STRAIN PARTITIONING IN THE FOOTWALL OF THE SOMIEDO NAPPE - STRUCTURALEVOLUTION OF THE NARCEA TECTONIC WINDOW, NW SPAIN

The Somiedo Nappe is a major thrust unit in the Cantabrian Zone, the e xternal foreland fold and thrust belt of the North Iberian Variscan or ogen. Exposed at the Narcea Tectonic Window are Precambrian rocks belo w the basal decollement of the Somiedo Nappe, which exhibit a differen t deformation style than the overlying Paleozoic rocks above the basal decollement. During Variscan deformation, folding and widespread sub- horizontal, bedding-parallel decollements were produced in the hanging wall within the Paleozoic rocks. Vertical folding, with related axial -planar cleavage at a high angle to the decollement planes, developed simultaneously in the upper Proterozoic Narcea Slates of the footwall, below the detachment. The relative magnitude of finite strain, measur ed in the footwall rocks, diminishes towards the foreland. These obser vations indicate that (1) significant deformation may occur in the foo twall of foreland fold and thrust belts, (2) the shortening mechanism in the footwall may be different from that of the hanging wall, and (3 ) in this particular case, the partitioning of the deformation implies the existence of a deeper, blind decollement surface contemporaneous with the first stages of the foreland development, that does not crop out in the region. This implies a significant shortening in the footwa ll, which must be taken into account when restoration and balancing of cross-sections is attempted. A sequential diagram of the evolution of the Narcea Tectonic Window with a minimum shortening of 85 km is prop osed, explaining the complete Variscan evolution of the foreland to hi nterland transition in the North Iberian Variscan orogen. Copyright (C ) 1996 Elsevier Science Ltd.