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

关于克拉通岩石圈还有许多有待了解的地方，包括区别于其下伏软流圈的地幔的物理和成分性质，其运动如何耦合到软流圈中流动，以及它的形成过程。为了深入了解这些主题，我们将结合散射波(Sp和Ps)的互补分辨率与波罗的海盾牌内部和边缘的各向异性面波层析成像，波罗的海盾牌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，在那里，EarthScope可移动阵列和其他站提供出色的采样。我们将用Sp和Ps相成像地壳和地幔的不连续性，用Love和Rayleigh波层析成像约束地壳和地幔的速度和衰减，并进行联合反演，综合面波、Sp、Ps和其他数据，得到岩石圈和软流层的三维方位各向异性模型。这项工作将有助于约束岩石圈和软流圈(地震岩石圈-软流圈边界)之间的剪切速度垂直梯度是否在克拉通和非克拉通大陆岩石圈之间存在根本差异。我们将通过岩石圈和软流圈之间的温度、体积成分、挥发分、熔体含量和粒度的对比来解释这些结果。在克拉通岩石圈内部，在岩石圈中部深度观察到了负的速度梯度，Sp和Ps相，经常与首次在远程地震剖面上看到的“8&amp；不连续”重合，在北美，与来自长周期波形层析成像的方位各向异性的快速方向上的垂直梯度重合。我们将获得美国中部克拉通岩石圈和波罗的海地盾内部结构的新分辨率，并将使用这些结果来测试形成克拉通地幔或促进其后续演化的过程的模型。最后，软流圈相对于岩石圈的剪切将导致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下的地幔和美国中部克拉通的方位各向异性的分布，使我们能够评估大陆次软流圈的剪切几何，以及大陆板块运动如何跨越岩石圈-软流圈边界耦合到软流圈。克拉通代表大陆岩石圈的古老、稳定的区域，在过去约5.5亿年或更长时间里没有经历过重大的构造活动。地球物理和地质证据表明，大多数克拉通被具有异常高地震波速的地幔覆盖，地幔岩石圈与软流圈在化学上是不同的，而且其厚度比周围较年轻的岩石圈大得多。然而，关于克拉通地幔岩石圈的物理和化学性质以及它与下面较弱的软流圈地幔有何不同，仍有许多需要了解。在这项研究中，我们正在改进北欧和美国中部克拉通地区克拉通地幔和软流层结构的模型；在后一地区，我们使用了来自地球望远镜可移动阵列以及其他站的数据。我们的方法结合了对地震速度结构中的局部梯度敏感的地震波和反映体积平均速度结构的地震波。对地幔结构的改进约束将有助于测试克拉通地幔岩石圈是如何形成的，它是如何随时间演变的，以及它的板块运动如何与软流圈中的流动和变形有关。

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.