TECTONIC IMPLICATIONS OF GEOLOGIC MAPPING, MEDICINE LAKE VOLCANO AND VICINITY, NORTHERN CALIFORNIA

Geologic mapping of the 0.5 Ma to Recent Medicine Lake volcano (MLV) indicates that vents for individual eruptions are most commonly aligned between N 35° W and N 35° E. The only significant exceptions are vents located (1) on the SW side of the volcano where vent trends of N 55° E are found, probably influenced by a highland of vents (and apparent crustal weakness) connecting Medicine Lake volcano to Mt. Shasta some 50 km to the WSW, and (2) within and immediately adjacent to the caldera, where vent trends are commonly tangential to the caldera rim. The center of the caldera is the focus of downwarping of MLV, currently occurring at a maximum rate of 0.9 cm/yr (Dzurisin and others, in press). Drill hole data indicate that this high rate of caldera floor subsidence is a recent phenomenon. The caldera was probably created by a combination of factors including explosive eruption, construction of rim vents, and subsidence in part related to flow of fluid rim lavas away from the center of the volcano and its magmatic focus. Fault trends on the volcano are similar to vent trends, but faults on the edifice are limited in number and extent. Many are likely buried by younger lavas. On the lower N and E flanks, open ground cracks are common. Other such cracks located on the upper NW rim of the caldera were apparently caused by a dike system that fed eruption of the late Holocene fissure rhyolite of Little Glass Mountain (Fink and Pollard, 1983). E-W extension is indicated by offset features on opposite sides of some of these open cracks. On the lower NNE flank, similar cracks probably opened during the 3 ka fissure eruption of Black Crater and Ross Chimneys. Regional dominantly N-S-trending Basin & Range-type faults with normal offsets project directly under MLV from both N and S. No lateral motion has been documented on these faults. Location of vents and faults at MLV is evidently controlled by both external (regional E-W extension) and internal (caldera-related) stress. The volcano coincides with the intersection of major N-S normal faults, the zone of weakness connecting to Mt. Shasta, and with the northwestern extension of the Walker Lane. It is interesting that to the east, in million-year-old basalts of the Devils Garden, some NNW-trending faults that are presumably part of the extended dextral-slip Walker Lane display both normal and left-lateral slip.