GP11D-0847 0800h
Study of Nearly Inviscid Flow in a Rotating and Precessing Spheroid
A numerical code for solving the time-dependent incompressible MHD equations with finite differences on overlapping grids in a spheroid is being developed. In our code, the momentum equation for the velocity and the induction equation for the magnetic field are solved together with the Poisson equation for the pressure. The velocity and the magnetic field are advanced explicitly in time using a Runge-Kutta scheme. The grids are chosen to overcome pole and origin problems, which limit the time-step size, and to enhance spatial resolution at places where high resolution is required such as at boundary layers. A new approach is used in evaluating the numerical fluxes for both the momentum and the induction equations. The method is based on modern shock capturing schemes for compressible fluids, such as ENO (Essentially Non-Oscillatory), WENO (Weighted-ENO), and Central schemes. The key idea of these schemes is to approximate the fluxes to high order and to avoid the creation of spurious oscillations in the solution. The new incompressible code conserves both momentum and the magnetic fluxes. It is found to be numerically stable without the need for explicit dissipation terms. This is significant when seeking flows in systems such as the Earth's core where the Ekman number and magnetic Prandtl number are very small. We are using this code to study the fluid motion in a rotating and precessing spheroid, a significant problem in astrophysics and geophysics. In the non-magnetic case, the numerical solutions for both viscous and pressure couplings are consistent with those of Poincar\'e (1910) and Stewartson and Roberts (1963). Including the magnetic field, we investigate dynamo action driven by the precession. We shall report the numerical method, the boundary conditions, and the computational results in the studies mentioned above.
GP11D-0848 0800h
Scale-Invariance in Geomagnetic Secular Variation
The historical geomagnetic field model {\it gufm1} (A.D. 1590-1990, spherical-harmonic expansion up to degree 14 inclusive) is based upon the largest-ever compilation of historical geomagnetic data, and captures geomagnetic change on time scales from years to centuries at the top of the outer core. This surface has been subdivided into grid cells for which the model provided (dipole-detrended) time series of first differences for each of the three orthogonal vector components. Analysis of these fluctuations in terms of various probability distribution functions suggests that power laws can adequately represent the observed, absolute, size-ranked changes over time over a range of (size) scales. Moreover, novel techniques from Complexity Theory allow the estimation of the Hurst parameter (per cell per component), which quantifies the intensity of systemic "memory." Subsequent cluster analysis of cells for a range of significant Hurst threshold values has additionally brought to light spatial power laws in cluster surface areas when ranked by size. Finally, spectral analysis of all power law residuals was used to investigate discrete scale-invariance, i.e., the possible existence of preferred physical length scales in the geodynamo process. Positive evidence in the historical model seems to correspond with similar traits found in the geomagnetic reversal record (present to Cretaceous superchron inclusive). The combined profile of spatial and temporal power laws, intense systemic memory, and characteristic length scales appears to suggest that the geodynamo may be operating as an out-of-equilibrium system just below the threshold of chaos (i.e., in a state of self-organised criticality). Reversals and historical secular variation could thereby be considered expressions of the same underlying process.
GP11D-0849 0800h
Dynamic Sub-grid Scale model for MHD simulations in a rotating plane layer
The Sub-grid scale modeling is required for the geodynamo simulations because the fluid motion and the magnetic field in the Earth's outer core have small scale components which cannot be resolved in numerical simulations because of the small Ekman number and large Rayleigh number. We have previously modeled the influence of sub-grid scale motion for the momentum and heat flux, the Lorentz force, and the induction term using the nonlinear gradient model by Leonard (1974), which is a form of the scale similarity model. The result suggests that the nonlinear gradient model can represent basic characteristics of the effects of the sub-grid scale motion, but some discrepancies are observed around the boundaries. In the present study, we implement a dynamic version of the nonlinear gradient model, where the model coefficients are allowed to evolve according to the procedure introduced by Germano (1992). We also allow the model coefficients to vary as a function of the vertical position to investigate effects of the boundary on the SGS terms. In the present study, we evaluate the model coefficients using a snapshot of a MHD simulation in a rotating plane layer model with the periodic boundary in the horizontal direction. The dynamic model predicts large variations in the model coefficients around the boundaries, while the coefficients in the interior are relatively constant. This result suggests that the amplitude the SGS terms are changed by the presence of boundaries. Furthermore, the coefficient for the SGS induction term is smaller than the other coefficients. This result suggests that the dynamic SGS induction term is smaller, which is consistent with the previous simulations because the magnetic energy was over-damped by the SGS induction term when the model coefficients were all set to 1. References: Leonard, A., Energy cascade in large-eddy simulations of turbulent fluid flows, Advances in geophysics, 18, 237--248, 1974. Germano, M., Turbulence: the filtering approach, Journal of Fluid Mechanics, 238, 325--336, 1992.
GP11D-0850 0800h
Thermal convection and magnetic field generation in a rotating spherical shell at high Rayleigh number
We have carried out 3D numerical simulations of MHD dynamo in a rotating spherical shell to investigate behavior of the convection structure and the generated magnetic field. Here, we focus on Rayleigh number ($Ra$) dependence of numerical dynamos to make extrapolation to the geodynamo regime, and thus, performed parameter survey varying $Ra$ up to about 53 times the critical value, $Ra_c$. To make wider parameter survey, we use moderate value of the Ekman number ($10^{-4}$). Using the other fixed parameters, the Prandtl number $Pr=1$, the magnetic Prandtl number $Pm=2$, and radius ratio $r_i/r_o=0.35$, dynamos undergo a series of bifurcations with increasing $Ra$ from well known dipolar dynamos without convection inside the tangent cylinder (TC), DP1, to the ones with active convection inside TC generating the toroidal magnetic field through the $\omega$-effect, DP2, non-dipolar dynamos, NDP, and then TC-dominating dynamos, TCD. Between DP2 and NDP transition, the magnetic energy shows a dramatic reduction, and then increases again, whereas the kinetic energy monotonically increases. TCD dynamos are also found in Glatzmaier and Roberts dynamo model using hyperdiffusivity. In this regime, advection term in the momentum equation plays a role in momentum balance, while other dynamos at lower $Ra$ are essentially in geostrophic state. The Lorentz force counteracts the advection term to inhibit disturbance in convection structure due to vortex stretching. It seems that viscous effects still play a role in momentum balance. Using the results obtained in this study, it is suggested that the magnetic Reynolds number similar to the Earth's core would be achieved at $Ra\sim 80Rac$.
GP11D-0851 0800h
Correlated Changes in the Length of a Day and the Magnetic Field with a Period of 60 Years
The length of the day changes on many scales. Long-term changes occur as the surface of the Earth rebounds from glaciation. Very short-term variations occur in response to changes in the outer fluid envelopes of the Earth, the oceans and atmosphere. In between there appears to be at least one significant quasi-periodic variation with a 60-year period. A similar 60-year period is present in the Earth's magnetic field, leading to the speculation that the length of day variation with a 60-year period is due to periodic motion of the core of the Earth. We examine the variation of the length of the day, using Hilbert-Huang transformation to deduce intrinsic mode function and to separate out the behavior of the length of the day at periods near 60 years. We repeat for terrestrial magnetic field records and see similar behavior. Finally, we deduce the relative phase of the two variations at the 60-year period.
GP11D-0852 0800h
Azimuthal flows in the Earth's core and changes in length of day at millennial timescales
In this work, we attempt to constrain a part of the dynamics in the Earth's fluid core at millennial timescales by using observed variations in the Earth's rotation and geomagnetic field over the last three Millennia. Variations in length of day reconstructed from ancient records of eclipses contain an oscillating component with a periodicity of roughly 1500 years. A part of the time-dependent variations observed in archaeomagnetic field models consist of episodic eastward and westward drifts with a similar periodicity. Using standard inversion techniques, we obtain time-dependent azimuthal flows at the surface of the core that can explain this part of the archaeomagnetic field variations. We show that these flows are consistent with the changes in core angular momentum required to produce the observed length of day variations provided there exists a shear in velocity in the axial direction inside the core. We interpret these time-dependent azimuthal flows as fluctuations in thermal and magnetic winds and discuss the implications for the geodynamo and core-mantle coupling at millennial timescales.
