LATE CENOZOIC RIVER INCISION IN THE ROCKY MOUNTAIN REGION AND POSSIBLE CONNECTIONS TO MANTLE-DRIVEN UPLIFT

Late Cenozoic histories of the Colorado, Green and Yampa Rivers suggest possible links between mantle processes and patterns and rates of river incision. New age estimates of fluvial terrace gravels and basin-fill volcanic units show that the onset of river incision has varied regionally in parts of Colorado, Wyoming, and Utah. These differences are attributable to Neogene differential uplift of the Rocky Mountain region driven by mantle flow and/or buoyancy.

Ages of Miocene basalt flows show that upper Colorado integration occurred ca. 16-11 Ma, prior to integration through Grand Canyon ca. 6-5 Ma. Colorado River bedrock incision began 9.5-7.8 Ma and resulted in 750 to 1500 m of incision at rates of 96 to 155 m/Ma since 10 Ma. Largest magnitudes of incision and fastest incision rates are associated with areas of Miocene basaltic magmatism. In the upper Yampa River valley, incision began later than 6.1 Ma, and the river has incised 700 to 850 m at rates of 90 to 115 m/Ma. It is possible that incision in the lower Yampa River valley began slightly earlier, ca. 8 Ma. Integration of the upper Green River is poorly constrained between 8 and 2 Ma, and probably post-dates the advent of the ancestral Yampa River ca. 8-6 Ma. Using Oligocene river gravels as a datum, the Green River has incised <400 m at rates of <20 m/Ma over the past ca. 30 Ma.

Comparison of Neogene river incision patterns with P-wave tomography suggests possible connections between mantle processes and landscape evolution. Upper Colorado regions have the largest incision magnitudes and fastest incision rates, and the slowest P-wave velocities. In contrast, the upper Green River basin has the smallest incision magnitudes and slowest incision rates, and fastest P-wave velocities. These broad differences are consistent with differential uplift between the Colorado Rockies and the upper Green River basin caused by differences in mantle buoyancy.