Common Structural Errors in Paleomagnetic Analysis of Fold-And Belts
Abstract
Paleomagnetism is essential to decipher vertical axis rotations (VAR) in orogens, an elusive variable in tectonic studies. While unraveling the significance of VAR in the internal parts of orogenic belts might be made difficult due to the complex superposition of deformation events, it has provided timely insights into the geometry and kinematics of the external parts; the fold-and-thrust belts (FAT belts). Accurate quantification of VAR demands a reliable recording of the Earth's magnetic field. Inclination flattening or non-dipolar geometries of the geomagnetic field are well known to affect the reliability of paleomagnetic directions. Other factors are related to the geometry and mechanisms of deformation and are often overlooked: 1) Overlapping of vectors has to do with the technical limitation to isolating components in the laboratory. Secondary remagnetizations may be a common post tectonic process. The orientation of the overlapped vector will change along the fold geometry. Their restoration will result in different DEC and INC errors depending upon the degree of overlapping, the fold axis orientation (FAO), dip of the beds and the polarity of the components. 2) Many paleomagnetic studies implicitly assume the rigid-body behavior of rock volumes during deformation (absence of internal strain). However, pure shear may appear at the fold hinges and simple shear is frequent at the fold limbs and around fault planes. The bedding correction (BC) will induce DEC and INC errors controlled by the FAO and shear magnitude. 3) Different stages of folding and thrusting may happen in FAT belts and may be responsible for the superposition of non-coaxial axis of deformation. Besides, folds are not always cylindrical. These factors are responsible for the generation of plunging, conical and superposed folds, etc… The BC, instead of the reverse sequential restoration, will produce declination errors that depend upon the non-coaxility angle and the folding and tilting magnitudes. All these errors will produce apparent synfolding magnetizations in the fold test and have to be considered with caution. The field test is the best tool not only to assess the timing of directions but also the best way to control "structural errors";
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2013
- Bibcode:
- 2013AGUSMGP42A..01P
- Keywords:
-
- 1525 GEOMAGNETISM AND PALEOMAGNETISM / Paleomagnetism applied to tectonics: regional;
- global