Bivergent extension in orogenic belts: The Menderes massif (southwestern Turkey) Ralf Hetzel Institut fu ¨ r Geowissenschaften, Johannes Gutenberg-Universita ¨ t, 55099 Mainz, Germany Cees W. Passchier Uwe Ring O ¨ zcan O. Dora Department of Geological Engineering, Dokuz Eylu ¨ l University, 35100 Bornova/Izmir, Turkey ABSTRACT The central Menderes massif is characterized by an overall dome-shaped foliation pattern and a north-northeast–trending stretching lineation. The asymmetry of shear bands and quartz c-axis fabrics on either side of the structural dome demonstrate a top to the north-northeast shear sense in the northern part and a top to the south-southwest shear sense in the southern part of the submassif, i.e., a bivergent downdip movement. This suggests a symmetric collapse of the Alpine Menderes orogenic belt along two extensional shear zones. Conjugate shear bands and symmetric quartz c-axis fabrics in the east-trend- ing transition zone demonstrate a coaxial deformation between the two extension domains. Bivergent extension in the Menderes massif is in contrast to asymmetric extension in the Aegean Sea. Here, the still-active Hellenic subduction zone evolved from an advancing plate boundary associated with crustal thickening into a retreating plate boundary in Oligo- cene-Miocene time. Southward rollback of the subducting plate during continuous north- ward subduction allowed asymmetric top to the north-northeast extension in the back-arc region during the exhumation of the Cycladic core complexes. In western Turkey, the arrival of the thick continental crust of the Menderes massif halted subduction and prob- ably caused the symmetric collapse of the massif because the high potential energy of the thickened crust was no longer supported by subduction. INTRODUCTION Late orogenic extension is a common phe- nomenon in orogenic belts (Dewey, 1988) and results in the exhumation of metamor- phic core complexes along extensional shear zones and detachment faults (Lister and Davis, 1989). On the basis of the geometry and kinematics of the extensional shear zones, two end-member types of metamor- phic core complexes can be distinguished (Malavieille, 1993). The more common type is asymmetric and bounded by a single low- angle normal shear zone, which is upwarped during tectonic denudation of the core com- plex (Lister and Davis, 1989). The second type is rare and is characterized by two low- angle shear zones with opposite vergence at both flanks of the core complex (Malavieille, 1993). This paper presents kinematic data which show that the Menderes massif in southwestern Turkey is a symmetric meta- morphic core complex. GEOLOGIC BACKGROUND The Menderes massif, an elongate culmi- nation of metamorphic rocks in southwest- ern Turkey, is the eastward continuation of the Attic Cycladic crystalline complex of the Aegean Sea (Fig. 1) (Dixon and Robertson, 1984). The massif has been subdivided into a core consisting mainly of Precambrian au- gen gneiss, schist, and migmatite, and an overlying metasedimentary cover that was deposited in Paleozoic to Mesozoic time (S ¸engo ¨r et al., 1984; Dora et al., 1990). In the study area the cover rocks are made up of greenschist facies mica schist, phyllite, quartzite, and marble, whereas the core rocks are characterized by a monotonous se- quence of pelitic amphibolite facies schist. Two east-trending Neogene grabens, the Gediz and Bu ¨yu ¨k Menderes grabens, sub- divide the massif into three submassifs (Fig. 1). The Oligocene-Miocene exhumation of the Attic Cycladic crystalline complex has been ascribed to core-complex type crustal extension. The movement direction of ex- tensional shear zones and detachment faults is generally top to the north-northeast and indicates asymmetric extension (e.g., Lee and Lister, 1992; Gautier and Brun, 1994) (Fig. 1). In contrast to the Attic Cycladic crystalline complex, structural studies in the Menderes massif revealed two opposite ex- tension directions. The exhumation of the southern submassif has been attributed to an extensional shear zone with a top to the south-southwest shear sense (Bozkurt and Park, 1994). In the northern submassif and in the northern part of the central submassif a top to the north-northeast shear during Miocene tectonic denudation has been doc- umented (Hetzel et al., 1995). This study de- scribes the structural boundary between the two extension domains, which is located in the central submassif (Figs. 1 and 2). STRUCTURAL DEVELOPMENT In the core and cover rocks, an S1 folia- tion is locally observed, overprinted by an S2 foliation that developed as a crenulation cleavage. In most places, however, a single S2 cleavage defined by the preferred orien- tation of micas is present and has destroyed all earlier fabrics. The overall orientation of S2 is dome shaped; there is a gentle north- Figure 1. Location of Men- deres massif (dot pattern) in Aegean region. Dashed line indicates Attic Cycla- dic crystalline massif. Ar- rows indicate movement direction of hanging-wall blocks along major exten- sional shear zones during Oligocene-Miocene ex- tension (data are from Gautier and Brun, 1994; Hetzel et al., 1995). Box in- dicates location of Fig- ure 2A. Geology; May 1995; v. 23; no. 5; p. 455– 458; 4 figures. 455 on December 8, 2014 geology.gsapubs.org Downloaded from