The continental collision process deduced from metamorphic pattern in the Himalayan collision belts

Abstract:

The following collision model of the Himalayan collision belt is proposed based on data published in previous studies. Collision between the Indian and Asian blocks started in the west before ca. 55 Ma. In the western part, the amount of oceanic slab subducted prior to continent collision was enough to pull the continental crust down to the depths of UHP metamorphism (750–770°C, 30–35 kbar), as a wide ocean existed between the Asian and Indian blocks prior to collision. Following UHP metamorphism, oceanic slab break-off started at ca. 46~55 Ma in the west due to the very strong buoyancy of the deeply subducted continental block. In contrast, the subduction of continental crust continued at this time in the middle and eastern parts of the belt. The zone of break-off migrated eastward, initiating a change from steep- to low-angle subduction. In the mid-eastern parts of the belt, the slab break-off occurred at depth of HP eclogite facies metamorphism (>780°C, 20 kbar) at ca. 30 Ma. Final slab break-off may have occurred in the easternmost part of the belt at ca. 22-25 Ma. The slab break-off in the eastern part occurred at the depth of high-pressure granulite facies metamorphism (890°C, 17–18 kbar).The depth of slab break-off decreased toward the east, indicating that the rate of uplift decreased eastwards due to a decrease in buoyancy of the continental slab. The slower uplift resulted in a longer period of thermal relaxation and a higher geothermal gradient. In the west, the high rate of uplift resulted the epidote amphibolite facies (580–610°C) retrograde metamorphic overprint on the UHP eclogites, whereas the relatively slow uplift in the mid-eastern part caused high-grade granulites (850°C) retrograde metamorphic overprint on the HP eclogites. The study indicates that the metamorphic pattern along the collision belt is strongly related to the amount of subducted oceanic crust between continents before collision and the depth of slab break-off. Therefore metamorphic pattern can be used to interpret the tectonic process during continental collision which disappeared now.

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