1. Late Devonian palaeomagnetism of the North Tien Shan, Kyrgyzstan: can secular variation vary on a short timescale?
- Author
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Bazhenov, Mikhail L., Van der Voo, Rob, Levashova, Natalia M., and Dominguez, Ada R.
- Subjects
PALEOMAGNETISM ,DEVONIAN Period ,GEOMAGNETIC secular variation - Abstract
We studied more than 80 lava flows from a ∼600-m-thick pile of Upper Devonian (Frasnian) basalts and andesites of the Aral Formation in the North Tien Shan (Kyrgyzstan, Central Asia). With the aid of stepwise thermal demagnetization, a high-temperature, dual-polarity component was reliably isolated from most flows. The primary origin of the high-temperature component is demonstrated by positive reversal, conglomerate and fold tests. The most prominent and intriguing feature of this Late Devonian data set involves a clear distinction in angular dispersion between the lower and upper parts of the studied section. A rather low concentration parameter (k = 13) and several directional anomalies characterize the lower section; in contrast, a much better grouping (k = 46) and a lack of directional outliers is observed for the dual-polarity vectors from the upper flow sequence of the formation. We analysed different mechanisms to account for this directional pattern and found that it is possible in just two ways. One is to assume that secular variation (SV) in the upper sequence is strongly underrepresented, and it is a coincidence that the mean directions of both polarities are statistically antipodal, and the corresponding concentration parameters are statistically equal. The other explanation is to hypothesize that the magnitude of SV can vary several-fold at the same palaeolatitude and over time intervals estimated as 105–106 yr. This is in sharp contrast with other models of SV, where this magnitude has been assumed to be rather time-independent (for a given latitude). Our hypothesis accounts for the observed irregularities in palaeomagnetic data, but makes attempts to establish a correlation between SV and other parameters (geographic latitude, reversal frequency, age, etc.) more difficult. We are aware, however, that more data are needed to refute or confirm it. [ABSTRACT FROM AUTHOR]
- Published
- 2013
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