Batı pontidlerde kretase ve eosen ada yayı volkanizmalarının paleomanyetik çalışması

Abstract

Pontidlerin paleotektonik evrimine ışık tutmak amacıyla Karadeniz bölgesinde Zonguldak ve Samsun arasında kalan ve güneyde Kastamonu ile sınırlanan çalışma alanından 1 Alt Kretase, 14 üst Kretase ve 3 Eosen yaşlı olmak üzere toplam 18 mevkiden kayaç örnekleri toplan mıştır. Paleomanyetik verilerin değerlendirilmesinden çıkan sonuçlar şunlardır: 1- Alt Kretase yaşlı mevkiden saptanan paleomanyetik eğim açısı 3 dir. 2- üst Kretase yaşlı mevkilerden saptanan ortala ma eğim açısı 34 dir. 3- Eosen yaşlı mevkilerden saptanan eğim açısı ise 33 bulunmuştur. 4- Çalışma alanı içinde paleomanyetik sap ma açılarının kendi içinde tutarlı olarak dağıldığı 4 blok ayırtlan- mıştır. Pontidlerin paleotektonik gelişimi günümüze kadar yapılmış güvenilir paleomanyetik verilerin ışığında Geç Permiyen'den Eosen 'e kadar olan jeolojik zaman içerisinde incelenmiştir. Geç Permiyen-Alt Triyas dönemine ait paleomanyetik veriler, Batı Pontidlerin Lavrasya'ya ait olduğuna işaret eder; Permo - Triyas zamanında 7 kuzey paleoenle- minde olan Batı Pontidler, Alt Triyas 'da 31 kuzey paleoenlemine doğru yer değiştirmiştir. Alt Triyas - Üst jura dönemi pontidlerin paleo tektonik evriminde çok kritik bir zaman aralığıdır. Jeolojik verile re göre Orta Jura' da Pontidlerde Paleotetis'in kapanımı olarak yorum lanan bir okyanus kapanması olmuştur. Bu okyanusun kalıntıları olan ofiyolitik kayaçlar, Küre civarında yüzeylerler. İstanbul Napı, sözü edilen ofiyolitik kayaçlar üzerinde tektonik olarak durur. Bu çalışma da Batı Pontidler olarak isimlendirilen bölge İstanbul Napı'na Karşı gelmektedir, üst Jura ve Alt Kretase dönemine ait paleomanyetik ve rilerden saptanan paleoenlem ekvator civarındadır. Alt Triyas zama nında 31 kuzey paleoenleminde bulunan Batı Pontidlerin Lavrasya'dan koparak Üst-Jura - Alt Kretase döneminde ekvatorda bulunması Batı Pon tidlerin bu yer değiştirme esnasında güneyinde bulunan Paleotetis' i kapatarak, Doğu Pontidlerle çarpışmasıyla açıklanabilir. Üst Jura-Alt Kretase'de Pontidler artık tek bir tektonik birim olarak ortaya çık mışlardır. Bunun jeolojik kanıtı da gerek Batı ve gerekse Doğu Pon- tidleri örten üst Jura - Alt Kretase yaşlı cökel kayaçların varlığıdır. Pontidlerin Üst Kretase 'deki paleoenlemi 20 civarındadır. Alt Kreta se - üst Kretase dönemi içinde gerçekleşen bu yer değiştirmenin nedeni, Pontidlerin güneyinde Alt Kretase zamanında açılmaya başladığı jeolo jik çalışmalarla belgelenen Neotetis okyanusudur. Pontidleri 20 paleo enlem civarına getiren Neotetis açılımıyla birlikte aynı okyanus kuzey de Pontidlerin altına dalarak, üst Kretase ada yayı volkanizmasını oluş turmuştur. Üst Kretase - Eosen dönemi içerisinde Pontidlerin kuzeye doğru hareket etmedikleri Eosen 'e ait paleomanyetik verilerden anlaşıl maktadır. Bu araştırmada paleomanyetik verilerden elde edilen paleoku- tup pozisyonları Alt Kretase için 47.0 K, 207.2 D; Üst Kretase için 66.7°K, 241.5°D; ve Eosen için 64.8°K, 215.6°D dir. Bütün bu paleoman yetik kutup pozisyonları Avrasya yerine, Afrika'nın kutup pozisyonları ile uyum halindedir.

