle origins for disseminated gold mineralization in mylonite gaocun gold deposit hetai goldfield guangdong proince south china gold mineralization in the hetai goldfield in south china is almost entirely confined to a series of steeply dipping ductile shear zones generated during hercynian indosinian orogenesis of sinian host rocks approximately half of the mineralization is in the form of disseminations in the mylonite and the remainder was clearly emplaced as a result of brittle deformation in einlets and as sulfide cement to brecciated mesoscopic quartz eins heaily influenced by the presence of disseminated mineralization all publications on hetai determine a major role for ductile deformation in the introduction of gold into the deposit the brittle deformation is typically regarded as redistributing within the host structures the gold initially introduced as a result of ductile deformation mechanisms howeer textures of the sulfides associated with the gold indicate an entirely postductile deformation timing as they display none of the features attributed to ductile deformation during the eident brittle deformation phase competence contrasts at the microscopic scale allowed fracturing to focus on the quartz sericite domain boundaries of the mylonite foliation thus permitting the introduction of auriferous fluid into the mylonite to create disseminated mineralization the control exerted by preexisting structures is further reflected in the restriction of disseminated mineralization to the quartz domains presumably because the sericite domains accommodated the deformation in a more distributed fashion seeral other lines of eidence support an origin of the hetai deposit in an upper crustal regime a similar style of disseminated mineralization to that seen in the mylonite surrounds microeinlet terminations in the brecciated quartz eins as well as oerprinting pseudotachylite which is associated with cataclasite there are also large olumes of unmineralized mylonite indicating no connection between the intensity of strain and gold grade we conclude that in structurally controlled deposits the textural category of disseminated gold mineralization can be generated either under entirely ductile or entirely brittle conditions and that microtextural eidence has to be carefully analyzed to determine the mode of formation of a deposit

structural rotation and volcanic source implications of magnetic data from eocene volcanic rocks sw idaho eocene crystal rich ash flow tuffs in the southwest part of the challis volcanic field in idaho yield a biotite kar isochron data near 45 38 0 19 ma this age determination is in good agreement with a previous result 1 from this locality near poison creek which corrected for modern decay constants is near 44 7 0 8 ma af and thermal demagnetization experiments on 113 oriented cores from sixteen sites in these challis ash flow tuffs provide a tilt corrected mean paleomagnetic direction with i 63 6 d 019 4 95 6 2 n 15 k 39 4 the corresponding paleomagnetic pole 74 9 n 313 3 e a95 8 9 lies clockwise 34 0 9 8 relative to a synthetic 40 ma reference pole for north america 2 the observed inclination 63 6 is close to that expected 61 8 for this locality and age therefore a clockwise tectonic rotation of about 34 is inferred to have affected these rocks in the interval 40 to 15 ma approximately accompanying basin and range regional extension ams analyses of these ash flow tuffs yield a predominant e w trend for the maximum susceptibility axis assuming that this axis reflects the primary flow fabric of the rock and that this direction was also rotated by the amount indicated in the paleomagnetic results a flow source along a trend n55 e s55 w is calculated a source region northeast of the study area in the main region of development of the challis volcanic field is consistent with these results

the northeast nevada volcanic field magnetic properties and source implications 1 magnetic susceptibility and remanence studies on late eocene ash flow deposits of the northeast nevada volcanic field nenvf were undertaken at 39 sites in four sampling localities dolly varden mountains 20 sites nanny creek area of the pequop mountains 11 sites eastern windermere hills 7 sites and southern snake mountains 1 site these deposits span a narrow age range from 41 08 to 39 08 ma based on 40ar 39ar data brooks et al 1995a 1995b and on new k ar ages presented here characteristic remanences reveal that all ignimbrites sampled are reversely magnetized the characteristic directions of remanent magnetization do not significantly deviate from the expected direction computed from the 42 ma mean pole for north america indicating that no major vertical axis rotations have affected the sampling regions since eocene time anisotropy of magnetic susceptibility studies undertaken to decipher flow trends and possible source areas for these ash flow tuff units of the nenvf are consistent with a source region near the toano range this places the ash flow source area east of nanny creek the type area for the nenvf the suspected source region lies close to the wells fault system however tertiary detachment faulting basin and range normal faulting erosion and sedimentation have obscured the suspected source region a more clearly defined zone of eocene igneous activity in the central pilot range to the east cannot be ruled out as a potential source area

