Geology Reference
In-Depth Information
rate change was due to a surface rupturing fault
or a change in the ramp angle of a deeply bur-
ied fault surface. Wobus et al. (2003, 2005) also
showed that a major discontinuity in the 40 Ar/ 39 Ar
cooling ages of detrital muscovite that was col-
lected from small tributary catchments straddles
the PT2 (Fig. 9.14C). North of the PT2, cooling
ages average 10 Ma or less, whereas south of the
PT2, cooling ages range from an average of
300 Ma to > 1000 Ma. These data imply that the
northern catchments have cooled at rates above
30 ° C/Myr, whereas the southern catchments
experienced rates averaging below 1 ° C/Myr.
This simplest, although not unique, explanation
for  the abrupt change in cooling ages is a
previously unrecognized, surface-rupturing fault
at the PT2.
Given that the cooling ages north of the PT2
imply rapid cooling since 10 Ma, the reader
might justifiably wonder why this example is
included under “intermediate time scales.” The
reason is that erosion rates north of the PT2 are
sufficiently high that many hundreds of meters
of erosion would occur during 500 kyr and, as a
consequence, the topographic indices examined
here (hillslope gradients and channel steepness)
would be sensitive to erosion of that magnitude.
Where terraces of different ages are preserved,
patterns of deformation across tens to hundreds
of thousands of years can be deduced, and
sometimes constraints can be placed on the
underlying fault geometry. Consider, for exam-
ple, a suite of terraces in which progressively
older terraces are increasingly back-tilted above
a thrust fault (Fig. 9.15A). After subtracting the
gradient of the modern channel, the magnitude
of tilting can be defined, as well as a narrow
zone in which each terrace transforms from
tilted to simple planar uplift (Fig. 9.15B). Based
on deformed terraces on New Zealand's South
Island, Amos et  al. (2007) argued that this
configuration of terrace deformation is most
consistent with an underlying, near-surface
listric thrust fault that transitions to a planar fault
at depth (Fig. 9.15C). For this model, if the width
of the tilted backlimb and its tilt magnitude, as
well as the position and dip of the frontal fault,
are known, then the radius of curvature of the
listric fault, the magnitude of fault slip, the depth
to the transition to a planar fault, and the dip of
the planar ramp can be calculated (Amos et al. ,
2007). If the vertical offset (d z in Fig. 9.15C) of
the non-tilted terrace treads is known, this offset
provides a check of  the dip prediction for the
planar fault at depth. It is important to note,
however, that listric faulting is not the only way
to produce progressive terrace tilting: both
detachment folding (Scharer et al. , 2006; Suppe
et al. , 2004) and simple-shear fault-bend folding
(Suppe et al. , 2004) can produce similar tilting.
Sometimes the local geology can rule out alter-
native models, but commonly subsurface imag-
ing is needed to test among them.
Fault-bend fold theory (Fig. 4.36A) (Suppe,
1983) predicts that, whenever a planar marker is
transported through an active axial surface, the
marker will deform in a geometrically predictable
way. Consequently, planar geomorphic features,
such as fluvial terraces, that are beveled or
deposited across an active axial surface can be
folded following the rules for fault-bend folds.
This deformation is particularly obvious and
useful on the backlimbs of folds where structural
advection of a terrace through an active axial
surface commonly forms a fold scarp (Fig. 9.16A
and Plate  6). The dip of the fold scarp is a
predictable function of (i) the dip of the original
terrace surface and (ii) the angular difference in
dip of the underlying fault plane on either side
of axial surface (Hubert-Ferrari et  al. , 2007;
Chen et al ., 2007). From a tectonic perspective,
the key aspect of the fold scarp is that its length
approximates the  slip on the fault since the
terrace was created (Fig. 9.16B). Clearly, if the
age of the terrace is known, then a fault slip rate
can also be determined. Where multiple dated
terraces are present, temporal changes in slip
rates can also be assessed.
A spectacular example of a fold scarp was
recently described by Hubert-Ferrari et  al.
(2007) from the southern margin of the Tien
Shan in western China (Box 9.2). In this
arid  setting, erosional modification of many
geomorphic features is modest, such that
large  triangular facets representing extensive
remnants of fold scarps are well preserved in
the landscape. In the cross-section at Quilitak
fold (Fig. 9.17A), a remarkable, planar facet
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