The spatial heterogeneity of fracture orientations in fault zones

The spatial heterogeneity of natural fracture networks in fault zones provides valuable information to understanding the evolution of fault systems. The damage zones and cores of many natural fault zones display complex geometric and kinematic patterns that are not easily explained by simple plane strain faulting models like those proposed by Anderson.

In this presentation, we illustrate this complexity with reference to spatial variations in the orientation and slickenline patterns of fracture systems that develop during single normal faulting episodes in wall rocks where the host lithologies are homogeneous aeolian sandstones . The fracture system examples used are related to the dextral transtensional 90-Fathom Fault, Cullercoats Bay, NE England and an inferred, but unexposed, dip-slip normal fault near Appleby, Cumbria, NW England. Both faults cut Permian New Red Sandstones and both are inferred to have developed during Permo-Traissic fault movements affecting northern England.

Our results show that the fracture orientations vary systematically from the outer parts of fault zones towards the fault cores. Many parts of both fault zones are characterised by distinctly non-Andersonian multimodal faulting patterns and, in the case of the Ninety Fathom Fault, by the development of mutually cross-cutting dip-slip normal and dextral strike-slip faults in the region proximal to the master fault and, in the case of Appleby, by the development of localized multimodal fracture orientation patterns with limited number of dip-slip slickenlines near the inferred but unexposed normal fault Compared to the multimodal fracture orientation patterns shown at Cullercoats, at Appleby, seemingly diffuse Andersonian fracture patterns (conjugate clusters) seen at the scale of the overall outcrop may be the result of bulk averaging of more localised, complex multimodal fracture patterns that developed in general 3D strains at sub-outcrop scales. In both examples, there is no compelling evidence to suggest that the complex fracture patterns are the product of superimposed deformation events affecting the Aeolian sandstones, although the Ninety Fathom Fault almost certainly reactivates an early Carboniferous-age structure at depth.

We propose that the fracture orientation and kinematic patterns reflect the spatial partitioning of strain in damage zones and/or the development of multimodal, non-Andersonian fracture systems during non-plane strain regional deformation. We suggest that substantial variations in the strain state will be particularly widespread in areas of basement-influenced oblique rifting where regional extension vectors lie significantly oblique to pre-existing structures at depth, causing rotation of the principle stress trajectories (e.g. strain partitioning).