1. Application to the Iberian Margin
IMEDL BENCHMARKING EXERCISE Integrated Structural, Thermal and Isostatic Modelling of Lithosphere Deformation:
Application to the Iberian Margin
The work carried out is based upon an interpretation of the data from the Iberian margin provided for this benchmark exercise (see
Forward kinematic models have been generated based upon parameters that can be extracted from that interpretation or parameters that have to be assumed.
Stuart Egan

2. Modelling Approach
Modelling approach:
The first step to extract as much information as possible about the structure to be modelled from the available data. For example, the upper section, derived from regional seismic data, shows fault positions and extensional heave values.
Other information, such as the depth to the Moho and the pre-deformational geotherm, are either determined from other data sources (e.g. publications) or are assumed.
This information is then used as input parameters for numerical modelling of lithosphere deformation and basin formation. The lower profile shows a typical starting condition for the modelling, which illustrates a regional cross-section of undeformed lithosphere with defined values for parameters such as crustal thickness, density, temperature, etc.
Deformational parameters (e.g. fault deformation) are obtained from data.
Physical constants and variables (e.g. geothermal gradient) are assumed for the lithosphere.

3. Integrated Model - extension
Modelling approach continued:
Once the lithosphere is defined it is then possible to model its deformation via a variety of geological and geodynamic processes.
The upper profile shows the effects of extending the lithosphere due to a coupled faulting-stretching process. The upper to middle crust has accommodated extensional deformation by movement along a sequence of faults, while the lower crust and mantle lithosphere has been deformed by regionally distributed pure shear or stretching mechanism.
The model shows a basement profile with a sequence of closely spaced half grabens with relative uplift of footwall blocks, mainly caused by isostatic flexure. Extension has also caused heating of the lithosphere temperature field, which subsequently has cooled to generate a gradual subsidence. The effects of this process can be seen in the model by the red shaded post-rift sequence that blankets the underlying fault blocks.
A similar model is presented in the lower profile where lithosphere extension has been modelled by an entirely pure shear process. All of the lithosphere has been stretched with the magnitude of deformation quantified by a sequence of Beta values (i.e. one plus the strain). This approach is useful when there is little information available on fault deformation.
In addition, the lithosphere temperature field can be thermally conditioned both before and during deformation. For example, it is possible to represent the effects of phenomenon such as hot-spots.

4. Iberia Seismic Experiment - 1
This slide shows seismic line ISE-1, provided for the benchmark exercise, which begins in the east at about 11 km from the Portuguese-Spanish coastline, crossing the Galicia Interior Basin, the Galicia Bank, the deep Galicia Basin, and the Peridotite Ridge.
The x-scale indicated shows CDP locations which are 12.5 m apart, giving the section a total length of about 325 km (note 50 km scale bar).
The y-axis is in Two-Way Travel Time, extending down to 10 s.

5. ISE-1 Interpretation Galicia Bank Galicia Interior Basin Deep Galicia
Peridotite
Ridge
The section has been interpreted to show:
the seabed (yellow line)
top basement in green
basement faults in red.

6. ISE-1 Fault Deformation
In order to start building the first model the magnitude of horizontal extension (i.e. heave) has been quantified for each of the faults that have been interpreted.
Each heave value has been measured as the distance from the footwall cut-off to the point of contact between basement and the fault. No attempt has been made to re-construct the footwall tops to take into account material lost through erosion, therefore the amount of heave may be underestimated.
Total extension (i.e. the sum of all of the heave values) is km, approximating to about 50% extension (Beta = 1.5).
Total extension = km
Overall Beta = 1.5

