Comparing Tibet-Himalayan and Caledonian crustal architecture, evolution and mountain building processes

The Himalaya-Tibet and Caledonide orogens are comparable in scale and are similar in various aspects. Regional suture zones are recognizable in both, although their identification is more problematic in the deeply eroded Caledonide orogen. Crustal-scale thrust belts, regional Barrovian metamorphism characterized by clockwise P–T paths, and migmatitic cores with crustally-derived leucogranite complexes are the dominant structural feature of both orogens. Both orogens also record calc-alkaline magmatism attributed to subduction activity prior to collision. Syn-orogenic extension accompanied crustal thickening in both orogens, however, the Caledonides also have a protracted record of late- to post-orogenic extension that is attributed to lithospheric delamination in combination with oblique plate divergence. The oblique nature of the Caledonian collision is also reflected in the development of regionally significant sinistral strike-slip faults and shear zones, whereas such structures are apparently not as significant within the Himalayan orogen. The major difference between the two orogens relates to their contrasting gross structure: the Caledonides has bivergent geometry with thrust belts developed in the pro- and retro-wedges, whereas the Himalaya has a thrust belt located only in the pro-wedge segment. These differing geometries are probably explicable with reference to pre-collision contrasts in rheology and/or inherited structures. As such, there is no reason to suggest that either example should be viewed as being a ‘typical’ product of collisional orogenesis – they likely represent end-members of a range of possible orogenic profiles.