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M

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2.1 Basins due to divergence - Rifts - Subsidence analysis - Passive continental margins

Lakes Tanganyika & Victoria Gulf of Suez & Red Sea USA Atlantic Margin

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a. Stretching

Extension => deformation = stretching

Stretching <=> thinning (volume conservation)

thinitial

linitial

lfinal

thfinal

Stretching factor (β): lfinal/linitial (always >1) Thinning factor: thfinal/thinitial (always <1)

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Stretching (extensional stress)

a)  Stretching => thinning b)  Deformation = Normal faults in upper crust

and uplift of Lithos/asthenos boundary c)  N. Faults => initial subsidence d)  LAB isotherm uplift =>increased geotherm

a)  Stop of extensional stress => and faults b)  Cooling of thinned lithosphere c)  Cooling => density increase => subsidence d)  Subsidence => sediment deposition

Lithospheric Stretching

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Rifting and subsidence processes

Cooling

Stretching/ thinning

Loading

Rifting (lithosphere stretching and thinning) involves the 3 basic subsidences processes,

simultaneously or successively. 0°C

1300°C

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Geological example of a rift : Viking Graben

Moho

Thinned cont. Crust (lithosph. also affected)

Synrift sequence (initial subsidence)

Postrift sequence Rift axis

Postrift basin

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How does it work ? -> the McKenzie model = Subsidence Analysis

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Computing the value of initial subsidence

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Sediment-fill = load => extra-subsidence

Initial Subsidence

without sediment

sediment

Initial Subsidence with sediment

Extra Subsid. due to

sediment load

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Computing the subsidence due to Sediment-load

3.15

2.3

1

Sload = 0.60 hs

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subsidence in rift basins

Temporal subdivision Processes Driving mechanisms

Syn-rift (intantaneous, initial)

+ Post-rift (long-term)

Fault activity

+ Thermal recovery

Sediment load + Geodynamics

(“tectonic”)

Total subsidence in rift basins (measured in borehole)

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Postrift subsidence = lithosphere thermal recovery

Theoritical evolution of thermal subsidence for different amounts of stretching

Following rifting, the stretched mantle lithosphere gets cooler. As it gets cooler, the density increases, and subsides (principle of isostacy) = « thermal subsidence »

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Backstripping : geodynamics from total subsidence

Basement burial = total subsidence

Remove effect of sediment loading = backstripping

Computed « geodynamic » subsidence

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Backstripping : 1D modeling softwares

Données pour Backstripping Le puits Chote Name Base Ageb SLb WDb Top Aget SLt Wdt c C o type

Kimmer 3.063 145 .150 .300 2.824 140 .250 .300 2.6*103 .71 .70 0 Cal-argi Potlan 2.824 140 .150 .300 2.752 134 .250 .200 2.6 .71 .70 0 Cal-argi Kti 2.752 134 .150 .2 00 2.668 110 .150 .100 2.6 .71 .70 0 calc Uncon 2.668 110 .150 .100 2.668 64 .200 .200 Pchi 2.668 64 .200 1.200 2.546 60 .200 1.200 2.42 .39 .56 0 Sab-arg Pchm 2.546 60 .200 1.200 2.320 54 .200 .900 2.42 .39 .56 0 sab>argi Pchs 2.320 54 .200 .900 2.136 49 .200 .400 2.42 .27 .56 0 sab Eg 2.136 49 .200 .400 1.240 39 .200 .200 2.38 .51 .63 0 Argil Chapo 1.240 39 .200 .200 .988 36 .225 .100 2.38 .51 .63 0 Argil Horco .988 36 .225 .100 .831 30 .225 .50 2.42 .27 .49 0 sable Pri .831 36 .225 .050 .639 30 .200 .025 2.42 .27 .49 0 Sabl m Prs .639 30 .200 .025 .368 25 .150 .025 2.42 .27 .49 0 Sabl g Coatzi .368 25 .150 .025 0 22 .100 .010 2.38 .51 .63 0 argi

Data for backstripping Chote well (W. Gulf of Mexico)

Form

atio

n

age

Dep

th B

ase

Sea

leve

l

Bath

ymet

ry

dens

ity

Poro

sity

at

de

posi

tion

Com

pact

ion

fa

ctor

litho

logy

age

Dep

th B

ase

Sea

leve

l

Bath

ymet

ry

Top of formation Base of formation Formation physical properties

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Backstripping

Burial history for each stratigraphic interval Chote well (W. Gulf of Mexico)

Geodynamic subsidence = backstripped total subsidence computed

total subsidence = basement burial observed

Removal of

Sediment load

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b- Kinematics of rifting

Passive uplift of asthensphere

1300°C isotherm

Lithosphere stretching & thining

Extensional stress

plume

melting of lithosphere

Thermal erosion

Active upwelling of mantle plume

(Advection from lower mantle)

Regional uplift Intense volcanism (LIP’s)

1300°C isotherm

Two extreme models: Passive rifting vs Active rifting

Intermediate cases (or indetermined ?) do exists

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b- Kinematics of rifting

-  Stretching in crust above stretching in the mantle, -  Postrift subsidence centred on the rift axis -  conjugate normal faults

-  Offset of stretching in crust and in the mantle -  surface uplift above mantle thining -  postrift subsidence offset -  Low-angle extensional detachment -  lower crust/mantle denudation

Symetrical (McKenzie, 1978)

Assymetrical (Wernicke, 1981)

uplift

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Viking Graben - seismic section

- Geometry ? - synrift vs postrift ?

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Viking Graben - depth section

- Geometry ? -  synrift vs postrift ? -  approx. thinning ratio ? -  Prerift?

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c. Rift basin architecture

Tectonic-sedimentation relationships -  synrift sediment geometry -  Fault profile

Parallel reflections

Divergent reflections

Horizontal, onlap reflections

Rift basin formation

Define prerift, synrift, postrift ?

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Active rift basin = Death Valley

www.marlimillerphoto.com/

normal F.

Basement erosion

Alluvial fan in the basin

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Active rift basin - Nevada

Small radius aggrading

alluvial fans

Sabkha or lake

Uplifted & eroded footwall

Hanging-wall

wide radius prograding alluvial fans

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Sedimentary facies distribution ⇒ syn-tectonic sedimentation • Rapid facies change: proximal -> distal • Breccia along active faults - mudstone at rift axis • Growth structures/ progressive unconformities

Breccia against border fault

Sand & conglomerates channels (fluvial)

Lacustrine limestone

Benedicto , 1996

Benedicto , 1996

Fossil rift basin = Les Matelles

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Architecture of rift basin infill

Downlap + thinning away from fault => Proximal breccia -> distal fine clastics

Define prerift, synrift, postrift ?

Prosser, 1993

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Active rift basins : Gulf of Corinth

Acc = Sed

Acc > Sed

Acc < Sed

Acc << Sed

Continent.

marine

Rohais & al 2007

d’ap

rès

Flot

té &

al 2

003

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Type sequence of a rift basin

Synrift unconformity

Fluvial or littoral

Lake delta

Lake turbidites

Lacustrine blackshales (anoxic => organic matter)

Alluvial fan

fluvial

postrift unconformity

prerift

Rift initiation (sediment against

active faults)

Rift climax Accom >> Sedim

Starved basin

Filling sequence Accom < Sedim

End of rifting

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Synthesis : Model of a rift basin

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Architecture of rift basin infill

Porcupine Basin

Rm: Geometry of post-rift sequence = « steer-head »