Areas in which there is a transition from diagenesis to metamorphism occur in a variety of tectonic settings, some of which are of an extensional character rather than the collision setting normally associated with regional metamorphism. In collision settings the diagenetic to metamorphic transition is typically followed by a continuum into the highest grades of metamorphism. The baric type is usually of an intermediate-pressure variety, and metamorphism occurs at a late stage in the tectonic cycle. In extensional settings, such as marginal basins, a restricted facies series is developed that ranges from diagenetic levels up to a low greenschist facies maximum. The metamorphism is normally of a low-pressure type and develops early in the tectonic cycle. These characteristics suggest that it is possible to subdivide broadly the field of low-grade regional metamorphism into compressional (orogenic) and extensional settings.
- Geological Society of America