Foliation boudinage
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Abstract
In this thesis foliation boudinage and related structures have been studied based on
field observations and numerical modeling. Foliation boudinage occurs in foliated rocks
independent of lithology contrast. The developing structures are called ‘Foliation boudinage
structures (FBSs)’ and show evidence for both ductile and brittle deformation. They are
recognized in rocks by perturbations in monotonous foliation adjacent to a central
discontinuity, mostly filled with vein material.
Foliation boudinage structures have been studied
in the Çine Massif in SW-Turkey and
the Furka Pass-Urseren Zone in central Switzerland. Four common types have been
distinguished in the field, named after vein geometries in their boudin necks in sections
normal to the boudin axis: lozenge-, crescent-, X- and double crescent- type FBSs. Lozengetype
FBSs are symmetric and characterized by lozenge-shaped veins in their boudin neck with
two cusps facing opposite sides. A symmetrical pair of flanking folds occurs on the two sides
of the vein. Crescent-type FBSs are asymmetric with a single smoothly curved vein in the
boudin neck, with vein contacts facing to one side. X- and double crescent- type FBSs are
asymmetric. The geometry of the neck veins resembles that of cuspate-lobate structures. The
geometry of flanking structures is related to the shape of the veins. The veins are mostly filled
with massive quartz in large single crystals, commonly associated with tourmaline, feldspar
and biotite and in some cases with chlorite.
The dominance of large facetted single quartz
crystals and spherulitic chlorite in the veins suggest that the minerals grew into open fluidfilled
space. FLAC experiments show that fracture propagation during ductile deformation
strongly influences the geometry of developing veins. The cusps of the veins are better
developed in the case of propagating fractures. The shape of the boudin neck veins in foliation
boudinage depends on the initial orientation and shape of the fracture, the propagation
behaviour of the fracture, the geometry of bulk flow, and the stage at which mineral filling
takes place.
A two dimensional discrete element model was used to study the progressive
development of foliation boudinage structures and the behavior of visco-elastic material
deformed under pure shear conditions. Discrete elements are defined by particles that are
connected by visco-elastic springs. Springs can break. A number of simulations was
Abstract vii
performed to investigate the
effect of material properties (Young’s modulus, viscosity and
breaking strength) and anisotropy on the developing structures. The models show the
development of boudinage in single layers, multilayers and in anisotropic materials with
random mica distribution. During progressive deformation different types of fractures develop
from mode I, mode II to the combination of both. Voids develop along extension fractures, at
intersections of conjugate shear fractures and in small pull-apart structures along shear
fractures. These patterns look similar to the natural examples. Fractures are more localized in
the models where the elastic constants are low and the competence contrast is high between
the layers. They propagate through layers where the constants are high and the competence
contrast is relatively low. Flow localize around these fractures and voids. The patterns similar
to symmetric boudinage structures and extensional neck veins (e.g. lozenge type) more
commonly develop
in the models with lower elastic constants and anisotropy. The patterns
similar to asymmetric foliation boudinage structures (e.g. X-type) develop associated with
shear fractures in the models where elastic constants and anisotropy of the materials are
relatively high. In these models boudin neck veins form commonly at pull-aparts along the
shear fractures and at the intersection of fractures.