Geoscience Reference
In-Depth Information
8.2.2 Determining past motion on
brittle structures
To determine the direction of
movement of slickenfi bres move
your hand up and down parallel
to the fi bres. The slickenfi bres
feel smoothest in the direction
of motion of the missing block.
With any fracture, a primary goal is fi nding out in which
direction the adjacent rocks moved, and how far. Accumulating
such data builds a picture of local, and subsequently regional,
brittle deformation that feeds into studies of earthquakes on
major faults, especially in plate boundary zones. Many faults
carry clues to their past motion. The following indicators are
the most common:
8
Slickenfi bres
(Figures 8.6a and 8.6b) are mineral fi bres that
grow during fault creep. The long axes of fi bres are parallel
to the fault slip direction, while steps in the fi bres may
reveal the sense of fault motion. The direction of motion is
down the steps.
Slickenside lineations
(slickenlines) are grooves and
striations on fault surfaces (slickensides) that also run
parallel to fault slip direction (Figure 8.6c), i.e. down dip
on thrust and normal faults, or subhorizontal on strike-slip
faults. Slickenlines observed at an intermediate orientation
between the dip and strike of a fault plane imply that the
fault moved by oblique slip, a combination of dip-slip and
strike-slip movement which is not uncommon. This
interpretation assumes the fault to be in its original
orientation. Note also that slickensides tend to record the
latter stages of fault movement, which in some cases may
differ from the main phase of slip on the fault.
(a)
Minor fractures
in fault zone breccias or gouges may also
bear slickensides, which can be measured to build up a
picture of movement throughout the zone.
In some cases, increments of slip (perhaps representing single
earthquakes) may be recorded by lines or ridges at a high angle
to the striations (e.g. the subhorizontal ridges in Figure 8.6c),
and variations in slip direction can produce curved
slickenlines, or multiple sets of slickenfi bres with different
orientations. Overlapping of these features can in some cases be
analysed to construct a relative chronology of fault slip.
(b)
Figure 8.6
(a) Mineral fi bres in this vein grew at right angles to the
vein walls, showing that it is a dilational vein. Vein is 15 mm wide at top
of image. Southwest Wales, UK. (b) Slickenfi bres of quartz showing
oblique slip (parallel to hammer shaft) on a minor thrust fault in the
Himalaya, north Pakistan. Direction of movement of the upper block is
right to left, parallel to handle of hammer. (c) Steep striations (i.e.
slickenside lineations) on a fault in limestone in the Swiss Alps indicate
dip-slip motion. Subhorizontal ridges may record successive slip episodes,
but not the sense of motion. (a-c: Tom W. Argles, The Open University, UK.)
(c)
