Western U.S. Uplift: Papers and Abstracts

Papers:

Abstracts:

Jones, C. H., B. P. Wernicke, G. L. Farmer, J. D. Walker, D. S. Coleman, L. W. McKenna, and F. V. Perry, Variations across and along a major continental rift: An interdisciplinary study of the Basin and Range Province, western USA, Tectonophysics, 213, 57-96, 1992.

Abstract. Geological, geochemical, and geophysical data gathered within the central part of the Basin and Range and adjacent areas of the western USA suggests that considerable heterogeneity characterizes Cenozoic extension in this region. Good exposure and an abundance of pre-rifting markers indicate 250 km of extension of the upper crust over the past 16 m.y. Extension of several hundred percent has occurred in two distinct deformational domains, Death Valley and Lake Mead, separated by a relatively unextended block, the Spring Mountains. The limited topographic differences between extended and unextended regions imply that material with a crustal density has been added to the extended regions. Although igneous activity can provide some of this added material, kinematics of extension within the Death Valley region suggest that lateral flow of the middle and lower crust into the extended areas accounts for much of the needed material. Such flow is consistent with geochemical analysis of intermediate to silicic volcanic rocks in the Death Valley area. These volcanic rocks contain isotopic and geochemical trends similar to Mesozoic plutonic rocks from the western part of the Sierra Nevada, about 150-200 km to the west, thus suggesting that the upper crust has moved by that amount relative to deeper crustal levels. Geochemical analyses of basaltic magmas in the region indicate that two mantle reservoirs are present: an OIB-type asthenosphere, and an old, Precambrian continental lithosphere. The ancient lithospheric mantle is preserved beneath the central Basin and Range, but to the west and north the basaltic rocks have a signature compatible with an asthenospheric origin. These differences indicate that the degree of thinning and removal of the mantle lithosphere varies considerably across the central Basin and Range. These differences are compatible with the inference from geological and geophysical arguments that thinning of the mantle lithosphere at the latitude of the central Basin and Range is localized beneath the Sierra Nevada. Geophysical measurements have shown that the thickness of the crust varies little from a mean of about 30 km over the entire Basin and Range; the crust under the high Sierra Nevada to the west might have about the same thickness. Estimates of the buoyancy of the crust and mantle based on P-wave crustal structures suggest that the most buoyant, and thus probably the warmest, mantle lies under the Sierra Nevada and not under areas of strongly thinned upper crust of the Death Valley and Lake Mead regions to the east. Similar analyses indicate that the extended upper crust of the northern Basin and Range overlies an upper mantle more buoyant than that of the southern and central Basin and Range; this is in accord with geochemical and seismological inferences. Thus, the style of lithospheric extension varies considerably both along and across the strike of the Basin and Range.

Map of the 1992 Rocky Mountain Front (RMF) experiment, 1994-5 Colorado Plateau-Great Basin (CPGB) experiment, permanent University of Nevada, Reno (UNR) broadband stations, U.S. National Net stations (USNSN), and other broadbands being used in studying the lithosphere of the southwestern U.S. by CU and UNR research groups.

Sheehan, A.F., G. A. Abers, C. H. Jones, and A. L. Lerner-Lam, Crustal thickness variations across the Colorado Rocky Mountains from teleseismic receiver functions,J. Geophys. Res., 100, p. 20,391-20,404, 1995.

Abstract Variations in crustal thickness from the Great Plains of Kansas, across the Colorado Rocky Mountains, and into the eastern Colorado Plateau are determined by receiver function analysis of broadband teleseismic P waveforms recorded during the 1992 Rocky Mountain Front PASSCAL experiment. The receiver functions are calculated using a time-domain deconvolution approach, and are interpreted in terms of a single crustal layer , with thickness determined by a grid-search comparison of observed receiver functinos with synthetics. The results compare favorably with refraction data, which indicate only slight crustal thickening from the Great Plains to the Rocky Mountains at these latitudes, and relatively thinner crust beneath the Colorado Plateau. Together with gravity data and mass balance calculations, these results indicate that the Rocky Mountains are not isostatically compensated by a simple Airy-type crustal root nor any other mechanism that restricts compensation solely to the crust, but require a significant amount of compensation from the mantle.

GIF image of RMF station map

A large version of the RMF station map (135k)

C. H. Jones, J. R. Unruh, and L. J. Sonder, The role of gravitational potential energy in active deformation in the southwestern United States, Nature, 381 (6577), 37-41, 1996.

Topic paragraph Orogenic belts arise when intrinsic (buoyancy) and extrinsic (boundary) forces act on the continental lithosphere; determining the relative importance of these forces is fundamental to understanding the dynamics of continental deformation. Buoyancy forces are proportional to differences in gravitational potential energy; our calculation of potential energy in the western United States shows that buoyancy forces are sufficient to produce the wide variety of rates and styles of observed deformation (including absence of deformation). This underscores the importance of gravitational potential energy for deformation in orogenic belts.

