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Non-technical summary: Hammond and Thatcher (2004) have measured the slow and gradual extension of the Basin and Range province, western United States, using ten years of repeat measurements with the Global Positioning System (GPS). The Basin and Range is the largest portion of the intermountain west, with a physiography that is dominated by repeating north-south oriented mountain ranges that are separated by broad, arid valleys. East to west tectonic extension (~4 mm/yr) is thought to cause normal faulting that builds the mountains and lowers the valleys over time, a processes that is thought to be caused by gravitational collapse of the high elevation of western North America. Right lateral shear occurs at a rate greater than the extension (~10 mm/yr) and is attributed to the Pacific plate moving by North America, and dragging the Sierra Nevada and Great Valley of California along. The recent GPS observations suggest, however, that the extension and shear occurring today is focused in the westernmost 150-200 km of the province. This part of the Basin and Range, known as the Walker Lane Belt, is exactly where the gravity changes are weakest and where the normal faulting is least regular. This suggests that the underlying cause for the focusing of deformation may be pervasive weakness in the North American plate in the vicinity of the Walker Lane Belt. These results also suggest that the extension that causes the archetypical Basin and Range topography occurs at a much slower rate than the deformation seen to the west, or possibly occurred at greater rates in the past.

Figure 1. (a) GPS velocity across the Basin and Range, western United States with respect to North America (blue vectors) with 95% confidence ellipses superimposed on topography (Lambert conic projection). Confidence ellipses include uncertainty in the North America reference frame. Click on image for the whole pdf article or on the link above.