GMU Geo: Mountains, Ice, and Earthquakes in central Idaho

Dr. Glenn Thackray: Chair, Dept. of Geosciences, Idaho State University http://geology.isu.edu/geology-web/gthac/thackray.htm

Monday, 28 January, 2:30pm

Research Hall Showcase

Mountains, Ice, and Earthquakes in central Idaho

The Sawtooth Mountains of central Idaho preserve dramatic evidence of glaciation and recent faulting in landforms and lake sediments.  A moraine belt at the mountain front documents several Late Pleistocene glacier advances and retreats; specific landform characteristics and limited numerical dating suggest that those events occurred during early phases of the last glaciation as well as during and after the Last Glacial Maximum, suggesting that climatic events recorded in the maritime Pacific Northwest also affected inland regions.  Glaciers retreated from the range front to major cirques ca. 14,000 yr BP and did not readvance dramatically, as evidenced by cirque lake sediments, although cosmogenic exposure dating of moraine boulders in one range-front drainage suggests that ice readvanced ca. 11,400 yr BP.

These ages are critical for interpretation of a young fault scarp, discovered and evaluated through high-resolution LiDAR topographic data.  The fault scarp cuts glacial landforms and Holocene alluvial sediments along 50-65 km of the range front.  The 4-9 m fault scarp in Late Pleistocene glacial landforms suggests 2-3 fault movements since deglaciation and fault slip rates of 0.5-0.9 mm/yr, similar to, or greater than slip rates of the most active regional faults. Turbidite sediments in a large lake in the fault hanging wall indicate major earthquakes ca. 4,100 yr BP and 7,000 yr BP.  The Sawtooth Fault is clearly a major fault capable of generating strong earthquakes.

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