DEFORMATION RATES IN THE CENTRAL ZAGROS (IRAN) INDICATED BY DISPLACEMENT OF GEOMORPHIC SURFACES

The Zagros fold belt results from active collision of the Arabian plate with central Iran. Although the architecture of this accretionary prism has been extensively studied because of its important implications for hydrocarbon exploration, aspects such as the kinematics of individual folds and the sequence of fold development remain to be investigated in detail. We have mapped deformed fluvial terraces along the Dalaki and Mand rivers in the central Zagros, as well as marine terraces along the Persian Gulf, in order to unravel the spatial pattern of vertical displacements and to analyse active deformation and its implications for seismicity. Using appropriate fold models based on structural data allows interpreting such vertical displacements in terms of horizontal shortening across the fold structures. Obtaining well-constrained rates of deformation depends on reliably dating deformed geomorphic markers; by combining different dating techniques we propose an internally consistent set of ages, which allow the first geomorphic estimates of shortening rates absorbed by individual structures in the central Zagros. Our results show that shortening on Late Pleistocene timescales is concentrated in the frontal part of the belt, consistent with recent GPS data. Three or four frontal structures appear to absorb practically all of the active shortening across the Zagros, broadly consistent with a normal forward-propagating deformation sequence but with local out-of-sequence activation of structures behind the forward-most folds. The localization, rate and direction of surface shortening across individual structures appear decoupled from basement deformation, as indicated by structural cross-sections and the distribution of seismicity. Such independent behaviour suggests that the sedimentary cover of the frontal Zagros is decoupled from the basement, most probably at the level of the Hormuz Salt. This weak basal detachment level, together with several intermediate d´ecollement levels, appears to be responsible for the overwhelmingly seismic deformation of the Zagros sedimentary cover, and also to control the development of a large panel of fold structures, from detachment to fault-propagation folds with varying wavelength and rooting depth.