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Rock Magnetism and the Interpretation of Anisotropy

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Rock Magnetism and the Interpretation of Anisotropy View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by University of Minnesota Digital Conservancy ROCK MAG N ETISM AN D THE INTERPRETATION OF AN ISOTROPY OF MAG N ETIC SUSCEPTI BI LITY P.Rochette, • M. Jackson,2 and C. Aubourg Laboratoirede G•ophysiqueInterne et Tectonophysique Observatoirede Grenoble, Grenoble, France Abstract.The conventional rules, derived from empirical and isotropyto makequantitative inferences on the intensityof theoreticalconsiderations, for theinterpretation of anisotropy the preferredorientation and consequently on strain.In fer- of magneticsusceptibility (AMS) in termsof microstructure romagneticgrains, AMS mayalso be influencedby themag- and deformationare subjectto numerousexceptions as a neticmemory of the grains(including natural remanence). resultof particularrock magneticeffects. Unusual relation- The effectof alternatingfield or thermaldemagnetization on shipsbetween structural and magnetic axes (so-called inverse AMS is brieflydiscussed. Various rock magnetic techniques, or intermediatemagnetic fabrics) can occurbecause of the specificto AMS interpretation,have to be developedfor a presenceof certainmagnetic minerals, either single domain betterassessment of thegeological significance of AMS data. magnetiteor variousparamagnetic minerals. When more than Thesetechniques mainly rely on measurementsof suscep- onemineral is responsiblefor magneticsusceptibility, various tibility versusmagnetic field and temperature, together with problemsappear, in particularthe impossibilityof usingan- anisotropyof remanence. 1. INTRODUCTION grain(maximum susceptibility parallel to the long axis and minimumsusceptibility parallel to the shortaxis). Magneticsusceptibility K is definedby M = [K] x H, The anisotropyof magneticsusceptibility (AMS) is a where M is the inducedmagnetization of the materialand physicalproperty of rockswhich is usedfor petrofabricand H is the inducingmagnetic field. As both M and H are ex- structuralstudies (see Owens and Barnford[ 1976], Hrouda pressedin amperesper meter,volumetric susceptibility K is [ 1982], Borradaile [ 1988], Lowrie [ 1989], and Jacksonand dimensionless(written as "SI"), while masssusceptibility X Tauxe[1991], for recent reviews). Its principle is simple: = K/p is in cubicmeters per kilogram.Susceptibility varies AMS arisesfrom the preferredorientation of anisotropic in the generalcase according to field andtemperature values magneticminerals, in otherwords, the magnetic fabric. The and may vary accordingto the measurementdirection re- preferredorientation of crystallographicaxes, themselves sultingin a nonparallelismbetween H and M vectors.K is often controllinggrain shape,determines the AMS for the most often measuredat room temperatureand low-field vastmajority of minerals.The shapepreferred orientation of strength (•<1 mT) because in such conditions, individualgrains or of grain clusters,the secondcase cor- (1) experimentalprocedures are the easiest,allowing high respondingto the "distributionanisotropy" introduced by sensitivityand rapid measurements,even in the field; (2) K Hargraveset al. [1991], playsa directrole in AMS essen- is a goodestimate of the in situ inducedmagnetization of a tially in the caseof magnetite. rock in the Earth's field, thereforeserving as a basis for The AMS techniqueis gainingnumerous users for a wide magneticanomaly interpretations; and (3) K canbe correctly range of applications in Earth sciences because of approximatedby a second-ordersymmetric tensor, a situation (1) applicabilityto practicallyevery rock and soft sediment which greatly facilitatesthe measurementof susceptibility type; (2)high sensitivity,which allows one to determine anisotropy.At the grain scale the anisotropyof magnetic fabricsin rockspreviously seen as isotropic(for example, susceptibility,as manyother physical properties such as op- rocks with a fabric due to flow of magma, bottom water tical refractive index, mechanicalconstant, conductivity, currents,weak deformation), therefore opening new research etc., is controlledby the type and directionsof the crystal- fields;(3) timely operation(about 15 min per specimenin- lographicsystem. However, other phenomena overcome the cludingfield samplingand orientation,preparation, meas- crystallographiccontrol in mineralswith very high suscep- urement, and processing) allowing statistical and tibility suchas magnetite.In sucha casethe anisotropyof cartographicfabric investigations, particularly useful to map a multidomaingrain is simplythe imageof the shapeof the complexstructures such as thosefound in magmaticbodies 'Permanentlyat Facult•St-J•r0me, Marseille, France. or in tectonicunits; (4) possibilitiesfor