Formation of the Tibetan Metamarphic Domes

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A series of metamorphic domes that occur in the boundary domain between the Himalayas and Tibet expose rocks originally formed in the mid crust before being brought to the Earth's surface and they offer a window into processes occurring in otherwise inaccessible regions. These domes potentially contain important information about how the structures of the Himalayas and the deep crust of Tibet are linked. However, they have been very little studied. We aimed to help fill this gap in our knowledge by carrying out field based and laboratory studies of the dome regions. We also carried out a number of related studies important for understanding the processes occurring deep in the crust in other related regions. The most important results directly related to the southern Tibet metamorphic domes are as follows.Definition of Domes The metamorphic domes are broadly elliptical domains of metamorphic and igneous rocks that occur in southern Tibet. The domes have a granite core surrounded by roug … More hly concentric Barrovian type metamorphism. The main deformation can be divided into two stages, D1 and D2. D1 is associated with crustal thickening and top-to-the-south sense of shear. D2 is associated with crustal thinning and top-to-the-north sense of shear. D2 can be correlated with the major normal fault the Southern Tibetan Detachment(STD) that lies along the northern margin of the Himalayas. The spatial association between the granites and the deformation and metamorphism suggests these features are all related.Age of the Domes The metamorphic domes are generally thought to be 20-10 Ma. In contrast, the central granites are considered to be around 500 Ma and unrelated to the Himalayan orogeny. This preconception has prevented consideration of models that link granite intrusion with the deformation and metamorphism of the domes. However, in the Malashan dome we show the granitic core formed around 18 Ma. This granite, therefore, intruded during the Himalayan orogen. The large number of similarities between the domes suggests that all of them contain similar young granites at their cores. Our proposal means that it is now possible to examine models that link granite intrusion with the formation of the domes.Dome Formation and Himalayan Tectonics We suggest the domes formed directly as a result of intrusion of granitic magma. Associated contact metamorphism produced the concentric metamorphic zones around the central grantie bodies. In addition, the thermal weakening of the rocks caused a change from crustal thickening during D1 to crustal thinning related to D2. The driving force for the extensional deformation in a compressive tectonic setting is the generated by the action of gravity on the surface slope of the Himalayan front. Less