Late Tertiary tectonic evolution of northern Iran: A case for simple crustal folding Bernard Guest b, ⁎ , Alice Guest a , Gary Axen b,1 a California Institute of Technology, Jet Propulsion Laboratory, Pasadena California, USA b University of California Los Angeles, Department of Earth and Space Sciences, Los Angeles, California, USA Received 12 May 2006; accepted 15 February 2007 Available online 11 April 2007 Abstract Here we present a crustal folding or buckling mechanism to explain the rootless 3–5 km high Alborz Mountains in northern Iran as well as ∼ 10 km of Late Miocene to recent subsidence in the south Caspian basin and ∼ 3–6 km of subsidence in the central Iranian basin in the context of the middle Miocene to recent Arabia–Eurasia collision. A key element of the mechanism is the presence of lateral and vertical lithospheric strength contrasts between the north Iranian continental and south Caspian oceanic crusts: when compression from the collision is applied across the region, the strong south Caspian oceanic crust, buried under N 10 km of premiddle Miocene sediment, interacts with the bottom of the mechanically strong continental upper crust of northern Iran, resulting in upward buckling of the continental crust and downward buckling of the oceanic crust. We test this mechanism using a finite-element numerical model with a Maxwell rheology and obtain results that are consistent with the geological and geophysical observations. The observations compiled here and the model results demonstrate the potential for using this region as a natural laboratory for studying the early stages of continent–oceanic collision, including processes like basin inversion, fault localization and, potentially, subduction initiation. © 2007 Elsevier B.V. All rights reserved. Keywords: lithospheric buckling or folding; fault localization; subduction initiation; basin inversion; Alborz; Iran; south Caspian basin 1. Introduction This paper is concerned with the lithospheric scale interaction of the Alborz Mountains with the adjacent south Caspian and central Iranian basins in the context of the ongoing collision between Arabia and Eurasia that is located 300–700 km to the south (Fig. 1). The region is unique in that over N 200 km, it exhibits ∼ 25 km of basement relief between the crest of the Alborz Mountains and the basement of the south Caspian basin, ∼ 15 km of which developed over the last ∼ 12 Ma (Nadirov et al., 1997; Devlin et al., 1999; Axen et al., 2001; Guest et al., 2006b). Furthermore, the Alborz have a crustal thickness of only 35–40 km which is unusually thin for a 100 km wide, 3–5 km high mountain range and is interpreted to indicate that it is not compensated by a lithospheric root (Dehghani and Makris, 1984; Rodgers et al., 1997; Seber et al., 1997). Explaining synchronous rapid south Caspian subsidence, Alborz uplift, subsidence in the central Iranian Global and Planetary Change 58 (2007) 435 – 453 www.elsevier.com/locate/gloplacha ⁎ Corresponding author. Ludwig-Maximilians-University Depart- ment of Geology and Environmental Sciences. Geology Section, Munich, Germany. E-mail address: b.guest@iaag.geo.uni-muenchen.de (B. Guest). 1 Now at the New Mexico Institute of Mining and Technology, Department of Earth and Environmental Science, Socorro, New Mexico, United States. 0921-8181/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2007.02.014