Thermochronology of the Miocene Arabia-Eurasia collision zone of southeastern Turkey

The Bitlis-Piitiirge collision zone of SE Turkey is the area of maximum indentation along the > 2400-km-long Assyrian-Zagros suture between Arabia and Eurasia. The integration of (1) fission-track analyses on apatites, (ii) (U-Th)/He analyses on zircons, (iii) field observations on stratigraphic and structural relationships, and (iv) preexisting U-Pb and Ar-Ar age determinations on zircons, amphiboles, and micas provides for the first time an overall picture of the thermochronometric evolution of this collisional orogen. The data set points to ubiquitous latest Cretaceous metamorphism of a passive margin sedimentary sequence and its igneous basement not only along the suture zone but across the entire width of the Anatolia-Tauride block north of the suture. During the early Paleogene the basement complex of the Bitlis and Piitiirge massifs along the suture was rapidly exhumed due to extensional tectonics in a back-arc setting and eventually overlain by Eocene shallow-marine sediments. The entire Oligocene is characterized by a rather flat thermochronometric evolution in the Bitlis orogenic wedge, contrary to the widely held belief that this epoch marked the inception of the Arabia-Eurasia collision and was characterized by widespread deformation. Deposition of a thick Oligocene sedimentary succession in the Mu-Hinis basin occurred in a retroarc foreland setting unrelated to continental collision. During the Middle Miocene, the Bitlis-Piitiirge orogenic wedge underwent a significant and discrete phase of rapid growth by both frontal accretion, as shown by cooling/exhumation of the foreland deposits on both sides of the orogenic prism, and underplating, as shown by cooling/exhumation of the central metamorphic core of the orogenic wedge. We conclude that continental collision started in the mid-Miocene, as also shown by coeval thick syntectonic clastic wedges deposited in flexural basins along the Arabian plate northern margin and contractional reactivation of a number of preexisting structures in the European foreland.