Burial and exhumation cycles tracked by 40Ar/39Ar and apatite (U-Th)/He thermochronology in a strike-slip fault zone, central Turkey

Abstract

Muscovite and K-feldspar 40Ar/39Ar ages from the eastern margin of the Niğde massif in central Anatolia document the timing of initial exhumation, reburial, and final exhumation and cooling of metamorphic rocks affected by deformation in the Central Anatolian Fault Zone, a long-lived strike-slip fault zone. Although the ages of initial and final cooling were known from previous studies, our new results document the timing of an intermediate thermal perturbation. Four of eight muscovite 40Ar/39Ar samples have Late Cretaceous ages that date the initial cooling of high-grade metamorphic rocks at ~75 Ma. The remaining four muscovite samples have perturbed spectra that climb to Late Cretaceous ages with increasing extraction temperatures. These four samples are located beneath a faulted unconformity overlain by Paleogene sedimentary rocks that were derived in part from the metamorphic massif, then buried and metamorphosed under greenschist facies conditions. Samples closer to the unconformity are more perturbed than structurally deeper samples. The age of the thermal perturbation is determined at 35-25 Ma using multi-diffusion domain modeling of 40Ar/39Ar data from two K-feldspar-bearing gneiss samples, one located close to the unconformity, and one at a structurally deeper level. Muscovite 40Ar/39Ar results and modeled K-feldspar temperature-time histories show that the eastern margin of the massif experienced a thermal pulse that peaked between 35-25 Ma after an initial cooling in the Late Cretaceous. Apatite (U-Th)/He (AHe) results track cooling from this perturbance, and indicate that the eastern margin of the massif had cooled to ~65°C by ~19 Ma. The thermal pulse has been attributed to the re-burial and heating of the eastern portion of the massif due to a period of transpression across the Central Anatolian Fault Zone, which runs along the massif's eastern margin. Slip on the Central Anatolian Fault Zone at this time may be related to major tectonic events that were taking place in Central Anatolia, particularly the collision of Arabia with the Anatolian microplate.