Cover photo (C. H. Jones photo): Late Palaeozoic and earliest Mesozoic sedimentary rocks of the Colorado Plateau stretch across Canyonlands National Park south of Dead Horse Point, Utah. The Abajo Mountains on the skyline are mid-Cenozoic intrusions -- a period during which the Colorado Plateau suffered little deformation. Analysis now reveals that this observation -- and the contrasting active deformation seen in neighbouring regions of the southwestern United States -- can be explained as the result of spatially varying bouyancy forces arising from differences in the thickness and density of the lithosphere.

To view the original at Nature you will have to register; the links below will get you to the relevant parts of their web page once you have registered. You can see the cover from Nature's site, or view their article summary or a summary of Phil England's News and Views article.

You can see an article in the San Francisco Chronicle about this work, or an article in the Colorado Daily (Boulder)

Other papers publishing articles from this Nature publication are the Rocky Mountain News, the Denver Post, the Sacramento Bee (all on 2 May 1996), the Boulder Daily Camera (3 May 1996), the Contra Costa (CA) Times (5 May), the Sunday San Francisco Examiner-Chronicle (5 May), the Eugene (OR) Register-Guard (6 May), the (Portland, OR) Oregonian (16 May), the Monument (CO) Tribune (23 May), the Flagler (CO) News (20 June) (most of the post-2 May stories are derived from the Sacramento Bee story). Small stories (~1 page) are have (or will) appeared in Geotimes (Sept. 1996), Discover (look for the bottom article), and Earth (Dec. 1996?).

C. H. Jones (on behalf of the CPGB Field Crew), The 1994-5 Colorado Plateau-Great Basin PASSCAL Experiment, IRIS newsletter, v. XV, no. 1, p. 1-4, 1996.

You can view a somewhat expanded web version of this article (there is no abstract).

Jones, C. H., L. J. Sonder, and J. R. Unruh, Lithospheric graviational potential energy and past orogenesis: Implications for conditions of initial Basin and Range and Laramide deformation, Geology, 26, 639-642, 1998.

Gravitational body forces (i.e., buoyancy forces) have come to be seen as critical to the evolution of orogens. Nevertheless, constraining the role of body forces in specific geologic scenarios is made difficult by the substantial number of poorly constrained physical parameters needed to fully relate forces to deformation. By separating the calculation of buoyancy forces from the calculation of the resulting deformation, models based on relatively simple descriptions of the lithosphere can yield geologically useful constraints. Among these are the importance of paleoelevation in driving syn- and postcontractional extension and in localizing contractional strain. Although such phenomena have been considered in more complex models of continental deformation, the simpler analysis presented here clearly establishes first-order limits on lithospheric structures and paleoelevations consistent with buoyancy-driven deformation. In the early Cenozoic Great Basin of the western United States, we show that the low elevations inferred in much of the geologic literature are inconsistent with a body-force origin for observed extensional tectonism. East of the Colorado Plateau, localization of Laramide deformation coincides with pre-Laramide subsidence of the Western Interior seaway. This subsidence prestressed the lithosphere, making the Southern Rocky Mountains the weak link in responding to regional compressional stress.

Jones, C. H., L. J. Sonder, and J. R. Unruh, Reply to Comment on "Lithospheric gravitational potential energy and past orogenesis: Implications for conditions of initial Basin and Range and Laramide deformation", Geology, 27, 475-476, 1999.

You can get (or view) a pdf file of the comment and reply directly from GSA

Sonder, L. J., and C. H. Jones, Western United States extension: How the West was widened, Annual Review of Earth and Planetary Sciences, 27, 417-462, 1999.

If you have a subscription to Annual Review, you can get the pdf file from them

Cenozoic extension in the western United States presents a complex interrelation of extension, volcanism, and plate boundary tectonics that defeats simple notions of "active" or "passive" rifting. Forces driving extension can originate at plate boundaries, through basal traction, basal normal forces, or from buoyancy forces internal to the crust and lithospheric mantle. The latter two are most responsible for driving extension where it is observed in the Basin and Range. The complex evolution of the northern Basin and Range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust. In contrast, crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 Ma to drive extension in the southern Basin and Range. The central Basin and Range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western United States.

Y.H. Chen, S. W. Roecker, C. Jones, and J. Gomberg, Three-dimensional elastic wave velocity structure of the crust and upper mantle beneath the southern Great Basin, abstract, EOS, 74 (34, suppl.), 419, 1993.

