Implications for backarc basin dynamics including a low viscosity wedge in simple 2 and 2.5D numerical flow models
Flow in the asthenospheric mantle wedge associated with backarc basins is often assumed to be driven by motions of the overlying and adjacent plates, suggesting a roughly 2D corner flow pattern with mantle flow rates comparable to plate velocities. Geodynamic models have a notoriously difficult time capturing and explaining some first order observations such as intermediate dips of subducting plates, and initiating and sustaining backarc spreading. In addition, arc-parallel shear wave splitting and geochemical observations imply along- strike flow in the Lau, Mariana, and other backarc basins, presenting a further challenge to geodynamic models of plate-driven subduction systems, as plate-driven flow models predict arc-perpendicular fast direction with A-type mineral texturing. Although B-type mineral texturing could rotate the fast directions 90 degrees relative to A-type and likely occurs in the forearc mantle, such texturing is unlikely in the asthenosphere wedge with its higher temperatures and weaker rheology, requiring a different explanation. Some of these outstanding issues in backarc dynamics may be addressed by considering a low viscosity region in the upper corner of the wedge. Recent work demonstrates that the existence and growth of a low viscosity wedge can control slab dip [Manea, 2006]. Here, I examine the effect of a low-viscosity wedge on asthenospheric flow in the along arc dimension and possible controls on the localization of backarc spreading. Along-arc flow rates through a low viscosity channel may be significantly faster than plate rates. Constraining along-arc flow velocity in the Lau subarc mantle by tracking mineral texturing development within model flow fields with a low viscosity wedge, show rates upwards of 50 cm/yr. Models without a low-viscosity channel beneath the arc fail to simultaneously satisfy the shear wave splitting and geochemical constraints. A low viscosity wedge also modifies the dynamic stress field within the mantle wedge, resulting in horizontal tensional stresses at the base of the overlying plate directly over the low viscosity region. These results may have implications on the localization of rifting in the backarc and the development of backarc spreading. In addition to rates of enhancing mantle flow to rates substantially faster than plate rates which may have profound implications for plate boundary mantle convection and mixing of mantle reservoirs, low viscosity regions in the wedge may contribute to the deformation and development of the basins themselves.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 3001 Back-arc basin processes;
- 8120 Dynamics of lithosphere and mantle: general (1213)