Rayleigh-Taylor Instability and Mantle Dynamics Beneath Mountain Belts

莫尔纳尔0106909在这个ABR项目中，PI将继续与格雷格·豪斯曼和他的同事们合作，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只有一层的简单情况，即模拟地幔岩石圈覆盖在无粘流体之上。他们计划在一个三层模型上更彻底地研究施加水平缩短的影响：在厚重的粘性层(如地幔岩石圈)上覆盖一个较轻的粘性层(如地壳)，然后覆盖一个低粘度且密度略低于地幔岩石圈的层(如软流层)。浮力地壳的作用可以深刻地影响不稳定的性质，以至于在某些情况下，向下涌出的致密层斑点形成邻近而不是低于覆盖层最厚的部分，因此下沉到相当于山带边缘的下方。不出所料，初步计算表明，定义一次或两次下降的粘度比也取决于上层的密度，较轻的层有利于两次下降。研究人员计划更深入地探索参数空间，特别是通过量化两层不同密度比和厚度比对下流形态的影响。他们将构建“相图”，显示发生一次或两次下沉的参数范围，并检查非牛顿粘度。他们将考虑与目前使用的边界条件略有不同的影响。特别是，他们计划在变形层的顶部使用不同应变率混合的条件，而是考虑横向变化的粘度可能会如何影响不稳定的增长。最后，他们将把计算与地幔岩石圈似乎已经变薄或完全消失的地带的地质和地球物理观测联系起来。推动这项研究的过程包括广泛的正断层作用和山带的造山后火山作用，这两者都必须与地壳下岩石圈结构的演变有关。

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.