Structures of the Zagros fold-thrust belt in Iran

TLDR

The Zagros fold‑thrust belt in Iran, the external part of the Zagros orogenic wedge, consists of a 7–12 km thick heterogeneous Neoproterozoic–Phanerozoic sedimentary cover over a Precambrian crystalline basement with a complex pre‑Zagros fabric, requiring both in‑sequence and out‑of‑sequence basement involvement for balancing structures. Six detailed balanced and retrodeformable cross‑sections built from geological and geophysical data across the belt reveal fault‑bend and fault‑propagation folds that formed by slip on underlying thrusts. These cross‑sections show that out‑of‑sequence, basement‑rooted thrusts breach the cover/basement interface, propagate through incompetent cover layers, and generate new folds superimposed on pre‑existing structures, producing a 200–300 km wide zone of complex, partly synchronous deformation, a still subcritical belt with internal shortening of 16–30 % and weak layers acting as detachment horizons.

Abstract

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.