1. Application to Deep Intra-Continental Basins
Integrated 2-D and 3-D Structural, Thermal, Rheological and Isostatic Modelling of Lithosphere Deformation:
Application to Deep Intra-Continental Basins
Stuart Egan

2. Contents Introduction:
Modelling lithosphere extension and basin formation - basic concepts and initial models
Importance of geological and geophysical data in model development
Processes and modelling theory:
Structural processes
Thermal effects - perturbation and re-equilibration
Isostasy
Surface processes and the development of basin stratigraphy
Case studies:
Eastern Black Sea
South Caspian basin

3. The McKenzie Model (Uniform Lithosphere Extension) Pure Shear
(stretching)
The McKenzie Model
(Uniform Lithosphere
Extension)
The model quantifies subsidence occurring due to crustal thinning and uplift caused by the raising of hotter material at depth nearer to the surface, along with the associated Airy isostatic compensation
Introduction: Modelling lithosphere extension and basin formation - basic concepts and initial models
One of the first numerical models to link deformation of the continental lithosphere to extensional basin formation was published in This model is generally referred to as the McKenzie or uniform lithosphere extension model.
This model assumes that the whole of the lithosphere deforms by a pure shear or stretching mechanism, which is quantified by the Beta factor, equivalent to one plus the strain.
In response to extension, the model quantifies thinning of crust and mantle lithosphere, thermal effects, infill of accommodation space and Airy isostatic compensation.
It also quantifies the post-rift thermal subsidence phase of basin evolution, primarily driven by the re-equilibration of the temperature field.
The second half of McKenzie's model simulates the thermal subsidence phase of basin evolution
(McKenzie, 1978)

4. Subsidence curve generated by McKenzie model
Results from the uniform lithosphere extension model are typically presented in the form of a 1-D subsidence curve like this example here. Although this a theoretical curve, it clearly shows the two phases of subsidence that characterise an extensional sedimentary basin:
the rift phase caused by the combined effects of crustal thinning, rift-induced heating, isostatic compensation and infill of accommodation space.
the post-rift or thermal subsidence phase primarily driven by the gradual cooling of the lithosphere temperature field back to an equilibrated state.
The McKenzie model, along with various derivatives, have been applied to many extensional basins in the geological record and can successfully explain the subsidence histories of those basins that have experienced a relatively uncomplicated evolution of rifting followed by gradual post-rift/thermal subsidence.

5. BIRPS* Seismic Data
Lewis basin
Outer Isles Fault
Orkney basin
Introduction: Importance of geological and geophysical data in model development
During the mid to late 1980s, one of the main catalysts for the development and the increased sophistication of lithosphere-scale modelling was the acquisition of deep seismic data. Several organisations were important in making this data available to the Earth Science community. For example, this slide shows two examples of deep seismic reflection data acquisitioned by the BIRPS group from the UK.
The SWAT line is located between SE Ireland and SW England, and shows major crustal and top mantle lithosphere reflectors. In particular, this section reveals the important role played by major crustal faults during basin formation, whereby the North Celtic Sea basin has been produced by extension along a crustal fault structure that terminates at mid to lower crustal depths.
Similar data to the North of Scotland, the MOIST seismic reflection line, shows the relationship between major crustal faults and the Lewis and Orkney basins.
These data, along with the study of other areas such as lithosphere rheology, have been very important in revealing the large scale layering and structure of the continental lithosphere within various tectonic regimes. These developments have been used to further develop models which, in turn, have become more accurate with regards to forward and reverse modelling the evolution of basin structures.
*British Institutions Reflection Profiling Syndicate

6. Integrated Model - parameters
Processes and modelling theory
This slide shows a typical starting condition for the kinematic modelling of lithosphere extension and basin formation.
It shows a regional cross-section of undeformed lithosphere.
The crustal component of the lithosphere is assumed to be 35 km thick with a density of 2800 kg.m-3, while the mantle lithosphere is 90 km thick with a density of 3300 kg.m-3.
The modelled lithosphere is thermally conditioned with a geotherm, which has a surface temperature of 00C and a temperature at the lithosphere-asthenosphere boundary of about 13000C.
All of the above parameters can be varied within the modelling environment.

