ABSTRACT. The Zagros fold - thrust belt in Iran forms the external part of theZagros active orogenic wedge. It includes a sequence of heterogeneous latest Neoproterozoic–Phanerozoic sedimentary cover strata, 7 to 12 km thick and composed of
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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 - ofsequence 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
