Topography and origin of the southern Rocky Mountains and Alvarado Ridge

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Abstract
Summary: The southern Rocky Mountains of the western United States and their structural continuation southward to the Mexican border represent the crest of a bilaterally symmetrical, continental feature of large dimensions, the Alvarado Ridge. It is characterized by long, gentle topographic rises with systematic, concave-upward slopes on which elevation declines in a quasi-exponential manner. The rises were originally blanketed with clastic sediments, a few tens to hundreds of metres thick; their erosional source being at the ridge crest. The blanket on the E rise is well preserved and has been undisturbed for nearly 5 Ma, save for regional Neogene warping and local, near-crest faulting associated with uplift of the ridge. A comparative study of variations in regional elevation, gravity and crustal thickness suggest that the Alvarado Ridge and its rises are isostatically compensated and that almost none of the compensation involves an Airy crustal root. The flexural rigidity of the lithosphere is likewise believed to play but a minor role in the origin of the regional topography. Instead, the data are interpreted as confirming the hypothesis of distributed lithospheric thinning continuous and non-continuous in nature, and related thermotectonic uplift. Elevation of the ridge crest took place above the axis of an elongate, rapidly developing, asthenospheric bulge along which extensional strain in the shallow crust was limited to a central corridor only 150–200 km wide. The relatively low density and geometrical configuration of that bulge supports the topography. Heat-flow data in the region are in accord with this model.