chronology of expansion and contraction of four great basin lake systems during the past 35 000 years abstract during the past 35 000 years lake bonneville lake russell and lake searles underwent a major period of lake level change the lakes were at moderate levels or dry at the beginning of the period and seem to have achieved highstands between about 15 000 and 13 500 yr b p the rise of lake lahontan was gradual but not continuous in part because of topographic constraints intrabasin spill lake lahontan also had an oscillation in lake level at 15 500 yr b p radiocarbon age estimations for materials that were deposited in the lake basins indicate that lake bonneville rose more or less gradually from 32 000 yr b p and had major oscillations in level between 23 000 and 21 000 yr b p and between 15 250 and 14 500 yr b p lake russell and lake searles had several major oscillations in lake level between 35 000 and 14 000 yr b p the timing and exact magnitude of the oscillations are difficult to decipher but both lakes may have achieved multiple highstand states all four lakes may have had nearly synchronous recessions between about 14 000 and 13 500 yr b p after the recessions the lakes seem to have temporarily stabilized or experienced a minor increase in size between about 11 500 and 10 000 yr b p these data provide circumstantial evidence that the younger dryas event affected climate on at least a hemispheric scale during the holocene the four lakes remained at low levels and small oscillations in lake level occurred an important aspect of the lake level data is the accompanying expansion of lake surface area at the time of the last highstand lake bonneville and lake lahontan had surface areas about 10 times larger than their mean historical reconstructed areas whereas lake russell and lake searles had surface areas about 5 times larger than their mean historical reconstructed areas differences in the records of effective wetness may have been due to the locations of the basins relative to the position of the jetstream or they may have resulted from lake atmosphere feedback processes

glacial interglacial variability in upwelling and bioproductivity off nw mexico implications for quaternary paleoclimate sedimentary accumulation of biogenic components organic carbon opal and biogenic barium on the northwestern mexican margin declined during every glacial interval of the past 140 kyr indicating decreases in upwelling induced productivity during cold periods the glacial interglacial contrasts in upwelling on this margin are attributed to reversals in land ocean thermal contrast the waxing and waning of the laurentide ice sheet and consequent responses of the western hemisphere wind fields this scenario is consistent with three independent lines of evidence terrestrial paleoclimatic data general circulation model results and our marine records this pattern of glacial interglacial variability in upwelling off nw mexico is opposite to that observed in other low latitude and midlatitude upwelling areas such as the eastern equatorial pacific these results add to a growing pool of observations that the response of oceanic upwelling to glacial climatic forcing has been regionally variable

late quaternary climate change record from two long sediment cores from guaymas basin gulf of california modern guaymas basin gulf of california mexico is a region of high diatom productivity where exceptional preservation factors maintain biannually alternating sediment deposition as annual varves new sediment cores from guaymas basin md02 2512 and md02 2515 present the opportunity to construct climate records from below the last glacial period a low resolution age model has been constructed from oxygen isotope analysis correlation with other dated short piston cores from guaymas basin and an estimate of sedimentation rate md02 2512 from eastern guaymas basin has an age range from the holocene to late marine isotope stage 6 mis 6 md02 2515 from western guaymas basin has an age range from 8000 to 40 000 yr shipboard analyses of colour reflectance magnetic susceptibility and sediment density are combined with continuous x ray fluorescence scans to reconstruct a picture of glacial climate in the gulf of california eastern guaymas basin is affected by glacial sea level fall which results in a drastic change in productivity rates and sediment type the laminated record of mis 5 allows comparison with the holocene showing a similarity of sedimentation patterns during deglaciation and a series of very rapid variations just prior to the last glaciation in western guaymas basin there are a series of younger dryas like events during the glacial typified by low productivity and high terrigenous input long term climate and productivity changes appear to be caused by the southward displacement of the subtropical high pressure zone copyright 2005 john wiley sons ltd

astronomic calibration of the late eocene early oligocene massignano section central italy we present an astronomically tuned polarity timescale for the late eocene early oligocene based on a cyclic limestone marls sedimentary succession exposed at massignano italy gssp for the eocene oligocene boundary here we applied spectral analysis techniques to the high resolution magnetic susceptibility record the results provide clear evidence of a regular and cyclic signal that allows us to discard a stochastic mechanism as controlling these fluctuations conversely these outcomes reveal the presence of stable and notable periodicities that are in tune with those associated with orbital forcing through a cyclostratigraphic analysis of the rhythmic sedimentary alternations and combination with the results of time series analysis of the proxy record we refine the late eocene portion of the gpts through astronomical tuning of the rhythmic sedimentary alternations to recent astronomically calculated variations of earth s orbit we recalibrated the polarity boundaries ages and the main stratigraphic events recognized throughout the sedimentary record among these an age of 33 714 ma is proposed for the e o boundary

