Surface Deformation and Shallow Dike Intrusion Processes at Inyo Craters, Long Valley, California

The Inyo craters are the two largest of four phreatic craters that lie within a 2.5-km-long, 500- to 700-m-wide N-S trend of faults and fissures at the south end of the Inyo volcanic chain in eastern California. The alignment of these features with dike-fed silicic volcanic centers of the same age a few kilometers to the north suggests that they were produced during intrusion of a dike at about 650–550 yr B.P. E-W extension south of south Inyo crater is ten to several tens of meters, suggesting that the dike is at least that thick. To understand how the faults and fissures developed, we mapped and studied the fault and fissure pattern; used a theoretical boundary element model to determine the surface strain profile above a shallow dike in a purely elastic medium; and conducted physical model experiments of fault and fissure growth. Results of the field studies and experiments indicate that deformation develops according to the following sequence: 1) extension fractures and perhaps other inelastic deformation develop immediately above the dike top and on the limbs of a shallow syncline which forms above the dike; 2) fissures form along two parallel trends at the surface on opposite sides of the dike plane, leaving a relatively unfractured region in between; 3) dip-slip movement on subsurface fractures, and linkage of these fractures with surface fissures produces inward-facing normal fault scarps that bound a nested graben above the dike. Experimental graben widths are up to several times narrower than predicted from the locations of theoretical strain maxima above a dilating crack in a linearly elastic medium. This fact is apparently due to the growth of a zone of inelastic deformation above the experimental dike top in the subsurface. If the ratio between the depth to the dike top and the distance between the outermost surface fissures at Inyo craters is within the range of ratios measured experimentally, then the depth to the dike top is between 250 m and several times that. A slant hole drilled in the summer of 1987 by the Department of Energy, Office of Basic Energy Sciences, intersected three breccia bodies of possibly intrusive origin directly below the center of south Inyo crater between 550 m and 610 m below the crater rim. No intact juvenile igneous rock was intersected, suggesting that the hole passed above the (magma-filled) dike top, perhaps passing through the zone of inelastic deformation which is inferred to exist from our model results.

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