GP21A-0098
The error of linear regression on demagnetizaion plots
Multi-component analyses of progressive demagnetization plots are now common routine in paleomagnetic studies. The direction of each component is found using the linear regression. An algorism formulated by Kirschvink (1980) calculating the least square direction of each component is widely used, and known as the PCA (Primary Component Analysis) method. He gives a measure of the precision of fitting as MAD (Maximum Angular Dispersion), but does not formulate the error. The statistical model that Kirschvink (1980) assumes is a set of linearly aligned points with isotropic error of a 3- dimensional normal distributon. The line that the PCA gives is that of least sum of square errors. Generality is not lost using a coordinate of the X-axis aligned to the true direction. Assuming the dispersion of the error of each point is small, the mean square of the angle between fitted lines and X-axis ({σa}2) is derived as {σa}2 = \frac{MADp2}{N}=\frac{MADs2}{N-2} where MADp and MADs denote the MAD of population and sample, respectively. The MAD that Kirschvink (1980) gives is MADs, which underestimates the dispersion when number of points are small, thus MADp(=MADs\frac{N}{N-2}) shall be used for a measure of the goodness of fitting. The 2-d normal distribution with this dispersion gives a confidence limit of significance level P as δ a P = \sqrt{-\ln(1-P)}δ a (1.73 σa when P=95%) This relationship is reproduced well by a numerical simulation.
GP21A-0099
Late Quaternary Geomagnetic Excursions at High Northen Latitudes in Marine Sediments: Reproducing Results for the Wrong Reasons?
Paleomagnetic records based on multiple sediment cores from the Arctic Ocean and Greenland Seas (e.g. Nowaczyk et al. 2003 Geophys. J. Int. 155) have suggested that several Late Quaternary geomagnetic excursions took place in this region, with the possibility of two excursions with calibrated radiocarbon ages younger than Laschamp (~40ka). During a study of the effect of the Laschamp geomagnetic excursion and the associated decrease in dipole moment on cosmogenic radionuclide production in the atmosphere, we have measured the palaeomagnetic properties of three sediment cores: JM05-31GC1 (from the Yermak Plateau), JM04-025PC2 (western Svalbard shelf) and LINK15 (Faroe-Shetland Channel). Magnetic susceptibility logs of the three cores show millennial-scale trends that can be correlated to the Dansgaard-Oeschger stadial/interstadial cycles recorded in the Greenland ice cores. Laschamp is the only geomagnetic excursion in marine isotope stage 3 that can be securely identified in the LINK15 core and it took place during D-O cycles 9 and 10, as determined from the record of magnetic susceptibility. This core was recovered from a sediment drift receiving detrital minerals from a mainly basaltic province and, therefore, the sediments contain relatively high concentrations of magnetic minerals and are characterized by stable natural remanent magnetizations. On the other hand, the two other cores were recovered from areas with a detrital source dominated by sedimentary bedrock. Thus, the sediments have a relatively low magnetic mineral concentration and relatively weak natural remanent magnetizations. In the two cores from the Arctic, a few apparent excursions were revealed by the application of successively stronger alternating field demagnetization steps. Thus, we were able to reproduce the results of Nowaczyk et al. (2003). However, we discovered that many of the apparent excursions in our cores, which are also characterized by low relative paleointensity estimates, were artifacts caused by the acquisition of anhysteretic remanent magnetizations perpendicular to the final demagnetization (vertical geographic) axis by samples with weak natural remanent magnetizations. We cannot claim that the earlier published reports of excursions were due to a similar systematic laboratory magnetization; however, we do not find conclusive evidence of an excursion younger than Laschamp (e.g. Mono) in these relatively high-resolution sediment records.
GP21A-0100
Effect of hydrothermal alteration on rock magnetic properties from basalts in the Krafla geothermal field, Iceland
The high-temperature Krafla geothermal field is situated within the caldera of the Krafla central volcano in NE Iceland. The last fissure eruptions (Krafla fires) occurred between 1975 and 1984. Aeromagnetic surveys from this area indicate a magnetic high corresponding to Mt. Krafla, whereas the magnetic low coincides with the caldera bottom where the Krafla geothermal field is located. The geothermal fluids are meteoric in origin and the Sudurhlídar field is boiling from depth until the surface. The permeability is higher in vertical than in horizontal profiles and the production of secondary minerals suggests a depth zonal distribution related to the temperature. The study of the magnetic properties of volcanic rocks affected by hydrothermal alteration is significant to understand magnetic anomalies related to MORB and its tectonic implications. Our study focuses in an area where the hydrothermal alteration diminishes the Ti-magnetite content of fissure subaerial lavas. The samples were taken from KH1 (200 m depth) and KH3 (400 m depth) drill cores, from the rim of the caldera. In our study we aim to correlate both, c-T curves and textural observations from the magnetic phases with the degree of hydrothermal alteration. NRM, field dependence of susceptibility (Fd) and Koenigsberg ratios (Q) from the samples are very low: NRM is < 3.1 A/m, Fd values range between 0.2 and 7.9, and Q between 0 and 6. Magnetic susceptibility varies with the magnetic mineral content. Typical textural features are shrinkage cracks from maghemitization together with exsolved textures in Ti-magnetite from high temperature oxidation. This texture is present in the deeper part of both cores (177 m in KH1 and 380 m in KH3), but KH1 samples show abundant ghost structures of Ti-magnetite, altered to a network formed by clays and Ti-oxide. A high quantity of sulphide precipitation accompanies the ghost structures. The magnetic phases strongly alter depending on the porosity of the rocks, but the alteration profile seems not to be dependent with depth. A higher degree of dissolved textures of Ti-magnetite is seen in KH1, which seems to be strongly affected by the recent activity in the geothermal field.
GP21A-0101
Late Cretaceous-Early Tertiary remagnetization recorded on sedimentary rocks in the Jinan Basin, Korea
In order to decipher the tectonic evolution of the Korean Peninsula, a paleomagnetic study has been carried out for the Cretaceous rocks in the Jinan Basin, southwestern part of the Ogcheon Metamorphic Belt (OMB), Korea. Demagnetization of 1089 independently oriented specimens successfully isolated characteristic remanent magnetization (ChRM) as a secondary overprint. A suite of rock magnetic experiments revealed that fine-grained hematite, magnetite and/or pyrrhotite are remanence carriers. Two distinctively different paleomagnetic poles were obtained: 111.8°E and 87.9°N ( A95 = 3.5°) for Dalgil and Sansudong formations; and 181.7°E and 80.5°N ( A95 = 3.1°) for Mandeoksan Formation. These pole positions lie fairly close to the Late Cretaceous and Tertiary pole, respectively, indicating a back-to-back remagnetization in Late Cretaceous and in Early Tertiary. During this period, igneous activities (Bulguksa Disturbance) had been pervasive throughout the Korean Peninsula due to a change in subduction-regime of the Kula-Pacific plate. As in previously reported sedimentary rocks in the OMB, ChRMs are overprinted by authigenically formed hematite that precipitated from iron-bearing hydrothermal fluids. It is likely that authigenic hematite had been formed during the early stage of igneous activity. On locking of Late Cretaceous paleofields by secondary hematite, the steady heat from the igneous body possibly generated secondary pyrrhotite and magnetite by consuming pyrite and abundant organic materials.