style and age of late oligocene early miocene deformation in the southern stillwater range west central nevada paleomagnetism geochronology and field relations paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late oligocene to early miocene episode of large magnitude extension in the southern stillwater range and adjacent regions of west central nevada the southern stillwater range was the site of an oligocene to early miocene volcanic center comprising 1 28 3 to 24 3 ma intracaldera ash flow luffs lava flows and subjacent plutons associated with three calderas 2 24 8 to 20 7 ma postcaldera silicic dikes and domes and 3 unconformably overlying 15 3 to 13 ma dacite to basalt lava flows plugs and dikes the caldera related luffs lava flows and plutons were tilted 60 70 either west or east during the initial period of cenozoic deformation that accommodated over 100 extension directions of remanent magnetization obtained from these extrusive and intrusive caldera related rocks are strongly deflected from an expected miocene direction in senses appropriate for their tilt a mean direction for these rocks after tilt correction however suggests that they were also affected by a moderate 33 4 11 8 component of counterclockwise vertical axis rotation paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24 8 to 20 7 ma postcaldera dikes and domes in detail an apparent decrease in rotation with decreasing age of individual isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24 4 and 24 2 ma the onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked deformation was not driven by magma buoyancy however because tilting equally affected the caldera systems of different ages including their plutonic roots it is more likely that regional extension was focused in the southern stillwater range due to magmatic warming and reduction of tensile strength of the brittle crust faults that accommodated deformation in the southern stillwater range initially dipped steeply and cut deeply to expose more than 9 km of crustal section the exposed crustal sections are probably rotated blocks above an unexposed basal detachment that lay near the early miocene brittle ductile transition

san agustin plains new mexico age and paleoenvironmental potential reassessed paleomagnetic analysis of samples from the original 600 m long core from the san agustin plains new mexico showed an unquestionable reversal stratigraphy dating the record back to at least 1 6 my analysis of pollen ostracodes and algae of a duplicate sample section in the vicinity of the original coring site and spanning the last 18 000 yr b p shows a much higher resolution than the earlier results

secular variation in numerical geodynamo models with lateral variations of boundary heat flow we study magnetic field variations in numerical models of the geodynamo with convection driven by nonuniform heat flow imposed at the outer boundary we concentrate on cases with a boundary heat flow pattern derived from seismic anomalies in the lower mantle at a rayleigh number of about 100 times critical with respect to the onset of convection the magnetic field is dominated by the axial dipole component and has a similar spectral distribution as earth s historical magnetic field on the core mantle boundary cmb the time scales of variation of the low order gauss coefficients in the model agree within a factor of two with observed values we have determined the averaging time interval needed to delineate deviations from the axial dipole field caused by the boundary heterogeneity an average over 2000 years the archeomagnetic time scale is barely sufficient to reveal the long term nondipole field the model shows reduced scatter in virtual geomagnetic pole positions vgps in the central pacific consistent with the weak secular variation observed in the historical field longitudinal drift of magnetic field structures is episodic and differs between regions westward magnetic drift is most pronounced beneath the atlantic in our model although frozen flux advection by the large scale flow is generally insufficient to explain the magnetic drift rates there are some exceptions in particular equatorial flux spot pairs produced by expulsion of toroidal magnetic field are rapidly advected westward in localized equatorial jets which we interpret as thermal winds

a record of holocene summer season palaeohydrological changes from the southern margin of chihuahua desert mexico and possible forcings proxy records of runoff lake water salinity and aeolian activity reconstructed by analyzing the concentrations of ti ca and zr ti in the sediments of palaeolake las cruces provide information a

time variations of the earth s magnetic field from daily to secular variations in time of the earth s natural magnetic field have a frightfully wide spectrum ranging over more than 20 orders of magnitude they extend from well over 103 hz where they merge into the radioelectric noise spectrum to probably more than 100 m y in rough summary the higher frequencies stem from external sources in the ionosphere and above into the magnetosphere whereas the longer periods arise from internal sources primarily fluid motions in the core at depths in excess of 2900 km methods for separating external and internal signals have been available since the time of gauss 1 838 but the accuracy the space distribution and the time duration of available data are often not sufficient to unambiguously solve the problem for all types of variations although periods shorter than 1 yr should clearly be assigned to external sources the extent to which the spectra of the external and internal parts overlap is the subject of recent debate and progress currie 1966 1 968 proposed that there was little overlap and a cutoff of 3 7 yr a long debated l l yr contribution due to the solar cycle was clearly demonstrated by currie 1973a courtillot le mouei 1976a b and alldredge 1976 and the cutoff period was thus rejected to at least 20 yr the situation seemed to be confused again when courtillot et al 1978a provided evidence for sharp impulses of 1 yr duration i n the internal so called secular variation the debate that followed over the reality of brief internal events is one of the topics addressed in this review we first provide a short historical account of successive discoveries

