Through thick and thin: A new model for the Colorado Plateau from seismic refraction data from Pacific to Arizona Crustal Experiment

Abstract

We have constructed a model for the crust and uppermost mantle beneath the Colorado Plateau by combining newly acquired seismic refraction/wide-angle reflection data from the 1989 Pacific to Arizona Crustal Experiment (PACE) cross profile with data collected by Roller (1965). A combination of forward modeling methods was used to model travel times and relative amplitudes of crustal and upper mantle phases. The salient features of the model for the Colorado Plateau (hereafter referred to as the plateau) are (1) an upper crust having an average velocity of 6.1 km/s with a low vertical velocity gradient, (2) midcrustal discontinuities at depths of 30 to 38 km beneath the plateau, (3) a lower crust that varies in thickness (5 to 16 km) and has an average velocity of 6.8 km/s, (4) a transitional lower crust/upper mantle boundary, and (5) a crustal thickness of 45 _+ 3 km. A comparison of our new model for the Colorado Plateau with recent models for the Basin and Range shows that the upper crusts of the two provinces are similar but that the lower crust of the plateau is thicker, particularly in the plateau's central region. These observations are consistent with the idea that the Basin and Range could have formed from crust like that of the Colorado Plateau. INTRODUCTION The Colorado Plateau (hereinafter referred to as the plateau) has long aroused scientific curiosity because of differences between it and neighboring geologic provinces. Since Proterozoic time, the plateau has remained relatively stable tectonically (except for cumulative subsidence represented by the deposition of sedimentary rocks over a period of about 600 Ma) amid the active lithospheric deformation of surrounding areas. The Pacific to Arizona Crustal Experiment (PACE), a multiyear, multi-institutional effort, was designed to investigate the structure of the crust from offshore California to Arizona and to elucidate factors that may control the tectonic behavior of these distinct geologic regions [Mc-Carthy et al., 1990]. The 1989 PACE seismic refraction/wideangle reflection experiment was a part of this larger effort. It expanded upon the work of previous years with the addition of two profiles: the 150-km SW-NE main line, which extended through the Transition Zone to the western portion of the Colorado Plateau, and the 150-km SE-NW cross line, which was located exclusively within the Colorado Plateau (Figure 1). The main profile better images the lower crust of the Transition Zone and its boundary with the westernmost portion of the Colorado Plateau; the cross profile, which images the crust of the plateau, is the focus of this paper. GEOLOGIC AND TECTONIC SETTING Numerous publications address the geology and tectonic history of the Colorado Plateau. General features of this tectonic province along with extensive references are presented by Lucchitta [1989], Hinze [1988], and Smith et al. [1989]. A concise tectonic description of the structural evolution of the Colorado Plateau and surrounding areas can 1Now at be presented in three stages: (1) ...

Hydrating laterally extensive regions of continental lithosphere by flat subduction: A pilot study from the North American Cordillera

“…3D and 5B), indicative a wet sub-lithospheric mantle. At the base of the sub-continentalmantle, at a depth of about 90 km (Wolf and Cipar, 1993;Beghoul et al, 1993), the S-wave velocity anomaly is even lower, as much as ∼500 m/s less than average S-velocity (4.5 km/s) in surrounding areas, suggesting a highly hydrated base of the continental lithosphere (Fig. 5C).…”

“…1). The crust in the central part of the Colorado Plateau is about 45 km thick and is underlain by a 35-50 km thick low velocity sub-continental-mantle (Wolf and Cipar, 1993;Beghoul et al, 1993). The most likely explanation for why the plateau has not experienced deformation, yet is surrounded on all sides by deformed belts, is that it is simply tectonically "strong".…”

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“…The presence of a layer characterized by high seismic wave speeds within the Colorado Plateau has previously been proposed to result from mafi c material that has underplated the Colorado Plateau (e.g., Wolf and Cipar, 1993;Zandt et al, 1995). If seismic velocities in the lower crust gradually increase toward mantle velocities, no abrupt velocity contrast would be present.…”

“…The identifi cation of a continuous mid-crustal layer extending from the southern Basin and Range to the edge of the transition to the Colorado Plateau led to suggestions that material was added to the crust during Basin and Range extension (McCarthy and Parsons , 1994). Alternatively, the lack of observations of continuous layering within the crust of both the Basin and Range and Colorado Plateau may indicate that the crust of the Basin and Range represents an extended version of the same crust that makes up the Colorado Plateau with only localized areas of magmatic intrusions (e.g., Wolf and Cipar, 1993). The receiver functions presented here exhibit stronger converted signals in the Basin and Range than in the Colorado Plateau, indicative of a smaller velocity contrast between the mantle and the base of the crust of the plateau.…”

The formation of volcanic centers at the Colorado Plateau as a result of the passage of aqueous fluid through the oceanic lithosphere and the subcontinental mantle: New implications for the planetary water cycle in the western United States

“…2a), and that of the entire lithosphere is up to 110 km (Fig. 2b-d;Gilbert et al, 2009;Alibert, 1994;Wolf and Cipar, 1993;Beghoul et al, 1993). During the Laramide episode (∼70-45 Ma), the subcontinental mantle was undifferentiated from the rest of the North American craton, because it was underlain by the Farallon plate (Ehrenberg, 1982a).…”