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

了解克拉通及其边缘：来自体波和表面波的见解

• 批准号: 1345143
• 负责人: Karen Fischer
• 金额: $ 23.14万
• 依托单位: Brown University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-02-01 至 2017-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1345143&HistoricalAwards=false
• 关键词: Understanding; Cratons; their; Margins; Insights

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.

关于克拉通岩石圈还有很多东西有待了解，包括区分其地幔与底层软流圈的物理和成分特性、其运动如何与软流圈中的流动耦合以及其形成过程。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断层扫描，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，EarthScope 可移动阵列和其他站在这里提供了出色的采样。 我们将利用 Sp 和 Ps 相位对地壳和地幔不连续性进行成像，利用 Love 和 Rayleigh 波层析成像约束地壳和地幔速度和衰减，并进行集成表面波、Sp、Ps 和其他数据的联合反演，以导出地壳和地幔的 3D 方位各向异性模型岩石圈和软流圈。 这项工作将有助于约束克拉通和非克拉通大陆岩石圈之间岩石圈和软流圈（地震岩石圈-软流圈边界）之间的剪切速度垂直梯度是否存在根本差异。 我们将根据岩石圈和软流圈之间的温度、总体成分、挥发物、熔体含量和粒度的对比来解释这些结果。 在克拉通岩石圈体内，在 Sp 和 Ps 阶段观察到岩石圈中部深度的负速度梯度，通常与首次在远程地震剖面中看到的“8˚不连续性”相一致，并且在北美，在长周期波形断层扫描的方位各向异性的快速方向上具有垂直梯度。 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，并将利用这些结果来测试形成克拉通地幔或促成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和美国中部克拉通下方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与整个岩石圈-软流圈的软流圈耦合边界。克拉通代表了大陆岩石圈古老、稳定的区域，这些区域在过去约 5.5 亿年中没有经历过重大构造活动，或者 更多的。 地球物理和地质证据表明，大多数克拉通下面都有地震波速异常高的地幔层，这代表地幔岩石圈寒冷，化学成分与软流圈不同，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的区别，还有很多东西有待了解。 在这项研究中，我们正在改进北欧和美国中部克拉通地区下方克拉通地幔和软流圈结构的模型；在后一个区域，我们使用来自 EarthScope 可移动阵列以及其他站的数据。 我们的方法将对地震速度结构中的局部梯度敏感的地震波与反映体积平均速度结构的地震波结合起来。 对地幔结构的改进约束将有助于测试克拉通地幔岩石圈如何形成、它如何随时间演化以及其板块运动如何与软流圈中的流动和变形相关的模型。