7. Uniform Lithosphere Extension
Galicia Interior
Basin
Galicia Bank
Deep Galicia
Peridotite
Ridge
Crust
Moho
Detachment
= 15 km
Pure shear
This slide shows initial model results based upon regional uniform lithosphere extension. The upper profile shows a lithosphere-scale section, whereas the lower section shows an amplification of the near-surface.
The fault positions and heave values interpreted from the data have been reproduced in the model. The fault detachment depth (or necking depth) has been set to 15 km, below which the lower crust and mantle lithosphere have been regionally stretched over a width of 600 km, with a Beta of 1 at the margins (i.e. no deformation) to a maximum 1.36 (i.e. 36% extension) at the western edge of the profile.
The modelled time has been assumed to be 200 Ma, starting with an instantaneous rift phase in the Lower Jurassic through to present day. The basin has been assumed to fill to sea level with sediment throughout all phases of basin evolution and shows rift-related subsidence in yellow and overlying thermal subsidence in red. The elastic thickness of the lithosphere is taken to be 5 km at rifting, gradually rising to 20 km at the end of the model evolution. In response, there is marked uplift of the footwall fault blocks covered by a broad blanket of thermal subsidence.
These initial model results show the correct pattern of relative uplift and subsidence across the fault blocks (compare with seismic section). However, the model fails to reproduce the overall pattern of subsidence across the profile, with deepening to the West.
In many basins it is impossible to use the observed magnitude of fault deformation to constrain deformation throughout the lithosphere. This is partly there is an underestimation of the deformation due to faulting arising from the data acquisition and interpretation processes. However, many basins in the geological record show a mismatch between the deformation due to faulting and overall thinning of the crust that is just too great to explain as being due to a lack of identification of fault deformation. This highlights the main problem with model results based upon uniform lithosphere extension. The maximum Beta value at the edge of the model is 1.36, which leads to post extensional crustal thickness of about 25 km, whereas published material indicates that the crust should be about 3 or 4 km thick (at most!) at the western edge of the profile.
Beta = 1.36
Beta = 1
Thermal subsidence
Syn-rift
Te = 5 – 20 km
Time = 200Ma

8. Reconciliation of fault deformation and overall thinning of the crust – depth dependent stretching
“Cool” Lithosphere:
One possible scenario that can reconcile the observed low magnitude of fault deformation with high attenuation of the crust is a migrating detachment or necking depth. This depth determines the relative importance of upper crustal thinning due to faulting to lower crustal thinning due to a more regionally distributed process.
Although there is a significant amount of upper crustal thinning due to faulting shown by the ISE-1 seismic section (equivalent to an overall Beta value of 1.5), it is not sufficient to account for the overall thinning of the crust in the vicinity of the western edge of the section where the crust has a thickness of 3 or 4 km, implying Beta values of 8 or 9.
It is possible that there has been a change from a deep to shallow detachment/necking depth as the margin has evolved.
The driving mechanism for this migration of the detachment depth can be explained by an increase in geothermal gradient and change in rheology as the rifted margin evolved.
“Warm” Lithosphere:
Adapted from Braun and Beaumont, 1989

9. Galicia Bank Galicia Interior Basin Deep Galicia Peridotite Ridge
Crust
Detachment
= 15 – 4 km
Moho
Pure shear
Beta = 8
Beta = 1
This slide shows a model based upon a migrating detachment depth. The initial deformation due to faulting is based upon that interpreted from the ISE-1 seismic section and assumes a fault detachment depth of 15 km.
The necking depth is allowed to migrate to a depth of 4 km during a second phase of deformation. In response, the lower crust and mantle lithosphere is gradually thinned from no deformation in the East to a maximum Beta of 8 at the western edge of the section in order to reproduce the proposed attenuation of the crust.
All other parameters have been kept the same as the previous model based upon uniform lithosphere extension, except that the thermal subsidence has been water filled to avoid overloading and, therefore, over-deepening the basin.
Model results show both a realistic basement geometry and overall pattern of subsidence across the margin (see seismic section).
Water infill
Sediment infill
Time = 200Ma
Te = 5 – 20 km

10. Depth Data (Pinheiro et al, 1996)
Published material has been used to gain information on depth across the section. This slide shows an interpretation of seismic data by Pinheiro et al ( see red lines for location) that has some overlap with line ISE-1 (see green line).

