Is this magnetic fabric normal? A review and case studies in volcanic formations

When studying weakly anisotropic rocks non-standard relationships between m agnetic and expected petrofabric axes are sometimes observed, in particular the so-called inverse or intermediate fabrics. A review of possible explan ations for these effects is presented and leads to the conclusion that a ma jority of abnormal magnetic fabrics, often encountered in magnetite-rich vo lcanic rocks, are due to abnormal preferred orientations or distributions o f magnetite grains. Through case studies in dikes (from Skye Island, Yemen and the Iberian Peninsula), and magma and ash flows (from the Ethiopian tra ps and southern Corsica) the various possible origins of such fabrics are e xamined. It is possible to retrieve reliable magma flow directions from the se sites provided proper data selection is performed, leading to non-unique interpretations. (C) 1999 Elsevier Science B.V. All rights reserved.