Alleghanian orogeny Alleghanian orogeny The Alleghanian orogeny or Appalachian orogeny is one of the geological mountain-forming events that formed the Appalachian Mountains and Allegheny Mountains. The term and spelling Alleghany orogeny was originally proposed by H.P. Woodward in 1957. The Alleghanian orogeny occurred approximately 325 million to 260 million years ago over at least five deformation events in the Carboniferous to Permian period. The orogeny was caused by Africa colliding with North America. At the time, these continents did not exist in their current forms: North America was part of the Euramerica super-continent, while Africa was part of Gondwana. This collision formed
Grenville orogeny here. Ages are approximated from the magmatic activity associated with the individual cycles of the orogeny. The gaps in the ages of the compression cycles and isotope analysis of hornblende, biotite, and potassium feldspar suggest that extension was occurring when compression had momentarily ceased. Rivers' 2008 paper has now examined the timing of the different periods of the orogeny and reconstructed the timeline based on the spatial and temporal metamorphism of the rocks present. According to this newer version of the timeline which is a composite of Rivers 1997 and Gower and Krogh 2002, the Elzevirian orogeny occurs from 1240