Rupture and relamination of continental lithosphere in a subduction system (Variscan Orogeny, SW Iberia)

Abstract

Relamination is closely connected to subduction zones. Despite the numerous high-P metamorphic belts recognized on Earth, relamination remains a poorly documented process. We present a continental subduction zone where the exhuming high-P rocks became a relaminant under the upper plate. This study focuses on the Variscan Orogeny (SW Iberia) and is based on geological mapping and structural, metamorphic, and geochronological analysis. A lithospheric-scale thrust within the subduction zone conducted the early exhumation of the high-P rocks. This thrust brought deeper sections of the slab onto shallower ones. The exhumation assisted by this thrust proceeded up to the base of the upper plate, which locked the exhumation path of the high-P rocks through the subduction zone. At this point, the high-P rocks became a relaminant that moved under the upper plate and away from the subduction zone. A combination of synthetic and anti-thetic shearing relative to the subduction polarity conducted relamination. The shear zones formed at this point coexisted with the narrowing of the subduction system upon a progressive arrival of a more buoyant lithosphere to the subduction zone. The combination of both processes resulted in large-scale recumbent folds, which affected the early thrusts and contributed to the relamination process by facilitating subhorizontal flow during fold amplification. The ensuing collisional stage affected the whole subduction system (lower and upper plate, collectively) and led to an out-of-sequence thrust system that duplicated the suture zone defined by the high-P rocks.