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The Yellowstone plateau volcanic field is less than 2 million years old, lies in a region of intense tectonic and hydrothermal activity, and probably has the potential for further volcanic activity. The youngest of three volcanic cycles in the field climaxed 600,000 years ago with a voluminous ashflow eruption and the collapse of two contiguous cauldron blocks. Doming 150,000 years ago, followed by voluminous rhyolitic extrusions as recently as 70,000 years ago, and high convective heat flow at present indicate that the latest phase of volcanism may represent a new magmatic insurgence. These observations, coupled with (i) localized postglacial arcuate faulting beyond the northeast margin of the Yellowstone caldera, (ii) a major gravity low with steep bounding gradients and an amplitude regionally atypical for the elevation of the plateau, (iii) an aeromagnetic low reflecting extensive hydrothermal alteration and possibly indicating the presence of shallow material above its Curie temperature, (iv) only minor shallow seismicity within the caldera (in contrast to a high level of activity in some areas immediately outside), (v) attenuation and change of character of seismic waves crossing the caldera area, and (vi) a strong azimuthal pattern of teleseismic P-wave delays, strongly suggest that a body composed at least partly of magma underlies the region of the rhyolite plateau, including the Tertiary volcanics immediately to its northeast. The Yellowstone field represents the active end of a system of similar volcanic foci that has migrated progressively northeastward for 15 million years along the trace of the eastern Snake River Plain (8). Regional aeromagnetic patterns suggest that this course was guided by the structure of the Precambrian basement. If, as suggested by several investigators (24), the Yellowstone magma body marks a contemporary deep mantle plume, this plume, in its motion relative to the North American plate, would appear to be "navigating" along a fundamental structure in the relatively shallow and brittle lithosphere overhead. The concept that a northeastwardpropagating major crustal fracture controls the migration path of the major foci of volcanisim is at least equally favored by existing data, as Smith et al. (19) noted.

The mountain peaks of the present-day Southern Rocky Mountains are the highest peaks in the Rocky Mountain system. They represent a second generation of mountains, one that originated from a different tectonic mechanism from that of the predecessor Laramide Rockies. Epeirogeny lifted the Laramide ranges in Colorado and New Mexico after their Late Cretaceous-early Cenozoic orogenic creation. The area was lifted tectonically from 1300 m to perhaps as much as 2000 m, the result of heating of the lithosphere stemming from its thinning, as well as infl ation of the crust by the intrusion of extensive, relatively low density batholiths and plutons of middle Tertiary age. This uplift produced an elongate northstriking crustal swell that cuts across major structural features in the crust, including the northeast-trending fundamental sutures that resulted from assembly of the North American plate, the northwest-striking trends of the Ancestral Rocky Mountains in Colorado, and the northeast-trending Colorado Mineral Belt. The contemporary Southern Rockies are unique in that their eastern piedmont slope is quite unlike that of other prominent orogenic mountain ranges around the globe owing to the presence of this supporting swell, or epeirogen. The lithosphere beneath the epeirogen's summit is characterized by a coincident geoid anomaly, diminished seismic velocities in the upper mantle, and a northtrending, elevated Curie isothermal surface in the lower crust, all suggestive of elevated temperatures. Surface heat fl ow on the summit is complex, revealing both shallow crustal heat sources and a much deeper, more profound source that strikes north. Uplift resulting from these factors was initiated in post-middle Eocene time. At the wavelength of topographic smoothing employed here, the epeirogen's regional topography makes it the highest general feature on the North American plate, individual mountain peak elevations, here and elsewhere, excepted. A fi rst-order, re-leveled survey line in southern Colorado suggests that the epeirogen is still rising today.

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.

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