Rayleigh-Taylor Instability and Mantle Dynamics Beneath Mountain Belts

Molnar 0106909在这个ABR项目中，PI将继续与Greg Houseman及其同事合作，研究与地球相关的瑞利-泰勒不稳定性，最终目标是了解增厚的地幔岩石圈如何变得重力不稳定。到目前为止，大多数工作都考虑了简单的情况，其中只有一个层，即模拟地幔岩石圈覆盖在无粘流体上。 他们计划更彻底地研究强加的水平缩短对三层模型的影响：轻粘性层（如地壳）覆盖在重粘性层（如地幔岩石圈）上，重粘性层又覆盖在低粘性层（如软流圈）上，密度略低于地幔岩石圈。 浮壳的影响可以深刻地影响不稳定性的性质，例如在某些情况下，密度层的下沉团块形成在上覆层最厚的部分附近，而不是下面，因此下沉到相当于山区边缘的地方。初步计算表明，毫不奇怪，定义一个或两个下行链路之间的过渡的粘度比也取决于上层的密度，较轻的层有利于两个下行链路。研究人员计划更彻底地探索参数空间，特别是通过量化两层的不同密度比和厚度比对下降流配置的影响。 他们将构建“相图”，显示出现一个或两个下降的参数范围，并将检查非牛顿粘度。 他们将考虑与目前所用的边界条件稍有不同的边界条件的影响。 特别是，他们计划使用不混合变形层顶部不同应变率的条件，而是考虑横向变化的粘度如何影响不稳定的生长。 最后，他们将把计算结果与地质和地球物理观测结果联系起来，这些观测结果来自地幔岩石圈似乎已经变薄或完全被移除的地带。激发研究的过程包括广泛的正断层作用和造山后火山作用在山区带，这两者都必须与不断演变的地壳下岩石圈结构。

Molnar 0106909 In this ABR project the PI will continue work with Greg Houseman and colleagues that examines Rayleigh-Taylor instability relevant to the earth, with the ultimate goal of understanding how thickened mantle lithosphere can become gravitationally unstable. Most of the work until now has considered simple situations in which only one layer, that simulating mantle lithosphere overlies an inviscid fluid. They plan to examine more thoroughly the effect of an imposed horizontal shortening on a three-layered model: a light viscous layer (like crust) over a heavy viscous layer (like mantle lithosphere), which in turns overlies a layer (like asthenosphere) of low viscosity and slightly lower density than mantle lithosphere. The effect of buoyant crust can profoundly affect the nature of the instability, such that in some situations downwelling blobs of the dense layer form adjacent, not below, the thickest part of the overlying layer and therefore sink beneath the equivalent of the margins of mountain belts. Preliminary calculations show, not surprisingly, that the viscosity ratio that defines the transition between one or two downwellings depends also on the density of the upper layer, with lighter layers favoring two downwellings. The investigators plan to explore the parameter space more thoroughly, in particular, by quantifying the effects of different density ratios and thickness ratios of the two layers on the configuration of downwelling. They will construct "phase diagrams" that show the parameter ranges for which one or two downwellings occur and will examine non-Newtonian viscosity. They will consider the effects of somewhat different boundary conditions from those used so far. In particular, they plan to use conditions that do not mix varying strain rates at the top of the deforming layer, and instead consider how laterally varying viscosity might affect unstable growth. Finally, they will relate the calculations to geological and geophysical observations from belts where mantle lithosphere seems to have thinned or been removed entirely. Processes that motivate the study include the widespread normal faulting and post-orogenic volcanism in mountain belts, both of which must be associated with the evolving sub-crustal lithospheric structure.