Where tectonic plates collide, the result is clear: the edge of a plate crumples and throws up a mountain belt. But where a plate is stretched rather than squeezed, the evidence is usually lost, because the thinned lithosphere subsides and the basins that form fill with sediment.The Basin and Range Province of the western United States and northwestern Mexico is the great exception. Between the Colorado Plateau and the Sierra Nevada, dozens of mountain ranges march north in near-parallel array, in a crust that has been pulled out to about twice its original width — and it is still stretching today.In this episode we look at how we know that: from GPS antennas bolted into bedrock that measure the province widening in real time, to the record of ancient earthquakes, to the dismembered thrust sheets that let the whole region be reassembled as it was 20 million years ago. We also discuss the low-angle detachment faults that have brought rock from fifteen kilometres down up to the surface and why a stretched province still stands a mile high.Brian Wernicke is an Emeritus Professor in the Division of Geological and Planetary Sciences at Caltech.For photos, figures, and further reading, see https://www.geologybites.com.

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