How strong is the United States?

It is possible to determine the strength of a piece of rock by applying a known force to it (squeezing or shearing) and measuring the amount of strain (deformation). This type of work is normally done in a lab using powerful machines, and rock samples a few centimetres in size.

Geophysicists from the State University of New York and from Cambridge have recently used the same principal to estimate the strength of part of the North American Plate (NAP) (Flesch et al., 2000). In this case the most of the "known" force is the shearing effect of the northward motion of the PP along the San Andreas Fault (SAF), and the measurements are primarily from a network of GPS stations in California, Nevada, Arizona and Utah.

The geological setting of this "experiment" is shown to the right. The motion of the Pacific Plate (PP) applies a right-lateral shearing force to the western part of the NAP. The existence of a relatively high plateau in the Basin and Range province of Nevada also contributes to the applied force because this thick crust has a tendency to spread laterally under the force of gravity. This causes some squeezing in California and Oregon and to the east of the Rockies, and some extension within the Basin and Range area.

The "flow field" for the study area is shown in the map to the left. These vectors are calculated from the gravity force and the PP/NAP shear force, and they closely resemble the actual plate motions in this region as determined from the GPS network.

The variations in lithospheric strength are shown on the map below. The strength is expressed in terms of viscosity, with values ranging over three orders of magnitude, from 10²⁰ to 10²³ Pa.s (Pascal seconds). The weakest part of the area is where the two plates meet (along the SAF) with values around 5 * 10²⁰. Values are also relatively low along the eastern California Shear Zone (along the border with Nevada) - at around 2 * 10²¹. The lithosphere is strong in the central part of California, and is particularly strong in central and eastern Nevada, where some areas have viscosities as high as 10²³.

This type of work provides a useful insight into how large crustal blocks respond to external and internal forces. We can expect to see similar studies carried out in other areas in the near future.

References

Flesch, L., Holt, W., Haines, A and Bingming Shen-Tu, Dynamics of the Pacific-North American plate boundary in the western United States, Science, V. 287, p. 834-836 (February 2000)

Houseman, G, The strength of a continent, Science, V. 287, p. 814-815 (February 2000)

Steven Earle, 2000. Return to Earth Science News