Investigation of upper-mantle anisotropy beneath İstanbul (ISK) broad-band station

Türkiyenin tektonik rejimi farklı levha hareketlerinden etkilenmektedir. Avrupa ve Arap levhalarının Zagros bölgesine bindirmesi, Anadolu Levhası’nın Ege’ye doğru olan kaçma hareketinin temel nedeni olmakla beraber, kaçış asıl olarak doğrultu atımlı Kuzey ve Doğu Anadolu fayları boyunca olmaktadır. Makaslama dalgası ayrımlanması analizi çalışmalarından elde edilen hızlı makaslama dalgası doğrultuları, farklı tektonik süreçlerle ilişkilendirilebilir. Gerilme deformasyon, mutlak ve göreceli levha hareketleri, astenosferik akış, geçmiş jeolojik devirlerden kalmış mineral yönlenmeleri bunların başında gelir. Böylelikle, sismik anizotropi çalışmaları, mevcut tektonik ve GPS verileri de göz önünde bulundurularak, bir bölgenin karmaşık tektonik yapısının araştırılmasında kullanılabilir. Marmara Bölgesi’nde üst mantoya ait sismik anizotropi İstanbul (ISK) geniş-bant istasyonunda (KRDAE-UDİM) kaydedilen verilere SKS ayrımlanması analizi uygulanarak incelenmiştir. Bölgenin sismik anizotropik yapısının incelenmesi amacıyla, ISK istasyonundan sağlanan 15 depreme ait kayıtlar veri analiz penceresini otomatik olarak seçen makaslama dalgası ayrımlanması analizi programı ile incelenmiştir. Analiz edilen verilerdeki SKS fazlarında belirgin makaslama dalgası ayrımlanması gözlemlenmiştir. ISK istasyonu için hızlı makaslama dalgası doğrultusu (43.7°) hesaplanmıştır. Elde edilen doğrultu Orta Anadolu’da yer alan Ankara (ANTO) geniş-bant istasyonundan (IRIS-GSN) elde edilen doğrultu (43°) ile karşılaştırılmıştır. Marmara Bölgesi’nin altındaki üst mantoya ait sismik anizotropi yapısının Ankara ve çevresi ile uyum gösterdiği görülmüş ve sözü edilen bölgelerin benzer tektonik rejimlerin etkisinde olduğu, bununla birlikte, önceki çalışmalarda hesaplanmış Pn anizotropisi doğrultularının, hesaplanan hızlı makaslama doğrultusu ile uyumlu olmaması nedeniyle, bölgedeki üst manto yapısının daha karmaşık olduğu sonucuna varılmıştır.  Anahtar Kelimeler: SKS ayrımlanması analizi, sismik anizotropi, Marmara Bölgesi.The seismic anisotropy contains information regarding the fabrics and deformation of deeper parts of the earth (Babuska etal., 1998). In an anisotropic medium, one component of a shear wave travels faster than the orthogonal component. The difference in speed causes the waves to separate; this phenomenon is called shear wave splitting (Savage, 1999). SKS splitting analysis is one of the most widely used methods to determine anisotropy (Silver and Chan, 1991). SKS splitting parameters; fast polarization direction (f) and delay time (dt), provide information about the dominant flow direction and thickness of the anisotropic layer. The study of anisotropy can shed light to the complex tectonic structure (or strain pattern) of the area when evaluated together with whole tectonic and GPS data. In the areas of recent convergence the fast direction of anisotropy generally aligns in the direction of the plate boundary (Vinnik etal., 1992). This correlation suggests that the shortening is the same in the crust and the upper-mantle. In the regions of rifting, direction of mantle flow generally aligns parallel to the extension in the crust. In the stable regions it is likely that combined effect of frozen anisotropy in the lithosphere and recent anisotropy in the asthenosphere are observed. The tectonic regime of Turkey is dominated by different plate motions. The collision of Eurasia and Arabia in the Zagros region is the driving force of west-ward tectonic escape towards the Aegean between a rigid boundary on the north, oceanic crust in the Black Sea and a weak boundary on the south, Aegean subduction zone (Şengör etal., 1985). The escape is mainly along the North and East Anatolian strike-slip faults. The fast shear wave polarization direction obtained from SKS splitting studies can be correlated with the absolute motion of the plates and the study of anisotropy can light up the complex tectonic structure (or strain pattern) of the area. In this study, seismic anisotropic structure of the upper-mantle beneath İstanbul (ISK) broad-band station (KRDAE-NEMC), Marmara Region, is investigated by using SKS splitting analysis. Fifteen earthquakes with magnitude greater than 5.0 occured at epicentral distances between the range of 85° and 120° are selected based on the criteria of existing of clear SKS phases on the records. Splitting correction method of Silver and Chan (1991) can be used for obtaining SKS splitting parameters (f, dt).  In this method, analysis window is chosen manually. But calculated parameters are sensitive to the manually selected shear-wave analysis window. Therefore, we used an automated shear wave analysis code (Teanby etal., 2004) to analyse the data. In this code, the analyse window is selected automatically by performing a grid search over different windows to find stable splitting measurements with small error. To stabilize the results and reduce cycle-skipping effects, start of the window should be chosen slightly before the onset of the shear wave. It is also important that over a wide range of different analysis windows splitting parameters are stable to provide robust measurements (Teanby etal., 2004). If anisotropy exist, the particle motion within the window should be elliptical. Also energy should be present in the transverse component of the data. When those conditions are provided, splitting analyse is performed. Splitting parameters are the parameters that correct the effect of the anisotropy. After the correction is applied with obtained splitting parameters, splitting should be corrected which means a linear particle motion instead of elliptical, no energy on the transverse component and no delay time. Following those data processing steps, SKS splitting parameters for the ISK station, from 15 teleseismic earthquakes are obtained. For ISK station fast polarization direction is calculated as 43.7°. This parameter is compared to the one that was obtained from Ankara (ANTO) broad-band station (IRIS-GSN) in Central Anatolia and existing results of Pn anisotropy studies (Al-Lazki etal., 2004) which is sensitive to uppermost mantle anisotropy. Splitting parameters obtained from ISK station exhibit parallelism with those obtained from ANTO station. This suggests that seismic anisotropy under ISK and ANTO are effected by similar tectonic regimes, however, due to the incoherent SKS fast polarization direction (f) and Pn anisotropy directions upper-mantle structure beneath ISK station is suggested to be more complex. Keywords: SKS splitting analyse, seismic anisotropy, Marmara region

