Geoscience Reference
In-Depth Information
Liu and Gurnis (
2010
) proposed a model of uplift based on Farallon slab evolution
and mantle convection to account for uplift of the south-west plateau. They concluded
that up to 1,200 m of uplift took place between the Late Cretaceous and the Eocene,
with an initial phase of subsidence followed by uplift in two stages, one in the
Late Cretaceous, the other in the Eocene. An apparently opposing model based on
late Cenozoic edge-driven convection along the plateau margins indicated several
hundred metres of late Cenozoic uplift (van Wijk et al.,
2010
). In fact, as Flowers
(
2010
) noted in her commentary on these papers, the two models are not mutually
exclusive, and both early and later uplift are possible. Flowers and Farley (
2012
)
later obtained apatite
4
He/
3
He and (U-Th)/He thermochronometric ages for both the
eastern and western Grand Canyon and concluded that the western Grand Canyon had
been incised to within a few hundred metres of its modern depths by around 70 Ma
ago, refuting the view that the entire canyon was cut in the last 5 to 6 Ma. It is even
possible that the ancestral Colorado River flowed north-east into the Labrador Sea
during the late Oligocene-early Miocene before its capture by a river flowing to the
Gulf of California (Sears,
2013
).
Hyndman and Currie (
2011
) observed that the crust beneath the North American
Cordillera is only 30-35 km thick, in contrast to the 40-45 km of the low-elevation
craton and other tectonically stable areas to the east of the Cordillera. They concluded
that the Cordillera is located in a former back-arc zone where temperatures are
uniformly hot, with thermal expansion responsible for about 1,600 m of the present
Cordillera elevation compared to the colder stable areas. However, uplift was not
synchronous along the entire length of the North American Cordillera. Mix et al.
(
2011
) examined around 3,000 stable isotope measurements that had been used as
an index of surface elevation and concluded that Eocene uplift propagated from
north to south, leading to the development of an Eocene-Oligocene highland 3-4 km
in elevation, by which time modern patterns of precipitation had been established.
Although there was continued uplift, faulting and local subsidence after that time,
continuing in to the Quaternary, themajor topographic elements governing the location
of the present-day arid and semi-arid areas in North America and northern Mexico
were well-established by the late Oligocene. Before proceeding to consider how these
tectonic events influenced the biota in the deserts and semi-deserts of the region, we
need first to consider Cenozoic climatic changes in the Arctic and their impact on
North America.
20.5 Cenozoic cooling of the Arctic
The early Cenozoic environment of the Arctic region was very different from that of
today (Grantz et al.,
1990
), culminating in the Palaeocene-Eocene thermal maximum
of around 55 Ma, when a brief period of rapid global warming took place at a
time when the world was already warm, most probably as a result of an increase in
atmospheric greenhouse gas concentrations (Pagani et al.,
2006
; Sluijs et al.,
2006
).
