Abstract - Fault zones are characterized by high internal complexity and heterogeneous deformation of the surrounding rock volume. In the past few decades significant progress has been made in understanding fault zone structure, and several models of fault zone growth and evolution have been developed on the basis of qualitative and quantitative analysis of faulting from outcrop and seismic data and from laboratory experiments and numerical models.
In this study, we present an exceptional dataset from an active opencast lignite mine in the Ptolemais basin, NW Greece, and discuss the similarities and differences observed in the 3D structure of five different fault zones. The Pliocene lignite/marl sequence is displaced by numerous well exposed sub-parallel normal faults with syn- and post-depositional throws up to 50 meters. The active mining operations allow 3D mapping of individual faults and other structures to be undertaken as the lignite is excavated. We have visited the mines and mapped the faults on 14 occasions over the last four years. In the three month intervals between field seasons each of six active mine faces is excavated a further 20 to 50 meters, resulting in about 80 samples through a fault strike-parallel sample area about 2km long and 100m high, for each of five fault zones studied in detail. Each newly-excavated fault zone outcrop is mapped in detail at the face, and these data are combined with semi-regional scale photographs in an oil-industry standard 3D structural interpretation package, allowing outcrop resolution characterisation of fault zones at the scale of seismic data.
3D characterisation of fault zone structure requires mapping the displacement distribution and partitioning along, within and around individual segments or splays that comprise a zone. Our analysis of mapped fault zones addresses the question of whether or not a specific form of fault zone structure, to which individual fault zones in a particular fault system conform, can be defined using a general and simple description. To this end, we compare and contrast the different styles of segmentation and of displacement partitioning observed in different co-genetic fault zones within the mine.
We find quantitative similarities in the style of displacement partitioning within these fault zones when they are compared to other fault systems. For example, fault segments have anomalously high fault displacement to fault rock thickness ratios compared to other segments in other fault systems. This factor fortuitously results in the preservation on large displacement faults of structures formed at low displacement, providing insight into the kinematic evolution of the zones. Another general observation is that displacement gradients on individual segments are larger here than in many other fault systems, an observation compatible with the overall sense that these faults are more highly segmented than usual.
Individually, at the scale of the dataset, each fault zone is different. For example, in Zone R, most of the displacement is accommodated within a dominant slip surface, while in the other zones it is more evenly distributed between different segments. In zone P, two main segments separated by a large relay ramp with fault-perpendicular bed rotations. In contrast, Zone S comprises multiple en-echelon fault surfaces with alternating areas of soft- and hard-linkage between the fault surfaces. In zone Q, an array of soft-linked fault segments occupy the
core of a monocline, and hence the dominant feature of this fault zone is pervasive normal drag. In terms of the timing of fault movements, cross-fault growth indicates that Fault P had significant syn-sedimentary movements during deposition of the earlier part of the sequence, while displacement across the other faults was dominantly post-depositional with only subtle syn-sedimentary movements confined to restricted intervals within the succession.
Our analysis of faults in this mine, as well as in three other similar mines in the same basin, provides unprecedented outcrop-scale access to the 3D geometry of oilfield-scale faults. Segmentation occurs in these faults at a range of scales and, like any complex hierarchical structure, different scales of structure require different sampling volumes for adequate characterisation. Our mapping shows that these faults, which have maximum throws of between 15 and 50m, are all significantly different within a 2km along-strike sample. It may be that given a larger sample area the faults might become statistically similar, or it may not. In other words, there may or may not exist a representative elementary volume for the structure of a fault zone that is smaller than the volume of the fault zone itself. Although this dataset cannot yet answer this important question, it does provide an unprecedented insight into the multiple scales of fault zone complexity that exist.
Abstract of talk given to:
Geometry and Growth of Normal Faults, Geological Society of London, June 2014.