Dating inset terraces and offset fans along the Dehshir Fault (Iran) combining cosmogenic and OSL methods

Kristell Le Dortz (1) , Bertrand Meyer (1) , Michel Sébrier (1) , Regis Braucher (2) , H. Nazari (3) , L Benedetti (2) , M. Fattahi (4, 5) , B. Bourlès (2) , M. Foroutan (3) , Lionel Siame (2) , A. Rashidi (6) , D. Bateman (5)

1 iSTeP - Institut des Sciences de la Terre de Paris
2 CEREGE - Centre européen de recherche et d'enseignement des géosciences de l'environnement
3 Research Institute for Earth Sciences
4 Tehran Institute of Geophysics
5 Sheffield Centre for International Drylands Research, Department of Geography
6 Geological Survey

¹⁰Be and ³⁶Cl cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of offset terraces have been performed to constrain the long-term slip-rate of the Dehshir fault. Analysis of cosmogenic ¹⁰Be and ³⁶Cl in 73 surface cobbles and 27 near-surface amalgams collected from inset terraces demonstrates the occurrence of a low denudation rate of 1 m.Ma⁻¹ and of a significant and variable inheritance from exposure prior to the aggradation of theses alluvial terraces. The significant concentrations of cosmogenic nuclides measured in the cobbles collected within the riverbeds correspond to 72 ± 20 ka of inheritance. The mean CRE age of the surface samples collected on the older terrace T3 is 469 ± 88 ka but the analysis of the distribution of ¹⁰Be concentration in the near-surface samples discard ages older than 412 ka. The mean CRE age of the surface samples collected on terrace T2 is 175 ± 62 ka but the ¹⁰Be depth profile discard ages older than 107 ka. For each terrace, there is a statistical outlier with a younger age of 49.9 ± 3.3 ka and 235.5 ± 35.4 ka on T2 and T3 respectively. The late sediments aggraded before the abandonment of T2 and inset levels, T1 b and T1a, yielded optically stimulated luminescence (OSL) ages of respectively 26.9 ± 1.3 ka, 21.9 ± 1.5 ka, and 10.0 ± 0.6 ka. For a given terrace, the OSL ages, where available, provide ages that are systematically younger than the CRE ages. These discrepancies between the CRE and OSL ages exemplify the variability of the inheritance and indicate the youngest cobble on a terrace, that minimizes the inheritance, is the most appropriate CRE age for approaching that of terrace abandonment. However, the upper bound on the age of abandonment of a terrace that is young with respect to the amount of inheritance is best estimated by the OSL dating of the terrace material. For such terraces, the CRE measurements are complementary of OSL dating and can be used to unravel the complex history of weathering and transport in the catchment of desert alluvial fans. This comprehensive set of dating is combined with morphological offsets ranging from 12 ± 2 m to 380 ± 20 m to demonstrate the Dehshir fault slips at a rate in the range 0.9 mm.yr⁻¹ - 1.5 mm.yr⁻¹. The variable inheritance exemplified here may have significant implications for CRE dating in arid endorheic plateaus such as Tibet and Altiplano.

Domaines: Sciences de la Terre

Kristell Le Dortz, Bertrand Meyer, Michel Sébrier, Regis Braucher, H. Nazari, et al.. Dating inset terraces and offset fans along the Dehshir Fault (Iran) combining cosmogenic and OSL methods. Geophysical Journal International, 2011, 185 (2), pp.1-28. ⟨10.1111/j.1365-246X.2011.05010.x⟩. ⟨hal-00590894⟩