The role of structural and metamorphic memory in the distinction of tectono-metamorphic units : the basement of the Como Lake in the Southern Alps

The concept of 'metamorphic field gradient' applied to the lithologically homogeneous deep crust of the Southern Alps suggests the existence of two metamorphic units. The comparison of P-T-d-t paths, derived in adjacent portions of this basement and supported by a continuous foliation trajectory map helps to distinguish three tectono-metamorphic units, corresponding to the Domaso-Cortafo, Dervio-Olgiasca and Monte Muggio zones. The degree of granular scale reorganisation of the prevailing planar fabric is considered in the three zones together with the relative chronology of the structural imprints and the metamorphic environments in which they developed. This approach emphasizes that the dominant metamorphic imprint of each unit coincides with that of the most pervasive fabric at the regional scale when the degree of fabric evolution is sufficiently high, and not with the T(max)-P(Tmax) recorded in each tectono-metamorphic unit. In terrains that underwent polyphase deformation and metamorphism the 'metamorphic field gradient' concept cannot therefore be utilised to discriminate tectono-metamorphic units. (C) 2000 Elsevier Science Ltd. All rights reserved.