Integrated 2-D and 3-D Structural, Thermal, Integrated 2-D and 3-D Structural, Thermal, Rheological and Isostatic Modelling of Rheological and Isostatic Modelling of

Lithosphere Deformation:Lithosphere Deformation:

Application to Deep Intra-Application to Deep Intra-Continental BasinsContinental Basins

Stuart Egan

Introduction:Introduction:• Modelling lithosphere extension and basin formation - Modelling lithosphere extension and basin formation - basic basic

conceptsconcepts and and initial modelsinitial models• Importance of Importance of geological and geophysical datageological and geophysical data in model in model

developmentdevelopment Processes and modelling theory:Processes and modelling theory:

• Structural processesStructural processes• Thermal effectsThermal effects - perturbation and re-equilibration - perturbation and re-equilibration• IsostasyIsostasy• Surface processes and the Surface processes and the development of basin stratigraphydevelopment of basin stratigraphy

Case studies:Case studies:• Eastern Black SeaEastern Black Sea• South Caspian basinSouth Caspian basin

ContentsContents

Pure Shear(stretching)

The McKenzie Model(Uniform Lithosphere

Extension)

The McKenzie Model(Uniform Lithosphere

Extension)

(McKenzie, 1978)

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

The second half of McKenzie's model simulates the thermal subsidence phase of basin evolution

Subsidence curve generated by Subsidence curve generated by McKenzie modelMcKenzie model

-4.000

-3.500

-3.000

-2.500

-2.000

-1.500

-1.000

-0.500

0.000

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Time (Ma)

De

pth

(k

m)

Post - rift (thermal)subsidence

Beta = 2

Syn - riftsubsidence

Rifting

BIRPS* Seismic DataBIRPS* Seismic Data

Lewis basin

Outer Isles Fault

Orkney basin

*British Institutions Reflection Profiling Syndicate

Integrated Model - parametersIntegrated Model - parameters

Integrated Model - extensionIntegrated Model - extension

CTFx = Fx - Fx-e

X

e

e e

Fx

Fx-e

CTFx

e e e e e e

Chevron (Vertical Shear) Construction

Moho

Co

Pure Shear

CTPx

CTPx = (Co-Zd).(1-1/x)

Zd

(Bx)

Temperature field perturbationsTemperature field perturbations

Flexural Isostasy

Integrated Model of Lithosphere Integrated Model of Lithosphere ExtensionExtension

• 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.

Integrated Model - shorteningIntegrated Model - shortening

Black Sea LocationBlack Sea Location

Black Sea TectonicsBlack Sea Tectonics

Uniform lithosphere extensionUniform lithosphere extension

……..followed by shortening at margins..followed by shortening at margins

Crustal thickness can be used to define a Crustal thickness can be used to define a Beta (“stretching”) profileBeta (“stretching”) profile

Uniform lithosphere extension based Uniform lithosphere extension based upon magnitude of crustal thinningupon magnitude of crustal thinning

Depth dependent stretchingDepth dependent stretching - -enhanced extension of lower crust and mantle enhanced extension of lower crust and mantle

lithospherelithosphere

Lithosphere strength distribution Lithosphere strength distribution and inferred depth of and inferred depth of necking/detachmentnecking/detachment

Necking depth

Crust

M antle

““Cool” Lithosphere:Cool” Lithosphere:

Necking depth

Crust

M antle

““Warm” Lithosphere:Warm” Lithosphere:

Adapted from Braun and Beaumont, 1989 Adapted from Braun and Beaumont, 1989

A)

A)

Beta P rofiles as Suggested byFaulting and Crusta l Thickness

Bet

aDifferences in C rusta l Thickness Produced byCoupled Faulting/P ure Shear and Pure S hearThinning of the W hole Crust

Brunet, et al, 2003

South CaspianSouth CaspianBasinBasin

MIDDLE EAST BASINMIDDLE EAST BASINEVOLUTION PROGRAMMEEVOLUTION PROGRAMME

Confidential DataConfidential Data

SW-NE Cross-SectionSW-NE Cross-SectionPart 1:

Part 2:

• 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 DataConfidential Data

Confidential DataConfidential Data

Fault Deformation –Fault Deformation –Model input parametersModel input parameters

• 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.

