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Thermal Subsidence Tool in Move
During the formation of rift basins, the continental crust is stretched and thinned. As it thins, the
asthenosphere rises to fill the space created by the thinned continental crust. Over time, the asthenosphere
cools causing subsidence, which then creates accommodation space for sedimentary infill. Tracking the extent
of subsidence through time has profound implications for source rock maturation and should be considered
when performing a restoration.
McKenzie (1978) developed a model for the effects of heat variation on subsidence as rift basins form. This is
implemented in the Thermal Subsidence tool in Move and can be used to account for the thermal uplift or
subsidence of basins in either 2D or 3D.
Using the Thermal Subsidence Tool
The Thermal Subsidence tool in Move can be used in either a forward or reverse sense to model the effects of
crustal subsidence (and provides a guide on the amount of accommodation space that will be created) or to
remove the effects of subsidence. It is composed of a number of sheets, including:
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•
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Collect Objects – where the objects involved in the restoration are added.
Thermal Subsidence – on this sheet, the subsidence diagram is displayed alongside timings of rifting.
Crustal Properties – parameters for calculations are defined on this sheet.
Before using the Thermal Subsidence tool, it is important to assign an age to each horizon in the Stratigraphy
database. The database lists all horizons in the model along with their assigned colour, age, thickness and rock
type. Any changes made to a horizon within the Stratigraphy database (i.e. colour or age), will automatically
update all horizon objects in the model. The ages defined in the database are used to populate the ages in the
Thermal Subsidence sheet for the upper stratigraphic limit (the youngest horizon in the model) and the lower
stratigraphic limit (the horizon which represents the end of the geological period being modelled). The age of
the lower stratigraphic limit is automatically chosen as the second youngest horizon in the model.
In order to calculate subsidence experienced at the time set by the stratigraphic limits, the age of rifting must
be defined. This is achieved by typing the age of rifting in the Rifting Time Table tab or by moving the red
vertical line on the Thermal Subsidence Diagram, Figure 1, to the correct age. The syn-rift duration must also
be specified, which is represented by the grey shaded area on the Thermal Subsidence Diagram.
The age of rifting and syn-rifting duration ages can be overwritten by typing in values in the Thermal
Subsidence sheet or selecting the relevant line on the Thermal Subsidence Diagram and moving it to the
required age. The colour assigned to each limit is consistent with the horizon colour in the Stratigraphy
database therefore can be quickly verified.
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Figure 1: Thermal Subsidence Diagram highlighting the rifting age and syn-rift duration. Also noted are the
Upper and Lower Stratigraphic Limits.
The solid blue line on the Thermal Subsidence Diagram represents change in subsidence over time. From the
graph, it is possible to establish how much subsidence was experienced during a specific time period. The
subsidence that is modelled between the stratigraphic limits is determined by subtracting the subsidence value
at the upper limit from the subsidence value at the lower limit.
The curve in the subsidence diagram is calculated from the initial rifting subsidence and the subsequent
thermal subsidence, both of which are defined by McKenzie (1978). Equation 1 lists the standard McKenzie
equation for initial subsidence, with the key parameters including densities and Beta value.
Si
ɑ
ρ0
ρc
ρw
α
T1
β
Tc
Initial subsidence
Thickness of lithosphere
Mantle density
Continental crustal density
Sea water density
Thermal expansion coefficient
Temperature of the asthenosphere
Beta value
Initial thickness of the continental
crust
Equation 1: Initial subsidence calculation with description of parameters.
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All parameters in the equation can be adjusted in the Crustal Properties sheet to reflect the region being
investigated. The only parameter that is not controlled on this sheet is the Beta value, which can be altered on
the Thermal Subsidence sheet. The Beta value represents the change in lithospheric thickness and can be
calculated using Equation 2 (summarised in Figure 2), if the initial thickness of the continental crust is known.