In this study we determined the three-dimensional elastic wave velocity structure of the crust and upper mantle beneath the Southern Great Basin using arrival times of 2710 P waves from 80 teleseismic events and 15409 P and 8875 S waves from 639 local earthquakes and 35 explosions recorded by 55 stations of the Southern Great Basin Seismic Network. We combined results from several previous studies of velocity structure to construct an a priori starting model, but found that the final result was similar to that using a layered structure as a starting model. Resolution tests suggest that the local event data is capable of resolving features on the order of 4 km dimension, while that from the teleseismic data is on the order of 20 km. The principal features in our model are the appearance of a pervasive low velocity (-5%) region in the center of the study area (roughly corresponding to the Nevada Test Site) that extends from the near surface to depths of about 70 km. Below this depth the dominant structure is high velocities in the center, with the lowest of the surrounding velocities located to the northeast. This pattern extends more or less intact to depths of about 250 km. The location of the high velocity "plug" is similar to that determined by Evans et al. (1992) but the contrasts are somewhat higher and the details of its shape are resolvably different.

Jones, Craig H.,A. F. Sheehan, L. J. Sonder, M. K. Savage, and S. Ozalaybey, Isolating Crustal Versus Mantle Sources of Isostatic Support of the Colorado Plateau, EOS, 76 (suppl. to no. 46), F619, 1995

Existing refraction estimates of the crustal structure of the Colorado Plateau fall into two main groups: (1) those with a crust thinner than about 45 km and lacking a thick (>10 km) high-velocity (6.8-7.3 km/s P velocity) lower crust, and (2) those with a thicker crust including a high-velocity lower crust. Simple application of velocity-density relations and use of local isostasy permits separation of a crustal contribution to buoyancy (Hc) from a mantle contribution (Hm) using e = Hc + Hm - 2.4 (e = mean elevation, in km; Lachenbruch and Morgan, 1990). For case (1) the mantle is much more buoyant (Hm = -1.1 to -1.7 km) than the decidedly antibuoyant mantle in case (2) (Hm = -2.0 to -2.5 km). Conversion of likely density-depth structures in the mantle results in a near-zero difference in gravitational potential energy relative to a pure asthenospheric column (ΔPE) for case (1) and a very negative Δ PE ( -2 x 1012 N/m) in case (2), which would predict horizontal deviatoric compressional stresses. Given the extensional state of stress in most of the Plateau, a negative & Delta;PE for the Plateau would indicate that either the assumptions underlying the use of PE for estimating strain rates are flawed, or that substantial boundary forces are presently being applied to the Plateau from the sides and/or the bottom.

To resolve between cases (1) and (2), we will review previous interpretations of refraction data in the Plateau, present new constraints from receiver functions at Colorado Plateau stations from the 9 month long 1994-5 Colorado Plateau- Great Basin seismic field experiment, and consider the impact of likely crustal compositions consistent with seismic and xenolith observations on the velocity-density relation. Resolving between these models not only improves our understanding of the modern force balance on the Colorado Plateau, but it also constrains models for the Cenozoic uplift of the Plateau.

(Special WWW caption for key figure from 1995 poster:) Receiver functions from two of the Colorado Plateau stations (located on map in Jones, 1996) compared with synthetic receiver functions generated from previous refraction interpretations of the Hanksville-Chinle line across the center of the Plateau. The best interpretation appears to be Prodehl's (1979) interpretation, which had a thinner crust than the later Wolf and Cipar interpretation. The new data are thus compatible with a thinner Plateau crust, implying that the elevation of the Plateau is supported in large part by a less-dense mantle; this in turn is consistent with the extensional stress state of the crust in the Plateau.

Jones, C. H.,L. J. Sonder, and J. R. Unruh, Implications of Topography for Tectonics of the SW U.S., Geol. Soc. Am. Abstr. Prog., 28 (7), A-513, 1996.

The age and origin of the high elevations of the southwestern U.S. (SWUS) have been longstanding geotectonic problems, as highlighted by recent paleobotanical estimates of SWUS paleoelevations that challenge the long-held belief that the uplift is late Cenozoic. Our recent analysis of the modern-day budget of lithospheric gravitational potential energy (GPE) for the SWUS (Jones et al., 1996) yields estimates ranging from -0.7 x 10^12 N/m to 2.6 x 10^12 N/m, sufficient to produce lithospheric stresses capable of driving strain rates of ~10^-15 s^-1. We can make inferences about uplift and paleotectonics using this technique. A 45 km thick crust's GPE increases by 10^ 13 N/m (3x the modern range of GPE values in the SWUS) if its mean elevation increases from sea level to 3 km due to density changes in the mantle. In contrast, thickening a 35-km thick crust to produce the same change in surface elevation increases the GPE by only 2.5 x 10^12 N/m. In both cases, the change in GPE changes the stress state of the lithosphere. For comparable values of average lithospheric strength, however, the resulting strain rates differ significantly. By constructing simple structures of paleolithosphere, applying these principals, and comparing the results with the observed tectonics, we can test different tectonic hypotheses for consistency with isostasy, the tectonic history, plausible lithospheric structures, and plausible lithospheric forces with a minimal number of free parameters. We will illustrate this potential with some simple examples.

A WWW version of the talk can be found here.

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