Crustal-scale rheological transitions during late-orogenic collapse

Abstract

Orogeny involves crustal thickening followed by thermal relaxation and radiogenic heat production in the thickened crust, culminating in crustal melting and magma intrusion which decrease the crustal viscosity by several orders of magnitude and cause late-orogenic collapse. Collapse of the Canadian Cordillera is expressed in the Early Tertiary Shuswap metamorphic core complex, British Columbia, which displays a three-layer crustal section separated by two fundamental rheological discontinuities: (1) the brittle–ductile transition, across which high-angle normal faults in the upper crust control basin formation merge into a low-angle detachment zone where leucogranite laccoliths ponded and deformed progressively under submagmatic to low-temperature conditions; and (2) the metatexite–diatexite transition across which the rocks lose their solid framework and behave like a viscous magma. This transition has the potential to mechanically decouple the upper crust from the rest of the lithosphere during a late-orogenic gravitational collapse.