Syn- to post-orogenic S-directed extension of the Kırşehir Massif, central Türkiye

The Alpine Orogeny resulted from the closure of the Neotethyan Ocean and the subsequent collision of Eurasia, Africa-Arabia and numerous microplates in between. The Kırşehir Massif is located at the NW corner of the Central Anatolian Crystalline Complex (CACC) and offers insights into the tectonometamorphic evolution of CACC during Alpine orogenesis. The tectonostratigraphy is defined by a migmatitic core that grades into a metasedimentary lower unit comprising gneiss, amphibolite, micaschist, calcschist and quartzite, transitioning into metasedimentary rock alternations and overlain by marbles with amphibolite intercalations in the upper unit. The metamorphic rocks are correlated to the Paleozoic-Mesozoic Tauride platform sequence. Purported southward obduction of Late Cretaceous (90-85 Ma) Neotethyan supra-subduction zone ophiolitic slices onto the massif resulted in Barrovian metamorphism of these rocks, increasing from greenschist (SE) to granulite (NW) facies. Prograde metamorphism is evinced by Cpx-Ttn-Plg-bearing melanocratic bands within migmatites and compositionally zoned Mn-rich spessartine garnets in high-grade metaclastic rocks. Preliminary published monazite U-Pb geochronology on a Grt-Sil gneiss indicates migmatization occurred at c. 85 Ma. Epidote and chlorite halos around clinopyroxene and partially chloritized mica indicate minor retrogression to greenschist facies. The presence of late- to post-tectonic garnets suggests a tectonic quiescence during the retrograde stage. Our mapping reveals a consistent structural architecture across the massif characterized by subhorizontal planar foliation. Early folds (F₁) are preserved in decimeter-scale microlithons. Two generations of recumbent folds are present in mylonites: dominant folds with NNW-SSE axes (F_2a) that are associated with strong stretching and mineral lineations marked by quartz, amphibole, and mica trails, and local folds with ~E-W axes (F_2b) in anisotropic layers with S-vergent drag folds. Sigmoids, flanking and asymmetric folds, and shear bands indicate a pervasive top-to-S sense of shear under ductile and brittle-ductile conditions, and boudinage, stretched veins, and extensional crenulation cleavage suggest synchronous layer parallel extension and vertical thinning. Brittle cataclasis is most obvious along the marble horizons, and the massif is dissected by numerous high-angle oblique faults related to post-Cretaceous/Paleocene wrench tectonics in the region. New in-situ white mica Rb-Sr geochronology from foliation-defining white mica yielded c. 75 Ma dates, and indicates deformation was coeval with c. 74-67 Ma calc-alkaline to alkaline intrusions in the western and northern margins of the massif. New zircon (U-Th)/He dates from the basement rocks are concordant with published apatite fission track dates, suggesting Paleocene rapid cooling, further confirmed by early Paleogene sedimentary basins unconformably overlying the basement. Notably, crustal thickness estimates in the CACC are ~35 km. Despite an earlier structural investigation linking exhumation to a top-to-W low-angle detachment fault along the western boundary of the massif, we instead prefer a model invoking syn- to post-orogenic extreme N-S extension and vertical thinning. This process triggers the collapse of ~55 km thick crust and subsequent uplift of the Kırşehir Massif since the Late Cretaceous.