Collaborative Research: Geophysical Investigation of the Mid-Continent Rift System

对中大陆裂谷系统的地球物理调查令人惊讶的是，美国中部是研究塑造我们星球的更重要过程之一的理想地点--大陆分裂形成新的海洋。纵观地球历史，大陆一直沿着裂谷、火山活动和伸展的长长地带分裂，成为扩张中心，新的海洋盆地沿着这些中心形成，并可以增长到大西洋和太平洋的大小。它甚至看起来类似的裂谷可能发生在其他星球上。了解裂谷是如何工作的对科学和社会都很重要，因为裂谷为碳氢化合物和其他矿产资源的沉积提供了条件。问题是，当裂谷成功地形成了一个新的海洋时，早期阶段留下的东西不多了，所以很难看到发生了什么。为了绕过这一点，研究人员正在研究一个失败的壮观裂谷--中大陆裂谷系统。这个系统大约在11亿年前演变而来，当时北美几乎分裂。然而，裂谷失败了，留下了一条2000公里长的火山岩和沉积岩带，这是早期裂谷阶段留下的。这个化石裂谷的长度与现在的东非裂谷和贝加尔裂谷相似，有两个主要的武器在苏必利尔湖地区相遇。一个至少向西南延伸到堪萨斯州，另一个向东南延伸穿过密歇根州。利用致密和高磁性火成岩产生的巨大重磁异常来识别这些手臂。它们大部分被较年轻的沉积物覆盖，但露出苏必利尔湖附近，并可在钻芯中进一步向南追踪。这项研究将NSF的EarthScope计划获得的地震和大地电磁数据与重磁数据结合在一起，形成了裂谷系统的完整三维图像，并限制了其演化。它的目标之一是了解裂谷是像东非裂谷通常所说的那样是对地幔下部融化的反应，还是像亚洲的贝加尔裂谷那样是由于岩石圈内传递的应力而形成的。

Geophysical Investigation of the Mid-Continent Rift System Surprisingly, the Central U.S. is an ideal place to study one of the more important processes shaping our planet - the breaking apart of continents to form new oceans. Throughout earth's history, continents have split along rifts, long zones of volcanism and stretching, which become spreading centers along which new ocean basins form and can grow to the size of the Atlantic and Pacific. It even looks like similar rifting may have happened on other planets.Understanding how rifting works is important for both science and society, because rifting provides conditions for the deposition of hydrocarbons and other mineral resources. The problem is that when rifting succeeds in forming a new ocean, not much remains from its early stages so it's hard to see what happened.To get around this, researchers are studying a spectacular rift that failed, the Mid-Continent Rift System. This system evolved around 1.1 billion years ago, when North America nearly split. However, the rift failed, leaving a 2000 km long belt of volcanic and sedimentary rocks left from the early rifting stage. This fossil rift, which is similar in length to the present East African and Baikal rifts, has two major arms meeting in the Lake Superior region. One extends southwestward at least as far as Kansas, and the other extends southeastward through Michigan. These arms are identified using the large gravity and magnetic anomalies resulting from dense and highly magnetic igneous rocks. They are largely covered by younger sediments but outcrop near Lake Superior and can be followed further south in drill cores. The study combines seismic and magnetotelluric data being acquired by NSF's EarthScope program with gravity and magnetic data to develop an integrated three-dimensional image of the rift system and constrain its evolution. One of its goals is to learn whether the rift formed as a response to melting in the underlying mantle, as commonly proposed for the East African Rift, or because of stresses transmitted within the lithosphere, as appears to be the case for Asia's Baikal Rift.