paleoecological studies at lake patzcuaro on the west central mexican plateau and at chalco in the basin of mexico abstract a 1520 cm sediment core from lake patzcuaro michoacan mexico is 44 000 yr old at the base all parts of the core have abundant pollen of pinus pine alnus alder and quercus oak with frequent abies fir the interval dated from 44 000 to 11 000 yr ago has a homogeneous flora characterized by abundant juniperus juniper pollen and frequent artemisia sagebrush it is believed to represent an appreciably drier and colder climate than at present the holocene at lake patzcuaro is characterized by a moderate increase in pinus pollen and the loss of juniperus pollen as the modern type of climate succeeded alnus was abundant until about 5000 yr ago its abrupt decrease with the first appearance of herbaceous weed pollen may reflect the cutting of lake shore and stream course alder communities for agricultural purposes or it may simply reflect a drying tendency in the climate pollen of zea corn appears at lake patzcuaro along with low peaks of chenopod and grass pollen at 3500 yr b p apparently recording a human population large enough to modify the natural environment as well as the beginning of agriculture a rich aquatic flora in this phase suggests eutrophication of the lake by slope erosion in the most recent period corn is absent from the sediments perhaps reflecting a change in agricultural practices the environment changes at lake patzcuaro are similar to and correlate with those in the cuenca de mexico where diatom stratigraphy from the chalco basin indicates fluctuations in lake levels and lake chemistry in response to variations in available moisture before 10 000 yr ago climates there were cool and dry and the chalco basin was occupied by a shallow freshwater marsh that drained north to lake texcoco where saline water accumulated by evaporation increases in effective moisture and possible melting of glaciers during the holocene caused lake levels to rise throughout the cuenca de mexico and lake texcoco flooded the chalco basin with brackish water after 5000 yr ago such flooding decreased and shallow freshwater ponds and marshes were restored in the chalco basin this environmental change coincides with the appearance of zea pollen and suggests cultural control of lake levels and salinity

archean magmatic granulites diapirism and proterozoic reworking in the northern marginal zone of the limpopo belt the northern marginal zone nmz of the limpopo belt southern africa is a high grade gneiss belt dominated by magmatic granulites of the charnoenderbite suite which intruded minor mafic ultramafic and metasedimentary rocks between 2 74 and 2 57 ga the intrusive rocks have crustal and mantle components and occur as elliptical bodies interpreted as diapirs peak metamorphism p 800 mpa t 800 850 c occurred at ca 2 59 ga the highly radiogenic nature of the rocks in the nmz supplemented by heat from mantle melts led to heating and diapirism culminating in the intrusion of distinctive porphyritic charnockites and granites horizontal shortening and steep extrusion of the nmz during which crustal thickening was limited by high geothermal gradients contrast with overthickening and gravitational collapse observed particularly in more recent orogens the granulites were exhumed by the end of the archean the pervasive late archean shortening over the whole of the nmz contrasts with limited deformation on the zimbabwe craton possibly owing to the strengthening effect of early crust in the craton in the southeast of the nmz strike slip kinematic indicators occur within the transition zone and the triangle shear zone where dextral shearing reworked the archean crust at ca 1 97 ga

chronology and stratigraphy of late quaternary sediments in the konya basin turkey results from the kopal project abstract the late quaternary environmental history of the konya plain in south central turkey is used to examine sediment facies changes in a shallow non outlet basin which has experienced major climatically driven changes in lake extent two principal types of sedimentary archive are used to reconstruct a palaeoenvironmental record namely alluvial sequences on the carsamba alluvial fan and sediments from residual lakes the latter have been used to investigate broader climatic and vegetational histories via palaeolimnological techniques including pollen diatom and stable isotope analysis these changes are dated here by radiometric techniques including radiocarbon ams and conventional osl and u th chronological agreement is generally good between the different dating techniques although typically there is greatly reduced precision beyond ca 25 ka lake sediment cores investigated have basal ages beyond the range of 14c dating but contain hiatuses as a result of subsequent alternation between phases of lacustrine sedimentation and aeolian deflation in contrast to most deepwater non outlet lake systems the konya basin may have been occupied by a single extensive lake for as little as 10 of late quaternary time mainly around the time of the lgm this lake highstand was followed by an important arid interval in the absence of unbroken chronostratigraphic sequences palaeohydrological investigation of shallow non outlet lakes may require analysis of basin wide changes in sedimentation rather than reliance on single core records stratigraphic continuity in such sedimentary environments cannot be assumed and requires independent chronological control through radiometric dating