GP21A-0102
Modelling the Hysteresis of Interacting Pseudo-Single-Domain Magnetite Particles
Recent studies have shown the importance of fine magnetite exsolution structures as remanence carriers in igneous rocks. These structures often form arrays of tightly spaced individual grains straddling the single domain (SD) to pseudo-single-domain (PSD) threshold. Due to the proximity of neighbouring particles, magnetostatic interactions are expected to play an important role for the magnetic properties of these particle assemblages. We have used an unconstrained, fully three dimensional finite element (FE) micromagnetic model to calculate hysteresis curves of 3x3 arrays of magnetite particles with aligned easy axes with grain sizes r=50 … 200nm and grain spacings d=0 … 3· r . The calculations show that the domain state of individual particles is not only dependent on their grain size but also on the grain separation. Closer d generally leads to an increased SD-PSD size threshold. Associated with that is a characteristic change in magnetic stability: For d<50nm MRS/MS decreases sharply for all modelled grain sizes. This decrease is related to the appearance of supervortex structures at zero external field. Our modelling approach allowed us to observe the formation of these supervortices in the course of a hysteresis cycle. As expected, SD sized particles retain a uniform particle magnetisation throughout the whole hysteresis cycle. The particle arrays, however, form intermediate supervortex structures even for relatively large d because the individual particles' magnetisation cannot collapse into a vortex state to reduce its magnetic stray field. Only when the spacing increases to 150nm (i. e. d=3· r), does the magnetisation of all particles in the array rotate coherently. In contrast, PSD particle arrays generally reverse their magnetisation by spin curling and the formation of individual vortex states. For almost touching particles, though, an SD like magnetisation structure of individual grains is maintained throughout the hysteresis cycle by forming supervortex states. The individual vortex state, however, is energetically more favourable as soon as d is only slightly increased. The paper will discuss the particle size and spacing dependence of the average interaction field and its influence on magnetic grain size determinations.
GP21A-0103
An improved algorithm for calculating first-order reversal curve (FORC) distributions using locally-weighted regression smoothing
First-order reversal curves (FORCs) are a powerful method for characterizing the magnetic hysteresis properties of natural and synthetic materials, and are rapidly becoming a standard tool in rock magnetic and paleomagnetic investigations. Here we describe a modification to existing algorithms for the calculation of FORC diagrams using locally-weighted regression smoothing (often referred to as loess smoothing). Like conventional algorithms, the FORC distribution is calculated by fitting a second degree polynomial to a region of FORC space defined by a smoothing factor, N. Our method differs from conventional algorithms in two ways. Firstly, rather than a square of side (2N+1) centered on the point of interest, the region of FORC space used for fitting is defined as a span of arbitrary shape encompassing the (2N+1)2 data points closest to the point of interest. Secondly, data inside the span are given a weight that depends on their distance from the point being evaluated: data closer to the point being evaluated have higher weights and have a greater effect on the fit. Loess smoothing offers two advantages over current methods. Firstly, it allows the FORC distribution to be calculated using a constant smoothing factor all the way to the Hc = 0 axis. This eliminates possible distortions to the FORC distribution associated with reducing the smoothing factor close to the Hc = 0 axis, and does not require use of the extended FORC formalism and the reversible ridge, which swamps the low-coercivity signal. Secondly, it allows finer control over the degree of smoothing applied to the data, enabling automated selection of the optimum smoothing factor for a given FORC measurement, based on an analysis of the standard deviation of the fit residuals. The new algorithm forms the basis for FORCinel, a new suite of FORC analysis tools for Igor Pro (www.wavemetrics.com), freely available on request from the authors.
GP21A-0104
En Bloc vs. Standard Tilt-tests and the Problem of Initial Dip in Paleomagnetic Studies.
Tilt-tests are standard paleomagnetic tests used to determine if the time of acquisition of remanence predated or postdated the tilting of strata. Partial untilting ('unfolding') tests seek to establish if remanence acquisition occurred during deformation of the strata. These kinds of tests assume that 1) tilting involved a rotation about the line of strike and 2) that the stratification (bedding) was horizontal at the time of deposition. If either one of these premises is incorrect, then incorrect conclusions may be drawn from the results of the tilt test. Although sedimentary environments on continental platforms or at abyssal depths tend to be dominated by quasi-horizontal strata, volcanic deposits commonly exhibit significant initial dip. This study explores initial dip effects in the early Tertiary Caetano Formation of Nevada, and compares en bloc tilt correction results with those of the standard tilt-test. Inferences about the degree of initial dip present can be drawn from the results. Caveats relevant to initial dip and the tilt-correction are summarized.
GP21A-0105
Magnetic Studies of coastal Early-Late Cretaceous mafic dike swarms from São Paulo State (Brazil): Preliminary Results.
The Mesozoic magmatism in Southern Brazil is represented mainly by the basaltic flows of the Serra Geral Formation, the dikes swarms from the Ponta Grossa Arch, Florianópolis, along the coast between São Paulo and Rio de Janeiro, and several alkaline complexes that lie along tectonic features associated with the evolution of the Paraná Basin. The emplacement of the dike swarms and the alkaline complexes is related to the processes of separation between South America and Africa and, consequently, with the opening of Atlantic Ocean. The studied swarms occur along the coast in the city of São Sebastião (NE of São Paulo State), and crosscut Archean and Proterozoic poilimetamorphosed rocks of the Costeiro Complex. The dikes are basaltic and alkaline (lamprophyric) in composition, and they crop out side by side in the beaches. They range from a few centimeters up to 2 m wide. Their trend is predominately N40-50E with vertical dips. Magnetic studies were performed in 32 dikes with both basaltic and alkaline compositions. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock magnetism properties indicate that pseudo-single-domain grains of almost pure magnetite carry the magnetic fabrics. In addition, paleomagnetic directions are also being determined. The analysis at the individual-dike scale defines three fabrics for the dikes according to the eingenvector orientations with respect to the dike plane, called normal, intermediate and inverse fabrics. Normal AMS fabric acquired during magma flow is dominant, and its Kmax-Kint plane is parallel to the dike plane while the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination permitted to infer that the dikes were fed by horizontal (Kmax < 30°), inclined (30° < Kmax< 60°), and vertical (Kmax > 60°) flows independent of their ages and composition. Intermediate and inverse AMS fabrics occur in a few dikes, and no direct relation with magnetic mineralogy and chemical composition of the dikes and the fabric type was observed.
