DISTINGUISHING AND CORRELATING DEPOSITS FROM LARGE IGNIMBRITE ERUPTIONS USING PALEOMAGNETISM

Paleomagnetic data have been used to correlate rhyolitic ignimbrites within the mid-Miocene Cougar Point Tuff succession in southwest Snake River Plain of Idaho, USA. The ignimbrites are intensely welded rheomorphic outflow sheets from large explosive eruptions from the Yellowstone hotspot. Developing a regional ignimbrite stratigraphy is essential to evaluate the causal relationship between volcanism, tectonics, and climate change. The Cougar Point Tuffs records an unusual pattern of geomagnetic field directions, providing the basis for robust stratigraphic correlations. Several pitfalls, however, can hinder determination of the remanence direction of volcanic deposits, including remanence deflection from large magnetic anisotropy and acquisition of gyroremanence magnetization during alternating field demagnetization (AFD).

Rheomorphic ignimbrites undergo intense flow-related simple shear and compaction strains at high temperatures during emplacement, producing a mylonite-like L-S fabric and associated thermal remanence anisotropy (ATRM). ATRM deflects the remanence closer to the rheomorphic flow direction and flattening fabric. Variation of the ATRM within different unblocking temperature (or coercivity) grain fractions produces curvilinear demagnetization trajectories, previously thought to represent sub-blocking temperature rotation of magnetic grains. Partial anisotropy of anhysteretic remanence measurements were used to correct for remanence deflection and to identify its relationship with coercivity.

The fine-grained magnetite in ignimbrites commonly also suffers from acquisition of gyroremanence (GRM) during AFD. GRM is produced in most Snake River ignimbrite samples, though it is more pronounced and problematic in weakly magnetized ignimbrites that cooled while the geomagnetic field was transitional. We used an efficient new method for removing GRM, involving permutation of the AFD axes order with progressively higher AFD steps and subsequent smoothing of demagnetization data. This procedure involves no extra remanence measurements or AFD applications than a typical 3-axis demagnetization, and therefore can be implemented with significant advantage as standard practice for alternating field demagnetization of rocks.