GP21A-0148 0800h
Anisotropy of Magnetic Susceptibility Study of Lacustrine Sediments Recovered From the Deschutes River (Pringle Falls, Oregon) Area.
We have studied a total of 827 samples drilled from five widely spaced profiles sampled along the Deschutes river Oregon . The five profiles sampled recorded a high-resolution paleomagnetic record of the Pringle Falls magnetic polarity episode (ca. 218+/- 10 ka) and are characterized by diatomaceous lacustrine sediments. This lacustrine sedimentary sequence that was sampled was part of an extensive prehistoric fluvial and lacustrine complex that formed east of the Cascade Mountains during the last 1.0 Ma. The lake appears to have resulted from a late Pliocene/Pleistocene rise in the base level to the east of the of the sampling area near the western margin of the Basin and Range structural province and is related to the development of the extensive volcanism associated with the Newberry volcano. Present outcrops and subsurface distribution of these lakes are to the west and north of the volcano, and their distribution appears to have been influenced by the development of streams and rivers systems draining the Newberry volcanic center as well as the Cascades themselves. We have conducted anisotropy of magnetic susceptibility (AMS) measurements in order to investigate if the acquisition of their fabrics during their formation was primary or secondary and also the characteristics of such fabrics. We conducted low-field vs susceptibility analysis to determine the magnetic carriers of the sediments and we found that the main magnetic carrier is pure magnetite (Curie point 575 $^{o}$C). The magnetic grain size indicated SD-PSD magnetite. The magnetic fabrics of the 5 different profiles indicated that the sediments were deposited with the minima axes (Kmin) perpendicular to the bedding plane of the sediments. The sediments are also characterized by a strongly oblate fabric that is confined to the bedding plane of the sediments, in addition since the fabric is entirely foliated without a superimposed lineation then the sediments were deposited almost on a horizontal surface and quiet and calm water conditions (current less than 1 cm/s).
GP21A-0149 0800h
The Whole AMS Matrix: Using the Owens Lake, Ardath Slump, and Gaviota Slide cores to explore classification of ellipsoid shapes
Categorizing sediment history using Anisotropy of Magnetic Susceptibility (AMS) has been a long standing challenge for the paleomagnetic community. The goal is to have a robust test of shape fabrics that allows workers to classify sediments in terms of being primary depositional fabric, deposition in with currents, or altered fabrics. Additionally, it is important to be able to distinguish altered fabrics into such classes as slumps, crypto-slumps, drilling deformation (such as fluidization from drilling mud and flow-in), and so forth. To try to bring a unified test scheme to AMS interpretation, we are using three example test cases. First is the Owens Lake OL92 core, which has provided previous workers with a long core example in a lacustrian environment. OL92 was classified into five zones based on visual observations of the core photographs. Using these groupings, Rosenbaum et al. (2000) was able to use the deflection of the minimum eigen vector from vertical to classify each individual AMS sample. Second is the Ardath Shale location, which provides a clear case of a lithified outcrop scale problem that showed success with the bootstrap eigen value test. Finally is the Gaviota Slide in the Santa Barbara Basin, which provides usage of 1-2 meter gravity cores. Previous work has focused on Flinn, Jelinek, and bootstrap plots of eigen values. In supporting the shape characterization we have also used a 95% confidence F-Test by means of Hext's statistical work. We have extended the F-Test into a promising new plot of the F12 and F23 confidence values, which shows good clustering in early tests. We have applied all of the available techniques to the above three test cases and will present how each technique either succeeds or fails. Since each method has its own strengths and weaknesses, it is clear that the community needs to carefully evaluate which technique should be applied to any particular problem.
http://schwehr.org/agufall2004
GP21A-0150 0800h
Combined Neutron Texture And Magnetic Analyses On "Undeformed" Clays: The Origin Of Tectonic Lineations In Extensional Basins
In extensional sedimentary basins it has been demonstrated that fine-grained sediments that do not appear deformed at the mesoscale can carry a magnetic fabric consistent with the regional deformation. In this study, carried out on clays from extensional basins in southern Italy, the magnetic lineation is tectonically controlled and oriented perpendicularly to the main normal faults. As the origin of such magnetic lineation is not yet well understood, we present combination of magnetic and textural analyses in order to unravel that phenomenon. A combination of low-field, high-field and low-temperature measurements was used to distinguish the ferrimagnetic and paramagnetic contribution to the magnetic susceptibility. The magnetic anisotropy of studied sediments is predominantly carried by the paramagnetic phyllosilicates. Neutron texture analysis was used to evaluate the spatial distribution of the chlorite basal planes. Results demonstrate that the orientation of the magnetic lineation is related to the spatial distribution of phyllosilicate minerals, lying parallel to the common axis of differently oriented basal planes. Independent of the chlorite basal planes distribution (from an axial symmetric to a girdle pattern in the bedding plane), magnetic lineation is always parallel to the minimum axis of the mineral fabric ellipsoid. In order to make a quantitative correlation between AMS and rock fabrics, the low- and high-field magnetic anisotropy (AMS, HFA) were compared to the theoretical anisotropy calculated from the neutron texture goniometry measurements. Quantitative correlation is presented in terms of the standard deviatoric susceptibility, k', and the difference shape factor, U, expressing anisotropy degree and shape, respectively. The degrees of the theoretical anisotropy, AMS, and HFP correlate very well implying nearly the same degree of anisotropy for all the employed methods.
GP21A-0151 0800h
Magnetic Fabrics, Rock Magnetism, Cathodo-luminescence and Petrography of ``undeformed'' Bambui Limestones from S\~{a}o Francisco basin (Minas Gerais State SE-Brazil): An Integrated Study
Magnetic fabric is a sensitive indicator of rock deformation, frequently developing before visible evidence. Apparently undeformed rocks may in fact be internally deformed. We have performed our studies on apparently undeformed limestone and carbonate shales from nearly-horizontal stratigraphic layers mainly from the basal units of the Neoproterozoic Bambui Group in the southern part of the S\~{a}o Francisco basin. Magnetic fabrics were determined applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence magnetization (AAR). The AAR tensor was less well defined than the AMS fabric, due to the low ferromagnetic mineral content. For almost all sites the AMS and AAR fabric orientations are different, mainly with respect to the lineations, and are also different from that expected for depositional-compaction fabric. Even though the magnetic foliation poles (minimum axes) are vertical and close to bedding poles the magnetic lineations (maximum axes) are very well clustered and compare favorably with the mesoscopic-scale fabrics. In general, the AMS lineation is well developed and follows the trend of the main thrusts and faults while the AAR lineation follows the trend of main regional fold axes. Rock magnetism, cathodo-luminescence, transmitted and reflected light microscopy, and X-ray diffraction analyses reveal that there is a mix of ferromagnetic minerals mainly magnetite and pyrrhotite in the majority of the sites, however, in some sites the ferromagnetic minerals are magnetite and hematite and goethite (?). Fine-grained pyrrhotite and pyrite accompany fine-grained graphite and probably amorphous carbon in some of stylolites, while pyrrhotite is present as larger interstitial masses outside the stylolite-containing areas. The magnetic fabrics of the apparently underformed Bambui limestones are typical of very weakly deformed sediments, in which the depositional-compaction fabric has been partly overprinted by a tectonic one, with minimum susceptibility direction remaining perpendicular to bedding
GP21A-0152 0800h
Magnetic Fabrics in Shear Zones from the Southern Appalachian Mountain Belt
Shear zones from the Piedmont Province in the Southern Appalachian Mountains have progressively modified a pre-existing fabric in biotite-hornblende gneisses. In the pre-deformed gneiss a sub-horizontal L-fabric of varying intensity is found. This fabric changes progressively to an S-fabric towards the center of the localized ductile shear zones. The anisotropy of magnetic susceptibility (AMS) was used to investigate how the mineral and magnetic fabric is modified by the ductile shear zones. The AMS was measured in low fields with an AGICO KLY-2 susceptibility bridge. High-field measurements were made with a torsion magnetometer to investigate the minerals responsible for the magnetic fabric. The magnetic fabric in the pre-deformed gneiss is characterized by a prolate ellipsoid with the maximum axes of the AMS ellipsoid lying sub-parallel to the mineral lineation, dominating the gneiss and which is to the NE-SW and sub-horizontal. The intermediate and minimum axes are distributed in a girdle with the intermediate axes lying predominantly in the SE quadrant and the minimum axes in the NW quadrant. Several sample profiles were taken across the shear zones. The magnetic fabric becomes progressively less prolate for samples closer to the shear zone and in some cases becomes oblate in the center of a shear zone, which is associated with a macroscopic foliation. The maximum axes of the AMS remain sub-horizontal but rotate progressively to a more N-S direction. High-field torsion magnetometry indicates that the mineral carriers of the magnetic fabric are not homogeneous. Some samples are dominated by the ferrimagnetic minerals, others are dominated by paramagnetic phases and still others have both, a ferromagnetic and paramagnetic contribution. The orientation of the maximum axes is in good agreement between the low- and high-field measurements. The intermediate and minimum axes are always in the plane normal to the maximum axes, and the low-field component coincides with the mineral component that dominates the high-field AMS.
GP21A-0153 0800h
Magnetic Anisotropy and Paleomagnetic Study of Dikes Emplaced in the Wai'anae Volcano, Oahu, Hawaii: a Re-evaluation of the AMS Data
The Wai'anae Volcano is the older of two shield volcanoes that make up the island of O'ahu. Previous age determinations suggest that the subaerial portion of the edifice erupted between approximately 3.7 and 2.7 Ma. The eroded Wai'anae Volcano had a well-developed caldera centered near the back of its two most prominent valleys, and two major rift zones: a prominent north-west rift zone, well defined by a complex of sub-parallel dikes trending approximately N52W, and a more diffuse south rift zone, trending between S20W to due south. In order to investigate the volcanic evolution, the plumbing and the triggering mechanisms of the catastrophic mass wasting occurred in the volcano we have undertaken a paleomagnetic and AMS study of 7 dikes from the volcano. We drilled the dikes paying special attention to the chilled margins were we recovered a minimum of 8 and up to 23 samples per margin. The width of the dikes ranges between 0.5 to 4 m. In terms of the paleomagnetic results at least 20 samples per intrusive were stepwise demagnetized by a.f. from 5 to 100mT. Companion specimens from the same core were demagnetized at 15 temperature steps. In both cases demagnetization diagrams obtained with each technique showed a stable Characteristic direction of remanence (ChRM) determined with no ambiguity. The ChRM was calculated using principal component analysis for the demagnetization diagrams with a well-defined component trending to the origin. In addition, low field susceptibility vs temperature (k-T) and SIRM experiments were able to identify magnetite (575$^{o}$C) and a low temperature mineral phase at about 250-300$^{o}$ C which probably reflects the presence of titanomagnetite. The determined directions of the intrusives resulted in normal and reversed polarities indicating that such dikes were emplaced at different periods of time covering a gap of 350 kyrs. Magnetic fabric studies of the dikes along a NW-SE section across the present southwestern part of the Waianae volcano have been conducted. The flow direction was studied using the imbrication angle between the dike walls and the magnetic foliation (e.g. Geoffroy et al., 2002). At the dike scale, the magnetic zone axis, which underlines the intersection of the magnetic foliation from the two borders of the dike (i.e. a direction perpendicular to flow), has yielded a precise orientation in three of the sites studied. The flow direction has been obtained in the seven studied dikes. For the majority of the cases, the maximum axis K1 appears to be perpendicular to the flow direction and in some cases with a partial axes permutation with respect to the intermediate axis K2 or even with respect to the minimum axis K3. In addition, in one of the sites studied, the minimum axis K3 is very close to the flow direction. In all the cases, the magma flowed along a direction with a moderate plunge. For six of the dikes, the interpreted flow was from the internal part of the volcano towards the volcano border and corresponds probably to the inflation phase of the volcano. In two cases (dikes located on the northwestern side of the volcano), the flow is slightly downwards, possibly related to the distal extension due to inflation of the central part of the volcano. The seventh dike is located closer to center of the volcano and is characterized by a slightly different orientation with respect to the other six dikes, and also revealed a downward flow that could correspond to another magma pulse that resulted from a flow-back during distension due to the collapsing of the Wai'anae volcano.
GP21A-0154 0800h
Magnetic Anisotropy of Hemo-ilmenite Single Crystals: Testing the Lamellar Magnetism Hypothesis
The anisotropy of magnetic susceptibility (AMS) of single crystals of hemo-ilmenite was investigated to examine the relationship between directions of the principal axes of the AMS ellipsoid in minerals considered to contain lamellar magnetism. The crystals, within slabs taken from the Pramsknuten massive hemo-ilmenite "dikes", South Rogaland, Norway, were oriented by electron backscatter diffraction (EBSD) and the c- and a-axes of seventeen individual crystals were identified by comparing both the front and back sides of the slabs. The anisotropy of magnetic susceptibility was measured in low fields with an AGICO KLY-2 susceptibility bridge and in high fields (maximum field: 1.8 T) with a torsion magnetometer. The principal axes of the AMS ellipsoids are k1 $>$ k2 $>$ k3. Low-field AMS shows a triaxial ellipsoid in which the k3 axis lies near to the crystallographic c-axes for 16 of the 17 crystals (13 crystals $<$ 11$\deg$, 3 crystals $<$ 17$\deg$). Discordance may be due to misalignment during mounting or contamination by minute amounts of magnetite. Eight crystals show agreement between the alignment of the k2 of the AMS ellipsoid and an a-crystallographic axis (5 crystals $<$ 11$\deg$, 3 crystals $<$ 17$\deg$). The significance of this relationship will be discussed. In these crystals the remanent magnetization is also oriented close to the same direction, consistent with the lamellar magnetism hypothesis. In 6 crystals the k1 of AMS ellipsoid is close ($<$ 15$\deg$) to an a-crystallographic axis but there is no correlation with the remanent magnetization. High-field AMS measurements confirm the orientation of the c-crystallographic axes and suggest that hematite is an important contributor to the observed AMS.