Search results for "Boudinage"
showing 6 items of 6 documents
Apparent boudinage in dykes
2004
Intrusive rocks may be arranged in the form of strings of lenses or beads, as found on the Cap de Creus Peninsula, NE Spain, and in the South Finland Migmatite-Granite Belt. These structures first appear to be the result of stretching and boudinage of intrusive sheets or dykes. However, closer examination reveals that they are not boudins, but are instead primary intrusive structures. A detailed study was performed on a swarm of pegmatite intrusions at Cap de Creus. Layering is often continuous between beads, and, in some cases, individual beads exhibit a very irregular shape. These observations are shown to be incompatible with an origin by boudinage. Analogue experiments were used to test…
The transition from single layer to foliation boudinage: A dynamic modelling approach
2012
Abstract Foliation boudinage is a deflection of foliation in the vicinity of a central discontinuity in foliated rocks, mostly filled with vein material. It shows evidence for brittle deformation and void-opening during ductile flow. We used a two-dimensional visco-elastic spring model based on a discrete element approach to study the dynamic development of foliation boudinage and the behaviour of anisotropic visco-elastic material deformed under pure shear conditions. The anisotropies are set by defining rheological heterogeneities in the models with (1) a single layer in a weaker matrix; (2) multi-layers with different elastic properties and (3) random-distributed “micas”, rows of horizon…
P-T path development derived from shearband boudin microstructure
2016
This work focuses on the development of a regional P-T-path from the Malpica-Lamego Ductile Shear Zone, NW Portugal, based on the microstructures of shearband boudins evolved during progressive simple shear. The combination of microstructural analysis, fluid inclusion studies, crystallographic pre- ffered orientation and fractal geometry analyses, allows to link several stages in the internal evolution of the boudin to regional P-T conditions. The boudinage process is initiated under differential stress after the original layer achieved sufficient viscosity contrast relative to the surrounding matrix. Two main transformations occur simultaneously: i) change in the external shape with contin…
Boudinage classification: end-member boudin types and modified boudin structures
2004
In monoclinic shear zones, there are only three ways a layer can be boudinaged, leading to three kinematic classes of boudinage. These are (1) symmetrically without slip on the inter-boudin surface (no-slip boudinage), and two classes with asymmetrical slip on the inter-boudin surface: slip being either (2) synthetic (S-slip boudinage) or (3) antithetic (A-slip boudinage) with respect to bulk shear sense. In S-slip boudinage, the boudins rotate antithetically, and in antithetic slip boudinage they rotate synthetically with respect to shear sense. We have investigated the geometry of 2100 natural boudins from a wide variety of geological contexts worldwide. Five end-member boudin block geome…
Numerical modelling of asymmetric boudinage
2002
Abstract Asymmetric boudinage structures are commonly used as shear sense indicators but their development is incompletely understood. This paper describes the influence of initial shape and kinematic parameters on the evolution of boudin trains using a numerical approach based on the finite difference code FLAC. Boudin trains are simulated as a series of competent objects embedded in a soft matrix subjected to general monoclinic ductile flow. Deformation of boudin trains includes heterogeneous stretching, rotation of boudins and offset along the neck regions. The sense of relative boudin offset is mainly influenced by the initial orientation of the interboudin plane in the boudinaging laye…
Asymmetric boudins as shear sense indicators—an assessment from field data
2003
Asymmetric boudins are potential but problematic shear sense indicators. They can be divided into two groups, with slip on the inter-boudin surface that is either synthetic (S-slip) or antithetic (A-slip) with respect to bulk shear sense. Since both groups have mirror-image symmetry, independent geometric criteria are needed to distinguish them if they are to be used as shear sense indicators. Investigation of asymmetric boudins in trains parallel to the main foliation from the Kaoko Belt in Namibia and elsewhere indicate that the geometry of both groups is in most cases different. Shearband boudins (formed by S-slip) have a long, curved lenticular shape and large relative displacement and …