Evidence from overprinting of magnetizations of Late Permian and Mesozoic rocks and from the rank of Permian coals and Mesozoic phytoclasts (coal particles) suggests that surface rocks in the Sydney Basin, eastern Australia, have been raised to temperatures of the order of 200°C or higher. As vitrinite reflectance, an index of coal rank or coalification, is postulated to vary predictably with temperature and time, estimates of the paleotemperatures in the Sydney Basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin. These estimates give maximum paleotemperatures of present day surface rocks in the range 60–249°C, depending on factors such as location in the basin, the thickness of the sediment eroded, and the maximum paleogeothermal gradient. Higher coal rank and, consequently, larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the Tasman Sea on the basin's eastern margin. A theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperatures in the Sydney Basin. This estimate places the maximum paleotemperature in the range 250–300°C along the coastal region. Both coalification and thermal activation of magnetization models provide strong evidence of elevated paleo‐temperatures, which in places exceed 200°C, and the loss of sediment thicknesses in excess of 1 km due to erosion.
Subsidence data from the Michigan and Illinois Basins in the USA, with a simple exponentially decreasing (or l/&, where t is time) subsidence with time, Subsidence in these basins is observed to occur in two consecutive stages which exhibit exponential-like decreasing subsidence with time. A model which entails a period of heating followed by a period of cooling is proposed to explain the observed subsidence. Subsidence due to deep crustal metamorphism occurs during the latter part of the heating period, and is followed immediately by subsidence due to thermal contraction during the cooling period.The metamorphism, which drives the initial stage of subsidence, is assumed to occur at the greenschist facies to amphibolite facies metamorphic boundary which occurs approximately along the 45OoC isotherm at depths in the order of 10-20 km. This boundary is petrologically very complex, and cannot be treated as a simple Clausius-Clapeyron phase transition curve.Loss of volume at the metamorphic facies boundary, which is proportional to subsidence, is approximated by determining the intersection of the changing geotherm (during the heating period) with the greenschist-amphibole facies boundary, defined by the curve: T = 52 t 350, where T is temperature in O C and z is depth in km. This approach is valid as perturbation of the temperature field in the vicinity of the facies boundary during metamorphism is shown to be negligible, and sufficient heat is shown to be available for metamorphism at crustal depths within 30 Myr of commencement of the heating period.The model indicates that subsidence of the form observed can be attributed to deep crustal metamorphism followed by thermal contraction of the lithosphere. The rapid reversal from heating to cooling assumed in this model is justified by the observation of very slow subsidence suddenly becoming very rapid, therefore a sudden change in the thermal regime is needed to produce a subsidence model concordant with the data. md && c o o p e r Mi Eroaag2 B2ZirZ ir &8i ??Zd '4#22?&2 2iv h&@82&)&2>
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.