Laramide orogeny

The Laramide orogeny was a massive period of mountain building in western North America. This tectonic event reshaped the continent over many millions of years. It began during the Late Cretaceous period, roughly 80 to 70 million years ago. The process ended much later, between 55 and 35 million years ago. Scientists still debate the exact dates for when it started and finished. The orogeny did not happen all at once. Instead, it occurred in a series of pulses. These pulses were separated by quiescent phases, which are quiet periods of little activity.

This mountain building was caused by a process called subduction. Off the west coast of North America, the Kula and Farallon plates were sliding under the North American Plate. Most scientists believe this was flat-slab subduction. In this process, the oceanic crust moves under the continent at a very shallow angle. This shallow angle might have happened because the plates were converging at a faster rate. Another theory suggests the subducted oceanic crust was thicker than usual. This shallow movement caused the underlying oceanic lithosphere to drag against the bottom of the continental lithosphere.

This specific type of subduction created a magmatic gap. Usually, subduction creates volcanic activity near the edge of the plate, like the Andes mountains. However, during the Laramide orogeny, no magmatism occurred in the central west of the continent. This gap happened because the subducted slab was touching relatively cool continental lithosphere. It was not touching the hotter asthenosphere. Instead, volcanic activity occurred much further east. This activity was found along the Colorado Mineral Belt. The drag from the shallow subduction helped create a broad belt of mountains. These mountains were the progenitors, or the early ancestors, of the Rocky Mountains.

The orogeny produced a distinct pattern of mountain blocks and structural basins. This is known as thick-skinned deformation. It is typical for continental plates near convergent margins that do not have continent-to-continent collisions. The process created compressive uplifts and deep basins. The deformation was mostly confined to the edges of these blocks. It is not uncommon to see 12 kilometers of structural relief between a basin and an uplift. The basins filled with thousands of meters of Paleozoic and Mesozoic sedimentary rocks. They also collected Cretaceous and Cenozoic sediments.

In the United States, these intermontane basins are most prominent in the central Rocky Mountains. They stretch from Colorado and Utah to Montana. The basins are best developed in Wyoming. The largest ones include the Bighorn, Powder River, and Wind River basins.

The floors of these basins look like the western Great Plains. However, they are surrounded by high mountain vistas. At the boundaries, rock units dip steeply into the basins. These eroded, steeply dipping units create shapes called hogbacks and flatirons. Many of these boundaries are formed by thrust or reverse faults. Some boundaries appear as monoclinal flexures, though deep faulting is suspected.

The landscape was very different during the Laramide orogeny. Basin floors and mountain summits were much closer to sea level than they are today. After the seas retreated from the Rocky Mountain region, the basins changed. They became filled with floodplains, swamps, and vast lakes. The drainage systems that formed then still exist today. Since the Oligocene, the entire region has undergone epeirogenic uplift. This means the land has been gradually raised to its present elevation. Most modern topography comes from Pliocene and Pleistocene events. These include more uplift, glaciation, and water erosion.

These geological changes had huge effects on biology. Paleontologist Thomas M. Lehman noted that this event triggered a dramatic turnover in dinosaur communities. This happened in the Late Cretaceous, just before the dinosaurs went extinct. In the south, specialized dinosaurs like centrosaurines and lambeosaurines were replaced by more basal upland dinosaurs. In the north, the biomes changed as well. The hadrosaur community was greatly reduced. Instead, the area became dominated by Triceratops. This shows how shifting mountains and basins can completely change the lives of animals.