25.01.2015 Views

IASPEI - Picture Gallery

IASPEI - Picture Gallery

IASPEI - Picture Gallery

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy<br />

(S) - <strong>IASPEI</strong> - International Association of Seismology and Physics of the Earth's<br />

Interior<br />

JSS012 Poster presentation 2215<br />

Instantaneous mantle flow induced by subduction of a freely sinking slab<br />

Dr. Claudia Piromallo<br />

Seismology and Tectonophysics INGV-Roma <strong>IASPEI</strong><br />

Thorsten W. Becker, Francesca Funiciello, Claudio Faccenna<br />

We conduct three-dimensional subduction experiments by a finite element approach to study flow<br />

around slabs, which are prescribed based on a transient stage of upper mantle subduction from a<br />

laboratory model. Instantaneous velocity field solutions are examined for a slab that sinks freely into the<br />

mantle, focusing on the toroidal vs. poloidal components as a function of boundary conditions (BCs),<br />

plate width, and viscosity contrast between slab and mantle. The 3-D approach shows that the toroidal<br />

flow plays a key role in affecting the circulation geometry in the vertical plane, compared to the 2-D<br />

case. In particular, the material resumed at surface in the back-arc wedge by the return flow cell below<br />

the slab tip is minimal with respect to 2-D models, in agreement with laboratory models. Furthermore,<br />

we find that circulation is characterized by a non-negligible upward flow component close to slab sides,<br />

that could have important implications to interpret local tectonic structure at slab edges. We show that<br />

BCs affect the magnitude as well as the pattern of the flow velocity field. In particular, in proximity of<br />

the slab the flow field is similar for no- and free-slip BCs, while strong variations exist elsewhere.<br />

Moreover, in our models of a transient subduction stage, the characteristic spatial length-scale<br />

influencing the flow pattern is given by the box height. By modelling different viscosity contrasts<br />

between slab and mantle (), we find that significant return flow around edges can only be obtained for<br />

stiff slabs and that the strength of the Toroidal/Poloidal Ratio increases with , nearly independent of<br />

slab width. We observe that for >=103 the toroidal flow components make up about 6070% of the<br />

poloidal ones, while we estimate about 4050% for lower viscosity contrasts (~102). We note in our<br />

models that the toroidal term peaks for slab/mantle viscosity ratios tmax=102, independent of slab w.<br />

This trend is found not only for transient but also for steady-state, rollback subduction. Estimates for<br />

effective viscosity contrasts in nature are comparable to, or somewhat higher than tmax.<br />

Keywords: subduction, toroidal flow, mantle

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!