(RE)ASSESSING THE APPLICATION OF VISCOUS REMANENT MAGNETISM TO DATING GEOMORPHIC EVENTS

Rock magnetics theory predicts a relationship between the time a magnetite-bearing rock has spent in a magnetic field and the temperature of (de)magnetization of that component of the rock’s natural remanent magnetism (NRM). A rock acquires its primary magnetization at the time of formation. If the rock is subsequently disturbed by a geomorphic event such that it becomes misaligned to the magnetic field, it may begin to acquire a viscous remanent magnetic (VRM) overprint. The longer a single-domain magnetite grain remains misaligned to the magnetic field, the higher the temperature required to remove its VRM overprint. In principle, this relationship can be exploited to determine the age of a geomorphic disturbance. We expect to distinguish among historical (<1ka), Holocene (~10ka), late Pleistocene (~100ka), and earlier Quaternary events.

We evaluate the potential of VRM to dating two geomorphic disturbances of known age that disrupt basalt: 1) The ~1 ka Bonneville Landslide on the Columbia River in Washington, and the ~20 ka Lake Bonneville outburst flood deposits in SE Idaho. We drilled 23 oriented, 2.5-cm cores from 13 boulders of 2 -4 m diameter m at three sites in landslide deposit, and 12 oriented cores from 12 boulders of 1 -3 diameter m in boulder bars at two sites in the outburst flood deposit. All specimens were treated with liquid nitrogen to remove the influence of larger multi-domain grains, prior to thermal demagnetization. The specimens were thermally demagnetized in 5° or 10° increments from 45°C to 240°C, holding the temperature for 30 minutes at each step.

For the landslide, of the specimens that responded to thermal demagnetization, most showed a single inflection in magnetic directions with progressive demagnetization. The temperatures of the inflection points yield a mean and standard deviation within the expected range for a <1ka event. For the flood boulders, most showed two inflections or continuously changing directions with progressive demagnetization. Selecting the temperature at which the greatest change in orientation is observed, one of the two sites yields temperatures consistent with a Late Pleistocene event; results from the other suggest younger disturbance. We see promise in this tool as an inexpensive technique to provide gross age constrains on recent tectonic and geomorphic events.