Epeirogeny in the Southern Rocky Mountains region: Evidence and origin

Abstract

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 inflation of the crust by the intrusion of extensive, relatively low density batholiths and plutons of middle Tertiary age. This uplift produced an elongate north-striking 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 epeirogen9s summit is characterized by a coincident geoid anomaly, diminished seismic velocities in the upper mantle, and a north-trending, elevated Curie isothermal surface in the lower crust, all suggestive of elevated temperatures. Surface heat flow 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 epeirogen9s regional topography makes it the highest general feature on the North American plate, individual mountain peak elevations, here and elsewhere, excepted. A first-order, re-leveled survey line in southern Colorado suggests that the epeirogen is still rising today.