Paleomagnetic studies have started elaborating the large-scale plate movements for 40 years. Thei first application of such a / work was to bring paleomagnetic evidence to the opening of the Atlantic Ocean. Since then paleomagnetic studies have provided information about the distribution and movement of continents with respect to each other throughout geological time. In the last 10-15 years paleomagnetic studies have especially tended to solve more local geological problems in land such as evolution of oro- genic belts and their deformation history. Turkey, which consists of more than one suture belt extending in east -west direction in general, takes place within the Alpine- Himalayan orogenic belt. The evolution of the suture belts, which may encircle some small-scale blocks, has been the subject of some debates among the geologists workins in Turkey. However, there is a mutual view that the main tectonic units of Turkey such as the Pont ides and the Kırşehir Massif, were independent in the past and brought into ane single unit by converging movements of African and Eurasian plates. To obtain a better understanding of the geodynamical model for the evolution of the Pontides, this paleomagnetic work was planned and carried out in the area between Zonguldak and Samsun. Paleomagnetic samples were collected from 1 Lower Cretaceous, 14 Upper Cretaceous, and 3 Eocene sites. Samples were oriented by both magnetic and solar compasses and taken from mostly volcanic and volcano-clastic rocks (For detail see: Chapter 2). Paleomagnetic measurments were carried out at the KANTEK laboratory, which is run by the Istanbul Technical University and Kandilli Ob servatory of the Boğaziçi University. First of all, each oriented sample was cut into 3 or 4 specimens each of which had 2.54 cm in radius and height. The orientation arrows were carefully transfer red onto the specimens and they became ready for magnetization measurements. After having measured natural magnetizations (NRM) on the Astatic magnetometer, two specimens from each hand sample were selected as pilot samples to be tested by alternative and thermal cleaning methods. As a result of these cleaning tests secondary and primary magnetization directions were successfuly obtained by using the computer programs coded for this purpose (Chapter 3, 4, 5).. ivNRM, RM (remanent magnetization after cleaning) and tilt corrected RM directions of hand samples belonging to the same site were cal culated and displayed by sterographic projections. From these one can easily observe changes in the direction of magnetization which may have taken place through demagnetization process and tectonic correction. In Chapter 7 reliability of the paleomagnetic results was questr. ioned. The questions put forward are the followings: 1-Was secondary magnetization eliminated? 2- Is the direction of pri mary magnetization stable? 3- What is the age of primary mag netization? 4- Are the magnetization directions in the same aged sites are compatible? 5- Were normal and reverse polarities obta ined in the sites? 6- How can the results of this work be compa red with previous paleomagnetic works done in the Pontides. After having made the analysis of paleomagnetic data by answering those questions the results of this work were considered fairly ; reliable. The results obtained in this study are discussed in Chapter 8. Theresults can be summarized as below: 1- The inclination angle of the Lower Cretaceous site is found to be of 3. 2- The mean inclination of the Upper Cretaceous sites is of 34. 3- The mean inclination of the Eosen sites is of 33. The study area can be divided into 4 blocks on the basis of paleo magnetic declination angles. The first block lies between Bartın and Samsun with the exception of İnebolu and Sinop sites. The declination angles obtained for this area do not indicate any rota tion. The second is the Sinop peninsula whose declination angles indicate ant i -clockwise rotation of 20. The third one extends from Bartın to eastwards covering the Ereğli and Devrek areas, which _olsa rotated 20 ant i -clockwise. The fourth block covers the area between Samsun and Gümüşhane which is also rotated 30 anti-clockwise. 5- The inclination angle provided from the İnebo lu site is of 78 being guite higher than the other sites. This anomalous value may have been produced by some tectonic activities in the area. For that reason, this site was not taken into account in the interpretation of paleomagnetic data. 6- Paleomagnetic di rections of the Bartın and Ereğli sites were used in the interpre tation without application of any tectonic correction. The incli nation values of the Bartın and Ereğli sites would be unreasonably high if tectonic correction was applied. Not applying tectonic correction can be justified with the reason that the dip angles of the beds underlying volcanics may represent original flow surface of the lavas. 7- In the Gürsökü site, which is located south of Sinop, the only one sample was measured out of 4 samples. The declination and inclination of the sample was found 166 and -24 respectively.The results of this work were compared with the previous paleomag- netic data acquired from the Pontides. On the basis of this com parison some similarities can be seen between this work and the previous paleomagnetic works: 1- The mean inclination value of 4 which is estimated from the work of Van der Voo (1968), Orbay (1979) and Lauer (1981) is almost the same as the inclination value of 3 obtained in this work. 2- The inclination values of previous Upper Cretaceous data vary between 36 and 50. The mean inclination value of the Upper Cretaceous sites of this work is of 34. 