Subduction initiation and back-arc opening north of Neo-Tethys: Evidence from the Late Cretaceous Torbat-e-Heydarieh ophiolite of NE Iran

Abstract

How new subduction zones form is an ongoing scientific question with key implications for our understanding of how this process influences the behavior of the overriding plate. Here we focus on the effects of a Late Cretaceous subduction-initiation (SI) event in Iran and show how SI caused enough extension to open a back-arc basin in NE Iran. The Late Cretaceous Torbat-e-Heydarieh ophiolite (THO) is well exposed as part of the Sabzevar-Torbat-e-Heydarieh ophiolite belt. It is dominated by mantle peridotite, with a thin crustal sequence. The THO mantle sequence consists of harzburgite, clinopyroxene-harzburgite, plagioclase lherzolite, impregnated lherzolite, and dunite. Spinel in THO mantle peridotites show variable Cr# (10–63), similar to both abyssal and fore-arc peridotites. The igneous rocks (gabbros and dikes intruding mantle peridotite, pillowed and massive lavas, amphibole gabbros, plagiogranites and associated diorites, and diabase dikes) display rare earth element patterns similar to MORB, arc tholeiite and back-arc basin basalt. Zircons from six samples, including plagiogranites and dikes within mantle peridotite, yield U-Pb ages of ca. 99–92 Ma, indicating that the THO formed during the Late Cretaceous and was magmatically active for ∼7 m.y. THO igneous rocks have variable εNd(t) of +5.7 to +8.2 and εHf(t) ranging from +14.9 to +21.5; zircons have εHf(t) of +8.1 to +18.5. These isotopic compositions indicate that the THO rocks were derived from an isotopically depleted mantle source similar to that of the Indian Ocean, which was slightly affected by the recycling of subducted sediments. We conclude that the THO and other Sabzevar-Torbat-e-Heydarieh ophiolites formed in a back-arc basin well to the north of the Late Cretaceous fore-arc, now represented by the Zagros ophiolites, testifying that a broad region of Iran was affected by upper-plate extension accompanying Late Cretaceous subduction initiation.

Neotethyan Subduction Ignited the Iran Arc and Backarc Differently

“…Cadomian exposures are scattered across Iran (Figure 1a). That Iran is mainly underlain by Cadomian crust is supported by exposures of these rocks in core complexes (Ramezani & Tucker, 2003) and by basement uplifts (Moghadam et al, 2019). Inherited zircons as well as bulk rock Nd and zircon Hf model ages from Cenozoic magmatic rocks also confirm that Iran crust mainly consists of Cadomian magmatic rocks (e.g., Moghadam et al, 2018; Sepidbar et al, 2018; Sepidbar et al, 2019).…”

Slab Folding and Surface Deformation of the Iran Mobile Belt

“…These episodes of upper-plate deformation and trench horizontal motion are reported in Figure 3. Numbers indicate the references used to build the chart: (1) Aghanabati and Rezaie (2009), Ali et al (2012), Allahyari et al (2014), Aswad et al (2011), Babaie et al (2006, Ghasemi et al (2002), Monsef et al (2010), Saccani et al (2014), Moghadam and Stern (2015); (2) Agard et al (2005), Moghadam and Stern (2015); (3) e.g., McQuarrie and van Hinsbergen (2013); (4) e.g., Azizi et al (2018), Azizi and Jahangiri (2008), Badr et al (2013), Chiu et al (2013), Mazhari et al (2009), Nouri et al (2016), Omrani et al (2008), Sepahi and Athari (2006); (5) Falcon (1974); (6) Agard et al (2005Agard et al ( , 2006 Allen and Armstrong (2008), Hadi et al (2019), Saidi et al (1997), Wilmsen et al (2005Wilmsen et al ( , 2009Wilmsen et al ( , 2015Wilmsen et al ( , 2018; (16) e.g., Mattei et al (2015Mattei et al ( , 2017, Morley et al (2009), Verdel et al (2007); ( 17) Omrani (2008), Moghadam et al (2014Moghadam et al ( , 2020; (18) Chiu et al (2013Chiu et al ( , 2017,…”

“…The compilation of structural, stratigraphic, and magmatic data in the WCD documents two episodes of upper-plate extension (from >100 to ∼70 Ma and from ∼55 to ∼35 Ma) alternating with two periods of upper-plate shortening (from ∼70 to ∼55 Ma and from ∼35 Ma to present) (Figure 2). The Cretaceous opening of the Nain-Baft and Sabzevar-Sistan (e.g., Figure 2; Moghadam & Stern, 2015;Moghadam et al, 2009Moghadam et al, , 2014Moghadam et al, , 2020Pirnia et al, 2020;Rossetti et al, 2010Rossetti et al, , 2014Verdel et al, 2011;Zarrinkoub et al, 2012) represents the first phase of extension and retreat of the Neotethys trench occurred from >100 to ∼75 Ma (Figure 2). The progressive closure of the different back-arcs and their suturing before the middle Eocene (e.g., Rossetti et al, 2014;Moghadam et al, 2009Moghadam et al, , 2014 suggests the occurrence of a period of upper-plate shortening with the advance of the trench from ∼75 to 55 Ma (Figure 2).…”

“…A second event of oceanic spreading in the Neotethys appears to have occurred during the Cretaceous. Indeed, most of the Neotethys ophiolites indicate that the late Cretaceous oceanic spreading is either MORB-related or associated with a supra-subduction zone (Moghadam & Stern, 2015;Moghadam et al, 2014Moghadam et al, , 2020. This Cretaceous oceanic spreading occurred most likely shortly before the obduction of the Neotethys ophiolites onto the Arabian margin (e.g., Agard et al, 2011).…”

“…compressional and extensional episodes associated with magmatic pulses (e.g., Barrier & Vrielynck, 2008;Omrani et al, 2008;Rossetti et al, 2010Rossetti et al, , 2014Saccani et al, 2010;Moghadam et al, 2014Moghadam et al, , 2016Moghadam et al, , 2020Tadayon et al, 2017Tadayon et al, , 2019Verdel et al, 2011). Each of these pulses of deformation has been so far related to different deep processes, such as slab break-off, flat-slab retreat, or convergence velocity variations (Agard et al, 2011;Rabiee et al, 2020;Tadayon et al, 2019;Verdel et al, 2011).…”