Reconstructing the volcanic history of the Ulukışla Caldera: A collapse structure within the Hasandağ Volcanic Complex, Central Anatolia (Turkey)

Abstract

The Ulukışla Caldera is a collapse structure within the active Hasandağ Volcanic Complex (Central Anatolia), situated along the southern branch of the Tuz Gölü Fault Zone (TGFZ). This study aims to reconstruct the volcanic history of the Ulukışla Caldera by characterising its associated pyroclastic deposits using tephrostratigraphy, glass chemistry (major and trace elements), and geochronological data (⁴⁰Ar/³⁹Ar and U-(Th)-Pb). Our findings reveal that the Ulukışla Caldera has undergone at least three major explosive rhyolitic eruptions, which formed the pyroclastic deposits of the Yenipınar Eruption (∼442 ka), the Belbaşhanı Eruption (∼400 ka), and the Ulukışla Eruption (∼326 ka). The Yenipınar unit was produced by an unsteady eruption column originating from paleo-Ulukışla volcanic structure, that deposited pumice fallout layers and interbedded pyroclastic density current (PDC) deposits. The Belbaşhanı Eruption began with a Plinian column that deposited the Belbaşhanı Pumice fallout. This phase was followed by the emplacement of thick PDC deposits and co-ignimbrite lithic lag breccias during the collapse, which ultimately led to the formation of the Ulukışla Caldera. The volume of the Belbaşhanı deposits, including the pumice fallout and the caldera-forming ignimbrite, could reach up to 10 km³ DRE (Dense Rock Equivalent), corresponding to an eruption of magnitude ∼6. The Ulukışla Pumice resulted from a post-caldera eruption, which did not generate PDCs. The Ulukışla Caldera exhibits an elongated morphology, which is strongly influenced by the NW-SE alignment of the TGFZ. Based on this morphology and the regional tectonic setting, we conclude that the Ulukışla Caldera is a strike-slip/graben caldera. Reconstructing the volcanic history of this newly identified caldera is essential for enhancing our understanding of the Hasandağ Volcanic Complex. Our findings offer valuable context for future eruptive behaviour, improving the hazards assessment for potential caldera collapses, and contribute to mitigating associated risks.