Two-step obduction of the Porvenir serpentinites: A cryptic Devonian suture in SW Iberian Massif (Ossa-Morena Complex)

Abstract

[EN] The Porvenir serpentinites are an ~600-m-thick body of meta-peridotites exposed in SW Iberia (Variscan Orogen). The serpentinites occur as a horse within a Carboniferous, out-of-sequence thrust system (Espiel thrust). This thrust juxtaposes the serpentinites and peri-Gondwanan strata onto younger peri-Gondwanan strata, with the serpentinites occupying an intermediate position. Reconstruction of the pre-Espiel thrust structure results in a vertical juxtaposition of terranes: Cambrian strata below, Porvenir serpentinites in the middle, and the strata at the footwall to the Espiel thrust culminating the tectonic pile. The reconstructed tectonic pile accounts for yet another major thrusting event, since a section of upper mantle (Porvenir serpentinites) was sandwiched between two tectonic slices of continental crust (a suture zone sensu lato). The primary lower plate to the suture is now overlying the upper plate due to the Espiel thrust. Lochkovian strata in the upper plate and the Devonian, NE-verging folds in the lower plate suggest SW-directed accretion of the lower plate during the Devonian, i.e., Laurussia-directed underthrusting for the closure of a Devonian intra-Gondwana basin. Obduction of the Porvenir serpentinites was a two-step process: one connected to the development of a Devonian suture zone, and another related to out-of-sequence thrusting that cut the suture zone and brought upward a tectonic slice of upper mantle rocks hosted in that suture. The primary Laurussia-dipping geometry inferred for this partially obducted suture zone fits the geometry, kinematics, and timing of the Late Devonian suture zone exposed in NW Iberia and may represent the continuation of such suture into SW Iberia.