Recent vertical crustal movements from precise leveling data in southwestern Montana, western Yellowstone National Park, and the Snake River Plain

Abstract

Repeated levelings in southwestern Montana, the western portion of Yellowstone National Park, and the Snake River Plain provide information on the pattern of relative vertical crustal movement throughout this region. Except for the coseismic deformation associated with the 1959 Hebgen Lake earthquake the most outstanding and best defined feature of the data is contemporary doming at a rate of 3–5 mm/yr involving approximately 8000 km2 including the epicentral area and aftershock zone of the 1959 Hebgen Lake earthquake. On the basis of observations over different time intervals, doming appears to have continued throughout the time the movements were monitored, beginning at least 25 years prior to the 1959 earthquake and continuing at least 1 year after the earthquake. The character of the coseismic deformation associated with the 1959 earthquake and the high regional elevation are consistent with the observed doming. It is suggested that doming preceded the earthquake for a considerable time (of the order of hundreds to thousands of years, perhaps longer), giving rise to tensional stresses in the upper crust. When these stresses, combined with the regional tectonic stresses, exceeded some critical value, faulting and collapse in response to gravity occurred, resulting in the 1959 earthquake. The voluminous Tertiary and younger volcanics in the vicinity of the doming region suggest that magma intrusion into the crust is the most likely cause of the observed uplift. The proximity of the doming region to the thermally active Yellowstone area supports this suggestion. Secondary features of the data include (1) a spatial correlation between tilting and historic seismic activity; (2) uplift within the Norris-Mammoth corridor in Yellowstone National Park relative to nearby bench marks to the north and south; (3) regional subsidence of the eastern Snake River Plain relative to points north and west of this physiographic province, including subsidence of the Pleistocene Island Park caldera floor relative to its rim fractures; and (4) rapid tilting in the vicinity of the Continental fault east of Butte, the intersection of the Gardiner, Mammoth, and Reese faults just north of Yellowstone National Park, and the Madison Range fault in eastern Idaho.