Multidisciplinary constraints on the Cadomian compression and early Cambrian extension in the Iberian Chains, NE Spain

Abstract

This paper presents a multidisciplinary approach (stratigraphic, sedimentologic, structural, and mineralogic) to document the evidences for Cadomian and earliest-Cambrian tectonic processes recorded in the Iberian Chains, which were strongly overprinted during Variscan and Alpine deformation episodes. The contact of the Neoproterozic Paracuellos Group and the lower Cambrian Bámbola Formation is commonly identifiable by the presence of a distinct erosive unconformity related to the onset of alluvial plain sequences, or associated with synsedimentary faults. A meso-structural analysis of the deformation recorded in the Paracuellos slates reveals the existence of an earlier slaty cleavage (S 1N) overprinted by Variscan contractional structures, some of them related to positive inversion of pre-Variscan normal faults. The comparative geometrical study with structures observed in the Bámbola Formation (essentially, a single, NW–SE to N–S striking cleavage S 1C) is not conclusive about the record of the Cadomian orogeny. However, micro-scale deformation associated with the earliest cleavage is much more intense in the Paracuellos slates. In addition, the metamorphic grade and the pressure conditions of the lower Cambrian anchizonal shales contrast with the Neoproterozoic epizonal slates. Both observations are consistent with a Cadomian contractional deformation associated with a low-grade, intermediate-pressure metamorphism, absent in the overlying rocks. Postdating both processes, stratigraphically marked by a tectono-thermal discontinuity located at the Paracuellos/Bámbola contact, the onset of the lowermost Cambrian Bámbola Formation is accompanied by strong palaeogeographic transformations in the sedimentary basin, including: (i) a sharp relative sea-level fall leading to deposition of alluvial plain sediments; and (ii) an increase in sedimentation rates and tectonically induced subsidence at fault-bounded depressions. All these changes, together with the presence of normal or transtensional faults at the base of Bámbola Formation, point to an earliest-Cambrian extensional regime similar to that described for other neighbouring post-Cadomian basins.