The theory of plate tectonics predicts that the outer layer of the Earth is comprised of lithospheric plates (0 - 250 km thick) that are in motion with respect to one another (at rates on the order of 1 - 20 cm/yr), with the majority of deformation concentrated at the plate boundaries. At the Earth's surface, this deformation is commonly manifested in the form of increased and localized seismicity, volcanism, and mountain building. The expression of plate boundary zone deformation underneath the plates, in the Earth's mantle, however, is not well understood. Geodynamic modeling is a valuable tool that can be used to predict the Earth's viscous mantle response to and interaction with the tectonic plates, as the mantle cannot be accessed directly. The proposed work will use high-resolution, three-dimensional geodynamic modeling as a tool to simulate how the Earth's viscous mantle responds to and interacts with the tectonic plates at subduction zones, regions where one tectonic plate slides beneath another and induces motion within the underlying viscous mantle.
Recent seismological observations of shear wave splitting indicate the portion of the mantle wedged between the overriding plate and subducting plate, the mantle wedge, is commonly characterized by seismic fast axes oriented oblique to plate motion, indicative of a complex mantle flow field in many subduction zones. However, far from the plate boundary, the seismic fast axes are commonly oriented sub-parallel to plate motion, indicative of coupling between the plate interior and underlying mantle flow field. A two-dimensional subduction regime cannot explain the along strike and across strike variations in mantle flow implied by the shear wave splitting, thus requiring a three-dimensional framework. Previous three-dimensional numerical simulations of subduction zones predict that trench-parallel flow can be driven by pressure gradients in the mantle wedge and that toroidal flow can be generated around lateral slab edges due to steepening of the subducting plate or rollback in the trench. In addition models of small-scale convection within the mantle wedge predict complex mantle flow and seismic anisotropy in the mantle wedge. However, what controls the transition from complex mantle-overriding plate motion near the subduction zone to aligned mantle-overriding plate motion in the plate interiors has not been investigated. Furthermore, although recent work indicates the rheological flow law governing deformation in the mantle may be instrumental in controlling the magnitude and length-scales of the subduction induced viscosity reduction and complex mantle flow field, this has not been quantified in three-dimensional models of subduction. For this research, three-dimensional numerical models will be constructed, run, and analyzed, to systematically test the controls on the lateral extent of the slab driven viscosity reduction in the mantle wedge due a strain-rate dependent viscosity. The results have implications for the length-scales of complex seismic anisotropy observed in subduction zones and may place constraints on the magnitude of coupling between the mantle and overriding plate. Moreover, understanding how the rheology modulates the viscous flow of the mantle has important implications for understanding the rates of tectonic plate motion, the length-scales of plate boundary zone deformation, as well as the three-dimensional transport of geochemical signatures within the volcanic front.
Intellectual Merit: In a subduction zone, the tectonics plates, which form the outer rheological layer of the Earth, converge with the denser of the two plates descending beneath the other into the warm viscously flowing mantle. As one plate descends into the mantle it, in essence, stirs the ambient mantle resulting in a local circulation within the uppermost mantle. This project investigates how the rate and character of this subduction induced mantle flow depends on the strength of the tectonic plates, the rheological formulation of the mantle, and the geometry of the descending subducting plate, or slab. To do this, a series of high-resolution computational fluid dynamics experiments were run using high-performance computing. The results show that using an experimentally derived flow law to define the viscosity structure, either a Newtonian viscosity for the diffusion creep deformation mechanism or a Composite viscosity for both the diffusion creep and dislocation creep deformation mechanisms, has a first order effect. Models using the composite viscosity produce a zone of subduction induced mantle weakening that results in reduced viscous support of the slab. The high-resolution two-dimensional models predict an increase in velocity magnitude with decreasing slab strength, due to the increased failure in the slab hinge. In all models, the magnitude of the induced mantle flow is greater in models using the composite viscosity, due in part to the weakening effects in regions of high strain rate. The models suggest that the slab steepening is a natural part of the evolution of a subduction zone, and the slab strength as well as viscous support of the slab can play a large role in modulating the rate and extent of slab steepening and consequently the magnitude of induced mantle flow. The slab driven zone of reduced mantle viscosity leads to lateral variability in the upper mantle viscosity and implies lateral variability of the coupling of the mantle to the base of the surface plates. Broader Impacts: The induced mantle weakening, increased mantle flow rates, and associated small scale circulation in the subduction zone have implications for the transport of volatiles in subduction zones and patterns of volcanism. In addition, the results predict lateral variability in the coupling of the mantle to the base of the surface plates, implying variability in the ability of the mantle to drive and resist tectonic plate motions. The small-scale mantle circulation enveloping the slab enhanced by the composite viscosity is consistent to a first order with the localized complex mantle flow associated with subduction zones as inferred from observations of shear wave splitting. Connecting rheology to the phenomenology of the subduction process, such as the slab evolution and slab driven mantle deformation, is an important avenue of research. Large viscosity variations in models of this kind pose a challenge for computational codes, and models with complex 3D geometries require substantially greater numbers of elements, increasing the computational demands. Thus, this work provides an example of a real world geophysical problem to motivate advances in fields outside of Earth Science, such as in computational science, mathematics, continuum mechanics, and fluid dynamics. This project made an impact on the professional development and retention of an early career female scientist in the STEM fields.
