Abstract

Geological evidence and theoretical models are presented to illustrate deformation paths followed by continental crust during underthrusting, subduction and collision. Fieldwork in an early Proterozoic collisional orogen indicates that a thrust belt and its autochthonous footwall basement are obliquely exposed in a crustal cross-section with significant structural relief. The collisional orogen records an evolution in the style and extent of involvement of the Archaean footwall from: (1) underthrusting without evidence of internal deformation; (2) limited thrusting and ductile deformation of footwall rocks; to (3) folding of the entire thrust belt and its footwall into cuspate-lobate, long wavelength structures. The structural evolution occurred during hydration of basement rocks coeval with collision-related metamorphism. Lithospheric strength profiles, constrained by simple numerical heat conduction models, suggest that the observed deformation path for the footwall crust was controlled by weakening due to heating of basement rocks, and by changes in the relative strengths of different crustal layers. The profiles predict a very weak lower crust and upper mantle at the time of footwall-involved folding, suggesting that the lower crust and upper mantle deformed by ductile flow concomitant with buckling of the stronger upper crust.

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