Summary: 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. Shaded relief map (NPS) The name basin and range is pretty descriptive. There are alternating narrow, high mountain uplifts separated from each other by long narrow valleys, or basins. In the core area of the Basin and Range Province, east-central Nevada and west-central Utah, there are dozens of mountain ranges and intervening valleys – 50, 60 or more. The topographic map of the region led one early geologist, Clarence Dutton, to compare the basin and range to an "army of caterpillars marching toward Mexico" – and that’s really not a bad way of thinking of it. 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
