Geology and thermochronology of Tertiary Cordilleran-style metamorphic core complexes in the Saghand region of central Iran

Abstract

An ~100-km-long north-south belt of metamorphic core complexes is localized along the boundary between the Yazd and Tabas tectonic blocks of the central Iranian microcontinent, between the towns of Saghand and Posht-e-Badam. Amphibolite facies mylonitic gneisses are structurally overlain by easttilted supracrustal rocks including thick (>1 km), steeply dipping, nonmarine siliciclastic and volcanic strata. Near the detachment (the Neybaz-Chatak fault), the gneisses are generally overprinted by chlorite brecciation. Crosscutting relationships along with U-Pb zircon and 40 Ar/ 39 Ar age data indicate that migmatization, mylonitic deformation, volcanism, and sedimentation all occurred in the middle Eocene, between ca. 49 and 41 Ma. The westernmost portion of the Tabas block immediately east of the complexes is an east-tilted crustal section of Neoproterozoic-Cambrian crystalline rocks and metasedimentary strata >10 km thick. The 40 Ar/ 39 Ar biotite ages of 150-160 Ma from structurally deep parts of the section contrast with ages of 218-295 Ma from shallower parts, and suggest Late Jurassic tilting of the crustal section. These results defi ne three events: (1) a Late Jurassic period of upper crustal cooling of the western Tabas block that corresponds to regional Jurassic-Cretaceous tectonism and erosion recorded by a strong angular unconformity below mid-Cretaceous strata throughout central Iran; (2) profound, approximately east-west middle Eocene crustal extension, plutonism, and volcanism (ca. 44-40 Ma); and (3) ~2-3 km of early Miocene (ca. 20 Ma) erosional exhumation of both core complex and Tabas block assemblages at uppermost crustal levels, resulting from signifi cant north-south shortening. The discovery of these and other complexes within the mid-Tertiary magmatic arcs of Iran demonstrates that Cordilleran-style core complexes are an important tectonic element in all major segments of the Alpine-Himalayan orogenic system.

Timing of uplift in the Zagros belt/Iranian plateau and accommodation of late Cenozoic Arabia–Eurasia convergence

“…The geological meaning of the crustal thinning is probably two-fold. First, the development of Eocene deep-water basins to the north of the Urumieh-Dokhtar volcanic arc has been already noticed (Vincent et al 2005 and references therein) and might be related to the regional backarc extension episode (Vincent et al 2005;Verdel et al 2007;Morley et al 2009). Second, a renewed episode of extension during Late Miocene time of unclear geodynamic origin (Morley et al 2009) surely contributed to the crustal thinning.…”

“…velocity estimate does not support mantle delamination (Kaviani et al 2007), and more generally there is no definitive evidence supporting the convective removal of lithosphere beneath the plateau. On the other hand, upwelling of asthenospheric mantle controlled by slab retreat may provide an explanation for such thin lithosphere as suggested by several geological constraints (Vincent et al 2005;Verdel et al 2007;Morley et al 2009).…”

The Sanandaj–Sirjan Zone in the Neo-Tethyan suture, western Iran: Zircon U–Pb evidence of late Palaeozoic rifting of northern Gondwana and mid-Jurassic orogenesis

“…These occurrences are consistent with the Sanandaj-Sirjan Zone having a Pan-African basement similar to that found in the Zanjan region of northwest Iran and elsewhere in Central Iran such as in the Yazd Block and in northern Central Iran (Figs. 2, 8) (Ramezani and Tucker, 2003;Verdel et al, 2007;Hassanzadeh et al, 2008;Saki, 2010a, b;RahmatiIlkhchiet al, 2011;Balaghi Einalou et al, 2014). Pan-African basement ages are common in the Cimmerian continental fragment in central and northern Iran but have not been identified as a basement component of the Turan Platform (Şengör and Natal'lin, 1995;Natal'in and Şengör, 2005).…”

“…5, 6). (Verdel et al, 2007;Hassanzadeh et al, 2008;Saki, 2010a, b;Rahmati-Ilkhchi et al, 2011;Balaghi Einalou et al, 2014). In the Lake Urumieh region of the northwest Sanandaj-Sirjan Zone are Carboniferous plutons (Khalifan Granite, Bea et al, 2011; Ghushchi granites and gabbronorites, Moghadam et al, 2015) and a Permian pluton occurs east of Golpaygan (Hasanrobat Granite, Hag, Alirezaei and Hassanzadeh, 2012).…”

“…2 for locations). References: (1) Nutman et al (2014), (2) Shakerardakani et al (2015), (3) this work, (4) Mahmoudi et al (2011), (5) Hassanzadeh et al (2008), (6) Bea et al (2011), (7) Alirezaei and Hassanzadeh (2012), (8) Ramezani and Tucker (2003), Verdel et al (2007), (9) Zanchi et al (2015) , (10) Balaghi Einalou et al (2014), and (11) Horton et al (2008). Mattei et al, 2015).…”