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

关于克拉通岩石圈还有很多有待了解的地方，包括区分地幔与底层软流圈的物理和组成特性，它的运动如何与软流圈中的流动耦合，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）的互补分辨能力和波罗的海地盾内部和边缘的各向异性表面波层析成像，在那里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，在那里地球范围可移动阵列和其他站提供了出色的采样。我们将利用Sp相和Ps相对地壳和地幔不连续面进行成像，利用Love波和Rayleigh波层析成像约束地壳和地幔的速度和衰减，并结合面波、Sp、Ps等数据进行联合反演，得到岩石圈和软流圈的三维方位各向异性模型。这项工作将有助于约束克拉通和非克拉通大陆岩石圈在岩石圈和软流圈之间的切变速度垂直梯度（地震岩石圈-软流圈边界）是否存在根本差异。我们将根据岩石圈和软流圈之间的温度、体积成分、挥发物、熔体含量和颗粒大小的对比来解释这些结果。在克拉通岩石圈体内，在岩石圈中部深度观测到Sp和Ps相的负速度梯度，通常与“8&#730；“不连续”首次出现在长程地震剖面中，并且在北美，从长周期波形层析成像中可以看出，在方位角各向异性的快速方向上存在垂直梯度。我们将获得美国中部和波罗的海地盾克拉通岩石圈内部结构的新分辨率，我们将利用这些结果来测试形成克拉通地幔或促进其后续演化过程的模型。最后，软流圈相对于岩石圈的剪切作用会在方位各向异性方向上产生垂直梯度。这项工作将限制波罗的海地盾和美国中部克拉通下地幔的方位各向异性分布，使我们能够评估次大陆软流圈剪切的几何形状，以及大陆板块运动如何在岩石圈-软流圈边界上与软流圈耦合。克拉通代表了大陆岩石圈中古老、稳定的区域，在过去的5.5亿年或更久的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，大多数克拉通的下面是具有异常高地震波速度的地幔层，这代表了地幔岩石圈是寒冷的，在化学上与软流圈不同，其厚度比周围的年轻岩石圈大得多。然而，关于克拉通地幔岩石圈的物理和化学性质，以及它与底层较弱的软流圈地幔有何不同，还有很多有待研究。在这项研究中，我们正在改进欧洲北部和美国中部克拉通地区的地幔和软流圈结构模型；在后一区域，我们正在使用地球范围可移动阵列以及其他台站的数据。我们的方法结合了对地震速度结构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模型，它是如何随着时间的推移而演变的，以及它的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.