Collaborative Research: Measuring the Earth's Lithospheric Deformation Field and Sub-Lithospheric Flow Field

在本项目中，研究人员计划构建与地幔动力学直接相关的两个重要的全球图像：1)变形大陆岩石圈内的应变场和2)岩石圈正下方的地幔流场。前者对允许的大陆变形和演化的物理机制提供了强有力的约束，而后者则构成了评估板块-地幔系统耦合以及驱动地幔流动和板块运动的相关力的最直接手段。作为解决这两个问题的一种方法，他们计划利用两个大型但很少组合的数据集：来自地质和大地测量学的地表变形测量，以及来自地震各向异性的地幔变形测量。在岩石圈变形方面，他们计划量化地表和岩石圈地幔之间的垂直相干变形程度。这种诊断是测试各种变形模型的有效手段，这些模型可以预测不同程度的垂直相干性。对于岩石圈像薄粘性薄片一样变形的模型，预计高度垂直相干性。对于涉及岩石圈剥离或弱下地壳内流动的模型，预计垂直相干性较低。对于地幔流动，他们将估算世界上尽可能多的海洋和大陆下岩石圈下的流动速度。通过对地表速度场的观测，结合地幔方位各向异性的取向，可以推断软流层底部的地幔流速。已经完成的两项区域研究表明了这种方法的可行性及其巨大潜力：在西藏，他们发现了西藏地下岩石圈变形的高度垂直相干性。如果这与本项目进一步的观察结果相一致，它将有力地限制该地区和类似地区的允许模型的范围。其次，他们首次对北美西部岩石圈下地幔流动速度进行了直接观测，在热点参考系中发现向东的地幔流动速度约为5厘米/年。利用这些新的观测约束，研究人员试图解决有关岩石圈下流场的基本问题。这个流场是否固定在热点坐标系或任何其他参考系中？或者是不同的子域，如不同的海洋盆地，以不同但相对均匀的流速为特征？由深部地幔密度驱动的预测流场是否与观测到的岩石圈下流场基本一致？流的速度通常需要岩石圈下面有一个弱软流层吗？虽然岩石圈变形或亚岩石圈流动可能在任何给定区域内占主导地位，但他们将假设两者通常作为两个不同的层起作用。他们的建模方法将明确地解释这一点，从而提供在任何地区分离这两种重要影响的能力。

SilverEAR-0215616In the course of this project the investigators plan to construct two important global images that are directly relevant to mantle dynamics: 1) The strain field within deforming continental lithosphere and 2) The mantle flow field directly beneath the lithosphere. The former provides a strong constraint on the allowable physical mechanisms by which continents deform and evolve, while the latter constitutes arguably the most direct means of evaluating the coupling of the plate-mantle system, and the relevant forces that drive both mantle flow and plate motion. As a way of attacking both problems, they plan to utilize two large but rarely combined data sets: surface deformation measurements from geology and geodesy, and mantle deformation measurements from seismic anisotropy. With respect to lithospheric deformation, they plan to quantify the degree of vertical coherence of deformation between the surface and lithospheric mantle. This diagnostic constitutes an effective means of testing a wide variety of deformation models that predict varying degrees of vertical coherence. High vertical coherence is expected for models in which the lithosphere deforms like a thin viscous sheet. Low vertical coherence is expected for models that involve lithospheric delamination or flow within a weak lower crust. For mantle flow, they will estimate the sub-lithospheric flow velocity beneath both oceans and continents in as much of the world as possible. Observations of the surface velocity field, together with the orientation of mantle azimuthal anisotropy, enable inference of the mantle flow velocity at the base of the asthenosphere. Two regional studies have been completed that illustrate both the feasibility of this approach and its significant potential: in Tibet they have found high vertical coherence of lithospheric deformation beneath Tibet. If this stands up with further observations to be made in this project, it will serve to strongly restrict the range of allowable models for this and similar regions. Second, they have made the first direct observation of sub-lithospheric mantle flow velocity beneath western North America, where they find eastward-directed mantle flow of about 5 cm/yr in a hotspot reference frame. Using these new observational constraints, the investigators seek to address basic questions about the sub-lithospheric flow field. Is this flow field fixed in a hotspot frame or any other reference frame? Alternatively are different subdomains, such as the various ocean basins characterized by distinct but relatively uniform flow velocities? Are predicted flow fields driven by deep mantle density generally compatible with the observed sublithospheric flow field? Do flow velocities in general require a weak asthenosphere beneath the lithosphere? While either lithospheric deformation or sub-lithospheric flow may be dominant within any given region, they will assume that both are contributing as two distinct layers in general. Their modeling methodology will explicitly account for this, thus providing the capability for separating these two important influences in any region.