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

ABSTRACT

The Almacık 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 İstanbul and the Sakarya Zones meet along a suture zone called the Intra-Pontide Suture Zone (IPSZ). The units within the İstanbul Zone are exposed in the eastern part of the Almacık 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 İstanbul 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 İstanbul Zone had an age of ~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 İstanbul 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 İstanbul Zone, the main ZrHe age peaks of the Sakarya Zone are ~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 ~63 Ma, which indicates that the final closure of the Intra-Pontide Ocean (IPO) took place during the Early Tertiary.

Graphical abstract

KEYWORDS:

• Western Pontides
• Almacık Block
• Sakarya Zone
• Istanbul Zone
• zircon U-Pb
• (U-Th)/He ages

Highlights

• The main exhumation of the basement rocks of the İstanbul Zone occurred during the interval between the early Cretaceous and the middle late Cretaceous period.

• Precambrian (559–556 Ma) basement rocks of the İstanbul Zone crop out in the Almacık Block

• A monzonitic granite in the Sakarya zone yielded 161.8 ± 0.82 Ma and a deformed granite 404.5 ±3.9 Ma zircon U-Pb ages.

• Campanian ZrHe ages (~62 Ma) derived from the Intra-Pontide Suture Zone support the Early Eocene closure of the zone, not Early Cretaceous.

7. Acknowledgments

This study was supported by TÜBITAK (The Scientific and Technical Research Council of Turkey, Project no: CAYDAG 109Y257) and the İstanbul Technical University (BAP Project No. MGA-2018- 41082). We are so grateful to Fatma Gülmez for sharing the knowledge obtained during her master’s degree study in the region (Gülmez 2009). The authors thank Dr. Cavazza and anonymous reviewer for their thoughtful reviews and constructive criticism of the manuscript. We are thankful to Dr. Stern for editorial handling.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2022.2084645

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work was supported by the TÜBİTAK [CAYDAG 109Y257]; İTÜ BAP [MGA-2018- 41082].