Nallıhan Kamasının Merkezi Kısmının Tektonik Evrimi (nallıhan-çayırhan, Ankara)

Abstract

Pliyo-Kuvaterner yaşlı Kuzey Anadolu Fay Zonu (KAFZ) ve Orta Miyosen – Alt Pliyosen yaşlı Trakya-Eskişehir Fay Zonlarının (TEFZ) arasında yer alan Nallıhan kaması İzmir-Ankara Okyanusu ve İntra-Pontid Okyanusunun kapanmasının etkilerinin görüldüğü yerdir. Nallıhan kaması kuzeybatıda KAFZ, güneybatıda TEFZ ve doğuda ise Beypazarı üzerinden geçen 160 km’lik bir hat ile sınırlandırılan 24000 km2’lik alandır. Nallıhan kaması içerisinde batıda kıvrımlar kapalı iken doğuya doğru açık hale gelirler. Çalışma alanı ise bu kamanın merkezinde bulunan 1440 km2’lik alanı kapsamaktadır. Nallıhan kamasının merkezi konumunda bulunan Nallıhan ve Çayırhan’da temel olarak olarak nitelendirilen, üç farklı tektonik üniteye ait ve ilişkileri arazide de tektonik olan bu birimler Sakarya Zonu’na ait oluşan Permo-Triyas yaşlı Karakaya kompleksi, İzmir-Ankara Kenedini temsil eden Kretase yaşlı Dağküplü melanjı, Tavşanlı Zonu’na ait olan Alt-Orta Triyas yaşlı Kocasu formasyonu ve Triyas-Kretase yaşlı İnönü mermerleri bulunmaktadır. Çalışma alanın kuzeydoğusunda fillat, mermer ve metabazitten oluşan Karakaya karmaşığının üzerine uyumsuzlukla kumtaşı, konglomera, kireçtaşı, tüf, lav akıntıları ile temsil edilen yamaç çökellerinden oluşan Jura yaşlı Mudurnu formasyonu bulunur. Mudurnu formasyonu ise düşey yönde tekdüze kireçtaşı ve killi kireçtaşı gibi şelf ortamında çökelmiş birimlerden oluşan Jura-Kretase yaşlı Soğukçam formasyonuna geçişlidir. Tabanda metamorfik ve kireçtaşı çakıllarından oluşmuş üst seviyelerde kumtaşı şeyl ardışımı ve kireçtaşı ile temsil edilen yamaç ortamında çökelmiş Üst Kretase yaşlı Yenipazar formasyonu tabanda düşey yönde Soğukçam formasyonu ile geçişlidir, tavanda ise çalışma arazisinin dışında ancak hemen kuzeyinde Alt-Orta Eosen yaşlı Kızılçay formasyonu tarafından paralel uyumsuzluk ile örtülmektedir. Çalışma alanının batısında Karakaya karmaşığını uyumsuzlukla örten, kumtaşı, çamurtaşı ve konglomera ile temsil edilen akarsu, alüvyal yelpaze ve göl çökellerinden oluşan Alt-Orta Eosen yaşlı Kızılçay formasyonu bulunur. Kızılçay formasyonu yanal ve düşey yönde tüf, aglomera ve andezitik lavlardan oluşan Orta Eosen yaşlı Meyildere volkanitleri ile geçişlidir. Fosilli kumtaşı, şeyl ve kireçtaşı ile temsil edilen plaj ve sığ denizel ortam çökeli olan Orta Eosen yaşlı Güvenç formasyonu Meyildere volkanitleri ile yanal ve düşey yönde geçişlidir. Çalışma alanın D-GD bölgelerinde geniş alanlarda yüzlek veren Alt-Orta Miyosen yaşlı şeyl, kumtaşı, linyit, konglomera, tüf, kireçtaşı ile temsil edilen gölsel çökellerden oluşan Hançili formasyonu kendisinden daha yaşlı birimleri uyumsuzluk ile örtmektedir. Kumtaşı, kiltaşı ve konglomeradan oluşan göl ve akarsu çökellerinden oluşan Üst Miyosen yaşlı Uruş formasyonu tabanda Hançili formasyonu ile uyumludur. Pliyosen yaşlı alüvyal yelpaze ve akarsu çökellerinden oluşan kumtaşı, kiltaşı ve konglomera ile ifade edilen Örencik formasyonu kendisinden daha yaşlı birimleri uyumsuzluk ile örtmektedir. Tüm bu birimler uyumsuzlukla güncel alüvyon ve yamaç molozu çökelleri tarafından örtülmektedir. Çalışma alanında B-D ve GB-KD doğrultulu, büyük ölçekli doğrultu bileşenli oblik bindirme fayları görülmektedir. Eosen ve Miyosen yaşlı birimlerde ise ana kıvrım eksenleri B-D ve BGB-DKD doğrultuludur. Bu durum bölgede K-G doğrultuludan KKB-GGD doğrultuya zamanla geçiş yapan sürekli bir sıkışma rejiminin hâkim olduğunun göstergesidir. Nallıhan kamasının geometrik yapısı ve oluşumu İntra-Pontid ve İzmir-Ankara okyanuslarının kapanması ile doğrudan ilişkilidir. Nallıhan ve Çayırhan bölgeleri yapısal özellikleri bakımından Nallıhan kamasının merkezi konumunda bulunmalarından dolayı kama ile aynı özellikleri göstermektedir. Kuzeyde Intra-Pontid okyanusunun kapanması esnasında Eosen döneminde bölge bir ramp havzası halini almış ve burada örgülü akarsu rejimi hâkim olur. Eosen boyunca sıkışma devam etmiş ve Miyosen dönemi başlarında ise havza artık dağlar arası havza halini almıştır. Söz konusu sıkışma rejimi Orta Miyosen sonlarına kadar devam etmiştir.

