Open Session - SWISS GEOSCIENCE MEETINGs

32
Symposium 1: Structural Geology, Tectonics and Geodynamics
developed on the basis of simple iterations. By taking iteration on several levels of coarser grids, the residual of the equations
is simultaneously relaxed over a wide range of wavelength. This usually speeds up iterative solvers to a large extent.
Additionally, the solution speed scales only linearly with grid resolution and therefore is especially attractive to use in high
resolution simulations.
Here, we compare a number of iterative and direct solver techniques (including multigrid) for solving the 2D Stokes equations
with a finite-element approach. In a first step, we use an iterative Gauss-Seidel smoother on only one grid level. As a
second step we employ this smoother in a multigrid solver. Performance tests are employed to evaluate the effectiveness of
the various techniques for a number of geodynamics test scenarios.
REFERENCES
May, D. A. & Moresi, L. 2008: Preconditioned iterative methods for Stokes flow problems arising in computational
geodynamics, PEPI (in press).
1.23
Change of deformation mechanism of quartz with increasing strain in
mylonites
Kilian Rüdiger, Heilbronner Renee, Stünitz Holger*
Geological Institute, Basel University, Switzerland, Bernoullistr. 32, 4056 Basel, ruediger.kilian@unibas.ch
*Geological Institute, University Tromsø, Norway
The microstructural and textural evolution of a small scale metagranodiorite shear zone from the Gran Paradiso nappe
(Western Alps) is studied in detail.
The shear zone formed at lowermost amphibolite facies conditions. Quartz deformed as the mechanically stronger phase in
a fine grained matrix of biotite and decomposed plagioclase grains with a grain size of approx. 5-15 µm. In the coarse aggregates,
quartz textures are measured using the CIP method (Panozzo Heilbronner & Pauli, 1993). EBSD is used for the analysis
of the fine grained phase mixtures.
In the core of the shear zones two microstructurally different parts can be defined.
In the mylonitic outer part of the shear zone, layers of recrystallized quartz and matrix are parallel to the shear zone
boundary. Quartz deforms by dislocation creep. Recovery is dominated by grain boundary migration recrystallization and
minor subgrain rotation recrystallization. Quartz c-axis crystallographic preferred orientations (CPO) typically show synthetically
rotated peripheral maxima and - occasionally - weak single girdles coinciding with the activity of basal and only
minor prism slip (Fig.1a).
Calculations using estimates of the shear strain accommodated in the quartz aggregates, recrystallized grain size piezometry
and quartz flow laws yield strain rates 5 to 10 times higher in the matrix than in quartz aggregates.
The ultramylonitic central part of the shear zone shows compositional layering consisting of feldspar layers, relict recrystallized
quartz aggregates and phase mixtures with single quartz grains dispersed in the matrix. Quartz aggregates are boudinaged
at the grain scale with primarily K-feldspar precipitating between torn apart grains. No fractures penetrating quartz
grains were observed. Large quartz grains still reflect the size of the quartz aggregates of the mylonitic part they originate
from but do not show subgrain structures. Small quartz grains below 10 µm are evenly dispersed in the ultramylonite and
spatially unrelated to the disrupted aggregates. The quartz CPO is generally weakened compared to the mylonitic part of the
shear zone (Fig.1b). The larger grains reflect the geometry of the original CPO in the mylonite with peripheral, inclined c-axis
orientations. The smallest grains (< 3µm) show a very different CPO with c-axes close to the centre of the polefigure (Fig.1c).
Intermediate grain sizes have a close to random orientation.
The disruption process and the randomization of the CPO is interpreted to be caused by dissolution-precipitation accommodated
granular flow. There is no evidence for fracturing or dynamic recrystallization driving further grain size reduction of
the quartz in the ultramylonite. Dissolution-precipitation and growth of quartz can account for its spatial distribution in
the ultramylonite. The peculiar CPO of the smallest grains can not be understood in terms of normal quartz growth but
could be explained by a preferred orientation of nuclei or faster growth perpendicular to the c-axis in the imposed stress
field.