ALBERT

Description: Fifty years have now passed since Graham (1954) published his seminal paper advocating the use of anisotropy of magnetic susceptibility (AMS) as a rapid and sensitive petrofabric tool. During these five decades, Graham's under-exploited' method has become standard, and AMS and related techniques are now routinely applied to characterizing fabrics in a wide variety of geological materials (e.g. the GEOREF database lists over 500 journal publications with magnetic anisotropy' as keywords). Magnetic anisotropy works as a petrofabric tool because individual grains of most minerals are magnetically anisotropic, i.e. easier to magnetize in certain orientations, which are governed primarily by crystallography and/or grain shape. Magnetic anisotropy at the bulk rock scale results from the preferred crystallographic orientation (PCO) and/or preferred dimensional orientation (PDO) of anisotropic mineral grains. AMS can also result from magnetostatic interactions among closely spaced, strongly magnetic grains that are heterogeneously distributed in a matrix of more weakly magnetic minerals. In either case, magnetic anisotropy is directly related to some aspects of rock fabric, and thus it provides a quick, simple and effective characterization tool, even though the relationship between magnetic fabric and petrofabric is quite complex in detail. The present collection of papers originated, in part, at a special session on magnetic fabrics at the Joint Assembly of the EGS-AGU-EUG (April 2003) in Nice, which highlighted recent methodological advances, theoretical and experimental studies, and characterization of flow and deformation fabrics in rocks and sediments. A similar session at the AGU Fall Meeting (December 2003) in San Francisco underscored the continuing breadth ... This 250-word extract was created in the absence of an abstract.

Description: The Martinsburg Formation at Lehigh Gap, Pennsylvania, undergoes a transition from shales to slates, reflecting local progressive deformation on an outcrop scale. The anisotropy of magnetic susceptibility (AMS) was measured in low and high fields. The high-field measurements show that the magnetic susceptibility is controlled by the paramagnetic minerals. X-ray goniometry was used to define the mineral fabrics of chlorite and mica. The phyllosilicates are initially oriented preferentially in the bedding plane and are gradually reoriented into the cleavage plane through rotation, microfolding and recrystallization. The AMS fabric mirrors this change in mineral fabric. The magnetic fabric is originally oblate in the least deformed site, with the plane of flattening parallel to bedding, and becomes prolate with increasing deformation, reflecting the development of pencil structure in the shales. In the most deformed site, shortening results in a tectonic cleavage fabric, which controls the magnetic fabric. A similar pattern of fabric development can be observed on a regional scale at other sites across the central Appalachian fold and thrust belt. The AMS and mineral fabric from the Martinsburg Formation has undergone bedding compaction in the foreland near the Allegheny Front. The AMS and textural analysis both show that, as the deformation increases towards the hinterland, prolate fabrics develop and in the most deformed sites slaty cleavage controls both the mineral and magnetic fabrics.