Physics
Scientific paper
Jul 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991pepi...67..374c&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 67, Issue 3-4, p. 374-388.
Physics
5
Scientific paper
We present the results of a palaeomagnetism and magnetic fabric study in six sampling sites of the lower Ordovician redbeds of the Cap de la Chèvre formation, Crozon Peninsula, France. Measurement of anisotropy of magnetic susceptibility (AMS) shows that these beds are affected by a weak but significant tectonic strain during the Hercynian folding of the Armorican Palaeozoic cover. Samples containing small elliptical reduction spots allowed us to check the control of strain on AMS. Site mean AMS data were then used to provide strain estimates at each site. Analysis of remanent magnetization through thermal and chemical stepwise demagnetization procedures allowed us to isolate three magnetic components. Component A, defined in the lower unblocking temperature range, is a recent overprinting which clusters around the present-day dipole field direction. Components B and C, defined respectively in the intermediate and higher unblocking temperature ranges, appear to be pretectonic in age. The fold test, however, is inconclusive for both, because of the significant dispersion still remaining after the simple tilt correction. Assuming that at least a part of this dispersion is due to strain, we have applied the strain removal technique using AMS-derived strains at each site. This results in a clustering of component C from four sites, allowing recovery of the primary Ordovician paleomagnetic direction (D = 261°, I = 71°, α95 = 11°). In contrast, unstrained component B shows a small circle distribution, containing the formation mean of unstrained component C. The hypothesis that component B and C distributions arise from beds being tilted during a long-term magnetization acquisition in the Ordovician field is discussed, taking into account the extensional geodynamic context during deposition of lower Palaeozoic redbeds in Brittany.
Bonhommet Norbert
Cogné Jean-Pascal
Kropacek Vladimir
Petrovski E.
Zelinka T.
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