Kinematics of deformation and high-temperature relict fabrics in the western strandja massif, nw Turkey

Abstract

The Strandja Massif is a prominent, polymetamorphic belt located in the northwestern part of Turkey and the southeastern part of Bulgaria. The NW-trending massif is exposed over approximately 300 km and is about 100 km wide. The massif records a prolonged tectonic history involving multiple episodes of metamorphism, magmatism, and deformation. It consists of a Neoproterozoic-Paleozoic metasedimentary complex that is intruded by plutons of different ages and compositions. All these units are overlain by a Mesozoic metasedimentary unit with a tectonic contact. Previous studies have indicated that the massif has experienced two distinct metamorphic events. It is known that the massif underwent greenschist- lower amphibolite metamorphism and significant deformation during the Middle Jurassic to Early Cretaceous time. The aim of this study is to constrain the P-T conditions of the Paleozoic metamorphism and to determine the deformation phases. The Neoproterozoic-Paleozoic metasedimentary complex is mainly composed of biotite gneiss, biotite-garnet gneiss, amphibolite, and quartzofeldspathic schist. Migmatization is widespread within this unit. This unit is cut by four different metagranites that are compositionally and structurally distinct. The oldest of these metagranites is the Büyünlü Complex. It has a monzogranitic composition and is characterised by highly strained K-feldspars. The unit exhibits a structurally complex appearance with two distinct foliations and lineations. The second unit is the Çatma Complex. This unit has a granodioritic-granitic composition. Migmatization is widespread in both the Büyünlü and Çatma complexes, indicating they have undergone Paleozoic metamorphism. The Hamzabeyli Complex is granitic in composition. This unit exhibits weak foliation and lineation. The Hamzabeyli Complex is overlain by the Mesozoic metasedimentary unit, which consists of limestones, marble, dolomite, metasandstone, schist, and phyllite. The Doğanköy Complex, which is compositionally similar to the Büyünlü Complex, differs from it by not showing migmatization and having a rather monotonous structure. As a result of structural analysis, D1 and D2 deformations are distinguished, representing Paleozoic and Mesozoic metamorphism, respectively. The D1 deformation is homogenous in the macro scale and is observed in all units except the Doğanköy and Hamzabeyli complexes and the Mesozoic metasedimentary unit. Structures associated with this deformation include S1 foliation, L1 lineation, and F1 folds. S1 foliation is defined by its parallelism with migmatization. In the study area, the L1 lineation is dominantly dipping to the southeast. The kinematic indicators indicate NW vergence of tectonic transport. F1 folds are divided into two groups: macro (F1-1) and meso (F1-2) scales. The macroscale F1-1 folds are observed in the southern and southwestern parts of the study area. The strict parallelism of the hinge line of the F1-1 folds, and L1 lineation suggests that these faults can represent a section from a sheath fold that indicates a northwest vergence of motion. The mesoscale F1-2 folds are isoclinal folds with axial planes parallel to the S1 foliation. Both fold systems indicate that the D1 deformation occurred under highly ductile conditions. Migmatization associated with Paleozoic metamorphism plays a significant role in the ductile behaviour of this deformation. The D2 deformation is observed in all units within the study area. This deformation is heterogeneous at the macro scale. Structures associated with the D2 deformation consist of S2 foliation, L2 lineation, and F2 folds. The type of S2 foliation depends on the intensity of the D2 deformation. If the intensity of D2 deformation is low, S2 foliation is weakly developed, and spaced foliation that is often defined by muscovite. Where D2 deformation is moderately intense, crenulation cleavage is observed. In these areas, the cleavage domains are represented by S2, while the microlithons are represented by S1 foliation. The areas of intense D2 deformation are defined by ductile shear zones. In these zones, S2 foliation is penetrative and continuous. S1 foliation can be traced in these rocks through compositional layers formed by migmatization. L2 lineation is observed as crenulation lineation in areas where the D2 deformation is moderately intense and as mineral lineation or aggregate lineation in areas where it is highly intense. Similar to L1 lineation, L2 lineation predominantly dips to the southeast, indicating northwestward tectonic movement. The F2 folds are predominantly represented by tight, asymmetric folds with northwest vergence. The F1 folds are commonly superimposed by the F2 folds. The D2 deformation marks the brittle-ductile transition. Even though the D1 and D2 deformations are of similar kinematic sense, they are distinguishable by their differences in terms of material behaviour. The D1 deformation suggests conditions of highly ductile behaviour, whereas the D2 deformation represents conditions of brittle-ductile transition. Besides, the relationship between structures and migmatitic zones is another distinguishing parameter. Structures associated with the D1 deformation are compatible with migmatitic zones, whereas the D2 deformation displays an oblique relationship. The peak metamorphic conditions of the Paleozoic event are constrained by three criteria: (1) The widespread migmatization in the basement units necessitates temperatures exceeding the wet solidus of granitic rocks that sets a minimum temperature boundary of approximately 650 °C, (2) The stable coexistence of hornblende and plagioclase in amphibolites confirms conditions within the amphibolite facies, (3) The discovery of partially preserved kyanite crystals provides a critical pressure constraint. Integrating these mineralogical and field observations, the peak metamorphic conditions for the Paleozoic event are restricted to a temperature range of 650–720 °C and a pressure range of 6–12 kbar. In this study, high temperature relict fabrics were investigated to characterise the Paleozoic metamorphism. The chessboard subgrain pattern in quartz is observed specifically within the leucosomes of the migmatitic gneisses provides evidence that peak metamorphism occurred during one of the episodes of a prologue structural history of the Strandja Massif. The absence of this subgrain pattern out of leucosomes supports that the Paleozoic metamorphism did not advance into the granulite facies. The grain boundary migration (GBM) and subgrain rotation (SGR) recrystallisations and the growth of deformation myrmekites along the high-stress sites of feldspar also require higher temperature conditions than the metamorphic conditions of the Mesozoic deformation. Thus, identifying these high-temperature relicts is the key to unravelling the pre-Mesozoic thermal history that was masked by later deformations.