Understanding Cratons and their Margins: Insights From Body and Surface Waves

关于地幔岩石圈还有很多东西需要了解，包括区分地幔和下面的软流圈的物理和成分特性，它的运动如何耦合到软流圈中的流动，以及它形成的过程。为了深入了解这些主题，我们将联合收割机将散射波（Sp和Ps）的互补分辨率与波罗的海盾内部和边缘的各向异性表面波层析成像结合起来，其中存在密集的永久和临时宽带站阵列，以及美国中部克拉通及其边缘，EarthScope可移动阵列和其他台站提供出色的采样。 我们将用Sp和Ps相位成像地壳和地幔不连续性，用Love和Rayleigh波层析成像限制地壳和地幔速度和衰减，并进行联合反演，将面波，Sp，Ps和其他数据整合在一起，以获得岩石圈和软流圈的三维方位各向异性模型。 这项工作将有助于限制岩石圈和软流圈之间的剪切速度的垂直梯度（地震学岩石圈-软流圈边界）是否根本上不同的岩石圈和非岩石圈的大陆岩石圈。 我们将解释这些结果的岩石圈和软流圈之间的温度，散装成分，挥发分，熔体含量和粒度的对比。 在超声波岩石圈的主体内，在岩石圈中部深度的负速度梯度已被观察到具有Sp和Ps相位，通常与首次在长距离地震剖面中看到的“8#730;不连续性”相一致，并且在北美，在长周期波形层析成像的方位角各向异性的快速方向上具有垂直梯度。 我们将获得新的分辨率的内部结构的岩石圈在美国中部和波罗的海盾，我们将使用这些结果来测试模型的过程中形成的岩石圈地幔或有助于其随后的演变。 最后，软流圈相对于岩石圈的剪切作用将引起方位各向异性方向的垂直梯度。 这项工作将限制波罗的海地盾和美国中部地幔中方位各向异性的分布，使我们能够评估次大陆软流圈中剪切的几何形状以及大陆板块运动如何通过岩石圈-软流圈边界耦合到软流圈。大陆岩石圈的稳定区域，在过去~ 5.5亿年或更长时间内没有发生重大构造活动。 地球物理和地质证据表明，大多数地幔岩的下面都有地幔层，这些地幔层的地震波速度非常高，代表了地幔岩石圈，它的温度和化学性质与软流圈不同，其厚度比周围的年轻岩石圈要大得多。 然而，仍有许多需要了解的物理和化学性质的软流圈地幔岩石圈，以及它如何不同于下伏的，较弱的软流圈地幔。 在这项研究中，我们正在改进北方欧洲和美国中部的地幔和软流层结构模型;在后一个区域，我们使用来自地球观测可移动阵列以及其他台站的数据。 我们的方法结合了地震波，是敏感的局部梯度的地震速度结构与那些反映体积平均速度结构。 对地幔结构的改进约束将有助于测试模型，以了解地幔岩石圈如何形成，它如何随着时间的推移而演变，以及板块运动如何与软流圈中的流动和变形相关。

Much remains to be learned about the cratonic lithosphere, including the physical and compositional properties that distinguish its mantle from the underlying asthenosphere, how its motion couples to flow in the asthenosphere, and the processes by which it formed. To gain insight on these topics, we will combine the complementary resolving power of scattered waves (Sp and Ps) and anisotropic surface wave tomography in the interior and across the margin of the Baltic Shield, where dense arrays of permanent and temporary broadband stations exist, and the central U.S. craton and its margins, where the EarthScope Transportable Array and other stations provide excellent sampling. We will image crust and mantle discontinuities with Sp and Ps phases, constrain crust and mantle velocities and attenuation with Love and Rayleigh wave tomography, and carry out joint inversions that integrate surface wave, Sp, Ps and other data to derive 3D azimuthally anisotropic models of the lithosphere and asthenosphere. This work will help to constrain whether the vertical gradient in shear velocity between the lithosphere and asthenosphere (the seismological lithosphere-asthenosphere boundary) fundamentally differs between cratonic and non-cratonic continental lithosphere. We will interpret these results in terms of contrasts in temperature, bulk composition, volatiles, melt content and grain size between the lithosphere and asthenosphere. Within the body of the cratonic lithosphere, negative velocity gradients at mid-lithospheric depths have been observed with Sp and Ps phases, often coinciding with the "8˚ discontinuity" first seen in long-range seismic profiles, and, in North America, with a vertical gradient in the fast direction of azimuthal anisotropy from long-period-waveform tomography. We will obtain new resolution of the internal structure of the cratonic lithosphere in the central U.S. and Baltic shield, and we will use these results to test models for the processes that formed the cratonic mantle or contributed to its subsequent evolution. Finally, shearing in the asthenosphere relative to the lithosphere will cause vertical gradients in the orientation of azimuthal anisotropy. This work will constrain the distribution of azimuthal anisotropy in the mantle beneath the Baltic Shield and central U.S. cratons, allowing us to assess the geometry of shear in the sub-continental asthenosphere and how continental plate motion is coupled to the asthenosphere across the lithosphere-asthenosphere boundary.Cratons represent ancient, stable regions of the continental lithosphere that have not undergone major tectonic activity for the last ~550 million years or more. Geophysical and geological evidence has shown most cratons are underlain by layers of mantle with anomalously high seismic wavespeeds that represent mantle lithosphere that is cold and chemically distinct from the asthenosphere, and whose thickness is much greater than the surrounding, younger lithosphere. However, much remains to be learned about the physical and chemical properties of the cratonic mantle lithosphere and how it differs from the underlying, weaker asthenospheric mantle. In this research we are improving models for the structure of the cratonic mantle and asthenosphere beneath cratonic regions in northern Europe and the central U.S.; in the latter region we are employing data from the EarthScope Transportable Array as well as other stations. Our approach combines seismic waves that are sensitive to localized gradients in seismic velocity structure with those that reflect volume-averaged velocity structure. The improved constraints on mantle structure will help to test models for how the cratonic mantle lithosphere formed, how it has evolved over time, and how its plate motion relates to flow and deformation in the asthenosphere.