Magnetic fabrics of Tertiary sandstones from the Arc of Fars (Eastern Zagros, Iran)

An analysis of magnetic fabric (45 sites, 514 samples) has been performed in folded but apparently not penetratively deformed Mio-Pliocene sedimentary rocks (mainly sandstones) from the Arc of Fars (Eastern Zagros, Iran). Different types of magnetic fabrics and shapes of susceptibility ellipsoids were observed from an oblate form reflecting a sedimentary fabric to a prolate form signifying a magnetic lineation of tectonic origin. Classically in such weakly deformed sedimentary rocks the magnetic lineation is perpendicular to the shortening direction and is interpreted as recording a pre-folding layer-parallel shortening. In the studied area, the trends of the magnetic lineation show two clusters oriented ENE-WSW and WNW-ESE. To a first approximation, the magnetic lineation follows the bending of the fold trend. However, a quite systematic obliquity is observed. According to regional geology and to other considerations arising from the magnetic analysis, we consider that the tectonic transport direction is NNW-SSE. The arc pattern observed in the fold and magnetic lineation trends may result either from an inherited structural framework or from post-folding clockwise rotations or both. In any case, our data rule out any interpretation of the whole Arc of Fars as resulting from the superimposition of two non-coaxial folding events.