Abstract -
The standard outcrop description of fault zones currently in vogue is a high strain fault core containing fault rock surrounded by a low strain halo termed a damage zone. These terms began to gain prominence about 15 years ago largely because they provided a convenient description of faults in high porosity sandstones where fault zones are frequently flanked by a wide zone of deformation bands. The terminology has subsequently become the standard description of faults in most outcrop studies, and is increasingly used in studies of fault zone evolution based on the assumption that core and damage zone have some generic or mechanistic significance. That this is not the case is highlighted by studies describing the many and varied processes which lead to the formation of structures included under the term ‘damage’; these include normal drag, fault segmentation and relay ramps. Similarly, recent articles have recognised ‘multi-cored’ faults suggesting that they are generically different to ‘single-cored’ faults despite the routine observation that a fault may branch and rejoin.
While the damage zone/fault core description may be convenient for 1D or effectively 1D samples through faults (core or small outcrops of large faults), usually in circumstances where displacements are not defined, it is clear when extensive outcrop or seismic datasets are available that this description does not capture the main features of fault zones. For example, segmentation is a fundamental feature of faults and linkage between segments is arguably the main process in fault zone evolution. However fault segmentation and the associated spatial variation in fault displacement is not incorporated in core/damage zone descriptions. Attempts to quantify fault zone structure in terms of merely core and damage zone dimensions are therefore not relevant to the study of segment linkage or the impact of segmentation on fluid flow within or across fault zones
Outcrop studies can best contribute towards an understanding of fault zones if they are set in the context of an appropriate 3D appreciation of faults, including quantitative definition of internal displacements and strain. Fault terminology should be guided by those datasets where 3D fault zone structure can be deciphered rather than by what is convenient in outcrops where it cannot. We suggest that the damage zone/fault core description promotes not only a simplified view of faults, but also a misleading one which is an obstacle to understanding them.
Abstract of talk given to:
Tectonic Studies Group Annual Meeting, Edinburgh, January 2012.