VARIATION IN DEFORMATION STYLE BOTH ACROSS AND ALONG STRIKE IN FOLD-THRUST BELTS: EXAMPLES FROM THE ZAGROS SIMPLY FOLDED BELT, IRAN AND THE SAWTOOTH RANGE, MONTANA

The Zagros Simply Folded Belt is ideal for the study of progressive deformation at a convergent margin using remote mapping. In the Zagros, lines of anomalously long, high-aspect ratio folds, crossed by multiple wind gaps, are clearly linked to movement along underlying thrusts and correlate with major steps in the landscape. The thrusts formed sequentially as the deformation front progressed towards the Persian Gulf.

In the Zagros, brittle and ductile deformation processes interact. Movement over a thrust ramp creates a fault-bend fold. Movement on the thrust becomes progressively more difficult and the compressive stresses build up, leading to serial folding in the cover behind the fault-bend fold. Eventually, movement along the thrust requires stresses in excess of those required to form a new thrust. The original thrust is abandoned, the footwall collapses and the process repeats.

A field study of a fold pair in the frontal region of the Sawtooth Range indicates that folds change character along strike. The Teton Anticline and adjacent Little Teton Anticline are incipient box folds with broad rounded hinges, with variable wavelengths related to the depth to the detachment. Teton Anticline is symmetric in the northern mapped section but becomes asymmetric when traced to the south. Little Teton Anticline also shows some asymmetry before dying out in the south of the mapped area. The change in geometry of the Teton Anticline implies that there is a corresponding change in the geometry of the subsurface detachments to the south. The spatial development of this complexity coincides with the location of a basement fault zone and may be accommodated by a transfer zone in the subsurface faults.

Folds in a fold-thrust belt display a spectrum of different geometries between detachment folds and fault-bend folds. These differences are determined by the relative amounts of brittle and ductile deformation and can occur both across the strike of a fold-thrust belt and along strike of individual folds.