GP11D-0853 0800h
The relationship between the difference of daily variation in geomagnetic total intensity and seismicity in Taiwan
The relationship between the difference of daily variation in geomagnetic total intensity and earthquakes were found. The Lunping (25.0\deg N, 121.2\deg E) station located in a seismically quiet area in Taiwan was used to be a reference. The ratios were calculated by other six observation stations data dividing with reference data. It was found the daily variation record 40 km near by observatories could be changed about 100 days before and after an earthquake occurrence. The statistics method was used to remove artificial determinations and define the anomaly in geomagnetic records. The result shows that the magnetic field strength become smaller 100 days prior the earthquake onset and back to normal value about 100 days later.
GP11D-0854 0800h
Solar Flare Effect and Storm Sudden Commencement Event in Geomagnetic H,Y and Z Fields at Euro-African Observatories.
Solar Flare effect and Storm Sudden commencement event in Geomagnetic H, Y and Z fields at Euro-African Observatories. Okeke, F. N. and Okpala K. C. Department of Physics and Astronomy P. O. Box 3238, University of Nigeria, Nsukka. Abstract Variations in the three components of geomagnetic field were observed at twenty-two Euro-African geomagnetic observatories, during Euro-African the solar flare that occured on the 6th of May 1998 8:05 UT and storm sudden commencement that took place on May 7th 1998 at 12.03UT. The geomagnetic field on 6, May 1998 was quiet, but correlations between the intensities of geomagnetic s.f.e. and that of Sq current system show that they are of two different origin. Strong responses of Z and Y fields to solar activity was noted. From the estimation of the contributions of D-and E-layer to the geomagnetic s.f.e it was inferred that both of these layers contribute to the geomagnetic s.f.e almost equally.
GP11D-0855 0800h
Taylor Expansion Method for Paleosecular Variation
Constable and Parker (1988) first suggested that auss coefficients can be treated as independent normal random variables with variances depending only on the degree. Kono et al. (2000) and Kono and Roberts (2002) demonstrated that this postulate is well satisfied in the long time behavior of various numerical dynamo models. However, significant deviations from the original model of Constable and Parker were also found by the analysis of the paleomagnetic data of the last 5 Ma; most notably the large amplitude in the fluctuation of the (2,1) harmonic. In order to express these facts better, Kono and Tanaka (1995) developed Taylor expansion method. But its performance was not satisfactory enough because the approximation used in the calculation was only up to the second order. This paper presents the Taylor expansion method extended to arbitrarily high approximation. We assume that the long term behavior of the geomagnetic field can be modeled as the sum of two parts; the mean field and the fluctuation $\boldsymbol{m=\mu+\Delta m}$, where all the elements of $\Delta\boldsymbol{m}$, $m_j$, are zero-mean normal variates with the variance $\sigma_j^2$. The mean value of a nonlinear quantity can be obtained by averaging the Taylor series about the mean model, with summation rules such as E$[\Delta m_j\Delta m_k] = \sigma_j^2\delta_{jk}$, etc. Summation of the series to high orders is made possible because the nonlinear quantities (or their first derivatives with respect to a Gauss coefficient) used in paleomagnetism (inclination $I$, declination $D$, virtual geomagnetic pole (VGP) position $\theta_p$, $\phi_p$, etc.) can be expressed as the product of the linear quantities and nonlinear quantities such as the total intensity $F$. These nonlinear quantities can always be expressed as the square root of the sum of squared linear quantities (e.g., $F = \sqrt{X^2+Y^2+Z^2}$). Because of this property, the general form of the derivatives of nonlinear quantities can be written down in a form that is convenient for calculation using the computers. The assumption of the normal distribution also make it possible to estimate quantities such as E$[\Delta m_j^2\Delta m_k^2\cdots\Delta m_n^2]$. The number of terms increase roughly $O(N^L)$, where $N$ is the number of fluctuating coefficients and $L$ is the truncation level. It is thus not possible to include more than a few parameters in the model. This method was used to obtain the statistics of the mean VGP and angular standard deviation. To make the problem simpler, only the axisymmetric models were studied. It is found that the truncation must be quite high to obtain a good convergence. The main features of the PSV can be surprizingly well represented by fluctuations of $g_2^1$ and $h_2^1$ only. The present results suggest that the exceeding importance of the (2,1) harmonic, rather than the equal partition of the energy as envisioned by Constable and Parker, may be the most important characteristic of the dynamo operating in the Earth.
GP11D-0856 0800h
Evaluation of Agreement Between Paleosecular Variation Models and Empirical Distributions at 20 Degrees Latitude
Several approaches have been employed to derive paleosecular variation (PSV) models of the magnetic field, one of which supposes that PSV can be represented as a Giant Gaussian Process (GGP). We examine the accuracy of models based on the GGP and the validity of their statistical and symmetry assumptions by direct comparison with paleomagnetic directional data drawn from latitudes around 20 degrees. We compiled paleomagnetic directional data from multiple published studies of volcanic provinces in four regions, Hawaii, Mexico, Reunion and French Polynesia. Sites were selected according to the following criteria: ages younger than 5 Ma, sampling latitudes approximately 20 degrees from the equator, and uncontaminated by tectonic rotation. If multiple directions were reported for the same lava flow we averaged those results. The resulting data set is large enough to justify more extended analyses than previously possible: namely statistical analyses of the distributions of magnetic directions and quantitative comparisons with proposed PSV models. For each region we calculate virtual magnetic poles (VGPs), paleomagnetic directions projected along the expected axial dipole field, and perform a principal component analysis, calculating shape and strength parameters for the data distributions. We determine the total angular dispersion of the calculated VGPs for each region. Data from all regions are merged to generate the largest possible data set for studying zonal (no longitudinal dependence) models. Sub-sets of the zonal data set are used to evaluate the influence of temporal and geographic variations in the PSV. We explore mechanisms for quantitatively evaluating agreement among empirical distributions of these data sets and distributions predicted from a variety of secular variation models. Using models proposed by Johnson and Constable (1999), CJ98, and Tauxe and Kent (2004), TK03, we examine the consequences of axial symmetry, or lack thereof, for the secular variation. We compare the observable geographic dependence of magnetic directional variation to a range of models with different variances in the axial-dipole and non-axial-quadrapole Gauss coefficients, and investigate whether the disparity between the modeled and observed distributions places limits on the values these parameters may take. Preliminary results suggest that the model parameters from CJ98 and TK03 cannot simultaneously satisfy data from all four regions. The tests outlined above will be used to determine whether this results from sampling bias in the data or an inadequacy of the model.
GP11D-0857 0800h
Low latitude geomagnetic field variation at the Banda Sea during the past 800 kyr
We reconstruct the geomagnetic field reversals at the Banda Sea by using paleomagnetic data from the core MD012380 during the International Marine Global Change Study (IMAGES) VII Cruise in 2001. Magnetic analysis is carried out for the whole core on the basis of discrete measuring with a spacing of 1 cm. Magnetic susceptibility ($\chi$), nature remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) are measured. Our results reveal the low latitude geomagnetic field variation at the Band Sea during the last 800 kyr. Except for the Brunhes/Matuyama boundary, there is no clear reverse signal in the paleo-direction (paleo-inclination and paleo-declination) pattern. This causes the difficulty in identifying reverse subchrons. However, the synthetic paleointensity curve displays asymmetrical saw-tooth pattern, available for indicating polarity reversals, and shows a maximum intensity drop at the Brunhes/Matuyama boundary. The characteristic of paleointensity provides a useful tool to determine the reverse events and modifies the problems arose from using the paleo-direction pattern, especially at low latitude. With the help of the paleo-direction pattern, we have identified seven reverse events, two excursions and three possible polarity subchrons during the Brunhes chron.
