Topography and origin of the southern Rocky Mountains and Alvarado Ridge

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.

History and causes of post-Laramide relief in the Rocky Mountain orogenic plateau

“…This event is earlier than, and separate from, the incision event described here. Thus, we agree with McKenna and Love (1972) that the Ogallala Formation and its equivalents mark a nearly continuous aggradational surface that covered most of the basins formed during the Laramide orogeny and merged with the lowrelief topography in the adjacent Great Plains and Colorado Plateau (Lillegraven and Ostresh, 1988;McKenna and Love, 1972;Eaton, 1987).…”

“…In the Colorado Plateau, maximum basin fill thickness is hard to calculate, because there are few occurrences of late Cenozoic sedimentary units. It is possible that very little deposition occurred on the Colorado Plateau during this interval, however, exhumed Oligocene-Miocene intrusives and perched Oligocene-Miocene basalt flows indicate that the land surface, even without basin fill deposits, remained relatively high and likely merged with the basin fill surface to the northeast and east (Eaton, 1987) and that timing of turnaround from net aggradation to net degradation was similar to that of the Rocky Mountains (Hunt, 1956;Nelson et al, 1992).…”

“…A misfit exists, however, between the width of incision (>250 km) on either side of the Rio Grande Rift and the characteristic wavelength of thermal surface uplift associated with continental rifting (10-100 km; Wemicke and Axen, 1988). Thus, the Rio Grande Rift may be a surface manifestation of, or superimposed upon, some long-wavelength, possibly dynamic, uplift related to upper-mantle and/or thermal effects (Eaton, 1986(Eaton, , 1987Heller et al, 2003;Roy et al, 2004).…”

“…Traditionally, this cycle of incision has been assumed to record late Cenozoic regional epeirogeny beginning ca. 5 Ma (Trimble, 1980;Epis and Chapin, 1975;Eaton, 1987). More recently, however, climate change as the cause of incision has been invoked (c.f.…”

Geophysical Constraints on the Flexural Subsidence of the Denver Basin

“…Lithospheric thinning can explain a buoyant subsurface load due to hot light asthenospheric material substituting denser mantle lithosphere (Reinke, 1991). Eaton (1987) argued that this may have coincided with regional uplift and lithospheric extension throughout the western portion of the basin which began in Miocene time. There are Miocene-age normal faults in much of north central Colorado and south central Wyoming (Izett, 1975).…”