3- For the Eosene time the inclination values of the previous data and this work are 40 and 33 respectively. Paleotectonic evolution of the Pontides has been further elaborated in 5 geological epoques between the Late Permian and Eocene in the light of the available paleomagnetic data. The Permo-Triassic (Gregor and Zijderveld, 1964) and the Lower Triassic (Saribudak et al., 1987) data showed that at least the western Pontides may have belonged to Laurasia in the Early Mesozoic time. The western Pontides were at 7 paleolatitude during the Permo-Triassic and then, they moved up to 31 paleolatitude in the Lower Triassic time. The Lower Triassic -Upper Jurassic period was a critical turning point in the development of the Pontides: the so-called Paleotet- hys was closed during the Early Jurassic time whose oceanic remnants were scattered in the Küre and Kastamonu areas. The continental assemlage of the western Pontides, which is equivalent to Şengör's Istanbul Nappe (Şengör et al., 1985), resides on the Paleotethys' oceanic remnants tectonically. The paleomagnetic data belonging to the Upper Jurassic and Lower Cretaceous times place the Pontides around the equator region. Taking all together the geological and paleomagnetic data, it can be concluded that the Paleotethys' oceanic remnants may be the result of a collision between the western and eastern Pontides before the Middle Jurassic. In other words, the Paleotethys was an ocean that separated the western and eastern Pontides before the Early Jurassic. The rifting off the western Pontides towards south closed the Paleotethys and collided with the eastern Ponti des. After the collision the Pontides as a single tectonic unit moved from equator to 20 paleolatitude between the Lower and Upper Cretaceous fimes. The driving mechanism for the Pontides may be the result of opening of the Neotethys ocean in the south of the Pontides. The Neotethys ocean had started subducting under the Pontides resulting in the formation of island are volcanism on which this paleomagnetic work is carried out. The Eocene paleolatitude shows no difference from the Upper Cretaceous paleo latitude, meaning that the northernly drift of the Pontides was stopped between the Upper Cretaceous and Eocene times. viA wide ocean would be necessary in order to allow the Pontides to move northerly over several thousands of kilometers between the Lower and Upper Cretaceous times. Available paleomagnetic data from the Caucasus (Westphal et al., 1986) also require and ocean between the Caucasus and the Pontides. In the basis of paleomag netic data, this interpretation indicates that the Black Sea basin which encircles the Pontides from the north is considered to be remnant of Early Mesozoic Tethys. However some researchers propose that the Black Sea basin was resulted from spreading behind a Late Cretaceous volcanic island arc (Adamia et al., 1974; Şengör and Yılmaz, 1981; Zonenshain and Le Pichon, 1986). Calculated paleomagnetic pole positions of the Lower and Upper Cretaceous, and Eocene times are 47.0°K, 207.2°D; 66.7°K, 241.5°D; and 64.8 K, 215.6 respectively. All these paleopole paritions fit well with the African paleopoles rather than Eurasian (Chapter 8). A paleolatitude curve was obtained using all the available data from the Pontides. A comparison of this curve with those of Africa and Eurasia (Westphal et al., 1986) showed the latitudunal changes of the Pontides with respect to Eurasia and Africa. During the Late Permian-Lower Triassic, the western Pontides were Posi tioned at the southern margin of Laurasia. Between the Upper Jurassic and Lower Cretaceous, the Pontides were located too far to the south of both the Eurasia and Africa continents. However, it seems physically impossible for the Pontides to take place in the south of Africa. For that the Pontides must have been in the east of Africa, possibly in the Proto-îndian ocean (Lauer, 1981). The paleolatitude' value estimated for the Upper Cretaceous time is similar to that of Africa. The Pontides almost kept the same lati tudinal position in the Eocene with respect to Africa (Chapter 8). The declinations of all the available paleomagnetic data and the main tectonic features (North Anatolian Fault, thrusts and folds) of the Pontides were correlated in order to substantiate any relation with each other. Four blocks are distinguished each of which has distinct declination angles in itself. The first one extending from Bartın to Samsun indicates almost no rotation with the exception of the Sinop sites (Sİ01, Sİ02). The axes of folds and thrusts have an E-W trending in this area indicating a N-S compression regime since the Upper Cretaceous time. The above- mentinoned observations are found to be compatible with the non- rotated declination angles of this inferred area. The second block starts from south of Samsun and continues toward Gümüşhane following the volcanic belt of the island arc. The declination angles in this block show 30 counter-clockwise rotation app roximately. The axes of folds and thrusts have also a NE-SW tren ding in this area supporting the counter-clockwise rotation of the block. The responsible forces for the rotation of the area Vllmay be explained by the post -tectonic activities which resulted from the collision of the Arabian platform with the eastern Turkey in Miocene time (Şengör and Yılmaz, 1981). The Sinop block being the third one also indicates a counter-clockwise rotation of some 20. It is known that the Sinop peninsula is bounded by normal and thrust faults from the south and north respectively (ilhan, 1976). These faults may have caused the Sinop peninsula to rotate in the counter-clockwise fashion. The deformation causing the change of declination angle in counter-clockwise in the fourth block (Ereğli, Devrek and Bolu areas) may have been resulted from the collision of the Pontides and Sakarya Continent which was probably occurred during or after the Upper Cretaceous time (Şengör and Yılmaz, 1981). It is worth to mention that the NE-SW orientation of the fold axes and thrusts also supports the counter-clockwise rotation of the block.