Rapid early-middle Miocene exhumation of the Kazdag Massif (western Anatolia)

Apatite fission-track analyses indicate that the Kazdag. Massif in northwestern Anatolia was exhumed above the apatite partial annealing zone between 20 and 10 Ma (i.e. early-middle Miocene), with a cluster of ages at 17-14 Ma. The structural analysis of low-angle shear zones, high-angle normal faults and strike-slip faults, as well as stratigraphic analysis of upper-plate sedimentary successions and previous radiometric ages, point to a two-stage structural evolution of the massif. The first stage encompassing much of the rapid thermal evolution of the massif-comprised late Oligocene-early Miocene low-angle detachment faulting and the associated development of small supradetachment grabens filled with a mixture of epiclastic, volcaniclastic and volcanic rocks (Kucukkuyu Fm.). The second stage (Plio-Quaternary) has been dominated by (i) strike-slip faulting related to the westward propagation of the North Anatolian fault system and (ii) normal faulting associated with present-day extension. This later stage affected the distribution of fission-track ages but did not have a component of vertical (normal) movement large enough to exhume a new partial annealing zone. The thermochronological data presented here support the notion that Neogene extensional tectonism in the northern Aegean region has been episodic, with accelerated pulses in the early-middle Miocene and Plio-Quaternary