7. Integrated Model - extension
Flexural Isostasy
Temperature field perturbations
Once the lithosphere is defined its deformation can be modelled using current understanding of geological and geodynamic processes. This slide illustrates many of these processes in the context of extensional tectonics.
The model approach is compatible with seismic data in that the crust has accommodated extensional deformation by movement along a sequence of faults. There are several methods available for modelling the faulting process, one of the most popular is the vertical shear or Chevron construction. It assumes that each vertical section or thickness of hanging wall is displaced laterally by the same amount of extension or heave (defined by the letter e on the diagram). It is also assumed that any unsupported part of the hanging wall following extension deforms/collapses directly downwards onto the underlying rigid fault and footwall.
Although the deformation caused by fault movement plays an important role during lithosphere deformation, seismic data and rheological studies indicate that the lithosphere, below the depths of about km, is mostly too ductile to support major fault structures. Instead the lower crust and parts of the mantle are thought to deform by movement along a dense network of shear zones that split and merge over relatively short distances. The mechanism that can be used to model this process is pure shear (i.e. stretching under extension and squashing under compression).
Lithosphere deformation also causes perturbation of the temperature field (i.e. heating or cooling). For example, the slide shows a model profile of the lithosphere temperature field following extension and graben formation (red shading indicates heating). The extension has caused relative heating at depth due to raising of the asthenosphere. Similarly, faulting causes more localised disturbances to the temperature field. This heating leads to thermal expansion and uplift at rifting followed by gradual subsidence as the temperature field re-equilibrates.
Another geodynamic process to consider when studying deformation at lithosphere scale is isostasy. Changes in crustal thickness, thermal perturbations, infill of accommodation space all impose loads upon the lithosphere, which have to pass through an isostatic filter to give a resultant surface topography and underlying crustal structure. For example, the slide shows the flexural isostatic response of the lithosphere to the negative loading induced by crustal thinning due to faulting. The top profile shows crustal thinning (i.e. the load) generated movement along a crustal fault. This represents a negative load upon the lithosphere, which induces isostatic rebound as shown by the next profile. If the top two profiles are added together, then the resultant profile consists of the isostatically compensated basin which shows not only uplift and shallowing within the basin, but also uplift of the basin flanks.
Chevron (Vertical Shear) Construction

8. Integrated Model of Lithosphere Extension
All of these different processes are combined into a single modelling environment to generate an integrated model of lithosphere extension that shows:
A basement profile with a sequence of closely spaced half grabens with relative uplift of the footwall.
Extension has also caused heating of the lithosphere temperature field, which subsequently has cooled to generate a gradual subsidence. The effects of this can be seen in the model by the red shaded post-rift stratigraphic sequence that blankets the underlying fault blocks and syn-rift sequences.
See comments on slide for further information.
Basins are generated by extension along a sequence of closely spaced faults, which flatten within the crust.
Pure shear/stretching is assumed to deform the lithosphere below the faults and is distributed regionally.
The large subsidence within the basin is partly attributable to the effects of sediment infill and isostatic loading.
The Footwall and Moho are raised beneath the basin mostly as an isostatic response to crustal thinning.
The stratigraphy in the basin shows post-rift thermal subsidence overlying syn-rift megasequences.

9. Integrated Model - shortening
This modelling approach is also sufficiently flexible to simulate the effects of lithosphere shortening. Movement along a sequence of crustal reverse faults has been modelled to generate a piggy-back style of thrusting. Lithosphere flexure, mainly in response to loading caused by the thrust sheets (i.e. crustal thickening), has been modelled to simulate foreland basin formation.

10. Case study: Eastern Black Sea
Black Sea Location
Case study: Eastern Black Sea
The kinematic modelling of lithosphere extension described above is being used, in combination with basin analysis techniques and fieldwork, to investigate the evolution of the Black Sea and South Caspian basins. These intracontinental basins are very deep and are, as yet, poorly understood in terms of the mechanisms that have controlled their subsidence and uplift history.
The Black Sea is about 1200 km E-W by about 600km N-S. It is surrounded by Turkey to the South, Bulgaria, Romania and Ukraine to the West, and Georgia and Russia to the East.

