Fuat Erkül

Tectonic significance of synextensional ductile shear zones within the Early Miocene Alaçamdağ granites, northwestern 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 Alacamdag 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 Alacamdag 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 the Izmir–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 hanging-wall 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 overlying Izmir–Ankara Zone. Mesoscopic- to map-scale folds in the shallow-dipping shear zones of the Alacamdag 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, the Izmir–Balikesir 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.

An extensional and transtensional origin of elongated magmatic domes and localised transfer faults in the northern Menderes metamorphic core complex, western Turkey

Abstract The northern Menderes metamorphic core complex has complex exhumation history and is one of the key localities to investigate the spatial and temporal relationships of extensional and compressional structures. Detachment faults and syn-extensional plutons are linked to a series of antiforms and synforms and the denudation of the northern Menderes Massif occurred in three stages. The first stage is related to the development of detachment faults under the consistent NE–SW-directed extension. The second stage is represented by a series of elongated magmatic domes that were oriented parallel, oblique and perpendicular to the regional extension direction. Emplacement of these asymmetrical magmatic domes appears to have been controlled by heterogeneous extension and post-dates the extensional Simav detachment fault. On the third stage, progressive heterogeneous extension that led to updoming of plutons has been finally accommodated by a localised and short-lived transfer zone, which was described as the Gerni shear zone for the first time in this study. The transfer zone is formed by a NE-striking, dextral ductile/brittle shear zone that accommodated the propagation of folds, conjugated strike-slip faults and normal- and oblique-slip faults. Mylonites associated with the transfer zone are related to the localisation of strain along the thermally weakened strike-slip fault systems by short-lived intrusions rather than to the development of regional-scale detachment faults. These structures are consistent with a transtensional simple shear model, which properly explains the evolution of extensional and compressional structures exposed in the northern Menderes core complex. Structural setting of the Eğrigöz region is somewhat similar to that of the NE-trending gneiss domes in the northern Menderes Massif and updoming of magma during late stages of detachment faulting appears to have played an important role in the exhumation of lower and upper plate rocks.

Evidence for Continuous Mixing of Individually Fractionated, Coeval Felsic and Mafic Magmas Forming Synextensional Plutons, the Menderes Core Complex, Western Turkey

The Eğrigöz and Koyunoba monzogranites in western Turkey contain igneous enclaves that provide important information concerning the magma-forming processes and their petrogenetic origin in a core-complex setting. The Eğrigöz monzogranite differs from the Koyunoba monzogranite in that it displays a porphyritic texture and contains abundant amphibole and allanite. In addition, two mineralogically contrasting groups of igneous enclaves have also been distinguished in the Eğrigöz and Koyunoba monzogranites; one group contains both hornblende and biotite, while the only mafic mineral in the other group is biotite. The host rocks and igneous enclaves are compositionally calc-alkaline and metaluminous to slightly peraluminous and belong to the I-type class of granites. The geochemical signatures of the host rocks and igneous enclaves are largely similar, but some enclaves contain high Al2O3, Fe2O3, MgO, CaO, TiO2, MnO, P2O5, Na2O, Mg#, Cu, Zn, Ni, Ga, Nb, V, Ti, and Zr and low SiO2, K2O, and Th contents. The igneous enclaves are strongly depleted in Ba and light rare earth elements (REEs) and moderately depleted in heavy REEs, Nb, P, and Ti with respect to their host rocks. The Koyunoba monzogranite and associated igneous enclaves represent both the most evolved magma and the late stages of crystallization. This study suggests that the progressive fractionation and continuous mixing/mingling of coeval crustal and mantle magmas are important in defining the near-final composition of these granitoid magmas and their igneous enclaves. The host rocks and igneous enclaves have partly overlapping 87Sr/86SrI, εNdI, and Pb isotope values that indicate a contribution from crustal assimilation. Development of these processes in synextensional granitoids is consistent with a geodynamic model involving slab rollback-induced asthenospheric upwelling as a heat source, which caused melting and mixing of lower crustal and lithospheric mantle. Rollback-induced extension also played a fundamental role in the generation of conduits for the entry of high-K, mantle-derived mafic melts into the large felsic-magma reservoirs.