Abstract
The source of buoyancy for the uplift of cratonic plateaus is a fundamental question in continental dynamics. The ~1.9 km uplift of the Colorado Plateau since the Late Cretaceous is a prime example of this problem. We used apatite (U-Th)/He thermochronometry (230 analyses; 36 samples) to provide the first single-system, regional-scale proxy for the unroofing history of the southwestern quadrant of the plateau. The results confirm overall southwest to northeast unroofing, from plateau margin to plateau interior. A single phase of unroofing along the plateau margin in Late Cretaceous to Early Tertiary (Sevier-Laramide) time contrasts with multiphase unroofing of the southwestern plateau interior in Early and mid- to Late Tertiary time. The Early Cretaceous was characterized by northeastward tilting and regional erosion, followed by aggradation of ≥1500 m of Upper Cretaceous sediments along the eroded plateau margin. Sevier-Laramide denudation affected the entire southwestern plateau, was concentrated along the plateau margin, and migrated from northwest to southeast. Following a period of relative stability of the landscape from ca. 50–30 Ma, significant unroofing of the southwestern plateau interior occurred between ca. 28 and 16 Ma. Additional denudation north of the Grand Canyon took place in latest Tertiary time.
Mid-Tertiary dates from the Grand Canyon basement at the bottom of the Upper Granite Gorge limit significant incision of the modern Grand Canyon below the Kaibab surface to <23 Ma. Modeling the age distributions of samples from the basement and Kaibab surface nearby suggests that the gorge and the plateau surface had similar Early to mid-Tertiary thermal histories, despite their >1500 m difference in vertical structural position. If these models are correct, they indicate that a “proto–Grand Canyon” of kilometer-scale depth had incised post-Paleozoic strata by the Early Eocene. Evidence for kilometer-scale mid-Tertiary relief in northeast-flowing drainages along the plateau margin, as well as the mid-Tertiary episode of plateau interior unroofing, imply that the southwestern plateau interior had attained substantial elevation by at least 25–20 Ma, if not much earlier. These observations are inconsistent with any model calling for exclusively Late Tertiary uplift of the southwestern plateau.
Sevier-Laramide plateau surface uplift and incision thus result from one or more processes that enhanced the buoyancy of the plateau lithosphere, expanding the Cordillera's orogenic highlands into its low-standing cratonic foreland. The onset of the Laramide slab's demise at ca. 40 Ma and the major pulse of extension in the Basin and Range from ca. 16–10 Ma appear to have had little influence on the denudation history of the southwestern plateau. In contrast, the post-Laramide unroofing episodes may be explained by drainage adjustments induced by rift-related lowering of regions adjacent to the plateau, without the need to otherwise modify the plateau lithosphere. Our data do not preclude a large component of post–Early Eocene elevation gain (or the geodynamic mechanisms it may imply), but they do point toward Laramide-age buoyancy sources as the initial cause of significant surface uplift, ending more than 500 m.y. of residence near sea level.
