Western Turkey has been dominated by N-S extension since the Early Miocene. The timing and cause of this N-S extension and related basin formation have been the subject of much debate, but new data from the Bigadiç borate basin provide insights that may solve this controversy. The basin is located in the Bornova Flysch Zone, which is thought to have formed as a major NEtrending transform zone during Late Cretaceous-Palaeocene collisional Tethyan orogenesis and later reactivated as a transfer zone of weakness, and which separates two orogenic domains having different structural evolutions.Volcanism in the Bigadiç area is characterized by two rock units that are separated by an angular unconformity. These are: (1) the Kocaiskan volcanites that gives K/Ar ages of 23 Ma, and (2) the Bigadiç volcano-sedimentary succession that yields ages of 20.6 to 17.8 Ma. Both units are unconformably overlain by Upper Miocene-Pliocene continental deposits. The Kocaiskan volcanites are related to the first episode of volcanic activity and comprise thick volcanogenic sedimentary rocks derived from subaerial andesitic intrusions, domes, lava flows and pyroclastic rocks. The second episode of volcanic activity, represented by basaltic to rhyolitic lavas and pyroclastic rocks, accompanied lacustrine-evaporitic sedimentation. Dacitic to rhyolitic volcanic rocks, called the Sındırgı volcanites, comprise NE-trending intrusions producing lava flows, ignimbrites, ash-fall deposits and associated volcanogenic sedimentary rocks. Other NE-trending olivine basaltic (Gölcük basalt) and trachyandesitic (Kayırlar volcanites) intrusions and lava flows were synchronously emplaced into the lacustrine sediments. The intrusions typically display peperitic rocks along their contacts with the sedimentary rocks. It is important to note that the Gölcük basalt described here is the first recorded Early Miocene alkali basalt in western Turkey.The oldest volcanic episode occurred in the NE-trending zone when the region was still experiencing N-S compression. The angular unconformity between the two volcanic episodes marks an abrupt transition from N-S collision-related convergence to N-S extension related to retreat of the Aegean subduction zone to the south along an extensional detachment. Thrust faults with top-to-the-north sense of shear and a series of anticlines and synclines with subvertical NE-striking axial planes observed in the Bigadiç volcano-sedimentary succession suggest that NW-SE compression was reactivated following sedimentation.Geochemical data from the Bigadiç area also support the validity of the extensional regime, which was characterized by a bimodal volcanism related to extrusion of coeval alkaline and calc-alkaline volcanic rocks during the second volcanic episode. The formation of alkaline volcanic rocks dated as 19.7 AE 0.4 Ma can be related directly to the onset of the N-S extensional regime in western Turkey.
Synextensional granitoids may have significant structural features leading to the understanding of the evolution of extended orogenic belts. One of the highly extended regions, the Aegean region, includes a number of metamorphic core complexes and synextensional granitoids that developed following the Alpine collisional events. The Alaçamdag area in northwestern Turkey is one of the key areas where Miocene granites crop out along the boundary of various tectonic units. Structural data from the Early Miocene Alaçamdag granites demonstrated two different deformation patterns that may provide insights into the development of granitic intrusions and metamorphic core complexes. (1) Steeply dipping ductile shear zones caused emplacement of syn-tectonic granite stocks; they include kinematic indicators of a sinistral top-to-the-SW displacement. This zone has also juxtaposed theİzmir-Ankara Zone and the Menderes Massif in the west and east, respectively. (2) Gently dipping ductile shear zones have developed within the granitic stocks that intruded the schists of the Menderes Massif on the structurally lower parts. Kinematic data from the foliated granites indicate a top-to-the-NE displacement, which can be correlated with the direction of the hangingwall movement documented from the Simav and Kazdag metamorphic core complexes. The gently dipping shear zones indicate the presence of a detachment fault between the Menderes Massif and the structurally overlyingİzmir-Ankara Zone. Mesoscopic-to map-scale folds in the shallow-dipping shear zones of the Alaçamdag area were interpreted to have been caused by coupling between NE-SW stretching and the accompanying NW-SE shortening of ductilely deformed crust during Early Miocene times. One of the NE-trending shear zones fed by granitic magmas was interpreted to form the northeastern part of a sinistral wrench corridor which caused differential stretching between the Cycladic and the Menderes massifs. This crustal-scale wrench corridor, theİzmir-Balıkesir transfer zone, may have controlled the asymmetrical and symmetrical extensions in the orogenic domains. The combination of the retreat of the Aegean subduction zone and the lateral slab segmentation leading to the sinistral oblique-slip tearing within the Eurasian upper plate appears to be a plausible mechanism for the development of such extensive NE-trending shear zones in the Aegean region.
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