CRENULATION CLEAVAGE DEVELOPMENT AND THE INFLUENCE OF ROCK MICROSTRUCTURE ON CRUSTAL SEISMIC ANISOTROPY

This thesis investigates the cause of mass transfer during crenulation cleavage formation
and the role that elastically anisotropic minerals and fabric development play in promoting
seismic anisotropy in the crust. Crenulation cleavage is the most common fabric in multiplydeformed,
phyllosilicate-rich metamorphic rocks. During its formation the originally planar fabric
gets crenulated, eventually leading to the differentiation of quartz- and feldspar-rich regions (QFdomains)
in the crenulation hinges, and phyllosilicate-rich regions (P-domains) in the crenulation
limbs. This differentiation is driven by the dissolution of quartz and feldspar in the P-domains,
and the precipitation of those minerals in the QF-domains.
Finite element models are created to investigate how the elastic interactions of quartz and
muscovite minerals affect the grain-scale stress and strain distributions at different stages of
crenulation cleavage development. Gradients in mean stress and volumetric strain develop
between the limbs and hinges of the microfolds during fabric formation and are sufficient to drive
mass transfer between the two domains.
To study the influence of different microstructural variables on seismic wave speed
anisotropy, simplified muscovite-quartz models are created with varying amounts of muscovite,
varying quartz and muscovite orientations, and varying spatial distributions. A new computer
program is used to calculate bulk stiffness tensors and seismic wave speeds. Muscovite’s
abundance and preferred orientation have significant influence of seismic wave speed anisotropy
due to the extreme anisotropic elasticity of the mineral.
The same method is employed to study the seismic behavior of rocks containing different
stages of crenulation cleavage. Mineral orientation maps of rock samples were created, using
electron backscatter diffraction, and used as input files for the computer program. Schists with a
planar foliation are highly elastically anisotropic, but a rock with a well developed crenulation
cleavage is much less anisotropic because the orientations of the micas cancel each other
effectively out. These results imply that regions with larger scale crustal structures, such as folds
and shear-zones, can be much more muted in their seismic signal than the schistose samples that
make up those structures.