8.2 Methods for Determining Subsidence
If the load remains in place for a long time period,
deformation of the lithosphere changes in a manner
as shown in Fig. 8.3b. After a first stage of rapid
downwarping, lithospheric material at depth under
the load relaxes stress, thereby creating a deeper central depression in conjunction with a narrower basin.
The peripheral bulge is progressively uplifted and
migrates toward the load. Ultimately, relaxation will
evolve toward astate of local isostatic equilibrium.
Such a time-dependent development is primarily controlled
by the mechanical state of the lithosphere. Although some
workers strongly believe that models based on elastic condition plates can fairly weil predict the processes observed
in nature, others favor a uniform visco-elastic (Maxwell)
model of the lithosphere. A third possibility is the assumption of a Maxwell layer with temperature-dependent viscosity overlying a low-viscosity fluid (e.g., Quinlan and
Beaumont 1984). Few of the models have taken into account additional local heat sources associated with special
magrnatic or volcanic events during the history of a continental margin or other flexure-influenced basins.
8.1.3 Mineral Phase Transitions
Some authors favor the phase transition from gabbro
(average density 3.0 g/cm 3 ) to the denser eclogite
(average density 3.5 g/cm 3 ) in the lower lithosphere
as a principal mechanism of subsidence. Dependent
upon depth (or pressure ) within the lithosphere, this
phase transition is accomplished at temperatures of
700 to 800°C and accelerated by the presence of
small quantities of water. Anomalous heat transfer
from the deeper mantle can raise this phase boundary
and thus reduce lithospheric thickness without
changing the mass of a given lithospheric column on
top of the isostatic compensation depth.
If, for example, the phase boundary migrates 10 km upward
and consequently a 10 km thick column of gabbro is transformed into eclogite, then the thickness of the lithosphere
is reduced by approximately 1.5 km (without taking into
account a possible additional sediment load or the simultaneous thermal expansion of the lithosphere). Because this
mechanism does not require horizontal extension of the
lithosphere, it is quoted as the main factor for the formation
of intracratonic basins (cf. Sect. 12.3). For summaries on
this topic see, e.g., Wyllie (1971), Artyushkov and Sobolev
(1983), Ismail-Zadeh et al. (1997).
Phase transitions due to deep crustal metamorphism
may explain unloaded subsidence (or uplift) up to the
order of 2 km. Such transitions take place over a very
long time period (order of 100 Ma) when small quantities of water are not available in the lithosphere to
speed the process. But it is doubtful whether phase
transitions alone can cause the strong and rapid subsidence observed in many rift settings.
393
8.1.4 Summary (Subsidence)
Subsidence (or uplift) of a sedimentary basin is
controlled by the following principal factors:
Thinning (or thickening) of the lithosphere due
to horizontal extension (or compression,
underplating).
- Upwdling of mantle material in response to
crustal thinning.
Increase (or decrease) in lithospheric density
due to (1) cooling (or heating), (2) pervasive
dike intrusion or other injections of magma,
and (3) melting of crust or magma crystallization, as well as mineral transitions (from low to
higher density or vi ce versa).
- Isostatic subsidence (or uplift) in response to
~ediment loading (or erosion) or flexuralloadmg.
8.2 Methods for Determining
Subsidence of Sedimentary Basins
8.2.1 Introduction
The subsidence history of sedimentary basins may be
described in different ways. Structural geologists and
geophysicists are primarily interested in that part of
subsidence which is controlled by crustal processes
(tectonic or thermo-tectonic subsidence), whereas
sedimentologists and stratigraphers deal with the progressive burial history of sediments (total or cumulative subsidence) and often want to interpret subsidence curves in terms of sedimentation rates, paleowater depth, and sea-level changes.
In order to determine the subsidence of a chosen
location in a sedimentary basin as weH as to discriminate between the various mechanisms mentioned
above, special techniques must be used. If the tectonically driven part of subsidence is the objective,
this technique is referred to as "backstripping" . The
purpose of backstripping is to calculate and remove
the effects of compaction, sediment loading, changing paleo-bathymetry, and sea-level variations. When
backstripping is combined with the search for information on the development of the depositional system, the term "geohistory analysis" or burial history
is commonly used.
For further discussion see, e.g., van Rinte (1978), Watts
and Steclder (1979), Gradstein et al. (1985), Guidish et al.
(1985), Regarty et al. (1988), Steckler et al. (1988).
