Tertiary Paleogeography, Southwestern US



Early Tertiary (Eocene) Paleogeography, Southwestern US (50 Ma). As the Rocky Mountains were uplifted, basins formed and subsided between major uplifts. Huge lakes filled the basins during the Eocene. Although the lakes were in basins, paleobotanical data suggests that their absolute elevation was near the current elevation of the deposits. This fact suggeste that much of the Rocky Mountain region and the Colorado Plateau were uplifted in the Eocene. Mountainous terrain existed in Nevada, western Utah, and central and southern Arizona. Streams drained these uplands onto the Colorado Plateau and probably into the lakes. The coastal region continued to have right-lateral translation of terrains northward. Some of these terrains lie outboard of marginal seas along the west coast and formed continental borderlands at the edge of the continent. The most famous of these, Salinia, probably started out in northern Mexico and was progressively translated northward; you can follow its progression on the four Tertiary maps



Early Tertiary (Oligocene) Paleogeography, Southwestern US (35 Ma). Much of the drainage of the Rocky Mountain and Colorado Plateau regions was poorly developed and probably internal to the region. However, some streams found outlets to lower elevations and carved canyons into the margins of the Colorado Plateau. Volcanism was widespread and extreme across much of the Western Interior. The first stages of the Rio Grande Rift developed as parts of western North America evolved from compressional tectonics to extensional tectonics. California borderlands continued their northward translation and a series of complex basins formed between and east of them. The East Pacific Rise, a Pacific spreading center, neared the coast of southwestern North America. Its impending collision would have profound effects on the evolution of the region



Middle Tertiary (Early Miocene) Paleogeography, Southwestern US (20 Ma). Drainage systems of the Western Interior were still not well integrated. Note the absence of a through going stream on the Colorado Plateau. Extensive volcanism continued. On the west coast, normal subduction resumed, but only north of where the East Pacific Rise had already collided with North America. Since the Permian or Triassic, the Farallon and related plates were subducted below North America. Once the East Pacific Rise collided with the continent, the westerly plate, the Pacific Plate, came into contact with the continent. Because both the Pacific Plate and North American Plate have westerly components of motion, a new stress regime was established. Extension dominated regions east of where North America contacted the Pacific Plate. A large transform fault system, the San Andreas, was created. Blocks moving northwest along the Pacific side of the fault were caught in a bend of the fault system in southern California. These blocks were rotated in clockwise fashion to form the Transverse Ranges



Latest Middle Tertiary (Late Miocene) Paleogeography, Southwestern US (10 Ma). The tectonic patterns of the earlier Miocene continued. Volcanism continued widespread and streams continued to gnaw at the Colorado Plateau. Extension spreads northward as the Mendocino triple junction, the point of contact between the Farallon, North American, and Pacific plates moved northward. Note that Salinia is now in central California. The Cordilleran arc shuts down south of the triple junction as subduction ceases and transform motion begins.



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