Paleomagnetism and paleothermometry of the Sydney Basin 2. Origin of anomalously high unblocking temperatures

The Milton Monzonite of southeastern Australia was thermoviscously remagnetized as a result of Cretaceous burial and uplift. Thermal demagnetization separates the low unblocking temperature (LT) overprint from the high unblocking temperature (HT) primary remanence, with a relatively sharp junction between LT and HT components in vector projections. For single-domain grains, the junction temperature T_L between two such vectors corresponds to the maximum blocking temperature T_r reactivated in nature, apart from a correction for the difference between natural and laboratory timescales. However, measured T_L values are distributed over an implausibly wide range (>250°C) for burial remagnetization of an untilted intrusion like the Milton Monzonite. Furthermore, many T_L values are anomalously high compared to the predictions of single-domain theory. Multidomain grains are the cause of these anomalies. Samples pretreated before thermal demagnetization by zero-field cycling to liquid nitrogen temperature, so as to erase multidomain remanence and isolate single-domain remanence, do have the theoretically expected T_L values. In these samples, realistic remagnetization time and temperature (t_r, T_r) conditions in nature are predicted using the t-T contours of Pullaiah et al. [1975]. The anomalously high T_L values before low-temperature treatment are due to multidomain grains, which carry ≥50% of the LT overprint. The LT thermal demagnetization curve in samples dominated by multidomain grains is quasi-exponential in shape with a high-temperature tail extending almost to the Curie point, as predicted by multidomain theory. These high LT unblocking temperatures, which are much greater than plausible remagnetization temperatures reached in nature, overlap and mask the lower part of the HT unblocking temperature spectrum, driving up T_L values and leading to inflated estimates of T_r. Although multidomain remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the Milton Monzonite, chemical overprinting may be a factor in other lithologies and tectonic settings.