Uniform Lithosphere ExtensionUniform Lithosphere Extension(based upon fault heave values)(based upon fault heave values)

• Subsidence in the basin is far too low.• Bmax = 1.11!

Time = 150MaTe = 5 – 10 kmi = 2500 kg.m-3

c = 2850 kg.m-3

…….followed by compression.followed by compression

• Subsidence in the basin is still far too low.

Estimation of Moho DepthEstimation of Moho Depth

• Moho depth based upon limited information (e.g. Mangino & Priestley 1998).• max = 3.5

Confidential DataConfidential Data

Reconciliation of fault-controlled Reconciliation of fault-controlled extension and attenuation of the crustextension and attenuation of the crust

Enhances syn-rift subsidence due to thinning of the lower crust.Enhances syn-rift subsidence due to thinning of the lower crust. Enhances post-rift subsidence through increased initial heating Enhances post-rift subsidence through increased initial heating

of the lithosphere (Bmax = 3.5).of the lithosphere (Bmax = 3.5). Overall subsidence is comparable to data. However, NE of Overall subsidence is comparable to data. However, NE of

section clearly not deep enough (more data required!).section clearly not deep enough (more data required!).

SW NE

Crust

Post-rift

Syn-rift

Subsidence due tocompression

Extensional and compressional faulting

SummarySummary The first numerical models of lithosphere extension were developed The first numerical models of lithosphere extension were developed

about 25 years ago. These models were successful in showing how about 25 years ago. These models were successful in showing how crustal attenuation, thermal perturbations crustal attenuation, thermal perturbations andand locallocal isostatic isostatic compensationcompensation control control basin subsidence basin subsidence and the and the evolution of syn- and evolution of syn- and post-rift post-rift stratigraphic sequences.stratigraphic sequences.

The acquisition of The acquisition of deep seismic reflection and refraction datadeep seismic reflection and refraction data has has played a key role in helping to understand the played a key role in helping to understand the structure and rheological structure and rheological layering of the continental lithospherelayering of the continental lithosphere. This led to the development of . This led to the development of more realistic models of continental lithosphere tectonics.more realistic models of continental lithosphere tectonics.

The most up to date models of lithosphere deformation take into The most up to date models of lithosphere deformation take into account the complex interaction, in 4-dimensions, of account the complex interaction, in 4-dimensions, of structural, structural, thermal, isostatic, rheological, metamorphic and surface processesthermal, isostatic, rheological, metamorphic and surface processes to to simulate the evolution of extensional basins and thrust belt-foreland simulate the evolution of extensional basins and thrust belt-foreland basin couplets.basin couplets.

Summary - Black Sea and South Summary - Black Sea and South Caspian Sea case studies Caspian Sea case studies

It is It is not possible to reproduce basin subsidence when the not possible to reproduce basin subsidence when the magnitude of lithosphere extension is based on the amount of magnitude of lithosphere extension is based on the amount of fault controlled deformationfault controlled deformation. .

The large magnitude of Tertiary ("post-rift") subsidence observed The large magnitude of Tertiary ("post-rift") subsidence observed in the basins in the basins cannot be explained by loading and flexure caused cannot be explained by loading and flexure caused by surrounding thrust beltsby surrounding thrust belts. .

Models in which the magnitude of deformation is constrained using Models in which the magnitude of deformation is constrained using crustal thinning/thickeningcrustal thinning/thickening generate amounts of total subsidence generate amounts of total subsidence that are comparable with that observed. These models rely upon a that are comparable with that observed. These models rely upon a depth dependent extensiondepth dependent extension mechanism to reconcile the observed mechanism to reconcile the observed (small) magnitude of faulting with overall attenuation of the crust.(small) magnitude of faulting with overall attenuation of the crust.

3-D modelling of the eastern Black Sea shows that the magnitude 3-D modelling of the eastern Black Sea shows that the magnitude of total subsidence is significantly reduced when accounting for a of total subsidence is significantly reduced when accounting for a realistic bathymetry, a late stage Upper Miocene - Quaternary infillrealistic bathymetry, a late stage Upper Miocene - Quaternary infill and and regional flexure.regional flexure. The observed subsidence can only be The observed subsidence can only be accounted for by the accounted for by the extension of thickened crustextension of thickened crust or or additional additional subsidence mechanisms (?)subsidence mechanisms (?)..