Page 159 - Fundamentals of Geomorphology

142 STRUCTURE

Time 1 along which no or minute differential movement has
taken place.

Dip-slip faults
Many tectonic forms result directly from faulting. It is
helpful to classify them according to the type of fault
involved – dip-slip or normal faults and strike-slip faults
and thrust faults (Figure 5.23). Dip-slip faults pro-
duce fault scarps, grabens, half-grabens, horsts, and tilted
Time 2
blocks. Strike-slip faults sometimes produce shutter
ridges and fault scarps (p. 144). Thrust faults tend to
produce noticeable topographic features only if they are
high-angle thrusts (Figure 5.23c).

Fault scarps
The fault scarp is the commonest form to arise from
faulting. Many fault scarps associated with faulting dur-
Figure 5.22 Gorge development in a snout of a resistant ing earthquakes have been observed. The scarp is formed
rock formation by stream persistence across a plunging on the face of the upthrown block and overlooking the
anticline. downthrown block. Erosion may remove all trace of a
Source: After Twidale and Campbell (1993, 354) fault scarp but, providing that the rocks on either side of
the fault line differ in hardness, the position of the fault
called active faults if movement was recent. Faults are is likely to be preserved by differential erosion. The ero-
commonly large-scale structures and tend to occur in sion may produce a new scarp. Rather than being a fault
fault zones rather than by themselves. A joint is a small- scarp, this new landform is more correctly called a fault-
scale fracture along which no movement has taken place, line scarp. Once formed, faults are lines of weakness,
or at least no differential movement. Joints arise from and movement along them often occurs again and again.
the cooling of igneous rocks, from drying and shrinkage Uplift along faults may produce prominent scarps that
in sedimentary rocks, or, in many cases, from tectonic are dissected by streams. The ends of the spurs are ‘sliced
stress. Many fractures described as joints are in fact faults off’ along the fault line to produce triangular facets.

Figure 5.23 Faulted structures. (a) Normal fault. (b) Strike-slip fault. (c) High-angle reverse or thrust fault. (d) Low-angle
thrust fault.
Source: Adapted from Ahnert (1998, 233)