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

1. A. Pérez-Estaún ¹
2. J. A. Pulgar ²
3. J. Álvarez-Marrón ¹
4. A. Marcos ²
5. F. Bastida ²
6. J. Aller ²
7. J. Marquínez ²
8. P. Farias ²
9. J. L. Alonso ²
10. G. Gutiérrez ³
11. J. Gallastegui ²
12. R. Rodríguez ⁴
13. N. Heredia ⁴
14. M. Bulnes ²
15. E. Banda ¹
16. J. R. Martínez ³
17. D. Córdoba ⁵
18. J. J. Dañobeitia ¹
19. M. C. Comas ⁶

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More publications in: Revista de la Sociedad Geológica de España

Abstract

The ESCI-N1 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 surface 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 Precambrian 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 synmetamorphic 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 antiform in the upper crust. The deeper structure of the Narcea antiform suggests that uplift of the Precambrian basement in this area was accomplished largely by thrusting rather than vertical doming

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