Sedimentary Response to Fault Evolution in Rift Basins: Insights from the Gulf of Suez, Greece, and the North Sea
The evolution and linkage of fault segments to form continuous, basin-bounding normal fault zones is recognized as a first-order control on the size, shape, and stratigraphy of sedimentary basins within areas of continental extension. Integrated structural and sedimentological studies of the late Jurassic of the North Sea and outcrop analogs from the Miocene of the Suez rift and the late Quaternary of Greece have allowed the evolution of fault populations to be investigated and the landscape and sedimentary response to fault evolution to be determined. These studies highlight marked temporal changes in structural style and sedimentary response that have a major impact on synrift reservoirs and the timing of trap development. Typically, the active faulting phase of rift basin evolution can be divided into three progressive stages: (1) rift initiation, (2) fault interaction and linkage, and finally (3) rift climax. Typically the rift initiation to rift climax evolution occurs on the time scale of several million years.
During rift initiation, fault activity is distributed on short (1-4 km long), low displacement (<1 km) segments. Major tilted fault blocks that characterize the structural style during the later rift climax stage are absent, and fault-propagation folding above blind normal faults is a prominent feature at the Earth's surface. Examination of stratal terminations around fault tips suggests that faults may attain their maximum length soon after the onset of rifting. Over the first several million years of rifting, early formed segments either begin to hard link, forming longer, segmented fault zones, or become inactive and die. Stress feedback between ruptures on adjacent fault segments is an important control on fault growth.
Following rift initiation, displacement becomes progressively localized onto greater than 25 km-long border fault zones that bound broad, tilted fault blocks. During this interaction and linkage phase, many of the early intra-block fault zones become inactive. Thus strain becomes progressively localized on fewer, but longer, fault zones and, as a result, displacement rate on the remaining "rift climax" faults is greater than during rift initiation times. The stratigraphic expression of this strain localization is a change in the basin fill from overfilled during rift initiation times, when sedimentation is dominated by continental to shallow marine/lacustrine deposits, to sediment-starved, deep-water facies during the rift climax phase when displacement rates outpace sedimentation. Furthermore, the breaching of relay ramps during linkage may cause major readjustments to the drainage networks feeding sediment into the rift.
The locus of fault activity continues to migrate following the development of a through-going, linked border fault zone. Commonly, this is associated with a narrowing of the rift zone such that old, inactive faults become progressively abandoned and uplifted in the footwalls of younger fault zones. This results in cannibalization of older basin fills.
The dynamics of fault population evolution illustrated here are comparable to those suggested by analog and numerical modeling studies. They have important implications for the tectono-stratigraphic evolution of rifts and for understanding complex and often subtle synrift plays and the structural compartmentalization of major fault blocks.