Magnetic and mineral fabric development in the Ordovician Martinsburg Formation in the Central Appalachian Fold and Thrust Belt, Pennsylvania
A. M. Hirt, W. Lowrie, C. Lüneburg, H. Lebit, T. Engelder, 2004. "Magnetic and mineral fabric development in the Ordovician Martinsburg Formation in the Central Appalachian Fold and Thrust Belt, Pennsylvania", Magnetic Fabric: Methods and Applications, F. Martín-Hernández, C. M. Lüneburg, C. Aubourg, M. Jackson
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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.
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Fabric is a ubiquitous and significant feature of geological materials. The processes involved in the formation and deformation of rocks and sediments leave their mark on the orientations of the constituent mineral grains. Petrofabrics thus provide essential keys to understanding the history of geological materials. Magnetic anisotropy is directly related to petrofabric, and has become one of the most rapid, sensitive and widely used tools for its characterization. The relationship between magnetic fabric and petrofabric is complex and depends on various factors including the composition, concentration and grain size of mineral grains. Ongoing research in geological applications is paralleled by studies of the fundamental mineral magnetic phenomena involved.
The papers in this book represent the current state of investigations in magnetic anisotropy studies as a discipline that integrates geological interpretations, mineral fabric development, technical advances and rock-magnetic properties.