ABSTRACT
Consideration of world-wide epicenter distribution has shown that
deformation
in continental lithosphere is not narrowly confined to
well-defined plate
boundaries but is present in wide, diffuse plate boundary zones.
Early
studies on the seismicity of the peri-Mediterranean area resulted
in the
division of the lithosphere in that region into a number of small
plates,
or microplates. Later studies in central Asia, which integrated
seismicity
with Quaternary geology, indicated, however, that a continuum
approach
may be more realistic to describe continental tectonics. This
study concentrates
on geometry and timing of continental deformation that resulted
from continental
collision in Central Europe and Eastern Mediterranean.
In Central Europe continental collision occurred along the Alps
during
the Lutetian/Priabonian boundary, Foreland deformation in the form
of rifting
at high angles to the orogen (the Upper Rhine Graben) and
strike-slip faulting
at about 45° to 60° to the orogen followed the collision.
Rifting
was nearly synchronous with the collision; strike-slip faulting
happened
about 20 m.y. after the collision.
In the Eastern Mediterranean the North Anatolian Transform and the
Turkish-Iranian Plateau were the main objects of study. The North
Anatolian
transform fault is a morphologically distinct and seismically
active strike-slip
fault which extends for about 1200 km from Karliova to the Gulf of
Saros
along the Black Sea mountains of N. Anatolia. It takes up the
relative
motion between the Black Sea and the Anatolian plates, thereby
connecting
the E. Anatolian convergent zone with the Hellenic Trench through
the complex
plate-boundary zone of the Aegean. For most of its length, the
transform
has a typical strike-slip fault zone morphology, characterized by
a narrow
'rift zone', offset, captured and dammed streams, sag ponds and
other deformed
morphological features. The fault zone is a broad region of
extensively
crushed country rock cut by a number of parallel and/or
anastomosing strike-slip
faults. The transform has periods of seismic activity the last of
which,
from 1939 to the present, is characterized by frequent 6<M<7
earthquakes;
these are separated by quiet periods of about 150 years. The crust
along
the fault zone is thinner than normal. The transform probably
originated
some time between the Burdigalian and the Pliocene and has an
offset of
about 85 km. Whether the offset of the fault changes
systematically along
its strike is not known. The North Anatolian transform fault seems
to have
originated as a consequence of the Arabia-Anatolia collision
during the
late (?middle) Miocene, when the Anatolian Plate originated and
was wedged
out into the oceanic tract of the E. Mediterranean from the
converging
jaws of Arabia and Eurasia to prevent excessive crustal thickening
in E.
Anatolia. The westerly motion of Anatolia with respect to Eurasia
and Africa
caused a great change in the tectonic evolution of the eastern
Mediterranean,
giving rise to the Aegean extensional regime and to internal
deformation
of Anatolia.
The Turkish-Iranian Plateau (Fig. 5.1) is a high region with an
average
elevation of about 1.5 km. During the late Miocene the last piece
of oceanic
lithosphere between the Eurasian and Arabian continents was
eliminated
at the Bitlis/Zagros suture zone. Continued convergence across the
collision
site resulted in the shortening of the plateau across strike by
thickening
and by sideways motion of parts of it. Predominantly calc-alkaline
vulcanism
is present on the highest portions of the area, despite the
absence of
a descending slab of lithosphere. Surface geology and vulcanism of
the
Turkish-Iranian Plateau resemble greatly those of the Tibetan
Plateau,
and both are underlain by a zone of seismic attenuation. From a
comparison
of these features and their tectonic setting, we argue that the
two plateaux
are homologous structures, albeit at different stages of their
evolution.
Both areas appear to be tectonically alive and actively
shortening. Available
evidence lends little support to the hypothesis of large-scale
underthrusting
of continental lithosphere and of plastic-rigid indentation where
such
high plateaux, located directly in front of the "rigid indenter,"
are considered
to be tectonically "dead." Their peculiar features are best
explained in
terms of shortening and thickening the continental crust whereby
its lower
levels are partially melted to give rise to calc-alkaline surface
vulcanism.
Minor associated alkaline volcanism may be due to local
longitudinal cracking
of the crust to provide access to mantle.
In conclusion, it appears that although the existing mechanical
models
of continental collision processes satisfy the first-order
properties of
collision zones they fail to predict the geological (particularly
the temporal)
details of these areas. Detailed field-mapping rather than
attempting to
refine the existing theoretical models seems necessary.
Sengor, A.M.C., 1979. Geometry and Kinematics of Continental
Deformation
in Zones of Collision: Examples from Central Europe and Eastern
Mediterranean.
Unpublished MSc. thesis, State University of New York at
Albany.
126pp., +x.
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