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An Uplift History of the Colorado Plateau and Its Surroundings from Inverse Modeling of Longitudinal River Profiles G

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An Uplift History of the Colorado Plateau and Its Surroundings from Inverse Modeling of Longitudinal River Profiles G TECTONICS, VOL. 31, TC4022, doi:10.1029/2012TC003107, 2012 An uplift history of the Colorado Plateau and its surroundings from inverse modeling of longitudinal river profiles G. G. Roberts,1 N. J. White,1 G. L. Martin-Brandis,2 and A. G. Crosby3 Received 10 February 2012; revised 22 June 2012; accepted 27 June 2012; published 16 August 2012. [1] It is generally agreed that a region encompassing the Colorado Plateau has been uplifted by sub-crustal processes. Admittance calculations, tomographic studies and receiver function analyses suggest that dynamic support is generated by some combination of convective upwelling and lithospheric thickness changes. Notwithstanding advances in our understanding of present-day setting, uplift rate histories are poorly constrained and debated: an improved history will aid discrimination between proposed models. Here, we show that a regional uplift rate history can be obtained by inverting longitudinal river profiles. We assume that the shape of a river profile is controlled by uplift rate and moderated by erosion. In our model, uplift rate is allowed to vary smoothly as a function of space and time, upstream drainage area is invariant with time. Simultaneous inversion of river profiles from the Colorado, Rio Grande, Columbia and Mississippi catchments shows that three phases of regional uplift occurred. The first phase occurred between 80 and 50 Myrs, when 1 km of uplift was generated at a rate of 0.03 mm/yr. A second phase occurred between 35 and 15 Myrs, when 1.5 km of uplift was generated at a faster rate of 0.06 mm/yr. A final phase of uplift commenced 5 Myrs ago. These distinct phases of Late Cretaceous and Oligocene uplift are corroborated by stratigraphic considerations, by thermochronometric data, and by stratigraphic evidence of periodic clastic efflux delivered into the Gulf of Mexico. An episodic uplift history is consistent with staged removal of thick lithospheric mantle beneath a large region, which is currently centered on Yellowstone. Citation: Roberts, G. G., N. J. White, G. L. Martin-Brandis, and A. G. Crosby (2012), An uplift history of the Colorado Plateau and its surroundings from inverse modeling of longitudinal river profiles, Tectonics, 31, TC4022, doi:10.1029/2012TC003107. 1. Introduction the thickness, temperature or composition of lithospheric mantle generate the required surficial elevation. Such chan- [2] The magnitude, timing and cause of Late Cretaceous ges could be produced by small-scale convection or by and Cenozoic regional uplift centered on western North erosion/delamination of the base of the plate [e.g., Bird, America has been debated for over 100 years [e.g., Burchfiel 1979; van Wijk et al., 2010]. Heating of the lithosphere, et al., 1992, and references therein]. In recent years, atten- thinning of the thermal boundary layer and melt extraction tion has focussed on the Colorado Plateau, which sits in the on the margins of the plateau might also play a role [Roy middle of the southern half of the uplifted zone (Figure 1). et al., 2009]. Chemical modification of lithospheric mantle The most popular models of uplift can be divided into could be facilitated by hydration during subduction of the crustal, lithospheric and sub-lithospheric categories. The Farallon plate [Humphreys, 1995; Humphreys et al., 2003]. first category of models suggests that elevation of a region A final category emphasizes the importance of sub-plate encompassing the Colorado Plateau can be explained by an processes such as large-scale convective upwelling and the increase in crustal thickness generated by, say, Late Creta- changing geometry of the subducting Farallon plate [e.g., ceous to Early Cenozoic lower crustal flow or magmatic Thompson and Zoback, 1979; Spencer, 1996; Lowry et al., underplating [e.g., Wolf and Cipar, 1993; McQuarrie and 2000; Moucha et al., 2009; Liu and Gurnis, 2010; Chase, 2000]. A second category argues that changes in Karlstrom et al., 2012]. [3] There have been significant advances in our under- 1Bullard Laboratories, Department of Earth Sciences, University of standing of the crustal, lithospheric and sub-lithospheric Cambridge, Cambridge, UK. structure beneath western North America. Crustal thickness 2Royal Opera House, London, UK. 3 measurements are a key constraint. The most reliable mea- BP Exploration, Sunbury-on-Thames, UK. surements come from modern wide-angle seismic surveys Corresponding author: G. G. Roberts, Bullard Laboratories, Department and from receiver function analyses. Crustal thicknesses are of Earth Sciences, University of Cambridge, Madingley Rise, Madingley 30–45 km across the Colorado Plateau [e.g., Gilbert and Road, Cambridge CB3 0EZ, UK. ([email protected]) Sheehan, 2004; Wilson et al., 2005] (Figure 1). The most ©2012. American Geophysical Union. All Rights Reserved. striking observation is that crust thicknesses beneath the 0278-7407/12/2012TC003107 TC4022 1of25 TC4022 ROBERTS