Control of fault geometry, interaction and mechanical stratigraphy on strain distribution in normal fault zones

Abstract

It is known that the development and distribution of strain associated with normal faulting is
influenced by the process of fault growth within mechanically layered and heterogeneous sedimentary
rocks. Fault displacement is often partitioned between discontinuous throw on slip surfaces and zones
of distributed strain, which in some cases can be the result of folding associated with normal faulting.
The amount of ductile deformation can vary significantly along the strike of a normal fault array as a
result of various processes, such as fault-tip propagation and fault interaction and linkage. In this
study we investigate the influence of mechanical stratigraphy, fault geometry and fault mechanical
interaction on the variability and distribution of ductile strain in the rock volume surrounding normal
faults. We show that mechanical competence contrasts can control the manner in which strain is
accommodated and, hence the overall patterns of secondary fault and fracture systems within normal
fault-related folds. This can have consequences on the way in which disruption of an associated shale
smear occurs, impacting the sealing properties of the fault zones. Also, we show that folding can be
generated by different mechanisms that vary in importance in time and space along a normal fault
array. Mechanical properties of the host rocks, together with the spatial configuration of the faults
control the mechanical interaction between faults, exerting an influence on the variability of ductile
strain within the volume of deformation surrounding normal faults. Specifically, conjugate normal
faults that intersect within layers with low compressibility have geomechanical characteristics
favorable for migration of stress concentrations near the upper fault tips which generate higher
propagation/slip ratios and the development of lower amplitude folds, or no folding. The host rock
lithology and the overlapping normal fault configuration at the time of interaction controls the three-dimensional
relay ramp geometries and associated strains within relay ramps. Normal faults within
mechanically competent rocks tend to develop relay ramps with tabular geometries, that have larger
aspect ratios and smaller fault-parallel shear strains compared to those developed in mechanically
incompetent rocks. Fault-normal shear strain within ramps can be the result of the development of
asymmetric displacement gradients on the overlapping faults as a result of mechanical interaction
between surface-breaking normal faults. The probability of a relay ramp bounded by surface-breaking
normal faults to be completely breached depends not only on the accumulated ramp shear strains and
the ratio between throw and separation of the bounding faults, but also on how the throw is partitioned
between the interacting faults. Also, we argue that the style of breaching, dominantly through the
upper part of the relay ramps, is influenced by the stress interaction between the overlapping faults
and the Earth’s free surface