Paleomagnetism of Cougar Point Tuff XII, Snake River Plane Idaho
Thursday, 18 December 2014
Yellowstone hotspot center migration during the mid-Miocene northeast along the Snake River Plain (SRP) resulted in a succession of explosive caldera-forming super-eruptions, often involving 1000’s of km3 of ejecta and covering 10,000’s of km2. The frequency and volume of the rhyolitic eruptions in the SRP are of both academic and societal interest, yet remain poorly known. Identification and correlation of individual eruption deposits are critical for evaluating eruption volumes and frequency over time and the relationship with climate and tectonics. Rhyolitic ash-flow deposits flank both the northern and southern margins of the SRP, but have not been successfully correlated because of their similarity in both outcrop appearance and chemical composition. Paleomagnetic correlation using the stable magnetic remanence direction has the advantage of very high temporal resolution, on the order of centuries because of the geologically rapid rate of geomagnetic secular variation and the high accuracy in which extrusive volcanic rocks may record the instantaneous direction of the magnetic field. The strength of a paleomagnetic correlation increases with the rarity of the field direction recorded by the ash flow. Here we demonstrate correlation of SRP ignimbrites by sampling the Cougar Point Tuff (CPT) Xll at three widely spaced (~25 km) locations. The transitional polarity recorded by titanomagnetite in CPT Xll enables a strong paleomagnetic correlation. CPT Xll, however, is weakly magnetized because it was erupted during a polarity transition of Earth’s magnetic field, and therefore is more susceptible to magnetic overprinting. Nonetheless, careful alternating-field demagnetization coupled with line- fit and great-circle analysis, yields well-constrained directions of stable remanence. Gyroremanent magnetization, a spurious component produced by demagnetization at the higher field steps, is dealt with satisfactorily by taking small alternating-field steps and by permuting the order of demagnetization along the three sample axes.