Late Devonian paleomagnetism of the North Tien Shan, Kyrgyzia: can secular variation vary on a short time scale?
Abstract
We studied more than eighty lava flows from a ca. 600-meter thick pile of Upper Devonian (Frasnian) basalts and andesites of the Aral Formation in the North Tien Shan (Kyrgyzia, 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 dataset involves a clear distinction in angular dispersion between the lower and upper parts of the studied section. A rather low concentration parameter (k = 14) 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. Unable to find any rock magnetic difference between upper and lower flow sequences, we favor a geomagnetic origin for this difference and advocate the hypothesis that the magnitude of secular variation can randomly vary several-fold at the same paleolatitude and over time intervals estimated as 100,000-1,000,000 y. This is in sharp contrast with other models of secular variation, where this magnitude has been assumed to be rather time-independent (for a given latitude). Our hypothesis accounts for the observed irregularities in paleomagnetic data but makes attempts to establish a correlation between secular variation and other parameters (geographic latitude, reversal frequency, age, etc.) more difficult or even rather futile.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGP43A..08B
- Keywords:
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- 1522 GEOMAGNETISM AND PALEOMAGNETISM / Paleomagnetic secular variation