GP21A-0106
Paleomagnetic Data from Tertiary Mafic Dikes and Pseudotachylites from the Sierra Ladrones, a Spatially Focused, Large-Magnitude Extension Rift-Flank Uplift, Rio Grande Rift, New Mexico: Implications for Footwall Exhumation
The Sierra Ladrones (SL) are a fault-bounded, isolated, uplift of Proterozoic rocks reaching over 2700 m in elevation located at a transition in the central Rio Grande rift (RGR), where it widens around the southeast Colorado Plateau (CP). Rapid, Neogene uplift of the SL may reflect a structurally complex, shear couple allowing conversion of a salient of the essentially unextended southeast CP from the highly extended RGR at this latitude. The Ladron footwall block, which comprises most of the SL, is bound to the north and east by the east-dipping low-angle (20 to 30° dip) Jeter normal fault, to the south by the sinistral oblique-slip Cerro Colorado fault and to the west by the steep, east dipping, (presently) reverse sense Ladron fault. The SL is interpreted as an isostatic uplift with west-side down tilting due to unloading of ~6 km of hanging wall rock along the Jeter fault and shallowing of this fault from an original moderate dip of ~48° to a low angle of ~20°. Apatite fission track (AFT) study of Proterozoic granitoids from the eastern and central SL yield dates from 9±2 to 14±2 Ma, indicating rapid, denudation controlled exhumation from depths of about 3 to 4.5 km based on the ~110°C annealing temperature for apatite. Two samples from the northwest side of the SL have less well-defined AFT dates of 32±12 and 19±10 Ma but may support a hypothesis involving appreciable west-side down tilting. To further test this hypothesis five mafic dikes and 12 pseudotachylite veins that cut Proterozoic rocks were sampled for paleomagnetic data in the SE corner of the SL footwall near the trace of the Cerro Colorado fault. These dikes trend NW to ENE and their dips range from steep to shallow. Most Tertiary (dominantly Oligocene) mafic dikes in central New Mexico strike north-south and are near vertical, thus sub-perpendicular to the RGR least principal stress axis. The off-vertical orientations of dikes in the SL and other nearby footwalls have been interpreted to be the result of post-emplacement west-side down tilting. The five dikes sampled yield a mean (D=358°, I=59°, α95=7.1°, κ=146; all normal polarity). Ten of the 12 pseudotachylite veins yield dual polarity paleomagnetic directions parallel to dike directions. Data from both dikes and pseudotachylites are statistically indistinguishable from mid-Cenozoic and younger NA reference directions and do not support significant structural reorientation following pseudotachylite and dike emplacement and remanence acquisition in this part of the SL. An alternative explanation for this relatively limited data set from the mafic dikes is that they are relatively old (i.e. Oligocene) and have been tilted after remanence acquisition by some 30° (west side down) and then the entire southwest part (if not all of) the Sierra Ladrones has been rotated in a counterclockwise fashion by about 20 to 30°. Further paleomagnetic work, as well as 40Ar/39Ar age dating of the dikes, together with apatite (U- Th)/He thermochronology of the Proterozoic rocks are underway to define the youngest phase of uplift.
GP21A-0107
Reflected Light Microscopy, SEM and Rock Magnetic Characterization of Magnetic Minerals Through an Intact Sequence of Oceanic Crust, IODP Hole 1256D
The identification of magnetic minerals throughout a complete oceanic crustal section yields important information about the carriers of the marine magnetic anomalies, one of the longest continuous archives of the behavior of the geomagnetic field. A study involving the determination of the magnetic hysteresis parameters, Curie temperatures, IRM acquisition curves, magnetic susceptibility, and the microscopic identification of magnetic minerals of samples from sections recovered during drilling of IODP Leg 206, Expedition 309 and 312. Identification of magnetic minerals was performed by reflected light microscopy and scanning electron microscopy and by classifying the oxidation state of titanomagnetites according to the oxidation stages of Haggerty (1976, 1991). The composition of Fe-Ti oxides has been determined semi-quantitatively by energy dispersive X-ray analysis (EDX). Analyses on these sections confirm that a number of downhole variations exist in the magnetic properties which can be attributed to variations in the composition and alteration state of the magnetic mineral phases. From the top of 1256D, the sheeted flows and massive basalts (Leg 206 and Exp. 309) have cruciform/dendritic titanomagnetite grain shapes typical for MORBs. There is huge variability in grain sizes from <1μm up to 100μm in this part of the section. The degree of alteration roughly increases with depth. This finding is mainly evidenced by microscopically visible shrinkage cracks that indicate a transition from titanomagnetite to titanomaghemite and by the continuously increasing Curie temperatures. Primary sulphides are present and in some cases are in contact with titanomagnetites. The magnetic minerals in these sections are likely to carry primary magnetization. In the sheeted dike section (Exp. 309), grain shapes are more ideomorphic, reflecting earlier stage crystallization and possibly a change of the primary composition of Fe-Ti- oxides. In some cases oxy-exsolution is seen with occurrences increasing with depth. An increase in the degree of alteration is evident by the kind of mineral replacement seen in these rocks (titanomagnetite - titanomaghemite-rutile/titanite/magnetite). Secondary sulphides are present in veins in addition to the primary sulphides found. In the plutonic section (Exp. 312), the gabbros contain massive titanomagnetite grains up to 2-3 mm in size. These grains are frequently oxy-exsolved to magnetite and ilmenite with the ilmenite lamellae being partially to totally replaced by secondary minerals like sphene and/or rutile. Hemoilmenite grains are also present as discrete crystals. In some cases, much smaller magnetite grains around 1μm in size are exsolved from silicate minerals. Sulphides are ubiquitous in the plutonic rocks. The present level of oxidation exhibited by the titanomagnetite grains in this section ranges from stage C2 to C3. The Curie temperature throughout the sheeted dikes and gabbroic units lies between 550 and 580°C confirming the presence of Ti-poor titanomagnetite either as the primary product of Fe-Ti-oxide crystallization or as a result of oxyexsolution. In both cases, the present composition would have been established during initial emplacement or shortly thereafter.
