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

correction to the 40ar 39ar ages and tectonic setting of the middle eocene northeast nevada volcanic field by william e brooks charles h thorman and lawrence w snee

distribution of reynolds stress carried by mesoscale variability in the antarctic circumpolar current satellite altimeter data collected by the u s navy s geosat exact repeat mission were used to investigate turbulent stress resulting from the variability of surface geostrophic currents in the antarctic circumpolar current the altimeter measured sea level along the subsatellite track the variability of the along track slope of sea level is directly proportional to the variability of surface geostrophic currents in the cross track direction we used the two components of the slope at points where the satellite ground tracks cross to calculate orthogonal components of the current s variability because the grid of crossover points is dense at high latitudes the satellite data could be used for mapping the temporal and spatial variability of the current two and a half years of data were used to compute the statistical structure of the variability leading to a better understanding of the role of variability in the dynamics of the current the statistics included the probability distribution functions for each component of the current the time lagged auto correlation functions of the variability and the reynolds stress produced by the variability the results demonstrate that stress is correlated with bathymetry in some areas the distribution of negative stress indicate that eddies contribute to an acceleration of the mean flow strengthening the hypothesis that baroclinic instability makes important contributions to strong oceanic currents

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

the 40ar 39ar ages and tectonic setting of the middle eocene northeast nevada volcanic field widespread middle to late eocene calc alkalic volcanism which formed the northeast nevada volcanic field marks the earliest tertiary volcanism in the northern basin and range the central part of this major field in northeast nevada and adjacent utah is herein defined by 23 40ar 39ar ages that range from 42 6 to 39 0 ma rock chemistry from 12 localities stratigraphic position of the volcanic rocks above a regional middle eocene unconformity volcanic setting and lithology the type area is at nanny creek in the northern pequop mountains nevada where rhyolite ash flow tuffs are overlain by a thick section of intercalated andesitic to dacitic flows and flow breccias and rhyolite ash flow tuffs the intermediate composition rocks are locally derived throughout the volcanic field whereas the sources for rhyolite ash flow tuffs are unknown the uniform and widespread occurrence of the andesitic and dacitic flows and flow breccias strongly suggests that the upper crust was perforated by intermediate composition magma across the entire region in the central part of the field the middle eocene volcanic rocks rest with depositional angular discordance on deformed middle paleozoic to triassic strata ostracode bearing limestone probably of early eocene age is locally present below the volcanic rocks in the western and southeastern parts of the field these middle eocene volcanic rocks rest with depositional angular discordance on lower eocene lacustrine strata of the elko and white sage formations respectively this angular discordance documents a middle eocene deformational event previously unrecognized in the region

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

synfolding magnetization in the jurassic preuss sandstone wyoming idaho utah thrust belt the jurassic preuss sandstone exposed in five thrust plates of the wyoming idaho utah thrust belt carries directions of remanent magnetization that group most tightly after only partial unfolding field petrographic and rock magnetic evidence indicates that the carrier of this magnetization is detrital low ti titanomagnetite the detrital titanomagnetite was remagnetized at low temperatures 75 150 c probably completely during folding anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain we suggest that low temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization the acquisition of which was enhanced by stress or a piezoremanent magnetization that involved stress induced movement of domain walls during intracrystalline strain or was a combination of the two mechanisms stress may promote remagnetization at temperatures much lower than predicted by current theoretical models other mechanisms such as acquisition of chemical remanent magnetization during folding deflection of a prefolding magnetization by internal strain or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the preuss the locus of acquisition of synfolding magnetization in the preuss migrated in conjunction with deformation in the thrust belt a model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps a lack of reverse polarity directions remains a puzzling feature of the remanence the remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in late cretaceous and early tertiary periods of predominantly normal polarity

the slot technique for rock magnetic sampling abstract we have developed a new technique for paleomagnetic sampling that allows one to sample extremely friable poorly to moderately lithified sedimentary and volcanic materials the method uses a portable electric drill and a dual blade masonry wheel or two sharp steel cutting blades with a 2 cm separation which cuts two parallel slots in the material being sampled a second set of cuts at right angles to the first produces a fixed cubic pedestal around which a plastic sample box can be inserted oriented and extracted the method has allowed us to take samples at localities where we were not able to sample using other methods to test the method we have sampled the bandelier tuff in northeast new mexico and have measured the anisotropy of magnetic susceptibility ams on boxes taken using the new method and on cylinders drilled at the same sites using standard techniques results show better ams within site precision for box samples taken using the new method we conclude that the method is convenient reliable and may be preferred in certain types of paleomagnetic studies

