Geology Reference
In-Depth Information
Lower Crustal Flow Against A Stro
ng Obstacle
positive pressure
negative pressure
(subsidence)
(uplift)
strong
obstacle
A
Vertical Deflection versus Effective Elastic Thickness
2500
viscosity = 2 x 10
18
Pa s
velocity = 100 mm/yr
Efective Elastic
Thickne
ss
2000
1500
Te
=
1 km
Fig. 10.48
Impact of a strong crustal
obstacle on lower crustal flow at a plateau
margin.
A. Strong obstacle causes positive pressure on
the upstream side of the flow and negative
pressure on the downstream side, particularly
in comparison to adjacent regions that lack
strong obstacles. B. Upward (downward)
deflection is predicted on the upstream
(downstream) side as a response to the
pressure gradient. The magnitude and
wavelength of deflection depends on
the effective elastic thickness (
T
e
). C. Example
of topographic and deflection profiles across
an inferred strong versus weak (or normal)
margin in eastern Tibet. Both the topography
and deflection mimic that predicted for a
strong obstacle to flow. The inferred “weak”
margin reveals a steadily descending ramp.
Te= 10 km
Te= 30 km
Te= 80 km
1000
positive
pressure
500
0
edge of obstacle
-500
B
0
200
400
600
800
1000
1200
1400 1600
1800 2000
Distance (km)
C
Eastern Tibet - Sichuan Basin Topography
West
East
strong margin
dynamic
topography
weak margin
weak margin
strong margin
edge of
Sichuan Bas
i
n
Eastern Tibetan Plateau
West
East
Distance
and vice versa (Fig. 10.49). Fourth, where
surface erosion at the plateau margin is intense,
the ductile, lower crustal channel could be
drawn toward the surface (Fig. 1.9C) and active
faults would likely bound such a channel
(Beaumont
et al
., 2001). Conversely, with less
intense erosion, surface faulting might be sig-
nificantly reduced, and the ductile lower crust
could remain deeply buried, despite a strong
topographic gradient across the plateau's
margin.
Many of these characteristics are observed
along the eastern margin of the Tibetan Plateau
(Clark and Royden, 2000; Royden
et al
., 1997).
From the southeastern corner of Tibet, a huge,
low-gradient ramp extends south and east to
the lowlands (Fig. 10.50). North of the Sichuan
Basin, a second ramp extends towards the
northeast. These two ramps are interpreted as
zones in which lower crustal flow is channeled
outward from the Tibetan Plateau. In between
these ramps, the Sichuan Basin appears to be
underlain by old, stiff crust that resists the
interpreted outward flow of lower crust from
beneath Tibet (Clark
et al
., 2005a). As suggested
by topographic profiles (Fig. 10.48C), the pla-
teau surface appears deflected upwards adjacent
to this more rigid Sichuan crust. In comparison
to the Himalayan front, erosion is generally
slower in eastern Tibet, and active surface fault-
ing is considerably more subdued, although cer-
tainly is not absent - as demonstrated by the
2008
M
=
7.9 Wenchuan earthquake (Kirby
et al
.,
2008). The inferred pattern of outward crustal