GP21A-0108
Magnetic Investigation of Ancestral Puebloan Rio Grande (New Mexico) Glaze Wares
In geologically heterogeneous regions, such as the Rio Grande, archaeologists typically rely on petrographic analyses to determine ceramic provenance and reconstruct prehistoric trade patterns. Even in these regions, other methods are useful for elucidating trade patterns and/or resolving ambiguities from the petrographic data. Magnetic properties of Ancestral central Rio Grande Puebloan ceramics are being acquired to assess their use in identifying provenance, trade patterns, composition, manufacturing techniques, and firing conditions of ceramics, before and during the early European contact period (ca. A.D. 1325-1700) in New Mexico. Similar to the study of Moskowitz et al. (1987), we use a combination of bulk susceptibility, NRM, ARM, and SIRM intensity, AF response by NRM, ARM, and SIRM, thermal demagnetization of NRM and SIRM, and coercivity of remanence, to study temporal change in Rio Grande glaze wares from four archaeological sites in the northern Rio Grande (approximately 90 sherds per site). Rio Grande glaze wares were widely traded among Ancestral Puebloan groups before and during the European contact period. The ceramics are from the two earliest Spanish administrative centers in New Mexico, San Gabriel del Yungue and Palace of the Governors, and two mission pueblos, Pecos Pueblo and San Marcos Pueblo. Magnetic property data are being compared with petrographic observations to test the effectiveness of several magnetic measurements to identify, among other things, ceramic provenance. A tentative observation in our study is that bulk susceptibility values correlate with different ceramic provenances. The mean bulk susceptibility values for Galisteo Basin ceramics, tempered with augite monzonite and hornblende latite, are significantly higher (5.56E-04 and 4.91E-04 SI mass, respectively) than those for Pajarito Plateau ceramics, tempered with glassy tuff, tuff rocks, and andesite, (1.79E-04, 2.53E-04, and 2.58E-04 SI mass, respectively). This study is the first of its kind on Native American ceramics.
GP21A-0109
The effect of surface roughness on the hysteresis properties of single-domain and pseudo- single-domain grains of magnetite.
Numerical micromagnetic modelling has provided huge advances in our understanding of the stability of paleomagnetic remanences in both single domain (SD) and pseudo-single-domain (PSD) grains. In mineral magnetism, the numerical model has usually employed finite difference (FD) method, which allows simulation of magnetic domain structures in relatively large grains (up to one micron). The FD models are constructed from regularly shaped cubic cells, and so the efficiency of computation is made at the expense of the accuracy with which a grain's geometry can be represented. However, one of the most important factors that affect the domain state, and the stability of the paleomagnetic recording, is the internal demagnetizing field. The demagnetizing field is in turn determined by both the grain size and its geometry. By taking a finite element/boundary element (FEBE) approach where the grain geometry is represented by arbitrary shaped tetrahedral elements, a much better representation of the grain geometry can be achieved. Thus a much more accurate determination of the demagnetizing field is possible. The FEBE approach, therefore, allows us to examine, for the first time, the influence of irregular grains shapes and in particular the grain surface roughness, on the nucleation of domain states. This initial study will look at the effect of surface roughness on spherical grains of magnetite between 30nm to 100nm in diameter, covering the SD to PSD grain size range. Spherical grains are chosen in order to eliminate any influence of grain shape other than the surface roughness. In addition, the effects of magnetocrystalline anisotropy are ignored. The roughness is defined both in terms of the average amplitude of the surface peaks above the mean diameter of the sphere, as well as the mean angular frequency of surface the peaks and troughs. The results demonstrate that, as expected, rough surfaces act to encourage nucleation of domain reversals in the SD grain size range. Thus coercivities will be greatly reduced. However it is observed that in the PSD grain size range the irregular internal demagnetizing field produces non-symmetric vortex domain states, which have a much higher coercivity and saturation remanence. The implication is that that while the truly SD grain size range maybe smaller, the PSD size range is capable of contributing a much larger and stable remanence than previously thought.
GP21A-0110
Paleomagnetic Data Bearing on the Eastern and Southern Boundaries of the Walker Lane Belt Transfer System
In west-central Nevada, a transfer zone, which initiated in the mid-Miocene, presently links, via the Mina Deflection, right-lateral faults of the Eastern California Shear Zone to the south and the Central Nevada Seismic Belt and Walker Lane to the north. This transfer zone, the early inception of which is characterized by moderate (20-30°) clockwise crustal rotations previously identified (e.g., Candelaria Hills and surrounding ranges), along with right-lateral structures to the south and north, are part of a diffuse zone of intracontinental deformation that accommodates some 25 percent of the motion between the Pacific and North American plates. Although the northern and western boundaries of the transfer zone are relatively well defined by paleomagnetic data, the eastern and southeastern boundaries remain poorly constrained. Additional paleomagnetic data are being obtained from mid-to-late Tertiary volcanic rocks, presumably lying within (e.g., Montezuma Range, Palmetto Mountains, Monte Cristo Range) and outside (e.g., Goldfield Hills, San Antonio Mountains, Slate Ridge) of the transfer zone. Areas outside of the transfer zone are inferred to have not undergone any appreciable rotation since its inception. Volcanic rocks as well as shallow intrusions ranging in age from Oligocene to mid-Pliocene have been sampled (N=187) from inside and outside of the inferred southern and eastern boundaries of the transfer zone. Overall, the collection responds very favorably to progressive demagnetization; initial results are tentatively interpreted as suggesting the absence of appreciable rotation of the San Antonio Range (Tonopah, Nevada area and farther north). The extent to which areas near the eastern and southeastern boundaries have been rotated is under investigation. These data will aid in a better understanding of differential block rotation and tilting throughout the development of the west-central Nevada transfer system from the mid-Miocene to late Pliocene.
GP21A-0111
The Magnetic Properties and Fabric of Granulite Facies Deep Crustal Rocks in the Mount Hay Region, southeastern Arunta Block, Central Australia
The granulite-facies deformation in the Mount Hay region of the Arunta Inlier in central Australia occurred at 816±27°C and approximately 800 MPa. Mount Hay is comprised primarily of mafic granulite plus lesser amounts of charnockitic, porphyroclastic, and quartzofeldspathic granulite. Magnetic fabric of these variably deformed, deep crustal rocks provide insights into the deformation processes occurring within high grade metamorphic shear zones. Samples were collected along a north-south trending 5.5 km traverse. Magnetic measurements include anisotropy of magnetic susceptibility (AMS), high and low temperature susceptibility measurements, saturation remnant magnetization as a function of low temperature, and hysteresis loops. AMS studies in the Mount Hay region show S>L to S>>L (stronger foliation than lineation) for mafic and porphyroclastic granulites, while S<L for charnockitic and quartzofeldspathic granulite. Hysteresis data suggest the room temperature magnetic grain size of all rock types are pseudo-single-domain to multidomain. The mafic granulites generally have the lowest coercivity of remanence to coercivity ratio, indicating a finer magnetic grain size, typically pseudo-single- domain. Low-temperature saturation isothermal remanent magnetization yielded Verwey transition temperatures ranging from 111-123 K, suggesting the magnetite is in some cases slightly oxidized or contains impurities. A weak correlation between lower Verwey transitions and lower coercivity of remanence to coercivity ratio is consistent with oxidation due to a larger surface area to volume ratio for finer grain magnetite grains. The dominant Curie temperatures are approximately 567°C, typical of magnetite with minor cation substitution. Most samples exhibit a secondary ordering transition of ~330°C and a 35 K low-temperature saturation isothermal remanence transition during warming which are both characteristic of pyrrhotite. AMS ellipsoid principle directions correlate well with the field orientations for lineation and foliation in the quartzofeldspathic granulites. The relationship between AMS and field fabrics is more variable for other rock types. This study suggests that multiple phases may contribute to the AMS of deep crustal rocks and that magnetic techniques may be a powerful tool to quantify variations in their deformation. Differences between AMS and field fabrics may be the result of variable contributions from multiple phases or that different lithologies record different strain events or that different rocks deform by different mechanisms.
