HR: 14:55h
AN: GP23B-06    [Abstracts]
TI: Complex Magnetic Fabrics in Igneous Rocks
AU: Plenier, G D
EM: gplenier@es.ucsc.edu
AF: U.C. Santa Cruz, 575 High Street , Santa Cruz, C A 95060
AU: * Glen, J M
EM: jglen@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025
AB: Since the end of the 1980's, studies on the anisotropy of magnetic susceptibility (AMS) of dikes and lavas have largely assumed a simple relation between the principal AMS directions and magma flow. In the so-called "normal" magnetic fabric case, the magnetic foliation mimics the flow plane. The AMS minimum -- Kmin, which is normal to the foliation plane, lies perpendicular to a dike's walls or to a lava's top and bottom, and the magnetic lineation, given by the AMS maximum - Kmax, parallels the magma flow direction. However, numerous studies report anomalous magnetic fabrics for which the principal AMS directions are exchanged. Recent studies of Oligocene lava flows from the Kerguelen archipelago and the present study of 48 mid-Miocene dikes from the Roberts Mountains, located along the eastern Northern Nevada Rift (NNR), indicate that complex magnetic fabrics are much more common than is generally expected. Indeed, of the 48 NNR-dikes studied so far, only 45% possess a normal fabric (assuming a vertical flow). The relatively low number of samples possessing a normal fabric and the existence of permuted (exchanged) AMS axes occurring at the cooling unit scale (all the samples of a flow or dike yielding the same abnormal fabric) as well as at the sample scale (only one or two permuted samples observed in a cooling unit) suggest that the magnetic lineation may not always indicate the flow direction. Even when all the samples from the same cooling unit display the same magnetic fabric, an alternative flow direction indicated by Kint (AMS intermediate axis) cannot be refuted, leading to an uncertainty in the interpretation of the AMS data. Therefore, it is of great interest to study in detail the permutations of the AMS axes to 1) determine what factors control the occurrence of the various abnormal fabrics, and 2) possibly distinguish an abnormal fabric and infer from it the true flow direction. For that purpose, we performed a suite of mineralogical experiments to characterize representative samples of the different AMS fabrics, including FORC distribution analyses at different temperatures and MPMS measurements. We also studied the evolution of magnetic fabric upon AF and thermal demagnetization, and induced remanent magnetization. In addition, measurements of the anisotropy of magnetic remanence and of AMS in a high DC-field are in-progress. Surprisingly, the results obtained so far, seem to indicate that the permutations of AMS axes are not related to mixtures of different grain sizes. However, additional results are needed to check if this interpretation is statistically robust and test the other permutation mechanisms like magnetic interactions of the grains, rolling of the elongated particles in strong velocity gradients, or crystallographic particularities.
DE: 5109 Magnetic and electrical properties
DE: 5139 Transport properties
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting