Precambrian continental freeboard and geological evolution: A time perspective

Continental crustal growth rates, crustal volumes/thicknesses, and continental freeboard are considered to be intimately related; however none are easy to quantify, least of all in the Precambrian. Additionally, crustal volumes/thicknesses and isostasy are related to changing mantle heat and the concomitant variation in thickness of ocean crust. Another set of variables would be the interaction of plate tectonics (and the onset of plate tectonics, a contentious issue in itself) and mantle plumes/superplumes. Further complication is provided by the ranges of freeboard elevations and eustatic sea level changes overlapping in scale, and sharing common genetic causes. The “constant freeboard model” suggests that the net results of the interaction of this complex set of variables has been that continental elevation relative to sea level, has remained essentially similar, on a global average basis, over much of geological time, assuming an essentially constant ocean volume since c. 3.8 Ga. Application of this model to the Precambrian geological record suggests that sigmoidal crustal growth rate models are probably the most reliable. A conundrum of the constant freeboard model would be that freeboard would tend to become lowered towards mean sea level for any specific craton over time; this “freeboard-equilibrium” would be interrupted for a particular craton by active geodynamic processes resulting from the interplay of plate tectonics and mantle thermal instabilities. In this paper we test the validity of this thesis of alternating approximate freeboard-equilibrium and active geodynamics, by examining the volcano-sedimentary record of a number of cratons: Kaapvaal, Singhbhum and Pilbara. For the Singhbhum craton (and bearing in mind its poor chronological data base), freeboard-equilibrium appears to have prevailed until c. 2.1 Ga, but for Kaapvaal and Pilbara, until c. 1.8 Ga, the geodynamic processes largely outweighed the tendency towards equilibrium, except for c. 200 My from c. 2.6 to 2.4 Ga when global sea level appears to have remained high and transgression was the norm. We thus tentatively add the plate tectonic cycle and mantle-thermal processes to the already complex association of concepts controlling continental freeboard over time.