Tertiary sequence of deformation in a thin-skinned/thick-skinned collision belt: The Zagros folded belt (Fars, Iran)

We describe how thin-skinned/thick-skinned deformation in the Zagros Folded Belt interacted in time and space. Homogeneous fold wavelengths (15.8 +/- 5.3 km), tectono-sedimentary evidence for simultaneous fold growth in the past 5.5 +/- 2.5 Ma, drainage network organization, and homogeneous peak differential stresses (40 +/- 15 MPa) together point to buckling as the dominant process responsible for cover folding. Basin analysis reveals that basement inversion occurred similar to 20 Ma ago as the Arabia/Eurasian plate convergence reduced and accumulation of Neogene siliciclastics in foreland basin started. By 10 Ma, ongoing contraction occurred by underplating of Arabian crustal units beneath the Iranian plate. This process represents 75% of the total shortening. It is not before 5 Ma that the Zagros foreland was incorporated into the southward propagating basement thrust wedge. Folds rejuvenated by 3-2 Ma because of uplift driven by basement shortening and erosion. Since then, folds grew at 0.3-0.6 mm/yr and forced the rivers to flow axially. A total shortening of 65-78 km (16-19%) is estimated across the Zagros. This corresponds to shortening rates of 6.5-8 km/Ma consistent with current geodetic surveys. We point out that although thin-skinned deformation in the sedimentary cover may be important, basement-involved shortening should not be neglected as it requires far less shortening. Moreover, for such foreland folded belts involving basement shortening, underplating may be an efficient process accommodating a significant part of the plate convergence.