2011 - Cooperative Institute for Research in Environmental Sciences ...

Craig Jones
Understanding the Origin of Mountains
in the Western U.S.
FUNDING: NATIONAL SCIENCE FOUNDATION (TECTONICS,
EARTHSCOPE AND CONTINENTAL DYNAMICS PROGRAMS)
Mountains in the western
U.S. form at times
and in manners that
seemingly disregard the
way North America interacts
with oceanic plates to
the west. At present, my
group’s research focuses
on two areas: the Sierra
Nevada in California and
the Southern Rockies in
Colorado and Wyoming.
Studies of the Sierra
Nevada have centered
around a major seismological
deployment from
2005–2007. Images of the
crust and mantle in the
range show that the crust
is actually thicker under
the low western foothills than the high part of the range.
It appears that the western part of the range has dense
material under it that has recently been removed from the
High Sierra, causing the High Sierra to rise up relatively
recently.
In the Rockies, colleagues Lang Farmer (CIRES),
Shijie Zhong (CU Physics), Brad Sageman (Northwestern
University) and I have proposed a new means to make the
Rockies. In essence, the idea is that thick, ancient conti-
38 CIRES Annual Report 2011
Longs Peak in Colorado.
nental plate in Wyoming was sucked down by subducting
oceanic plate that got unusually close to the plate’s base.
In pulling down the Colorado-Wyoming region, this created
a hole filled by the Pierre Shale, a marine sedimentary
rock responsible for bowing up basements around the
Boulder area. By pulling down on this area, the sucking
causes the side to crowd in to fill the hole, and this creates
the forces that deform the crust and build the Rockies.
Illustrations contrasting
elements of existing and
new explanations
for geologic events
about 70 million years
ago that led to creation
of the Southern Rockies.