PALEOMAGNETISM AND PALEOTHERMOMETRY OF THE SYDNEY BASIN .2. ORIGIN OFANOMALOUSLY HIGH UNBLOCKING TEMPERATURES

The Milton Monzonite of southeastern Australia was thermoviscously rem agnetized as a result of Cretaceous burial and uplift. Thermal demagne tization separates the low unblocking temperature (LT) overprint from the high unblocking temperature (HT) primary remanence, with a relativ ely sharp junction between LT and HT components in vector projections. For single-domain grains, the junction temperature T-L between two su ch vectors corresponds to the maximum blocking temperature T-r reactiv ated in nature, apart from a correction for the difference between nat ural and laboratory timescales. However, measured T-L values are distr ibuted over an implausibly wide range (>250 degrees C) for burial rema gnetization of an untilted intrusion like the Milton Monzonite. Furthe rmore, many T-L values are anomalously high compared to the prediction s of single-domain theory. Multidomain grains are the cause of these a nomalies. Samples pretreated before thermal demagnetization by zero-fi eld cycling to liquid nitrogen temperature, so as to erase multidomain remanence and isolate single-domain remanence, do have the theoretica lly expected T-L values. In these samples, realistic remagnetization t ime and temperature (t(r), T-t) conditions in nature are predicted usi ng the t-T contours of Pullaiah et al. [1975]. The anomalously high T- L values before low-temperature treatment are due to multidomain grain s, which carry greater than or equal to 50% of the LT overprint. The L T thermal demagnetization curve in samples dominated by multidomain gr ains is quasi-exponential in shape with a high-temperature tail extend ing almost to the Curie point, as predicted by multidomain theory. The se high LT unblocking temperatures, which are much greater than plausi ble remagnetization temperatures reached in nature, overlap and mask t he lower part of the HT unblocking temperature spectrum, driving up T- L values and leading to inflated estimates of T-r. Although multidomai n remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the Milton Monzonite, che mical overprinting may be a factor in other lithologies and tectonic s ettings.