Nallıhan wedge which is bounded by Plio-Quaternary aged North Anatolian Fault Zone (NAFZ) and Thrace-Eskişehir Fault Zone (TEFZ) shows the effects of closure of the Intra-Pontid and İzmir-Ankara oceans. Nallıhan wedge is 24000 km2 area and the boundaries of this area are NAFZ at the northwest, TEFZ at the southwest and eastern boundary is at the Beypazarı which has 160 km length. In this wedge, the folds are tight in the western side but they become to gentle towards the east. The study area is at the center of this wedge and its size is 1440 km2. Nallıhan and Çayırhan are located at the center of the Nallıhan wedge. There are three different tectonic units in this region. These tectonic units are Permo-Triassic aged Karakaya Complex that consists of phyllite, metabasite, marble and is the basement of the Sakarya Zone, Cretaceous aged Dağküplü Mélange which represents the İzmir-Ankara Suture, Lower-Middle Triassic aged Kocasu Formation and Triassic-Cretaceous aged İnönü Marbles which represent the Tavşanlı Zone and these units were evaluated as basement in this study. Jurassic aged Mudurnu Formation is composed of slope deposits; sandstone, conglomerates, limestone, tuff, and lavas and covers the Karakaya Complex unconformably in the northeastern side of the study area. Mudurnu Formation shows vertical transition to Jurassic-Cretaceous aged Soğukçam Formation that includes shelf deposits; limestone and clayey limestone. This formation shows vertical transition to Upper Cretaceous aged Yenipazar Formation that consists of slope deposits; conglomerate with metamorphic and limestone blocks, sandstone, shale and limestone. Lower-Middle Eocene aged Kızılçay Formation lies on the Yenipazar Formation with parallel unconformity outside of the study area. In the western side of the study area, Lower-Middle Eocene aged sandstone, mudstone and conglomerate sequence is known as Kızılçay Formation that indicates alluvial fan, braided river and lacustrine environments. This unit covers the Karakaya Complex with an unconformity and shows vertical transition to Middle Eocene aged Meyildere Volcanics. Meyildere Volcanics consist of tuff, agglomerates and andesitic lavas and shows both vertical and lateral transition to Middle Eocene aged Güvenç Formation that is composed of beach and shallow marine deposits; sandstone with fossils, shale and limestone. In the southern and southeastern part of the area, all these units are unconformably overlained by Lower-Middle Miocene aged Hançili Formation that includes lacustrine deposits; shale, sandstone, lignite, conglomerate, tuff and limestone is cover the area. Hançili Formation shows vertical transition to Upper Miocene aged Uruş Formation that involves alluvial fan and river deposits; sandstone, claystone and conglomerates. Pliocene aged river and alluvial fan deposits cover the sequence unconformably. These sediments are sandstone, claystone and conglomerates of Örencik Formation. On the top of the sequence, recent alluvium deposits lay unconformably. The study area represents an example of a fold and thrust belt. The folds are close and tight in the Eocene units located in the western side, however they become to open, gentle, asymmetric and overturned towards to the east. In Miocene units, the folds are mainly open, gentle and asymmetric and the fold axes are ESE-WNW-trending. These folds were formed under the influence of NNW-SSE-trending compression. The folds in the Eocene units are both W-E-trending and NE-SW-trending. In order to define the direction of the deformation during Eocene-Oligocene, we eliminated the Miocene deformation from the Eocene units by using structural analysis methods. In addition to that, we separated the Eocene units as east and west, because the folds have different behaviour in eastern and western sides of the study area. Eocene units were deformed by N-S-trending compressional regime not only in the eastern side but also in the western side of the study area. The longest major fault of the study area is the 44 km-long right-lateral oblique Nallıhan thrust, which extends from the northwestern part to the northeastern part of the study area. In the eastern part, the thrust divides into two branches. The SW-NE-trending northern branch is situated between Lower-Middle Jurassic unit and Upper Cretaceous rocks, and E-W-trending southern branch is situated between