ORIGIN OF RIFTED CRATONIC BASINS: TESTING THE SLOW STRETCHING MODEL
Oyepeju, Alli Oluwaseun 1980-
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Cratonic basins are large depressions filled with sediments that are located on cratonic shields. They are huge repositories of hydrocarbons, fresh water aquifers, and other important resources, and therefore are of important economic importance. Cratonic basins are characterized by having a long history of subsidence, a saucer-oval shape and 2-9 km of sediment thickness. Despite this economic and geodynamic importance, the process (or processes) that form these basins are still debated. One widely accepted hypothesis suggests that cratonic basins are formed by slow rifting of the lithosphere followed by a long period of thermal subsidence. In this work I investigated this hypothesis by using 2D geodynamic numerical models of the lithosphere. These models are developed to understand the thermal subsidence during and following slow rifting, the deformation of the crust and mantle lithosphere, and the lithospheric stress field. Several models were tested in which I varied the lithosphere thickness, strain rates, and stretching factors. The model allows the lithosphere to rift, thereby creating accommodation space, shallowing of the Moho, and allowing passive upwelling of mantle material. I found that minor rifting is followed by a phase of ~175 million years of cooling of the lithosphere, and thermal subsidence. So, according to the models, the slow rifting hypothesis for the origin of cratonic basins can account for the long subsidence phase experienced by these basins. For cratonic basins where there is no record of minor rifting, other processes such as downgoing mantle flow, dynamic topography, and phase changes drive the slow subsidence.