| Summary: | During the Middle Miocene through the Pliocene the Appalachian Mountains underwent continued erosion and approached modern elevations. The Rocky Mountains had undergone uplift to half or more of their present elevation during the Late Cretaceous to Middle Eocene Laramide Revolution; after a lull during the Middle Eocene through the Early Miocene, there was increased tectonic activity beginning ~12 Ma and especially between 7 and 4 Ma. Locally some highlands may have approached or attained modern elevations. The increasingly high mountains and plateaus of Asia and North America deflected the major air streams southward, bringing colder polar air into the middle latitudes of North America. An extensive Antarctic ice sheet further cooled ocean waters and contributed to the spread of seasonally dry climates. The elimination of most of the Asian exotics from the North American flora dates to the Late Miocene-Pliocene as a result of a decline in summer rainfall. The Sierra Nevada attained about two-thirds of their present elevation within the past 10 Ma. They were appreciably elevated at ~5 Ma, stood at ~2100 m at 3 Ma, and have risen ~950 m since 3 Ma (Huber, 1981). The California Coast Ranges and Cascade Mountains attained significant heights by 3 Ma, and there was a rapid rise of the Alaska Range at ~6 Ma. Temperatures increased between ~18 and 16 Ma. In the absence of major plate reorganization and intense volcanic activity and with increased erosion from continued replacement of the dense evergreen forest by deciduous forest and shrubland (increasing albedo), atmospheric CO2 concentration decreased and a sharp lowering of temperature occurred in the Middle Miocene between 15 and 10 Ma. Eolian dust deposits increased in the Late Cenozoic, suggesting greater aridity (Rea et al., 1985). This is supported by kaolinite records from North Atlantic deep sea sediments (Chamley, 1979). At ~4.8~4.9 Ma global cooling and a marine regression of ~40~50 m combined to isolate the Mediterranean Basin from the ocean and to ...
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