What causes the Basin and Range?

Dave,
I drove across US 50 in Nevada this summer. Beside being hotter than blazes, I was struck by the repeated miles-long straightaways and then steep climbs over mountain ranges. This sequence was repeated over and over. This has got to be geology, right?
Joe DeMarsh,
Dallas

Dear Joe,

You have successfully learned Doctor Rock's First Law of Everything: GEOLOGY MADE THAT! Nevada is in the Basin and Range physiographic province. This huge area is characterized by alternating tilted mountain ranges and flat valleys. These are oriented more-or-less north-south. US 50 runs right across the middle of the state, from east to west, against the grain of the Basin and Range.

Characteristic Basin and Range topography- mountains rising
several thousand feet separated by 10-20-mile-wide sediment-
filled basins. Click to enlarge any figure.

 The Basin and Range is formed by stretching of the crust in the arid US west. The crust is thin here, only 18-20 miles thick. It is spreading laterally on top of the weak, ductile mantle. This motion causes the crust to fracture along faults that are angled downward at around 60 degrees. The rock on one side of these fractures slides down relative to the other, so these fractures become faults that occasionally slip as earthquakes.The movement along these angled faults allows the crust to 'extend' laterally, east to west. There has been so much extension in the Basin and Range that the crust has stretched until it is twice as wide as it was before extension began.

Development of the Basin and Range Province
http://www.iris.edu/hq/inclass/lesson/brittle_vs_ductile_rocks

The blocks of rock that drop down along the faults form the long valleys that separate the parallel rotated mountain ranges. This topography is the signature of the Basin and Range province.



Two ways to get alternating ranges and valleys.
The Basin and Range developed via the
model on the right.



The Hanaupah fault scarp in Death Valley National Park.

The fault cuts across an alluvial fan, uplifting the mountain

range relative to the valley floor. Many thousands of small

uplifts and downdrops along faults like this are needed to

create the basins and ranges.Photo by Marli Miller
http://geomaps.wr.usgs.gov/parks/province/basinrange.html

 

Extension in the Basin and Range began around 17 million years ago and continues today. You can read about a 1983 magnitude 7 Basin and Range earthquake in Idaho here.

Basin an Range earthquakes in the first week of August, 2013
http://earthquake.usgs.gov/earthquakes/map/.

A must-read laypersons book about the geology here is "Basin and Range" by John McPhee. The book is also included in his later anthology, "Annals of the Former World". Wonderful mind expanding reading by a nongeologist.