An integrated AMS, structural, palaeo- and rock-magnetic study of Eocene marine marls from the Jaca-Pamplona basin (Pyrenees, N Spain); new insights into the timing of magnetic fabric acquisition in weakly deformed mudrocks
Juan C. Larrasoaña, Emilio L. Pueyo, Josep M. Parés, 2004. "An integrated AMS, structural, palaeo- and rock-magnetic study of Eocene marine marls from the Jaca-Pamplona basin (Pyrenees, N Spain); new insights into the timing of magnetic fabric acquisition in weakly deformed mudrocks", Magnetic Fabric: Methods and Applications, F. Martín-Hernández, C. M. Lüneburg, C. Aubourg, M. Jackson
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In this paper, we present a combined magnetic anisotropy (AMS), structural, palaeo- and rock-magnetic study of Eocene marine mudrocks from the western sector of the Jaca-Pamplona Basin (southwestern Pyrenees, north Spain). Comparison of structural, AMS and palaeomagnetic data reveals a subtle, but evident tectonic overprint affecting the phyllosilicate matrix of the mudrocks and the remanence carriers. The particular structural setting of the studied rocks has allowed us to combine palaeomagnetic and structural data with the AMS results in order to establish a relative chronology between sedimentation, blocking of magnetic fabrics, acquisition of magnetic remanence and deformation. Our data suggest that the blocking of the magnetic fabrics and the lock-in of the remanence occurred simultaneously during the early stages of gentle warping that affected the Jaca-Pamplona basin throughout the Mid-Late Eocene. According to the origin of the remanence carriers and the synsedimentary nature of the Mid-Late Eocene warping, the blocking of the magnetic fabrics and the lock-in of the remanence can be bracketed to a very short time span, of a few (<15?) ka, after sediment deposition. Our findings confirm previous interpretations claiming a very early origin of the magnetic fabric blocking in mudrocks undergoing weak deformation. They therefore validate the use of magnetic fabrics as palaeostress indicators and suggest that AMS data might also provide a useful tool for detecting preferred paths for the migration of geofluids during the early stages of deformation.
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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.