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YELLOWSTONE NATURE NOTES


Vol. XXXIII June, 1960 Special Edition

YELLOWSTONE'S STRUCTURAL FRAMEWORK

Important times of faulting (fig. 3) have been shown to occur with the early mountain building and later in the Tertiary after the emplacement of the welded tuffs. The more recent rhyolite flows are also severed by faults. This complex pattern of faulting is of extreme significance to both the geologist and the seismologist.

Pre-volcanic deformation has been largely obscured by the later flows of rhyolite and welded tuff. Consequently structural control in the underlying rocks can only be inferred in an indirect way by projecting structural trends from the surrounding area as they pass beneath the volcanic cover. Occasional mountain peaks within the park that stand above the flows permit a direct observation of folding and faulting.

One of the most exciting discoveries of the past year is the realization that post-glacial faulting has occurred in many places in the park. Love (1959, p. 1782) has found more than 50 faults on the Mirror Plateau where ice scoured valleys have been disrupted by displacements of 200 to 400 feet. He describes another area five miles west of Yellowstone's South Entrance where Beula, Hering and South Boundary Lakes were created by disruption of drainage along faults.

Plate 5 illustrates a spectacular interruption of a youthful drainage system north of Ice Lake and east of the Norris Geyser Basin. Here a series of valleys trending northeast-southwest have been intersected by a north-south trending normal fault that is downdropped to the east. The waters of Wolf Lake have been impounded against another smaller fault scarp that can be seen trending in a northwesterly direction.

fault scarps
PLATE 5. Fault scarps on the Solfatara Plateau central Yellowstone Park. Park Aerial Survey photos 1-144 and 1-145.

Displacements in excess of 400 feet in the past 10,000 years can lead only to the conclusion that Yellowstone must have been one of the more active seismic areas in the United States and with this comes the realization that earth tremors must have played an important part in the evolution of the hot springs and geyser basins.

Prior to the 1959 earthquakes it was possible to observe some fracturing of the geyserite deposits in the thermal basins.

geyserite
PLATE 6. Fractured geyserite in Potts Geyser Basin near West Thumb, Yellowstone Park. Note sinuous crack caused by an early unrecorded earthquake that has controlled alignment of thermal activity. Photo taken in August 1955 by W. A. Fischer.

Since the tremors of last summer extensive rifting of the geyserite cover can be seen in the Midway and Lower Geyser Basins.

Normal faulting of the plateau surface is probably caused by a foundering of the crustal cover or by adjustment of this rigid mass to the contracting magma below.

The position of known and suspected faults in the park is shown on Plate 36.

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31-Mar-2006