GNGTS 2022 - Atti del 40° Convegno Nazionale

176 GNGTS 2022 Sessione 1.3 THERMAL STRUCTURE OF THE UPPER MANTLE IN THE ZAGROS COLLISION ZONE AND SURROUNDINGS M. Tesauro 1,2 , A.Pastorutti 1 , T. Pivetta 1 , C. Braitenberg 1 , I. Koulakov 3,4 1 Trieste University, Italy 2 University of Utrecht, Utrecht, Netherlands 3 Trofimuk Institute of Petroleum Geology and Geophysics, SB RAS, Prospekt Koptyuga, Novosibirsk, Russia 4 Novosibirsk State University, Novosibirsk, Russia Late Mesozoic convergence between the Arabian plate and Eurasia generated subduction of the Neo-Tethys ocean beneath Central Iran, and the onset of the closure of the oceanic domain occurred in Late Cretaceous. The final closure of the Neo-Tethys ocean (~12 Ma) formed the Zagros collision zone composed of different parallel tectonic features extending from southwest to northeast, from the Turkish-Iranian border in the NW, to the Makran area in the SE (where oceanic subduction is still active): the Zagros Fold and Thrust Belt (ZFTB), the Sanandaj–Sirjan Metamorphic Zone (SSZ), and the Urumieh-Dokhtar Magmatic Assemblage (UDMA) (Mouthereau et al. , 2011). The ZFTB is the young and seismically active zone of the Zagros Mountains and is separated from the SSZ by the Main Zagros Thrust (MZT), which is considered to be the suture zone between the Arabian lower plate external zones to the SW and the Eurasian upper plate internal zones to the NE. The SSZ consists mainly of Precambrian metamorphic rocks and igneous rocks, whose formation is related to the subduction of the Neo-Tethyan slab. The UDMA hosts abundant Tertiary magmatism, dominantly of arc or island-arc type. Central Iran, the UDMA and SSZ can together be considered to represent the upper plate domain during most of the recent convergence history leading to the Zagros orogeny (e.g. Agard et al. , 2011). The changes in arc magmatism during Eocene time, with a major shift from the SSZ to the UDMA, and with extensive magmatism affecting much of Iran have been attributed to a change in the slab dynamics: from a change in the slab dip angle to lateral slab tearing/break-off to slab folding at the mantle transition zone (e.g. Agard et al. , 2011). Currently, the convergence is accommodated across the Iranian Plateau and the surrounding mountain ranges, at a rate of 10–20 mm/yr, resulting in different styles of deformation in this active collision/subduction zone (e.g. Khorrami et al. , 2019). Several models, based on the interpretation of seismic tomography and receiver function data, revealed high velocities beneath the Zagros in the upper mantle down to depths exceeding 200 km, implying a relatively thick lithospheric mantle (e.g. Priestley and McKenzie, 2006). However, some studies, based on the lack of deep seismicity in the Zagros and absence/ reduction of high-velocity anomalies below 200 km depth, support the hypothesis of the slab break-off and delamination of the lower part of the Arabian lithosphere. In contrast, other studies reveal the slab continuity up to the depth of the transition zone (e.g., Wortel and Spakman, 2000; Koulakov et al. , 2011). Furthermore, according to some authors, thick high- velocity lithosphere of the Arabian Plate is extended beneath UDMA and southeastern Central Iran (e.g., Motaghi et al. , 2017; Mahmoodabadi et al. , 2020), while based on some others, thin lithosphere characterizes the UDMA (Mohammadi et al. , 2013 ) and the SSZ (e.g. Manaman et al., 2011). Therefore, up to now, a consensus has not been reached on the maximum depth and neither on the lateral extension of the subducting Arabian lithosphere. Other uncertainties are related to the dip and nature of the slab. In order to address these controversial issues, we analyze global and regional seismic tomography models and convert their absolute velocities in temperature, assuming the composition of a Phanerozoic mantle, taken from studies on xenolith samples. Indeed, the conversion in temperatures allows us to better identify the shapes of the upper mantle features. To this purpose, we use Perple_X (Conolly, 2005) that computes physical properties for a given mineralogical model, expressed by the main mantle