Deciphering extensional dynamics within the Menderes Massif, Western Turkey

The Menderes metamorphic core complex in southwestern Turkey exposes amphibolite-facies rocks with ages of prograde metamorphism ranging from the Cambro-Ordovician to the Eocene-Oligocene. Sometime in the mid- to late Cenozoic, blueschist- and amphibolite-facies compression shifted to large-scale extension and core complex development in this,(arguably) the largest core-complex of the Alpine-Himalayan orogen. The timing of the transition from compression to extension provides a key test of three possible mechanisms for producing widespread extension in orogens: orogenic collapse (extension initiates in the Oligocene), back-arc spreading (Miocene), and tectonic escape (pliocene to present). Previously published results imply that initiation could have occurred anytime between the late Eocene and late Miocene. In the northern (Alasehir) detachment, two syn-extensional plutons occur, the Salihli and Turgutlu granites. Their crystallization ages must postdate initial extension and thus potentially distinguish tectonic mechanisms. Thirty-four Th-Pb ion microprobe monazite (REEThPO4) ages are indistinguishable between the two granites, and date their intrusion at 15.0±2.0 Ma (±1σ, MSWD=2). Biotite 40Ar/39Ar ages reported elsewhere are 13.1±0.2 Ma (Turgutlu) and 12.2±0.4 Ma (Salihli). These data are most consistent with orogenic collapse dynamics, initiating prior to 15 Ma, with possible rapid mid-Miocene slip. Trends on Harker diagrams and REE patterns imply similar sources for both granites, and show interesting similarities to orthogneisses exposed 10-20 km southeast in the central area of the massif (Odemis-Kiraz). This work has implications for Aegean tectonics, the transition from compression to extension in orogens overall, and for development of monazite as a geochemical and chronologic monitor of extension.