Open Session - SWISS GEOSCIENCE MEETINGs

0
Symposium 1: Structural Geology, Tectonics and Geodynamics
1.30
Evolution of a mantle shear zone and the influence of second phases,
Hilti massif, Oman.
Linckens Jolien*, Herwegh Marco*, Müntener Othmar**, Mercolli, Ivan*
*Institute of Geological Sciences, Baltzerstrasse 1-3, CH-3012 Bern (linckens@unibe.ch)
**Institute of Geology and Paleontology, BFSH-2, CH-1015 Lausanne
Localization of deformation in shear zones is a common feature in the Earth’s crust and upper mantle (e.g., thrusts, strike
slip faults). The evolution of such high strain zones is often long lasting incorporating variations in physical (e.g. P, T) and
chemical conditions with time. To learn more about the distribution and evolution of deformation in mantle rocks as a
function of variable physico-chemical conditions, detailed mapping and microstructural investigations of a large-scale peridotite
shear zone in the Hilti massif (Oman ophiolite) was performed.
Detailed mapping of the shear zone shows that the largest part of the shear zone experienced moderate deformation, whereas
high deformation is localized in meso-scale shear zones. In some parts of these meso-scale shear zones the deformation
is further localized in ultramylonitic bands.
In terms of tectonites, four different types can be discriminated: (i) astenospheric flow related tectonites (outside of the shear
zone), (ii) weakly-moderately deformed protomylonites, (iii) mylonites and (iv) ultramylonites.
In the area investigated, both dunites and harzburgites occur. Particularly in case of the harzburgites, both mineralogy and
modal compositions can vary. In these rocks, secondary minerals like (opx, cpx, spinel) can affect the grain size of the predominating
olivine in the matrix, which can have an influence on the deformation mechanisms (e.g. diffusion vs. dislocation
creep). In order to test the effect of such secondary phases on microstructure, deformation mechanisms and strain localization
behaviour, microstructures of the different tectonites were quantitatively investigated. For this purpose, grain size
(dp) and volume fractions (fp) of olivine and the second phases (opx, cpx and spinel) as well as CPOs and deformation temperatures
were analyzed. In addition, geothermometry, using opx (Ca and Al-Cr in opx) has been performed to gain information
on the deformation temperature.
Similar to previous studies performed on calcite mylonites, each of the different mantle tectonites can be subdivided into
two different microfabric types: (a) second phase controlled microstructures, where the olivine grain size increases with
increasing Zener parameter (Z = dp/fp, second phase grain size/second phase volume fraction e.g. see Herwegh and Berger,
2004). (b) Recrystallization controlled microstructures, where the olivine grain size shows no dependence on the second
phase content and remains constant with increasing Z.
The ultramylonites show weak CPOs at all Zener parameters, indicating that diffusion creep was the mean mechanism for
deformation. For the mylonites, the second phase controlled microstructures show weaker CPOs, indicating a decrease in
intracrystalline plasticity with increasing second phase content. Furthermore, the CPOs suggests deformation of both monoand
polymineralic layers under the presence of water (Jung and Karato, 2001).
The comparison of the Zener trends for the different tectonites gives the following results:
(i) Three different meso-scale shear zone mylonites give the same Zener trend and yield the same deformation temperatures
(≈800°C) indicating that these different shear zones were formed under the same deformation conditions (e.g. strain rate,
stress and temperature). It can therefore be postulated, that the variations in second phase type and content was no major
parameter for strain localization. In contrast, the microstructures adapt to the local Zener parameter, enabling specific
combination of dislocation creep and diffusion creep to maintain an overall homogeneous deformation within the mesoscale
shear zones.
(ii) The Zener trend of the ultramylonites is shifted to lower olivine grain sizes compared to the mylonites and the geothermometry
results show that they were formed under lower temperatures (≈690°C). Therefore the shift in the Zener trend is
most likely caused by the lower deformation temperatures and is eventually accompanied by a higher strain rate.
(iii) The Zener trend of the weakly deformed protomylonites show, compared to that of the mylonites, larger olivine grain
sizes but the same deformation temperatures. Indicating that the shift in the trend is only caused by the lower strain rate
of the protomylonite compared to that of the mylonite.
(iv) The asthenosphere flow fabrics show a shift to even larger olivine grain sizes. These fabrics formed at higher temperatures
(≈1150°C) and the shift can be explained by a combination of higher deformation temperature probably accompanied
by lower strain rates.
The Zener trend is different for each tectonite and can be explained by variations in both strain rate and deformation temperature.
From a geodynamic point of view, Zener trends can therefore be applied as a tool to determine the deformation
conditions and their variations in space and time in case of mantle rocks.
REFERENCES
Boudier, F., Ceuleneer, G., Nicolas, A., 1988: Shear zones, thrusts and related magmatism in the Oman ophiolite: initiation
of thrusting on an oceanic ridge, Tectonophysics 151: 275-296.