Turkish Journal of Earth Sciences

Geology and the Tectonics of the Gediz Graben


Tahir EMRE




The Gediz Graben is a part of a Basin and Range type graben system of the western Anatolia. In this study, a 40 km lenght of the Gediz Graben between Sart Mustafa (Salihli) and Dereköy (Alaşehir) is examined and especially the stratigraphic and the structural characteristics of the region are discussed. The Salihli Alaşehir plane forms the infill of the Gediz Graben and in the center of the study area, the plane extends WNW-ESE direction. The northern and southern borders of the Gediz Graben lithologically, morphologically and structurally are dfferent. Along the southern border, the topography between the Salihli-Alaşehir plane and the basement rocks of the Menderes metamorphics is very rugged with high elevations. On the other hand, the northern border of the graben is characteristically nearly flat or it has a very undluting surface. The contrasting morphology between the northern and southern boundaries reflects directly the structural differences in these two areas. Along the southern border of graben, the Salihli group unconformably overlies the Menderes metamorphics and the granadioritic stocks that cut this metamorphic sucsession . The Salihli group is dominantly represented by coarse continental detritic rocks and is nearly 2000 m in thikness. Three different formations are separated in this group and they are named as the Acıder, Göbekli and Asartepe formations. The beds of the Salihli group generally dip southward againist the basement rocks and they are cut by a series of high-angle normal faults. In only one area along the southern border, Gastropoda fossils were found in the Göbekli formation and they yielded an age of the Early- Pliocene (Dasien). Along the northern border of the graben, The Adala group crops out and it is composed of river and lake sediments of upto 400 m in thickness. Two different formations are separated in this group and they are named as the Filiztepe and Mevlütlü formations. The Adala group rests uncformably above the Menderes metamorphics along a low-angle to flat erosional surface. There is no foult along that border, and the layers of the sedimentary rocks are either flat or dip with a low-angle. The Gediz Graben started from by a regional tensional forces and first the low-angle boundary fault (Karadut faults) was formed. This fault has characteristics of a detachment fault and in the fault zone cataclastic rocks were formed. the thickness of the cataclastic zone ranges from 10 to 60 m and the lithology of the rocks inside this zone depends upon the ubderlying Menderes Metamorphics. Away from the fault zone, there is a gradation from gauge breccia to mylonite-ultramylonite. On the other hand, the mylonitic rocks also grade into undeformed granodioritic or metamorphic rocks. Just after and during the movement og the detachment fault, along the southern border of the graben, a series of high angle normal faults were formed. These high-angle faults were active during rhe deposition of the graben fill and some of them are even presently active. However, along the northern border, only the Mevlütlü fault and the Pleistocene-Holocene Kula volcanism were formed by the tectonic activity. The Gediz Graben has been evolved as s half graben and seismicity along this graben is still going on.

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