estimates of flow direction for calc alkaline welded tuffs and paleomagnetic data reliability from anisotropy of magnetic susceptibility measurements central san juan mountains southwest colorado abstract flow directions are estimated from the measurement of the magnetic fabric of 106 samples collected at 18 sites in four welded tuff units in the central san juan mountains of southern colorado the estimates assume that the tuffs generally flowed directly away from the extrusive vents and that the lineations of magnetic grains within the tuffs represent the flow direction at individual sites errors in the estimation may arise from topographic variation rheomorphism post emplacement mass flow within the tuff and other factors magnetic lineation is defined as the site mean anisotropy of magnetic susceptibility maximum azimuth a test on the flow directions for individual units is based on the projection of lineation azimuths and their intersection within or near the known source caldera for the tuff this test is positive for the four units examined paleomagnetic results for these tuffs are probably reliable indicators of the geomagnetic field direction in southwest colorado during the time 28 2 26 5 ma of emplacement

directly dated mis 3 lake level record from lake manix mojave desert california usa abstract an outcrop based lake level curve constrained by 70 calibrated 14 c ages on anodonta shells indicates at least 8 highstands between 45 and 25 cal ka bp within 10 m of the 543 m upper threshold of lake manix in the mojave desert of southern california correlations of manix highstands with ice marine and speleothem records suggest that at least the youngest three highstands coincide with dansgaard oeschger d o stadials and heinrich events 3 and 4 the lake level record is consistent with results from speleothem studies in the southwest that indicate cool wet conditions during d o stadials notably highstands between 43 and 25 ka apparently occurred at times of generally low levels of pluvial lakes farther north as interpreted from core based proxies mojave lakes may have been supported by tropical moisture sources during oxygen isotope stage 3 perhaps controlled by southerly deflection of pacific storm tracks due to weakening of the sea surface temperature gradient in response to north atlantic climate perturbations

secular variation of the late archean early proterozoic geodynamo 1 we report new paleomagnetic data from 2 5 ga dikes of the superior province and derive estimates of paleosecular variation when combined with data from coeval dikes of the karelia craton russia and older 2 7 ga volcanics of the pilbara craton these estimates suggest a latitudinal variation of secular variation similar to that of the last 5 million years but with potentially smaller quadrupole family contributions the observations are consistent with some numerical models of the field that suggest a more dipolar field when the earth had a much smaller solid inner core

large holocene lakes and climate change in the chihuahuan desert lake level variations preserved as beach ridges in the laguna el fresnal and laguna santa maria subbasins northern mexico record millennially spaced episodes of increased precipitation during the holocene epoch we find that the early middle and late holocene were punctuated by periods wet enough to establish large pluvial lakes in currently dry basins in the chihuahuan desert the largest dated pluvial lake covered 5650 km2 during the early holocene constructional beach ridges in these subbasins are 221 33 14c yr b p little ice age equivalent 3815 52 to 4251 59 14c yr b p early neoglacial 6110 80 to 6721 68 14c yr b p mid holocene and 8269 64 to 8456 97 14c yr b p early holocene dates that correlate with other millennially spaced wet or cold events in the northern hemisphere we attribute these wet episodes to increased precipitation cooler temperatures and reduced evaporation following southward shifts in winter storm tracks which are related to long term el nino southern oscillation variability during the holocene

egan range volcanic complex nevada geochronology paleomagnetism and magnetic fabrics abstract in a relatively brief interval near the eocene oligocene boundary trachydacite lavas were erupted over an area of about 300 km2 to form the egan range volcanic complex in the southern butte mountains and adjacent egan range of east central nevada the mean radiometric age for this volcanic episode is 34 1 0 2 ma based on whole rock and biotite k ar ages detailed comparisons were made between anisotropy of magnetic susceptibility ams and the macroscopic petrofabric at 31 sites great variation in flow structures is attributed to complex flow in viscous lava over a surface of considerable relief the magnetic fabric as expressed by ams axes agrees well with the flow petrofabric typically flow lineations are parallel to k1 and flow foliation is perpendicular to k3 as expected extension cracks and joints are normal to k1 late stage deformation of pipe vesicles also impressed a measurable magnetic fabric because of significant initial dip and complexity of flow fabric paleohorizontal is not generally demonstrable paleomagnetic studies indicate an exclusively normal polarity the mean in situ characteristic direction is relatively well grouped d 350 i 52 95 8 for 28 selected sites and provides a mean pole at 124 e 81 n which is not significantly different from the coeval cratonic reference pole these results indicate that large oligocene or younger rotations have not affected this sector of the basin and range province the combination of age and magnetic polarity suggests a correlation with a normal polarity subchron of chron 13 or 15

a 164 ka record of environmental change in the american southwest from a carlsbad cavern speleothem abstract a horizontal core 2 8 m in length drilled from the georgia giant column in carlsbad cavern provides climate information for the last 164 ka forty six alpha spectrometric u series ages determined at intervals of 7 6 cm along the core indicate five periods of deposition and five hiatuses the longest from 136 to 110 ka variations in growth rate 0 to 70 mm ka in the abundance of aragonite chalcedony and fe bearing phases and in 13c indicate that glacial intervals of the last 164 ka ois 6 4 and 2 were much wetter than today as were the colder substages 5d and 5b of ois 5 by contrast during the two warmest periods of the past 164 ka namely ois 5e and 1 there was no deposition on either side of the speleothem suggesting conditions as dry or drier than today the record from carlsbad parallels data from many other sites in the southwestern usa and northern mexico and data from marine sediments and ice cores demonstrating the extent to which ice sheet fluctuations influenced conditions in southern new mexico detailed correlation of 13c values in the georgia giant which range from 6 6 to 0 9 relative to pdb with distant speleothem records and with data from ice cores further documents the linkage of southwestern climate with global scale extent of glaciation values of 18o in the georgia giant core range from 9 7 to 4 7 relative to pdb and average 6 6 18o depleted carbonate in the georgia giant during ois 6 at a time when the world s oceans were enriched in 18o suggests that precipitation during cold intervals was brought largely by pacific air masses in fall winter and spring as a result of the southward displacement of the polar jet stream by the growth of the laurentide ice sheet termination ii marking the end of the penultimate glaciation is well defined in the core s 18o data analytical uncertainties in the radiometric ages do not preclude a start of termination ii as late as 128 ka as suggested by specmap data however data from the georgia giant core are more compatible with an earlier start like that obtained from devils hole and vostok data raising the possibility that early warming was widespread in the u s southwest region by 145 ka

paleomagnetic evidence for the age and extent of middle tertiary counterclockwise rotation dixie valley region west central nevada paleomagnetic data obtained from oligocene to lower miocene igneous rocks and middle miocene basaltic rocks at fifteen localities from a region surrounding dixie valley in west central nevada indicate that parts of the area experienced counterclockwise vertical axis rotation and these data provide constraints on the extent and timing of rotation counterclockwise vertical axis rotation probably exceeding 30 is indicated for oligocene to lower miocene rocks in the central part of the study area calculated rotations increase with increasing age of the oligocene to lower miocene rocks indicating that vertical axis rotation was concurrent with ash flow deposition about 23 33 ma paleomagnetic data indicate that middle miocene basaltic rocks about 10 17 ma postdate significant counterclockwise vertical axis rotation although this interpretation is complicated because the rocks were erupted episodically in pulses that apparently were short lived with respect to geomagnetic secular variation counterclockwise rotation was related to a deformation event that predated development of the present basin and range physiography of the area the total amount of crustal rotation in tertiary time in the area is poorly known rotation estimates calculated from time averaged mean directions that incorporate data from a broad range of the oligocene to lower miocene units probably underestimate total tertiary rotation because the mean directions include data from units that postdate much of the rotation for example a rotation estimate for a composite sequence from the central part of the study area is 23 15 whereas estimates from older and younger halves of the sequence are 37 21 and 11 16 respectively paleomagnetic data indicate that oligocene to lower miocene rocks at some localities in the northern and southern parts of the study area e g the golconda canyon locality probably did not experience significant tertiary counterclockwise rotation although the paleomagnetic data alone are insufficient to fully define the geographic area subjected to tertiary counterclockwise rotation the data suggest that a discrete subregion was affected rather than the entire basin and range province