Geoscience Reference
In-Depth Information
Tectonics, denudation and basins
APPLICATIONS
Following tectonic-climate links in the New Developments box on p.294, we now outline the impact of uplift on
geomorphology and sedimentology in active continental rift basins. The basin-range province is an asymmetric, half-
graben system generated by uplift, where the south-western United States has overridden the northern Pacific plate
MOR, sliding off the north-west side of the Colorado plateau towards the Pacific Ocean. Small basaltic volcanoes
and cinder cones (
Plate 13.12
)
in basin floors attest to pull-aparttrans-tensional tectonics. Steep normal faults, above
the major low-angled detachment fault, form western-facing fault footwall scarps overlooking the hanging wall dip
slope of the adjacent block across an intervening basin (see
Figure 10.7d
)
. The entire system of north-south basins
and alternating mountain ranges - each 10-15 km long and wide - dominates Nevada and south-eastern California,
representing active, syn-rift environments developed during the past
f
20 Ma. One suite, Death Valley and its flanking
ranges, is featured here (
Figure 13.12
).
The ranges move as huge rotational landslides, elevating and steepening footwalls by faulting in brittle crust. This
continually exposes unweathered rock and stimulates aggressive weathering, erosion and sediment transfer to the
basin. Canyons, narrowing downslope across active fault zones, spread erosion products as debris fans on to the
basin floor, producing an overall wine-glass shape (
Plate 13.13
).
Rock slopes with concave sections (turtlebacks),
expose Proterozoic basement rocks far older (
f
1 Ga) than those accumulating today in the basins. Offset streams
cross the footwall, indicating strike-slip movement along the scarp in addition to normal faulting; small fault scarps
reflect the latter and related seismic activity (see
Plate 13.13
).
By contrast, wider opposite hanging walls subside
more slowly and the geomorphic environment is less active, with greater regolith cover on Cenozoic surface rocks,
larger debris fans and fewer dislocated streams (
Plate 13.14
).
Fan lobe sequences also differ between the two slopes,
with successively younger hanging-wall lobes travelling downslope across older ones ('steady' erosion mode), whereas
footwall rotation decants younger lobes at the canyon mouth (active uplift mode).
Huge sediment accumulations, typically
f
5 km thick, line the intervening basins below ranges rising 2-3 km above
their modern surfaces, attesting to dramatic denudation rates and localized isostatic subsidence under their weight.
Badwater Basin, the lowest point in continental North America, lies 86 m below sea level and
f
3,435 m below the
highest summits in the Panamint range to the west. Ephemeral annular/parallel (axial) streams usually drain these
interior deserts today, with centripetal drainage locally into seasonal saline lakes where the former are blocked (
Plate
13.15
and see p.331). However, they were often the sites of large Pleistocene lakes which drained, sometimes
catastrophically, leaving shorelines and evaporate rocks behind (
Plate 13.16
).
These provide further strong evidence
of tectonic control of climate. Rapid and continuing tectonic uplift in parts of the coastal ranges and Sierra Nevada
to the west enhance orographic precipitation and raised them high enough for Pleistocene glaciation. Cirque glaciers
survive today - just! The Basin Ranges lie in their rain shadow and hence are arid, exacerbated further by the cold
Californian current offshore. The Pleistocene lakes were starved and drained, to be replaced by playa lakes and saline
basins today (see
Chapter 16).
which is too close for comfort in the engineering world.
A compromise is struck between increased stability and
cost. Hence
F
s
1 is measured in expensive stabilization
schemes or loss of space/resources in road cuttings or
quarries by reducing slope angles.
F
s
= 1 is measured by
the costs of failure during the design life of a structure in
human lives, resource value, etc. We simplify the balance
of forces on natural slopes to the relationship between
shear
strength
, shear
stress
and slope angle - and explore
Mohr-Coulomb (see box, p. 303).
Slope stability
In its simplest form, slope stability depends on the ratio
of stabilizing forces resisting movement to destabilizing
forces encouraging it and is reflected in the engineering
term
factor of safety
,
F
s
. A state of
limiting equilibrium
or incipient failure exists when
F
s
= 1, as we see in the
Mohr-Coulomb equation later. Movement can be
triggered by any further deterioration. The natural angle
of rest in granular slope materials approximates
F
s
= 1,
