Geology Reference
In-Depth Information
that, if the piggyback basin is breached and
drained of sediments, deformation is likely to
step back toward the hinterland. Such a scenario
suggests a climate sensitivity: a more erosive
climate would lead to a retraction of the leading
edge of deformation. Each of these scenarios
poses questions that can be at least partially
answered with tectonic-geomorphic studies that
assess which faults have been active and when,
how basin filling, basin emptying, and faulting
interrelate, and whether the plateau margin is
progressively extending in the absence of faulting.
rates of faulting, erosion, and deposition (Ellis
et al
., 1999). Rivers flowing across the fault from
the footwall uplift will tend to dissect and embay
the mountain front, whereas active faulting will
tend to restore its linear character (Fig. 10.4).
Hence, both facet geometry and the linearity
of the range front are clues to the activity of
the bounding fault. Facets initially develop as
footwall scarps in bedrock. Given the steep
dip of most normal faults (
∼
60
°
), however, the
facets degrade and lay back over time due to
weathering and erosion. Consequently, steep,
high facets are signatures of rapidly slipping
faults, whereas gentle, low facets are typical of
Range fronts, basins, and normal faults
Commonly, ruptures on active normal faults can
be characterized as a series of linear fault
segments separated by transfer zones with more
complex geometries (Figs 4.9 and 4.16). The
linear trends result from the fact that most
normal faults intersect the surface at high angles
(
Rapid Normal-Fault Slip
small fans
large, slightly
degraded facets
mountain-
piedmont
junction
∼
°
), such that the trace of the fault is
only slightly deflected by surface topography.
In many cases, normal faults also approximately
define a boundary between an erosional domain
in the uplifted footwall and a depositional, nearly
horizontal, domain above the downthrown
hanging wall. As seen earlier, a predictable
pattern of co- and interseismic vertical motions
exists (Figs 4.25 and 4.26), with the greatest
magnitude of footwall uplift and hanging-wall
subsidence proximal to the fault. Enhanced
hanging-wall subsidence near the fault tends to
steer regional river systems toward the fault, and,
subsequently, their deposits tend to fill the space
available to accommodate sediment to the extent
that the sediment supply permits. Despite such
deposition, coseismic footwall uplift commonly
results in positive topography that bounds the
depositional basin along the fault. One would,
therefore, expect that recurrent normal faulting
would produce a relatively linear mountain front
delineating the footwall-hanging wall boundary.
60
footwall
block
A
Slow Normal-Fault Slip
entrenched
fanheads
degraded,
dissected
facets
large, low-
gradient fans
footwall
block
distal axial
river
B
Fig. 10.4
Range-front fans and facets in normal
faulted mountain ranges.
A. Rapid footwall uplift and hanging-wall subsidence
create a linear range front, large facets, small piedmont
fans, and a proximal axial river. B. Slower deformation
leads to large, low-gradient fans, small facets,
entrenched fanheads, and distal axial rivers.
Facets and drainage spacing
The topographic evolution of normal-faulted
mountain fronts depends strongly on the relative