11. Black Sea Tectonics Northwestern Shelf Crimean Peninsula Moesian
The basin overall is divided into two sub-basins, the western and eastern Black Sea basins, which are separated by the mid-Black Sea high.
It forms a large region of subsidence surrounded by Alpine-Himalayan mountain belts, including the Pontides, Great Caucasus, Balkanides and Crimean mountain-belts. In addition, this basin has undergone about 12 km of subsidence, most of which occurred during the Tertiary whilst surrounding regions were experiencing compression and uplift.
Crimean
Peninsula
Dolna -
Kamchia
depression
Moesian
Platform
Caucasus
Western
Black Sea Basin
Eastern Black Sea
Basin
Balkanides
Mid - Black Sea High
W. Pontides
E. Pontides

12. Current understanding of the tectonic evolution of the Black Sea is summarised by the two schematic cross-sections in this slide.
The very origins of the western part of the basin go back to a phase of back arc extension during the Mid to Late Cretaceous when northward subduction was occurring beneath modern day Turkey representing the closure of Tethys ocean.
Extension continued throughout the Cretaceous and it has been suggested that this lead to continental separation and the formation of oceanic crust (see pink shading).
During the Tertiary period (see lower cross-section) there was a switch in tectonic regime to one of general compression with the formation of thrust belts around the Black Sea margins. During this time the Black Sea experienced most of its subsidence.

13. This section of the presentation will focus on the eastern Black Sea with a regional cross-section that covers a distance of just over 500 km from onshore Turkey, near the city Samsun, into the centre of the eastern Black Sea and finally onto the Russian shelf in the North-East of the region.
The cross-section has been constructed from a set of regional seismic lines acquisitioned by BP and the Turkish oil company TPAO. Superficially, the section is dominated by major extensional faults. For example, the Archangelsky and Shatsky ridges represent regionally uplifted footwall blocks to extensional faults that form the southern and northern continental slope regions.
Thickness variations across the oldest horizons in the section suggest that rifting started some time in the very Late Cretaceous to Palaeocene.
The magnitude of extension associated with this rift phase was not very great and amounts to about 60 km of heave on the faults observed in the section (i.e. 13% extension).
The onshore and shelf regions of the section become more complicated with compressional deformation that was most intense in the late Eocene to Oligocene and represents the formation of the Pontides in the South and the Great Caucasus mountain belt to the NE.
The section also shows the typical structural and stratigraphic style in the central part of the basin, which is characterised by a large thickness of flat-lying Base Tertiary to Quaternary sedimentary sequences. Occasionally these layers are disrupted by extensional faults, but these are minor compared to the structures observable towards the northern and southern margins of the basin.

14. Uniform lithosphere extension
This slide shows a model representation of the section in the previous slide.
The upper profile shows crust and mantle lithosphere structure down to a depth of 70 km.
The lower profile shows the top 15 km of the crust to show the basin in more detail.
The extensional faults observed in the cross-section, along with the movement along them, have been reproduced in the model. Deformation by faulting is balanced to regionally distributed stretching within the lower crust and mantle lithosphere. In addition, accommodation space is filled to sea level throughout basin evolution, which has been simulated for a period of 65 Ma to present day.
Model results show clearly that it is not possible to reproduce Black Sea subsidence with extensional deformation uniformly distributed throughout the lithosphere. Maximum observed subsidence within the basin is over 12 km, whereas the model shows a maximum basin depth of approximately 8 km.

15. …..followed by shortening at margins
Models that include compressional deformation at the edge of the basin show very little deepening in the central region.

16. Crustal thickness can be used to define a Beta (“stretching”) profile
A potential flaw with the modelling approach used to generate the previous models is that the deformation, and therefore subsidence, predicted by the modelling is constrained by the magnitude of fault-controlled deformation determined from the seismic data. It is very likely that this data does not show all of the deformation that has occurred within the eastern Black Sea region (i.e. the observed faulting is not a true indication of deformation throughout the whole of the lithosphere).
In order to counter this potential problem a modelling approach has been used in which the magnitude of deformation has been calculated using crustal thickness rather than basement faulting.
The cross-section presented in this slide has been adapted from the regional seismic line presented earlier to show an estimation of the depth to basement. Moho topography is also displayed and is based upon commercial gravity data and published material.
The magnitude of crustal thinning or thickening relative to a regional average has been determined and used calculate a sequence of Beta values across the basin.

