Crustal structure of the Cantabrian Zone: seismic image of a Variscan foreland thrust and fold belt (NW Spain)

espanolEl perfil sismico de reflexion profunda ESCI-Nl, de 140 km de longitud, muestra la estructura de Ja zona externa de la Cordillera Varisca Europea (Zona Cantabrica) en la Peninsula Iberica y su transicion a las zonas internas (Zona Asturoccidental-leonesa). Las reflexiones existentes en la parte superior y oriental de la linea (Zona Cantabrica), corresponden a las rocas paleozoicas. El contraste litologico entre las diferentes formaciones de la secuencia paleozoica da lugar a la existencia de bandas reflectivas y trasparentes. La superficie de despegue de la Zona Cantabrica se evidencia por la existencia de reflexiones dispuestas en una estrecha banda inclinada hacia el oeste, situada entre los 4 s (tiempo doble) al este y los 6 s al oeste. Esta superficie pone de manifiesto al caracter de tectonica epitelial de la zona externa del orogeno. La corteza por debajo del despegue, entre los 6 y los 14 s de profundidad, presenta reflexiones subhorizontales, cortas y poco coherentes. La Moho puede situarse a 14 sen esta zona. Este fuerte grosor cortical se debe a la deformacion alpina. En la parte oeste de la linea, en la transicion entre las zonas Cantabrica y Asturoccidental- leonesa, existen varias bandas reflectivas, entre 5 y 10 s de profundidad, inclinadas al oeste. La mas profunda une Ja superficie de despegue de la Zona Cantabrica, con la corteza inferior. La deformacion varisca afecta a la totalidad de la corteza en este sector. Una corteza inferior reflectiva esta presente en la parte occidental del perfil. La estructura profunda en la transicion a las zonas internas del orogeno, parece mostrar una indentacion del basamento precambrico de la Zona Cantabrica en la corteza de las zonas internas. EnglishThe ESCI-Nl normal incidence seismic reflection line (140 km long) shows much of the shallow structure of the foreland thrust and fold belt (Cantabrian Zone) of the European Variscan Belt in the Iberian Peninsula (NW Spain), as well as the deep structure of the transition to the hinterland (Westastur-leonese Zone), and allows a direct comparison with surface geology. Reflections, on the eastern upper part of the line (Cantabrian Zone), correspond to Palaeozoic rocks mappable on surface. The Palaeozoic sequence provided strongly reflecting and transparent zones. The general decollement slirface beneath the thrust belt is visible as a set of reflections dipping westward and placed between 4 and 6 s two-way travel time (TWTT), establishing the thin-skinned character of the deformation. In the central part of the line, the reflectivity decreases in the upper crust, above the decollement, due probably to the strong deformation of the area. The crust underneath the decollement shows, in the eastern part of the line, short, subhorizontal and numerous reflections from 6 to 14 s. The thickening of the crust in this area is interpreted to be related to Alpine reworking. On the western part of the line, at the transition between the Cantabrian and Westastur-leonese zones (the Narcea antiform, cored by Precarnbrian rocks), a set of strong reflections dipping west joint the decollement (at 6 s TWTT) with the lower crustal levels (9 s). These reflections die out on the top of the lower crust (horizon of decoupling). The lower crust, in this region, is characterised by a zone of high reflectivity situated between 9 and 12 s TWTT, that seems to occur only in the area with thick-skinned tectonics, and with synmetarnorphic deformation. The whole deep crustal structure of the transition zone may be interpreted as an indentation of the foreland basement into the hinterland crust producing a duplication of the lower crust and a complex antiforrn in the upper crust. The deeper structure of the Narcea antiform suggests that uplift of the Precarnbrian basement in this area was accomplished largely by thrusting rather than vertical doming