Collaborative Research: Deep Structure of Three Continental Sutures in Eastern North America

我们世界的大陆被比作冰山和洋葱。 它们就像冰山，因为它们由漂浮在更深更密物质海洋上的浮力固体组成，而且它们大部分都是看不见的，龙骨延伸到地球表面以下250公里（150英里）。 它们就像洋葱一样，由一个中心核心组成，核心周围是一系列围绕它生长的年轻层。大陆形成的过程仍然是一个地质之谜。解开这一谜团的两个关键在于中央核心（或地质术语“克拉通”）最深处的结构和各层之间的边界（“缝合线”或“地形边界”）。 Earthscope的可移动阵列（TA）是研究地球结构的地震学家的巨大资产，因为它为美国整个陆地地区提供了极高质量的数据，该地区包括许多重要的地质特征，包括板块边界，裂谷带和热点。 然而，它对“大陆起源”问题的影响是有限的，因为大陆的核心并不在它的足迹之下，而是位于稍北的魁北克。 我们的项目，定时发生时，TA是在美国东部的操作，是协同作用，并提供了一个配套的数据集，特别是针对在地球的核心和周围的地形边界。 我们正在从沿海的缅因州（大陆边缘）到北方魁北克（其核心）部署地震仪。 测站的密度是可变的，在地形边界交叉的三个地方最高。 我们使用地球成像技术来测试三个重要的假设：1）大陆龙骨的底部是一个明显的界面，向东太平洋核心加深; 2）大陆的属性是区域化的（也就是说，洋葱的每一层都有自己的可识别属性），地形边界具有向下延伸到底部的签名; 3）大陆底部以下的地球部分（“软流圈”）正在围绕龙骨流动（就像海水围绕冰山的龙骨流动一样）。 该项目支持两名研究生，为他们提供一个场所，以发展他们作为研究人员和教师的专业技能。地震仪操作和数据分析涉及本科生研究助理，包括夏季实习和学期就业模式。在夏季，教育功能被纳入实地考察，以更好地向这些本科生介绍该项目的科学基础，并更普遍地为他们在科学事业做好准备。这些活动包括日常科学讨论、参观重要地质特征以及与合作机构的科学家进行访问。

Our world's continents have been compared to both icebergs and onions. They are like icebergs in that they consist of a buoyant solid floating on a sea of deeper denser material, and also in that they are mostly unseen, with keels extending down to 250 km (150 mi) beneath the Earth's surface. They are like onions in that they consist of a central core surrounded by a sequence of younger layers that has grown around it. The process by which continents were formed remains a geologic mystery. Two of the keys to that mystery lie in the structure of the deepest part of the central core (or "craton", to use the geologic term) and of the boundaries between the layers ("sutures" or "terrain boundaries"). Earthscope's Transportable Array (TA) is a tremendous asset to seismologists studying Earth structure, because it provides extremely high-quality data for the entire land area of the US, a region that includes many important geological features, including plate boundaries, rift zones and hot spots. However, its impact on the "origin of the continents" question is limited, because the core of the continent is not beneath its footprint, but rather lies a little to the north, in Quebec. Our project, timed to take place when the TA is operating in eastern US, is synergistic and provides a companion data set specifically targeted at the cratonic core and surrounding terrain boundaries. We are deploying seismometers in a line stretching from coastal Maine (the edge of the continent) to northern Quebec (its core). The density of stations is variable, and is highest in the three places where terrain boundaries are crossed. We use earth-imaging techniques to test three significant hypotheses: 1) that the bottom of the continental keel is a distinct interface that deepens towards the cratonic core; 2) that the properties of the continent are regionalized (that is, each layer of the onion has its own recognizable properties), with terrain boundaries having a signature that extends right down to the bottom; and 3) that the part of the Earth below the bottom of the continents (the "asthenosphere") is flowing around the keel (in the same sense that ocean water flows around the keel of an iceberg). The project is supporting two graduate students, providing them with a venue to develop their professional skills as both researcher and teachers. Seismometer operations and data analysis involves undergraduate research assistants, in both summer internship and academic-semester employment modes. During the summer, educational features are incorporated into the fieldtrips to better introduce these undergraduates to the science underpinning the project and more generally to prepare them for a career in science. These include daily science discussions, stops to view important geological features and visits with scientists from collaborating institutions.