GP21A-0112
Paleomagnetism of the Stanislaus Group, CA reveals revised stratigraphy, Walker Lane kinematics, and radio-isotopic constraints on C5 magnetic subchrons
Paleomagnetic study of the c.a. 9.2-10.3 Ma Stanislaus Group of intercalated latite (trachyandesite) lavas, ignimbrites and accessory sediments at three localities in Mono county California reveals: 1) a detailed, revised stratigraphy for the Stanislaus group, 2) kinematic constraints on the part of the Walker Lane since Stanislaus group emplacement, and 3) two age-constrained magnetic subchrons during chron C5N recorded by latites that had previously only been identified in seafloor magnetic anomalies. The revised stratigraphy results from detailed magnetostratigraphy combined with previous 40Ar/39Ar geochronologic constraints and stratigraphic studies. We find the lowermost unit, Table Mountain Latite, to consist of 23 or more individual lava flows falling into 5 magnetic (mostly normal) polarity zones, indicating that these rocks span at least 40,000 years of geologic time, based on the expected duration of magnetic reversals. Overlying Table Mountain Latite is the reversed-polarity Tollhouse Flat member of the Eureka Valley Tuff as described by previous authors. In the Sweetwater Roadless Area, thought to be proximal to the Stanislaus eruptive center, latite lava of both normal and reversed polarity are emplaced ontop of the Tollhouse Flat Member. Normal-polarity By Day member and normal-polarity Upper Member lie at very top of entire sequence. We find no field evidence for the normal polarity Dardanelle Formation latite flow at the top of the Group as had been previously reported by other workers. Instead, the Dardanelle formation member likely corresponds to the latite lava(s) between the By Day and Tollhouse Flat Eureka Valley Tuff. Based on previous 40Ar/39Ar dating, the two reversed zones within our magnetostratigraphy correspond to two of the proposed reversed subchrons/excursions during chron C5N. Direct dating of these reversed units may lead to future improvements to the magnetic polarity timescale for C5N. Our paleomagnetic results from three study localities also reveals kinematic constraints for the boundary between the Sierra Nevada and central Walker Lane belt. The section at Sonora Peak, CA, within the stable Sierra Nevada microplate, yields a locality mean direction for these rocks indistinguishable from the expected direction for the late Miocene. The two other study localities, Grouse Meadow and Burcham Creek, lie within the western Walker Lane Belt, east of the Sierra Nevada range front fault zone. Results from these latter localities reveal 10°-20° of vertical axis rotation of fault-bounded blocks of the western Walker Lane since emplacement of the Stanislaus Group. These results are obtained relative to a variety of datums – by averaging secular variation, by comparing locality mean directions from one place to another, and by comparing unit mean directions for the extensive e ignimbrites from one place to another. These results allow explicit delineation of the kinematic boundary between stable Sierra Nevada and the Walker Lane and geometrical calculation of total strain accommodated by these rotations.
GP21A-0113
Paleomagnetic Investigation of the Late Carboniferous-Early Triassic Sedimentary Rocks in the Yeongnam Massif, Korea
A paleomagnetic study has been conducted for the Upper Carboniferous-Lower Triassic Pyeongan Supergroup exposed in the Danyang area on the Yeongnam Massif, Korea to provide a clue to local and regional tectonic problems in Korea. Among the total of 475 samples from 41 sites of five formations, the characteristic remanent magnetization (ChRM) components were obtained from 263 samples (31 sites) of three formations (the Late Carboniferous Manhang Formation, the Early Permian Jangseong Formation and the Earth Triassic Donggo Formation). Tilt tests and reversal test in this study reveal that the ChRM of the Manhang and Donggo formations are primary in origin while the Jangseong Formation was completely remagnetized during the Early Tertiary. The Late Carboniferous and the Early Triassic paleomagnetic pole positions calculated from the site-mean directions of the primary magnetizations in this study are at 39.9°N, 2.2°E (A95 = 3.5°) and at 40.7°N, 17.8°E (A95 = 5.3°), respectively. These poles are close to the contemporary poles of the NCB rather than to those of the SCB, suggesting that the Yeongnam Massif has been connected to the NCB. Paleomagnetic paleolatitudes (1.5°N for the Manhang Formation; 10.2°N for the Donggo Formation) indicate that the Yeongnam Massif has moved northward from the proximate of the equator during Late Carboniferous-Early Triassic times. Paleomagnetic directional results in this study also yield an important interpretation on the local geologic problem in the northern part of the Yeongnam Massif. The Pyeongan Supergroup distributed in the Danyang area and the Taebaek area (Baekunsan Syncline) forms a curved geometry (e.g., arcuate belt). The paleomagnetic declinations of the primary remanences in the Danyang area are rotated counter-clockwise by about 50-60° from those in the Taebaek area. The change in the paleomagnetic declination correlates closely with the variation in structural trend (i.e., strike of the strata) between two areas. This result suggests that the curvature is a secondary feature due to relative vertical-axis rotation between the Danyang and Taebaek areas from the originally straight configuration.
