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the zircon PAZ.
The apparent apatite fission track age from granitoid sample TS1 may indicate that
exhumation following the India-Asia collision may have initiated around 11 Ma in the
southernmost Tien Shan. Similar fission track ages are known from the northern Tien
Shan, where detrital fission track thermochronology indicates that the north-western
Tien Shan was tectonically calm for much of the Cenozoic (Bullen et al. 2001). In
contrast, Sobel (2000) reported several Oligocene-Miocene apatite cooling ages (both
basement rocks and sediments) at different places across the Tien Shan, which he
interpreted to slightly post-dating the initiation of Cenozoic thrusting. The estimated
11-10 Ma onset of exhumation is in agreement with the initiation of compressive range
building deducted from GPS measurements (e.g. Abdrakhmatov et al. 1996, Sobel
2000). Sobel (2000) suggests that during the earlier Oligocene to Early Miocene
exhumation event the samples did not reached the surface. He concludes that during
the Middle to Late Miocene the exhumation rate decreased until rapid final exhumation
to the surface in Late Miocene.
The southernmost Tien Shan was the distal, northern edge of the Alay foreland basin,
which developed in front of the rising Pamir mountains; its deposits thicken towards
the south. As the Tien Shan began to rise in the Late Miocene, the old foreland basin
was deformed and uplifted along its northern margin.
N-Kunlun
Geochronological studies within the Northern and Central Pamirs traced the
Palaeozoic and Mesozoic Kunlun belt from Tibet into the Pamirs (chap. 3, Schwab et al.
in press). In the northernmost Pamirs (Altyndara section), U/Pb zircon ages of
metavolcanic rocks range between ~370 and 320 Ma and give evidence for
Devonian/Carboniferous arc activity (north Kunlun arc). Devonian and Carboniferous
zircon fission track grain ages are badly represented in the Tertiary intramontane
basins within the northernmost Pamirs (Appendix C, Tab. C7, Fig. C4). More distinct is
a Permian/Triassic grain age population at ~243 Ma (sample AD7d). Like in the Central
Pamirs, this age population is most likely related to rifting processes, namely the
opening of the Sonpan-Ganze ocean. Although this ocean was subducting in the
Triassic along its northern margin beneath the Kunlun arc, and along its southern
margin beneath the Qiangtang block, the widespread batholith occurrence in the
Karakul-Mazar and south Kunlun range is post-dating (~220 Ma) the peak-ages, and
therefore are excluded as source for this ages. Nevertheless, Pan (1996) reported
Permian arc related volcanic rocks in the Kunlun mountains, which could also be the
source for such fission track peak ages. Consequently, the Permian/Triassic grain age
population gives evidence for low-T cooling of zircons related to the second arc stage,
proposed for the late Palaeozoic-Triassic (south Kunlun arc; see chap. 3). The youngest
zircon fission track grain ages of sample AD7d cluster around 131 Ma, whereas the
detrital zircons of sample AD8a seem to consist of only one grain population with an
age of ~152 Ma, pointing to a mid-Jurassic thermal event. As the stratigraphic age of
both samples is badly constrained, it is unclear whether the age difference between
samples AD7d and AD8a is due to different levels of erosion in the hinterland or
whether the two samples derived from different catchment areas with different thermal
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