Experimental evaluation of thermal recording of successive polarities during uplift of metasediments

Abstract

The discovery of a post-tectonic natural remanent magnetization (NRM) carried by pyrrhotite in the Dauphinoise Zone of the Western Alps allows us to test the ability of these rocks to record a sequence of magnetic polarity versus temperature acquired during their slow cooling. Several laboratory experiments [composite partial thermal remanent magnetizations (pTRMs), Thellier-Thellier experiments, TRM thermal demagnetization] as well as NRM demagnetization were performed. This shows that the pyrrhotite grains are single-domain-sized and that they are able to record successive independent pTRMs. The NRM thermal demagnetization analysis demonstrates the occurrence of several anti-parallel components. A comparison between NRM and TRM thermal demagnetizations is attempted in order to highlight small pTRMs naturally recorded in a temperature interval of only a few degrees. We attempt to retrieve the temperature at which each reversal occurs during the post-metamorphic cooling. This method, called thermopalaeomagnetism, is discussed and developed in order to delineate the sequence of polarity chrons recorded versus temperature.