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Two Palaeozoic sutures divide the Chinese Tien Shan into three tectonic entities: the
South Tien Shan (STS), Yili-Central Tien Shan (YCTS), and North Tien Shan (NTS) (e.g.,
Windley et al. 1990, Allen et al. 1993). From the Late Ordovician to the Early
Carboniferous, the STS documents the evolution of an oceanic basin, established
between the passive N-Tarim continental margin and the active STS margin. The STS
ocean subducted towards the north underneath the YCTS and created an island arc
with volcanic rocks of Late Silurian to Early Carboniferous age. Palaeozoic high T/P and
high P/T metamorphic belts with syncollisional granites formed on the southern side of
the YCTS block (Gao et al. 1995). The YCTS consists of Precambrian basement and was
separated from the Junggar plate in the north by the Late Cambrian to Early
Carboniferous palaeo-Junggar ocean, which is suggested by the rock associations of
the NTS and the Junggar and Turpan basins (e.g., Carroll et al. 1990). This ocean was
subducted southward under the northern margin of the YCTS where early Palaeozoic
island arc volcanic rocks and granitoids occur (Chen et al. 1985, Hopson et al. 1989,
Allen et al. 1993). The closure of the Junggar ocean was in Late Carboniferous time
(e.g., Carroll et al. 1990). The NTS is dominated by Devonian to Carboniferous calcalkaline
volcanic rocks, deep marine volcanogenic sedimentary rocks, and scattered
ultramafic rocks and radiolarian cherts.
The western segment of the South Tien Shan is part of the Gissar-, Zeravshan-, and
Turkestan-Alay ranges (Fig. 3.1 and 3.2) and consists mainly of Palaeozoic sedimentary
strata, Precambrian basement, remnants of oceanic crust, and volcanic arcs. Here, a
Silurian to Lower or Middle Devonian calc-alkaline volcanic arc developed during the
successive closure of an Early Palaeozoic ocean basin (Turkestan ocean) (Brookfield
2000). Further to the south, adjacent to the Tadzhik basin, Brookfield (2000) reported a
second island arc of Early Carboniferous age. He suggested that the so-called Gissar
island arc had been active only for the short time period of the Early Carboniferous and
thus the Gissar basin, which was subducted northward along the Gissar arc, could
have been only a very small basin.
No post-Palaeozoic igneous rocks are found in the Tien Shan and north of it. South of
the Tien Shan, in the Tadzhik depression, Leith (1982) documented a south-facing
mainly Mesozoic passive continental margin.
The South Tien Shan (STS) is separated from the Pamirs by the Alay basin. Currently,
this basin being closed from W to E by an anticlockwise rotation of India against
Eurasia (Le Pichon et al. 1992). The Alay basin developed from a foreland basin of the
rising Pamir orogen to an intramontane molasse basin.
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