MODES OF EXTENSION IN COLLAPSING OROGENS

Lithospheric extension commonly initiates in orogens either trailing or many eons after the end of orogeny. Using thermo-mechanical models constrained by geological and geophysical observations, we studied extensional collapse of orogens. Assuming a weak mid-crustal shear zone strengthens over time due to cooling and annealing, we showed that variable intensities of decoupling induced by the shear zone between upper and middle crust of variable strength can generate a wide range of extensional structures. Simulations of young orogens with weak to strong ductile middle crust predict three modes of extension dominated by middle crustal extrusion, detachment faults and metamorphic core complexes formation, and upper crustal thinning, respectively. Simulations of old orogens instead predict core complex, diffusive and localized rift mode. We found that depth-dependent stretching, middle crustal flow and regional stress rotation are critical for the unique styles of extension in collapsing orogens. Our models consistently predict diverse geological and geophysical observations in young Cenozoic/Mesozoic orogens such as US Cordillera, the Aegean, and Papua New Guinea as well as old Caledonian to Variscan orogens along the North Atlantic margins.