Paleozoic paleogeography of the south western part of the Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the world's
largest accretionary orogens, which was active during most of the
Paleozoic. In recent years it has again moved into focus of the
geological community debating how the acrreted lithospheric
elements were geographical arranged and interacting prior and/or
during the final amalgamation of Kazakhstania. In principal two
families of competing models exist. One possible geodynmaic setting
is based on geological evidence that a more or less continuous
giant arc connecting Baltica and Siberia in the early Paleozoic was
subsequently dissected and buckled. Alternatively an archipelago
setting, similar to the present day south west Pacific was
proposed. This thesis collates three studies on the paleogeography
of the south western part of the CAOB from the early Paleozoic
until the latest Paleozoic to earliest Mesozoic. It is shown how
fragments of Precambrian to early Paleozoic age are likely to have
originated from Gondwana at high southerly paleolatitudes (~500
Ma), which got then accreted during the Ordovician (~460 Ma),
before this newly created terrane agglomerate (Kazakhstania)
migrated northwards crossing the paleo-equator. During the Devonian
and the latest Early Carboniferous (~330 Ma) Kazakhstania occupied
a stable position at about ~30°N. At least since this time the area
underwent several stages of counterclockwise rotational movements
accompanying the final amalgamation of Eurasia (~320 - ~270 Myr).
This overall pattern of roughly up to 90° counterclockwise bending
was replaced by internal relative rotational movements in the
latest Paleozoic, which continued probably until the early Mesozoic
or even the Cenozoic. In Chapter 2 a comparison of declination data
acquired by a remagnetization process during folding in the
Carboniferous and coeval data from Baltica and Siberia lead to a
documentation and quantification of rotational movements within the
Karatau Mountain Range. Based on this results it is very likely
that the rotational reorganization started in the Carboniferous and
was active until at least the early Mesozoic. Additionally, the
data shows that maximal declination deviation increases going from
the Karatau towards the Tianshan Mountains (i.e. from North to
South). This observation supports models claiming that Ural
mountains, Karatau and Tianshan once formed a straight orogen
subsequently bent into a orocline. The hinge of this orocline is
probably hidden under the sediments of the Caspian basin. In
chapter 3 we show that inclination shallowing has affected the red
terrigenous sediments of Carboniferous age from the North Tianshan.
The corrected inclination values put this part of the Tianshan in a
paleolatitude of around 30°N during Carboniferous times. These
results contradict previously published paleopositions of the area
and suggest a stable latitudinal position between the Devonian and
the Carboniferous. Chapter 4 presents paleomagnetic data from early
Paleozoic rocks from within the North Tianshan. They imply a second
collisional accretion event of individual terranes in the
Ordovician. To further constrain the dimensions of these early
Paleozoic terranes, chapter 5 presents a compilation of all
available paleomagnetic data from the extended study region of
southern Kazakhstan and Kyrgyzstan. Apart from a broad coherence of
paleolatitudes of all studies at least since the Ordovician and the
exclusive occurrence of counterclockwise declination deviations, no
areas with the same rotational history can be detected. Also a
clear trend caused by oroclinal bending can not be observed. We
conclude that first order counterclockwise oroclinal bending, shown
in chapter 2, resulted in brittle deformation within the mountain
belt and local block rotations. In order to improve our
understanding of intra-continental deformation a study combining
the monitoring of recent deformation (Global Positioning System,
GPS) with a paleomagnetic study of Cenozoic age in the greater
vicinity of the Talas-Ferghana fault has been undertaken in chapter
6. The major task was to distinguish between continuous versus
brittle deformation. As it turned out the GPS signal indicates
rather continuous and consistent counterclockwise rotational
movements of the order of ~2° per Myr. This is in contrast to our
paleomagnetic results, where even within fault bounded areas the
error intervals of the rotations do always overlap. This indicates
that a pure block model seems not appropriate even to explain
Cenozoic paleomagnetic data. If this means that also Paleozoic
rocks have been affected by complex recent deformation, and that
the Paleozoic rotational pattern has been obscured by this, can not
be decided based on the present data set. It means, however, that
interpreting Paleozoic rotational data from this area has to be
done with great caution.