ONGOING MANTLE-DRIVEN UPLIFT OF THE COLORADO PLATEAU-ROCKY MOUNTAIN REGION

Geomorphic studies of the Colorado Plateau - Rocky Mountain (CP-RM) region show that river systems on opposite sides of the continental divide were integrated to near- present base levels after 6 Ma. We hypothesize that this “young” integration was primarily caused by Neogene mantle-driven uplift rather than by climatic influences and drainage reorganization events acting alone upon a landscape uplifted prior to the Neogene. This was the most recent of 3 episodes of uplift of the CP-RM region that are widely recognized but with relative magnitudes that have been long debated. Conceptual approaches to quantifying “young” (after 10 Ma) uplift are as follows. 1) Climate forcings are predicted to drive episodic river incision (especially since ca. 3-2 Ma onset of glacial-interglacial cycles) and extend beyond watershed boundaries. 2) Mantle-driven uplift/subsidence predicts long-wavelength surface response to mantle flow and buoyancy variation that should be compatible with geophysical and geologic data. 3) Mantle-driven uplift is predicted to produce temporally steady but spatially variable bedrock incision such that observed patterns of differential incision may indicate differential uplift related to structural boundaries, locii of magmatism, and zones of sharp upper mantle velocity transitions. 4) River integration and stream piracy events can also result in differential incision but are predicted to result in non-steady (in time and space) incision patterns due to transient knickpoints which propagate upstream. Observed incision magnitudes are > 1 km in the Rocky Mountains and southwestern edge of the Colorado Plateau indicating that the RM are uplifting relative to the CP, and the CP relative to the Basin and Range (BR) in the past 10 Ma. Incision rates averaged over the past 0.5 to 4 Ma plotted on mantle tomography at 100 km depth show that highest long-term rates and magnitudes correspond to low velocity domains and sharp mantle velocity transitions. Thus, differential river incision is interpreted to reflect km-scale mantle-driven differential uplift. Uplift mechanisms include a combination of mantle flow, magmatism, delamination/isostatic support, isostatic rebound due to differential erosion, and footwall uplift during extension.