Membrane deformation rate and geometry of subducting slabs

Membrane deformation rate and geometry of subducting slabs
Chiao, L.-Y.
University of Washington, Seattle
157 p.
Geophysical Institute Library: THESIS NOT UAF
The subduction process forces the oceanic lithosphere to change its geometric configuration from a spherical shell to the geometry delineated by the Wadati-Benioff seismicity. This change induces lateral membrane deformation within the slab in addition to the bending deformation typically analyzed in two dimensional cross sections. Observations including the along-arc variations of slab geometry, seismic activity, and orientation of earthquake focal mechanisms suggest that this membrane deformation is an important mechanism in controlling the evolution of the subduction zone structure and seismic generation pattern. To quantify this type of slab deformation, we assume that subducting slabs behave like thin viscous sheets with either Newtonian or Power-Law rheology flowing into a mantle with significantly lower viscosity. A non-linear optimization scheme is developed to find the slab geometry and the subduction flow field minimizing the integrated total dissipation power by fixing boundary conditions constrained by the Wadati-Benioff seismicity and the relative plate convergence. The rationale behind this optimization is that since the subducted slab has strong resistence to membrane deformation and relatively little strength to respond to slab normal forces, finding the optimal configuration with the least amount of membrane deformation rate will thus provide insights on both the slab structure and the pattern of slab deformation. Experiments on the Cascadia subduction zone suggest that the proposed arch structure is a natural consequence of the subducted slab responding to the concave-oceanward trench. The arch also provides a plausible explanation for the origin of the Olympic Mountains accretionary prism in the context of the Critical Taper Theory. The concentration of seismicity beneath the Puget Sound area may be the result of bending the already arched slab. The computed deformation rate is dominated by along-arc compression under Puget Sound and the peak compressional strain-rate is around 2 $\times$10$\sp{-16}$sec$\sp{-1}$ which is comparable to the value estimated from the total intraplate seismic moment release during the last century. In both the Alaska-Aleutian and NW-Pacific subduction zones, preliminary experiments predict similar arch structures. Also, modelling results provide plausible explanation for along-arc variations of the deformation regime within slabs that are not resolvable by 2-D approaches.
Ph.D. dissertation
Minerals Data and Information Rescue in Alaska (MDIRA)