The Roza flow of the Columbia River Basalt group in
Washington State U.S.A. extruded at about 15 Ma during
intermediate geomagnetic polarity. The Roza is underlain
by normal polarity flows and overlain by reversed units.
The Roza is an extensive flow, up to 60 in thick. As the
remanence-blocking isotherms progressed into the flow, it
recorded a short continuous segment of the transitional
geomagnetic field.
Our studies show limited thermal remagnetization in
the underlying flow, and we infer that groundwater was
effective in extracting the heat. The extent of basement
heating was further reduced by the insulating scoria
immediately beneath Roza. Hence, the Roza flow cooled
symmetrically from its top and base.
Samples from the drilicores acquired drilling induced
remanent magnetization (DIRM), shown to be well modelled as
an isothermal remanent magnetization produced in nonuniform
fields of the order of 10 mT at the rim of the drillstring.
Alternating field demagnetization was usually successful in
removing the DIRM.
The remanence stability is higher in the top third of
Roza, due to smaller magnetic particles, than in the lower
two thirds of the flow, where the magnetic properties are
nearly uniform. The stability profile corresponds to the
entablature/colonnade subdivision. High temperature
subsolidus oxidation of the titanomagnetites increased with
height in the flow, altering the primary symmetric
intraflow distribution.
The declination of the Roza flow sampled at numerous
outcrops is consistently about 189°. In a 54 m drillcore
section, Roza inclinations become more negative towards the
flow interior, consistent with its magnetostratigraphic
position. Superimposed is a symmetric, rapid change in
inclination from -2 to -15° and back to -5°. The flow's
thermal history predicts that these fluctuations have a
characteristic time between 15 and 60 years, such that the
inclination changed at a rate of 1/2 to 2° per year,
showing that the directions of this transitional field
fluctuated several times more rapidly than the present
geomagnetic field. However, considering the generally
reduced intensity during transitions, these fluctuations
might not be unusual. / Graduation date: 1990
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29359 |
| Date | 06 July 1989 |
| Creators | Audunsson, Haraldur |
| Contributors | Levi, Shaul |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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