GP21A-0114
Uploading, Searching and Visualizing of Paleomagnetic and Rock Magnetic Data in the Online MagIC Database
The Magnetics Information Consortium (MagIC) is commissioned to implement and maintain an online portal to a relational database populated by both rock and paleomagnetic data. The goal of MagIC is to archive all available measurements and derived properties from paleomagnetic studies of directions and intensities, and for rock magnetic experiments (hysteresis, remanence, susceptibility, anisotropy). MagIC is hosted under EarthRef.org at http://earthref.org/MAGIC/ and will soon implement two search nodes, one for paleomagnetism and one for rock magnetism. Currently the PMAG node is operational. Both nodes provide query building based on location, reference, methods applied, material type and geological age, as well as a visual map interface to browse and select locations. Users can also browse the database by data type or by data compilation to view all contributions associated with well known earlier collections like PINT, GMPDB or PSVRL. The query result set is displayed in a digestible tabular format allowing the user to descend from locations to sites, samples, specimens and measurements. At each stage, the result set can be saved and, where appropriate, can be visualized by plotting global location maps, equal area, XY, age, and depth plots, or typical Zijderveld, hysteresis, magnetization and remanence diagrams. User contributions to the MagIC database are critical to achieving a useful research tool. We have developed a standard data and metadata template (version 2.3) that can be used to format and upload all data at the time of publication in Earth Science journals. Software tools are provided to facilitate population of these templates within Microsoft Excel. These tools allow for the import/export of text files and provide advanced functionality to manage and edit the data, and to perform various internal checks to maintain data integrity and prepare for uploading. The MagIC Contribution Wizard at http://earthref.org/MAGIC/upload.htm executes the upload and takes only a few minutes to process tens of thousands of data records. The standardized MagIC template files are stored in the digital archives of EarthRef.org where they remain available for download by the public (in both text and Excel format). Finally, the contents of these template files are automatically parsed into the online relational database, making the data available for online searches in the paleomagnetic and rock magnetic search nodes. During the upload process the owner has the option of keeping the contribution private so it can be viewed in the context of other data sets and visualized using the suite of MagIC plotting tools. Alternatively, the new data can be password protected and shared with a group of users at the contributor's discretion. Once they are published and the owner is comfortable making the upload publicly accessible, the MagIC Editing Committee reviews the contribution for adherence to the MagIC data model and conventions to ensure a high level of data integrity. http://earthref.org/MAGIC
GP21A-0115
Magnetic Mineralogy, AMS, and Paleomagnetism of the mid-Tertiary Three Peaks Laccolith, Iron Axis Province, Southwest Utah
The Three Peaks Laccolith is part of the mid-Tertiary Iron Axis magmatic province, defined by several shallowly emplaced, intermediate to silicic composition intrusions in southwest Utah. The intrusion is exposed over about 3.5 km 2, and crops out about 30 km west of Cedar City. The laccolith is nearly 100 percent exposed. New AMS and paleomagnetic data from 24 sites (13 samples per site), distributed across the eastern-most part of the intrusion within the interior and selvage joint zones of the intrusion complement existing paleomagnetic/AMS data from 47 sites. Rock magnetic experiments fully characterize the magnetic mineralogy, domain states, grain size distributions, and mineral composition of the rocks. The ChRM of the intrusion is defined by essentially two types of demagnetization responses, yet both yield northwest declination (300 to 320°) and moderate positive inclinations (40 to 50°). Demagnetization behavior between sites is extremely varied. For about half of the sites, AF demagnetization isolates well-defined/grouped magnetization. For the other sites thermal demagnetization revealed laboratory unblocking temperatures below about 560 C; reflected light petrography shows that these very high coercivity sites are dominated by hemoilmenite/ ilmenohematite intergrowths. Both magnetic assemblages yield indistinguishable remanence directions, suggesting rapid magnetization acquisition relative to paleosecular variation. AMS results reveal an orderly pattern of both oblate and triaxial fabrics. The magnetic foliations generally follow the contacts with the country rock and dip moderately to the northwest on the west while on the east foliations dip moderately to the southeast. We argue that the intrusion was emplaced as a moderately dipping sill from the northwest to the southeast. The AMS fabric data do not appear to show any dependence on the principal magnetic phase present at the site level. Preliminary high temperature hysteresis and susceptibility experiments yield very narrow laboratory curie point estimates between 546 C to 551 C consistent with hemoilmenite and ilmenohematite as a principal magnetic phase. We are currently reducing the remaining rock magnetic data (high - low temperature hysteries, low-temperature susceptibility, and high-field susceptometer experiments) and should be able to fully characterize the magnetic properties of all rock types in order to define the exact mineralogy and magnetic grain size of the iron oxides and better understand the AMS and paleomagnetic data.
GP21A-0116
Electron Holography and Magnetic Properties of Exsolved Synthetic Titanomagnetites
Natural titanomagnetites frequently exhibit nano-scale exsolution structures, but the contribution to bulk magnetic properties of these assemblages of iron-rich ferromagnetic titanomagnetites and titanium-rich paramagnetic ilmenite and ulvospinel is poorly understood. To investigate the role of these two-phase systems, a suite of single-phase polycrystalline and single-crystal samples were synthesized from the magnetite-ulvospinel solid solution. They were characterized using magnetic properties and subsequently resintered under oxidizing conditions to create two-phase systems of titanomagnetite and ilmenite. These systems exhibited a variety of geometries depending on the oxygen fugacity and temperature of the oxidizing run, including lamellar exsolution which mimics that found in natural magnetite-ulvospinel assemblages. These exsolved samples were examined using electron holography to quantitatively measure the magnetic flux in and around the magnetic grains and the collective behavior of interacting magnetic grains. A more thorough understanding of these interaction effects and the domain states of the magnetic grains can inform our interpretation of both remanence measurements and bulk rock properties.
GP21A-0117
Paleomagnetism and Magnetostratigraphy of the Upper Triassic Chinle Group in North- Central, Western and Eastern New Mexico
The Upper Triassic Chinle Group spans most of the Late Triassic and was deposited by a large scale fluvial system. Chinle Group strata are composed of predominantly red to purple mudstones with lesser orange siltstones and buff to red sandstones. In the Chama Basin, north-central New Mexico, both lower and upper Chinle strata are well-exposed and sections at several localities have been sampled to develop a more complete magnetic reversal chronology for the Late Triassic of the American Southwest. Localities in eastern and west- central New Mexico were also sampled for comparative purposes. Sampling at all sections concentrated on hematitic mudrocks and these materials typically carry a well-defined, well-grouped, dual polarity magnetization dominated by pigmentary hematite with laboratory unblocking temperatures about 660C we interpret as a primary, Late Triassic remanence (e.g., Painted Desert Mbr., corrected grand mean: D = 182.9°, I = 4.4°, α95 = 2.9°, k = 61.7, N/No = 40/45 sites (14 levels of N polarity, 26 levels of R polarity). The Shinarump Formation (lowest unit of the Chinle Group) failed to yield interpretable magnetizations. The Salitral, Poleo, Petrified Forest and Rock Point formations all yielded magnetizations with either south or north-seeking declinations and shallow inclinations, comparable to those of the Painted Desert Member(e.g.: Poleo Formation grand mean: D = 183.1°, I = 0.3deg, α95 = 5.7°, k = 33.9, N/No = 20/30). Although the Chinle Group of New Mexico was sampled at a relatively coarse sampling interval, the composite, yet incomplete,magnetic reversal chronology derived from these sections compares, at a gross scale, to similar age strata from Arizona, eastern North America and the Tethyan region of southern Europe. Chinle Group strata of Carnian age (based on palynostratigraphy) are of mixed polarity. Lower Norian strata are dominantly of reverse polarity and upper Norian strata are of mixed polarity. Paleopoles calculated from averaged VGPs show very little motion along the apparent polar wander path throughout the Carnian (Salitral Fm. Pole: 56.9°N, 62.0°E, A95 = 4.8°) and early Norian (Petrified Forest Fm., Painted Desert Member: 55.3°N, 67.3°E, A95 = 3.2°), followed by a shift in the pole position in the Late Norian (Rock Point Fm.: 58.5°N, 42.4°E, A95 = 14.8deg).
