STRAIN ANALYSIS IN JURASSIC ARGILLITES OF THE MONTE-SIRINO AREA (LAGONEGRO ZONE, SOUTHERN APENNINES, ITALY) AND IMPLICATIONS FOR DEFORMATION PATHS IN PELITIC ROCKS

Analysis of strain in Jurassic argillites forming part of the folded a nd thrusted sedimentary succession of the Lagonegro basin (southern It alian Apennines) has been carried out using ellipsoid-shaped 'reductio n' spots as strain markers. Most of the determined finite strain ellip soids are of oblate type and show a peculiar distribution of the maxim um extension direction (X), with maxima either subparallel or sub-perp endicular to the local fold axes. Using the strain matrix method, two different deformation histories have been considered to assist the int erpretation of the observed finite strain pattern. A first deformation history involved vertical compaction followed by horizontal shortenin g (occurring by a combination of true tectonic strain and volume loss) , whereby all strain is coaxial and there is no change in the intermed iate axis of the strain ellipsoid. By this type of deformation sequenc e, which produces a deformation path where total strain moves from the oblate to the prolate strain field and back to the oblate field, prol ate strain ellipsoids can be generated and may be recorded where tecto nic deformation has not been large enough to reverse pretectonic compa ction. This type of deformation history may be of local importance wit hin the study area (i.e. it may characterize some fold hinge regions) and, more generally, is probably of limited occurrence in deformed pel itic rocks. A second deformation sequence considered the superposition of pre-tectonic compaction and tectonic strain consisting of initial layer-parallel shortening followed by layer-parallel shear (related to flexural folding). Also in this instance, volume change during tecton ic deformation and tectonic plane strain have been assumed. For geolog ically reasonable amounts of volume loss due to compaction and of init ial layer-parallel shortening, this type of deformation history is cap able of producing a deformation path entirely lying within the oblate strain field, but still characterized by a changeover, during deformat ion, of the maximum extension axis (X) from a position parallel to the fold axis to one perpendicular to it. This type of deformation sequen ce may explain the main strain features observed in the study area, wh ere most of the measured finite strain ellipsoids, determined from the limb regions of flexural folds, display an oblate shape, irrespective of the orientation of their maximum extension direction (X) with resp ect to the local structural trends. More generally, this type of defor mation history provides a mechanism to account for the predominance of oblate strains in deformed pelitic rocks.