17. Uniform lithosphere extension based upon magnitude of crustal thinning
The calculated Beta distribution shown in the slide has been used to constrain the magnitude of extension and compression evident in the model.
Model results show an overall basin depth that is almost comparable with that across the eastern Black Sea. Maximum subsidence predicted by the model is about 10 km, which is near to that observed in the centre of the basin.

18. Depth dependent stretching - enhanced extension of lower crust and mantle lithosphere
The model profile presented in this slide reconciles the magnitude of observed faulting with the overall thinning of the crust by assuming depth dependent stretching.
The faulting configuration is based on the regional seismic line presented earlier, but extension has been increased in the lower crust and mantle lithosphere to reproduce a realistic attenuation of the crust.
The overall magnitude of subsidence has been increased to a maximum of 12 km. Additionally, the pattern of subsidence showed in this model is more similar to that exhibited by the real data with a very thin syn-rift phase and a large thickness of post-rift deposition.
The next slide indicates possible ways as to how this scenario be explained.

19. Lithosphere strength distribution and inferred depth of necking/detachment
“Cool” Lithosphere:
The division between upper crustal and lower lithosphere deformation, the necking or detachment depth, can be predicted from modelling the rheological structure of the lithosphere.
Lithospheric strength can be represented by a yield stress envelope, which defines the tensile or compressive stresses that the lithosphere can endure before failure.
The upper schematic diagram shows a typical yield stress envelope for lithosphere with a quartz-feldspathic crust and olivine mantle with a relatively low geothermal gradient. Under these relatively cool conditions the brittle upper crustal has a relatively high thickness relative to the low strength ductile regions in the lower crust, which generates a relatively deep detachment depth. In other words, the region of fault controlled upper crustal deformation will be much thicker than the underlying thinning of the ductile lower crust.
In contrast, relatively hot lithosphere (e.g. lithosphere that has experienced rifting) will have a much thinner brittle layer and therefore a shallower detachment or necking depth (see lower diagram). In this situation thinning of the lower crust will have more importance than the deformation by faulting in the upper crust.
“Warm” Lithosphere:
Adapted from Braun and Beaumont, 1989

20. The model results presented in this slide investigate the implications of a migrating detachment or necking depth:
The initial or early stage of rifting is represented by the upper left basin profile and is characterised by balanced brittle faulting and lower crustal pure-shear with a constant detachment depth.
With progression of extension and temperature elevation, rheological modelling suggests that the detachment depth (or necking depth) will be subject to vertical migration allowing penetrative ductile shear to become more prominent and shallower with continued extension.
Extensional beta increases from 1.13 during the initial rift phase to 1.5 during the latter rift stage. The enhanced thinning of the crust at its base reproduces enhanced lower lithosphere extension, especially beneath the flat central basin where seismically visible faulting appears absent.
The mechanism, when combined with realistic infill of the basin, accounts for the thin syn-rift and thick post-rift sequences and overall architecture of the basin.

21. The modelling presented so far has been two-dimensional
The modelling presented so far has been two-dimensional. One of the weaknesses with 2-D modelling is that it tends to under- or over-estimate subsidence and uplift because it assumes that deformation and loading is constant out of the plane of section.
A 3-D modelling approach has been developed that concentrates upon processes such as regional flexure.
Data constraints for the 3-D modelling have been provided by regional section described earlier in the talk (section A in the slide) and three additional cross-sections (B, C and D in slide).
Section B has the most westerly location, and shows structure and stratigraphy from offshore Turkey, near to the city of Sinop, to the central region of the basin some 200 km to the NNE. The Archangelsky and Andrusov ridges are apparent on this section and represent a structure that divides the Black Sea into western and eastern sub-basins. This section suggests that this major structural feature has primarily resulted from upward flexure of the footwall blocks of a sequence of extensional faults that mostly dip towards the North.
Section C is located very near to the regional section described at the beginning of the case study, starting from near to the port city of Samsun on the Turkish coast. Similar to section B, the Archangelsky ridge forms an uplifted footwall block to a northerly dipping extensional fault.
Section D has the most easterly location. This cross-section is dominated by a steeply dipping extensional fault across which there is considerable thickening of Tertiary sequences into the central part of the basin.

