Abstract The Zagros fold–thrust belt (ZFTB) extends for c . 2000 km from Turkey in the NW to the Hormuz Strait in the SE. This belt results from the collision of the Arabian and Eurasian plates during Cenozoic times and constitutes a morphological barrier (with some peaks exceeding 4000 m) separating the Arabian platform from the large plateaux of central Iran. To the east a pronounced syntaxis marks the transition between the Zagros collision belt and the Makran accretionary wedge. In the ZFTB, the Proterozoic to Recent stratigraphic succession pile of the southern Tethys margin is involved in huge folds detached from the Pan-African basement and offers the opportunity to study the stratigraphic and tectonic evolution of the Palaeo-Tethyan margin over large time periods. Few recent data are widely available on the southern Tethys margin as preserved in the Zagros Mountains. Since the classical works of James & Wynd (1965) and Murris (1980) , the most recent synthesis is the palaeogeographical reconstruction of the Arabian platform published by Ziegler (2001) . Many petroleum data have been acquired during the last 10 years, but few of these have been published. The Middle East Basins Evolution (MEBE) Programme, coordinated by P. Barrier and M. F. Brunet, in close relationship with colleagues of the Geological Survey of Iran, was an excellent opportunity to go back to the field and to collect new data to better constrain the evolution of this margin. In this volume, the structure of the Zagros Mountains is explored through different scales and using different approaches.

Abstract In the Zagros Fold–Thrust Belt (ZFTB) of Iran it is firmly established that the basement is involved in the deformation. The strongest line of evidence for this assertion comes from the relatively intense mid-crustal seismic activity. On one hand, the main basement structures such as the Main Zagros Fault (MZT) and High Zagros Fault (HZF) reach the surface and are therefore well identified. On the other hand, basement faults south of the HZF are hidden by sedimentary cover and their location is uncertain. In the Eastern Zagros, basement control on surface structures occurred only at a late stage of the tectonic evolution. In other words, the current thick-skinned style of Zagros deformation succeeded a more general thin-skinned phase of orogeny. This chronology is particularly well illustrated by spectacular interference patterns, in which early detachment folds are cut by late oblique basement faults. We present a combined morphological and structural analysis of such structures, and we explore their impact on the river network. We confirm that basement involvement occurred at a late stage of deformation and we show that thick-skinned deformation progressively propagated towards the foreland. An overview of basement steps throughout the Zagros based on published cross-sections allows us to conclude that although basement deformation is concentrated on two major faults in the Central Zagros, it is distributed on several segmented faults in the Fars Arc. This segmentation increases to the SE towards the so-called Oman line and the transition to the Makran accretionary prism.

ABSTRACT The kinematics and rates of displacement along single faults of the Outer Belt and through the entire thrust belt of the Himalayas of western Nepal have been estimated by structural field work, balanced cross sections, fluviatile terrace-deposit studies, and geodesy. GPS geodetic studies indicate that the shortening is 15 ± 2mm/yr and is perpendicular to the trend of the western Nepal Himalayas and compatible with elastic strain accumulation. This surface shortening would be induced by a 19 mm/yr slip rate, at depth, on the basal detachment beneath the Great Himalayas, whereas the detachment is locked beneath the Lesser Himalayas. In the outer zone, most of the displacement presumably occurs during major seismic events, and the 2.5- to 5-m thrust movements observed locally for imbricate fans branching off the major thrusts of the Sub-Himalayas could reflect surficial rupture during these events. During the Holocene, the Main FrontalThrust (MFT) has been active, but portions of the piggyback thrusts of the Outer Belt also show episodes of activity that vary from a few meters to several tens of meters of displacement. The ratio of MFT shortening to total Himalayan shortening varies laterally, from 1 to less than 0.5, and out-of-sequence thrusting occurs in the Sub-Himalayas and possibly in the Lesser Himalayas. This pattern of episodic out-of-sequence reactivation fits with the evolution of a brittle thrust wedge affected by sur-ficial mass transport and/or fluid pressure variation due to fault-valve behavior. The total shortening rate through the Himalayas of western Nepal following the Miocene is 19 mm/yr, with present-day, Holocene, and long-term shortening-rate uncertainties of 2, 6, and 5 mm/yr, respectively.

ABSTRACT The Sub-Himalayan Zone constitutes a tectonic wedge of synorogenic sediments along the southern edge of the Himalayan Belt. Sediments are incorporated into the prism from the foreland Indo-Gangetic plain, undergo a tectonic cycle within it, and eventually are eroded. The structural sketch map exhibits westward-plunging arcuate structures on the foremost location of the Outer Belt. Investigations from spatial imagery and digital elevation modeling (DEM), together with kinematic data, allow us to calculate velocities for the geomorphologic development. Four velocities rule the general evolution of the wedge. The foremost geomorphic structure (ridge) is the assemblage of elementary structures. The lateral ridge propagation velocity is estimated to be 40 cm/yr, which supports a general cylindrical development of the Outer Belt, in spite of the asymmetrical development of each independent elbow-shaped structure. The sediment’s burial history can be quantified from geometric and kinematic data. We emphasize that because of the cylindrical behavior of the prism, extrapolation of the sediment transfer to the entire western Nepal Siwalik is valid. Burial in the foreland basin takes two times longer than the entire tectonic cycle, which only lasts for about 6.5 m.y. Sediments reaching 6 x 10– 5 km 3 per year and per linear kilometer accrete along the Siwalik range. About 21% of the flowing material within the wedge is captured and withdrawn from it as subducted duplexes. Assuming a steady-state development of the wedge, and according to the Coulomb-wedge theory for the Siwalik, mean erosion rates are estimated to be about 1.9 mm/yr, which is in accord with previous estimates. We emphasize that this consistency supports the overall estimates for the general development of the wedge.