IGCP 480: New Opportunities for Focussing on the Altaids

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• IGCP 480 New Opportunities for Focussing on the
  Altaids
• A. M. C. Sengör
• ITÜ Maden Fakültesi, Jeoloji Bölümü,
  Ayazaga 34469 Istanbul, TURKEY
• Truth comes only through error
• Friedrich August Quenstedt, 1858

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The Altaids are the orogenic system formed by the
Turkic-type growth and the collisional edifices
that grew around the Angaran Craton and finally
consolidated by the collision with them of the
Uralides and the Intermediate units of Asia.
3
The most significant advance in our understanding
of the Altaids in the last two decades has been
the re-recognition of the continuity of its
constituent structures and the unity of its
tectonic history.
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This has been a revival of the ideas of the great
Russian geologists of the past and their foreign
friends, such as Vladimir Afanasiyevich
Obruchev and Eduard Suess
Eduard Suess map of the major part of the
Altaids (from Suess, 1901)
The Altaids according to Obruchev (1926)
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One remarkable development has come from the
palaeomagnetic observations by Rob van der Voo
and his colleagues.
The geological setting in Kazakhstan shows
evidence for thick-skinned oroclinal bending ....
Naturally, the idea that the horseshoe-shaped
belts are the result of oroclinal bending has
occurred to scientists studying the area and has
led to models of complex oroclinal deformation
(e.g., Sengör and Natal'in, 2004 ), whereas
others have proposed amalgamation and collision
of arc segments without large-scale rotations
(Didenko et al., 1994 Filippova et al., 2001
).
Our recent paleomagnetic results (Bazhenov et
al., 2003 Collins et al., 2003 Levashova et
al., 2003 ) have indicated that rotations in the
Kazakh belts are significant, however. ...
Positive tilt and conglomerate tests for several
of these results indicate that the magnetizations
are primary or, at the very least, were acquired
early moreover, the directions do not resemble
magnetization directions observed in younger,
post-Silurian rocks. Because the drift of the two
sampling areas must generally have been northward
during much of the Paleozoic, in order for them
to end up as part of the northern-hemisphere
Laurasia assembly by Permian time, the polarity
of the two sets of results can reasonably be
assumed to have been normal ... Field and
laboratory work in progress is designed to test
our polarity assignments and to complement the
data distribution in a spatial sense with new
results from the hinge areas of the arcs.
Assuming for the time being that our polarity
assignments are correct and that the northern
Chingiz area is rotated about 180 with respect
to the southern area in the North Tien Shan, we
have a ready explanation for the geometrical
oddity of the tightening arcs around Lake
Balkhash. Late Permian directions on both sides
of the orocline appear to be more or less
parallel, indicating that the rotations were
largely completed by that time. Middle Devonian
paleomagnetic directions (Levashova et al., 2003
) are in need of confirmation, but suggest that
the rotations occurred afterwards. The most
likely time span for oroclinal bending is late
CarboniferousEarly Permian
(van der Voo, 2004)
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Fig. 7 from Collins et al. (2003) showing their
comparison of two reconstructions.
Palaeolatitude
60
40
20
0
250
350
450
550
Ma
-20
Age
Sengör Natalin (1996)
-40
Didenko et al. (1994)
Observed motion of part of Boshchekul-Tarbagatay
arc system of Sengör Natalin
(Fig. 8c of Collins et al., 2003)
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Fig. 7 from Collins et al. (2003) showing their
comparison of two reconstructions.
Fig. 14 of Bazhenov et al. (2003)
Palaeolatitude
60
40
20
0
300
400
500
600
-20
-40
-60
Sengör and Natalin
Djezkazgan-Kirgiz arc observed palaeolatitudes
Didenko et al.
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Sengör and Natalin
Djezkazgan-Kirgiz arc observed palaeolatitudes
Sengör and Natalin
Boshchekul-Tarbagatay arc observed palaeolatitudes
Didenko et al.
Angaran Craton
Didenko et al.
Russian Craton
Angaran Craton
Russian Craton
Western Tien Shan follows the Russian Craton
eastern Kazakhstan follows the Angaran Craton.
These units have not behaved as independent
terranes
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Fortey Cocks (2003) with coloured additions by
Sengör
Sengör Natalin (1996)
All the palaeobiogeographic data on early
Palaeozoic trilobites and brachiopods presented
by Fortey Cocks are as easily explicable in
terms of the reconstruction by Sengör Natalin
(1996) except the Altai-Sayan terrane of Fortey
and Cocks. This last is more easily explicable
on the basis of Sengör Natalins hypothesis
than Fortey and Cocks!
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Continuous units with clearly identified genetic
labels have proven a more fruitful, because more
restrictive and predictive hypothesis than
nameless terranes in studying the Altaids.
This has been also the experience in the North
American Cordillera The terrane methodology has
retrograded Codilleran studies for at least two
decades! (B. Clark Burchfiel) Twice the same
experience was made in the Alps Once, in the
mid-nineteenth century, when fault-bounded
block mapping gave way to mapping of continuous
folds. Then, in the later twentieth century, when
nameless nappes began to be identified with
actualistic environments. In all cases, mapping
of mute fault-bounded blocks hindered progress.
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Very large shear zones have implications for
structural styles of later Altaid evolution. Are
they corroborated?
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Late Altaid basins of Alakol, Junggar and Turfan
(from Allen et al., 1995)
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Formation of the Alakol, Junggar and the Turfan
basins as shear-related structures and its
palaeomagnetic corroboration (Allen et al., 1995)
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Rotating oblique-slip fault-bounded blocks
Onset of oblique-normal separation
faulting-related subsidence here
REVISION OF THE TIMING OF REVERSE SHEAR ON THE
GORNOSTAEV KEIROGEN BY ALAN CARROLL AND COLLEAGUES
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The Altaid evolution cannot be considered in
isolation. If a model is proposed for it, that
model must be compatible with all the structures
surrounding the Altaids.
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The evolution of the Scythides
(Natalin and Sengör, 2005,
Tectonophysics, v. 404, pp. 174-202)
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Notice how the reverse motion on the Gornostaev
Keirogen stacks up the Silk Road Arc is inferred
from the independent study of the Scythides
(Natalin and Sengör, 2005)
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Altaids is one of the prime sites in the world
where there was significant crustal growth in the
Phanerozoic
P. V. Ermolov (2002)
Juvenile crustal source accretionary complexes
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• The Altaid research has now entered a phase
  comparable to that during the last quarter of the
  nineteenth century that had culminated in Suess
  great synthesis in 1901.
• Apart from traditional mapping including
  stratigraphy and structural geology (which must
  be continued unabated on scales ranging from
  1/25,000 to 1/10,000), the most promising areas
  of research seem to be
• Palaeomagnetism
• Palaeobiogeography
• Basin stratigraphy
• Geochemistry

NO EARTH SCIENCE RESEARCH CAN BE FRUITFULLY
UNDERTAKEN WITHOUT A SOUND BASIS OF GEOLOGICAL
MAPPING.