Collaborative Research: Deciphering the Structure and Evolution of North America's Cratonic Core

这个项目试图阐明美国中大陆的形成、组装和随后的演变。两所大学的研究人员及其研究生和本科生将研究中大陆地壳和上地幔的组成。一种新的重磁数据组合以及从地震接收函数数据估计的地震速度比Vp/VS和由面波和体波层析成像得到的速度结构将被用来建立地壳内多层的温度变化、厚度和体积组成。这些工作的新元素包括开发一种使用参数域互相关和叠加的接收函数成像的新方法，以及通过似然滤波和速度-密度关系将地震图像与潜在野外数据耦合。磁学研究还将侧重于了解磁探测到的边界的物理性质和构造意义，特别强调分隔北美克拉通核的隐藏的中元古代地球化学和磁性边界，以及田纳西-伊利诺伊州-肯塔基州线(或TIKL)及其不同寻常的磁化带状模式。地球物理模型的结果将与稀疏采样的基底同位素地球化学和年龄数据相结合，以解释岩石圈地块的形成和吸积历史以及随后的构造岩浆改造。关于地壳的物理性质及其构造作用和火山活动的历史的知识是一系列固体地球科学主题的核心。对未来地震可能性和深层经济矿藏潜力的评估只是这类知识在其中发挥重要社会作用的两个应用实例。为了更好地评估这些风险和经济潜力，一个由两所大学的教授、研究生和本科生参与的多学科项目将寻求了解构成美国中大陆地质核心的关键结构、组成和过程。在美国西部山区，物理性质的测绘变化和推断构造历史相对简单，因为深部地壳和最上地幔的地球物理图像可以通过对地质过程暴露和/或带到地表的岩石的研究来证实。然而，在美国的中大陆地区，厚厚的沉积岩序列覆盖了岩浆活动时期产生的古老岩石。到达沉积盖层以下的钻孔很少，这使得理解可能控制地震和隐伏矿藏位置的地质过去变得具有挑战性。因此，该区域的构造和地震灾害图更多地依赖于地球物理线形：在地球物理数据集的地图视图中发现的线形特征，表示地质边界和薄弱地带。这项研究试图结合几种不同类型的地球物理数据集，包括重磁异常以及由EarthScope？S地震仪阵列提供的图像。从这些数据中得出的中大陆地壳以前未知的特征将被用来更好地描述地质边界的含义，以全面了解地球-S外层的历史和性质。

This project seeks to illuminate the formation, assembly and subsequent evolution of the US mid-continent. Investigators at two universities and their graduate and undergraduate students will examine composition of the mid-continent crust and upper mantle. A novel combination of gravity and magnetic data with seismic velocity ratios vP/vS estimated from seismic receiver function data and velocity structure from surface wave and body wave tomography will be used to establish variations in temperature as well as thickness and bulk composition of multiple layers within the crust. The novel elements of these efforts include development of a new approach to receiver function imaging using parameter-domain cross-correlation and stacking, and coupling of the seismic images to the potential field data via likelihood filters and velocity-density relationships. Magnetic studies will focus also on understanding the physical properties and tectonic implications of magnetically detected boundaries, with particular emphases on a hidden middle Proterozoic geochemical and magnetic boundary dividing the cratonic core of North America and also on the Tennessee-Illinois-Kentucky lineament (or TIKL) and its unusual banded pattern of magnetization. The geophysical model results will be combined with sparsely sampled basement isotope geochemistry and age data to interpret the history of formation and accretion of lithospheric blocks as well as their subsequent tectonomagmatic modification. Knowledge about physical properties of the crust and its history of tectonism and volcanism is central to a wide array of solid Earth science topics. Assessments of the likelihood of future earthquakes and the potential for economic mineral deposits at depth are just two examples of applications in which such knowledge plays a vital societal role. In order to evaluate better these risks and economic potential, a multi-disciplinary project involving professors and graduate and undergraduate students at two universities will seek to understand key structures, composition and processes that formed the geologic core of the US mid-continent. In mountainous regions of the western US, mapping variations in physical properties and inferring tectonic history are relatively straightforward because geophysical images of the deep crust and uppermost mantle can be corroborated by studies of rocks exposed and/or brought to the surface by geologic processes. In the mid-continent region of the US however, thick sequences of sedimentary rocks cover the ancient rocks produced by episodes of magmatism. Boreholes reaching below the sedimentary cover are few and far between and that has made it challenging to understand the geologic past that may control the locations of earthquakes and hidden mineral deposits. Consequently, tectonic and seismic hazard maps of this region rely more heavily on geophysical lineaments: linear features found in map-views of geophysical data sets signifying geologic boundaries and weak zones. This study seeks to combine several different types of geophysical data sets, including gravity and magnetic anomalies as well as images made possible by EarthScope?s array of seismometers. The previously unknown characteristics of the mid-continent crust derived from these data will be used to better characterize implications of the geologic boundaries for understanding comprehensively the history and properties of the Earth?s outer layers.