Tectono-stratigraphic evolution, regional structure and fracture patterns of the Zagros fold-thrust belt in the Duhok region, Kurdistan, northern Iraq

Abstract

This study is the first detailed investigation of the tectono-stratigraphy, regional structure and mesoscopic fractures of the Zagros fold-thrust belt in the Duhok region, Kurdistan, Northern Iraq, and is based on field studies, remote sensing-GIS analyses, cross-section restorations, and tectonic analysis of key structures. The Duhok fold belt is a frontal zone of the north-western part of the Zagros fold and thrust belt in the Kurdistan region near the north-eastern margin of the Arabian Plate. It is characterized by WNW-ESE striking, doubly-plunging, upright symmetric to asymmetric detachment folds that involve ~8.2–10.9 km of Paleozoic to Quaternary strata. These folds vary from 5 to 75 km long, 4–20 km wide and have a surface relief (caused by basement uplift) up to 3.2 km. Five regional and twelve local structural cross-sections were constructed parallel to the regional SSW tectonic transport and perpendicular to the fold axial trends. Section lengths vary between 4.94 km and 101.55 km. Two main detachment levels have been inferred for the Duhok fold belt. A deep basal detachment was placed at the base of the Paleozoic successions within the Ordovician shales of the Khabour Formation at depths from 8 km to 10 km. Shallower, intermediate detachment levels occur within the shales and evaporites of the Mid-Upper Permian and Upper Jurassic strata at depths 1.7 km in the frontal sectors to around 5.5 km in the hinterland. The thickness of the Paleozoic succession in the Duhok fold belt is ~2 km. Deformation by thin-skinned detachment folding together with thick-skinned thrusting is inferred to have occurred in the Duhok region from the Mid Miocene through to the Present-Day. The maximum NNE-SSW-directed shortening produced by detachment folding was about 18.62 km (19.71%) over a section length of 94.45 km. The maximum shortening along thrust faults was around 1.98 km (1.95%) over a section length of 101.55 km. The mesoscopic fracture patterns in the study area were grouped into two major extension sets, four major shear-hybrid sets and eight minor shear-hybrid sets. Field characteristics and relative chronology indicate that most of the fractures are tectonic in origin, and they developed before (i.e., pre-folding fractures) and during (i.e., early- and late-folding fractures) the Mid Miocene folding in the Duhok fold belt.