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

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

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?

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)

Where do thrust belts form on passive margins?

1. Lying at the toe of slope Fold and thrust belt at slope toe (FATB@ST)

2. Lying at the regional decollement edge Fold and thrust belt at regional decollement edge (FATB@RDE)

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

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

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

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

~10km 10km

Contraction Extension 2km

Extension overprinting contraction

Structural elements

Structural elements

Structural elements Thrusts Extensional faults Strike slip faults

Main late linked systems

Early linked systems

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%)

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

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

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

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

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

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

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

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

10km Ploughshare ramp

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

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

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

10km Simple imbricates Ploughshare ramp Ramp anticline

Mismatch of extension and contraction 10 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.

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.

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

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

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%.

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.

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

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

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

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

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

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

Mismatch of extension and contraction