Quantifying cooling and exhumation patterns in a bivergent metamorphic core complex: the central Menderes Massif, western Turkey

Abstract

Metamorphic core complexes are commonly associated with an asymmetric exhumation pattern owing to a simple shear mode of extension along the core-bounding extensional detachment fault. In contrast, a symmetric exhumation pattern was proposed for the central Menderes Massif (western Turkey), because it comprises two detachment faults with opposite dip and shear sense, which accommodated N-S extension since the middle Miocene. However, available constraints on the evolution of the central Menderes Massif are mainly based on data from the northern part including the north-dipping Gediz detachment. Exhumation of the southern part along the south-dipping Büyük Menderes detachment was assumed to occur more or less synchronously. New thermochronological and structural data (Wölfler et al., 2017; Nilius et al., 2019) show, however, that extension in the southeastern part of the central Menderes Massif was accommodated along the Büyük Menderes detachment during the middle Miocene and shifted in the Pliocene to the Demirhan detachment, which is situated in the hanging wall of the eastern Büyük Menderes detachment. Together with new zircon and apatite (U-Th)/He and fission track ages from the Gediz detachment, our data set allows constraining temporal and spatial variations in cooling and exhumation patterns in the entire central Menderes Massif. Our new data from the Gediz detachment imply that faulting commenced in the middle Miocene and continuously propagated to the east until the late Pliocene. Exhumation rates in the detachments footwalls were constrained by 1D thermokinematic modelling. The results show that the southern part of the Menderes Massif was exhumed at rates of ~0.5 km/Ma in the middle Miocene and at ~0.4 km/Ma in the Pliocene. In contrast, the western part of the northern central Menderes Massif was exhumed at rates of 0.3-1 km/Ma in the middle to late Miocene; the eastern part shows even higher exhumation rates of ~1.5 km/Ma during the Pliocene. Faster exhumation of the northern central Menderes Massif compared to slower exhumation of the southern part is also expressed in the slip rates derived from cooling ages in the footwalls of the detachments. We obtain the highest slip rate 5.6 km/Ma along the eastern Gediz detachment in the Pliocene, ~2.5 km/Ma along the western Gediz detachment in the middle Miocene, and ~2 km/Ma during the Pliocene along the Demirhan detachment. Consequently, the Gediz detachment is responsible for about twice as much extension and exhumation as the Büyük Menderes and Demirhan detachments.