Structures of the Zagros fold-thrust belt in Iran

The Zagros fold-thrust belt in Iran forms the external part of the Zagros active orogenic wedge. It includes a sequence of heterogeneous latest Neoproterozoic--Phanerozoic sedimentary cover strata, ∼7 to 12 km thick and composed of alternating incompetent and competent layers, overlying Precambrian crystalline basement with a complex pre-Zagros structural fabric. Balancing structures of the cover statigraphic units in the Zagros fold-thrust belt requires in-sequence and out-of-sequence involvement of the Precambrian basement in the deformation. Six detailed balanced and retrodeformable cross-sections, which are constructed based on geological and geophysical data across various sectors of the belt, show fault-bend and fault-propagation folds interpreted to have formed by slip on their subjacent thrusts. Out-of-sequence, basement-rooted thrusts, as the interpretations in the constructed cross-sections suggest, have breached the cover/basement interface and, using incompetent cover strata for propagation, cut across the cover structures and have created associated new folds superimposed upon the pre-existing structures. This style of deformation, which has resulted in structural complexity of the belt, characterizes a ∼200 to 300 km-wide zone of distributed, partly synchronous, deformation with along-strike and across-strike variations. It also implies that the Zagros fold-thrust belt, as the external part of the orogenic wedge, is still in its subcritical condition with internal deformation to achieve a critical taper. In creating structural complexity, in addition to out-of-sequence thrusting, mechanically weak layers (evaporites and mudstones) of the cover strata have played a significant role by providing several detachment horizons. Shortening estimates across the belt are variable; based on the constructed cross-sections and their restorations, minimum shortening estimates range from 16 percent to 30 percent in different sectors of the belt.