A tectonic redefinition of the Southern Rocky Mountains

Abstract

Abstract The Southern Rocky Mountains of the Western United States, physiographically defined and described by N.M. Fenneman nearly 60 yrs ago, are tectonically redefined and extended. They are shown to constitute the crestal range of a mammoth, continental, arch-like feature here named the Alvarado ridge. Its axis trends south from Casper, Wyoming at least as far south as El Paso, Texas, beyond which the ridge begins to lose morphological identity. Maximum elevations along the crest of the ridge exceed 4.2 km. The summit ranges are bordered on either side by gently sloping rises that extend outward for at least 1300 km, falling to elevations of less than 400 m. Modest rift structures along the ridge axis continue beyond the southern terminus of the mountains before playing out. A major sediment-filled axial graben exists over the southern two-thirds of the ridge, but equivalent parts of it farther north were stripped of their Neogene fill by erosion in the headwaters areas of the Colorado and North Platte rivers. The maximum elevation of earlier Laramide mountains in this area has been estimated to have been no more than 2 km in Colorado, half that of the present range. Related topography was nearly obliterated by erosion prior to late Eocene time. Lateral stream planation produced a southeast-sloping, major late Eocene erosion surface across the region that had what was probably an isostatically adjusted, average maximum elevation of less than 900 m. Present day elevations and relief on the ridge crest are the result of steep crestal normal faulting, pronounced block uplift, and regional arching, with extensional strain limited to an axial corridor less than 200 km wide. This episode of mountain building began in middle Miocene time (17 to 12 Ma), culminating in latest Miocene and early Pliocene time, between 7 and 4 Ma ago. Debris from the newly elevated range (the Southern Rocky Mountains, sensu lato) was shed along the full length of the Neogene Alvarado ridge down parts of both rises, as well as into its medial graben. The uplifted and tilted east rise (the Great Plains ramp) has an extensive aggradational cover that spread eastward to distances of 700 km and more in middle Miocene to earliest Pliocene time. Its fluviatile, lacustrine and eolian sediments are undergoing dissection today, but a sufficient portion remains to define the unmodified profile of the east rise, as well as the original length of the ridge. It is coincidentally similar in configuration and scale to the west rise of the mid-Atlantic ridge. The topography of the crestal mountain ranges on both structures is also remarkably similar. The Southern Rocky Mountains are as high as the Alps, but are not the product of lithospheric compression and crustal thickening. Rather they reflect profound epeirogenic uplift driven by lithospheric thinning and accompanied by extensional strain and differential vertical jostling along the crest of the ridge.