A GUIDE TO DOME IMPROVEMENT, LESSON 1: IS YOUR DOME BUILT ON GRANITE OR GNEISS?

Domes of high-grade metamorphic and plutonic rocks commonly occur in orogenic cores. This paper uses the Himalayan orogen and considers two regions where young "gneiss domes" are well-developed; the syntaxes (the western & eastern tips of the Himalayan arc) and the central & eastern Himalaya (Southern Tibet, structurally above Nepal, Sikkim and Bhutan). These areas suggest that there may be two alternative or end member types of domes; (1) those "cored" or formed mostly by granitic rocks that are associated with extrusive flow of a partially molten middle crust, and (2) those formed by polymetamorphism and extensive reworking & exhumation of the subducted Indian cover & basement.

In the central Himalaya, domes typically consist of large volumes of granitic rock that mostly (but not always, e.g., Kangmar) has crystallisation ages that are young with respect to the collision. On the eastern flank of the Yadong-Gulu Rift System, numerous leucogranite domes of a few 100 sq. km (e.g., Karo La, Kangmar, & Mangda Kangri) poke through a thin (<15 km thickness revealed by INDEPTH profiling data) cover of Tethyan phyllite that acted as the principal thin-skinned decollement during early Himalayan convergence. This layer of non-coaxially strained phyllite has acted as a partial thermal sink that impeded granite ascent while at the same time forming the upper margin of the slab (or "channel") through which the partially molten middle crust is being extruded southwards. The domes are a minor by-product of the slab extrusion and there is no major re-working of basement. It is unlikely that these domes would be recognisable in the roots of the future fossil Himalayan orogen.

In the syntaxial regions of the Himalaya (Namche Barwa & Nanga Parbat) are polymetamorphic terranes involving very young, multiple re-working of older Himalayan- to Proterozoic-aged crust. In the most recently active portions of the syntaxes, relative uplift of the dome core is via steep crustal scale shear zones. There is no extensive partial melt body; plutonism and migmatization was via localised and protracted multi-stage addition. The Dome architecture seems to be central (!) to the localised late orogenic activity and is likely a modern analogue for gneiss domes found in ancient orogens (i.e. these domes would be recognisable in the future fossil Himalaya).