Paper No. 2
Presentation Time: 1:25 PM


REED Jr, John C., U. S. Geol Survey, MS 980 Federal Center, Denver, CO 80225,

The Colorado Rocky Mountains lie within the largest area in North America with an average elevation of more than 10,000 feet. Some 54 peaks more than 14,000 feet high cluster within this elevated area, more than half the named peaks over 14,000 feet in North America. They are carved in a variety of resistant rocks, including Proterozoic basement rocks, Late Paleozoic sedimentary rocks, and Tertiary intrusive and volcanic rocks. The youngest rock from which any of them is carved is 23 Ma welded tuff.

All but two of the 14ers are in the Colorado Mineral Belt or on the flanks of the Rio Grande Rift, and almost half (including most of the highest) lie within 50 miles of their intersection.

Opening of the Rio Grande Rift began at about 26 Ma and continues today. The Colorado Mineral Belt has no obvious topographic expression, but is marked by high heat flow and by some of the lowest Bouguer gravity anomalies in North America. It is marked by ductile shear zones that were active in the Proterozoic, and by plugs and dikes of felsic intrusive rocks ranging in age from Laramide to Early Miocene.

The Colorado mountain ranges sit atop a broad epeirogenic uplift in which the roots of the Laramide mountains were elevated thousands of feet beginning in the Oligocene, accelerating in the Miocene, and probably continuing to the present. Fault blocks of Miocene strata and interleaved basalt flows have been tilted, and streams on both sides of the uplift deeply incised during Miocene and later uplift. Ten million year old basalt flows have been uplifted by as much as 5000 feet. The distribution of the 14,000 foot peaks along the Colorado Mineral Belt and the Rio Grande Rift suggest that these structures focused later stages of the regional uplift and that the peaks were sculpted by erosion largely during the Late Miocene and the Pliocene. The fact that elevations of the 54 major summits lie within a range of less than 450 feet suggests that the pre-elevation topography was relatively flat, but may have included scattered monadnocks of resistant rocks a few hundred feet high. The close accordance of summit elevations resembles that of high peaks in the Sierra Nevada, but is quite different from that of the peaks elsewhere along the active plate margin.