Today we’re going to a geological province that is almost unique in its nature – at least in terms of its size. The Basin and Range in Nevada and Utah is a region of broken, extended crust nearly 450 miles wide and even longer in a north-south direction. Areas of basin and range topography extend north into Oregon and Idaho, into southeastern California, southern Arizona and New Mexico, and cover a broad swath or northern Mexico.
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| Shaded relief map (NPS) |
The alternating uplifts and basins, technically called horsts and grabens, are the result of extension of the earth’s crust over this wide area. Take something brittle – continental crust – and pull it from the two sides, and it will break. The breaks are mostly steep normal faults – sometimes more than one – that separate the basins from the ranges. As with any mountain uplift, as soon as there is a variation in mountain relief, erosion starts, and the eroded material was shed into the adjacent basins. In some places, there is more than 10,000 feet of sediment filling the basins, all eroded from the adjacent mountains, which may stand 6,000 feet or more above the valleys. I’ve actually done quite a lot of work on this region because my specialty, gravity and magnetic data, is useful in figuring things out here. The sediments in the valleys are typically much less dense than the rocks in the ranges, so that density contrast is easy to see in gravity data – the denser stuff has a stronger gravitational pull than the less dense stuff.
This extension started in the Early Cenozoic or maybe even in very late Cretaceous time. It’s not as if these breaks all just happened suddenly – faulting, while it may generate catastrophic earthquakes, typically only offsets rocks by a few centimeters at a time – or a few meters in really huge quakes. That motion over millions of years can add up to a lot. The early phases of extension in Nevada produced low areas along low hills – nothing like today’s ranges. But the beginning basins were low enough for sediment and even lakes to form. For sure by Eocene time there were at least a few lakes in the region. It’s the Oligocene when the action starts to pick up, with ranges and basins starting to have higher relief, and more movement on faults. There was enough breaking to allow for some pretty vast volcanic activity as well – much of the region today is covered by sheets of volcanic ash falls and ash flows. Most of the volcanism is older than the most recent phase of mountain uplift, because the volcanics are cut by the faults that form the boundaries between basins and ranges, but there has been some volcanic activity in Nevada as recently as the past 5 million years or so.OK, so stretching broke the crust into these long, narrow basins and ranges. What caused the stretching? This is a really big question, and we really don’t have a definitive answer. As with many complex processes, it’s likely to be a combination of diverse origins. One thought has been that the continent-scale uplift of the Rocky Mountains, centered to the east of the Basin and Range, was enough for gravity to drag the western slope of the mountains down to the west, like a gargantuan landslide, and the crust broke as it slid. But the details of the faults show that many of them are not simple straight line breaks dipping steeply into the earth. They are like that near the surface, but then they often curve at depth, merging into a possible deep, flat zone called a detachment. This is a hypothetical surface that would be a boundary above which the blocks – the basins and ranges – would tilt and slide into their present-day geometry. There’s quite a lot of support for some variation on this theme.
But still, what’s the ultimate driving force? That gravitational sliding idea doesn’t really work because the scales involved are too small. Is there something else that could drive uplift, and therefore the extension? At about the time the basin and range faulting got going, the North American continent was overriding the oceanic spreading center in the Pacific Ocean, the rest of which is the East Pacific Rise mid-ocean ridge today. The spreading center itself subducted, and the whole tectonic framework changed. The San Andreas fault formed in California as a result – and it formed in Oligocene and Miocene time, about the same time as the Basin and Range started to form. Conceptually, it’s kind of easy to visualize that spreading center down there underneath the continental crust, subducting, but still with the pulling apart happening. Those forces might have translated up into the overlying crust, breaking it. You can think of it as an incipient continental rift, like the East Africa Rift system – but then we have to explain why the breaking is so widely distributed. You can maybe do that by saying the subducting East Pacific Rise has different properties than a normal rift, because it’s subducting, and maybe the nature of the crust in Nevada and Utah was such that it broke the way it did. The continental crust there is a lot thinner than normal continental crust, and heat flow is quite a bit higher than normal, but we’re getting into the realm of speculation now. There might also be consequences of a change in the angle of subduction that could have affected things here. You recall that back in the Cretaceous we called on a change in subduction angle to perhaps explain the breaking of the continental crust well into the continent, in the Laramide Orogeny. Maybe something similar happened here, even though the breaks in Nevada and Utah are mostly – but not entirely – within the Paleozoic and Mesozoic sedimentary cover.
Another idea is that as the San Andreas fault developed, it put a new kind of stress on this part of western North America. Instead of the fairly straightforward collision subduction produced, now we had a strong shear stress, essentially wrenching the continent so that dozens of breaks formed. Imagine a pile of wet napkins – that’s the sedimentary cover in Nevada and Utah. Put your right hand on the right side of the pile – that’s the strong, stable core of North America. Put your left hand on the left side, and push your left hand away from you, simulating the movement of the San Andreas Fault. All the country between your hands will wrinkle – and if you could do this with something brittle, it would break in many places. That’s the concept of this regional shear pattern generating the basin and range.
So obviously it’s complicated and there is no strong consensus as to how the Basin and Range formed. It is even more complicated by things that were going on as the Miocene phase of basin-range faulting got going, about 17 million years ago. Things like the opening of the Rio Grande Rift, in New Mexico, the eruption of vast flows of basalt in the Columbia River country of Washington and Oregon, and the first interactions between North America and the Yellowstone Hot Spot. We’ll tackle some of those things later this month.
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Two geological birthdays today. Richard Alexander Fullerton Penrose Jr. was born December 17, 1863, in Philadelphia. R.A.F. Penrose studied the mining district at Cripple Creek, Colorado, for the U.S. Geological Survey, and invested in mining ventures that made him wealthy. He endowed the Geological Society of America with a gift of almost $4 million at his death in 1931 – a bequest that to this day funds significant grant programs for the Geological Society of America. The Penrose Medal, the highest award given by the GSA, is named for him.
Nelson Horatio Darton was born December 17, 1865, in New York City. His long career with the USGS was quite varied, including important works on the hydrogeology of the Great Plains and Black Hills, the geology of the Big Horn Mountains, and paleontology studies, resulting in more than 200 publications. He received the GSA’s Penrose Medal in 1940.
—Richard I. Gibson
LINKS
Basin and Range (Idaho State U.)
Basin & Range aquifers
Basin & Range (USGS)
Basin & Range (NPS)
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