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4.4 Conclusions
This low-temperature fission track study of the southernmost Tien Shan and eastern
Pamirs resulted in two major conclusions:
(1) Unreset Tertiary sediment samples trace the Palaeozoic and Mesozoic thermal
history of the hinterland of the intramontane basins of the southern Tien Shan and
eastern Pamirs. The reconstruction of the cooling and denudation histories constrain
the thermal events, which were affecting the different tectonic units, and support the
conclusions which were drawn in chapter 3.
Four major phases of tectonic activity could be traced by the decomposed apatite and
zircon fission track ages:
(a) Age populations of about ~370 Ma were found in the Tertiary sedimentary basins of
the northern Pamirs/Kunlun belt and northern Central Pamirs/Qiangtang block. These
ages are interpreted to be related to the first, northern Kunlun arc stage, which was
defined e.g. by the metavolcanic rocks of the Altyndara section (~370-330 Ma).
(b) A major Permo-Triassic peak ranging from ~266 to 242 Ma (depending on the
region) was detected in all determined regions from the southern Tien Shan to the
eastern Pamirs. Induced by the break-up of Pangea, the Triassic is a dominant phase of
rifting and basin formation. Such large basins with thick sediment accumulations are
widespread in Central Asia. Examples are the Ferghana, Turfan, Junggar, and Tadjik
basins. The basins were fed from the rising Tien Shan and Kunlun mountains.
Increased heat flow during basin formation is thought to be the source for the
dominant occurrence of the Permo-Triassic grain ages. A further source for Triassic
zircons may be derived from rocks thermally influenced by Triassic southward
subduction along the Jinsha suture.
(c) Jurassic peak ages range from 170-145 Ma. They occur in the Tertiary sedimentary
rocks of the South Tien Shan, Kunlun, southern Qiangtang, and Rushan Pshart zone.
They are attributed to the Rushan Pshart arc and Rushan Pshart basin closure in the
Mid-Jurassic, leading to the collision of the Central Pamirs/Qiangtang and SE
Pamirs/Lhasa blocks. This event was coeval with Mid-Jurassic deformation determined
in the Kunlun mountains.
(d) Late Cretaceous fission track ages were determined in the South Tien Shan
(Tertiary sedimentary rocks) and in the Central Pamirs (Cretaceous intrusions). The
occurrence of late Early Cretaceous intrusions along the southern margin of the
Qiangtang block may be explained by flat-slab subduction along the Shyok suture and
the Late Cretaceous zircon fission track ages as cooling ages to the emplacement ages.
Late Cretaceous intrusions along the northern margin of the Shyok suture are probably
induced by intracontinental renewed subduction of Karakul-Mazar and Jinsha
lithosphere beneath the Qiangtang block, induced by proceeding compression due to
collisional stages further to the south (Karakorum and Kohistan-Ladakh).
(2) Reset sediment samples and low-T cooling ages of basement rocks reflect the
cooling history related to Tertiary tectonics, which is responsible for the modern shape
and topographic pattern of the southern Tien Shan and eastern Pamirs. At least since
the Tertiary (in some areas since the Late Cretaceous) the Pamir orogen was emerged
above sea-level and was eroded at rates that can be approximated by fission track
data:
(a) Apatite fission track ages of the Karakul-Mazar belt decrease from 56 Ma in the
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