Abstract: Quaternary Tectonics and Historical Seismicity of the Upper Madison Valley, Southwest Montana

J. M. O'Neill, T. H. Leroy, M. C. Stickney, P. E. Carrara

The upper Madison River valley in the Cliff Lake 15^prime quadrangle, about 40 km west of Yellowstone Park, is a north-trending, 10-12 km wide depression divisible into three geomorphic segments. The south and north segments are broad, gently sloping alluvial plains of a Middle Pleistocene fill inset by younger terraces along the river. In the central segment the river is confined in a 2 km wide, 300 m deep canyon. Much of the valley and the adjacent Gravelly Range are underlain by the 2.01 Ma Huckleberry Ridge Tuff that in places is overlain by alluvial gravels. Structure contours on top of the tuff show nearly 1100 m of relief of which more than 700 m is related to Quaternary uplift of the southern Gravelly Range. Well defined fault scarps and abundant landslides in the quadrangle reflect contemporary tectonic activity. Tilted trees on many landslides indicate that the slides are presently active. Tree-ring analyses indicated > 25% of the trees at one or more of three slides sampled recorded landslide movement in response to the 1983 Borah Peak, 1982 Centennial Valley, 1973 Yellowstone, 1959 Hebgen Lake, 1954 Mammoth, 1935 Helena, 1926 Clarkson, 1909 Virginia City, and 1895 Butte earthquakes. In addition, evidence prior to good historical seismic records indicate disturbance in 1893 and 1865. From 1908 to 1977, 34 earthquakes of magnitude greater than 5.0 have occurred within 100 km of the quadrangle. Between Jan. 1992 and Nov. 1994, 370 earthquakes with magnitudes less than 1.0 to 3.7 occurred in the quadrangle. This activity is part of an east-tren ing belt of intense seismicity extending from the Centennial Valley on the southwest, through the quadrangle, and into Yellowstone National Park. The seismic zone coincides with the area of greatest structural relief in the quadrangle as recorded by the Huckleberry Ridge Tuff.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana