370
In arc� related basins, volcanic control of the sediment supply may overprint the effects of sea-level
change. For example, Winsemann and Seyfried
(1991) stated:
"Sediment supply and tectonic activity overprinted the
eustatic effects and enhanced or lessened them. If large
supplies of clastics or uplift overcame the eustatic effects,
deep marine sands were also deposited during highstand
of sea level, whereas under conditions of low sediment
input, thin-bedded turbidites were deposited even during
lowstands of sea level."
A fluvial example of the volcanic control of sediment supply was given by Kuenzi et a!. (1979). Fluvial styles in this area seem unusual but may, in fact,
be typical of nonmarine arc-related environments
characterized by active intermediate volcanism. Explosive eruptions, coupled with very high rainfall,
lead to the catastrophic transportation and sedimentation of huge quantities of coarse volcaniclastic debris. This may be confined to the trunk rivers, and
the result is that the tributaries are dammed at their
junction with the main river and back up to form
lakes (Fig. 7.15). These then fill with mud and small
Gilbertian deltas. Lobate, river-dominated deltas
form at the coast when sediment supply is high, but
between major eruptions these may be reworked by
waves into narrow, arcuate deltas. An ancient example of this fluvial style was described by Ballance
(1988).
In more general, regional terms, sediment supply
in volcanic terrains is markedly influenced by
the uplift and subsidence generated by the thermal
effects of volcanic activity. A good example is
described by Dorsey and Burns (1994). Magmatic
intrusion . generates regional uplift, resulting in
steepening of regional slopes, and the sediment supply is vastly increased by accompanying extrusive
activity. Cessation of magmatic activity leads to thermal relaxation, subsidence, and transgression, with
a reduction of sediment supply and the drowing of
paleovalleys.
Other examples of the tectonic control of major
sedimentary units are provided by the basins within
and. adj acent to the Alpine and Himalayan orogens.
Sediments shed by the rising mountains drain into
foreland basins, remnant ocean basins, strike-slip
basins, and other internal basins. But the sediment
supply is controlled entirely by uplift and by the
tectonic control of dispersal routes, as described in
several places earlier in this chapter. Brookfield
(1992) reported on the beginnings of a project to
investigate the shifting of dispersal routes through
basins and fault valleys within the Himalayan orogen
of central and southeast Asia. Some of the major
Tectonic Control of Fluvial Sedimentation
rivers in the area (Tsangpo, Salween, Mekong) are
known to have entirely switched to different basins
during the evolution of the orogen. Much work remains to be done to relate the details of the stratigraphy in these various basins to the different controls
of tectonic subsidence, tectonic control of sediment
supply, and eustatic sea-level changes.
11.3.5 Intraplate Stress
<(Ridge-push" and «slab-pull" are informal terms
that have been used for some time in the platetectonics literature to refer to the horizontal (inplane) forces associated with, respectively, the
horizontal compressional effects resulting from the
injection of magma at a spreading center and the
tensional effects on an oceanic plate generated by the
downward movement into a subduction zone, under
gravity, of a cold slab of oceanic crust. In-plane
(horizontal) stress is a central theme of the paper by
Molnar and Taponnier (1975) describing a model of
Himalayan collision, in which it was demonstrated
that most of the Cenozoic structural geology of west
China, extending for 3000 km north of the Indus
suture to the edge of the Siberian craton> including
that of many hinterland basins filled with nonmarine sediments (see Sect. 11.4), could be explained as the product of deformation resulting
from intraplate stresses transmitted into the continental interior from the India-Asia collision zone. It
came to be recognized that tectonic plates are not
rigid and can store and transmit horizontal forces
many thousands of kilometers from zones of platemargin stress. The presence of residual stress fields
in continental interiors has long been known from
such evidence as the development of active joints
C'break-otits") in exploration holes (summary in
Cloetingh 1988). Cloetingh (1988, p. 206) suggested
that
«the observed modern stress orientations show a remarkably consistent pattern [in northwest Europe], especially
considering the heterogeneity in lithospheric structure in
this area. These stress-orientation data indicate a propagation of stresses away from the Alpine collision front over
large distances in the platform region."
Cloetingh et al. (1985) were the first to recognize
the significance of these ideas for their possible effects on sedimentary basins. They argued
«that variations in regional stress fields acting within inhomogeneous lithospheric plates are capable of producing
vertical movements of the Earth's surface or the apparent
se(;l. -level changes ... of a magnitude equal to those deduced
from the stratigraphic record."
In arc� related basins, volcanic control of the sediment supply may overprint the effects of sea-level
change. For example, Winsemann and Seyfried
(1991) stated:
"Sediment supply and tectonic activity overprinted the
eustatic effects and enhanced or lessened them. If large
supplies of clastics or uplift overcame the eustatic effects,
deep marine sands were also deposited during highstand
of sea level, whereas under conditions of low sediment
input, thin-bedded turbidites were deposited even during
lowstands of sea level."
A fluvial example of the volcanic control of sediment supply was given by Kuenzi et a!. (1979). Fluvial styles in this area seem unusual but may, in fact,
be typical of nonmarine arc-related environments
characterized by active intermediate volcanism. Explosive eruptions, coupled with very high rainfall,
lead to the catastrophic transportation and sedimentation of huge quantities of coarse volcaniclastic debris. This may be confined to the trunk rivers, and
the result is that the tributaries are dammed at their
junction with the main river and back up to form
lakes (Fig. 7.15). These then fill with mud and small
Gilbertian deltas. Lobate, river-dominated deltas
form at the coast when sediment supply is high, but
between major eruptions these may be reworked by
waves into narrow, arcuate deltas. An ancient example of this fluvial style was described by Ballance
(1988).
In more general, regional terms, sediment supply
in volcanic terrains is markedly influenced by
the uplift and subsidence generated by the thermal
effects of volcanic activity. A good example is
described by Dorsey and Burns (1994). Magmatic
intrusion . generates regional uplift, resulting in
steepening of regional slopes, and the sediment supply is vastly increased by accompanying extrusive
activity. Cessation of magmatic activity leads to thermal relaxation, subsidence, and transgression, with
a reduction of sediment supply and the drowing of
paleovalleys.
Other examples of the tectonic control of major
sedimentary units are provided by the basins within
and. adj acent to the Alpine and Himalayan orogens.
Sediments shed by the rising mountains drain into
foreland basins, remnant ocean basins, strike-slip
basins, and other internal basins. But the sediment
supply is controlled entirely by uplift and by the
tectonic control of dispersal routes, as described in
several places earlier in this chapter. Brookfield
(1992) reported on the beginnings of a project to
investigate the shifting of dispersal routes through
basins and fault valleys within the Himalayan orogen
of central and southeast Asia. Some of the major
Tectonic Control of Fluvial Sedimentation
rivers in the area (Tsangpo, Salween, Mekong) are
known to have entirely switched to different basins
during the evolution of the orogen. Much work remains to be done to relate the details of the stratigraphy in these various basins to the different controls
of tectonic subsidence, tectonic control of sediment
supply, and eustatic sea-level changes.
11.3.5 Intraplate Stress
<(Ridge-push" and «slab-pull" are informal terms
that have been used for some time in the platetectonics literature to refer to the horizontal (inplane) forces associated with, respectively, the
horizontal compressional effects resulting from the
injection of magma at a spreading center and the
tensional effects on an oceanic plate generated by the
downward movement into a subduction zone, under
gravity, of a cold slab of oceanic crust. In-plane
(horizontal) stress is a central theme of the paper by
Molnar and Taponnier (1975) describing a model of
Himalayan collision, in which it was demonstrated
that most of the Cenozoic structural geology of west
China, extending for 3000 km north of the Indus
suture to the edge of the Siberian craton> including
that of many hinterland basins filled with nonmarine sediments (see Sect. 11.4), could be explained as the product of deformation resulting
from intraplate stresses transmitted into the continental interior from the India-Asia collision zone. It
came to be recognized that tectonic plates are not
rigid and can store and transmit horizontal forces
many thousands of kilometers from zones of platemargin stress. The presence of residual stress fields
in continental interiors has long been known from
such evidence as the development of active joints
C'break-otits") in exploration holes (summary in
Cloetingh 1988). Cloetingh (1988, p. 206) suggested
that
«the observed modern stress orientations show a remarkably consistent pattern [in northwest Europe], especially
considering the heterogeneity in lithospheric structure in
this area. These stress-orientation data indicate a propagation of stresses away from the Alpine collision front over
large distances in the platform region."
Cloetingh et al. (1985) were the first to recognize
the significance of these ideas for their possible effects on sedimentary basins. They argued
«that variations in regional stress fields acting within inhomogeneous lithospheric plates are capable of producing
vertical movements of the Earth's surface or the apparent
se(;l. -level changes ... of a magnitude equal to those deduced
from the stratigraphic record."