Mechanism of subsidence of ancient cratonic rift basins

Abstract

Cratonic basins commonly occur over ancient rift zones. These inactive rift basins are recognizable by a positive linear Bouguer gravity anomaly that may correspond to the axial gravity high found in modern rift valleys. Many of these basins undergo discrete periods of increased subsidence rates, or reactivations, long after the mass excess responsible for the linear gravity high was emplaced. Furthermore, the reactivation of many cratonic basins occurs simultaneously with large-scale compressional tectonics. It is suggested that the driving force for subsidence is the isostatically uncompensated ancient mass excess. The subsidence of these basins is modelled by a lithospheric flexure model with a nonlinear Maxwell viscoelastic rheology. Solutions to this model indicate that basins may experience a low subsidence rate throughout geologic time. The subsidence of a basin will stop only when isostatic compensation of the mass excess is achieved. Since ancient rift mass excesses may be uncompensated over long geologic time intervals, the early thermal and structural evolution of rifts may not significantly influence later basin subsidence. The models suggest that basins may be reactivated by any mechanism which lowers the effective viscosity of the lithospheric material, allowing the uncompensated basin to settle toward an isostatic-compensation depth faster than normal. Since viscosity is a strong function of temperature, reactivation by a world-wide increase in heat flow is suggested as a possible mechanism for the synchroneity of basin subsidence throughout a continent. An increase of 15% in the geothermal gradient, for example (from 16.5°–18.9°K/km), will cause about a 5% increase in subsidence. This increase in heat flow, however, seems unlikely of producing by itself the magnitude of basin subsidence during a reactivation phase that is observed in the geologic record where up to 100% increase in subsidence might occur. Since the rheology of the lithosphere is nonlinear, effective viscosity is also a strong nonlinear function of stress. The presence of a regional compressive stress during periods of tectonism of 1.1 · 10 8 Pa (about 2.8% of the buckling strength of the lithosphere) produces a short period of reactivated subsidence (≅ 10 5 yr). During the reactivated subsidence, the newly-imposed regional stress relaxes sufficiently in the lower lithosphere to restore the effective viscosity to values similar to that before reactivation. This suggests that reactivated subsidence caused by regional compression can be maintained as long as the stress level remains high in the lower lithosphere. This may be accomplished by an intermittent application of the regional stress over time.