Kuzeybatı Anadolu'nun Tektoniği Ve Paleomanyetik Sonuçlar

Abstract

Batı Anadolu'nun tektonik evrimine ışık tutmak amacıyla; Çanakkale- Foça arasında kalan ve doğuda Bergama hizasından sınırlanan çalışma alanından 8 üst Oligosen, 35 alt-orta Miyosen ve 4 üst Miyosen olmak üzere 47 paleomanyetik mevkiden numune toplanmış ve bu mevkilerden sadece ikisi duraylı sonuç vermediği için değerlendirme dışı tutulmuştur. Numune toplama, mıknatıslanma doğrultularının ölçülmesi ve ikincil mıknatıslanmaların temizlenmesinde paleomanyetik teknikler kullanılmıştır. Paleomanyetik verilerin değerlendirilmesinden çıkan sonuçlar şunlardır: 1-) Üst Oligosen yaşlı mevkiler saatin ters yönünde 20°-30° arasında bir dönmenin olduğunu göstermektedir. 2-) Çalışma alanında en yaygın olarak bulunan alt-orta Miyosen yaşlı volkanizmaya sahip mevkilerde; Edremit Körfezi'nin kuzeyi için saatin ters yönünde 25°'lik bir dönme, güneyi için ise yine saatin tersi yönünde 15°-40° arasında dönmelerin olduğu tespit edilmiştir. Yine aynı yaşlı kayaçlarda; Assos ve civarında, listrik faylanmadan ötürü oluşan bir çok mikrobloğun farklı doğrultu ve eğime sahip olacak şekilde düşerek yerdeğiştirdikleri saptanmıştır. Edremit Körfezi'nin güneyinde ise Kozak Dağı diğer mevkilere göre ters yönde (saat yönünde 31°) bir dönme gösterdiği için bir blok olarak ayırtlanmıştır. 3-) Üst Miyosen yaşlı mevkilerin ise, saatin tersi yönünde 16°'lik bir dönmeye sahip olduğu bulunmuştur. Bu verilerin ışığı altında; kuzeybatı Anadolu'nun Neojen'den bu yana saatin tersi yönünde bir dönme hareketi geçirdiği ve bu dönme esnasında, Edremit Körfezi'nin kuzey ve güney kesimindeki levhaların birbirlerine göre farklı hızla hareket etmesi sonucu Edremit Körfezi'nin açıldığı kanısına varılmıştır.

Neotectonic of Anatolia has started with the closure of neotethis during the latest Cretaceous-earliest Tertiary. During late Cretaceous, compressional regime sequized plates and ophiolithic naps were formed. During late Eocene-early Miocene this compression continued and thickness of the crust riched 60 km. This regime caused partial melting of lower crust. The results of partial melting can be seen in silisic volcanic activities in western Anatolia during this period. Until early Miocene, thickening of the crust continued and because of this, the gravitational force (Menderes Massive) subsided. Thus caused extensional regime in western Anatolia and the grabens were formed possibly mid-Miocene. During mid and late Miocene Çüngüş Basin closed complety and collision was started between Arabian and Eurasia plates. This collision caused the crust thickening in Eastern Anatolia. Force exerted from thickening of the crust, converted into the lateral forces, produced strike slip faulting on the weak parts of the Anatolian block, namely North Anatolian Fault (NAF), East Anatolia Fault (EAF). Along this faults, Anatolian blocks moved westwards. This movement of Anatolian block was stopped by Greece Shear Zone, producing a movements of plates towards Southwest, and producing extensional regime in North-South direction. This extensional regime generated volcanic activity in western Anatolia, Tertiary volcanism of Biga peninsula has started in Eocene. These volcanics consisting generally of tuffs, agglomerates and lava flows of dacites and green-gray andesites are locally intercalated with sedimentary rock of Middle Eocene age, cropping out between Biga and Çanakkale. From Upper Oligocene onwards, a new episode of volcanism has started to be effective, yielding rocks of andesitic, dacitic, trachy-andesitic and rhyodacitic composition, in the medial and eastern parts of the like peninsula and locally in the vicinity of Gökçeada and Altınoluk. The rocks have mostly been altered and locally have been silicified. Radiometric dating have yielded K/Ar ages of 28.2-23.6 My. Most of the metallic ores of NW Anatolia are related to this event. Miocene volcanism of the Biga peninsula has occurred in two episodes occupying a very large area. It has been effective Northwest of the peninsula, Bozcada and Gökçeada in Lower Miocene slowing down Late Middle-Miocene. Lava, tuff and ash of