Wasatch Fault, Little Cottonwood Stock, Utah
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
South of Salt Lake City a westward jog of the Wasatch Fault offsets the fault trace. On the edges of the Little Cottonwood Stock (green, above) a carapace of sheared and altered phyllonite (micaceous mylonite) is found. Many localities on the Wasatch Fault reveal details of recent surface ruptures and near-surface brittle fracture. This locality displays evidence of deep deformation.
The jog in the fault coincides with a small horst, the Traverse Mountains.
New construction in the area has substantially altered roads and topographic contours. New roads (gray) are approximately located and the topography of the built-up area is significantly altered.
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Looking north. The flat bench at the base of the Traverse Mountains looks artificial but is actually a Lake Bonneville shoreline. |
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The transverse fault runs approximately up the valley. The Little Cottonwood Stock is on the opposite side of the valley and the lower slope of the mountain is approximately a dip slope of the fault. |
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The rocks on the slope grade from nearly intact granite to sheared granite to micaceous mylonite (phyllonite). Petrographic studies suggest the rocks were deformed at a depth of around 11 km and at temperatures of about 350-400 C. The stock is Oligocene in age. |
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Below: looking west along the north front of the Traverse Mountains. The Oquirrh Range is in the distance. |
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Left: the GSA field trip group. Below: Green phyllonite with a dikelet of pseudotachylite. |
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New construction. The rumpled hillslope is a former landslide. |
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The valley halfway up the hillslope was interpreted by G.K. Gilbert as a valley incised into the stock and then upliifted by later fault movements. |
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Looking north along the Wasatch Fault. |
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Left and below: looking south along the Wasatch Fault from north of the phyllonite locality. The Wasatch Fault is famous for its faceted spurs, but how much of the smoothness of these spurs is actually due to wave action by Lake Bonneville? |
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Left: a sharply-defined shoreline at the base of the mountains. |
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Created 18 November 2005, Last Update 06 June 2020