Finite strain patterns and their significance in Permian rocks of the Alpes Maritimes (France)

More finite strain data has been obtained from autochthonous Permian mudstones of the Alpes Maritimes, S.E. France. These new data were computed from field measurements of green spots on all available sections, deformed mudcracks and from the quantitative correlation between magnetic susceptibility anisotropy and finite strain in these rocks. Previously published finite strain data and the new results are presented on a series of structural maps and cross-sections for the Dôme de Barrot, the Tinée and Vionène region and the Roya region. As in previous studies difficulties arise in explaining the apparently variable extension parallel with the 100°, subhorizontal bedding-cleavage intersection: either this is real or there were large volume changes during the tectonic deformation. Study of quartz fibres, developed in deformed mudcracks in the Tinée valley, suggest that early in the tectonic history incremental stretching directions were parallel with the bedding—cleavage intersection, while later they were down-dip in the 100° trending cleavage. Since these Permian rocks have remained stuck to the Argentera basement they also record displacements and deformations in the basement. The early 100°, subhorizontal stretching is consistent with NW-SE dextral, strike-slip basement faulting, while later, down-dip stretching in the cleavage is consistent with contraction faults in the basement. This information and new palaeomagnetic data on the same samples are combined with recent geophysical evidence and regional tectonic studies, to provide a new precision to the tectonic history of this part of the Western Alpine External Zone.     

Some limitations of the Rf/φ technique of strain analysis — Discussion

A recent note by De Paor (1980) suggests that finite strain cannot be decomposed into tectonic and compaction components on the basis of asymmetry of the R_fφ data about the mean φ trace. This is true if only the shape of the R_fφ data field is considered, but shown to be incorrect when the distribution of data within this field is considered.     

Analysis of three-dimensional, homogeneous, finite strain using ellipsoidal objects

It has been suggested (Oertel, 1971, 1972;Owens, 1974; Shimamoto and Ikeda, 1976) that some methods for analysis of finite homogeneous strain from deformed ellipsoidal objects (Ramsay, 1967; Dunnet, 1969a; Elliott, 1970; Dunnet and Siddans, 1971; Matthews et al., 1974) require sections to be cut in principal planes of the finite strain ellipsoid. A mathematical model is presented which enables the homogeneous deformation of a randomly oriented ellipsoid to be investigated. In particular the elliptical shapes that result on any three mutually perpendicular sections through the ellipsoid, in the deformed state, can be computed, together with the corresponding strain ellipses. The resulting ellipses can be unstrained in the section planes by applying the corresponding reciprocal strain ellipses. It is shown that these restored ellipses are identical with the elliptical shapes that result on planes through the original ellipsoid when the planes are parallel to the unstrained orientation of the section planes.The model is extended to investigate the finite homogeneous deformation of a suite of 100 randomly oriented ellipsoids of constant initial axial ratio. The pattern of elliptical shapes that result on any three mutually perpendicular section planes, in the deformed state, is computed. From this data the two-dimensional strain states in the section planes are estimated by a variety of methods. These are combined to recalculate the three-dimensional finite strain that was imposed on the system. It is thus possible to compare the results of the two- and three-dimensional analyses obtained by the various methods. It is found that providing all six independent combinations of the two-dimensional strain data are used to compute a best finite strain ellipsoid, the methods of Dunnet (1969a), Matthews et al. (1974) and Shimamoto and Ikeda (1976) provide accurate estimates of the three-dimensional finite strain state.It is concluded that measurement of the two-dimensional data on section planes parallel to the principal planes of the finite strain ellipsoid is not necessary and that all six independent combinations of the two-dimensional strain data should always be made and used to compute a best finite strain ellipsoid.     

Arcuate fold and thrust patterns in the Subalpine Chains of Southeast France

The arcuate pattern of folds and thrusts is examined in the Subalpine Chains of Southeast France. Five segments are recognised; (1) a NNE-SSW trending zone between Grenoble and Die, (2) an E-W zone between Die and Sisteron, and (3); (4), & (5) the Arcs de Digne, Castellane, Nice, trending NNW-SSE, E-W, and N-S, respectively. Folds of three ages are known: pre-Senonian (recognised in Dévoluy), Paleocene to early Eocene and late Oligocene to late Miocene. The last phase was accompanied by low-grade metamorphism and cleavage development. Thrusts were formed during the last two phases.Examination of the age of folding and thrusting, directions of tectonic transport and the continuity of structures reveals that segments (1), (2) & (3) are not continuous. Segment (2) has a northward sense of tectonic transport and is thought to be of pre-Senonian and Paleocene age. It is crossed in Dévoluy by the late Oligocene structures which are continuous from segment (3) into segment (1). Segments (3), (4) & (5) are continuous. Their curvature is thought to be primary and to have resulted from gravity gliding.A section through the Digne Thrust Sheet is examined in terms of models of thrusting proposed by Elliott and by Chapple. It is found that the compressional component in Chapple's model was completely dominant in overcoming the resistance to sliding during the Miocene phase, the glacier spreading component having a negligible effect. Other thrusts, in particular those of the Arc de Castellane, do not appear to fit the model. It is thought that gravity gliding may have been the mechanism of thrusting in these areas.