HR: 1340h
AN: U33A-0031    [Abstracts]
TI: Toward Defining Archean Plate Motion
AU: * Bauch, D G
EM: db007k@mail.rochester.edu
AF: Dept. of Earth & Environmental Sciences, University of Rochester 227 Hutchison Hall, Rochester, NY 14627 United States
AU: Tarduno, J A
EM: john@earth.rochester.edu
AF: Dept. of Earth & Environmental Sciences, University of Rochester 227 Hutchison Hall, Rochester, NY 14627 United States
AU: Cottrell, R D
EM: rory@earth.rochester.edu
AF: Dept. of Earth & Environmental Sciences, University of Rochester 227 Hutchison Hall, Rochester, NY 14627 United States
AU: Watkeys, M K
EM: watkeys@nu.ac.za
AF: Geological Sciences Programme, School of Geological and Computer Sciences University of KwaZulu-Natal, Durban, 4041 South Africa
AB: Paleomagnetic data from previous paleomagnetic studies of mid-Archean rocks ($\sim$3.0-3.5 Ga) have been interpreted as reflecting a plate tectonic style similar to that of the Cretaceous to recent. Times of rather slow motion (a few centimeters per year) are punctuated by very rapid motion (over 14 cm/yr). However, prior results have been based on whole rock samples, which have been subjected to prolonged low grade metamorphic conditions. In some rocks (mafic lavas and sedimentary rocks) this has resulted in a crystallization remanence of unknown age. In others (plutonic rocks) it has the potential to produce a complex history of overprinting. Here we revisit these issues by approaching the paleomagnetic study at a mineral scale, following our prior work and studies in the 1960's and 1970's indicating the presence of magnetic inclusions in silicate minerals. In rock magnetic investigations we find that hornblende carries a multidomain signal and therefore we expect it to record magnetic overprints. In contrast, quartz and microcline yield single domain to pseudosingle domain behavior, suggesting that the magnetic inclusions they contain could preserve primary magnetizations. To explore the paleomagnetic signals of these crystals, we are developing a CO$_{2}$ laser/SQUID magnetometer experimental approach which allows us to obtain directional and paleointensity information from oriented single mineral grains. Preliminary data from 3.2 Ga plutons of the Kaapvaal craton (southern Africa) suggest that prior indications of very high plate velocities may be premature. We will discuss alternative interpretations, including overprinting of whole rock samples and the possibility of mid-Archean non-dipolar fields.
DE: 8155 Plate motions--general
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1527 Paleomagnetism applied to geologic processes
SC: Union [U]
MN: 2004 AGU Fall Meeting