In accretionary convergent margins, the subduction interface is formed by a lower plate décollement above which sediments are scraped off and incorporated into the accretionary wedge. During subduction, the basal décollement is typically located within or at the base of the sedimentary pile. However, the transition to collision implies the accretion of the lower plate continental crust and deformation of its inherited rifted margin architecture. During this stage, the basal décollement may remain confined to shallow structural levels as during subduction or re-localize into the lower plate middle-lower crust. Modes and timing of such re-localization are still poorly understood. We present cases from the Zagros, Apennines, Oman, and Taiwan belts, all of which involve a former rifted margin and point to a marked influence of inherited rift-related structures on the décollement re-localization. A deep décollement level occurs in the outer sectors of all of these belts, i.e., in the zone involving the proximal domain of pre-orogenic rift systems. Older—and shallower—décollement levels are preserved in the upper and inner zones of the tectonic pile, which include the base of the sedimentary cover of the distal portions of the former rifted margins. We propose that thinning of the ductile middle crust in the necking domains during rifting, and its complete removal in the hyperextended domains, hampered the development of deep-seated décollements during the inception of shortening. Progressive orogenic involvement of the proximal rift domains, where the ductile middle crust was preserved upon rifting, favors its reactivation as a décollement in the frontal portion of the thrust system. Such décollement eventually links to the main subduction interface, favoring underplating and the upward motion of internal metamorphic units, leading to their final emplacement onto the previously developed tectonic stack.
Supplemental Material: Diversification of Archean tonalite-trondhjemite-granodiorite suites in a mushy middle crust