GP11D-0858 0800h
First Paleo- and Rock Magnetic Results From Erlongwan Maar Lake, Long Gang Volcanic Field, NE China
Two 23 m long sediment cores were recovered from the Erlongwan maar lake in 2001. The crater is sitting within Archean migmatites and the mostly minerogenic sediments are finely laminated. Graded layers with thickness from 5 mm up to 1.5 m are intercalated into the sedimentary sequence especially in the uppermost 6 m and below 17 m depth. Both cores were continuously subsampled and all samples were subjected to detailed paleo- and rock magnetic investigations. The inclination and declination records are similar in both cores indicating that the sediments recorded the paleosecular variations of the Earth magnetic field. Based on this assumption a provisional chronology for the profile was created, using published PSV records from the region. The profile probably covers the last 30 kyr. A statistical analysis of the rock magnetic results confirmed the division into lithozones based on a visually inspection of the cores. High temperature runs of saturation magnetizations revealed that magnetite is the dominant magnetic carrier mineral. Further investigations of the sediments will include geochemical and sedimentological analysis in order to reveal additional information about the magnetic recording behaviour of the sediments as well as the influence of post-depositional processes.
GP11D-0859 0800h
Refinement of the Magnetostratigraphy of the Chinle Group (Late Triassic) of North-Central and Central New Mexico
The Upper Triassic Chinle Group spans most of the Late Triassic, as indicated by vertebrate biostratigraphy and palynostratigraphy. The Chinle Group was deposited by a fluvial system and consists predominantly of red to purple mudstones with some red to orange siltstones and red to buff sandstones. The Chama basin, north-central New Mexico, and the Zuni Mountains, central New Mexico, contain excellent exposures of both lower and upper Chinle Group strata that can be used to develop a magnetic reversal chronology for the Late Triassic from these vertebrate-bearing, continental, fluvial-dominated sequences to compare to the Newark as well as to marine magnetostratigraphic records. Our sampling concentrated on mudrocks, using an intricate block sampling approach. These typically carry a well-defined, well grouped magnetization dominated by pigment hematite that is unblocked below about 660\deg C (e.g., for a single horizon, with six independent samples, Decl. = 185.5\deg, Incl. = 0.3\deg, a95= 6.6\deg, and k = 102.6). Sandstones and siltstones contain pigment hematite and, based on preliminary experiments, contain both detrital hematite and some magnetite. An initial reversal chronology has been developed for Poleo and Petrified Forest strata (middle and upper Chinle Group) in the Chama basin and the Bluewater Creek Formation and Blue Mesa Member, Petrified Forest Formation (lower Chinle Group) in the Zuni Mountains. The Petrified Forest Formation in the Chama basin is characterized by fairly regular reversals, and the Poleo Formation is dominated by reverse polarity. In the Zuni Mountains, a reverse polarity chron is defined by strata at the boundary between the Bluewater Creek Formation and the Blue Mesa Member.
GP11D-0860 0800h
Palaeomagnetic Secular Variations, Polarity Transitions, and Magnetostratigraphy Visual Databases
Geographical Informational Systems (GIS) provide a variety of possibilities to visualize geophysical databases. ESRI ArcView 3.x software is a very useful GIS tool for palaeomagnetists. The dBASE table format make the process of transition of the database into visual form very quick and easy. The object-oriented programming language Avenue dramatically reduces time for creating user-friendly utilities and applications. Recently we transferred the Global Palaeomagnetic Database into GIS form (Pisarevsky and McElhinny, 2004), now I want to present the next achievement - visual Geomagnetic Secular Variation databases, visual Polarity Transitions database, and visual Magnetostratigraphy database. These databases contain exactly the same information, as the corresponding IAGA databases, but provide a much more friendly interfaces and a wide range of utilities. This strongly simplifies the process of searching, analysis, synthesis, and presentation of data. Additionally there are several additional possibilities, such as geographical search, quick selection of data subsets, quick calculations, graphics and others, that were impossible in previous formats of these databases. All utilities are combined into menu items, buttons and tools. They also include printing reports and storing subsets of data in formats for other popular software, including Microsoft Excel, Microsoft Word and some others. This approach also provides a lot of possibilities to combine the palaeomagnetic data with geochronolgical, geochemical, petrological, and other geographically oriented databases and with digital maps.
GP11D-0861 0800h
Magnetostratigraphy and Rock Magnetism of the Neogene Yaha Section (Northwest China) and Possible Late Cenozoic Uplift of the Tianshan by 11 Ma
In order to better constrain the timing of the Cenozoic uplift history of the Tianshan due to the India-Asia collision, we carried out a magnetostratigraphic study at the Yaha section, on the southern flank of the Tianshan range (northwest China). 1069 samples were collected from a 2814-m-thick section composed mainly of fluvio-lacustrine sandstone and conglomerate mapped as the Neogene Kuqa Formation. Rock magnetic experiments show that the magnetic remanence is principally carried by magnetite yet hematite is also abundant . Stepwise thermal demagnetization isolated a linear magnetization component that decays univectorally toward the origin. Based on a positive fold test and the equal proportion of magnetic polarities, the isolated direction likely represents a primary magnetization. Eighteen magnetic polarity changes were identified. They correlate between ~5.2 and ~12.6 Ma with the reference magnetic polarity time scale. We also measured the anisotropy of magnetic susceptibility of the samples. Before ~ 11 Ma the average shape parameter T of the AMS is around 0 meaning the grains are spherical in shape and far-traveled. By ~11 Ma, T strongly increases to values strictly greater than 0 meaning that the magnetic grains are oblate in shape and traveled less far. The timing of this change in the T parameter is strongly correlated to a decrease in the bulk susceptibility that likely signals a modification of the source rock. From the rock magnetic parameters, together with the acceleration of the sedimentation rate, we conclude that the uplift of the Tianshan mountains started by ~ 11 Ma.
GP11D-0862 0800h
Preliminary Magnetostratigraphic Study of the Split Mountain and Lower Imperial Groups, Split Mountain Gorge, Western Salton Trough, CA
We present preliminary results of a magnetostratigraphic study of Miocene-Pliocene sedimentary rocks of the Split Mt. and lower Imperial Groups exposed in Split Mt. Gorge and eastern Fish Creek-Vallecito basin, western Salton Trough. Precise age control for the base of this thick section is needed to improve our understanding of the early history of extension-related subsidence in this region. The geologic setting and stratigraphic framework are known from previous work by Dibblee (1954, 1996), Woodard (1963), Kerr (1982), Winker (1987), Kerr and Kidwell (1991), Winker and Kidwell (1986; 1996), and others. We have analyzed Upper Miocene to lower Pliocene strata exposed in a conformable section in Split Mt. Gorge, including (in order from the base; nomenclature of Winker and Kidwell, 1996): (1) Split Mt. Group: Red Rock Fm alluvial sandstone; Elephant Trees alluvial conglomerate; and lower megabreccia unit; and (2) lower part of Imperial Group, including: Fish Creek Gypsum; proximal to distal turbidites of the Latrania Fm and Wind Caves Mbr of Deguynos Fm; upper megabreccia unit; marine mudstone and rhythmites of the Mud Hills Mbr (Deguynos Fm); and the basal part of the Yuha Mbr (Deguynos Fm). Measured thickness from the base of the Elephant Trees Cgl to the base of the Yuha Mbr is about 1050 m, consistent with previous measurements of Winker (1987). Paleomagnetic samples were collected at approximately 10 m intervals throughout this section. The upper portion of our sampled section overlaps with the lower part of the section sampled for magnetostratigraphic study by Opdyke et al. (1977) and Johnson et al. (1983). They interpreted the base of their section to be about 4.3 Ma, and calculated an average sedimentation rate of approximately 5.5 mm/yr for the lower part of their section. Good-quality preliminary results from 15 paleomagnetic sites distributed throughout our sampled section permit preliminary identification of 6 polarity zones. Based on regional mapping and stratigraphic correlations (Winker and Kidwell, 1996) and extrapolation of the Johnson et al. (1983) sedimentation rates, we tentatively correlate two normal-polarity intervals in the lower part of our section to Chrons C3An.1n and C3An.2n of the geomagnetic polarity time scale of Cande and Kent (1995). If correct, this result would suggest that the lowest reversed-polarity portion of the Red Rock Fm near the base of the section should be correlated to Chron C3Ar.2r, and would be between 6.57 and 6.94 Ma in age. An alternative, but less likely, correlation for the reverse-polarity interval at the base of the section would be with Chron C2Br (7.09-7.43 Ma). This has significant implications for the timing of earliest extension-related basin formation in the western Salton Trough, and potentially raises new questions about the relationship of the Red Rock and Elephant Trees Formations to the Alverson Volcanics (22-14 Ma; Ruisaard, 1979; Kerr, 1982). These preliminary results will be tested and refined with a complete analysis of the remaining samples.
