Mechanical Stratigraphy Controls Normal Fault Growth and Dimensions, Outer Kwanza Basin, Offshore Angola

David Redpath, Christopher Aiden-Lee Jackson , Rebecca E. Bell

Mechanical stratigraphy controls the growth patterns and dimensions of relatively small normal faults, yet how its influences the development of much larger structures remains unclear. Here we use 3D seismic reflection data from the Outer Kwanza Basin, offshore Angola to constrain the geometry and kinematics of several normal faults formed in a deep-water clastic succession. The faults are up to 6.3 km long, 1.9 km tall, and have up to 44 m of throw. Aspect ratios and lower-tip throw gradients are greater for faults that terminate downwards at a c. 100 m thick, mass-transport complex (MTC) (up to 5.2 and 0.12) than for those that offset it (up to 2.7 and 0.01). Faults that offset the MTC invariably have >30 m of throw. Based on their geometric properties and throw patterns, we interpret that the faults nucleated above the MTC and propagated down towards it. Upon encountering this unit, which we infer was weaker and behaved in a more ductile manner than encasing strata, tip propagation was halted until tip stresses were sufficiently high (corresponding to minimum throw of c. 30 m) to breach it. Faults with smaller throw were unable to breach the MTC. We argue that using only geometric criteria to determine fault growth patterns can mask the not insignificant control mechanical stratigraphy has on fault kinematics. Mechanical stratigraphy therefore has a key control on the growth of large, seismic-scale normal in a similar way to that observed for far smaller structures.