Geoscience Reference
In-Depth Information
opening of the Rio Grande Rift. Later workers have disputed this interpretation, with
Schellart et al. (
2010
) offering an alternative interpretation involving the Cenozoic
slowdown of the Farallon plate and the ensuing decrease in the rate of subduction. They
concluded that the change from the Sevier-Laramide orogenic regime to the Basin and
Range extensional tectonic regime was related to the width of the subducting Farallon
slab in western North America. Uplift and deformation in the Rocky Mountains
began in the Jurassic some 160 Ma ago, culminating in uplift of the Sevier Mountains
and Canadian Rockies around 125 Ma ago. Schellart et al. (
2010
) postulated that
these orogenic events were associated with the subduction of a wide slab and crustal
shortening, whereas extension began in the Eocene around 45 Ma ago when there was
a phase of intermediate to narrow slab extension and slab-driven trench retreat.
Regardless of the precise causes of uplift and extension, our primary concern here
is with the timing and amount of uplift or subsidence, which is often surprisingly
hard to establish from existing data. Determining how and when the Grand Canyon
formed illustrates this point well. There are several schools of thought in regard to
what Rebecca Flowers (
2010
) has called 'the enigmatic rise of the Colorado Plateau'
and the equally enigmatic history of the Colorado River and its incision into the
Colorado Plateau (
Figures 20.1
and
20.2
). One view is that uplift of the plateau
and corresponding river incision is geologically recent, possibly no more than about
5 million years. The opposing view is that uplift began at least 25 million years ago. An
arsenal of highly ingenious techniques has been used in an effort to solve the enigma.
Sahagian et al. (
2002
) observed that the size of the uppermost vesicles within
basalt flows reflects surface atmospheric pressure at the time that they formed, and
they postulated that the inferred atmospheric pressure can then be used to estimate
the surface elevation at the time when the lava flows were laid down. Most of the
lava flows in the Colorado Plateau were emplaced over the past 25 Ma at what are
now elevations of 800-2,000 m. The results of the vesicle analysis showed that uplift
was already underway by 25 Ma, with 800 m of uplift before 5 Ma at an average
rate of 40 m/Ma and 1,100 m of uplift after 5 Ma at a mean rate of 220 m/Ma. They
concluded that initial Miocene uplift was relatively slow, with an order of magnitude
increase in the rate of uplift during the Pliocene and Quaternary.
Polyak et al. (
2008
) used a special type of speleothem called 'cave mammillaries'
to determine past elevations of the water-table at nine sites in the Grand Canyon.
They obtained uranium-lead ages for the speleothems and assumed that incision into
the Grand Canyon was accompanied by a fall in the local groundwater-table, which
seems reasonable. They further assumed that the rate of incision was comparable to
the rate of water-table fall. On this basis, they concluded that incision in the western
Grand Canyon amounted to 55-123 m/Ma over the past 17 Ma, while incision was
more rapid in the eastern Grand Canyon, amounting to 166-411 m/Ma. They went
on to propose that the Grand Canyon was formed by headward erosion from west
to east, with an acceleration in incision in the eastern sector during the past 3.7 Ma
or so.
