Tectono-stratigraphy and general structure of the northwestern Zagros collision zone across the Iraq-Iran border

Abstract

Tectono-stratigraphic units within the Zagros Orogen in northeast Iraq (foreland) and northwest Iran (hinterland) are correlated to provide an integrated map along the collision zone. Access to this part of Iraq during the past four decades has been limited due to geopolitical situation. Structural cross-sections across the Zagros Suture Zone in this area reveal the relationships between the tectonic terranes of various ages and different origins. Terranes of oceanic affinity have accreted onto the Arabian plate during collision-accretion events that started in the Late Cretaceous. The collision resulted in closure of the Neotethys Ocean and the construction of a structurally complex suture zone. Jurassic-Cretaceous deep ocean radiolarites of the Qulqula-Kermanshah terrane and ophiolitic mélange serpentinites were structurally accreted against the Arabian passive margin during an ophiolite arc-continent collision event in the Late Cretaceous. The overthrust radiolarites and ophiolitic mélange terranes initiated the development of the foreland basin overlapping flysch (turbidites) and molasse assemblages on the now active Arabian margin. Eocene-Oligocene volcano-sedimentary rocks of the Walash-Naopurdan-Kamyaran terrane developed as an intra-oceanic island-arc within the intervening Neotethys Ocean. They now structurally overlie the older ophiolite mélange and radiolarite terranes as a result of continued convergence onto the margin of the Arabian Plate. These younger thrust sheets and nappes have been transported over the Miocene molasse unit of the Tertiary Red Beds in the flexural foreland basin and covered the Late Cretaceous accretionary complex terranes and foreland basin assemblages. The Qulqula-Kermanshah terrane is exposed in a tectonic window in the northeastern part of the mapped area indicating that the Late Cretaceous accreted terranes occur below the Walash-Naopurdan-Kamyaran thrust sheet. A Late Cretaceous ophiolite-bearing terrane named the “Upper Allochthon” (i.e. Gemo-Qandil nappe, 97-118 Ma) was emplaced by younger thrust sheets over the Eocene-Oligocene Walash-Naopurdan-Kamyaran terrane by out-of-sequence thrusting. Mesozoic metamorphic rocks from the hinterland, including volcanic and intrusive rocks of the active Iranian continental margin (Sanandaj-Sirjan zone), were emplaced during continent-continent collision and occur in the youngest nappes and klippes along the Zagros Suture Zone.

Keywords: Iraq; Iran; Neotethys; Tectonic; Zagros