Geology and tectonic evolution of the southeastern part of the Istranca zone (Büyükçekmece-Çatalca)

This study covers the region between Çatalca center and Çiftlikköy (eastern parts of Istranca Massif) within the borders of Çatalca district of Istanbul province. The detailed stratigraphy of the region, macro and micro deformation structures, principal stress axes and tectonic regime characteristics, metamorphic conditions and petrology, geochemistry and geochronology, and geodynamic evolution of the basic units are presented using field and laboratory data. The Istranca Massif basement is composed of the Çamaşırdere Metagranodiorite aged 639.7 ±11.3 My, which is traced as angularly unconformable and faulted in places and named for the first time in this study and whose radiometric age has been revealed for the first time, and between 534.5 ±4.7 My and 546.0 ±3.9 My (Ediyakaran) aged İhsaniye Metagranite and Çatalca Metagranite crystalline units, as well as Triassic (?) aged metacrastic rocks included in the Kocabayır Group and Jurassic aged tuff interbedded, metachertstone-metagrovac units. The igneous and metamorphic basement units and the Tertiary-aged clastic and carbonate sediments of the Thrace Basin are unconformably surfaced in the study area, which forms the southeastern end of the Rhodope-Istranca massif extending along the Romania, Bulgaria and NW Turkey line. The sedimentation that started with the Eocene transgression turns into a regressive sedimentation in the Oligocene, and the region becomes a terrestrial-transitional environment with shallowing. This angularly unconformably overlying the Eocene-Oligocene transgressive carbonate and clastic units, characterized by fish fossils in places, is covered by a river deposit characterized by poorly consolidated, gravelly levels that unconformably overlies all other units in the Miocene following a major erosional phase at the end of the Oligocene, especially in the upland parts of the study area. Petrographic and geochemical investigations of the Çamaşırdere, Çatalca and İhsaniye plutons show that they are Precambrian aged crystalline basic units with granitic and granodioritic characteristics and that these intrusions of similar age in the same belt present features reflecting different geodynamic environmental conditions as a product of active continental margin arc and intraplate magmatism. The crystalline units are overlain by well-developed SC structures, sigmoidal and asymmetric shear direction markers S-C, S-C' type shearing, σ-type shearing, stair-stepping, sigmoidal grains and buden structures, mica fish, strain shadows and deltaic structures, as well as slate, phyllate, metagneolite and groves reflecting low to moderate metamorphic conditions. These levels, which are characterized by their highly deformed and folded structure, are widespread outcrops on both sides of the Istranca escarpment, and the present lithologic structure, as well as the data on the primordial texture and grain composition of the rock in places, as well as the change towards the top in the stack, reveal that the protoliths of these metamorphic rocks are sedimentary rocks belonging to two separate transgressive series. From oriented samples collected from low to intermediate milonitic metamorphites, microtectonic investigations carried out on meta sediments overlained unfconformaby by jurassic metaconglomerates in the region revealed shear orientation, which generally generally a indicate a top-to-the SW sense of shear. With the fracture, fault and fold data collected from the basic units and overlying Tertiary sedimentary rocks in the Istranca uplift, which forms the most SW end of the massif and is bounded by faults on both sides, the active deformation faults in the region were tried to be revealed and it was observed that superimposed structures developed under multi-phase deformation phases related to the compression reflecting the orogenic conditions of the Cadomian, Variscan and Alpine periods and the tensile and shear tectonic regime related to the intermediate periods.