This article is about the geological concept. Most rifts consist of a series of separate segments that together form rifts world book 7 pdf linear zone characteristic of rifts.
The individual rift segments have a dominantly half-graben geometry, controlled by a single basin-bounding fault. Segment lengths vary between rifts, depending on the elastic thickness of the lithosphere. Areas of thick colder lithosphere, such as the Baikal Rift have segment lengths in excess of 80 km, while in areas of warmer thin lithosphere, segment lengths may be less than 30 km. Segment boundaries often have a more complex structure and generally cross the rift axis at a high angle. These segment boundary zones accommodate the differences in fault displacement between the segments and are therefore known as accommodation zones. Accommodation zones take various forms, from a simple relay ramp at the overlap between two major faults of the same polarity, to zones of high structural complexity, particularly where the segments have opposite polarity.
Accommodation zones may be located where older crustal structures intersect the rift axis. In subaerial rifts, drainage at this stage is generally internal, with no element of through drainage. As the rift evolves, some of the individual fault segments grow, eventually becoming linked together to form the larger bounding faults. Subsequent extension becomes concentrated on these faults. The longer faults and wider fault spacing leads to more continuous areas of fault-related subsidence along the rift axis. Significant uplift of the rift shoulders develops at this stage, strongly influencing drainage and sedimentation in the rift basins. Once rifting ceases, the mantle beneath the rift cools and this is accompanied by a broad area of post-rift subsidence.
Some rifts show a complex and prolonged history of rifting, with several distinct phases. They form within post-rift sequences when hydrothermal fluids associated with magmatic activity are expelled at the seabed. Just over half of estimated oil reserves are found associated with rifts containing marine syn-rift and post-rift sequences, just under a quarter in rifts with a non-marine syn-rift and post-rift, and an eighth in non-marine syn-rift with a marine post-rift. Rift-basin structure and its influence on sedimentary systems”. Forward and reverse modelling of rift basin formation”.
Syn-rift sedimentary architectures in the Northern North Sea”. Volcanoes and the geological cycle”. This page was last edited on 22 January 2018, at 17:34. Midcontinent Rift, here labeled Keweenawan Rift. Lake Superior occupies the apex of the rift.
The rift failed, leaving behind thick layers of rock that are exposed in its northern reaches, but buried beneath later sedimentary formations along most of its western and eastern arms. Later compressive forces from the Grenville Orogeny likely played a major role in the rift’s failure and closure. The Midcontinent Rift appears to have progressed almost to the point where the ocean intruded. 22 million years the rift failed.
Lake Superior occupies a basin created by the rift. Iowa magnetic anomaly map showing the rift curving from the north center to the southwest of the state. Oil identified as Precambrian has been found seeping out of the Nonesuch Shale in the White Pine mine in Michigan. A few deep wells were drilled to explore for oil and gas in rift rocks as far southwest as Kansas. No oil and gas were found, but the explorations did make some deep rock samples available.
Keweenawan-age rocks associated with the rift. Precolumbian times, reactivated in the 1840s and continued for more than a century. Minneapolis, Minnesota: University of Minnesota Press. Iowa Department of Natural Resources Geological Survey. University of Wisconsin at Eau Claire. Gas Journal, 18 September 1995, pp. Iowa Geological Survey Resource Information Fact Sheet 2006-2.
This page was last edited on 29 January 2018, at 14:13. Latest breaking news, including politics, crime and celebrity. Find stories, updates and expert opinion. Unlimited access to exclusive stories.