Paleo-exhumation histories of the Sakarya and the Istanbul Zones of the Western Pontides, the Almacik Block and its surroundings, NW Turkey

Abstract

The Almacik Block is a tectonic sliver formed during the activity of the North Anatolian Fault Zone (NAFZ). It includes two important tectonic zones of the Western Pontides (NW Anatolia): the Istanbul and the Sakarya Zones meet along a suture zone called the Intra-Pontide Suture Zone (IPSZ). The units within the Istanbul Zone are exposed in the eastern part of the Almacik Block, whereas the rocks of the Sakarya Zone are exposed in the west. In the presented study, some of the magmatic rocks have been analysed using the zircon U-Pb method: these rocks have previously been either incorrectly or not dated. The U-Pb results have shown that some of the granitic rocks in the Istanbul Zone, which intruded into the amphibolites of the Intra-Pontide Suture Zone, have a crystallization age of 559-556 Ma. A granitoid block in the Upper Cretaceous wild flysch formed over the units of the Istanbul Zone had an age of similar to 566 Ma. Two distinct granitoid bodies in the Sakarya Zone were dated at 404.5 +/- 3.9 Ma (Early Devonian) and 161.8 +/- 0.82 Ma (Late Jurassic) ages. The palaeo-exhumation evolutions of the zones were obtained using the zircon (U-Th)/He technique (ZrHe) to understand the differences in the geological evolution of the individual zones. ZrHe ages of the samples obtained from the Istanbul Zone indicate a pronounced Early Cretaceous palaeo-exhumation period. The Middle-Upper Jurassic Mudurnu Formation in the Sakarya Zone gave ZrHe ages close to its sedimentation age, which indicates that the major part of the unit was not buried too deep to open the ZrHe closure system. Unlike those the Istanbul Zone, the main ZrHe age peaks of the Sakarya Zone are similar to 202, 160, 98, 74, and 52 Ma. Finally, a sample of the metamorphic rocks that were assigned to the IPSZ yielded a peak age of similar to 63 Ma, which indicates that the final closure of the Intra-Pontide Ocean (IPO) took place during the Early Tertiary.