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A number of field observations suggest that sliding on fault planes may occur at very shallow dip in the brittle field. The existence of active low angle normal faults is much debated because (1) the classical theory of fault mechanics implies that faults are locked when the dip is less than 30° and (2) shallow dipping fault planes do not produce large earthquakes ( M > 5.5). To reconcile observations and theory, we propose a new model for fault reactivation by introducing an elasto-plastic frictional fault gouge as an alternative to the classical dislocation models with frictional properties. Contrary to the classical model which implies that the dilation angle equals the friction angle , our model accounts for and permits in the fault gouge as deduced from laboratory and field observations. Whilst the predicted locking angles differ in most cases by less than 10° from the classical model, a significant amount of plastic strain (strain occurring in elasto-plastic regime) is predicted to occur on badly oriented faults prior to locking in a strain-hardening regime. We describe four modes of reactivation which include complete/partial reactivation with/without tensile failure in the surrounding medium. This paper presents analytical solutions to separate those four different modes as well as numerically computed estimates of the amount of plastic strain accumulated before locking. We conclude that plastic strain on badly oriented faults is favoured by compaction of the fault gouge and that those faults are probably aseismic because this strain always occurs in a hardening regime. [ABSTRACT FROM AUTHOR]

Geological and geomorphologic offsets are used to constrain the total displacement and estimate the slip rate of the right-lateral Deshir Fault. Cumulative morphologic offsets have been found along the southern portion of the fault, on the Iranian Plateau. High-resolution SPOT5 images (pixel size 2.5 m) combined with SRTM DEM document right-lateral offsets of rills, terrace risers and successive alluvial fan systems. Cumulative offsets range from 25 to 900 m and increase with relative elevation, hence relative age, of offset markers. The smallest cumulative offsets are most probably of Holocene age and suggest that the fault slips at an average slip rate close to 2 mm yr−1. Coeval normal faulting is associated to strike-slip motion at several places along the southern portion of the fault. Normal and probably active faults are encountered within the High Zagros 70 km ahead the tip of the Deshir Fault and may suggest the fault is propagating southwards. The total displacement of the Deshir fault, estimated from right-lateral offset of the Nain-Baft suture, is . The total displacement would have accumulated over the last 25–40 Ma at an averaged Holocene slip rate of 2 mm yr−1. Placing these observations within the framework of the tectonics of Iran allows a discussion of the timing of the transition between collision and subduction in Eastern Iran. [ABSTRACT FROM AUTHOR]