Contents GSA TODAY publishes news and information for more than 17,000 GSA members and subscribing libraries. GSA Today lead science articles should present the results of exciting new research or summarize and synthesize important problems or issues, and they must be understandable to all Vol. 12, No. 8 August 2002 in the earth science community. Submit manuscripts to science editor Karl Karlstrom, [email protected]. GSA TODAY (ISSN 1052-5173) is published monthly by The Geological The Great Sand Dunes Colorado Photo by Martin Miller Society of America, Inc., with offices at 3300 Penrose Place, Boulder, Colorado. Mailing address: P.O. Box 9140, Boulder, CO 80301-9140, U.S.A. Periodicals postage paid at Boulder, Colorado, and at additional mailing offices. Postmaster: Send address changes to GSA Today, Member Services, Colorado Plateau uplift and erosion evaluated using GIS P.O. Box 9140, Boulder, CO 80301-9140. Joel L. Pederson, Rob D. Mackley, and James L. 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On the cover: Obique three-dimensional view looking northeast onto the Colorado Plateau showing estimated erosional exhumation since ca. 30 Ma. Note the high erosion following Grand Canyon at bottom left of image. The canyon appears inverted because of high erosion along its axis and low values in the surrounding rim country. Negative values shown in blue are places of net deposition since ca. 30 Ma, such as the Quaternary San Francisco volcanic field near Flagstaff, Arizona, at the bottom center of image. Erosional unloading of the Colorado Plateau over the middle-late Cenozoic should cause significant rock uplift through isostatic rebound. 50% Total Recovered Fiber Image by J.L. Pederson. See “Colorado Plateau uplift and erosion evaluated using 10% Postconsumer GIS,” by J.L. Pederson et al., p. 4–10. Colorado Plateau uplift and erosion evaluated using GIS fact, less uplift on the plateau than pro- Cenozoic epeirogeny (e.g., Hunt, 1956; Joel L. Pederson, Rob D. Mackley, posed sources can supply. This suggests Morgan and Swanberg, 1985; and James L. Eddleman, Department Laramide uplift of the plateau was signifi- Humphreys, 1995; Spencer, 1996; of Geology, Utah State University, cantly less than that of the Rocky McQuarrie and Chase, 2000). The earliest Logan, Utah 84322-4505, USA Mountains, consistent with its prevalent researchers visiting the Colorado Plateau sedimentary basins, and/or that there has saw the deep incision of its spectacular been little or no post-Laramide uplift be- canyons and concluded that erosion has ABSTRACT yond erosional isostasy. been driven by recent—and ongoing— Study of the interaction between uplift uplift (e.g., Powell, 1875; Dutton, 1882; and erosion is a major theme of our sci- INTRODUCTION Davis, 1901; Hunt, 1956). This conclusion ence, but our understanding of their in- Pioneering geologists such as John was in keeping with W.M. Davis’ influen- terplay is often limited by a lack of Wesley Powell, Clarence Dutton, G.K. tial model that large-scale cycles of uplift quantitative data. A classic example is Gilbert, and William Morris Davis pon- and erosion end with landscapes de- the Colorado Plateau, for which the dered the Colorado Plateau landscape nuded to a peneplain near sea level, and starting and ending points are well evolution, and questions still puzzle re- with the related assumption that incision known: The region was at sea level in searchers today. In particular, what is the must be driven by subsequent uplift the Late Cretaceous, and now, the explanation for the plateau’s mild struc- rather than other means of lowering base deeply eroded land surface is at ~2 km. tural deformation compared to surround- level for streams. The concept of a The path of the landscape between ing areas, its high average elevation, and plateau denuded to near sea level after these endpoints is less clear, and there its dramatically incised landscape (Fig. 1)? Laramide time, and then epeirogenically has been longstanding debate on the Hypotheses for uplift of the Colorado uplifted later in the Cenozoic to account mechanisms, amounts, and timing of Plateau include mechanisms such as flat- for canyon incision, has persisted, but uplift and erosion. We use a geographic slab subduction, crustal thickening, and most workers recognize that the eleva- information system to map, interpolate, anomalous mantle properties during two tional history and the timing of erosion of and calculate the Cenozoic rock uplift stages of activity: (1) early Cenozoic the Colorado Plateau are still unknown. and erosional exhumation of the (Laramide) uplift; and (2) middle-late Colorado Plateau and gain insight into its landscape development through time. Initial results indicate uplift and erosion are highly spatially variable with mean values of 2117 m for rock uplift and 406 m for net erosional exhumation since Late Cretaceous coastal sandstones were deposited. We estimate 843 m of erosion since ca. 30 Ma (a larger value because of net deposition on the plateau over the early Cenozoic), which can account for 639 m of post-Laramide rock uplift by isostatic processes. Aside from this iso- static source of rock uplift, paleobotanical and fission-track data from the larger re- gion suggest the early Cenozoic Laramide orogeny alone should have caused more than the remaining rock uplift, and geo- physical studies suggest mantle sources for additional Cenozoic uplift. There is, in Figure 1. Physiographic extent of the Colorado Plateau according to Hunt (1956), which is used here for all discussion and analyses. Mean elevation is taken from merged 90 m digital elevation models (NAD83). 4 AUGUST 2002, GSA TODAY Isostatic response to the erosional ex- humation of the plateau over the middle- late Cenozoic should itself result in sig- nificant rock uplift, but this has not been adequately considered in evaluating the above ideas. Several well-known studies have investigated this effect in other areas using approaches different from those used here (e.g., Molnar and England, 1990; Montgomery, 1994; Tucker and Slingerland, 1994; Small and Anderson, 1995, 1998; Whipple et al., 1999). Here Figure 2. Illustrations of terms as described in text: U = rock uplift, U = we use a straightforward approach of di- R S ε rectly measuring rock uplift and exhuma- surface uplift (surface lowering when negative), = exhumation. A: The case of “active” tectonic rock uplift. B: “Passive” rock uplift due to isostatic tion using information preserved in the response to exhumation.