Structural and stratigraphic interactions during rift basin evolution : insights from outcrop and subsurface studies

The aim of this project is to integrate structural and stratigraphic data from major normal fault zones in the Suez Rift (Thal Fault Zone) and the northern North Sea Rift (Murchison-Statfjord North Fault Zone), to investigate the structural evolution of normal faults and the controls exerted on the temporal and spatial development and variability of the adjacent syn-rift stratigraphy. Analysis of well exposed syn-rift sediments from the hanging wall of the Thal Fault Zone has enabled a succession of continental to fully marine, clastic-dominated facies to be identified. Key, regionally correlatable stratal surfaces within the syn-rift define time-equivalent stratal units that exhibit considerable dip and strike variability. Integrating this stratigraphic data with the structure reveals that: (1) during initial rifting (<6 Myr), antithetic faults were most active and controlled deposition, whilst the Thal Fault (or its precursor strands) were insignificant, (2) a displacement gradient existed between fault strands at the centre and tip of this early fault population, controlling gross stratal thickness and implying that interaction was occurring between the fault strands, (3) during later rifting (>6 Myr) fault activity switched to the main [Thal] fault, (4) local, along-strike variability in stratal thickness near the fault tip was related to a persistent fault strand boundary (relative high) that also provided a transport pathway (relay ramp) for sediments into the basin, and (5) in general, low rates of faultcontrolled subsidence and accommodation development at the tip, coupled with a high sediment supply produced a strongly progradational delta. Interpretation of structural and stratigraphic subsurface data from the North Sea, indicates that the Murchison-Statfjord North Fault Zone has grown by fault linkage. Isolated fault strands (<4 km in length) with local hanging wall depocentres developed during the initial c. 13 Myr of rifting, and controlled mudstone-dominated deposition. The fault strands progressively linked along-strike forming two segments (10 km in length) separated by a relay ramp, that controlled mudstone and turbidite deposition during the ensuing 10.5 Myr. A regionally significant footwall island became exposed, supplying turbidites that were transported down the relay ramp and captured within structurally controlled hanging wall depocentres, and (3) the fault zone hard-linked during the final 7 Myr of rifting, and the breached relay ramp no longer acted as a major sediment transport pathway. Thus, fault evolution controlled the size and distribution of depocentres, the generation of local accommodation space, the creation of sediment source areas, and the creation and destruction of sediment transport pathways. These key observations from the Suez and North Sea rifts suggest that: (1) patterns of fault growth and linkage can only be fully determined and temporally constrained using syn-rift stratigraphy, and not by structural data alone, (2) major blockbounding faults may not have always experienced the greatest subsidence throughout rifting or have persistently had the main syn-rift depocentres in their hanging walls; in fact, a considerable thickness of early syn-rift strata can be controlled by intra-block faults that do not develop into the major, block-bounding faults, (3) the stratigraphic development of a half-graben is often influenced by adjacent fault zones, affecting sediment supply and distribution. Thus, it is essential to consider adjacent and regional structures, to gain a complete insight into tectono-stratigraphy during rifting, (4) alongstrike variations in stratal thickness and stratigraphic architecture occur on a large scale (i. e. 30 km) between the centre and tips of major normal faults, due to significant differences in fault-controlled subsidence and sediment supply, and (5) in addition, small-scale (<10 km) stratal variability may occur along the entire length of major basin-bounding fault segments (including the tip region), related to the persistence of strand boundaries (intra-basin highs) during fault growth. The results of this study have general implications for tectono-strati graphic development and variability during rift basin evolution.