Thermal Subsidence Tool in Move

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Doris Adams
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1 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: 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.

Figure 1: Thermal Subsidence Diagram highlighting the rifting age and syn-rift duration. Also noted are the Upper and Lower Stratigraphic Limits.

From the graph, it is possible to establish how much subsidence was experienced during a specific time period.

2 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 Initial subsidence ɑ Thickness of lithosphere ρ 0 Mantle density ρ c Continental crustal density ρ w Sea water density α Thermal expansion coefficient T 1 Temperature of the asthenosphere β Beta value T c Initial thickness of the continental crust Equation 1: Initial subsidence calculation with description of parameters.

3 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 t c Initial crustal thickness t f 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.

4 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.

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.

5 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 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.

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.

6 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 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 If you require any more information about Thermal Subsidence in Move, then please contact us by enquiries@mve.com or call: +44 (0)

Backstripping sediments progressively through geological time.

Backstripping sediments progressively through geological time. The purpose of backstripping is to use the stratigraphic record to quantitatively estimate the depth that basement would be in the absence