Asymmetrical fold test in the case of overfolding: two examples from the Makran accretionary prism (Southern Iran)

Abstract

We present paleomagnetic data from two overturned kink folds with horizontal axes, located NW of the Makran accretionary prism, southern Iran. In each one of these structures, the subvertical and the overturned limbs have been sampled in order to perform a fold test. The rocks are Upper Oligocene to Middle Miocene finely laminated sandy flysch without visible cleavage nor internal deformation. They have been folded during the Mio-Pliocene phase of shortening. The fold test performed on the intermediate-temperature characteristic remanent magnetization (ChRM) ( 200 ° C⩽T ub ⩽400 ° C ) is negative in both structures. The results suggest a synfolding remagnetization acquired in the last stage of folding. However, the synfolding directions obtained by proportional untilting of the two limbs are not consistent with the regional geodynamics. Therefore we differentially untilted the two folds using the small-circle method [Geophys J. Int. 130 (1997) 405]. Furthermore, we noticed that the ChRM in each subvertical limb is very close to both the mean Pliocene direction of Europe and the overprinted magnetization observed in other sites in the Makran, while the ChRM direction is significantly offset in the overturned limbs. This led us to suggest an alternative use of Shipunov’s test by progressively untilting the inverted limb, while holding the subvertical one in its present orientation. The ChRM clustering is significantly improved, compared to the classical fold tests, and its direction is more realistic. This asymmetrical untilting confirms that the remagnetization was acquired during the late stage of shortening, just before the now inverted limbs overturned. This typical case exemplifies some errors that can be induced by the use of standard fold tests when applied to a remagnetization acquired during asymmetrical folding, if the chronology of the folding process relative to the magnetization acquisition time is not taken into account.