GP11D-0863 0800h
Magnetostratigraphy and geochronology of the upper Balsas Group, Guerrero, Mexico
The Balsas Group in the Guerrero-Morelos platform consists of continental redbeds, of fluvial, alluvial, and lacustrine origin that post-date Laramide deformation in southern Mexico. Although this unit is widespread in the region, its stratigraphy, thickness and facies variations, and tectonic significance are largely unknown. It lacks fossils, but it is sandwiched between the radiometrically dated Tetelcingo Formation (ca., 65 Ma) and the Tilzapotla caldera products (ca. 33 Ma.). Red sandstone, siltstone, and mudstone, intercalated with abundant gypsum beds, outcrop along Federal Highway-Autopista del Sol, north of the Balsas River. These are interpreted as part of the Balsas Group, but its position within the sequence is uncertain. They appear to overlie typical limestone boulder conglomerate alluvial facies and underlie buff-colored lacustrine deposits. The section is approximately 110 m in thickness. A pink-colored ignimbrite occurs within the sequence and has been dated using Ar-Ar techniques. The ignimbrite is about 4 m thick and is associated with an altered rhyolitic flow. The ignimbrite includes lithic fragments, green altered pumice, as well as sanidine and biotite crystals in an aphanitic matrix. 40Ar-39Ar age determinations provide relatively imprecise ages and spectra do not provide strict plateaus; interpreted pseudo-plateau ages of 35.7+/-0.5 Ma (biotite) and 31.3+/-1.3 Ma (sanidine), indicate a late Eocene-early Oligocene age. Samples collected for magnetostratigraphy from this section and a section exposed within the Amacuzac river valley, north of Atenango del Rio, are consistent with deposition during the chron sequence C17-C12, within the resolution allowed by the sampling interval. This result shows that this part of the Balsas Group partly contemporaneous with the main ignimbrite emplacement episode in the region. Paleomagnetic directions are concordant, suggesting that previously determined post-Laramide tectonic rotation had ceased by Early Oligocene time.
GP11D-0864 0800h
Paleomagnetic and magnetic fabric studies of the San Gaspar ignimbrite, western Mexico - constraints on emplacement mode and source vents
Paleomagnetic and magnetic fabric data for the 4.8 Ma San Gaspar ignimbrite, one of the largest in western Mexico, are used to investigate on the source vents and emplacement mechanism. Rock-magnetic properties are similar, suggesting relatively homogeneous mineralogy of the unit. Isothermal remanence and continuous susceptibility-temperature experiments point to low to medium-Ti titanomagnetite as the main ferromagnetic mineral. Hysteresis parameters of most samples fall in the pseudo-single-domain grain size region; wasp-waisted hysteresis loops were identified corresponding to high Hcr/Hc values. Stepwise thermal and alternating field demagnetization shows that secondary components are negligible, and always completely removed below 20 mT or 400°C. Thereafter, the characteristic component is isolated with small within-site dispersion of mean direction (95<10° at 7 out of 10 sites). The within-site consistency of directional features of magnetic fabric supports that the ignimbrite welded and cooled in situ and was probably emplaced in NW-SE rift zone from a single eruption center located south or east of a younger Pleistocene caldera. Paleomagnetic, rock-magnetic and ore microscopy data support the hypothesis that the extensive and widespread ignimbrite deposits in western Mexico correspond to a major explosive volcanic phase in the Pliocene.
GP11D-0865 0800h
Paleomagnetic Reversal Frequency in the Lorne Lavas of Western Scotland and their Implications for Eruption Duration
The Lorne Lavas of Western Scotland are a sequence of approximately 400 million year old basalts and basaltic andesites. A previous paleomagnetic study, undertaken in 1974, of randomly selected sites within the lava pile found the same polarity in all samples. This current work aims to resample the lava flows in a temporal sequence and to apply modern paleomagnetic techniques to look for paleofield reversals. The aim of this study is to constrain the time period over which these lavas were erupted. These results will also be integrated with a geochemical study of the lavas currently being undertaken in the department. A sub-study of anisotropy of magnetic susceptibility (AMS) will aim to deduce paleoflow directions in an attempt to determine whether the flows were erupted from a central vent or from numerous fissures. This poster will present preliminary data and interpretations from thermal demagnetisation and AMS measurements from approximately 20 sampling sites throughout the lava pile.
GP11D-0866 0800h
Investigating the Timing and Extent of Early Yellowstone Hotspot Volcanism While Expanding the Steens Mountain Magnetic Reversal Record.
The lower Steens Basalt flows of the Oregon Plateau appears to have been erupted earlier than the oldest Columbia River Basalt flows (Hooper et al., 2002), although some contest this conclusion (Baksi, 2004). The extent and timing of the Steens Basalt eruptions are not well understood and knowledge gained in this area will enhance understanding of the Yellowstone Hotspot and Large Igneous Provinces. Volcanic flow stacks in this area show at most one reversal and when present always show a reversed to normal change. This fact suggests that the lavas all record the same reversal and, consequently, that the eruptions occurred over a short period of time. By using high precision Ar/Ar dating and the path of the VGP during the transition, we can test whether the reversals recorded in this area are contemporaneous. At the same time, we hope to find new transitional directions of the well documented Steens Mountain reversal. VGP paths that we have obtained from Summit Springs suggest that this section is indeed Steens equivalent. In addition, we have dated two flows from the transition found at Steens Mountain to 16.69 +- 0.19 Ma and 16.48 +- 0.18 Ma using Ar/Ar dating techniques. Initial portable fluxgate analysis at Catlow Peak show around 30 transitional flows present. We will present paleomagnetic results from this section and several others in the area.
GP11D-0867 0800h
Paleomagnetic Paleolatitudes of the Ontong Java Plateau From 120 Ma to 55 Ma: Implications for the Apparent Polar Wander Path of the Pacific Plate.
Paleomagnetic analyses of Late Cretaceous- early Tertiary deep-water carbonates obtained during ODP Leg 192 drilling have been used to document the latitudinal motion of the Ontong Java Plateau (OJP). Paleolatitude estimates were obtained for the following periods: magnetochrons 25n-25r (55.9-57.5 Ma), 27r-31n (61.3-68.7 Ma), 32n-32r (71.1-73.6 Ma) and 33n (73.6-79.1 Ma). The broad agreement between our new sedimentary paleomagnetic data and those recently obtained from OJP basement rocks suggests that possible compaction-induced inclination shallowing in the studied sediments is negligible. Anisotropy of anhysteretic remanent magnetization measurements further support this argument, yielding magnetic fabrics that appear poorly correlated with individual characteristic remanent magnetization inclinations. \ Our results when combined with other OJP paleomagnetic data yield an internally consistent data set that describes the northward motion of the plateau from its formation $\sim$120 Ma until 55 Ma. Between 120 Ma and 76 Ma the plateau is located near $24\deg$S and is either stationary or exhibits a slow northward movement. From 76 Ma until 68 Ma the plateau moves rapidly through approximately $10\deg$ of latitude to $\sim10\deg$S. From 68 Ma until 56 Ma the plateau is again almost stationary before moving slowly northward. Our data therefore support previous suggestions for a Cretaceous "standstill" of the Pacific plate and provide additional constraints upon proposed episodes of rapid plate movement. Our new OJP paleomagnetic paleolatitudes are generally lower than those predicted by indirect data from seamount magnetization studies and deskew analyses for chron 27r/31n but are higher than those predicted from deskew analyses for chrons 32 and 33n. More direct paleomagnetic data are necessary to define Pacific plate motion, drift of hotspots, and possible true polar wander events. We suggest that deep-water carbonates, if carefully analysed, can yield reliable paleomagnetic inclinations.
