TECTONIC CONTROLS ON SEDIMENT ACCOMMODATION WITHIN A HALF GRABEN: AN INVESTIGATION OF THE INFLUENCE IMPOSED ON SYN-RIFT ARCHITECTURE BY EXTENSION AND SUBSIDENCE KINEMATICS

Local accommodation rates due to fault-slip within hanging wall blocks have been recognized to significantly control the distribution of regional depositional centers within rift basins. Thin layers of sediment are generally preserved or pinch out on top of horst structures and rift flanks. Adjacent to these deposits are thicker units that were deposited within deep hanging wall depressions. However, past quantitative research has assumed vertical subsidence and has not focused on the effects of sediment accommodation due to fault block extension. Fault block rotation involves both an increase in hanging wall dip and a decrease in footwall dip through time as the hanging wall is translated away from the footwall. This kinematic model leads to a non-linear growth in cross-sectional area within the basin, which has a profound effect on the basin ratio between sediment input and accommodation space. In order to keep a basin filled throughout extension the sediment input from upstream drainage basins must increase at a similar rate. If the ratio between sediment input and accommodation is less than unity the syn-rift fill will contain enhanced distal unconformities/off lap surfaces and laterally restrictive footwall gravel deposits that express an enhanced back-stepping trend. In addition to basin fill effects, extension of the hanging wall block away from a relatively static upstream boundary of deposition creates phase shifts within the timing of grain-size boundary movements and between instantaneous fluvial parameters. Basins with low dip magnitude faults (high extension rates) express an earlier onset of gravel retreat and a delayed initiation of incursion within the preserved record when compared with basins with high dip magnitude faults (slow extension rates). This leads to the generation of asymmetric gravel progradational bodies within the basin cross-section. Phase shifts between sinusoidally varied parameters (sediment flux, subsidence, gravel fraction and diffusivity), proximal accumulation rates and gravel progradation maximum/minimums are not significant when basin bounding fault dip is greater than 45 degrees. However, decreasing fault dip to 20 degrees causes observable phase shifts between sinusoidally varied parameters, accumulation rates and gravel progradation timing.