CRUSTAL DOMING MECHANISMS FROM NUMERICAL MODELS OF LARGE HOT OROGENS: IMPLICATIONS FOR GNEISS DOMES AND METAMORPHIC CORE COMPLEXES

Gneiss domes, metamorphic core complexes, and similar structures are common in large continental orogens. Examples include the North Himalayan gneiss domes of the Himalayan-Tibetan system and the Cordilleran core complexes of western North America. Many of these features, notably core complexes, are associated with normal faults and have been widely interpreted to form in post-orogenic tectonic settings characterised by orogen-scale, gravitationally induced extension. Results from coupled thermal-mechanical model experiments suggest that several different processes can lead to the formation of a range of dome styles while the orogen is in a state of overall contraction. Styles observed in the models include near-symmetric exhumation structures, asymmetric ductile extrusion zones associated with hinterland domes, and domes triggered by underthrusting of a strong lower crustal layer. Domes formed by any of these mechanisms may be extruded at the orogenic front or transported from their original tectonic setting into another. An essential component of the models is hot, tectonically thickened crust containing a weak mid-crustal layer that behaves as a fluid on the timescales required to form these structures. This layer is pumped by the dynamical pressure regime into locally extending and contracting regions of the model crust and also acts to generate the instabilities that create these structures. The style of doming and subsequent exhumation is also sensitive to upper crustal strength and surface denudation. In the models, a density contrast between the weak layer and the overlying crust is not required to form these structures. The model domes are characterised by bounding normal and/or thrust sense shear zones and strongly condensed metamorphic sequences between dome cores and flanks. The models predict that a range of dome-like structures, including gneiss domes and core complexes, may form in either contractional or extensional settings, raising the possibility that similar features in demonstrably extended orogens may have started evolving while the orogen was in a state of overall contraction.