If the load remains in place for a long time period,
deformation of the lithosphere changes in a manner
as shown in Fig. 8.3b. After a first stage of rapid
downwarping, lithospheric material at depth under
the load relaxes stress, thereby creating a deeper central depression in conjunction with a narrower basin.
The peripheral bulge is progressively uplifted and
migrates toward the load. Ultimately, relaxation will
evolve toward astate of local isostatic equilibrium.
Such a time-dependent development is primarily controlled
by the mechanical state of the lithosphere. Although some
workers strongly believe that models based on elastic condition plates can fairly weil predict the processes observed
in nature, others favor a uniform visco-elastic (Maxwell)
model of the lithosphere. A third possibility is the assumption of a Maxwell layer with temperature-dependent viscosity overlying a low-viscosity fluid (e.g., Quinlan and
Beaumont 1984). Few of the models have taken into account additional local heat sources associated with special
magrnatic or volcanic events during the history of a continental margin or other flexure-influenced basins.
8.1.3 Mineral Phase Transitions
Some authors favor the phase transition from gabbro
(average density 3.0 g/cm 3 ) to the denser eclogite
(average density 3.5 g/cm 3 ) in the lower lithosphere
as a principal mechanism of subsidence. Dependent
upon depth (or pressure ) within the lithosphere, this
phase transition is accomplished at temperatures of
700 to 800°C and accelerated by the presence of
small quantities of water. Anomalous heat transfer
from the deeper mantle can raise this phase boundary
and thus reduce lithospheric thickness without
changing the mass of a given lithospheric column on
top of the isostatic compensation depth.
If, for example, the phase boundary migrates 10 km upward
and consequently a 10 km thick column of gabbro is transformed into eclogite, then the thickness of the lithosphere
is reduced by approximately 1.5 km (without taking into
account a possible additional sediment load or the simultaneous thermal expansion of the lithosphere). Because this
mechanism does not require horizontal extension of the
lithosphere, it is quoted as the main factor for the formation
of intracratonic basins (cf. Sect. 12.3). For summaries on
this topic see, e.g., Wyllie (1971), Artyushkov and Sobolev
(1983), Ismail-Zadeh et al. (1997).
Phase transitions due to deep crustal metamorphism
may explain unloaded subsidence (or uplift) up to the
order of 2 km. Such transitions take place over a very
long time period (order of 100 Ma) when small quantities of water are not available in the lithosphere to
speed the process. But it is doubtful whether phase
transitions alone can cause the strong and rapid subsidence observed in many rift settings.
393
8.1.4 Summary (Subsidence)
Subsidence (or uplift) of a sedimentary basin is
controlled by the following principal factors:
Thinning (or thickening) of the lithosphere due
to horizontal extension (or compression,
underplating).
- Upwdling of mantle material in response to
crustal thinning.
Increase (or decrease) in lithospheric density
due to (1) cooling (or heating), (2) pervasive
dike intrusion or other injections of magma,
and (3) melting of crust or magma crystallization, as well as mineral transitions (from low to
higher density or vi ce versa).
- Isostatic subsidence (or uplift) in response to
~ediment loading (or erosion) or flexuralloadmg.
8.2 Methods for Determining
Subsidence of Sedimentary Basins
8.2.1 Introduction
The subsidence history of sedimentary basins may be
described in different ways. Structural geologists and
geophysicists are primarily interested in that part of
subsidence which is controlled by crustal processes
(tectonic or thermo-tectonic subsidence), whereas
sedimentologists and stratigraphers deal with the progressive burial history of sediments (total or cumulative subsidence) and often want to interpret subsidence curves in terms of sedimentation rates, paleowater depth, and sea-level changes.
In order to determine the subsidence of a chosen
location in a sedimentary basin as weH as to discriminate between the various mechanisms mentioned
above, special techniques must be used. If the tectonically driven part of subsidence is the objective,
this technique is referred to as "backstripping" . The
purpose of backstripping is to calculate and remove
the effects of compaction, sediment loading, changing paleo-bathymetry, and sea-level variations. When
backstripping is combined with the search for information on the development of the depositional system, the term "geohistory analysis" or burial history
is commonly used.
For further discussion see, e.g., van Rinte (1978), Watts
and Steclder (1979), Gradstein et al. (1985), Guidish et al.
(1985), Regarty et al. (1988), Steckler et al. (1988).