ET AL.: UPLIFT HISTORY OF COLORADO PLATEAU TC4022 Figure 1. Topographic map of North America, which summarizes present-day crustal and lithospheric thickness data. Colored circles = crustal thickness estimates based upon receiver function analyses com- piled from literature (see text); solid numbered lines = lithospheric thicknesses (LT) from Priestley and McKenzie [2006]; heart-shaped dashed line marks CP = outline of Colorado Plateau where crust is 35– 45 km thick; BR = Basin and Range province; RG = Rio Grande rift; GP = Great Plains where crust is 35–60 km thick; RM = Rocky Mountains; YS = Yellowstone; A-A′ = location of schematic cross-section shown in Figure 15. Great Plains are similar to, or exceed, those beneath the compensation occurs in regions such as the Rocky Colorado Plateau, even though their respective elevations Mountains where thickened crust is encountered. Nonethe- are < 500 m and > 2000 m (Figure 2). This elevation dif- less, the admittance value at long wavelengths is indicative of ference can only be maintained by crustal isostasy if crust regional sub-crustal support. This support encompasses beneath the Great Plains is 0.15 Mg/m3 denser than crust Yellowstone, the Colorado and Rocky Mountains plateaux, beneath the elevated plateaux. Magmatic underplating is one and the Basin and Range province (Figures 3 and 4) obvious mechanism for generating a dense lower crust but [McKenzie and Fairhead, 1997; Lowry et al., 2000; Li et al., the regional history of magmatism suggests that under- 2002b; Roy et al., 2005]. Seismic tomographic models show plating is much more likely to have occurred beneath the that regional dynamic support is generated by convective elevated plateaux. The required density difference also upwelling beneath the western edge of the North American means that crustal velocities should be faster by 1 km/s. plate. For example, the S40RTS shear velocity model shows Neither wide-angle seismic surveys nor receiver function that a large negative velocity anomaly sits beneath western analyses indicate that sufficiently large differences in crustal North America [Ritsema et al., 2011]. This widespread velocity exist [Thompson and Zoback, 1979; Braile, 1989; anomaly is largely confined to the upper mantle, although in Sheehan et al., 1995; Snelson et al., 1998; Li et al., 2002a; places it appears to extend beneath the 670 km discontinuity. Ramesh et al., 2002; French et al., 2009; Rumpfhuber et al., The anomaly also spreads horizontally beneath the thickened 2009; Wilson et al., 2010]. edge of North American lithosphere. Surface wave tomo- [4] These isostatic constraints suggest that the topographic graphic models suggest that 240 km thick lithosphere elevation of western North America is supported by density occurs beneath the Great Plains, thinning westward [West variations within the lithospheric and/or the sub-lithospheric et al., 2004; Priestley and McKenzie, 2006]. Beneath the mantle (Figure 2). This conclusion is corroborated by large- Colorado Plateau, receiver function analyses indicate that the scale geophysical observations. A +40 mGal long wave- 410 km discontinuity is often deeper than the global average length free-air gravity anomaly is centered on Yellowstone [Gilbert et al., 2003]. The depth to the 670 km discontinuity (Figure 3). Spectral analysis of free-air gravity and topo- also varies. Although the topography of these two dis- graphic signals show that the admittance, Z, at wavelengths continuities does not correlate, it is consistent with patches of of > 2000 km is 14 Æ 3 mGal/km (Figure 4). This value is higher than average ambient temperatures [Shen et al., 1998]. smaller than the +30 mGal/km expected for dynamic These and other seismological observations suggest that a support, which suggests that partial crustal isostatic region encompassing the Colorado Plateau overlies thinner 2of25 TC4022 ROBERTS ET AL.: UPLIFT HISTORY OF COLORADO PLATEAU TC4022 notably the Gulf of Mexico, provides a tangible means for estimating uplift history [Galloway et al., 2000, 2011; Alzaga-Ruiz et al., 2009]. [6] Our principal objective is to show that longitudinal profiles of the trunk rivers and their tributaries, which drain North America (e.g. Colorado, Rio Grande, Columbia, Mississippi) can be jointly inverted to determine a mean- ingful spatial and temporal pattern of uplift rate of a region which surrounds and includes the Colorado Plateau. We suggest that the observed drainage pattern of western North America provides useful clues about the temporal evolution of uplift, which can be integrated with incision rate records, Figure 2. Crustal thickness estimates plotted as a function of elevation above mean sea level. Black, grey and white cir- cles show where LT ≥ 200 km, 200 > LT > 120 km and LT ≤ 120 km, respectively. Labeled gray bands = relation- ship between crustal thickness and elevation which is calcu- lated by equalizing lithostatic pressure at base of 30 km thick (band a) and 40 km thick crust (band b). Width of gray band reflects range of crustal density (2.8 Æ 0.05 Mg/m3). Litho- spheric mantle density is 3.3 Mg/m3. Poor correlation sug- gests that topography is partially supported by sub-crustal density variations.
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