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

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Symposium 1: Structural Geology, Tectonics and Geodynamics
Our first order model results are, therefore, in agreement with (1) geometrical constrains for the shape of the Morcles
nappe, (2) kinematic constrains for the finite shear strain distribution within the nappe and (3) physical constrains for
rheology (flow law) and temperature.
We use the best fit parameters to retro-deform the reference line which provides estimates for the horizontal movement
of the Morcles nappe between 10 and 15 km. We further apply an elaborated 1D shear zone model in which the deformation
is driven by a horizontal pressure gradient. The results indicate that for standard calcite flow laws and temperature
profiles constrained from field data the shear zone thickness is of the order of a few kilometers in agreement with the
size of the Morcles fold nappe. Horizontal pressure gradients in the order of 1 to 10 MPa/km are necessary to yield horizontal
velocities in the order of centimeters per year.
Our simple thermo-mechanical model provides important results for a better understanding of the dynamics of fold nappe
formation and yields the basis for further, more elaborated, 2D models for the formation of the Morcles nappe.
REFERENCES
Schmid, S., Boland, J. & Paterson, M. 1977: Superplastic flow in fine-grained limestone. Tectonophysics 43(3–4), 257–291.
Schmid, S., Paterson, M. & Boland, J. 1980: High temperature flow and dynamic recrystallization in Carrara marble.
Tectonophysics 65, 245–280.
Dietrich, D. & Casey, M. 1989. A new tectonic model for the Helvetic nappes. In: Alpine Tectonics (edited by Coward, M. P.,
Dietrich, D. & Park, R. G.). Geological Society special publication, 47-63.
Epard, J. L. & Escher, A. 1996. Transition from basement to cover: A geometric model. Journal of Structural Geology,
18(5), 533-548.
Kirschner, D. L., Sharp, Z. D. & Masson, H. 1995. Oxygen-isotope thermometry of quartz-calcite veins - unraveling the
thermal-tectonic history of the subgreenschist faces Morcles nappe (Swiss-Alps). Geological Society of America
Bulletin, 107(10), 1145-1156.
1.4
The role of seamount subduction in the evolution of convergent margins:
constraints from the Paleo-Tethys suture zone in Iran
Buchs David 1 , Bagheri Sasan 2 , Martin Laure 3 , Hermann Joerg 3 & Arculus Richard 3
1 IFM-GEOMAR, Kiel, Germany (dbuchs@ifm-geomar.de)
2 University of Sistan and Baluchestan, Iran
3 Research School of Earth Sciences, Australian National University
Seamounts of volcanic origins are very abundant in modern oceans and may play a significant role in the evolution of
convergent margins. Integrated satellite and ship-track bathymetry data show that >200’000 seamounts occur today in the
world, covering >12% of the ocean floor (Hillier & Watts, 2007). Seamounts are morphologically and compositionally distinct
from the “normal” ocean floor, and geophysical and geochemical observations along modern convergent margins
show that seamount subduction is an important process, which can notably trigger: (1) mass-wasting of the forearc wedge
and subduction erosion by abrasion of the upper plate (e.g. Ranero & von Huene, 2000); and (2) compositional changes of
the arc magmatism (e.g. Hoernle et al., 2008). In addition, field observations indicate that accreted seamount material is
abundant along some margins (e.g. Buchs et al., 2011), indicating that seamounts can contribute to the construction of
convergent margins at shallow depth. However, processes of seamount subduction or accretion are still poorly constrained
in space and time, and poorly documented in deeper levels of the subduction zone. We have investigated a Variscan accretionary
complex exposed along the Paleo-Tethys suture zone in Iran, which provides new constraints on the role of
seamount subduction in the long-term evolution of convergent margins.
The studied accretionary complex belongs to the Anarak-Jandaq composite terrane, which formed along the northern
Paleo-Tethyan margin between the Devonian and Triassic and was exhumed during the closure of the Paleo-Tethys in
Triassic times (Bagheri & Stampfli, 2008). Combined field observations, satellite multispectral data and geochemical analyses
allow recognition of four types of lithologic assemblage in the complex: (1) meta-igneous rocks and marble that represent
accreted fragments of seamounts; (2) arc-derived meta-igneous rocks; (3) meta-greywacke considered to represent
accreted forearc or trench-fill sediment; and (4) serpentinized peridotite bodies of undefined origin, which locally contain
altered volcanic intrusives and pillow lavas in blueschist facies conditions. These lithologies occur as kilometer-sized slices
preserved in low, greenschist and blueschist facies conditions. Most of the fragments of seamounts and arc-derived metaigneous
rocks are embedded within a matrix of accreted meta-greywacke. This suggests that subducting seamounts were
Swiss Geoscience Meeting 2011
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