A model of intracratonic basin formation, entailing deep crustal metamorphism

Abstract

Summary. Subsidence data from the Michigan and Illinois Basins in the USA, and the Cooper Eromanga Basing in central Australia are inconsistent with a simple exponentially decreasing (or l/t, 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 450°C 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= 5z+ 350, where T is temperature in °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 n 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. Copyright © 1980, Wiley Blackwell. All rights reserved