Abstract - Normal faults contained in multilayers are generally segmented both in map view and
in cross-section. Fault dip variations in different lithologies are well documented and
are due to different mechanical properties of the layers. Changes in fault strike, however,
are poorly understood. From a purely geometrical point of view it can be shown
that a continuous refracting normal fault requires strike changes in different lithologies
(’zig-zag’ geometry) if the intersection lineation of fault and bedding is non-horizontal. These strike changes are however geometrical compromises,
whereas the optimal failure angles and strikes can be maintained by the generation of
segmented faults exhibiting en ´echelon geometry. The degree of fault segmentation increases
with increasing dip of bedding, and a higher degree of segmentation is expected
in the relatively less abundant lithology.
The geometrical model is verified using sandbox modelling that shows that normal
faults in sedimentary overburden above a pure dip-slip basement fault exhibit systematic
stepping of fault segments if the fault/overburden intersection is non-horizontal.
Systematic stepping and segmentation of faults in overburden above basement faults is
not, as is often assumed, diagnostic of oblique-slip reactivation. Furthermore, for the
same geometrical configuration, multiple sets of faults, i.e. a pseudo-orthorhombic
fault system, can develop even under plane strain conditions. Our geometric and analogue
models support the notion that departures from simple geometric configurations
of faults, layers and stresses, can give rise to complex and discontinuous fault geometries
(Mandl 1987, JSG 9, 105-110; Treagus and Lisle 1997, JSG 19, 997-1010; Walsh
et al. 2003, JSG 25, 1251-1262).
Abstract of talk given to:
Tectonic Studies Group Annual Meeting, Durham, January 2004