Extensional collapse of orogens

TLDR

Lithospheric extension preferentially occurs along orogenic belts because their thicker crust, structural inhomogeneities, and isostatic body forces promote extensional collapse. Collapse is driven by rapid advective thinning of the shortened thermal boundary layer beneath an orogen, producing uplift, and is modulated by boundary forces that can generate radial thrusting and oroclinal loops, continue extension during rapid compressional‑tensional shifts, or be enhanced by subduction rollback even during ongoing convergence. Extensional collapse explains cyclic ocean closure and reopening, preserves high‑pressure metamorphic rocks and supracrustal sequences, restores normal crustal thickness without extensive denudation, and accounts for high‑temperature metamorphic assemblages and a minimum‑melting granite suite.

Abstract

Lithospheric extension is sited, preferentially, along orogenic belts because they have a thicker continental crust, contain structural inhomogeneities, and suffer extensional orogenic collapse caused by body forces resulting from isostatically compensated elevation and sharp elevation gradients. Collapse occurs especially where rapid advective thinning of the shortened thermal boundary conduction layer occurs beneath an orogen and causes rapid uplift. Where boundary forces are compressional, extension is balanced by radial thrusting to form oroclinal loops around collapsed extensional basins. Where, as in the disruption of Pangea, boundary forces change rapidly from compressional to tensional, body force collapse is continued by general extension which may lead to continental splitting. Even where overall convergence is continuing, orogenic collapse may be enhanced by subduction rollback into small remnant oceans. The extensional collapse of orogens offers a partial explanation for why oceans cyclically close and reopen in roughly the same places, preservation of very high pressure metamorphic rocks, for the return of orogenic large crustal thicknesses to normal without very much erosional denudation with the widespread preservation of supracrustal sequences, high temperature metamorphic assemblages and the minimum‐melting granite suite.