Williams, Helen Myfanwy
Magmatic and tectonic evolution of Southern Tibet and the Himalaya.
The Open University.
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The Himalaya-Tibetan orogen has become the paradigm for continental collision and is central to deciphering continental tectonics. Neogene extension in the orogen is not predicted by plate tectonic theory, and its significance is widely debated.
In the Himalaya, north-south extension is restricted to the Southern Tibetan Detachment System (STDS), which juxtaposes the High Himalayan Crystalline Series (HHCS) against the Tibetan Sedimentary Series (TSS). 40Ar-39Ar ages from HHCS and TSS of the Garhwal Himalaya indicate that STDS movement initiated between 17.3 ± 0.4 to 24.3 ± 1.6 Ma (2σ), synchronous with Main Central Thrust (MCT) movement. One-dimensional thermal modelling suggests that the STDS is a reactivated thrust, implying a fundamental change in Himalayan tectonics in the early Miocene.
The onset of east-west extension in southern Tibet is constrained by north-south trending shoshonitic dykes to be 13.3 ± 0.8-18.3 ± 2.7 Ma. Trace-element modelling indicates that the shoshonitic dykes and associated lavas in southern and northern Tibet were derived by ≤2% melting of enriched sub-continental lithospheric mantle (SCLM) at 65-85km. The northern and southern shoshonites have distinctive isotopic (εNd(i), north, -5.5 to-10.3; south -8.8 to - 18.1) and major element signatures that relate to distinct SCLM sources corresponding to the tectonically accreted terranes of the plateau. The trace-element compositions of these sources, determined by inverse modelling, suggest subduction-related metasomatism. 40Ar- 39Ar dating of xenocrystic phlogopites indicates metasomatism of the southern SCLM occurred at 62±2 Ma, synchronous with collision.
These data link Neogene extension to a thermally perturbed lithosphere. SCLM thinning following slab detachment explains magmatism, extension and uplift in southern Tibet. Episodic convective removal of the SCLM is proposed for northern Tibet. In view of these models, initiation of extension at 18.3±1.6 Ma in southern Tibet places a minimum constraint on plateau uplift. This overlaps with STDS and MCT movement, implying that changes in Himalayan tectonics are controlled by plateau uplift.
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