GP11D-0868 0800h
Paleomagnetic and Rock Magnetic Signature of Upper Oceanic Crust Generated by Superfast Seafloor Spreading: Results from ODP Leg 206
During Ocean Drilling Program (ODP) Leg 206, Site 1256 ($6.736\deg$N, $91.934\deg$W) was cored deep into a 15-Ma section of oceanic crust that is part of the Cocos Plate formed by superfast spreading ($>$200 mm/yr) at the East Pacific Rise. Three holes penetrated through 250 m of sediment and into igneous basement, with the two deepest holes, Holes 1256C and D, reaching 88.5 and 502 m sub-basement, respectively. The igneous section consists of an uppermost massive ponded flow that is $>$70 m thick, underlain by thin flows ($<$3 m thick), massive flows ($>$3 m thick; some of which may be dikes), pillows, and hyaloclastites. The uppermost units, particularly the massive ponded flow, record an anomalously steep paleomagnetic direction for this near equatorial site and commonly retain only a few percent of their natural remanent magnetization (NRM) after being demagnetized in peak alternating fields (AF) of only 20 mT. The steep direction appears to be the characteristic remanent magnetization (ChRM) as it can be separated from an even steeper drilling overprint. Units below this have shallow inclinations and retain 5% to 15% of their NRM after 20 mT AF, both characteristics being more typical of what would be expected for near equatorial oceanic basalts. Rock magnetic results based on hysteresis, FORC, and thermomagnetic measurements indicate that there are insignificant differences in the paleomagnetic carriers for the units with steep and shallow inclinations. In both cases, pseudo-single domain titanomagnetite and titanomaghemite are the main carriers. This supports the interpretation that the uppermost basalts were extruded in a weak transitional or excursional field. Given that the site was cored 5 km east of the transition zone between marine magnetic anomalies 5Bn.2n and 5Br, the massive ponded lava most likely recorded this transitional field after traveling $\sim$5 km from the spreading axis.
GP11D-0869 0800h
Paleomagnetic and Rock Magnetic Investigation of Leg 210 Cores, Newfoundland Basin
Leg 210, the final leg of the ODP, was devoted to studying the history of rifting and postrift sedimentation in the Newfoundland-Iberia rift. Two sites (Sites 1276 and 1277) were drilled in the central Newfoundland Basin that are along a transect exactly conjugated to the ODP Leg 149/173 drilling sites on the Iberia margin. Site 1276 was cored from 800 to 1739 m below seafloor with excellent recovery of Albian-lower Oligocene sediments and sills (estimated to be 100-200 m above basement). Site 1277 is located about 40 km southeast of Site 1276. Recovered cores from the upper part of basement at this site represent an assemblage of basalt flows, slivers of gabbro, serpentinized peridotite, and sediments. Below these rocks, basement is serpentinized peridotite with veins of gabbro. Stable components of magnetization are revealed in the results of detailed thermal and AF demagnetization on the sediments and diabase sills from Site 1276 and serpentinized peridotites from Site 1277. Azimuths of the maximum anisotropy axis of magnetic susceptibility are used to evaluate the fidelity of the natural magnetic memory of the serpentinized peridotites. Combining the age information available from other Leg 210 studies, we estimated paleolatitudes vs. time for the Newfoundland drill sites. Results from a series of rock magnetic measurements show that (titano)magnetites are present in the diabase sill and serpentinized peridotites, whereas maghemite is mainly responsible for the magnetic signatures displayed in the sedimentary and volcanic succession at Site 1277. These results, in concert with previous paleomagnetic characterization of serpentinized peridotite samples from ODP Legs 149 and 173, provide an inter-basin correlation of the magnetic properties of the peridotite rocks. The magnetic signatures of the serpentinized peridotites recovered from both sides of the Newfoundland-Iberia rift appear not to conflict with the notion that conjugate margins will have generally similar crustal structure and evolution history.
GP11D-0870 0800h
Magnetic Properties and Paleointensity of a Mid-Miocene Gabbro from the Costa Rica Accretionary Wedge, ODP Leg 170
Samples of gabbro from cores recovered by the Ocean Drilling Program Leg 170 have been analyzed for magnetic mineralogy and paleointensity using a Thellier-Thellier method. The cores are from the Costa Rica Accretionary Wedge, Site 1039, Hole C at a minimum depth of 423 meters below the sea floor. This gabbroic unit is younger than 16.49 Ma based on nannofossils in the intruded oozes. This makes it ideal for a paleointensity study because few paleointensity data exist for this time period in this region. Rock magnetic investigations included continuous low field thermomagnetic analysis, alternating field (AF) demagnetization, optical microscopy, scanning electron microscopy (SEM), and element analysis. AF demagnetization paths are straight to the origin on orthogonal vector endpoint diagrams suggesting that the magnetization is single component. Optical microscopy and SEM observations identified titanomagnetites as the most abundant magnetic mineral in the samples; occurring as either coarse- or fine-grains with euhedral shape. Titanomagnetite composition, estimated from weight percents provided by element maps from the backscattered electrons, are both low and high titanium, TM22 and TM74. Reversible susceptibility vs. temperature curves from low field thermomagnetic analysis to 710$\deg$C suggests that the minerals do not alter in an argon atmosphere. Curie temperatures below 200$\deg$C and above 400$\deg$C calculated from these curves are consistent with the SEM results. The modified Thellier technique used a 0.4 uT field and an argon atmosphere. Low temperature demagnetization before and after some temperature steps was used to look for different behavior of the large and small magnetite grains. Based on Thellier and thermal experiments, most of the samples have about equal contributions from low and high Curie temperature titanomagnetite. Although alternating field demagnetization produced straight line paths on demagnetization diagrams, thermal results show that the two magnetites carry different components. We are currently exploring the reasons for the difference, e.g., the contribution of drill string remanence, and whether either magnetite can be used to determine the paleomagnetic field intensity.
GP11D-0871 0800h
Paleosecular variation and time-averaged field recorded in lavas flows from Mexico
An analysis of secular variation and time-averaged paleomagnetic field in the Mexico, using data both newly obtained and from the literature is presented. Interpretation regarding time-averaged field and secular variation can best be constrained after excluding from the dataset all the sites from the western part of the Trans-Mexican Volcanic Belt and the sites older than Quaternary from the central and eastern part of the TMVB, which appear to be affected by counterclockwise tectonic rotations. The mean direction among the remaining sites of Quaternary age is Dec = 359.1\deg, Inc = 31.3\deg, \alpha$_{95}$= 2.0\deg, N = 157. This direction doesn't overlap the expected GAD but is consistent with a geocentric axial dipole field (GAD) with a 5% quadrupole. The VGP scatter of this group of sites (12.3\deg, with lower and upper 95% confidence limits of 11.4\deg and 13.3\deg) is close to the value expected from Model G (13.5\deg).
GP11D-0872 0800h
The Time-Averaged Field at the Equator: Results from Ecuador
Seventy sites were collected from Pliocene to recent lavas in Ecuador at latitudes of less than 1\deg south. Lighting affected many of the sites sampled and the final data set consisted of 49 sites with \alpha$_{95}$ of 10\deg or less. Nineteen flows yielded normal (Dec = 355.8\deg, Inc = -5.8\deg,\alpha$_{95}$ = 7.1\deg) and 29 reverse (Dec = 182.5\deg, Inc = 5.7\deg,\alpha$_{95}$ = 4.9\deg) directions of magnetization. These sites pass a reversal test at a high level. The combined data give a direction of Dec = 359.8\deg, Inc -5.8\deg,\alpha$_{95}$ = 4.1\deg, which is just significantly different from the GAD field. These results are however what is expected if a 5% quadrupole component is added to the dipole field. The dispersion of the selected site mean directions is 13.2\deg which is not significantly higher than the value expected from Model "G".
