Geometry, evolution and scaling of fault relay zones in 3D using detailed observations from outcrops and 3D seismic data

Abstract

A new surface attribute was developed during the course of the thesis, which enables fault-related deformation – specifically, the apparent dip of mapped horizons measured in a direction perpendicular to the average strike of a fault array (here termed “fault-normal rotation”, or “FNR”) – to be quantitatively analysed around imaged faults. The new utility can be applied to any 3D surface and was used to analyse centimetre-scale to kilometre-scale fault-arrays, interpreted from laser scan point clouds, digital elevation models, and 3D seismic datasets. In all studied examples, faults are surrounded by volumes of fault-related deformation that have variable widths, and which can consist of faults, fractures and continuous bed rotations (i.e. monoclines). The vertical component of displacement calculated from the areas of fault-related deformation on each horizon act to “fill-in” apparently missing displacements observed in fault throw profiles at fault overlaps. This result shows that complex 3D patterns of fault-related strain commonly develop during the geometrically coherent growth of a single fault-array. However, if the component of continuous deformation was not added to the throw profile, the fault-array could have been misinterpreted as a series of isolated fault segments with coincidental overlaps.
The FNR attribute allows the detailed, quantitative analysis of fault linkage geometries. It is shown that overlapping fault tip lines in relay zones can link simultaneously at multiple points, which results in a segmented branch line. Fault linkage in relay zones is shown to control the amount of rotation accommodated by relay ramps on individual horizons, with open relay ramps having accommodated by larger rotations than breached relay ramps in the same relay zone. Displacements are therefore communicated between horizons in order to maintain strain compatibility within the relay zone. This result is used to predict fault linkage in the subsurface, along slip-aligned branch lines, from the along-strike displacement distributions at the earth’s surface.
Relay zone aspect ratios (AR; overlap/separation) are documented to follow power-law scaling relationships over nine orders of magnitude with a mean AR of 4.2. Approximately one order of magnitude scatter in both separation and overlap exists at all scales. Up to half of this scatter can be attributed to the spread of measurements recorded from individual relay zones, which relates to the evolution of relay zone geometries as the displacements on the bounding faults increase. Mean relay AR is primarily controlled by the interactions between the stress field, of a nearby fault, and overlapping fault tips, rather than by the host rock lithology. At the Kilve and Lamberton study areas, mean ARs are 8.60 and 8.64 respectively, which are much higher than the global mean, 4.2. Scale-dependent factors, such as mechanical layering and heterogeneities at the fault tips are present at these locations, which modify how faults interact and produce relatively large overlap lengths for a given separation distance. Despite the modification to standard fault interaction models, these high AR relay zones are all geometrically coherent.