NEOTECTONISM IN THE COLORADO FRONT RANGE AS SHOWN BY PATTERNS OF EROSION

Neotectonism in the Front Range east of the concurrently developing Rio Grande rift was reflected by paleotopographies developed on shifting areas of local deformation. A ROUNDED UPLAND TERRAIN on Precambrian rocks in the core of the Front Range was flanked outwardly by progressively lower foothills to widespread plains on sedimentary rocks. The rounded uplands terrain bevels Oligocene intrusions and volcanic rocks, indicating an early Miocene age. The present mountain area was uplifted irregularly in middle Miocene time as a rising Front Range block tilted eastward; low domes formed in the southern and northern parts of the Front Range while the central area remained low. Major block faulting took place within the southern dome. Local uplift caused erosion to cut a hilly terrain on the flanks earlier domes (INCISED TERRAIN), which fed debris to the middle and upper Miocene Ogallala Formation on the adjacent High Plains. This erosion progressed slowly at first, and formed a moderately hilly topography (Rocky Mountain peneplain of Lee). Mountain uplift quickened at the end of the Miocene and extended eastward onto the western High Plains; concurrent erosion cut flaring- to precipitously-walled canyons (CANYON TERRAIN) in the mountains, deposition of the Ogallala ceased, and excavation of the modern broad valleys of the western High Plains began. Sequential patterns of erosion both in the mountains and plains indicate that uplift and erosion took place in a developing mosaic of local blocks, and not from a single brief epirogenic episode.