2.13 Rocky Mountains, Formation of the Southern Rockies: Still an Enigma being Debated by Geologists

The portion of the Rocky Mountains that extends from Southern Montana to New Mexico was produced by a period of deformation known as the Laramide Orogeny. This event, which created some of the most picturesque scenery in the United States, peaked about 60 million years ago. The mountain ranges generated during the Laramide orogeny include the Front Range of Colorado, the Sangre de Cristo of New Mexico and Colorado, and the Big Horns of Wyoming.

Playlist of Geology and Earth Science Videos from Snow Mountain
https://www.youtube.com/watch?v=owlPSr39Nn8&list=PL6taRb3I0WLhYc8g1cuBk5crqUtLs3oXZ

These mountains are structurally much different from the northern Rockies, which include the Canadian Rockies and those portions of the Rockies found in Idaho, Western Wyoming, and Western Montana. The northern Rockies are compressional mountains, composed of thick sequences of sedimentary rocks that were deformed by folding and low-angle thrust faulting. Most investigators agree that the collision of one or more microcontinents with the western margin of North America generated the driving force behind the formation of the northern Rockies.

The southern Rockies, on the other hand, formed when deeply buried crystalline rock was lifted nearly vertically along steeply dipping faults, upwarping the overlying layers of younger sedimentary rocks. The resulting mountainous topography consists of large blocks of ancient basement rocks that are separated by sediment filled basins. Since their formation, much of the sedimentary cover has been eroded from the highest portions of the uplifted blocks, exposing their igneous and metamorphic cores. Examples include a number of granitic outcrops that project as steep summits, such as Pikes Peak and Longs Peak in Colorado Front Range. In many areas, remnants of the sedimentary strata that once covered this region are visible as prominent angular ridges, called hogbacks, flanking the crystalline cores of the mountains.

It was once assumed that like other regions of mountainous topography, the southern Rockies stood tall because the crust had been thickened by past tectonic events. However, seismic studies conducted across the American southwest revealed a crustal thickness no greater than that found below Denver. These data ruled out crustal buoyancy as the cause for the abrupt two kilometer, 1.2 mile, jump in elevation that occurs where the Great Plains meet the Rockies.

Although the southern Rockies have been extensively studied for more than a century, there is still a good deal of debate regarding the mechanisms that led to uplift. One hypothesis proposes that this period of uplift started with the nearly horizontal subduction of the Farallon plate eastward beneath North America as far inland as the Black Hills of South Dakota. As the subducted slab scraped beneath the continent compressional forces initiated a period of tectonic activity. As the comparatively cool Farallon plate sank, it was replaced by hot rock that upwelled from the mantle. But, according to this scenario, the hot mantle provided the buoyancy to raise the southern Rockies, as well as the Colorado Plateau in the mountains of the Basin and Range.

Others disagree, maintaining that there is no need to invoke the process of buoyant subduction. Rather, they suggest that plate convergence and the collision of one or more microcontinents to the western margin of North America generated the driving force behind the Laramide orogeny.

It should be pointed out that neither of these proposals has gained widespread support. As one geologist familiar with this region put it, “We just don’t know”. Sounds like a great challenge for future geologists to study.

Playlist of Geology and Earth Science Videos from Snow Mountain
https://www.youtube.com/watch?v=owlPSr39Nn8&list=PL6taRb3I0WLhYc8g1cuBk5crqUtLs3oXZ