Faults in the field, part two
Fault patterns and field analyses
Strike-slip pull apart basin
Thrust duplex
Contraction (thrust) faults
Characteristics
Thrust faults:
• Low-angle dip-slip faults, along which the hangingwall has been upthrown.
• In imbricate thrust zones, the thrusts bring more deeply buried rocks up and
over shallower rocks.
• The hangingwall of a thrust is commonly called a thrust sheet.
• The shape of individual thrust is not usually planar, but it is curved as:
• Listric: the thrust plane curve smoothly from steeper dip at its upper end
to shallow dip at depth so that it becomes parallel to bedding, then defines
the detachment or décollement. This concave upward is called listric.
• The ramps: the thrust develop a kind of staircase shape. The hangingwall
over the ramps bends upwards taking on an antiformal shape.
Thrust symbol on maps is defined by barbs on the hangingwall/thrust sheet side.
If displacement lines on thrusts plunges to S, the movement is top to N; and the
structure is described as oblique thrust.
The fold related fault is drag fold, and formed on the hanging wall.
Recognitions
of thrusts on
maps
A group of thrusts on a map will have the effect of repeating the stratigraphy several
times, and if imbricate zones are present too, the outcrop pattern can be extremely
intricate.
TAKE CARE imbrications and outcrop repetitions can be developed in extensional
terrane BUT in case of thrust terrane, the older rocks is locate over the younger
rocks as thrust imbricated nappes.
Recognitions
of thrusts on
maps
Autochthons exposed within allochthons napps through tectonic window.
Note autochthons/ allochthons thrust contact surrounds the exposed periphery of
autochthons.
Remnants of allochthons on the autochthons appear as klipps.
Recognitions
of thrusts on
maps
If thrusts tend to be shallow, they are amenable to the methods developed in
horizontal and dipping beds. The map trace of a thrust will tend to follow
topographic contours, and will parallel them if the thrust is horizontal.
Thrusts will be upsetting the stratigraphic sequence due to repeating rock packages
across the imbricate thrust napps.
Recognitions
of thrusts on
maps