Open Session - SWISS GEOSCIENCE MEETINGs
Open Session - SWISS GEOSCIENCE MEETINGs
Open Session - SWISS GEOSCIENCE MEETINGs
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Journal of Structural Geology 26: 1483-1498<br />
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1.31<br />
Influence of rheological weakening by fluids and melts on subduction at<br />
an active margin: Numerical modelling<br />
Löw Felicitas Irena*, Gerya Taras V*<br />
*Institut für Geophysik, ETH Zürich, Schafmattstrasse 30, CH-8093 Zürich (irenaloew@web.de)<br />
The dynamics of subduction under an active margin is analyzed by using a 2D coupled geochemical-petrological-thermomechanical<br />
numerical model of an oceanic-continental subduction process. This model includes spontaneous slab retreat and<br />
bending, dehydration of the subducted crust, aqueous fluid transport, partial melting of both crustal and mantle rocks and<br />
melt extraction processes resulting in magmatic arc crust growth. The innovation in this model is the consideration of rheological<br />
weakening effects by fluids and melts.<br />
The numerical experiments revealed that rheological weakening by fluids and melts controls the mode of subduction. By<br />
varying the weakening effects, five different regimes can be induced showing the following characteristics: (1) formation of<br />
a backarc basin, occurance of plumes that (2) ascend and intrude into the continental crust or (3) extend horizontally beneath<br />
the continental plate (underplating) or (4) remain above the subducting plate and (5) neither formation of backarc basins<br />
nor occurance of plumes. The transition between the different tectonic regimes of subduction at an active margin is caused<br />
by the concurrence of rheological weakening by (1) aqueous fluids percolating from the subducting slab into the mantle<br />
wedge and (2) melts propagating from the mantle wedge toward the surface.<br />
The aqueous fluids mainly affect the forearc region: strong fluid-related weakening of rocks atop the slab promotes the<br />
stacking of sediments and the development of an accretion wedge. Since the material in the subduction channel is weak the<br />
coupling of the plates in these regimes of subduction is low facilitating extension in the subduction channel. In contrast, a<br />
small weakening effect by fluids results in strong coupling of the plates inducing collision-like subduction and subduction<br />
erosion. Lithospheric thickening and large sedimentary plumes are the consequence.<br />
Extracted melts rheologically weaken the lithosphere below the arc. Strong rheological weakening by melts in combination<br />
with low coupling of the plates allows for necking of the continental lithosphere and leads to the formation of a backarc<br />
basin. In the case of sedimentary plumes, weakening of the continental lithosphere by extracted melts generates a weak<br />
channel in which the positive-buoyant plumes may ascend. That way silicic intrusions are emplaced in the continental crust.<br />
If the continental lithosphere is not sufficiently weakened by melts, the plumes cannot ascend but may extend horizontally<br />
leading to underplating.<br />
In general, the degree of coupling of the plates is of great importance as it influences the topographic evolution and the<br />
crust production rate of subduction zones. Strong coupling leads to an extreme topography whereas low coupling enhances<br />
the production of new crust. The fact that the coupling is strongly influenced by fluid- and melt-related weakening emphasizes<br />
the significance of fluids and melt in subduction.<br />
Symposium 1: Structural Geology, Tectonics and Geodynamics<br />
1