Spatial variation in fault curvature and slip patch segmentation

Fault plane morphology is often oversimplified at all scales in most fault models despite its influence on the mechanical behaviour of a fault. Existing failure criteria for rocks typically assume that fractures are planar and there is little available theory to explain the mechanics of fracture surfaces that are curved. This is at odds with field observations of naturally occurring fracture systems, in which individual surfaces are often observed to be significantly non-planar.

Our observations of curved fractures include examples recorded at a wide variety of scales, from small-scale fractures seen in outcrop, to regionally significant active faults studied in the field and imaged with 3D seismic reflection data. Outcrop examples encompass a range of different lithologies and include opening mode joints, shear fractures and major faults with large measurable offset.

We use terrestrial laser scanning (ground-based LiDAR) to carry out detailed measurements of the 3D geometry of well exposed fracture surfaces. The data provide unprecedented detail and allow spatial variation in various curvature parameters (such as normal, mean and Gaussian curvature) to be quantified. Recent laser-scanning work confirms the following qualitative observations made during previous field studies: many fractures are significantly curved; fracture curvature can include areas of cylindrical, elliptical and hyperbolic geometry (where Gaussian curvature is zero, positive and negative, respectively); fractures can curve repeatedly through the mean orientation, to give sinuous fault traces, in which the dip direction changes along the length of the fault.

The Arkitsa fault zone is located in an area of active extension along the North Evia Gulf in Greece, and has well preserved exhumed fault planes and large cumulative slip. Laserscan data suggest that some areas of high curvature on fault planes can be caused when smaller slip patches coalesce to form larger fault panels. There is also evidence that the intersection of separate fracture patches can cause slip to be concentrated upon a curved composite surface that combines sections of both the individual intersecting patches, leaving large sections of relict patch remaining in the hanging wall of the newly developed fault. However, some slip still clearly localises on these relict patches, causing disruption of the through-going fault plane in the region around the branch-line. Furthermore, the presence of multiple slicken-line vectors on the exposed surface of the faults emphasises the 3D complexity of deformation in this region, and also raises the possibility that non-plane strain is accommodated by a range of slip events that occur on a number of possible combinations of fault patches, different permutations of which are linked at any given time.