Geoscience Reference
In-Depth Information
receives warm, moist air from both the Atlantic and Pacific, as well as torrential rains
from tropical cyclones, which can extend well inland to Oklahoma and beyond.
20.4 Mesozoic and Cenozoic tectonism, cooling and desiccation
in North America
Several seemingly unrelated factors were responsible for the origin of the arid and
semi-arid regions of North America and northern Mexico. First and most important
were the tectonic movements that gave rise to the Cordillera or Rocky Mountains in
the west, the Basin and Range Province, the Colorado Plateau and the Great Plains.
Next in importance was the progressive cooling of the Arctic, which culminated in
the expansion of sea-ice and the accumulation of land-ice across the Laurentian shield
region of North America. As temperatures fell and the climate became drier during
the late Cenozoic, forests gave way to grasslands and the fauna changed accordingly.
Superimposed on these long-term cooling and drying trends were the relatively rapid
climatic fluctuations associated with the waxing and waning of the great ice sheets
of North America and Greenland. The arrival of periodic pulses of meltwater into
the North Atlantic via the St. Lawrence River and down the Mississippi River into
the Gulf of Mexico caused changes in sea surface temperatures and local climate.
More fundamental were alterations to the deep-water circulation in the North Atlantic
and concomitant changes in what has been termed the global 'thermohaline' oceanic
circulation system (Broecker,
1992
; Williams et al.,
1998
), leading on occasion to
severe droughts in North Africa and the Near East. The adjective 'thermohaline'
simply means that the ocean currents are driven by density differences linked to
temperature and salt content, with warm freshwater being less dense than cold, salty
water. The denser water sinks and becomes part of the deep-water circulation, while
the less dense water remains at the surface.
The tectonic history of North America is complex and has been reviewed in detail
elsewhere. For comprehensive summaries of the earlier work, see Bally and Palmer
(
1989
) and Bally et al. (
1989
). Our aim here is simply to summarise the major tectonic
events relevant to the inception of the present-day arid lands of North America. The
origin of the Rockies and of the Colorado Plateau and the Basin and Range Region is
tied up with the history of the Farallon oceanic plate, which began to move beneath
the west coast of North America during the Jurassic. The western margin of this plate
coincides with the East Pacific Rise (see
Chapter 3
).
In a widely cited paper, Dickinson and Snyder (
1979
) argued that once the San
Andreas transform fault had developed, thereby separating the Farallon plate from
the North American plate, the lack of subduction at the transform plate boundary
led to the growth of a slab-free zone beneath the continental block near the San
Andreas transform. They went on to argue that diapiric upwelling of magma from the
asthenosphere led to uplift of the adjacent Sierra Nevada and Colorado Plateau and the