dacitic, rhyodacitic, andesitic, latitic and trachy-andesitic composition are observed. Lava flows domes, volcanic necks, ignimbrites and lahars are characteristic elements of the Lower-Middle Miocene volcanism. K/Ar ages of 13.6-22.3 have been obtained. The most important centre of eruption is the Behram caldera situated in the marine area between Behram and the Midilli island. A new episode has been effective in Upper Miocene with initial eruptions of trachy-andesitic dare coloured rocks followed by alkali olivine basalts. These crop out as dykes intruding the previous Tertiary volcanism or as lava flows in the vicinity of Ezine, Ayvacık, Çanakkale, Çan and Tavşan island. The radiometric dating yields an age of 9 My. The Eocene volcanism of the region is of calcalkaline character, ascribed to a subduction zone. Upper Oligocene, Lower-Middle Miocene volcanism is calcalkaline and hybride and ascribed to crustal thickening following the Eocene collision between the Anatolids and Pontides. The alcaline volcanism of Upper Miocene has presumably originated from the mantle. Tectonic movements can produced a rotation of blocks which can be determined from palaeomagnetic studies. Although there are some research undertaken in Western Anatolia (Kissel etal, 1986 ; Orbay etal, 1993), they deal mainly on global scale. In this study, palaeomagnetic samples collected from 47 sites (eight of them late ageing Oligocene, 35 of them early Miocene, 4 of them late Miocene), around Çanakkale-Foça and Bergama, have been analysed in order to understand the tectonics of Western Anatolia. Only two of sites were rejected because they exhibit unreliable magnetization. 386 core samples were collected and cut intol inch in length making 835 cylindrical samples, prepared for palaeomagnetic measurements. In the laboratory, first Natural Remanent Magnetization (NRM) were measured for each sites, then two pilot samples were chosen for alternative demagnetization and thermal demagnetization. Using alternative and thermal demagnetization methods, primary magnetization of the samples were obtained. If the coercivity of magnetization is high enough 25, 50, 75, 100,..., 600 (Oe or °C) steps were used, otherwise demagnetization steps were chosen as 10, 20, 30, 40,..., 250 (Oe or °C). For each step of cleaning, Magnetization magnitude, declination and inclination were measured using a spinner magnetometer, and XI cleaning steps of samples were determined from As-Zijderveld projection, Stereografic projection and Normalised magnetization diagram. In the second stage of cleaning, remaining samples of each site were cleaned using demagnetization field (or temperature) decided during the first stage of cleaning, then mean direction of magnetization of each site were calculated using Fishers statistics. The declination, inclination, precision parameter (k) and circle of confidence of 0.05 probability level (a95 ) for Natural remanent magnetization (NRM) and Remanent magnetization (RM) are given in Table 5.1. The palaeomagnetic results of two sites have not been included for analysis since they have precision parameter less than 20. In the calculation of mean direction of magnetization for a locality, firstly reverse polarities converted to normal polarities then location mean was obtained. The calculated mean palaeomagnetic pole position of this work is not significantly different from axial dipole field in 0.05 probability level. This means that, secular variation of geomagnetic field in sampled area has been companseted. Following conclusions are deduced from this study: 1-) Two sites taken from Kirazlı Village (BA16-BA17, upper Oligocene in age) on Çanakkale-Çan highway, in the Northern part of the working area, has a mean direction of magnetization of D=-36°, 1=46°. 2-) Four sites around Ezine (B1-B2-BA5-BA6, upper Miocene in age) has a mean direction of magnetization of (D=-16°, 1=64°, k=69.3, a95 =3.7). 3-) Two sites, one of which (BA4) West of Ezine and the other North of Tuzla (BA7, lower Miocene in age) a mean direction of magnetization of D=+32°, 1=57°. 4-) Five sites around Assos (B10-B11-B12-B13-B14, early Miocene) a mean direction of magnetization of (D=-25°, 1=50°, k=64.8, a95=2.7). 