On the rootless nature of a Devonian suture in SW Iberia (Ossa-Morena Complex, Variscan Orogen): geometry and kinematics of the Azuaga Fault

Abstract

Suture zones are key to understand collisional orogens, but not all the remains of subduction leading to collision occur in the root of the suture. The Azuaga Fault bounds a Devonian suture zone known as Central Unit. This fault is a steeply NE-dipping, Variscan strike-slip fault with left-lateral and reverse oblique slip components formed during sinistral transpression in the Pennsylvanian. Motion along this fault was coeval with folding and fabric development in both its hanging wall and footwall and also with the Matachel Fault. Tectonic flow associated with the Azuaga Fault shows high-vorticity, explaining the exhumation of a flat-lying Devonian suture zone via WNW-plunging extrusion from the upper-middle crust under inclined triclinic transpression during ENEWSW convergence. The exposed basal contact of the Central Unit is not the root zone of a Variscan suture zone, but instead is a NE-dipping breaching fault that cuts across the suture zone that is contiguous to the SW under the upper section of the footwall. The peri- Gondwanan terrane between the Central Unit and the South-Portuguese Zone of the Iberian Massif (most of the Ossa-Morena Zone) is underlain by a Devonian suture, implying it is a continental allochthon. Variscan suture zones in Europe are affected by strike-slip faults. In our case, this pattern implies the location of suture zone exposures and location of its root are different. Suture zones and strike-slip faults are common in orogens and analysis of their relationships may lead to relocation of suture zone roots and re-thinking of upper and lower plates.