Open Session - SWISS GEOSCIENCE MEETINGs

Herwegh, M., Berger, A., 2004: Deformation mechanisms in second-phase affected microstructures and their energy balance.
Journal of Structural Geology 26: 1483-1498
Herwegh, M., Berger, A. and Ebert, A., 2005: Grain coarsening maps: A new tool to predict microfabric evolution of
polymineralic rocks, Geology 33: 801-804
Jung, H. and Karato, S.,2001, Water induced fabric transitions in olivine. Science 293, 1460-1463
Michibayashi, K. and Mainprice, D., 2004: The role of pre-existing Mechanical anisotropy on shear zone development within
oceanic mantle lithosphere: and example from the Oman ophiolite. Journal of Petrology 45: 405-414
Nicolas, A., Boudier, F., Ildefons, B., Ball, E., 2000: Accretion of Oman and United Arab Emirates ophiolite-Discussion of a
new structural map. Marine Geophysical Researches 21: 147-179.7
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Influence of rheological weakening by fluids and melts on subduction at
an active margin: Numerical modelling
Löw Felicitas Irena*, Gerya Taras V*
*Institut für Geophysik, ETH Zürich, Schafmattstrasse 30, CH-8093 Zürich (irenaloew@web.de)
The dynamics of subduction under an active margin is analyzed by using a 2D coupled geochemical-petrological-thermomechanical
numerical model of an oceanic-continental subduction process. This model includes spontaneous slab retreat and
bending, dehydration of the subducted crust, aqueous fluid transport, partial melting of both crustal and mantle rocks and
melt extraction processes resulting in magmatic arc crust growth. The innovation in this model is the consideration of rheological
weakening effects by fluids and melts.
The numerical experiments revealed that rheological weakening by fluids and melts controls the mode of subduction. By
varying the weakening effects, five different regimes can be induced showing the following characteristics: (1) formation of
a backarc basin, occurance of plumes that (2) ascend and intrude into the continental crust or (3) extend horizontally beneath
the continental plate (underplating) or (4) remain above the subducting plate and (5) neither formation of backarc basins
nor occurance of plumes. The transition between the different tectonic regimes of subduction at an active margin is caused
by the concurrence of rheological weakening by (1) aqueous fluids percolating from the subducting slab into the mantle
wedge and (2) melts propagating from the mantle wedge toward the surface.
The aqueous fluids mainly affect the forearc region: strong fluid-related weakening of rocks atop the slab promotes the
stacking of sediments and the development of an accretion wedge. Since the material in the subduction channel is weak the
coupling of the plates in these regimes of subduction is low facilitating extension in the subduction channel. In contrast, a
small weakening effect by fluids results in strong coupling of the plates inducing collision-like subduction and subduction
erosion. Lithospheric thickening and large sedimentary plumes are the consequence.
Extracted melts rheologically weaken the lithosphere below the arc. Strong rheological weakening by melts in combination
with low coupling of the plates allows for necking of the continental lithosphere and leads to the formation of a backarc
basin. In the case of sedimentary plumes, weakening of the continental lithosphere by extracted melts generates a weak
channel in which the positive-buoyant plumes may ascend. That way silicic intrusions are emplaced in the continental crust.
If the continental lithosphere is not sufficiently weakened by melts, the plumes cannot ascend but may extend horizontally
leading to underplating.
In general, the degree of coupling of the plates is of great importance as it influences the topographic evolution and the
crust production rate of subduction zones. Strong coupling leads to an extreme topography whereas low coupling enhances
the production of new crust. The fact that the coupling is strongly influenced by fluid- and melt-related weakening emphasizes
the significance of fluids and melt in subduction.
Symposium 1: Structural Geology, Tectonics and Geodynamics
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