5-) It has been reported that (Y. Yılmaz, personal communication, 1994) there are some listric faults running E-W direction parallel the seashore of Edremit Bay (from Gülpmar to Edremit). Samples collected from 15 sites although gave reliable direction of magnetizations, they also exhibit the effect of listric faulting. xn 6-) There seems to be 2 main groups of directions in the palaeomagnetic results around Ayvalık (Southwest of Edremit Bay). The rocks consisting of each group have the same age (upper Oligocene) but they indicate different direction of rotations: The first group consist of three sites (A3-A4-BA15) and shows 15° rotation in counter clockwise. The other group indicates 29° rotation in clockwise. 7-) The samples taken area (B15, B16, B17) from Karadere region, south of Edremit has a mean direction of magnetization of D=+19°, 1=39°. 8-) The samples have taken around Altinova streem (lower Miocene in age) a mean direction of magnetization of D=-54°, 1=56°. 9-) Eigth sites (B18-B19-B20-B21-B22-B23-B24-B25, lower Miocene in age) located between Aşağıbey Village and Gökçeağıl Village, Southwest of Kozak Dağı, has a mean direction of magnetization of D=+31°, 1=57°, k=28.6 and ag5-3.7. 10-) Two sites from Dikili (BA12, BA13, lower Miocene in age) gave the following direction of magnetizations respectively 1° and 17°, and inclination 45° and 20°. 11-) Site (BAH, lower Miocene in age) from North of Bergama has a denclination of 29° in anticlockwise direction and inclination of 40°. Northwestern Anatolian Volcanics have been investigated palaeomagnetic directions by Kissel etal, (1986) and Orbay etal, (1993). Results obtained from this study are in good agreement with the results obtained above mentioned authors. Which are plotted in Figure 7.1. In this figure reverse polarities are converted into normal polarities in order to get an easy looking. In Çanakkale-Edremit Region, results obtained from this study shows a good correlation with Orbay's results, indicating 25°-34° rotation in counter clockwise direction but there is no correlation with Kissel's results except Ezine region. On the other hand, results obtained in three studies correlated with each other for the region of South of Edremit Bay. Rotation of 10°-30° in counter clockwise direction has been calculated around Alibey adası, Şeytan Sofrası, the East part of region shows 20°-30° rotation in clockwise direction. xm This region surrounding by Ayvalik-Dikili and Bergama is rotated 15°- 40° in counter clockwise direction, only exception Kozak Dağı. The result obtained from Kozak Dağı shows that there has been 31° rotation in clockwise direction. This result is significantly different than the results obtained from the area surrounding Kozak Dağı. When all the results are taken together it leads to that Northwest Anatolia has been rotated in counter clockwise direction since Neogene. This is also in agreement with GPS measurements taken during 1990-1992 which suggest that the movement of the block is in (WWS) direction (Figure 8.1). Taymaz (1990) investigated tectonic movements in Aegean Sea. He used seismologic data collected from earthquakes around Greece, Aegean sea, Bulgaria and Nortwest Anatolia (38°,42 N / 22°,28 E). He concluded that there are four major blocks separated by three faults and they extend up to the Hellenic Trench. He explained that East- West compression of Aegean Sea causes the plates to move Southwards. During this motion central part moves faster than Western and eastern parts. This movement causes a rotation of plates, clockwise direction in Western parts, counter clockwise direction in eastern parts. Kissel's results from Greece indicates a clockwise rotation, which proves Taymaz's model for the western part. The results from Orbay etal, (1993) and this study supports Taymaz's model for the Eastern part of the area. Palaeomagnetic result obtained from the area surrounding Edremit Bay, lead to a conclusion that during anticlockwise rotation of Northwest Anatolia, south of Edremit Bay rotated faster than the Northern part, as a result of this Edremit Bay was opened.