11. Water infill Sediment infill
Time = 200Ma
Te = 5 – 20 km
Comparison of model results and data interpretation at the western part of the profile show that modelled basement depth is reasonably accurate with the tops of the fault blocks at depths of 4 – 5 km and basement depth within the half graben structures at depths of 6 – 8 km.
There are some mismatches between the model results and interpretation. In particular, the water depth is too low and the sediment thickness is too great. However, this mismatch can be removed by reducing the amount of infill of the basin. Therefore, a model is presented in which the basin has been half-filled with sediment at rifting and allowed to flexurally subside, with any remaining subsidence filled with water.
Moving across the section to the East, the Galicia Bank is too deep in the model, possibly due to over-estimating the magnitude of extension across the faults or by inaccurately reproducing the flexural strength of the lithosphere.
The eastern part of the section shows a reasonably accurate geometry and depth profile across the fault blocks, apart from the uplift structure at the edge of the model. The magnitude of uplift could be removed by either erosion, changing the flexural rigidity of the lithosphere or by the occurrence of additional extension off-section to the East.
The modelling could be advanced, and used to validate interpretations, by reproducing separate phases of rifting. This would allow simulation of stratigraphy across the margin. However, constraint on parameters such as palaeo-water depth would be needed to carry out this modelling.
This kinematic modelling approach can also be used in a “what if” situation to obtain constraint on specific parameters such as crustal thickness, detachment depth, etc.

12. Changing Flexural Rigidity
Te = 3 km
Changing
Flexural Rigidity
Te = 10 km
Galicia Interior
Basin
Galicia Bank
Deep Galicia
Peridotite
Ridge
Application of modelling to investigate the flexural rigidity of the lithosphere across the Iberian margin:
The upper profile shows a possible end-member with flexural rigidity given by an elastic thickness, Te, of 3 km, whilst keeping all other parameters the same as in the previous models. Low flexural rigidities, or low Te values, promote high amplitude, low wavelength deflections in response to the various loads imposed by extensional tectonics. In turn, this produces relatively shallow basins, with high, “spiky” uplift of the footwall blocks. Additionally, post-rift subsidence is relatively narrow and deep (see model profile) when flexural rigidity is low.
The model in the middle profile has been generated with a Te of 10 km. Raising Te (i.e. raising flexural rigidity) will tend to deepen a basin. Equally, it will broaden the uplift of the footwall blocks, but will shallow and widen the thermal subsidence.
The lower model profile has been generated with a relatively high Te of 25 km. Model results show that part of the profile is deepened while other parts are shallower. In particular, the Galicia Bank is reduced in depth and more realistic in this high flexural rigidity model. These effects are due to the complex interference of flexural uplift and subsidence generated in response to the combination of negative and positive loads produced by extending the lithosphere. For example, extensional faulting thins the crust, which, in turn, produces a negative load upon the lithosphere that responds by uplift or rebound. For high Te values that rebound will have a low amplitude so it will produce a deep basin. In contrast, positive loads (e.g. infill of accommodation space, thermal subsidence, etc) will be reduced in amplitude but distributed over a wider area.
Which of these Te values are most applicable to the Iberian margin?. The high Te profile is the most realistic in terms of overall geometry and overall subsidence across the profile.
Te = 25 km

13. Summary
24 crustal faults have been interpreted in section ISE-1, exhibiting a total horizontal extension of about 105 km (Beta = 1.5)
Kinematic modelling based upon uniform lithosphere extension constrained by fault deformation generates a realistic basement geometry across the Iberian margin, but fails to reproduce overall subsidence due to insufficient thinning of the crust.
The magnitude of fault controlled deformation has to be reconciled with the overall attenuation of the crust to generate a realistic magnitude of subsidence. Can this be explained by a gradually shallowing necking depth?
Kinematic modelling can be used to assess the values and importance of a number of deformational and physical parameters in margin evolution. For example, model results suggest that the flexural rigidity of the Iberian margin is relatively high.