GP11D-0873 0800h
A Late Cretaceous Reversal Recorded in Lavas of the High Canadian Arctic
We report new directional data from Late Cretaceous basaltic lavas of northern Ellesmere Island, Canada (present day latitude 81-82$^{o}$ N). Analyses of a preliminary field sampling suggested the presence of a reversed to normal polarity transition. We collected new sites (drilled as standard paleomagnetic cores oriented with a Sun compass) from the same stratigraphic section as the preliminary sampling to confirm the reversal. A portable magnetometer (a three axis fluxgate magnetometer in Mu-metal shields) was used to check sample polarity in the field and guide sampling. In addition to the lavas, we sampled dikes from an adjacent area; the two areas are separated by faults. Detailed thermal and alternating field demagnetizations of cores from 34 sampling sites yield characteristic remanent magnetization directions that can be grouped into the following: reversed polarity lava flows, normal polarity lava flows, transitional lava flows, and normal polarity dikes. Based on these results, the lavas are considered to be at least 15 million years younger (corresponding to chron 33r or younger) than the widespread $\sim$95 Ma volcanic rocks of the Strand Fiord Formation. Fisher averages of the preliminary normal and reversed polarity flows are nearly antipodal. Sites from the dikes, however, represent a distinct group of directions and therefore may differ in age from the lavas. The preliminary paleomagnetic data and reconstructions based on other data indicate that the lavas were erupted near the tangent cylinder, that imaginary cylinder parallel to the spin axis and tangent to the solid inner core. Hence, directional and paleointensity results from these and future sites may ultimately be useful for understanding the dynamo.
GP11D-0874 0800h
A Preliminary Paleomagnetic Result of the Lower Triassic Sedimentary Rocks in the Danyang Area, Korea: its Tectonic Implications
Southern Korean Peninsula is tectonically divided into Gyeonggi Massif, Okcheon Belt and Yeongnam Massif from northwest to southeast. In recent decades, many geologists and paleomagnetists have endeavored to elucidate the tectonic affinity of the Gyeonggi and Yeongnam massifs with adjacent Chinese blocks, such as North China Block (NCB) or South China Block (SCB), before the collisional event of East China, but it is still controversial. A paleomagnetic study has been carried out for the Lower Triassic Donggo Formation, exposed in the Danyang area on the Yeongnam Massif, to provide reliable paleomagnetic constraints on the origin of the Yeongnam Massif. The characteristic remanent magnetization (ChRM) components were isolated from a total of 120 samples in 13 sites. The ChRMs in this study reveal both normal and reversed polarities and pass a reversal test at the 95% confidence level. Stepwise untilting test shows that the maximum clustering of ChRM directions occurs at 100% untilting, indicating that the ChRMs isolated in this study are of primary in origin. The paleomagnetic pole position calculated from the site mean directions of the primary remanence in the Danyang area is at $38.3\deg$N, $18.5\deg$E (A$_{95}$ = $6.5\deg$). This pole is displaced westward by $49.4\deg$$\pm$$11.5\deg$ with respect to the Early Triassic paleopole reported from the Taebaek area which is located at approximately 50 km distant to the northeast of the study area. The Donggo Formation in the Danyang area is distributed with NE-SE trend, parallel to the general trend of the Okcheon Belt, while that in the Taebaek area is distributed with E-W trend. Furthermore, Paleomagnetic direction (D/I = $354.9\deg$/$61.5\deg$, k = 58.0, \alpha$_{95}$ = $4.7\deg$) of the Cretaceous overprint component in the Taebaek area shows significant eastward declinational deflection (c.a. $60\deg$) with respect to that (D/I = $55.8\deg$/$54.8\deg$, k = 18.3, \alpha$_{95}$ = $9.5\deg$) of the Late Cretaceous-Early Tertiary remagnetization component in the northeastern Okcheon Belt (Jeongseon area) with an statistically insignificant inclinational difference. It is therefore interpreted that the Taebaek area rotated clockwise by about $50\deg$-$60\deg$ with respect to adjacent areas, such as Danyang and Jeongseon areas, after the Early Triassic. Korean paleomagnetic poles that satisfy more than four reliability criteria of Van der Voo (1990), since the Late Carboniferous, have been selected and synthesized for comparison with Chinese apparent polar wander (APW) paths. Comparisons of Korean paleopoles with the APW paths of the NCB and SCB show that the Yeongnam Massif belongs to the NCB, while the Gyeonggi Massif has been a part of the SCB before the completion of suturing between the NCB and SCB. This tectonic correlation based on paleomagnetic data agree well with that based on the paleontologic data. In addition, the relative tectonic movements of Korean blocks with respect to the NCB and SCB since the Late Carboniferous will be further discussed in the presentation.
GP11D-0875 0800h
Paleomagnetism of Silurian and Devonian rocks from Central and Northern Kazakhstan - comparison with South Kazakhstan directions.
Central and Northern Asia consist of a set of continental blocks, which were amalgamated and subsequently welded to Europe during Carboniferous and Permian times. Within this mosaic, Kazakhstan takes a central position. Understanding the tectonic evolution of this area is the key to understanding the tectonic processes to the east of the Urals as well as for the understanding of the geological history of Eurasia as a whole during the Paleozoic. Here we present the results of a detailed paleomagnetic study of Silurian and Devonian rocks from the Teniz depression (Central Kazakhstan) and the Chingiz Range (Northern Kazakhstan). Overprint components were isolated for Lower Silurian redbeds of Northern Kazakhstan(D=352.2\Grad, I=+87.2\Grad, k=20.3, $\alpha_\mathrm{95}$=6.4\Grad, 11 sites) 2),Lower Silurian redbeds from the Chingiz Range(D=193.0\Grad, I=-65.2\Grad, k=25.2, $\alpha_\mathrm{95}$=15.5\Grad, 5 sites) Middle - Upper Devonian redbeds from Central Kazakhstan (D=005.0\Grad, I=+62.6\Grad, k=37.9, $\alpha_\mathrm{95}$=5.9\Grad, 10 sites. Both, prefolding directions published elsewhere and secondary magnetizations, presented here identified in rocks covering in time the Ordovician to Carboniferous indicate counterclockwise rotations of Southern Kazakhstan with respect to Baltica (80\Grad) and to Siberia (140\Grad) since Ordovician times. These rotations are not in accord with tectonic models based on the existence of the Kipchak arc (Seng{\"o}r et al., 1993, Seng{\"o}r and Natal'in, 1996) during the Paleozoic. These models postulate the existence of an Early Paleozoic volcanic arc, dconnecting Baltica in the South to Siberia in the North with the edges of the arc being tied to these two cratons. The suggested geometry of the original arc puts very strict kinematic constraints on relative motions and rotations of individual blocks within this framewor. In particular it implies, that since the Early Devonian, southern Kazakhstan has experienced clockwise rotation of about 90° relatively to Baltica and about $30\Grad$ clockwise rotation with respect to Siberia (Seng{\"o}r and Natal'in, 1996). This is in strong contrast to the observed paleomagnetic results indicating counterclockwise rotation.of a consolidated Kazakhstan independent of both Baltica and Siberia.
