Some comments on two‐layer extensional models for the evolution of sedimentary basins

Abstract

Simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves. In this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform. We review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous, that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks, that subcrustal heat input during rifting occurs over an area much broader than the rift proper, and that there may be broad regional uplift before and/or during rifting. To explain the uplift of the flanks during rifting and their postrift subsidence, we invoke two mechanisms: subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks. The uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis, but uplift due to both sources can, in certain cases, be incorrectly attributed to regional uplift or doming. We present a simplified analysis of the two‐layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere. We provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history. Application of this method to the central part of the Pattani Trough in the intracratonic Gulf of Thailand shows that extension and thinning of the lithosphere of a factor of ∼2 since late Eocene can account for much of the subsidence history and the present‐day heat flow. Our analysis suggests that thinning of the crust in this area is ∼20% greater than thinning of the whole lithosphere. Assuming that none of the lithosphere is removed from the region of extension during rifting, we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere. Such an extensional process produces significant uplift of the flanks of the rift, and as a result of erosion of the uplifted areas, the effective cross‐sectional area of the basin can be increased by as much as 10–20%, depending on the rate of erosion, compared to the area that would have been created by a process of uniform extension. Finally, we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins.