Crustal-scale rheological transitions during late-orogenic collapse

Orogeny involves crustal thickening followed by thermal relaxation and radi ogenic heat production in the thickened crust, culminating in crustal melti ng and magma intrusion which decrease the crustal viscosity by several orde rs of magnitude and cause late-orogenic collapse. Collapse of the Canadian Cordillera is expressed in the Early Tertiary Shuswap metamorphic core comp lex, British Columbia, which displays a three-layer crustal section separat ed by two fundamental rheological discontinuities: (1) the brittle-ductile transition, across which high-angle normal faults in the upper crust contro l basin formation merge into a low-angle detachment zone where leucogranite laccoliths ponded and deformed progressively under submagmatic to low-temp erature conditions, and (2) the metatexite-diatexite transition across whic h the rocks lose their solid framework and behave like a viscous magma. Thi s transition has the potential to mechanically decouple the upper crust fro m the rest of the lithosphere during a late-orogenic gravitational collapse . (C) 2001 Elsevier Science BN. All rights reserved.