Geology Reference
In-Depth Information
with any study seeking to define rates, key con-
straints are provided by dating of the formation
and abandonment of each terrace.
Across actively deforming zones in which mul-
tiple faults are closely spaced, a single terrace
may be displaced by several faults. Even without
knowledge of the terrace age, the variable dis-
placement of the marker surface by each fault
will indicate how deformation has been parti-
tioned among the active structures, and relative
rates of displacement can be defined. Because
flexural slip faults (Plate 6) exploit weak inter-
beds as slip surfaces, the limbs of tightening syn-
clines sometimes display fairly closely spaced
faults (Figs 4.34 and 9.7). Studies along the
Ventura River in southern California of terraces
offset across flexural slip faults in the Canada
Larga syncline (Rockwell
et al.
, 1984) provide
well-calibrated examples of both progressive ter-
race displacement and differential partitioning
of displacement among several faults (Fig. 9.7D).
In the case of growing folds, warped fluvial
terraces can provide unique insights into the
two-dimensional geometry of the fold and its
rate of growth. Antecedent streams that maintain
their courses across growing folds will often
produce strath terraces that may or may not
be mantled with a veneer of alluvial debris.
The terraces develop during intervals when
lateral abrasion dominates over vertical
incision (Figs 2.12 and 7.13). In cases in
which (i) terraces have extensive, down-valley
continuity and (ii) the deformed treads within a
rising structure, such as a fold, appear to grade
into undeformed treads beyond the structure, it
is likely that climatic fluctuations controlled the
periods of major terrace formation. Alternatively,
if the growth of the structure itself was
tectonically pulsed, then terraces may have
formed during intervals of reduced deformation
rates (e.g., Lu
et al.
, 2010). Most published work
(Medwedeff, 1992; Suppe
et al.
, 1992; Vergés
et al.
, 1996; Hubert-Ferrari
et al.
, 2007) in which
rates of fold growth are well calibrated, however,
is inconsistent with a pulsed deformation model.
It is worth stressing that a crucial component
in the analysis of both marine and fluvial
terraces is the correlations that are drawn
between physically isolated terrace remnants.
Often, erosion makes it impossible to trace terrace
surfaces confidently, even along smoothly folded
structures. Whenever faults are encountered,
correlation of terraces across the fault becomes
even more difficult. Because interpretations of
offsets are entirely dependent on such correla-
tions (Fig. 2.17), characterization of the terrace
surface and its subsurface stratigraphy is often a
major element in any such study (Merritts
et al.
,
1994). Soil development, loess stratigraphy,
tephra layers, and relative and absolute dating
techniques can all be used to distinguish
between and correlate among terraces.
Landscape responses at intermediate
time scales
We distinguish here between landscape features
that permit a direct calibration of deformation
rates, such as terraces, and features that
represent part of the landscape response to
deformation. Calibration features primarily
comprise displaced geomorphic markers whose
initial shape is quite well known. The initial
geometries of most other elements in the
landscape, ranging from stream channels to
hillslopes, are less easily traced backward in
time, because these geometries represent an
integrated response to ongoing deformation,
base-level variation, and climate change. These
features, therefore, only indirectly calibrate rates
of deformation.
Stream gradients
River networks represent a hierarchical
organization of tributary streams (lower order)
routing flow into trunk streams (higher order).
For a graded river flowing across uniform rock
types and experiencing uniform uplift, the
downstream channel gradient gets systematically
gentler as a function of increasing discharge,
which itself tends to vary as a function of
catchment area (Fig. 8.5). Departures of the river
gradient from this idealized, smooth shape may
reflect variations in the rock strength of the river
bed or variations in rock-uplift rate. Numerical
models of tectonically perturbed rivers predict
