|Western United States, then and now.|
It’s absolutely true. The Great Basin of western North America is expanding at a rate of one centimeter per year and if I were traveling to the West Coast 30 million years ago instead of today, the trip would be 250 miles shorter!
Note: the Great Basin and the Basin and Range Province are frequently confused. They are not completely synonymous, though there is substantial overlap. The Great Basin is a topographic or hydrographic region, defined by internal drainage. In contrast, the Basin and Range Province is defined based on landform type and arrangement, dominated by parallel north-to-northeast-trending ranges and basins. I use both terms here, as the region of interest -- western Utah and most of Nevada -- is in the area of overlap
Below: Great Basin on left; from Kmusser via Wikimedia Commons. Basin and Range Physiographic Province on right, by Kathleen Smith via Wikipedia.
The Basin and Range Province, and therefore much of the Great Basin, has been undergoing east-west extension since late Eocene time. How curious ... it was just a few million years earlier (40 Ma) that western North America still was being compressed, as the shallowly-subducting Farallon plate scraped and bumped its way eastward under the North America plate, pushing up the Rocky Mountains and Colorado Plateau. But something happened to reverse the trend, and western North America began to stretch.
Do we know why deformation changed so radically? There are some ideas. Perhaps the subduction angle of the Farallon plate steepened. That would also explain the concurrent flare-up of igneous activity in the Great Basin, which had been quiet during Mesozoic compression when the Farallon plate did not descend deep enough to generate magma. With cessation of compression, perhaps the continental plate “relaxed” and bounced back a bit. But much more is needed to explain the large amount of extension across the Basin and Range Province during the last 30 million years.
Perhaps the major tectonic event to the west was to blame. At 20 Ma, roughly the same time that extension increased significantly, the convergent plate boundary along the west coast of North America began to change to a transform boundary, as the Pacific Plate encountered the North American plate while the Farallon Plate was consumed under the continent. Shear along the growing transform boundary may have provided enough stress and strain to stretch (pull apart) the Great Basin, continuing into the present.
“These four diagrams illustrate the shrinking of the formerly very large Farallon Plate, as it was progressively consumed beneath the North American and Caribbean Plates, leaving only the present-day Juan de Fuca, Rivera, and Cocos Plates as small remnants. Large solid arrows show the present-day sense of relative movement between the Pacific and North American Plates. (Modified from USGS Professional Paper 1515).” From This Dynamic Earth by the US Geological Survey. Note the elongating transform fault, now infamously represented by the San Andreas fault in California.
American geologist Clarence Dutton famously described the topography of the Great Basin as “an army of caterpillars marching south into Mexico” (or north from it, both versions exist). He was referring to the regular pattern of generally north-trending uplifts. (Update: see this entertaining post, leading up to the actual quote and source).
|Caterpillars on the march, upper left quadrant; from Geologic Provinces|
of the United States: Basin and Range Province (USGS).
Dutton’s caterpillar army was one of the results of regional extension. Most of the mountain ranges (caterpillars) and intervening basins are blocks uplifted and downdropped along normal faults, and movement along normal faults is associated with extension. The Basin and Range Province is often described as classic “horst and graben” topography (German for heap and trench). But if these ranges and basins were only simple horsts and grabens, there would not be enough lateral movement to explain the amazing extension of the Great Basin over the last 30 Ma.
|Illustration from USGS showing horsts and grabens produced by|
normal faulting associated with extension.
Many of the basins are actually half-grabens. A half-graben is bounded by a listric fault -- a normal fault that starts out with the usual high angle at the surface, but flattens with depth, becoming nearly horizontal. The hanging wall (dropping block) is increasingly tilted as it moves along the curving fault, and the lateral component of motion is greater. Voilá ... more extension accounted for!
|Horsts, graben and half-grabens. Modified from University Idaho|
Structural Geology, online materials. Also in DeCourten 2003.
|Metamorphic core complex; from Basin and Range (source not given).|
Above: Northern Snake Range décollement or detachment fault, part of a metamorphic core complex; courtesy Looking for Detachment. LFD currently has 21 posts tagged "detachment", most recently Top 10 reasons I love detachment faults / core complexes.
"Geologists dispute the formation and development of these things, allowing for great arguments over beers, great field trips and arm-waving sessions, and exceptional reason for practising down-to-earth to wild-eyed geo-speculation."
The large amount of extension associated with MCCs can cause severe thinning of overlying strata. Right: highly-attenuated capping strata, East Raft River Mountains, Utah. From Utah Geological Survey Survey Notes, January 2012.
“These two stratigraphic columns show a typical section of Paleozoic rock in northwest Utah (left), and the same section of rock where it has been highly attenuated to less than 1/8 its original thickness as part of the Albion–Raft River–Grouse Creek metamorphic core complex (right). Modified from Wells (2009).”
Below: black blobs are metamorphic core complexes of western North America (click to view). From Metamorphic Core Complexes complied by V. L. Rystrom; no source given. At this website Rystrom presents six theories of core complex development.
Obviously this has been an absurdly brief overview of the geologic story of the amazing Expanding Great Basin. I’ve omitted the widespread igneous conflagrations entirely, as well as the possible role of shallow ductile rock layers. But I have other obligations -- my trusty steed is saddled and waiting, and the trail west calls.
DeCourten, Frank. 2003. The broken land: adventures in Great Basin geology. Salt Lake City, Utah: University of Utah Press. Much for the information in this post comes from DeCourten's enjoyable book.