Short Reversal of the Palaeomagnetic Field about 280 000 Years Ago at Long Valley, California
A reversal of the palaeomagnetic field is recorded in exposed lake sediments at Long Valley and Mono Basin in east-central California. The reversal is estimated to be several thousand years long and 280 000 years old. The chronology is based on correlation of volcanic ash beds at Long Valley and Mono Basin with ones at Summer Lake, Oregon, and correlation of ash beds at Summer Lake with ash beds in a core at Tulelake, California, where age control is provided by tephrochronology and magnetostratigraphy.
The path of the virtual geomagnetic poles forms a large clockwise trending loop in the hemisphere of Long Valley and is below 45°S latitude for approximately 1000 years. An alternate interpretation of the field behaviour is that it is an excursion, i.e. an aborted reversal, as proposed by Hoffman (1981). In that possibility the record at Long Valley is inconsistent with the examples selected in favour of aborted reversals during times of stable normal polarity.
KeywordsThermal Demagnetization Mono Lake Virtual Geomagnetic Pole Geological Survey Professional Paper Valley Site
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