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Tectonically active Aalenian in the Northwestern Iberian Basin (Spain)

García-Frank A., Ureta S., Mas R.

Volumina Jurassica

2006

Vol. 4, no. 1

42-42

The Alpine NW Iberian Range (N Spain), formed due to the tectonic inversion of the Iberian Basin. Four major episodes constituted the extensional development of the Iberian Basin during the Mesozoic: Permo-Triassic rifting; Jurassic post-rift subsidence; Upper Jurassic - Lower Cretaceous rifting; and Upper Cretaceous post-rift subsidence. Thus, the Aalenian in the Iberian Range was considered as a tectonically inactive period. However, the Aalenian in Northern Europe has been interpreted as a period of general regional regression, related to tectonic events. The Aalenian-Bajocian succession from the Northern Iberian Range is related with periods of low sedimentation rates and prolonged submarine exposure. The stratigraphic sequences defined in this area, highlight a progressive change in the sedimentary conditions of the basin. These sequences are established as precisely dated discontinuity-bounded sequences, defined by ammonoids. The complex depositional architecture of the basin, recognized on the basis of changes in isopach maps, is interpreted to be tectonically controlled, containing areas, ith low and high topographic platforms as a result of semigraben structures. Thus, two main paleogeographic settings are proposed for the studied region. The NW-SW-SE area is characterized by condensed sedimentation with five discontinuity-bounded sequences. The NE area is characterized by more expanded sedimentation without major discontinuity bounding sequences (except at the base of the succession) and with 10 coarsening-upwards sequences. The NE area corresponds to a different sedimentation setting, with more affinity with coeval rocks in the Cantabrian Range than those in the Iberian Range. In addition, the volcanism in nearby areas of the Iberian Basin within this period, is interpreted to be related to a crustal stretching phase. Tectonic instability during this period has been previously underestimated, although rarely documented in southern parts of this basin, and elsewhere in the Iberian Peninsula. Furthermore, to reinforce the synsedimentary tectonic activity, hydrothermalism related to exhalative events is recognized in the transitional beds of the Upper Toarcian - Lower Bajocian. All these data suggest that the Iberian Basin developed in a distensive context, and that the transition between Lower and Middle Jurassic was a period in which a certain tectonic instability is recognised.