Β
Beta value
tc
Initial crustal thickness
tf
Final crustal thickness
Equation 2: Beta value calculation
Figure 2: Illustration of the Beta value calculation highlighting the change in crustal thickness before and after
extension.
By default, a uniform Beta value is applied across the complete cross-section. However, differences in crustal
thickness may mean that it is more appropriate to set a lateral Beta value variation along the cross-section,
which can be done by adding beta locations. Beta locations are added by holding down Ctrl and left clicking on
the cross-section, which adds a new row for each location to the Lateral Beta Value Variation table in the
Thermal Subsidence sheet. Each row is assigned a different colour and, if Lateral Beta Value Variation is
toggled on, this colour corresponds to a different subsidence curve, as shown in Figure 3. The Beta value for
each location can be changed by typing directly in the table.
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Figure 3: Thermal Subsidence Diagram with multiple Beta Values drawn as subsidence curves in different
colours.
The Beta values are represented graphically, as a light blue coloured box in the Section View and will display
the amount of subsidence which has been experienced during the defined time interval.
If working in 3D, a surface can be used to represent the variable Beta value if it has an attribute called Beta
value. For this to be successful, each point on the surface must contain a Beta value that corresponds to the
specific point of the model. This surface has to be collected into the 3D Thermal Subsidence toolbox when
Variable Beta is toggled on.
Combining with Decompaction
When performing a backstripping restoration in Move, it is common to have multiple tools open at the same
time in order to avoid repeating the input of parameters. If the Decompaction tool is open alongside the
Thermal Subsidence tool, the burial history of a specific point on the cross-section can be plotted alongside the
subsidence curve. To do this, click Pick on the Thermal Subsidence sheet and select a location in the Section
View. The position is displayed as a vertical dashed line, and can be moved interactively in order to view the
relationships at different points on the model.
This plot of burial history on the Thermal Subsidence Diagram allows the relationship between basement level
(as taken by the Decompaction tool) and subsidence to be analysed and information extracted regarding
deposition and erosion. If the subsidence curve plots higher than the basement, the area can be considered to
have experienced deposition, which is highlighted by the background of the plot being coloured pink, figure 4.
If the basement plots higher than subsidence, then erosion is considered to have occurred and with no
deposition (as uplift is greater than subsidence). This is typically highlighted by a number of horizons not being
present in the sedimentary succession.
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Figure 4: Burial History plotted on thermal subsidence. Burial history is plotted as a black dashed line whereas
subsidence is solid blue line. Pink shaded are indicates period of deposition.
Figure 5 highlights how plotting the burial history and subsidence through time alongside each other can be
important. In Figure 5A, the burial history position (black dashed line) is at the left of the cross-section and
subsidence is always greater than the basement position, which means that deposition always occurred in that
area. In contrast, Figure 5B shows the basement greater than subsidence between 100–150 Ma, which
suggests erosion in the area. This correlates with the fact that horizons are not present on the cross-section. If
the plots do not correspond with the stratigraphic history, then the crustal properties may need to be redefined.
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Figure 5: Example of subsidence and burial history curves from different areas of the cross-section. A) All
horizons in the sequence are present therefore the area is always in deposition. B) There is erosion between
150-100 Ma, which is supported by the fact that certain horizons are not present in the cross-section.
Summary
The Thermal Subsidence tool in Move provides the means of accurately removing the effects of temperature
changes in the asthenosphere, which can make a significant difference when considering the restoration of a
basin and cumulative subsidence experienced over the period being restored. It is especially important when
analysing depositional depths and maturation windows. The ability to combine thermal subsidence with
decompaction functionality provides a useful means of analysing the evolutionary history of an area.
References
McKenzie, D., 1978, Some Remarks on the Development of Sedimentary Basins: Earth and Planetary Science
Letters, 40, p.25-32.
If you require any more information about Thermal Subsidence in Move, then please contact us by email:
[email protected] or call: +44 (0)141 332 2681.
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