tübinger geowissenschaftliche arbeiten (tga) - TOBIAS-lib ...

76
Carboniferous-Early Permian time. The oldest peak age of 333±56 Ma, determined from
the above mentioned sediments, is from apatite sample TS8a (Appendix C, Tab. C7,
Fig. C4). The peak contains only 13.4% (2 grains) of the whole grain fraction and
interpretations should be taken with care. Nevertheless, this age may mirror the Early
Carboniferous collisional phase along the southern margin of the Tien Shan (e.g. Allen
et al. 1993). Collision ceased in the Tien Shan by latest Permian times (see chapter 3).
Permian cooling, most likely related to erosive denudation, is demonstrated at mid-T
ranges by Ar/Ar biotite cooling ages of granitoids at 277 Ma (samples TS12b and
TS20a, Appendix A, Tab. A3), and at low-T ranges by sandstone fission track peak
ages at 266 Ma (TS5a zircon) and 245 Ma (TS5a apatite). During the Triassic, the whole
Central Asian region was under extension, initiated by the break-up of Pangea. The
opening of several Permo-Triassic basins with huge sediment accumulations suggests
several high lands along the margins of these basins, which shed a vast amount of
sediments. The Tarim, Tadjik and Sonpan-Ganze regions were surrounded by the rising
Tien Shan and Kunlun mountains. Basin opening may have caused a high heat transfer,
producing the widespread Triassic apatite and zircon fission track ages. Similar fission
track results were reported from the Chinese Tien Shan (e.g. Dumitru et al. 2001).
All sediment samples from the southern Tien Shan show Middle to Late Jurassic peak
ages (Appendix C, Tab. C7, Fig. C4). In case of sample TS5a the zircon and apatite
peak ages are nearly identical, indicating fast hinterland cooling during Late Jurassic
times. The deposition of thick Triassic and Jurassic strata in the Turfan and Junggar
basins in the eastern Tien Shan and in the northern Tarim basin is associated with
faulting (Burtman 1980, Hendrix et al. 1992, Burtman et al. 1996, Bullen et al. 2003).
Jurassic deposits are also documented in the southwest Tarim and Ferghana basins
(Sobel 1999). Other coeval deposits are restricted to isolated accumulations in a few
intramontane basins (Bullen et al. 2003). From these sediment accumulations, it can be
inferred that the Tien Shan experienced a modest amount of Jurassic exhumation, an
event that left a discernible thermochronological imprint. Synkinematic biotite from a
north vergent thrust in the western Kunlun mountains south-west of Wuyitake yield an
Ar/Ar age of 146±0.7 Ma (Arnaud et al. 1993). This suggests a compressional setting
during Late Jurassic time. Sobel (2000) published apatite fission track data of a N-S
transect across the whole Tien Shan. Palaeozoic sedimentary and magmatic rocks from
the Atbashi range 60 km north of the Chinese border yield Jurassic ages and belong to
a single grain age population. This confirms the relevance and extent of this Mid to
Late Jurassic depositional and cooling event. Here, it is suggested that this thermal
event is geodynamically related to the amalgamation of the Qiangtang and Lhasa
blocks to the Eurasian margin. The docking to the south, gave a major exhumation
impulse in the Tien Shan range, most likely under compressional setting like indicated
in the Kunlun and Tien Shan mountains.
To determine the significance of the Late Cretaceous peak of apatite sample TS5a,
more Neogene sediment samples must be analysed. Coeval Late Cretaceous tectonism
in the hinterland might be inferred from the closure of the Shyok suture (~80 Ma) with
widespread magmatic intrusions. So far, only Cretaceous apatite fission track age
populations could be detected in this study, but no Cretaceous zircon fission track
grain age population. Probably the Tertiary erosion level was not deep enough to reach