Geology Reference
In-Depth Information
mean spacing of the mouths of the basins along
the range front (Wallace, 1978). For tectonically
active, extensional fault blocks, length-to-outlet
spacing ratios are typically between 1.8 and 4
(Hovius, 1996; Wallace, 1978) with a mean of
about 2.5 (Talling
et al
., 1997). In “well-behaved”
ranges bounded by half-grabens, when opposing
flanks are compared, differences in the distance
to the main divide are clearly reflected in
differences in the spacing of the major rivers
draining each flank, such that shorter drainages
are associated with more closely spaced rivers
(Fig. 10.6). In terms of trying to decipher the
tectonic regime from the drainage patterns, the
key attribute of these patterns is that shorter
and steeper drainages are commonly found on
the more tectonically active flank of the range.
In more complex geological uplifts and in older
mountains, where drainages have had more
time to integrate or cannibalize other nearby
drainages, basin shapes are more irregular and
river-outlet spacing can be wider, yielding
spacing ratios as low as 1.2 (Wallace, 1978).
Facets in the Baikal Rift
600 m
Fig. 10.5
Facet spacing and shape along an active
normal fault in the Baikal rift.
Active range-front faulting in the Barguzin Basin
produces triangular facets. These facets have an average
slope of
∼
°
, heights up to 900 m, and estimated slip
rates of
∼
1 mm/yr. Modified after Petit
et al
. (2009b).
35
Drainage Spacing versus Range Half-Width
Stillwater Range,
Nevada
half-width
of range
Geomorphology and quantitative assessment
of normal-faulted range fronts
N
White Mountains,
California
A normal-faulted range front that has
experienced persistent rock uplift typically
displays a related suite of geomorphic
characteristics. Mountains in the footwall block
may rise more than 1000 m above the adjacent
basins. Rapid footwall uplift tends to result in
a linear mountain front characterized by high
relief and deeply incised streams (Fig. 10.5).
Within the footwall uplift, these V-shaped
valleys have steep gradients and small to non-
existent floodplains. Within the hanging-wall
basin, the length and gradient of transverse
streams or fans, as well as the relative distance
from the depocenter to the mountain front in
comparison to the width of the basin, provide
indicators of the balance between tectonically
driven subsidence and sediment supply. Lower
transverse river gradients, longer fan lengths,
and increasingly distal depocenters should
indicate lower rates of active deformation and
subsidence along the basin-bounding fault;
whereas short, relatively steep alluvial fans, the
10 km
active flank
(shorter
half-width)
Fig. 10.6
Asymmetric flank widths on fault-block
mountains in the Basin and Range.
Note that the average spacing of the outlets scales with
the average length of the drainages, such that more
closely spaced outlets are associated both with shorter
tributary basins and with the more rapidly uplifting
flank of the range. Modified after Talling
et al
. (1997).
slower slip rates (Petit
et al
., 2009a). Although
facet steepness correlates with slip rate, facet
widths tend to scale with drainage-basin spacing
(Figs 10.5 and 10.6). Spacing can be measured
as the ratio between the mean length of the
basins, i.e., the mean distance from the main
drainage divide to the mountain front, and the
