Abstract - Cross-cutting faults of the type illustrated
by John Castagna (May issue, TLE) are common in normal fault systems and
can occur on a range of scales and in a variety of tectonic settings. Intersecting
faults which produce the characteristic 'X'-shape in cross-section are
generally termed conjugate faults (e.g. Horsfield 1980; Nicol et al., 1995)
or 'hourglass' structures (e.g. Woods 1988; 1992). John's article prompted
several letters (December issue, TLE) explaining how such fault geometries
may arise. These models, like many others, are based on the implicit assumption
that the rock volume between the faults act as rigid blocks, undergoing
only translations. Anthony Gangi suggested that the geometry results from
strike-slip movement offsetting dipping beds whereas Harm Frikken argued
for strike-slip movement, on non-parallel faults, coupled with block rotation.
The third model, by Bernie Schwartz, suggested that the geometry is simply
a geometric consequence of the intersection of contemporaneous, non-parallel
faults. As John suggested, the principal problems posed by conjugate fault
geometries include: (i) why Ôthe faults do not seem to offset each
otherÕ and (ii) what Ôis happening where the horst and graben
meetÕ? These problems are addressed in some detail in a recent article
on the development of conjugate fault arrays, based on a study of conjugate
faults from high quality 2D and 3D seismic from offshore NW Australia (Nicol
et al. 1995).
Leading Edge, 812-816, 1996.