TIMING OF POST-OROGENIC EXTENSION IN THE GöRDES MIGMATITE DOME, WESTERN TURKEY: INSIGHTS FROM U-PB AND RB-SR GEOCHRONOLOGY

The Menderes Massif in western Turkey is a crustal-scale metamorphic core complex and is conventionally subdivided into northern (Gördes), central (Ödemiþ-Kiraz) and southern (Çine) submassifs, where the seismically active E-W-trending Gediz Graben in the north and Büyük Menderes Graben in the south are taken as dividing lines, respectively. The evolution of the massif occurred during two distinct but successive events: (1) continental collision and consequent crustal thickening of pre-Alpine basement during the closure of Neotethys along the Ýzmir-Ankara suture zone and (2) subsequent post-orogenic Neogene extension during which metamorphic rocks of the massif exhumed progressively in the footwall of first ductile, then brittle detachment faults detachment faults. Simav detachment fault around the Simav Graben forms one of the most spectacular structures of the massif. The footwall rocks are composed of high-grade metamorphic rocks, including the migmatites, while the hanging wall is made up of low-grade metamorphics and rocks of the Ýzmir-Ankara suture zone. The supradetachment basins (e.g., Gördes, Demirci, Selendi, Akdere basins) ) in the immediate hanging wall hosted sedimentation of upper Oligocene to middle Miocene continental clastic rocks attesting the surficial and erosion of footwall rocks. While extensive volcanic activity has been prevailing in the upper plate, the lower plate was intruded by two large bodies of Egrigöz and Koyunoba granites (ca. 20 Ma).

In the present paper we present the results of a recent geochronological campaign (U-Pb on zircon and monazite; Rb-Sr on muscovite and biotite) on the migmatitic, granitic, pegmatitic and cataclastic rocks of the Gördes migmatite dome. The aim was to date the timing of faulting / the extensional deformation and to constrain the history and therefore the evolution of core complex formation in western Turkey. The results argue that extension in the region commenced by ca. 30 and continued till ca. 8 Ma and that core-complex formation in the northern Menderes Massif is complex, occurred in pulses and involved periods of alternating rapid and slow cooling/denudation rates.