GP11D-0876 0800h
Palaeomagnetism of the Late Neoproterozoic of Ella O, North-East Greenland
Neoproterozoic to lower Ordovician sediments outcrop in a N-S trending band in the fjord region of North-East Greenland. The sequence comprises, in ascending order, the Eleonore Bay Group, the Tillite Group and the Canyon and Spiral Creek Formations. The Eleonore Bay Group is thought to be Upper Riphean in age and consists of cherty limestone and dolomite capped by red siltstone. The overlying Tillite Group contains two tillite packages separated by an intertillite which comprises marine siltstone and sandstone. The Canyon and Spiral Creek Formation consist of evaporitic red siltstone, with chert-rich horizons and some dolomite. The Spiral Creek Formation is overlain by a basal Cambrian quartzite. 500 samples were collected from the late Precambrian succession on the island of Ella O in Kong Oscars Fjord. Sampling was aimed in particular at red beds and other likely magnetic targets. The specimens were demagnetised using progressive alternating frequency and thermal techniques and typically revealed a multi-component remanence structure. A majority of the specimens carry a low stability (generally $<$ 20mt, $<$ 250$\deg$) component, directed north and steeply down. This closely resembles present Earth's field. Demagnetisation of the Eleonore Bay Group reveals a high stability component directed south and shallow down, with an opposing component north and up. After tilt correction the mean direction yields a palaeolatitude of 4$\deg$. This component passes field tests, and is interpreted as primary. Magnetic characteristics in the Tillite Group are distinct from those of the Eleonore Bay Group. The Lower Tillite Formation carries an east directed shallow down component. The palaeolatitude derived from this direction indicates low latitude deposition for the glacial rocks above the Eleonore Bay Group. The Upper Tillite Formation carries a high stability component directed steeply upwards. Specimens from a limited pilot study pass reversal and fold tests, but further experiments are required to verify the result. If confirmed, this would indicate a hiatus between the lower and upper tillites. It also suggests that the Upper Tillite, which potentially correlates with the 630Ma 'snowball' Earth event, was deposited at high latitude. This would be the first confirmation that the late Neoproterozoic Marinoan glaciation extended over a broad range of latitudes.
GP11D-0877 0800h
Remagnetization of Lower Carboniferous Carbonates, Northeastern Ireland: Preliminary Paleomagnetic Results
Conodont color alteration index (CAI) values provide indirect paleotemperature estimates for rocks that have been subjected to a thermal event either by burial metamorphism or by hot hydrothermal fluids. Extensive CAI studies have been conducted on the Lower Carboniferous carbonates of Ireland that are host to the major sedimentary lead and zinc deposits in the Irish ore field. The CAI values show that these rocks have been affected by a regional thermal event with a decreasing trend from south (CAI=7.0) to north Ireland (CAI=1). In addition, the relative timing of the regional metamorphic event and ore mineralization in the Irish ore field, critical to the much debated genetic theories for ore genesis, has remained elusive. A paleomagnetic study, that includes thermal and alternating-field demagnetization and isothermal remanence procedures, has been conducted on 235 specimens representing 18 sites from Lower Carboniferous carbonates (CAI $<$ 3.0) of northeastern Ireland to date the metamorphism. Preliminary results indicate that the characteristic remanent magnetization (ChRM) in 144 specimens from 12 sites (sites 7-18) is carried by single- to pseudo-single domain pyrrhotite and/or magnetite with a mean direction (component A) at D=192.8$\deg$, I=3.0$\deg$ ($\alpha$$_{95}$=4.5$\deg$). In contrast, 67 specimens from 6 sites (sites 1-6) with fine to coarse hematite as the chief magnetic remanence carrier have a mean ChRM direction (component B) at D=17.8$\deg$, I=32.9$\deg$ ($\alpha$$_{95}$=9.4$\deg$). Both the A and the B component ChRM directions were corrected for the tilting of the beds. The A component direction is similar to the ChRM direction isolated for the Lower Carboniferous host carbonates of the Navan lead and zinc deposit in the Irish Midlands. This suggests that a large-scale hydrothermal fluid event may be responsible for a regional remagnetization of the carbonates. Conversely, the B component direction is significantly different and younger in age than the A component and may indicate a secondary magnetization, likely the result of weathering of the carbonates by oxidizing fluids. In addition, this regional hydrothermal event may also be related to the observed CAI values in Lower Carboniferous carbonates of northeastern Ireland.
GP11D-0878 0800h
Paleomagnetic Evidence on a Volcanological Mystery - The 1631 Eruption of Mount Vesuvius, Italy
Was or was not effusive activity part of the 1631 eruption of Mount Vesuvius? We report on a detailed rock magnetic, paleomagnetic and absolute paleointensity study of lava flows from Mount Vesuvius and on its evidence concerning the enigmatic 1631 eruptive episode. Characteristic magnetization directions determined from detailed stepwise alternating field and thermal demagnetization give four new well-defined flow unit mean directions with a95 ranging from 0.7° to 2.4°. Paleodirections for eight lava flows from twenty-two flows studied appear related to the 1631 eruption, as indicated by their correlation to the early seventeen century segment of the Italian paleosecular variation reference curve. This provides new evidence supporting that the 1631 episode was an explosive-effusive eruption.
GP11D-0879 0800h
Anomalously Shallow Paleomagnetic Inclinations of the mid-Turonian Turbidites From Baja California Sur, Mexico
Paleomagnetic inclinations of sedimentary rocks tend to be biased toward shallower than expected values, probably largely due to post-depositional burial compaction that is controlled by the intrinsic sedimentologic factors such as clay content, grain size, and sedimentary facies. As part of our continued efforts to quantify the sedimentalogical controls on inclination shallowing and thus to correct the shallow inclinations, we have conducted a paleomagnetic and rock magnetic study of the mid-Turonian ($\sim$92 Ma) turbidites from Arroyo San Lorenzo ($27.0\deg$N, $113.7\deg$W), Baja Californina Sur, Mexico. Fifteen block samples were collected from two 25 cm thick beds, which contain mudstone and siltstone/sandstone layers, within a deep marine sandstone-mudstone turbidite sequence. The two beds are stratigraphically $\sim$15 m apart and the mudstone layer in the stratigraphically lower bed displays faint laminations. These fine-grained beds were likely deposited from waning turbidity currents. All samples were subjected to either stepwise alternating field (af) demagnetization up to 120 mT or progressive thermal demagnetization up to $650\deg$C. Both the susceptibility and remanence anisotropies were measured to identify and correct shallow inclinations, if any. The 19 mudstone samples from the two beds show similar directions with a mean of Dec=$323.3\deg$, Inc=$29.3\deg$, k=46.4, $\alpha$$_{95}$=$5.0\deg$ and the 44 siltstone/sandstone samples from the two beds yield directions with a mean at Dec=$322.4\deg$, Inc=$47.1\deg$, k=43.8, $\alpha$$_{95}$=$3.3\deg$ in stratigraphic coordinates. The most prominent features of the paleomagnetic results are that both beds yield inclinations shallower than the expected cratonic inclination ($\sim$$57\deg$) for the sampling site and the inclinations of the mudstone samples are $\sim$$17\deg$ shallower than those of the siltstone/sandstone samples. The disparate inclinations of the mudstone and siltstone/sandstone layers suggest that these rocks carry a primary remanence but the inclinations were likely biased by depositional and/or postdepositional processes and need to be corrected prior to being used for tectonic interpretations. The magnetic susceptibility and remanence data show a predominately oblate fabric with minimum axes perpendicular to the bedding, indicating that the shallow inclinations were caused by compaction and can be corrected by their remanence anisotropies. The ongoing laboratory experiments focus on determining the {\it a} factor, the anisotropy of individual magnetic particles, for mudstones and siltstones/sandstones required by the theoretical correction model. The correction of the shallow inclinations will be discussed in the presentation.