22. The cross-sections described in the previous slide have been used to provide 'seed-lines' for a 3-D model that covers an area of km2 of the Turkish and central regions of the eastern Black Sea.
The modelling approach used is a true 3-D representation of the processes included in the 2-D modelling, as well as accounting for bathymetry and periods of high/low deposition.
The model results presented in this slide show basement and Moho surfaces extending from the Turkish margin into the central part of the eastern basin
In addition, cross-sections showing basin geometry and stratigraphy are provided. In the left hand column are sections B, C and D (i.e. the real data), and on the right are cross-sections extracted from the 3-D model at the exact positions of the real data.
The model shows that the basement, as well as relative proportions of syn- and post-rift sequences, are comparable with those observed across the eastern Black Sea.
It was necessary to assume a pre-rift crustal thickness of 40 km in order to achieve this close match between observed data and model results. A thickened crust was necessary in order to generate a large enough initial accommodation space to later house the great thicknesses of post-rift sediments.

23. Case study: South Caspian Basin
In terms of tectonic setting, the South Caspian basin appears to be very similar to the neighbouring Black Sea in that it is a Mesozoic basin surrounded by thrust belts.
The Great Caucausus mountain belt is located to the North-West and continues across the basin in the form of the Apsheron Sill.
To the south there is the Alborz orogenic belt in Iran.
Despite the apparent similarities in regional structure, there are enormous differences between the two basins. For example, the South Caspian is a smaller structure, but it is much deeper.
Brunet, et al, 2003
MIDDLE EAST BASIN EVOLUTION PROGRAMME

24. Confidential Data
BP have provided depth maps for a number of stratigraphic horizons covering about a third of South Caspian basin to the East of Azerbaijan.
The raw data that has been used to generate these maps consists of 14 regional seismic lines that cover down to 20s TWT.
The basement, which is assumed to be mid to late Jurassic in age, shows a gradual deepening from about 14 km in South to over 24 km in the North of the area, so overall the South Caspian twice a deep as the Black Sea basin and is one of the deepest basins in the world.

25. SW-NE Cross-Section Confidential Data Confidential Data Part 1:
Sections produced from interpretation of seismic data by BP geoscientists.
Note depth is in TWT
There is an overlap and slight offset where sections intersect (see next slide)
Confidential Data
Interpretations of two seismic lines which run from the SW to NE part of the study area show some similarities to the Black Sea.
There is evidence of initial rifting, but the magnitude of fault controlled deformation does not appear to be very great.
Later compression is most intense to the NE in the region of the Apsheron sill where it affects horizons from top Jurassic to top Surakhany (Upper Pliocene) and is undoubtedly basement controlled.
The compressional deformation continues to the SW but is more thin skinned and particularly affects Base Maikop (Late Oligocene) and younger units.
Part 2:
Confidential Data

26. Fault Deformation – Model input parameters
The next sequence of slides show some initial modelling results of overall basin subsidence based on the sections in the previous slide.
An attempt has been made to quantify the magnitude of deformation by faulting. The fault heave values measured are very approximate as they are difficult to estimate from data. However, it is clear that extension due to faulting is very low.
Fault heave values are very approximate as they are difficult to estimate from data.
However, extension due to faulting is very low
Also, difficult to quantify the compressional deformation, which intensifies to NE.

