Paleomagnetic direction obtained by strain removal in the Pyrenean Permian redbeds at the “Col du Somport” (France)

Abstract

In order to investigate the possibility and limitations of paleomagnetic works within strained regions, a paleomagnetic study, related with strain analysis has been conducted in the deformed Pyrenean Permian redbeds in the “Col du Somport” area. Paleomagnetic sampling together with strain estimates have been conducted in 6 sites through a fold. The results obtained by measuring the orientation and axial ratios of elliptical reduction spots show that (1) the shale beds have undergone a penetrative strain, (2) the sandy beds can be regarded as tectonically unstrained with reduction spots flattened in the bedding, showing that they recorded the compaction. It is shown that the total strain recorded in the slaty beds probably results from the superimposition of tectonic strain upon the compaction fabric. The paleomagnetic study shows that the primary pretectonic magnetization is widely overprinted by a secondary syn- or post-tectonic magnetic component. As both components appear to be carried by hematite pigment, their separation using classical demagnetization procedures has been difficult. A characteristic remanent magnetization (ChRM) has however been determined, when possible, as the hardest component in demagnetization curves. Then, the ChRM direction distributions are represented in stereographic density plots. Although these ChRM directions exhibit a clear tendency towards SE declinations and shallow inclinations, characteristic of Permian paleomagnetic field direction for the Iberian plate, the tilt correction does not induce a clustering of these directions. Strain is inferred to be responsible for this situation. Assuming that both pretectonic magnetization directions and bedding planes closely follow the material plane and line strain response model of March [1], an attempt has been made to remove the effect of strain upon the remanent magnetization. It is shown that when using a reconstructed tectonic strain tensor (i.e., the total strain tensor as measured in the field, corrected for an estimated compaction) we obtain a significant clustering of ChRM directions. The computation of the relevant VGP, gives a pole position (210.5°E, 42.0°N) compatible with the reference APWP for the Iberian plate. It is therefore inferred that the strain removal technique is a usable tool in order to obtain paleomagnetic results within such strained rocks.