MESOZOICA ERA SERIES

The supercontinent Pangaea existed for approximately 100 million years, reaching its largest size during the Triassic period. Despite the discovery of Glossopteris and other geologic evidence, the theory of continental drift was not accepted for decades, until the mechanisms of continental movement were discovered and reformulated under the modern theory of plate tectonics.

Yet Glossopteris fossils are found in South America, Africa, Australia, India, and Antarctica! The mountain belts along the margins of North America, Africa, and Europe line up as well and have similar rock types, an indication that the continents at one time were joined as Pangaea. For example, the Permian-age fossil plant Glossopteris had seeds too heavy to be blown across an ocean. How do we know that Pangaea existed 250 million years ago? Fossil evidence and mountain belts provide some of the clues. The rising ancestral Rocky Mountains provided sediment that filled the seaway, and uplift from the ongoing orogeny finally caused the water to split in the Dakotas and retreat south. The largest and most important evidence of this is the Idaho Batholith- three major lobes of granitic material intruded beneath large areas of Idaho between 100 and 65 million years ago. While most of the magmatic activity at this time occurred on the western edge of the continent in the volcanic arc of the Sierra Nevada, some did take place farther inland.

The Sevier Orogeny (100 - 72 million years ago) raised the portion of the Rocky Mountains in Montana, Wyoming, and Utah known as the вЂњOverthrust Belt.вЂќ The second event, the Laramide Orogeny, peaked around 68 - 65 million years ago, when the angle of the subducting plate became shallower, uplifting the Rocky Mountains in Colorado and New Mexico. The compressional forces of subduction faulted the crustal rocks of western North America and uplifted the Rocky Mountains in two major pulses. The Farallon plate continued to collide with western North America, thrusting layers of rock up over each other and causing increasing volcanism to the west of the Western Interior Seaway. During the very latest stages of the Cretaceous period, around 70 million years ago, the Western Interior Seaway was displaced by slow uplift of the continent.ДЇigure 1.11: Landmasses following the breakup of Pangaea. This downwarped the center of the continent and created a basin that allowed the waters of the Gulf of Mexico to meet with those in the north, forming the Western Interior Seaway ( Figure 1.12), which inundated a 1000-kilometer (620-mile) wide swath from Mexico to Alaska. Around 85 million years ago, when the Farallon plate began to subduct at an unusually shallow angle, it slid farther inland beneath western North America before finally sinking into the asthenosphere.

As the new Atlantic Ocean widened, sea levels began to rise.

MESOZOICA ERA SERIES

Far to the west, oceanic crust (the Farallon plate) had been subducting under western North America for tens of millions of years, causing a series of volcanic island complexes to collide with and become accreted to that margin of the continent, forming the Sierra Nevada of California. Figure 1.10: The Northwest Central US during the Jurassic, approximately 170 million years ago.Дuring the early Cretaceous, Pangaea entered its final stages of breakup ( Figure 1.11).