GP21A-0118
Paleomagnetic Baked Contact Test of a Permo-Carboniferous Dike Intruding the Ross of Mull Granite, Isle of Mull, NW Scotland
The Caledonian Ross of Mull Granite (ROMG) forms the western-part of the Ross of Mull peninsula, NW Scotland, and consists of three roughly concentric zones that vary in composition from diorite, granodiorite, to granite. The western contact of the ROMG with Iona Group metasedimentary rocks is exposed on the skerries off the east coast of Isle of Iona. The eastern contact and floor of the intrusion is exposed on the west side of Ardalanish Bay on the south coast of the isle. Field relations suggest that magma ascent may have occurred along the Sound of Iona fault with magma flowing laterally away from the structure to the east-southeast. At least two later magmatic pulses of intermediate to mafic composition intrude the ROMG as dikes and dike swarms of late Caledonian (Silurian) to Permo-Carboniferous age. One hundred thirty-two anisotropy of magnetic susceptibility (AMS) and paleomagnetic sites were established across the ROMG in order to test the magma emplacement model. In addition, seventeen mafic dikes were sampled to evaluate post-emplacement deformation of the intrusion and to evaluate the antiquity of the magnetization of the granite. A detailed paleomagnetic baked contact test involving one of the Permo-Carboniferous dikes hosted by ROMG country rock is underway to determine if the characteristic magnetization (ChRM) of the granite is a thermoremanent magnetization acquired at time of cooling or if the ROMG was remagnetized since the emplacement of the dikes. The baked contact test includes 10 paleomagnetic samples from a lamprophyre dike to define its ChRM. The results from the dike are being compared with results from those from 22 samples collected from the ROMG at progressively farther distances from the contact with the dike. The results should provide critical information to evaluate the antiquity of the magnetization of the ROMG and aid in interpreting the paleomagnetic and AMS data. Preliminary hysteresis and susceptibility experiments on the ROMG yield very narrow laboratory Curie point estimates between 558 C to 565 C consistent with a low-Ti oxide phase (magnetite) as a principal magnetic phase. Additional rock magnetic experiments are underway and the results should allow us to fully characterize the magnetic properties of all rock types and define the exact mineralogy and magnetic grain size of the iron oxide phases.
GP21A-0119
Frequency Dependent Susceptibility Analysis of Magnetic Carriers: Application to Fe-Oxides on Mars surface
On Mars, Fe-oxides mineral phases (inferred/detected) are mainly magnetite, pyrrhotite, and hematite. Kletetschka et al., 2005 suggested that the grain size dependent potential may contribute to the Mars surface magnetic anomaly. Grain size of Fe-oxides may play a role for the magnetic signature and anomaly on Mars. According to Kletetschka et al., 2005, the larger the grain size, the larger the magnetization (in this case hematite's TRM). Weather they are magnetite, pyrrhotite or hematite, nano-phase or superparamagnetic grains may contribute to the absence of remanent magnetization on the surface of Mars. In this contribution we tackle how to resolve grain size variations by frequency dependent susceptibility measured on terrestrial hematite samples such as hemo-ilmenite from Allard Lake, Canada, Mars analogue concretions from Utah and Czech Republic, and hematite aggregates from Hawaii. The magnetic characteristics of hematite-goethite mineralogies of Utah and Czech concretions suggested (Adachi et al., 2007) that they contain super paramagnetic (SP) to single domain (SD) magnetic states. Coercivity spectra analysis from acquisition of isothermal remanent magnetization (IRM) data showed the distinct behaviors of hematite, goethite, and mixed composition of both. The estimated magnetic states are analyzed with the frequency-dependent susceptibility instrument (500-250,000 Hertz). The frequency- and size-dependent susceptibility for hematite, goethite, and magnetite are calibrated using the known size powdered (commercial) samples. http://lep694.gsfc.nasa.gov/gunther/gunther/publications.html
GP21A-0120
Development of Magnetic Fabrics in Experimental Shear Zones During Plastic Deformation
Magnetite-bearing rock analogues were synthesized and deformed in simple shear to observe the development of magnetic fabrics under metamorphic conditions. Deformation was carried out using a gas-medium pressure apparatus at elevated temperatures (500 ° C) and confining pressures around 300 MPa in order to achieve plastic deformation. Samples composed of 2-micron, high-blocking-temperature (TB > 500 ° C) magnetite in a calcite matrix were pressed and annealed under isostatic conditions so as to produce a nearly isotropic starting fabric. The samples were then given weak-field thermoremanent magnetizations (TRM) perpendicular to the shear plane and deformed below the blocking temperature of pseudo-single domain magnetite. Intensity of post-deformation magnetic anisotropy as determined by AMS and AARM increases systematically with amount of shear strain. In all deformation experiments, intermediate-coercivity overprints were acquired with variable orientations and magnitudes, and the initial magnetization direction was preserved as a stable, high-coercivity component. The original TRM therefore withstands differential stresses up to 300 MPa and indicates that no systematic rotation of the magnetite grains takes place during shear, yet the magnetic fabric ellipsoid produced by deformation is oriented oblique to the shearing direction.
GP21A-0121
Magnetic Fabric Techniques are used to Characterize Deformation of Deep Crustal Granulites of the Arunta Block Central Australia
Granulites of the Capricorn Ridge shear zone of the Arunta Inlier of central Australia record deformation at 776 ± 38 °C and 800 MPa (~ 30 km depth). The planarity and continuity of compositional bands that parallel the mesoscopic foliation vary spatially, recording variations in finite strain magnitude within the lithologically heterogeneous shear zone. As is typical of granulite facies terranes, strain markers are absent. We have investigated the degree to which magnetic foliation and lineation track strain in different rock types by comparing magnetic anisotropy to field fabrics, with the goal of quantifying strain across this deep crustal shear zone. Samples collected along a 700m transect include mafic (most common), porphyroclastic, charnockitic, and quartzofeldspathic granulites. The field foliation generally strikes ESE and dips steeply to the SW and lineation plunges steeply to the SSE in all rock types. We measured anisotropy of magnetic susceptibility (AMS) and anisotropy of remnant magnetization (ARM) to determine the shape and orientation of the magnetic anisotropy ellipsoids of these deep crustal rocks. The mafic granulites' ARM principle directions show a tighter grouping than do the same samples' AMS principle directions. The other rock types have a tighter grouping of data points with AMS; however, the difference between the ARM and AMS ellipsoids is not as substantial as that exhibited by the mafic granulite samples. The maximum AMS axes of the porphyroclastic, charnockitic, and quartzofeldspathic granulites plunge steeply to the SSE, similar to the observed field lineation. In contrast, the mafic granulites generally show a shallowly plunging, NW-trending maximum ARM direction. The mafic granulite ARM ellipsoids have a relatively consistent anisotropy orientation, but shapes range from highly oblate to triaxial to slightly prolate. The charnockite AMS ellipsoid exhibits a consistent triaxial shape. The porphyroclastic and quartzofeldspathic granulite AMS ellipsoid shape varies across the transect. The charnockites generally show the highest degree of anisotropy whereas the mafic granulites show the lowest. Field observations and magnetic fabrics are consistent with lower crustal strain localization within the charnockites relative to the mafic granulites. These observations suggest lithology-specific deformation processes, fundamentally reflecting rheology, are responsible for the magnetic fabrics in the shear zone.
