Pre-Earthscope Workshop Proposal: Tectonic Targets for Earthscope in the Midcontinent

这笔赠款资助了一个讲习班，以发展与EarthScope有关的研究项目，涉及美国中大陆中部之下地壳和地幔的地质特征和构造演化。具体的焦点区域包括两个主要的大陆区块，奥扎克高原和伊利诺伊州盆地。讲习班上的互动可以促进熟悉目标区域(包括伊利诺伊州、印第安纳州、密苏里州和肯塔基州)地质背景的研究人员、熟悉覆盖该地区的现有地球物理数据集的研究人员以及那些在设计和实施地球物理实验方面具有经验的研究人员之间的伙伴关系，这些实验可以利用与地球望远镜相关的仪器。参与者来自研究型大学、州地质调查局、四年制学院和美国地质调查局。感兴趣的区域是北美克拉通平台的一部分。克拉通地台是一个长期存在的大陆省，古生代沉积岩层覆盖在变质岩和火成岩之上。(该地区古生代岩石的年龄不到5亿年，而前寒武纪的岩石则有10到30亿年的历史。)美国中大陆提供了世界上最引人入胜的克拉通地台岩石圈之一。具体地说，这个区块包括一个主要的隆起(欧扎克高原)和一个主要的盆地(伊利诺伊盆地)。欧扎克高原顶部古生代与前寒武纪岩石的接触高度在海平面以上700m，而伊利诺伊盆地底部的接触深度在海平面以下约7公里。换句话说，隐藏在今天中西部玉米地下的地质运动产生了与山脉地带相当的地形。感兴趣的区域还包括密西西比河北端(在过去1亿年中一直保持异常低的区域)，三个主要的前寒武纪缝合带，新地壳沿着这些缝合带与世界上最大的花岗岩省之一北美相连，以及几个重要的断层和褶皱带的交汇处。这些带可能起源于裂谷，由于前寒武纪地壳的伸展，自那以来一直保持脆弱和易受位移的影响。因此，目标区是克拉通地台岩石圈地震活动最活跃的地区之一，历史上中大陆中部曾发生过多次地震。对目标区域的研究可以解决克拉通的形成和演化、板内地震活动以及区域盆地和穹隆的形成等问题。在克拉通地台中，相干和耦合运动从前寒武纪一直延续到现在。令人惊讶的是，在板块构造出现几十年后，还没有普遍接受的理论来解释克拉通地台构造作用的控制，尽管这些地区占大陆岩石圈的25%以上。认识上的局限性在很大程度上是因为缺乏关于这些区域之下地壳和地幔更深层次特征的数据。这些数据将通过与地球望远镜有关的测量获得，因此不容错过为研究克拉通平台的特别例证而组织协调科学议程的独特机会。增进对美国这一地区的了解，对于了解远离板块边界的人口聚居区的地震灾害、能源和碳固存储藏库的演化以及对大陆地质演化的一般理解都是重要的。由于中大陆没有大量暴露在地表的地质特征，与地球望远镜有关的工作也提供了吸引公众对地质过程和现象感兴趣的机会。

This grant funds a workshop to develop EarthScope-related research projects concerning the geologic character and tectonic evolution of the crust and mantle beneath the central Midcontinent of the United States. The specific region of focus includes two major continental blocks, the Ozark Plateau and the Illinois Basin. Interactions at the workshop can foster partnerships among researchers familiar with the geologic context of the target region (which encompasses Illinois, Indiana, Missouri, and Kentucky), those familiar with existing geophysical data sets covering the region, and those with experience in design and implementation of geophysical experiments that can take advantage of the instrumentation associated with EarthScope. Participants come from research universities, state geological surveys, four-year colleges, and the U.S. Geological Survey. The region of interest is part of North America's cratonic platform. A cratonic platform is long-lived continental province in which a veneer of Paleozoic sedimentary rocks overlies metamorphic and igneous rocks. (The Paleozoic rocks in the region are less than half a billion years old, whereas the Precambrian rocks are 1 to 3 billion years old.) The U.S. Midcontinent provides one of the most fascinating examples of cratonic-platform lithosphere anywhere in the world. Specifically, this block includes a major uplift (the Ozark Plateau) and a major basin (the Illinois Basin). The contact between Paleozoic and Precambrian rocks on the crest of the Ozark Plateau is 700 m above sea level, whereas the same contact at the base of the Illinois Basin is about 7 km below sea level. Put another way, hidden beneath the Midwestern cornfields of today, geologic movements have produced relief comparable to that of mountain belts. The region of interest also includes the northern end of the Mississippi Embayment (a region that remained anomalously low during the past 100 million years), three major Precambrian sutures along which new crust attached to North America, one of the world's largest granite provinces, and the intersection of several significant fault-and-fold zones. These zones possibly originated as rifts, due to stretching of the crust in the Precambrian, and have remained weak and susceptible to displacements ever since. As a result, the target region is one of the most seismically active regions of cratonic platform lithosphere anywhere?numerous earthquakes have happened in the central Midcontinent during historic time. Study of the target region can address issues of craton formation and evolution, intraplate seismicity, and the formation of regional basins and domes. In cratonic platforms, coherent and coupled motions have continued from the Precambrian to the present. Surprisingly, decades after the advent of plate tectonics, there is no universally accepted theory to explain controls on tectonism in cratonic platforms, even though these regions account for over 25% of continental lithosphere. The limitation on understanding comes largely from lack of data on the deeper character of the crust and mantle beneath these regions. Such data will become available through EarthScope-related measurements, so the unique opportunity to organize a coordinated scientific agenda for studying a particularly illustrative example of cratonic platform is not to be missed. Improved understanding of this region of the United States is important for understanding seismic hazard in populated areas away from plate boundaries, for the evolution of reservoirs for energy resources and for carbon sequestration, and for the development of a general understanding of the geological evolution of continents. Since the midcontinent does not have abundant surface exposures of geologic features, EarthScope related work in the midcontinent also provides the opportunity to attract public interest in geologic processes and phenomena.