2. Mineralogy – Petrology – Geochemistry - SWISS GEOSCIENCE ...

22
Symposium 1: Structural Geology, Tectonics and Geodynamics
1.8
3-D numerical modelling of oceanic spreading initiation
Gerya Taras 1
1 Institut of Geophysics, ETH-Zurich, Sonneggstrasse 5, CH-8057 Zurich (taras.gerya@erdw.ethz.ch)
Mid-ocean ridges sectioned by transform faults represent prominent surface expressions of plate tectonics. A fundamental
problem of plate tectonics is how this orthogonal pattern has formed and why it is maintained. Gross-scale geometry of
mid-ocean ridges are often inherited from respective rifted margins. Indeed, transform faults seem to nucleate after the
beginning of the oceanic spreading and can spontaneously form at a single straight ridge.
Due to the limited availability of data, detailed interpretations of nucleation and evolution of the orthogonal spreading
pattern are difficult and controversial. Analogue and numerical modeling have to be additionally employed. Two main
groups of analogue models were implemented (Gerya, 2011 and references therein): (i) themomechanical freezing wax
models with accreting and cooling plates and (ii) mechanical models with viscous mantle and brittle lithosphere. The
freezing wax models reproduced characteristic orthogonal ridge - transform fault patterns but often produced open spreading
centers with exposed liquid wax, which is dissimilar to nature. On the other hand, in the mechanical models, new
lithosphere is not accreted in spreading centers, which is conflicting with oceanic spreading. Numerical models of transform
fault nucleation (Gerya, 2011 and references therein) mostly focused on short-term plate fragmentation patterns and
strain reached in these numerical experiments was too small to test the long-term evolution of transform faults. Recent
large-strain numerical experiments (Gerya, 2010) studied spontaneous nucleation of transform faults at a pre-existing
single straight ridge but the initiation and maturation of the orthogonal pattern after continental plate breakup remained
unaddressed.
I present new 3-D numerical thermomechanical model of oceanic spreading initiation suggesting that orientation and
geometry of transform faults and spreading centers changes with time as the result of accommodation of new oceanic
crust growth. The resulting orthogonal ridge-transform system is established on a timescale of millions of years from an
arbitrary plate breakup pattern. By its fundamental physical origin, this system is a crustal growth pattern and not a
plate fragmentation pattern. In particular, the characteristic extension-parallel orientation of oceanic transform faults is
a steady state orientation of a weak strike-slip fault embedded in between simultaneously growing offset crustal segments.
REFERENCES
Gerya, T., 2010. Dynamical instability produces transform faults at mid-ocean ridges. Science, 329, 1047-1050.
Gerya, T. (2011) Origin and models of oceanic transform faults. Tectonophysics, doi: 10.1016/j.tecto.2011.07.006
1.9
Linking titanium-in-quartz thermometry and quartz microstructures:
strong evidence of continued vein formation during strain localization
Härtel Mike 1 , Herwegh Marco 1
1 Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern
Detailed microstructural examinations on mylonites from the Simplon Fault Zone (SFZ) in southern Switzerland revealed
the transition of different recrystallization processes in quartz veins of the footwall, ranging from Grain Boundary
Migration Recrystallization (GBM) over Subgrain Rotation Recrystallization (SGR) to Bulging Nucleation (BLG), from the
rim towards the centre of the shear zone. GBM-microstructures can be recognized at distances of more than 4400 m from
the centre and at around 1200 m distance they start to be progressively overprinted by SGR (in the northern part), producing
a kilometer-wide mylonite zone comprising ribbon textures in the quartz veins. Near the centre of the SFZ, coremantle-structures
develop and bands of bulging grains transect older quartz ribbons. This microstructural sequence displays
the cooling-related localization of strain from initial temperatures over 600 °C down to temperatures lower than 350
°C with respect to their dynamically recrystallized grain sizes. However, the age relations between all the quartz veins
remained unknown and some microstructures do not fit into this pattern. The application of Ti-in-quartz geothermometry
revealed highest temperatures of 535 ± 17 °C as formation temperatures of GBM-samples, which is lower than the ex-
Swiss Geoscience Meeting 2011
Platform Geosciences, Swiss Academy of Science, SCNAT