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Lithospheric Extension Near Lake Mead, Nevada: A Model for Ductile Flow in the Lower Crust

Kruse, Sarah and McNutt, Marcia and Phipps-Morgan, Jason and Royden, Leigh and Wernicke, Brian (1991) Lithospheric Extension Near Lake Mead, Nevada: A Model for Ductile Flow in the Lower Crust. Journal of Geophysical Research B, 96 (B3). pp. 4435-4456. ISSN 0148-0227. doi:10.1029/90JB02621.

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Small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions. In this study we examine the boundary between the unextended Colorado Plateau and a strongly extended domain in the Basin and Range Province in the Lake Mead (Nevada) region. Bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small. Analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly dependent on the channel thickness and on the length scale of flow required. Finite element modeling of Newtonian flow and power law creep shows that flow over the length scale of the eastern Basin and Range (500 km or more) corresponding to upper crustal extension by a factor of 1.4–3 over 10 m.y. requires effective viscosities less than 10^(18)–10^(20) Pa s for ductile channels 10–25 km thick. Flow over shorter length scales (150 km) may be accommodated with effective viscosities as high as 10^(21) Pa s. Modeling suggests that these effective viscosities may be sustained by lower crustal material deforming at laboratory-derived power law creep rates. The longer-scale flow may require elevated crustal temperatures (more than 700°C), depending on the composition and material properties assumed. Under the boundary conditions assumed in this study the linear viscous flow models yield a satisfactory approximation to deformation by power law creep. This work suggests that flow in the lower crust may be a viable mechanism for producing small variations in total crustal thickness between strongly extended and less extended regions, and thereby explaining the relative uniformity in gravity and topography between such regions.

Item Type:Article
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Wernicke, Brian0000-0002-7659-8358
Additional Information:© 1991 American Geophysical Union. Received February 13, 1991; revised November 12, 1990. Roger Buck was generous with his time and made suggestions that considerably strengthened this work. His support is gratefully acknowledged. Constructive reviews by Jon Spencer and Kevin Furlong significantly improved the manuscript. This work was supported in part by NASA NAG 5-1084 to M.M.
Funding AgencyGrant Number
NASANAG 5-1084
Issue or Number:B3
Record Number:CaltechAUTHORS:20140806-115436547
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Official Citation:Kruse, S., M. McNutt, J. Phipps-Morgan, L. Royden, and B. Wernicke (1991), Lithospheric extension near Lake Mead, Nevada: A model for ductile flow in the lower crust, J. Geophys. Res., 96(B3), 4435–4456, doi:10.1029/90JB02621.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48082
Deposited On:06 Aug 2014 21:05
Last Modified:10 Nov 2021 18:03

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