For permission to copy, contact editing@geosociety.org © 2008 Geological Society of America 571 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 Creta- ceous 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 quad- rant of the plateau. The results confirm over- all 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 inte- rior 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 pla- teau 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, signifi- cant 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 sur- face nearby suggests that the gorge and the plateau surface had similar Early to mid-Ter- tiary 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 inte- rior 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 con- trast, 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 buoy- ancy sources as the initial cause of significant surface uplift, ending more than 500 m.y. of residence near sea level. Keywords: Colorado Plateau, (U-Th)/He, Grand Canyon, unroofing, incision, uplift, ther- mochronometry. INTRODUCTION Like most of the North American craton, the Colorado Plateau remained near sea level for 500 m.y. during slow subsidence and deposi- tion of Paleozoic and Mesozoic sediments (Hunt, 1956). However, unlike most of the craton, the plateau was uplifted to its current elevation of ~1.9 km with little internal upper crustal strain (<1%), requiring the acquisition of significant lithospheric buoyancy, sometime after widespread deposition of Upper Creta- ceous marine sediments. Models of how this buoyancy was acquired are numerous (e.g., McGetchin et al., 1980; Morgan and Swanberg, 1985), and can be broadly subdivided into three groups on the basis of the predicted timing of uplift. Late Cretaceous to Early Tertiary uplift mechanisms related to Sevier-Laramide contractional defor- mation (80–40 Ma) include crustal thickening due to channel flow (e.g., McQuarrie and Chase, 2000), convective removal of lithospheric man- tle (England and Houseman, 1988), or chemical modification of the lithosphere by volatile addi- tion from the Laramide flat slab (Humphreys et al., 2003). Mid-Tertiary buoyancy addition (40–20 Ma), perhaps driven by the demise of the Laramide flat slab, could be due to partial removal of the plateau lithosphere and replace- ment with hot asthenosphere (Spencer, 1996), or post-Laramide chemical modification through melt extraction along the plateau margins (Roy et al., 2004). Late Tertiary uplift models (post– 20 Ma) associated with regional extensional tectonism involve heating the lithosphere from below (Thompson and Zoback, 1979) possibly aided by a mantle plume (Parsons and McCarthy, GSA Bulletin; May/June 2008; v. 120; no. 5/6; p. 571–587; doi: 10.1130/B26231.1; 10 figures; 1 table; Data Repository Item 2008027. Unroofing, incision, and uplift history of the southwestern Colorado Plateau from apatite (U-Th)/He thermochronometry R.M. Flowers † Division of Geological and Planetary Science, California Institute of Technology, Pasadena, California 91125, USA B.P. Wernicke K.A. Farley Division of Geological and Planetary Science, California Institute of Technology, Pasadena, California 91125, USA † E-mail: Rebecca.Flowers@colorado.edu Present address: Department of Geological Sci- ences, University of Colorado, Boulder, Colorado 80309, USA