MÉLANGES AND OTHER TYPES OF BLOCK-IN-MATRIX FORMATIONS IN THE CANTABRIAN ZONE (VARISCAN OROGEN, NW SPAIN): ORIGIN AND SIGNIFICANCE

The block-in-matrix formations in the Variscan foreland of Spain (Cantabrian Zone) occur in two different geological settings. The major block-in-matrix formations are mélanges, which appear as carpets beneath submarine thrust systems and overlying flysch units. These mélanges may reach up kilometric thickness and are mostly composed of broken formations (tectonosomes) of Late Carboniferous age and scattered “exotic” blocks derived from older Palaeozoic formations. The source of the mélange blocks was the front of advancing nappes, chiefly the upper part of the nappe stacks. Therefore, the Cantabrian mélanges are interpreted as originated by submarine sliding and slumping associated with steep slopes at the orogenic front. The different types of rock bodies of these mélanges may be related to the degree of lithification of the sediments or rocks during slumping, since it depends on their age and composition. So, broken formations are boudinaged sequences where the boudins or blocks resulted from extensional faults developed in lithified or semilithified limestones and sandstones whereas the unlithified shaly matrix was undergoing continuous deformation. The scattered “exotic” blocks with a range of ages from Early Cambrian to Early Carboniferous came into the basin as individual slip blocks from competent well-lithified formations, originally located at the lower part of the nappe stacks. The mélanges in the Cantabrian Zone also intercalate minority debris flows with chaotic block-in matrix fabric (olistostromes). However, most of the olistostromes of the Cantabrian Zone occur in a different geological setting. They are usually intercalated into the normal marine deposits of the Variscan foreland basin. In contrast with the Cantabrian mélanges, these olistostromes are mostly related to the margins of carbonate platforms, ahead of moving nappes, and consist of intrabasinal (native) components.