(04)Basin&RangeOne - GEOLOGY 20 - LECTURE 4 - BASIN...

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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!) Basin and Range - Mountain-Building 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, with the mountains rising and the basins falling episodically during thousands of individual earthquakes along the faults Extension is essentially “stretching” where tectonic stresses are directed away from each other - 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 - 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 stress: (these stress orientations can occur at all scales, from plate boundaries to the scale of individual crystals) 1) extension : as above . . . 2) compression : squeezing, or pushing together where stresses point toward each other (e.g., convergent margins like subduction zones and many mountain belts are large- scale versions of compression) 3) shear : lateral motion where stresses move past each other across a plane or zone (e.g., transform margins are commonly major faults that connect other tectonic boundaries – we’ll discuss transform plate margins later in the course) - the San Andreas Fault was created by shear forces Rocks break along characteristic faults that develop in response to these stress orientations. Each type of deformation produces a specific type of fault. A fault is simply a planar fracture along which displacement of the rocks on either side has occurred. - the orientation of the fault plane may vary from vertical to near-horizontal - faulting is scale-independent. offset (used synonymously with displacement ) can occur at the scale of inches to the scale of hundreds of miles.
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2 - the trace of a fault is its surface expression, its path across the landscape (remember that faults are planar features and extend downward into the crust, typically for a few kilometers) Movement along a fault is commonly abrupt, resulting in an earthquake - as stress (extensional, compressional, or shear) accumulates along the fault plane, the frictional strength of the rocks keeps the fault from breaking - once the stress builds enough to exceed the frictional strength, the fault ruptures, resulting in seismic energy released outward away from the fault Shear stress creates strike-slip faults (aka ‘ lateral’ faults) where blocks of rock slide past each other laterally, without any appreciable upward or downward motion
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This note was uploaded on 04/22/2008 for the course GEL 20 taught by Professor Osleger during the Spring '08 term at UC Davis.

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(04)Basin&RangeOne - GEOLOGY 20 - LECTURE 4 - BASIN...

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