GP11D-0880 0800h
The Achilles Heel of Red Bed Paleomagnetism
To obtain a paleomagnetic pole position from a group of red bed samples requires the extraction of the single-component `characteristic' remanence (a direction generally considered to be acquired at or within a few hundred years after deposition). The accepted method of extraction of this remanence, which has been in vogue for the last 40 years, involves total thermal demagnetization of the samples in a series of steps. Usually, at some temperature during this procedure, a sample's remanence (as displayed on a vector diagram), will begin to follow a straight-line path to zero intensity. This path direction is identified as that of the `characteristic' remanence of that sample; the temperature of initiation of straight-line decay is designated as the point at which all secondary components have been preferentially removed. The mean 'characteristic' remanence is then obtained from all of the sample `characteristic' directions. The underlying, significant assumption in this technique is that any secondary components reside in grains with lower maximum blocking temperatures than those carrying the `characteristic' remanence. Straight-line decay of remanence to zero, however, can also arise in another, highly plausible, way that has been completely excluded from consideration by red-bed adherents. As a result of long-term remanence acquisition, a sample may contain multiple remanence components that reside in grains of overlapping blocking temperatures. In this case, stepwise thermal demagnetization will produce proportional decay of all components, such that remanence decay will also follow a straight-line path to zero; a `characteristic' remanence component, therefore, cannot be distinguished from the others. During standard red-bed analysis, once straight-line remanence decay begins, the researcher, in reality, loses all ability to distinguish single- from multiple-component behavior. Yet, because of the restriction of the method, he/she will interpret any straight-line remanence decay to be the unique result of single-component, `characteristic' remanence behavior. This fundamental problem in analysis technique primarily will be demonstrated through the study of a fabricated two-component sample and a section of Triassic red beds in Wyoming that previously has been reported, in two separate classic red bed studies, to contain only single-component `characteristic' remanence but that unequivocally can be shown to contain three, greatly divergent major components.
GP11D-0881 0800h
Evolution Of The Alpha Ride, The Arctic Ocean, On The Basis Of The Geohistorical Analysis Of The Magnetic Anomalies
A new magnetic anomaly map of the Amerasian Basin has been created owing to a joint reprocessing of the Russian and American aeromagnetic data [Glebovsky, Kovacs at all., 2000]. This model produced the base for the magnetic data interpretation on the more qualitative level. As a result three series of seafloor spreading-type magnetic anomalies have been identified within the area of the Alpha Ridge and the adjacent part of the Canada Basin [Gurevich et all, 2003]. Their sources were formed from three spreading centers (SC). Two spreading centers: the western and the eastern, are situated at the axial part of the Alpha Ridge, the third one - the southern, is located on the southern slope of the Alpha Ridge and on the adjacent part of the Canada Basin. The triple junction of these SC had been located in the central part of the recent Alpha Ridge. The geohistorical analysis of these magnetic anomalies is fulfilled using an original computer programs. In consequence of this analysis: the geochronological characteristics are specified; the kinematic characteristics of the oceanic floor movement are determined and the main stages of the area evolution are found. The magnetic anomalies M16r (140 Ma), which signify the position of all three SC, and pair anomalies M20r (146.5 Ma) and M23r (151.5 Ma) are identified enough sure for all three SC and pair anomalies M30r (157.5 Ma) - fore the eastern and the southern SC. Finite and differential Euler poles of the lithospheric plates rotation were calculated for all three SC from best-fit pair magnetic anomalies. All the poles are concentrated around the Nares strait and at the northeastern part of the Ellesmere island. Angle and linear spreading rates were calculated using Euler poles. The calculation has showed that all three SC had low spreading rates. Three stages of the area evolution are found on the basis of the plate tectonic reconstruction for the periods 146.5, 151.5 and 157.5 Ma ago. The first stage, slightly earlier 157.5 Ma ago: the initiation of the oceanic crust formation to the north-west from the recent shelf of the Prince Patrick island. The second stage, from about 157.5 Ma ago: SC had advanced to the north-east; the oceanic crust was forming from one SC. The third stage, from slightly earlier 151.5 Ma ago to 140 Ma ago: the oceanic floor spreading from three SC took place, 140 Ma ago spreading ceased in this area. During the third stage the triple junction of the spreading centers was not stable and changed from type "ridge - ridge - ridge" to type " ridge - ridge - transform". The intraplate volcano-tectonic activity of the oceanic floor that created the Alpha Ridge was t8he fourth stage of the area evolution The kinematic characteristics of the spreading imply of crustal compression in the north of the Greenland and in the north-east of the Ellesmere island and of crustal stretching in the area of the Queen Elizabeth Islands, that agrees with their geological structure. The main stages of the Amerasian Basin evolution correspond by age to unconformities that A. Embry determined in the Mesozoic strata of the Sverdrup Basin [Embry, 1991]. The work has been supported by the Russian Foundation for basic Research (Grant 01-05-65481).
GP11D-0882 0800h
Application of CM3 Model in Compilation of Marine Magnetic Anomaly Data of North Pacific
Marine magnetic data available from the NGDC have a long time span of data collection from 1950s to 2002, and external magnetic fields generally have not been removed from the data. Based on satellite and observatory data, Sabaka et al. (2002) developed a new comprehensive model, CM3, which defines not only the main field but also quiet time external magnetic fields for the period from 1960 to 2002. In this study, the CM3 model was utilized in the compilation of the NGDC marine magnetic data set in the Pacific area east of 140 E of the northern hemisphere. The whole data set amounts to about 6.3 million records with a total line length of about 2.5 million nautical miles. Before applying the CM3 model, the data set was edited. Magnetic anomalies were recalculated by subtracting DGRF reference fields from observed total magnetic intensity values, then bad data were removed by checking all the cross-over errors (COEs) $>$ 300 nT. This data editing reduced total number of data records by 0.5 % (or 1% of track line length), and 10 (including 4 cruises before 1960) out of 870 cruises were excluded from the data set. In order to clarify the effect of CM3 model, two types of new magnetic anomalies were calculated by replacing the DGRF model by: (case 1) only the CM3 main field model and (case 2) both of the main and external field models. Cross-over analysis reveals that the standard deviation (SD) of COEs reduced from about 60 nT for the DGRF model to about 58 nT for the CM3 main field model. The secular variation of main field before 1970 and after 1995 is significantly different between the DGRF and CM3 models in the areas east of 160 W and north of 50 N. Utilization of the CM3 main field model is particularly effective in these areas. Application of CM3 external field model further reduces the SD of COEs to about 50 nT. The external fields have large amplitudes in the area south of 20 N and west of 180, where the magnetic equator passes. Removal of the external fields has a marked effect on reducing COEs particularly in this area: the SD of COEs reduced by about 15 nT. Application of CM3 main and external field models to data collected from various kind of research cruises makes a good compilation of magnetic anomaly map promising, although there are still many data sparse areas.
GP11D-0883 0800h
A Possibility of the Aeromagnetic Survey by a Small Unmanned Aerial Vehicles, Ant-Plane
 Magnetic surveys by helicopters and airplanes are a useful technique to estimate the geological structure under the ice sheets in Antarctica. However, it is not easy to employ this due to the transportation of the planes, logistic supports, security, and financial problems. Members of Ant-Plane Project have investigated the unmanned aerial vehicles (UAV, Ant-Plane) for the solution of the problems. Recently the aeromagnetic survey is verified by a model airplane navigated by GPS and a magneto-resistant (MR) magnetometer. The airplane (Ant-Plane) consists of 2m wing length, 2-cycles and 2-cylinder 85cc gasoline engine, GPS navigation system by microcomputer and radio telemeter system. The total weight is 15kg including 2 litter fuels, the MR magnetometer, a video camera and an emergency parachute. The speed is 130 km/h and maximum height is 2000m. The magnetometer system consists of a 3- component MR magnetometer, GPS and data logger. Three components of magnetic field, latitude, longitude, altitude, number of satellite and time are recorded in every second during 3 hours. The sensitivity of the magnetometer is 7 nT and we use a total magnetic field intensity for magnetic analysis due to unknown heading of the plane. November 2003 we succeeded the magnetic survey by the Ant-Plane at the slope of Sakurajima Volcano, Kyushu, Japan. The plane rotated 9 times along the programmed route of about 4x1 km, total flight distance of 80 km, keeping the altitude of 700 m. Consequently we obtained almost similar field variation on the route. The maximum deviation of each course was less than 100 m. Therefore, we concluded that the aeromagnetic survey in the relatively large anomaly areas can be performed by Ant-Plane with the MR magnetometer system. Finally the plane flew up 1400m with a video camera to take the photo of active volcano Sakurajima (1117m). It succeeded to take photos of craters through steam from the volcano.