3D fault zone architecture in Kardia Mine, Ptolemais Basin, Greece

 

PhD Candidate: Efstratios Delogkos

Supervisor: Dr. Conrad Childs, Dr. Tom Manzocchi, Prof. Spyros Pavlides, Prof. John J. Walsh

Funded by: This work was carried out as part of the Earth and Natural Sciences Doctoral Studies Programme, funded by the Higher Education Authority (HEA) through the Programme for Research at Third Level Institutions, Cycle 5 (PRTLI-5), co-funded by the European Regional Development Fund (ERDF). Part of the fieldwork expenses were also covered by the multi-company QUAFF project

 

Abstract:

Research on normal faults derives mainly from outcrop- and seismic-based studies. Outcrops are characterized by their high resolution but often lack a 3D context, in contrast with seismic-based data that can be fully 3D but with limited resolution compared to outcrop data. The novelty of this study is the examination of a truly 3D dataset of seismic scale fault zones at outcrop resolution.  The dataset has been acquired in the active, opencast Kardia lignite mine in the Ptolemais Basin, NW Greece. The basin is affected by two fault systems related to two extensional episodes. The first, Late Miocene episode resulted in the formation of the basin in response to NE-SW extension. The normal faults that occur in the Kardia mine formed during the second, Quaternary episode in response to NW-SE extension. Repeated visits at 3-monthly intervals over a 5-year period have allowed serial sections through the faults to be examined. These sections have been analysed in three dimensions, providing a unique insight into the structure of normal faults. The faults in the Ptolemais mines are unusual in that they are associated with little or no fault rock generation but instead detailed internal fault zone structure, which would normally be comminuted to fault rock, is preserved at very high strains. This feature of the faults allows detailed study of the geometric evolution of the faults and, in the case of the Kardia mine, interaction with synchronous bed-parallel slip surfaces.

The total throw on a fault can be considered to be partitioned onto three components 1) throw on the main fault surface, 2) throw on subsidiary fault surfaces and 3) throw accommodated by continuous deformation. Measurement and analyses of these components for the fault zones in Kardia mine demonstrates that the first of these becomes more important with increasing throw consistent with progressive strain localisation during fault growth. Rapid lateral variations in the degree of throw partitioning over a fault zone reflect the range of scales of segmentation of the initial fault. Continuous deformation constitutes an integral element of fault structure at all stages of fault growth and its contribution to total throw decreases with increased throw suggesting that continuous deformation, such as normal drag, develops during the early stages of a fault zone development.

Detailed three-dimensional mapping of a seismic scale normal fault shows various degrees of fault linkage with a gradual transition from hard- to soft-linkage with increasing scale of segmentation. Average shear strains measured across the normal fault zone at 81 locations vary by four orders of magnitude. Fault zone geometrical features indicative of linkage between fault segments occur over the full range of shear strains encountered supporting a model in which fault segment linkage is the primary control on the internal structure of the fault zone. By analogy this conclusion suggests that fault segment linkage may be the main control on the thickness and distribution of fault rock in areas where the details of fault zone structure are not preserved but are comminuted to fault rock.

Bed-parallel slip within the multilayer sequence in Kardia mine occurred towards the beginning of the second phase of extension and overlapped in time with Quaternary normal faulting.  Bed parallel slip, which is attributed to flexural-slip caused by reverse-drag folding in the hanging wall of a major fault, has a persistent top to the north slip direction. Bed-parallel slip surfaces occur throughout the excavated section and individual slip surfaces have slip up to 4.5 metres. 3D mapping of bed-parallel slip surfaces demonstrate that they display many of the features of dip-slip faults, for example, bed-parallel slip surfaces can be segmented both parallel and normal to the slip direction and on a wide range of scales. The displacement to length ratios derived for bed-parallel slip surfaces are typical of those for normal faults but are significantly lower than for the normal faults in Kardia mine. Backstripping of fault zone evolution at the locations of mutually offsetting bed-parallel slip surfaces and normal faults demonstrate how complex fault zone structure arises in the presence of synchronous bed-parallel slip. Bed-parallel slip surfaces formed during fault growth effectively introduce new displacement markers that can be used to examine the growth history of blind faults.