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Magnetic and mineral fabric development in the Ordovician Martinsburg Formation in the Central Appalachian Fold and Thrust Belt, Pennsylvania

By
A. M. Hirt
A. M. Hirt
1
Institute of Geophysics, ETH-Hönggerberg, CH-8093 Zürich, Switzerland e-mail: hirt@mag.ig.erdw.ethz.ch
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W. Lowrie
W. Lowrie
2
Institute of Geophysics, ETH-Hönggerberg, CH-8093 Zürich, Switzerland
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C. Lüneburg
C. Lüneburg
3
Department. of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148, USA
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H. Lebit
H. Lebit
3
Department. of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148, USA
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T. Engelder
T. Engelder
4
Department of Geosciences, Pennsylvania State University, State College, PA 16802, USA
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Published:
January 01, 2004

Abstract

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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Contents

Geological Society, London, Special Publications

Magnetic Fabric: Methods and Applications

F. Martín-Hernández
F. Martín-Hernández
Faculty of Geosciences, Utrecht University, The Netherlands
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C. M. Lüneburg
C. M. Lüneburg
Department of Geology and Geophysics, University of New Orleans, USA
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C. Aubourg
C. Aubourg
Laboratoire de Tectonique, Université de Cergy-Pontoise, France
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M. Jackson
M. Jackson
Institute for Rock Magnetism, University of Minnesota, USA
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Geological Society of London
Volume
238
ISBN electronic:
9781862394865
Publication date:
January 01, 2004

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