Geology Reference
In-Depth Information
Erosional Unloading
Tectonic Loading
A
B
asymmetric depositional units
tabular depositional units
mass lost
inactive thrust
front
asymmetric subsidence:
wedge-shaped units
asymmetric rock uplift
tabular units
active thrust
front
rock
uplift
rock
uplift
reference
point
reference point
basement uplift
longitudinal rivers dominant
transverse rivers dominant
inactive
th
rust
m
ass lost
progradation
active thrusts
basement uplift
time line
time line
Fig. 7.27
Effects of tectonic loading or erosional unloading on subsidence and depositional patterns
in foreland basins.
Either tectonic loading or erosional unloading can cause uplift of individual summits, but, during the latter, mean
surface elevations decrease. A. Tectonic loading is typified by asymmetric subsidence in the foreland basin, driven
primarily by crustal thickening in the hinterland. In this scenario, transverse rivers are short and join a longitudinal
river in a proximal to medial part of the foreland basin. Coarse-grained deposits tend to be restricted to near the
mountain front. B. Erosional unloading in the hinterland drives in rock uplift in the proximal foreland. In this
scenario, transverse rivers flow across much of the foreland, longitudinal rivers are in a distal position, and coarse
facies may prograde far across the basin. Modified after Burbank (1992).
stream a short distance from the mountain front
and turn to flow parallel to the mountains. Coarse-
grained sediment tends to be trapped in the prox-
imal setting by the rapid subsidence (Heller
et al.
,
1988). In the subsurface, a prism of sediments
that thickens toward the mountains provides
evidence of persistent tectonic loading in and
preferential subsidence toward the hinterland
(Flemings and Jordan, 1989).
If the regime switches to one in which ero-
sion prevails over tectonic thickening, such that
the crust actually thins, then the reduced mass
of the mountains will drive an isostatic uplift
within the proximal foreland as well, as a func-
tion of the finite flexural rigidity of the crust
(Fig. 7.27). Erosion will occur in the proximal
foreland, and coarse-grained sediments may
prograde as tabular, thin sheets across the
medial and distal foreland (Heller
et al.
, 1988).
Transverse rivers can flow across nearly the
entire foreland basin before merging with a lon-
gitudinal trunk stream, which may itself be
pushed toward the far edge of the basin by the
proximal rock uplift (Burbank, 1992).
Such a pattern appears to be consistent with the
geometry of modern rivers in the Himalayan fore-
land (Burbank, 1992). Whereas, during Miocene
times, coarse strata remained near the mountain
front and axial rivers dominated the medial fore-
land basin, during Quaternary times, tabular
sheets of strata extend across the medial and
distal foreland, and transverse drainages stretch
across the foreland. These changes suggest that
the relative balance between crustal thickening
and thinning has changed during the past few mil-
lion years. If true, then uplift of summits observed
today would represent an isostatic response to a
reduction in the overall thickness of the crust.