GP21A-0122
Insights Into Magma Ascent During Shallow-Level Crustal Shortening From Magnetic Fabrics of the Philipsburg Batholith, SW Montana
Latest Cretaceous development of the Sevier fold and thrust belt in SW Montana overlapped spatially with silicic magmatism. In the fold thrust belt, large volumes of magma were emplaced well east of the main magmatic arc, now exposed as the Idaho Batholith. Hypothesized mechanisms for emplacement of magma within the overthrust belt often involve magma ascent along shallow, west-dipping faults. The ~ 74 Ma (K-Ar method) Philipsburg Batholith is a 122 km2 tabular granodiorite emplaced into deformed Precambrian Belt Supergroup through Cretaceous strata. The Philipsburg Batholith lies in the upper plate of the Georgetown- Princeton Thrust, NW of Anaconda, Montana and cross-cuts two other previously mapped faults. Anisotropy of magnetic susceptibility (AMS) measurements of 122 sites from the Philipsburg Batholith define magnetic foliations and/or lineations to test magma ascent along the Georgetown-Princeton Thrust. AMS fabrics in the Philipsburg Batholith, dominantly defined by magnetite, are generally oblate or triaxial and are typically very consistent at the site level. Preliminary fabric data show subhorizontal foliations across most of the batholith, with steeply dipping foliations near the margins and a minor increase in foliation dip near the inferred fault trace. The hypothesis of magma ascent along fault surfaces will be supported if further data confirm the concentration of relatively steep foliation orientations across the trace of the Georgetown-Princeton thrust.
GP21A-0123
ULF Pc 3-4 Pulsations and Their Parameters in the California Region as Measured with a Network of Search Coil Magnetometers
The common characteristics of the signal found in the 0.0025-0.025 Hz portion of the geomagnetic spectrum are presented as measured by the CalMagNet high-resolution search coil network. Coherence, azimuth ratio, and timing of wave characteristics are depicted, showing their statistical variances, and potential for revealing residual local signals. The low coherence frequency band near 0.1Hz is compared with higher coherence signals at lower frequencies. Also presented are the results of comparing the characteristics of these signals around the time of earthquakes near the sensors as compared to control periods of data during non-earthquake periods.
GP21A-0124
Magnetic Modeling of Buried Basalt Near the Potential Repository at Yucca Mountain, Nevada, Constrained by New Paleomagnetic, Rock Magnetic, and Petrographic Studies
Probability estimates for igneous disruption of the potential repository at Yucca Mountain, Nevada, are affected by uncertainties in the number and age of basaltic volcanoes possibly buried in the area. To reduce these uncertainties, the U.S. Department of Energy (DOE) sponsored studies, including a high-resolution aeromagnetic survey of the Yucca Mountain region, to identify potential sites of buried basaltic intrusions and their characteristics. The survey was conducted using a helicopter with an average sensor elevation of 40–50 m [131–164 ft] above terrain. Based on the resulting anomaly map, a subset of seven anomalies (A, G, I, JF5, JF6, O, and Q) was identified for additional testing. These seven anomaly sites were cored to determine whether buried basalt was the source of the anomalies. Basalt was encountered in four of the seven boreholes—A, G, JF5, and Q. Basalt samples from these four boreholes were collected for additional analyses, including radiometric age determinations and mineral identification. This paper reports petrographic, paleomagnetic, and geophysical modeling results from an independent review of the DOE aeromagnetic data and analysis of core samples. Experiments included measurements of (i) natural remanent magnetization, (ii) alternating field demagnetization and thermal demagnetization to isolate the inclination of the characteristic remanent magnetization,(iii) room-temperature bulk susceptibility, (iv) temperature dependence of low-field susceptibility to 700 °C [1,292 °F], and (v) hysteresis and coercivity. The paleomagnetic measurements characterized the magnetic properties of the samples and were used as input for the models. The modeling approach included development of two-dimensional forward models along optimally oriented profiles for each of the four magnetic anomalies. Magnetic source bodies were developed as geologically reasonable polygons with known or inferred magnetic properties for each forward model. Geometry of the source polygons was derived from a combination of known subsurface geologic conditions from the boreholes, extrapolation of nearby geologic structures to the subsurface, analogs to similar features in the region, and general geologic principles. Forward modeling consisted of trial and error alteration of the polygons (both properties and geometries) until the models produced a magnetic response curve that closely matched the observed magnetic profile. Care was taken to preserve aspects of the subsurface geometries that were known (e.g., depth to the top of the body based on borehole information) or were likely (e.g., projection of a nearby fault to the subsurface) and to only alter aspects of the source geometry or magnetic parameters that were uncertain (e.g., lateral dimensions of the body in the subsurface or average remanent magnetization intensity). Results from our geophysical modeling indicate that the profiles of anomalies G, JF5, and Q in the Yucca Mountain region can be modeled as two-dimensional bodies of relatively thin (10–140 m [33–459 ft]) basaltic rock. The study of Anomaly A coupled with observations from the core and from thin sections suggests that this anomaly is best modeled as a relatively thick (>500 m [1640 ft]) basaltic rock suggestive of a sill. This abstract is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the NRC.
GP21A-0125
A Geophysical Study on the Hwasan Caldera in the Euisung Sub-basin, Korea, Using Magnetotelluric Survey
Among the calderas of the southeastern region of the Korean Peninsula, the Hwasan caldera has a comparatively large spatial size and its ring fault system accompanied by forming caldera structure has been preserved well until now. Several previous gravity and magnetic surveys on this area have provided regional geophysical knowledge such as depth of basin basement and spatial distribution of volcanic rocks, but it was difficult to reveal the detailed subsurface structure of internal and marginal part of the caldera. To extend our detailed knowledge for the Hwasan caldera, we carried out magnetotelluric (MT) survey, which is pretty sensitive to electrical property variation in both horizontal and vertical direction of subsurface, across the Hwasan caldera with the direction of EW. The 2-D inversion results of observed MT data lead to following conclusions. Firstly, the depth of the basin basement inferred by the MT inversion results matches well with that suggested by previous potential studies, but the basement resistivity seems fairly low when compared to that of general case. This feature might be related with the large-scaled, highly conductive layer beneath the Euisung Sub-basin suggested by Lee (2006). Secondly, the high resistivity zones reaching to 4000 ohm-m are imaged around two external ring fault boundaries. These zones are thought of as the response of the rhyolitic dykes intruding along the ring fault, and in the previous gravity data correspond to relatively high density anomalies. Thirdly, low resistivity zone reaching to 200 ohm-m is detected around a depth of 1km beneath the central part of the caldera, which has been not yet reported in korean geophysical literatures. If we take account of the evolution model of the Hwasan caldera, this zone is regarded as the past sedimentary layer that subsided during the period of forming external ring fault system. In addition, the relatively low density anomaly observed in the central part of the caldera may be attributed to this sedimentary layer.