Boudinage

Abstract: Preface Acknowledgements 1. Fundamental Principles 2. Minor fractures - their nomenclature and age relationships 3. Concordant and discordant intrusions 4. Diapirs, related structures and circular features 5. Faults - nomenclature, classification and basic concepts 6. Strike-slip faults 7. Overthrusts and thrust nappes 8. Normal faults and associated structures 9. Development of systematic fractures in slightly deformed sedimentary rocks 10. Introduction to folding 11. Surface and single layer buckling 12. Multilayer folds and associated structures 13. The buckling of anisotropic rocks 14. Initiation of large buckle folds and fracture-fold relationships 15. The life and times of a buckle fold 16. Boudinage and pinch-and-swell structures 17. Rock cleavage and other tectonic fabrics 18. Structural analysis Index.

Abstract: This paper is concerned with the geometry of homogeneous strain in rocks. Equations relating rotation of planes and lines to the deformation of the rock containing them are derived with the aid of the deformation ellipsoid. The Fresnel construction is borrowed from optical mineralogy to determine from the deformation ellipsoid the principal axes of compression and extension in any plane, and an expression is derived for determining the lengths of these axes. Further equations are derived which define the surfaces of no infinitesimal strain and no finite strain for all possible ellipsoids, and it is shown that these surfaces may be used for rapidly determining whether the principal axes in any plane have suffered shortening or extension. With these tools the rather complex geometry of three-dimensional homogeneous strain is examined in detail. It is shown, for instance, that during deformation planes and lines develop preferred orientations reflecting the symmetry of the deformation, and pre-existing folds rotate bodily in space and either open or close or both during the deformation. In order to draw further conclusions of general interest the condition of homogeneous strain is relaxed in order to consider the deformation of layered rocks with competence differences between the layers. It is argued that during the deformation of such rocks folds and boudinage form parallel to the principal directions of strain in the layers. The conditions that determine whether folds or boudinage are formed are examined and the subsequent development of the structures after their generation is followed. These methods of analysis are used in the construction of models of superimposed fold-systems which are shown to be similar to some recently described field examples. It is argued that the tectonic axial cross must be replaced by the deformation ellipsoid. The structures treated in the paper are usually derived by shear-plane hypotheses, but it is considered that such hypotheses are superfluous.

Abstract: In northern California, the central belt of the Franciscan subduction complex of late Mesozoic to early Tertiary age is largely a zone of chaotically mixed pelitic-matrix melange. The melange contains a diverse assemblage of rocks of various sizes and degrees; of metamorphism. The most abundant rock types are graywacke and greenstone, but the exotics such as blueschist and eclogite are the most distinctive rocks in the belt. The melange belt is traceable for hundreds of kilometres along strike and is up to tens of kilometres wide. Elongate fragments of most rock types show pinch and swell structure that grades into boudinage. This suggests they behaved as relatively rigid bodies in a ductilely deforming matrix. The general planar preferred orientation of elongate blocks, development of foliation in the matrix, wide separation of blocks, and chaotic mixing indicate that large amounts of flow occurred during melange formation. The major mineralogy of the matrix is quartz + albite + chlorite + white mica and rarely kaolinite. Minor amounts of pumpellyite and lawsonite are present. Comparison with burial metamorphic sequences and other blueschist terranes indicates that most of the blocks and matrix are compatible with conditions of metamorphism in the range of 100 A model is proposed in which the pelitic matrix melanges of the Franciscan represent zones where flow, driven by the movement of the descending plate, occurred in the accreted sediment pile. When flow occurs in a low-angle corner, a forced convection drives material back to the surface and transports blocks of blueschist and eclogite upward. A varied assemblage develops because fragments of various lithologies and grades of metamorphism are plucked from many points along the walls of the melange wedge and added to subducted material. Laminar flow causes particles to be dispersed. Most large clasts are broken into smaller, rounded fragments by boudinaging as the melange turns the corner to flow back towards the surface. Mixing is enhanced because differences in block size and density cause differential settling of large blocks within the nonuniform velocity field. Oblique convergence cause;s blocks to take helical paths and may explain the dispersal of blocks such as eclogite along the length of the Franciscan from a few localized sources. The thermal regime in a mature flow melange is such that the downward and upward P-T paths of contained material almost coincide. This accounts for the retrograde blueschist facies metamorphism of the high-grade blocks such as eclogite. The Franciscan melange belt was probably initially produced by the subduction and deformation of a thick continental margin sequence with a high shale/sandstone ratio (the “Knoxville”). Similar chaotic zones that do not contain exotic blocks in other subduction complexes, may have originated as small flow melanges that developed in wide-angle wedges that did not go to great depth.