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Paleomagnetic Correlation of Yellowstone Hotspot Related Rheomorphic Ignimbrite in the Snake River Plain of Southern Idaho, USA


Large-volume explosive volcanic eruptions from the Bruneau-Jarbidge region of southwestern Idaho are thought to have impacted mid-Miocene environments across continental USA and probably perturbated global climate. They are recorded by widely dispersed tephras and a proximal succession of welded rhyolitic ignimbrites known as the Cougar Point Tuffs (CPT). Ignimbrite successions similar to the CPT in age, chemistry, and physical characteristics are present along both the southern and northern margins of the central Snake River Plain (cSRP). Identification of individual eruption-units spanning between distant locations is essential to understand the true scale and frequency of volcanism. Fortunately, the CPT record an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations. Paleomagnetic characterization of eruption-units based on geomagnetic field variation has a resolution on the order of a few centuries or less, providing the means for strong tests of whether two deposits could have been emplaced from the same eruption or from temporally separate events. In this thesis, I present paleomagnetic, geochemical, mineralogical, and geochronologic evidence for correlation of the CPT eastward to the Brown’s Bench escarpment (6 common eruption-units) and Cassia Mountains (3 common eruption-units) regions of southern Idaho. The new stratigraphy presented here significantly reduces the frequency and increases the scale of known cSRP ignimbrite eruptions.

Individual ignimbrite cooling-units, however, display significant variation of magnetic remanence directions and other magnetic properties. This complicates paleomagnetic correlation. The ignimbrites are intensely welded and exhibit mylonite-like flow-banding produced by rheomorphic ductile shear during emplacement, prior to cooling below magnetic blocking temperatures. This results in a large anisotropy of thermal remanent magnetization, which in turn results in large deflections of the stable remanence direction. To obtain reliable paleomagnetic directions, the anisotropy of anhysteretic remanence was measured in the CPT to correct for magnetic anisotropy. In addition to magnetic anisotropy, the strong preferential alignment of anisotropic grains results in the acquisition of a significant component of gyroremanence (GRM) during alternating field demagnetization. The accepted method proposed by Dankers and Zjiderveld (1981) for excluding GRM affected measurements requires nearly triple the amount of lab work, and by consequence, is almost never regularly implemented on large batches of samples. Here, I present a laboratory procedure and subsequent analysis (SI method) that removes the effects of GRM in static AF demagnetization without requiring extra laboratory work.

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