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

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.

A transtensional basin model for the Organyà basin (central southern Pyrenees) based on magnetic fabric and brittle structures

“…Internal deformation accompanied with the development of a close-packed foliation planes is found to impose a compressional magnetic fabric orientation in the North Pyrenean Zone near the boundaries of the Mauléon basin, or in the Basque-Cantabrian basin where compressional structures are present (Soto et al 2007;Oliva-Urcia et al 2010). This probably occurs due to the new formation of phyllosilicate grains on the cleavage plane, as described, for example, in marly sediments (Housen & van der Pluijm 1990;Hirt et al 2004;Debacker et al 2004).…”

“…Some works demonstrate the development of the magnetic fabric at the time of deposition in both extensional (magnetic lineations are parallel to the extension direction; Mattei et al 1997; Cifelli et al 2005; Soto et al 2007) and compressional regimes (magnetic lineations are perpendicular to the compression direction; e.g. Housen et al 1993; Lüneburg et al 1999; Parés et al 1999; Hirt et al 2004; Soto et al 2009). In an inverted extensional basin, the magnetic fabric depends greatly on the time of development of the magnetic fabric and the intensity of the deformation during the inversion of the basin (Soto et al 2007).…”

“…The study of AMS in tectonically inverted basins helps to constrain basin models, especially when the magnetic fabric acquisition occurs at the time of the formation of the basin (Soto et al 2007) and is not altered during the later compressional events. One common cause of alteration of the primary fabric is the new formation of phyllosilicates or ferromagnetic minerals during the compressional event (Housen & van der Pluijm 1990; Aubourg et al 1999; Chadima & Hrouda 2000; Debacker et al 2004; Hirt et al 2004; Oliva‐Urcia et al 2009). In addition, the primary mineral fabric can also be affected by the physical rotation of existing grains into the plane of flattening or cleavage plane (Lüneburg et al 1999; Parés et al 1999).…”

On the reliability of AMS in ilmenite-type granites: an insight from the Marimanha pluton, central Pyrenees

“…Techniques to separate the magnetic fabrics such as low temperature AMS (LT-AMS; that isolates the paramagnetic fabric) or low field compared to high field susceptibility measurements that help to discriminate between paramagnetic and ferromagnetic (s.l.) These studies show a very good correspondence between the strain and the AMS ellipsoids in strongly deformed rocks: from sedimentary rocks with well-developed tectonic foliation and synthetic aggregates (Housen & van der Pluijm 1990;Hirt et al 2004;Parés & van der Pluijm 2004;Schmidt et al 2008Schmidt et al , 2009Almqvist et al 2011) up to gneisses, shear bands and migmatites (Siegesmund et al 1995;Zhou et al 2002;Tomezzoli et al 2003;Gage et al 2004;Kruckenberg et al 2010). These studies show a very good correspondence between the strain and the AMS ellipsoids in strongly deformed rocks: from sedimentary rocks with well-developed tectonic foliation and synthetic aggregates (Housen & van der Pluijm 1990;Hirt et al 2004;Parés & van der Pluijm 2004;Schmidt et al 2008Schmidt et al , 2009Almqvist et al 2011) up to gneisses, shear bands and migmatites (Siegesmund et al 1995;Zhou et al 2002;Tomezzoli et al 2003;Gage et al 2004;Kruckenberg et al 2010).…”

“…contributions (Borradaile & Jackson 2004), together with classical petrological studies allow to determine the carrier of the magnetic fabric, which is mostly related to paramagnetic minerals such as phyllosilicates. These studies show a very good correspondence between the strain and the AMS ellipsoids in strongly deformed rocks: from sedimentary rocks with well-developed tectonic foliation and synthetic aggregates (Housen & van der Pluijm 1990;Hirt et al 2004;Parés & van der Pluijm 2004;Schmidt et al 2008Schmidt et al , 2009Almqvist et al 2011) up to gneisses, shear bands and migmatites (Siegesmund et al 1995;Zhou et al 2002;Tomezzoli et al 2003;Gage et al 2004;Kruckenberg et al 2010).…”