Mesozoic Tectonics and Paleogeography

Pangea started breaking up (in a region that would become eastern Canada and the United States) around 210 million years ago in the Late Triassic. Clear evidence includes the age of the sediments in the Newark Supergroup rift basins, the Palisades sill of the eastern part of North America, and the age of the Atlantic ocean floor. Due to sea-floor spreading, the oldest rocks on the Atlantic floor are along the coast of northern Africa and the east coast of North America, while the youngest are along the mid-ocean ridge.

This age pattern shows how the Atlantic Ocean opened as the young Mid-Atlantic Ridge began to create the seafloor. This means the Atlantic ocean started opening and was first formed here. The southern Atlantic opened next, with South America separating from central and southern Africa. Last (after the Mesozoic ended) was the northernmost Atlantic, with Greenland and Scandinavia parting ways. The breaking points of each rifted plate margin eventually turned into the passive plate boundaries of the east coast of the Americas today.

In western North America, a plate boundary had started with the subduction of ancient oceanic crust, controlling most of the tectonics of that region in the Mesozoic. Another possible collision with oceanic crust created the Sonoman Orogeny (orogeny= mountain belt) in Nevada during the latest Paleozoic to the Triassic. In the Jurassic, another collision caused the Nevadan Orogeny, a large volcanic belt similar to the Andes mountains today. The Sevier Orogeny followed in the Cretaceous, mainly a volcanic arc to the west and a thin-skinned fold and thrust belt to the east, meaning stacks of shallow faults and folds built up the topography. Many of the structures in the Rocky Mountains today date from this orogeny.

Tectonics influenced one more important geographic feature in North America: the Cretaceous Western Interior Foreland Basin, which flooded during high sea levels forming the Cretaceous Interior Seaway. Subduction of Farallon Plate, an oceanic plate connected to the Pacific Plate (seen today as remnants such as the Juan de Fuca Plate, off the coast of the Pacific Northwest), occurred on the West coast of Northern America. Subduction was shallow at this time because a very young, hot, and less dense portion of the Farallon plate was subducted. This shallow subduction caused a down warping in the central part of North America, which created the basin that flooded to become an interior sea.

High sea levels due to shallow subduction, increasing seafloor spreading and subduction rates, high temperatures, and melted ice also contributed to the high sea levels. These factors allowed a shallow epicontinental seaway that extended from the Gulf of Mexico to the Arctic Ocean to divide North America into two separate land masses, Laramidia to the west and Appalachia to the east, for 25 million years. Many of the coal deposits in Utah and Wyoming formed from swamps along the shores of this seaway. By the end of the Cretaceous, cooling temperatures caused the seaway to regress.