EXPERIMENTAL DEFORMATION OF 2-COMPONENT IRM IN MAGNETITE-BEARING LIMESTONE - A MODEL FOR THE BEHAVIOR OF NRM DURING NATURAL DEFORMATION

Experimental, room temperature, triaxial deformation at 200 MPa (2 kbar) and at a controlled strain rate of 10(-5) s-1 is performed on three limestones with magnetic behaviour dominated by single-domain magnetite. Permanent deformation is achieved mainly by twinning of calcite. Isothermal remanent magnetisation (IRM) in the samples is reduced in magnitude, especially when parallel to compressive stress. The decrease in magnitude correlates slightly better with peak stress than with strain, in accord with previous work in uniaxial tests, indicating the role of stress in deformational 'demagnetisation'. Remanences have their vector plots condensed after deformation, particularly in the range 0-30 mT. A two-component IRM is used as a simple model of a multi-component natural remanent magnetisation (NRM). The resultant vector is deflected by the action of stress towards whichever components are least reduced by deformation; in the case of magnetite-bearing rocks this appears to be the most coercive components. In the case of a natural deformation, it is predicted that the effects of stress at any stage should be to remove progressively any low-coercivity components added to an NRM by remagnetisation during tectonism, especially those parallel to compressive stress.