DEFORMATION BY FOOTWALL COLLAPSE AND SERIAL FOLDING IN A FOLD-THRUST BELT

The Zagros Simply Folded Belt is ideal for the study of progressive deformation at a convergent margin. This deformation is being analysed using processed satellite images, digital elevation models and synthetic river networks.

Detachment folds, characterised by a periclinal geometry and fault-bend folds, characterised by long, linear hinges occur in the Zagros Belt. The amplification rate of periclinal folds can be defined from theoretical folding analyses and is found to vary in time and space. At a specific point on the fold the uplift rate varies with time and at a particular time the uplift rate varies along the fold hinge. In contrast, fault-bend folds result from relatively uniform uplift of a basement block and do not show the same variation in uplift rate. This is reflected in the different patterns of wind and water gaps controlled by the different uplift histories and which characterise the two fold types.

River networks in the Zagros form a trellis pattern where the prominent direction parallels the fold hinges. Major river diversions are spatially associated with folds crossed by multiple wind gaps and with high aspect ratios, considered to be fault-bend folds. Fold Front Sinuosity measurements, used as a proxy for age, show that fold structures young to the SW, with anomalously old folds near the major thrust faults.

Major steps in the landscape correlate with the long folds and major thrust faults. These faults formed sequentially as the deformation front migrated to the SW, generating the long overlying folds and causing the diversion of river channels.

In the Zagros Belt brittle and ductile deformation processes interact. Movement up a thrust ramp creates a fault-bend fold. Compressive stresses build up leading to serial folding in the cover behind the fault-bend fold. Eventually deformation of the block requires stresses in excess of those required to form a new thrust. The original thrust is abandoned, the footwall collapses and the process repeats.