Paleosecular variation and GAD studies of 0–2 Ma flow sequences from the Aleutian Islands, Alaska



[1] A study was undertaken in the late 1960s to investigate paleosecular variations of the geomagnetic field as recorded in volcanic rocks from the Aleutian Islands. The early results were internally consistent but to be wholly credible by today's standards needed more detailed demagnetization. Complete thermal demagnetization protocols have been applied to the unmeasured archived samples from the six flow sequences used in the initial study, and 40Ar/39Ar techniques have been used to improve the time resolution. The flow sequences have ages ranging from about 50 ka to 2 Ma, and the number of sequential flows in the individual sequences varies from 8 to 21. After strict selection criteria were applied (MAD < 5°, α95 < 5°) for both demagnetization data from samples and samples within one flow, the number of acceptable flows per flow sequence dropped to between 5 and 15. With the exception of a sequence showing transitional field behavior, the between-flow dispersion and the α95 values for the other sequences were notably low with respect to secular variation models, and their mean directions were very close to the GAD field. Since the time represented by the individual sequences is not well determined, the low dispersion could represent very short eruption times. In contrast, the lack of dispersion with respect to the GAD field can be taken to indicate good time averaging. Since the locations of the sampled flows are at roughly the same latitude (about 50°N) but are spread over about 10° of longitude, the dispersion was calculated for both the locality-means and the flow-means. These data represent the whole 2 Myr and give a dispersion which is lower than current secular variation models predict. A similar data set published for locations in western Canada that are at roughly the same latitude and overlap in age with the Aleutian sites gives dispersions that are close to the model predictions. At face value this can be interpreted as indicating low secular variation for the Aleutian sites.