The Rocky Mountain Region: An Evolving Lithosphere Tectonics, Geochemistry, and Geophysics

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The stable cratonic region is defined by the Rocky Mountain Front grey dashed line to the west, and the continent rift margin red thick line to the east. Depth cross-section locations are shown as thick black lines with white circles for better correspondence with Figs 7 , 10 , 11 , Right-hand panel: physiographic provinces of the western US. Labels follow Burchfiel et al. The thick grey dashed line is the Rocky Mountain Front.

The two thin grey lines are the Sevier Thrust and Fold belt, and the Strontium isotope ratio 0. In Figs 3 a — c , we present the average 1-D depth profiles for our model region, as well as separate averages for the cratonic part of North America and the tectonically active west. We present two mean profiles, one for the entire region of our study, which includes both continental and oceanic parts, and the other for the continental part only.

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The continental and regional scale average V s profiles are similar, and faster than the global average 1-D reference in Fig. The craton average is significantly faster than the regional average from the crust down to depths of — km. This profile shows two minima, one centred around km depth, the other around km, separated by a velocity maximum around km depth.

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In continental Australia, Fichtner et al. The WUS V s profile, on the other hand, shows a pronounced minimum between and km depth, followed by a strong positive gradient down to km depth, marking the bottom of the low-velocity zone LVZ. The profiles converge below km to values that are slightly slower than the global 1-D reference down to transition zone depths. Because we have poor resolution in the first 40 km of the mantle due to the long period character of our waveforms, we cannot resolve a maximum in V s associated with the thin lithosphere in the WUS.

Compared to radial anisotropy, the profiles of azimuthal anisotropy strength G show much more structure with depth Fig. The strongest azimuthal anisotropy is under the WUS and is confined primarily to the first km of the upper mantle.

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Between and km depth, G is stronger in the WUS than in other parts of the continent. In contrast, under the craton, the G profile has two maxima, around 80— km depth, and around km depth. There is also a marked minimum in G around km. Based on our resolution tests Figs S12 and S13 , the maximum at km is well resolved in our data, although its amplitude may be underestimated and its exact position in depth is not constrained to better than 50 km. The fact that the fast axis direction of azimuthal anisotropy is roughly parallel to the APM everywhere below the LAB, as we will see below, strengthens this argument.

Our 3-D isotropic velocity model of the NA continent Fig. The spatial similarities among the different models indicate that the first order V s features found are robust. Isotropic velocity V s perturbations, plotted with respect to the NA regional average shown in Fig. Black dashed line approximately delineates the continent cratonic region, which approximately follows the Rocky Mountain front RMF to the west, and the Ouachita and Appalachian fronts to the south and east.

The Palaeozoic continent rift margin in the western US, which spatially correlates with the Sevier thrust and fold belt in the western US, is shown as a black line for reference.

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  8. At shallow depths i. In the Precambrian regions, the fast velocities extend below km depth, except for a trend of slow velocity at about km beneath the Great Fall Tectonic zone in north Wyoming province and beneath the Trans-Hudson Orogen separating the Superior and Hearne provinces to the north. This slow velocity anomaly is spatially associated with an upper-mantle low-conductivity zone revealed from electromagnetic inversions Jones et al. Above km, velocities on the continental margins show much larger spatial variations. At km depth, this anomaly connects to the EPR.

    Slow velocities are also present beneath the Juan de Fuca JdF and Gorda plates, albeit in a narrower depth range 70— km. This whole system of low velocities seems to be connected from south EPR to north, through the Basin and Range and into the JdF and Gorda ridge system.

    This is best seen in a 3-D rendering of the velocity structure in the WUS as presented in Figs 5 a and b. The Gulf of Mexico, on the other hand, is generally slow above km, while below that depth, a fast anomaly is present and extends into the centre of the continent. Further north, the velocity is slow beneath the extinct Labrador Sea rift system down to km depth.

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    The isosurfaces in a and b are drawn at —1. The velocity variations are with respect to the NA regional average. Green lines at the top of each plot show the western US physiographic regions.

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    Thick black line shows the plate boundary between the North American and the Pacific plates. The most striking feature in the tomographic V s maps is the sharp boundary between the fast and slow velocities across the RMF Figs 4, 5a and b. Thinning of the cratonic lithosphere towards the WUS is confined between the Laramide front and the Strontium ratio 0. As seen in some smaller scale studies e.

    Dueker et al. This is most prominently seen in our model at a depth of km Fig. Between and km depth, the sharp boundary between the fast and slow velocities across the RMF disappears. Significantly faster velocities are present under the Archean Superior province, the Palaeo-Proterozoic Buffalo Head terrane in northern Alberta, and the Archean Rae province.

    Under Alaska, high velocities appear in this depth range, and extend deeper into the transition zone, likely related to the Alaskan subduction. Large slow velocities are present under the Labrador Sea, northeastern Pacific, and the Rio Grande rift system. Figs 4 and 5b.

    Green dashed line approximately delineates the continent cratonic region, as in Fig. Colour scale is the same for all depths. At shallow depths 70— km , these negative anomalies correlate with surface geological features: for instance, along the RMF in the western Cordillera; under the Rio Grande rift in central Colorado; and along the rifted continent margin in the south and southeastern US.

    This feature coincides with a quasi-cylindrical low velocity feature which is interpreted as resulting from either mantle plumes or asthenospheric instabilities in Bank et al. The Colorado Plateau stands out as a unique feature. It is separated from the stable cratonic region to the east by the Rio Grande rift Fig. According to Griffin et al. The km root thickness estimated from the thermobarometric analysis is consistent with our seismic results.

    The tilting of the LAB to larger depths towards the Northwest is also inferred from body wave tomography images Sine et al. Levander, personal communication, The S -wave RFs of Abt do not sample the Plateau, but indicate much thinner lithosphere beneath the Basin and Range in agreement with the velocity model Fig. This result is consistent with GPS velocities that indicate that only the centre of the Colorado Plateau behaves as a stable block and that extension is modifying its eastern and western margins Kreemer et al.

    The cross-sections in Fig. An eastward and upward flow from beneath central Nevada is proposed in geodynamic modelling Moucha et al.

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    Wilson et al. V s is plotted with respect to the western US average in Fig. Labels follow Fig. Azimuthal anisotropy shows a strong depth dependence under both the Precambrian regions and the active WUS.

    Cenozoic Landscape Evolution of the Southern Rocky Mountains

    This has been discussed extensively in two other papers YB10a and YB10b. In Fig.

    Maps of: a Variations of azimuthal anisotropy strength G. Green and grey dashed line approximately delineates the continent cratonic region as in Fig. The strongest azimuthal anisotropy is found in the WUS between depths of 70— km.