David Peacock

Fault interactions and associated damage

D.C.P. Peacock¹, C.W. Nixon¹, A. Rotevatn¹ & D.J. Sanderson²

¹ Department of Earth Science, University of Bergen, Allégaten 41, 5007 Bergen, Norway

² Engineering and the Environment, University of Southampton, Highfield, Southampton, SO17 1BJ, UK.

The way that faults interact with each other will control fault geometries, displacements and strains, which in turn affects fluid flow and mineralisation. This is important as faults rarely occur individually but as networks of numerous faults. The arrangement of these faults (i.e. the topology) and their development can produce a variety of different fault interactions, including but not limited to fault splays, abutments, relay zones and cross-cutting relationships. What are the characteristics of these different interactions? How do we interpret them? What are their implications for fault behaviour? To answer these questions, we describe and analyse different fault interactions, focussing on improving our understanding of their development and the resulting fault damage.

Fault interactions can occur between two or more faults of any orientation or relative age. They are defined by the geometric and kinematic relationships that form between the interacting faults. For example the faults may or may not be geometrically coupled, when fault planes share an intersection line (or branch line); and/or kinematically coupled, where the displacements, stresses and strain of one fault influences those of the other. Thus fault interactions are analysed in terms of the following: 1) Geometry and topology – describing the arrangement of the faults and whether the faults share an intersection line or not. The topology describes the geometric relationships between the faults and can be linked to connectivity. 2) Kinematics – describing the displacement distributions of the interacting faults and whether the slip directions are parallel, perpendicular or oblique to the intersection line, and if the faults have the same or opposite slip directions. 3) Stress and strain – whether extension or contraction dominates in the acute bisector between the faults. 4) Chronology – the relative ages of the faults.

We explore a variety of fault interactions that illustrate the use of this classification scheme in understanding fault-related structures, discussing the implications for the analysis of damage zones. We introduce the terms intersection damage zone for structures created around the intersection line of geometrically coupled faults, and approaching damage zone for the zone between geometrically uncoupled faults.