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Geophysics Department, Lakehead University, Thunder Bay, Ont., Canada, P7B 5E1
Undisturbed Mesozoic limestones of eastern England show univectorial magnetizations of high thermal stability that appear, on first inspection, to be primary geological remanences. However, viscous remanent magnetization (VRM) of anomalously high thermal stability is an alternative possibility. VRM acquired elsewhere in laboratory experiments, or in a few hundred years as in the examples studied here, may require unblocking temperatures >300°C to demagnetize. Normally, such demagnetization temperatures are required to remove only very ancient geological remanences. The classical ‘conglomerate test’ of palaeomagnetism fails to discriminate between a VRM and a primary remanence. Any lithology sensitive to VRM will yield a negative result, suggesting that the remanence is secondary. However, tests of physical phenomena that caused multiple reorientations of clasts, at rotation rates comparable with the viscous remagnetization rate, provide a more suitable field test for VRM. In this study, these latter kinds of test reveal that some in situ Mesozoic limestones of eastern England have VRM, despite stability to temperatures >300°C. The high thermal stability VRM of the bedrock must have been acquired in the last 0.7 Ma, since the last geomagnetic reversal. Two new field tests detected the viscous nature of the high stability remanence. The first was a ‘debris flow test’, in which limestone blocks spin slowly with respect to the geomagnetic field. The blocks record variably tilted or helicoidal, multicomponent vectors. The second was a ‘masonry test’, in which samples of limestone from buildings of known age were exposed to the geomagnetic field for known periods, up to 1600 years in this study. This shows that unusually stable VRM may be common in carbonates. Finally, the masonry test permits an empirical estimate of viscous remagnetization rates over hundreds of years. This established chronometric curves that date ground disturbances of geotechnical or geomorphological interest, and archaeological features of unknown age.