27. Uniform Lithosphere Extension (based upon fault heave values)
This slide shows a model representation of the section, assuming evolution by uniform lithosphere extension by a coupled faulting-pure shear process.
The overall dimensions of the model are based upon the NE-SW section in the previous slide, whereby major extensional faults observed in the cross-section, along with the movement along them, have been reproduced in the model.
Deformation by faulting is balanced to a regionally distributed stretching within the lower crust and mantle lithosphere to represent uniform lithosphere extension.
In addition, accommodation space is filled to sea level throughout basin evolution.
The flexural isostatic response of the lithosphere is constrained by an effective elastic thickness of 5 km during rifting and 10 km during thermal subsidence, which has been simulated for a period of 150 Ma.
Like the Black Sea, model results show clearly that it is not possible to reproduce subsidence in the South Caspian basin with extensional deformation uniformly distributed throughout the lithosphere. Maximum observed subsidence within the basin is over 20 km, whereas the model shows a maximum depth of approximately 3 or 4 km.
Time = 150Ma
Te = 5 – 10 km
ri = 2500 kg.m-3
rc = 2850 kg.m-3
Subsidence in the basin is far too low.
Bmax = 1.11!

28. ….followed by compression
Additionally, overall subsidence in the basin is not being driven by the compression to the NE, which has been modelled simplistically by loading the lithosphere just off section.
The compression deepens the basin in distal region but has little affect upon overall subsidence to the SW
Subsidence in the basin is still far too low.

29. Estimation of Moho Depth
In order to begin to understand what is driving the overall subsidence in the basin it is necessary to consider the overall attenuation of the crust.
The section in this slide shows an estimation of Moho depth beneath the section. It is now possible for the observed deformation by faulting to be combined with the observed thinning of the crust to produce a more realistic model in terms of overall subsidence.
Confidential Data
Moho depth based upon limited information (e.g. Mangino & Priestley 1998).
Bmax = 3.5

30. compressional faulting
Reconciliation of fault-controlled extension and attenuation of the crust SW NE
Crust
Post-rift
Syn-rift
Subsidence due to
compression
Extensional and
compressional faulting
This best fit model (to date) shows an enhanced syn-rift subsidence phase due to a combination of thinning of the upper crust by faulting and thinning of the lower crust by stretching.
In addition, there is a thick post-rift subsidence phase through increased initial heating of the lithosphere.
The resultant model shows an overall subsidence that is comparable to data as far as the SW of the study area is concerned. However, the NE of the section is not deep enough, suggesting subsidence mechanisms are not being modelled properly in this region.
Extended data coverage to the NE will enable more accurate modelling of the loading effects due to the compressional deformation that has formed the Apsheron sill.
Enhances syn-rift subsidence due to thinning of the lower crust.
Enhances post-rift subsidence through increased initial heating of the lithosphere (Bmax = 3.5).
Overall subsidence is comparable to data. However, NE of section clearly not deep enough (more data required!).

31. Summary
The first numerical models of lithosphere extension were developed about 25 years ago. These models were successful in showing how crustal attenuation, thermal perturbations and local isostatic compensation control basin subsidence and the evolution of syn- and post-rift stratigraphic sequences.
The acquisition of deep seismic reflection and refraction data has played a key role in helping to understand the structure and rheological layering of the continental lithosphere. This led to the development of more realistic models of continental lithosphere tectonics.
The most up to date models of lithosphere deformation take into account the complex interaction, in 4-dimensions, of structural, thermal, isostatic, rheological, metamorphic and surface processes to simulate the evolution of extensional basins and thrust belt-foreland basin couplets.

32. Summary - Black Sea and South Caspian Sea case studies
It is not possible to reproduce basin subsidence when the magnitude of lithosphere extension is based on the amount of fault controlled deformation.
The large magnitude of Tertiary ("post-rift") subsidence observed in the basins cannot be explained by loading and flexure caused by surrounding thrust belts.
Models in which the magnitude of deformation is constrained using crustal thinning/thickening generate amounts of total subsidence that are comparable with that observed. These models rely upon a depth dependent extension mechanism to reconcile the observed (small) magnitude of faulting with overall attenuation of the crust.
3-D modelling of the eastern Black Sea shows that the magnitude of total subsidence is significantly reduced when accounting for a realistic bathymetry, a late stage Upper Miocene - Quaternary infill and regional flexure. The observed subsidence can only be accounted for by the extension of thickened crust or additional subsidence mechanisms (?).
I'll finish with two conclusion slides, and as I’ve emphasised these points whilst going through the talk. I'll leave you to read through them.