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_04_Basin_RangeOne - GEOLOGY 20 LECTURE 4 BASIN RANGE Part...

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1 GEOLOGY 20 - LECTURE 4 - BASIN & RANGE: Part 1 - Old Rocks, Young Faults, Building Mountains (Ch. 7 & 1 in Harden - read selectively!) I urge you to print off all the notes and images in Resources and start reading them in concert with the text. The midterm will be coming up soon, so firm up your understanding of everything we’ve covered till now by putting time into it outside of class. The best way to begin is by building yourself a notebook with the notes and images organized as best you can. Basin and Range Province Basin and Range (B/R) extends N-S from eastern Oregon into Mexico, and E-W from the eastern front of the Sierra Nevada, across Nevada, to central Utah. In California, B/R includes the region east of the Sierra Nevada, the Mojave sub- province in southeastern CA, and the Modoc Plateau in the northeast. - the B/R consists of a swarm of north/south-trending elongate mountain ranges alternating with broad elongate valleys (aka “ basins ”) - many of the basins lie 4000-7000’ above sea level, with some ranges reaching over 13,000’ Most of the Basin and Range Province (including the portions of the province in California) is a desert, where evaporation exceeds precipitation. - this sparsely populated expanse exists due to a rainshadow effect caused by high mountain ranges to the west such as the Sierra Nevada, Cascades, San Gabriels and San Bernardinos that capture moist air masses that move in from the Pacific. - as moist air from the Pacific rises over the Sierra Nevada and other mountain ranges, it cools. Colder air cannot hold as much moisture, so the water condenses out as rain or snow on the mountains. By the time the air mass reaches the east side of California and the rest of the Basin & Range, it is usually dry - windswept, sagebrush-covered, mostly exposed rock and loose sandy sediment Basin and Range – building mountains one earthquake at a time Basin & Range Province (B/R) originated due to extensional forces that have pulled apart the area, mostly over the past 16 m.y. or so. As the lithosphere was stretched, faults developed. Mountains rose along the faults and basins dropped episodically along the faults during thousands of individual earthquakes Extension is essentially “stretching” where tectonic stresses are directed away from each other
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2 - the direction of extension in the B/R is basically E-W, which pulls apart the brittle upper crust into N-S trending basins and ranges, separated by faults - the total amount of extension in the B/R is difficult to determine, but careful mapping and analysis suggests that the distance between Reno and Salt Lake City has doubled since before stretching began Three main types of deformation, related to orientation of tectonic stress: (these stress orientations can occur at all scales, from plate boundaries to the scale of individual crystals) 1) extension : stretching, where tectonic stresses are directed away from each other 2) compression
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_04_Basin_RangeOne - GEOLOGY 20 LECTURE 4 BASIN RANGE Part...

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