Isostacy of the Rocky
Mountains
Introduction
A prominent example of the effects of isostacy can be seen in the Rocky Mountain
region which has the highest mean elevation in the United States (Heller 2006).The
Rocky Mountains form the backbone of North America running from Canada to near
Mexico. The Rocky Mountains formed early in the Laramide orogeny which occurred
around from 75-50 ma (Lam, 1998). The uplift was thought to cease around the
Cenozoic and the associated volcanism reappeared in the west, east and north of the
original orogeny. The modern Rockies are thought to then been uplifted in the Cenozoic
after the Ancestral Rockies were basically leveled only leaving sedimentary formations
such as the fountain formation of the flatirons as clues to their existence (Lam, 1998).
The second episode of uplift in the Rockies took place in the Eocene when they were
once again up lifted 1-2km (Lam, 1998). There are many hypotheses to explain the
second uplifting of the Rocky Mountains. The Colorado Rocky Mountains have an
average height of around 3.2 km, this elevation was produced by a variety of events
some of the most supported theories are low angle slab subduction that eroded the
base of the lithosphere, the breaking off of the subducting slab that caused additional
uplift, and the upwelling of the asthenosphere to support the higher topography
(Karlstrom et. al, 2012). The fascinating aspect of the Rockies is the lack of a large
lower density crustal root below the mountain range which usually accompanies high
topography, the lack of the root and the forces that lift the Colorado Rockies will be
explored. The root of the Rockies is not thick enough to support their soaring elevations
at an average of 3.2 km this support is believed to be achieved through a combination
of buoyancy in the crust and upper mantle which was observed through a variety of
geophysical survey methods (Karlstrom et al 2012).
Longs Peak Rocky Mountain National Park
Fountain Formation
Eldorado Springs, Co
14,295 ft.
Discusion on structure
Without a large crustal root to support the height of the Rocky Mountains there
must be some other forces at play supporting the Rockies other than the standard Airy
model of isotactic compensation. There has been a lot of debate over the driving force
that accounts for uplift in the Colorado Rockies to the highest mean elevations of the
Rocky Mountains. The crust underlying the Rockies is only about 50 km on average and
therefore the roots of the mountains are not nearly deep enough to support the
elevation in the airy style isostacy model were a large low density root underlies the
mountain range (Karlstrom et. al, 2012). Many have suggested that over half of the
height of the Rockies is supported by buoyancy in the mantle which correlates well with
the low seismic velocities and gravity surveys taken in the Rockies (Lam, 1998). The
areas of low shear and compressional wave are concentrated around the Rio Grande
rift in southern Colorado and correlate well to the highest velocities (Lam, 1998). The
large scale of the seismic anomaly observed is likely due to a thermal difference in the
mantle and could in a smaller part be compositional in nature (Lam, 1998). The near
complete isostatic compensation seen in the Rockies is another factor that suggest that
the topography is related to interactions in the mantle (Lam, 1998). The isostatic
rebound of the Rockies due to erosion in the Rockies has been estimated through the
use of paleo-surfaces and the erosion rates of rivers. In one such study Karlstrom et al
2012 the study found that the height of the Rockies in the last 10 ma is only about 25%
percent related to isostatic rebound so much of the topography is achieved through
other mechanisms, a map of the isostatic rebound can be seen in figure 1. The
correlation between the mantle and the high elevation in the Rockies has been
observed in many studies. Karlstrom, in a 2012 study found a consistent link between
the highest elevations, lowest seismic velocities of crust and mantle, the highest crustal
rebound. There are several models that have been proposed for the differential uplift of
the Colorado Rockies relative to the surrounding areas. Here are some of the most
current models that have been proposed for the deep structure of the Rocky Mountains.
Model of eroded thickness vs. isostatic rebound
Model of mantle upwelling from 410km
unified hypothesis
Figure 1.
Models From DataDelamination of the lithosphereOne model for the anomalies seen is the delamination on the lithosphere and it
subsequent replacement by up welling asthenosphere (Karlstrom et al 2012). The
delamination of the lithosphere is likely to have occurred in with the magmatism around
the CMB magmatism and the San Juan volcanism which altered the lithosphere and
thinned the crust creating a lower MOHO boundary (Karlstrom et al 2012). The removal
of lithosphere would result in surface up lift predicted to be up to a km in geophysical
models (Karlstrom et. al, 2012). The volcanism and the creation of plutons in SW
Colorado could also be explained through this process.
Mantle Upwelling-
a second model is that mantle may be upwelling under the Colorado Rockies as a result
of melting of the Farralon plate which in turn causes the increased buoyancy seen in the
Colorado portion of the Rocky Mountains. Seismic imaging has revealed images of what
could be the sections of the Farralon plate at the 410 discontinuity boundary (Karlstrom
et al 2012). The rising of the diapers would correlate well with the volcanism, uplift and
isostatic rebound seen in the Colorado Rockies.
-Those two models explain the the relationship between highest topography, thinnest
crust, and the lowest seismic velocities velocities seen in the crust and the mantle
(Karlstrom et al 2012).
Geophysical Data Acquisition MethodsThere have been many methods used in the past to determine just what gives the
Rockies there soaring height in this final section I will look into the various geophysical
techniques used and what they have helped resolve.
Siesmic Refractionseismic refraction investigations of the Rockies began as early as the sixties, many of
the early surveys were centered around the Rio Grande Rift and were used to
determine the amount of extension (find). Seismic refraction has also been used by
various researchers to resolve mantle and crust thickness and structure(find). Siesmic
refraction has allowed the idea that the mantle must be involved in isostatic
compensation to be revealed (find). Seismic refraction has also revealed the low mantle
velocities seen in the low density mantle underneath Colorado (find).
Siesmic Reflectionseismic reflection has also been used as a tool be researchers to resolve the structure
and isostacy of the Rocky Mountains (find). Many of the seismic reflection surveys have
been the result of petroleum exploration and in turn many are not released to the
public(find). One find of seismic reflection is a thrust fault dipping through the crust to
nearly 25km in the Wind River Range of Wyoming.
Gravity Studiesgravity studies have also been used in studying the Rocky Mountains usually in
combination with seismic surveys. Gravity surveys can be used to estimate thickness
and composition of structures (find). Crustal thickness has also been looked at through
gravity surveys and low density bodies such as large batholiths (find).
References
Karlstrom, K. E., Coblentz, D. D., Dueker, K. K., Ouimet, W. W., Kirby, E. E., van Wijk, J.
J., & ... Cole, R. (2011). Mantle-driven dynamic uplift of the Rocky Mountains and
Colorado Plateau and its surface response; toward a unified hypothesis.
Lithosphere, 4(1), 3-22. doi:10.1130/L150.1
Lerner-Lam, A. L., Sheehan, A. F., Grand, S., Humphreys, E. D., Dueker, K. G., Hessler,
E., & ... Savage, M. (1998). Deep structure beneath the Southern Rocky
Mountains from the Rocky Mountain Front broadband seismic experiment. Rocky
Mountain Geology, 33(2), 199-216.
Keller, G. R., Snelson, C. M., Sheehan, A. F., and Dueker, K. G., 1998, Geophysical studies of crustal
structure in the Rocky Mountain region: A review, Rocky Mountain Geology, v. 33, no. 2, p. 217-228.