Paleomagnetic eruption duration: Ash Creek Butte - Anthony Pivarunas - 2021 MagIC Workshop (1/20/21)
Channel: Magnetics Information Consortium (MagIC)
Date: 2021-01-27

"Paleomagnetic Determination of Eruption Duration at Ash Creek Butte, California" by Anthony Pivarunas, U.S. Geological Survey: Geology Minerals Energy and Geophysics Science Center.

Lightning poster talk presented at the virtual 2021 MagIC Workshop (https://earthref.org/events/MAGIC/2021/) - Rock and Paleomagnetism through Time and Space. Jan 19th-21st, 2021.

Abstract:
Anthony F. Pivarunas(1,2), Dawnika L. Blatter(2), Andrew T. Calvert(2)

U.S. Geological Survey: (1)Geology Minerals Energy and Geophysics Science Center, (2)Volcano Science Center

Although the prototypical image of a continental arc volcano is a captivating, snow-capped stratovolcano, only a limited number of stratovolcanoes exist in the Cascade arc. Most Cascade volcanoes are basaltic andesite to andesite shields and stratocones. In contrast to the well-studied major volcanic centers, background regional volcanism in the Cascade arc remains poorly understood despite that its eruptive style dominates igneous activity along the arc. For instance, the simple question – how long does it take to build a typical regional volcano? – is unconstrained by anything other than intelligent guesswork.

Ash Creek Butte is an ~11 km3 shield volcano along the Cascades arc axis between the major eruptive centers at Mount Shasta and Medicine Lake. It is a typical basaltic-andesite shield like its companions The Whaleback and Goosenest. Ash Creek Butte is considered monogenetic in that it lacks visible unconformities and its flows have similar petrology and chemistry. One flow has a whole-rock Ar-Ar age of 227 ± 9 ka, and the shield structure was heavily eroded during MI6 glaciations providing excellent exposures of the lava flows comprising the edifice.

Volcanic eruptive histories are constructed from a combination of field geology, geochemistry, and absolute radiometric dating. However, as seen at Ash Creek Butte, uncertainties of Ar-Ar dates can be thousands of years. Paleosecular variation (PSV) is the rock-encoded record of a constantly shifting geomagnetic field that can be resolved by paleomagnetic directions on the order of decades to centuries. Thus, the paleomagnetic approach can provide the relative temporal resolution needed to delineate the rapid eruption and construction of volcanic edifices.

We sampled oriented cores at Ash Creek Butte to determine a temporal framework relative to PSV for the construction of this “typical” regional volcano. Thirteen sites in total were taken from flows on the flanks of the shield, and from stacked flows exposed along the cirque ridges, to provide a relative stratigraphic ordering. A feeder dike in a ridge saddle also provided an opportunity to sample the intrusive portion of this volcanic system.

Specimens from each sample were cleaned by AF demagnetization. Thermal demagnetization was performed on selected twin specimens, and this behavior was identical to that from AF demagnetization except for specimens with major IRM overprints. We failed to recover good characteristic directions from 2 sites: both had substantial high-coercivity overprints and yielded poorly grouped directions substantially different from the other characteristic directions. We recovered a single directional population from 11 well-determined paleomagnetic sites with a mean of D=7.6°, I=+60.6° (a95=2.1°, k=486; Fig. 2). The known stratigraphic order of the lava flows does not correlate with any linear trends in these data. Directions for younger flows sampled from the lower flanks of the shield also scatter around the mean direction with somewhat poorer precision, and these directions likely reflect minor post-cooling deformation of the outcrops. Our results suggest that the individual site directions are from sampling a singular geomagnetic field during a truly monogenetic eruption. The only inconsistency in this scenario comes from a few dike (S209) samples which have more easterly declinations than the other sites. 

The angular difference between the mean paleomagnetic direction of early flows (within the glacially-eroded edifice) and late flows (at lower elevations on the volcano flanks) is minimal at ~3°. Common mean tests indicate the early/late mean directions are indistinguishable from one another at 95% confidence. With the 2.1° radius of our confidence interval on the pooled (early/late) mean, and an average Holocene rate of PSV in the western United States of 0.05°/year (Hagstrum and Blinman, 2010), the duration of Ash Creek Butte’s emplacement was likely no longer than several decades. This is a substantially reduced duration than the centuries to millennia currently estimated for such volcanic edifices. Comparison of these paleomagnetic data with direct measurements from geomagnetic observatories, considering various sources of experimental error, is probably the best tool for obtaining a more quantitative estimate of the eruption duration at Ash Creek Butte.