HR: 1400h
AN: T43A-02    [Abstracts]
TI: Instantaneous mantle flow induced by subduction of a freely sinking slab
AU: Piromallo, C
EM: piromallo@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
AU: * Becker, T W
EM: twb@usc.edu
AF: University of Southern California, Department of Earth Sciences MC0740 3651 Trousdale Pkwy, Los Angeles, CA 90089-0740, United States
AU: Funiciello, F
EM: ffunicie@uniroma3.it
AF: Dipartimento di Scienze Geologiche Universita` degli Studi Roma Tre, Rome, Italy
AU: Faccenna, C
EM: c.faccenna@uniroma3.it
AF: Dipartimento di Scienze Geologiche Universita` degli Studi Roma Tre, Rome, Italy
AB: We conduct three-dimensional (3-D) subduction experiments by a finite element approach to study flow around slabs which are prescribed based on a transient subduction stage from a laboratory model. Instantaneous velocity fields are examined for a slab that sinks freely into the mantle, focusing on the toroidal vs. poloidal components as a function of boundary conditions (BCs), plate width, and viscosity contrast between slab and mantle. Results show that the toroidal flow is important for the circulation geometries in the vertical plane. In particular, the material resumed at surface in the back-arc wedge by the return flow cell below the slab tip is minimal with respect to 2-D models, in agreement with laboratory models. Furthermore, we find that circulation is characterized by an upward flow component close to slab sides that could be important for local tectonic structures at slab edges. We show that BCs affect the magnitude and pattern of velocities. In particular, in proximity of the slab the flow field is similar for no- and free-slip BCs, while strong variations exist elsewhere. Moreover, the characteristic spatial length-scale is given by the box height. By modeling different viscosity contrasts between slab and mantle (η'), we find that significant return flow around edges can only be obtained for stiff slabs and that the strength of the toroidal/poloidal Ratio increases with η', nearly independent of slab width. For η' ≥ 10³, the toroidal is ~60-70% of the poloidal component, while we estimate about 40-50% for lower viscosity contrasts. In our models, the toroidal term peaks for slab/mantle viscosity ratios η'_max~ 10². This trend is found not only for transient but also for steady-state, rollback subduction. Estimates for effective viscosity contrasts in nature are comparable to, or somewhat higher than η_max.
DE: 8150 Plate boundary: general (3040)
DE: 8155 Plate motions: general (3040)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Joint Assembly