A strong negative magnetic anomaly, caused by an intense natural remanent magnetization (NRM) approximately opposite today's geomagnetic field, is observed above the MCU IVe' unit of the Bjerkreim Sokndal Layered Intrusion. The anomaly is strongest in the east, close to Heskestad, and decreases when following the layer toward the north and west. This study investigates how the NRM changes along the layer and how its direction and intensity are affected by magnetic fabrics in the intrusion. NRM, low-field anisotropy of magnetic susceptibility, and anisotropy of anhysteretic remanence have been measured on 371 specimens from 46 sites. The orientation of both the magnetic fabrics and the NRM changes for different locations along the layer, and it appears that the NRM is tilted away from the mean paleofield and toward the direction of maximum susceptibility and maximum anhysteretic remanence. When NRM directions are corrected for magnetic fabrics, the angle between the NRM and mean paleofield direction generally decreases for specimens with a single-component NRM. No correlation was found between the NRM intensity and the directional relationship between NRM, magnetic fabric, and mean paleofield.
Bibliographical noteFunding Information:
Alexander Michels, NTNU, is thanked for help during fieldwork. Bjarne S.G. Almqvist, Uppsala University, kindly provided access to their MFK1 susceptibility bridge for initial AMS measurements, and Morgan Ganerød, Norwegian Geological Survey (NGU), is thanked for access to their JR6 spinner magnetometer. We are grateful to Peat Solheid for assistance during sample preparation and measurements at the Institute for Rock Magnetism (IRM), University of Minnesota. This study was funded by the Swiss National Science Foundation (project P2EZP2-155517 to A. Biedermann), NTNU, Research Council of Norway (grant 222666 to S. McEnroe), and a visiting fellowship at the IRM, which is supported by the Instruments and Facilities program of the U.S. National Science Foundation, Division of Earth Sciences. Fátima Martín-Hernández and Martin Chadima are acknowledged for their constructive reviews which helped to improve the manuscript and Michael Walter for editorial handling of the manuscript. Additional data tables and figures are available in the supporting information.
©2017. American Geophysical Union. All Rights Reserved.
- NRM deflection
- lamellar magnetism
- layered intrusion
- magnetic fabric