Tertiary cooling history of the Gore Range: a northern Rio Grande Rift flank uplift, central Colorado

Despite extensive study, there is no consensus on why or when the Rocky Mountains experienced the uplift that created the region's dramatic topography. Some infer that the region experienced regional uplift as recently as the last few million years, possibly attributable to mantle upwelling associated with the Rio Grande Rift. Whether the Rift propagated northward in Tertiary time as implied by its tapering northward character or experienced a similar history along most of its length bears on the role of the Rift in the evolution of the Rockies. The Gore Range and adjacent Blue River Valley of central Colorado represent the northernmost significant fault-related manifestation of the Rift in the heart of the Rockies. The Blue River normal fault separates the basin from the range and accommodates a minimum vertical displacement of 1.4 km, creating 1.4 km relief in the entirely crystalline southern half of the range. A cross fault separates the southern from the northern Gores. The rolling hills north of the cross fault have a maximum relief of 700 m, vertical displacement on the Blue River fault of up to 800 m, and retain a partial cover of Mesozoic sedimentary and mid-Tertiary volcanic rocks.;Apatite (U-Th)/He dates for 15 samples in the southern Gore Range are younger eastward toward the Blue River fault. In the western Gore Range, Eocene dates are preserved at elevations >3600 m while Oligocene dates occur at lower elevations. Eastern Gore samples from an elevation range of 4000-3000 m yield Miocene dates. The youngest date of 6.9 Ma occurs at an elevation of 2790 m at the eastern edge of the range. These results suggest two separate cooling and unroofing episodes in Oligocene and Miocene time. The youngest date implies ~2 km of unroofing in the easternmost Gore Range since the late Miocene. It is inferred that the entire range underwent late Oligocene unroofing. The geomorphic contrast between the northern and southern Gores is likely due to the restriction of significant displacement along the Blue River fault to the southern Gores, inducing Miocene unroofing, creating significant relief, and stripping any remaining sedimentary cover from this area. This two-phase unroofing history is similar to that inferred for other rift basins to the south, implying the broadly synchronous onset and evolution of a >700 km segment of the Rio Grande Rift.