G.C. Willis | Geology Reports

USGS2007

Holocene slip rate and revised characteristic earthquake parameters for the western segment of the Castle Mountain fault, Alaska

The western segment of the Castle Mountain fault poses a significant seismic hazard to the most populated region of south-central Alaska. We identify a previously unrecognized margin of a postglacial outwash channel that is offset right laterally 36 ± 4 m across the western segment of the Castle Mountain fault. This offset occurred after glaciers withdrew from the lowland 11,300–15,380 cal yr b.p. and after outwash channel margins were cut and stabilized 11,210–13,470 cal yr b.p. Using these ages and the measured separation, we obtain a maximum slip rate of 3.0 ± 0.6 mm yr −1 and a minimum slip rate of 2.8 ± 0.7 mm yr −1 . These are the first lateral slip rates for the Castle Mountain fault established by a field measurement. Based on timing of the most recent earthquake, 670 ± 60 yr b.p. , the Castle Mountain fault could have accumulated an average single-event slip of about 1.9 m (extremes range from 1.3 to 2.6 m). The fault consists of two segments; a surface-rupturing earthquake likely will be limited to the 62-km-long western segment. Area-magnitude regression calculations suggest that such an earthquake on the western Castle Mountain fault would have a moment magnitude of 6.9 to 7.3.

Alaska↗

USGS1996

Early miocene bimodal volcanism, Northern Wilson Creek Range, Lincoln County, Nevada

Early Miocene volcanism in the northern Wilson Creek Range, Lincoln County, Nevada, produced an interfingered sequence of high-silica rhyolite (greater than 74% SiO2) ash-flow tuffs, lava flows and dikes, and mafic lava flows. Three new potassium-argon ages range from 23.9 ?? 1.0 Ma to 22.6 ?? 1.2 Ma. The rocks are similar in composition, stratigraphic character, and age to the Blawn Formation, which is found in ranges to the east and southeast in Utah, and, therefore, are herein established as a western extension of the Blawn Formation. Miocene volcanism in the northern Wilson Creek Range began with the eruption of two geochemically similar, weakly evolved ash-flow tuff cooling units. The lower unit consists of crystal-poor, loosely welded, lapilli ash-flow tuffs, herein called the tuff member of Atlanta Summit. The upper unit consists of homogeneous, crystal-rich, moderately to densely welded ash-flow tuffs, herein called the tuff member of Rosencrans Peak. This unit is as much as 300 m thick and has a minimum eruptive volume of 6.5 km3, which is unusually voluminous for tuffs in the Blawn Formation. Thick, conspicuously flow-layered rhyolite lava flows were erupted penecontemporaneously with the tuffs. The rhyolite lava flows have a range of incompatible trace element concentrations, and some of them show an unusual mixing of aphyric and porphyritic magma. Small volumes of alkaline, vesicular, mafic flows containing 50 weight percent SiO2 and 2.3 weight percent K2O were extruded near the end of the rhyolite volcanic activity. The Blawn Formation records a shift in eruptive style and magmatic composition in the northern Wilson Creek Range. The Blawn was preceded by voluminous Oligocene eruptions of dominantly calc-alkaline orogenic magmas. The Blawn and younger volcanic rocks in the area are low-volume, bimodal suites of high-silica rhyolite tuffs and lava flows and mafic lava flows.

Brigham Young University Geology Studies↗