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，因为它们是由漂浮在更深密度物质海洋上的浮力固体组成的，而且它们几乎是看不见的，龙骨延伸到地球表面下250公里(150英里)。它们就像洋葱，因为它们由一个中心核心组成，周围环绕着一系列生长在它周围的年轻层。大陆的形成过程在地质学上仍然是个谜。解开这个谜团的两个关键在于中央核心最深处的结构(用地质学术语来说就是“克拉通”)和各层之间的边界(“缝合线”或“地形边界”)。对于研究地球结构的地震学家来说，地球望远镜的可移动阵列(TA)是一笔巨大的财富，因为它提供了美国整个陆地地区的极高质量的数据，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，而是在稍北一点的魁北克。我们的项目是在TA在美国东部运作时进行的，具有协同性，并提供了专门针对克拉通核心和周围地形边界的配套数据集。我们正在部署从缅因州沿海(非洲大陆的边缘)到魁北克北部(其核心)的地震仪。站点的密度是可变的，在地形边界交叉的三个地方最高。我们使用地球成像技术检验了三个重要的假设：1)大陆龙骨底部是一个向克拉通核心加深的独特界面；2)大陆的属性是区域化的(即洋葱的每一层都有自己可识别的属性)，地形边界具有向下延伸到底部的特征；3)地球在大陆底部以下的部分(“软流圈”)围绕龙骨流动(与海洋围绕冰山龙骨流动的意义相同)。该项目支持两名研究生，为他们提供了一个发展他们作为研究人员和教师的专业技能的场所。地震仪操作和数据分析涉及本科生研究助理，既有暑期实习，也有学期就业模式。在暑假期间，教育特色被纳入实地考察中，以更好地向这些本科生介绍支撑该项目的科学，更广泛地说，为他们在科学领域的职业生涯做好准备。这些活动包括日常的科学讨论，停下来观看重要的地质特征，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.