Evidence for the Permo-Triassic transtensional rifting in the Iberian Range (NE Spain) according to magnetic fabrics results

摘要

Anisotropy of magnetic susceptibility (AMS) techniques are applied to Permo-Triassic red beds from the Castilian Branch (Iberian Range, NE Spain) that were deposited in an extensional basin inverted during Cenozoic times. The main goal of this work is to characterize the tectonic evolution of the basinal stage by differentiating synsedimentary to early diagenetic magnetic fabrics from the secondary tectonic fabrics related to compression, which are scarcely developed because no penetrative structures related to compression have been recognized. Oblate magnetic fabrics, with kmin axes perpendicular to bedding ,are observed in most cases. Magnetic lineations are variable, showing a dominant ENE-WSW maximum, which fits with a dextral transtensional regime acting on NW-SE master faults during the Triassic. We propose that variations in the orientation of the magnetic lineation are associated with transfer faults which fragment the basin and trigger strain partitioning in different areas. Magnetic fabrics are locally modified by Cenozoic compression, with intermediate and minimum axes distributed along girdles perpendicular to fold axes. Comparing all these results with macrostructures and mesostructural kinematic indicators, we conclude that the fine-grained hematite-bearing rocks carry a consistent magnetic fabric which can be used to reconstruct the basin history.