semiquantitativeand 2Permanentlyat Institutefor RockMagnetism, University of quantitativeapplication in termsof fabricand deformation Minnesota,Minneapolis. intensityand symmetry; and (5) useas a newtool to constrain Copyright1992 by the AmericanGeophysical Union. Reviewsof Geophysics,30, 3 / August1992 pages 209-226 8755-1209/92/92 RG-00733 $15.00 ß 209. Papernumber 92RG00733 210 ß Rochetteet al.' ROCKMAGNETISM 30, 3 / REVIEWSOF GEOPHYSICS paleomagneticinterpretations in termsof age of the natural the relative lengthof the susceptibilityaxes) and the inten- remanentmagnetization (NRM), better definitionof struc- sitiesof linear or planarpreferred orientations, respectively. tural corrections,possible deviation of NRM from the geo- In the case of solid-statedeformation this implies a direct magneticfield, etc. relationshipbetween AMS and strain. Thereforea quanti- The outputof AMS measurementsis an ellipsoidof mag- tativeapplication of AMS is possibleonce a calibrationwith netic susceptibilitydefined by the lengthand orientationof strainhas been performed [e.g., Borradaile, 1991].In many its threeprincipal axes, K• •> K2 •> Ks, i.e., the threeeigen- cases,only a semiquantitativeinterpretation is possible;i.e., vectorsof the susceptibilitytensor. The parametersusually a more anisotropicrock is more strained. presentedare the meansusceptibility Km = (K• + K2 + Ks)/ 3. The AMS measurementis not affectedby naturalor 3 and the anisotropyratios L = K•/K2, F = KdKs, and P artificialremanent magnetizations. = K•/Ks (lineation,foliation, and degreeof anisotropy,re- Theseassumptions can be comparedto the basicrequire- spectively).The choiceof theseparameters and a standard- mentsto obtaina paleomagneticpole: The naturalremanent izedway of presentingAMS data,used throughout this paper, magnetization(NRM) of a rock is parallel to the field in are discussedby Ellwood et al. [1988]. The sensitivityof which it is acquired;the age of NRM is the formationage the individualmeasurements depends on the instrumentused of the rock; the ancientfield is representedby a geocentric (eithera low-fieldtorquemeter, a spinnermagnetometer (both axial dipole. In fact, as in paleomagnetism,the basic as- measuringdirectly a susceptibilitydifference) a suscepti- sumptionsof AMS interpretationare not alwaysfulfilled, bility bridge, or an inductivemagnetometer), the value of and one needsvarious techniques to assesstheir validity in Km, and the anisotropyratio L for the sensitivityon K• di- eachcase study. rection and F for Ks (see discussionby Ellwood, [1984a] Comparisonswith casestudies or modelsare not sufficient andSchmidt et al., [ 1988]). In the authors'opinion the most by themselvesto establishtruly scientificAMS interpreta- reliable, sensitive,and practical instruments presently avail- tions.Models are usuallytoo simpleto accountfor the com- able appear to be the torquemeterand the susceptibility plexity of naturalrocks, and empiricallybased deductions bridgeKLY-2 (manufacturedby GeofyzikaBrno). They al- stronglylimit the field of AMS applications.One can only low one to determinereliable AMS directionsin practically concludein termsof likelihoodwhen dealing with AMS data everyrock type down to an anisotropyratio of 1.002. that are not supportedby structuralmeasurements. When Statisticaltreatment of AMS datafrom a setof specimens suchstructural measurements exist, the needfor AMS study to define a site mean AMS tensorrequires specific tech- is minor becauseit bringsfew new information.Finally, a niques,either analytical(the tensorialmean method of Je- strongbias in the generalityof an empiricallaw may arise linek [1978]) or numerical (the bootstrap method of from the natural tendencyof workersin the field to only Constableand Tauxe[1990]). Thesetechniques must be pre- publishresults that fit in the usualinterpretation grid. ferred becausethey weigh each sampleby its anisotropy Amongthe independentpieces of evidenceused to estab- ratios,provide mean anisotropyratios more significantthan lish the validity of paleomagneticresults, rock magnetic the arithmeticmean of specimenvalues, and provide ellipses criteriaplay a major role, in particularthrough the identifi- of confidenceon the mean directions.Recently, Lienert cationof magneticminerals responsible for the NRM (i.e., [ 1991]has demonstrated by Monte-Carlosimulations that the magneticmineralogy). As in paleomagnetism,where it has ellipsesgiven by Jelinek's[ 1978] simplermethod are very become standardpractice to produce supplementaryrock goodestimates of the dispersion.However, when differently magneticevidence, AMS interpretationshould also be based orientedfabrics are superposedin a large set of samples, on rock magneticcriteria. their orientationcan be betterseparated by usingprocessing One of the main breakthroughsin that directionin the methodsfor
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