Geology Reference
In-Depth Information
the surface since it was created. As a conse-
quence, multiple lines of evidence commonly
need to be assembled to justify the recon-
struction of a formerly contiguous surface. In
places, a low-relief surface stands in contrast to
a nearby tectonically active front characterized
by high relief, such that the uplifted surface
provides a useful marker for estimating tectonic
displacement at long time scales. For example,
in the San Bernardino Mountains of southern
California, the Big Bear plateau is distinguished
by an undulating, deeply weathered surface
that covers ~1500 km
2
and averages about 2 km
in elevation. The plateau sits atop a 600- to
1000-m-high, north-facing escarpment that is
underlain by a north-vergent, active thrust fault
(Spotila and Sieh, 2000). The erosion surface is
interpreted to have developed since late
Cretaceous times and to have been at much
lower elevations in the Miocene when basalt
flows were erupted across parts of it. Where
preserved, these basalts fossilize the former
erosion surface. Elsewhere, however, deep
weathering (up to 30 m) has obliterated the
actual Miocene erosion surface across most of
the plateau. Nonetheless, the lateral continuity
of this weathered surface, its clear spatial
relationship to sites where the erosion surface
is preserved, and consistent cooling ages across
the surface all support its interpretation as an
uplifted erosion surface (Spotila and Sieh,
2000).
Similarly, in the Tien Shan of Central Asia, a
regionally extensive erosion surface at least
100 000 km
2
in extent was beveled across
Paleozoic and Mesozoic rocks and buried by
Cenozoic sedimentary rocks (Chediya, 1986;
Sadybakasov, 1990). Wherever basal Cenozoic
strata are exposed above this unconformity,
their bedding parallels the dip of the uncon-
formity at kilometer scales, implying very low
relief on the unconformity surface when it
was buried. This surface has been recently
exhumed due to rock uplift. The striking con-
trast in erodibility of the rocks above and
below the unconformity has caused the
Cenozoic sediment to be rapidly eroded,
revealing the unconformity surface (Plate 1D),
which provides an excellent marker for
recording folding and faulting of ranges that
rise as much as 2 km above the surrounding
terrain (Burbank
et al.
, 1999). Not only do ero-
sion surfaces like those in the San Bernardino
Mountains or the Tien Shan serve to define the
three-dimensional pattern of differential rock
and/or surface uplift, but the unconformity
surface itself forms a reference for calibrating
the amount of erosion that has occurred
beneath it and for assessing the processes by
which such uplifted bedrock surfaces are dis-
sected (Oskin and Burbank, 2005, 2007;
Goode and Burbank, 2011).
Linear geomorphic markers
Whereas the previously described geomorphic
markers represent areally extensive surfaces, it
is also possible to use linear geomorphic and
man-made features to determine deformation.
Although displaced planar features are more
suitable for defining regional tilting, linear
features, such as glacial moraines (Plate 1E),
can provide ideal piercing points from which
an offset can often be unambiguously measured.
Unlike many two-dimensional surfaces, such
as marine or fluvial terraces, many linear geo-
morphic features can be formed by individual
events, some of which may have occurred
instantaneously from a geological perspective,
for example, the levees that form on the
margins of a debris flow (Plate 1F). Such
features often have no direct relation to climatic
variations, so that ages need to be determined
for each event in order to determine rates of
deformation.
Rivers and ridge crests
The courses of rivers and ridge crests that are
displaced across strike-slip faults can clearly
record lateral offsets (Fig. 2.16A). It is important
to ascertain, however, that the deflection of a
stream is due directly to differential displace-
ment of its course by faulting and is not the
result of the intersection between a regionally
sloping surface and a fault scarp. If streams are
offset in directions that oppose the regional
