4-Concluding Summary
Geological field observations and the geophysical data suggest the
presence of an ophiolitic mélange-accretionary complex under the cover
sequence of eastern Anatolia (A in fig 11-I). The eastern Anatolian
volcanic and sedimentary cover units were piled up during the closure of
the NeoTethyan Ocean that was located between the Pontide arc to the
north, and the continental slivers drifted away from the Arabian Plate
to the south (e.g., Şengör and Yılmaz1981). The Upper
Cretaceous-Lower-Middle Eocene deep-sea sedimentary rocks, associated
genetically with the ophiolitic mélange, indicate that the NeoTethyan
oceanic lithosphere survived during this period and was finally
eliminated from entire eastern Turkey by the Late Eocene. Continental
fragments of various sizes were tectonically incorporated into the
mélange prism, possibly during the growth of the accretionary complex.
The northward advance of the Arabian Plate continued after the
elimination of the oceanic environment. The mélange-accretionary prism
that occupied a large terrain behaved like a wide and thick buffer unit,
which did not allow a head-on collision of the bordering continents
(cf., the Turkic type orogen of Şengör and Natalin 1996). The northward
advance of the Arabian Plate that continued after the initial stage of
the collision caused shortening deformation. It began squeezing the
eastern Anatolian accretionary complex. As a result, eastern Anatolia
was elevated above the sea during the Late Eocene-Oligocene. An
irregular topography was developed on the elevated land as indicated by
coarse‐grained thick Upper Eocene-Oligocene terrestrial conglomerates
deposited in irregularly developed narrow depressions) (B in Fig 11-I).
The rough topography was smoothened, and the region subsided steadily
during the Late Oligocene-Early Miocene when an epeiric sea invaded the
region again (Yılmaz 2017 and the references therein). This is evidenced
by the low-energy marine sediments laid down on the Upper
Eocene-Oligocene terrestrial sedimentary rocks. The smooth topography
survived during the Early-Middle Miocene. Shallow marine limestones (the
Adilcevaz Limestone) were deposited above the fine-grained marine
sediments (C in Fig 11-I) (Şaroğlu and Yılmaz 1986; Bedi and Yusufoğlu
2018). The limestones graded upward into evaporates and lacustrine
limestones during the Late Miocene (Fig 3 and D in Fig 11-I). The
gradual transition observed in the entire eastern Anatolian region
reveals that interconnected lakes were developed over the elevated land
following the disappearance of the sea (Şaroğlu and Yılmaz 1986; Yılmaz
2017). This event may also be interpreted that the eastern Anatolia
began to rise as a coherent block (en mass) during the Late Miocene
(Phase I, Fig.11-I). Following the disappearance of the interconnected
lakes, the elevated land underwent a severe denudation phase, which
formed a flat‐lying erosional surface above the Upper Miocene lacustrine
limestones (ES in Figs.11-I and the accompanying photo A1) (Yılmaz
2017). The smooth topography disappeared after this period. Various
sediment packages were formed in separate depressions from this Late
Pliocene onward (Phase II, Figs 11-II).
Entire eastern Turkey, including the Pontide and the Arabian Platform
has behaved as an interconnected tectonic entity since their tectonic
amalgamation. Paleomagnetic studies and the GPS data support this
conclusion, which show that eastern Anatolia has been deformed together
with the surrounding tectonic entities since the Late Miocene following
the collision of the Arabian plate with the Anatolian blocks (Reilenger
et al. 2006; Şengör et al 2008; Çinku et al. 2014; 2016; Gürer et al
2017; Bakkal et al 2019).
The continuing N-S compressional stress caused a complex pattern of
structures in the Eastern Turkey (Fig. 4A). The rigid continental crust
underlying the peripheral mountains accommodated the N-S compression by
elevating faster (0.2-0.3 mm/y) than the eastern Anatolian plateau
(0.1-0.2 mm/y). This is possibly because of the blocks and matrix of the
ophiolitic mélange underlying eastern Anatolia partly absorbed the
compression.
Starting from the Late Pliocene-Pleistocene big scale folds and thrusts
began to form in the eastern Anatolia (phases III and IV; Figs 11-III
and 11-IV) and the accompanying photos C1 and D1). The peripheral
mountains were thrust over the eastern Anatolian plateau (Fig 2; 5A and
5C). Two narrow, fault-bound chains of E-W trending depressions were
formed along the thrust fronts as intermountain or ramp basins (Fig1 and
Figs 5A; 5B and 5C). The boundary faults give the young basins their
distinct rhombohedral or parallelogram geometrical patterns (Fig 5B).
When the N-S compression and associated shortening reached an excessive
stage, which could no longer be accommodated within the volume of
eastern Anatolia, the stress permutation occurred. This led to the
development of two transform faults, the North Anatolian Transform Fault
(NATF) and the East Anatolian Transform Fault (EATF) (Figs 1 and 2)
(Şengör 1979; Şengör and Kidd 1979; Şengör and Yılmaz 1981; Çemen et
al.,1993 and Yılmaz 2017). They defined an independent tectonic entity,
the Anatolian Plate, which began escaping away from the area of
convergence to transfer part of the north‐south compressional stress to
the west (Mc Kenzie 1972; 1978; Şengör 1979; Şaroğlu and Yılmaz 1991;
Şengör and Yılmaz 1981). The escape tectonics and associated lateral
extrusion initiated a new tectonic regime in Anatolia and the
surrounding regions known as the Neotectonics, which determined the
development of the major morphotectonic entities in the peripheral
mountains and the eastern Anatolian High Plateau (Yılmaz 2017). This
event also caused anticlockwise rotations of the semi-independent
fault-bounded blocks of Central Anatolia (Yılmaz 2017).
According to geophysical data lithospheric mantle under East Anatolia is
very thin. Almost the whole thickness of the mantle lithosphere was
removed from the overlying thickened crust (e.g., Barazangi et al.,
2003). The space created was filled with a hot, upwelling asthenosphere,
which produced mantle-derived magmas. The volcanic activity began
sporadically during the Late Miocene and intensified about 5–6 Ma ago.
The volcanoes were commonly developed above the extensional openings
associated with the basin boundary faults. The volcanic edifices covered
the entire plateau as a thick blanket (Yılmaz, 1990; Yılmaz et al.,
1987, 1998; Pearce et al., 1990; Keskin, 2007; Keskin et al., 2012).
The north-directed compressional stress is actively deforming eastern
Turkey. This is evidenced by GPS measurements (e.g., Reilenger et al.,
2002) indicating that the high plateau and the peripheral mountains are
still elevating, and the Anatolian Plate’s westward escape is continuing
at an about 20 mm/y rate. This continuing deformation may be regarded as
the late-post tectonic phase of the orogenic development.