DISTINGUISHING AND CORRELATING DEPOSITS FROM LARGE IGNIMBRITE ERUPTIONS USING PALEOMAGNETISM
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.