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

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.