1. What deep water, passive margin thrust belts tell us about thrust tectonics

2. Acknowledgements
The author gratefully recognizes the permission to present this data given by BHP Billiton Petroleum and our partners Mitsui, PetroSA and Namcor.

3. Deep water passive margin thrust belts
Existing paradigms of how thrust systems behave are based mostly on classic outcrop based studies (NW Scotland, Rockies, Alps)
Pre-1990s seismic and well information on thrust belts was almost exclusively on plate-convergence driven (orogenic) thrust belts
Since 1990, abundant 3D & 2D seismic data plus well information has become available on gravity-driven toethrust belts on passive margins (e.g. GoM, West Africa, Brasil)
Marine seismic gives better definition of the 3D geometry of these toethrust systems than we have of most orogenic thrust belts
What can they tell us?
Are they just small, wet thrust belts? Are they scale models of orogenic thrust belts? Are they fundamentally different? Can we take any lessons from them to the orogenic thrust belts?

4. Scope of this presentation
Only deep water thrust belts
Only passive-margin, gravity-driven belts (not offshore Colombia, Trinidad, accretionary prisms
Only shale/mud floored systems (not salt-floored systems like Angola or Atwater Fold Belt)

5. Where do thrust belts form on passive margins?

6. 1. Lying at the toe of slope
Fold and thrust belt at slope toe

7. 2. Lying at the regional decollement edge
Fold and thrust belt at regional decollement edge

8. 3. Composite Near-Shelf Linked Systems
Composite Fold and thrust belt near shelf edge

9. 3. Composite Near-Shelf Linked Systems
Composite Fold and thrust belt near shelf edge

10. 3. Composite Near-Shelf Linked Systems
Composite Fold and thrust belt near shelf edge

11. Namibian margin – a type example toe thrust system Geometries are visible on an extensive, well-imaged 3D survey
Namibia
South Africa

12. ~10km
10km

13. Contraction
Extension
2km

14. Extension overprinting contraction

15. Structural elements

16. Structural elements

17. Structural elements
Thrusts
Extensional faults
Strike slip faults

18. Main late linked systems

19. Early linked systems

20. Documented structural styles in passive margin thrust belts
Simple imbricate fan is dominant style (>95% of examples)
Ramp anticlines are rare (<5%)
Duplexes are extremely rare (<1%)

21. Dominant structural style in compressional belt
10km
Simple imbricates
Simple imbricates
Ramp anticline
Ramp anticline

22. Why is this? Single or dominating easy-slip surface
Multiple easy-slip surfaces at different stratigraphic levels
In typical deep water passive margin thrust belts;
Mechanical stratigraphy is fairly uniform (mud/sand)
No massive strong layers (platform carbonate)
Weakest link is overpressured mud
Overpressure increases downwards
Effective competence tends to increase uniformly upwards
All factors tend to give a single basal decollement
Overburden weight
Effective competence
Fluid pressure
Decollement layer
depth

23. Competence (qualitative)
GOM Shelf Well Data
16800-TD
Competence (qualitative)
1000
2000
3000
4000
5000
6000
7000
8000
9000
TVDrkb (ft)
10000
11000
12000
13000
14000
15000
16000
17000
Overpressured, weak, likely decollement layer
18000
19000
20000
Data courtesy of Glen Bowers

24. Differences in structural style
The structural style of passive margin fold belts is broadly similar to known orogenic thrust belts.
However, there ARE significant differences in structure, especially in upper slope composite linked systems.
Newly documented styles of thrust geometry and structural evolution will require expansion of the paradigm of thrust tectonics

25. Conventional thrust geometries
Plan view of thrust surface
Conventional thrust geometries
Oblique view of thrust surface

26. Conventional thrust geometries
Frontal ramp
Lateral ramp
flat
ramp
Plan view of thrust surface
Conventional thrust geometries
Oblique view of thrust surface

27. Thrust geometries in composite linked system thrust belt
Plan view of thrust surface
Thrust geometries in composite linked system thrust belt
Oblique view of thrust surface

28. Thrust geometries in composite linked system thrust belt
Ploughshare ramp
Frontal ramp
Lateral ramp
Plan view of thrust surface
Thrust geometries in composite linked system thrust belt
Oblique view of thrust surface

29. 10km
Ploughshare ramp

30. Ramp anticlines can develop over ploughshare ramps
Simple imbricates
Ramp anticlines can develop over ploughshare ramps
Plan view of thrust surface
Oblique view of thrust surface

31. Especially where the ploughshare ramp is modified
Plan view of thrust surface
Oblique view of thrust surface

32. Local ramp-flat-ramp geometry
Plan view of thrust surface
Oblique view of thrust surface

33. 10km
Simple imbricates
Ploughshare ramp
Ramp anticline

34. Mismatch of extension and contraction10 20 30 40
50km
2.9km cumulative extension
0.6km contraction on thrust fault 5
TWT (seconds)
Tracing of a well-imaged reflection seismic section (SE India continental margin); horizon correlations are robust.
Data extending further downdip confirm that no additional contractional structures exist.
80% of observed extension (2.3km) is unaccounted for.
Most likely explanation: shortening by lateral compaction. If distributed over 15km, this represents ~15% bulk compaction.

35. Mexican Ridges present day depth section 10:1 vertical exaggeration
20-25km cumulative extension
5-10km contraction on folds and thrusts
20-25km updip extension. Restoration of visible downdip contraction features (folds and thrusts) gives maximum of 5-10km shortening; 10-20km of contraction is missing.

36. Mexican ridges at 1:1 Updip extensional zone
Downdip fold belt at true scale, indicating low limb dip angles, hence very low apparent shortening by folding

37. Mexican Ridges End Miocene geometry 10:1 vertical exaggeration
5-10km of updip extension occurred BEFORE development of the downdip contractional structures

38. Mexican Ridges - where did the shortening go?
Interpretation: 10-20km (50-80%) of the extension is balanced invisibly by lateral compaction, representing a bulk shortening by layer-parallel compaction of ca. 15%.

39. Lateral compaction aka Layer Parallel Shortening: conclusions
Documentable lateral compaction occurs and some cases it is the main means of accommodation of updip extension. Bulk shortening of 15% appears to be feasible Total shortening depends on width of LPC zone .
Significant lateral compaction is predicted wherever thrusting affects unlithified, mud-rich sediments, whether these are in a passive margin or a foreland basin setting
Section balancing in future will need to incorporate this. Line-length and area balance needs to be used with caution.
Significant implications for fluid flow and fluid pressure, seismic velocity, etc.

40. distributed lateral compaction
Lateral compaction application to the intercutaneous wedge puzzle (Skuce 1992, 1996)
Outer thrusts (e.g. Edson)
Triangle zone
Outer thrusts
Observed structures in Alberta Triangle zone (schematic, after Skuce 1996)
Upper panel
Inexplicable pip
Middle wedge
Intercutaneous layer
Lower wedge
Intercutaneous wedge model favored by most publications
distributed lateral compaction
Lateral compaction model is best solution. Not favoured in 1990s due to lack of evidence of LPC. Amount of LPC required is less than proven in passive margin settings

41. Regional section, Krishna-Godvari Basin, India
Is it a duplex?

43. Could it be a duplex?
Roof thrust?
floor thrust?

44. Sliding back the “roof thrust” puts thrust heads back on shoulders

45. Sliding back the “roof thrust” puts thrust heads back on shoulders
Mass transport complex

46. This is a “pseudo duplex” produced by decapitating an imbricate fan
NB this model was discussed and rejected in the Moine Thrust!

47. Mismatch of extension and contraction