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

identifying ash flow sources with directional data an application to the kizilkaya ignimbrite central anatolia clast shape preferred orientation or anisotropy of magnetic susceptibility ams data are increasingly used to constrain the source location of ash flow tuffs little attention has been paid to processing of these directional data for the accurate and objective identification of the sources this topic is the focus of this paper as applied to locating the source of the 5 ma kizilkaya ignimbrite central turkey first geological criteria allowing a rough identification of source vent location are discussed second ams data are processed to more accurately locate the source of this widespread deposit hillhouse and wells 1991 present a method which consists of determining the areal overlapping density function of the ams error fans defined by the kmax confidence ellipse applications to the kizilkaya ignimbrite show various possible source positions however a unique anomaly occurs in every model suggesting a source in the derinkuyu basin another method is proposed here and is named the average radial angular difference arad function it consists of calculating the average angular difference of lines that join a given source point to the sampling sites with the lineations corresponding to the ams data interpreted in terms of kinematic axes application with the data obtained at the 44 ams sampling sites shows that the function decreases to 35 in the derinkuyu basin if the analysis is narrowed to the presumed 12 most proximal sites the function provides values of 20 in the central part of the derinkuyu basin the identified source area occurs near the source zone of older miocene ignimbrites and coincides with a gravity anomaly and with a basaltic shadow zone

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

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

the mobility of radium 226 and trace metals in pre oxidized subaqueous uranium mill tailings the exchange of 226ra and trace metals across the tailings water interface and the mechanisms governing their mobility were assessed via sub centimetre resolution profiling of dissolved constituents across the tailings water interface in cell 14 of the quirke waste management area at rio algom s quirke mine near elliot lake ontario canada shallow zones 2 m water depth fosters stagnant bottom waters and permits the development of anoxia above the benthic boundary these anoxic tailings are characterized by substantial remobilization of 226ra resulting in a relatively large flux of 226ra from the tailings to the water column the strong correlation between the porewater profiles of 226ra and ba r2 0 99 as well as solubility calculations indicate that the mobility of ra is controlled by saturation with respect to a poorly ordered and or impure barite phase ra ba so4 in the anoxic zones severe undersaturation with respect to barite is sustained by microbial so4 reduction flux calculations suggest that the increase in 226ra activity in the water cover since 1995 from 0 5 to 2 5 bq l 1 can be attributed to an increase in the spatial distribution of anoxic bottom waters caused by increased density of benthic flora the anoxic vegetated areas also exhibit minor remobilization with respect to dissolved as ni and zn the removal of trace metals in the anoxic bottom waters appears to be limited by the availability of free sulphide collectively the data demonstrate that while the water cover over the u mill tailings minimizes sulphide oxidation and metal mobility anoxic conditions which have developed in deeper areas have led to increased mobility of 226ra

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

the 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

crossover plots a useful method for plotting sirm data in paleomagnetism the effective grain size of magnetic minerals and therefore domain type controls the saturation isothermal remanent magnetization acquisition and subsequent demagnetization response and can be used to create crossover plots for paleomagnetic specimens where sirm is plotted as a function of the field applied using a logarithmic scale this paper presents templates of such plots for magnetite pyrrhotite hematite and goethite onto which sample data can be plotted for conventional paleomagnetic specimens to characterize their magnetic mineralogy and effective grain size

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

single domain particles in rocks and magnetic fabric analysis magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes we have recently demonstrated however in laboratory produced samples that as theoretically expected susceptibility anisotropy is dependent on particle size multidomain particles of magnetite or similar strongly magnetic particles with shape anisotropy exhibit a maximum susceptibility parallel to their easy remanent magnetization axes whereas uniaxial single domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state by demonstrating uniaxial single domain anisotropy characteristics for the first time in a rock specimen we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro or ferrimagnetic particles in rocks in particular apparently foliated rocks can actually be lineated and vice versa moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment

chronology of polyphase extension in the windermere hills northeast nevada fission track and 40 ar 39 ar dating and chemical correlation of volcanic strata exposed in the windermere hills and northern pequop mountains northeast nevada indicate a protracted polyphase history of tertiary late eocene late miocene extension along the northern margin of a major cordilleran metamorphic core complex early extension is recorded by a west tilted half graben filled with early oligocene 34 79 0 18 39 18 0 12 ma sedimentary rocks in the eastern windermere hills above the lowangle black mountain detachment fault the early oligocene half graben conformably overlies a widespread suite of late eocene 39 18 0 12 40 38 0 06 ma calc alkaline volcanic rocks reflecting a temporal link between early extension at a high structural level and the end of the ignimbrite flare up these strata are cut by east west striking normal faults which are exposed along and parallel to the northern margin of the metamorphic complex available age data e g between 14 93 0 08 and 34 79 0 18 ma permit the interpretation that the east west striking faults formed at the same time as or after large magnitude unroofing of highgrade rocks we interpret the east west striking faults to accommodate differential uplift of greenschist grade metamorphic rocks in the upper crust above a lateral ramp in a west northwest directed mylonitic shear zone subsequent extension in the windermere hills is defined by deep rapidly filled half grabens of middle miocene 7 42 2 0 to 14 93 0 08 ma age that unconformably overlie older faults and synextensional deposits these are the youngest half grabens in the region and are inferred to be initiated by extensional stresses imparted to the base of the lithosphere by a laterally spreading mantle plume e g the yellowstone hotspot located in southeastern oregon at this time