Fault-Propagation Folding in Extensional Settings: Examples from the Suez Rift and Comparisons with Numerical Models

Christopher Aiden-Lee Jackson¹, Rob Gawthorpe², Ian Sharp³, and Emma Finch^2
1 Imperial College London, London, United Kingdom
² University of Manchester, Manchester, United Kingdom
³ Norsk Hydro Research Center, Bergen, Norway

Fault-propagation folding is an important process during the early stages of normal fault growth. Exceptional exposure of several block-bounding fault zones in the Suez rift, Egypt, allows analysis of the distribution, geometry and scale of secondary structures, and permits reconstruction of the style and sequence of deformation during fault-propagation folding. These observations are then used to test predictions made by numerical models.

The hangingwalls to the faults are characterised by fault-parallel synclines, which may be deformed by reverse faults that display progressively steeper dips up-section. The footwalls are characterised by fault-parallel anticlines that are deformed by fault-parallel, macro-scale (100-250 m displacement) and meso-scale (1-10 m displacement) extensional faults, the latter being restricted to highly-attenuated mudstone-dominated units within 200 m of the fault zone, and showing progressive rotation with increasing proximity to the fault zones.

Based on these geometric relationships it is interpreted that during the initial stages of folding, reverse faults, which progressively young up-section, form in the hangingwall. Macro- and meso-scale normal faults accommodate footwall extension and layer-parallel slip along mudstone horizons. During the latter stages of folding, both hangingwall and footwall structures become inactive and are passively rotated in the monoclinal limb of the fold prior to it being breached.

The style and sequence of secondary deformation interpreted from the field examples is similar to that predicted by numerical models. In addition, these models suggest that cover strength, fault angle and propagation-to-slip ratio are key controls on the timing, location and magnitude of secondary deformation in fault-propagation folds.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005