Upper Cretaceous units and Eocene rocks. This faults is the reason of the folding in the Eocene rocks, therefore the age of the thrusting is supposed to be Lower-Middle Eocene. Atça Thrust Fault is located in the eastern part of the study area and at the south of Atça village. This 8 km-long W-E-trending thrust fault indicates the thrusting of the Eocene rocks to Lower-Middle Eocene and Lower-Middle Miocene units. The fault could be formed in the Lower-Middle Miocene. The 10 km-long NE-SW-trending Sekli Thrust is a right-lateral oblique thrust between Permo-Triassic unit and Eocene rocks. It prolongs from the south of Karaköy village to northeast of the Sekli village. The fault is observed as a blind thrust under the Lower-Middle Miocene rocks on the eastern side of the fault plane. The lower part of the sequence was tilted by this fault) and the age of the fault is supposed to be Upper Eocene-Middle Miocene. The 11 km-long E-W-trending Davutoğlan (Çayırhan) thrust is situated north of Çayırhan around Davutoğlan village. This right-lateral oblique thrust fault can be observed as thrusting itself in the Lower-Middle Miocene units (Fig 10F). The fault became blind thrust on the eastern side of the fault plane, and it is observable as an antiformal structure. This fault is dated as Lower-Middle Miocene. The geometric structure and evolution of the Nallıhan Wedge is directly related to closure of the Intra-Pontide and İzmir-Ankara oceans. Nallıhan and Çayırhan regions display same structural properties with Nallıhan Wedge because they are located at the center of this wedge. In the wedge, the folds which were formed in Eocene are tight in the western part and they become gentle towards the east. This is because NAFZ superimposed on the Intra-Pontid Suture, TEFZ superimposed on the İzmir-Ankara Suture and the block between them deformed during the closure of both oceans. In the study area, the Neo-Tethys Ocean had started to open on the metamorphic basement in Jurassic. Shelf and sloped deposits, red basal conglomerates, volcanic lavas, sandstones and limestone of Mudurnu Formation deposited on this metamorphic basement. After the opening of the ocean developed related to rifting, the basin began to deepening and the neritic limestones of the Soğukçam Formation deposited. Ocean became deeper and deep sea shales of the Yenipazar Formation were deposited. At the end of Upper Cretaceous, the basin started to become shallow and even the region locally eroded due to the uplift. These events indicate the closure of the Neo-Tethys Ocean. The data about the Cretaceous-Eocene period is limited in the study area. The study area is situated in the south of the Intra-Pontide Ocean. The Karakaya Complex exists between the Intra-Pontid and İzmir-Ankara oceans. There is no Paleocene unit in the study area. Because of the collision, the sea had almost disappeared at the end of the Lower Eocene and erosion and sedimentation had started at the same time. The conglomerates of Kızılçay Formation contains metamorphic pebbles which indicates the uplift of the basement units during the closure of Intra-Pontide Ocean. In the Middle Eocene, the region had become a ramp basin and the andesitic and rhyolitic volcanism increased related to the collision activity. The shallow sea environment was still in the northwestern part of the basin in this period. This basin had tightened, uplifted and become folded by the reason of the N-S compression and eroded until Early Miocene. During this event, unconformable contact between Karakaya Complex and Kızılçay Formation was structurally dislocated. At the same time, eastern part of the basin had become an intermontane basin. N-S-trending compressional regime had turned into NNW-SSE-trending regime during Lower-Middle Miocene and in the meantime the basin was a swamp/lake. Associated with this compressional regime, the Eocene units continued to uplift and they gave material to the Miocene rocks by means of erosion. The Neogene basin started to uplift and the environment evolved to a river in the Upper Miocene. For this reason, Lower-Middle Miocene and Upper Miocene units display local unconformities in the region. Open folds had formed in the Lower-Middle Miocene rocks and this unit started to erode in Upper Miocene because of the compression. In the Pliocene, the basin was dominated by alluvial fans and river.