The Colorado Rocky Mountains Awaken: Understanding Topographic Rejuvenation in Postorogenic Mountain Belts

Tectonically inactive postorogenic mountains enigmatically display landscape instability through fluctuations in fluvial incision, relief production, and sediment flux long after mountain building ends. The Colorado Rocky Mountains (CRM), situated in the interior of the North American plate, exemplify this phenomenon. Primarily constructed during the Laramide orogeny (ca. 80–40 Ma), the CRM maintain high-relief terrain, while the fluvial network at their ancient foreland basin, the High Plains, records a shift from net deposition to net erosion in the Pliocene to early Pleistocene. Geodynamic and climatic drivers have been proposed to explain this evidence of a geologically recent landscape rejuvenation, yet conclusive evidence favoring a given mechanism remains elusive. In this study, we analyze bedrock channels draining the CRM to the High Plains using new and existing ¹⁰Be-derived basin-averaged erosion rates and bedrock incision rates from luminescence dating of fluvial terraces to determine whether the rejuvenation of the Colorado Rockies is due to regional geodynamic forces or climatic changes in the last ~5 m.y. Within fluvially dominated portions of the CRM, our results reveal two distinct geomorphic zones separated by a series of upstream-migrating knickpoints across the entire ~350-km-long mountain front: (1) a slowly eroding, lower-gradient landscape at higher elevations, and (2) a steep, rapidly eroding landscape at lower elevations. The lower and steeper landscape below knickpoints indicates a regional increase in the rate of base-level lowering relative to the CRM mountain front, implying geologically recent relief production. Additionally, our results detect a gradual increase in channel steepness below knickpoints from north to south, consistent with previous geomorphic evidence and geodynamic models for the CRM region, which predict a southward increase in rock uplift rates in the last ~5 m.y. On a broader scale, our findings demonstrate that geodynamics, along with climate, lithology, and autogenic factors, can influence the evolution of postorogenic mountain belts long after mountain building has ceased.