Determination of molten magma parts of the Köroğlu caldera system using two-dimensional magnetotelluric resistivity models

Subsurface magmatic conditions beneath the Köroğlu Caldera remain poorly understood, particularly regarding the presence of partial melt zones and potential residual magma reservoirs. To address this gap, the present study aims to identify subsurface geological structures such as calderas and domes, estimate melt fraction and reservoir geometry using resistivity models derived from magnetotelluric (MT) inversion, and evaluate the role of deep magmatic systems as potential geothermal heat sources. A total of 36 broadband MT stations were deployed along four northeast–southwest-oriented profiles at intervals of approximately 2–3 km. Time-series data covering periods between 0.032 and 1000 seconds were processed to construct two-dimensional resistivity models. These models revealed four near-surface conductive zones (C1–C4) and one deeper conductive body (C5). The shallow conductive features are interpreted as hydrothermal fluid pathways associated with alteration zones. The deeper conductor, referred to as C5, was identified in profiles 1 to 3 at depths between 6 and 11 km, with resistivity values ranging from 5 to 10 Ω·m. Thermodynamic modeling using the Rhyolite-MELTS software indicates that this zone likely represents a felsic melt reservoir undergoing partial crystallization, with melt fractions of 8.6–25%, temperatures between 740 and 846 °C, and dissolved water contents ranging from 1% to 6% by weight. It is thought that graben structures in the region provide permeable pathways for the ascent of deep-sourced hydrothermal fluids. In this context, the C5 zone is considered a potential heat source for the active geothermal system in the Afyonkarahisar region.