Cinématique de l'Iran central et oriental : Morphotectonique et datations cosmogéniques et OSL

This study presents the results of geomorphological analysis on faults kinematics in central and eastern Iran. According to GPS measurements, the right-lateral shear between central Iran and Afghan block amounts to 16 mm/yr (Vernant et al., 2004 ; Masson et al., 2007) and might be accommodated by strike-slip faulting limited to the western and eastern edges of the Lut block (Walker et al., 2004). However the geological slip-rate of these faults is poorly constrained as for the faults located within the Iranian plateau, where limited activity is inferred. We demonstrate the ongoing activity of the Anar and Deshir right-lateral strike-slip faults located on the plateau and establish their geological slip-rate averaged on several seismic cycles. We combined cumulative offset of alluvial fan surfaces measurements with their ages obtained by cosmic ray exposure dating (10Be et 36Cl) and optically stimulated luminescence dating (OSL). The comparison of surface and near-surfaces ages and concentrations reveals the occurrence of a variable and significant inheritance in the alluvial material and we suggest a method to estimate the minimum amount of inheritance. We derived minimum slip-rates of 0.8 mm/yr and 0.85-2 mm/an for the Anar and Deshir faults, respectively. These results suggest that the right-lateral shear between central and eastern Iran might affect a wider region than the Lut block and its boundaries and is accommodated by a larger number of faults than initially suggested. Geological and GPS data are in insufficient number to determine if there are incompatibilities between finite and instantaneous deformation.Ce travail de thèse présente les résultats d'études géomorphologiques sur la cinématique de failles dans le centre et l'est de l'Iran. Dans cette région, des observations géodésiques associées à l'analyse d'un réseau GPS à large maille (Vernant et al., 2004 ; Masson et al., 2007) suggèrent que le cisaillement dextre entre l'Iran Central et le bloc Afghan est d'environ 16 mm/an. Un modèle cinématique (Walker et al., 2004) propose que ce cisaillement dextre soit pris en compte depuis les 5 derniers Ma par les grands décrochements limitant le Lut à l'Ouest et à l'Est. Leur vitesse géologique est cependant mal contrainte tout comme celle de décrochements situés à l'intérieur du plateau iranien, généralement considérés peu ou non actifs. Nous démontrons l'activité de deux décrochements dextres situés sur le plateau iranien, les failles d'Anar et de Deshir, et déterminons leur vitesse de glissement moyennée sur plusieurs cycles sismiques. Pour cela, des décalages de bords d'incision de terrasses alluviales ont été mesurés et des datations par mesure de la production in-situ en isotopes cosmogéniques (10Be et 36Cl) et par luminescence stimulée optiquement (OSL) ont été effectuées. L'analyse des données de surface et de profil, obtenues par les mesures d'isotopes cosmogéniques, a permis de mettre en évidence l'existence d'un héritage variable et important du matériel composant les terrasses alluviales. Nous avons proposé une méthode pour estimer la quantité minimum d'héritage variable, dans le but de déterminer l'âge d'abandon des surfaces abandonnées puis les vitesses de glissement sur les failles. Les datations par isotopes cosmogéniques et OSL combinées aux mesures de décalages morphologiques impliquent des vitesses de glissement géologique court-terme de 0.8 mm/an pour la faille d'Anar et de 0.85 – 2 mm/an pour la faille de Deshir. Ces résultats nous permettent de discuter le rôle de ces failles dans l'accommodation de la déformation dans le centre et l'est de l'Iran, suggérant que la déformation cisaillante se répartit sur une zone plus large que le Lut et ses bordures, et est prise en compte par un plus grand nombre de failles, qu'initialement supposé. Les données géologiques et GPS disponibles sont encore en nombre insuffisant pour déterminer s'il y a ou non des incompatibilités entre déformation finie et déformation instantanée

Seismic swarms in Tricastin, lower Rhône Valley (France) : review of historical and instrumental seismicity and models

International audienceThe Tricastin region in the lower Rhône Valley (France) is affected by an atypical seismic activity characterised by the development of long-lasting and recurrent seismic swarms. Indeed, since the 16th century, hundreds of seismic events sometime associated with underground noises of the explosion have been reported by local inhabitants. However, to date, none of the many scenarios of earthquake generation proposed for the area, involving either tectonics and/or hydrological forcings, appears consensual. To overcome that lack of comprehension, we compile and analyse an 880 seismicevents catalogue derived from both historical macroseismicity and instrumental records. The earthquakes appear to occur at shallow depths similar to those determined below a local network in 2002-2003. We confront to this catalogue models involving hydrological mechanisms, including aquifers elastic loading and karst-drains responses, as well as tectonic mechanisms, including transient aseismic processes and their related effects on the fold hinges or on the local fault planes. Most of the earthquakes are located at short distances from karst drains and fractured fold hinges, possibly affected by transient hydrological changes

Interpreting scattered in-situ produced cosmogenic nuclide depth-profile data

International audienceModelling the evolution of the concentration of in-situ produced cosmogenic nuclides as a function of depth (depth-profile) has been developed to allow determining both the exposure duration and the denudation rate affecting geomorphic features. However, material sampled through surficial deposits may exhibit an inherited component resulting from exposure to cosmic rays before deposition. In case of homogeneous inheritance, this inherited component may be estimated through sampling at increasing depths and subsequently subtracted. In case of variable inheritance, the measured concentrations are scattered and the random distribution of the depth-profile concentrations prevents modelling confidently a depth-profile and precludes constraining an exposure duration. Often observed in desert and endorheic regions, this greatly restricts the possibilities to determine an accurate abandonment age of alluvial surfaces in such environments. Provided the denudation is demonstrated negligible, a method for determining a more accurate range of minimum inheritance, hence a more accurate maximum abandonment age for a given alluvial surface, is proposed. This method, based on the rejuvenation of depth-profile samples, relies on the simple hypothesis that at least one of the depth-profile samples would be emplaced with no or negligible inherited component and on the obvious principle that none of analyzed sample has been emplaced with a negative cosmogenic nuclide concentration. The method consists then in determining which of the measured depth-profile sample may have been emplaced with a null CRE concentration; i.e., with a zero inheritance value. This requires to calculate the in-situ duration of exposure needed to reach the concentration measured for each depth-profile sample and to retain the one that provides the smallest in-situ exposure duration. Several examples from alluvial surfaces of central Iran illustrate the profile rejuvenation method and highlight a variable inheritance ranging between 1.5x10⁵ and 16.1x10⁵ at/g (SiO₂) for terraces whose abandonment ages range from ten to several hundreds of ka

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

International audience¹⁰Be 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

Holocene right-slip rate determined by cosmogenic and OSL dating on the Anar fault, Central Iran

International audienc

Late Pleistocene-Holocene right-slip rate and paleoseismology of the Nayband fault, western margin of the Lut block, Iran

International audienceThe 290-km-long, Nayband strike-slip fault bounds the western margin of the Lut block and cuts across a region thought to have been quiescent during the last few millennia. Cl-36 cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of cumulative geomorphic offsets are used to derive the long-term slip rate. The measured offsets at two sites along the fault range between 9 ± 1 m and 195 ± 15 m with ages from 6.8 ± 0.6 ka to ∼100 ka, yielding minimum and maximum bounds of late Pleistocene and Holocene slip rates of 1.08 and 2.45 mm yr-1, respectively. This moderate slip rate of 1.8 ± 0.7 mm yr-1, averaged over several earthquake cycles, is compared to the paleoseismic record retrieved from the first trench excavated across the fault. Combining the paleoseismic evidence with 18 OSL ages obtained from this trench site demonstrates the occurrence of at least four large (M ∼7) earthquakes during the last 17.4 ± 1.3 ka and of two older wearthquakes, one before ∼23 ka and another before 70 ± 5 ka. The exposed sediment succession also indicates a significant gap at the end of MIS-2 and the beginning of MIS-1. The age of the most recent regional incision is accurately bracketed between 6.1 ka and 7.4 ka. Sediments from the last ∼7 ka contain evidence of the three younger earthquakes. Interestingly, the penultimate and antepenultimate events occurred between 6.5 ± 0.4 ka and 6.7 ± 0.4 ka within a time interval lasting at most 1 ka whereas the most recent earthquake occurred within the last millennium. Such an irregular earthquake occurrence suggests the seismic behavior of the Nayband fault is not strictly time dependent but possibly related to clustering. From this and taking into account the occurrence of the most recent earthquake within the last 800 years, the imminence of an earthquake along the Nayband fault cannot be discarded. Although the most recent surface-rupturing event seems to have occurred after AD 1200, this event went unnoticed in the historical records. This provides a marked illustration of the incompleteness of the historical seismic catalogs in Central Iran, challenging any assessment of regional seismic hazard without appropriate geologic and geochronological information. Large and infrequent earthquakes are characteristic of the seismic behavior of the slow-slipping strike-slip faults slicing Central and Eastern Iran. Also, the slip rates summed across Central and Eastern Iran from the Iran Plateau up to the Afghan lowlands appear in agreement with the most recent GPS data