Rocky+Mountains

Isostacy of the Rocky Mountains

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).

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. upper left Model of eroded thickness vs. isostatic rebound upper right mantle up welling 410 km



__Delamination of the lithosphere__- __Models From Data-__ One 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 Methods-__ There 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. __Seismic Refraction-__ seismic 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 (Keller 1998). Seismic refraction has also been used by various researchers to resolve mantle and crust thickness and structure(Keller 1998). Seismic refraction has allowed the idea that the mantle must be involved in isostatic compensation to be revealed (Keller 1998). Seismic refraction has also revealed the low mantle velocities seen in the low density mantle underneath Colorado (Keller 1998). __Seismic Reflection-__ seismic reflection has also been used as a tool be researchers to resolve the structure and isostacy of the Rocky Mountains (Keller 1998). Many of the seismic reflection surveys have been the result of petroleum exploration and in turn many are not released to the public(Keller 1998). One find of seismic reflection is a thrust fault dipping through the crust to nearly 25km in the Wind River Range of Wyoming. __Gravity Studies-__ gravity 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 (Keller 1998). Crustal thickness has also been looked at through gravity surveys and low density bodies such as large batholiths (Keller 1998).