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M
aste
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éser
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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 »