Sources and Flow Directions of Dikes at Sinker Butte Volcano, Western Snake River Plain, Idaho

At least thirteen basaltic dikes are exposed in a 140° arc along the south and east sides of the Pleistocene Sinker Butte Volcano. The dikes are exposed for up to 500 m along strike and are between 28 and 115 cm in width. Most have nearly vertical dips. Greater abundances of plagioclase and olivine phenocrysts in the interiors of many of the dikes suggest they were concentrated by flow differentiation, although complex zoning and internal chilled borders in a few of the dikes are attributed to multiple magma pulses. The dikes are best exposed along the walls of the Snake River Canyon, where they intrude a thick series of phreatomagmatic tuffs sandwiched between sequences of lava flows. Some of the dikes cut the lower sequence of lavas, and several merge with the capping flows; however, the relationship between the dikes and the overlying lavas is not yet clear. Anisotropy of magnetic susceptibility (AMS) was determined for about 80 oriented drill cores collected from nine of the Sinker Butte dikes. Seven showed strong anisotropies (up to 9.7%). Average magnetic lineations, which are assumed to reflect flow directions, are nearly horizontal for two of the dikes, plunge about 45° downward and away from the presumed vent area in two others, and are nearly vertical for the other three. The AMS data indicating horizontal and inclined flow directions are consistent with the propagation of radial dikes outward from a magma column or lava lake located in the upper part of the edifice. Vertical flow directions may have been upward in response to the impermeable barrier at the base of the tephra section, or may have been downward in response to draining of magma from the central conduit or lava lake. Our model for the emplacement and transport of magma in the radial dikes at this center should be further refined by detailed field mapping and studies of crystal alignments currently in progress.