Contrasting structural styles in northern and southern Zagros: transpressional collision caused by oblique closure of Tethys

There is widespread evidence along the Makran-Zagros-Bitlis fold belt that deformation has involved a combination of shortening and strike-slip (i.e. transpression) in response to the oblique closure of Tethys in Cenozoic times. This is strongly supported by field observations, present-day plate vector velocities derived from geodetic GPS measurements, and seismological data.

The way in which transpressional collision is accommodated varies along the fold belt, and a number of regional deformational domains can be delineated. First-order variations in structural style are strongly influenced by the local angle of incidence between the direction of plate motion and the orientation of the colliding plate boundaries. A key factor is the degree to which transpressional deformation is partitioned into constituent strike-slip (simple shear) and shortening (pure-shear) components, and the scales at which partitioning occurs.

In the Makran, NW Kurdistan and SE Turkey sectors, plate motion is approximately perpendicular to the orientation of the plate boundary, and the structural style is dominated by folds and thrusts with little evidence of significant strike-slip. In the Fars and Dezful sectors, plate motion is oriented ca. 45-55 relative to the plate boundary, and the strike-slip component is accommodated along a number of regional-scale right-lateral faults, each with many kilometres or tens of kilometres of displacement. In Lurestan and eastern Kurdistan the angle of incidence of plate motion is more oblique, marked by major plate-scale strain partitioning in which the simple shear component of deformation is localised onto a major active strike-slip fault in NW Iran, the Main Recent Fault.

The effective plate-scale partitioning of strain and localisation of simple shear in the NE Zagros explains the apparent paucity of field evidence for strike-slip in Kurdistan. An exception is seen in the Bradost region (Dinarta block) situated at the apex of the oroclinal bend (i.e. where the NW-SE orientation of the Zagros changes into the WNW-ESE structural trend of northern Kurdistan and the Bitlis zone). The Bradost anticline shows strong evidence for significant right-lateral displacement (see presentation by Kilsdonk et al., this conference). Further evidence of a component of strike-slip may lie in the asymmetry of fracture systems relative to a number of anticlinal axes observed in Kurdistan.