Quantitative modelling of multiphase tectonic subsidence mechanism and lithospheric stretching controls on the evolution of the Central Kotla Graben, Sirt Basin, Libya

Abstract

The Kotla Graben remains a poorly investigated region. Understanding the architecture of the graben and adjacent basement highs through time is critical for resolving due to the impacts of transitional accelerated subsidence to thermal sagging and basin-bounding fault reactivation mechanisms that create such a graben. This study attempts to restore the Late Cretaceous-Middle Eocene geometry of the Central Kotla Graben, using a combination of well data from 47 boreholes and twenty-six 2D seismic reflection profiles, nine of which are selected to show examples of the new information on the present-day architecture of the deeply buried graben. This geological and geophysical integration has enabled many variables to be involved in backstripping and lithospheric stretching calculations. The shape of backstripped subsidence curves reveals four discrete tectonic (I–IV) phases of subsidence-uplift that are recognised with variable rates and separated by three different types of unconformities. The rifting initiated during the Cenomanian, and both highs were uplifted during the Santonian. The rifting peaked during the Palaeocene and was terminated by a thermal sagging phase during the Eocene. Seismic interpretation indicates that the acoustic basement is generally characterised by three main intra-basement reflection (IRP) packages, located within the Dahra Platform basement and referred to as the Dahra Shear Zone in this study. However, seismic analysis has also enabled the recognition of four different seismic facies units. These facies units are integrated with three recognised gamma-ray log trends to correlate lithological variations within the distinctive boundaries of the tectonic subsidence phases. The tectonic subsidence maps of the graben geometry indicate that during the syn- and post-rift phases, the depocentre of the Central Kotla Graben is located in the south. Two different techniques used to estimate the stretching factor (β): The subsidence-derived stretching revealed lithospheric stretching ranged from 1.0 to 1.5. Moreover, the fault-derived stretching across the Central Kotla Graben ranged from 1.5 to 2, suggesting that the maximum crustal stretching in the graben axis ranges from 50 % to 100 %. This discrepancy is described as “extension discrepancy”.