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

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

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

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

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

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

forecasting exposure to volcanic ash based on ash dispersion modeling a technique has been developed that uses puff a volcanic ash transport and dispersion vatd model to forecast the relative exposure of aircraft and ground facilities to ash from a volcanic eruption vatd models couple numerical weather prediction nwp data with physical descriptions of the initial eruptive plume atmospheric dispersion and settling of ash particles three distinct examples of variations on the technique are given using era 40 archived reanalysis nwp data the feb 2000 nasa dc 8 event involving an eruption of hekla volcano iceland is first used for analyzing a single flight results corroborate previous analyses that conclude the aircraft did encounter a diffuse cloud of volcanic origin and indicate exposure within a factor of 10 compared to measurements made on the flight the sensitivity of the technique to dispersion physics is demonstrated the feb 2001 eruption of mt cleveland alaska is used as a second example to demonstrate how this technique can be utilized to quickly assess the potential exposure of a multitude of aircraft during and soon after an event using flight tracking data from over 40 000 routes over three days several flights that may have encountered low concentrations of ash were identified and the exposure calculated relative changes in the quantity of exposure when the eruption duration is varied are discussed and no clear trend is evident as the exposure increased for some flights and decreased for others a third application of this technique is demonstrated by forecasting the near surface airborne concentrations of ash that the cities of yakima washington boise idaho and kelowna british columbia might have experienced from an eruption of mt st helens anytime during the year 2000 results indicate that proximity to the source does not accurately determine the potential hazard although an eruption did not occur during this time the results serve as a demonstration of how existing cities or potential locations of research facilities or military bases can be assessed for susceptibility to hazardous and unhealthy concentrations of ash and other volcanic gases

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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