Chlorite control of correlations between strain and anisotropy of magnetic susceptibility

• DOI: 10.1016/0031-9201(90)90114-d
• Publication Date: 2002-11-12T10:46:34Z
• Authors: Bernard A. Housen; Ben A. van der Pl...
• Keywords: Principal axis theorem; Cleavage (geology); Finite strain theory; Strain (injury); SLATES

Abstract Anisotropy of magnetic susceptibility (AMS) has in the past been used as an indicator of finite strain; mathematical relationships between the measured AMS ellipsoid and the finite strain ellipsoid in slates are numerous. The Ordovician Martinsburg Formation at Lehigh Gap, Pennsylvania, provides a unique opportunity to test this correlation, as a complete shale-to-slate transition is preserved. Magnitudes of the principal susceptibility axes appear to reflect a strain gradient at this location. Apparent strain values derived from AMS, using equations from Rathore and Hrouda, have X : Y : Z ratios from 2.79 : 1.87 : 0.19 (shale) to 2.92 : 0.82 : 0.42 (slate). The orientations of the susceptibility axes characterize two fabrics: (i) k int k max planes at 20° to bedding and (ii) k int k max planes parallel to cleavage. Anisotropy of magnetic susceptibility fails to show any intermediate orientations of the principal susceptibility axes in the zone in which cleavage is developed. Comparison with phyllosilicate crystallographic orientations indicates that the AMS intermediate and maximum susceptibility axes are parallel to chlorite basal planes. Because chlorite reoriented via dissolution and new growth as cleavage developed in this location, and because models correlating AMS and strain instead assume that strain is reflected by the rotation of the magnetic susceptibility carriers, the apparent strain values calculated from AMS reflect only the degree of dissolution and neocrystallization in these rocks and not the finite strain.