Thinned crust should sink, yet the American West stands a mile high
The evidence for plate stretching is usually buried under sediment. The Basin and Range Province in the United States is the rare exception: the crust has already been pulled to roughly twice its width and is still extending, and GPS, the paleoseismic record and detachment faults let us see it happening.
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The argument · timestamps estimated from transcript position
The evidence for stretching is almost always buried under sediment
When plates collide, the result is legible: the margins are crumpled and thrown up into mountain belts. When plates are pulled apart, the evidence usually goes missing. The chain of loss has two steps: thinned lithosphere subsides, and the basins that subsidence creates are then filled with sediment. What is left at the surface is a flat blanket of cover, with no sign of the rifting that happened earlier. This opening contrast sets up the suspense for the whole episode — to study extension, you have to find a window like the Basin and Range, one that sediment has not erased.
Dozens of parallel ranges were not squeezed up by compression
The original calls the Basin and Range Province the great exception outright — a region of the western United States and northwestern Mexico, wedged between the Colorado Plateau and the Sierra Nevada, where dozens of mountain ranges line up northward almost in parallel. They are not isolated peaks but the surface relief left behind after an entire tract of crust was lengthened. Both the spacing and the arrangement indicate that what happened here was not a single episode of compression but large-scale extension. Precisely because it has preserved intact basin-and-range topography, it has become a natural field site for studying the extensional process.
The crust has already been pulled to twice its width, and it is still pulling
The episode offers one hard number: the crust of the Basin and Range has already been pulled apart to roughly twice its original width. This is not a static relic of deep antiquity, because the description is immediately followed by still stretching today — it is continuing to widen right now. The rate of extension is invisible to the eye, but the scale is enough to reshape the topographic pattern of the whole region. The factor of two settles its geological significance, while reaching today turns it from a historical question into a dynamical one that can be monitored in real time.
GPS antennas can measure the West widening in real time
The episode lists three lines of evidence for extension. First, GPS antennas anchored in bedrock, which can measure the Basin and Range widening in real time. Second, the paleoseismic record, which preserves the historical rupture events that extension has produced. Third, dismembered thrust sheets, which let researchers reassemble the whole region back into what it looked like 20 million years ago. The three work on different timescales: GPS sees the present, the earthquake record sees geological history, and the thrust sheets see the ancient structure at the regional scale. Only by overlaying them do you arrive at the conclusion that the crust has been pulled to twice its width.
Rocks from fifteen kilometers down can reach the surface without erosion
Low-angle detachment faults are the most counterintuitive structure in this passage. One of their main functions is to bring rock from fifteen kilometers depth up to the surface — not by lifting a mountain range as a whole, but by sliding and detaching along a low-angle fault plane. Fifteen kilometers depth corresponds roughly to the middle-to-upper crust, and rock at that level would take an extremely long time to be exposed by erosion; detachment faults provide a more direct route. Being able to look at deep-crustal rock directly is key to understanding the internal structure of the extensional process.
Thinned crust ought to collapse, and this crust has not
At the end, the episode flips the question onto the topography: if the crust of the Basin and Range has been thinned, why is it still standing about a mile high? Intuition says rifting goes with subsidence, yet this highland has not collapsed. The opening introduction only poses the paradox and gives no answer, which is plainly a hook left for the body of the interview. For an extensional province, the coexistence of high elevation and thinned crust is itself a fact that demands explanation.
In their own words · checked verbatim
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.
Figures
| Amount of crustal extension in the Basin and Range | roughly twice its original width | 0:00 |
| Depth from which detachment faults raise rock | 15 kilometers | 0:00 |
| Time the regional reconstruction corresponds to | 20 million years ago | 0:00 |
| Topographic elevation of the extensional province | about a mile high | 0:00 |
Glossary
- Basin and Range Province
- A large zone of crustal extension in the western United States and northwestern Mexico, with parallel mountain ranges alternating with basins.
- lithospheric extension
- The process by which crust is lengthened and thinned under tensional stress.
- low-angle detachment faults
- Large normal faults with very shallow dips, capable of raising deep-crustal rock to shallow levels.
- dismembered thrust sheets
- Rock bodies emplaced by thrusting that were later broken up and offset by extension and block faulting.
How to listen
Graduate students in structural geology, anyone working with GPS crustal-deformation measurements, and physical-geography enthusiasts who want to grasp quickly why the Basin and Range differs from a compressional mountain belt.