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

令人惊讶的是，美国中部是一个理想的地方来研究塑造我们星球的一个更重要的过程——大陆分裂形成新的海洋。在整个地球历史上，大陆沿着裂谷、火山活动和伸展的长带分裂，这些裂谷成为扩张中心，新的海洋盆地沿着裂谷形成，并可以增长到大西洋和太平洋的大小。看起来类似的裂谷也可能发生在其他行星上。了解裂谷作用对科学和社会都很重要，因为裂谷作用为碳氢化合物和其他矿产资源的沉积提供了条件。问题是，当裂谷成功形成一个新的海洋时，早期阶段留下的东西并不多，所以很难看到发生了什么。为了解决这个问题，研究人员正在研究一个壮观的断裂，即中大陆裂谷系。这个系统大约在11亿年前进化，当时北美洲几乎分裂。然而，裂谷破裂了，留下了一条2000公里长的裂谷早期留下的火山岩和沉积岩带。这个化石裂谷的长度与现在的东非裂谷和贝加尔湖裂谷相似，在苏必利尔湖地区有两个主要的分支。一条向西南延伸，至少延伸到堪萨斯州，另一条向东南延伸，穿过密歇根州。这些旋臂是利用致密和高磁性火成岩造成的大重力和磁异常来识别的。它们大部分被较年轻的沉积物覆盖，但在苏必利尔湖附近露头，可以通过钻芯进一步向南追踪。该研究将美国国家科学基金会EarthScope项目获得的地震和大地电磁数据与重力和磁场数据结合起来，形成了裂谷系统的综合三维图像，并对其演化进行了限制。它的目标之一是了解裂谷的形成是由于底层地幔的融化，就像人们通常认为的东非裂谷一样，还是因为岩石圈内部传递的压力，就像亚洲贝加尔湖裂谷的情况一样。

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.