Damage Zone
Evolution in Coal Measures Strata From the Northumberland
Wightman, Ruth1, Jonathan
Imber1, Richard R. Jones2, David Healy1,
Robert E. Holdsworth1, Kenneth J.W. McCaffrey1 (1) Durham
University, Durham, United Kingdom (2) Geospatial Research Ltd. University of
Durham, Durham, United Kingdom
We develop kinematic models to describe
damage zone evolution during fault growth and test their predictions against
measurements of damage zone attributes within siliciclastic sand/shale
sequences from the Carboniferous Northumberland Basin, NE England. These data,
obtained from faults with throws spanning 0.1-20 m, were measured from detailed
(cm-resolution) digital outcrop models captured using terrestrial laser
scanning techniques. Study locations include coastal sections and areas of
active open-cast mining that provide good 3D exposure of faults during
progressive coal extraction.
The damage zones comprise: fault splays
and oversteps; drag folds; rotated fault-bound blocks; sub-parallel fracture
sets and ductile shear zones; cataclasite lenses; and intensely deformed scaly
gouge. We propose two kinematic models to explain the observed structural
relationships. Firstly, cataclasite lenses develop from fault-bounded blocks in
contractional oversteps. In this scenario, damage zone width remains
approximately constant as throw increases and is defined by the initial fault
separation. The second model describes the space incompatibility that develops
between discrete fault planes in coherent sandstone layers and adjacent shales
where slip is distributed along ductile shear zones. In this case, damage zone
width may increase with increasing fault throw. Alternatively, the width of the
damage zone may be controlled by thickness of the rheologically weaker shale.
These geologically-based models highlight the importance of bed thickness and
host-rock rheology - in addition to fault throw - in controlling damage zone
evolution and provide a basis for predicting the likely sizes of different
damage zone elements associated with seismically-imaged faults in the
subsurface.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California