Taconic orogeny Taconic orogeny The Taconic orogeny was a mountain building period that ended 440 million years ago and affected most of modern-day New England. A great mountain chain formed from eastern Canada down through what is now the Piedmont of the East coast of the United States. As the mountain chain eroded in the Silurian and Devonian periods, sediments from the mountain chain spread throughout the present-day Appalachians and midcontinental North America. Beginning in Cambrian time, about 550 million years ago, the Iapetus Ocean began to close. The weight of accumulating sediments, in addition to compressional forces in the crust, forced
Colorado orogeny Central Plains orogeny. The boundary between the Colorado orogeny and the Wyoming craton is the Cheyenne belt, a 5-km-wide mylonitic shear zone that verges northward. The Cheyenne belt transects and cuts off the south edge of the older Trans-Hudson orogeny. The Paleoproterozoic volcanic and sedimentary rocks that resulted from the Colorado orogeny underwent metamorphism followed by plastic folding under moderate pressure and temperature (PT) conditions (temperature about 500 °C and pressures in excess of 1.2 GPa). The metamorphism was accompanied by intrusion of intermediate calc-alkalic rocks, such as the granodiorites of the Boulder Creek batholith. The accompanying amphibolite facies metamorphism